Paging optimization1-130313230537-phpapp01

Ericsson Confidential 1 (23)

Prepared (also subject responsible if other) No.

EAL/O/EO Hafida Sadeuk ben abbes Approved Checked Date Rev Reference

EAL/O/EO Mohamed Abd Rahmen Mohamed 2007-09-20 PA1

Paging Optimization study





Contents 1 Abstract...................................................................................................3 2 Introduction ............................................................................................3 3 Paging capacity......................................................................................5

3.1 Channel configurations ..............................................................5 3.2 Paging block structure................................................................6 3.3 Paging groups ............................................................................7 3.4 Queuing in the BTS....................................................................8 3.5 Paging Response.......................................................................9 3.6 Random Access .........................................................................9

4 Paging strategy ....................................................................................10 4.1 Paging parameters...................................................................10

5 Practical case .......................................................................................14 5.1 Network Information .................................................................14 5.2 Main point affecting paging ......................................................15 5.3 SDCCH congestion ..................................................................15 5.4 Timers ......................................................................................16 5.5 COVERAGE GAPS..................................................................19 5.6 RACH Level Access Min tuning over Huawei ..........................20

6 Conclusion............................................................................................21 Abbreviation ..................................................................................................23





1 Abstract

The aim of this report is to show factors driving paging success rate in order to improve our paging KPI in ATM-Mobilis the network.

ATM-Mobilis network include 20 MSCs and 66 BSCs 30 are Ericsson and the reset are shared between ZTE and Hawaii. The paging performance of ATM-Mobilis is a major KPI that affects the accessibility of the system.

2 Introduction

When a Mobile Station (MS) is paged, a Paging message is sent from the MSC to each Base Station Controller (BSC) belonging to that MSC's service area (global page), or to those BSCs serving at least one cell belonging to the LA where the MS is registered (local page).

Figure 1: General description of paging

For each Paging message received by the BSC, Paging Command messages have to be sent to all cells belonging to the LA where the target MS is registered. The number of cells in an LA ranges from a few tens up to perhaps one hundred cells, sometimes even more. This means that one incoming Paging message to the BSC leads to a considerably larger number of outgoing Paging Commands from the BSC.

All BSC in the MSC

BSC with cells using

specified LA

MSC

Local

Global

Paging message Cells with specified LA

Paging command

Paging command

Paging Request

Paging Request





Figure 2: Paging commands

The BTSs have to broadcast all the incoming pages. The Paging Request messages are sent on the Paging Channel (PCH) on the Common Control Channel (CCCH). Too large LAs may lead to a too high paging load in the BTS resulting in congestion and lost pages due to capacity limitation on the air interface.

Smaller LAs reduce the paging load in the BTSs as well as in the BSCs. However, smaller LAs also mean a larger number of LA border cells in the network. Each time an MS crosses the boarder between two LAs, a Location Updating is performed. The Location Updating affects the load on the signaling sub-channels, SDCCH, in the LA border cells. The SDCCH signaling capacity depends on the SDCCH configuration in the cell and is further described.





3 Paging capacity

3.1 Channel configurations

The paging is done on timeslot zero on the Broadcast Control Channel (BCCH) frequency. Timeslot zero consists of several channels; Broadcast Channel (BCH), CCCH and, if combined BCCH/SDCCH is used, also Dedicated Control Channel (DCCH). This is given in Figure 2.

• The BCH consists of Frequency Correction Channel (FCCH), Synchronization Channel (SCH) and BCCH.

• The CCCH consists of two sub-channels; the PCH and the Access Grant Channel (AGCH). The PCH is used to page the MSs with Paging Request messages.

• The DCCH consists of SDCCH, Slow Associated Control Channel (SACCH) and (if cell broadcast is used), Cell Broadcast Channel (CBCH).

Figure 2: Channel combinations on timeslot 0. The dashed squares are used in a combined BCCH/SDCCH configuration.

