Interference Reduction in Multi-Cell Massive MIMO Systems with Large-Scale Fading Precoding

A wireless massive MIMO system entails a large number (tens or hundreds) of base station antennas serving a much smaller number of users, with large gains in spectral-efficiency and energy-efficiency compared with conventional MIMO technology. Until recently it was believed that in multi-cellular massive MIMO system, even in the asymptotic regime, as the number of service antennas tends to infinity, the performance is limited by directed intercellular interference. This interference results from unavoidable re-use of reverse-link training sequences (pilot contamination) by users in different cells. We devise a new concept that leads to the effective elimination of inter-cell interference in massive MIMO systems. This is achieved by outer multi-cellular precoding, which we call Large-Scale Fading Precoding (LSFP). The main idea of LSFP is that each base station linearly combines messages aimed to users from different cells that re-use the same training sequence. Crucially, the combining coefficients depend only on the large-scale fading coefficients between the users and the base stations. These coefficients change slowly and they are independent of antenna index and OFDM frequency subcarrier index [1, Ch.3], [2]. Each base station independently transmits its LSFP-combined symbols using conventional linear precoding that is based on estimated small-scale fading coefficients. Further, we derive estimates for downlink and uplink SINRs and capacity lower bounds for the case of massive MIMO systems with LSFP and a finite number of base station antennas. In this operation regime mitigation of all types of interference, not only the pilot contamination, is required. We consider optimal and suboptimal LSFP precodings optimized according to the max-min SINR criterion that takes into account all sources of interference. Our simulations results show that this optimization also gives very good results for moderate number of base station antennas with respect to the 5%-outage rate performance criterion, which is the minimum rate among 95% of the best users.