BSS PARAMETERSAND FUNCTIONS

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Groups of ParametersCell (re)selectionPower ControlHandover ControlRadio LinkSignalling timers

Areas of ImprovementMinimization of InterferenceHandover behaviour ImprovementTraffic DistributionIncrease Call Completion

Cell (Re)selectionCell reselection is done using C1 path loss criterion.The purpose is to ensure that the MS is camped on to the cell with the best transmission quality.The MS will camp on to the cell with the highest C1 value if C1 > 0.

The following parameters are used to calculate the C1 criterionThe received signal at the MS side.Rxlev_access_min - broadcast on the BCCH - The minimum received level at the MS required for access to the network.Ms_txpwr_max_cch - the maximum power that an MS may use when initially accessing the network.The maximum power of the MS

C1 = A - Max(B,0)

A = Received level Average - Rxlev_access_min.

B = MS_txpwr_max_cch - maximum output power of the MS

Cell Reselect HysteresisCell reselection on the border of two location areas result in a location update. When an MS moves on the border of two location areas lots of location updates take place. To avoid these location updates, the reselect hysteresis is introduced.A location update is performed only if:The C1 value of the new location area is higher than the C1 value in the current location area andThe received signal strengths have at least a difference of the reselect hysteresis.

LA1 LA2 LA1 LA2Location Update to LA1

Location Update to LA2

Power ControlPower control enables the MS and/or BTS to increase or decrease the transmit power.The main purpose is to reduce interference.Power control also saves MS battery power.If power control is disabled the MS/BTS will always transmit at maximum power.

Power Control ProcessThe BTS measures the uplink and receives downlink measurements from MS every 480 ms (every SACCH multiframe, 104 TDMA frames). The following measurement parameters are used in the power control processRXLEV_DL (receive signal level measured by MS)RXLEV_UL (receive signal level measured by BTS)RXQUAL_DL (receive signal quality measured by MS)RXQUAL_UL (receive signal quality measured by BTS)The BTS reports the measurements to the BSC.The BSC calculates the foll.. averaged parameters using a sliding window to control the speed of the power control process.AV_RXLEV_XL_PC & AV_RXQUAL_XL_PCwhere XL means either UL (uplink) or DL (downlink)Averaged parameters are used to remove the effect of Rayleigh fading on measurements.The BSC evaluates whether a power control increase or decrease action should be initiated. It compares the averaged parameters to the defined thresholds.

DecidePowerControlEvaluate MeasurementsCalculateAveragedParametersBSSParametersMS BTS BSC Um AbisInterface Interface MSmeasurementsBTSmeasurementsPowerControlProcess

Before averagingAfter averagingSignal

levelTimeSignal Level Averaging

RXLEVRXQUALThe signal strength is measured in dBm. There are 64 levels of signal strength that can be reported, with 0 indicating a weak signal and 63 a strong signal.The signal quality is measured in bit error rate (BER), which represents the percentage of incorrect bits received (after decoding). The signal quality can be poor due to inter symbol, co-channel and/or adjacent-channel interference.There are 8 levels of signal quality. 0 indicates good quality and 7 indicates bad quality.

Signal Levels

Signal level

Range (in dBm)

Signal Quality

Range (in BER)

0

RXLEV < -110

0

BER < 0.2

1

-110 < RXLEV < -109

1

0.2 < BER < 0.4

2

-109 < RXLEV < -108

2

0.4 < BER < 0.8

3

0.8 < BER < 1.6

4

1.6 < BER < 3.2

62

-49 < RXLEV < -48

5

3.2 < BER < 6.4

63

RXLEV > -48

6

6.4 < BER < 12.8

7

BER > 12.8%

W_QUAL_PCBSS parameter defining the weighting factor used to minimize the influence of Discontinuous transmission (DTX) on the quality averaging procedure. W_QUAL_PC has a domain value of 1, 2 or 3. When DTX is not applicable to a specific measurement, W_QUAL_PC has the value 1.

Power Control EvaluationIncreaseA power control increase is initiated whenAV_RXQUAL_XL_PC > L_RXQUAL_XL_PorAV_RXLEV_XL_PC < L_RXLEV_XL_P

where L_RXQUAL_XL_P is the BSS parameter defining the lower receive quality for the Power control, and L_RXLEV_XL_P is the BSS parameter defining the lower receive level for the power control.DecreaseA power control decrease is initiated when (AV_RXQUAL_XL_PC < U_RXQUAL_XL_PandAV_RXLEV_XL_PC >L_RXLEV_XL_P + POW_RED_STEP_SIZE)orAV_RXLEV_XL_PC > U_RXLEV_XL_PC

where POW_RED_STEP_SIZE defines the RF power reduce step size for power control.

