CA2009234C

CA2009234C - Base station emulator

Info

Publication number: CA2009234C
Application number: CA002009234A
Authority: CA
Inventors: John David Kaewell, Jr.; Scott David Kurtz
Original assignee: InterDigital Inc
Current assignee: InterDigital Technology Corp
Priority date: 1987-11-20
Filing date: 1990-02-02
Publication date: 1994-10-04
Anticipated expiration: 2010-02-02
Also published as: DE4003820A1; NL9000331A; DE4003820C2; BE1003825A3; SE9201163L; FR2658372A1; AU4915890A; AU617030B2; SE9201163D0; US4935927A; CA2009234A1; FR2658372B1; SE470393B

Abstract

A wireless digital telephone system containing at least one emulated base station plus one or more subscriber stations, the emulated base station comprising a station similar to the subscriber station but having the capability of initi-ating a synchronization process whereby it is enabled to assign time slots to the subscriber station within the frame pattern of an amplitude signal by means of monitoring for positive edges in the signal.

Description

a~qa31 BAC~GnOUNt) 0~ TtlE lNVENTlON

In general, pee~ent day telepllone sy~tems are lncreas-lngly uslng wlrele~s technology ~or long dl~tancQ calls and. 1ll some lnstances, have begun tne use of dlg~tal technology: how-ever, no system ln general ~Q today has been capable of ,1 - .

` 20Q~2~4 providing effective and efficient wireless digital technology for local calls to and from individual subscribers. Such tech-nology has been disclosed in various recent patents commonly owned by the present applicants' assignee, as, for example, in U.S. Patent No. 4,644,561, dated February 17, 1987 and U.S.
Patent No. 4,675,863, dated June 23, 1987. The technology disclosed in these patents provides base stations in communi-cation with both a central office and a plurality of subscriber stations utilizing digital wireless time division circuits wherein there are repetitive sequential slot positions in a transmit channel bit stream, each slot being associated with a particular subscriber.
The base stations used in the above time division system are relatively complex and expensive but economically feasible for a large system serving a large number of subscri-bers: however, for relatively small systems serving a relatively small number of subscribers it may be economically infeasible.
In addition, such a system utilizes a pair of frequencies, one for transmission and one for reception, and, in view of the limited amount of channels available in the spectrum, it would be highly advantageous if only one frequency could be effec-tively used.
It is, therefore, an object, of the present invention to provide what may be called a simulated or emulated base station which can be effectively substituted for an actual base station in certain situations.
Another object is to provide a system that can be utilized for plural subscribers but which is operable on only a single frequency.

2~0!~4 ~, Other objects will become apparent from the following description and claims:

SUMMARY OF THE INVENTION
In essence, the system of the present invention utilizes what is, in effect, a modified subscriber station to act as a simulated or emulated base station, thereby consid-erably decreasing the total cost and complexity of the qystem.
This emulated base station essentially differs from the subscriber station only in being able to initiate the synchroni-zation process, whereas the subscriber unit only acts to scanthe RF signals sent out by the emulated base station until it finds the frequency and slot assigned to it. In the intervals between transmissions of the RF signals the emulated base station is adapted to receive RF signals from the ~ubscriber units. In this manner, the subscriber unit may either talk to the emulated base station which then acts as another subscriber station, or it may talk to another subscriber station that has been synchronized therewith by the emulated base station.
BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a block diagram showing an overall system embodying the present invention.
Fig. 2 is a diagramatic illustration of the RCC wave-form used in the standard base station.
Fig. 3 is a diagramatic illustration of the RCC wave-form used in the present invention.

20~g234 Fig. 4 is a diagramatic illustration showing the posi-tive edges of the amplitude of the received signal used in course synchronization of the pre~ent invention.
Fig. 5 is a block diagram of the circuit for obtaining course synchronization in the present invention.
Fig. 6 is a block diagram of the received AGC circuit used in the present invention.
Fig. 7 is a block diagram showing the frequency acqui-sition circuit used in the present invention.
Fig. 8 is a diagramatic illu~tration of a wireless phone system configuration embodying the present invention.
Fig. 9 is a diagramatic illustration similar to Fig. 8 but ~howing a dual subscriber sy~tem.
Fig. 10 i9 a diagramatic illu~tration of the frame format of the dual subscriber system of Fig. 9.
Fig. 11 is a diagramatic illustration of the frame format of a plurality of dual subscriber systems.
Fig. 12 is a diagramtic illustration of a system embodying the present invention which is used for monitoring one or more functions.
Fig. 13 is a diagramatic illustration of a repeater system embodying the present invention.
Fig. 14 is a diagramatic illustration of a systent embodying the present invention utilizing multiple repeaters.
Fig. 15 is a diagramatic illu~tration of a sy~tem embodying the present invention where a single repeter is used to drive a plurality of other repeaters as well as subscriber units.

