CA1235459A - Load balancing for cellular radio telephone systems - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Balancing of loading of cells in a cellular mobile radio telephone system is performed by periodically determining the channel utilization of each cell, computing a representative voice channel occupancy level, and attempting to hand-off calls from cells with higher voice channels occupancy levels to adjacent cells with lower voice channel occupancy levels. Voice channel occupancy levels of cells are measured and compared with threshold values, and the results of the comparisons are used to direct cells to enter predetermined states. In one state, complete cell blockage is prevented by directing cells to hand-off calls to adjacent cells. In another state, voice channels are preserved for incoming hand-offs by directing the cell to deny access to mobile transceivers initiating new calls. Cells may assume a combined stage wherein both of these functions are performed simultaneously. Cells are selected as hand-off candidates for hand-offs initiated to more evenly distribute loading throughout the cellular system in accordance with cell state (i.e., voice channel occupancy level) and measured signal strength at the cells of the calls attempted to be handed off.

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Description

~;~35~S~

LOAD VALANCING FOR CELLULAR
MOBILE RADIOTELEPHONE
SYSTEMS

Field of the Invention The present invention is related to cellular radio communication systems, and more particularly, to techniques for using radio frequency spectrum and hardware resources in a cellular mobile radiotelephone communications systems more efficiently and in a manner which improves system reliability.

BACKGROUND OF TIRE INVENTION
. . .

Cellular mobile radiotelephone systems, a recently-devel~ped technology useful for public motile telephone service, can provide service quality and features comparable to those provided by the Public Switched Telephone Network (STEINWAY) for a large number of mobile users Cellular radiotelephone systems are capable of using the same radio frequency (of) communications channel for communications by multiple users (geographically separated from one another) independently and on a non-interfering basis, and thus use the portion of the radio frequency spectrum available for radiotelephone communications far more efficiently I than ever before . Ever the ever-increasing demand for radiotelephone communications service make techniques permitting still more efficient use of of spectrum and equipment resources extremely valuable .

I.

isle Before cellular radiotelephone systems came into wide use, single high-power repeater stations were typically used to provide mobile-to-mobile and mobile-to-land communications. In such systems, a single repeater station is often located in a centralized position of high elevation within a predetermined geographical area to be serviced. All communications within the service area are routed directly through the repeater. The repeater typically provides full duplex communications simultaneously and independently over a plurality of fixed-frequency communications channels. Mobile stations within the repeater's service area can access the repeater to communicate with any other mobile station within the service area or to establish communications with a land line (i.e., conventional telephone line) coupled to the repeater. The repeater also is typically capable of calling a mobile station and initiating communications with the called station in response to control by a central dispatch Operator or in response to signals received by the repeater from a land line. ~ixed-frequency channels are allocated to calls using conventional trucking techniques such as those described in U.S. Patent No. 4,360,927 to Bowmen et at ~1982) and U.S. Patent No. 4,347,625 to Williams (1982). This type of communications system is extremely useful due to its flexibility and relatively low cost.
While systems of the type described above are very useful for servicing areas in which there are relatively few mobile stations or in which the urea' demands are relatively low (ego due to the nature of the communications briny conducted), system performance may be adversely affected as call so traffic (volume) increases. The number of conversations capable of being sustained simultaneously with a single repeater-type system is equal to the number of repeater channels available or communications. Channel blocking can occur during periods of peak usage even using a repeater provided with a relatively large number of repeater channels ego., 20 or more), degrading system reliability and causing great frustration to users who attempt to initiate communications only to find that all channels are already in use. Allocation of additional repeater channels can reduce blocking potential but is sometimes not possible in congested metropolitan areas where many other services compete for available spectrum space.
Cellular mobile radiotelephone communications systems avoid toe limitation of a one-to-one correspondence between the number of available channels and the number of independent conversations capable of being unstained simultaneously by reusing channels already in use in one portion of the service area on a non-interfering basis in another portion of the service area. Cellular systems divide a large service area into a number of smaller discrete geographical areas called cells zones) each typically ranging in size from about to about 20 kilometers in diameter.
Each cell is contiguous with adjacent cells to provide continuous coverage throughout the service area. Each of the cells is served by a base station installation ("base station including plural low-power transceivers which ore capable of operating independently on different radio frequency (RF) channels. Each of the cell site base station installations is thus capable of participating in ~L~3~5~

communication simultaneously with plural mobile radio transceivers operating within the associated cell.
The cell site base stations also communicate via a data link (and voice trunks) with a central control station called a Mobile Telephone Exchange (MIX) the function of which is to selectively connect cell voice trunks to other cell voice trunks and/or to land lines and to coordinate activities of the cells.
The coverage range and capacity of a cellular lo system is potentially unlimited. Additional cells may be added to increase the size of the area served by the system.
Moreover, existing cells can be split or sectored (e.g. by providing additional cell site transceivers coupled to omnidirectional or directional antennas) to accommodate additional communications traffic within particular cells. The frequency reuse concept (whereby the same set of frequencies can be used independently in non-contiguous cells) as well as the flexibility of accommodating increased traffic demands through cell splitting or cell sectoring has made cellular mobile radio systems the radiotelephone system of choice in North America as well as in Europe and Japan.
The following references provide additional general background information concerning cellular radio systems and techniques:
U.S. Patent No. 4,398,063 to Hess et at (1983);
U.S. Patent No. 4,308,429 to Kay et at (1981);
U.S. Patent No. 4,242,538 to It et at (1980)i U.S. Patent No. 4,127,744 to Yoshikawd et at ~1978)i U.S. Patent No. 4,125,808 to Graham (1978)i U.S. Patent No. 3,663,762 to Joel 9 Jr. (1972);
Broody et at, application of Digital Switching in d Cellular Mobile Radio System"
International Switching Symposium May 7-11, 1984, Florence, Italy);

ISLES

Ma et at, "DMS-MTX Turnkey System For Cellular Mobile Radio Application", Institute of Electrical and Electronic Engineers 1984 Vehicular Technology Conference May 21-~3, 1984, Pittsburgh, Pennsylvania);
Panda et at, "Performance Modeling for An Automated Public Mobile Telephone System"
International communications Conference (June 13-17, 1982, Philadelphia); and SWISS ETA Interim Standard Cellular Mobile Station-Land Station Compatibility Specification (July 1981).
In a cellular mobile telephone system, a central lo mobile telephone exchange (MIX) supervises the cell site base stations to allow calls in progress to continue without interruption when mobile stations move from one cell to another. When a mobile transceiver participating in a call is about to exit a cell, the MIX automatically hand-off or transfer the call to a free channel in an adjacent cell into which the mobile transceiver has moved (mobile transceivers can be in two or more cells simultaneously due to the overlap between cells). Cell site base stations typically measure the received signal strength (RSSI~ for each ongoing call and request the MIX to hand-off a call when the RSSI of the call falls below a predetermined threshold.
The cell to which the call is handed off may be selected in accordance with a voting process initiated by the MIX at the time the MIX is notified the RSSI in the transfer or cell (the cell from which a hand of is necessary) has dropped below the predetermined threshold. The MIX may at that time direct the cell site base stations of the cells adjacent to the cell serving the call to monitor the strength of the signal transmitted by the mobile transceiver on the channel in use (monitoring receivers may be provided at each cell site solely for this monitoring purpose, or receivers of unused cell site transceivers may be used). The MIX typically receives the RSSI information back from the adjacent cells and orders cells issue by signal strength intensity. The MIX may select the cell in which the highest signal level was received and, if d service channel is available in that cell, the MIX may direct the mobile transceiver through the first cell to begin operating on that available service channel. If a service channel is nut available in the cell determined do having the highest receive signal level, other cells with received signal levels above a predetermined minimum threshold may be checked for available service channels until a new service channel is located and lo allocated to the call. If no service channel is available after all possible cells have been checked, the call cannot be handed-ofF and the mobile transceiver may be signaled accordingly (in which case the mobile transceiver operator must either quickly complete his call or halt his vehicle before it lo exits the cell it is presently in).
A description of this inter cell hand-off technique is found in, for example, U.S. Patent No. 3,898,390 to Wells et at (l975). A variation on the Wells et at hand-off technique is disclosed in U.S. Patent No. 4,475,010 to Honshu et at (l984), which describes a hand-off technique wherein cell sites themselves perform hand oils in order to conserve MIX
processing resources. When a signal from a specific mobile unit associated with a controlling cell site in the Honshu et at system drops below a prespecified threshold, the controlling cell site itself selects the group of nearby cell sites which are to measure the signal strength of transmission on the radio channel currently being used by the mobile unit. The MIX
simply passes signal strength measurement request messages from the requesting cell site to each cell site in a list of nearby cell site addresses identified by the controlling cell site.
The controlling cell site uses the signal strength reports and antenna/channel availability also provided by the addressed cell sites) to generate a list of cell cites which are candidates for a hand-off. The controlling cell site further detects when the signal from a specific mobile unit drops below a second prespecified threshold, and selects a more limited list of nearby cell sites which are to measure received signal strength if this occurs.
As mentioned above, it may sometimes be desirable to direct a cell other than the one registering the highest received signal strength to handle a hand-off. For instance, U.S. patent No. 4,144,412 to It et at (1979) lo describes a cellular radio system having overlapping cells.
When communication is to be established with a mobile station, the mobile station transmits a call signal over a control channel receivable by each of the cell site transceivers. The installations at each cell site measure the call signal lo strength, and the cells are ordered in accordance with received signal strength. A search is then made to determine if the cell site base station which received the signal of maximum intensity has a channel free to handle a call. If such an idle channel exists, information designating the idle channel is transmitted to the mobile station and communication is established between the cell receiving the highest signal strength and the mobile station. On the other hand, if the search for an idle channel reveals that the cell site base station which received the signal having the maximum intensity has no available channels, it is determined whether the cell site base station receiving the next strongest signal intensity has a channel available for communications. If this second cell site installation likewise has no available channels, a third cell site base station having the next highest received signal strength is checked to determine if it has an available channel. The number of searches and designations to be repeated is determined in accordance with the intensity of the received signal, the degree of congestion of the communications ~235~

and other conditions.
A vehicle in a congested cell may thus be included in an adjacent cell by designating d communications channel of a cell which has received a signal having an intensity next to maximum, with the result that the equivalent area of the congested cell is effectively narrowed and the area of the cell adjacent to the congested cell is effectively increased. Even when the channels of a cell site installation of the cell in which a calling vehicle is located are all busy, lo it is possible to complete a call by using an idle channel assigned to an adjacent cell, thereby decreasing the number of call failures, increasing the quantity of traffic which can be handled, and improving the efficiency of channel utilization.
U.S. Patent No. 49435,840 to Kojima et at (1984) lo discloses a somewhat similar technique in which cell size is not merely effectively, but actually selectively enlarged or reduced (e.g., by controlling the output power of the cell site transceivers) in accordance with a value representing current or instantaneous volume of traffic handled by the cell (i.e., a utilization factor of equipment of each cell site base station). If the traffic in a first cell is much greater than the traffic in an adjacent cell, the size of the first cell is reduced and the size of the second cell is increased (by adjusting cell site transceiver and/or mobile transceiver power outputs and/or receiver sensitivities) to permit the adjacent cell to handle traffic which the first cell would otherwise have been expected to handle.
U.S. Patent No. 4,144,496 issued to Cunningham et at (1979) discloses an adaptive channel assignment technique for use in a cellular radio system having cell site transceivers which are remotely tunable by the MIX. Each cell site transceiver may be selectively tuned to any one of the frequencies allocated to the system under MIX control.

