Friday, February 11, 2011


Mobile WiMAX specifications support five types of accesses that reflect five mobility-related usage scenarios. First, fixed broadband access assumes a subscriber being in the same geographic location during the whole duration of access to the network services. Second, nomadic access supports the movement between different cells without managing handoff. Third, portable access provides nomadic access to a portable device with the expectation of a BE handoff. Fourth, simple mobility access supports subscribers moving at speeds up to 60km/hour; it provides service continuity despite mobility and fulfils brief interruptions during handoff. Both portable and simple mobility accesses implement the hard handoff concept. Finally, full mobility access guarantees service continuity at high speeds up to 160km/hour and achieves seamless handoff with less than 50 ms latency and less than 1 percent packets loss ratio. To support full mobility access, IEEE 802.16e specifications define three handoff methods, which are the hard handover (HHO), the fast base station switching (FBSS) handover, and the macrodiversity handover (MDHO). HHO is mandatory while both FBSS and MDHO are optional. Initially, HHO is the only type required to be implemented by certified mobile WiMAX equipments.

1: Hard Handoff

Hard handoff results in a sudden connection transfer from one managing BS to a second one as the MS can communicate with only one BS each time. Therefore, all connections with the serving BSs are broken before a new connection with the target BS is established. In Figure 1, the red thick line at the border of the cells indicates the place where HHO is executed. The threshold level hysteresis is used in practice to avoid the repeated switching of neighbors BS during a movement lengthwise to the cell boundaries. HHO is a less complex handoff type but it induces high latency; therefore, it is used for data as it is not suitable for real-time latency-sensitive applications such as VoIP.

Figure 1: The HHO process.

The handoff decision may be taken by either the BS, the MS, or a third entity in light of the periodic measurements done by the MS. In fact, each MS periodically processes a radio frequency scan during scanning intervals allocated by the serving BS and measures the signal quality of the neighboring BSs. Scanning consists in monitoring each possible frequency until a DL signal is received. The number of scanned frequencies depends on the regulatory-provisioned bandwidth, the physical specification, and the chosen bandwidth per channel, which depends on the physical specification. The MS is allowed to perform the initial ranging process to associate with one or more neighboring BSs. Once the handoff is decided, the MS starts the synchronization with the DL transmission of the target BS; it then performs ranging if this was not done during scanning, and it finally ends the connection with the previous BS. The undelivered MAC protocol data unit (MPDU) is stored at the BS until the timer expires.

2: Macrodiversity Handoff

MDHO is a form of soft handoff as the MS is allowed to maintain a valid connection simultaneously with more than one BS. When the MDHO is supported by both the MS and the BS, a diversity set, also referred to as active set, is maintained. The active set is a list of BSs that may be involved in the handoff procedure. Such BSs list is updated through the exchange of MAC management messages. These messages are sent based on the long-term CINR of BSs, which depends on two threshold values broadcasted in the DCD: the Add Threshold H_Add_Threshold and the Delete Threshold H_Delete_Threshold. A serving BS is dropped from the diversity set when the long-term CINR is less then H_Delete_Threshold. A neighbor BS is added to the diversity set when its long-term CINR is higher than H_Add_Threshold. The MS continuously monitors the BSs in the diversity set and defines an anchor BS among them. The MS synchronizes and registers to the anchor BS and performs ranging while monitoring the DL channel for control information. The MS communicates with anchor BS and active BSs in the diversity set as depicted in Figure 2. Two or more BSs transmit data on the DL so that multiple copies are received by the MS which needs to combine them using any of the well-known diversity-combining techniques.

Figure 2: The MDHO process.

To evaluate the performances, suggest the implementing of a handover delay timer that adds a short delay between the time when the handover conditions are met and the handover initialization is started. They assumed a MDHO handover and a periodic reporting of 1 s and then evaluated the number of handovers between 4 BSs and 42 MSs. The principle was to select the BS with the best signal strength for each MS and then create the diversity set for each MS based on defined thresholds and signal strengths. Three different cases were simulated. In the first case (variety I), the probability of affected CINR value is 0.5 percent (it means that the 99.5 percent of values are unaffected). In the second case (variety II), the probability of affected value is 0.05 percent (99.95 percent of values are unaffected). In the third case (variety III), no values were affected (100 percent of values are unaffected). There has been simulated 30 min. time interval and the values were evaluated with 1 s step for all varieties. Results are depicted in Figure 3. 

Figure 3: Number of initialized handovers in function of HDT duration.

3: Fast Base Station Switching

The fast base station handoff is also a form of soft handoff which is supported by both the BS and the MS. The MS maintains a list of BSs referred to as the active set and then continuously monitors it. As in the case of MDHO, the MS may perform ranging and maintain a valid CID with the BSs of the active set. Nevertheless, the MS is allowed to communicate with only one BS called the anchor BS as depicted in Figure 4. In fact, the MS is registered and synchronized with the anchor BS; both entities exchange UL and DL traffic including management messages. The anchor BS may be changed from frame to frame with respect to the BS selection scheme. In that case, the connection is switched to the new anchor BS without performing explicit handoff signaling as the MS simply reports the ID of the newly selected BS on the CQICH. Note that every frame can be sent via a different BS belonging to the active set.

Figure 4: Fast base station switching.

The anchor BS may be updated by implementing two mechanisms: the handover MAC management method and the fast anchor BS selection mechanism. The first updating mechanism is based on the exchange of five types of MAC management messages while the second updating mechanism transmit anchor BS selection information on the fast feedback channel. The new anchor BS should belong to the current diversity set; its selection is based on the signal strength reported by the MS. Adding BSs to the diversity set and removing others is done on the basis of the comparison of their long-term CINR to H_Add_Threshold and H_Delete_Threshold. Note that FBSS and MDHO have many similarities and achieve better performance compared to HHO. Nevertheless, they are more complex as they require the BSs of the active set or the diversity set to be synchronized, use the same carrier frequency, and share the network entry information.

4: Seamless Handoff

Seamless mobility intends to achieve a seamless handoff between different data networks and access technologies. Examples of such handoff occur between WiMAX and WiFi networks, WiMAX and UMTS networks, WiFi and 3G mobile networks including CDMA 2000 and UMTS, etc. Subscribers, who are becoming more and more demanding, wish to have voice, video, and data connections anywhere and anytime and expect to roam within and between networks at low cost while preserving the initial security level and QoS guarantees. To take up such challenges, the used handsets should integrate multiple radio interfaces and support a wide range of applications while implementing power management. Such mixed-network devices must be able to automatically detect and select the best wireless network. On the other hand, networks should provide high bandwidth while implementing intelligent networking mechanisms including seamless roaming and cross-network identity and authentication. For instance, an efficient and usage-model appropriate means of establishing identity should be provided. When considering seamless mobility between WiFi and WiMAX, it is evident not to initiate handoff when the WiFi coverage is available as WiFi provides high bandwidth and achieves good performance. The received signal strength level, provided by the physical layer and the bandwidth of network layer, may be the guiding parameters that determine when to initialize handover and how to choose the best WiFi access point. Link-layer triggers should be defined to help IP handoff preparation and execution while cross-layer information exchange should fasten the handoff process. To guarantee an interrupted user connection during handoff between different networks, IEEE proposes a new standard referred to as IEEE 802.21.

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