Abstract:
Advantage is taken of the fact that downlink quality is always known at a mobile station. Thus, a base station may use preference information from the mobile station as a basis for assigning a channel, rather than requiring the details of channel conditions. In one embodiment, the base station pre-selects orthogonal beam-forming vectors for subcarriers and broadcasts the channels into different sectors of the region served by base station. The mobile stations then determine a priority (based for example on received quality) order of the codes of the received vectors. This priority order is sent uplink to the base station and the base station then, based on a priority listing of vectors from the mobile station, selects the downlink sub-channel. The vectors may be established with some degree of randomness, or may be based on a desired beam coverage profile.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates generally to wireless communication, and more particularly, to systems and methods for overhead reduction in wireless networks using space division multiple access (SDMA). 
       BACKGROUND OF THE INVENTION 
       [0002]    Space division multiple access (SDMA) is being used in wireless communication systems to improve the system&#39;s spectral efficiency. However, to enable SDMA, a base station has traditionally required information regarding the quality of the communication from the base station to the mobile user (downlink channel). That is, for existing SDMA implementations, the base station must be able to estimate the quality of the signal received by the remote subscriber unit so that a proper channel can be allocated for a particular air interface between a transmission point and a particular mobile user. For example, in a traditional SDMA implementation, the base station obtains the downlink channel information, such as magnitude and phase information, in order to form the beamforming vector so that the signal targeted to one user can be directed toward that particular user without interfering with other users. 
         [0003]    Common methods for estimating downlink channel conditions, such as magnitude and phase, include: (1) assumption of downlink/uplink channel reciprocity; and (2) closed-loop feedback. The first method provides for estimating downlink quality using uplink quality, which the base station can determine from the incoming subscriber signal. However, due to possible differences in transmit and receive channels, the antenna array may need to be calibrated to compensate for phase inconsistencies. Not only may the calibration be expensive, but it may not even provide a solution in many implementations, since channel reciprocity does not hold for FDD systems. Closed-loop feedback of downlink channel information from a subscriber unit may require the use of a significant portion of the system bandwidth. Rapidly changing channel conditions, such as may be common in mobile applications, may drive the bandwidth cost even higher due to frequent channel quality reports. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    Advantage is taken of the fact that the downlink quality is always known at the mobile station. Thus, the base station uses preference information from the mobile station as a basis for assigning an appropriate channel, rather than requiring the same degree of detail regarding channel conditions as would be required by a traditional SDMA system. In one embodiment, the base station pre-selects orthogonal beam-forming vectors for subcarriers and broadcasts the channels (subcarriers with different beamforming vectors) into different sectors of the region served by base station. The mobile stations then determine a priority (based for example on received quality) order of the codes of the received vectors. This priority order is sent uplink to the base station and the base station then, based on a priority listing of vectors from the mobile station, selects the downlink channel. The vectors may be established with some degree of randomness, or may be based on a desired beam coverage profile. 
         [0005]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
           [0007]      FIG. 1  shows a wireless communication system adapted to provide SDMA according to an embodiment of the invention; 
           [0008]      FIG. 2A  shows a method for reusing an SDMA subcarrier according to an embodiment of the invention; 
           [0009]      FIG. 2B  shows one embodiment of the control within a mobile device for determining beam preferences; and 
           [0010]      FIG. 3  shows one embodiment of the base station beam-forming controller. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]      FIG. 1  shows one embodiment of wireless communication system  10  adapted to provide SDMA. Base station  100  comprises a plurality of antennas, shown here as antennas  101  and  102 , although base station  100  may have any number of antennas in an array, or any number of arrays. Although embodiments of the invention may utilize any number of antennas and beams, the illustrated embodiment will be discussed with reference to the two antenna beams to simplify the discussion herein. As used herein, the term antenna means a phase center, and the term array means a collection of two or more phase centers. 
         [0012]    Users  103  and  104  receive signals from base station  100 , which is transmitting signals s 1  (t) and s 2  (t) using beam-forming vectors w 1 =[w 11  w 12 ] and w 2 =[w 21  w 22 ]. Signals s 1  (t) and s 2  (t) represent a single subcarrier that is to be transmitted in two different directions on two different beams. Base station  100  is shown transmitting two signals on the same subcarrier using the two beam-forming vectors, but may transmit any number of signals using an appropriate number of beam-forming vectors. For example, a base station may use N beam-forming vectors with N antennas to reuse a subcarrier by transmitting N signals on N beams. This allows reuse of a single subcarrier N times in a single cell. 
