Handoff mechanism in Mobile WiMAX

For implementing a mobile network, a handoff mechanism must be defined to maintain uninterrupted user communication session during his/her movement from one location to another. Handoff mechanism handles subscriber station (SS) switching from one Base Station (BS) to another. Different handoff techniques have been developed. In general, they can be divided into soft handoff and hard handoff.

Picture. Soft Handoff
A SS maintains multiple connections. Delay is very minimal

Soft handoff is used in voice-centric cellular networks such as GSM or CDMA. It uses a make-before-break approach whereas a connection to the next BS is established before a SS leaves an ongoing connection to a BS. This technique is suitable to handle voice and other latency-sensitive services such as Internet multiplayer game and video conference. When used for delivering data traffic (such as web browsing and e-mail), soft handoff will result in lower spectral efficiency because this type of traffic is bursty and does not require continues handover from one BS to another.

Picture. Hard Handoff
A SS maintains a connection to a single BS at any given time.

Mobile WiMAX has been designed from the outset as a broadband technology capable of delivering triple play services (voice, data, video). However, a typical Mobile WiMAX network is supposedly dominated by delay-tolerant data traffic. Voice in Mobile WiMAX is packetized (what is called VoIP) and treated as other types of IP packets except it is prioritized. Hard handoff (HHO) is therefore used in Mobile WiMAX. In hard handoff, a connection with a BS is ended first before a SS switches to another BS. This is known as a break-before-make approach. Hard handoff is more bandwidth-efficient than soft handoff, but it causes longer delay. A network-optimized hard handoff mechanism was developed for Mobile WiMAX to keep a handoff delay under 50 ms.