WO2007107090A1

WO2007107090A1 - Method and apparatus for enabling soft handoff in an ofdma-based communication system

Info

Publication number: WO2007107090A1
Application number: PCT/CN2007/000848
Authority: WO
Inventors: David Comstock; Jianmin Lu; Anthony C.K. Soong; Yunsong Yang; Zhigang Rong; Jung Woon Lee
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2006-03-17
Filing date: 2007-03-16
Publication date: 2007-09-27
Also published as: CN101366205A; CN101366205B; WO2007107090A8

Abstract

A method of enabling a soft handoff in a communication system including a plurality of base stations and at least one mobile station, and base station includes at least one sector. The method includes the steps of: utilizing the base station to group sectors from a list of potential soft handoff sectors to form a soft handoff group; utilizing the mobile station to measure combined pilot signal strength of each soft handoff group; selecting a desired soft handoff group based upon combined pilot signal strength; and updating member sectors in the mobile station's current soft handoff group responsive to the desired soft handoff group.

Description

METHOD AND APPARATUS FOR ENABLING SOFT HANDOFF IN AN OFDMA-BASED COMMUNICATION

SYSTEM

Field of the Technology

The present invention relates generally to wireless communications systems, and more particularly, to novel methods and apparatus for providing soft handoff in a wireless communication system. Background of the Invention

In a cellular communication network based on orthogonal frequency division multiplexing (OFDM), a base station communicates with mobile stations that are within the base station's coverage by using signals that are orthogonal in frequency. Moreover, current third generation (3G) systems achieve a significant increase in throughput over second generation (2G) systems by taking advantage of multi-user diversity gain. That is, for point to multipoint systems such as forward link systems, all resources of a base station are dedicated to a single user (also referred to as a mobile station).

Whenever possible, a scheduler within the base station chooses the user with the best radio reception from among a set of users. If the set of users is large enough and if the channel fading of each user is independent, there is almost always a user with good radio reception. Consequently, the base station avoids the expense of sending information to a user with poor radio reception.

Furthermore, to facilitate mobility (the movement of a mobile station through a service area), a fast sector selection is employed. This technology allows the mobile station to quickly switch the transmission of data from one sector to another. It is important to realize that although the mobile station switches from sector to sector, the mobile station only receives a signal from only one sector.

For a mobile station at an edge of a cell (which is the boundary region between two or more sectors), even though the base station transmits to this mobile station with maximum power, the received signal is often received with very low power. As a result, this mobile station receives very poor radio reception and thus its data throughput is very low.

This has several effects on system performance. One effect is that if that particular mobile station requires a certain QoS, the base station must expend significant resources to serve this mobile station. The result of which is a significant decrease in total system throughput. Another effect is that the perceived user experience for that mobile station is very poor due to the fact that the data rate sustained with that link is very low. This is a significant issue because users expect to have the level of service regardless of where they are located in the sector.

Summary of the Invention

Given the previously described state of the art, the present invention contemplates that methods and apparatus to increase the throughput of users at the edge of a cell are now needed. In response, the present invention discloses novel methods and apparatus for soft handoff in a communication system - particularly an OFDMA-based communication system.

In accordance with the present invention, resources that are used to serve a mobile station, whether these resources are in the distributed assignment zone or localized assignment zone, must be the same. In addition, an identical hopping pattern of sub-carriers is set, especially in an OFDM system for resources assigned by a scheduler. The methods and apparatus of the present invention also provide a variety of schemes of using pilot signals to detect single or combined soft handoff signals, grouping sectors into a plurality of soft handoff groups, creating a list of potential soft handoff sectors, and using a channel quality information feedback for managing sectors that participate in the soft handoff.

The present invention provides soft handoffs for mobile stations at the edge of a sector with reliable and minimal signaling overhead, and thus improves the performance of the system.

The present invention provides a system to enable soft handoff for mobile stations in order to improve performance, particularly for mobile stations at an edge of a sector with reliable and minimal signaling overhead. The present invention provides that base stations may avoid transmitting soft handoff data where the soft handoff data would occupy same resources as a common pilot channel for any of sectors of a Soft Handoff Group.

The present invention further provides that base station may determine which sectors are in a Soft Handoff Set and which Soft Handoff Groups a mobile station may use at a given time. A base station may identify available Soft Handoff Sets and Groups when an Active Set for a mobile station is assigned. In a message assigning the Active Set, message record fields for sectors in a same Soft Handoff Set may be placed one right after another. A field in a sector's record may indicate whether the sector is a start of a new Soft Handoff Set.

The present invention still further provides that a base station may identify members of its own Soft Handoff Set that is available for Soft Handoff Group operation to facilitate a mobile station requesting soft handoff operation when a data connection is initially setup.

The present invention still further provides that a mobile station may determine Soft Handoff sets and Soft Handoff Groups, and request service from a Soft Handoff Group. The Soft Handoff Group is identified by the mobile station using a scrambling code on a reverse link control channel in conjunction with a message field. The present invention also provide a method of using a scrambling code to identify a Soft Handoff Group ID in a forward link.

The present invention also provides that Channel Quality Information (CQI) of a combined channel for a Soft Handoff Group may be fed back to a base station from a mobile station in a sector. In one embodiment, difference between a combined channel CQI and a serving sector CQI is described.

The present invention also further provides that Soft Handoff Group transmission for the 3GPP2 Strictly Backward Compatible (SBC) mode of IxEV-DO Rev C may be performed on a traffic data channel, especially in OFDM portion. In a Soft Handoff Group (SHOG) transmission, a macro antenna in the group may serve as a single antenna in the sector. Thus a group with multiple macro antennas may serve an Access Terminal (AT) in a Multiple-Input Multiple-Output (MIMO) scheme. The following description and drawings set forth in detail a number of illustrative embodiments of the invention. These embodiments are indicative of but a few of the various ways in which the present invention may be utilized.

Brief Description of the Drawings

For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:

FIG. 1 shows an illustrative example of one embodiment of a base station with a main/remote architecture in a communication system according to the present invention; and

FIG 2 shows illustrative examples of managing schemes for soft handoff sectors in a communication system according to the present invention.

FIG. 3 is a flow chart depicting determination of a sector's Soft Handoff Groups (SHOGs) and corresponding SHOG IDs based on the information received according to the present invention; and

FIG. 4 is a flow chart providing a Soft Handoff Group ID of an active Soft Handoff Group using a scrambling code on a forward link control channel according to the present invention.

Detailed Description of the Invention

The present invention provides unique methods and apparatus for soft handoffs in an OFDMA-based communication system. It is understood, however, that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components, circuits, signals, messages, protocols, and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to limit the invention from that described in the claims. Well known elements are presented without detailed description in order not to obscure the present invention in unnecessary detail. For the most part, details unnecessary to obtain a complete understanding of the present invention have been omitted inasmuch as such details are within the skills of persons of ordinary skill in the relevant art.

In a soft handoff, a mobile station receives data simultaneously from more than one sector. The same data is sent from multiple sectors. The sectors that send the same data are called the sectors that participate in the soft handoff. The signals from the participating sectors are combined in a radio channel for an OFDM system.

Consequently, in accordance with one aspect of the present invention, the resources that are used to serve the mobile station are the same in either a distributed assignment zone or a localized assignment zone. In one embodiment, where the soft handoff participating sectors are collocated in one cell site, a scheduler located at the cell (called a distributed scheduler) assigns the same resources to serve the mobile station and transmits the same data with the resources at each participating sector. As long as two signals in an OFDM transmission arrive at a mobile station within the period of the cyclic prefix, the mobile station can detect a combined signal with significant improvement in signal quality and consequently achieve higher throughput.