Timeslot 0 is logically divided into multi-frames, where each multi-frame is 235.4 ms. one multi-frame equals to 51 frames. The multi-frames have different contents for the different channel configurations, and therefore the paging capacity varies:

• Non-combined BCCH/SDCCH can fit 9 paging blocks per multi-frame (Figure 3). The signaling sub-channels are mapped on one or more separate timeslots (8 signaling sub-channels, SDCCH/8, is fitted on one timeslot).

Figure 3: Non-combined mapping of BCHs and CCCHs on timeslot 0, downlink. One multi-frame can fit 9 CCCH blocks.





• Combined BCCH/SDCCH can fit 3 paging blocks per multi-frame (Figure 4), which means that the maximum BTS paging capacity is reduced to one third.

Figure 4: Combined mapping of BCHs, CCCHs and SDCCHs on timeslot 0, downlink. One multi-frame can fit 3 CCCH blocks.

The parameter BCCHTYPE is used to choose between the combined and the non-combined configuration.

The AGCH is used to assign an SDCCH (Immediate Assignment) to an MS during the call set-up phase. The AGCH shares the same resources as the PCH. The CCCH can be set up to have either dedicated blocks for Access Grant, or Access Grant can work in stealing mode, which means replacing paging blocks with Access Grant blocks, if required. If dedicated blocks are used, each multi-frame contains two paging blocks (for combined) or eight paging blocks (for non-combined).

3.2 Paging block structure

Each multi-frame contains 3 (combined BCCH/SDCCH) or 9 (non-combined BCCH/SDCCH) paging blocks. Each paging block can fit up to four Paging Requests.

Each paging block can fit either: • 2 IMSI Paging Requests (IMSI = International Mobile Subscriber

Identity) • TMSI Paging Requests (TMSI = Temporary Mobile Subscriber

Identity) • 1 IMSI + 2 TMSI Paging Requests

IMSIIMSI

T = TMSI T T T T

Figure 4: A paging block can fit two IMSI pages or four TMSI pages or a combination of one IMSI and two TMSI pages.

IMSI T T





3.3 Paging groups

After an MS tunes to the BCCH carrier and decodes the System Information, it performs an evaluation to which paging group it belongs, and hence, which particular paging block of the available blocks on the paging channel that is to be monitored.

Figure 5: Paging group The operator can set the number of paging groups for each cell. • A high number of paging groups means that the MSs have to wait for a

longer time than the low number case before the right paging block arrives. This increases the time for paging.

• A low number of paging groups shortens the call setup time due to that the MS listens to the paging block more frequently. The drawback is that the MS power consumption is higher.

There are two main parameters that determine the number of paging groups in a cell (besides BCCHTYPE). The two parameters are AGBLK and MFRMS: • AGBLK: This parameter determines how many of the paging blocks per

multi-frame are reserved for the AGCH. • MFRMS: is the multi-frame interval and is defined as the transmission

interval of paging messages to the same paging group.

Number of paging group is subject to the setting of MFRMS and AGBLK. The formula to calculate the number of paging group is as below:

Combined BCCH/SDCCH cells: Number of paging groups = (3 - AGBLK) * MFRMS





None combined BCCH/SDCCH cells: Number of paging groups = (9 - AGBLK) * MFRMS

3.4 Queuing in the BTS

Incoming Paging Commands are buffered in a queue (one for each paging group). The queue for each paging group in the BTS can fit between 6 and 14 pages, depending on the number of paging groups. The BTS distributes the Paging Commands as Paging Request messages on the radio path when paging blocks are available. A too high rate of incoming Paging Commands to the BTS increases the queuing time, something that leads to an increase of the average time for the paging response. If the queue is full, the incoming pages are "thrown away". If a page is queued for a too long time in the BTS, the page may also be lost due to the fact that the MSC does not receive the paging response before the timer (PAGTIMERREP1LA or PAGTIMEREPGLOB) has expired. The risk of having an excessive delay in the BTS increases if the time between the transmission of each paging group (set by the parameter MFRMS) is long.

