Presentation on Drive Test

in GSM Networks

DRIVE TESTING

Drive testing is the most common and maybe the best way to analyze Cellular Network

performance by means of coverage evaluation, system availability, network capacity,

network retainibility and call quality. Although it gives idea only on downlink side of the

process, it provides huge perspective to the service provider about what’s happening

with a subscriber point of view.

While statistics give an idea about the real behavior faced by all end users regardless of

their geographical location, drive testing or walk testing bring a simulation of end user

perception of the network on the field from one call perspective. Drive tests give the

‘feel’ of the designed network as it is experienced in the field. The testing process starts

with selection of the ‘live’ region of the network where the tests need to be performed,

and the drive testing path. Before starting the tests the engineer should have the

appropriate kits that include mobile equipment (usually three mobiles), drive testing

software (on a laptop), and a GPS (global positioning system) unit.

Primary Motives behind Drive Test Every alive Network needs to be under continues control to maintain/improve

the performance.

Optimization is basically the only way to keep track of the network by looking

deep into statistics and collecting/analyzing drive test data.

Drive test helps operation and maintenance for troubleshooting purposes.

HW Requirements Involving Drive Test Drive Test, as already mentioned, is the procedure to perform a test of Cellular Network

performance while driving. Following are the list of tools required while performing

drive test.

1. A Laptop - or other similar device

2. Data Collecting Software installed

3. Security Key - Dongle - common to these types of software

4. At least one Mobile Phone

5. one GPS

Figure 1 Drive Test HW Components

Where,

GPS: collecting the data of latitude and longitude of each point / measurement data,

time, speed, etc.. It is also useful as a guide for following the correct routes.

MS: mobile data collection, such as signal strength, best server, etc .

Thus, the main goal is to collect test data, but they can be viewed / analyzed in real time

(Live) during the test, allowing a view of network performance on the field. Data from all

units are grouped by collection software and stored in one or more output files.

Procedure Involved in Drive Testing DRIVE TEST ROUTES

DRIVE TEST ROUTES ARE THE FIRST STEP TO BE SET, AND INDICATE WHERE TESTING WILL OCCUR. THIS

AREA IS DEFINED BASED ON SEVERAL FACTORS, MAINLY RELATED TO THE PURPOSE OF THE TEST.THE

ROUTES ARE PREDEFINED IN THE OFFICE. A PROGRAM OF A LOT OF HELP IN THIS AREA IS GOOGLE EARTH. A

GOOD PRACTICE IS TO TRACE THE ROUTE ON THE SAME USING THE EASY P PATHS OR POLYGONS.

Figure 2 Drive Test Routes using Google Earth

Some software allows the image to be loaded as the software background (geo-

referenced). This makes it much easier to direct routes to be followed.

It is advisable to check traffic conditions by tracing out the exact pathways through

which the driver must pass. It is clear that the movement of vehicles is always subject to

unforeseen events, such as congestion, interdicted roads, etc.. Therefore, one should

always have on hand - know - alternate routes to be taken on these occasions.

Avoid running the same roads multiple times during a Drive Test (use the Pause if

needed). A route with several passages in the same way is more difficult to interpret.

Drive Test Schedule Again depending on the purpose, the test can be performed at different times - day or

night.

A Drive Test during the day shows the actual condition of the network - especially in

relation to Traffic loading aspect of it. Moreover, a drive test conducted at night allows

you to make, for example, tests on transmitters without affecting most users.

Typically takes place nightly Drive Test in activities such System Design, for example with

the integration of new sites. And Daytime Drive Test apply to Performance Analysis and

also Maintenance.

Important: regardless of the time, always check with the responsible area which sites

are with alarms or even out of service. Otherwise, your job may be in vain.

Types of Calls

The Drive Test is performed according to the need, and the types of test calls are the

same that the network supports - calls can be voice, data, video, etc.. Everything

depends on the technology (GSM, CDMA, UMTS, etc. ...), and the purpose of the test, as

always.

A typical Drive Test uses two phones. A mobile performing calls (CALL) for a specific

number from time to time, configured in the Collecting Software. And the other, in free

or IDLE mode, ie connected, but not on call. With this, we collect specific data in IDLE

and CALL modes for the network.

The calls test (CALL) can be of two types: long or short duration.

Short calls should last the average of a user call - a good reference value is 180 seconds.

Serve to check whether the calls are being established and successfully completed

(being a good way to also check the network setup time).

Long calls serve to verify if the handovers (continuity between the cells) of the network

are working, ie calls must not drop.

