EFFECTIVE FREQUENCY PLANNING TO

ACHIEVE IMPROVED KPI's, TCH AND

SDCCH DROPS FOR A REAL GSM

CELLULAR NETWORK

1 Prabhjot Singh, 2 Mithilesh Kumar, 3 Ambarish Das 1,2 Electronics Engineering Department, University College of Engineering, Rajasthan Technical University

Kota, India

3 Institute of Radio Physics and Electronics, University of Calcutta Calcutta, India

1 prabhsingh007@gmail.com , 2 mith kr@yahoo.com, 3 ambar 64@yahoo.com

Abstract--- The drastic development in the field of

wireless communications has resulted huge demand for

voice and data communication in public domain but

spectrum is the major concern and scarce resources

which imposes a high cost on the data transmission.

Our objective is to provide a quality communication to

maximum number of users. The number of users

supported by system can be increased by using more

frequencies, hence efficient RF (radio frequency)

planning is required to maximize the limited spectrum

resources. One has to look for optimum compromise

between dense reuse with least interference. To

establish the quality communication, we investigate KPI

(key performance indicators), TCH (traffic channel)

and SDCCH (stand alone dedicated control channel)

drop on 93 GSM sites for a cellular operator in a city of

India. We design an efficient frequency planning

approach practically with limited spectrum availability

and BCCH-BSIC (broad cast channel- Base station

Identity code) set of frequencies, MAL (mobile

allocation list) and DCHNO (TCH) frequency with

NCCPERM (national color code permitted) parameter

are used for least interference. We investigate with the

help of Google Earth and simulation software to

practically implement these change values in field data.

We have found a significant improvement in KPl's,

TCH drop and SDCCH drop which out perform the

previous frequency plan. TCH drop in NBH period is

improved by 15 %, SDCCH drop in NBH (national

busy hour) period is improved by 6 %. We also

observed that during BBH (bouncing busy hour) period

as per TRAI (telecom regulatory authority of India)

guideline TCH drop � 2 % is improved by 47 % and

SDCCH drop � 2 % is improved by 43 % with respect

978-1-4799-3140-8/14/$31.00 ©2014 IEEE 673

to previous frequency plan. It is relevant with customer

satisfaction and enhances revenue for the cellular

operator.

Keywords: - Frequency planning, Cell planning; BCCH,

BSIC, HSN, MAIO, TCH Drop, SDCCH Drop, NBH,

BBH

1. INTRODUCTION

Wireless communication plays a key role to

transmit large amount of information to the

longer distance, now a days research in wireless

communication is growing towards effective

frequency planning for a cellular network in a

city. A cellular or mobile network is a wireless

network distributed over land areas called cells,

where each served by at least one fixed-location

transceiver. This is known as macro cell site or base station (BTS) where each cell uses a

different set of frequencies from neighboring

cells, to avoid interference and to provide

guaranteed bandwidth within each cell. This

macro cell system provides radio coverage over a wide geographic area which enables a large

number of mobile phones to communicate with

each other anywhere in the network, via base

stations [1-3]. This also enables to use voice

communication, data communication with high

speed and less interference.

Minimization in interference is a key objective for any cellular operator to provide smooth coverage maintaining network KPI's and customer satisfaction which is linked with the revenue of the company, therefore effective frequency planning is off utmost importance. Recently a new approach had been represented based on large scale optimization to study FFR in network with irregular cell layout which is important for real life network [4]. They have considered number of sub-bands as a parameter, where in conventional FFR scheme three sub­bands have been used. This parameter enables significant throughput improvement at cell-edge zones, and also splits the cell edge band into more than the standard three sub-bands.

A theoretical approach has been studied to improve KPI's for a GSM cellular network such as call set up success rate, call drop rate, traffic channel congestion rate and handover success rate [5]. They have discussed the steps involved in RF optimization and parameter retuning which is useful in improving network performance. The solution for overshooting problem in GSM network by increasing antenna down-tilt and improving KPI's is demonstrated [6]. TCH call rate is improved by 0.1 % and SDCCH call drop rate is decreased by 0.2 %. Uplink quality is improved by 0.3 % and downlink quality improved by 0.2 %. They had also discussed another mechanism to reduce overshooting. A generic cell configuration scheme using antenna orientation rotation technique is used [7]. It is demonstrated that the clover-leaf cell configuration which is widely used in wireless industry for radio network planning is a special case of this generic cell planning method with antenna orientation rotation with 30° for all sectors. Applying this novel cell configuration scheme, alternate row antenna rotation (ARAR) 4x12 GSM frequency reuse is proposed. In ARAR scheme half the antennas are rotated by 60°. It improves the C/I ratio at weak coverage link by 2.5 dB. Data throughput had been improved in wireless packet system with link adaption and efficient frequency reuse [8]. They employs link adaption between numbers of modulation in coding schemes as means of providing several hundred kbls peak rates in macro cellular environment. He also introduced a simple but effective slot re-assignment method for improving packet data throughput. He shows that high peak rates achieved by link adaption can provide high throughput but the user QoS is strongly impacted by tight frequency reuse. His

