08 mn1788eu08mn 0001 interference reduction

Interference Reduction Siemens

MN1788EU08MN_00011

Contents

1 Power Control 3

1.1 General Information 4

1.2 Measurement Preprocessing for Power Control 6

1.3 Power Control Decision 8

1.4 Power Control Execution 12

1.5 Parameters for Power Control Execution 15

1.6 Power and Quality Measurements on Uplink/Downlink TCHs 16

2 Discontinuous Transmission DTX 19

3 Channel Allocation Due to Interference Level 23

3.1 General 24

3.2 Parameters for Channel Allocation due to Interference Level 28

3.3 Interference Level Measurements on Idle TCHs 30

4 Exercise 33

Interference Reduction

Siemens Interference Reduction

MN1788EU08MN_00012


MN1788EU08MN_00013

1 Power Control


MN1788EU08MN_00014

1.1 General Information

The objective of power control is to adapt the transmit power of the MS as well as ofthe BTS to the reception conditions.

For example a mobile station MS 1 located near the BTS can use a lower transmitpower than a mobile station MS 2 at the edge of a cell to achieve the required uplinkquality.

Advantages of power control:

� reduction of the average power consumption (especially in the MS),

� reduction of the interference experienced by co-channel (for adjacent) channelusers.


MN1788EU08MN_00015

BTS

MS 1

MS 2

T

X

P

W

RT

X

P

W

R

Fig. 1

Power control is applied separately for the uplink and the downlink and separately foreach logical channel. It can be enabled/disabled using the following flags(administered within the PWRC object):

Specification Name DB Name Meaning

EN_MS_PC EMSPWRC Flag to enable/disable uplink power control.

EN_BS_PC EBSPWRC Flag to enable/disable downlink powercontrol.

Remark:

Downlink power control is not applied for downlink bursts using the BCCH fre-quency.


MN1788EU08MN_00016

1.2 Measurement Preprocessing for Power Control

Any control loop is based on measurements of the controlled system parameters. ForPC purposes, for each call in progress and for both links (downlink/uplink), meas-urements of:

� received signal level

� received signal quality

are carried out over each SACCH multiframe,

Every SACCH multiframe the MS sends in the next SACCH message block thedownlink measurements on the dedicated channel (averaged over one SACCH multi-frame) via the Measurement Report message to the serving TRX of the BTS.

This means that any SACCH message will report the averaged data for the previousreporting period only. The TRX performs the uplink measurements on the dedicatedchannel. The measurement reports (uplink and downlink) referring to the sameSACCH multiframe are used as input parameters for the Measurement Report aver-aging procedures, implemented within the BTS.

The adjustment of the transmit power of the MS and the BTS is based on followingmeasurement values:

� RXLEV_DL_FULL / SUB- RXLEV_UL_FULL / SUB

� RXQUAL_DL_FULL / SUB- RXQUAL_UL_FULL / SUB


MN1788EU08MN_00017

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The measurement values are preprocessed within the BTS in the same way as forthe handover process, i.e. a gliding average window and a weighting of FULL andSUB values is used. The parameters for measurement preprocessing for powercontrol are administered in the object PWRC.

SpecificationName

DB Name Range Meaning

A_QUAL_PC PAVRQUAL

AQUALPC

1-31 Averaging window size forRXQUAL values, used forpower control decisions

W_QUAL_PC PAVRQUAL

WQUALPC

1-3 Weighting for RXQUAL_FULLvalues

A_LEV_PC PAVRLEV

ALEVPC

1-31 Averaging window size forRXLEV values, used for powercontrol decisions

W_LEV_PC PAVRLEV

WLEVPC

1-3 Weighting factor forRXLEV_FULL values


MN1788EU08MN_00018

1.3 Power Control Decision

For power control decision the average values of RXLEV_UL/DL andRXQUAL_UL/DL are compared with some preset thresholds (O&M parameters).