Types of Drive Test The main types of Drive Test are:

1. Performance Analysis

2. Integration of New Sites and change parameters of Existing Sites

3. Antenna Redesign

4. Benchmarking

1. Performance Analysis

Tests for Analysis Performance is the most common, and usually made into clusters

(grouping of cells), ie, an area with some sites of interest. They can also be performed in

specific situations, as to answer a customer complaint.

2. Integration of New Sites and change in parameters of Existing Sites

In integration testing of new sites, it is recommended to perform two tests: one with the

site without handover permission - not being able to handover to another site - thus

obtaining a total visualization of the coverage area. The other, later, with normal

handover, which is the final state of the site.

Depending on the type of alteration of the site (if any change in EIRP) both tests are also

recommended. Otherwise, just perform the normal test.

3. Antenna Redesign

This activity perform mainly for those cells/sites which are overshooting that means

providing coverage beyond there desired coverage limit. In this type of Drive Test

Mobile Station first camp on the desired cell and locked itself with the BCCH frequency

of that particular cell and than it start observing the coverage limit until its level

completely diminished.

4. Benchmarking Tests

Benchmarking tests aims to compare the competing networks. If the result is better, can

be used as an argument for new sales. If worse, it shows the points where the network

should be improved.

Important observing Indicators in Drive Test Drive Test Indicators plays a vital role in analyzing, optimizing and troubleshooting the

cell/site radio end issues. Following are the list of indicators that needs to observe in

Drive Test in order to properly analyze and investigate the known issues emerging from

radio end site.

1. Bit Error Rate (BER)

2. Rx-level

3. Rx-Quality

4. Frame Erasure Rate

5. Speech Quality Index (SQI)

1. Bit Error Rate (BER)

The BER is an estimated number of bit errors in a number of bursts to which corresponds a

value from 0 to 7 (best to worst) of the RX QUALITY. After the channel decoder has decoded a

456 bits block, it is coded again using the convolutional polynom in the channel coder and the

resulting 456 bits are compared with the 456 input bits. The number of bits that differs

between these two 456 bits blocks corresponds to the number of bit errors in the block. The

number of bit errors is accumulated in a BER sum for each SACCH multi frame and the result is

classified from 0 to 7 according to the BER-RX QUALITY conversion Table shown below.

RX QUALITY is still considered as a basic measurement. It simply reflects the average BER over a

period of 0.5s. However, listener speech quality evaluation is a complex mechanism that is

influenced by several factors. Some of these factors that RX QUALITY does not consider are:

Time distribution of BER. For a given BER, if the rate fluctuates a lot, the perceived

quality is less than if the BER is constant over the time.

When entire frames are lost, speech quality is negatively impacted.

Handovers generate some frame losses. It is not evident in RX QUALITY measurements

since, during handovers, BER measurements are skipped.

Overall quality depends closely on the type of codec used.

In conclusion, RX QUALITY does not capture many phenomena that affect the listener’s

perception of speech quality. That is why other metrics are defined.

BER to RX QUALITY Conversion table

2. Rx-Level

Rx-Level is define as The power level corresponding to the average received signal level

of the downlink as measured by the mobile station. The range of Rx-level is between -55

to -110.It is been further classified as Rx-Level Sub and Rx-level Full. Where Rx-Level sub

is based on the mandatory frames on the SACCH multi frame. These frames must always

be transmitted which means that they carry intelligent signaling data. Whereas The

FULL values are based upon all frames on the SACCH multi frame, whether they have

been transmitted from the base station or not. This means that if DTX DL has been used,

the FULL values will be invalid for that period since they include bit-error measurements

at periods when nothing has been sent resulting in very high BER.

Figure 3 A Typical RX-Level Plot

3. Rx-Qual

Rx-Qual is define as the level corresponding to the mobile station's perceived quality of

the downlink signal. Rx Quality is a value between 0 and 7, where each value

corresponds to an estimated number of bit errors in a number of bursts. The Rx Quality

value presented in TEMS is calculated in the same way as values reported in the

measurement report sent on the uplink channel to the GSM network.

Each Rx Quality value corresponds to the estimated bit-error rate according to the

following table, which is taken from GSM technical specification shown in BER table.

Figure 4 A Typical RX-Qual Plot

4. Frame Erasure Rate (FER)

A speech quality degrade factor that indicates fading and interference. Voice quality

is judged upon the Frame erasure rate. Even while experiencing bad Received

Quality, the voice Quality still could be maintained. FER (frame erasure rate) range

goes from 0 (being the best performance) to 100%. This represents the percentage

of blocks with an incorrect CRC (cyclic redundancy check). Since the BER is calculated

before the decoding with no gain from frequency hopping, the FER is then used in

this case. Being even more stable than the BER, the FER also depends on codec type.

