Switch-Based Hybrid Analog/Digital Channel Estimation for mmWave Massive MIMO

This paper addresses the problem of channel estimation using pilots in hybrid analog/digital massive multiple-input multiple-output (MIMO) systems for future millimetre wave (mmWave) communications. To further reduce system cost and implementation complexity, we consider an alternative architecture derived from RF switches as opposed to the phase shifters in the conventional literature. The channel estimation is formulated as a combinatorial optimization problem where the aim is to minimize the mean square error (MSE) between the real and estimated channels over a finite set of allowed values for the precoder switches. A genetic algorithm (GA) is developed for solving this problem and obtaining the MIMO channel estimates. Simulations show that the proposed scheme can estimate channels as accurately, if not more, as an existing solution using phase shifters.

I. Introduction

Massive multiple-input multiple-output (MIMO) is one of the key enabling technologies advanced to meet the exacting target performance requirements of the fifth generation (5G) of wireless systems [1]. Due to the use of millimetre-wave (mmWave) signals, the antenna arrays of massive MIMO systems will be smaller and include more antenna elements than previous technologies [2]. However, using more antennas requires having more radio-frequency (RF) chains, which in turn is problematic because the latter are expensive and power-hungry [2], [3]. One way to cope with this constraint is to employ a hybrid architecture, wherein an analog beamformer is used to reduce the effective number of RF signals prior to the application of baseband processing [4], [5].