Spectral and Energy Efficiency of Multi-Pair Massive MIMO Relay Network With Hybrid Processing

Abstract

Abstract-We consider a multi-pair massive multiple-input multiple-output (MIMO) relay network, where the relay is equipped with a large number, N , of antennas, but driven by a far smaller number, L, of radio frequency (RF) chains. We assume that K pairs of users are scheduled for simultaneous transmission, where K satisfies 2K = L. A hybrid signal processing scheme is presented for both uplink and downlink transmissions of the network. Analytical expressions of both spectral and energy efficiency are derived with respect to the RF chain number under imperfect channel estimation. It is revealed that, under the condition N > 4L2 /π , the transmit power of each user and the relay can be respectively scaled down by 1/ √ N and 2K/ √ N if pilot power scales with signal power, or they can be respectively scaled down by 1/N and 2K/N if the pilot power is kept fixed, while maintaining an asymptotically unchanged spectral efficiency (SE). While regarding energy efficiency (EE) of the network, the optimal EE is shown to be achieved when Pr = 2KPs, where Pr and Ps respectively refer to the transmit power of the relay and each source terminal. We show that the network EE is a quasi-concave function with respect to the number of RF-chains which, therefore, admits a unique globally optimal choice of the RF-chain number. Numerical simulations are conducted to verify our observations. Index Terms-Massive MIMO relay, hybrid processing, spectral efficiency (SE), energy efficiency (EE), limited RF chain.

Weighted Spectral Efficiency Optimization for Hybrid Beamforming in Multiuser Massive MIMO-OFDM Systems

“…To simplify the problem, we would like to avail ourselves of some experience from the designs employed in single carrier systems which approach the performance of the pure digital beamforming using zero-forcing (ZF) precoding in massive MIMO [7]. Similarly to the beamforming methods in [6]- [8], [29], the hybrid beamforming design for multiuser MIMO could be divided into two stages: (1) First maximize the signal power on the entire band for each user utilizing the analog precoder ignoring the interference among users; (2) Second, design the digital beamformers according to the effective channels using the linear digital beamforming schemes such as MMSE to cope with the multiuser interference.…”

“…Hybrid beamforming designs have been considered for single-user MIMO systems [4], [5] and multiuser MIMO systems [6]- [8] over single-carrier channels. However, hybrid beamforming designs over frequency selective channels are more challenging.…”

Rethinking Uplink Hybrid Processing: When Is Pure Analog Processing Suggested?

“…Generally, analog processing is implemented by phase shifters which conduct rotations to signal phases. In this stage, the analog processing is designed by selecting the optimal angles to maximize the signal power of each user, like the designs in [10]- [12]. Interference among users is left to be addressed via the following digital processing if hybrid processing applies.…”

“…However, as sub-optimal design without optimal performance, hybrid beamforming based on linear digital beamforming designs is not proved to outperform analog processing in all cases. Serving as circumstantial evidence, from [10]- [12], the analog beamforming approximately transferred the channel matrix into a diagonal matrix in which no interference among users may asymptotically exist. Therefore, it is reasonable to investigate whether the analog beamforming could sometimes beat some popular linear, but not mathmatically optimal hybrid beamforming.…”

Hybrid Block Diagonalization for Massive MIMO Two-Way Half-Duplex AF Hybrid Relay

“…III. PROPOSED DESIGN For a massive MIMO relay, consisting of large number of antennas, it is costly to connect each antenna to a separate RF chain [5]. A hybrid relay, where each RF chain is connected to multiple antennas, can significantly reduce the system cost [5].…”