PC Decision

Process

RXQUAL_XX<

U_RXQUAL_XX_P

RXLEV_XX<

L_RXLEV_XX_P +

2* POW_RED_

STEP_SIZE

power decrease

X

power increase

power increase

power increase

RXQUAL_XX>

L_RXQUAL_XX_P

RXLEV_XX>

U_RXLEV_XX_P

RXLEV_XX<

L_RXLEV_XX_P

X

X

X

no

yes

yes

yes

yes

yes

no

no

no

no

Fig. 2 Power Control Decision Process


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Power Control Decision

RXQUAL

63

L_RXQUAL_XX_P

RXLEV

U_RXQUAL_XX_P

0

7

2 * POW_RED_STEP_SIZE

L_RXLEV_XX_P U_RXLEV_XX_P

Power Decrease

(good quality)

Power

Decrease

(good level)

Power Increase

(bad level)

Power Increase

(bad quality)

Fig. 3


MN1788EU08MN_000110

The power control decision is primarily based upon the received signal quality, ratherthan on the received signal level. The reason for this is that the transmitter powerdirectly affects the quality of the radio link regardless of the overall received signallevel, which may be dominated by co-channel interference.

If the controlled variable (its average value) lies in the tolerance defined by thethresholds, then no control action is taken, i.e. a “deadband” type of control responseis produced. This introduces stability into the control process and guarantees anadequate speech quality.

Note, that the controlled variables involved in the algorithm are used in accordancewith their coding, e.g. RXQUAL_XX = 0 corresponds to the least BER (best signalquality: BER<0.2%).

The test

RXLEV_XX < L_RXLEV_XX_P + 2 * POW_RED_STEP_SIZE

should prevent the control loop from oscillating, i.e. a power decrease decision forquality reasons should not be followed by power increase decision for signal levelreasons.

Note, that the O&M parameter POW_RED_STEP_SIZE is defined in terms of adifference between two transmit power levels. An unit power level step correspondsto a nominal 2 dB step in the variation of the transmit power.

Field measurements have shown that even at very low received power levels a goodquality for a radio link can be obtained with a relative high probability.

Because sudden fades may deteriorate the quality very rapidly, if the above conditionis satisfied, the received power level is then compared with the corresponding lowerthreshold to ensure a required minimum power level on the radio link.


MN1788EU08MN_000111

Parameters for Power Control Decision

The parameters for power control decision are administered in the object PWRC

SpecificationName

DB Name Range Meaning

L_RXLEV_DL_P

L_RXLEV_UL_P

LOWTLEVD

LOWTLEVU

0...63 RXLEV threshold ondownlink/uplink for powerincrease

U_RXLEV_DL_P

U_RXLEV_UL_P

UPTLEVD

UPTLEVU

0...63 RXLEV threshold ondownlink/uplink for powerdecrease

L_RXQUAL_DL_P

L_RXQUAL_UL_P

LOWTQUAD

LOWTQUAU

0...7 RXQUAL threshold ondownlink/uplink for powerincrease

U_RXQUAL_DL_P

U_RXQUAL_UL_P

UPTQUAD

UPTQUAU

0...7 RXQUAL threshold ondownlink/uplink for powerdecrease

The following integrity conditions on the power control thresholds have to beobserved:

U_RXQUAL_XX_P < L_RXQUAL_XX_P XX = UL, DL;

L_RXLEV_XX_P < U_RXLEV_XX_P XX = UL, DL.

Furthermore the power control thresholds have to harmonize with the ones forhandover, i.e.:

U_RXQUAL_XX_P < L_RXQUAL_XX_P < L_RXQUAL_XX_H

XX = UL, DL;

L_RXLEV_XX_H < L_RXLEV_XX_P < U_RXLEV_XX_P XX = UL, DL.


MN1788EU08MN_000112

1.4 Power Control Execution

Having detected within the decision process that a change of transmit power shouldbe carried out, the value for this change has to be fixed. If power control would onlybe based on RXLEV, the difference between RXLEV and the level threshold could beused as the step size for power control decision. However, since the main objective isto maintain an acceptable speech quality and since there is no direct simple relationbetween a change of the transmit power level and the resulting change of RXQUAL,the strategy is to change the transmit power in small steps and then analyze theresulting RXQUAL. This means that fixed step sizes for power increase and powerdecrease are used given by the parameters:

� POW_INCR_STEP_SIZE: 2, 4 or 6 dB

� POW_RED_STEP_SIZE: 2 or 4 dB,

To be able to react fast enough on sudden drops of link quality it is recommended:

POW_INCR_STEP_SIZE > POW_RED_STEP_SIZE.

