1_03802

GSM Radio Network Tuning

1_03813 LZU 108 3298 Rev R7A 1

1_03813 LZU 108 3298 Rev R7A GSM Radio Network Tuning1

Field Experience & MethodologyField Experience & MethodologyField Experience & Methodology

Managing the Quality of a Radio Network


1_03813 LZU 108 3298 Rev R7A 2

• How shall the radio network be managed to meet specified quality criteria.• The described procedures and tools can be applied in general.• The described functionality and tools are related to Ericsson systems.


Agenda

• What is quality

• The network tuning process

• How to monitor the radio network performance


1_03813 LZU 108 3298 Rev R7A 3

Terms are defined in the CCITT Recommendation E.880

• Quality of service: How the subscriber is satisfied with the overall service • Accessibility: To be able to get in contact with the network• Retainability: To continue the connection with the network until all tasks are successfully terminated• Service Integrity: To be able to perform a service and to keep the quality of the connection on a level,where the information can successfully be exchanged in the shortest possible time•Support Perf.: The ability of the operator to provide the service and to use its utilization •Operability Perf.: The ability of a service to be successfully and easily used by the user•Security: EIR, secure billing, no unauthorized monitoring, TMSI, etc.


How is Network Quality defined?User (subscriber)

Serviceintegrity

Qualityof service

Service support

performance

Serviceoperability

performance

Serviceaccessibilityperformance

Serviceretainabilityperformance

Serveability performance

Provider (operator)

Service securiity

performance

Quality of Service

Network Performance


1_03813 LZU 108 3298 Rev R7A 4

Which performance can be achieved with the individual network configuration (architecture, terrain, etc.)?

• Experience from the own network => Reference Network => Performance Requirements• Experience from other similar networks => General performance requirements

In the flow chart above, the start point is a good network.Of course it is possible to start with a badly performing network. In this case the procedure starts with the box performance analysis, as it is already known, that the performance is bad.In this case the reference network for performance comparison is not available.


General Tuning Process

Monitor Performance

Performancesatisfying?

Reference Network

Changes

Monitor Performance Performancesatisfying?

Yes

Yes

No

Performance Analysis

Collect additionalinformation

ChangeDecisionPossible?

No

Analysis

No

Start:good network

Yes


1_03813 LZU 108 3298 Rev R7A 5

•Performance Monitoring: - Does the network fulfil the performance requirements- Has the performance changed compared to earlier- Key performance indicators (KPIs) are needed since too much information is available


Monitoring the Network Performance

Goal:Perceive the performance

at a glance!

Solution:• Extract and calculate, of all available information,

key indicators, which are directly connected to the quality• Information about why the performance is as it is, does not

need to be provided during this step

Good? Bad?

InformationOverflow!!!


1_03813 LZU 108 3298 Rev R7A 6

•Test drives are expensive, therefore it is beneficial to get theinformation to monitor the radio network performance from the networkelements

• Most of the information is available in the different network elements.Information, which is only available from the mobiles is:

- idle mode information- geographical information (position)- information about subjective speech quality


Where can Information about the Network Quality be collected?

MS

MSMS

MSC MSC

BSC BSC BSC

BTSBTSBTSBTSBTSBTSNetwork Information Mobile Information

Sources for Performance Monitoring Information are:• Statistical information from the network elements• Information from a single test mobiles• Information from several special mobiles distributed to selected users • Customer complaints


1_03813 LZU 108 3298 Rev R7A 7

• For call set-ups at originating calls, information is only available in the network elements, if information has been received by the system.For originating calls the best way to collect call set-up performance data isrepeated short call set-ups with a test mobile.

• For monitoring purposes it can be assumed that the information for originating call set-ups, available in the network elements, is exact enough.


Performance Indicators describing the Radio Network Performance in a GSM System

Accessibility:– Call set-up success rate

• For mobile originated calls, this information is only partly availablein the network elements

– Paging success rate– SDCCH establishments out of all random accesses– SDCCH drops (also retainability)– Assignment success rate– Congestion on SDCCH and TCH

Retainability:– Dropped calls


1_03813 LZU 108 3298 Rev R7A 8

• Due to the specification of GSM, it is not possible to get information above the real speech quality, as perceived by the subscribers. It is only possibleto estimate the subscriber perceived speech quality.

• In R8, speech quality supervision was introduced which gives statistics based on SQI samples reported to the BSC


Performance Indicators describing the Radio Network Quality in a GSM System

Service Integrity:– Subjective speech quality and number of retransmissions

• for the downlink this information is only available on the mobile side

Indicators collected from the network which give informationabout the speech quality:– Dropped calls due to bad quality– Call releases due to bad quality– Handover failures– Handover, quality controlled– Intra-cell handover, quality controlled– RXQUAL distribution– FER measurements/distributions


