On Downlink Interference Decoding In Multi-Cell Massive MIMO Systems

In this paper, the downlink of a multi-cell massive MIMO system is considered where the channel state information (CSI) is estimated via pilot symbols that are orthogonal in a cell but re-used in other cells. Re-using the pilots, however, contaminates the CSI estimate at each base station (BS) by the channel of the users sharing the same pilot in other cells. The resulting inter-cell interference does not vanish even when the number of BS antennas M is large, i.e., M → ∞, and thus the rates achieved by treating interference as noise (TIN) saturate even if M → ∞. In this paper, interference aware decoding schemes based on simultaneous unique decoding (SD) and simultaneous non-unique decoding (SND) of the full interference or a part of the interference (PD) are studied with two different linear precoding techniques: maximum ratio transmission (MRT) and zero forcing (ZF). The resulting rates are shown to grow unbounded as M → ∞. In addition, the rates achievable via SD/SND/PD for finite M are derived using a worstcase uncorrelated noise technique, which are shown to scale as O(logM). To compare the performance of different schemes, the maximum symmetric rate problem is studied, where it is confirmed that with large, yet practical, values of M , SND strictly outperforms TIN, and also that PD strictly outperforms SND.