The maximum range for uplink power control is given by:

13 dBm ... Min (MS_TXPWR_MAX, P) for a GSM-MS Phase 1

5 dBm ... Min (MS_TXPWR_MAX, P) for a GSM-MS Phase 2

0 dBm ... Min (MS_TXPWR_MAX, P) for a DCS1800-MS

where P is the maximum RF output power of the MS (power class) andMS_TXPWR_MAX the maximum transmit power allowed in the respective cell. Theminimum step size for transmit power adjustment is 2 dB.


MN1788EU08MN_000113

For downlink power control the range is determined by the maximum output powerPBTS of the BTS and the static reduction BS_TXPWR_RED of the BTS outputpower:

BS_TXPWR_MAX = PBTS - 2 * BS_TXPWR_RED

BS_TXPWR_RED = 0, 1, ... 6, Unit: 2 dB.

The range for downlink power control is then given by:

BS_TXPWR_MAX - 30 dB ... BS_TXPWR_MAX with a step size of 2 dB.

In the case of a power control decision a MS/BS Power Control message is createdwherein the MS/BS is requested to adjust its transmit power level to:

REQ_TXPWR = CONF_TXPWR + POW_INCR_STEP_SIZE (Power Increase)

REQ_TXPWR = CONF_TXPWR - POW_RED_STEP_SIZE (Power Decrease)

where CONF_TXPWR is the confirmed power level used by the MS or BTS on theconcerned channel. If these values of REQ_TXPWR are not within the range forpower control, the nearest value within the range is used instead.

Having requested a transmit power REQ_TXPWR, the power control decision proc-ess is suspended and it is waited for a confirmation that the transmit power of theMS/BTS is adjusted to requested value, i.e.

CONF_TXPWR = REQ_TXPWR.

If such a confirmation is not received within an interval of P_CONFIRM SACCHmultiframes, the power control decision process is immediately resumed using themost recently reported confirmed value.

If a confirmation is received, the power control decision process is suspended for acertain number of SACCH multiframes given by the parameter P_CON_INTERVAL.The reason for this is to allow an observation of the effect of one power control de-cision before initializing the next one; by this means the power control process isstabilized. Thus it is recommended to set

P_CON_INTERVAL > A_QUAL_PC

in terms of a number of multiframes.


MN1788EU08MN_000114

Time relations for power control execution:

� case 1: requested transmit power confirmed

� case 2: requested transmit power not confirmed

(1) requested transmit power confirmed

(2) requested transmit power not confirmed

time

resume

Power Control

P_CONFIRM

suspension of PC decision

P_CON_INTERVAL

CONF_TXPWR

= REQ_TXPWRrequest

TXPWR

resume

Power Control

(1) (2)

Fig. 4


MN1788EU08MN_000115

1.5 Parameters for Power Control Execution

SpecificationName

DB Name/Object

Range Meaning

MS_TXPWR_MAX MSTXPMAX

/ BTSB

0...19 GSM0...15 DCS

* 2 dB

maximum TXPWR an MS may usein the serving cell

0 = 43 dBm, 15 = 13 dBm (GSM,phase 1)

0 = 43 dBm, 19 = 5 dBm (GSM,phase 2)

0 = 30 dBm, 15 = 0 dBm (DCS)

Range for uplink power control:

5 dBm ... MAX (GSM, phase 2)

0 dBm ... MAX (DCS)

MAX = Min (MS_TXPWR_MAX, P)

P: power class of the MS

BS_TXPWR_RED PWRRED

/ TRX

0, 1...6

* 2 dB

Static reduction of the TRX outputpower:

BS_TXPWR_MAX

= PBTS - 2 * BS_TXPWR_RED

Range for downlink power control:

BS_TXPWR_MAX - 30 dB ...

BS_TXPWR_MAX

POW_INCR_

STEP_SIZE

PWRINCSS

/ PWRC

DB2, DB4,DB6

Step size for power increase

POW_RED_

STEP_SIZE

PWREDSS

/ PWRC

DB2, DB4 Step size for power reduction

P_CONFIRM PWRCONF

/ PWRC

1...31

* 2 TSACCH

Maximum interval for waiting for aconfirmation of the new transmitpower level.

