Impact of Underlaid Multi-antenna D2D on Cellular Downlink in Massive MIMO Systems

Abstract

In this paper, we consider a massive multiple-input multiple-output (MIMO) downlink system underlaid with a network of multi-antenna D2D user equipments (UEs). Each D2D transmitter (Tx) uses all its antennas to beamform information towards its desired D2D receiver, which uses only a single antenna for reception. While beamforming at the D2D Tx reduces D2D interference to the neighbouring cellular UEs (CUEs), the cellular-to-D2D interference is also negligible due to highly directional beamforming at the massive MIMO base station. For the above proposed system, we analyze the average per-user spectral efficiency (SE) of CUEs (\(R^{c,d}\)) as a function of the D2D area spectral efficiency (ASE). Our analysis reveals that for a fixed D2D ASE (\(R_0^{(d)}\)) and fixed number of D2D antennas (\(N\)), with increasing density of D2D Txs (\(\lambda\)), \(R^{c,d}\) increases (for sufficiently large \(\lambda\)) and approaches a fundamental limit \(R_{\infty}^{c,d}\) as \(\lambda \to \infty\). Also, \(R_{\infty}^{c,d}\) depends on \(R_{0}^{(d)}\) and \(N\), only through the ratio \(\frac{R_{0}^{(d)}}{N-1}\), i.e, for a given fundamental limit \(R_{\infty}^{c,d}\), the D2D ASE can be approximately doubled with every doubling in \(N\).