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We investigate the performance degradation of basic modulation schemes in a rapidly time varying channel using a first order auto regressive channel model. We propose a partially coherent detector for both noncoherent frequency shift keying (FSK) and differential phase shift keying (PSK) that exploits partial channel knowledge to enable the receiver to operate effectively in both fast and slow fading. The maximum likelihood rule (ML) obtained for the partially coherent FSK turns out to be a linear combination of coherent and noncoherent detection rule. Results demonstrate that significant performance improvement can be achieved over the best of coherent and noncoherent FSK detection in fast fading. We also propose a few adaptive schemes that vary the modulation scheme in response to degrading quality of the channel estimate between successive training symbols

We investigate the performance degradation due to rapidly time varying channels in a repetition based coherent cooperative system. We demonstrate that mobility of source affects the performance much more than the mobility of destination, for both amplify and forward (AF) and demodulate and forward (DF) relays, despite the symmetry of the network. Exploiting the property of FSK modulation that allow us to detect either coherently or noncoherently or even semi-coherently, we develop ML detection rules for a variety of mobile scenarios. The detection rules that take into account the mobility of the nodes, are mostly hybrids of partially coherent detectors and noncoherent detectors. The performance of these detectors is better than the best of coherent and noncoherent detectors in fast fading and a gain of about 2 dB is obtained over a wide range of SNR and a gain of almost 3 dB is achieved at the crossing of coherent and noncoherent curves. As energy efficiency is one of the main objectives for pursuing cooperation and relaying, these hybrid detectors assume significance in fast fading scenarios