Multiply-and-Forward and Phase-Forward Two Way Relays

Relay/cooperative communication has been an active area of research in the past few years. In most of the investigations, the amplifiers used in the system are assumed to be linear. In this thesis, we first investigate the effect of amplifier non-linearity has on the bit-error-rate (BER) of a two-way cooperative communication system that employs linear modulation and amplify-and-forward (AF). It was found that irreducible error floor arises, caused by the inter-modulation effect with non-linear amplification and the subsequent imperfect self-interference cancellation. As a result of this finding, we propose two signal forwarding techniques that offer MPSK modulations immunity against non-linear amplifier distortion. The first technique, termed multiply-and-forward (MF), scales the product of the two received signals at the relay for downlink transmission. The second technique, termed phase-forward (PF), has a similar product structure, except that the amplitude information in the product signal is discarded. We show that both schemes do not exhibit the irreducible error characteristic found in AF when the amplifier is non-linear. The PF scheme, in particular, can attain a BER lower than that of AF even when the amplifier is linear. The conclusion is reached that MF and PF are suitable signal forwarding strategies for two-way cooperative communication in the presence of amplifier non-linearity, with PF being the more robust of the two. Due to the multiplicative nature of the MF and PF, self-informantion cancellation becomes simple when differential modulation is used in the two-way cooperative relay system. With differential modulation, there is no explicit channel state information (CSI) estimation and and this leads to a saving of pilot symbol overhead. In the second and third parts of this thesis proposal, we extend the MF and PF protocols to DPSK and CPFSK modulations. Specifically, we demonstrate that the proposed forwarding methods has low computational complexity at the relay as well as simple self-information cancellation at the receiver in the last phase of the cooperative communication. Based on the results we have obtained on the new relay forwarding protocols, we can see that the MF and PF are two promising forwarding techniques and they worth further study and refinement.