It is generally known that handoff delay degrades the QoS by packet drop, packet delay and jitter. Moreover, handoff highly effects on QoS in beyond 3G system because not only micro cells for achieving high capacity and fast moving nodes induce the frequent handoff but also hard handoff must be carried out in OFDM system. Therefore study on seamless handoff for QoS is required. Related works on seamless handoff mainly focused on reducing the L3 handoff delay or packet loss. That is, these schemes try to compensate L2 delay rather directly eliminate it. So, remained L2 delay lowers QoS, especially delay-sensitive realtime trac. So we proposed the seamless handoff scheme which can minimize the L2 handoff delay.
L2 handoff delay mainly comes from the contention among nodes to access to the BS(Base Station) through random access channel. It means that delay can be reduced by avoiding the contention. Our main idea is channel reservation for access to the new base station before handoff. It makes possible for node to access and reregister to new BS seamlessly without contention after disconnecting with current BS. In our scheme, it is required that mobile node achieves uplink synchronization with reserved channel before handoff decision to exploit the reserved channel directly. So, we also proposed uplink estimation method called pseudo synchronization. We performs simulation to show whether uplink estimation method is reliable or not. As a result, we showed the feasibility of proposed scheme. In addition, we compared the handoff delay of proposed scheme with random access scheme and the result reflected that proposed scheme gurantees the lowest L2 handoff delay. Therefore, propsed seamless handoff can support QoS of delay-sensitive realtime traffic.
