The Research On Pilot Contamination In Massive MIMO Multi-cell TDD Systems

Massive MIMO systems equipped with a lot of base station antennas can raise the spectral efficiency and energy efficiency substantially and it has become one of the key techniques for next generation communication systems.The performance of massive MIMO systems depends largely on the fact that the base station can estimate channel state information accurately.However,the channel coherence interval is short and the number of users is large,it's impossible to transmit orthogonal pilot sequences for all users.The pilots have to be reused,but it will result in the channel estimation suffer from interference from users that employ the same pilot sequence,which is also called pilot contamination.Pilot contamination seriously restricts the performance of massive MIMO and becomes an urgent problem to be solved in communication systems.This thesis studies the schemes to mitigate the effect of pilot contamination in detail.At first,this thesis elaborates the system model,channel estimation methods,uplink and downlink data transmission of massive MIMO.Then,the common schemes and algorithms of pilot decontamination are summarized.At last,the pilot contamination problem and its influence are analyzed emphatically,which pave the way for the future research.This thesis researches the pilot contamination problem from three aspects: pilot reuse and schedule,pilot sequences transmission control and fractional pilot reuse.The interference reduces greatly after pilot reuse because the distance between users that employ the same pilot is increased.Two different schedule schemes are studied and their targets are maximizing the sum rate of cell users and providing fairness for all users.The effectiveness is verified by simulations.For pilot sequences control scheme,a new pilot transmission mechanism is designed,where users repeatedly send the same pilot to eliminate interference.It can acquire relatively accurate channel estimation,however the pilot transmission time accounts for a high proportion,which is suitable to the condition that there are not too many users in a cell.The main idea of fractional pilot reuse scheme is that users suffered from mild pilot contamination are allocated the same set of orthogonal pilots,while users suffered from severe pilot contamination in adjacent cells all use orthogonal pilots.A criteria of pilot contamination serverity is proposed,an optimal reuse factor is obtained and closed-form approximate expressions of uplink achievable rate for MRC and ZF receivers are derived.The simulation results show that uplink sum rate is improved significantly.