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.