In accordance with another aspect of the present invention where participating sectors are from different cells, a centralized scheduler is used to assign the same resources to serve a mobile station in each sector. Consequently, a fast cell to cell communication is used to minimize the scheduling delay. In one embodiment, a base station includes a main module with a plurality of remote modules. The main module usually contains all base band processing of an entire cell while each of the plurality of remote modules usually contain all radio frequency processing for one sector of the cell.

FIG. 1 depicts a base station with such an architecture. The architecture includes one main module 120 and three remote modules 111, 112, and 113 for a cell configured with three sectors 101, 102, and 103. In one embodiment, the main module 120 groups multiple cells together in a centralized location. This allows inter- cell communication to be performed via a backplane communication among the main module 120 and other main modules in an OFDM system. It should be clear to those skilled in the art that once the main module 120 and each of the other main modules are centrally located, a variety of other inter-cell communications can be performed including, but not limited to, a fast Ethernet connection between the main modules.

In another embodiment, this inter-cell communication is accomplished through traditional backhaul communication. In another embodiment of the invention, dedicated inter-cell communication links are used to enable fast communication. In a cellular deployment, where it is not feasible to centrally locate all the main modules in one deployment, more than one of the previously described methods may be used simultaneously to achieve fast inter-cell communication.

In order to ensure that the same resources are used to serve a mobile station, hopping patterns in all participating sectors are similar in structure. In one embodiment of the invention, all sectors that belong to a same cell have the same hopping pattern regardless whether these sectors are or are not participating. Consequently, the hopping pattern is predetermined and can thus be communicated to the mobile station with minimal overhead. In another embodiment, a hoping controller is used such that the hopping pattern associated with the resources that are participating in soft handoff, is synchronized and dynamically communicated to the mobile station. In yet another embodiment, the resources associated with the soft handoff are excluded from hopping. It should be clear to those skilled in the art that one or more of these embodiments can simultaneously exist in a system.

According to another aspect of the invention, pilots are sent to aid the detection of the combined soft handoff. In one embodiment, a common pilot is sent from each base station. However, the phases of the common pilot from different base stations are different and are known at each of the base stations when these base stations are in soft handoff with each other.

For ease of notation, an embodiment will now be described for a case of two sectors participating in a soft handoff. A first Sector transmits pilot Pi on a particular pilot subcarrier as well as data signal Si on a particular data subcarrier. A second sector transmits pilot P₂ on the same pilot subcarrier as well as data signal S₂ on the same data subcarrier. The phase rotation, denoted as A, is between pilot Pl and pilot P2. The first Sector is designated as a reference base station. A base station can obtain an estimate of a phase rotation, denoted as A', for a data subcarrier from interpolation of the A quantities of two pilot subcarriers that are adjacent to the data subcarrier. In the case when the second Sector transmits the same data symbol but with a phase shift, S₂ = Si* A', a received pilot signal at the mobile station is given by:

Rpiiot = H₁P₁ + H₂P₂ = (H₁-J-H₂A)Pi,

where Hj and H₂ are the channel frequency responses for the transmissions from the first Sector and second Sector respectively. It is clear that the mobile station can now estimate the quantity H=(H ₁+H₂A) from the received pilots and estimate the quantity H'=(Hi+H₂A') via interpolation. Furthermore, H' is precisely the channel estimate that is necessary for the detection of the data signal because the received data signal, Rdata is given by:

Rdata = H₁S₁ + H₂S₂ = (H,+H₂A')Si.

More than two sectors may participate in soft handoffs in a similar manner. For the extension to more than two sectors in a system, the system chooses one of the sectors as a reference sector. In one embodiment, the reference sector is designated as an anchor base station in the soft handoff. Other choices of which a sector acts as the reference base station are possible. For example the sector with the best channel to the mobile station can be the reference base station.

If Aj is a phase difference between the pilot signal from the i-th sector and first Sector, which is the reference sector, and A'j is a phase rotation at a data subcarrier obtained via interpolation as described above for the i-th sector, then a mobile station can estimate

H=(H₁+H₂A₂+H₃A₃+H₄A₄+...)

from the pilot subcarriers and for data subcarriers

H' = interpolation(H) =(H₁+H₂A₂'+H₃A₃'+H₄A₄'+...)

Where interpolation() is the interpolation function.

It is now clear that if the i-th base station transmits the data signal Sj = Aj' Si, then H' is precisely the channel estimate that is needed to detect the data. In another embodiment, a dedicated pilot is sent by participating sectors. The pilot pattern used for the dedicated pilot is the same for all participating sectors. Furthermore, the location of the pilot by both frequency and time are also identical. Consequently, the mobile station can estimate the composite pilot channel response directly.

In yet another embodiment of the invention, a mobile station makes use of a standard pilot that is already transmitted in any OFDM systems. In this embodiment, the mobile station estimates a channel from each of the participating sectors independently and then combines them to form a channel estimate for detection.

Although the different pilot structures are described independently, one or more of the embodiments described above may exist simultaneously in a system.

According to another aspect of the present invention, a setting of the transmit power in each of the participating sectors may be employed. In one embodiment of the invention, the transmit power from each of the sectors in a mobile station that are participating in the soft handoff are set to be the same. This setting of the transmit power is used to achieve equal gain combining. In this embodiment, no feedback is needed from the mobile station. In another embodiment, maximum ratio combining can be achieved by weighting the transmit power of the transmission from each sector by Channel Quality Indicator (CQI) feedback from the mobile station for that sector.

In this embodiment, more power is transmitted by the sectors that have a better link to the mobile station. In addition, the mobile station measures CQI from each of the participating sectors and reports that CQI back to the base station. Other types of combining can be achieved by various setting of the transmit powers from different sectors participating in the soft handoff. For example, combining can be achieved by setting the power of a sector with the best link to the mobile station to a maximum value while setting all other links to have a transmit power of zero. Although each power setting scheme above is described as a separate embodiment, a plurality of these schemes may exist in a system.

According to another aspect of the present invention, a management scheme for the participating sectors can be employed. In one embodiment, a mobile station keeps a list of potential handoff sectors. These sectors can potentially participate in the soft handoff of the mobile station. A decision to add a sector to the potential handoff sector list can be based upon the strength of the pilot from that sector. Thus, a sector is added to the potential handoff sector list if the pilot strength of the sector is above a certain threshold. The addition of the sector to the potential handoff sector list is communicated between the mobile station and base station via signaling messages.

As an illustrative example, a mobile station receives pilots and measures the strength of each of these pilots from each of sectors, then determines whether each pilot strength of each sector is above a certain threshold. The mobile station then sends a message to the base station indicating that a particular pilot strength of a sector is above the threshold. Upon receiving that message from the mobile station, the base station sends a message back to the mobile station to add this sector to the potential soft handoff sector list.

Furthermore, the base station groups sectors from this list together to form one or a plurality of soft handoff groups. That is, a soft handoff group is a subset of the potential soft handoff sector list. A soft handoff group may include one or more base stations. The number of soft handoff groups is determined by the base station. Many factors may be used to determine how a base station or an access network groups a plurality of sectors into soft handoff groups. These factors include, but are not limited to, sector location, and sector loading.

For example, a base station or an access network may group all possible combinations of sectors in the potential soft handoff list. An example is illustrated in Fig. 2. In this example, a mobile station 250 is in a five way soft handoff region 200. This handoff region 200 includes five sectors, 201, 202, 203, 204, and 205. Sector 201 and Sector 202 are grouped as a soft handoff group 210, Sector 203 alone as soft handoff group 220, and Sector 204 and Sector 205 as soft handoff group 230. This grouping is done in such a fashion that sectors that belong to a same soft handoff group communicate with the mobile station 250 using the soft handoff procedure.

