Research On Precoding Techniques And Pilot Contamination For Massive MIMO Wireless Systems

Massive MIMO wireless systems with large array antennas are able to improve the system capacity and reliability without increasing the system bandwidth; however, it produces the multiuser interferences. In the downlink channel, precoding techniques can not only reduce the interferences and enhance the spectral efficiency effectively, but also simplify the complexity of receivers. Moreover, in the uplink channel, the base station estimate channel after getting t he pilots from users, which are contained by other users. The so-called pilot contamination has a great negative impact on the system performance. Therefore, research on precoding techniques and pilot contamination is very valuable and practical.This paper builds a general massive MIMO system model of TDD protocol, which considers both large-scale and small-scale fading. Then it simulates the Matched Filter(MF), Zero Forcing(ZF), Regularized Zero Forcing(RZF), and Minimum Mean Square Error(MMSE) precoding algorithms and compares their bit error rates. It turns out that MMSE has the best performance, then RZF and ZF, at last the MF.Besides, the paper also analyzes the channel coherence coefficient which proves to decrease with antennas of the base station in a single cell. As a result, the system achievable rates improved greatly as long as the antennas grow. In the multi-cell environment, when SNR is high, ZF precoding of vector normalization has better performance than ZF precoding of matrix normalization and MF precoding; however, ZF performs worse than MF precoding in low SNR.In the conventional aligned pilot scheme, the pilot sequence should be as long as the number of users at least in a cell, since the pilots are orthogonal. In this case, the expected signal contains the interference from all other users in the system. In order to reduce the pilot co ntamination, this paper studies a novel time-shifted pilot scheme, which reuses the same pilot sequence in all cell groups; at the same time, this me thod removes the interference from other cell groups and improves the system capacity.