GO_NA22_E1_1 GSM UL-DL Unbalance & Solutions-31

GSM UL-DL Unbalance & Solutions

Zte university

Training goals

To know definition of link balance To master calculation process of link balance To understand the factors influencing link balance

and the checking process of link balance

Contents

Definition of link balance

Theoretical calculation of link balance

Test method of link balance

Analysis and elimination of link unbalance

Practical cases


What is link balance?

In radio communications, UL and DL, they each have their own transmitting power, equipment loss, pathloss and receiving sensitivity. Strictly speaking, link balance means that the UL-DL radio pathloss difference is “0”.

For each network, link balance is one of the key factors to setting up network and ensuring network performance.

UL-DL unbalance may result in network problems, such as failure in starting a call, handover failure and silent call, etc..

Deviation range of link balance

link balance judgment actually refers to judging the deviation range. In UL-DL calculation, a deviation within ±10dB is allowed.

If this range is exceeded, we can say that the link is in unbalanced status, hence some network optimizations or engineering checking procedures should be considered so as to solve the problem.

Explanations on engineering margin

Origin of the engineering margin±10dB: UL/DL power control

Since we set it according to the more conservative threshold, the cell marginal power control won’t take effect. So we don’t need to consider it here.

BTS UL measurement calibration deviation --±3dB Difference in carrier output power --±1dB Measurement difference value under DTX --±0.5dB Difference in combiner loss --±0.5dB


The difference between MS actual power and the reported power class--±2dB;

As for power control process, the difference between MS actual power and power control class can reach to ±5db.;

Take these factors into consideration, we set the MS actual power deviation range to be ±3dB .

normal extreme

1 ‑ ‑ ‑ ‑ ‑ ‑ 1 W (30 dBm) ±2 ±2.5

2 8 W (39 dBm) 0.25 W (24 dBm) ±2 ±2.5

3 5 W (37 dBm) 4 W (36 dBm) ±2 ±2.5

4 2 W (33 dBm) ±2 ±2.5

5 0.8 W (29 dBm) ±2 ±2.5

Tolerance (dB) forconditionsPower class

GSM900 NominalMaximum output

power

DCS1800 NominalMaximum output

power


Influence from radio environment--±2 dB

In practice, call quality is affected by environment and many other factors, such as fluctuation in signal level, which can be very big sometimes, so we accept a deviation of ±2dB in engineering.

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Practical cases

Calculation formula of link balance

PB(link balance value) ＝ RxLevDL-RxLevUL=BtsTxPwrMax-MsTxPwrMax

RxLevDL ： DL level of MS receiving BTS signalRxLevDL=BtsTxPwrMax-RadioPathLossDLRxLevUL ： UL level of BTS receiving MS signalRxLevUL=MsTxPwrMax-RadioPathLossUL

BtsTxPwrMax ： rack-top output power

RadioPathLoss ： radio pathloss, UL and DL are the same

Equipment parameters —Transmitting power

Product transmitting power

Terminal transmitting

powerPower class

GSM 900 Nominal Maximum output

power

DCS 1800 Nominal Maximum

output power

PCS 1900 Nominal Maximum output

power

1 - - - - - - 1 W (30 dBm) 1 W (30 dBm)

2 8 W (39 dBm) 0.25 W (24 dBm) 0.25 W (24 dBm)

3 5 W (37 dBm) 4 W (36 dBm) 2 W (33 dBm)

4 2 W (33 dBm)

5 0.8 W (29 dBm)

Product seriesMax transmitting power

GMSK modulation 8PSK modulation

BTS V3_DTRUBTS V2_ETRM

60W 47.8 dBm 40W 46 dBm

BTS V2_TRM 40W 46 dBm -- --

Equipment parameters—typical values for combiner loss

Combiner loss

Module Condition Combining loss (dB)(typical value)

CDUG Jump combiner 0.8

2 in 1 4.2

CEUG 2 in 1 3.4

3 in 1 5.5

CENG 6 in 1 5.3

CDUD Jump combiner 0.8

2 in 1 4.4

CEUD 2 in 1 3.6

3 in 1 5.5

CEND 6 in 1 5.5

CDUP Jump combiner 0.8

2 in 1 4.4

CEUP 2 in 1 3.6

3 in 1 5.5

Standard value for link balance

Adopt BTS V3 product series 8PSK modulation method BTS static power class=0 common BTS type (PA output port is directly connected with CDU

combiner input port) common antenna system (excl. TA, repeater and indoor distribution

system) Exclude various kinds of coverage enhancement techniques

Frequency band PB standard

900M 46-4.4-33 ︽ 9dB

1800M 46-4.2-30 ︽ 12dB

Normal deviation range of link balance

Taking into consideration the engineer margin of ±10dB, the normal deviation range of link balance should be:

Frequency band Normal PB deviation range

900M -1~19dB

1800M 2~22dB

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Practical cases

Measurement method of link balance—measurement task at OMCR

UL/DL power control is counted into the statistics at OMCR; the statistics gathered are about cells but not carriers.

Counter name BTS_V2 BTS_V3

Total value of uplink signal strength of wireless channels

C10710 C901120010

Number of received uplink signal strength samples of wireless channels

C10707 C901120007

Total value of downlink signal strength of wireless channels

C10714 C901120014

Number of received downlink signal strength samples of wireless channels

C10711 C901120011

UL level statistical average C10710/ C10707 C901120010/ C901120007

DL level statistical average C10714/ C10711 C901120014/ C901120011

PB calculation formula

Influence from dynamic power control 10dB shall be deducted in the PB calculation formula for 900M

cells

Influence from UL channel grain BTS V2_TRM shall deduct UL channel gain of 14dB

Frequency band

Product series

BTS V3(dB)/ BTS V2_ETRM(dB) BTS V2_TRM(dB)

900MDL level statistical average –(UL level statistical average -10 )

DL level statistical average –(UL level statistical average -10 -14)

1800M

DL level statistical average –UL level statistical average

DL level statistical average –(UL level statistical average -14)

Influencing factors in link-balance calculation

In order to obtain precise PB value, the following factors shall be taken into consideration in calculation:

Influencing factors Explanations

Frequency band of existing network

There is a difference of 3dB in judgment criteria due to different frequency bands;

Model of BTS products used & modulation method adopted

Judgment criteria for different BTS products are different for the BtsTxPwrMax is different.

