Joint Subcarrier Matching and Power Allocation for OFDM Multihop System

Relay networks have recently attracted extensive attention due to its potential to increase coverage area and channel capacity. In a relay network, a source node communicates with a destination node with the help of the relay node. The performances of improving the channel capacity and coverage area have been explored and evaluated in the literature (Sendonaris et al., 2003)-(Laneman et al., 2004). There are two main forwarding strategies for relay node: amplify-and-forward (AF) and decode-and-forward (DF) (Laneman et al., 2004). The AF cooperative relay scheme was developed and analyzed in (Shastry & Adve, 2005), where a significant gain in the network lifetime due to node cooperation was shown. Power allocation is studied and compared for AF and DF relaying strategies for relay networks, which improves the channel capacity (Serbetli & Yener, 2006). However, DF means that the signal is decoded at the relay and recoded for retransmission. It is different from AF, where the signal is magnified to satisfy the power constraint and forwarded at the relay. This has the main advantage that the transmission can be optimized for different links, separately. In this chapter, the relay strategy DF is used. In wideband systems, orthogonal frequency division multiplexing (OFDM) is a mature technique to mitigate the problems of frequency selectivity and intersymbol interference. The optimization of power allocation for different subcarriers offers substantial gain to the system performance. Therefore, the combination of relay network and OFDM modulation is an even more promising way to improve capacity and coverage area. However, as the fading gains for different channels are mutually independent, the subcarriers which experience deep fading over the source-relay channel may not be in deep fading over the relay-destination channel. This motivates us to consider adaptive subcarrier matching and power allocation schemes, where the bits on the subcarriers from the source to the relay are reassigned to the subcarriers from the relay to the destination. The system architecture of OFDM two-hop relay system is demonstrated in the Fig.1. A fundamental analysis of cooperative relay systems was done by Kramer (Kramer et al., 2006), who has given channel capacity of several schemes. Relaying for OFDM systems was considered theoretically in (Shastry & Adve, 2005). Multi-user OFDM relay networks were studied by Zhu (Zhu et al., 2005), where the subcarrier was allocated to transmit own information and forward other nodes’ information. Relay selection in OFDM relay networks was studied by Dai (Dai et al., 2007), which indicated the maximum diversity by selecting different relay for the different subcarrier. Radio resource allocation algorithm for relay