Cell Selection And Reselection 5uq61

GSM RF OPTIMIZATION BASICS www.huawei.com

HUAWEI TECHNOLOGIES CO., LTD.

Huawei Confidential

GSM

Contents 1. Introduction to RF Optimization

2. Coverage and Quality Analysis 3. Derive Test Cases and TA

4. Summary

Introduction to RF Optimization • Radio Frequency Optimization is process through which different Process of changing soft(CRO,Power,etc) and hard(Azimuth and Tilt)of BTS in order to improve coverage area and quality of the signal In GSM Technology it is highly required to: • Control Signal Level Coverage(Rx Level) • Control Interference • Control HO(Dedicated mode) and CRO(idle mode) factors( based on DT)

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Introduction to RF Optimization • Some KPI Thresholds Parameter Rxlevel Rx Quality CSSR CDR CSHO Average MOS call set up time

Threshold Remarks >-90dBm 0 to 4 >98% <0.2% >98% >3 <4ms

• Depends on scenarios(Urban,suburban and rural) Page 4

GSM

Contents 1. Introduction to RF Optimization

2. Coverage , Quality and Other Network Issues 3. Drive Test Cases, and TA

4. Summary

Coverage , Quality and Other Network

Issues

• Issues faced while network performance analysis • • • • • •

Coverage problem Lack of dominant server Sudden decrease on signal level Cell overshooting Cross feeder Missing Neighbor Relation

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Coverage , Quality and Other Network

Issues

• Coverage Issue • Weak signal can be caused by attenuation of the signal due to long distance travel or path loss. Solution: • • • • •

New site proposal Sector Addition Repeater Antenna parameter adjustment(azimuth and tilt) Loss or attenuation check(feeder ,connectors jumpers etc)

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Coverage , Quality and Other Network

Issues

• Lack of Dominant Server • More than one signals of strength from different cells with little margin can be reaching a spot causing Ping pong handovers. Solution: • Antenna parameter adjustment(azimuth and Tilt) • Increase Tx Power of the cell needed to be dominant. Decrease others.

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Coverage , Quality and Other Network

Issues

• Sudden Decrease of signal strength • This could be caused by blockage or scatterers Solution: • Antenna parameter adjustment(azimuth and Tilt) • Increase Tx Power of the cell.

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Coverage , Quality and Other Network

Issues

• Overshooting cells • When site cell serves from far (bigger TA)can cause bigger level co-channel interference (if same band) causing poor Rx quality Solution: • Antenna parameter adjustment(azimuth and Tilt) • Decrease Tx Power of the cell.

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Coverage , Quality and Other Network

Issues

• Cross feeder problem • When the feeder cables of two different sectors completely crossed. It causes a lot of HO failures and call drops Solution: • Exchange the feeder cables so that all sectors serves in the right direction.

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Coverage , Quality and Other Network

Issues

• Missing Neighbors • A good handover candidate in the neighbor list can be noticed but HO will not take place and call will drop.Note that optimum of neighbor cell lists save BS and MS transmission power. Solution: • Exchange the feeder cables so that all sectors serves in the right direction.

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GSM

Contents 1. Introduction to RF Optimization

2. Coverage and Quality Analysis 3. Derive Test Cases and TA 4. Summary

Cross Feeder Analysis • When different sectors serving direction is swapped as shown below, the feeder cable is crossed.

The feeder connection has been shifted clockwise direction as shown in the above figure. Change the RI cable for DCS anti-clockwise to solve the cross connection (i.e. Sector 3 to sector 2, sector 2 to sector 1 and sector 1 to sector 3). Page 14

Call Setup Failure Due to Poor Coverage • call set up failures shown below has occurred due to weak coverage. The area is covered with dense forest and also no site for road coverage

• Adding

GSM site

for the coverage will resolve the coverage issue. Page 15

Call Drop due to Poor Coverage • The above events has occurred due to poor coverage from site 111440. this is due to mountain blockage. The UE sent many MRs but no response from the network since the link has broken.

• adding new site in the weak coverage area will resolve this issue Page 16

Hand over failure due to poor quality • The serving site 111402G1 is serving from far location and this caused abnormal release due to time out.TA=3

•111059G2 and 111036G1 are not neighbors for the serving cell since far cells. Hence limiting the coverage for 111402G1 will solve the problem. Page 17

BCCH Interference • If frequency not planned properly, interference will occur between same frequency band. Red arrow is interfering BCCH and Green ones are proper. Same for TCH also

• Needs to check frequency planning if proper or not.Note that interference can be: Adjacent channel, co-channel and intermodulation

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Time Advance(TA) • Main purpose is to allows GSM BTS to control signal delays in their communication with the mobile (for sync and other purposes).Transmited in SACCH • Overshooting Sites Coverage can be optimized using TA. • For TA_0 the MS is between 0 and 550 meters from BTS. • For TA 1 0 to 1100m •……………… • For TA>63 is 35km(max) Example Sector 112142_ G1

M Tilt Before

M Tilt After

90% TA before(m)

90% TA after(m)

2

6

7150

4950

Before

After

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GSM

Contents 1. Introduction to RF Optimization

2. Coverage and Quality Analysis 3. Derive Test cases and TA 4. Summary

4. Summary • Neighbour definition is prior importance in GSM network for timely handover in order to keep signal coverage and quality at good level. • Handovers can take place based on signal level, signal quality, Load and other factors(ex. UL HO RX level margin is 10 and HO DL margin is 20 for Huawai algo. 1.) • CRO usually set not more than 25dB. Reasonable setting of this value reduce frequent cell reselection. • Frequency Planning is very important in GSM for Good network performance.

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