The smaller the speech codec bit rate, the more sensitive it becomes to frame erasures.

FER plays a major role in troubleshooting of Interference.

Formula for Calculating FER:

FER (%) = (no. of blocks with incorrect CRC / total no. of blocks)*100

Whereas,

Block represents 456 bits.

Figure 5 A Typical FER Plot

5. Speech Quality Index (SQI)

SQI is an estimate of the perceived speech quality as experienced by the mobile user, is based

on handover events and on the bit error and frame erasure distributions. The need for speech

quality estimates in cellular networks have been recognized already in the GSM standard, and

the Rx Quality measure was designed to give an indication of the quality.

However, the Rx Quality measure is based on a simple transformation of the estimated average

bit error rate, and two calls having the same Rx Quality ratings can be perceived as having quite

different speech quality. One of the reasons for this is that there are other parameters than the

bit error rate that affects the perceived speech quality. Another reason is that knowing the

average bit error rate is not enough to make it possible to accurately estimate the speech

quality. A short, very deep fading dip has a different effect on the speech than a constant low

bit error level, even if the average rate is the same.

Generally Speech Quality Index, which is an estimate of the perceived speech quality as

experienced by the mobile user, is based on handover events and on the bit error and frame

erasure distributions. The quality of speech on the network is affected by several factors

including what type of mobile the subscriber is using, background noise, echo problems, and

radio channel disturbances. Extensive listening tests on real GSM networks have been made to

identify what type of error situations cause poor speech quality. By using the results from the

listening tests and the full information about the errors and their distributions, it is possible to

produce the Speech Quality Index. The Speech Quality Index is available every 0.5 second in and

predicts the instant speech quality in a phone call/radio–link in real–time.

Figure 6 A Typical SQI Plot

Drive Testing Through TEMS TEMS stands for Test Mobile System is considered as the most reliable tool across the

globe while dealing with drive test performing analysis, investigation and successful

troubleshooting in 2nd Generation based networks. Its primary task is to to read and

control information sent over the air Interface between the base station and the mobile

station in GSM/Cellular system. It can also used for radio coverage measurement.

A Quick look at TEMS In this session we take a quick look of TEMS and its Interface. The information provided

by TEMS is displayed in status windows. This information includes cell identity, base

station identity code, BCCH carrier ARFCN, mobile country code, mobile network code

and the location area code of the serving cell.

There is also information about RxLev, BSIC and ARFCN for up to six neighboring cells;

channel number(s), timeslot number, channel type and TDMA offset; channel mode, sub

channel number, hopping channel indication, mobile allocation index offset and hopping

sequence number of the dedicated channel; and RxLev, RxQual, FER, DTX down link,

TEMS Speech Quality Index (SQI), timing advance (TA), TX Power, radio link timeout

counter and C/A parameters for the radio environment.

The signal strength, Rx-Qual, C/A, TA, TX Power, TEMS SQI and FER of the serving cell

and signal strength for two of the neighboring cells can also be displayed graphically in a

window.

Figure 7 A Typical TEMS Interface

MODES OF DRIVE TEST 1. Dedicated / Continuous / Long Call Mode

2. Idle Mode

3. Frequency Scan Mode

1. Dedicated / Continuous / Long Call Mode In this kind of Drive Test we Make a continuous call along drive test activity Before

starting the route, call the drive test number and only stop the call when the route (drive

test) finish.

Figure 8 An Example of Continuous Long Mode Drive Test.

2. Idle Mode

A drive test activity in which, the MS is “ON” but no call occur. A powered on mobile station

(MS) that does not have a dedicated channel allocated is defined as being in idle mode (see

Figure 3). While in idle mode it is important that the mobile is both able to access and be

reached by the system. The idle mode behavior is managed by the MS. It can be controlled

by parameters which the MS receives from the base station on the Broadcast Control

Channel (BCCH). All of the main controlling parameters for idle mode behavior are

transmitted on the BCCH carrier in each cell. These parameters can be controlled on a per

cell basis.

Moreover, to be able to access the system from anywhere in the network, regardless of

where the MS was powered on/off, it has to be able to select a specific GSM base station,

tune to its frequency and listen to the system information messages transmitted in that

cell. It must also be able to register its current location to the network so that the network

knows where to route incoming calls.

Figure 9 An Example of Idle Mode Drive Test

3. SCAN Mode

• one of TEMS feature

• scan all or selected frequencies on the selected spot or route

• to find the clearest frequency

Its main application in frequency plan Application of frequency plan is to find the

best frequency to be use in the site and to identify interference adjacent channel

and co channel.