~23~ 9 Additional transceivers are provided at cell sites to permit shifting of channels from cell to cell as needed to accommodate heavier traffic loading in busier cells by diverting channels from lightly loaded cells, An algorithm designed to minimize interference is used to control the channel assignments.
U.S. Patent No. 3,764,915 to Coy et at (1973) is also of interest in disclosing a cellular radio system wherein an MIX dynamically allocates communication channels in response to requests for channels by mobile users. The determination of the channel which should be allocated is made in accordance with channel reuse and allocation optimization criteria to insure that it is the preferred allocation From the standpoint of desired system performance. More particularly, channels are allocated to cells on the basis of close spatial proximity to the requesting mobile transceiver, relative velocity of the mobile transceiver, etc. in order to prevent co-channel interference and to avoid "wasting" channels by uneconomically assigning them.
SUMMARY OF THE INVENTION
Known cellular systems use communications channels very efficiently, but are nevertheless subject to cell blocking during periods of high demand. Because traffic distribution in a cellular system is variable and, to a large extent unpredictable, it is possible for some cells to become completely "blocked" (i.e., fully occupied without capacity to handle any additional calls) while nearby cells remain only lightly loaded. This condition results in decreased system performance, since blocked cells cannot be used to handle new calls or to act as transferees for calls being handed off.
In accordance with the present invention, loading of cells is dynamically redistributed by selectively transferring ongoing calls to adjacent cells in accordance with traffic level in order to reserve channels for Han doffs and 10 ~L;23~9 for new calls. Communications is established between a first set of mobile radio transceivers contained within a first predetermined geographical area (cell) and a subset of a first plurality of stationary radio transceivers serving the first area. A channel occupancy level for the first geographical area (indicating the number of the first plurality of stationary transceivers which are in communication with a mobile radio transceiver with respect to the number of the first plurality of stationary transceivers not in communications with a mobile transceiver) is periodically determined. If the channel occupancy level exceeds a predetermined threshold level, at least one call is transferred from a stationary transceiver serving the first geographical area to a stationary radio transceiver serving another predetermined geographical area overlapping the first area and also containing the mobile transceiver.
The channel occupancy level associated with the first geographical area may be compared with a first predetermined threshold level DHTHRESH. A state associated with the first geographical area is set to a first predetermined value if the channel occupancy level exceeds the first threshold level, and is set to a second predetermined value if the channel occupancy level is less than the first threshold level. If the channel occupancy level exceeds the first threshold level, the number of communications between the subset of stationary radio transceivers serving the first area and engaged in communications with mobile transceivers which must be terminated to decrease the channel occupancy level to below the first threshold value is calculated, and the number of communications so calculated is transferred to plural stationary radio transceivers serving the other area.
Channel occupancy level may also be compared to a second predetermined threshold value DRTHRESH. If the 11 ~23S~S~

channel occupancy level exceeds the second threshold value, the state associated with the first area is set to a third predetermined value and any available stationary transceivers serving the first area are not permitted to accept new communications (thus reserving them for use as transferees of ongoing calls).
An example embodiment of the invention will now be described in conjunction with the drawings in which:
Figure 1 is a graphical illustration of a 13-cell cellular mobile radiotelephone system;
Figure 2 is a schematic block diagram of a cellular mobile radiotelephone system in accordance with the present invention;
Figure 3 is a schematic state transition diagram of the various states a cell site base station shown in Figure 2 may assume and the conditions which cause transitions between such states;
Figure 4 is a schematic diagram of information maintained in the random access memory of the mobile telephone exchange (MIX) shown in Figure 2;
Figures PA and 5B form a flow chart of the load balancing routine performed by the MIX shown in Figure 2;
Figures 6A-6F are schematic diagrams of -the formats of various messages communicated between the MIX and the cell site base stations shown in Figure 2;
Figure PA and 7B are together a flow chart of another routine executed by the MIX shown in Figure 2;
Figures PA and 8B together form a flow chart of steps performed by each of the cell site controllers of the cell site base stations shown in Figure I
Figure 9 is a detailed flow chart of the steps performed by the "process load balancing message" block shown in Figure 8;

~235~59 Figure lo is a 3-dimensional graphical illustration of a typical mobile traffic density used in a simulation of the system shown in Figure 1;
Figure 11 is a graphical illustration of probability that a call hand-off receives unsatisfactory services plotted against percent load increase of the system shown in Figure 1 with and without load balancing in accordance with the present invention;
Figure 12 is a graphical illustration of overall blocking perceived by mobile transceivers in the system shown in Figure 1 with respect to system overload with and without load balancing in accordance with the present invention, and Figure 13 is a graphical illustration of probability that a hand-off attempt must wait for a free RF
lo channel, overall system blocking perceived by mobile transceivers, and average cell blocking with respect to the number of channels reserved for hand-off in the system shown in Figure 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1 is a schematic representation of the geographical layout of a mature 13-cell cellular system 10.
System 10 includes 13 cells C1 through C13 each having a cell site base station (T1-T13~ respectively) associated therewith (each base station including at least one base station transceiver). System 10 may be considered an isolated piece of a larger system which has been fragmented. The contour or boundary of each cell is defined as a circle along which the received radio signal strength of signals transmitted by the cell site transceiver is equal to a predetermined minimum signal threshold. Such "bad service" contours characterize the minimum received signal strength indicator (RSSI) required to maintain adequate service within the cells.
Mobile radio transceivers within the contour of ~2354~i9 a cell can participate in communications of acceptable quality with the cell site base station associated with that cell (i.e., a minimum desired signal-to-noise ratio without excessive signal fading is maintained throughout the geographical area defined within the cell keynoter The cell site contour may thus be considered dependent upon cell site transceiver effective radiated output power (err) and mobile transceiver receiving sensitivity (as limited by cell site transceiver receiving sensitivity and mobile transceiver output lo power). Mobile transceiver output power may be selected so that the contour also corresponds to the signal strength of signals received by the cell site transceiver (i.e., the cell contours may also represent the area a transmitting mobile transceiver must be operating within for the cell site lo transceiver to receive the signals transmitted by the mobile transceiver at useful signal strengths). In this way, the range of the mobile transceivers may be made to correspond to the range of the cell site transceivers to avoid waste of resources and undue co-channel interference.
Cell site installations Tl-T13 are positioned relative to one another such that there is a degree of overlap between contiguous cells. Hence, for instance, a mobile transceiver positioned in the area 15 of overlap between contiguous cells C5, C6 and C12 of system 10 could participate in communications with transceivers of any one of cell site base stations To, To and T12. Some degree of overlap is important to permit transfer of calls from one cell to another, as will become more apparent shortly.
The great power and flexibility of cellular system 10 results in part because transceivers of multiple ones of cell site base stations Tl-T13 may operate independently and on a non-interfering basis on the same RF channel frequency).
For instance, a mobile transceiver My located in cell C9 may 3~i9 communicate with a transceiver of cell site base station To via a communications channel XI while, at exactly the same time, another mobile transceiver My located in another cell (for example cell Clue may communicate with a transceiver of associated cell site base station (Toll) via the same communications channel XI (i.e. the same frequencies are used for each call). Because cells C9 and Oil are geographically separated from one another (e.g., they do not overlap), the transmissions of mobile transceiver MI and cell site base lo station To cannot interfere with the communications between mobile transceiver My and cell site base station Toll, and vice versa. The same channel XI could also simultaneously be used in several other cells of system lo so long as such other cells do not overlap the cells C9 and Oil in which the channel is already in use (for example, channel XI might also simultaneously be used in cells SUE, SUE, C8, I, C7 or C6, or even in combinations of such cells such as cells C8 and I, cells SUE and C7~ etc.). In this way, cellular system IO is capable of handling many more independent conversations (calls) simultaneously than could a single repeater-type system allocated the same number of channels.
Although some mobile transceivers served by system IO may always remain in the same cell, it is generally desirable to provide continuous communications for mobile transceivers in transit between any two arbitrary points within the geographical area served by system IO. For instance, mobile transceiver MI may belong to a business executive having a home located in cell C9 and an office located in cell C6 who wishes to use his or her mobile transceiver while commuting between home and the office. A conversation may be initiated while mobile transceiver MI is in cell C9, but may continue as the mobile transceiver exits cell C9 and moves through, for ~;~35~

