Abstract:
A radio access network having multiple sectors comprises two or more sector transmitters serving respective sectors for transmitting data to mobile stations; a plurality of antennas, each antenna assigned to a respective sector transmitter; and a antenna management processor controlling sector antenna assignments and operative to dynamically reassign a sector antenna associated with a sector transmitter in a first sector to a sector transmitter in a second sector.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 11/108,323 filed on Apr. 18, 2005, and also claims priority from Patent Application PCT/US2006/009722 filed on Mar. 17, 2006, the disclosures of each of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to multiple input, multiple output (MIMO) systems and, more particularly, to a multi-sector MIMO system for a high speed packet data channel. 
         [0003]    The demand for wireless data services, such as mobile Internet, video streaming, and voice over IP, have led to the development of high speed packet data channels to provide high data rates needed for such services. Revision C of the IS-200 standard introduced the forward packet data channel (F-PDCH) for high speed packet data services. The F-PDCH takes advantage of multi-user diversity by opportunistically scheduling users to receive data on the forward packet data channel when the channel conditions are favorable. Subject to predefined fairness criteria, the throughput is maximized if all forward link resources, such as Walsh codes and power, are allocated to the mobile station with the best channel conditions and, hence, the highest supportable data rate. In general, a mobile station receives data only in good radio conditions. The mobile stations transmit channel quality information over reverse link overhead channels. This channel quality information is used at the base station to schedule the mobile stations and to select the most efficient modulation and coding scheme. 
         [0004]    Currently, use of high speed forward packet data channels has been limited to systems with a single transmit antenna. Multiple input, multiple output (MIMO) communication systems that employ communication links with multiple transmit and receive antennas can achieve significantly higher data rates and/or increase reliability. These gains are realized by exploiting spatial multiplexing in which data is multiplexed across the transmit antennas. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention relates to the transmission of packet data from one or more sectors in a radio access network to a plurality of mobile stations over a shared forward link channel, such as the Forward Packet Data Channel (F-PDCG) in IS-2000 systems and the High Speed downlink Shared Channel (HS-DSCH) in HSDPA systems. Each sector has a sector transmitter and multiple sector antennas. The sector transmitter may, for example, comprise a spatial multiplexing transmitter that divides a data stream for a mobile station into two or more substreams for transmission to the mobile station from respective antennas. A scheduler in each sector selects one or more mobile stations to receive packet data from the base station at any one time based on a predetermined scheduling criteria. In one embodiment, the scheduler selects one or more mobile stations to serve at any one time so as to maximize data throughput. The scheduler may also take into account fairness criteria, quality of service, or other factors. A selected mobile station may be served by all of the sector transmitters in the serving sector, or by less than all of the sector transmitters in the serving sector. If less than all the sector transmitters in the serving sector are used to transmit data to a selected mobile station, the scheduler determines the best combination of antennas to use. 
         [0006]    Sector antennas may be dynamically and temporarily reassigned from one sector to another. Reassignment of a sector antenna from one sector to another may be appropriate, for example, where the channel conditions between a mobile station in a boundary region between sectors are favorable. A sector antenna may also be reassigned when the loading of cross sectors is unbalanced. In one embodiment of the invention, an antenna management processor at the base station handles the reassignment of sector antennas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a block diagram illustrating components of an exemplary mobile communication network. 
           [0008]      FIG. 2  is a schematic diagram illustrating a multi-sector cell in the mobile communication network, wherein each sector has multiple transmit antennas. 
           [0009]      FIG. 3  is a block diagram illustrating relevant portions of a base station for a mobile communication network. 
           [0010]      FIG. 4  is a schematic diagram illustrating one scenario in which an antenna in one sector is reassigned to another sector. 
           [0011]      FIG. 5  is a schematic diagram illustrating another scenario in which an antenna in one sector is reassigned to another sector. 
           [0012]      FIG. 6  is a schematic diagram illustrating combined transmit diversity and spatial multiplexing in one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]      FIG. 1  illustrates logical entities of an exemplary wireless communication network  10  that provides packet data services to mobile stations  40 .  FIG. 1  illustrates a wireless communication network  10  configured according to the cdma2000 (IS2000) standards. Other standards, including Wideband CDMA (W-CDMA) could also be employed. The wireless communication network  10  is a packet-switched network that employs a high-speed forward packet data channel (F-PDCH) to transmit data to the mobile stations. Wireless communication network  10  comprises a packet-switched core network  20  and a radio access network (RAN)  30 . The core network  20  includes a Packet Data Serving Node (PDSN)  22  that connects to an external packet data network (PDN)  16 , such as the Internet, and supports PPP connections to and from the mobile station  40 . Core network  20  adds and removes IP streams to and from the RAN  30  and routes packets between the external packet data network  16  and the RAN  30 . 
