基于IEEE 802.11g标准的CBTC车地通信系统设计

The Design of the CBTC Train-Ground Communication System Based on IEEE 802.11g Standard

根据城市轨道交通的实际环境,设计基于IEEE 802.11g标准的冗余CBTC车地通信系统结构。根据IEEE 802.11g标准关于不同传输速率的接收机灵敏度的规定,采用ISM 2.4 GHz频段接近地面信号的传播模型,计算列车接收信号电平低于灵敏度的概率,由此确定接入点的布置间距为200 m。根据IEEE 802.11g标准的链路层协议参数计算链路层的延迟时间,分析计算结果可知,延迟时间受传输速率和数据包长度的影响不大,但随着重传次数的增加而迅速增加。在重传10次、延迟时间40 ms、数据处理时间50 ms时,设计通信周期为200 ms。采用仿真软件OPNET对周期偏移量进行仿真可知,当通信周期为200 ms时,只要将周期偏移量控制在65～100 ms范围内,就能使车地双方在每个周期收到实时数据,保证行车的安全和效率。

Combined with the actual environment of urban rail transit,the redundant CBTC train-ground communication system based on IEEE 802.11g standard was designed.According to the IEEE802.11g standard concerning the receiver sensitivities with different transmission rates,ISM 2.4 GHz band propagation model close to the ground signals was adopted to calculate the probability that the received signal level was lower than the sensitivity.Accordingly,the spacing between the access points for the urban light rail environment was set to be 200 m according to the calculated probability.Based on the protocol parameters of the link layer specified in the IEEE 802.11g standard,the delay time of the link layer was calculated.Analysis results show that the transmission rate and the data packet size don′t have much effect on the packet transmission delay.However,the packet transmission delay will increase rapidly with the increase of the retransmission times.The train-ground communication cycle is calculated to be 200 ms with a retransmission time of 10,the packet transmission delay of 40 ms,and packet processing time of 50 ms.OPNET based simulation results shows that the cycle offset between 65～100 ms is enough for both the train and the ground to send and receive real-time data in every single 200 ms communication cycle,which can guarantee the traffic safety and efficiency.