example, cells C2, C1, C7 to finally reach a destination within cell C6. As mobile transceiver My approaches the "bad service"
contour of cell C9, system 10 must somehow transfer ("hand-off") the ongoing communications to the new cell the mobile transceiver has entered or is about to enter (such as cell C2) if the conversation is to be continued. Overlapping of adjacent cells makes it possible for such Han doffs to occur without interrupting the cell being handed-off (since a mobile transceiver is typically in more than one cell at once when lo approaching the "bad service" contour of the cell). It is important that such Han doffs are accomplished very rapidly and reliably if conversations are to continue without interruption as mobile transceivers exit one cell and enter another.
A central mobile telephone exchange (MIX) (not lo shown) of system 10 supervises the cell site base stations Tl-T13 to allow calls in progress to continue without interruption when mobile stations move from one cell to another. When a mobile transceiver participating in a call is about to exit a cell, the MIX automatically hands-off the call to a free channel in an adjacent cell into which the mobile transceiver has moved. As mentioned above, cell site base stations typically continuously measure an indication of received signal strength (RSSI) for each ongoing call and request the MIX to hand-off a call when the RSSI of the call falls below a predetermined threshold.
Figure 2 is a schematic block diagram of the presently preferred exemplary embodiment of a cellular mobile radio telephone system 10 in accordance with the present invention. System 10 includes a mobile telephone exchange (MIX) 20, a public telephone central office 22, d plurality of cell site base stations (e.g. To, To, To) and a plurality of mobile radio transceivers (only one transceiver Ml is shown). Public telephone central office ("central Office") 2Z is a conventional telephone exchange providing communications to a plurality of hard-wired telephones via conventional land lines. Central office 22 is accessible by MIX
20 via a plurality of voice trunks ANN.
Telephone calls may be initiated by MIX 20 and handled by central office 22, or may be initiated by the central office (e.g., by telephones connected to the central office) and directed to the MIX. Central office 22 and voice trunks ANN thus permit system 10 to establish communications between mobile transceivers (Ml) and conventional land-based communications facilities.
As discussed previously, cell site base stations To, To and To are installed at fixed locations separated from one another within a geographical area to be served. Each of the cell site base stations defines a geographical cell (e.g., base station To serves cell C9 shown in Figure 1, base station To serves geographical cell C2 shown in Figure 1, etc.). Cell site base stations To, To and To establish bidirectional communications with mobile transceivers within the service area of system 10.
MIX 20 controls the operations of each of cell site base stations To, To and To, and also selectively routes voice information through system 10. More particularly, voice information is communicated between MIX 20 and each of cell site base stations To, To, and To via a plurality of 4-wire voice trunk lines in the preferred embodiment (for example, voice trunk lines Ann connect cell site base station To with MIX 20). Each of voice trunks Ann are bidirectional signal paths ~7~35~

carrying analog voice information (or digitized voice information if appropriate analog-to-digital and digital-to-analog converters are provided). MIX 20 includes a switching network 24 which is capable of selectively connecting any voice trunk line of any cell site base station to any other voice trunk line (of the same or different cell site base station or central office 22). Switching network 24 operates under control of a processor 26 dedicated to MIX 20.
Processor 26 executes programs stored in a random access memory (RAM) 28, which is also available for data storage by the processor.
MIX 20 also includes an input/output controller-interface 30 which permits the MIX to communicate control information to each of cell site base stations To, To and To via dedicated high-level data links and associated controllers (for example, a high-level data link 56 connects MIX 20 with cell site base station To). Such high-level data links are bidirectional signal paths (conventional balanced wire pairs may be used) which communicate control information in digital form from MIX 20 to the cell site base station dedicated to the data link and vice versa. In the preferred embodiment, the data communications protocol used to transmit control information via the high-level data links dedicated to cell site base stations To, To, and To is conventional and is described in "Data Communications-High Level Data Link control wreck. -Elements of Procedure (Independent Numbering), International Standard IS 43 35 ~1976~.

~23S4S9 Cell site base stations To, To and To are identical to one another in structure, and only base station To will now be described. Cell site base station To includes a cell site controller 32, a control channel transceiver 44, a plurality of voice channel transceivers assign a locating receiver 56 (which may ho be a transceiver with an unused transmitter section) and an antenna 38. Control information is communicated between cell site controller 32 and transceivers 44, 56 and Ann via data links 58, 60 and Ann, respectively.
Cell site controller 32 controls the operations of each of transceivers 44, 56, and Ann via information conveyed over the data links.
Cell site controller 32 itself includes a digital signal processor ego., a microprocessor) with owe amount of information storage capacity, and an input/output controller (either hardware or outwore) which communicates information Jo and from TO 20 via data link 56. Each of transceivers 44, 56 and Suzanne include associated full duplex radio transceiving device (i.e., a transceiver and a receiver capable of operating simultaneously on different frequencies) the input/output of which is connected to shared antenna 38 via a conventional matching/multiplexing network 66 (separate antennas may be used if desired. Cell site controller 32 selects the frequencies a which transceivers 44, 56 and assign operate under software control (although; these frequencies are fixed upon initialization of cell site controller 32 in the preferred embodiment, dynamic frequency allocation under software Conner} is also possible). The frequencies of operation of transceivers 44, 56 and 12~

Ann are selected so that all of these transceivers may operate simultaneously on a non-interfering basis.
Voice channel transceivers Ann are used by cell site base station To to establish voice communications with mobile transceivers within geographical cell C9. For instance, voice channel transceiver aye may be engaged in communication with mobile transceiver My at the same time that voice lo channel transceiver 50b is communicating with another mobile transceiver, etc. Thus the number of mobile transceivers cell site base station To is capable of communicating with simultaneously is limited by the number of voice channel transceivers Ann (this number may range from 10 to about 40 in the preferred embodiment).
Control channel transceiver 44 is used to transfer control information between cell site controller 32 and each of the mobile radio transceivers operating in cell C9. The exchange of such control information may be conventional and of the protocol and format described in "SWISS ETA
Interim Standard Cellular System Mobile Station-Land Station Compatibility Specification" (July 1981).
Briefly, mobile transceivers within cell C9 not actually engaged in voice communications via one of voice channel transceivers Ann continuously monitor the transmit frequency of control channel transceiver 44 awaiting a call. If MIX 20 receives a call designating a particular mobile transceiver as the recipient of the call, the MIX will transmit control information (via the high-level data links) to each of the cell site base stations corresponding zoo ~l23591~

to the cells in which the called mobile transceiver might be expected to be found (typically, particular mobile transceivers are only authorized to operate in some subset of the colic of system 10). To call a mobile transceiver, c211 site controller 32 causes control channel transceiver 44 to transmit a "paging" message which designates the mobile transceiver intended to receive the call by a unique identification code, The mobile transceiver monitors the output frequency of the control channel transceiver 44, and when it receives a call designating it, responds by transmitting access commands over the receive frequency of the control channel transceiver.
Additional hand-shaking back and forth between the felled mobile transceiver and the control channel transceiver ensures reliable channel acquisition.
When cell site controller 32 is prepared to dedicate one of voice channel transceivers Ann to the called mobile station, the cell site controller transmits indict of the voice channel frequencies to be used to the called mobile transceiver via control channel transceiver 44 along with a command instructing the mobile transceiver to begin operating on the voice channel. Cell site controller 32 also actuates the one of voice channel transceivers assign being dedicated to the called mobile transceiver. The mobile transceiver begins operating on the voice channel frequencies indicia of which it received via the control channel, end the new call proceeds. Similar exchanges may be initiated when a mobile transceiver within cell C9 transmits an "origination" message indicating it wishes to make a call. Cell site controller 32 21 ~;23S~9 controls all such channel acquisition transacting under the supervision of processor 26 of MIX 20.
As mentioned above, the number of mobile transceivers capable of communicating via any particular cell site base station (e.g., cell base station To) is limited by the number of voice channel transceivers ego., Ann) the base station is provided with. Depending on its location, a mobile transceiver may be capable of communicating with more than one cell site base station (i.e., when the mobile transceiver may be located in an area of overlap between two or more cells). More often, however, a mobile transceiver will be positioned within only one cell, and therefore must communicate, if at all, via the cell site base station of that cell. It is therefore important that some voice channel transceivers ox each cell site base station are always available (i.e., not in use) if new calls are to be processed without delay.
A more critical situation arises when a mobile transceiver already engaged in communications with a iris cell ego., cell C9) is about to exit the first cell and move into a second cell (e.g., cell C8). If communications are Jo be continuously maintained as the mobile transceiver moves out of one cell and into another, it is necessary for the first cell to transfer the ongoing call to a cell into which the mobile transceiver is moving. Such transfer Han doffs must occur reliably if ongoing communications are to continue without interruption. If the cell into which the mobile transceiver is moving has no voice channel transceiver available to handle the call hand-off, no hand-off is possible and the call will be lost as 22 I So the mobile transceiver requiring the hand-off moves out of the effective service area of the cell site base station it is communicating with. In accordance with the present invention, instantaneous cell loading is periodically monitored and ongoing calls are transferred in order to redistribute or balance loading among the various cells in system I .
Figure 3 is a graphical illustration of a lug state transition diagram for each of the cell site base stations shown in Figure 2. The cells of system 10 may enter one of four possible predetermined states in the preferred embodiment depending upon cell loading the "normal"
state; I the "directed hand-off" state; lo) the "directed retry state"; and (4) the "combined"
state. Transitions of cells between the various states are controlled in accordance with voice channel occupancy level (VCO) in measure of I instantaneous cell loading. Two (independent) predetermined threshold values DHT~RES~ and DRTHRESH
are assigned to each cell in system 10. These values are determined by data stored in random access memory (RAM) 28 of MIX 20, and may be preset upon initialization of system 10. MIX 20 periodically computes the VC0 of each cell, which is determined by comparing the number of voice channel transceivers of the cell site base station engaged in communications with respect to the total number of the voice channel transceivers the base station is provided with. Thus, the VCO of a cell rises as the cell handles more communications traffic.

23 ~2~S4~

The station of all cells of system lo are set at time of initialization to the normal state.
If cell loading increases so that cell VCO DH~HRESH, the cell enters the directed hand-off mode. The cell will remain in the directed hand of mode until either its VCO decreases to less than (or equal Jo) DHTHRESH, or until cell VCO increases further to DRTHRESH (in which case the cell enters the combined mode).
Similarly, if the VCO of a cell in the normal state increases to DRTHRESH, the cell enters the directed retry mode. If cell loading decreases so that cull VCO s DRTHRESH, the cell will return to the normal mode. On the other hand, if cell loading increases further so that cell VCO 2 DHTHRESH, the cell enters the combined mode. Additional transitions between cell staves are shown in Figure 3. As can be seen, the state of a cell is determined by instantaneous cell loading (i.e., the amount of communications traffic it is presently handling with respect to the total amount of traffic it is capable of handling).
Cell site base stations are directed to perform certain functions and are inhibited from performing other junctions depending upon cell state. Cell site base stations in the normal state are permitted to perform any of the functions a base station is capable of performing. For instance, cell site base stations for normal cells can accept call Han doffs from adjacent cells initiated either because a mobile transceiver is about to exit an adjacent cell and enter the normal cell or because balancing of loading of cells is desired. Normal cells are also permitted to handle new calls 24 12354~j~

originated by mobile transceivers ("originations or call originated by ventral office 22 pages").
Cells in the directed hand-off state are so loaded that they are in danger of becoming completely blocked if traffic levels increase further. In accordance with the present invention, cells in the directed hand-off state are directed to hand-off some of the calls they are handling to adjacent cells in order to reduce cell loading to prevent complete cell blockage from occurring. MIX
20 instructs the cell site base stations of cells in the directed hand-off state to attempt to hand-off the number of calls necessary to reduce fell VCO to below the DHTHRESH threshold. Moreover, cell site base stations of cells in the directed hand-off mode are not permitted by MIX 20 to accept Han doffs due to load balancing from other cells. However, cell site base stations of cells in the directed hand-off mode will accept Han doffs necessary to maintain an ongoing call as a mobile transceiver exits an adjacent cell and enters the cell in the directed hand-off state (since a very high priority is placed upon maintaining ongoing calls without interruption in accordance with the present invention).
Moreover, cells in the directed hand-off state will handle new calls (originations or page responses) in order to permit additional mobile transceivers to access system 10 seven though handling of new calls does increase the loading of cells in the directed hand-off state, such new calls once established may become candidates for Han doffs to decrease cell loading ) .

