Cooperative Transmission with Constant Envelope Modulations and Phase-Only Forward Relays

We propose the concept of Phase-Only Forward (PF) relays for cooperative communication involving constant envelope modulations. The technique enables the relaying nodes to maintain constant envelope signaling without the need to perform decoding and signal re-modulation. The bit-error-probability (BEP) of this PF technique, in conjunction with constant envelope CPFSK modulation in a time-selective Rayleigh fading environment and discriminator detection, was analyzed. Both cases of with and without antenna selection at the relay were considered. In the former case, a clever phase-adjustment scheme that allows the relay to maintain phase continuity was proposed. Our results indicate that without antenna selection, PF has a lower BEP than decode-and-forward (DF). It also delivers the same performance as amplify-and-forward (AF) when fading is quasi-static. In a time-selective fading environment, PF is not as efficient as AF without antenna selection. However with dual-antenna selection at the relay, PF has practically the same performance as AF even in a time-selective fading environment. We thus believe that PF is a cost-effective alternative to AF and DF, since signal re-modulation and expensive linear amplifiers are no longer needed at the relay to support non-constant envelope transmission. Finally, this phase-forward concept can be generalized to other continuous-phase modulation schemes as well as to PSK.