This paper investigates a cooperative uplink system where two users wish to send messages to a base station with the help of a buffer-aided relay. Transmission modes in terms of both non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) are considered. For the considered system, the probability for the user-to-relay channels to successfully perform NOMA is first theoretically derived in two scenarios of dynamic and fixed power controls (PCs) at users. Then, an efficient buffer-aided hybrid NOMA/OMA based mode selection (MS) scheme is proposed, which adaptively switches between the NOMA and OMA transmission modes according to the instantaneous strength of wireless channels and the buffer state. The state transmission matrix probabilities of the corresponding Markov chain is also derived, and the performance of the proposed hybrid NOMA/OMA scheme is analyzed with closed-form expressions, in terms of sum throughput, outage probability, diversity gain, and average packet delay. For both the dynamic and fixed PCs, the proposed scheme is proved to achieve a diversity gain of two when the buffer size is not smaller than three, which means that fixed PC will not lead to a loss of diversity gain.

Introduction

Non-orthogonal multiple access (NOMA) technology, using successive interference cancellation (SIC) and multiplexing in power-domain, can improve spectrum efficiency, which is considered to be a promising fifth-generation mobile communication technology [1]. NOMA allows multiple users to share the communication resources simultaneously, i.e., time, frequency and code domain, then achieves better performance than conventional orthogonal multiple access (OMA) [2]–[۴]. Cooperative communication can further enhance the efficiency and reliability, and expand the coverage of wireless communication networks. In order to enhance the performance of NOMA, the work in [5] first exploited cooperation between the users, i.e., the stronger users help the other weaker users by using the decode-and-forward (DF) scheme, so that the optimal diversity gain can be achieved. On the other hand, the work in [6], [7] investigated cooperative NOMA with a single relay, where multiple users were helped by a dedicated relay node using the DF and amplify-and-forward (AF) schemes. In addition, relay selection (RS) scheme has also been proposed for cooperative NOMA networks with multiple relays in some existing works (e.g., [8]–[۱۱]). Recently, the uplink cooperative communication has also been investigated in [12], where a coordinated direct and relay transmission was proposed.