Adaptive channel assignment in SDMA-based wireless LANs with transceiver resource limitations

Beamforming with adaptive antenna arrays is the most promising means for increasing data rates of wireless systems, since it enables channel reuse by several users in a cell through space division multiple access (SDMA). In SDMA, multiple beams are formed towards different users, each beam by a dedicated transceiver. However, the use of adaptive antenna arrays at the physical layer mandates significant modifications for higher layers. Joint consideration of beamforming and higher layer issues is required in order to fully exploit the benefits of SDMA. Moreover, adoption of the popular orthogonal frequency division multiplexing (OFDM) technique creates additional challenges when the number of beams that can be formed at the transmitter is bounded. This issue is attributed to transceiver resource limitations and gives rise to a coupled resource allocation problem, that of assigning transceiver hardware units and OFDM subcarriers for transmission to users. Different users can be served either with the same beam from a transceiver and different subcarriers or with different beams and the same subcarriers. We characterize the problem and propose meaningful heuristic algorithms for beamforming and assignment of subcarriers and transceivers to users. The objective is to increase achievable system rate and ensure QoS in the form of minimum rate guarantees. The criteria for resource assignment and beam formation are based on spatial separability properties of users, beam vector cross-correlations and induced interference. Numerical results quantify the performance benefits of these cross-layer techniques and provide useful insights and design guidelines for realistic systems.