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.

Mobility, Handover and Power-Save Modes

Handover Considerations

One of the major goals of the 802.16e amendment is to introduce mobility in WiMAX. Consequently, mobile WiMAX profiles are based on 802.16e. Mobility is based on handover. Handover operation (sometimes also known as ‘handoff’) is the fact that a mobile user goes from one cell to another without interruption of the ongoing session (whether a phone call. data session or other). The handover can be due to mobile subscriber moves, to radio channel condition changes or to cell capacity considerations. Handover is a mandatory feature of a cellular network. The handover (HO) is described as defined in 802.16e.

In 802.16e, the two known generic types of handover are defined:

§  Hard handover, also known as break-before-make. The subscriber mobile station (MS) stops its radio link with the first BS before establishing its radio link with the new BS. This is a rather simple handover.

§  Soft handover, also known as make-before-break. The MS establishes its radio link with a new BS before stopping its radio link with the first BS. The MS may have two or more links with two or more BSs, which gives the soft handover state. The soft handover is evidently faster than the hard handover.

Two types of soft handover are then defined in 802.16e:

§  Fast BS Switching (FBSS). This is a state where the MS may rapidly switch from one BS to another. The switch is fast because the MS makes it without realising the complete network entry procedure with regard to the new BS.

§  Macro Diversity HandOver (MDHO). Transmissions are between the MS and more than one BS.

In the mobile WiMAX profiles, only the hard handover is mandatory. The FBSS and MDHO are optional. The 802.16 standard also indicates that the support of the MDHO or FBSS is optional for both the MS and the BS.

Handover has challenging objectives. First, it has to be fast enough, of the order of 50 ms or 150ms. There are also security requirements, as some attacks are possible at the occasion of the handover procedure. Finally, the handover does not have only Layer 2 considerations. Layer 3 considerations are also needed. Hence, the handover is not independent of the architecture.