Combining of Loop Signals in Frequency Offset Amplify-and-Forward Relay

The fourth generation mobile communication system is expected to be implemented on frequency bands higher than the bands already assigned for IMT-2000. Therefore, to compensate propagation loss, relay schemes have been investigated. Amplify-and-Forward (AF) relay has advantages in terms of lower complexity and time slot usage. However, the coupling effect causes the loop signal between transmit and receive antennas in the relay station and it can degrade the performance. Thus, in this research, a combining scheme of loop signals and original signals in the frequency offset AF relay scheme is proposed. It is shown that diversity gain and bit error rate (BER) performance vary according to the set of frequency offsets in the relays and modulation schemes. The system with 64 quadrature amplitude modulation (QAM) suffers from interference owing to frequency offsets.