Bit-error-rate performance improvement of mobile dual-hop relaying systems using directional antennas

The bit-error-rate (BER) performance of orthogonal frequency division multiplexing (OFDM) systems in mobile multihop relaying (MMR) system is severely degraded by the effect of Doppler shift and the severity of this degradation increases with the number of hops traversed by the OFDM signal. In this study, the authors propose a method to mitigate the effect of Doppler shift in MMR system (such as the IEEE 802.16j system) using directional antennas. It is shown that the effect of the resulting inter-carrier interference (ICI) because of the phase noise generated by the Doppler shift over multi-hop relaying channels, can be reduced by employing directional antennas at both the mobile and relay stations. Consequently, the BER performance of MMR system is significantly enhanced. Analysis and simulation results show that the BER enhancements using the proposed approach are strongly related to the orientation and beamwidth of the directional antenna employed. As the antenna beamwidth is reduced, the BER enhancement increases for both the perpendicular and parallel antenna orientations, and comparing these two orientations, the parallel orientation case provides slightly better BER enhancements.