Spectral efficiency enhancement in multi-channel systems using redundant transmission and diversity reception

We first consider conventional multi-channel systems with fixed-power variable-rate transmission of independent data streams as a function of channel variations. In these conventional systems, no data is transmitted through the sub-channels whose received signal strength is below a certain cutoff threshold. We then propose new hybrid adaptive modulation and diversity schemes for multi-channel transmission systems. The proposed hybrid schemes attempt to increase the overall spectral efficiency by (i) transmitting redundant information symbols through the deeply faded sub-channels that are not to be used in conventional multi-channel systems and then (ii) diversity combining these replicas at the receiver end. We evaluate the performance of the newly proposed hybrid schemes and compare their spectral efficiency with that of (i) the conventional scheme and (ii) a benchmark scheme which maximizes the overall spectral efficiency but is too complex for practical implementation. We show using some selected numerical examples that the proposed hybrid schemes offer spectral efficiency very close to that of the optimal scheme with much reduced complexity.