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In this paper, we investigate the coexistence of two technologies that havebeen put forward for the fifth generation (5G) of cellular networks, namely,network-assisted device-to-device (D2D) communications and massive MIMO(multiple-input multiple-output). Potential benefits of both technologies areknown individually, but the tradeoffs resulting from their coexistence have notbeen adequately addressed. To this end, we assume that D2D users reuse thedownlink resources of cellular networks in an underlay fashion. In addition,multiple antennas at the BS are used in order to obtain precoding gains andsimultaneously support multiple cellular users using multiuser or massive MIMOtechnique. Two metrics are considered, namely the average sum rate (ASR) andenergy efficiency (EE). We derive tractable and directly computable expressionsand study the tradeoffs between the ASR and EE as functions of the number of BSantennas, the number of cellular users and the density of D2D users within agiven coverage area. Our results show that both the ASR and EE behavedifferently in scenarios with low and high density of D2D users, and thatcoexistence of underlay D2D communications and massive MIMO is mainlybeneficial in low densities of D2D users.

Energy Efficiency and Sum Rate Tradeoffs for Massive MIMO Systems with Underlaid Device-to-Device Communications