Energy Efficiency Maximization for Hybrid TDMA-NOMA System With Opportunistic Time Assignment

In this paper, we consider an energy efficient resource allocation technique for a hybrid time division multiple access (TDMA) - non-orthogonal multiple access (NOMA) system. In such a hybrid system, the available time for transmission is divided into several sub-time slots, and a sub-time slot is allocated to serve a group of users (i.e., cluster). Furthermore, signals for the users in each cluster are transmitted based on the NOMA approach. With NOMA, multiple users can be served simultaneously through utilizing power domain multiplexing at transmitter and successive interference cancellation (SIC) at receiver. In this paper, to maximize the energy efficiency (EE), we jointly allocate both the available time slots and the available transmit power in the hybrid TDMA-NOMA system. In particular, we formulate an EE maximization (EE-Max) problem aiming to maximize the overall EE of the system with a per-user minimum rate and transmit power constraints. However, this joint optimization problem is non-convex in nature, and thus, cannot be solved directly. Therefore, we develop an iterative algorithm by approximating the original problem into a convex one with sequential convex approximation (SCA) and a novel second-order cone (SOC) approach. Simulation results demonstrate that the performance of the proposed hybrid TDMA-NOMA system with joint resource allocation outperforms the system with equal time allocation in terms of the overall EE. Simulation results further confirm that the proposed iterative approaches with SCA and SOC techniques converge within a few number of iterations while yielding the solution to the original non-convex problem.

I. Introduction

Non-orthogonal multiple access (NOMA) has been envisioned as a promising multiple access technique to significantly improve the spectral efficiency, user fairness and support massive connectivity for the fifth generation (5G) and beyond wireless networks [1]. Different from the conventional orthogonal multiple access (OMA) technologies, such as time division multiple access (TDMA) and orthogonal frequency division multiple access (OFDMA), multiple users in downlink NOMA simultaneously share the same radio resources, namely time and frequency resources [2]. At the transmitter, this simultaneous resource sharing is carried out by exploiting power domain multiplexing, which is referred to as a power-domain superposition coding (SC) technique in the literature [4]. With this multiplexing technique, multiple signals intended for the corresponding users are encoded with different power levels and transmitted simultaneously. The successive interference cancellation (SIC) technique is employed at the receiver end to decode the signals transmitted to multiple users. In the SIC technique, the signals with stronger channel conditions are first decoded and subtracted from the multiplexed received signal prior to decoding their own signals [3]–[5]. Furthermore, NOMA can accommodate much more users than OMA by employing non-orthogonal resource allocation, which addresses the dramatically increasing demand for user access required for the Internet-of-Things in future wireless networks [1], [2], [8].