Research On Pilot Decontamination In Massive MIMO Systems

Multiple-input Multiple-output(MIMO)is a multiple antenna technique that allows for transmission of multiple data streams between base station and user terminals.In long term evolution(LTE),singleuser MIMO(SU-MIMO)was adopted to increase spectral efficiency,both in the time division and frequency division multiplexing modes.To meet the international mobile telecommunication–advance requirement(IMT-A),LTEAdvance introduces multi-user MIMO(MU-MIMO)technology with a maximum of 8*8 MIMO.MU-MIMO transmission scheme compared to SU-MIMO provides better spatial multiplexing gains that are effective in improving the average throughput,cell coverage,and reliability of mobile communication systems.However,with an explosion of wireless mobile devices and services,there are still some challenges that cannot be accommodated even by 4G,such as the spectrum crisis and high energy consumption.Wireless system designers have been facing the continuously increasing demand for high data rates and mobility required by new wireless applications and therefore have started research on fifth generation(5G)wireless systems that are expected to be deployed beyond 2020.So,5G cellular network is coming.Massive MIMO has been identified as a promising technology in 5G research,and offers orders-of-magnitude improvements in spectral-efficiency over 4G technologies.Massive MIMO proposes utilizing a very high number of antennas to multiplex messages for several devices on each time-frequency resource,focusing the radiated energy toward the intended directions while minimizing intra and intercell interference.In Massive MIMO system,several studies are being carried out to explore the limitations and challenges through theoretical analysis,simulations and prototyping.One of the major limiting factors is pilot contamination when TDD protocol is considered.Different methods have been proposed in literature to mitigate effect of pilot contamination in multi-cellular systems.In this paper,we analyze the reason of pilot contamination in Massive MIMO system when TDD protocol is considered and propose several approaches for pilot decontamination.Firstly,we give a brief description on TDD scheme,and then we describes the massive MIMO system model.Based on this model,the effect of pilot is discussed and communication scheme which include the uplink and downlink phase is also explained.we simulate the average throughput when using orthogonal and non-orthogonal pilot sequences in the same cell,and show the impact of cell radius and the number of BS antennas on pilot contamination.The result shows that orthogonal pilot sequences can eliminate pilot pollution.Besides,linear precoding and detection schemes in downlink of massive MIMO systems were investigated.The capacity performance analysis correspoding to MF,ZF and MMSE schemes in downlink uplink of massive MIMO systems were given respectively.And the formula of capacity of the above linear precoding detection schemes in some communication scenarios were derived.The simulations show that for the massive MIMO of a large number of antennas,the capacity performance of MF precoding detection is similar with that of the MMSE.Secondly,in order to dealt with Pilot Contamination,a new Time Shifted-Cell Sectorization Pilot Assignment(TS-CSPA)joint scheme is proposed.This scheme can mitigate Pilot Contamination by reduce the reuse of pilot sequence through the time domain and space domain.And we give the closed-form expression for the achieved rates and signal to interference plus noise ratio(SINR)to verify the advantages of the proposed scheme,Numerical results show significant performance gains to mitigate pilot contamination problem compared traditional methods.Lastly,a pilot allocation scheme based on user location information is proposed.The proposed scheme arranges the users according to the user location and the polar angle of the user location in the polar coordinate system with the base station of the cell as the pole,and assigns the pilots sequentially.the scheme can reduce the pilot contamination and improve the achievable rate of the system by controlling the multiplexing distance of the pilot to a far range.Simulation results show that the proposed scheme can not only reduce the pilot pollution between users,but also reduce the performance gap between different users and improve the fairness of the system.