On the optimality of multiantenna broadcast scheduling using zero-forcing beamforming

• Published in: IEEE journal on selected areas in communications Vol. 24; no. 3; pp. 528 - 541
• Main Authors: Taesang Yoo, Goldsmith, A.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.03.2006; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Algorithms; Array signal processing; Asymptotic properties; Base stations; Beamforming; Broadcast channel; Broadcasting; Channels; dirty paper coding (DPC); Downlink; downlink scheduling; fair scheduling; H infinity control; imperfect channel state information (CSI); MIMO; multiple-input/multiple-output (MIMO); multiple-input/multiple-output capacity; multiuser diversity; Optimization; proportional fair; Receiving antennas; Scheduling; Strategy; Studies; Throughput; Time division multiple access; Transmitting antennas; zero-forcing beamforming (ZFBF)
• ISSN: 0733-8716; 1558-0008
• DOI: 10.1109/JSAC.2005.862421

Although the capacity of multiple-input/multiple-output (MIMO) broadcast channels (BCs) can be achieved by dirty paper coding (DPC), it is difficult to implement in practical systems. This paper investigates if, for a large number of users, simpler schemes can achieve the same performance. Specifically, we show that a zero-forcing beamforming (ZFBF) strategy, while generally suboptimal, can achieve the same asymptotic sum capacity as that of DPC, as the number of users goes to infinity. In proving this asymptotic result, we provide an algorithm for determining which users should be active under ZFBF. These users are semiorthogonal to one another and can be grouped for simultaneous transmission to enhance the throughput of scheduling algorithms. Based on the user grouping, we propose and compare two fair scheduling schemes in round-robin ZFBF and proportional-fair ZFBF. We provide numerical results to confirm the optimality of ZFBF and to compare the performance of ZFBF and proposed fair scheduling schemes with that of various MIMO BC strategies.