The Design Of MAC Protocol Which Supporting Cooperative Communication In Ad Hoc Network

In the wireless communication environment, the signal suffers from serious chan-nel fading and interference such as frequency selective fading or time selective fading.How to reduce the in?uence of fading and improve the signal quality is a big ques-tion. Diversity is an effective technology. However, the traditional diversity technol-ogy need multiple antennas, which is impractical for single-antenna terminals. Then,user cooperation becomes a very popular topic for research. Cooperative diversity isa transmission technique, where multiple terminals form a virtual antenna array thatrealizes spatial diversity gain in a distributed fashion. In IEEE 802.11 system, theconcept of cooperation has already been introduced to MAC layer to design MACprotocol. But it is kind of different with that mentioned in User Cooperation. So, it ismeaningful to design a MAC protocol to support user cooperation.We first analyze the performance and complexion of three TDMA-based cooper-ative protocols. Then, we present a novel MAC protocol based on IEEE 802.11, calledC-MAC, that is able to support the basic building block of cooperative system. In otherwords, in C-MAC, a source would invite a relay node into data transmission if thereexits an available one. The data transmission is split into two time slots. The sourcesends the signal to destination in the first time slot. The relay node will retransmit theoverheard information to the destination in the second time slot while the source nodekeeps silence. The destination combines two signals from the source and the helper tocreate the spatial diversity and robustness against channel fading.It's easy to find that the performance of C-MAC not only depends on MAC pro-tocol itself, but also depends on the physical layer's performance as it just providesthe support for cooperation at the MAC layer. The user cooperation itself works inphysical layer and the decode of two signals at the destination node is very cruel as it determines the packet error rate(PER).As CMAC is IEEE 802.11b-based MAC protocol and it utilizes several featuresof 802.11b, the CMAC is backward compatible to the legacy IEEE 802.11 system. Inour work of design CMAC, according to the relay information, CMAC can degenerateto 802.11b。Later, we switch CMAC and 802.11b due to the value of packet errorrate(PER).We also perform extensive simulation using ns2 with assumptive physical param-eters. The results show that C-MAC would outperform 802.11 if packet error rate(PER) is over some threshold, e.g, when PER is 0.4, C-MAC can achieve ip to 11.5%higher throughput than IEEE 802.11.An important part of our work is to realize the CMAC coding and run corre-sponding simulation for CMAC and IEEE 802.11b. Since CMAC evolves from IEEE802.11b,we dig into the structure and codes of IEEE 802.11b in NS-2 simulatorsoftware and modify lots of lines within different classes to make CMAC mechanismworks. We compare the performance difference between CMAC and IEEE 802.11with the results got from logical scenario. And the result shows CMAC would get bet-ter performance than IEEE 802.11b while in the case of high packet error rate whichmeans the fading or interference is very serious.