Fractionally spaced equalization for broadband amplify-and-forward cooperative systems

In this paper, we revisit the concept of fractionally spaced equalization (FSE) for broadband single-carrier amplify-and-forward (AaF) cooperative systems. Particularly, we investigate fractionally spaced frequency domain equalization (FS-FDE) for cooperative multi-relay systems. Our motivation stems from the elegant properties reported for the FSE in the point-to-point communication systems (i.e., its robustness to sampling phases and potential in achieving the optimum performance) and the scalability of the FDEs. In particular, we propose a T_S/2-spaced equalizer that transforms the temporal sample sequence of the received signal to the frequency domain, applies linear/decision-feedback equalization, and returns the resulting signal back to the time domain for detection. The vital importance of using FS-FDE method in cooperative systems is disclosed in practical scenarios where the transmitted signals have nonzero roll-off components and sampling phase error may occur in relay(s) and destination terminals. Our results demonstrate that, under specific channel realizations and sampling errors, the cooperative systems with symbol spaced FDE (SS-FDE) fail to harvest the available cooperative diversity and the performance approaches to that of no relay scenario. On the other hand, the performance of cooperative system with FS-FDE method becomes independent of sampling phase errors and full benefit of cooperation is retained.