GSM P&O Guidebook-Application of ZXG10 BSS Power Control Algorithms V1.1 (2)

Internal Use Only▲

Application of ZXG10 BSS Power Control Algorithms

Special Subject Guidebook

Version: V1.0

Released by:

GSM Network Planning & Optimization Dept.

Engineering Service Division

ZTE Corporation

This document contains proprietary information of ZTE Corporation and is not to be disclosed or used

except in accordance with applicable agreements.


GSM Network Planning & Optimization Dept.

Special Subject Guidebook

Version introduction:

Versio

n

Date Writer Assessor Translator Amendment records

V1.

0

2009-05-

12

Chang

HaiJjie

Zheng

Hao

None

V1.

0

2009-05-

26

Chang

HaiJjie

Zheng

Hao

Feng XiaoYing Parameters and application

scenarios were added; figure

description of some

problems was added.




Key words:

Power control algorithms, Application of algorithms

Abstract:

This guidebook mainly introduces ZTE power control algorithms and the application of

these algorithms.

References:

《Preprocessing of FUC and Design of Power Control Module in ZXG10-BTS(V2)》

《Training Materials of Power Control》

《Power Control Adjustment Scheme》

《BTSV6.20.102e Power Control Guidebook》




Contents

1 Overview.............................................................................................................................................1

2 Features of Power Control................................................................................................................2

3 Application Scenario of Power Control...........................................................................................3

4 Theories of Power Control................................................................................................................4

4.1 Location of power control module in the system............................................................................4

4.2 Overall flow of CS power control...................................................................................................5

4.3 Descriptions of power control process............................................................................................7

4.3.1 Storage of measurement data.................................................................................................7

4.3.2 Calculation of averages.........................................................................................................7

4.3.3 Window, average and threshold...........................................................................................10

4.3.4 Power control step size........................................................................................................12

4.3.5 Performance measurement..................................................................................................14

4.3.6 Power control indication......................................................................................................14

4.4 Improvement on power control algorithms...................................................................................14

4.4.1 Comparison of algorithms before/after improvement..........................................................14

4.4.2 Initial state of power control................................................................................................15

4.4.3 UL level and quality decision, step size..............................................................................16

4.4.4 Power rank control..............................................................................................................18

4.4.5 Conversion of power control states.....................................................................................20

4.4.6 Clear MR queues and counters............................................................................................21

4.5 PS power control theories.............................................................................................................22

5 CS Power Control Parameters & Reference Values.....................................................................26

5.1 Uplink /downlink power control...................................................................................................26

5.2 Received signal level & quality threshold.....................................................................................26

5.2.1 Received signal level...........................................................................................................27

5.2.2 Received signal quality........................................................................................................27


except in accordance with applicable agreements. I


5.3 Power control period.....................................................................................................................29

5.3.1 Sample count（window）& Weight（weight）................................................................29

5.3.2 Number of power control samples N & P...........................................................................29

5.4 Power adjustment step..................................................................................................................30

5.5 Criteria of power control state conversion-N1, N2.......................................................................31

5.6 Max power rank of MS & BTS for initial access..........................................................................31

5.7 Rapid power control......................................................................................................................32

5.8 Fast averaging...............................................................................................................................33

5.9 Setting of power control parameters.............................................................................................34

6 Setting of Power Control Parameters in Different Scenarios......................................................36

6.1 Setting of signal quality threshold (under poor DL radio environment).......................................36

6.2 Setting of uplink/downlink power control period.........................................................................36

6.3 Handover threshold.......................................................................................................................37

6.4 Highway/ railway..........................................................................................................................37

6.5 Building........................................................................................................................................37

7 Examples of Power Control...............................................................................................................38


except in accordance with applicable agreements. II


1 Overview

Power control is divided into static power control and dynamic power control. Static

power control is to restrict the max transmitting power of MS or BTS; dynamic power

control is for the network to dynamically decide the max transmitting power of MS or

BTS according to radio environment around subscribers. The power control discussed

in this manual belongs to the scope of dynamic power control.

The main function of power control is to optimize the transmitting power of MS and

BTS without affecting radio transmission quality, so as to improve frequency efficiency

as well as reducing the average transmitting power of MS and BTS, and to reduce

interference to other communications. In mobile systems, to reduce interference means

to have high spectrum efficiency, which means increase of capacity. In the system,

power control of uplink and downlink is independent to each other. MS power control

is to adjust the output power of MS, so that BTS can get stable receive signal level,

interference from other MS using the same channel will be restricted, MS power

consumption will be lowered, and MS average useful time will be extended; BTS

power control is to make MS get stable receive signal level, to restrict interference

from MS using the same channel, and to lower BTS power consumption.


except in accordance with applicable agreements. 1


2 Features of Power Control

According to the amount of measurement data, power control makes weighted average

of the data respectively, then compares the averages with corresponding thresholds, and

then makes MS and BTS power control according to result of the comparison. In order

to meet requirements of power control speed on site, ZTE provides fast averaging,

rapid power control, etc. to accelerate power control speed.

Uplink and downlink power control is performed separately.

CS and PS power control is performed separately.

In Version 6.20.101e and the subsequent versions, dynamic power control algorithms

have been optimized. Having combined the speed and stability of power control, power

control is divided into two states: initial state and stable state. Initial state is to adjust

power to the proper level; stable state is to prevent frequent adjustment of frequency, so

as to keep power output stable.




3 Application Scenario of Power Control

UL ordinary power control is usually enabled; other types of power control are mainly

used in areas with dense sites, for only in areas with dense sites, can power control

work well to reduce interference. Here is a table of application scenario of power

control for reference.