- 20~3234 DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENTS

The overall internal operation of the system, gener-ally designated 10, i9 shown in block diagram form in Fig. 1.
In this system, during a telephone conversation, a person speaks into the telephone 12 and the ~peech signal i8 sent to the local telephone interface unit 14. The signal is digitized by the codec 16 and the resultant digital data stream is then fed to the speech processor 18 which compresses the speech data to a lower data rate. The compressed data is then fed to the modem 20 via line 22 and double-throw switch 24, the modem acting to convert the data stream to a spectrally efficient analog signal.
This analog signal is fed to the radio 26 via line 28. The radio upconverts the signal to a radio frequency (RF) signal and then transmits this RF signal via the antenna 30.
In the interval~ between transmi~ions of the RF
signals, the unit is adapted to receive RF signal~ from a sub-scriber unit. The radio 26 downconverts each of these RF
signals to an IF signal and feeds this IF signal to the modem 20 via line 32. The modem 20 demodulates the IF ~ignal to form a digital signal which is then fed to the speech proces~or via switch 24 and line 36. The speech proces~or thereupon act~ to expand the signal to a digitized speech signal and this digitized signal is then fed into the codec 16 which outputs an analog ~peech signal to the telephone 12 via the telephone inter-face 14.
The data transmission mode is similar to that de~cribed above except that the telephone is replaced by a data ` ` -` ` ~009234 i~ terminal or computer 38 and the telephone, codec and speech processor are bypas~ed by means of the alternate position of the switch 24 that is then coupled to the terminal 38 by line~ 40 and 42.
5The modem 20 and radio 26 are both coupled to a control unit 44. The control unit 44 is initially set to a predetermined slot, modulation and training mode for the modem and to a predetermined RF frequency and power level for the radio. However, these parameters can be adjusted by the lO9ub9criber unit in the event they are not adequate to provide a satisfactory reception at the subscriber station.
In a system utilizing an actual base station, such as, for example, the system described in the aforesaid Patent No.
4,675,863, the transmitted waveform is divided into a multi-15plicity (i.e. 45) msec. frames. Each frame is, in turn, divided into four ll.25 msec. slots. The base station transmits on all four slots to produce a 100% duty cycle modulation waveform, the lone exception being the radio control channel (RCC). The RCC
slot is slightly shorter than 11.25 msec and this causes a small gap in the modulation at the beginning of every frame. This gap is known as an AM hole. A diagram of the waveform of the RCC
channel in the actual base station format is shown in Fig. 2.
In the system of the present invention, however, there is no transmission of a 100~ duty cycle waveform. Instead, there is a transmission on only one slot per frame (a 25~ duty cycle waveform), as shown in Fig. 3. This modified frame format neces-sitates changes in coarse synchronization, automatic gain control (AGC) and frequency acquisition. These changes are indicated in the following description:

~`9~2 3 ~

~ Coarse Synchronization Since the system of the present invention utilizes only a 25% duty cycle waveform, it monitors the amplitude of the received signal and searches for positive edges in the amplitude signal. These positive edges are illustrated in Fig. 4. The ~ubscriber unit adjusts its frame timing to align with the occur-rence of these positive edges.
The circuit for obtaining the above type of coarse synchronization is shown in block diagram form in Fig. 5 where the received signal is shown a~ being fed into an amplitude computation device 50 which produces a computer amplitude signal that is then passed to a comparator 52 where it is compared to a predetermined threshold signal, thereby forming a digital signal (1 = signal present, O = no signal present). This digital signal is fed into an edge detector 54 that outputs a strobe to indicate the detection of a positive edge.