method improves QoS also. An analytical method to optimize an existing cellular network [10] had been discussed and practical approach of planning for efficient GSM network users had derived. They showed that optimization is tradeoff between quality, traffic revenues and investment. He performed this along with drive test. This study is helpful for operators to enhance coverage of network and improved quality which is important for company revenue. A frequency analysis method is used to determine the performance of 3 GSM networks in Turkey and analyze with the help of drive test [11]. Drive test is an important activity to measure correctness of frequency planning design for a mobile network. A case study about evaluation and optimization of GSM network in Jenien city of Palestine discussed along with drive test reports [12]. Through this optimization, TCH drop rate reduce from 0.76 % to 0.62 % whereas the quality samples is increased from 65.5% to 76.8%.

In this paper we propose a scheme of frequency planning for a real GSM network for an operator and its implementation over existing frequency plan to improve KPI's. KPI's are used to judge the network performance and evaluating the quality of service which results in customer satisfaction and the business revenue for an operstor. We have collected RF field data practically and perform frequency planning by commercial tool. By carefully assigning BCCH frequency, BSIC allocating the MAL frequency along with HSN (hoping sequence no) and MAIO (mobile allocation index offset) as per current BTS site configuration. Later on in­consistencies like CO-BCCH, CO-BSIC, cells retune in order to minimize co-channel interference to achieve desire KPI's.

This paper is organized as follows. In section II Reasons for degraded KPI's has been discussed. Section III contains performance evaluation of system. Section IV contains frequency planning design. Section V contains result and summary and section VI contains conclusion followed by references.

II REASONS FOR DE-GRADED KPI's

There are many reasons for de-graded KPI's which are as follows,

1. Wrong frequency planning of an eXIstIng system or due to addition of a new BTS site.

674 20 J 4 International Conference on Signal Propagation and Computer Technology (ICSPCT)

2. TCH drop reasons (Fig. 1) are following as

excessive timing advance, low signal strength, bad quality and sudden loss of

connection. 3. SDCCH drop reasons are low signal

strength, bad quality, sudden loss of

connection, faulty hardware and ambient conditions like temperature.

4. Definition issue (BTS/ BSC/ MSC). 5. Any BTS performance alarm like diversity

alarm and path loss.

6. Errors on the transmission medium.

Dropped call counters

Fig 1 Reasons for drop call

III. PERFORMANCE EV ALUA nON

The network system is constantly monitored for system performance to maintain a sufficient level

of call quality and coverage of the network with good set of KPI's. To measure the performance of the

network one has to take care of switch or NMS (network monitoring section) data, past drive test reports, traffic reports, KPI's reports and customer

complaint data. NMS data is most useful entity in this context. This helps us to know the current status

regarding network status and site configuration. This represents information of site fault alarms registered, call attempts / failures, traffic reports, TCH and

SDCCH drop status and handover statistics.

In order to measure the network performance, the daily reports from NMS team has to be considered, while for performance evaluation call

drop issue, congestion situations should also be analyzed. Following KPIs are more important for

GSM radio network to achieve remarkable KPIs such as TCH drop (traffic channel drop means ongoing call drop), SDCCH drop (standalone dedicated

control channel drop responsible for initial call set up

or SMS) and HOSR (handover success rate). Table 1 demonstrates formula table used for calculation of

TCH drop and SDCCH drop by using mentioned counters. These counters have been obtained by reports.

Table I

Parameter Description

No of Channels No. ofTCH defined for each cell

TCHDrop TNDROP I TCASSALL %

TNDROP No ofTCH drops

CMSESTAB No of SDCCH connections

TCASSALL Total complete TCH assignment attempts

CNRELCONG No of dropped SDCCH connections due to TCH

& tTanscoder congestion

No of released TCH signaling connections due

TNRELCONG to transcoder resource congestion during

immediate assignment on TCH

CNDROP Total no of dropped SDCCH connections.