Table 3-1 Application scenario of power control

Algorithm

Scenario

UL ordinary power control

UL rapid power control

DL ordinary power control

DL rapid power control

Urban area ▲ ▲

Dense urban area ▲ ▲ ▲ ▲

Suburb ▲ ▲

Wide coverage area

▲




4 Theories of Power Control

4.1 Location of power control module in the system

Power control is performed on BTS. Location of CS power control module POWCTRL in

the system is shown in Figure 4.1 (marked in red).

Figure 4-1 Location of POWCTRL in system

Refer toTable 4-2 for explanations on components & interfaces related to POWCTRL.

Table 4-2 Explanations on components & interfaces related to POWCTRL

Components Functions Interfaces

CMM It’s the manager of BTS and the proxy of remote operation & maintenance of BSC, it performs operation & maintenance of the BTS it belongs to:

① configuration of parameters② management of status and alarms③ management of software versions④ test of equipment

It completes configuration of PwrCtrl parameters, receives and transmits PwrCtrl performance measurement reports.

CHP It realizes processing of all baseband channels in BTS and controlling of some related parts.

It reports UL measurement statistics, and receives orders SET MS POWER and SET BS




Components Functions Interfaces

UL: de-modulation, equalization, de-interleaving, channel decoding, rate adaptation；DL: rate adaptation, channel coding, interleaving, etc.

POWER of POWCTRL module.

DCHM It’s the special channel processing sub-module of FURRM, handling message processing of SDCCH or TECH.

It invokes POWCTRL module, and transmits UL/DL MRs to POWCTRL.

CS power control module POWCTRL is located in the application layer of FUC

subsystem. According to the amount of measurement data, power control makes

weighted average of the data respectively, then compares the averages with

corresponding thresholds, and then makes MS and BTS power control according to

result of the comparison.

4.2 Overall flow of CS power control

After power control parameters are set at OMCR, they will be transmitted to the

OAMM module on FUC through CMM; OAMM configures related parameters at the

start or during the operation of FURRM process; FURRM saves these parameters in its

global variables.

During calls, FURRM periodically receives CHP MEASUREMENT IND message

reported by CHP and MS MEASUREMENT REPORT reported by LAPDm. DCHM

decides whether to perform power control according to the parameters saved in

FURRM. If power control is to be carried out, after the number of received CHP

MEASUREMENT IND from DCHM reaches Hqave, POWCTRL will make weighted

average of UL measurement data, then compares the average with the corresponding

threshold, then according to result of the comparison confirms whether to control MS

power. If MS power control is necessary, then send the new power to CHP through

CHP SET MS POWER message. After the number of received MS MEASUREMENT

REPORT reaches Hqave, POWCTRL starts weighted average of DL measurement

data, then compares the average with the corresponding threshold, then according to

result of the comparison it confirms whether to control BTS power. If BTS power

control is necessary, then send the new power to CHP through CHP SET BS POWER

message. These processes can be repeated again and again. In the meantime,




POWCTRL module can also conduct power control performance measurement

according to configuration of related parameters. A simple flowchart of power control

is shown bellow.




Figure 4-2 Simple flowchart of power control

4.3 Descriptions of power control process

4.3.1 Storage of measurement data

MR data are saved in the circular table structured with arrays. When the circular table

is filled with data for the first time, new MR data will cover the old data. We can use a

counter to record the number of received MRs, and get the location of latest MR data in

the table. The counter will be refreshed after power control is completed successfully,

and won’t be started until next power control (a certain number PCMinInterval of MRs

in the process will be abandoned; and a certain number PCMinInterval of MRs at the

beginning of the process will also be abandoned, because the first several MRs are not

accurate at the beginning of channel activation). Measurement data and averages of

uplink and downlink are stored separately. According to protocol 05.08, the number of

measurement data and averages of uplink and downlink can at most reach 32.

4.3.2 Calculation of averages

We make weighted average but not the simple averaging calculation of MRs, because

non-consecutive transmission (DTX) exists. In order to increase MR weight when

DTX is off, we can set the weight to be 1 when DTX is on; the weight of DTX Off can

be set at OMCR. These averages are saved in the circular arrays, and use of the data is

controlled by counters. When fast averaging is adopted, the first average is the first

datum, the second average is calculated from the first and the second data, and so on. If

rapid isn’t adopted, the first average is obtained only when the number of data reaches

Hqave.

Suppose the number of MRs in the window is n during DTX Off, receive signal level is

Levoff; and the number of MRs in the window is m during DTX On, and receive signal

level is Levon:

Sum of receive levels is=∑Levoff*Weight+∑Levon*1；

Weighted average=Sum/（n*weight+m*1）.




For example, the current window size is 3, its weight is 2; take receive signal level as

an example, the levels reported by the three MRs are -80dBm（ DTX off）， -

70dBm（DTX on），-82dBm（DTX off）;

Weighted average=[(-80)*2+(-82)*2+(-70)*1]/(2+2+1)=-78.8dBm

Comparison of ordinary averaging and fast averaging is shown bellow:




Figure 4-3 Comparison of MR processing

The comparison in Figure 4-3 shows that 7 MRs are needed for obtaining 4 averages in

ordinary averaging process, while only 4 MRs are needed in fast averaging process.




4.3.3 Window, average and threshold

Power control decision can be made according to the status of MS/BTS signal level

and quality. As shown in the following figure, lower signal Rxqual level on the

horizontal ordinate leads to lower signal error rate and better signal quality. For

example, when the receive signal level and quality values are within the range of

corresponding thresholds, the result of power control decision is not to make power

control adjustment; when the receive signal level value is lower than corresponding

threshold, and the receive signal quality is within the range of corresponding threshold,

the power control decision result is to increase power due to level; when the receive

signal level is higher than the “High level”, and receive signal quality is higher than the

“High Rxqual level”, the power control decision result is to increase power due to

quality.