In the network with paging discard problem at the cell level, it’s recommended to reduce the number of paging groups via reducing the MFRMS value which will also reduce call setup time.

There are two reasons why the paging discarded in the BTS: • Paging queue is full and the new paging message from BSC will

be discarded.

• Paging messages will be discarded if waited too long in the BTS, paging queue. Default is 5 sec.

There is one paging queue available for each of the paging group. The length of the paging queue is calculated based on the following formula:

Qlength = 14 – (PQmax / 10)

Where PQmax is the number of paging groups in the cell.

So, we will have a larger paging queue / buffer in the cell if we reduce the number of paging group.

Also, MS will listen to its paging group more often. The same paging group will be paged more frequent or shorter multi-frame period. The possibility of paging messages discarded in the queue will become minimal.

When there are no paging messages to be transmitted to MSs in a certain paging group, dummy pages are sent instead.





3.5 Paging Response

Upon receipt of a paging request message and if access to the network is allowed, the addressed MS shall initiate within 0.5 s the immediate assignment procedure. The establishment of the main signaling link is then initiated by use of an SABM with information field containing the PAGING RESPONSE message

Upon receipt of the PAGING RESPONSE message the network stops timer PAGTIMERFRST1LA (PAGTIMEFRSTGLOB).

If timer PAGTIMERFRST1LA (PAGTIMEFRSTGLOB) expires and a PAGING RESPONSE message have not been received, the network may repeat the paging request message and start timer PAGREP1LA (PAGREPGLOB).

Before going further in this report, we need to introduce one aspect which is closely related to the paging. It’s the Random Access.

3.6 Random Access

When the paged MS receives a Paging Request message it responds by sending a Channel Request message, thereby requesting a signaling channel for call set-up. The Channel Request message is sent on the Random Access Channel (RACH) and consists of 8 information bits: a random number (5 bits) and an establish cause (3 bits). In GSM specifications phase 2, the number of bits used to establish cause can be 3 through 6. Accordingly, the random number will be described using 5 down to 2 bits. The establish cause can be: • Answer to page • Originating call • Location updating • Emergency call • Others, for example IMSI detach, SMS, supplementary services management





4 Paging strategy

Paging strategy in the MSC is determined by the AXE parameters or the exchange property settings, the chart below show the relation between AXE parameter settings and the paging strategies:

Figure 3: Paging Strategies via A-interface

4.1 Paging parameters

The paging strategy implemented in the network depend on the setting of the MSC exchange property settings; while the cell level parameter settings will determine the paging capacity in the Air Interface.

4.1.1 MSC parameters

The following MSC parameters and exchange properties are relevant for paging and Location Updating:

BTDM implicit detach supervision should be equal (or longer) than T3212 in the BSC. If T3212 is increased, BTDM must also be increased. Note that BTDM is set in minutes and T3212 is set in deci-hours.

GTDM is an extra guard time in minutes before the subscriber is set to detached.

TDD sets the time (in days) that an inactive IMSI is stored in the VLR before it is removed.





PAGTIMEFRST1LA is the time supervision for the page response of the first page. The MS is paged in the LA with the first page if the Location Area Identity (LAI) information exists in the VLR. The parameter is set according to the default values.

PAGETIMEFRSTGLOB is the time supervision for the first global page. It is used instead of PAGTIMEFRST1LA if the LAI information does not exist in the VLR.

PAGEREP1LA decides how the second page is sent: • 0 Paging in LA is not repeated • 1 Paging is repeated in LA with either TMSI or IMSI • 2 Paging is repeated in LA with IMSI • 3 Paging is repeated as global paging with IMSI

PAGEREPGLOB defines how global paging is repeated according to: • 0 Global paging is not repeated • 1 Global paging is repeated with IMSI

PAGTIMEREP1LA is the time supervision for the second page to LA. This is the timer used for the second page when PAGEREP1LA is set to 1 or 2.

