Impact of pilot sequence contamination in massive MIMO systems

Time-division duplex (TDD) is the most efficient technique for acquiring channel state information in massive multiple-input multiple-output (MIMO) systems where the reciprocity between the uplink and downlink channels is utilised by the pilot signals to extract the channel parameters. In this study, the authors consider the pilot contamination problem in TDD multicell multiuser massive MIMO systems and examine two different pilot signal allocation schemes for which they derive the lower bounds on the achievable rate on the uplink for the cases of maximum-ratio combining (MRC) and zero-forcing (ZF) detectors. To achieve further performance enhancements, they propose a new algorithm for pilot sequences allocation in which the multiplicity of the pilot sequences over the number of users in each cell is exploited. The authors' results show that when pilot contamination is severe, allocating more system resources for channel estimation results in a better system performance especially in limited mobility environments. Moreover, they show that when the signal-to-interference plus noise ratio is low, MRC is superior to ZF, and vice versa. Finally, they demonstrate that their proposed allocation algorithm can significantly improve the spectral efficiency of the network compared to the conventional pilot allocation method.