A Measurement-Based Study of WLAN to Cellular Handover

Real-time vertical handover is an important capability for multimode WLAN/cellular handsets. In many cases however, seamless handover can be very difficult to achieve, since WLAN coverage may be lost long before a cellular call leg can be triggered and established. Worse-case handovers of this kind occur when mobile users walk from indoor building WLAN coverage to outdoors during voice connections. In this paper we report on a measurement-based study of WLAN-to-cellular handover. Our results are based on extensive IEEE 802.11 measurements that were made on the McMaster University campus during the summer of 2005. Our methodology involved traversing many indoor-to-outdoor paths for a large number of campus buildings and exits while monitoring multi-AP Wi-Fi coverage. The collected data was then processed to determine the probability of seamless handover using classical vertical handover algorithms. The results presented give important insights into the difficulty of this problem, and relate to issues such as Wi-Fi deployment type, handover triggering, and Wi-Fi link loss threshold. The results provided enable handset designers and WLAN administrators to better understand the sensitivity of vertical handover performance to these parameters and how they can be optimized