# Distributed User Clustering and Resource Allocation for Imperfect NOMA in Heterogeneous Networks

## TL;DR

This paper introduces a distributed framework for user clustering and resource allocation in imperfect NOMA systems within heterogeneous networks, addressing the impact of non-ideal SIC and optimizing spectral and energy efficiency.

## Contribution

It presents a novel distributed clustering and resource allocation approach that accounts for NOMA imperfections, including fractional error factors and QoS constraints.

## Key findings

- Higher spectral and energy efficiency compared to basic NOMA cluster sizes.
- Imperfect NOMA does not always outperform orthogonal access under certain conditions.
- NOMA gain is maximized with downlink/uplink decoupled UE association.

## Abstract

In this paper, we propose a distributed cluster formation (CF) and resource allocation (RA) framework for non-ideal non-orthogonal multiple access (NOMA) schemes in heterogeneous networks. The imperfection of the underlying NOMA scheme is due to the receiver sensitivity and interference residue from non-ideal successive interference cancellation (SIC), which is generally characterized by a fractional error factor (FEF). Our analytical findings first show that several factors have a significant impact on the achievable NOMA gain. Then, we investigate fundamental limits on NOMA cluster size as a function of FEF levels, cluster bandwidth, and quality of service (QoS) demands of user equipments (UEs). Thereafter, a clustering algorithm is developed by taking feasible cluster size and channel gain disparity of UEs into account. Finally, we develop a distributed alpha-fair RA framework where alpha governs the trade-off between maximum throughput and proportional fairness objectives. Based on the derived closed-form optimal power levels, the proposed distributed solution iteratively updates bandwidths, clusters, and UEs' transmission powers. Numerical results demonstrate that proposed solutions deliver a higher spectral and energy efficiency than traditionally adopted basic NOMA cluster size of two. We also show that an imperfect NOMA cannot always provide better performance than orthogonal multiple access under certain conditions. Finally, our numerical investigations reveal that NOMA gain is maximized under downlink/uplink decoupled (DUDe) UE association.