When setting receive signal level and quality thresholds, we must pay attention to the

relation between power control threshold and handover threshold. The low level

threshold of power control must be larger than level handover threshold, while the high

quality threshold of power control must be smaller than quality handover threshold, as

the stripped area shown in Figure 4-4.




Figure 4-4 Power control decision

Note: low Rxqual level represents good signal quality; high Rxqual level represents

poor quality.

The strategy of MS power control is the same with that of BTS power control. Take

MS power control as an example, suppose LEVELCAUSE records the current level

situation, and QUALCAUSE records the current situation of error rate, and:

1，Normal level：LEVELCAUSE = 0

2，Low level：LEVELCAUSE = 1

3，High level： LEVELCAUSE = 2

4，Normal error rate：QUALCAUSE = 0

5，Low error rate：QUALCAUSE = 1

6，High error rate：QUALCAUSE = 2

Suppose the level rank is from 0 to 63 (low to high), L_RXLEV is tending to be 0, U_RXLEV is tending to

be 63; error rate class (0~7) represents signal quality (high to low), L_RXQUAL is tending to be 7,

U_RXQUAL is tending to be 0. The average of UL measurement data and corresponding threshold values

and the comparison between them area as follows:




If at least a certain number PCULIncrLevP among a certain number PCULIncrLevN of

RXLEV_UL are lower than the low limit L_RXLEV_UL, LEVELCAUSE = 1;

If at least a certain number PCULDecrLevP among a certain number PCULDecrLev of

RXLEV_UL are higher than the high limit U_RXLEV_UL, LEVELCAUSE = 2;

For other situations, LEVELCAUSE =0

(1) If at least a certain number PCULDecrQualP among a certain number

PCULDecrQualN of RXQUAL_UL are lower than the high limit

U_RXQUAL_UL, QUALCAUSE=1;

(2) If at least a certain number PCULIncrQualP among a certain number

PCULIncrQualN of RXQUAL_UL are higher than the low limit

L_RXQUAL_UL, QUALCAUSE=2;

(3) For other situations, QUALCAUSE=0

Compassion of DL data is similar to the description above, just the eight parameters are

marked with DL, like PCDLIncrLevN.

Table 4-3 MS power control strategy

LEVELCAUSE QUALCAUSE Conclusion

0 0 MS_POWER_STAY

0 1 DECREASE_BYQUALITY

0 2 INCREASE_BYQUALITY

1 0 INCREASE_BYLEVEL

1 1 INCREASE_BYLEVEL

1 2 INCREASE_ BYQUALITY

2 0 DECREASE_BYLEVEL

2 1 DECREASE_BYLEVEL

2 2 INCREASE_BYQUALITY

BTS power control strategy: similar to MS power control strategy.

4.3.4 Power control step size

4.3.4.1 MS power control step size

According to parameter bRapidPCInd from OMU, MS decides whether to adopt rapid

power control.




If ordinary power control is adopted, STEP is INCREASESTEP or DECREASESTEP.

If rapid power control is adopted, it should be performed according to the following

principles. Specific conditions are still needed in deciding control over level

increase/decrease. If the conditions are not satisfied, only ordinary power control can

be performed.

1、 INCREASE_BYLEVE：

If LEV_UL + 2* INCREASESTEP < L_RXLEV_UL，then STEP = L_RXLEV_UL -

LEV_UL，LEV_UL is the current value, but not the average value.

2、 DECREASE_BYLEVEL：

If LEV_UL - 2* DECREASESTEP > U_LEV_UL，then STEP = min ( PwrDecrLimit,

LEV_UL- U_RXLEV_UL ), LEV_UL is the current value, but not the average value.

3、 INCREASE_BYQUALITY：

If LEV_UL + 2* INCREASESTEP < L_RXLEV_UL

STEP = max ( (1+max(0,Qa)) * INCREASESTEP , L_RXLEV_UL - LEV_UL )

Or STEP = (1+max(0,Qa)) * INCREASESTEP

Qa = QUAL_UL - L_RXQUAL_UL ； QUAL_UL is the current signal

quality，LEV_UL is the current signal level，and neither of them is the average value.

4、DECREASE_BYQUALITY：

For power decrease caused by signal quality, we should be conservative with the step.

size.

Therefore:

If LEV_UL - 2* DECREASESTEP > U_LEV_UL，

STEP = min ( PwrDecrLimit, LEV_UL – U_ RXLEV_UL, (1 +max(0, Qa) ) *

DECREASESTEP); or STEP = DECREASESTEP。

LEV_UL is the current signal level, but not the average value.

Qa = U_RXQUAL_UL – AV_QUAL_UL。

AV_QUAL_UL is the signal quality average.




4.3.4.2 BTS power control step size

The calculation and adjustment of BTS power control step size is the same as that of

MS, only change UL in parameters to DL.

4.3.5 Performance measurement

During BTS power control, it needs to report related performance data to BSC, and all

the statistical data is stored in PowerMeasData.

4.3.6 Power control indication

In order to optimize handover algorithms, after power control of MS or BTS, BSC will

be notified the direction (downlink or uplink) of power control by a message POWER

CONTROL IND, and then BSC will clear the corresponding buffer area based on this

message. After each time of power control, the queue of MRs reported will be cleared

to 0. The first average value will not be obtained again until the number of MRs

reported reaches Hqave (non-rapid power control).