         [0014]    RAN  30  connects to the core network  20  and gives mobile stations  40  access to the core network  20 . RAN  30  includes a Packet Control Function (PCF)  32 , one or more base station controllers (BSCs)  34  and one or more radio base stations (RBSs)  36 . The primary function of the PCF  32  is to establish, maintain, and terminate connections to the PDSN  22 . The BSCs  34  manage the radio resources within their respective coverage areas. The RBSs  36  include the radio equipment for communicating over the air interface with mobile stations  40 . A BSC  34  can manage more than one RBSs  36 . In cdma2000 networks, a BSC  34  and an RBS  36  comprise a base station  38 . The BSC  34  is the control part of the base station  38 . The RBS  36  is the part of the base station  38  that includes the radio equipment and is normally associated with a cell site. In cdma2000 networks, a single BSC  34  may comprise the control part of multiple base stations  38 . In other network architectures based on other standards, the network components comprising the base station  38  may be different but the overall functionality will be the same or similar. 
         [0015]    Referring to  FIG. 2 , each RBS  36  is located in and provides service to mobile stations  40  in a geographic region referred to as a cell  12 . The cell  12  is divided into sectors  14  to reduce interference. The individual sectors  14  are denominated by S 1 , S 2  and S 3  respectively in  FIG. 2 . In  FIG. 2 , the RBS  36  is located near the center of the cell  12  though other arrangements are possible. 
         [0016]      FIG. 3  illustrates components in one sector  14  of the RBS  36 . The RBS  36  includes a sector transceiver  50  having a plurality of sector antenna  56 , a scheduler  58 , and an antenna management processor  60 . The transceiver transmits signals to and receives signals from mobile stations  40  in the sector  14 . Each sector  14  includes multiple sector antennas  56 . The scheduler  58  schedules packet data transmissions from each sector antenna  56 . The scheduler  58  may employ any predetermined scheduling criteria. In one embodiment, the scheduler  58  schedules mobile stations to maximize data throughput. In other embodiments, the scheduler  58  may take into account other factors such as quality of service and fairness. An example of a scheduler  58  that incorporates a fairness criteria is a proportionally fair scheduler. The antenna management circuit  60  determines the number of sector antennas  56  assigned to the sector  14 , which the sector transceiver  50  can use. 
         [0017]    In the exemplary embodiment shown in  FIG. 3 , the transceiver  50  comprises a receiver  52  and a spatial multiplexing transmitter  54 . The spatial multiplexing transmitter  54  may comprises, for example, a per antenna rate control (PARC) transmitter. The spatial multiplexing transmitter  54  divides a data stream for a given mobile station  40  into multiple substreams and transmits each substream to the mobile station  40  using a different sector antenna  56  for each substream. 
         [0018]    The spatial multiplexing transmitter  54  is used in the exemplary embodiment to transmit packet data to the mobile stations  40  over a high-speed packet data channel, such as the forward packet data channel (F-PDCH) in cdma2000 systems. The scheduler  58  takes advantage of multi-user diversity to increase system throughput by opportunistically scheduling mobile stations  40  to receive data on the forward packet data channel when the channel conditions are favorable. Subject to predefined fairness criteria, the throughput is maximized if all forward link resources, such as Walsh codes and power, are allocated to the mobile station  40  with the best channel conditions and, hence, the highest supportable data rate. In general, a mobile station  40  receives data only in good radio conditions. The mobile stations  40  transmit channel quality information over reverse link overhead channels to the RBS  36 . The scheduler  58  at the RBS  36  uses this channel quality information to schedule the mobile stations  40  and to select the most efficient modulation and coding scheme. 