Power Control for uplink0 L_RXLEV_UL_PU_RXLEV_UL_P63+++++Optimum---Signal level AV_RXLEV_UL_PCSignal QualityAV_RXQUAL_UL_PC (in BER)U_RXQUAL_UL_PL_RXQUAL_UL_P07

Time between power control commands.The minimum time between two power control commands is represented by the BSS parameters P_CON_INTERVAL and P_CON_ACK. After any power control action the BSS will wait for the specified time before performing the next power control action.Acknowledge Timer - When the BSC sends a power control command the timer P_CON_ACK starts. It stops when a power update message is received from the BTS. If the timer expires, the BSC can decide to send a new power control command.Interval timer. As soon as an acknowledge message is received from the BTS the P_CON_INTERVAL timer starts. The BSC has to wait for this period of time before it can send another power control command.

BSCBTSPower Control commandNew BTS powerPower Control CommandP_CON_ACKP_CON_INTERVALMINIMUM TIME BETWEEN TWO COMMANDS

Handover ControlThe handover process allows to change the serving cell without losing the call in progress. Handover process will normally only be started if power control is not helpful anymore.The reasons to perform a handover are:Distance (or propagation delay) between MS and BTS becomes too big.Receive signal quality becomes too bad.Receive signal level becomes too bad.Path loss situation for the MS to another cell is better.

Power Budget HandoverA power budget handover takes place as soon as a better cell with respect to power budget is available to handle the call.PBGT(i) = AV_RXLEV_NCELL(i) - AV_RXLEV_SCELL + (MS_TXPWR_MAX(i) - MS_TXPWR_MAX)*2The list of neighbouring cells is ordered using the value of the expressionPBGT(i) - HO_MRGN(0,n)

AV_RXQUAL_XL_HO > L_RXQUAL_XL_HandAV_RXLEV_XL_HO > RXLEV_XL_IHIntra Cell HandoverAV_RXQUAL_XL_HO > L_RXQUAL_XL_HAV_RXLEV_XL_HO >L_RXLEV_XL_HPBGT(i) >HO_MARGIN(0,i)HANDOVER CONTROL ALGORITHMNo HandoverAV_RXLEV_NCELL (i) >RXLEV_MIN(i) +2 x Max(0,P-MS_TXPWR_MAX(i))& Traffic load CriteriaInter Cell HandoverSelect the best fromneighbouring cells fulfillingthe following condition

Handover ParametersL_RXLEV_XL_HThe higher it is set, the more handover attempts are made.L_RXQUAL_XL_HThe lower it is set, the more handover attempts are made.RXLEV_XL_IHThe lower it is set, the more intra-cell handover attempts are made.

Free factor_XClassifies the traffic load to different bands depending upon no. of free traffic channels.FREELEVEL_XAnalyzes the number of free traffic channels in a cell to adjust the cell order.Link factorDefines the order preference of the concerned adjacent cell.

Radio linkAccess Grant BlocksOn a BCCH a number of blocks are reserved as CCCH (Common Control Channel). These common control channels can be used as Access grant channel or as paging channel. The no. of blocks reserved for access grant is specified by BS_AG_BLKS_RES. It is also possible to set the value to 0 in which case the network decides how many are used as AGCH. The rest are used as PCH.

Maximum number of retransmissionsMAX_RETRAN specifies the number of retransmissions an MS may perform on the RACH when no AGCH is available from the BTS side. The retransmission is performed after a random no. of time slots the maximum of which is specified by TX-INTEGER. After this max no of retries, the MS is required to reselect another cell.

Radio Link ..contd..No of spread slots trans (TX_INTEGER)TX-INTEGER is used for random spreads of the random access bursts when the Immediate assignment is not received within a given time.No. of Multi framesBS_PA_MFRMS specifies no. of multiframes between paging messages to MSs of the same paging group.This parameter is also used as Downlink Signalling failure counter. (DSC). When an MS camps on to a cell, DSC is set to a value equal to the nearest integer of 90/BS_PA_MFRMS for that cell. If a message is successfully decoded DSC is increased by 1, otherwise it is decreased by 4. When DSC

Radio Link FailureThe criterion for determining Radio Link Failure in the MS is based on the success rate of decoding messages on the downlink SACCH.

Radio Link Time OutIf the MS is unable to decode a SACCH message, the time out counter is decreased by one, else it is increased by two. If the counter reaches 0 a radio link failure is declared and the connection is cut off.

Radio Link Failure Warning.This counter is set lower than the time out counter and works in a similar fashion. If the counter reaches 0, the BSC directs the BTS and MS to transmit at maximum power.

