Pilot distribution optimization in multi-cellular large scale MIMO systems

Salient characteristics of a wireless communication system deploying a great number of antennas at the base station (BS), namely a massive MIMO system, are investigated in this work. The asymptotic performance of the linear zero-forcing precoding scheme is found, both in terms of signal-to-interference-plus-noise ratio (SINR) and bit-error rate (BER), and shown to be equivalent to the matched-filter beamforming performance. Furthermore, analysis of the massive MIMO system downlink is carried out from the viewpoint of uncoded BER performance, including some realistic adverse effects, such as interference from neighbouring cells, channel estimation errors due to background thermal noise, and pilot contamination. The latter has been shown to be the only impairment that remains in the MIMO multicell system with infinite number of BS antennas. For such scenario, we derive expressions for the asymptotic BER, i.e. in the limit of infinite number of antennas at BS. A quite simple and efficient method for optimizing the massive MIMO system performance under different optimization metrics is proposed, which consists of simply distributing the pilot sequences among the users of the cell in an efficient manner. As a result, a user rate gain of six times regarding the random strategy has been achieved for the downlink with unitary reuse factor, while the user rate increases twice for reuse factor of three. These benefits are achieved by only knowing the powers and the long-term fading coefficients of users in adjacent cells, for each pilot sequence.