Figure 10 SCAN Mode Drive Test Example

The scanning can be performed by a TEMS mobile station or by a dedicated frequency

scanner mobile (TEMS Scanner). The mobiles supported in this version of TEMS

investigation are capable of all scanning tasks handled by TEMS Scanners, including CW

scanning. As measurement devices, Network Scanners are rigorously designed for the

challenges in network optimisation and trouble shooting. They include a high-end RF

front-end and sophisticated algorithms to quickly and accurately scan the air interface

and reliably detect all base stations and their signal components. In contrast to mobile

phones, they do not face the limitations of a consumer product in precision, processing

power and size.

In trouble shooting scenarios, Network Scanners come into play when a mobile phone

for example cannot register to the network, drops the call or faces degradations in its

voice or data quality. The Network Scanner can provide network information in

situations which are beyond the capabilities of a mobile phone.

3.1 Frequency Scan Using TEMS at a Glance:

• The Tems MS as an input device for detecting frequencies can reach 50x higher

measurement speed than test mobiles can deliver.

• The measurements are independent from the network and the operator.

• Spectrum analysis function and Channel Impulse Response (CIR) provide additional

information.

• Higher level and timing accuracy compared to mobile based measurements.

• Higher sensitivity and dynamic range than test mobiles.

• Use of only one unit for multiple networks and technologies.

• No costs and no capacity will be allocated.

• It can also perform Neighborhood frequency Analysis.

• Detection of RF problems in base stations like faulty clock, missing synchronization or

intermodulation.

Problem Identification and Resolution Using Drive Test

As mentioned earlier drive test plays a vital role in identifying, analyzing and

Troubleshooting issues arises specially at radio end. In this session we will analyze

identify and resolve issues that cause degradation in cellular networks. Following are

the major issues faced by network in day to day analysis of network performance.

1. Coverage Problems

2. Lack of Dominant Server

3. Sudden Decrease on Signal Level

4. Cell Overshooting Problem

5. Cross Sector and Cross Feeder Problem

6. Missing Neighbor Relation

1. Coverage Problems Low signal level is one of the biggest problems in a Network. The coverage that a

network operator can offer to customers mostly depends on efficiency of network

design and investment

plans. This problem usually pops up when building a new Network or as the number of

subscribers increases by the time resulting in new coverage demands.

Low signal level can result in unwanted situations that could directly lower the network

performance. Poor coverage problems are such problems that are really hard to solve,

because it is impossible to increase coverage by optimizing network parameters. Any

hardware configuration changes might improve the coverage a little. This is mainly

effect mainly by problems as shown in mentioned figure.

Figure 11 Depicting causes of Coverage issues

2. Lack of Dominant Server Signals of more than one cell can be reaching a spot with low level causing ping

pong handovers. This might happen because the MS is located on the cell borders

and there is no any best server to keep the call.

Figure 12 An Example of Lack of Dominant Server

3. Sudden Decrease on Signal Level It may be notice sudden decrease on signal level when analyzing the log files emerges.

This will result in excessive number of handovers. Before suspecting anything else,

check if the test was performed on a highway and that particular area was a tunnel or

not. Signal level on the chart will make a curve rather than unstable changes. Tunnel

effect will most likely result in ping pong handovers. The other reason that It may

happen for example that some peculiar propagation conditions exist at one point in

time that provide exceptional quality and level although the serving BTS is far and

another is closer and should be the one the mobile should be connected to if the

conditions were normal.

It may then happen that these exceptional conditions suddenly drop and the link is

lost, which would not have happened if the mobile had been connected to the closest

cell. So for these reasons, this cause does not wait for the power control to react.

Figure 13 An Example of Sudden decrease in Signaling Level

4. Cell Overshooting Problem When we get the signal from the site that not close to the current area drive test.

Usually we get bad RxQual and long/bigger TA.

We can suspect this as a overshoot case. This case happen when a site/cell is serving

far away from its area. This cause is used when a dominant cell provides a lot of

scattered coverages inside other cells, due to propagation conditions of the

operational network. The consequence of these spurious coverages is the probable

production of a high level of co-channel interference.

It may happen for example that some peculiar propagation conditions exist at one

point in time that provide exceptional quality and level although the serving BTS is far

and another is closer and should be the one the mobile should be connected to if the

conditions were normal.

It may then happen that these exceptional conditions suddenly drop and the link is

lost, which would not have happened if the mobile had been connected to the closest

cell.

5. Cross Sector and Cross Feeder Problem As the name suggests, this happens when the feeder cables of two different sectors are

completely crossed, which in turn leads to the fact that the coverage areas of the two

adjacent cells are swapped. Drive tester may observe a lot of HO failures and call drops.

