Abstract: GSM network consists of two parts one is mobile switching and other one is radio. The radio part is a critical factor which effects quality of services in GSM network. Radio network optimization refers to modification according to some rules in order to get best quality of services and best utilization of resources. Optimization can be divided into three parts, initial optimization, primary optimization and maintenance optimization. The main focus throughout the research will be on optimization by drive test and how to overcome the network problems such as call failure, handover drop rate, non-working TRX, over shooting sites, PING PONG handover i.e. a place where user gets same signal power from two different BTS, interference problems, missing neighbours, faulty installation of equipment, not having handovers, analysing of call flow ,Rx level, and Rx quality. The following parameter will be considered in order to improve the network performance. These parameters are such as Defining missing neighbour relations, Suggesting new sector, Recommending antenna azimuth changes, proposing antenna tilt changes, Reâ€"tuning of interfered frequencies, BSIC changes, changing power parameters antenna height change, azimuth changes, BTS equipment modification. The Drive Test (DT) is use in RF optimization GSM network to assure the availability and reliability of the network.
Keywords RF optimization, GSM network optimization, Drive Test
1 Introduction
Network optimization is necessary for the improvement of performance and for best utilization of resources .It is a way in which network can be modify on the basis of collecting data, usually done by Drive Test .Network optimization can be analyse by different steps like 1 collecting basic data,2 drive test ,3 OMCR data statistical Reports.1 Collecting Basic Data ,Basic information from network planning like tower height, antenna height and type, azimuth, BSC and location area code(Lac),antenna tilt, frequency band.2 Drive Test This test is done to check whether the requirement of site is fulfilled or not. It is a method of determining about network performance by means of accessibility, capacity, and congestion rate. If the results of this test do not match with requirement than some changes are suggested in order to get desire goal. 3 OMCR data statistical Reports It includes the traffic reports about call drop, call failure, handover failure and so on.
RF OPTIMIZATION OF GSM
Procedure of GSM network optimization is as shown in figure
Problem Statement
Optimization of BTS in good way is really a challenge .if the optimization is successfully done than we will be achieve best quality of services
2 Optimization Equipment
2.1 LAPTOP
All the parameters are analyse on special type of software installed in laptop
2.2 TEMS Software
TEMS software by Ericsson, software which is used for optimization, this is installed in laptop and all RF parameters can be analyse. The software is also connected with TEMS mobile
2.3 Global Positing System (GPS)
GPS is connected with laptop .its basic function is to locate position of drive test path.
2.4 Compass
It is used to check the tilt of antenna on the tower
3 Drive Test Process
3.1 Basic Information about Network
Before staring optimization, we need to get basic information to make optimization plan .Different information like frequency planning, engineering parameters .These information are helpful in optimization process.
After the complete installation of site, RF engineer reach on site by using MapInfo software. In first step, DT engineer check the antenna height, azimuth and tilt in order to confirm that all these equipment installation is according to planning. After connecting equipment, engineer start recording of log files and establish call from TEMS mobile to check intra site handovers, both clock wise and anticlockwise. In TRX (Transceiver) test, he make 10 calls for 15 sec duration from each TRX to check the TRX performance. Now engineer make long log files to check INTER site handover between neighbouring BTS in idle and dedicate mode, also analyse a numbers of parameters like RX level, RX signal quality, Co channel interface, adjacent channel interface, coverage of BTs in each sector. GPRS services are also checked from each sector from different location. If RX signal level is drop due to obstacle like mountain, building, forest, RF engineer must record the latitude and longitude and show the reason of weak signal at particular place. The Drive Test process from TEMS can be shown in figure
3.1 TEMS Software Information
It provides different information in separate windows like cell identity, base station identity code, ARFCN for up to seven neighbours, Mobile country code(MCC),Mobile network, code(MNC), .code(LAC). These information are displayed in static windows, information about RX level, Quality, C/I, handover are displayed in graphical window .Layer 2 and layer 3 messages can also be displayed in different windows.
