Ultra Reliable and Low Latency Non-Orthogonal Multiple Access: A Cross-Layer Approach

Non-orthogonal multiple access (NOMA) is recognized as one of the promising techniques in wireless communications. How to reduce the delay violation probability in NOMA systems is a challenging issue. In this paper, a cross-layer scheduling scheme is presented for NOMA systems. The scheduling scheme is designed to jointly determine the scheduling in the network layer and superposition coding process in the physical layer. In order to find the optimal scheduling scheme, we model the queue states of the users as a Markov chain, based on which the delay violation probability and the average power consumption can be analyzed. Then, we minimize the delay violation probability given constraint on average power consumption by formulating and solving a cross-layer optimization problem. We convert the optimization problem into an equivalent linear programming problem via variable substitution, which allows us to obtain the optimal delay-power tradeoff as well as the optimal scheduling policy. One of the optimal superposition coding policies can be determined directly, which can significantly reduce the computational complexity of the linear programming. Theoretical analyses and simulation results show that our approach achieves a better performance over the Longer Queue Highest Possible Rate (LQHPR) policy.

I. Introduction

The explosive growth of mobile multimedia and social networking applications poses challenging multiple access and quality of service (QoS) requirements for 6G [1]. NOMA as a significant technology holds the promise to meet massive machine type communications (mMTC) application scenarios. Hard delay constraint is considered as the critical performance metric of QoS for ultra-reliable and low latency communications (URLLC). URLLC and mMTC are the key functions enabled by 5G. In 6G, an even more critical mission is to rely on URLLC in mMTC [2]. In other words, the scheduling policy to minimize the delay violation probability for NOMA is desired.