Power-Efficient Ultra-Reliable Communication in Rayleigh–Rayleigh Fading Environment Using ARQ-I and CC-HARQ

We propose optimal power allocation for a transmitter that operates with either the ARQ-I or CC-HARQ protocol in an interference-limited environment where both the desired and interfering signals experience Rayleigh fading, forming an analytically tractable non-line-of-sight scenario, to achieve ultra-reliable communication with minimal power. We formulate the power allocation problem as a minimization of the average retransmit power under a given target outage probability for both retransmission schemes. Using the Karush–Kuhn–Tucker method, we derive protocol-specific equations solved numerically for retransmit powers. Unlike prior interference-limited numerical studies and noise-limited closed-form results, we derive closed-form analytical solutions for optimal retransmit power expressions for ARQ-I and CC-HARQ in a Rayleigh–Rayleigh interference-limited environment for two retransmissions, which act as computationally efficient benchmarks for real-time ultra-reliable applications and establish a baseline for extending to more general fading environments. This constitutes the primary contribution of this work. We show through simulations that the proposed schemes achieve significant power savings compared to conventional open-loop transmission, particularly in the ultra-reliable region. Benchmarking against prior Rician–Rayleigh results shows that, as expected, the Rayleigh–Rayleigh case requires higher power because it lacks the line-of-sight component that improves reliability, while confirming that CC-HARQ consistently outperforms ARQ-I in power efficiency.