         [0013]    Antenna  101  transmits signal  105 , which is a complex weighted combination of w 11 xs 1 (t) and w 21 xs 2  (t), combined by signal combiner  1050 . (As used herein, “x” denotes either scalar or vector multiplication.) Antenna  102  transmits signal  106 , which is a complex weighted combination of w 12 xs 1 (t) and w 22 xs 2  (t), combined by signal combiner  1060 . Signal combiner  1050  comprises summer  1051  and weighting elements  1052  and  1053 . Weighting element  1052  scales signal s 1  by w 11 , while weighting element  1053  scales signal s 2  by w 21  prior to  1051  combining the weighted signals. Similarly signal combiner  1060  comprises summer  1061  and weighting elements  1062  and  1063 , and operates similarly to combiner  1050 . 
         [0014]    User  103  receives signal  105  from antenna  101  through downlink channel  107 , having transfer function h 11  and signal  106  from antenna  102  through downlink channel  107 , having transfer function h 12 . User  103  then has a vector channel having transfer function h 1 =[h 11 h 12 ] T . User  104  receives signal  106  from antenna  102  through downlink channel  109 , having transfer function h 22  and signal  105  from antenna  101  through downlink channel  110 , having transfer function h 21 . User  104  has a vector channel having transfer function h 2 =[h 21 h 22 ] T . 
         [0015]    User  103  receives: 
         [0000]    s 1 (t)xw 1 xh 1 +s 2 (t)xw 2 xh 1 =s 1 (t)xw 11 xh 11 +s 1 (t)xw 12 xh 12 +s 2 (t)xw 21 xh 11 +s 2 (t)xw 22 xh 12 . 
         [0016]    Similarly, user  104  receives: 
         [0000]    s 1 (t)xw 1 xh 2 +s 2 (t)xw 2 xh 2 =s 1 (t)xw 11 xh 21 +s 1 (t)xw 12 xh 22 +s 2 (t)xw 21 xh 21 +s 2 (t)xw 22 xh 22 . 
         [0017]    For downlink transmission in an orthogonal frequency division multiple access (OFDMA) system, where base station  100  is equipped with multiple antennas, random orthogonal beam-forming vectors may be applied to each subcarrier or groups of subcarriers. Different subcarriers, or groups of subcarriers, may adopt different orthogonal beam-forming vectors. This results in a method of wireless communication which allows space division multiple access (SDMA) without requiring either downlink-uplink reciprocity calibration or closed-loop feedback of downlink channel information. Embodiments of the invention form a plurality of beams for downlink transmission and assigning one of the beams to a subscriber based on information received from that subscriber. Beams may be pre-formed, including random parameters, each with its own pilot data. Orthogonality among vectors reduces interference between different beams. Subscribers may determine the signal-to-interference ratios for one or more subcarriers and its associated beam-forming vector to feed back a subcarrier and beam preference. In this manner, two or more subscribers may use a signal subcarrier from a signal base station simultaneously. 
         [0018]    Applied to the system shown in  FIG. 1 , beam-forming vector w 1  may be determined in any suitable manner, including some degree of randomness. Beam-forming vector w 2  may then be formed to be orthogonal to vector w 1 . 
         [0019]    Each user  103  and  104 , being served by base station  100 , may then provide preference information for specific subcarriers and beam-forming vectors back to a scheduler managing the communication of base station  100 . Preference information may be based on signal-to-interference ratio (SIR) or signal-to-noise ratio (SNR), and may be abbreviated as compared with a closed-loop feedback system, as previously described. For example, feedback information may identify subcarriers and beam-forming vectors using only indices identified on pilot transmissions, rather than the same amount of vector channel information that would be required by a traditional closed-loop system. Also, no calibration is necessary to validate an assumption of reciprocity, since users  103  and  104  do provide at least some amount of feedback. 
         [0020]    Even though beam-forming vectors w 1  and w 2  may be determined randomly, rather than calculated for any particular user, a typical cellular system may have enough different users that there should be a high probability that some users will align well with at least one of the beam-forming vectors. Since w 1  and w 2  are orthogonal, alignment with one of the beam-forming vectors, either w 1  or w 2 , should result in low interference from the other. If a second user aligns well with the other beam-forming vector, two different users may share a single subcarrier, providing the benefits of SDMA. With an OFDMA channel scheduler at the base station which assigns subcarriers to users, at least in part, on user preferences, both OFDMA system multi-user diversity gain and SDMA gain may be achieved. 
