Abstract
This paper investigates the pilot length optimization to improve the system energy efficiency (EE) and spectral efficiency (SE) for the Device-to-device (D2D) communications underlay Massive multiple-input multiple-output (MIMO) cellular networks. In order to reduce the channel estimation error caused by the pilot reuse among cellular users (CUEs) and D2D users (DUEs), a heuristic pilot optimization and pilot allocation (PA) scheme is formulated through minimizing the channel-estimation-interference-strength (CEIS). By taking into account the interference among both CUEs and DUEs, the optimal pilot length is obtained with the purpose of maximizing the system EE and SE. Intensive simulations are done to investigate the relation between the pilot length and the number of D2D pairs in terms of EE and SE. The results shown that the optimal pilot length always exists with the variation of the number of D2D pairs and the PA with optimal pilot length can significantly improve the performance of both EE and SE compared with the completely reused or orthogonal pilot scheduling strategy.