4.4 Improvement on power control algorithms

4.4.1 Comparison of algorithms before/after improvement

The existing power control strategy can not simultaneously satisfy the following

situations: 1) make rapid power control of MS, and make MS UL level and quality

meet requirements in the shortest time; 2) after required UL level and quality are

satisfied, MS power control can perform stably, and no adjustment of power is needed

only because of one or two minor problems.

The improved power control algorithm: power control falls into two states—initial

state and stable state. Power control is carried out in over channels. The initialization of

channel power control status is initial state. After a number of power controls, MS UL

level and quality satisfy the expected values, the channel power control status enters

stable state, and it remains in stable state until the channel is released, and then the

status returns to initial state.




4.4.2 Initial state of power control

After MS accesses into channel (SDCCH or TCH), if power control period is too long,

it will bring interference to subscribers using other channels. Therefore, in order to

realize fast control of MS power, we carry out power control decision to every uplink

MR, and make adjustments of MS and BTS power rank to make MS UL level and

quality meet the requirements.

When MS or BTS initially accesses into channels, it transmits signals in the max power

rank. In this case, power control usually tends to decrease MS or BTS transmitting

power, until UL level and quality reach the expected values and enter stable state.

According to MS uplink receive signal level and quality contained in the MRs reported

by CHP, BTS judges the uplink signal state and sends the new power rank to MS after

power control decision. Due to coding problems and air interface delay, MS will not

immediately report the next MR with the new power rank, but report the MR with the

new power rank after a delay of three MRs. As shown in Figure 4-5, if BTS makes

power control decision only after it receives the new power rank that it sends to MS,

the power control is of poor efficiency and timeliness.

Figure 4-5 UL/DL power control I

In order to perform power control decision each time an MR is received, we add a

variable （MS PowerSet） to record the latest MS power rank value, and then make

power adjustment based on the value. For example: as shown in Figure 4-6, if the new

power rank sent by BTS (after receiving the last MR) is P1 (MS PowerSet), while the

current received MS power rank is P0, then power adjustment will be performed from

P1 to P2, while will be sent to MS. When another MR is received, power value will be




adjusted with P2 as the benchmark, and then a new power rank value P3 will be sent to

MS.

Figure 4-6 UL/DL power control II

Each adjustment of MS power is based on the latest power rank value, but not the

actual power value that MS uses to report. Because of the complexity and instability of

radio environment, the power rank sent by BTS may be lost if downlink signal is bad.

When there is great difference between the new power rank and the actual one, we take

the new one as benchmark in power adjustment, or power control adjustment will be

invalid. With the aim to ensure the validity of new power rank, we impose specific

decision conditions before new MS power rank is sent.

Power adjustment of BTS is usually effective and in time, and air interface delay and

loss of power rank won’t happen, therefore each time an MR is received correctly,

adjustment of power control decision can be performed.

4.4.3 UL level and quality decision, step size

In the initial state of power control, before making power control of each UL MR

reported by MS, we need to judge the situation of UL signal level and quality.




【 1 】 Compare each signal level in the UL MR with the threshold

（ byL_RXLEV_UL ） which causes increase of UL power and the threshold

(byU_RELEV_UL) which causes decrease of UL power:

byRxLevel < byL_RXLEV_UL；

byUpLevCause = 1;

or：

byRxLevl > byU_RELEV_UL；

byUpLevCause = 2;

if the level value is within the threshold range, byUpLevCause = 0;

【 2 】 Compare each signal quality in the UL MR with the threshold

（ byL_RXQUAL_UL ） which causes increase of UL power and the threshold

(byU_REQUAL_UL) which causes decrease of UL power:

byRxQuality > byL_RXQUAL_UL;

byUpQuaCause = 2；

or：

byRxQuality < byU_RXQUAL_UL;

byUpQuaCause = 1;

if the quality value is within the threshold range, byUpQuaCause = 0。

MS power control decision is performed based on the conditions of signal level and

quality abyMSPowDecision[byUpLevCause][byUpQuaCause]. In order to realize

effective and stable power control in the initial state, regardless power control mode

(INCREASE/DECREASE), the adjustment step size should be set:

STEP = DECREASESTEP。

Adjustment method for step size of BTS power control is the same with that of MS

power control.




4.4.4 Power rank control

In the adjustment procedure of MS/BTS transmitting power rank

(changeMS/BSpower), the same power rank control strategy is used in both the initial

state and stable state.

In adjustment of MS power, a variable (byMSpowerSet) is added to save the power

control adjustment value which is sent by BTS. Power control adjustment should be

based on the last power control adjustment value (byMSpowerSet), but not the

currently received MS power rank （byMSpower）.

During adjustment, no matter to increase or decrease power, a comparison with the

max and minimum power values allowed in the serving cell. For power increase, power

adjustment value=

MIN2(awGSMPowerCtrlLevel[byMSpowerSet]+byStep, byMS_TXPWR_MAX);

byStep is power adjustment step size;

byMS_TXPWR_MAX：max MS transmitting power (dbm) allowed in the serving

cell.

For power decrease, power adjustment value=

MAX2(awGSMPowerCtrlLevel[byTempMSpowerSet]-

byStep, byMS_TXPWR_MIN);

byMS_TXPWR_MIN：min MS transmitting power (dbm) allowed in the serving cell.

After power adjustment value is confirmed, we need to judge its practicability. When

uplink signal is bad, BTS can not receive the power rank reported by MS. In this case,

a variable (bReceiveMSpower) is added to display whether BTS has received the

power rank reported. If MS power rank is successfully received, set the variable to 1,

and clear it to 0 after each power control.