25 ~23~i45~

Cells in the directed retry state behave differently from cells in the directed hand-off state in that they behave in a manner which preserves some voice channels for incoming Han doffs due to either load balancing or in order to preserve ongoing call about to be lost as mobile transceivers exit adjacent cells. Cells in the directed retry state refuse to accept new calls in order to preserve channels for hand-off attempt.
The cell site base stations of cells in the directed retry mode turn away new calls and direct mobile transceivers involved in the new calls to try the cell site base stations of adjacent cells. Cells in the directed retry state do not attempt to hand-off calls to balance loading among adjacent cells (although they will, of course, hand-off calls which are in danger of being interrupted because a mobile transceiver is about to exit the cell). Cell site base stations of cells in the directed retry mode will a opt call Han doffs inflated due to either load balancing or due to mobile transceivers exiting adjacent Swahili Cells in the directed retry mode thus function somewhat as "overflow" cells which oven though they are heavily loaded themselves, are capable of receiving Han doffs from other cells.
Cells in the combined mode behave similarly to both cells in the directed hand-off mode and to cells in the directed retry mode. Cells in the combined mode will not accept new calls like cells in the directed retry model and will attempt to hand-off call in progress (like cells in the directed hand-off mode). Cell in the combined mode are unavailable for call hand-o~fs due to load balancing (like cells in the directed hand-off mud although they are available to accept fall 26 ~.23S~59 Han doffs necessary because a mobile transceiver is about to exit an ad jacent cell .
By varying the DHT~RESH and DRUTHERS values (which are programmable on a cell-by-cell basis), the behavior of each cell of system 10 under different degrees of loading can be controlled.
Typically/ the D~THRES~ value of a cell is set to be higher than the DHT~RESH value of the cell to cause cells to enter the directed hand-off mode at lower VCO levels than necessary for the cells to enter the directed retry mode. The levels of DRT~RESH and DHTHRESH may be chosen in accordance with observed performance of system 10 (e.g., by gathering statistical information of actual system behavior under varying degrees of loading) and/or in accordance with cell size, cell location, and a number of other different factors.
Information concerning cell mode and cell VCO level is stored in RAM 28 of MIX 20 and is periodically updated by processor 26 (in the manner which will be described shortly).
Figure 4 is a schematic diagram of some of the information processor 26 stores in I and uses to perform load balancing and other functions. Each cell of system 10 has associated with it a Table 80 called LBSTATUS Lydia Balancing Status) and a Table 94 called "Adjacent Cell Table". The LBSTATUS Table 80 of each cell stores information concerning cell mode and cell VCO as well as the DHTHRESH and DRTHRESH values assigned to the cell. The LBSTATUS Table 8Q includes: a present cell mode field 82 (whir stores indicia of the present mode of the cell, either normal, directed hand-off, directed retry or combined); a field 84 storing the cell mode the last time the cell status 27 1 2 3 So S

was updated; a VC0 level field 86 (which stores the current voice channel occupancy level of the cell);
a field 88 containing the number of unoccupied (seizable) voice channels of the cell (i.e., the number of voice channel transceivers of the cell site base station which are not presently in use); a field 90 continuing the total number of cell of voice channels (i.e., the total number of voice channel transceivers of the cell site base station);
a DHTHRES~ field 9Z; and a DRTH~ESH field 94.
Fields 90, I and 94 typically remain constant after initialization of system 10 while fields 82-8~ are periodically changed in accordance with instantaneous cell loading Of course, it may be desirable to change the values of fields 90-94 under certain circumstances. For instance, field 90 should be updated if any of the voice channel transceivers of a cell site base station malfunction or otherwise are placed out of service Moreover, it might be desirable to dynamically change the values of the DHTHRESH field 92 and THRESH field 94 depending upon, e.g., overall system loading, changes in cell site base transceiver power output, the time of day, etc.
adjacent cell Table 94 is duplicated for each cell (as is the LBSTATUS Table 80), end contains information specifying the cells adjacent to the cell associated with the Table. Adjacent cell Table 94 is indexed by a cell number f told 96 which specifies the cell associated with it A
field 98 contains the number of cells adjacent to the cell associated with the table (this number can vary depending upon, for example, whether a cell is within the center of the service area of system 10 or on its periphery). Following the field 9B is a 35~5~

list of all of the cells adjacent to the cell associated with the adjacent cell Table 94 (for example, the adjacent cell table 94 of cell C12 of system 10 Gould include a list of lo having entries corresponding to cells C5, C6 and C13). List 100 may contain up to 14 adjacent cell designations in the preferred embodiment. Adjacent cell Table 94 is used to specify which cells are candidates for call hand-off or for hand-off retries, as will be explained.
Figures PA and 5B are together a flow chart of the routine executed by MIX 20 to periodically update the state (mode) of each cell in system 10 to reflect current system traffic loading. The routine shown in Figures PA and 5B is executed periodically (i.e. once every 10 seconds in the preferred embodiment for each cell, although the routine might be executed more or less frequently depending upon MIX processing resource availability, the rate messages are transferred between the MIX and the cell site controllers, and desired accuracy in the correspondence of cell state with instantaneous cell loading). After the predetermined wait time has elapsed (block 152), MIX 20 computes the voice channel occupancy (VCO) of the cell the state of which is being updated (block 154) according to the following equation:

vc0=Total number of cell channels-unoccupied chinless Total number of cell channels VCO may hence be computed from the fields 88 and 90 of the LBSTATUS table 80 associated with the cell.
MIX 20 then retrieves the DHTHRESH and DRTHRESH
values from corresponding fields 92 and 94 of the LBSTATUS table 80 associated with the cell (block) ~235~59 156~, and compares the computed VCO level with these two threshold values. If VCO is less than DRTHRESH
and THRESH ( tested for by decision block 158) the current mode of the cell is normal. MIX 20 rewrites the present mode field 82 of LBSTATUS table 80 to normal (block 160) and sends a load balancing message to the cell via the high-level data link dedicated to the cell) instructing the cell to set its mode Jo normal (block 162). Figures 6 (a) schematically shows the format of the message sent by block 162 to instruct the cell to assume the normal state. This message includes only one field 202 (in addition to a conventional header field 200) indicating the new cell mode is normal. Routine 150 then returns to wait block 152 (after all other cells have been processed) to wait for the next time cell state is to be updated.
If VCO is not less than both DRTHRESH and THRESH decision block 164 determines whether VCO
is 2 DRTHRESH and c DHT~RESH. If VCO falls within this range of values, the current cell mode is directed retry, and present mode field 82 is updated accordingly (block 166). us will be recalled, cells in the directed retry mode will refuse to participate in new calls with mobile transceivers, and will instead refer calling or called mobile transceivers to other cells not in the directed retry or combined modes. MIX 20 accesses the adjacent cell table 94 associated with the cell the state of which is being updated and obtains the list 100 of adjacent cells from this table (block 168~.
MIX 20 then removes all of the cells from this list which are currently in the directed retry or combined states, and sends the remainder of the list to the cell site controller of the cell in a message 30 I So I

informing the cell to update its mode to directed retry (block 170). Figure 6 lo) is a schematic diagram of the format of the directed retry message sent by block 170 to the cell site controller. The directed retry message includes a header 200, a mode field 2Q2 instructing the cell to update its mode to directed retry, a field 204 indicating the number of entries in a list which follows, and a list 206 of frequencies of control channels of cells adjacent to the cell the mode of which is being updated. was will be explained shortly, the cell site base station transmits this list 206 of control channels to the mobile transceiver in a message directing the mobile transceiver to try another cell.
If the cell VCO is not within the range tested for by decision block 164, decision block 172 determines whether VCO < DRT~R~SM and > or equal to DHTHRESH. If VCO falls within the range tested for by decision block 172, the cell mode is set to directed hand-off (block 174) by rewriting the present mode field 82 of LBST~TUS table 80 of the cell accordingly MIX 20 then calculates the number of calls which the cell must hand off to adjacent cells in order for the c211 VCO to fall below DHTHRESH once again block 176). The contents of fields 90 and 92 of the LBSTATUS table 80 of the cell determine a maximum number of calls which the cell can handle while still remaining in the normal state, which can be used to obtain the number of 30 - calls to be handed off MIX 20 then transmits a directed hand-off load balancing message (schematically shown in Figure 6 (b)) to the cell including a field 202 instructing the cell to change 31 ~235~5~

its mode to directed hand off and filed 20B storing the number of calls calculated in block 176 which the cell is to attempt to hand off (block 178).
If VCO fails the decisions of blocks 15&, 164 and 172, then VCO > or equal to THRESH and >
or equal to DHT~RESH, and the c211 is in the combined mode. The present mode field 82 of the LBSTATUS table 80 is updated accordingly (block 180) and a list of adjacent cells is obtained in the manner described in conjunction with block 168 (block lB2). The number of calls which need to be handed-off to reduce the cell VCO to the normal level are calculated as described for block 176 (block 134), and a load balancing message is sent to the cell site controller instructing it to change its mode to combined (block 186). Figure 6 (d) is a schematic representation of the format of the load balancing message sent to the cell in block 186.
This combined load balancing message contains a field 202 instructing the cell to change its mode to combined, a field 204 and list 206 containing information corresponding to fields of the directed retry load balancing message shown in Figure 6 (c), and a field 208 containing the same information as the corresponding field in the directed hand-off message shown in Figure 6 by After the cell state has been updated, it will not be changed until after the time counted by wait block 152 has clasped (although the unoccupied voice channel field 88 of the LBST~TVS table 80 ox the cell is updated continuously as cell loading changes and thus always reflects instantaneous cell load).

