Handover Considerations
One of the major goals of the 802.16e amendment is to introduce mobility in WiMAX. Consequently, mobile WiMAX profiles are based on 802.16e. Mobility is based on handover. Handover operation (sometimes also known as ‘handoff’) is the fact that a mobile user goes from one cell to another without interruption of the ongoing session (whether a phone call. data session or other). The handover can be due to mobile subscriber moves, to radio channel condition changes or to cell capacity considerations. Handover is a mandatory feature of a cellular network. The handover (HO) is described as defined in 802.16e.
In 802.16e, the two known generic types of handover are defined:
§ Hard handover, also known as break-before-make. The subscriber mobile station (MS) stops its radio link with the first BS before establishing its radio link with the new BS. This is a rather simple handover.
§ Soft handover, also known as make-before-break. The MS establishes its radio link with a new BS before stopping its radio link with the first BS. The MS may have two or more links with two or more BSs, which gives the soft handover state. The soft handover is evidently faster than the hard handover.
Two types of soft handover are then defined in 802.16e:
§ Fast BS Switching (FBSS). This is a state where the MS may rapidly switch from one BS to another. The switch is fast because the MS makes it without realising the complete network entry procedure with regard to the new BS.
§ Macro Diversity HandOver (MDHO). Transmissions are between the MS and more than one BS.
In the mobile WiMAX profiles, only the hard handover is mandatory. The FBSS and MDHO are optional. The 802.16 standard also indicates that the support of the MDHO or FBSS is optional for both the MS and the BS.
Handover has challenging objectives. First, it has to be fast enough, of the order of 50 ms or 150ms. There are also security requirements, as some attacks are possible at the occasion of the handover procedure. Finally, the handover does not have only Layer 2 considerations. Layer 3 considerations are also needed. Hence, the handover is not independent of the architecture.
