A Study of WLAN-to-Cellular Handover Using Measured Building-Exit Data

Seamless wireless local area network (WLAN)-to-cellular handover can often be very difficult to achieve. A particularly bad scenario may occur when mobile users exit from indoor WLAN coverage to outdoors during active voice connections. In this paper, we report on a measurement-based study of this type of vertical handover (VHO) situation. The presented results come from measurements taken by traversing many indoor-to-outdoor paths for a large number of building exits while measuring multi-access-point (AP) WLAN coverage. The collected data was then processed to determine handover characteristics when optimal threshold-based handover triggering is used. Our results show that it is very difficult to successfully complete the handover unless the WLAN deployment has carefully been engineered. For WLAN deployments which restrict the use of lower data rates, we propose a limited data rate usage (LDU) algorithm that improves VHO performance without adversely affecting the WLAN capacity. Analytic models and simulations using the experimental data are presented, which show the improvements in call blocking when the LDU algorithm is used.