By Mahmoud Metwally

Introduction for path imbalance for KPI problem

The purposes of analyzing and calculating the balance between the uplink and the downlink of the GSM system are as follows:1. Obtaining the maximum available transmit power of the BTS2. Avoid the invalid downlink coverage3. Minimize the interference and system noise4. Provide a basis for obtaining a good network service quality.

The KPIs(Key Performance Indicator) related to the balance between the uplink and the downlink help engineers to check whether the network planning is reasonable and whether the BTS hardware or antenna system is faulty

Limited UplinkUplink signals on the existing network reach the

coverage edge, whereas downlink signals do not reach the coverage edge. In this case, the downlink coverage range is greater than the uplink coverage, and thus the following phenomenon may occur: The strong signal strengths are displayed on MSs that are located between the uplink and downlink coverage edges, but MSs cannot make calls. Even if calls are successfully initiated, the uplink discontinuous voice may occur. That is, the calling party/called party can hear the voice of the counterpart, but the counterpart cannot hear the voice of the calling party/called party. This may lead to a great number of call drops.

Based on the previous phenomenon, the limited uplink may cause user complaints and affect user experience. Therefore, during the network design, the transmit power of the BTS is decreased on purpose to prevent the limited uplink.

Limited DownlinkDownlink signals on the existing network

reach the coverage edge, whereas downlink signals do not reach the coverage edge. In this case, the uplink coverage range is greater than the downlink coverage range, and thus the following phenomenon may occur: The coverage range of the BTS is relatively small. If the BTS can guarantee seamless coverage and no blind area, MSs are not affected by the limited downlink, and thus user complaints are not received.

Thus, when the downlink on the existing network is limited, certain downlink power may be wasted to maximize the transmit power of the BTS, and thus the network security is ensured

Downlink Path

MSRX

Path Loss

BSTX

CombinerTX feeder TX antenna gain

The downlink path. Here, PA specifies the transmit power of the TRX (The static power level, dynamic power control level, and fine-tuned power level need to be considered). Combiner specifies the path loss of the combiner. TX feeder specifies the loss of the transmission feeder. Tx antenna gain specifies the gain of the Tx antenna. MSRX specifies the downlink receive level. Path loss specifies the downlink path loss

Uplink Path

MS RX

Path Loss

RX antenna gainTX feeder

Antenna diversity gain

BSRX

The uplink path. Here, MSTX specifies the transmit power of the MS (The maximum transmit power and dynamic power control level need to be considered). Rx antenna gain specifies the gain of the Rx antenna. RX feeder specifies the loss of the receive feeder. BSRX specifies the uplink receive level. Path loss specifies the uplink path loss.

Formula 1: Transmit power of the BTS + Antenna gain – Downlink path loss = Downlink receive level

Formula 2: Transmit power of the MS + Antenna gain – Uplink path loss = Uplink Receive level

Formula 3: Uplink and downlink path loss difference balance = Uplink path loss – Downlink path loss = (Transmit power of the MS – Uplink receive level + Antenna gain) – (Transmit power of the BTS – Downlink receive level + Antenna gain) = (Downlink receive level – Uplink receive level) – (Transmit power of the BTS – Transmit power of the MS)

The transmit power of the BTS is the cabinet-top power of the TRX. When you calculate the cabinet-top power of the TRX, the following factors need to be considered:

         TRX power type         Static power level         Dynamic power control level         Loss of the transmission feeder         Combination loss         Gain of the Tx antenna         Gain of the transmission mode         Fine-tuning of the power         Dynamic PBT gain

In the previous factors, the loss of the transmission feeder is equal to the gain of the Tx antenna. Thus, the two factors are balanced off during the calculation. The gain of the Tx antenna is not frequently used in common scenarios and is closely related to the installation of the antenna. As a result, it cannot be accurately estimated. Thus, the gain of the Tx antenna is neglected in the formula. The dynamic power control level can be obtained from the measurement reports (MRs). The dynamic PBT gain needs to be obtained through the invoking of the operating status of the dynamic PBT during the channel allocation. The values of other parameters can be obtained from the parameter configuration. In this way, the cabinet-top power of the TRX can be calculated

Cabinet-top power of the TRX = Initial power of the TRX – Static power level x 2 + Fine-tuning of the power – Combination loss – Downlink dynamic power control level x 2 + Dynamic PBT gain

If the multi-path is not considered, the uplink and downlink path loss at each point of the system is the same. In this case, you can infer that the uplink and downlink path loss are balanced, and thus the uplink and downlink are balanced. Because the multi-path exists in the radio environment, the uplink and downlink path loss difference exists. The uplink and downlink path loss difference of the entire system follows a normal distribution according to the related KPIs. The uplink and downlink path loss difference is zero when the values of the KPIs reach the maximum.

Standards of Theoretical Balance Level Judgment

Regardless of the balance between the uplink and the downlink specified in the edge coverage balance concept or the uplink and downlink path loss difference balance concept, the balance point should be 0 dB, that is, at level 6. The KPIs related to the balance level follow a normal distribution.

