Patent Application
20080020772
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:
Users 103 and 104 receive signals from base station 100, which is transmitting signals s1 (t) and s2 (t) using beam-forming vectors w1=[w11 w12] and w2=[w21 w22]. Signals s1 (t) and s2 (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.
Antenna 101 transmits signal 105, which is a complex weighted combination of w11xs1(t) and w21xs2 (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 w12xs1(t) and w22xs2 (t), combined by signal combiner 1060. Signal combiner 1050 comprises summer 1051 and weighting elements 1052 and 1053. Weighting element 1052 scales signal s1 by w11, while weighting element 1053 scales signal s2 by w21 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.
User 103 receives signal 105 from antenna 101 through downlink channel 107, having transfer function h11 and signal 106 from antenna 102 through downlink channel 107, having transfer function h12. User 103 then has a vector channel having transfer function h1=[h11h12]T. User 104 receives signal 106 from antenna 102 through downlink channel 109, having transfer function h22 and signal 105 from antenna 101 through downlink channel 110, having transfer function h21. User 104 has a vector channel having transfer function h2=[h21h22]T.
User 103 receives:
s1(t)xw1xh1+s2(t)xw2xh1=s1(t)xw11xh11+s1(t)xw12xh12+s2(t)xw21xh11+s2(t)xw22xh12.
Similarly, user 104 receives:
s1(t)xw1xh2+s2(t)xw2xh2=s1(t)xw11xh21+s1(t)xw12xh22+s2(t)xw21xh21+s2(t)xw22xh22.
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.
Applied to the system shown in
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.
Even though beam-forming vectors w1 and w2 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 w1 and w2 are orthogonal, alignment with one of the beam-forming vectors, either w1 or w2, 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.
For the purposes of discussing
s1(t)xw1xh1+s2(t)xw2xh1=s1(t)x1+s2(t)x0=s1(t).
Similarly, the signal received by user 104 is:
s1(t)xw1xh2+s2(t)xw2xh2=s1(t)x0+s2(t)x1=s2(t).
Even without perfect alignment between h1 and w1, or between h2 and w2, user 103 will still receive s1(t) at a considerably higher level than s2(t), and user 104 will receive s2(t) at a considerably higher level than s1(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.
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's changed circumstances. This reduces the computational burden for providing SDMA.
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'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.
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.
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.