         [0019]    In a multi-antenna system, allocating all of the antennas to a single mobile station  40  at any one time does not exploit the degrees of freedom in the channel. If the scheduler  58  is constrained to select a single mobile station  40  to be served by all sector antennas  56  at any time, it is possible that the channel conditions between the selected mobile station  40  and one or more of the sector antennas  56  will not be favorable even if the selected mobile station  40  is the one with the best average channel conditions. When the number of users is large, further improvements in throughput can be achieved by allowing the RBS  36  to transmit to more than one mobile station  40  at any one time and using a technique referred to herein as antenna selection. With antenna selection, the scheduler  58  may choose to serve a mobile station  40  with less than all of the sector antennas  56 . The scheduler  58  chooses the sector antennas  56  with the best channel conditions to the mobile station  40 . One interpretation of best is the sector antennas  56  that can support the highest data rate. Any remaining sector antennas  56  can then be used to serve another mobile station  40 . With M sector antennas  56 , up to M mobile stations  40  can be served simultaneously. 
         [0020]    When antenna selection is employed, the scheduler  58 , must determine which mobile stations  40  to serve at any one time, the number of sector antennas  56  to use for each of the selected mobile stations  40 , and which sector antennas  56  to use for transmission to each of the selected mobile stations  40 . If all of the decisions are made at the scheduler  58 , the mobile stations  40  need to feedback channel information for the propagation channel between each transmit antenna at the RBS  36  and each receive antenna at the mobile station  40 . The channel information feedback may include the signal to interference plus noise ratio (SINR) of the propagation channel, channel quality indicator (CQI), channel coefficients, or rate indicator. 
         [0021]    In some embodiments, part of the antenna selection process can be performed by the mobile station  30  to reduce the amount of channel information feedback that is required. The antenna selection process may be broken down into two steps. In the first step, the number of sector antennas  56  to use for transmission, referred to herein as the mode, is selected for the mobile station  40 . In the subsequent discussion, the selected mode is denoted by the notation mode-N, where N refers to the number of sector antennas  56  selected for transmission to the mobile station  40 . In the second step, the particular sector antennas  56  that will be used are selected. 
         [0022]    In a first divided approach to antenna selection, the mobile station  40  estimates the SINRs for all possible antenna combinations for each mode and chooses an antenna combination for each possible mode that results in the maximum sum data rate. The scheduler  58  at the RBS  36  selects which mobile stations  40  to serve and the mode based on the channel information feedback. In this approach, the mobile station  40  needs to feed back channel information for the propagation channels corresponding to the selected sector antenna(s)  56  in each mode. The feedback may comprises a channel quality indicator (CQI) or rate indicator. In the example of M=4 transmit antennas, the feedback load would be 1 CQI for mode- 1 , 2 CQIs for mode- 2 , 3 CQIs for mode- 3 , and 4 CQIs for mode- 4 , resulting in a total of 10 CQIs. 
         [0023]    In a second divided approach to antenna selection, the mobile station  40  estimates the SINRs for all possible antenna combinations and selects the best antenna combination for the best mode. It then feeds back a CQI or rate indicator for each selected sector antenna  56 , i.e., if mode-N is selected, then N CQIs are fed back. It also needs to signal the antenna selection from the 2 M -1 possibilities, which may require M bits. 
         [0024]    In one exemplary embodiment, further improvements in system throughput can be attained by extending the antenna selection concept to include sector antennas  56  in other sectors  14  when the mobile station is in soft or softer hand-off. Referring to  FIG. 4 , a mobile station  40  designated by the letter M is in a first sector S 1  near the boundary with second sector S 2 . In this example, it is assumed that the mobile station M is currently being served by sector S 1 . It is further assumed in this example that the channel conditions between mobile station M and sector antenna  56  designated by the letter A located in sector S 2  are very favorable. In this scenario, mobile station M may report the favorable channel conditions between antenna A in sector S 2  and mobile station M to the RBS  36 . This can be done by feeding back channel information for the propagation channel between antenna A and mobile station M, or by means of a request by mobile station M to be served by antenna A. The RBS  36  may elect to reassign antenna A from sector S 2  to sector S 1  due to the favorable channel conditions to improve system throughput as indicated by dotted lines in  FIG. 4 . Thus, the number of available antennas in sector S 2  decreases from 4 to 3, and the number of sector antennas  56  in sector S 1  increases from 4 to 5. 
         [0025]    In one embodiment of the invention, the RBS  36  includes a centralized antenna management processor  60  for all sectors  14 . The antenna management processor  60  reassigns sector antennas  56  between sectors  14 . The antenna management processor  60  may be implemented in one or more programmable processors. A centralized antenna management processor  60  could also be located at the BSC  34  and may support sectors  14  at more than one RBS  36 . Locating the antenna management processor at the BSC  34  allows reassignment of sector antennas  56  between sectors  14  in different cells  12 . In yet other embodiments, a distributed approach could be taken in which an antenna management processor  60  is located in each sector  14 , and the separate antenna management processors  60  coordinate their actions by signaling between sectors  14 . 
