Abstract:
A method is provided for selecting multiple customer premises equipments (CPEs) to share a channel in a wireless communications network. The method includes calculating a correlation between spatial signatures of every two CPEs subscribing to a base transceiver station (BTS) of the wireless communications network, wherein two CPEs constitute channel-sharing candidates if their correlation is lower than a predetermined threshold, creating a first set of CPEs of all the member of channel-sharing candidates, counting a number of channel-sharing candidates a CPE relates to for every CPE, selecting a first CPE related to the fewest number of channel-sharing candidates, creating a second set of CPEs by identifying all the CPEs that the first CPE relates to, and selecting a second CPE related to the fewest number of channel-sharing candidates from the second set of CPEs, wherein the first and second CPEs become two selected CPEs to share the channel.

Description:
CROSS REFERENCE 
     The present application claims the benefit of U.S. Provisional Application Ser. 60/836,719, which was filed on Aug. 10, 2006. 
    
    
     BACKGROUND 
     Spatial division multiple access (SDMA) increases the capacity of a wireless communications network by allowing more than one wireless station to access a communication channel on the same frequency at the same time. One example of channel sharing is that wireless stations, such as customer premises equipments (CPEs), transmit signals on the same frequency at different times or on different frequencies at the same time. 
     How to select a set of CPEs to share a communication channel is a very complicated and computationally intensive process. This is true primarily because not every CPE can share a communication channel with other CPEs in a wireless communications network employing SDMA. 
     There are a number of reasons why some CPEs cannot share a communication channel. One of the reasons is co-channel interference (CCI), which occurs when a CPE receives unintended signals from other CPEs sharing the same communication channel. This type of inter-user interference is the major drawback of an SDMA system. To suppress inter-user interference, an SDMA system must provide a means to isolate the spatial signatures of one CPE from those of the rest of the CPEs sharing a channel. Otherwise, inter-user interference may cause a communication channel to be unusable for the entire set of CPEs. 
     A second reason is that different CPEs may subscribe to the services of a base transceiver station (BTS) at any given time. In other words, a set of CPEs sharing a communication channel at one point in time may not be the same set of CPEs sharing the same communication channel at another time. A third reason is that the characteristics of the radio link may change over time. 
     In a conventional method, a BTS selects an optimal set of CPEs to permanently share a communication channel in a wireless communications network employing SDMA, based on certain predetermined parameters. The optimal set of CPEs chosen to share a communication channel at one time might not be optimal at a later time. Therefore, the permanent grouping of CPEs to share a communication channel is ineffective and inefficient. 
     What is desired is a system and method for improving the grouping of CPEs dynamically in a wireless communications network employing SDMA that addresses the dynamic nature of the radio link and participants of communication sessions. 
     SUMMARY 
     A method is provided for selecting multiple customer premises equipments (CPEs) to share a channel in a wireless communications network. The method comprises calculating a correlation between spatial signatures of every two CPEs subscribing to a base transceiver station (BTS) of the wireless communications network, wherein two CPEs constitute channel-sharing candidates if their correlation is lower than a predetermined threshold, creating a first set of CPEs of all the members of channel-sharing candidates, counting a number of channel-sharing candidates a CPE relates to for every CPE, selecting a first CPE related to the fewest number of channel-sharing candidates, creating a second set of CPEs by identifying all the CPEs that the first CPE relates to, and selecting a second CPE related to the fewest number of channel-sharing candidates from the second set of CPEs, wherein the first and second CPEs become two selected CPEs to share the channel. 
     The construction and method of operation of the techniques described herein, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The drawings accompanying and forming part of this specification are included to depict certain aspects of the techniques described herein. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. 
         FIG. 1  is a block diagram of a wireless communication network and depicting channel sharing techniques among customer premises equipment that share a communication channel for services with a base transceiver station. 
         FIG. 2  is a flow diagram illustrating a method to have a channel shared by two customer premises equipments based on the isolation of spatial signatures. 
         FIG. 3  is a flow diagram illustrating a second method to have a channel shared by two CPEs based on both the isolation of spatial signatures and path loss. 
     
    
    
     DESCRIPTION 
     The following detailed description refers to the accompanying drawings. The description includes exemplary embodiments, not excluding other embodiments, and changes may be made to the embodiments described without departing from the spirit and scope of the subject matter described herein. 
     The method and system described herein reduces inter-user interference and improves the bit error rate (BER) for a group of CPEs in a wireless communications network employing SDMA. The selection of a group of CPEs to share a communication channel is based on the isolation of spatial signatures and path loss differences. The method and system described herein is applicable to any wireless communications network and the term channel refers to any of the conventional multiple access channels such as frequency, time, code or any combination of them. The method can be extended to include more than two CPEs, though the techniques described herein are directed to how to select two CPEs to share a communication channel. 
     With reference to  FIG. 1 , a wireless communication network  5  is shown comprising a base transceiver station (BTS)  10  and multiple customer premises equipments (CPEs) C 1 -C K . The CPEs C 1 -C K  wirelessly communicate with the BTS  10 . Assume that there are K CPEs that subscribe to the services of the BTS  10  in a cell of the wireless communications network  5  employing SDMA. By using the method described herein, the BTS  10  dynamically decides which CPEs would share a communication channel. 
       FIG. 2  is a flow diagram illustrating a method to have a channel shared by two CPEs based on the isolation of spatial signatures. 
     Let {C 1 , C 2 , . . . , C K } denote the set of K CPEs subscribing to the services of the BTS equipped with M antennas. Let the spatial signature of CPE C i  be denoted as 
                 h   i     =     [           h     i   ,   1                 h     i   ,   2               ⋮             h     i   ,   M             ]       ,         
where m ∈(1,2, . . . ,M) and h i,m  is a spatial signature associated with antenna m.
 
