Pilot hopping in multi-cell massive MIMO systems: Pilot assignment and new results

Abstract

Massive MIMO promises large spectral and power efficiency. However, pilot contamination, a well known challenge in massive MIMO, limits these aspired gains. In this paper, we propose a pilot hopping sequence along with a pilot reuse strategy for pilot decontamination. A simple hopping scheme is proposed and simulated in an uplink L-cells system where every user is assigned distinct pilot sequences in consecutive time slots during the channel coherent time in order to diversify the contaminating users. The base station uses LS channel estimation and receive beamforming for data detection. The covariance matrix of the channel estimation error is obtained and it is shown that the number of antennas can be greatly reduced when using the pilot hopping compared to the conventional system. It is also shown that the gain achieved by using pilot hopping depends on the pilot reuse factor β. This gain is not monotonic function with β and therefore brings to attention that a careful cell planing is required when applying pilot hopping. Monte Carlo simulations are used to illuminate our results.