Cooperative Communications in Random Access Networks - Adaptive Signal Processing in Wireless Communications - page 334

11.7 Conclusion

Cooperative communications is a promising technique that has been proposed to combat
fading without requiring multiple antennas on each device. Most studies in the literature
focus on the signal processing aspects in the physical layer while assuming strict syn-
chronization among the cooperating users. This strict synchronization requirement is
perhaps the most difficult problem to overcome when realizing a cooperative network.
In this chapter, we studied the advantage of cooperative communications in a ran-
dom access scenario where the users transmit based only on simple local rules. Some
efficiency will be lost inevitably due to the lack of coordination, but significant improve-
ments can be attained with cooperation. Two concepts have been repeatedly explored
in this chapter: the user's ability to relay the information from the other users and the
diversity-combining technique at the destination. With proper design of the transmis-
sion protocol, these added functionalities may significantly improve the throughput
of the system. In this chapter, these advantages have been exploited in (1) the slotted
ALOHA system, (2) the CSMA/CA system, (3) the collision resolution, and (4) multihop
ad hoc networks. These discussions highlight the importance of cross-layered consider-
ations when designing cooperative systems.

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