Specifically, Sector 201 and Sector 202 communicate with the mobile station 250 using the soft handoff procedure. However, Sector 203 does not communicate with the mobile station 205 using the soft handoff procedure since group 220 includes only one member, Sector 203. In addition, Sector 204 and Sector 205 communicate with the mobile station also using the soft handoff procedure. However, at any one time, the mobile station 250 is served by only one soft handoff group. On the other hand, the soft handoff group that serves the mobile station 250 may switch from time to time. This ensures that the best group is used to serve the mobile station 250.

The soft handoff group that is used to serve the mobile station 250 may be switched in a fashion similar to that of fast sector selection. In one embodiment of the invention, the mobile station 250 measures the combined pilot strength of the sectors in each soft handoff group. In this example, mobile station 250 measures the combined pilot strength in soft handoff groups 210, 220, and 230, then chooses a soft handoff group with the largest combined pilot strength as the soft handoff group that the mobile station 250. This soft handoff group is called the desired soft handoff group. The mobile station 250 then communicates the desired soft handoff group to the base station via signaling.

For example, at a first time instance, the mobile station 250 determines that soft handoff group 210 has the highest combined pilot strength 215, the mobile station 250 then chooses soft handoff group 210 as the desired soft handoff group. The mobile station 250 then communicates to the base station via messaging. The base station that serves the mobile station 250 then uses soft handoff group 210. If the desired soft handoff group contains only one member, the data is transmitted only from that one sector. However, if the desired soft handoff group contains more than one sector, then data is transmitted from all sectors that are members of the group.

At a second time instance, the mobile station 250 determines that soft handoff group 230 has the highest combined pilot strength 235, chooses soft handoff group 230 as the desired soft handoff group, and gets service from soft handoff group 230. At a third time instance, the mobile station 250 determines that soft handoff group 220 has the highest combined pilot strength and chooses soft handoff group 220 as the desired soft handoff group and gets service from soft handoff group 220. A fast sector switch occurs if the mobile station notified the base station that its desired soft handoff group changed and that the base station desires to switch service from the old desired soft handoff group to the new desired soft handoff group. In one embodiment of the invention, the communication of the desired soft handoff group is done via a Walsh code on the CQI feedback channel. That is, a base station associates a unique Walsh code with every soft handoff group and communicates this unique Walsh code to the mobile station via signaling messages. The mobile station than signals the base station which soft handoff group is the desired soft handoff group by covering the transmission from the CQI feedback channel with the Walsh code associated with the desired soft handoff group. This embodiment is one method of communicating the desired soft handoff group and that other methods may also be used. These other methods may include, but are not limited to, using a layer three signaling message.

A base station, because of various reasons, may not be able to serve a mobile station with the desired soft handoff group signaled by the mobile station. In one embodiment of the invention, the base station may determine whether to accept or deny a desired soft handoff group via signaling messages. In this embodiment, the base station, upon receiving a new desired soft handoff group from the mobile station, decides whether the base station will serve the mobile station from the new desired soft handoff group.

If the base station accepts to serve the mobile station with the new desired soft handoff group, the base station sends an acknowledgement message to the mobile station and begins serving the mobile station from the new desired soft handoff group signaled by the mobile station. The base station may delay the time from which it begins serving the mobile station from the new desired soft handoff group by some period until it is sure that the mobile station has received the acceptance message.

For example, the base station may put an action time into the acceptance message. That action time means when the base station will start serving the mobile station from the new desired soft handoff group. In this way, the mobile station and the base station are synchronized to a time when the switch occurs. If the base station detects, for example, the loading on the new desired soft handoff group is too high, and decides to deny serving the mobile station from the new desired soft handoff group, the base station sends a denial message to the mobile station. In this case, the base station will not accept serving the mobile station from the new desired soft handoff group and that any service from the base station continues from the old desired soft handoff group.

According to another aspect of the present invention, CQI feedback may be employed in a system. In one embodiment of the invention, a composite CQI that represents the CQI of the combined soft handoff signal for the desired soft handoff group is sent in as feedback from a mobile station to a base station. This composite CQI is the strength of the total composite pilot from each sector in the soft handoff group. Other measures of the composite CQI may also be used.

For example, the composite CQI can be the weighted sum of the received powers of a pilot from each of the sectors in the soft handoff group. The base station then uses this CQI to select the modulation and coding scheme (MCS) for the data transmission. In another embodiment of the invention, an MCS or an MCS set based upon the combined pilot strength is sent as feedback to the base station by the mobile station. The base station then transmits the data with an MCS chosen from the MCS set if an MCS set is sent in the feedback.

In yet another embodiment of the invention, a set of individual pilot strengths for each sector in the soft handoff group is sent as feedback from the mobile station to the base station. In this case, the base station combines the pilot strength with, for example a weighted sum, and then based upon the combined pilot strength, chooses an MCS for the data transmission.

In yet another embodiment of the invention, the pilot strength of each of the sectors on the potential soft handoff list is sent as feedback from the mobile station to the base station. The base station, based upon that feedback, chooses an MCS or an MCS set.

For a user in cell edge, where the user is in the boundary region between two or more sectors, even though a base station transmits to a mobile station with maximum power, a received signal is often received with very low power. As a result, the mobile station is in very poor radio condition and thus its data throughput is very low. This has several effects on system performance: First, if the mobile station requires a certain Quality of Service (QoS), the base station must expend significant resources to serve the mobile station. The result of which is a significant decrease in total system throughput; second, the perceived user experience for the mobile station is very poor due to the fact that data rate that can be sustained with that link is very low. This is a significant issue because users expect to have a same user experience regardless of where users are located in the sector.

Therefore, there is a need to provide a system to increase throughput of users at an edge of a sector. In addition, there is a need to provide a system to enable soft handoff in an OFDM system to improve the performance of users at the edge of the sector.

To facilitate the description of the present invention, following terms are defined. A mobile station's Active Set is a set of sectors that may be used for a period of time for its data transmissions. A mobile station's serving sector is a member of its Active Set from which it receives air interface resource assignments. A Soft Handoff (SHO) Transmission is transmission of identical data from more than one base station where air interface resources relating to transmission time and frequency are the same, which means the transmissions are synchronous, the base stations use a same forward link hopping pattern for Soft Handoff subcarrier assignments, and the base stations use a same scrambling sequence. A Soft Handoff Set (SHO Set) is a subset of the Active Set whose members meet requirements to be used for Soft Handoff transmissions. A Soft Handoff Group (SHOG) is a subset of a Soft Handoff Set.

In accordance with the present invention, pilot signals are sent to aid the detection of a combined soft handoff signal. In one embodiment, when only dedicated pilots are in use, each Soft Handoff Group sector may transmit a dedicated pilot using same air interface resources relating to frequency and time. In this embodiment, both data and the dedicated pilot are combined in an air, and the received dedicated pilot may be used for detection. In another embodiment, when only common pilots are being used, each Soft Handoff Group sector may transmit a common pilot. Each sector may use a different offset for common pilot so that common pilot symbols may occur in different frequency/time positions. Channel estimates for each sector must be determined separately in a receiver and then combined. Each of the combined channel estimates is used to decode combined data. When common pilot is in use, each soft handoff sector must transmit a common pilot according to its offset. Other Soft Handoff Group sectors may transmit data in these positions. In a case where a sector is transmitting a common pilot, this sector should be excluded from the combined channel estimate and combined data for a SHOG.

In yet another embodiment, a base station may avoid transmitting soft handoff data where the base station may occupy same resources as a common pilot channel for any of sectors in a Soft Handoff Group. Resources used for any common pilot in the Soft Handoff Group are removed from available resources for soft handoff data transmission. These resources may be used for non-soft handoff data transmission within individual sectors.