P_CON_INTERVAL

PCONINT

/ PWRC

0...31

* 2 TSACCH

Minimum interval between changesof the RF transmit power level (timefor suspension of a power controldecision after a power controlexecution)


MN1788EU08MN_000116

1.6 Power and Quality Measurements onUplink/Downlink TCHs

Overview

Long name Power and quality measurements on uplink/downlinkTCHs

OMC name powerAndQualityMeasOnTCHsUplinkDownlink

BR3.0 short name PWRUPDW

Function group CHANMEAS

Number of counters (64+8) *2 = 144

64 values, one for each RXLEV band

8 values, one for each RXQUAL band

2 times of these values (uplink, downlink)

Kind of counters CC, Integer

Unit of counters ---

Observed object TRX

Measurement depth per TCH

First release implemented BR3.0

GSM/CCITT reference ---

Description

These performance measurements are based on the radio link measurementsRXLEV and RXQUAL (as standardized in GSM 05.08), used in the handover and RFpower control processes. This measurements considers the kind of configuration ofthe Traffic Channel (Fullrate, Halfrate).


MN1788EU08MN_000117

Trigger Events

For the selected Traffic Channel in use, the MS received signal level in dBm(reported as RXLEV value) and received signal quality, (equivalent to BER, reportedas RXQUAL value) are reported to the BTS at 480 ms intervals. The performancemeasurements to be processed with these raw data provide for each busy trafficchannel:

a) the number of times the received signal level exists, as the correspondingRXLEV value (0 to 63),

b) the number of times the received signal quality exists, within the correspondingRXQUAL bands

The counters per RXLEV/RXQUAL band are implemented in the BTS. One"scanReport" notification is generated at the end of the granularity period and sent bythe BTS to the BSC, containing the accumulated value for each RXLEV/RXQUALband.

The measurements on uplink and downlink idle TCHs are always reported together.

The RXQUAL- / RXLEV-Bands are separated as follows:

RXQUAL_0: BER < 0.2% RXLEV_0: <-110 dBm

RXQUAL_1: 0.2% < BER < 0.4% RXLEV_1: -110 dBm... -109 dBm


RXQUAL_3: 0.8% < BER < 1.6%

RXQUAL_4: 1.6% < BER < 3.2%


RXQUAL_6: 6.4% < BER < 12.8% RXLEV_63: > -48 dBm

RXQUAL_7: 12.8% < BER

BER = Bit error rate


MN1788EU08MN_000118


MN1788EU08MN_000119

2 Discontinuous Transmission DTX


MN1788EU08MN_000120

The goal of DTX is to reduce MS power consumption and to reduce the interferencein a cell. Each direction of transmission is occupied about 50% of time. DTX is amode of operation where the transmitters are switched on only for those framescontaining useful information. The difficulty is to find techniques to distinguish noisyspeech from real noise even in a noisy environment.

These algorithms are implemented in the VAD (Voice Activity Detection) function.

The background acoustic noise has to be evaluated in order to transmit characteristicparameters to the receive side. The receive sides generates a similar noise calledcomfort noise during periods where the radio transmission is cut.

SBS parameter for DTX / VAD administration:

SpecificationName

Object/Package

DB Name Meaning

DTX indicator uplink BTS/BTSO

DTXUL 0: MS may use DTX (if possible)

1: MS shall use DTX

2: MS shall not use DTX

DTX indicatordown-link

BTS/BTSO

DTXDL FALSE: downlink DTX disabled at BTS

TRUE: downlink DTX enabled at BTS


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The goal of DTX is to reduce the speech data rate from 13 kbit/s (user speaking) to500 bit/s. This low rate is enough to encode the background noise.

This means instead of one frame of 260 bits per 20 ms only one frame per 480 ms issent. These so called SID frames (Silence Descriptor Frames) are sent at the start ofevery inactivity period, then all 480 ms, as long as the inactivity lasts between BTSand MS.

