Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

WiMAX 253NormaloperationTS measures the SNRduring scanning periodTS receives the scanningtime periods grantsScanning/Choiceof the Base StationAssociationmodeScanningmodeDecision for HandoverDL synch.of new BSRead ULparametersActivateranging processCapabilitynegotiationsAuthenticationRegistration & newconnection estab.Figure 5-31Hard handover processby removing the requirement for a handoff and other normal operations, henceeconomizing battery and radio resources while still offering a simple and timelymethod (paging) for alerting the terminal station about the current downlink traffic.– Handover. Three handover possibilities are supported within IEEE 802.16e: (a) hardhandoff (HHO), (b) fast base station switching, and (c) macro diversity/soft handover.The hard handoff is mandatory and the others are optional. The requirementfor an HHO to switch from one base station to another is less than 50 ms. Theprocess of a hard handover is illustrated in Figure 5-31.Note that a decision for a handover will result in a similar process as described above fora new registration. In the normal operation mode, the terminal station asks the base stationthrough dedicated signalling information for a scanning period grant. After receiving thescanning grant, the terminal station initiates a frequency scanning mode, leaves the normalservice mode and transmits a signal to the neighbor base station. The parameters of theneighbor base station are transmitted by the base station through the dedicated grant ACKmessages in the frequency scanning period. If a new base station is found, the terminalstation tries to synchronize to the downlink of this new base station and to estimate theactual reception quality in the form of an SNR. Note that during a scanning interval theso-called ‘association’ process (optional) can be started, where an initial ranging with thenew base station can be agreed. The terminal station can make a decision on handoverby comparing the SNR value of the new base station with the actual SNR. Then a similarprocess as for a registration can be started (see Figure 5-31). Note that the decision of ahandover can also be triggered by the base station.SecurityWiMAX supports the following security managements [23]: (a) user and device authentication,(b) key management protocol, (c) traffic encryption, (d) control and managementplane message protection, and (e) security protocol optimization for fast handovers.