PAGTIMEREPGLOB, the time supervision for the second page, if it is global.

TMSIPAR indicates if TMSI should be used or not: • 0 TMSI is not allocated. Note that this setting this means that TMSI is not

used. The paging capacity will be decreased if TMSI is not used. • 1 TMSI is allocated only on encrypted connection • 2 TMSI is allocated

TMSILAIMSC states if a new TMSI shall be allocated at a change of LAI within the MSC/VLR. Only applicable if TMSIPAR is not equal to 0. • 0 Allocation only once • 1 Allocation on every change of LAI

If TMSI is used it will be used (at least) in the first page. Then, depending on how PAGEREP1LA is set the page is repeated with either TMSI or IMSI.

However, there will always be some pages that are sent out globally in the first page. The reason for this is that information about the MS did not exist in the VLR. Normally, this is due to that the MS was removed from the VLR, due to being inactive for too long (see parameter TDD above). At an incoming call, the HLR has information about the most recent location, i.e. VLR, where the MS was registered. Then, when the call is connected to the VLR a global page will be sent out due to that no information exists in the VLR about this particular MS. If the MS would have been registered in the VLR but not active, no page would have been sent out.

The following LATA exchange properties are valid only if the function Equal Access and Transit Network Selection in MSC/VLR and Gateway MSC (GMSC) is implemented. This is an optional GSM 1900 function.





LATAUSED defines the usage of LATA administration: • 0 LATA administration is not used • 1 LATA administration is used

PAGLATA defines if LATA paging is used for mobile terminating calls or not: • 0 LATA paging is not used • 1 LATA paging is used

PAGREPCT1LA defines how the paging in one location area is repeated, if the first Paging Attempt was local. This parameter is only valid when PAGLATA is set to 1. • 0 Paging in one LA is not repeated • 1 Paging in one LA is repeated with either TMSI or IMSI • 2 Paging in one LA is repeated with IMSI • 3 Paging is repeated as call delivery LATA paging with IMSI

PAGTIMEREPLATA defines the time supervision for page response of repeated LATA paging. After expiration of this timer no new paging repetition for this call is done.

TIMPAGINGM defines the timer for supervision over the Gs-interface. At expiry of the timer, according to the parameter SECPAGEPATH.

SECPAGEPATH defines over which interface • 0 No paging. • 1 Paging over the Gs-interface. • 2 Paging over the A-interface. TIMNREAM determines the time supervision period between the sending of the PAGING message and successful assignment of traffic channel using the Not Reachable timer. Default value is 12 secs.

Parameter Name Default Recommended Value Range BTDM OFF T3212 * 6 6 – 1530 (steps of 6) minutes or OFFGTDM - - 0 -255 minutes TDD OFF - 1 – 255 days or OFF

PAGTIMEFRST1LA 4 - 2 – 10 sec PAGTIMEFRSTGLOB 4 - 2 – 10 sec

PAGREP1LA 2 - 0 – 3 PAGREPGLOB 0 - 0 – 1

PAGTIMEREP1LA 7 - 2 – 10 sec PAGTIMEREPGLOB 7 - 2 – 10 sec

TMSIPAR 0 1 or 2 0 – 2 TMSILAIMSC 0 - 0 – 1 LATAUSED** 0 - 0 – 1 PAGLATA** 0 - 0 – 1

PAGREPCT1LA 2 - 0 – 3 PAGTIME REPLATA** 7 - 2 – 10 sec

SECPAGEPATH*** 1 - 0 – 2 TIMPAGINGM*** 9 - 2 – 20 sec

Table 1: MSC parameter table.





4.1.2 BSC parameters

In this section some of the most important BSC parameters for paging performance are explained:

MFRMS is the multi-frame period and defines the transmission interval of paging messages to the same paging group.

AGBLK sets the number of CCCH blocks in each multi-frame that will be reserved for access grants. Setting AGBLK to a value other than 0 will reduce the paging capacity.

