Design and analysis of Large Scale Antenna Systems

A Large Scale Antenna System (LSAS) entails a large number (tens or hundreds) of base station antennas serving a much smaller number of terminals, with large gains in spectral efficiency and energy efficiency compared with conventional multiuser MIMO technology. However, enabling multiuser MIMO requires very accurate channel state information at the transmitter (CSIT), which can be acquired via uplink pilots in Time Division Duplexing (TDD) systems and via downlink pilots and uplink feedback in Frequency Division Duplexing (FDD) systems. In conventional cellular technology, where FDD is employed, acquiring CSIT becomes prohibitive due to the presence of a large number of antennas. In this work, we propose Joint Spatial Division and Multiplexing (JSDM) and show that it achieves significant savings both in the downlink training and in the CSIT uplink feedback, thus making the use of large antenna arrays at the base station potentially suitable also for FDD systems. JSDM is a two stage beamforming scheme, and relies on serving groups of users with approximately similar covariances.;We prove a simple condition under which JSDM incurs no loss of optimality with respect to the full CSIT case and that such condition is approached in the large number of antennas limit with uniformly spaced linear arrays. We extend these ideas to the case of a two-dimensional base station antenna array, with 3-dimensional beamforming, including multiple beams in the elevation angle direction. We provide guidelines for optimization and calculate the system spectral efficiency under proportional fairness and max-min fairness criteria, showing extremely attractive performance. We also show that JSDM with simple opportunistic user selection is able to achieve the same scaling law of the system capacity with full channel state information, and propose a simple scheme for grouping users in a realistic setting. We propose a low-overhead probabilistic scheduling algorithm that selects these users at random with certain probabilities. As a result, only the pre-selected users are required to feedback their channel state information, realizing important savings in the CSIT feedback. We study the applicability of JSDM to mm-Wave channels and analyze its performance in some realistic propagation channels. Evaluations in propagation channels obtained from ray tracing results, as well as in measured outdoor channels show that JSDM performs surprisingly well in mm-Wave channels. Finally, we study the performance of JSDM in a heterogenous network consisting of a large number of small cells deployed under a macro-cellular "umbrella". We propose efficient intertier interference management schemes using JSDM as a sort of "spatial blanking", that is significantly more efficient than isotropic slot blanking (enhanced Inter-Cell Interference Coordination, eICIC) currently proposed in LTE standardization. Our numerical results are obtained via asymptotic random matrix theory, avoiding lengthy Monte Carlo simulations.