Air Timers - UmT3101 - Immediate assignment timersThis is used to complete the channel assignment after an IMMEDIATE ASSIGNMENT. The timer is started on receipt of the CHN_ACT_ACK message. If no Establish_Ind is received during this time, the R_CHN_REL message is sent to the BTS.T3107 - Assignment Command TimerThis timer is used to guard the seizure of the MS on the new channel. The timer is started by the sending of an handover ASSIGNMENT COMMAND message and is normally stopped when the MS has correctly seized the new channel. If the timer expires before an ASSIGNMENT_COMPLETE or an ASSIGNMENT_FAILURE message has been received from the MS, the allocated resources on the new channel have to be released and an ASSIGNMENT_FAILURE is expected on the old channel.

Air Timers - contd.T3109 - CHANNEL RELEASE TIMERThis timer is used to guard the reception of the RELEASE INDICATION. It is used in the channel release procedure. Its purpose is to release channels in case of loss of communication. This is to be set higher than the Radio Link Time Out counter, to ensure that the network will not put two MSs on the same dedicated channel.If no RELEASE INDICATION is received during this time, the message RF_CHANNEL_RELEASE is sent to the BTS.

T3111 - CHANNEL DELAY TIMERThis timer is used to delay the release of the channel resources in the BTS after the disconnection of the main signalling link. It is started on receiving a RELEASE INDICATION MESSAGE. When the timer times out the RF_CHANNEL_RELEASE is sent to the BTS.

Other TimersT10 - BSS MAP TIMERTime to return Assignment complete or Assignment Failure message. The purpose of the timer is to keep the old channel sufficiently long for the MS to be able to return to the old channel. The value should be greater than T3107.

T3212 - Periodic Location Update timer.The timer that governs the periodic location update procedure of the MS. Too low values will increase SDCCH traffic in the network. Too high values will make the timer ineffective. The timer should be less than the Implicit Detach Timer in the MSC.

Paging LoadCCCH Load Indication PeriodUsed for the load indication of CCHs. This parameter specifies the periodicity ofthe message load indication CCH which is sent from the BTS to the BSC. Theload indication is calculated with the parameters RACH_MEAS_PERIOD andRACH_BUSY_THRES. If the received signal level is greater than or equal to thevalue of RACH_BUSY_THRES then the RACH burst in question will beindicated as busy (one or more MSs have tried to access the network).CCCH_LOAD_IND_PERIOD >= RACH_MEAS_PERIOD

CCCH Load Indication ThresholdPaging load threshold. Defines the number of busy slots to start/stop sending messages between BTS and BCE.

RACH slots measurementRACH_Busy _ThresholdDetermines the threshold of the RACH receive level in the BTS. The level of eachRACH timeslot is evaluated. All the RACH timeslots that are above the definedlevel are used for the load measurements, and are forwarded to the BSC.

RACH_MEAS_PERIODThis parameter specifies the period of RACH measurements. It is used to calculatethe total number of RACH bursts for which a measurement is performed andcompared to RACH_BUSY_THRES.

Location Area Code - LACUsed to define the location area of a cell within a PLMN. A number of cells will share a LAC.Small groups will mean increased location updates and hence more SDCCH traffic.Large groups will increase the paging load.

Parameter values

Parameter

Escotel Value

Recommended value

RXLEV_ACC_MIN

-110 dBm

-102 dBm

MS_TXPWR_MAX_CCH

33 dBm

33 dBm

Reselect Hysteresis

6 dBm

6 dBm

MS_TXPWR_MAX

33 dBm

33 dBm

HO_MRGN

6 dB

6dB

RACH busy threshold

-110 dBm

-110 dBm

RACH load avg slots

255 SACCH frames

255 ( 240 s)

Load indication enabled

True

True

Threshold CCCH load ind

63

63

CCCH load ind. period

255 SACCH frames

255 ( 240 s)

Parameter values - contd.

Parameter

Escotel Value

Recommended

L_RXLEV_XL_P

- 90 dBm

- 90

U_RXLEV_XL_P

- 86 dBm

- 80

L_RXQUAL_XL_P

3

3

U_RXQUAL_XL_P

1

1

L_RXQUAL_XL_H

4

4

L_RXLEV_XL_H

- 102 dBm

- 102 dBm

RXLEV_XL_IH

- 74 dBm

- 74 dBm

Averaging Period

5 SACCH frames

5 (2.4 s)

P_CON_ACK

6 SACCH frames

6 (2.88 s)

P_CON_INTERVAL

4 SACCH frames

4 (1.92 s)

Parameter values - contd.

Parameter

Escotel value

Recommended value

MAX_RETRANS

4

4

TX-INTEGER

20 TDMA frames

10 TDMA frames

BS_AG_BLKS_RES

0

0

BS_PA_MFRMS

7

4

T3212

4 hours

12 hours

Radio Link Time out

24 SACCH frames

24 (11.52 s)

Radio Link Fail Warning

16 SACCH frames

16 (7.68 s)

T 3101

2 s

2 s

T 3107

23 s

23 s

T 3109

12 s

12 s

T 3111

1 s

1 s

Why