14.2 Network Topology Acquisition

14.2 Network Topology Acquisition

14.2.1 Network Topology Advertisement

A BS broadcasts information about the network topology using the MOB_NBR-ADV (Neighbour ADVertisement) MAC management message [2]. This message provides channel information about neighbouring BSs normally provided by each BS's own DCD/UCD message transmissions. The MOB_NBR-ADV does not contain all the information of neighbouring BSs, UCD and DCD. The standard indicates that a BS may obtain that information over the backbone and that availability of this information facilitates MS synchronisation with neighbouring BS by removing the need to monitor transmission from the neighbouring (handover target) BS for DCD/UCD broadcasts. The BSs will keep mapping tables of neighbour BS MAC addresses and neighbour BS indexes transmitted through the MOB_NBR-ADV message, for each configuration change count, which has the same function as for the DCD message.

BSs supporting mobile functionality must be capable of transmitting a MOB_NBR-ADV MAC management message at a periodic interval to identify the network and define the characteristics of the neighbour BS to a potential MS seeking initial network entry or handover. The standard indicates that the maximum value of this period is 30 seconds.

14.2.2 MS Scanning of Neighbour BSs

A scanning interval is defined as the time during which the MS scans for an available BS [2]. A BS may allocate time intervals to the MS for the purpose of MS seeking and monitoring suitability of neighbour BSs as targets for a handover. MS scanning of neighbour BSs is based on the following MAC Management messages: MOB_SCN-REQ, SCaNning interval allocation REQuest, MOB_SCN-RSP, SCaNning interval allocation Response. MOB_SCN-REP and SCaNning result REPort.

The MOB_SCN-REQ message is sent by the MS to request a scanning interval for the purpose of seeking available BSs and determining their suitability as targets for HO. In the MOB_SCN-REQ message the MS indicates a group of neighbour BSs for which only Scanning or Scanning with Association are requested by the MS. The Neighbour_BS_Index of the MOB_SCN-REQ message corresponds to the position of BSs in the MOB_NBR-ADV message. In this message, the MS may also request the scanning allocation to perform scanning or noncontention Association ranging. Association is an optional initial ranging procedure occurring during the scanning interval with respect to one of the neighbour BSs (see the following section).

Upon reception of the MOB_SCN-REQ message, the BS responds with a MOB_SCN-RSP message. The MOB_SCN-RSP message can also be unsolicited. The MOB_SCN-RSP message either grants the requesting MS a scanning interval that is at least as long as that requested by the MS or denies the request. In the MOB_SCN-RSP message the BS indicates a group of neighbour BSs for which only Scanning or Scanning with Association are recommended by the BS.

Following reception of a MOB_SCN-RSP message granting the request, an MS may scan for one or more BSs during the time interval allocated in the message. When a BS is identified through scanning, the MS may attempt to synchronise with its downlink transmissions and estimate the quality of the PHY channel.

The BS may negotiate over the backbone with a BS Recommended for Association (in the MOB_SCN-REQ message) the allocation of unicast ranging opportunities. Then the MS will be informed on Rendez vous time to conduct Association ranging with the Recommended BS. When conducting initial ranging to a BS Recommended for Association, the MS uses an allocated unicast ranging opportunity, if available.

The serving BS may buffer incoming data addressed to the MS during the scanning interval and transmit that data after the scanning interval during any interleaving interval or after exit of the Scanning mode. When the Report mode is 0b10 (i.e. event-triggered) in the most recently received MOB_SCN-RSP, the MS scans all the BSs within the Recommended BS list of this message and then transmits a MOB_SCN-REP message to report the scanning results to its serving BS after each scanning period at the time indicated in the MOB_SCN-RSP message. The MS may transmit a MOB_SCN-REP message to report the scanning results to its serving BS at any time. The message will be transmitted on the Primary Management CID.

14.2.3 Association Procedure

Association is an optional initial ranging procedure occurring during the scanning interval with respect to one of the neighbour BSs [2]. The function of Association is to enable the MS to acquire and record ranging parameters and service availability information for the purpose of proper selection of a handover target BS and/or expediting a potential future handover to a target BS. Recorded ranging parameters of an Associated BS may be further used for setting initial ranging values in future ranging events during a handover.

Upon completion of a successful MS initial ranging of a BS, if the RNG-RSP message (sent by the BS) contains a service level prediction parameter set to 2, the MS may mark the BS as Associated in its MS local Association table of identities, recording elements of the RNG-RSP to the MS local Association table and setting an appropriate ageing timer.

There are three levels of Association as follows:

  • Association Level 0: Scan/Association without coordination. The serving BS and the MS negotiate the Association duration and intervals (via MOB_SCN-REQ). The serving BS allocates periodic intervals where the MS may range neighbouring BSs. The target BS has no knowledge of the MS. The MS uses the target BS contention-based ranging allocations.

  • Association Level 1: Association with coordination. Unilaterally or upon request of the MS (through the MOB_SCN-REQ message), the serving BS provides Association parameters to the MS and coordinates Association between the MS and neighbouring BSs. The target BS reserves a CDMA initial ranging code and an initial ranging slot (transmission opportunity) in a specified dedicated ranging region (rendezvous time). The neighbouring BS may assign the same code or transmission opportunity to more than one MS, but not both. There is no potential for collision of transmissions from different MSs.

  • Association Level 2: network assisted association reporting. The MS may request to perform Association with network assisted Association reporting by sending the MOB_SCN-REQ message, including a list of neighbouring BSs, to the serving BS with scanning type = 0b011. The serving BS may also request this type of Association unilaterally by sending the MOB_SCN-RSP message with the proper indication. The serving BS will then coordinate the Association procedure with the requested neighbouring BSs in a fashion similar to Association Level 1. With Level 2, the MS is only required to transmit the CDMA ranging code to the neighbour BSs. The MS does not wait for RNG-RSP from the neighbour BSs. Instead, the RNG-RSP information on PHY offsets is sent by each neighbour BS to the serving BS over the backbone. The serving BS may aggregate all ranging information into a single MOB_ASC_REPORT, MOB_ASC-REP, Association result REPort, message.

14.2.4 CDMA Handover Ranging and Automatic Adjustment

For OFDMA PHY, 802.16e defines the handover ranging [2]. An MS that wishes to perform handover ranging must take a process similar to that defined in the initial ranging section with the following modifications. In the CDMA handover ranging process, the CDMA handover ranging code is used instead of the initial ranging code. The code is selected from the handover ranging domain. The handover ranging codes are used for ranging with a target BS during the handover. Alternatively, if the BS is pre-notified for the upcoming handover MS, it may provide bandwidth allocation information to the MS using Fast_Ranging_IE to send an RNG-REQ message.