Fast BS Switching (FBSS) and Macro Diversity Handover (MDHO)

1: Diversity Set

There are several conditions that are required to the diversity BSs featured in FBSS and MDHO procedures. These conditions are listed below :

§  The BSs are synchronised based on a common time source and have synchronised frames.

§  The frames sent by the BSs from the diversity set arrive at the MS within the prefix interval, i.e. transmission delay

§  The BSs operate at same frequency channel.

§  The BSs are required to share or transfer MAC context. Such context includes all information MS and BS normally exchange during Network Entry, particularly the authentication state, so that an MS authenticated/registered with one of the BSs from the diversity set BSs is automatically authenticated/registered with other BSs from the same diversity set. The context also includes a set of service flows and corresponding mapping to connections associated with the MS, current authentication and encryption keys associated with the connections. There are also BS conditions specific to MDHO (see below).

An MS may scan the neighbour BSs and then select BSs that are suitable to be included in the diversity set. The MS reports the selected BSs and the diversity set update procedure is performed by the BS and the MS. After an MS or BS has initiated a diversity set update using MOB_MSHO/BSHO-REQ, the MS may cancel the diversity set update at any time. This cancellation is made through transmission of an MOB_HO-IND with proper parameters. The BS may reconfigure the diversity set list and retransmit the MOB_BSHO-RSP message to the MS.

In an MS diversity set, a member identifier, TEMP_BSID, is assigned to each BS in the diversity set.

2: Different Types of BS for a Given MS

Before getting into the details of make-before-break handover algorithms, FBSS and MDHO, the different types of BS for a given MS are summarized:

§  Serving BS. The serving BS is the BS with which the MS has most recently completed registration at the initial Network Entry or during a handover.

§  Neighbour BS. A neighbour BS is a BS (other than the serving BS) whose downlink transmission can be (relatively well) received by the MS.

§  Target BS. This is the BS that an MS intends to be registered with at the end of a handover.

§  Active BS. An active BS is informed of the MS capabilities, security parameters, service flows and full MAC context information. For a Macro Diversity HandOver (MDHO), the MS transmits/receives data to/from all active BSs in the diversity set.

§  Anchor BS. For MDHO or FBSS supporting MSs, this is a BS where the MS is registered, synchronised, performs ranging and monitors the downlink for control information (see Figure 1). For an FBSS supporting MS, this is the serving BS that is designated to transmit/receive data to/from the MS at a given frame. Hence, it can be verified that an anchor BS is a specific case of a serving BS. An MS is required continuously to monitor the signal strength of the BSs that are included in the diversity set. The MS selects one BS from its current diversity set to be the anchor BS and reports the selected anchor BS on the CQICH or MOB_MSHO-REQ message. The MSs and BSs may use the fast-feedback method to update the diversity set: when the MS has more than one BS in its diversity set, the MS transmits fast anchor BS selection information to the current anchor BS using the OFDMA fast-feedback channel.

Figure 1: Illustration of an anchor BS in a diversity set

3: FBSS (Fast BS Switching)

An FBSS handover begins with a decision for an MS to receive/transmit data from/to the anchor BS that may change within the diversity set. An FBSS handover can be triggered by either MOB_MSHO-REQ or MOB_BSHO-REQ messages.

When operating in FBSS, the MS only communicates with the anchor BS for uplink and downlink messages (management and traffic connections). The MS and BS maintain a list of BSs that are involved in FBSS with the MS. This is the FBSS diversity set. The MS scans the neighbour BSs and selects those that are suitable to be included in the diversity set. Among the BSs in the diversity set, an anchor BS is defined. An FBSS handover is a decision by an MS to receive or transmit data from a new anchor BS within the diversity set.

The MS continuously monitors the signal strength of the BSs of the diversity set and selects one of these BSs to be the anchor BS. Transition from one anchor BS to another, i.e. BS switching, is performed without exchange of explicit handover signalling messages. An important requirement of FBSS is that the data are simultaneously transmitted to all members of a diversity set of BSs that are able to serve the MS.

The FBSS supporting BSs broadcast the DCD message including the H_Add Threshold and H_Delete Threshold. These thresholds may be used by the FBSS-capable MS to determine if MOB_MSHO-REQ should be sent to request switching to another anchor BS or changing diversity set.

4: MDHO (Macro Diversity Handover)

An MDHO begins with a decision for an MS to transmit to and receive from multiple BSs at the same time. An MDHO can start with either MOB_MSHO-REQ or MOB_BSHO-REQ messages. When operating in an MDHO, the MS communicates with all BSs in the diversity set for uplink and downlink unicast traffic messages (see Figure 2). The use of this transmission diversity is not the same in the two different communications:

§  For a downlink MDHO two or more BSs provide synchronised transmission of MS downlink data such that diversity combining can be performed by the MS.

§  For an uplink MDHO, the transmission from an MS is received by multiple BSs such that selection diversity of the information received by multiple BSs can be performed.

Figure 2: Illustration of an MDHO operation mode

The BSs involved in an MDHO or equivalently a member of an MS MDHO diversity set must use the same set of CIDs for the connections that have been established with the MS. The same MAC/PHY PDUs should be sent by all the BSs involved in the MDHO to the MS.

