QoS Performance Analysis And Improvement Schemes Of CSMA/CA Wireless Systems

Recent years, great interest is focused on wireless communication system because its convenient and fast accessing way, and CSMA/CA is widely accepted as the basic MAC mechanism of wireless channel accessing, such as IEEE 802.11 protocols in WLAN. However, along with the development of wireless communicaton technologies, people are expecting better QoS performance as well. The main purpose of this paper is to analyze CSMA/CA QoS performance and suggest improvement schemes, especially under IEEE 802.11 DCF mode, where CSMA/CA is used. Its results and conclusions are applicable to other similar wireless systems. Presently, wireless systems based on CSMA/CA are widely used, such as IEEE 802.11, but due to absent consideration on QoS content at the beginning of its design phase, IEEE802.11 protocal lacks effective QoS mechanism to support services which need higher request on delay performance.Based on the understanding of the classical analysing methods to the CSMA/CA performance, the paper focuses on the statistical characteristics analyzing of delay in CSMA/CA, provide the references for better QoS solution in wireless communication system based on CSMA/CA. The main contribution of this article includes:First, in this article, we make a deeply theoretical analysis to delay of IEEE 802.11 DCF by constructing a three-dimensional Markov chain model. Some important statistical characteristics of delay in WLAN have been achieved : the expression of statistical average sending delay, the expression of statistical average jitter, the expression of distribution range of delay; the expression of probability of collisions and freezing states.Second, through abundant research on IEEE DCF mode, we work out an improved scheme: TSDCF(Threshold_Slow Decrease Control Function). In this algorithm, a simple method is used to judge the state of channel and stations will back-off according to the congestion state. The simulation results show that the collisions in CWmin caused by increasing number of nodes are minimized effectively, and throughput and delay of system are enhanced, the fairness of accessing to the channel is also advanced.Third, with the understanding of classification method in IEEE 802.11e, an amendatory back-off algorithm named EADCF is brought forward. In EADCF(Enhanced Adaptive DCF), service priority is emphasized and distinguished by the CWmax and counter of resending, differentiate service is achieved in EADCF by differentiate priority of accessing to the channel. The results of simulation prove that high priority services have better delay and throughput performance than low priority ones in EADCFFinally, a noval method of forecasting station numbers is put forward, it uses Weighted First-order Local Area theory in chaos theory, forecasting results can be achieved through collecting enough foregoing datas, calculating parallel dimension numbers and then reconstructuring state space. With this method, we analyse the influence of history data on reliability of forecasting results. It is meaningful for future research.