Strategies for Handoff Detection

Strategies for Handoff Detection Three handoff detection strategies have been proposed for PCS networks. The schemes whereby the MS controls the handoff are called mobile-controlled handoff (MCHO) in literature. Schemes whereby the network exercises control are called network-controlled handoff (NCHO). A third class whereby the network controls the handoff but the MS assists with measurements of the links is called mobile-assisted handoff (MAHO). The evolution of mobile communication is toward decentralization, implying that both the management and setup of handoff procedures will be partially entrusted to the MS. Mobile-Controlled Handoff MCHO is both most popular technique for low-tier radio systems, and is employed by both the European DECT and the North American PACS air interface protocols. In this method, the MS continuously monitors the signal strength and quality from the accessed BS and several handoff candidate BSs. When some handoff criteria are met, the MS checks the "best" candidate BS for an available traffic channel and launches a handoff request. The combined Control of automatic link transfer (ALT, handoff between two BSs) and time slot transfer (TST, handoff between channels on the same BS) by the MS is considered desirable in order to:    Offload the handoff task from the network Ensure robustness of the radio link by allowing reconnection of calls even when radio channels suddenly become poor Control both automatic link transfer and time slot transfer thus preventing unhelpful, simultaneous triggering of the two processes Automatic link transfer control requires the MS to make quality measurements of the current and candidate channels in the surrounding BSs. The MS's handoff control between channels on the same BS is made possible by passing uplink -quality information in the form of a word error indicator, back, the to the MS on the Downlink Ongoing measurements and processing of measurement data which allow the MS to monitor quality. The trigger decision mechanism, whereby the MS uses processed measurement data to determine that some action, such as automatic link transfer or time slot transfer, is required. The choice of new frequency carrier for automatic link transfer or the new time slot for time slot transfer, which is a process closely allied with the trigger decision Execution of automatic link transfer or the time slot transfer or the time slot transfer via a signaling protocol between the MS and network equipment. Network -Controlled Handoff In this strategy, the surrounding BSs, the MSC or both monitor the radio signal. When the signal's strength and quality deteriorate below a predefined threshold, the network arranges for a handoff to another channel. NCHO is used in CT-2 Plus and AMPS. The BSs supervise the quality of all current connections by making measurements of RSSI. The mobile switching center (MSC) will command surrounding BSs to occasionally make measurements of these links. Based on these measurements, the MSC makes the decision when and where to effect the handoff. Because of heavy network signaling traffic needed to collect the information, and the lack of adequate radio resources at BSs to make frequent measurements of neighboring links, the handoff execution time is in the order of seconds. Since measurements cannot be made very often the accuracy is reduced. To reduce the signaling load in the network, neighboring BSs do not send measurement reports continuously back to the MSC: therefore, comparisons cannot be made before the actual RSSI is below a predetermine threshold. The required handoff time for NCHO can be up to 10 seconds or more. Mobile-Assisted Handoff It is a variant of NCHO strategy. In this strategy, the network directs the MS to measure the signal from the surrounding BSs and to report those measurements back to the network. The network then uses these measurements to determine where a handoff is required and to which channel. MACHO is used in GSM and IS-95 CDMA. In MAHO, the handoff process is more decentralized. Both the MS and the BS supervise the quality of the link for example the RSSI and WEI values. RSSI measurements of neighboring BSs are done by the MS. In GSM, the MS transmits the measurement of neighboring BSs are done by the MS. In GSM the MS transmits the measurement results to the BS twice second. The decision as to when and where to execute the handoff is still made by the network, that is the BS and the MSC or BSC. The GSM handoff execution time is approximately 1 second. IN both MAHO and NCHO systems, network, that is required to inform the MS about the handoff decision made by the network that is on which new channel to begin communicating is transmitted on the failing link. There is some probability that the link will fail before this information can be transmitted to the MS in this case the call will be forced to terminate.