MS power adjustment takes the latest power rank value （MS PowerSet）sent by BTS

as benchmark. The new MS power rank sent by BTS can be reported by MS only after

an interval of three MRs, so there is a difference (sub) between the new power rank to

be sent and the MS power rank（MS Power） in the currently reported MR. In order

to prevent Sub becoming too large, a limit value of 8db is added in the adjustment of

power decrease; if Sub value exceeds 8db, power adjustment won’t be performed.




In the process of power control, the following situations may be encountered:

1、When the received signal quality is good, and the received signal level is larger

than and close to the high level threshold, power decrease will be performed

according to power control decision. Only after an interval of three MRs, the new

power rank value (P) can be reported, at this time, the received signal level may be

lower than the low level threshold. In this case, power needs to be improved,

which leads to Pingpong power control (increase-decrease).

Figure 4-7 Power control process

2、UL received signal level is within the threshold range and close to the low level

threshold, while the received signal quality is very good and lower than the low

level threshold. In this case, power control decision is: power decrease due to

quality. After the power control, UL received signal level will be lower than the

low level threshold. In this case, power control decision is: power increase due to

level. In these cases, Pingpong power control (increase-decrease) can also be

caused.

In order to avoid this kind of Pingpong power control, a Margin value is added when

power control decision is to decrease power, which means to make compensation to the

actual power value and judge whether it will be lower than the threshold after power




control decision is made. The algorithm is: current received signal level value minus

the power difference value (sub), if the result is lower than the low level threshold,

power control will not be performed; if not, continue the power control adjustment.

if (byRxLevel < byL_RXLEV_UL + bySub)

byL_RXLEV_UL: low threshold of RXLEV, which cause increase of UL power.

If the condition is satisfied, power control will not be performed.

For example: suppose current received level (byRxLevel) =-79dbm,

byL_RXLEV_UL=-88dbm, bySub=8dbm; if power adjustment is made under this

situation, the result will be lower than the threshold. Therefore, Margin value is added

to prevent the unfavorable result.

When power adjustment value is finally confirmed, a new power control rank will be

sent to MS.

In BTS power measurement, the current power rank value of BTS transmitting signal is

used as the benchmark value of power adjustment. The calculation method is the same

as that in the stable state of power control.

4.4.5 Conversion of power control states

No matter it is MS or BTS power control, they need to enter the stable state from the

initial state. In order to judge the conversion, we set two values (Num1—the number of

“no power decrease”, Num2—total number of MRs) and a counter

（ dwPCToStableCount） to MS and BTS. In most cases, the power is adjusted

downward when MS or BTS initially access into channels (SDCCH or TCH). When

the required UL level and quality are achieved, the downward adjustment will be

ended. After each power control decision (byPowDecision), if it’s not needed to

decrease MS or BTS power value, the counter （ dwPCToStableCount） will be

increased by 1. If Num1 is achieved, it is decided that power control enters the stable

state. In order to ensure that all power controls will enter stable state from initial state,

we decide that if the number of reported MRs reaches Num2, power control directly

enters stable state, and Num1<Num2. Meanwhile, change power control state

indication byPCStateInd = INIT_PC_STATE to byPCStateInd = INIT_PC_STATE.

Hereafter, when the next MR is received, UL power control will be performed

according to the power control strategy of stable state.




Figure 4-8 Conversion of power control states

4.4.6 Clear MR queues and counters

When power control enters stable state, MR queues and counters will be cleared to 0.

The reason is when power control initially enters stable state, MRs reported before

stable state will be used in the sliding-window averaging, and the average value

obtained is not accurate, which can not be used in the measurement judgment of stable

state.

In the original strategy, the reported MR queues and counters are cleared to 0 after each

power control, and the first average value can not be obtained until the number

reported MRs reaches Hqave (non-rapid power control). In the improved strategy, each

reported MR is saved. After one time of power control, with the first MS MR reported

to BTS and the Hqave-1 MRs saved before, a weighted average can be performed

immediately, and the first average value needed in the power control will be obtained;

after the second MR is received, with the Hqave-1 MRs saved before this one,

weighted average will be continued and the second average value will be obtained.

When byPCDLIncrLevN MRs are received, byPCDLIncrLevN average values will be




obtained, with which the first power control decision can be performed. In this way,

power control decision period is greatly shortened. As shown in the following figure,

after power control enters stable state, the time for the first power control decision is

the time used for reporting W+N-1 MRs; from the second power control decision,

power control interval is the time used for reporting N MRs.

Figure 4-9 Window mechanism of power control

4.5 PS power control theories

（一） MS power control algorithm:

MS calculates its output power according to each UL PDCH (which means if MS

occupies more than one UL PDCHs, the transmitting power for each PDCH can be

different). The principle of power control is the output power of all channels should

remain lowest on condition that good communication quality is ensured.

MS calculates its output power according to the power control parameters provided by

network. Besides, MS output power is also decided by the max received power for

access to the cell, MS power rank and its received signal level. The power control

algorithm adopted by MS on each independent UL PDCH is as follows:




--MS transmitting power on each UL PDCH, its unit is dBm;

－GSM900为 39dBm；DCS1800为 36dBm;

--power control parameter of MS and some specific channel, which is sent to MS

through control message of RLC/MAC (ie. Packet UL assignment), its unit is dBm;

--system parameter, which can be broadcasted on PBCCH and sent to MS through

control message of RLC/MAC;

--standard MS received signal level, which is the average of the received levels on

the four common burst pulses (which make up the message block);

--max output power allowed in the cell; if PBCCH exists,

it=GPRS_MS_TXPWR_MAX_CCH; if not, it=MS_ TXPWR_MAX_CCH, its unit is

dBm.

and are power control parameters provided by PCU.

