Wireless transmissions are inherently errror-prone due to the random characteristics of the fading channel. If an erroneous packet cannot be repaired, e.g., by using forward error correction (FEC), it must be retransmitted. Unfortunately, retransmission is infeasible for real-time voice and video streams due to their strict time constraints.
Recently, cooperative diversity techniques were proposed [3-6] to make wireless transmissions more robust. In a multiuser scenario, every packet transmitted from a source (A) to a destination (D) can be overheard by other users nearby (B). These nearby users can provide the destination with additional information obtained from the overheard packet. With this additional information provided by cooperating users or partners, the destination may be able to decode a destroyed packet that would have to be retransmitted otherwise. Thus, cooperation exploits multiuser diversity.
- Cooperative multiple access -- Conventional approaches prevent interference caused by hidden users through collision avoidance with RTS/CTS. Exploiting additional stations as cooperation partners leads to further hidden users. If these terminals do not refrain from transmissions, they may cause interference at cooperation partners. Cooperative multiple access schemes must provide means to avoid this interference.
- Multi-hop routing -- In wireless ad hoc networks, transmissions may involve several hops. The impact of cooperation, e.g., partner selection, on conventional routing protocols must be studied. Two fundamental approaches exist that may or may not exploit cooperation-specific information at the network layer. Both approaches must be investigated and compared with each other.
The new cooperative communication protocol will be prototyped on the Sorbas 101 software-defined radio testbed, which has already been used to prototype IEEE 802.11a extensions.