~235~S~

Figures 7 pa) and 7 (by are together are a flowchart of a hand-off process routine 250 performed by MIX 20. Hand-off routine 250 receives hand-off request messages from the cell site base stations, determines whether a hand-off is possible, and computes which cells are the best ones to attempt to hand-off to (based on received signal strength indicator RUSS measurements and channel availability). Hand-off routine 250 then controls the cell site controllers to actually hand off calls from one cell to another.
Hand-off routine 250 is normally asleep, and wakes up periodically (once a second in a preferred embodiment) to process two queues of data: an incoming (hand-off request message) queue 252 and and an outgoing (SUE measurement response message) queue ~54 (see Figure 4). Input/output controller-interface 30 of MIX 20 writes hand of requests received from cell site controllers into the incoming queue 252 even when hand-off process 250 is asleep, and likewise removes messages deposited into outgoing 254 by routine 250 and transmits them to the appropriate cell site controller. Conventional mechanisms (such as rotating pointers) are provided to prevent hand-off routine 250 and input/output controller-interface 30 from attempting to access the same locations in incoming and outgoing queues 25~ and 254 at the same time. In the preferred embodiment, a cell site controller writes an entry into incoming queue 252 whenever it wishes to request a call hand off (either because it was instructed to do so by the load balancing routine 150 described above, or because one of the calls it is handling is about to be lost as a mobile transceiver exits the cell).

33 lZ35~!L59 an doff request messages deposited into incoming queue 2S2 include the following information in the preferred embodiment: channel designation (the channel the call to be handed of is presently occupying); designation of the supervisory audio tone (SAT) of the Han doff candidate call; the reason for the hand of (either load balancing or because a mobile transceiver is about to exit the cell); identification of the cell requesting the hand off; and identification of the mobile transceiver participating in the call to be handed-off Upon waking up (block 256), hand-off routine 250 determines whether there are any hand-off request messages in the incoming queue 252 (block 258). If the are messages in the incoming queue 252, routine 250 reads the next message (block 260), determines which cell is requesting the hand-off, and reads the adjacent cell list 100 from the adjacent cell table 94 associated with the cell (block 262). Routine 250 then obtains the present state of each cell in the adjacent cell list (from the LBSTATUS tables of the cells; block 264) and determines it a hand-off is possible (block 266).
Whether or not a hand-off is possible depends upon the reason fro the hand-off and the states of the adjacent cells. A hand-off necessary because a mobile transceiver is about to exit a cell will be handled by all cells except those completely unavailable (i.e., completely blocked), while a hand-off due to load balancing will only handled by those cells in the directed retry or normal states If none of the adjacent cells will accept the hand-off, hand-off routine 250 sends a hand-off retry message to the requesting cell (block I

which simply instructs the cell to wait a period of time and then re-request the hand-off.
If, however, there are adjacent cells which can accept the hand-off, routine 250 removes all those cells which will not handle the hand-off from the adjacent cell list obtained by block 264 (to reduce message traffic in system lo). Thus, all cells in the directed hand-off or combined modes are removed from the adjacent cell list when the reason lo for the hand-off request is load balancing (blocks 270 and 272), since cells in these states will refuse a hand-off due to load balancing All cells which haze no channels at all available (i.e., those cells with a zero value stored in field 88 of their lo LBSTATUS table 8) are also removed from the adjacent cell list block 274), since these cells have no available channels and cannot handle any type of hand-off. Routine 250 then sends a message requesting each cell in the list (as modified by blocks 272 and 274~ to measure the RSSI of the voice channel of the call desired to be handed-off by depositing appropriate messages in outgoing queue 254 (block 276).
The messages sent by block 276 simply request the cell site base stations of specified cells to monitor of the voice channel of the call being handed-off with their locating receivers, measure the signal strength of the voice channel signal, produce a RSSI value corresponding to this measured signal, and report the RSSI back Jo MIX
20. Such RSSI measurements request messages include the following information in the preferred embodiment: a list of each of the cells which are to measure RSSI Tithe list including the cell requesting the hand-off for which the RSSI is being I

measured); information identifying the call being handed-off; and additional information needed to keep track of the status of the RSSI measurement request. Routine 250 reserves space (in outgoing queue 254 in the preferred embodiment) into which input/output controller-interface 30 may write the responses received back from the cell site base stations. Entries in outgoing queue 254 also include information indicating the time the RSSI
measurement requests were sent, since routine 250 waits a predetermined period of time after the sending time for the cell site base stations to respond and only then looks for the responses.
Once RSSI measurement requests are deposited to outgoing queue 254 by block 276, routine 250 processes the remainder of the entries of the incoming queue 252 by returning to decision block 258. When incoming queue 252 is empty, routine 250 determines whether any RSSI response message have been received in outgoing queue 254 (block 278) (as has been explained, the routine in the preferred embodiment will only look for these messages after giving the cell site base stations sufficient time to respond to the associated RSSI
measurement requests). If the outgoing queue is empty, routine 250 goes to sleep (block ~79) to conserve MIX processor resources, and wakes up again after a predetermined period of time has elapsed.
If, however, there are RSSI response messages in outgoing queue 254, routine 250 reads the first response (block 280) and then retrieves the VCO of each responding cell from the LBSTATUS table 80 associated with the cell. us will be explained, cell site base stations do not respond Jo RSSI
measurement request in the preferred embodiment ~23~5~

unless the measured RSSI is above a predetermined minimum threshold value in order to reduce message traffic cells measuring RSSI less than this minimum threshold value could not provide adegua~e hand-off service and therefore would have to be eliminated from the list of possible cells to hand of to by MIX 20). Routine 250 then orders responding cells from "best" to "worst" based upon RSSI value and VCO. Block 284 produces a list of possible cells for handing-off the call in order, from top to bottom, of relative qualifications US candidates for handling the hand-off.
The top portion of the list produced by block 284 includes all cells responding to the RSSI
request message which have a VCO less than DRTHRESH. These cells are in turn ordered from strongest to weakest RSSI level. The remainder of the responding cells are added to the bottom of the list in order of VCO beginning with the lowest VCO. No cells which are completely (i.e., not having at least one available channel) are included in the list. If the reasons for the hand-off is load balancing, block 284 also removes from the list all those cells responding to the SUE measurement request message which have a SUE level below a second predetermined threshold value. Even though all responding cells are guaranteed to have a RSSI
value above a first predetermined threshold value (as described above), hand-off routine 250 will not attempt to hand-off a call due to load balancing to cells reporting a RSSI level which is below a slightly higher, second predetermined threshold value which may be assigned on a cell-by-cell basis if desired) in order to prevent uneconomical hand-oils from occurring. This second predetermined 37 354S~3 value is chosen to be high enough to prevent continuous hand-off attempts back and forth between adjacent cells neither of which can provide substantially better service to the call than the other. This second predetermined threshold value is not used if the reason for the hand-off is a mobile transceiver exiting a cell, since any cells responding to the RSSI request message will almost certainly have an RSSI value above that of the cell requesting the hand-off in such cases If no cells are listed in the ordered list produced by block 2B4 (as tested for by decision block 286), a Han doff retry message is sent to the cell requesting the hand-off (block 288) instructing the cell to wait a short period of time and then no-request the hand-off if it is still desired at that time. If, however, there is at least one cell in the ordered list, routine 250 determines if the same call is still in progress (block 290). If the call which was the subject of the hand-off request has terminated, there is no need to hand the call off and nothing is done.
If the same call is still in progress, routine 250 determines whether the cell requesting the hand-off is in the ordered list (block 292). As will be recalled, routine 250 sends an RSSI request the cell site base station requesting the hand-off as well as to the cell site base stations of cells adjacent to the cell requesting a hand-off. This is to ensure the call should in fact be handed-off (e.g., the mobile transceiver may have moved behind an obstruction) and to continuously optimize the cells handling calls. If the requesting cell is in the ordered list, routine 250 determines whether the cell is at the top of the list (block 294). Even Lowe though a cell requests a hand-off, it may nevertheless be the best cell to handle the call, and routine 250 sends a hand-off retry message to the cell in this event (block 296). If the cell requesting the hand-off is in the list but is not at the top of the list, all entries in the list which are worse candidates for handling the hand-off than the cell requesting the hand-off (i.e., those cells listed below the requesting cell) are eliminated from the list (block 298). If the requesting cell is not in the list (or has been removed from the list by block 298), routine 250 attempts to hand-off the call to cells in the list, one at a time, beginning with the cell at the top of the list (block 300). The process of block 300 in the preferred embodiment is actually performed by a call processor conventional design which operates independently of routine 250.
This call processor is responsible for initiating as well as actually handing-off calls by supervising cell site base stations through signaling protocol of the type described in ETA compatibility specification IS-3-B. The call processor has the additional responsibility of updating field 88 of the LBSTATUS table 80 of each cell as new calls are begun and calls are terminated or handed-off.
Figures PA and 8B together are a flowchart of a process continually executed independently by each of the cell site controllers of the cell site base stations of system 10. The function of routine 350 is to decide when a hand-off should be requested due to insufficient RSSI, to provide communications with MIX 20, and to interact with mobile transceivers engaged in calls in progress or attempting to access a communications channel. Routine 350 (expected by, 39 us e.g., cell site processor 32) continuously monitors the RSSI of all calls in progress (such as by measuring AGO voltage of each of the receiver portions of voice channel transceiver Ann), and determines whenever one of these RSSI values falls below a predetermined minimum threshold level (decision block 352) This event indicates that a mobile transceiver engaged in a call in progress is about to exit the cell and that the call must be handed of to an adjacent cell if it is to continue without interruption. Routine 350 sends a hand-off request to MIX 20 specifying low RSSI as the reason for the hand-off (block 354). Figure I is a schematic diagram of the format of the hand-off request message sent by block 354. The hand-off request message includes a header 356, a count field 358 specifying the number of Han doffs requested (usually only one for a low RSSI hand-off request), a reason field 360 specifying the reason for the hand-off low RSSI or load balancing), and a list 362 of voice channels (and other call designation information) specifying the call which the hand-off request applies to.
Next, routine 350 determines if a message has been received from MIX 20 (decision block 364). If a message has not been received, routine 350 returns to monitor voice channel signal levels (decision block 352). If a message has been received however, routine 350 determines if the message requires the cell site base station to hand-off a call to another cell (such messages would be generated by block 300 shown in Figure 7 (b)) (block 366). If a handcuff instruction message it received, routine 350 transmits a hand of command to the mobile transceiver engaged in the call to be Sue ~10 handed-off (typically specifying the new control and voice channel and other information), and sends a confirmation message to the MIX after receiving appropriate hand-shaking information from the mobile transceiver block 368~. Conventional hand-shaking mechanisms are included in this process to ensure calls are not lost during hand-off attempts If message has been received from MIX 20 but it is not an instruction to hand-off a call to another cell, routine 350 determines whether the received message is a request to measure RSSI
(decision block 368). If this type of message is received, routine 350 measures RSSI on the voice channel specified by MIX 20 block 370) and determines if the measured RSSI is greater than a predetermined threshold value (block 372~. Cell site base stations measure RSSI on specified voice channels in the preferred embodiment by simply tuning their locating receivers to the specified channel to obtain a measurement of signal strength. Of course, an unused voice channel transceiver could be used instead, if desired, in order to reduce equipment cost. If the measured RSSI
value is below the threshold (indicating the cell cannot adequately service the call), no response to the RSSI request is sent to MIX 20, and routine 350 returns to decision block 352. On the other hand, measured RSSI levels above the threshold value are reported to MIX 20 (block 373~.
If a message has been received which fails the test of decision blocks 366 and 368, routine 350 determines if the message is a hand-off retry message (block 374). If a hand-off retry message is received, routine 350 waits (block 376) before generating another hand of request. after waiting 41 ~L235~59 a predetermined period of time block 376) (a few seconds in the preferred embodiment), routine 350 may remeasure voice signal level (decision ok 352) to determine if a hand-off due to low RSSI is necessary if the reason for the hand-off requests was low RSSI. If the reason for the hand-off requests was load balancing, routine 350 may simply give up and wait for MIX 20 to request it to originate additional load balancing messages if necessary, If a received message fails the test of decision blocks 366, 368 and 374, routine 350 determines if the received message is a load balancing message (decision block 378). If a load balancing message has been received, it is processed (block 380) depending upon the type of load-balancing message. Figure 9 is a detailed flow chart of the steps performed to process a load balancing message. If the load balancing message commands the cell site base station to update its current mode to normal (such a message is sent in block 162 shown in Figure 5), routine 350 simply sets a storage location (flag) to reflect a new current mode of normal (blocks 382 and 384). If the new mode is directed retry (decision block 386) such a load balancing message is sent by block 170 shown in Figure 5), routine 350 updates the current cell mode to directed retry (block 388) and stores the list 206 of channels transmitted in the load balancing message (see Figure 6 (c)).
If, on the other hand, the load balancing message specifies the new cell mode as being directed hand-off decision block 392) (such a message is sent by block 178 shown in Figure 5), routine 350 set the current cell mode to directed ~235~5~

