Cross-layer Design of Combining Cooperative Diversity and Truncated ARQ
The use of multiple antennas at both the transmitter and receiver can bring significant capacity gains. Unfortunately, this could be impractical in an ad-hoc wireless network, due to the size of the node or the mobile unit. In order to overcome this limitation, a new form of spatial diversity, whereby diversity gains are achieved via the cooperation of nodes, has been proposed. The main idea behind this approach, which is called cooperative diversity, is to use orthogonal relay transmission to achieve diversity gain. In particular, each node has one or several partners. The node and its partner(s) are responsible for transmitting not only their own information, but also the information of their partner(s). Therefore, a virtual antenna array is obtained through the use of the relays' antennas without complicated signal design or adding more antennas at the nodes.
In most of the present cooperative protocols, no restrictions are imposed on the selection of relays. Therefore, when the channel between the source node and the relay node (s-r channel) is poor, cooperative diversity may result in even worse performance than the non-cooperative case due to severe error propagation. In , a selection relaying protocol with two nodes cooperation was proposed, where the relay forwards the source node's information only if the s-r channel fading coefficient is above a given threshold. In other words, the node is selected to be a relay only when its corresponding s-r channel is good enough. Obviously, such selective protocol can achieve better performance than the fixed ones . However, it is usually not trivial to select a suitable threshold since it depends on the actual value of the channel fading coefficients. A higher threshold will reduce the possible performance gain while a lower one will allow more error propagation which also degrades the performance.
ARQ protocol at the link layer is an effective means to overcome the channel fading, where CRC is usually used for error check and retransmissions are requested if the packet is received erroneously. In our work, we propose a novel cross-layer design which combines truncated ARQ at the data link layer and cooperative diversity at the physical layer . We will show that through this combination, adaptive cooperative diversity gain can be achieved without any specific threshold and error propagation can be therefore avoided. In this new scheme, Q idle nodes around the source node are defined as relay candidates. These nodes also receive the packet transmitted from the source node to the destination node and check the CRC results. Only the ones who detect the correct CRC are selected to be relays and involved in the possible retransmission. Specifically, if the destination node fails to detect the packet correctly, retransmission will start where both the source node and the relays utilize a suitable orthogonal space-time block code to retransmit this packet. It can be seen that this new scheme is adaptive to the s-r channels by virtue of the CRC bits instead of some specific threshold and so no error propagation will be incurred by relaying. Besides, high efficiency can be achieved since node cooperation is adopted only when the destination node fails to detect the nodes correctly. As a result, this scheme, which is referred to as Selective Cooperative diversity with ARQ (SCA), can be expected to bring significant performance gain over the previous ARQ-only or fixed cooperative diversity schemes.
 J. N. Laneman, G. W. Wornell, and D. N. C. Tse, "Cooperative diversity in wireless networks: efficient protocols and outage behavior," IEEE Trans. Inf. Theory, vol. 50, no. 12, pp. 3062-3080, Dec. 2004.
 Lin Dai and Khaled B. Letaief, "Throughput Maximization of Ad-hoc Wireless Networks Using Adaptive Cooperative Diversity and Truncated ARQ," IEEE Trans. Commun., vol. 56, no. 11, pp. 1907-1918, Nov. 2008.