According to the scope of node movement, mobility can be divided into micro-mobility and macro-mobility. On the link layer, most access networks provide mobility by having an access router keep track of the specific AP to which a MS is attached. The localized mobility between pico-cells (probably heterogeneous cells) in the same subnet and the mobility between subnets in one domain is called micro-mobility, whereas the mobility between domains in wide-area wireless networks is called macro-mobility. The mobility solutions like Mobile IP are classified as macro-mobility. But Mobile IP is not suitable for micro-mobility due to its signaling overhead, handover latency, and transient packet loss.
1 Hard Handover and Soft Handover
Hard handover is mandatory to be supported in mobile WiMAX networks. Hence, break-before-make operations may happen during the handover process. In other words, link disconnection may occur and throughput may degrade. Therefore, various levels of optimization are demanded to reduce association and connection establishment with the target BS. These optimization methods are not clearly defined in the IEEE 802.16e specification, so they should be supported on specific WiMAX systems and products.
On the contrary, soft handover is optional in mobile WiMAX networks. Two schemes, macro-diversity handover (MDHO) and fast Base Station switching (FBSS) are supported. In case of MDHO, MS receives from multiple BSs simultaneously during handover, and chooses one as its target BS. As for FBSS, the MS receives from/transmits to one of several BSs (determined on a frame-by-frame basis) during handover, such that the MS can omit the decision process of selecting the target BS to shorten the latency of handover.
2 MAC Layer Handover Procedure
The handover procedure in IEEE 802.16e-2005 is divided into MAC- and PHY-layer handover. Looking at the MAC-layer handover procedure, it is divided into the network topology acquisition phase and the handover process phase according to its performing sequence.
In the network topology acquisition phase, as illustrated in Figure 1, three functions are performed, namely network topology advertisement, MS scanning for neighboring BSs, and association procedure. After receiving a neighbor advertisement message broadcast from the serving BS, the MS gets all the neighboring BSs of its current serving BS. The MS can then perform synchronization with each neighboring BS, and then continue to the handover process phase.
Figure 1: Network topology acquisition phase for handover.
During the handover procedure, the process includes handover decision, handover initiation, and ranging procedures, followed by authorization and registration procedures. These procedures include cell reselection, handover decision and handover initiation, synchronization with new DL, acquisition of UL parameters, ranging, MS reauthorization, reregistration, and termination with the serving BS. These are shown in Figure 2 and Figure 3.
Figure 2: Handover decision, handover initiation, and ranging procedures.
Figure 3: Authorization and registration procedure.
In the ranging process, the MS can synchronize to the DL of the target BS and obtain DL and UL parameters using the DCD/UCD message. Then RNG_REG/RNG_RSP messages are exchanged to complete the initial ranging process. It may be done in a contention-based or non-contention-based manner.
If the RNG_REG contains the serving BSID, the target BS can obtain the MS information from the serving BS through the backbone network. If the MS is already associated with the target BS at the previous stage, some steps may be omitted. Therefore, the neighboring BS scanning and association should be done right after the handover initiation by utilizing preobtained information before the channel condition changes.
If all physical parameter adjustments are done successfully, the network re-entry process is initiated. Figure 3 shows this procedure. It includes MS authorization and new BS registration. The target BS requests MS authorization information via its backbone network. The new BS registration is performed by REG_REQ and REG_RSP messages. This includes capabilities negotiation, MS authorization, key exchange, and registration. After successful registration with the target BS, the MS can send a MOB_HO-IND message to the serving BS to indicate that handover is completed.