3.2 Fundamental Factors of Network Performance
3.2.1 Accessibility
To check accessibility, RF engineer make short call .The accessibility is measured in following way
Accessibility=SDCCH serviceability*TCH service ability
3.2.2 Retain ability
For checking retain-ability, long calls are establish during drive test. Retain-ability is required 100 per cent. The formula for retain-ability is
Retain-ability=1-TCH call drop rate
TCH call drop rate= (number of drop call)/ (total number of TCH attempt)
Numbers of drop call consists of all drops due to Abis fault, A interface losses or BTS failure
4 Idle Mode Behaviour
When no dedicated channel is assigned to mobile, the mobile is said to be in idle mode. In idle mode mobile is able to access network at any time. The idle mode behaviour of mobile is controlled by base station. Parameters which are necessary to control idle mode are transmitted on BCCH carrier. The mobile is able to access the system after power on.it is also able to update its location so that the network will be able to forward a call to specific user mobile. Idle mode behaviour of mobile in TEMS is shown in diagram.
In idle mode, mobile is sent message to network about its change in position and visitor location register update new location of mobile.
4.1 Location Update
The MS continuously analyse the system information received from the network and compare it with location area identity (LAI) which is store in mobile station. If LAI is different from stores LAI than location update is performed. The MS send location update request which include new LAI and older LAI and cell identity. The authentication parameters are sent to MS if VLR is same, if the visitor location register (VLR) is not same new MSC/VLR sent request to older MSC/VLR in order to authenticate the MS .If the authentication is successful new VLR is updated and older VLR is also updated. New MSC order BTS to assign SDDCH to MS. The MS then release SDCCH and goes to idle mode. If MS is changing its location area rapidly then every change in location needs to update location area identity (LAI)
4.2 Call Setup
In cell selection procedure, the MS tried to get cell. Once a cell is selected, mobile station start to search best RX level cell from neighbouring cell list in order to make cell reselection. After establishment of radio connection between mobile station and network, The MS send call setup request (including number of destination). The cell identity and IMSI are analysed, also authentication are requested from VLR .After this process, MSC receive a call setup request from mobile. MSC then analyse the services for MS like call barring for calls. These services are activated by user or from network. If MS is not barred MSC send a message to BSC for traffic channel request. BSC check about free TCH and allocate it to call and ask base station (BTS) to activate channel. The MS send confirmation message and BSC assign a TCH to mobile and inform to MSC about TCH assignment. After that call is connected and a ringing message is sent to calling number ,When called number pick up call, a message is send to indicate that a call setup process in completed
Signalling for call setup C/I for Hopping cell C/I for nonhopping
5 Analysis of Log Files
5.1 Coverage
One of the important parameter in optimization is coverage. There are several factors which effects on coverage, if there is any place between two different BTS where signals cannot reach, that particular place is known as blind zone. This problem can be resolved by increasing transmission power .Blind zone in tunnels can be removed with help of repeaters. The top solution of coverage problem is installation of new sites or repeater. If customer is not willing to improve the situation by adding new sites or repeaters than RF engineer will have to detect possible reason of attenuation by checking connectors, jumpers or others equipment on site, after checking equipment on site, coverage can be improve by swapping antenna type and height, replacing faulty equipment or removing attenuation.
5.2 Quality Problems
The collected indicators those give information about the speech quality are Calls dropped because of poor quality, RX level drop, release call because of poor quality, handover drop.
5.2.1 Bit error rate (BER) and Frame erasure rate (FER)
BER and FER have also effect on speech quality. In order to reduce BER and FER, number of factors is considered like co channel interference and adjacent channel interference. A good cell planning is required to overcome interference problems.
Bad Rx Level, Rx Quality and SQI
As signal level drop as shown in above TEMS graph, Rx quality and SQI are also dropped
5.2.2 Speech Quality Index (SQI)
SQI is another parameter when quality is under consideration. Using TEMS software SQI can also be estimated which depends on handover, BER and FER. The quality of speech can also depends on radio channel, echo, and type of mobile using by subscriber. Different test are made in order to check which parameter is affecting on quality.
5.2.3 CO-Channel Interference (C/I)
Co channel interference is produced due to the same carrier used in others cells. According to the GSM standards, C/I must have larger than 9dB for hopping frequency. i e C/I>9dB and for non-hopping frequency C/I requirement is greater than 12DdB.Interference is present not only in downlink frequency but also on uplink frequency but interference in downlink will create serious problem as compared on uplink frequency interference.