After receiving the message block with new and values, after a delay of two

radio blocks, will be updated based on the above formula.

Open-loop power control: at the initial period of GPRS network construction, MS may

choose open-loop power control mode. Implementation of the mode is to set to be 1,

and keep the continuity of values. BTS power attenuation value Pb can be set 0

(ie. DL power control is not adopted). MS power control algorithm is changed to:

Value of is calculated with BTS received signal level SSb. MS received signal

level is:

--max BTS output power;

--BTS power attenuation value for power control;

L—path loss

Then value of (quantification of received signal level):




So MS output power

So value of is:

MS uses the same output power for the four burst pulses contained in one radio block.

After entering a new cell, MS uses the output power defined by PMAX before it

receives the first new power control parameter.

（二） Power control algorithm of BTS:

In GPRS network, DL power control will be enabled only when the PDCH used by MS

and BCCH are on the same carrier. On PDCH which contains PBCCH or PCCCH,

BTS will use the constant output power, which may be lower than the output power

used on BCCH. The power attenuation value of PCCCH relative to BCCH is Pb, which

is broadcasted on PBCCH.

As for PTCCH/D, BTS usually uses the same output power as that used on PBCCH or

BCCH (if PBCCH is not available).

DL power control can also be used on other PDCH blocks. Except for the burst pulses

on PBCCH carriers, BTS uses the same output power for the four burst pulses

contained in one radio block on other carriers.

DL power control falls into two modes: mode A and mode B. Mode A is applies to all

allocation models; while mode B only applies to the fixed allocation model. Parameter

BTS_PWR_CTRL_MODE defines which mode to be applied in the network.

Both mode A and mode B use parameter P0, which is the power attenuation value

(relative to BCCH) and is contained in the packet channel assignment message.

Usually P0 is not allowed to be changed under packet transmission mode. MS can have

only one P0 value at some point of time.

On each PDTCH/D, the PR of MAC head indicates the power decrease rank of current

RLC data block. Coding of PR domain depends on the value of parameter

BTS_PWR_CTRL_MODE. Because of different power control modes, coding of PR

domain is different. Besides, value of PR is calculated based on P0 of the target MS.




If power control mode A is adopted, BTS will restrict the block power sent to MS and

keep MS level within （BCCH level－P0－10）dB～（BCCH level－P0）dB.

Output power for other blocks shall not exceed （BCCH level－P0）dB.

If power control mode B is adopted, the overall output power range of BTS will be

involved. In this case, BTS will take （BCCH level－P0）dB as its initial downlink

output power, and all the blocks it sends to a multi-slot MS shall use the same power

within one TDMA frame.




5 CS Power Control Parameters & Reference Values

Power control parameters are set with cells as controlled object; each cell is configured

with a series of adjustable power control parameters.

5.1 Uplink /downlink power control

We can choose power control object according to network demands. If adjustment of

MS transmitting power is needed, enable Uplink power control (PweControlUl); if

control over BTS power is needed, enable downlink power control (PweControlDl).

This parameter is set in “Others”, as shown in the figure.

5.2 Received signal level & quality threshold

To judge whether to make power control of the current received signal, to increase or

decrease its power, we need criteria, which can be used to measure whether the

received signal condition is as what we expect. The criteria consist of two aspects:

received signal level, received signal quality.

Setting of this parameter is shown in the figure:




5.2.1 Received signal level

For MS uplink received signal level, there are two parameter values: power increase

uplink level and power decrease uplink level, which are corresponding values to the

low level threshold and high level threshold of the expected signal level range.

Expected signal level=parameter value -110dbm, as shown in the figure, ie. [-88dbm, -

82dbm].

Similarly, for BTS downlink received signal level, there are also two parameter values:

power increase downlink level and power decrease downlink level. The expected

received signal level=parameter value-110dbm, as shown in the figure, ie. [-84dbm,-

78dbm].

5.2.2 Received signal quality

For uplink/downlink received signal quality, there are power increase uplink quality

and power decrease uplink quality. Higher received Rxqual level means poorer

received signal quality. The corresponding relationship between signal Rxqual level

and error rate is shown bellow:

Rxqual level Error rate0 <0.2%

1 [0.2%, 0.4%）2 [0.4%, 0.8%）




3 [0.8%, 1.6%）4 [1.6%, 3.2%)

5 [3.2%, 6.4%)

6 [6.4%, 12.8%)

7 >=12.8%




5.3 Power control period

5.3.1 Sample count（window）& Weight（weight）

5.3.2 Number of power control samples N & P

As shown in the figure, this parameter is configured under “Power adjust threshold”.

The precondition of power control is to collect N averages (number of averages reaches

N); then compare the averages with the expected threshold values to judge the state of

uplink/downlink received signal.




N represents the number of averages needed in power control decision;

P represents the threshold used for judging the level and quality state of the

averages.

5.4 Power adjustment step

If power control is needed after power control decision, make adjustment of MS and/or

BTS power. Except for rapid power control, the range of each power adjustment is a

fixed value, which is referred to power control step.

This parameter is set in “Others”, as shown in the figure:

“Power decreasing step” is usually set 2. Fast power decrease may cause received

signal level to become lower than the threshold or cause quality to deteriorate, which

may lead to call drops due to sharp decrease in power, therefore, power decreasing step

shall not be set a large value. “Power increasing step” is usually set to be 4dBm or

6dBm. It’s not appropriate to set it with a large value, or the signal level may be larger

than the level threshold after power control, which may lead to “Pingpong” power

control.




