| In some respects, the wireless LAN is much better than the traditional Local Area Network, and it has been the main direction of the development of access networks. Throughout the years, a self-organized network with no infrastructure has drawn more and more attention——Ad hoc network. Ad hoc network is more suitable to the military, relief, meetings, etc. But Ad hoc is a multi-hop network with no infrastructure, these characteristics have become the main challenges of Ad hoc network. Hidden terminal and exposed terminal problem seriously affect the performance of Ad hoc network. At present, there are some solutions. Such as: power control, double channel or multi-channel and directional antenna.According to the hidden terminal problem, this paper's mainly research based on the power control solutions. Analyses the current typical power control MAC protocols - BASIC protocol and PCM protocol, and points out the deficiency, and puts forward an improved MAC protocol -- APCM protocol. By changing the transmit power of the CTS frame to coding the Dur/ID field, which makes the stations within the carrier sense zone can also aware of the transmission in the wireless channel. Through its NAV to concede, avoid interference. By dynamic adjusting the physical carrier sensing threshold, APCM solved the hidden terminal problem better.To exposed terminals problem, this paper mainly studied the single-channel parallel transmission protocols. After analysis the present protocols, pointing out the shortcomings of MACA-P protocol, this paper proposed an improved protocol -- CMACA - P protocol. CMACA-P divided the surrounding area of the main nodes more meticulous, which is important in avoiding conflict between the nodes. Power control is also been used in CMACA-P, this is effective in resolving the cumulative interference.The two protocols are proposed in this paper to solve the hidden terminal and expose terminal problems, and they are better than the former protocols. The new protocols bring a higher network throughput and a less transmission delay. Finally, through the simulation software proves the effectiveness of the new agreement. Key words: Ad hoc, Hidden terminal, Exposed terminal, Carrier sense, Power control... |
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