Adaptive Modulation for Half-Duplex Cooperative Non-Orthogonal Multiple Access (C-NOMA)

Cooperative Non-Orthogonal Multiple Access (C-NOMA) is a promising paradigm for extending connectivity to cell-edge users; however, existing fixed-rate implementations fail to exploit the full spectral potential of time-varying channels. In this letter, we propose a joint adaptive modulation framework for Half-Duplex (HD) C-NOMA that dynamically adjusts modulation orders to maximize system throughput. To solve the non-convex optimization problem arising from the coupled relaying constraints, we develop a low-complexity greedy heuristic that identifies the optimal modulation pair with negligible computational overhead. Simulation results confirm that the proposed scheme significantly outperforms fixed-modulation C-NOMA benchmarks in terms of throughput, achieving near-optimal performance virtually identical to exhaustive search bounds. Crucially, this throughput gain is achieved while strictly adhering to pre-defined Bit Error Rate (BER) constraints, ensuring robust reliability even in the presence of practical hardware impairments such as imperfect Successive Interference Cancellation (SIC).