Mobility Management in IEEE 802.16m
The IEEE 802.16m amendment distinguishes between four types of handover:
- Serving R1 BS to target R1 BS;
- Serving ABS to target R1 BS;
- Serving R1 BS to target ABS; and
- Serving ABS to target ABS.
The first type is performed as per the IEEE 802.16-2009 standard. The amendment details description for types 2–4, in addition to inter-RAT handovers.
ABS to ABS Handovers
Similar to how legacy BS operate in IEEE 802.16-2009, an AMS acquires network topology either through periodic advertisements from the ABS or through scanning. An ABS advertisement contains information for neighboring ABSs and R1 BSs, but not neighboring CSG femtocells. A serving ABS may also unicast neighbor advertisement messages. In .16m, an AMS need not be assigned specific allocations by the serving ABS to perform scanning, and need not interrupt its communications with the ABS if such capability is supported. An AMS can prioritize the neighboring BSs to be scanned based on various metrics. Upon reporting these measurements to the network, either the AMS or the network can select a target BS to handover with. Conditions and rules for sending the AMS report are set by the ABS.
A handover can either be initiated by an AMS or commanded by an ABS, and either initiation or the command can include more than one Target-ABS (target ABS). If the ABS's command message contains only one target ABS, the AMS must adhere to this selection. An AMS's handover indication to the ABS results in stopping the serving ABS's downlink data and cancelling uplink allocations. If the command messages include more than one target ABS, the AMS would indicate its selection to the ABS. The serving ABS would define conditions with which the AMS would consider the target ABS(s) unreachable. If all recommended ABSs are unreachable, the AMS would select a new ABS and indicate this choice to serving ABS.
There are three distinct phases to a handover procedure in the IEEE 802.16m amendment: initiation, preparation and execution. The amendment also defines procedures for handover cancellation.
Either the AMS or the ABS can initiate a handover. Handover conditions and triggers are defined by the serving ABS. An AMS's handover request begins the handover initiation, while an ABS's handover command begins both the initiation and the preparation phases.
The handover preparation phase is completed by the selection of a single target ABS. For example, a serving ABS command with a single target ABS completes the preparation phase. If the serving ABS's command includes more than one target ABS, the AMS's indication of the target ABS to the serving ABS completes the preparation phase. Preparation involves communication between the serving ABS and target ABS through the backbone to transfer context and to optimize the handover. A handover command signaling indicates which context information is transferred to the target ABS, in addition to information on the disconnect time with the serving ABS and the multiplexing schemes utilized if the AMS is to maintain simultaneous connections with the serving ABS and the target ABS during the handover procedure.
Handover execution starts at the time specified in the serving ABS command message. At that time, the AMS begins network re-entry procedures at the target ABS. If simultaneous communication is not supported, the serving ABS will stop downlink allocations at the disconnect time. Otherwise, the AMS stops communicating with the serving ABS once the network re-entry completes.
A handover can be cancelled at any phase during handover procedures. Cancellation would return both the serving ABS and the AMS to normal operations. Conditions for handover cancellation can be advertised by the network.
Network re-entry in IEEE 802.16m follows that of IEEE 802.16-2009. If a dedicated ranging code and/or a dedicated ranging channel are provided by the target ABS in the handover preparation phase, the AMS should utilize such setup during re-entry. CDMA-based handover ranging can be omitted when the AMS performs the handover to the target ABS.
Mixed Handover Types
Mixed handovers refers to when a handover takes place between a serving R1 BS and a target ABS, or from a serving ABS to a target R1 BS. In mixed these settings, network topology is acquired as follows. An R1 BS advertises the system information to its neighboring R1 BSs (per the IEEE 802.16-2009) and the LZones of its neighboring ABSs. An ABS advertises the system information for its neighboring R1 BSs in boths its MZones and LZones, the LZones system information for its neighboring ABSs in its LZone, and system information for its neighboring ABSs in its MZones. The ABS may indicate its Advanced capability through its LZone.
