Optimized Handover Schemes for Mobile WiMAX

Mobility support in Mobile WiMAX is realized by three schemes of handoff: Hard Handoff (HHO), Fast Base Station Switching (FBSS) and Macro Diversity Handover (MDHO), where only the HHO is mandatory and the other two schemes are optional

1 FBSS

When the FBSS is supported, the Mobile Station (MS) and BS maintain a list of BSs that are involved in FBSS with the MS. This set is called an Active Set. In FBSS, the MS continuously monitors the BSs in the Active Set.

Among the BSs in the Active Set, the anchor BS is defined. When operating in FBSS, the MS only communicates with the anchor BS for uplink and downlink messages including management and traffic connections. Transition from an anchor BS to another (i.e., BS switching) is performed without invocation of explicit HO signaling messages. Anchor update procedures are enabled by communicating signal strength of the serving BS. An FBSS begins with a decision by an MS to receive or transmit data from the anchor BS that may change within the Active Set.

2 MDHO

For MSs and BSs that support MDHO, the MS and BS maintain an Active Set of BSs that are involved in MDHO with the MS. Among the BSs in the Active Set, the anchor BS is defined. When operating in MDHO, the MS communicates with all BSs in the Active Set for uplink and downlink unicast messages and traffic. MDHO begins when an MS decides to transmit or receive unicast messages and traffic from multiple BSs in the same time interval.

3 HHO

When the Mobile WiMAX unit switches from one Active Set area to another, it performs an HHO. Where the Break-Before-Make HO scheme is applied, service with the target BS starts after disconnection of service with the previous serving BS. Meanwhile, service starts before disconnection if the Make-Before-Break HO scheme is used

4. HARQ

HARQ is the function that integrates the existing Automatic Repeat Request (ARQ) function and an error correction technique (i.e., Chase combining (CC) and optionally Incremental Redundancy (IR)) that combines an error-detected packet with retransmission packets.

Error detection in the HARQ system is implemented by allocating a dedicated ACK channel in the uplink to provide feedback (i.e., ACK or NACK signaling) for fast retransmission in case the packet is in error. The receiver will keep the error packet and implement CC or IR to jointly process the packets in error and new transmission to improve the packet reception.

Mobile WiMAX supports multi-channel HARQ operation. Multi-channel stop-and-wait ARQ with a small number of channels is an efficient, simple protocol that minimizes the memory required for HARQ and stalling. Mobile WiMAX provides signaling to allow fully asynchronous operation where the packet retransmission after receiving an NACK is determined by the Base Station scheduler. The asynchronous operation allows variable delay between retransmissions which gives more flexibility to the scheduler at the cost of additional overhead for each retransmission allocation.