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In a device-to-device (D2D) underlaid massive MIMO system, D2D transmittersreuse the uplink spectrum of cellular users (CUs), leading to cochannelinterference. To decrease pilot overhead, we assume pilot reuse (PR) among D2Dpairs. We first derive the minimum-mean-square-error (MMSE) estimation of allchannels and give a lower bound on the ergodic achievable rate of both cellularand D2D links. To mitigate pilot contamination caused by PR, we then propose apilot scheduling and pilot power control algorithm based on the criterion ofminimizing the sum mean-square-error (MSE) of channel estimation of D2D links.We show that, with an appropriate PR ratio and a well designed pilot schedulingscheme, each D2D transmitter could transmit its pilot with maximum power. Inaddition, we also maximize the sum rate of all D2D links while guaranteeing thequality of service (QoS) of CUs, and develop an iterative algorithm to obtain asuboptimal solution. Simulation results show that the effect of pilotcontamination can be greatly decreased by the proposed pilot schedulingalgorithm, and the PR scheme provides significant performance gains over theconventional orthogonal training scheme in terms of system spectral efficiency.

Pilot Reuse Among D2D Users in D2D Underlaid Massive MIMO Systems