The 4G tracking area provides a way to track UE location in idle mode. MME uses TA information when paging idle UE to notify them of incoming data connections.
Tracking area planning is an important tasks to ensure great RF performance, for additional information regarding RF optimization tasks, please check this link.
4G 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 RBSs in the TA list. The concept of TA lists is shown in the following figure:
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. This is achieved by including the old TA in the new TA list received at TA update.
In old MME releases, the MME includes only the current TA in the TA list. More recent MME releases, the operator can specify up to 15 TAs for each TA to include in the TA list.
With the exception of the TAs specified by the operator, the MME automatically includes the old TA in the TA list to avoid ping-pong updates.
4g tracking area dimensioning
TA dimensioning is the process of finding a suitable number of RBSs to include in a TA list. A small number of RBSs in a TA list may require frequent TA updates. Frequent updates increases the MME load and UE battery consumption. In addition, frequent updates may reduce the paging success rate. As a result, the UE cannot respond to paging during the 4G tracking area update procedure.
By increasing the number of RBSs in the TA list, the TA update frequency reduces. The drawback of adding more RBSs to the TA list is that the paging load increases. The paging capacities of the MME and RBS determines the upper limit of the number of RBSs in a TA list.
4G tracking area paging Capacity
The paging capacities of the MME and RBS have an impact on TA dimensioning. This chapter describes how to estimate the MME and RBS paging capacities.
– MME Paging Capacity
MME paging capacity depends on the number of SCTP/S1 boards in the MME. For TA dimensioning, the recommendation is to not exceed 1500 outgoing pages/s per SCTP board. The MME paging capacity calculation follows this formula:
Cmme is the paging capacity of the MME for outgoing pages per second.
Nsctp is the number of SCTP boards
As an example, an MME configured with 5 SCTP boards has a paging capacity of 1500 x 5 = 7500 outgoing pages/s.
– RBS Paging Capacity
The RBS paging capacity depends partly on Central Processing Unit (CPU) constraints and also on the amount of resources that the paging traffic is allowed to consume. The more resources used for paging, the higher the paging capacity.
To calculate the RBS paging capacity use the following criteria:
Ccpu is the consumption of CPU resources due to paging traffic must be reasonably low for the CPU to handle other tasks.
Cpdschload is the consumption of PDSCH resources due to paging must be reasonably low. Paging traffic has higher priority than user data and a high paging traffic may reduce downlink capacity and achievable bit rates
Cblocking is the fraction of paging records blocked due to PDSCH limitations. Blocking introduces delays in the paging procedure and in the set-up of the data connection
Cpdcchload is the consumption of PDCCH load due to paging must be reasonable low. Paging traffic has higher priority than user data, and high paging traffic may reduce the PDCCH ability to carry other signalling traffic such as downlink scheduling assignments and uplink scheduling grants.
Paging Capacity and CPU Load
CPUs in the RBS handle the incoming pages. To ensure that paging traffic does not have an adverse effect on the ability of the CPU to handle other tasks, paging traffic must be reasonably low. For the purpose of TA dimensioning, the average paging intensity should not exceed 200 pages/s.
Paging Capacity and PDSCH Load
Paging Capacity and Blocking
Rmax is the value of maxNoOfPagingRecords which is the number of paging records that can be transmitted during a subframe is limited
Paging Capacity and PDCCH load
4G tracking area planning
Configuring TAs and TA lists helps to avoid excessive TA update signalling. 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 RBS 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.
TA Lists with Multiple TAs
For every TA, the operator can specify a list of up to 15 TAs (this can change by vendor and RAN release) to include and the MME always adds the old TA to the list to reduce the risk of ping-pong updates.
If the escenario uses TA lists with multiple TAs, the average number of RBSs per TA should not exceed:
where:
Nrbs,talist is the number of RBSs to include per TA list, calculated in the dimensioning process.
Nta,talist is the number of TAs in the TA list. When TA lists with multiple TAs are used, the minimum value to use 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:
With this strategy, the TA update load spread 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.
Early strategies for 4G tracking area lists
Planning without Considering Future Migration to TA Lists with Multiple TAs
With one TA in the TA list it is possible to allocate all Nrbs,talist RBSs from the dimensioning exercise to the same TA, as shown in the following figure:
With this strategy, the TAs are likely to be large, and large parts of existing RA boundaries can be re-used. The drawback to this strategy is that TAs must be replanned once TA lists with multiple TAs are introduced to the network.
Planning without Considering Future Migration to TA Lists with Multiple TAs
To avoid replanning TAs when introducing TA lists with multiple TAs, the option exists to dimension the TA size. A drawback is that this strategy can lead to more frequent TA updates, increasing the MME load. The additional update signalling is not a major problem if the number of subscribers is small. In the following figure you can find an example of planning for multiple TAs per TA list:
Estimating Paging Traffic per Subscriber
The paging intensity per subscriber is a key parameter in the TA dimensioning process. Indications on what to expect are obtained by studying the paging intensity in 3G networks.
Experience from previous systems shows that the paging intensity differs greatly between networks, depending on service, tariffs, types of terminals , and so on.
Measurements have also shown that malicious software can generate large amounts of paging traffic. Operators should counteract these applications to reduce the paging load.
The paging intensity also depends on the setting of parameter InactivityTimer, controlling the length of time that users must be inactive before release it to idle mode. The default setting of the timer is 61 seconds. A shorter value leads to a faster return to idle mode, and a higher paging intensity.
4G tracking area dimensioning example
In this example, the MME has 5 SCTP/S1 boards, so let’s calculate the number of RBSs in the TA list. The data used for the example is:
- In a city with 700,000 attached subscribers there are two MMEs.
- 50% of the subscribers have PC cards and 50% have handheld terminals.
- Let’s assume that there are 0.39 paging requests for PC card users and 1.68 paging requests for handheld users per busy hour.
- There are approximately 5000 users per RBS and the RBS paging capacity has been determined to be approximately 200 pages/s (based on formula review previously).
The following list shows the tasks in finding a solution for TA dimensioning:
- The first step is to calculate the paging intensity per subscriber and second:
2. The second step is to perform 4G tracking area dimensioning in relation to the MME:
Assuming that the number of attached users in the two MMEs are equally split between the two MMEs, Nsau,mme becomes:
The MME paging capacity with 5 SCTP boards is:
The number of RBS per TA list for MME paging capacity:
3. The third step is to perform TA dimensioning in relation to RBS paging capacity:
So for this example, the limit of the 4G tracking area list size is because of the MME capacity:
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