The system according to the present invention also manages Soft Handoff Sets and Groups. That is, a base station may determine and identify Soft Handoff Sets and Groups to a mobile station. In one embodiment, a base station may determine which sectors are in a Soft Handoff Set and which Soft Handoff Groups a mobile station may use at a given time. The base station may identify available Soft Handoff Sets and Groups when a mobile station's Active Set is assigned. In the message assigning the Active Set, message record fields for sectors in the same Soft Handoff Set may be placed one right after another. A field in a sector's record indicates whether the sector is a start of a new Soft Handoff Set. Table 1 provides an example of an use of such a field.

Table 1

Number of sectors = 6

Sector 1 Sector 1 RCQICHScramblingSeq SHOSetStart = 1

Sector 2 Sector2RCQICHScramblingSeq SHOSetStart= 0

Sector 3 Sector3RCQICHScramblingSeq SHOSetStart:= 0

Sector 4 Sector4RCQICHScramblingSeq SHOSetStart= 1

Sector 5 Sector5RCQICHScramblingSeq SHOSetStart= 0

Sector 6 SectorόRCQICHScramblingSeq SHOSetStart= 1 Each sector record includes a Scrambling Sequence field used to identity the sector and a field to indicate whether it is start of a new Soft Handoff Set. FIG. 3 illustrates a flow chart (100) of a procedure by which a mobile station may use the SHOSetStart field to determine SHOG ID mapping. A mobile station may use this procedure to determine a sector's Soft Handoff Groups and the corresponding Soft Handoff Group IDs. At an initial state, the mobile station receives a message containing sectors in the Active Set ordered in the message by Soft Handoff Sets. A field may be included associated with each sector indicating the start of a new Soft Handoff Set, as illustrated in Table 1. Variables used in flow chart (100) are described in the following. ActiveSetlndex is an index for sectors in an Active Set. ActiveSetlndex is initialized to 0, and incremented when a new Sector (AssignedSector) is retrieved from the message. It is used to identify sectors in the Active Set, such as when SHOG IDs are mapped to sectors. It is a record that includes an array SHOGID[O..3], where each array element contains a list of sectors associated with a corresponding SHOG ID for a sector. AssignedSector is a sector record most recently retrieved from the received Active Set assignment message. It is a record that includes a field SHOSetStart. A SHOSetStart field is associated with each sector. It indicates whether a sector is a start of a new Soft Handoff Set. SHOSetSectorlndex is an index for sectors in a same Soft Handoff Set. It is initialized to 0 at the start of a new Soft Handoff Set, and incremented when a new Sector (AssignedSector) is retrieved that is not the start of a new Soft Handoff Set. NumPilots is the number of sectors included in the Active Set.

The procedure includes a sequence of activities. ActiveSetlndex is initialized to 0 at the start of the procedure in step (S 102). Then step (SI lO) checks whether all sectors are retrieved. If all sectors of the Active Set are retrieved from the message, the procedure ends in (Sl 12). If not all sectors are retrieved, then retrieve another sector from the message in (Sl 14). For all sectors, SHOGID of "00" is the Soft Handoff Group with only the sector as a member. The current sector SHOGID[O] is set to itself in (S116).

Step (S 120) determines whether the current sector is start of a new Soft Handoff Set or not. If the current sector is the start of a new Soft Handoff Set, then set the SHOSetSectorlndex to 0 in (S 122), proceed to step (S 140) to increment the ActiveSetlndex, and retrieve a next sector by repeating step (SI lO). If the current sector is not the start of a new Soft Handoff Set, then increment SHOSetSectorlndex in (S 124).

Step (S 130) determines SHOG ID according to number of sectors in the current Soft handoff Set. If SHOSetSectorlndex is 1, then the current Soft Handoff Set contains at least 2 sectors: the current sector and a previous sector. In (S 132), each SHOGID[I], associated with the 1st and 2nd sectors in the Soft Handoff Set, is a set containing both of these sectors. Then proceed to step (140) to increment the ActiveSetlndex, and retrieve a next sector by repeating step (SI lO). If SHOSetSectorlndex is 2, then the current Soft Handoff Set contains 3 sectors: the current sector and previous 2 sectors. Set in (S 134) each SH0GID[2] associated with the 2nd and 3rd sectors in the Soft Handoff Set to be a set containing the 2nd and 3rd sectors. Set the SHOGIDβ] of the 1st sector and the SHOGID[I] of the 3rd sector to be a set containing the 1st and 3rd sectors in (S136). Set in (S138) SHOGID[3] of all 3 sectors in the Soft Handoff Set to be a set containing all 3 sectors. Then proceed to step (S 140) to increment the ActiveSetlndex, and retrieve a next sector by repeating step (SI lO).

Table 2 shows the resulting SHOGs for each sector and the corresponding SHOG IDs for the example provided in Table 1.

Table 2

Sector 1

Sector 1 R-CQICH scrambling sequence

SHOG

SHOG ID

0 Sector 1

1 Sector 1+2

2 Sector 1+3

3 Sector 1+2+3

Sector 2

Sector 2 R-CQICH scrambling sequence

SHOG SHOG ID

Sector 2 O

Sector 1+2 1

Sector 2+3 2

Sector 1+2+3 3

Sector 4

Sector 4 R-CQICH scrambling sequence

SHOG SHOG ID

Sector 4 O

Sector 4+5 1

The first sector in the message has a field set to '1 ', to indicate it is the start of a new Soft Handoff Set. The next 2 sectors are part of the same Soft Handoff Set, so the field is set to '0' . Sector 4 and 5 are in a new Soft Handoff Set, so the field is set to ' 1' for sector 4 and '0' for sector 5. Sector 6 is in a Soft Handoff Group consisting of 1 sector, so the field is set to T. If another sector were included in the Active Set, the field would be set to T, since sector 6 is a single-sector Soft Handoff Group. In this embodiment, for any message and at any particular time, Soft Handoff Sets are limited to three sectors. If more than three sectors may form a Soft Handoff Set (such as a six sector cell), the three sectors of a Soft Handoff Set may be changed by sending a new message. It should be clear to those skilled in the art that although Soft Handoff Groups are limited to 3 sectors in this embodiment, the present invention does not impose such a restriction. A person of the ordinary skill in the art will understand other methods may be used, such as, using a bitmap to indicate which sectors form a Soft Handoff Set.

Also, in this embodiment, the base station may identify members of its own Soft Handoff Set, that are available for Soft Handoff Group operation, for mobile stations without a data connection, to facilitate the mobile station requesting soft handoff operation when a data connection is initially setup. This may be done using a broadcast sector message.

In this embodiment, the base station may identify serving sector channel quality threshold at which the mobile station may request soft handoff operation. The base station may also identify a threshold on a maximally combined channel quality for a Soft Handoff Group. The base station may include these parameters when an Active Set is assigned.

In accordance with the present invention, the mobile station may determine Soft Handoff sets and Soft Handoff Groups, and request services from a Soft Handoff. In one embodiment, a mobile station may request soft handoff operation using a reverse link channel message, such as a Reverse Channel Quality Index Channel (R-CQICH). Table 3 illustrates a message that may be used to request Soft Handoff Group operation. Table 3

Table 4

In Table 3, a Soft Handoff Group identifier (SHOG ID) may identify which Soft Handoff Group associated with a sector identified by a R-CQICH scrambling sequence which is being requested. In this embodiment, a R-CQICH channel is scrambled with the R-CQICH scrambling sequence to identify the sector. The SHOG ID may be determined from information received in the Active Set assignment message. If service is requested from only one sector (the serving sector), the SHOG ID field is set to "00", and the requested Soft Handoff Group may include only the sector identified by the R-CQICH scrambling sequence. To request a Soft Handoff Group including the serving sector and one of the other two sectors in a three-sector Soft Handoff Set, the SHOG ID is set to "01" to request to include the sector that is in a higher position in the Active Set assignment message than the other sectors, and is set to "10" to request to include the other sectors. The SHOG ID is set to "11" to request an SHOG including all three sectors of a three-sector Soft Handoff Set.