The decision to update the diversity set and the process of anchor BS update begin with notifications by the MS (through the MOB_MSHOREQ message) or by the BS (through the MOB_BSHO-REQ message).

The Handover Process | WiMAX

The 802.16 standard states that the handover decision algorithm is beyond its scope. The WiMAX Forum documents do not select a handover algorithm either. Only the framework is defined. The MS, using its current information on the neighbour BS or after a request to obtain such information, evaluates its interest in a potential handover with a target BS. Once the handover decision is taken by either the serving BS or the MS, a notification is sent over the MOB_BSHO-REQ (BS Handover REQuest) or the MOB_MSHO-REQ (MS Handover REQuest) MAC management messages, depending on the handover decision maker: the BS or MS. The handover process steps are described in the following.

The handover process is made of five stages which are summarized in Figure 1. The HO process stages are described in the following sections

Figure 1: Illustration of handover process stages.

1: Cell Reselection

Cell reselection refers to the process of an MS scanning and/or association with one or more BS in order to determine their suitability, along with other performance considerations, as a handover target. The MS may use neighbour BS information acquired from a decoded MOB_NBR-ADV message or may make a request to schedule scanning intervals or sleep intervals to scan, and possibly range, the neighbour BS for the purpose of evaluating the MS interest in the handover to a potential target BS.

2: Handover Decision and Initiation

A handover begins with a decision for an MS to make a handover from a serving BS to a target BS. The decision may originate either at the MS or the serving BS. The handover decision results in a notification of MS intent to make a handover thruugh the MOB_MSHO-REQ (MS HO REQuest) message (handover decision by the MS) or the MOB_BSHO-REQ (BS HO REQuest) message (handover decision by the BS).

The BS may transmit a MOB_BSHO-REQ message when it wants to initiate a handover. This request may be recommended or mandatory. In the case where it is mandatory, at least one recommended BS must be present in the MOB_BSHO-REQ message. If mandatory, the MS responds with the MOB_HO-IND message, indicating commitment to the handover unless the MS is unable to make the handover to any of the recommended BSs in the MOB_ BSHO-REQ message, in which case the MS may respond with the MOB_HO-IND message with proper parameters indicating HO reject. An MS receiving the MOB_BSHO-REQ messsage may scan recommended neighbour BSs in this message.

In the case of an MS initiated handover, the BS transmits an MOB_BSHO-RSP message upon reception of the MOB_MSHO-REQ message.

3: Synchronisation to a Target BS Downlink

Synchronisation to a target BS downlink must be done. If the MS had previously received a MOB_NBR-ADV (MAC management) message including a target BSID, physical frequency, DCD and UCD, this process may be shortened. If the target BS had previously received hanndover notification from a serving BS over the backbone, then the target BS may allocate a non-contention-based initial ranging opportunity.

4: Ranging and Network Re-entry

The MS and the target BS must conduct handover ranging. Network re-entry proceeds from the initial ranging step in the Network Entry process: negotiate basic capabilities, PKM authentication phase, TEK establishment phase, registration (the BS may send an unsolicited REG-RSP message with updated capabilities information or skip the REG-RSP message when there is no TLV information to be updated) and the other following Network Entry optional steps (lP connectivity, etc.).

Network re-entry may be shortened by target BS possession of MS information obtained from the serving BS over the backbone network. Depending on the amount of that information, the target BS may decide to skip one or several of the Network Entry steps (Figure 2). Handover ranging can then be a simplified version of initial ranging. To notify an MS seeking handover of possible omission of re-entry process management messages during the current handover attempt (due to the availability of MS service and operational context information obtained over the backbone network), the target BS must place, in the RNG-RSP message, an HO Process Optimisation TLV indicating which re-entry management messages may be omitted. The MS completes the processing of all indicated messages before entering Normal Operation with the target BS.

Figure 2: Summary of network re-entry steps

Regardless of having received MS information from a serving BS, the target BS may request MS information from the backbone network.

5: Termination of MS Context

This is the final step of a handover. Termination of the MS context is defined as the serving BS termination of the context of all connections belonging to the MS and the discarding of the context associated with them, i.e. information in queues, ARQ state machine, counters. timers, header suppression information. etc. This is accomplished by sending the MOB__HO-IND message with the HO_IND_type value indicating a serving BS release.

6: Handover Cancellation

An MS may cancel HO at any time prior to expiration of the Resource_Retain_Time interval after transmission of the MOB_HO-IND message. Resource_Retain_Time is one of the parameters exchanged during the registration procedure (part of Network Entry). The standard indicates that Resource_Retain_Time is a multiple of 100 milliseconds and that 200 milliseconds is recommended as default.

Network Topology Acquisition

1 Network Topology Advertisement

A BS broadcasts information about the network topology using the MOB_NBR-ADV (Neighbour ADVertisement) MAC management message. 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.

2 MS Scanning of Neighbour BSs

A scanning interval is defined as the time during which the MS scans for an available BS. 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.

3 Association Procedure

Association is an optional initial ranging procedure occurring during the scanning interval with respect to one of the neighbour BSs. 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.

4 CDMA Handover Ranging and Automatic Adjustment

For OFDMA PHY, 802.16e defines the handover ranging^]. 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.