Cooperative transmission among nodes of a given wireless (cellular, ad hoc or mesh) network capitalizes on the broadcast nature of the radio medium by allowing multiple single node terminals to form virtual antenna arrays. This technology enables energy-efficient transmission, coverage extension and performance improvement through micro or macrodiversity. An relevant example is the Collaborative Automatic Repeat reQuest (ARQ) protocol, whereby terminals (source and relays) are assumed to collaborate through Space-Time-Block-Coding (STBC) during retransmissions, upon request by the destination.
In particular, as exemplified in the figure, in the first time-slot, the source broadcasts a packet that is decoded at the destination and available relays. If erroneous decoding at the destination is detected through Cyclic Redundancy Check (CRC), a Not-Acknowlegde (NACK) message is sent by the destination requesting retransmission. In this case, relays that have successfully decoded the packet signal their availability to the source and switch to the transmitting mode. During the retransmission slot, the activated relays and the source form a distributed transmitting antenna array, sending a STBC codeword that is decoded at the destination and the remaining receiving relays according to the chosen HARQ protocol. This procedure repeats with possible activation of currently receiving relays, until final successful transmission is confirmed via an Acknowledge (ACK) message from the destination, or until predefined maximum number of retransmission is reached.

Cooperative relaying in Ad-Hoc and Sensor Networks is a promising scheme using already existing network to provide cooperative transmission from source to destination. These schemes exploit diversity gained by multiple transmissions. The role the cooperative relays play in the transmission could be: amplify-and-forward, decode-and-forward or a hybrid combination of both. In a basic amplify-and-forward scheme the cooperating relay amplifies the received message and retransmits it while in decode-and forward the received message is decoded, processed and coded again before it is retransmitted. Unlike the decode-and-forward, the amplify-and-forward approach offers simplicity and achieves full diversity gain, providing global knowledge of the channel state information (CSI) is available. Obtaining channel information in a cooperative system places a significant overhead on system operation and therefore is sometimes impractical.