In another embodiment, a base station may provide an R-CQICH scrambling sequence for each of Soft Handoff Groups which the mobile station uses to request Soft Handoff Group operation. In this embodiment, a Soft Handoff Group Sector Identifier is included in the R-CQICH message to identify the Soft Handoff Group sector that is associated with Forward Link Channel Quality report and, during a handoff, a handoff target.

In yet another embodiment, a mobile station may request that a sector of a Soft Handoff Group be added by indicating the sector in a reverse link control channel and setting a one bit soft handoff field. In this embodiment, no additional overhead may be required to specify the Soft Handoff Group in the request. A serving sector may decode the R-CQICH message directly and add the Soft Handoff Group sector, or may receive a message over a backhaul from the indicated sector (in a similar way as for handoff) that requests the sector be added to the soft handoff transmission. A mobile station may request that a sector of a Soft Handoff Group be removed by indicating the sector in a reverse link control channel, and setting a one bit Soft Handoff field. Since the sector is already in the Soft Handoff Group, this bit means to remove the sector. In the case of handoff when a Soft Handoff Group is active, a handoff flag is set to ' 1 ' . The new soft handoff bit may be used to indicate whether or not the mobile station wants to keep the current soft handoff group or to operate with the target sector only.

In accordance with the present invention, a base station may indicate a Soft Handoff Group that is active. This is important when common pilot is used for channel estimation because an AT needs to know which sectors should be used to estimate a channel for a combined data. In one embodiment, the base station may provide a SHOG ID to identify a Soft Handoff Group when making a transmission resource assignment. A scrambling code may be used to identify the SHOG ID in the forward link. This is illustrated in FIG. 4 where a function block (360) is added to apply a scrambling code to the message data after the CRC is added. The interpretation of the SHOG ID in this embodiment is the same as in the mobile station request.

In another embodiment, the SHOG ID information may be provided as a field in a message that assigns transmission resources as illustrated for the mobile stations request. In yet another embodiment, the SHOG ID scrambling may be combined with the scrambling function shown in Function Block 330 in FIG. 4.

In accordance with the present invention, a mobile station may feed a Channel Quality Information (CQI) of a combined channel for a Soft Handoff Group back to a base station. In one embodiment, the mobile station sends the CQI for one sector and also sends the difference between this CQI and the CQI for the combined channel.

Table 3 illustrates a message containing these fields, and Table 4 illustrates possible encoding for a field that represents the CQI difference.

In another embodiment, individual CQI reports for sectors of a Soft Handoff Group may be sent to a base station, either simultaneously or at different times. The base station may use the CQI reports to estimate CQI for a combined channel.

In accordance the present invention, a Soft Handoff Group may be changed while maintaining a same serving sector, the serving sector may be changed while maintaining a same Soft Handoff Group, and both the Soft Handoff Group and the serving sector may be changed, In one embodiment, the message shown in Table 3 may be used to satisfy these scenarios. To change the serving sector but maintain the same Soft Handoff Group, the R-CQICH scrambling sequence is changed to one associated with a target sector, Forward Link (FL) Channel Quality for the target sector is included, a SHOG ID is set to a value associated with the target sector that corresponds to the current serving Soft Handoff Group, and a handoff request bit is set to ' 1 '. To also change the Soft Handoff Group, the SHOG ID may be set to a desired Soft Handoff Group.

In accordance with the present invention, a base station with more than one transmitting antenna may also constitute a Soft Handoff Group. In this case, a set of antennas from group members may constitute a macro antenna for the group transmission. As an example, consider that sector A has two antennas, Al and A2, and sector B has two antennas, Bl and B2. Consider a group, G, comprises sectors A and B. The same waveform (or some form of diversity may be used, such as CDD, etc.) is transmitted through both antenna Al and Bl. Due to soft combining feature of OFDM, the transmission may appear to the mobile station as a single non- distinguishable Macro Antenna. As a result, antennas Al and Bl may constitute a Macro Antenna Gl . Similarly, antennas A2 and B2 may constitute a Macro Antenna G2. Therefore, the mobile station may recognize that group G is the same as a sector with 2 antennas, Gl and G2. As an alternative, group G may silence one set of Macro Antennas, e.g., G2. In this case, only one Macro antenna may serve the mobile station.

In Strictly Backward Compatible mode of 3GPP2 framework, handoff operation may remain the same as IxEV-DO, that is, an AT may only monitor a control channel sent by its serving sector and based on a conventional CDM pilot, the AT makes a handoff request to change the serving sector by DRC cover. The handoff operation may be independent with the SHOG operation.

In addition to the conventional handoff operation, a SHOG transmission on a traffic data channel, especially in an OFDM portion may be defined. In the SHOG transmission, a macro antenna in a group serves as a single antenna in a sector. Thus the group with multiple macro antennas may serve an AT in a MIMO scheme. SHOG set information, such as which sector may constitute a group and the Group ID, etc. must be communicated to the AT. The SHOG set information may either be broadcasted in a common control channel or unicasted in a traffic channel. One example for using broadcast is to put the information in a neighboring sector list so that every AT may be aware of current SHOG setting. One example for unicast is to have an Access Node (AN) and the AT negotiate after a call setup in upper layer signaling.

Feedback for conventional IxEV-DO handoff may remain same as IxEV-DO. Feedback for MIMO in a group may be that a macro antenna for a group transmission equals antenna in a sector. The AT may regard the group with multiple macro antennas as a virtual sector supporting MIMO. A MIMO operation in the group may be the same as in one sector. Feedback for SHOG selection may be based on measurement, an AT may request a SHOG service from an AN. To feedback a preferred group and preferred serving sector, a data rate is necessary, A new physical channel may be defined to carry this information. Also, an original IxEV-DO feedback channel may be reused. For example, the AN and the AT may negotiate an additional DRC channel as if the DRC is for a virtual forward link carrier in asymmetric mode of IxEV-DO rev B. This channel may be named the Group Rate Control (GRC) channel with the GRC_Cover (3 bits) indicating the preferred Group ID and the GRC content (4 bits) carrying data rate control information for that group.

In an embodiment for a SHOG operation, an AT may measure an individual sector channel quality from a Common Spatial Pilot and calculate a combined channel quality for a potential group. Once criteria is met, an AT may report to an AN by switching cover of GRC to a desired one. After the AN is aware of this switching, the AN may serve the AT through the requested SHOG.

This disclosure also describes techniques that are needed to facilitate soft handoff in wireless mobile OFDM communication systems, especially in Strictly Backward Compatible (SBC) mode of 3GPP2 Air Interface Evolution. Before we go on this description, we first give some background information.

[Multi-user diversity in DO] The current, mobile 3 G communication system, say, DO rev A and rev B, achieves its significant increase in sector throughput by taking advantage of multi-user diversity gain. That is, for point to multipoint system such as that in the forward link, all resources of a base station are dedicated to a single user at a time. The scheduler, as much as possible, chooses the user with the best radio condition from among a set of users to send data to. If the set of users is large enough and that the channel fading of each user is independent, there is almost always a user in good radio condition to serve. Consequently, the base station avoids the expense of sending information to a user in poor radio condition resulting in a significant improvement of sector throughput when compare with 2G wireless systems.