1_03813 LZU 108 3298 Rev R7A 9

RxQual is obtained by transforming the bit error rate (BER) into a scale from 0 to 7. RxQual is the average BER over a certain period of time (0.5 s). By contrast, a listener's assessment of speech quality is a complex process which is influenced by many factors. Some of these, all of which RxQual fails to take into account, are the following: • The distribution of bit errors over time. For a given BER, if the BER fluctuates very much, the perceived quality is lower than if the BER remains rather constant most of the time. Different channel conditions give rise to radically different BER distributions. However, since RxQual just measures the average BER, it cannot capture this. (In fact, the logarithmic scale of RxQual gives rise to the opposite effect: a high BER variance gives a better RxQual than a low variance does. This is completely misleading from a speech quality point of view.) •Frame erasures. When entire speech frames are lost, this affects the perceived quality in a very negative way. •Handovers. Handovers always cause some frames to be lost, which generally gives rise to audible disturbances. This does not show at all in RxQual, however, since during handovers BER measurements are suppressed. •The choice of speech codec. The general quality level and the highest attainable quality vary widely between speech codecs. Moreover, each codec has its own strengths and weaknesses as regards types of input and channel conditions.


Monitoring Quality together with Frequency Hopping

0

0.1

0.2

0.3

0.4

0.5

0.6

0 1 2 3 4 5 6 7

RXQUAL

FER

FER: no hop, no DTX

FER: hop, no DTX

FER: hop, DTX


1_03813 LZU 108 3298 Rev R7A 10

SQI can be measured with e.g TEMS Investigation. SQI has been designed to take into consideration all the phenomena discussed in the preceding section. This ensures that it will produce an unbiased prediction of the speech quality, independently of channel conditions and other circumstances. Somewhat roughly, the computation of SQI involves • the bit error rate (BER) • the frame erasure rate (FER) • data on handover events • statistics on the distributions of each of these parameters.

Furthermore, for each speech codec, SQI is computed by a separate algorithm which is tuned to the characteristics of that codec.

Like RxQual, SQI is updated at 0.5 s intervals.


SQI – Speech Quality Index

TEMS Speech Quality Index (SQI):• Based on Handover and BER/FER distributions• Measured continuously• Scale -15 to +30• Over 90% correlation for a 5s sentence


1_03813 LZU 108 3298 Rev R7A 11


SQI and RxQual relation (FR codec)

The curve represents the average RxQual-to-SQI relation for different channels. The figure is based on simulations. The following conditions have been examined:• Four different 900 MHz channels:

• tu0ifh (typical urban, 0 km/h, ideal frequency hopping)• tu30mphnfh (typical urban, 30 mph, no frequency hopping)• ff12nfh (flat fading, 12 km/h, no frequency hopping)• tu3nfh (typical urban, 3 km/h, no frequency hopping).

• The C/I levels have been varied between 2 and 20 dB • For each channel and C/I level a 60 second simulation has been done.• The data from the simulations have been divided into 2.5 second long sentences, and the corresponding SQI values (for both the FR and the EFR codecs) have been calculated for each sentence.• RxQual have been calculated for each 0.5 second interval, i.e. there are fiveRxQual values for each sentence.• For each simulation, average parameter values has been calculated for each parameter (SQI, RxQual etc.). The graphs in this document are all based on these average parameter values.


1_03813 LZU 108 3298 Rev R7A 12

Depending on the status of the network, the methods to be used for the performance monitoring can vary. At the beginning, before there are even users in the network, drive testing is really the only way to handle the performance monitoring. As the number of users increases, it is possible to start using various complaints as well as network statistics (STS).


Performance Monitoring with TEMS (I)

Procedure:

• Specify a fixed test route, covering the areaswhich shall be monitored

• Perform periodic test drives on this route

• For accessibility:Repeated short call set-ups• For retainability and integrity: Long continuous calls


1_03813 LZU 108 3298 Rev R7A 13

Three mobiles are normally used for drive testing.One test mobile could be used for the test of coverage and should work in “scanning mode”.A second mobile could be used for the test of accessibility using short call test. A third mobile could be used for the tests of speech quality and retainability using long call test.


Performance Monitoring with TEMS (II)

Evaluation:• Produce statistics covering

– the call set-up success rate– the dropped call rate– the handover success rate– distribution for signal strength and speech quality

• Present graphically on a street map (GIMS, Map Info, etc.):– signal strength– speech quality– traffic events, such as dropped calls and handover


1_03813 LZU 108 3298 Rev R7A 14

Detect problems => Quality requirements are not fulfilled.Locate problems => Which area, which cell, which TRX or which time slot shows the problem.Detect areas for expansion => Where do hardware needs to be added.Detect “money makers” => How important is it to react against the problem.


Results of the Performance Monitoring

• Detect problems in the radio network• Locate problems geographically• Detect areas for expansion• Detect “money makers”


1_03813 LZU 108 3298 Rev R7A 15

Common problems when implementing a high capacity solution are:• Dimensioning and architecture of the radio network• Interference


Performance Analysis

• What creates the problem• How can the problem be solved

Problems related to the radio network:• Dimensioning and Architecture High Capacity• HW Problems• Interference High Capacity• Neighbour Cell Definitions• Locating Parameters• Idle Mode Parameters• Other Cell Parameters• Power Balance


1_03813 LZU 108 3298 Rev R7A 16


Final Remarks

• The best optimization tool is experience• The performance can not be better than allowed by the

cell configuration and the cell plan• Radio network optimization usually means fixing a poor

frequency plan

About performance and performance optimization:

=!

Documents

1_03802