Between TRAU and BTS these comfort noise frames are sent all 20 ms. The timebehavior looks like following:

TRAU <-> BTS

SSSSCCCCCCCCCCCCCCCSSCCCCCCCS

BTS <-> MS

SSSSC C C C SSC C S

where S = speech frame

C = comfort noise frame


MN1788EU08MN_000122

The so called TRAU frame contains not only these 260 bit:

number of bits, UL number of bits, DL

frame synchronization 35 35

discrimination: speech,data, full rate, half rate

5 5

time alignment 6 6

bad frame indication 1

DTX mode 1 1

other info 3 1

speech 260 260

spare 5 9

Contents of a speech block for transmission at 16 kbit/s, details in GSM 08.60


MN1788EU08MN_000123

3 Channel Allocation Due to Interference

Level


MN1788EU08MN_000124

3.1 General

The BTS performs measurements of the (uplink) received level on all its idle trafficchannels (TCHs) in the same way as for the busy channels, i.e. the level values in[dBm] in each TDMA frame are averaged over one SACCH period. The averagedvalue is mapped on an RXLEV value (0, ..., 63).

Since these measurements are related to idle channel the measured level is inter-preted as interference level caused by a mobile in another cell using this channel oran adjacent channel (or by an external source of interference).

The information on the interference level can be taken into account at channel selec-tion to assign the channel with the lowest interference level which is expected toguarantee the best link quality.

An improvement of link quality by this mechanism is only possible if there is a choice,i.e.

� in cells with low traffic load where some idle channels are available for assignment

� if the interference level on idle channels differs significantly (this is not the case ifrandom frequency hopping is applied).

The following section describes the classification and selection of traffic channels dueto interference level in more detail.

Having measured the interference level for some SACCH multiframes, an arithmeticmean RXLEV_IDLE of a certain number of successive interference level samples,given by a parameter INTAVE, is calculated if the channel has been idle for the wholemeasurement period INTAVE * TSACCH.


MN1788EU08MN_000125

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Using this averaged interference level, an idle traffic channel is classified as achannel of

Interference Band 1 if 0 < RXLEV_IDLE < X1

Interference Band 2 if X1

< RXLEV_IDLE < X2


< RXLEV_IDLE < X3


< RXLEV_IDLE < X4


< RXLEV_IDLE < X5 = 63

where X1, X2, ... X5 denote the boundaries (O&M parameter) of the five interferencebands.

0 < X1 < X2 < X3 < X4 < X5 = 63


MN1788EU08MN_000126

RXLEV_IDLE low quality

high quality

X5

X4

X3

X2

X1

0

Interference Band 5

Interference Band 4

Interference Band 3

Interference Band 2

Interference Band 1

Fig. 5

If a traffic channel has not been idle during the whole measurement period, it is nottaken into account in the classification procedure.

Having classified all the traffic channels which have been idle during the wholemeasurement period, the TRX immediately sends the result of this classification tothe BSC (RF Resource Indication message) where it is used in the channel allocationprocess.

The period for sending the RF Resource Indication message can be set by aparameter RF_RES_IND_PERIOD.


MN1788EU08MN_000127

The time relations between measurements in the BTS and information sending to theBSC is illustrated in the figure below.

In the case of a call setup as well as in the case of a handover a traffic channel out ofthe best non-empty interference class is allocated.

A condensed form of the information on the “quality” of the idle traffic channels at acertain BTS is sent from the BSC to the MSC using the RESOURCE INDICATIONmessage. This message contains, for a specified cell (BTS), the number of idle TCHswhich are available separately in each of the five interference bands.

RF RES IND

(TRX -> BSC)

time

SACCH_Multiframe

averaging

period

INTAVE = 5

RF resource indication period

RF_RES_IND_PERIOD = 15

averaging

period

INTAVE = 5

RF RES IND

(TRX -> BSC)

Fig. 6


MN1788EU08MN_000128

3.2 Parameters for Channel Allocation due toInterference Level

The parameters are administered in the object BTS in the package BTSI and arelisted in the table below.

Specification Name DB Name Range Meaning

EN_INT_CLASS INTCLASS TRUE/FALSE

Flag to enable the classificationprocedure in the BTS

INTAVE INTAVEPRaveragingPeriod

1...31

* TSACCH

Period for averaging the measuredinterference level. A traffic channelswhich has not been idle during thewhole measurement and averagingperiod, it is not take into account inthe classification procedure.

X1, X2, ..., X5 INTAVEPR

interferenceThresholdsBoundaries

0...63 Boundaries of the interferenceclasses/ bands.

