Research On Performance Of OFDM Relay System With Subcarrier Mapping

The future wireless mobile communication systems are expected to pro-vide high-rate, high-quality and efficient data transmission. Orthogonal fre-quency division multiplexing (OFDM) is known as an effective scheme to cap-ture multi-path energy. mitigate the inter-symbol interference. and offer high spectral efficiency in broadband communications. On the other hand, coopera-tive and relayed transmission would also play a key role in IMT-Advanced sys-tems since it can extend the radio coverage and improve the capacity of wireless networks. In future wireless communications, relay network combining with OFDM technique will be a potential way to further enhance the performance of wireless communication network.The main research work is OFDM technique combined with relay, named OFDM-Relay system. Implementing subcarrier mapping which rearranges the subcarriers in OFDM packets according to channel state information (CSI) such that an optimum coupling between subcarriers of the first and the second hop occurs is an excellent scheme for improving end-to-end system performance. This paper firstly evaluate two typical mapping schemes, named order mapping and reversed order mapping from the perspective of total system ergodic capac-ity and average system symbol error rate (SER). In ordered subcarrier mapping, the subchannels of two hops coupled into each other in a sorted way according to the subchannel gain, in order to maximize capacity. To minimize the SER, the subchannels of two hops should be coupled into each other in a reversed sorted way. Specifically, the subchannel with the biggest channel gain of the first hop maps to the subchannel with the smallest channel gain in the second hop, the subchannel with the second biggest channel gain of the first hop maps to the subchannel with the second smallest channel gain in the second hop, and so forth. Through theoretical analysis we derive approximated ergodic capac-ity and SER formula. The results show that the current two schemes have their advantages and disadvantages. The ordered mapping which maximizing the capacity could also make the SER worse. While, the reverse ordered mapping which minimizing the SER leads to the loss of spectrum. Thus, we propose a modified ordered mapping scheme in this paper. Both of theoretical analysis and simulations show this scheme can not only increase capacity, but also can greatly reduce the SER.In the end of this paper, we also investigate power allocation in OFDM-Relay system. Besides, the future application and other research directions of relay technology are described.