5.5 Criteria of power control state conversion-N1, N2

Parameter N1 is a fixed value 4. Parameter N2 is adjustable, which is currently

adjusted through parameter (PcMinInterval) in OMCR.

The default of minimum interval of power control (PcMinInterval) is 2. When

PcMinInterval<4, accordingly the value of N2 is 11 MRs; when PcMinInterval>17,

accordingly the value of N2 is fixed to be 17 MRs; for other situations, N2 is the value

configured in OMCR. Therefore, N2 can be set within 5~17 MRs. The time length

power control entering the stable state can be adjusted according to actual power

control situations. Suggestion: PcMinInterval=2.

5.6 Max power rank of MS & BTS for initial access

Usually, for GSM900, the max power rank of MS is 5, and the minimum is 16; for

DCS1800, the max power rank of MS is 0, and the minimum is 11. No matter in

GSM900 or DCS1800, the max transmitting power of BTS is 0.

The parameter of power rank is set in “Others”, as shown in the figure:

GSM900:

DCS1800：




5.7 Rapid power control

Rapid power control is to make immediate adjustment of power according to the

different between the received signal level/quality and the corresponding threshold

values; each power control step is not a fixed value, but a integral multiple of power

increasing/decreasing step. Rapid power control can satisfy the needs of dynamic

control of MS power, but it can also lead to sudden increase or decrease of MS

transmitting power and affect signal stability. This parameter is set in “Power control”,

as shown in the figure.

The power decreasing limit: the max limit of power decrease corresponding to different

Rxqual level.




5.8 Fast averaging

This parameter is set in the “Service process additional parameter”, as shown in the

figure:

Fast averaging works only when power control enters stable state from initial state,

waiting for the first average of window samples, and Window>1.

As shown in the figure, suppose W=5, N=3. After power control enters stable state,

average the first MR, then the first average value is obtained; after the second MR is

received, make weighted average of the first two MRs, then the second average value is

obtained; then the third MR is received and the third average value is obtained. After

N(3) average values are obtained, make the first power control decision. After the

fourth MR is received, average of the four MRs. When the fifth MR is received, ie.

window size W=5, return to the common power control algorithm (sampling average),

make weighted average of these (W) MRs, and the second average value is obtained.




After power control enters stable state, if the first power control decision takes along

time, it may lead to sudden deterioration in signal; in this case, fast averaging can make

prompt adjustment on power.

5.9 Setting of power control parameters

Recommended values of power control parameters are shown in the table, appropriate

adjustment can be made according to actual network conditions.

Parameter

Recommended value

versions

previous to 6.20.101e6.20.101e&subsequent versions

Average window size 4 3

UL/DL RQ increase threshold 3 2

UL/DL RQ increase P/N value 2/3 2/3

UL/DL RQ decrease threshold 1 0

UL/DL RQ decrease P/N value 2/3 2/3

UL/DL level increase /

decrease thresholddefault default

UL/DL level increase /

decrease P/Nvalue2/3 3/4




Increase step 4db 4db

Decrease step 2db 2db

Rapid averaging Enabled Disabled

Rapid power control Enabled Disabled

Power decrease limit 10，8，6，4，2，2，2，2 default

Power control min interval 1 default

MS min power rank 18（900M）、14（1800M) 18（900M）、14（1800M)

BS min power rank 15(900M)、14（1800M） 15(900M)、14（1800M）




6 Setting of Power Control Parameters in Different Scenarios

6.1 Setting of signal quality threshold (under poor DL radio environment)

During operation of power control, what affects network performance seriously is the

distribution of uplink and downlink RQ (=0～7). Due to the complexity of network,

uplink /downlink received signal quality threshold is usually set 【0, 2】, which can help

increase the proportion of network performance indicator RQ(=0～3).

Signal quality threshold range can be changed according to the actual uplink/downlink

radio environment. If radio environment is not good, we can change the quality

threshold range to 【0, 1】. However, increase in quality threshold may lead to increase

of uplink/downlink signal power, and hence the interference between uplink and

downlink signals will increase, especially the interference in downlink. Because of

power control, the proportion of network performance indicator RQ(=0～3) remains

high. When optimizing related parameters, we need to weigh increase of the proportion

of RQ=0~3 against increase of interference.

6.2 Setting of uplink/downlink power control period

Because the interference of uplink and downlink signals is different, we can adjust

UL/DL power control parameters separately. The new power control strategy features

fast frequency and timely adjustment. If radio environment is good, the downlink

power control frequency can be appropriately slowed down, that is to increase power

control decision time N. When making sliding-window averaging, if too many MRs

reported with the last sent power rank are used, the proportion of MRs reported with

new power rank will be low, thus the averaged level and quality values will be affected,

and the accuracy of new power control decision will be lowered, therefore, the number

of samples (window) should be reduced. The recommended values of related downlink

parameters are:

Sample count（W） Value N Value P




2 5 3

6.3 Handover threshold

When adjusting the range of uplink/downlink received signal level and quality

thresholds, we must ensure that there should be no conflicts with thresholds related to

BSC handover algorithms. The level threshold which causes power increase should

NOT be lower than the handover level threshold; the quality threshold which causes

power increase should be lower than the quality handover threshold. If quality

handover threshold =4, N=1, then the high limit of power control threshold should not

exceed 2. If power control can be performed, optimize signal level through power

control; if not, carry out handover. Besides, power control parameters can be adjusted

according to network performance indicators, like handover success rate, call drop rate,

etc..