BCCHTYPE can be either: • COMB = Combined; the cell has a combined BCCH, CCCH and

SDCCH/4. • COMBC = Combined with CBCH; The cell has a combined BCCH, CCCH

and SDCCH/4 with a CBCH sub-channel. • NCOMB = Not combined; The cell does not have a SDCCH/4.

If COMB is used the maximum capacity of the CCCH will decrease to a third (If AGBLK set to 0), compared to the NCOMB case.

T3212 is the time between the periodic registration.

ATT determines if attach/detach is allowed.

MAXRET defines maximum number of retransmission of MS may do when accessing the system on RACH. The settings parameter must be always in the optimum level because this is the trace off between call success and the paging capacity in the cell.

CRH is the hysteresis value used when the MS in idle mode crosses an LA border. As default this parameter is set to 4. A higher setting might be advantageous in areas with many LA borders and thus problems with many Location Updating.

PAGLIMIT defines the max number of paging orders allowed to be sent to the TRH per second. Set per BSC as a BSC Exchange Property. Since there are also other mechanisms in the BSC that prevents overload due paging, there should normally not be any need to change the parameter value. The only case when reducing it can be useful is o prevent congestion on the paging channel PCH.

Parameter name Default value Recommended value Value range MFRMS 6 4 2 - 9 CCCH multi-frames AGBLK 1 0 0 - 7 (0-1 for RBS 200, 2000) BCCHTYPE NCOMB see section 3.6 COMB, COMBC, NCOMB T3212 40 40 0 - 255 ATT YES YES YES, NO CRH 4 4 0 - 14 (in steps by 2) dB PAGLIMIT 0 (no restriction) 0 (no restriction) 0 to 10

Table 2: BSC parameter table





5 Practical case

The aim the study was to improve the paging success rate which was around 85% on system level. For that the first thing must be done was looking for the major point affecting the paging and after that find the optimum setting

5.1 Network Information

The network of Mobilis is composed by 66 BSCs : 30 Ericsson, 17 HUAWE and 19 ZTE

Country Algeria

Operator Mobilis

MSC 17

BSC 66

LAC 78

All MSCs are Ericsson equipments over the network

The strategy applied in the Mobilis network is described in the table below which contains the setting parameters value in MSC and BSC level

1-BSC level BSC Parameter Name Current Value Remark T3212 20 Periodic registration timer [BSC Level] ATT Yes Allow Attach/detach using IMSI 2-MSC level MSC Parameter Name Current Value Remark TMSIPAR 2 TMSI is allocated All connection PAGREP1LA 3 paging is repeated as global paging with IMSI PAGREPGLOB 1 global paging is repeated with IMSI PAGREPCT1LA 2 PAGLATA 1 PAGTIMEREPGLOB 9 the second paging timer [all LA of the MSC] PAGTIMEFRSTGLOB 6 the First paging timer [MSC level] PAGTIMEREP1LA 10 PAGTIMEFRST1LA 5 the First paging timer [LA] PAGTIMEREPLATA 7 BTDM 120 Periodic registration timer [MSC level] TMSILAIMSC 1 TIMNREAM 12

Table 3 paging strategy over the network





5.2 Main point affecting paging

The first step of the project was the determinations of the raison effecting paging witch are related to following point:

• SDCCH congestion

• MSC & BSC parameter and timers setting

• Coverage gaps

• RACH Level Access Min (huawei).

5.3 SDCCH congestion

The paging capacity are related to SDCCH configuration and by the way to SDCCH capacity since the paging response is performance on SDCCH

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System.paging Success Rate SDCCH congestion system

Figure 4: the paging Success Rate VS SDCCH congestion

As shown in the chart above the congestion of the SDCCH is inversely proportional to the paging performance.





5.4 Timers

The aim of the study was looking for tuned value of the timers controlling paging over the network. The timers involved in the study are: TIMNREAM, PAGTIMEFRST1LA and T3212.