[FCS in DO] Moreover in current 3G systems, when a data user move from the service area of one sector to another, fast sector selection is employed to quickly switch the transmission of data from one sector to another. This is unlike the case for voice users in CDMA systems where soft handoff is employed; that is both sectors that are involve in the handoff simultaneously transmit the same information to the mobile station. The mobile station takes advantage of this additional diversity in is reception. Since data users can only use fast sector selection, the throughput for the data user at the edge of the cell is significantly less than that for the users in the center of the cell. It is therefore desirable to improve the performance and user experience for the users at the edge of the cell. This maybe so even at the expense of sector throughput because operators may decide to satisfy the desire of most users to have an equal grade of service regardless of where they are located.

[Multiplexing DRC in DO r B] To indicate forward link channel quality, DRC is feedbacked to AN. From the DRC feedback, the AN determines which AT it should schedule a transmission to and with what packet format the packet shall be transmitted. DO rev B also have support for an asymmetric mode, where a single reverse carrier may be associated with multiple forward carriers. Therefore, one reverse link may carry multiple DRC feedbacks to its associated forward links. The multiplexing scheme of multiple DRCs can be long PN code scrambled, I/Q branch multiplex and code division multiplex (CDM).

[DSC in Handoff] To shorten the latency of handoff, DSC is introduced since DO rev A. DSC carries the preferred sector information, from which AN knows AT's handoff request and starts to route the data to the AT desired sector.

[RL in Handoff of DO] DO allows each sector in the active set to receive and acknowledge the RL traffic data, as well as sending the RL power control bit. It is AT obligation to collect and interpret these signals from different sectors.

[Intro for SBC] In the strictly backward compatible (SBC) mode of 3GPP2 Air Interface Evolution, the CDM data portion, pilot and FL MAC channel are basically, left intact as in DO rev A and B. The evolution part is the OFDM symbols may occur in the data portion to facilitate high data rate transmission, especially with the help of MIMO. To support forward link MIMO utility, some additional reverse link control channels, such as Spatial Signature Channel (SSC) and Spatial Rank Channel (SRC), are introduced, as well as the forward link Common Spatial Pilot (CSP) and Dedicated Spatial Pilot (DSP).

[SHOG in SBC] For the OFDM systems, since it is immune to self interference, soft handoff can be used to significantly improve the throughput of the user in the handoff region at the edge of the cell. As long as the sectors involved in the handoff transmit the same signal and that the signals from the different base stations arrives within the length of the cyclic prefix, the receiver can take advantage of this to improve the detection without any drawback of self interference. This set of coordinated sectors in this handoff scheme is known as a Soft HandOff Group (SHOG). [AS, AGS and serving group] The active set (AS) management is same as the conventional standard. On top of active set, we introduce the concept of Active Group Set (AGS), which is the set of potential serving SHOG that the AT shall actively monitor. The group may have one or more sectors as its member. At a time, the traffic is always transmitted from one SHOG which is known as the serving SHOG.

[AGS management solution 1: adding group into AGS] Expanding the size of AGS only occurs at the time a new sector being added into active set. Once a particular pilot strength has been determined to be above a threshold for a certain period, the AT reports the pilot strength and that pilot PN (sector) to AN as a handoff request. If permitted, AN will add this sector into the active set. At this point, AN will inform AT the updated group information after adding this sector. For example, sector A is already in Active Set, now the sector B grows stronger to meet the qualification so that AT reports sector B and AN grants it. At this point, if sector A and sector B can constitute a group, AN will, at least, regard two additional groups as sector B and sector A + sector B respectively, and assign the group ID. Here, "at least" means if sector B can constitute a new group with the legacy members of the active set, then more group may be assigned. If sector B is totally independent and cannot link with other member in the active set, AN will assign one group ID for sector B only. By this means, AT and AN keep a list of potential groups, which is Active Group Set (AGS).

[AGS management solution 1: removing group from AGS] Shrinking the size of AGS only occurs at the time a sector being dropped from active set. Once a pilot strength in active set has been weaker than a threshold for a certain period, AT reports that pilot PN (sector) to AN as a remove request. If permitted, AN will drop this sector from the active set and inform AT about that. At this point, the AN will reconfigure the SHOG's in the AGS to remove this sector and inform the AT of the new AGS configuration. If a SHOG is completely removed from the AGS, its group IDs will be released for the future use.

[AGS: solution 2] There is also an alternative to form the AGS, which is described in the following three paragraphs: informing the group information, adding group into AGS and removing group from AGS. In the above solution, AT and AN maintain the conventional active set management, on top of that, AT and AN will maintain an active group set (AGS) which is derived directly from active set. In this alternative, there is no active set operation, instead AT reports group strength and request group management (adding and removing) directly.

[AGS solution 2 Group possibility inform] Firstly AN has to inform which sectors are possible to constitute a SHOG so that AT can monitor or predict the composite channel quality from that group to make a handoff request or switch request. One way to inform is to advertise that group information through the broadcasting overhead channel. Another way is by inband unicast signaling after the traffic channel is established.

[AGS solution 2 Group adding to AGS] After having that list of potential groups, AT keeps monitoring and predicting the channel quality from each potential group through conventional pilot or CSP. Once the qualification is match, e.g. the channel quality is above some threshold for a certain period, AT makes a handoff request to AN asking for adding that group into Active Group Set (AGS). If permitted, AN will send a message to AT to assign that group with an ID, as well as the MAC ID. Moreover, a group specific scrambling code may be assigned or predefined by specification.

[AGS solution 2 Group removing from AGS] On the contrary, there is also a qualification for dropping a group from AGS. For example, if the composite channel quality of one particular group is below some threshold for a certain period, AT could send out a drop request. Responding to that, AN may grant it by acknowledge message.

[Serving Group] At any instant in time, there is one and only one group among AGS serving the AT. The sectors in group send the same signal in the same time- frequency resource. This needs the help of smart scheduler. The same signal sent by multiple group members can be in the form of cyclic delay diversity or phase swap diversity. As an alternative, AN may keep some group members silence to avoid the intra-group interference, specially when those group members are short of transmission power.

[CE in AT] Due to the existence of dedicated pilot in OFDM portion, it is easy for AT to estimate the composite channel state and decode the traffic. However, estimating the individual group member's channel and combine them into the group channel through calculation is also feasible. [Composite DRC for data serving] To have accurate forward link packet format control, the composite DRC (C-DRC), which is according to the composite group channel quality, is preferred. The C-DRC cover is pointing to the preferred group, from AT perspective.

[DSC for group switch] DSC is pointing to the preferred serving group from AT point of view. Once receiving a DSC different to the before, AN regards it as a group switch request.

[Group Switch option 1] One way of the group switch is as follow. Once AT intends to make a switch request, it carries desired group ID on DSC. After a certain time, automatically, AT will point the DRC/SRC/SSC to the desired group. If the switch is granted, the AN will set DRC_Lock and transmit packet in new serving group. Otherwise, AN won't set DRC_Lock, which imply the AT is refused to enter the new group.

[Group Switch option 2] Another way of the group switch is as follow. Once AT intends to make a switch request, it carries desired group ID on DSC. If permitted, AN will send AT a message. After that, AT switch the DRC/SRC/SSC to the desired group. If the AT did not received that message after a certain time, the AT regards it as a refuse. Also, an explicit refuse message is feasible.

