Car Wifi Applications Fast Ap Switching Between A Key Technology Research

Mobile Internet applications have been got wide attention all the world, and WiFi is one of the potential appropriate technologies for wireless access onboard. But because of the nomadic scenario aimed scanning mechanism, it needs further improvement to provide more reliable broadband link for the mobile Internet onboard. Hence, the scanning and handoff mechanism for typical mobile scenarios are studied in this thesis.Firstly, the scanning mechanism is studied based on simulation in this thesis, especially for the typical mobile scenarios of 60 Km/h in urban-road,120 Km/h in highway,250 Km/h and 360 Km/h in railways. The simulation results show that:(1) for the urban road, the number of radio channel can be deployed three orthogonal channels or all 11 channels, and the corresponding scanning interval can be designed for 180 or 160 ms;(2) for the highway, the radio channel number can be deployed two or three orthogonal channels, and the corresponding scanning interval can be designed for 100 ms;(3) for 250 Km/h railways, the radio channel number can be deployed two orthogonal channels, and the corresponding scanning interval can be designed for 60ms;(4) for 360 Km/h railways, the radio channel number can be deployed only one channel, and the corresponding scanning interval can be designed for 20 ms.Secondly, based on the embedded system,a WiFi scanning testbed is built up, and serials of experiments are carried out for fix,30Km/h, 60Km/h and 90Km/h scenarios. The experimental results show that acceptable credibility of scanning record for mobile scenarios can be realized by optimizing system parameters, i.e., scanning interval and etc. And for the speed of 30Km/h,60Km/h and 90Km/h, the scanning interval should be less than 1 second,0.5 second and 0.35 second.Finally, a hard inter-AP handoff testbed is built up and serials of experiments are carried out, because the WiFi link is inevitably disrupted suddenly with the current AP by the obstacles and etc in the fast moving scenario. During the experiment, the hard inter-AP handoff process is triggered by changing the status of the current AP. The experimental results show that the multi-radio based handoff approach can decrease the hard handoff latency to 300-400ms, and the negative influence on the throughput can almost be ignored during the hard handoff process.