In the last several years, there has been growing interest in multihop wireless networks, either infrastructure-based or ad hoc. In previous work on routing in wireless networks, however, the fading characteristics of wireless channels are not taken into full consideration. The channel is usually simplified as ˇ°ONˇ± or ˇ°OFFˇ± according to some specific SNR threshold and the whole network is modeled as a graph. Most routing protocols were developed based on error-free links aiming at the shortest path or the minimum number of hops. The broadcast property of wireless transmissions is also ignored.
We investigate the routing issue from the link layer point of view in . We focus on a multihop network with multiple relays at each hop, and aim at minimizing the end-to-end (or source-to-destination) outage. Routing strategies are designed to fully exploit the diversity gain provided by the cooperation among relays. In particular, the optimal routing strategy is proposed which chooses the path with the minimum end-to-end outage among all the possible paths. Despite the superior performance, it requires the channel state information of all the links and a joint optimization needs to be performed. To reduce the amount of required information, an ad-hoc routing strategy is further proposed, where the relay selection is performed in a per-hop manner so that only L-link information is needed at each hop. Not surprisingly, there will be a performance gap between these two routing strategies. To achieve a good complexity-performance tradeoff, an N-hop routing strategy is finally proposed, where a joint optimization is performed every N hops.
In , we further focus on the OFDM-based relaying. We show that in an OFDM-based network, no diversity gain can be achieved by selective relaying if the entire OFDM block is transmitted over the same path, i.e., the relay with the highest combined SNR is selected at each hop. Instead, the relay selection should be performed in a per-subcarrier manner. In particular, each subcarrier chooses the best relay independently at each hop, and so may go through a distinct path from others. In addition, joint selection of the last two hops is necessary to guarantee that L-fold diversity gain can be achieved at the destination. The above two types of relaying are referred to as Selective OFDM Relaying and Selective OFDMA Relaying, respectively. It is proved that full diversity gain can only be achieved with Selective OFDMA Relaying. Coding among subcarriers is further considered, and it is shown that the performance of Selective OFDM Relaying can be significantly improved through the coding gain achieved at each hop. Nevertheless, Selective OFDMA Relaying still has much better performance and requires only symbol detection at each hop, which implies that much lower processing complexity and decoding delay are incurred than the Selective OFDM Relaying case.
 Bo Gui, Lin Dai, and Leonard J. Cimini, Jr, "Routing Strategies in Multihop Cooperative Networks," IEEE Trans. Wireless Commun., vol. 8, no. 2, pp. 843-855, Feb. 2009.
 Lin Dai, Bo Gui and Leonard J. Cimini, Jr, "Selective Relaying in OFDM Multihop Cooperative Networks," in Proc. IEEE WCNC'07 (won the Best Paper Award).