In cooperative communications different network nodes share their

antennas and resources to form a virtual antenna array and improve

their performance through spatial diversity. This thesis contributes

to the advancement of cooperative communications by developing and

analyzing new multiple access cooperation protocols that leverage

the benefits of cooperation to upper network layers.

For speech communications networks, we propose a cooperative

multiple access protocol that exploits inherent characteristics of

speech signals, namely, long periods of silence, to enable

cooperation without incurring bandwidth efficiency losses. Using

analytical and simulation results we show that the proposed protocol

achieves significant increase in network throughput, reduction in

delay, and improved perceptual speech quality.

In TDMA networks, we investigate the problem of sharing idle time

slots between a group of cooperative cognitive relays helping

primary users, and a group of cognitive secondary users. Analytical

results reveal that, despite the apparent competition between relays

and secondary users, and even in case of mutual interference between

the two groups, both primary and secondary users will significantly

benefit in terms of maximum stable throughput from the presence of

relays.

For random access networks, we find a solution to the problem of

achieving cooperation gains without suffering from increased

collision probability due to relay transmissions. A novel

cooperation protocol is developed and analyzed for that purpose.

Analytical and simulation results reveal significant improvements in

terms of throughput and delay performance of the network. Moreover,

collision probability is decreased.

Finally, in the framework of a cognitive radio network, we study the

negative effects of spectrum sensing errors on the performance of

both primary and secondary networks. To alleviate those negative

effects, we propose a novel joint design of the spectrum sensing and

channel access mechanisms. Results show significant performance

improvement in the maximum stable throughput region of both

networks.