Multiuser MIMO Systems: From Rate Adaptation to User Selection

Associate Research Fellow
Research Center for Information Technology Innovation
Academia Sinica

Abstract:

Multi-user multiple input and multiple output (MU-MIMO) is one predominate approach to improve the wireless capacity. However, the aggregate capacity of MU-MIMO heavily depends on the channel correlations among the mobile users served concurrently. This means that the optimal bit rate of a user will be highly dynamic and change from one packet to the next. This breaks traditional bit rate adaptation algorithms, which rely on recent history to predict the best bit rate for the next packet. We introduce a rate adaptation scheme customized for MU-MIMO LANs. Our design allows clients in a MU-MIMO LAN to adapt their bit rate on a per-packet basis by simply passively learning two variables: its SNR when it transmits alone to the access point, and the direction along which its signal is received at the AP. Another important issue is how to select a proper set of concurrent clients to maximally realize the MU-MIMO gain. The fundamental challenge for user selection is the large searching space, and hence there exists a tradeoff between search complexity and achievable capacity. Previous works have proposed several low complexity heuristic algorithms, but they suffer a significant capacity loss. To remedy this inefficiency, we propose an adaptive user selection scheme that leverages a knob to control the aggressiveness in searching the best beamforming group. Our design keeps tracking the channel and the coherence time for each mobile user, and largely avoids unnecessary computing with a progressive update strategy. A prototype implementation in USRP-N200 shows that our rate adaptation improves the throughput gain by 1.7x and 2.3x for 2-antenna and 3-antenna APs, respectively. By further enabling adaptive user selection, we can achieve around 90% of the capacity compared to exhaustive search.

Bio:

Kate Ching-Ju Lin received the BS degree from the Department of Computer Science, National Tsing Hua University in 2003, and the PhD degree from Graduate Institute of Networking and Multimedia, National Taiwan University in 2009. She was a visiting scholar at CSAIL, MIT, from March 2007 to March 2008 and from October 2010 to March 2011. After her graduation, she joined Research Center for Information Technology Innovation at Academia Sinica, Taiwan. She is currently an associate research fellow. Her current research interests include wireless systems, wireless multimedia networking and visible light communications.

Dr. Lin received the Exploration Research Award from Pan Wen Yuan Foundation in 2012, the Research Project for Excellent Young Scholars from Taiwan’s Ministry of Science and Technology in 2013, the K. T. Li Young Researcher Award from ACM Taipei/Taiwan in 2014, and the 23rd Ten Outstanding Young Women Award in 2015. She is now the Editor of Wireless Networks, and has served as the TPC member of several major conferences, including IEEE INFOCOM, ACM MOBICOM, USENIX NSDI, IEEE ICC. She is a member of the IEEE and ACM.