Massive multiple input multiple output (MIMO) is a recent technological advancement which has been demonstrated to provide promising gains both in terms of spectral and energy efficiency in cellular systems. In this talk, we address the problem of decentralized multi-cell downlink beamforming in massive MIMO systems. Specifically, we formulate a multi-cell beamforming algorithm for massive MIMO systems which requires limited amount of information exchange between the BSs. The design objective is to minimize the total transmit power across all the BSs subject to satisfying the user signal to interference noise ratio (SINR) constraints. With our algorithm, the BSs need to exchange parameters which can be computed solely based on the channel statistics (e.g. the path loss information) rather than the instantaneous CSI. We make use of tools from random matrix theory to formulate the decentralized algorithm. We show that the performance of our algorithm asymptotically matches the performance of the centralized algorithm with full CSI sharing. Finally, we investigate the impact of imperfect CSI and pilot contamination effect on the performance of the decentralized algorithm, and propose a heuristic extension of the algorithm to accommodate these issues. Simulation results illustrate that our algorithm closely satisfies the target SINR constraints and achieves minimum power in the regime of massive MIMO systems, i.e., hundreds of antennas per BS serving tens UTs per cell. In addition, it also provides substantial power savings as compared to zero-forcing beamforming when the number of antennas per BS is of the same orders of magnitude as the number of UTs per cell.
Subhash Lakshminarayana currently holds a joint post doctoral research fellow appointment at Princeton University and the Singapore University of Technology and Design (SUTD). Previously, he received his M.S. degree in Electrical and Computer Engineering from The Ohio State University in 2009, and his Ph.D. from the Alcatel Lucent Chair on Flexible Radio and the Department of Telecommunications at École Supérieure d'Électricité (SUPELEC), France in December, 2012. He has also been has been an undergraduate student researcher at the Indian Institute of Science, Bangalore during 2007. His work has been selected among the "Best 50 papers" of Globecom - 2014 conference. His research interests include massive MIMO systems, cross layer design in wireless systems as well as design of smart energy systems and stochastic network optimization theory.