AGC
The 25% duty cycle modulation requires a distinct type of receive AGC circuit which avoids tracking when there is no 9ignal present. A slow rise fast decay AGC is, therefore, provided. This is shown in Fig. 6 where the received signal is fed into an amplitude computation device 56, which may take the form of a pre-programmed ROM, from which a resulting amplitude signal is fed into a comparator 58 in which it is subtracted from a predetermined threshold value to form a difference signal. This difference signal is fed through one of two scaling multiplier~, shown at 60 and 62, into a low pass filter 2Q092~4 -comprising an adder 64 and a delay means 66 connected through a loop 68. One or the other of the two multipliers is used in accordance with the sign of the difference signal. If the difference signal is positive, the slow decay in the AGC control signal is implemented. If the difference signal is negative, a fast rise in the AGC control signal is implemented. The output of the filter i8 the gain signal which is then fed to the gain control unit 44 shown in Fig. l.

Coarse Frequency Acquisition Since in the 25~ duty cycle frame format it is not required to perform frequency acquisition during the off time (75~ null time) and since the frame timing is not known at the time when frequency acquisition is performed, a modified form of frequency acquisition circuit has been provided, as shown in Fig. 7. In this circuit the received signal is fed into a Discrete Fourier Transform (DFT) computation device 70 which outputs the high band energy (energy in the frequency band above the center frequency) and the low band energy (energy in the frequency band below the center frequency). The high band energy output is subtracted from the low band energy output at the adder 72 and the output thereof is fed to a mixer or multi-plier 74. The received RF signal is also passed to a stripping means 76 which strips off the sign of the signal (negative or positive), thereby determining only the amplitude of the signal.
The stripped signal is then fed to a filter 78 which smooths the signal by averaging it out. The output from the filter 78 i~
fed, via amplifier 80, to the multiplier 74.

~ 2~û9~4 The primary purpose of the circuit through 76, 78, and 80 is to prevent the action of noise on the output signal while accentuating the signal itself. In this respect, since noise generally has a small amplitude, it is effectively filtered out during the smoothing process. On the other hand since the actual signal generally has a relatively large amplitude it is, in effect, highlighted by adding the smoothed or filtered signal to the mixer 74.
The scaled signal leaving the mixer 74 is balanced between the high and low energy frequencie~, and this balanced signal, that is proportional to the short term average amplitude of the received signal, is fed into a lowpass filter comprising an adder 82, and a delay means 84 which are looped at 86. The delay means 84 causes the output signal 88 to the VCXO control to represent the output immediately prior to the output actually fed into the lowpass filter. The VCXO control is used to adjust the frequency of the master oscillator in the system.
After initial or course synchronization has been effected, the system is in an idle voice mode but is fully set up for voice operation. If the phone at either end goes off-hook, the phone at the other end will ring until the ringing phone i8 answered or the ;nitiating phone goe~ on~hook.
The calls are set up by a voice code word (VCW) at the beginning of every voice slot, this code word indicating an off-hook condition at the initiating station. When this occurs, the station acting as an emulated base station then appears to itself go off-hook to the central office-(CO) thereby making a connection to the central office. The initiating subscriber station then proceeds to complete the call by dialing the desired number. When the initiating subscriber unit goes on-hook, the emulated base station is so informed by the VCW and presents an on-hook appearance to the central office.
When the emulated base station detects a ring signal from the central office, the subscriber unit is caused to ring by means of the corresponding VCW from the emulated base station. When the subscriber unit thereafter goes off-hook, the emulated base station is so informed via the corresponding VCW
and it then pre~ents an off-hook appearance to the central office.
The above type of wireless phone ~ystem configuration is exemplified in Fig. 8 where the subscriber unit 90 is shown in wireless communication via antennas 92 and 94 with the emu-lated ba~e station 96. The station 96 is in wireline communi-cation via line 98 and interface 100 with the central office.

Dual Subscriber System The above-described system can be employed with a dual subscriber arrangement as shown in Fig. 9. In thi~ system each channel is capable of supporting two complete conversations without the necessity of using a duplexer. In this respect, a dual subscriber unit 102 i~ connected by wires 104 and 106 to a pair of sub~criber telephone sets 108 and 110. The subscriber unit 102 i~ in wirele~s communication via antennas 112 and 114 with an emulated dual base station 116. The unit 116 i9 connected to the central office by wire lines 118 and 120.