[Group Switch option 3] The third way of the group switch is as follow. Once AT intends to make a switch request, it carries desired group ID on DSC. At this time, AT points SRC/SSC to the desired group, but the DRC value and cover remain for the current group. The reason is that there is no spatial information for new sector so far and spatial information affects channel quality a lot so that the spatial information shall be sent to new group as soon as possible. Before the new group takes charge, the original group may keep its former spatial information and still serve the AT. As an alternative, making DRC/SRC/SSC targeting at desired group or old group is also feasible. After receiving DSC, AN will manage the routing within the network. Once the new group is ready, it will send the data following the preamble which is scrambled by the group specific code. The AT simultaneously monitors the current and desired preambles which are scrambled by current scrambling code and the desired scrambling code respectively. Once it receives desired preamble, the SHOG switch is granted. To improve the transmission efficiency during the transition, more SRC/SSC/DRC may be created on the expense of reverse transmission power and interference. For example, the system may have two sets of SRC/SSC/DRC targeting at old group and new group respectively.

[RL representative sector] In order to be backward compatible with DO, all the sectors participating in AGS are trying to decode and acknowledge the reverse link traffic data, and to power control the reverse link through the FL MAC channel (DRC_Lock/ACK/RPC). To save transmission power and reduce the AT complexity, it is not necessary for all the sectors sending these control signal, given all the sectors in one BTS probably share with the same information in softer handoff case. Therefore, it is valuable to have only one representative sector sending the FL MAC channel on behalf of that sectors sharing with the same information. In reality, it is probable for each BTS/Cell in AGS to have one representative sector.

[R-DRC for power control FL MAC] To facilitate the power control of FL MAC channel, it is also valuable to send DRC (called as R-DRC) targeting at RL representative sector. Therefore, probably multiple DRCs have to be sent for different purpose. For simplicity, this invention regards each DRC as a DRC pointing to a virtual FL carrier in asymmetric mode in DO rev B. Therefore, different DRCs can be multiplexed through Long PN code scrambling, I/Q branch multiplex or Code division Multiplex (CDM). And the multiplex scheme shall be negotiated between AN and AT when a RL representative sector is added. Similar to DO, the R-DRC cover is pointing to the preferred representative sector.

[RL representative switch] It is possible to change the representative sector. AT measures the FL channel qualities and chooses its representative sector per BTS/cell. If AT intends to change the RL representative sector, AT will change the R-DRC cover to its desired sector. Noticing that, AN will begin to send FL MAC channel through the new sector, given the MAC ID of that new sector has been assigned. Once receiving that, AT knows the RL representative sector switch has been accepted. The alternative way may go through this procedure by signaling. But obviously, signaling is more expensive.

The switch of RL representative sector may be independent of the switch of serving group.

The same concept applies to multiple carriers system, where the SHOG comprises of a group sectors which can transmit in the same time-frequency resource. And this invention is not limited by to SBC in 3GPP2.

According to an embodiment, given 3 bit DRC cover and DSC cover, the group number is limited by 8. Therefore, the idea of active cell set is given up because in that case the number of group can easily blow up. Instead, the active group set is proposed, where each group is added or removed by L3 signalling. It is AT who chooses the anchor sector as the representative of each cell. And AN shall assign one additional DRC for each anchor sector report. Also, the anchor switch is supposed. Option 3(802.20) is proposed for group switch.

To enhance the performance of the cell edge users, SHOG is proposed in SBC mode. The AN conducts SFN transmission within the group to a handoff AT with various forms of diversity, e.g. delay diversity, etc. can be considered, and the AN ensures that no interference will be seen by a handoff AT from group members. AT keeps a list of potential SHO group which is advertised by AN and assigned GroupID, and makes measurement accordingly (from CSP). Once the channel quality thresthold is hit, AT reports to AN for the SHO request. AN makes the final decision of granting the SHOG. DSP will make the SFN transmission smoothly.

As to SHOG FL, there are two level concepts: AGS and serving SHOG, only one group serves at a time. Accordingly, there are two level procedure: AGS operation and SHOG operation. The AGS operation includes call initiation, group adding and removing. The SHOG operation includes SHOG switch. Group ID is assigned by AN. MAC ID is assigned on group basis. Group specific scrambling code is possibly the combination of the scrambling codes of its group members.

As to SHOG RL, all the sectors in active groups are trying to decode RL traffic and monitoring the RL quality. However, for each active cell, there is only one anchor sector sending the MAC channel (RPC/DRC_lock/ACK) using the MAC ID for the group who has the anchor sector only. AT measures the FL qualities and chooses its anchor sector per cell. Meanwhile AT reports its anchor sectors ID and FL quality by DRC as each anchor sector belongs to a virtual carrier in DO rev B. Anchor DRC (ADRC) is used to report anchor sector ID and forward link channel quality.

In an example, AGS includes B2, B3, B2+B3, Al. Serving group includes B2+B3. Anchor sector includes A2 and B2. If AT moving from B2+B3, AT will switch DSC/SSC/SRC to A2. DRC is remained in B2+B3 and ADRC is remained same. AT will switch DRC once receiving new preamble. AN will manage network routing, keep sending data from B2+B3 according to current DRC and the former SRC/SSC, and send data from A2 according to current SRC/SSC and DRC once ready.

In call initiation, the call is always set up to a sector (the group with only one number). Group ID, MAC ID and ARDRC are assigned by AN. Similar to DO rev B, DRC/SRC/SSC is pointing to serving SHOG and DRC value is the combined one, while DSC pointing to preferred SHOG. Moreover, as aforementioned, AT reports its anchor sectors ID and FL quality through ARDRC.

One of AGS operation is group adding and removing. Comparing to conventional DO rev A, the difference is that the operation unit is not a sector but a group. AT keeps a searching list of neighboring sectors and groups which is advertised through broadcasting message. Once a group is good enough, AT reports to AN through message like PilotStrengthReport today. AN grants the request and assigns Group ID as well as MAC ID through message like Traffic Channel Assignment today. Furthermore, if this group belongs to a new cell, AN shall assign one ADRC. The removing procedure is similar.

In anchor sector change, for each cell, AT is assigned one ADRC to report its anchor sector ID and FL quality. The anchor sector sends the MAC channel (RPC/DRC_Lock/ACK) as the representative of the cell. Once AT prefers to change the anchor, it will switch its ADRC cover to the desired one and monitor the MAC channel from the desired anchor sector. Once detecting the desired MAC channel, AT regards anchor sector switch honored.

In group switch, AT carries desired group ID on DSC and points SRC/SSC to desired group, while the DRC value and cover remain for the current group and original SRC/SSC. The reason is that there is no spatial information for new sector so far and SSC will affect channel quality a lot. AN will manage the routing within network. During the transition, source group keeps the original SRC/SSC information and continues to transmit data w.r.t the current DRC. Once the source group ready, it will send the data with the desired SRC/SSC. AT simultaneously monitors the current and desired preambles which are scrambled by current scrambling code and the desired scrambling code respectively. Once it receives desired preamble, the SHOG switch is granted. The previous description of the disclosed embodiments is provided to enable those skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art and generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein..

Claims

1. A method of managing data transmission during a soft handoff in a communication system, wherein the communication system includes a plurality of base stations, and each of the plurality of base stations includes at least one sector and at least one mobile station, the method comprising the steps of: utilizing the mobile station to generate a pilot signal strength report for each sector; utilizing the base station to maintain a list of potential soft handoff sectors for a mobile station that communicates with the base station, based upon a pilot signal strength report received from the mobile station; utilizing the base station to group sectors from the list to form a soft handoff group for the mobile station; communicating the soft handoff group to the mobile station; utilizing the mobile station to measure combined pilot signal strength of each soft handoff group; selecting a desired soft handoff group based upon combined pilot signal strength; and updating member sectors in the mobile station's current soft handoff group responsive to the desired soft handoff group.

2. The method of claim 1, wherein a soft handoff group contains at least one sector.

3. The method of claim 1 wherein the base station determines the grouping of sectors from a list of potential soft handoff sectors based on physical location of each sector.