A better understanding can be done while observing coverage level b/w Swap Sectors E

and F of Site NFZ0378 in below mention snap.

Figure 14 Drive Test results showing cross sector b/w two Cells

As can be observed from above snap that on the coverage area of sector F of NFZ0378 its

adjacent cell NFZ0378 of Sector E is serving whereas same condition applies for Sector E where

NFZ0378F is serving instead of Sector E of NFZ0378. As mentioned earlier that because of sector

NFZ0378F

NFZ0378E

swap increase in call drop at radio end with sharp rise in Handover drops observed because the

cells are serving in opposite direction of there coverage.

6. Missing Neighbor Relation Sometimes it is noticed that a good handover candidate in the neighbor list but

handover will not take place and call will drop. Although that neighboring cell with a

very good signal level appears to be a neighbor, It is because of missing

adjacencies/Neighbors is considered as common issue while monitoring network on

day to day basis in which the problem arises of which serving Site/Cell neighbors are

not properly assigned. By assigning neighbors it means that certain adjacencies should

be defined at OMC in order to carry out successful handover. Correct adjacency

definitions are the basic requirement for mobility. Optimization of neighbour cell lists

saves BS and MS transmission powers, since MSs are connected to optimal cells. Also,

the number of dropped calls is reduced.

Figure 15 Drive Test results showing cross sector b/w two Cells

As can be observed from above snap that on the coverage area of sector F of NFZ0378 its

adjacent cell NFZ0378 of Sector E is serving whereas same condition applies for Sector E where

Serving Cell

Defined Neighbor

Missing Neighbor

NFZ0378F is serving instead of Sector E of NFZ0378. As mentioned earlier that because of sector

swap increase in call drop at radio end with sharp rise in Handover drops observed because the

cells are serving in opposite direction of there coverage.

Solutions for Problems Concerning Cell Coverage, Lack of Dominant Server

and Sudden Decrease on Signal Level

Possible solution ways can be listed as below:

1. New Site Proposal

2. Sector Addition

3. Repeater

4. Site Configuration Change (Antenna Type, height, azimuth, tilt changes)

5. Loss or Attenuation Check ( Feeders, Connectors, Jumpers, etc..)

The best thing to do in case of low signal strength could be recommending new site

additions. A prediction tool with correct and detailed height and clutter data

supported with a reasonable propagation model could be used to identify the best

locations to put new sites. If client is not eager to put new sites because of high costs

to the budget or finds it unnecessary because of low demand on traffic, then

appropriate repeaters could be used to repeat signals and improve the coverage.

Adding repeaters always needs extra attention because they can bring extra

interference load to the network. The received level in the repeater should be above –

80dBm (or desired limits) so that it can be amplified and transmitted again. The mobile

should not receive both the original and the repeated signals at the same area, cause

signal from the repeater is always delayed and it will interfere with the original signal.

A repeater should not amplify frequencies outside the wanted band.

If none of the above recommendations are accepted by the client, then cheaper and

easier ways should be followed. First things to be checked would be possible

attenuation on the cells. Faulty feeders–jumpers–connectors or other faulty

equipment, high combiner loss, reduced EIRP, decreased output power, the

orientations and types of antennas, unnecessary down tilts, existence of diversity and

height of the site should be deeply investigated. Putting higher gain antennas,

increasing output power, removing attenuations, changing antenna orientations

towards desired area, reducing down tilts, replacing faulty equipment or usage of

diversity gain could improve the coverage.

Solutions for Problems Concerning Cell Overshooting

Following are some actions that can be performed to overcome the problem concerning

overshooting.

1. Change the azimuth of antenna to restrict its coverage.

2. Adjust the antenna down tilt.

3. Add missing adjacent cells for the cell to allow MS to camp on that cells while

leaving serving cell boundaries.

4. Both Electrical and Mechanical can be fine tuned to achieve desired coverage.

Solutions for problems concerning Cross Sector & Cross Feeder

After confirming Sector Swap issue through drive results the mentioned case was

escalated to operations departments to physically intervene the site and resolve issue

as most of the time it was found that semi rigid cables of two or three sector are partly

swapped. Sometimes TX cables are corrects but RX main and Diversity cables of two

sectors are swapped. Some time TX cables are swapped but RX cables are in their place.

Finally, physical intervention is required in site to solve the issue Concerning

Sector/Feeder Swap.

Solutions For Problem Concerning Missing Neighbors

The best solution is to perform visual neighbor audit while facing with missing

adjacencies/Neighbor issue. On field drive results would also help in identifying

potential suitable neighbors.