5.2.4 Adjacent channel Interference (C/A)
The distance between uplink and downlink frequency in GSM is 200KHZ which is called channel separation. This is necessary to reduce interference between frequencies. Adjacent frequencies are not used in same cell, these are used in different cells, GSM standards shows that carrier to adjacent channel ratio must have larger than -9dB. Adjacent channel interference reduces C/I performance. During GSM cell planning C/A must have larger than 3dB .i e C/A>3dB.
5.2.5 Inter System Interference (ISI)
Sometime interference is produced by other wireless networks operating on same adjacent frequency or military network are operating on the same frequency. This kind of interference is very difficult to avoid .If interference from other sources are affecting on quality of services than a comparison drive test is made to analyse to get clear situation.
5.2.6 Signal Propagation
Many factors effects on propagation like reflection, diffraction, and absorption .Usually, the height of mobile antenna is low as compare to building or trees, so signal comes across mobile antenna through different ways with delays. Amplitude and phases, sometime the signals are completely out of phase and the cancel each other. Rayleigh fading also effects on signal quality. Different diversity techniques are used to overcome these problems depending on the situation.
5.2.7 Time Dispersion
Due to different terrain like mountain, high building, hilly villages, time dispersion problems are produced. If the time delay is less than 15ms than equalizer manages the problem .In poor quality, time dispersion is considered in order to check the actual cause of problem. There are several factors which kept in mind when dealing with time dispersion. Some reflection is useful and some are harmful, only those reflection are harmful which causes delay in signal more than equalizer capacity. To avoid time dispersion, site must be suggested to locate near reflection object.
5.2.8 Handover
Rx level of severing and neighbouring cell are continuously measured by network, whenever signal level of serving cell is low and neighbour cell is high, a handover is attempt from weak signal level site to stronger signal level site. There are different types of handovers in GSM system.
1 INTER BSC HANDOVER
VLR is same but cells belong to different BSC. In this type of handover MSC/VLR help BSC to take handover
2 INTRA BSC HANDOVER
In this type of handover, subscriber remains in the same BSC and handover take place in different cell of same BSC.
3 INETR CELL HANDOVER
Handover between different sectors of same cell
4 INTER MSC HANDOVER
The handover between cell of different MSC, In this case serving MSC/VLR helps to new MSC/VLR in order to handover take place.
During call, serving BSC decide about handover depending on RX level and position of subscriber. The handover procedure follows some basic rules like, BSc sends a handover request to MSC including cell identity(CI)of target cell, The serving MSC and new MSC communicate with each other for handover request than new MSC assign a handover number and send message to BSC. The new BSC release a TCH to target cell in order to make handover, the new MSC receive message about handover with reference number.
5.2.9 Types of Handover
5.2.9.1 Power Budget Handover
In handover parameters, power budget handover are defined, if the signal level between serving and neighbour cell is exceeding to a specific level (define in power budget handover which is set to be 3 to 6dB), the call is transfer to neighbour cell by handover process. If there is Ping-Pong handover, than the problem can be solve by increasing power budget handover margin.
5.2.9.2 Quality Handover
An immediate quality handover is happen when down link quality is worse than threshold quality level.
5.2.9.3 Rapid field Handover
When the subscriber is travelling ,RX level will changes with the position of user, at some places, signal level will sudden decrease so rapidly that BSC will not able to perform handover and call will be drop. This type of call drop is known as rapid field drop. The problem can be solved by defining signal level margin i.e. if signal level drop up to -110dbm in certain areas than call will be handed over to next best Rx level cell.
5.2.9.4 Ping Pong Handover
In some places subscriber gets same signal level from different base station and result is rapid handover from one BTS to another BTS .The solution of Ping-Pong handover is to change the antenna tilt, azimuth angle or reduce transmission power of one BTS.
5.2.10 call Drops
There is several reason of call drop. If the radio link is fail than call is drop. User drops the call during talking due to TCH drop. These kinds of drop depend on different reasons like drop due to timing advance (TA).User is at a long distance from tower. The possible solution is installing a new BTS. Call may also be drop due to interference, which may be co channel or adjacent channel. To remove interference, Optimizer should find reason for interference.
6 CONCLUSIONS
This paper describe the optimization of GSM network .It is important for every network to be monitor on regular basis in order to detect the problems. This is the only way in which we can increase customer satisfaction by improving the performance of network. One of the cheap and most popular methods is drive test .By detecting the problem; we can easily find the solution to improve quality of services.
