Research On Wireless Anti-interference Of CBTC Systems In Urban Rail Transit

Communication Based Train Control(CBTC) systems have many characteristics such as high accuracy positioning and bidirectional train-to-wayside data communication, which can better realize the train control under moving block and meet the demand of train automatic control in current urban rail transit. Train-to-wayside data communication system is an important part in CBTC systems, which transmits data by using WLAN technology with the standard of IEEE802.11. The wireless standard works on the open ISM band, so the train-wayside communication processes are easily disturbed by other wireless signals in urban environment nowadays. Hence, after the analysis of interference causes of CBTC wireless communication systems, the corresponding anti-interference schemes have been put forward to improve the ability of anti-interference, which can ensure the safety and efficiency of train running and are significant to the further development in urban rail transit.Firstly, according to the Free Space Loss Mode and the special circumstance of CBTC,the wireless propagation model applying to urban rail transit environment has been constructed. After the detailed analysis of CBTC systems, the interference has been divided into two parts: internal interference and external interference. The problems of Access Point(AP) bottleneck and wireless mobile device interference are mainly difficult in the anti-interference research of CBTC systems. Some existing anti-interference measures has also been expounded on the basis of anti-interference schemes in CBTC systems.Secondly, after the analysis and improvement of the traditional Distributed Coordination Function(DCF) model protocols by the two-dimensional Markova chain, the influence of vehicle-mounted antenna nodes number, contention window size, Mobile Wireless Fidelity(MiFi) nodes number and other parameters on the throughput and transmission delay of CBTC systems have been given. To reduce the internal interference in CBTC systems, a DCF improvement scheme has been proposed, which can increase the priority of AP by developing different algorithms for AP and vehicle-mounted Antenna(VA). An adaptive scheme that sets the reasonable competition window value according to the number of MiFi nodes has been proposed, which can reduce the external interference and improve the anti-interference ability of CBTC systems. Meanwhile, based on the perspective of backoff mechanism, another scheme with slowly increase and decrease algorithm has also been proposed to further improve the anti-interference ability.Finally, according to the above analysis, the different train operation senses have been built by OPNET. The network module has been built with working parameters, such as communication protocol, data flow, and movement path. Then the different train operation scenes have been built to analyze and simulate the suffering interference of CBTC systemsfrom both inside and outside. By calculating throughput and transmission delay, the improved anti-interference scheme has been compared with the traditional DCF protocol. The results show that the improved anti-interference algorithm has a good effect in the performance improvement of CBTC systems and can improve the anti-interference ability of CBTC systems.