5.4.1 TIMNREAM

Since TIMNREAM determines the time supervision period between the sending of the PAGING message and successful assignment of traffic channel which mean at least TIMNREAM must include total paging time = First paging time (PAGTIMEFRST1LA) + Second paging time (PAGTIMEREPGLOB).

For that TIMNREAM was increased from 12 the default value to 18 on 22nd July in MSCBCH1

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Figure 5: The paging performance over MSCBCH1

After the implementation of the new value of TIMNREAM 4% improvement achieved in all LA of MSCBCH1.

TIMNREAM was increased from 12 the default value to 18 on 30th July for all network.

TIMNREAM implementation





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TIMNREAM implementation over network

Figure 6: The paging performance after increased TIMNREAM

5.4.2 PAGTIMEFRST1LA

The second timer involved in the study is PAGTIMEFS1LA. The strategy used in the network is the first paging local during time PAGTIMEFS1LA and the second paging global PAGTIMEREPGLOB.

The idea of study was to improve the First paging in the LA before trigger of the global paging

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Figure 7: The paging performance after increased PAGTIMEFS1LA

The timer PAGTIMEFS1LA increased from 5s to 8s since in the all BSCs had problem of

PAGTIMEFS1LA implementation over network





5.4.3 T3212

The T3212 parameter controls the Periodic Registration interval. A shorter interval, together with implicit detach supervision initiated in the MSC, will reduce the number of unnecessary paging for that The timer T3212 was reduced from 20 to 10 in order to have the accurate information in VLR which means more possibility for paging successful

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Figure 8: The paging performance after reduced T3212

But it leads to more signaling on the SDCCH, thereby affecting the SDCCH dimensioning

Implementation of the new T3212 value





5.5 COVERAGE GAPS

In order to show the contribution of the coverage gaps for the deterioration of paging success rate new LA 6030119102 was created in Setif city center where the coverage distribution of the city

Figure 9: the position of the new LA over SETIF city center

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6030119101 6030119102

Figure 10: The paging performance of the new LA

As seen in the paging success new LA 6030119102 of setif city center achieved 94% which confirm that the coverage gap is one of the reasons affecting the paging performance.

Position of the new LA 60301-19102




EAL/O/EO Mohamed Abd Rahmen Mohamed Rafik 2007-09-20 PA1

5.6 RACH Level Access Min tuning over Huawei

This parameter indicates the threshold level at which the system determines MS random access. Only when the level on RACH exceeds this threshold will BTS regard the access to be successful. In contrast to “Rx Level Access min” “RACH level Access min” is the accessibility on uplink. To improve the accessibility the value of this parameter can be made lower to 1 (-109) as the BTS sensitivity is (-110)

Range value 0~255 where 0 represents -110

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Figure 11 The paging performance after RACH Level Access Min tuning

The implementation date of RACH Level Access Min over huawei BSCs was on 4th august after tuning to (-109) the paging improved by 5%.

Tuning of RACH Level Access Min





6 Conclusion

So, in order to improve paging it is recommended to fix the following issues:

• Remove SDCCH congestion

• Good setting for MSC & BSC parameter and timers

• Improve Coverage gaps

The paging success achieve 4.5% enhancement and reach the level 89.5% paging success as shown in the chart below.

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Figure 12 : The paging performance after on the system Level

This enhancement achieved after the new setting on of the following parameter and timer:





1-BSC level

BSC Parameter Name New Value Remark

T3212 10dm Periodic registration timer [BSC Level]and ZTE

Rach acmin -109 Huawei parameter 2-MSC level

MSC Parameter Name New Value Remark

PAGTIMEFRST1LA 8s

TIMNREAM 18s Table 4 : the new value tuned





Abbreviation

BSC Bas Station Controller

BTS Radio Base Station

SACCH Slow Associated Control Channel

SDCCH Stand alone Dedicated Control Channel

Date post:	06-Aug-2015
Category:	Technology
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