The bandwidth deficit problem in loosely coupled WLAN-to-cellular vertical handover

Seamless WLAN-to-cellular handover is often difficult to achieve, since WLAN coverage can often be lost very quickly. Recent results have shown that successful handover may require that the WLAN link be allowed to rate-adjust itself to very low values (e.g., 1-2 Mbps) before the vertical handover is completed. When this occurs the call temporarily occupies a WLAN bandwidth far higher than the value for which it was originally provisioned. This effect is referred to as the vertical handover bandwidth deficit problem. In this paper we consider the effects of the bandwidth deficit problem on system performance, and how the problem can be mitigated. It is shown that this bandwidth demand can result in significant vertical handover dropping due to a lack of bandwidth on the WLAN access point. A static bandwidth reservation scheme motivated by the classical cellular guard channel approach is first considered and is found to result in an unacceptable increase in the WLAN new call blocking rate. A novel transient bandwidth reservation scheme accompanied by a momentary forced handover mechanism is proposed to overcome the shortcomings of the static approach in multiple AP WLANs. Our presented results show that almost two orders of magnitude reduction in the vertical handover dropping rate can be achieved while maintaining an acceptable new call blocking rate at the WLAN AP.