SDCCHDrop (CNDROP-CNRELCONG)/CMSESTAB % Rate

IV FREQUENCY PLANNING DESIGN

Frequency planning is a necessary step to meet out our new targeted KPI. Meanwhile, it has been also observed that addition of some of new BTS

sites in a city can change holistic network perception of the city (if wrongly planned). Improper planning

of only 5 towers is quite sufficient to disturb overall KPI of a city. It is necessary to go for new frequency planning, if we have added 5-6 towers in last 5-6

month. This activity is done by RF Engineers keeping in view call drops, customer complaint and

benchmarking in last 5-6 months. One has to follow tools specific rules for implementation.

We have taken 93 GSM 1800 MHz BTS sites for a service provider and practical field data is

collected by visiting all these 93 sites physically. We have collected latitude-longitude with the help of GPS, azimuth orientation with the help of compass,

noted the height of GSM Antenna with the help of scale, measuring electrical or mechanical tilt. It is

very rigorous and important activity for this implementation. Existing carrier data is collected with the help of simulation software or by NMS team

representing current state of BCCH, BSIC, LAI

2014 International Conference on Signal Propagation and Computer Technology (ICSPCT) 675

(location area identity), CI (cell identity) and TCH

frequency, MAL Frequency and HSN. Now we can

make a format required as per commercial tool. We have to fetch neighbor details from simulation

software or by NMS team. By collecting these three

reports formats and using commercial tool, we get a

pictorial presentation of sites map representing current data base.

Spectrum frequency is 6.2 MHz, GSM

carrier frequency is of 200 KHz, therefore total available frequency are 31 and as per AFRCN

(automatic radio frequency channel number) from

TRAI spectrum allocation list, out of 31, 14 number

of frequencies will be used as BCCH Frequency, any one frequency as a guard band and rest 16

frequencies as TCH (MAL) frequencies. Hopping

effective frequency planning of these available

frequencies is only option for any cellular operator to

design the network. We have to assign 5 sites with 15 BCCH frequencies to 15 cells & continue by

carefully designing for all sites. Each cell uses its

own BCCH, BSIC, HSN and MAL frequency as per

its configuration in live network. MAIO is given and channel group is also defined keeping in view of least

interference and we make sure that cell is practically

apart using same set of frequency. This is irregular

cell layout which depends on geographical clutter availability. These implementations are implemented

by simulation software in system by NMS (network

monitoring section) team. We need to check whether

proper implementation is done and all the parameters have been implemented as per planned values.

Afterwards, we check any inconsistencies like CO­

BCCH and CO-BSIC relations in system. If required,

frequency retune is done again, neither interference may occur. This process is commonly follow by

cellular operators in India.

We have analyzed the traffic reports conslstmg of

this new frequency plan. We have checked all the KPI's that must be in the desired range. This activity

has been done in 2014 and for analysis we consider

four days report as a pre data and four days after the

activity as a post data. KPI perfonnance has been

studied over NBH and BBH. The obtained results are very promising which are as follows.

I. TCH Drop (NBH) improved by 15 % over

previous frequency plan.

TCH Drop-NBH

14

1.2

0.8

0.6

0.4

0.2

TCH Drop

I Pre 1.31

1M 1.11

I % Improvement 15%

M

IPre 1M 1% Improvement

% Improvement

The proper & correct value of MAIO (mobile

allocation index offset), HSN (hopping sequence number), BCCH (broadcast channel), BSIC (base station identity code), DCHNO (MAL frequency), Fig 2. Plot ofTCH drop improvement pre and post

NCCPERM (NCCPERMIT), CHGR (channel group

define in BSC) will help us to grow the design of frequency planning & similarly planning will be done

by specific tool to avoid manual mistake and to

obtain better result. Handover success rate also affect

the TCH drop and SDCCH drop. One has to see for proper HOSR neighbor addition-deletion to reduce

call drop and increase customer satisfaction.

V SUMMARY AND RESULTS

2. SDCCCH Drop (NBH) Improve by 6% over

previous frequency plan.