         [0026]    The antenna management processor  60  determines the assignment of sector antennas  56  based on factors such as current channel conditions, sector loading, net impact on throughput, etc. For example, the antenna management processor  60  may reassign a sector antenna  56  from one sector  14  to another if the antenna management processor  60  determines that reassignment will result in an increase in overall system capacity. When the antenna management processor  60  decides to reassign a sector antenna  56  from one sector  14  to another, it notifies the schedulers  58  in the affected sectors  14 . This embodiment involves minimal change to the scheduler  58 . The schedulers  58  operate as previously described, except for a change in the number of possible antenna combinations to be considered. 
         [0027]    In other embodiments of the invention, a mobile station  40  in soft or softer handoff may be requested to report channel conditions between the mobile station  40  and sector antennas  56  in one or more sectors  14 . The mobile station  40  may report the channel conditions to the scheduler  58  in one sector  14 . In this embodiment, the scheduler  58  serving the mobile station  40  may request temporary reassignment of a sector antenna  56  from another sector  14  if doing so is advantageous. For example, the scheduler  58  could request reassignment of a sector antenna  56  if the reassignment would improve sector throughput. The antenna management processor  60  could balance the expected increase in throughput of the target sector  14 , i.e., the sector gaining a sector antenna  56 , against any decrease in throughput from the source sector, i.e., the sector losing a sector antenna  56 . 
         [0028]    The antenna management processor  60  may also decide to reassign a sector antenna  56  for reasons other than improving system throughput.  FIG. 5  illustrates a scenario where the load across sectors  14  is unbalanced. In  FIG. 5 , the sector  14  designated by S 1  is heavily loaded while sectors S 2  and S 3  are lightly loaded. In this scenario, the antenna management processor  60  may reassign one or more sector antennas  56  to achieve load balancing. If one sector  14  is heavily loaded and another sector  14  is lightly loaded, the antenna management processor  60  may reassign a sector antenna  56  from the lightly-loaded sectors  14  to the heavily-loaded sector. The reassignment of the sector antenna  56  to the heavily-loaded sector increases the throughput of the heavily-loaded sector, thereby enabling the heavily-loaded sector to serve more users. In  FIG. 5 , two sector antennas  56  are shown being reassigned, one from sector S 2  and one from sector S 3 . 
         [0029]    The present invention provides better opportunistic scheduling of mobile stations by allowing concurrent transmissions to two or more mobile stations  40  and by using antenna selection in the scheduling process. Further, the present invention achieves higher system throughput by allowing antennas in one sector to be temporarily reassigned to a different sector. Such reassignment may be done because of favorable channel conditions between the reassigned sector antenna  56  and a mobile station  40  in a different sector  56 . Also, reassignment may be employed as one means of balancing the load across all sectors  14 . 
         [0030]    The present invention can be also used in combination with a transmit diversity scheme as shown in  FIG. 6 . As shown in  FIG. 6 , a mobile station  40  is in communication with two sectors  14  denoted as Sector A and Sector B. Sectors A and B may be served by the same base station (i.e. softer handoff) or by different base stations (soft handoff). Two sector antennas  56  are selected from Sector A and one sector antenna is selected from Sector B. The two selected sector antennas  56  in Sector A transmit symbols s 0  and s 1  using a space-time code, such as the well-known Alamouti code. In a first transmit period, the two sector antennas  56  in Sector A transmit symbols s 0  and −s 1 * respectively. Sector antenna  56  in Sector B transmits symbol s 2 . In a second transmit period, the two sector antennas  56  in Sector A transmit symbols s 1  and s 0 * respectively, while sector antenna  56  for Sector B transmits symbol s 3 . The use of transmit diversity in Sector A may enhance the reliability of the symbols from Sector A, due to increased diversity order. This diversity gain may be useful in sectors  14  that have relatively low average SNR. 
         [0031]    In the embodiment shown in  FIG. 6 , there may be one or more additional sector antennas  56  in either Sector A and/or Sector B communicating with the mobile station  40 . For example, a second sector antenna in Sector B or a third sector antenna in Sector A may transmit symbols s 4  and s 5  to the mobile station  40  in the first and second symbol periods respectively. As previously described, sector antennas  56  may be reassigned from one sector  14  to another  14  depending on channel conditions and/or sector loading. 
         [0032]    The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the spirit and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.