Capacity and Performance of Adaptive MIMO System Based on Beam-Nulling

TL;DR

This paper introduces 'beam-nulling', a novel MIMO adaptation scheme that improves capacity and performance by exploiting the weakest subchannel, and extends it to multi-dimensional schemes with significant gains at high data rates.

Contribution

The paper proposes a new beam-nulling scheme for MIMO systems, analyzes its capacity and BER, and extends it to multi-dimensional schemes with superior performance at high data rates.

Findings

Beam-nulling achieves capacity close to water-filling at medium SNR.

Beam-nulling combined with linear dispersion code outperforms beamforming at high data rates.

Multi-dimensional beam-nulling with LDC significantly outperforms with STBC at high data rates.

Abstract

In this paper, we propose a scheme called "beam-nulling" for MIMO adaptation. In the beam-nulling scheme, the eigenvector of the weakest subchannel is fed back and then signals are sent over a generated subspace orthogonal to the weakest subchannel. Theoretical analysis and numerical results show that the capacity of beam-nulling is closed to the optimal water-filling at medium SNR. Additionally, signal-to-interference-plus-noise ratio (SINR) of MMSE receiver is derived for beam-nulling. Then the paper presents the associated average bit-error rate (BER) of beam-nulling numerically which is verified by simulation. Simulation results are also provided to compare beam-nulling with beamforming. To improve performance further, beam-nulling is concatenated with linear dispersion code. Simulation results are also provided to compare the concatenated beam-nulling scheme with the beamforming scheme at the same data rate. Additionally, the existing beamforming and new proposed beam-nulling can be extended if more than one eigenvector is available at the transmitter. The new extended schemes are called multi-dimensional (MD) beamforming and MD beam-nulling. Theoretical analysis and numerical results in terms of capacity are also provided to evaluate the new extended schemes. Simulation results show that the MD scheme with LDC can outperform the MD scheme with STBC significantly when the data rate is high.