Adaptive hybrid M-FSK M-QAM modulation

We propose a variable-rate variable-power scheme which employs both noncoherent M-ary frequency shift keying (M-FSK) and M-ary quadrature amplitude modulation (M-QAM). The idea is that adaptive M-FSK is used when the M-QAM system declares an outage. The goal is to provide a better channel utilization and a reliable link of communication even at very bad channel conditions. We consider this hybrid system design by maximizing its spectral efficiency for given constraints on average power transmission and bit error rate. As a result, we obtain power and bit loading functions for both the M-FSK and M-QAM schemes which are mutual and channel-gain dependents.