Considering the differences of TRXs, the RSSI compensation is applied to the production, but the difference of 2 dB is still allowed.

The balance point allows certain difference, and it is considered that the uplink and downlink are balanced if the balance level ranges from level 5 to level 7

Version Statistics Difference (Running Condition of the Existing Network)

The The uplink and downlink path loss difference balance is implemented on the BSC6000V900R008C12B195SP12.

edge coverage balance concept is applied to versions earlier than BSC6000V900R008C12B195SP12 that run on the existing network. Because of the diversified design of the power levels of TRXs, the values of related KPIs indicate that the uplink and the downlink are not balanced. Customers are highly concerned about this problem and raise doubts about the reliability of Huawei devices. We use the following formula to modify the balance level judgment related to the KPIs.

Balance level = (Power of the TRX at the top of the cabinet – Actual transmit power of the MS) – (Sensitivity of the MS – Sensitivity of the BTS)

You can calculate the balance level according to the previous formula, and then query Table 1 to obta

in the balance level of the algorithm of the modified KPIs

Influencing Factors of the Balance Between the Uplink and the DownlinkThis section describes the main factors that

influences the uplink and downlink receive levels according to the complaint cases from multiple sites, experience of tests performed on site or in laboratory, and the analysis on abnormal uplink and downlink receive levels.

The main factors are as follows:

Installation of the antenna feederInstallation of the TMAParameter settingBTS hardwareRepeaterAntenna compatibilitySubscriber behaviorNote: The previous factors are considered

from the antenna system, BTS hardware, and software.

Installation of the Antenna Feeder The components from the cable outlet at the top of

the cabinet to the antenna are the jumper, lightning arrester, conversion connector, grounding soldering point, and antenna. In some cases, power splitter is also included. The installation engineering quality of the components may affect the signal transmission and reception of the BTS.

For example, if the connector of the jumper is loosened, this has different impacts on the uplink and downlink levels. The transmit signal strength is great. For example, the transmit signal strength of the feeder is 30 dBm. The receive signal strength, however, is small. In normal cases, the receive signal strength is –80 dBm. Thus, if the connector of the jumper is loosened, the uplink receive level may be decreased, whereas the downlink receive level is not affected

BTS Hardware The faults of the TRX TX and RX modules may cause uplink

and downlink failures, and thus lead to the imbalance between the uplink and the downlink.

In addition, the faults of the TX and RX modules at the RF front-end may also affect the receive levels of the uplink and downlink, and thus lead to the imbalance between the uplink and the downlink.

In normal cases, uplink and downlink MRs can be measured on a TRX basis. Therefore, you can analyze the status of the balance between the uplink and the downlink of the TRX according to the KPIs related to the balance between the uplink and the downlink at the TRX level.

The faults of the BTS hardware mainly lie in the RF front-end module and the TRX module. In normal cases, the faults lie in the board hardware.

If the uplink and downlink imbalance occurs on a BTS with a certain software version on the entire network, check the software before checking the hardware. Generally, the problem may lie in the software.

If the problem lies in the BTS hardware that is produced in a batch, the hardware problem can be determined.

Note: In the case that a problem occurs on multiple software versions or boards that are produced in a batch or multiple batches, this may lead to the imbalance between the uplink and the downlink on a large scale. In this case, after the RF cables and antenna system are checked and even the comparison test is performed, the problem cannot be located in the hardware or software.

The hardware that is produced in a batch is always installed with the same software version on the existing network, so we can perform the following operations to locate the problem:

(1) Replace the software version and perform a test.(2) Check the balance between the uplink and the downlink

on the hardware that is produced in a batch on the entire network. 

The abnormal uplink gain needs to be tested through the special analyzer. Therefore, field engineers need to check the hardware one by one to prove that the abnormal uplink gain occurs on the hardware that is produced in a batch. Then, the faulty hardware should be returned for analysis.

Antenna Compatibility For some antennas, the gains on the uplink and

downlink are different. That is, the patterns on the uplink and downlink of the antenna are different. If the BTS is installed with some antennas, the uplink gains may be greater than the downlink gains. In this case, the related KPIs may indicate that the uplink and downlink are not balanced.

You can slightly adjust the antenna azimuth or tilt angle to change the spatial distribution of the uplink and downlink patterns of the antenna to solve the imbalance between the uplink and downlink. Alternatively, you can replace the antennas to solve this problem

Subscriber Behavior Some subscribers always put the MS on the table and

answer calls by using a headset during calls. If MSs are not moved, the uplink and downlink propagation features are different. This leads to the different distribution of the uplink and downlink receive levels.

That is, the receive level of the uplink and downlink varies with the location of the MS. The uplink receive level may be greater than or smaller than the downlink receive level because of the location of the MS.

If a large number of MSs are not moved for a long period during calls, the related KPIs are affected. Such cases, however, occurs only in a certain cell.

The imbalance between the uplink and the downlink is caused by the radio propagation feature of the place where the subscriber is located. You can adjust the antenna azimuth of the nearby BTS to change the radio propagation feature of the place where the subscriber is located so that the uplink and downlink are balanced.

Thanks