676 20 J 4 International Conference on Signal Propagation and Computer Technology (ICSPCT)

IPre

IpM

1.2

0.8

0.6

0.4

0.2

1% Improvement

SDCCH Drop·NBH

SOCCHOrop

113

106

6%

I Pre I pM I % Improvement

Fig 3. Plot of SDCCH drop (NBH) improvement pre

and post

3. No of cells BBH SDCCH Drop :'S 2.0 % is improved by 43 % over previous frequency plan. The existing system was 87.5 % therefore we have scope of improvement over rest 12.5% cells. The new frequency plan reduces this count upto 7.08% with the post value 92.92 %. This leads to 43%

improvement over existing system.

SDCCH Drops cells::; 2% - BBH

100 90 80 70 60 50 40 30 20 10

SDCCH Drop �2 % 1 Pre 87.5 • Po'/: 92.92 1 % Improvement 43%

Po'/:

1 Pre 1 Po'/: 1 % Improvement

% Improvement

Fig 4. Plot of SDCCH drop cells :::2% --BBH pre and

post

4. No of cells BBH TCH Drop :'S 2.0 % is improved by 47 % over previous frequency plan. The existing system was 75.52% cells therefore we have scope of improvement over rest 24.28% cells. With new frequency plan implementation, this count reduces upto 13.03% with post value 86.97% therefore it leads to 47% improvement over existing system.

TCH drops cells S 2% - BBH

100 7 80

60

40

20

o

86.97

TCH Drop $ 2%

• Pre 75.52

• Post 86.97

• % Improvement 47%

Pre

% Improvement Post

• Pre • Post • % IrnJJrovernent

.Fig 5. Plot ofTCH drop cells:s 2% --BBH pre and post

2014 International Conference on Signal Propagation and Computer Technology (ICSPCT) 677

678

5. Figure 6 represents SDCCH & TCH drops-­

NBH period over common scale. While

implementing new frequency plan, SDCCH

drop is improved by 6% and TCH drop is

improved by 15 % over existing system.

I Pre

IpM

14

12

0.8

0.6

0.4

0.2

SDCCH & TCH Drops-NBH

SDCCH Drop 1.13

1.06

1% Improvement 6%

TCH Drop 1.31

1.11

15%

• Pre • p09: l%.!mprover..,nt

Fig 6. Plot of SDCCH and TCH drop in NBH period over

common scale

6. Figure 7 represents SDCCH & TCH drop

¥> .1!4c --BBH period over common scale.

With new frequency plan implementation,

SDCCH drop is increases by 43% and TCH

drop increases by 47%.

SDCCH & TCH Drop<=2% BBH

100

80

60

40

20

SDCCH

Drop

TCH

Drop

<=2.0% <=2%

Ipre 87.5

I post 92.92

• % improvement 43%

75.52

86.97

47%

I pre I post • % improvement

post

%improve

Fig 7. Plot of SDCCH and TCH drop lrb.� in BBH period over

common scale

VI CONCLUSIONS

A practical approach for frequency planning of real

live GSM network for a service provider has been

shown. Our aim is to maintain efficient network for

customer benefit along with the company revenue. In

this paper, it has been practically proved that one can

optimize an eXlstmg cellular network using

optimization tools and fine parameter tuning. One

can take information from customer complaint, drive

test and traffic statistics. Here we have implemented

by taking view of 93 BTS sites participating

practically over entire network. This is qualitative

KPI's analysis and trend of the graph is obtained of

this form only. We achieved decreased TCH drop and

SDCCH drop over BBH and NBH period, which

have shown on common scale in Fig 6 & 7. Fine

approach is only assured by careful implementation.

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REFERENCES

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3. http://www.telecomsource.netlshowthread.php?86-Ericsson-training-material

4. Chen Lei and Yuan Di, "Beyond conventional fractional frequency reuse for networks with irregular cell layout: An optimization approach and performance evaluation", The 5th Annual ICST Wireless Internet Conference (WICON), 2010, 1-7,2010.

5. Sh. O. Seytnazarov, "RF Optimization issues in GSM networks",4th International Conference on Application of Infonnation and Communication Technologies (AICT), 2010,1-5,2010.

6. Yunxia Liu, Jianing Wang, Qiang Wang, Guangfeng Qin , Luping Zheng, Chunsheng Dong, Li Sun , Ping Zhang, Lei Song, "Centralized antennas adjustment of overshooting and its influence on network indexes" International Conference on Automatic Control and Artificial Intelligence (ACAI 2012),631-634,2012.

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8. Justin C-I Chuang, "Improvement of data throughput in wireless packet systems with link adaption and efficient frequency reuse", 49th Vehicular Technology Conference, 1999 [{Volume: I ) ,821-825,1999.

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