For a serving R1 BS to target ABS handover, the IEEE 802.16m amendment indicates that it will be possible for an R1 MS to handover to a target ABS's LZone using IEEE 802.16-2009 signaling and procedures. An AMS may also seek the same handover procedure as an R1 MS and switch zones (LZone to MZone) after handover. If the AMS is able to directly scan the Advanced-only target ABS's or the target ABS's MZone, it can perform such handover as well. At the moment, the IEEE 802.16m amendment does not detail how these handovers will be realized.
For a serving ABS to target R1 BS handover, R1 MS will proceed as if undergoing a IEEE 802.16-2009 handover. An AMS, however, would follow signaling and procedures of the Advanced system, but would perform network re-entry as per the IEEE 802.16-2009 procedures. A serving ABS would oversee the necessary mappings required between the Advanced and the IEEE 802.16-2009 for the context transfer.
Inter-RAT Handovers
An IEEE 802.16m network advertises information about other RATs through either solicitation or broadcast. The network acquires such information through a certain information server. Boundary information can also be broadcast by the IEEE 802.16m system through network boundary indication. An AMS, upon receiving this information, can perform measurements on the respective interfaces.
The IEEE 802.16m amendment discusses the possibility of generic handovers to variety other technologies such as 802.11, 3GPP and 3GPP2, but presumes that signaling details will be covered elsewhere, for example, the IEEE standard for Media Independent Handovers (802.21). The IEEE 802.16m amendment also discusses the possibility of enhanced inter-RAT handover procedures whereby the use of single or dual interfaces can be utilized.
Handovers in Relay, Femtocells and Multicarrier IEEE 802.16m Networks
For IEEE 802.16m with relay support, the ABS shall oversee the AMS handover procedures including scanning network topology advertisement. An ARS would only relay MAC control signaling between the AMS and the ABS. If the same AMS context is utilized between the ABS and its subordinate, no context transfer is necessary.
For systems with Femtocells, macrocell-Femtocell as well as Femtocell-Femtocell handovers are supported. A Femtocell going out of service due to network management instructions or by accident must initiate handover for its subordinate MSs to other macro or Femtocells. An MS should be able to prioritize the choice of accessible macro and Femtocells.
Handovers from macrocells to Femtocells would not be allowed to CSG-Femtocells if the MS is not a member, and to OSG unless it is critical for the MS's operation. In both cases, handovers are allowed in instances of emergency. Femtocell information can be advertised by the network and may be cached by the MS for future handovers. Information about CSG Femtocells are not broadcast, but either unicast or multicast to its members during handover preparation to a target Femtocell. Triggers and conditions for macro to Femtocell handovers are dictated by the network.
Meanwhile, for Femtocell to macrocell or other Femtocell BSs, network topology information can be either unicast or multicast depending on target BS accessibility to the AMS. Such handovers would proceed as a regular handover described above. Upon a successful Femtocell to macrocell handover, either the MS or network can cache handover information in case required for a reverse direction handover.
Under multicarrier operation, handover procedures are as described in Section 7.3.1. An ABS may broadcast/multicast/unicast its neighbors' multicarrier information to its subordinate AMSs. Management messages, however, are exchanged over the AMS's primary carrier. Network re-entry with the target ABS is performed on an assigned fully configured carrier at action time while fully communicating with the serving ABS. All primary and secondary carrier communication with the serving ABS cease once an AMS's network re-entry completes at the target ABS. An AMS capable of processing multiple carriers at the same time can seek a different primary carrier than the one specified in the serving ABS handover command. Once a handover is complete, network re-entry is performed through a target primary carrier. Once re-entry completes, the AMS may proceed to communicate over its primary and/or secondary carriers. Regardless of an AMSs multicarrier support, it may perform scanning and HO signaling with neighboring ABSs over multiple radio carriers while maintain normal operation with the serving ABS if the AMS is capable of concurrently processing multiple radio carriers.