Thursday, 29 October 2015

FDD-FL-Tracking Area Planning

1、Introduction
 
      The LTE specification allows for different cells within one eNodeB to belong to different TAs. In the current implementation, all cells within one eNodeB must be allocated to the same TA. 


2、Technical Background
2.1、Tracking Areas
      A Tracking Area corresponds to the Routing Area (RA) used in Wideband Code Division Multiple Access (WCDMA) . The TA consists of a cluster of eNBs having the same Tracking Area Code (TAC). The TA provides a way to track UE location in idle mode.TA information is used by the MME when paging idle UE to notify them of incoming data connections.
 
2.2、Tracking Area Lists
      In LTE, the MME provides the UE with a list of tracking areas where the UE registration is valid. When the MME pages a UE, a paging message is sent to all or a subset of the eNBs in the TA list. The concept of TA lists is shown in the following figure:
Figure 1 Concept of TA Lists
      The MME sends the TA list to the UE during the TA update procedure. TA updates occur periodically, and when a UE enters a cell with a TAC not in the current TA list. The TA list makes it possible to avoid frequent TA updates due
to ping-pong effects along TA borders.
There are three options for how the SGSN-MME creates a TAI list:
• TA lists only containing the current TA.
• TA lists containing the current TA and the last visited TA.
• TA lists containing the current TA, last visited TA and a operator specified list of up to 14 TAs.
 
2.3、Tracking Area Dimensioning
     TA dimensioning is the process of finding a suitable number of eNBs to include in a TA list.
     A small number of eNBs in a TA list may require frequent TA updates. Frequent updates increases the UE battery consumption. In addition, frequent updates may reduce the paging success rate or delay the paging response, because the UE cannot respond to paging during the TA update procedure.
     By increasing the number of eNBs in the TA list, the TA update frequency is reduced. The drawback of adding more eNBs to the TA list is that the paging load increases. The upper limit of the number of eNBs in a TA list is determined by the paging capacities of the MME and eNB.
 
2.4、Tracking Area Planning
     TA planning is the task of configuring TAs and TA lists so that areas with excessive TA update signalling are avoided. 
 
3、Tracking Area Planning
When planning borders between TAs and TA lists, the following two general rules apply:
       • TAs and TA lists should be planned so that areas with frequent TA update signalling are located in low traffic areas. This makes it easier for the eNB to cope with the additional signalling caused by the TA update procedure.
       • TAs and TA lists should be planned so that the need for TA updates is minimized. This is accomplished by considering how users travel within the network. Busy roads, railways, and so on, should cross as few TA list borders as possible.
3.1、TA Lists with Operator Specified List of TAs
       16 TAs per TA list is supported. For every TA, the operator can specify a list of up to 14 TAs to include and the MME always adds the current and old TA to the list to reduce the risk of ping-pong updates.
       In case TA lists with multiple TAs are employed, the average number of eNBs per TA should not exceed:
N eNB,TA = N eNB,TAlist /N TA,TAlist
Equation 1 Number of eNBs per TA
       N eNB,TAlist  is the number of eNBs to include per TA list, calculated in the dimensioning process.
       N TA,TAlist   is the number of TAs in the TA list. When TA lists with multiple TAs are used, the minimum value to use for N TA,TAlist   is equal to 2 . This is due to the fact that the MME always adds the current TA as well as the old TA to the list.
       Besides reducing the risk for ping-pong updates, TA lists with multiple TAs can help resolve the issue of a few cells having to handle a high number of TA updates.
       This is done by including many TAs in the TA list and defining the TA lists in a sliding window, as shown in the following figure:
Figure 2 TA Lists Planned in Sliding Window
       With this strategy, the TA update load is distributed over a large number of cells.
       The operator can determine the number of TAs to include per TA list, taking into account the characteristics of the cell plan. The larger the number the more even is the distribution of TA update signalling. One drawback with a larger TA list is that it requires more planning and administration.
 
3.2、TA Lists without Operator Specified List of TAs
        With this strategy the TA list will contain only the current TA or the current TA together with the last visited TA. There can be fewer TAs, which can contain more eNBs and large parts of existing RA boundaries in WCDMA/GSM can be re-used. This will decrease the work with planning and administration compared to the method using operator specified list of TAs.
        It is recommended to configure the MME to create a TA list consisting of both current TA and last visited TA. By this ping-pong updates will be reduced at the cost of doubling the number of TAs.
Figure 3 All eNBs Allocated to Same TA

Budi Prasetyo

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