Walsh–Hadamard Transform Based Non-Orthogonal Multiple Access (NOMA) and Interference Rejection Combining in Next-Generation HetNets

Abstract

In heterogeneous networks (HetNets), non-orthogonal multiple access (NOMA) has recently been proposed for hybrid-access small-cells, promising a manifold network capacity compared to OMA. One of the major issues with the installation of a hybrid-access mechanism in small-cells is the cross-tier interference (intercell interference (ICI)) caused by the macrocell users (MUs) that are unable to establish a connection to the small-cell base station (SBS). In this paper, a joint strategy is proposed for hybrid-access small-cells using the Walsh–Hadamard transform (WHT) with NOMA and interference rejection combining (IRC) to achieve high performance gains and mitigate intercell interference (ICI), respectively. WHT is applied mathematically as an orthogonal variable spreading factor (OVSF) to achieve diversity in communication systems. When applied jointly with NOMA, it ensures better performance gains than the conventional NOMA. It reduces the bit error rate (BER) and enhances subsequent throughput performance of the system. IRC is used at the receiver side for managing the cross-tier interference caused by MUs that are unable to connect to the small-cell base station (SBS) for hybrid-access. The work considers both ideal and nonideal successive interference cancellation (SIC) conditions for NOMA. Mathematical modeling is provided for the proposed joint strategy for HetNets and the results validate it in terms of BER and subsequent user throughput performance, compared to the conventional NOMA approach.