0~9~23~

The two separate subscribers 108 and 110 utilize a time slot arrangement, such as disclosed in the aforesaid Patent No. 4,675,863, wherein each ~ubscriber is assigned a separate slot. The frame format for this arrangement is shown in Fig. 10 where four slots are shown, numbered 1, 2, 3 and 4. the first two slots are used for the emulated base station and the last two are used for the two subscribers.
A plurality of dua] subscriber systems may be operated on different channels without duplexers by synchronizing all of the emulated base station transmi9sions. This i9 illustrated by the frame format shown in Fig. 11 where channel 1 is shown above and channel n (indicating any desired number of channels in between) is shown below. On each channel, the first two slots are for transmis~ion and the last two are for reception.

Paged Remote Service One emulated base station may be used with a plurality of different subscribers, one at a time. In such arrangement, for reception, the subscribers continuously monitor the transmis-sions of the radio control channels (RCC), described more fully in the aforesaid Patent No. 4,675,863, until a particular sub-scriber is paged by the emulated base station by means of the subscr;ber's ID Number (SID). After receiving a page, the subscriber initiates a transmission back to the emulated base station using the synchronization process described above. For initiating a call, the subscriber transmits on the RCC using the previously described synchronization process.

`'`' ' 20Qg2~4 Monitoring Function The present system may be used for monitoring one or more functions. In this respect, using a computer as a controlling/data logging device, a plurality of subscribers may be periodically polled to report on some function such as temperature, weather conditions, security, water/flood warnings, low fuel warnings, remote gas, electric or water meter readings, etc. This is illustrated in Fig. 12 where an emulated base station 122 is in wireless communication with a plurality of subscriber units respectively designated 124, 126 and 128. The unit 122 is in wire line connection with both a telephone 130 for voice communication and a computer or data terminal 132 for data input. Similarly, each subscriber unit i9 connected both to a respective telephone 134, 136 or 138 for voice communication and to a data device, as at 140, 142 or 144 respec-tively.

Repeater System An important use of the present system i8 a~ a repeater to extend the range of the system. In this arrange-ment, the emulated base station may be used to overcome inter-fering obstacles such as mountains and the like. Fig. 13 illustrates this function, showing a subscriber unit 146 in wireless communication with an emulated base station 148 on the summit of a mountain. The unit 148 is also in wireless communi-cation with a standard base station 150 connected to a centraloffice.

~ ~'.,'` f' 20~9~34 The relative simplicity and inexpensiveness of the emulated base station makes it very cost effective as a repeater unit. It can also be used a~ a repeater to extend the long distance range of the system regardless of the presence or absence of obstructions. By utilization of the time slot arrangement, the repeater unit, without the use of any duplexer, fits into the complete system while remaining transparent to both the standard base station and the subscriber. It can, of course, also be interposed between the subscriber and another emulated base station instead of a standard base station. This can be provided in multiple stages from one emulated base station to another to greatly increase the range of the system in a relatively inexpensive manner. This is illustrated in Fig.
14 where a series of repeater units 152 are interposed between the subscriber 154 and the base station 156.
In addition to extending the range of the system, the repeater unit serves to clean up the actual base station signal via equalization before retransmission to the subscriber.
One repeater can also be used in what may be termed a repeater star system to drive multiple repeaters and/or subscrib-ers. This i~ illustrated in Fig. 15 where the single repeater unit 158 ;9 in wireless communication with ancillary repeater~
160 and 162 as well as with one or more subscribers such as at 164. The ancillary repeater8 are themselves in wireless communi-cation with subscriber~ such as shown at 166, 168, 170, 172 and 174 a~ well as with other ancillary repeaters such as at 176.
Any one of the ancillary repeaters, such as repeater 162, may be used as the final repeater in direct communication with the base station ;ndicated at 178.

-~ ~û09~34 -Multiple repeater~ may be placed at one location, on different channels and synchronized so that their transmissions and receptions occur simultaneously, thereby avoiding the use of duplexers. In such a configuration, a master repeater is used to monitor the RCC channel of the base station and relays the monitored information to the various subscribers via the emu-lated base station's RCC. In such a configuration, on call setup, the subscriber~ are each assigned a repeater channel.

Claims

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

1. In a wireless digital communication system that includes at least two stations in RF communication with each other and wherein one station initiates the timing of RF signals and the other station synchronizes its timing in accordance with the initiated timing, a synchronization means at said other station comprising a course frequency acquisition circuit, said circuit comprising:
computation means which separates the signal received from said one station into high band and low band energy frequencies;
means to subtract the high band energy output from the low band energy output to obtain a resultant signal;
means to strip off the sign of the resultant signal to determine only the amplitude thereof;
means to accentuate the stripped signal while substantially filtering out noise; and means to feed the stripped signal to a voltage controlled crystal oscillator (VCXO) which provides the timing at said other station.

