An Efficient OFDM with Adaptive Guard Interval for Amplify and Forward Relay Systems

In amplify-and-forward (AF) relay systems, multi-hop transmissions over multiple frequency selective fading channels result in an accumulation of delay spread, which could lead to strong intersymbol interference (ISI) at the destination since there is no proper channel compensation at the relay nodes. If orthogonal frequency division multiplexing (OFDM) with fixed-length guard interval (GI) is used in these systems, then the length of the GI has to be extended to the longest possible channel delay spread to avoid ISI. However, using such a long GI lowers the effective data transmission throughput. To this end, we propose a novel OFDM system with adaptive GI for AF relay networks to reduce the overall transmission overhead by dynamically choosing the suitable GI length. More specifically, the GI is adaptively selected at the transmitter from a variable-length orthogonal code set, which provides a GI sequence with proper length, depending on the accumulated channel duration. By exploiting the orthogonal property between different GI sequences, the proposed adaptive GI scheme can be implemented without any extra control signal transmitted by the source to notify the destination about the GI used. This differs from existing adaptive GI studies which require such an additional control signal and thus introduce extra system overhead. Numerical results show that the proposed adaptive scheme (without additional control signal) can achieve the same symbol error rate (SER) performance as the conventional adaptive GI approaches (with control signal). This implies that the proposed scheme can further save the control signaling overhead without any SER performance loss.