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