hand-off (block 394), and sends the number of hand-off requests specified in field 208 of the directed Han doff message (see Figure I) to MIX 20. These hand-off requests are sent together in list 363 (as many as four at a time in the preferred embodiment) of a hand-off request message (see Figure I) to reduce message transfer overhead. Although routine 350 could arbitrarily choose ongoing calls to be handed-off, optimization is provided in the preferred embodiment by selecting those calls with the weakest RSSI levels (in the hope that the adjacent cells to which the calls are handed-off may provide somewhat better service and will not need to hand-off the call to yet another cell anytime soon).
If the received load balancing message fails decision blocks 382, 386 and 392, then the new cell mode is updated to combined (block 398), the list 206 contained in the combined load balancing message (Figure I) is stored (block 400) and routine 350 originates the number of hand-off requests specified in field 208 of the load balancing message to MIX 20.
Referring once again to Figures PA and 8B, a received message which is not any of the messages already tested for is tested to determine whether it is an origination or page response message (block 404~. If the message is not of this type, it is processed according to type (block 406) for example, the message may be a diagnostic or initialization message and will be processed accordingly). If the received message is an origination or page response message, however, routine 350 polls the current cell state to determine if it is normal or directed hand-off 43 5 So decision block 406). If the mode it normal or directed hand-off, the cell is permitted to handle new calls, and routine 350 establishes communications with the mobile transceiver on the channel specified by MIX 20 (transactions between the cell site base station and the motile transceiver initiated in response to origination and page response message are conventional and communications with mobile transceivers are established in response to these messages in a conventional fashion) (block 40~).
If the current cell mode is directed retry or combined, however, the cell is not permitted to participate in new calls and instead instructs the mobile transceiver involved to try another cell by transmitting the directed retry command to the mobile transceiver. Routine 350 obtains the list of control channels for adjacent cells sent in the most recent directed retry or combined load balancing message (see blocks 390 and 400 shown in figure 9) (block 410~ and sends a directed retry command to the applicable mobile transceiver via the cell site control channel transceiver (44, 46, 48~ (block 412). Upon receiving the directed retry command sent in block 412, the receiving mobile transceiver will scan the control channels specified by the transmitted list to find the one with the highest signal strength, will lock on to that control channel, and will attempt to establish communications with the cell site base station transmitting that control channel. Since block lo of routine 150 shown in Figure 5 never includes cells in the directed retry or combined state in this list of control channels, mobile transceivers are never directed to cells which will refuse them 44 12~S~S9 access due to traffic loading although some of the listed control channels may be for cells which are not within the Lange of useful communications f the mobile transceiver).
the mode of each cell for load balancing purposes is controlled by messages sent by routine 150 executed by MIX 20, as has been described.
There is a possibility that a message could be lost or gargled during transmission, and that MIX 20 and the applicable cell site base station (To, To, To) would not agree on the load balancing mode of the cell. To remedy this problem, an audit capability is provided which periodically ensures that all cell site controllers (32, 34, 36) agree with the MIX 20 on cell mode. A query load balancing status message is sent periodically from the MIX to each cell site controller (32, 34, 36) in response to which the cell site controllers transmit back to the MIX a load balancing reply message specifying current cell mode. The audit process executed by MIX 20 compares the mode stored in the LBSTATUS table 80 of each cell with the mode information received from the cell to determine if the two values agree. If the mode values do not correspond, the audit process sends a new load balancing message to the disagreeing cell site controller and thus causes it to place itself into the proper mode. In this way, much additional data transfer overhead, which might required Jo transmit load balancing messages is avoided in the preferred embodiment.

Lowe As has been described, system 10 in accordance with the present invention gives priority to calls requiring Han doffs by using a dynamo load sharing algorithm which reserves a given number of voice channels in each cell for calls being handed-off. A key feature of cellular systems is the ability to hand-off an ongoing call from one cell to another by balancing traffic load between cells. In the preferred embodiment, blockage of calls and loss of calls during Han doffs are avoided by taking advantage of the overlapping cell coverage areas designed into a typical multi-cell system. Load balancing. pence, helps to maintain some open channels for calls requiring hand-off due to weak signal strength.
In order to assess the effectiveness of the hand-off and dynamic load balancing techniques in accordance with the present invention, different systems were modeled and studied with the help of the Cellular System Traffic Simulator (SHUTS
computer-aided simulation tool developed to provide comprehensive call-by-call simulation for mobile systems. The simulation's key parameters are specified by the user through a set of input files. Some of these parameters are: the length of simulation run, the number of cells and their coverage areas, the average and holding time, and the traffic distribution. The cellular system's geographic service area is overlaid with a rectangular grid used to locate the positions of the cell size antennas and mobile units with calls currently in progress. The typical statistics collected are: breakdown of call jet ups according to call type and call disposition; breakdown of calls according to whether they were completed normally, blocked on hand-of~s or left the service area; blocking on the RF channels and the land line trunks, etch This software tool has been used extensively by others for analyzing the performance of actual systems during planning stages.
The results of the simulation which was conducted are based on the 13-cell mature system shown in Figure 1. The parameters considered in the specification of this system include holding times, traffic mix, contours, voice channel grade of service, cell coverage, cell overlapping, mobile transceiver transit speed and mobile traffic distribution.
Cellular mobile radio systems typically have relatively short holding times ranging from about 70 seconds to 140 seconds. The call holding time for purposes of the simulation was set at a 12 seconds. Typical systems normally carry far more mobile-to-land traffic than land to-mobile traffic. Moreover, mobile-to mobile traffic is typically relatively low. A distribution of 65%
mobile-to-land traffic, 30% land-~o-mobile and 54 mobile-to-mobile traffic was assumed. The "bad service" contour of the cell in the simulation was set at -85 dim, which identifies when a hand-off attempt should be initiated. The RF channels in the simulation were dimension for 2% blocking under normal load and no load balancing. The number of RF
channels for the system shown in Figure 1 are listed in Table I below:

