Robust and High Data Rate Wireless Link for Security Between a Bus and a Control Centre

For transport operators today, making passengers and crews feel safer on urban mass transportation systems, and curbing damage to rolling-stock, are two of the main priorities. To satisfy these needs, they rely more and more on surveillance systems based on audio and video sensors which are able to detect automatically potentially dangerous situations and to send an alarm signal, with the corresponding audio and video streams, to the security staff at the control centre. When the system is embedded in a public transport vehicle, the transmission system must be a wireless one and must be able to offer high data rate and high quality of service in order not to cause a deterioration to the audio and video stream. The work presented in this paper concerns the development of such a transmission system from an urban bus to a control centre. Existing telecommunication standards offering large radio coverage generally offer asymmetrical links and none of them is able to support very high data rate in up-link. The new IEEE802.16 or WiMAX (worldwide interoperability for microwave access) standard will offer 3 to 5 Mbps on both transmission directions. The paper will present a 2x4 MIMO transmission system based on the physical layer called PHY-OFDM of IEEE802.16-d with STBC code and turbo processing to increase robustness. Different signal processing solutions were evaluated in simulation with measured propagation channels with various correlation degrees (suburban and urban canyon) representative of urban bus environments. A real transmission system has been built on the basis of the simulation one. Experiments in real conditions were conducted in the centre of Lille in the north of France.