Practical MIMO-NOMA: Low Complexity and Capacity-Approaching Solution

MIMO-NOMA combines multiple-input multiple-output (MIMO) and non-orthogonal multiple access (NOMA) techniques to address heterogeneous challenges, such as massive connectivity, low latency, and high reliability in the 5G cellular communication system and beyond. In this paper, a coded MIMO-NOMA system with capacity-approaching performance and low implementation complexity is proposed. Specifically, the proposed MIMO receiver consists of a linear minimum mean-square error (LMMSE) multi-user detector and a bank of single-user message-passing decoders, which decompose the overall NOMA signal recovery into distributed low-complexity computations with iterative processing. An asymptotic extrinsic information transfer analysis is employed to model the overall performance, and a novel class of multi-user irregular repeat-accumulate channel codes that match with the LMMSE multi-user detector in the iterative decoding process are constructed for the system. As a result, the proposed coded MIMO-NOMA system achieves asymptotic performance within 0.2 dB from the theoretical capacity. Simulation results validate the reliability and robustness of the proposed system in practical settings that include different system loads, iteration numbers, code lengths, fast/block fading, and imperfect channel estimation.

I. Introduction

With the popularization of Internet and intelligent technology, the number of communication devices is predicted to reach 40.9 billion in 2020 [1], which includes new communication scenes, such as machine-to-machine communications [2], [3], Internet of things [4], and vehicle-to-vehicle (V2V) communications [5]. Due to the fact that available spectrum resources are limited, orthogonal multiple access technology in the fourth generation (4G) communication system cannot satisfy the massive access demands. As a result, Non-Orthogonal Multiple Access (NOMA) [6]–[19] emerges to support heavily overloaded communications, which allows multiple users to share the same time and frequency resources. To further improve spectral efficiency and reduce latency, NOMA combining with Multiple-Input Multiple-Output (MIMO) [20], [21], termed MIMO-NOMA [22]–[39], is considered as a key air interface technology in the fifth-generation (5G) communication system [40], [41].