     Each CPE C i  is a candidate for sharing a communication channel with another CPE C j . A partner set χ C     i   , which includes all the CPEs that could share a communication channel with the CPE C i , is created for each CPE C i . The number of CPEs in the set χ C     i    is denoted as n C     i   . A number set, denoted as χ pre     —     number ={n C     1   , n C     2   , . . . , n C     k   }, is formed to show the number of partners of each CPE C i . 
     In step  110 , for each C j , where 1≦j≦K and i≠j, the correlation s i   j  between CPE C i  and CPE C j  is calculated according to the following equation: 
               s   i   j     =              h   i   H     ⁢     h   j            =              ∑     m   =   1     M     ⁢       h     i   ,   m     *     ⁢     h     j   ,   m                .           ⁢       (   …   )     H               
denotes a Hermitian operator. If s i   j             γ 0 , where γ 0  is a predetermined threshold and 0≦γ 0 ≦1, then C j  is included in the partner set χ C     i    of CPE C i .

     The number of CPEs in the partner set of CPE C i  is denoted as n C     i   . If n C     i   &gt;0, C i  is included in a set χ pre , i.e., χ pre =χ pre ∪{C i } and n C     i    is included in the χ pre     —     number . The set χ pre  contains the CPEs that are pre-qualified to share a communication channel with another CPE. Step  110  is repeated for every CPE in the set {C 1 , C 2 , . . . , C K }. 
     In Step  120 , the smallest element of χ pre     —     number  is selected. If more than one n C     i    has the same smallest value, a predetermined tie-breaker rule is employed to select only one n C     i   . For example, the first one of at least two CPEs that have the same smallest number in the χ pre     —     number  is selected. In other words, CPE C i  with the smallest number of partners is chosen to be one of the two CPEs selected to share a communication channel. 
     In step  130 , let χ peer =χ C     j   . For each CPE C j  in the χ peer , the partner set of C i , n C     j    is included in the set χ peer     —     number . 
     In step  140 , the smallest element of χ peer     —     number  is selected. If more than one n C     j    has the same smallest value, a predetermined tie-breaker rule is employed to select only one n C     j   . In other words, CPE C j  with the smallest number of partners, is chosen to be the other CPE that is to share a communication channel. A pair of CPEs (C i , C j ) is identified to share a communication channel. 
     In Step  150 , CPEs C i  and C j  are removed from the χ pre ; n C     i    and n C     j    are removed from the χ peer     —     number . If there is more than one element in χ pre , the pairing process repeats from step  120 . The process continues until no more CPEs could share a communication channel. All feasible CPE pairs in SDMA are identified. 
       FIG. 3  illustrates a second method to have a channel shared by two CPEs based on the isolation of spatial signatures and path loss. 
     Let {C 1 , C 2 , . . . , C K } denote the set of K CPEs subscribing to the services of the BTS equipped with M antennas. Let the spatial signature of CPE C i  be denoted as 
                 h   i     =     [           h     i   ,   1                 h     i   ,   2               ⋮             h     i   ,   M             ]       ,         
where m ∈(1,2, . . . ,M) and h i,m  is a spatial signature associated with antenna m. Let α i  denote the path loss of CPE C i . Each CPE C i  is a candidate for sharing a communication channel with another CPE C j .
 
     Two predetermined thresholds γ 1  and Δγ, where 0≦γ 1 ≦1 and 0≦Δγ≦γ 1 , are chosen. The Δγ is the marginal threshold of γ 1 . A good guideline for the selection of Δγ is 0.1γ 1 ≦Δγ≦0.2γ 1 . 
     In Step  210 , for each C i , where 1≦i≦K, if α i ≧γ 1 +Δγ, then CPE C i  is included in the set χ pre , i.e., χ pre =χ pre ∪{C i }. Let L denote the number of CPEs with a path loss that satisfies the above condition. 
     In step  220 , CPE C i  with the smallest α i  is selected to be one of the two CPEs to share a communication channel. If more than one CPE has the same smallest path loss, a predetermined tie-breaker rule is employed to select only one. For example, the first of the CPEs that have the smallest path loss is selected. 
     In step  230 , for each C j  in the χ pre , where 1≦j≦L and i≠j, the correlation s i   j  between CPE C i  and CPE C j  is calculated according to the following equation: 
               s   i   j     =              h   i   H     ⁢     h   j            =              ∑     m   =   1     M     ⁢       h     i   ,   m     *     ⁢     h     j   ,   m                .           ⁢       (   …   )     H               
denotes a Hermitian operator. CPE C j  that has the smallest correlation with CPE C i  is identified to be a potential partner.
 
     In step  240 , for CPEs C i  and C j , a value based on spatial signatures and path loss is calculated according to the following equation: α i (1−|h i   H h j | 2 ). 
     In step  250 , if α i (1−|h i   H h j | 2 )≧γ 1 , then CPE C j  is the other CPE that is to share a communication channel. A pair of CPEs (C i , C j ) is identified to share a communication channel and then CPEs C i  and C j  are removed from the set χ pre . 
     If α i (1−|h i   H h j | 2 )&lt;γ 1 , CPE C 1  cannot be the one to share a communication channel with CPE C j . CPE C i  is then removed from the set χ pre . If more than one CPE remains in the set χ pre , the selection process is repeated from step  220 . 
     The process continues until no two CPEs could share a communication channel. All pairs of CPEs that could share a communication channel in SDMA are identified. 
     The above illustration provides many different embodiments or embodiments for implementing different features of the subject matter described herein. Specific embodiments of components and processes are described to help clarify the subject matter described herein. These are, of course, merely embodiments and are not intended to limit the subject matter described herein. 
     Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the subject matter described herein and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.