         [0021]    For the purposes of discussing  FIG. 1 , user  103  will be assumed to align perfectly with w 1 , while user  104  aligns perfectly with w 2 . This means that w 1 xh 1 =1, while w 2 xh 1 =0. Similarly, w 2 xh 2 =1, while w 1 xh 2 =0. Under this assumption, the signal received by user  103  is: 
         [0000]    s 1 (t)xw 1 xh 1 +s 2 (t)xw 2 xh 1 =s 1 (t)x1+s 2 (t)x0=s 1 (t). 
         [0022]    Similarly, the signal received by user  104  is: 
         [0000]    s 1 (t)xw 1 xh 2 +s 2 (t)xw 2 xh 2 =s 1 (t)x0+s 2 (t)x1=s 2 (t). 
         [0023]    Even without perfect alignment between h 1  and w 1 , or between h 2  and w 2 , user  103  will still receive s 1 (t) at a considerably higher level than s 2 (t), and user  104  will receive s 2 (t) at a considerably higher level than s 1 (t). Each user  103  and  104  may then have a relatively high SIR, allowing the scheduler at base station  100  to assign the same subcarrier to both. 
         [0024]    When a user moves, such that the assigned subcarrier and beam-forming vector is no longer suitable, the base station scheduler may change the assignment, rather than adapting a beam-forming vector to the user&#39;s changed circumstances. This reduces the computational burden for providing SDMA. 
         [0025]      FIG. 2A  shows one embodiment of a method, such as method  20 , for assigning a subcarrier to a particular mobile station. Process  201  establishes beam-formed vector w 1  in any suitable manner. Similarly, beam-forming vector w 2  is established by process  202  such that w 2  is orthogonal to w 1 . In process  203 , the beams, along with pilot data, are transmitted to any mobile stations in the coverage area. 
         [0026]    In process  204 , a mobile station user enters the coverage area and, as shown by process  205 , the user determines a preference hierarchy. This hierarchy can be based on many factors, such as SIR and SNR, but in any case represents a listing of best to worse beams for transmission purposes. In process  206 , the user provides preference information to a scheduler or controller at the base station which then assigns a subcarrier and beam-forming vector combination to the user via process  207 . The user&#39;s reception may change, as controlled by process  208 , resulting in a return to process  205  to determine a new preference and thereby obtain a new beam assignment. 
         [0027]      FIG. 2B  shows one embodiment of a mobile device, such as device  21 , adapted for determining beam preferences and for communicating that information to the base station. Device  21 , for example, contains processor  222 , and working in conjunction with algorithms contained in memory  223  controls the reception of beam data via receiver  220  and determines the list of qualities of the beams via  221 . The list can be according to coded identities for each beam and/or subcarrier. The ordered list of identities can then be transmitted uplink by transmitter  220 . 
         [0028]      FIG. 3  shows base station  100  comprising beam former  31 , beam-forming controller  32 , and assignment controller  33 . Beam former  31  comprises signal combiners  1050  and  1060 , discussed above. Beam-forming controller  32  provides beam-forming vectors w 1  and w 2  to beam former  31 . Assignment controller  33  associates signals, such as s 1 (t) and s 2 (t), with the proper beam-forming vector. 
         [0029]    For many cells, sets of beam-forming vectors may be selected based on historical or predicted user location densities. In some situations, a particular beam-forming vector may be unsuitable for use if there is no user in need of service in the area served by that beam-forming vector. That is, with pre-formed beams, a particular beam may only find use when a user needing service is in the correct location. For a traditional SDMA system using custom-formed beams, however, while there may be a potential for more efficient reuse, it comes at the cost of increased user feedback requirements that use system bandwidth. One possible way to pre form the beamforming vector is to let the direction of beams on different subcarriers be uniformly cover all possible directions uniformly or evenly-spaced. Another possible way is to randomly choose orthogonal vectors for each subcarrier. When the number of subcarriers in the system is large, this should provide good coverage for all directions. When the number of users is large, each subcarrier will likely be acceptable for some users, providing SDMA without the bandwidth requirements of traditional implementations. 
         [0030]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.