2. A wireless digital telephone system comprising at least two stations including a first station and a second station in communication with each other by means of RF frequency signals, said RF frequency signals comprising waveforms divided into a multiplicity of time frames, each frame comprising a single time slot;
a control means at each station;
amplitude monitoring means at each station that is controlled by said control means to monitor the amplitude of signals from another station and to determine the location of positive edges in the waveforms of such signals by comparing the amplitude of each signal from another station with a predetermined threshold signal;

frame adjustment means at each station that is controlled by said control means to adjust the frame timing therein to align the frame with occurrences of the positive edges;
said first station having frame synchronization initiating means and said second station having frequency acquisition means to synchronize the timing of signals received from said first station with the timing initiated by said first station;
said first station differing essentially from said second station only by the inclusion in said first station of said synchronization initiating means;
said system further including a slow rise, fast decay automatic gain control (AGC) circuit that avoids tracking in the absence of a signal, said circuit comprising an amplitude computation means into which a received signal is fed and which outputs an amplitude signal;
a comparator for receiving said amplitude signal and subtracting it from a predetermined threshold value to form a difference signal; and means to determine the positive or negative sign of said difference signal to selectively implement a slow decay or a fast rise in the AGC signal; and means to feed the implemented signal to said control means.

3. A telecommunication system for duplex communication between a primary station and one or more secondary stations over one of a plurality of available frequencies, the system comprising:
the primary station having:
(i) means for transmitting synchronization information including the assignment of time slots on a selected frequency, at least two time slots for transmission from said primary station and at least two time slots for reception by said primary station;
(ii) means for transceiving a plurality of duplex communications on the selected frequency including:

(a) means for transmitting a signal for a first duplex communication on the selected frequency in a first assigned transmit slot;
(b) means for transmitting a signal for a second duplex communication on the selected frequency in a second assigned transmit slot;
(c) means for receiving a signal for said first duplex communication transmitted from a secondary station on the selected frequency in a first assigned reception slot;
(d) means for receiving a signal for said second duplex communication transmitted from a secondary station on the selected frequency in a second assigned reception slot; and the secondary stations having:
(i) means to receive the synchronization information from the primary station and to identify the selected frequency and assigned transmit and receive time slots for signals of a duplex communication with said primary station;
(ii) means for receiving the signals for the duplex communication with said primary station on the selected frequency in the assigned primary station transmit slot; and (iii) means for transmitting the signals for the duplex communication with said primary station on the selected frequency in the assigned primary station receive slot.

4. A method for duplex communication between a primary station and one or more secondary stations over one of a plurality of available frequencies, the method comprising:
(i) transmitting synchronization information from the primary station to the secondary stations including the assignment of time slots on a selected frequency, at least two time slots for transmission from said primary station and at least two time slots for reception by said primary station;
(ii) transmitting from the primary station a signal for a first duplex communication on the selected frequency in a first assigned transmit slot;

(iii) transmitting from the primary station a signal for a second duplex communication on the selected frequency in a second assigned transmit slot;
(iv) receiving by the primary station a signal for said first duplex communication transmitted from a secondary station on the selected frequency in a first assigned reception slot;
(v) receiving by the primary station a signal for said second duplex communication transmitted from a secondary station on the selected frequency in a second assigned reception slot.

5. A wireless digital telephone system comprising a base station emulator and at least one subscriber station in communication with each other by a single radio frequency channel for both transmission and reception, said channel comprising waveforms divided into a multiplicity of frames, each frame comprising a single slot;
both the base station emulator and the subscriber station being in time frame synchronization with each other; and said base station emulator being substantially identical with said subscriber station except that the base station emulator provides synchronization signals and the subscriber station has acquisition means to adjust the time frames thereof in accordance with said synchronization signals.

6. The system of claim 5 wherein said emulated base station is in communication with a central station.

7. The system of claim 5 wherein there are at least two subscriber stations, the time frame having a separate slot for each subscriber station.

8. The system of claim 5 wherein said emulated base station is in wireless communication with a series of similar emulated base stations, the last emulated base station in the series being in wireless communication with a subscriber station.