Pilot Contamination Mitigation for Wideband Massive MMO: Number of Cells vs Multipath

This paper proposes novel joint channel estimation and beamforming approach for multicell wideband massive multiple input multiple output (MIMO) systems. Using our channel estimation and beamforming approach, we determine the number of cells Nc that can utilize the same time and frequency resource while mitigating the effect of pilot contamination. The proposed approach exploits the multipath characteristics of wideband channels. Specifically, when the channel has L multipath taps, it is shown that N_c ≤ L cells can reliably estimate the channels of their user equipments (UEs) and perform beamforming while mitigating the effect of pilot contamination. For example, in a long term evolution (LTE) channel environment having delay spread T_d = 4.69μ second and channel bandwidth B = 2.5MHz, we have found that L = 18 cells can use this band. In practice, T_d is constant for a particular environment and carrier frequency, and hence L increases as the bandwidth increases. The proposed channel estimation and beamforming design is linear, simple to implement and significantly outperforms the existing designs, and is validated by extensive simulations.