CELL SELECTION

Cell selection/resection enables a correct network topology acquisition and guides the handover process in selecting the best target BS.

1:  Neighbor Advertisement from Serving Base Station

Each BS in the network should broadcast information describing the network topology via MOB_NBR-ADV messages. As stated earlier, these messages carry channel information for neighboring BSs normally provided by each BS’s own DCD/UCD messages. The serving BS may obtain such neighbors-related information over the backbone before broadcasting it to the managed MSs. Thanks to MOB_NBR-ADV messages, MSs are able to synchronize with the neighboring BSs without being obliged to monitor transmission for individual DCD/UCD broadcasts. The standard specifications fix the maximum period of sending the MOB_NBR-ADV message to 1 s so that an MS moving at high speed through the coverage area of each BS may get the message and perform handoff. It is valuable to note that the period of sending the MOB_NBR-ADV message determines the maximum speed, with which an MS is allowed to move through the network; therefore, it must be carefully chosen. Optimizing the handoff process requires a selection of the most suitable target BS that fits mobility path and application needs. To achieve that goal, MSs have to scan multiple channels to discover neighboring BSs and then select the best target. That selection may be based on different parameters such as the measured signal strength, the packet delay, the error ratio, the throughput, and the security level . Cell reselection is achieved when the MS scans and/or associates with more than one BS to evaluate their suitability as handover target. The MS may integrate information obtained from a MOB_NBR_ADV message to give insight into available neighboring BSs for cell reselection. Note that the serving BS may allocate scanning intervals or sleep intervals for the cell reselection activity. However, such activity does not require terminating the ongoing connection with the serving BS.

2:  Periodic Intervals for Scanning Neighbor

Usually, the serving BS allocates time intervals known as scanning intervals to the MSs. Unfortunately, channel scanning can be a relatively time-consuming activity; therefore, MSs should process it and obtain the neighboring BSs list before performing handoff. The duration and frequency of scanning should be carefully determined to interleave scanning period and normal operations without affecting the network performances and the provided QoS. It is clear that a long scanning period increases the packets jitter and the end-to-end delay while imposing large buffer sizes. 

Contrarily, a short scanning period requires multiple iterations and increases the overall scanning duration. The scanning procedure depicted in Figure 1 begins when an MS sends a MOB-SCN_REQ message to its serving BS to request the allocation of a group of scanning intervals while indicating the estimated duration of time required for the scan. The serving BS replies by a MOB-SCN_RSP message denying the request or stating the scanning interval duration that should be at least as long as requested by the MS. If no MOB-SCN_RSP message is received within a timer, the MS may retransmit the MOB-SCN_REQ message. The serving BS may also send an unsolicited MOB-SCN_RSP message with a value of zero associated with the scan duration to trigger the MS to report scanning result. 

Upon the receipt of a positive MOB-SCN_RSP message, the MS may begin scanning for one or more BSs during the time interval stated in that message. The MS may attempt to synchronize with the DL transmission of the scanned BS and estimate the quality of the PHY channel. IEEE 802.16e specifies a default scanning strategy requiring that each MS keeps a nonvolatile storage where it saves the last set of operational parameters. When the MS intends to acquire a DL channel, it should use its stored information. 

However, if that MS fails to obtain the DL channel, it will continuously scan the possible channels of the DL frequencies until it finds a DL signal. IEEE 802.16e specifications support temporarily suspending the communication between the BS and the MS during the scanning period. The exchange of MOB-SCN_REQ and MOB-SCN_RSP messages enables each entity to buffer packets while the normal communication is temporarily suspended. An MS may end scanning and return to the normal operation mode anytime during any scanning interval; this is achieved by sending a MAC PDU message such as a bandwidth request to the target BS. At the end of scanning, the MS should report the scan status to its serving BS via a MOB-SCN_REP message.

Figure 1: IEEE 802.16e scanning operation.

Network-Initiated Handoff | WiMAX HANDOFF CONTROL

The network can initiate handover depending on its current status. Such a decision can be made after the evaluation of the payload of different BSs or the data throughput at the reference points. In profile A, ASN GWs may initiate handover of MSs under their control. The network-initiated handoff procedure depicted by Figure 1 begins by a prehandover operation, during which the serving ASN GW collects status information from the BSs and the MSs to decide whether a network-initiated handover is required. If it is the case, the ASN GW sends a HO_Directive message to the serving BS to order it to handoff some MSs to other BSs while providing it with a list of recommended BSs and starting a timer. 

The ASN GW may also specify how many payload should be migrated to other BSs to achieve load balancing as it may indicate the list of the recommended MSs that need to be handed over. The serving BS should respond by a HO_Directive_Rsp message to make the ASN GW stop the timer. The serving BS selects some candidate MSs based on the information maintained by it and the list given by the HO_Directive message and then it may order some candidates to achieve scanning to get their neighbors’ information. The serving BS will then select some suitable MSs for handover and send separately a HO_Req message relative to each MS to the serving ASN-GW. The following procedure is the same as the process of MS-initiated handoff described earlier. When the process of handover preparation is finalized by the network, the serving BS will send a MOB-BSHO_REQ message to each MS to order it to hand over to the target BS.

Figure 1: The preparation phase of a network-initiated handoff.

Base Station Initiated Handoff | WiMAX HANDOFF CONTROL

The serving BS may decide to no longer manage an MS and initiate handoff for it. This occurs generally when the serving BS can no longer provide the required QoS or when it detects that the MS is moving out of its coverage area. Although the causes of a BS-initiated handoff are similar to the causes of an MS-initiated handoff, it is useful to let the BS decide to centralize the handoff procedure. In fact, the MSs are generally tiny equipments with limited power and computing resources; therefore, it is important to implement the handoff process at the BS level. 

The serving BS continues broadcasting the MOB_NBR-ADV message for the served MSs, but it orders the MS that needs to perform handoff via a MOB_BSHO-REQ message to start scanning the neighboring BSs. The MOB_BSHO-REQ message transmitted on the basic connection defines a list of recommended target BSs along with service level predictions and channel details. Upon receiving that message, the BS starts the scanning procedure and sends back a MOB_BSHO-RSP message to the serving BS indicating a list of recommended BSs. 

The rest of the handoff process is similar to the MS-initiated handoff case. In fact, the MS waits for the list of the target BSs and then sends a HO-IND message to its serving BS. Upon receiving the fast ranging IE, the MS sends the RNG-REQ ranging request message to the target BS to register with it. The BS-initiated handoff process, described earlier, is depicted by the flow chart in Figure 1.

Figure 1: The BS initiated handoff at the MS level.