Spectrally Efficient Uplink Cooperative NOMA With Joint Decoding for Relay-Assisted IoT Networks

We propose a spectrally efficient uplink cooperative relaying protocol with <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> multiple half-duplex relay stations (RNs) for wireless Internet of Things (IoT) networks, where two IoT devices (IDs) and an access point (AP) cannot directly communicate with each other. The two IDs simultaneously transmit signals over the same spectrum resource using the nonorthogonal multiple access (NOMA) technique. In the proposed technique, at a certain time slot, a single RN is selected to send the received packets from two IDs in the previous time slot, while the remaining <inline-formula> <tex-math notation="LaTeX">$K-1$ </tex-math></inline-formula> RNs receive new packets from two IDs. Thus, the proposed relaying protocol can send <inline-formula> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> packets from an ID to the AP during <inline-formula> <tex-math notation="LaTeX">$N+1$ </tex-math></inline-formula> time slots without a duty-cycle loss in conventional half-duplex relaying schemes. We call the proposed protocol a spectrally efficient cooperative NOMA. The main highlight of our results is that we mathematically derive a closed-form outage probability of the proposed protocol by considering inter-relay interference, assuming an optimal joint decoding technique instead of successive interference cancelation (SIC) decoding at each RN. To the best of our knowledge, this is the first theoretical result in the literature. Extensive computer simulations are used to validate the mathematical results. Furthermore, it is shown that the proposed protocol significantly outperforms the existing two-hop half-duplex cooperative NOMA technique and the SIC decoding scheme in terms of outage probability.