Research On The Medium Access Control Technologies In IEEE 802.11 Wireless Local Area Networks

With the development of information technologies,the demands of people to the supportable mobility,high rate and real-time communication service of wireless networks are phenomenally growing.IEEE 802.11 wireless networks have been widely deployed and used to meet these demands.However,under the complex channel environment,due to the overall effect of the background noise,the burst interference,the random nodes mobility and the internal shortcomings of IEEE 802.11 protocol,the performance of IEEE 802.11 wireless networks has not met the users demands and expectation.As a reliable,convenient,economical and flexible method of the information network construction,and an important part of the future5 G mobile communication network,IEEE 802.11 wireless networks have become a hot spot in the field of the communication technology research and development,and have acquired a lot of significant results.In order to further improve the performance of the wireless network,our work focuses on some of the medium access control technologies to IEEE 802.11 wireless local area networks,which includes the following aspects:First,motivated by solving the over protection problem of the RTS/CTS handshake mechanism in IEEE 802.11 wireless networks,an interference tolerance aware multiple access protocol(ITAMA)is proposed.In ITAMA,the transceivers send the MAC RTS/CTS frames with an interference tolerance indicator,which is used to make more precise assessment of the silence range of the RTS/CTS frames by their neighbors.Based on the assessment each neighbor node can make better choices on whether or not to initiate a concurrent transmission,thereby releasing the concurrent transmission opportunities of the non-interference links.In addition,aiming at the problem of the bidirectional link interference caused by the control frames in wireless networks,according to the characteristics of the data transmission between the transceivers,a novel cross-layer MAC protocol based on the physical expected frame detection(EFDMA)is proposed.Using the physical layer frame detection technology,EFDMA protocol can detect the expected frame from the collisions at a very low signal noise ratio(SNR)level according to the semantic relevance of the data transmission between the transceivers,thereby achieving the link concurrent transmission.Simulation results show that both the proposed concurrent MAC protocols,i.e.,ITAMA and EFDMA,can increase the network concurrent transmission and effectively improve the throughput performance of IEEE 802.11 wireless networks.Secondly,in IEEE 802.11 wireless networks,the number of the aggregatable subframes(i.e.,the aggregation level)of the A-MPDU frame aggregation scheme will be inevitably and sharply reduced due to the effect of the lost subframes,which severely affect the throughput performance of the A-MPDU transmission.To overcome this problem,an enhanced position-information-control-based(PIC-based)A-MPDU scheme and an adaptive A-MPDU retransmission scheme with the two-level frame aggregation compensation are proposed.In the PIC-based A-MPDU scheme,the position information of each aggregated subframe is carried by the A-MPDU and a novel position information control is correspondingly used to acknowledge the A-MPDU.In addition,in the two-level aggregation compensation-based A-MPDU scheme,when the aggregation level of the retransmitted A-MPDU frame dramatically decreases,one of the appropriate two-level aggregation strategies is adaptively selected to compensate the length of the retransmitted A-MPDU frames.In addition,an enhanced two-level frame aggregation with the optimized aggregation level is proposed.In order to maximize the throughput,the aggregation level used for the two-level frame aggregation is optimized and adjusted dynamically according to the subframe size,the maximum aggregation level(MAL)allowed by the current frame aggregation,and the real-time channel bit-error-rate(BER).Simulation results show that both the proposed enhanced A-MPDU schemes can stabilize and improve the transmission efficiency and throughput performance of the A-MPDU frame aggregation scheme.Meanwhile,the results from simulations show the superiority of the proposed optimized two-level aggregation scheme in respect of transmission efficiency and throughput performance.Finally,aiming at the problems that the current link adaptation algorithms is not sensitive enough to the channel change,and their adaptability and convergence speed are respectively poor and slow,an adaptive rate control and the frame length adjustment algorithm and an AP downlink adaptive multi-rate and aggregation level retransmission scheme(i.e.,AMRAL)are proposed.The main ideas of the two link adaptation algorithms are that the data rate and the transmission frame length are organically jointly adjusted to fit the fluctuant channel condition.ARCLA uses the subframe error rate(SFER)of the A-MPDU transmission to estimate the link quality and adaptively adjust the data rate,and dynamically turns the aggregation level of the A-MPDU to improve the utilization of the channel air-time and the sensitivity to the channel variation.AMRAL jointly uses the goodput and the SFER analysis of the A-MPDU transmission to estimate the current link quality and predict the link change trend,and adaptively control and adjust the data rate and the aggregation level to improve the sensitivity and convergence speed of the link adaptation algorithm.We developed a theoreti-cal analysis to the link adaptation algorithm of jointed data rate and frame length adjustment of ARCLA through building a semi Markov process-based system model,and performed the simulation experiments to evaluate the performance of AMRAL.Theoretical analysis and simulation results show that the proposed link adaptation algorithms achieve excellent performance in respect of the adaptability and the throughput,and it is highly responsive to the time-varying link conditions.