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Handover and RRM procedures

2.2 Overview of the WiMAX Forum Access Network Architecture

2.2.2 Handover and RRM procedures

based on which the MS will decide where it will handover to.

The MS will send a MOB HO-IND message to indicate its final decision which initiates the action phase. The selected TBS will be informed of the incoming MS with a HO conf message with a ”confirm” value, so that the TBS has time to com- plete datapath registration and other required tasks. Also the context information of the MS can be exchanged between the SBS and the selected TBS to enable even faster handover execution during network re-entry procedures. HO conf message with value ”rejected” will be sent to the other candidate TBSs so that they can release the reserved resources.

Figure 2.9: BS initiated handover [ASN2].

The BS initiated directed handover preparation phase is preceded by a procedure where load information of all the BSs is exchanged in the form of spare capacity reports. There reports are used to determine when the BS is overloaded and trigger load balancing with directed handovers. If a BS is overloaded and the MSs that reside in overlapping areas are known, the directed handover procedure for a single MS can be initiated by the BS by sending HO req messages to the other less con- gested TBSs that cover the overlapping area. From there on the preparation phase and the action phase of the handover continue as in the MS initiated case. Framework for load balancing

What kind of a framework does Mobile WiMAX offer for load balancing? The spec- ification defines many RRM messages that are used in the communication between the RRCs and the RRAs. The core load balancing tool in the ASN is the Spare

Capacity Reporting procedure. This procedure can be used to keep all the Base Stations in the ASN up to date of the resource usage of their peers. A procedure to report physical RRM parameters is also provided and can be used as supplementary information for making the load balancing decisions.

Spare Capacity Report (SCR)

The spare capacity reporting procedure, preceding the BS initiated directed han- dover, has many similar qualities as the scanning procedure conducted in the MS initiated rescue handover. In both cases a handover is conducted as a result of a lack in resources, but where signal quality and strength is compared in the rescue handover, resource utilization is compared in a directed handover.

The Per-BS Spare Capacity Report (SCR) can be sent by request or configured to be sent periodically after passing a threshold in resource usage. Spare capacity is described with the Available Radio Resources-indicator. It describes the average number of available slots per frame. The value is averaged over a predefined inter- val11 and given in percentages.

The spare capacity is reported for both UL and DL and is defined as the set of free slots not used by any non Best Effort service flow class. The reason for omit- ting the resource usage of BE service flows apparently lies behind the idea that MSs with only BE services should conduct and trigger load balancing themselves. The SCR also includes a Radio Resource Fluctuation field that describes traffic fluctua- tion. The field describes the degree of fluctuation in channel data traffic throughputs for the Base Station and is dependent on the variability of the served traffic and channel fluctuations. This value could be used to eliminate unnecessary load bal- ancing handovers. Based on this information the BS can make the decision whether to trigger load balancing.

Physical Parameters Report

The Physical Parameters Report is conducted per MS and is done by request. The report includes the Carrier to Interference and Noise Ratio (CINR) and Received Signal Strength Indicator (RSSI) for both UL and DL for the given MS. It also includes the Physical Service Levels (PSL) for both UL and DL which describe the channel rate available for the MS. The PSL value corresponds to the MCS that can be used with the MS under the channel conditions. This information can be utilized when choosing the optimal TBS from the set of candidate TBSs and could also possibly be used in the process where the SBS tries to find out which MSs are in the overlapping areas.

11The interval is 200 frames by default, which corresponds to one second if frame length is 5 ms.

Spare Capacity Report per QoS profile

The specification defines also a way to report Spare Capacity per QoS profile. The basic idea is that a TBS could pre-calculate the number of MSs, with a specific QoS profile and PSL level, it could admit. The calculations would be based on the current channel and traffic conditions. This information could enhance the accuracy of the load balancing decisions.

However in release 1 of the WiMAX Forum Network Architecture, Spare Capac- ity report per QoS profile will not be used directly for load balancing12. In release 1 it can only be used in the handover preparation phase in the HO req and HO rsp messages. In other words, in the case of load balancing with directed handovers, it could be utilized to decide how many handovers should be conducted to each lightly loaded TBS. Resource reservation for handover connections

Be it an MS initiated rescue or a BS initiated directed handover, after a decision to do a handover has been made, resources have to be reserved from the candidate Target Base Stations.

The resource reservation procedure in the TBS is controlled with the HO req and HO rsp messages mentioned above. When sending the HO req message to the TBS, the SBS specifies the QoS parameters guaranteed for the service flows of the MS.

After receiving the HO req, the admission control function of the TBS will calculate whether it has enough resources to admit the connection and if not it will indicate failure due to insufficient resources. The TBS may also suggest a lower QoS profile in the HO rsp message.

If no prioritization of handover calls is done, the same admission control procedure used for new service flows, will be used for the handover service flows. As stated earlier in the case, where none of the candidate TBSs have sufficient resources, this will result in a handover drop. Prioritization of handover calls could be implemented by keeping a guard band for the rescue handovers coming into the TBS. It would mean that if resource reservation13, would go over this guard band limit, admission control would start to block new calls but would still accept rescue handovers.

12It will be supported in later releases.

13The minimum resources reserved for the service flows.

Background Research

Having gone through the key system aspects of IEEE 802.16e and WiMAX Forum network architecture, we can start considering how the actual load balancing with handovers could be conducted. Also we can start to examine the handover and traffic prioritization aspect in a more concrete way to get an understanding of its role.

A good place for us to start is to see what kind of prior research has been conducted regarding these issues and to examine how the existing ideas could be applied to Mobile WiMAX.

3.1 Load balancing with handovers

Load balancing with handovers will be the way system wide load balancing will be conducted in Mobile WiMAX. In this section we will first classify the different ways load balancing can be conducted in telecommunications systems, see what kind of a relationship Mobile WiMAX has with them and finally dig into the theory behind load balancing from the point of view of Mobile WiMAX.

Secondly, having gained a good understanding of the balancing method used, we will do a literary review of the previous research conducted and do some initial speculations of their feasibility to be applied in Mobile WiMAX.