6.4 Highway/ railway

On highways and railways, MS/BTS power control should be performed in fast

frequency and timely manner, so value N should not be a large value; meanwhile,

number of samples (W) should be reduced to improve accuracy; and we should not use

too many MRs reported with the last sent power rank in the averaging process.

Sample count（W） Value N Value P

1 3 2

6.5 Building

In office buildings and apartment buildings, MS moves in a slow speed, or stays still.

In this case, the basic setting of power control parameters can be adopted, and power

control duration N can also be extended.




7 Examples of Power Control

Figure 7-10 Graph of UL ordinary power control

Figure 7-11 Graph of UL ordinary power control with fast averaging




Figure 7-12 Graph of UL rapid power control

Figure 7-13 Graph of DL ordinary power control




Figure 7-14 Graph of DL ordinary power control with fast averaging adopted

Figure 7-15 Graph of DL rapid power control




Figure 7-16 Graph of DL rapid power control with fast averaging adopted

Experiment 1, see Figure 7-10, Graph of UL ordinary power control

Conditions: ordinary power control over uplink, step=2dB; no power control

over downlink; MS stays 5m away from the BS after the call finished.

Result analysis: the graphs shows that MS power is adjusted from 39dBm

(time-16:01:34.577) to 13dBm (time-16:02:38.934), the adjustment time is

64.357s, adjustment period is 4.801s/2dB. Besides, the graph shows that MS

power remains steady after the adjustment.

Experiment 2, see Figure 7-102, Graph of UL ordinary power control with fast

averaging

Conditions: ordinary power control with fast averaging is adopted over

uplink; no power control over downlink; MS stays 5m away from the BS

after the call is finished.

Result analysis: the graph shows that MS power is adjusted from 39dBm


35.001s, adjustment period is 2.399s/2dB. Compared with experiment 1, the




adjustment period in the experiment is only half of that in experiment 1;

power control speed is greatly increased.

Experiment 3, see Figure 7-12, Graph of UL rapid power control

Conditions: rapid power control over uplink; no power control over

downlink; MS stays 5m away from the BS after the call is finished.

Result analysis: the graph shows that MS power is adjusted from 39dBm


5.745s. Compared with experiment 1 and 2, the adjustment time is noticeably

shorter than that in the previous experiments.

Experiment 4, see Figure 7-13, Graph of DL ordinary power control

Conditions: ordinary power control over downlink; no power control over

uplink; MS stays 5m away from the site when the call starts; after a while,

MS moves far away from the BS.

Result analysis: the graph shows that BS power is adjusted from 0dB (time-

15:52:52.833) to -30dB (time-15:54:07.261), the adjustment time is 74.428s,

adjustment period is 4.799s/2dB. When MS stays near the BS, BS power is

gradually adjusted to the minimum value; when MS moves far away from the

BS, its received signal level drops and voice quality deteriorates, so BS keeps

increasing its transmitting power.

Experiment 5, see Figure 7-14, Graph of DL ordinary power control with fast

averaging adopted

Conditions: ordinary power control with fast averaging over downlink; no

power control over uplink; MS stays 5m away from the site when the call

starts; after a while, MS moves far away from the BS; finally, MS returns to

BS.


16:52:35.329) to -30dB (time-16:53:15.631), the adjustment time is 40.302s,

adjustment period is1.934s/2dB. When MS stays near the BS, BS power is

gradually adjusted to the minimum value; when MS moves far away from the

BS, its received signal level drops and voice quality deteriorates, so BS keeps

increasing its transmitting power; when MS returns to stay near BS, BS




adjusts its power back to the original value. Compared with experiment 4,

both the adjustment time and period are shortened to a great extent

experiment 5.

Experiment 6, seeFigure 7-15, Graph of DL rapid power control

Conditions: rapid power control over downlink; no power control over

uplink; MS stays 5m away from the site when the call starts; after a while,

MS moves far away from the BS; finally, MS returns to BS.


17:08:00.626) to -30dB (time-17:08:16.964), the adjustment time is 16.338s.

When MS stays near the BS, BS power is gradually adjusted to the minimum

value; when MS moves far away from the BS, its received signal level drops

and voice quality deteriorates, so BS keeps increasing its transmitting power

and makes compensation to the fast attenuation of signal level; when MS

returns to stay near BS, BS adjusts its power back to the original value and

keeps MS received signal level and quality stable. Compared with

experiment 4 and 5, the adjustment time is noticeably shortened in

experiment 6.

Experiment 6, see Figure 7-16, Graph of DL rapid power control with fast

averaging adopted

Conditions: rapid power control with fast averaging over downlink; no power

control over uplink; MS stays 5m away from the site when the call starts;

after a while, MS moves far away from the BS; finally, MS returns to BS.


17:19:57.659) to -30dB (time-17:20:07.259), the adjustment time is 9.6s.

When MS stays near the BS, BS power is gradually adjusted to the minimum

value; when MS moves far away from the BS, its received signal level drops

and voice quality deteriorates, so BS keeps increasing its transmitting power

and makes compensation to the fast attenuation of signal level; when MS

returns to stay near BS, BS adjusts its power back to the original value and

keeps MS received signal level and quality stable. Especially at around

17:21:03, when fast attenuation occurs to MS RxLev, and higher error rate

occurs to MS RxQual, BS immediately increases its power to save the call.




Compared with the salvage of call at 17:09:30 in experiment 6, the one in

experiment 7 is even faster. Compared with experiment 4, 5 and 6, the

adjustment time is noticeably shortened in experiment 7.



Documents

GSM P&O Guidebook-Application of ZXG10 BSS Power Control Algorithms V1.1 (2)