Analysis of Relaxed User Orthogonality for Wireless Multi-user MIMO Downlink Transmission
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Efficient resource allocation at the physical layer of wireless communication systems is closely linked to performance. Time and space are examples of such resources. In a densely-deployed multi-antenna, multi-user wireless downlink, finding a low-interference group of spatially distributed users in conjunction with spatial-domain multiple access beamforming represent techniques for efficient use of spatial resources. However, in practice, finding a perfectly orthogonal interference-free group of users to receive concurrent service is unlikely, thus wasting the transmission period or temporal resource. In this work, we set out to analyze the allocation of competing spatial and temporal resources in the context of the wireless downlink. The intention of this analysis is to investigate the orthogonality criteria that underpin many practical user selection algorithms. Deeper understanding of such criteria has potential for designing improved interference-mitigating algorithms in this sense, and in other related scenarios. A relaxed definition of orthogonality between users in group is investigated for practical amplitude and quadrature modulation schemes. Motivated by widely-linear processing techniques, new relaxed user orthogonality on the complex hyper-sphere illustrates temporal benefits and trade-offs associated with various system parameters. Beamforming and user selection are analyzed jointly for key scenarios of interest to gain insights into the interaction between these spatial resource management techniques. System throughput and reliability performance analysis is also developed, and applied to these scenarios to gain further insights.