47 isles Cell Total Number of Channels Of 24 I I

Oil 19 C12 lo ~13 18 TALE I

For purposes of the simulation, the small cells (cell Of) was assumed to have a hand-off radius equal to two kilometers, the medium-size cells ego., cell C2) were assumed to have hand off radii of 3.2 kilometers, and the larger cells (e.g., cell C9) were assumed Jo have hand-off radii of 5.0 kilometers.
Cell overlapping is a very important factor for hand-off reliability because a cellular system layout with only small amounts of overlap has less flexibility in handling shifts in mobile user density. The classic hexagonal minimum coverage layout results in only 5.7~ overlap between any two cells if the hexagons are replaced by circle. This amount of overlap is not enough for reliable hand-office On the other hand, excessive overlap may have negative co-channel interference effects, not to mention the extra cost of either increased I 3 So transmitter power or antenna height. Overlapping to the extent shown in Figure 1 was used for purposes of the simulation. however, optimum overlapping in an actual cellular system may be determined from an actual traffic statistics measured by the system in order to minimize co-channel interference and increase hand-off reliability.
For purposes of the simulation which was conducted, mobile transceivers were assumed to have Gaussian distributed velocities with means that vary according to specific sectors of the service area.
Mobile transceivers were assumed to travel in the small cell Of with an average velocity of 30 kilometers per hour with a standard deviation of 20 kilometers per hour. In the peripheral cells ego., cell Cog), mobile transceivers were assumed to have an average speed of 90 kilometers per hour with a standard deviation of 30 kilometers per hour.
Mobile traffic distribution, an important factor for the design of cellular systems, is usually highly concentrated within certain sectors of the service area (which often centers around a major metropolitan area). Figure 10 is a 3-dimensi~nal depiction of mobile traffic density used for purposes of the simulation.
The improvement of hand-off performance provided by dynamic load balancing in accordance with the present invention is illustrated in Figure 11, which shows the probability of a mobile transceiver receiving unsatisfactory hand-off service SUE below -97 dim) with respect to percentage overload in system traffic. Figure 12 is a graphical illustration of overall system blocking perceived by mobile transceiver users as a function of percent traffic overload on the system with and 49 ~L~35~5~

without load balancing in accordance with the present invention. Both of these graphs show that load balancing in accordance with the present invention provides much lower system blocking and result in a 10 to 20% increase in system traffic handling capacity.
The effect of reserving RF channels is illustrated in Figure 13, where the following parameters are plotted against the number of RF
channels reserved for Han doffs (1) probability that a hand-off attempt has to wait for a free RF
channel; I overall system blocking perceived by mobile transceiver users; and (3) average cell blocking for calls in a particular cell. As the number of reserved RF channels it increased (by decreasing the DRTHRESH and DHTHRESH values discussed previously), the blocking on first choice cells increases buy overall system blocking is much lower. As the number of reserved channels increases, the blocking of calls within a particular cell increases. However, the overall system blocking curve exhibits a minimum so that the number of reserved channels can be chosen to minimize overall system blocking. The probability that a hand-off attempt directed to a cell must wait for a free channel decreases with an increase in number of reserved channels. In accordance with the present invention, the DRT~RESH and DHTHRESH values may be varied on a cell-by-cell basis according to user specifications, and may be dynamically varied according to system loading if desired. The DRTHRESH and DHTHRESH may be pumiced according to statistical information gathered by system 10 during actual operation.

A load balancing technique for use in a cellular mobile radio telephone system has been described which causes the system to behave as n alternative routing or progressive grading system with an increased effective traffic-handling capacity of about 10-20%. Load balancing in accordance with the present invention ensures a negligible probability of call cut off due to unsuccessful hand-ofs without increasing system cost or the number of channels allocated to the system. Although only one embodiment of the invention has been described, person skilled in the art will appreciate the many modifications that may be made. For example, although cells have been described as being capable of entering four possible states (normal, directed hand-off, directed retry and combined), only two of these two states normal and directed hand-off) are. necessary for load balancing to be effective. Hence, the DRTHRESH
value may be set to an extremely high value if desired to prevent cells from entering the directed retry or combined modes, and the system will still produce very satisfactory results. Additional reduction in message transfers may be possible while accomplishing the same functions. Moreover, load balancing in accordance with the present invention is not limited to land-to-mobile cellular radio telephone communications systems, but can be used in any alternative trunk routing arrangement in which some trunks may be overloaded while others may be only lightly loaded. Therefore, while the invention has been described with reference to a particular preferred embodiment, it is to be understood that modifications may be made without departing from the spirit of the invention or from the scope of the claims.

Claims (25)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of communicating with mobile radio transceivers including the steps of:
(1) establishing communications between mobile radio transceivers located within a first predetermined geographical area and a first plurality of stationary transceivers serving said first area;
(2) determining a channel occupancy level associated with said first geographical area indicating the number of said first plurality of stationary transceivers which are in communication with mobile radio transceivers with respect to the number of said first plurality of stationary transceivers not in communication with mobile transceivers; and (3) if said channel occupancy level determined by said determining step (2) exceeds a predetermined threshold level, transferring an ongoing communication with a selected mobile transceiver from a stationary transceiver serving said first area to a further stationary radio transceiver serving a further predetermined geographical area distinct from and overlapping said first area and also containing the selected mobile transceiver.

2. In a cellular mobile radio telephone communications system of the type including plural geographically-separated radio base stations serving corresponding plural discrete geographical cells, adjacent ones of said cells overlapping one another, each of said base stations including radio transceiver means for communicating over a prespecified plurality of voice communications channels allocated to the cell served thereby simultaneously and independently with a corresponding plurality of mobile radio transceivers located within the cell, an improvement comprising:
determining, for at least one cell, a voice channel occupancy level indicating the number of voice communications channels allocated thereto being used for communications with respect to the total number of voice channels allocated thereto;
comparing said determined voice channel occupancy level of said cell with a predetermined threshold level; and if said comparison reveals the voice channel occupancy level of said cell exceeds said threshold level, transferring communications from the base station serving the cell to the base station serving an adjacent cell.

3. A central controller adapted for communicating control signals to/from plural stationary geographically-separated radio transceivers, said plural stationary transceivers of the type serving respective corresponding overlapping geographical areas and communicating radio signals to/from mobile radio transceivers located in said areas over radio communications channels reallocated thereto, said central controller including a digital signal processor connected to communicate said control signals and programmed so as to perform the following functions:
monitor the number of radio communications channels being used for communications by each of said plural stationary transceivers;
determine, for each stationary transceiver, a measure of the number of communications channels being used for communications with respect to the number of communications channels preallocated thereto; and control stationary transceivers having high determined measures to shift communications to stationary transceivers having low determined measures and serving geographically-neighboring areas.

4. In a cellular mobile radiotelephone communications system of the type including plural stationary geographically-saparated radio transceiving stations serving corresponding discrete geographical areas, adjacent ones of said areas overlapping one another, said system further including plural mobile radio transceivers communicating radio signals with the stationary transceiving stations serving areas said mobile radio transceivers are located within, an improvement comprising:
means for determining, for each stationary transceiving station, an indication of the instantaneous capacity of the transceiving station to communicate radio signals with additional mobile radio transceivers independently of the radio signals said transceiving station is already communicating with mobile radio transceivers; and control means, operatively connected to said plural stationary transceiving stations and to said determining means, for controlling stationary transceiving stations with low determined instantaneous capacities to terminate communications with selected mobile radio transceivers located at positions also served by further stationary transceiving stations having high determined instantaneous capacities and for controlling said further stationary transceiving stations to communicate with said selected mobile radio transceivers.

5. In a cellular radiotelephone communications system of the type including first and second geographically-separated stationary radio transceivers, said first stationary transceiver for communicating radio signals over a first plurality of communications channels preallocated thereto with physically-distinct mobile radio transceivers located in a first geographical area, said second stationary transceiver for communicating radio signals over a second plurality of communications channels preallocated thereto with physically-distinct mobile radio transceivers located in a second geographical area, said first and second geographical areas overlapping one another, an improvement comprising a method of balancing the relative loading of said first and second stationary transceivers comprising the steps of:

(1) determining the number of said first plurality of channels being used for communicating signals between said first stationary transceiver and said mobile radio transceivers;
(2) determining the number of said second plurality of channels being used for communicating signals between said second stationary transceiver and said mobile radio transceivers; and (3) if said first-mentioned number exceeds a first predetermined threshold associated with said first stationary transceiver and said second-mentioned number is less than a second predetermined threshold associated with said second stationary transceiver directing a selected mobile transceiver located in both said first and second geographical areas and tuned to one of said first plurality of channels to retune to one of said second plurality of channels and begin communicating radio signals with said second stationary transceiver.

6. A method as in claim 5 further including the step of:
(4) if said first-mentioned number is less than said first predetermined threshold and said second-mentioned number exceeds said second predetermined threshold, directing a selected mobile transceiver located in both said first and second geographical areas and tuned to one of said second plurality of channels to retune to one of said first plurality of channels and begin communicating radio signals with said first stationary transceiver.

7. A method as in claim 5 wherein:
said method further includes the step of repeating said determining steps (1) and (2) periodically;
and said directing step (3) includes the steps of:

(a) periodically comparing said first-mentioned number with said first threshold, (b) setting the value of a first state variable associated with said first stationary transceiver to a normal state whenever said comparing step (a) reveals said first-mentioned number is less than said first threshold, (c) setting the value of said first state variable to a directed hand-off state whenever said comparing step (a) reveals said first-mentioned number exceeds said first threshold, (d) periodically comparing said second-mentioned number to said second threshold, (e) setting the value of a second state variable associated with said second stationary transceiver to said normal state whenever said comparing step (d) reveals said second-mentioned number is less than said second threshold, (f) setting the value of said second state variable to said directed hand-off state whenever said comparing step (d) reveals said second-mentioned number exceeds said second threshold, (g) whenever the values of said first and second state variables are unequal, testing whether any mobile transceiver tuned to a channel preallocated to the one of said first and second stationary transceivers associated with a state variable having a directed hand-off value is located in both of said first and second geographical areas, and (h) it said testing reveals at least one mobile transceiver is located in both said geographical areas, directing said one mobile transceiver to retune to a communications channel preallocated to the stationary transceiver associated with the state variable having a normal value.

8. A method as in claim 5 further including the steps of:

(a) measuring the RSSI value at said first stationary transceiver of radio signals transmitted by each of said mobile transceivers tuned to one of said first plurality of channels;
(b) determining if any of said measured RSSI
values is less than a predetermined first RSSI threshold associated with said first stationary transceiver;
(c) if said determining step (b) reveals a mobile transceiver transmitting signals having an RSSI
value which is less than said predetermined RSSI threshold, measuring the RSSI value of radio signals transmitted by said mobile transceiver and received by said second stationary transceiver; and (d) if said RSSI value measured by said measuring step (c) exceeds a second predetermined RSSI
threshold associated with said second stationary transceiver, directing said mobile transceiver to retune to one of said second plurality of channels and begin communicating radio signals with said second stationary transceiver.

9. A method as in claim 8 further including the steps of:
(a') measuring the RSSI value at said second stationary transceiver of radio signals transmitted by each of said mobile transceivers tuned to one of said second plurality of channels;
(b') determining whether any of said RSSI
values measured by said measuring step (a') is less than a predetermined first RSSI value associated with said second stationary transceiver;
(c') if said determining step (b') reveals a mobile transceiver transmitting signals having an RSSI
value which is less than said predetermined RSSI value, measuring the RSSI value of radio signals transmitted by said mobile transceiver and received by said first stationary transceiver; and (d') if said RSSI value measured by said measuring step (c') exceeds a second predetermined RSSI
value associated with said first stationary transceiver, directing said mobile transceiver to retune to one of said first plurality of channels and begin communicating radio signals with said first stationary transceiver.