Combining method adopted Combiner level 1 is adopted in the judgment criteria. CduLoss difference caused by other combining methods shall be eliminated.

If external TA installed? In the judgment criteria, default is NO TA; if external TA is installed, its influence on UL-DL shall be eliminated.

If coverage enhancement technique adopted?

The gain produced by coverage enhancement technique shall also be eliminated.

Measurement method of link balance—RMA signaling analysis

Record Abis signaling for th problem cells, choose “measurement report”;

import signaling data to RMS, then use “TRX equation” to automatically generate the scattergram of link balance vs TA ;

The benefits of using “TRX equation” are as follows: When RMA calculates UL-DL difference, it

considers influence from UL-DL power control; investigate carriers, which is good for locating

problem carriers in the cell.

Note: When RMA signaling analysis tool is adopted, we need to choose the cell’s frequency band according to actual

situations.

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Practical cases

Performance analysis-link balance analysis

Static power control

Filter out cells with link unbalance

No

Investigate system equipment

Adjust parameters

Yes

Indoor distribution

System &repeater

No

Investigate

repeater system

Yes

No improvement

No improvement

No improvement

Antenna system problem

Abnormal installation of antenna

CDU problem

Abnormal carrier output

Abnormal carrier UL calibration

Analysis of link-unbalance causes

Link unbalance caused by external TA connecting to antenna, repeater or indoor distribution system The UL-DL of these external devices can be unbalanced, which can cause

the whole BTS system link to be unbalanced. Cable connection problem

If the cable connection between feeder, jumper or antenna and RF equipment is not tight, increase UL-DL level loss and UL-DL level difference can be caused, thus resulting in link unbalance.

Carrier hardware problem Carrier output power deviates from normal value; If the gain of carrier UL receive channel is small, it causes UL-DL level

difference to increase. CDU problem

The increased insertion loss in DL channel will cause DL level to decrease and reduce UL-DL level difference, thus coverage will be smaller;

Problem with UL receive channel will cause DL level to decrease, and reduce UL-DL level difference, thus silent call or call drop will be resulted.

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Practical cases

Case 1: Link unbalance caused by repeaterSubscribers complained about frequent call drops.

The problem cell was located after call tests were made; traffic statistics analysis showed in this 900M cell, (DL-UL) level difference was 25dB, which was far beyond the normal link balance range .

while the daily average of DL-UL level difference in Cell 2 (under the same BTS) was 2dB.

checked CDU and carrier, but no fault was found.

connected the problem cell’s antenna (its antenna is connected with

repeater) to the normal Cell2.

PB value in cell2 increased due to change on antenna .

The receive tributary with repeater has caused UL-DL level difference.The receive tributary with repeater has caused UL-DL level difference.

Problem description

Handling process

Case 2: Cable connection problemCustomers complained that MS displayed signal available, but calls couldn’t be established.

After call tests were made, problem cell was located. We discovered from traffic analysis that the UL-DL level difference in the cell was 30dB, which exceeded the normal range.

On-site checking was needed.

Changed carrier first.

During changing the carrier, we found the cable connector was not tightened.

Checked statistical result the next day, UL-DL level difference dropped to 6dB, which was within the normal range.

After test, we found that UL-DL level difference of carrier was improved by about 1dB, which could not be the main cause for the improvement. Considering the loose connector, we could confirm that poor cable connection was the main cause for link unbalance.

Loose cable connection could result in PB unbalance.Loose cable connection could result in PB unbalance.

Problem description

Handling process

Equipment maintenance

Bad PB statistical result may be caused by broken connector or cable, bad antenna VSWR or drop of equipment.

Regular maintenance schedule is recommended, in order to locate the parts with problem before the situation grows worse.

Case 3: Carrier problem Problem description:

Subscribers complained that handover happened frequently when they made calls around a cell. Test was made at the site being complained about. Sectional drawing of the test is shown bellow:

Test phenomena Handover from 1800M to 900M

cells happened after MS occupied a 1800M cell;

Handover from 900M to 1800M cells happened after MS occupied a 900M cell;

Handover between 900M and 1800M cells happened frequently.

Case 3: Carrier problem

Problem analysis Handover cause analysis

1800M900M handover cause: UL level handover 900M1800M handover cause: PBGT handover Number of UL level handover in 1800M cells takes 90% of total.

PB unbalance may exist; After traffic statistics analayis, we found UL-DL level

difference of the 1800M cells was all above 25dB. It’s confirmed to be UL-DL unbalance.

Case 3: Carrier problem

Handling process After KPI analysis, voice service in 1800M cells was confirmed

normal. Checked power control settings, no errors found. Checked cable connections, no loose connectors, no damaged

heads. Changed carrier, PB value decreased to 10dB. Result of retest displayed frequent handover problem was

sloved.

Carrier problem resulted in PB unbalance.Carrier problem resulted in PB unbalance.

Questions for thinking

When we calculate link balance, what factors need to be taken into consideration? Why?

As for link unbalance caused by too low UL level, what methods can be employed to make adjustment?

Documents

GO_NA22_E1_1 GSM UL-DL Unbalance & Solutions-31