4. The method of claim 1 wherein the base station determines grouping of sectors from a list of potential soft handoff sectors based on capability of fast backhaul communications between the sectors

5. The method of claim 1 wherein the base station determines grouping of sectors from a list of potential soft handoff sectors based on traffic load condition of each sector.

6. The method of claim 1 wherein the mobile station communicates the desired soft handoff group to the base station using a reverse channel quality indicator (CQI) channel.

7. The method of claim 6 further comprising the steps of: assigning a covering sequence for reverse CQI channel for each soft handoff group that the base station forms for a mobile station; communicating to the mobile station the covering sequence for each soft handoff group; covering reverse CQI channel by the mobile station with the covering sequence associated with the selected desired serving soft handoff group; and transmitting reverse CQI channel by the mobile station setting a transmit power for the designated group of base stations.

8. The method of claim 7 wherein the covering sequence is a Walsh code.

9. The method of claim 1 wherein the base station updates member sectors in the current soft handoff group based on the desired soft handoff group indicated, and its capability to support such a group.

10. The method of claim 9 wherein the base station accepts the desired soft handoff group as the current serving soft handoff group if the base station can support it and denies it and continues to use the existing current serving soft handoff group as the current soft handoff group if the base station can not support the desired serving soft handoff group.

11. The method of claim 10 further including communicating the decision of acceptance of the desired soft handoff group to the mobile station by the base station.

12. The method of claim 1 further including using the same hopping pattern among the member sectors of the same soft handoff group that the base station forms.

13. The method of claim 1 further including: reporting the channel quality indicator (CQI) with the combined pilot strength from member sectors of the selected soft handoff group to the base station by the mobile station; and selecting the modulation and coding scheme (MCS) by the base station for the data transmission to the mobile station based on the combined CQI report from the mobile station.

14. A base station transmitting apparatus for transmitting data to a mobile station in soft handoff, comprising; a scheduler adapted to assign resources of a plurality of sectors that participate in soft handoff for transmission of particular data to a particular mobile station; a plurality of remote modules, each containing a radio frequency processing unit for at least one sector of base station; a main module that comprises base band processing units for a plurality of remote modules of at least one cell; and a communicative connection between the remote module and the main module.

15. The apparatus of claim 14, further comprising a backplane that connects one main module with at least another main module.

16. The apparatus of claim 14, further comprising a fast inter-base station connection that one main module at a first base station with another main module at a second base station.

17. The apparatus of claim 14, further comprising a singular hopping pattern for the same sectors that participate in the soft handoff for a mobile station.

18. A method for a base station and mobile station to collaborate on the control and management of resources used for soft handoff transmission on the forward link where the mobile station uses a scrambling sequence on a reverse link control channel in conjunction with protocol message fields to request soft handoff transmissions from specific sectors.

19. A method as in claim 18 where in a message used by the base station to assign the Active Set, each sector of the Active Set has a scrambling sequence field associated with it that is used to scramble a reverse link control channel, which is used to identify Soft Handoff Groups.

20. A method as in claim 18 wherein a message used by the base station to assign the Active Set includes fields associated with sectors in the Active Set and the fields for sectors in the same Soft Handoff Set are placed one right after the other in the message.

21. A method as in claim 18 where in a message used by the base station to assign the Active Set, each sector in the Active Set has a message field that indicates whether the corresponding sector is the start of a new Soft Handoff Set. The new Soft Handoff Set includes the consecutive sectors in the message where a sector's associated field does not indicate that the sector is the start of a new Soft Handoff Set.

22. A method as in claim 21 where a field associated with an Active Set sector that indicates the start of a new Soft Handoff Set is used to determine the Soft Handoff Groups associated with each sector in the Active Set.

23. A method as in claim 18 for a mobile station to request soft handoff operation using a reverse link channel message, such as a Reverse Channel Quality Index Channel (R-CQICH). The Soft Handoff Group identifier (SHOG ID) message field identifies which Soft Handoff Group associated with the sector identified by the R-CQICH scrambling sequence.

24. A method as in claim 18, wherein the mobile station reports the Channel Quality Index for a sector in a reverse link control channel and also includes a report of the difference between the CQI of the one sector and the CQI for the combined Soft Handoff Group channel.

25. A method as in claim 18, wherein individual CQI reports for the sectors of the Soft Handoff Group can be sent to the base station, either simultaneously or at different times.

26. A method as in claim 18 where the base station uses a broadcast message to identify the members of its own Soft Handoff Set that are available for Soft Handoff Group operation to facilitate the mobile station requesting soft handoff operation when a data connection is initially setup.

27. A method as in claim 18, wherein the resources used for any common pilot in a sector that is a member of the Soft Handoff Group are removed from the available resources for soft handoff data transmission.

28. A method as in claim 18 wherein certain conditions must be met before a mobile station may request a soft handoff transmission.

29. A method as in claim 26 wherein the base station identifies a serving sector channel quality threshold at which the mobile station may request soft handoff operation.

30. A method as in claim 26 wherein the base station identifies the maximum combined channel quality allowed with Soft Handoff Group operation, which restricts when a mobile station may request Soft Handoff Group operation.

31. A method as in claim 18, wherein the base station provides a reverse link control channel scrambling sequence for each Soft Handoff Group which the mobile station uses to request Soft Handoff Group operation, a Soft Handoff Group sector identifier is included in the reverse link control channel message to identify the Soft Handoff Group sector that is associated with the FL Channel Quality report and the handoff target.

32. A method as in claim 18 wherein the mobile station requests that a sector of a Soft Handoff Group be added for soft handoff transmission by identifying the requested sector using a scrambling sequence on a reverse link control channel and using a one bit soft handoff field.

33. A method as in claim 18, wherein a scrambling code is used in the forward link to identify the active soft handoff group, the scrambling code is applied to the message data after the CRC is added.

34. A method as in claim 18 the message field is used in the forward HnIc to identify the active soft handoff group.

35. A method as in claim 18 wherein a scrambling code is used in the forward link to identify the active soft handoff group, in this case the scrambling code is applied to the message data after the CRC is added.

36. A method as in claim 18 wherein the Soft Handoff Group may be changed while maintaining the same serving sector, the serving sector may be changed while maintaining the same Soft Handoff Group, and both the Soft Handoff Group and the serving sector may be changed.

37. A method for facilitating Soft Handoff Group operation in the 3GPP2 Strictly Backward Compatible mode of IxEV-DO Rev. C, especially for the OFDM portion, the method comprising: the AN telling the AT the SHOG set information such as SHOG member and ID; the AT regarding the group with multiple macro antennas as a sector supporting MIMO; the AT making a SHOG switch decision based on its measurement and calculation of the composite channel quality of the group; the AT feeding back the SHOG request through the GRC channel, which may be one of the DRC channels in IxEV-DO Rev. B.

37. A method for facilitating a soft handoff in a wireless mobile Orthogonal Frequency Division Multiplexing (OFDM) communication system, the method comprising: creating an Active Group Set (AGS) comprising a plurality of sectors; adding new groups to the AGS when the pilot strength of the new sector exceeds a threshold value for a predetermined period; removing a group from the AGS when the pilot strength of the sector falls below the threshold value for the predetermined period; wherein at any instant of time, there is only one group serving the access terminal, with the members of the group transmitting the same signal in a SFN fashion or some diversity scheme; wherein a terminal may report the group DRC to indicate which group and which FL transmission format it prefers in the next available transmit slot; wherein only one sector in a cell transmits the forward link MAC channels to a wireless access terminal, this representative of the cell can be requested by the terminal; and wherein the terminal reports the DRC targeting at this representative to assist access network to ensure the transmission quality of FL MAC channels.