10. A method as in claim 6 further including the steps of:
(5) initiating communication of radio signals between said first stationary transceiver and additional mobile transceivers located within said first area over unused ones of said first plurality of channels;
(6) initiating communication of radio signals between said second stationary transceiver and mobile transceivers located within said second area over unused ones of said second plurality of channels;
(7) if said first-mentioned number exceeds a third predetermined threshold greater than said first threshold, inhibiting said initiating step (4) but not said directing step (3); and (8) if said second-mentioned portion exceeds a fourth predetermined threshold greater than said second threshold, inhibiting said initiating step (5) but not said directing step (3).

11. A method as in claim 5 wherein said directing step (3) includes the steps of:
(a) measuring a first RSSI value indicating the amplitude of radio signals transmitted by said selected mobile transceiver and received by said first stationary transceiver;
(b) measuring a second RSSI value indicating the amplitude of radio signals transmitted by said selected mobile transceiver and received by said second stationary transceiver;
(c) determining the difference between said first measured RSSI value and a first preprogrammed absolute RSSI value associated with said first stationary transceiver;
(d) determining the difference between said second measured RSSI value and a second preprogrammed absolute RSSI value associated with said second stationary transceiver; and (e) if said first-mentioned difference is less than said second-mentioned difference, said first-mentioned number exceeds said first predetermined threshold and said second-mentioned number is less than said second predetermined threshold, directing said selected mobile transceiver to retune to one of said second plurality of channels and begin communicating radio signals with said second stationary transceiver.

12. A method as in claim 11 wherein: said method further includes the steps of:
(c1) calculating a threshold modifier value in response to said first measured RSSI value, said first and second preprogrammed RSSI values, and a further preset threshold delta factor associated with said first stationary transceiver, (c2) modifying said second threshold value with said calculated threshold modifier to obtain an adjusted threshold value, and (c3) comparing said second measured RSSI
value with said adjusted threshold value and with said second preprogrammed absolute RSSI value; and said directing step (e) only directs said selected mobile transceiver to retune to one of said second plurality of channels if said comparison reveals said second measured RSSI value exceeds both said adjusted threshold value and said second preprogrammed absolute threshold value.

13. A method as in claim 5 further including the step of repeating said directing step (3) for additional selected mobile transceivers located within both said first and second areas until said first-mentioned number is less than said first threshold.

14. A method as in claim 5 wherein said directing step (3) further includes the steps of:
measuring the RSSI value of radio signals transmitted by mobile transceivers communicating signals with said first stationary transceiver over said first plurality of channels and received by said first stationary transceiver; and selecting the mobile transceiver located in both said first and second areas and transmitting radio signals having the weakest measured RSSI value.

15. In a cellular radiotelephone communications system of the type including first and second geographically-separated stationary radio transceivers, said first stationary transceiver for communicating radio signals over a first plurality of radio communications channels preallocated thereto with physically-distinct mobile radio transceivers located in a first geographical area, said second stationary transceiver for communicating radio signals over a second plurality of communications channels preallocated thereto with physically-distinct mobile radio transceivers located in a second geographical area, said first and second geographical areas overlapping one another, a controller means connected to transmit control signals to and receive control signals from said first and second stationary radio transceivers, said controller means for:
(1) determining the number of said first plurality of channels being used for communicating signals between said first stationary transceiver and mobile radio transceivers, (2) determining the number of said second plurality of channels being used for communicating signals between said second stationary transceiver and mobile radio transceivers, and (3) directing a selected mobile transceiver located in both said first and second geographical areas and tuned to one of said first plurality of channels to retune to one of said second plurality of channels and begin communicating radio signals with said second stationary transceiver if said first-mentioned number exceeds a first predetermined threshold associated with said first stationary transceiver and said second-mentioned number is less than a second predetermined threshold associated with said second stationary transceiver.

16. A system as in claim 15 wherein said controller means also directs a selected mobile transceiver located in both said first and second geographical areas and tuned to one of said second plurality of channels to retune to one of said first plurality of channels and begin communicating radio signals with said first stationary transceiver if said first-mentioned number is less than said first threshold and said second-mentioned number exceeds said second threshold.

17. A system as in claim 15 wherein said controller means includes a digital signal processor preprogrammed to perform the following functions:
(a) periodically determine said first-mentioned and second-mentined numbers;
(b) periodical compare said first-mentioned number with said first threshold;
(c) set the value of a first state variable associated with said first stationary transceiver to a normal state whenever said comparison reveals said first-mentioned number is less than said first threshold:
(d) set the value of said first state variable to a directed hand-off state whenever said comparison reveals said first-mentioned number exceeds said first threshold;
(e) periodically further compare said second-mentioned portion with said second threshold;
(f) set the value of a second state variable associated with said second stationary transceiver to said normal state whenever said further comparison reveals said second-mentioned number is less than said second threshold;
(g) set the value of said second state variable to said directed hand-off state whenever said further comparison reveals said second-mentioned number exceeds said second threshold;
(h) test whether any mobile radio transceiver communicating with the one of said first and second stationary transceivers associated with a state variable having a directed hand-off value is in both said first and second geographical areas whenever the values of said first and second state variables are unequal; and (i) direct a mobile transceiver revealed by said test to be in both areas to retune to communications channels preallocated to the stationary transceiver associated with the state variable having a normal value.

18. A system as in claim 15 wherein:
said first stationary transceiver includes means for measuring the RSSI value of radio signals transmitted by each of said mobile transceivers communicating with said first stationary transceiver and received by said first stationary transceiver;
said second stationary transceiver includes means for measuring the RSSI value of signals transmitted by a selected mobile transceiver and received by said second stationary transceiver; and said controller means includes a digital signal processor preprogrammed to perform the following further functions:
(a) determine if any RSSI value measured by said first stationary transceiver RSSI measuring means is less than a predetermined first RSSI value;
(b) control said second stationary transceiver measuring means to measure the RSSI value of signals transmitted by mobile transceivers revealed by said determination to be transmitting signals having an RSSI

value less than said predetermined first RSSI value as measured by said first stationary transceiver measuring means; and (c) direct said mobile transceiver to retune to a communication channel preallocated to said second stationary transceiver if said RSSI value measured by said second stationary transceiver measuring means exceeds a predetermined second RSSI value associated with said second stationary transceiver.

19. A system as in claim 15 wherein said controller means includes a digital signal processor preprogrammed to perform the following functions:
control said first stationary transceiver to initiate communication of radio signals with additional mobile transceivers located within said first geographical area over unused ones of said first plurality of channels;
control said second stationary transceivers to initiate communication of radio signals with additional mobile transceivers located within said second geographical area over unused ones of said second plurality of channels;
inhibit said first-mentioned control function while continuing to redirect communications of said selected mobile transceiver if said first-mentioned number exceeds said first predetermined threshold and said second-mentioned number is less than said second predetermined threshold whenever said first-mentioned number exceeds a third predetermined threshold greater than said first predetermined threshold; and inhibit said second-mentioned control function while continuing to redirect the communications of said selected mobile transceiver whenever said second-mentioned number exceeds a fourth predetermined threshold greater than said second predetermined threshold.

20. A system as in claim 15 wherein:
said first stationary transceiver includes means for measuring a first RSSI value for radio signals transmitted by said selected mobile transceiver and received by said first stationary transceiver;
said second stationary transceiver includes means for measuring a second RSSI value for radio signals transmitted by said selected mobile transceiver and received by said second stationary transceiver; and said controller means includes a digital signal processor connected to receive said first and second measured RSSI values and preprogrammed to perform the following functions;
(a) determine the difference between said first measured RSSI value and a first preprogrammed absolute RSSI value associated with said first stationary transceiver, (b) determine the difference between said second RSSI value and a second preprogrammed absolute RSSI
value associated with said second stationary transceiver, and (c) direct said selected mobile transceiver to retune to a communication channel allocated to said second stationary transceiver if said first-mentioned difference is less than said second-mentioned difference, said first-mentioned number exceeds said first predetermined threshold, and said second-mentioned portion is less than said second predetermined threshold.

21. A system as in claim 20 wherein said digital signal processor is preprogrammed to perform the following further functions:
(c1) calculate a threshold modifier value in response to said first measured RSSI value, said first and second threshold values and a further preset threshold delta factor associated with said first stationary transceiver, (c2) modify said second threshold value with said calculated threshold modifier to obtain an adjusted threshold value, (c3) compare said second measured RSSI value with said adjusted RSSI value and with said second absolute RSSI value, and (c4) condition said directing function (c) on said second measured RSSI value exceeding both said adjusted threshold value and said second absolute RSSI
value.

22. A system as in claim 15 wherein said controller means directs additional selected mobile transceivers to retune to communications channels preallocated to said second stationary transceiver until said first-mentioned number is less than said first threshold and/or until said second-mentioned number will exceed said second threshold if a further mobile transceiver is so directed.

23. A system as in claim 15 wherein said first stationary transceiver includes:
means for measuring the RSSI value of radio signals transmitted by a mobile transceiver tuned to one of said first plurality of channels; and means for selecting said mobile transceiver in response to measured RSSI values.

24. A system as in claim 23 wherein said selecting means includes means for choosing the mobile transceiver transmitting radio signals having the weakest measured RSSI value.

25. In a cellular radiotelephone communications system of the type including first and second geographically-separated stationary radio transceivers, said first stationary transceiver for transmitting radio signals over a first plurality of radio communications channels preallocated thereto to a first geographical area and for receiving radio signals transmitted over said first plurality of channels, said second stationary transceiver for transmitting radio signals over a second plurality of radio communications channels preallocated thereto and for receiving radio signals transmitted over said second plurality of channels, a mobile radio transceiver located in both said first and second geographical areas and including:
tunable transceiving means for transmitting and receiving radio signals over a radio communications channel;
tuning means connected to said transceiving means for tuning said transceiving means to one of said first plurality of channels; and control means connected to said tuning means for controlling said tuning means to retune said transceiving means to one of said second plurality of channels if a measure of the number of said first plurality of communications channels being used by said first stationary transceiver for transmitting/receiving radio signals exceeds a predetermined threshold and the number of said second plurality of communications channels being used by said second stationary transceiver for transmitting/receiving radio signals is less than said predetermined threshold.