Strategies for Handoff Detection Three handoff detection strategies have been proposed for PCS networks. The schemes wherebythe MS controls the handoff are called mobile-controlled handoff (MCHO) in literature. Schemeswhereby the network exercises control are called network-controlled handoff (NCHO). A thirdclass whereby the network controls the handoff but the MS assists with measurements of thelinks is called mobile-assisted handoff (MAHO). The evolution of mobile communication istoward decentralization, implying that both the management and setup of handoff procedureswill 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 theEuropean DECT and the North American PACS air interface protocols. In this method, the MScontinuously monitors the signal strength and quality from the accessed BS and several handoffcandidate BSs. When some handoff criteria are met, the MS checks the "best" candidate BS for anavailable traffic channel and launches a handoff request. The combined Control of automatic link transfer (ALT, handoff between two BSs) and time slottransfer (TST, handoff between channels on the same BS) by the MS is considered desirable inorder to: Offload the handoff task from the network Ensure robustness of the radio link by allowing reconnection of calls even when radiochannels 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 currentand candidate channels in the surrounding BSs. The MS's handoff control between channels onthe same BS is made possible by passing uplink -quality information in the form of a word errorindicator, back, the to the MS on the Downlink Ongoing measurements and processing of measurement data which allow the MS to monitorquality. The trigger decision mechanism, whereby the MS uses processed measurement data todetermine 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 slottransfer, 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 asignaling 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 thesignal'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 thequality of all current connections by making measurements of RSSI. The mobile switchingcenter (MSC) will command surrounding BSs to occasionally make measurements of theselinks. Based on these measurements, the MSC makes the decision when and where to effect thehandoff. Because of heavy network signaling traffic needed to collect the information, and thelack 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 veryoften the accuracy is reduced. To reduce the signaling load in the network, neighboring BSs donot send measurement reports continuously back to the MSC: therefore, comparisons cannot bemade before the actual RSSI is below a predetermine threshold. The required handoff time forNCHO 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 thesignal from the surrounding BSs and to report those measurements back to the network. Thenetwork then uses these measurements to determine where a handoff is required and to whichchannel. MACHO is used in GSM and IS-95 CDMA. In MAHO, the handoff process is more decentralized. Both the MS and the BS supervise thequality of the link for example the RSSI and WEI values. RSSI measurements of neighboring BSsare done by the MS. In GSM, the MS transmits the measurement of neighboring BSs are done bythe MS. In GSM the MS transmits the measurement results to the BS twice second. The decisionas to when and where to execute the handoff is still made by the network, that is the BS and theMSC or BSC. The GSM handoff execution time is approximately 1 second. IN both MAHO andNCHO systems, network, that is required to inform the MS about the handoff decision made bythe network that is on which new channel to begin communicating is transmitted on the failinglink. There is some probability that the link will fail before this information can be transmittedto the MS in this case the call will be forced to terminate.