RF_RES_IND_PERIOD

RFRSINDP 1...254

* TSACCH

Period for sending the classificationinformation to the BSC.

Relations:

0 < X1 < X2 < X3 < X4 < X5 = 63

INTAVE < RF_RES_IND_PERIOD


MN1788EU08MN_000129

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MN1788EU08MN_000130

3.3 Interference Level Measurements on Idle TCHs

Overview

Long name Interference level Measurements on idle TCHs

OMC name interferenceLevelMeasOnIdleTCHs

BR3.0 short name ILUPLKIC

Function group TRXMEAS

Number of counters 5 * No. of idle TCHs

one value for each of the 5 interference bands

Kind of counters CC, Integer

Unit of counters ---

Observed object TRX

Measurement depth per idle TCH, per interference band

First release implemented BR3.0

GSM/CCITT reference ---

Description

The measurement "Interference measurements on uplink idle TCHs" provides thenumber of times the measured RXLEV value on the idle TCHs exists within theappropriate interference bands. This measurement is executed per selected cell.


MN1788EU08MN_000131

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Trigger Events

This performance measurement is based on the radio link measurement RXLEV onidle traffic channels (standardized in GSM 05.08, GSM 08.58), used for definition ofthe channel allocation algorithm.

Interference measurements have to be performed on all idle TCHs of a BTSseparately by each TRX. Measurements of the received level are made over everySACCH multiframe period. An arithmetic mean of a certain number of successiveinterference level measurements shall be calculated. This number is given by theparameter INTAVE. Values are calculated for those TCHs which have been idleduring the whole measurement period INTAVE x TSACCH (480ms). The idle TCHsare classified using 5 interference bands. The BTS maps the averaged interferencelevel measurements into these 5 interference bands. The BTS transmits the result ofthis classification regularly to the BSC using the RF RESource INDication message.

These radio link measurement data are used to calculate this proprietary perform-ance measurement within the BSC described below.

'They are also used for execution of the GSM 12.04, Vers. 4.3.0 defined performancemeasurement Annex B, B.2.1.13: "Mean Number of Idle TCHs per InterferenceBand".


MN1788EU08MN_000132


MN1788EU08MN_000133

4 Exercise


MN1788EU08MN_000134


MN1788EU08MN_000135

Exercise 1

Title: Power Control

Task

Consider a mobile station (of GSM phase 1) of maximum output power PMS = 39dBm. Within the cell serving that MS the following parameters are set:

MS_TXPWR_MAX = 33 dBm

BS_TXPWR_MAX = 43 dBm

POW_INCR_STEP_SIZE = 3 (6 dB)

POW_RED_STEP_SIZE = 2 (4 dB)

Furthermore, the following thresholds for the uplink power control decision are set:

L_RXQUAL_P = 4 L_RXLEV_P = 20

U_RXQUAL_P = 3 U_RXLEV_P = 26

In the table below some examples for the measured and average (uplink) RXLEVand RXQUAL as well as for the last confirmed TXPWR are given. What are the corre-sponding values for the new commended TXPWR?

Example last confirmedTXPWR (dBm)

RXQUAL RXLEV new com-mandedTXPWR

1 25 2 23 ?

2 21 4 28 ?

3 15 2 30 ?

4 31 5 25 ?

5 17 5 34 ?


MN1788EU08MN_000136


MN1788EU08MN_000137

Exercise 2

Title: Channel Allocation due to Interference Level

Task

Consider a cell with the following parameters or interference classification:

INTAVE = 2

RF_RES_IND_PERIO = 4

X1 = 10

X2 = 20

X3 = 30

X4 = 40

X5 = 63

The following levels have been measured by the BTS on its channels each SACCHframe

B: busy channel

Last RF Resource indication message

1 2 3 4 5 6 SACCH frame

23 15Bchan 1 19 21 15

B Bchan 2 B B BB

chan 3 12 14 14 16 14 14

chan 4 22 18 18 17 1820

To which interference bands do channel 1-4 belong at SACCH frame 7?

Which channel is allocated this time in the case of an assignment request?


MN1788EU08MN_000138


MN1788EU08MN_000139

Exercise 3

Enable the power control mechanism and create the related scanner


MN1788EU08MN_000140

08 mn1788eu08mn 0001 interference reduction

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