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The seamless vertical handoff from a wireless local area network (WLAN) to a cellular system yet poses a big challenge. While serving by a WLAN, a mobile node (MN) may enter a region with a low data rate before completing a handoff towards a cellular network. To maintain quality of service, a large amount of WLAN bandwidth must be allocated to compensate for the weakness of MN link. However, this mechanism does not work properly in dense hotspots because of the lack of WLAN bandwidth. An alternative is to employ relay nodes (RN) to remain connected to the AP. This scheme also does not operate well in a low-density hotspot because the chance of finding a suitable RN is low. In this paper, a hybrid solution that uses both WLAN bandwidth and potential RNs is proposed. It uses the trade-off between the WLAN bandwidth and the potential RNs to reach an optimum handoff performance in terms of call dropping probability. Initially, the probability of call dropping in relay mode is theoretically evaluated and compared with the simulation results. Next, the proposed Hybrid approach is modelled by assuming 5% and 10% of nods as the relay nodes. The results show the proposed method can significantly reduce call dropping probability in both low and high-density hotspots when compared with a relay or bandwidth-based approach. It also reduces new call blocking probability of the bandwidth only but slightly worse than the relay only methods. [ABSTRACT FROM AUTHOR]

Loosely coupled interworking WLAN/cellular system has attracted a lot of attention recently, since the WLAN bandwidth is not restricted by the cellular network. In this kind of systems, a handoff from the WLAN to the cellular network is considered as a new call and therefore incurs a significant delay. Hence, the mobile station (MS) has to accomplish the handoff ahead of losing the WLAN coverage. In many cases the MS enters a region with a low bit rate before completing the handoff. This paper tries to aid the handoff by allocating the requested WLAN bandwidth to maintain the MS link with the serving access point as much as possible. In the case of WLAN bandwidth shortage, the handoff is assisted by using MS's as ad hoc relay stations (RS's) to extend the WLAN coverage. In a low-density WLAN, usually the handoff is accomplished by the aid of WLAN bandwidth. In contrast, in a high-density WLAN, the chance of finding a proper RS is high while the amount of available WLAN bandwidth is low. As a result, generally the handoff is accomplished by the assistance of potential RS's. Both analytical and simulation results are provided to validate the effectiveness of the proposed method in terms of decreasing the call dropping probability. [ABSTRACT FROM AUTHOR]

Handover from WLAN to a cellular network in loosely coupled WLAN/Cellular systems is considered as a new call and therefore is not given any priority in the cellular network. Hence, it induces a remarkable delay that makes loosely coupled WLAN/Cellular systems inappropriate for online services. Due to this fact, during a handover from a WLAN to a cellular network, a mobile station (MS) may be trapped in a low bit rate region before completing the handover. This paper suggests using relay stations (RS's) during a handover to extend WLAN coverage until the handover is completed. When RS's are not available, especially in sparse hotspots, allocating a large amount of the WLAN bandwidth to the MS can assist the handover process. In contrast, in a dense hotspot, the probability of finding an appropriate RS is high. Thus, the handover could be completed by using the potential RS's inside the WLAN coverage. It is confirmed, by analytical modeling and simulation, that the proposed method can reduce the call dropping probability and improve the handover performance. [ABSTRACT FROM AUTHOR]