Abstract
Multiple-Input Multiple-Output (MIMO) diversity techniques are widely employed to improve communication reliability in wireless systems; however, their energy-efficiency benefits under realistic propagation and hardware constraints remain insufficiently understood. This paper investigates the energy–reliability trade-off in MIMO diversity systems by jointly considering distance-dependent path loss, spatial correlation, retransmission effects and implementation-related energy costs. Representative diversity configurations, including Single-Input Single-Output (SISO) (1 × 1), Single-Input Multiple-Output (SIMO) (1 × 2) and Alamouti-based schemes (2 × 1 and 2 × 2), are evaluated across varying transmission distances and operating Signal-to-Noise Ratio (SNR) conditions. The results reveal a strongly operating-regime-dependent and non-monotonic relationship between diversity order and energy efficiency. Higher-order diversity is beneficial mainly in intermediate regimes, where reducing retransmissions can outweigh the additional implementation energy cost. Under favorable propagation conditions, however, implementation overhead dominates and lower-complexity configurations are more energy efficient. Conversely, under severely degraded conditions, all schemes enter a reliability-limited regime in which additional diversity provides diminishing energy returns. Furthermore, comparison with conventional Independent and Identically Distributed (i.i.d.) Rayleigh fading shows that simplified channel models systematically overestimate the achievable energy benefits of diversity techniques. The analysis identifies distinct operating regimes governing diversity-related energy efficiency and highlights the combined influence of propagation characteristics, reliability requirements, and hardware constraints. Overall, the results demonstrate that diversity is advantageous only under specific operating conditions, emphasizing the importance of operating-point-aware antenna configuration in practical wireless systems.