Cooperative hybrid automatic repeat request (C-HARQ) scheme based on multi-hop relaying has been adopted as an efficient strategy in underwater acoustic sensor networks (UASNs) to extend network coverage and to enhance the performance by utilizing spatial diversity gain. In this letter, we develop a stochastic model for multi-cluster transmissions in UASNs, considering the underwater-specific characteristics such as frequency-dependent signal attenuation, acoustic spreading, multi-path fading and underwater noises. From this generalized stochastic model, we derive accurate analytical expressions to analyse the end-to-end packet error rate (PER) and energy efficiency. Analytical results demonstrate that the C-HARQ scheme can significantly improve the performance of UASNs, especially with the increase in number of relay nodes. Analytical results are corroborated with extensive simulation studies.

In this article, we propose non-orthogonal multiple access (NOMA) based restricted access window (RAW) mechanism for IEEE 802.11ah compliant dense Internet of Things networks. The proposed scheme utilizes the principles of NOMA at the physical layer and distributed coordination function with theRAW mechanism at the data link layer. Using an analytical model, we evaluate the throughput of IEEE 802.11ah wireless local area network (WLAN) under the proposed RAW-NOMA integrated mechanism. From the results, we establish that the throughput and scalability of IEEE 802.11ah WLAN are significantly improved under the proposed channel access scheme. Finally, analytical findings are corroborated with ns-3 simulations.