BSS PARAMETERS

AND FUNCTIONS

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Groups of Parameters

Cell (re)selection Power Control Handover Control Radio Link Signalling timers

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Areas of Improvement

Minimization of Interference Handover behaviour Improvement Traffic Distribution Increase Call Completion

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Cell (Re)selection

Cell 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.

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The following parameters are used to calculate the C1 criterion

The 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

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C1 = A - Max(B,0)

A = Received level Average - Rxlev_access_min.

B = MS_txpwr_max_cch - maximum output power of the MS

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Cell Reselect Hysteresis

Cell 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 and– The received signal strengths have at least a difference

of the reselect hysteresis.

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LA1 LA2 LA1 LA2

Location Update to LA1

Location Update to LA2

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Power Control

Power 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.

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Power Control Process

The 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 process

– RXLEV_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_PC

• where 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.

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DecidePower

Control

Evaluate Measurements

CalculateAveragedParameters

BSSParameters

MS BTS BSC

Um AbisInterface Interface

MSmeasurements

BTSmeasurements

PowerControlProcess

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Before averaging

After averaging

Signal

level Time

Signal Level Averaging

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RXLEV RXQUAL

The 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.

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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%

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W_QUAL_PC

BSS 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.

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Power Control Evaluation

Increase– A power control increase is

initiated whenAV_RXQUAL_XL_PC > L_RXQUAL_XL_P

or

AV_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.

Decrease– A power control decrease is

initiated when (AV_RXQUAL_XL_PC < U_RXQUAL_XL_P

and

AV_RXLEV_XL_PC >

L_RXLEV_XL_P + POW_RED_STEP_SIZE)

or

AV_RXLEV_XL_PC > U_RXLEV_XL_PC

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

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Power Control for uplink

0 L_RXLEV_UL_P U_RXLEV_UL_P 63

+

+

+

+ +

Optimum

- -

-

Signal level AV_RXLEV_UL_PC

Signal QualityAV_RXQUAL_UL_PC (in BER)

U_RXQUAL_UL_P

L_RXQUAL_UL_P

0

7

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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.

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BSC BTS

Power Control command

New BTS power

Power Control Command

P_CON_ACK

P_CON_INTERVAL

MINIMUM TIME BETWEEN TWO COMMANDS

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Handover Control

The 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.

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Power Budget Handover

A 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)*2

The list of neighbouring cells is ordered using the value of the expression– PBGT(i) - HO_MRGN(0,n)

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AV_RXQUAL_XL_HO > L_RXQUAL_XL_Hand

AV_RXLEV_XL_HO > RXLEV_XL_IH

Intra Cell Handover

AV_RXQUAL_XL_HO > L_RXQUAL_XL_H

AV_RXLEV_XL_HO >L_RXLEV_XL_H

PBGT(i) >HO_MARGIN(0,i)

HANDOVER CONTROL ALGORITHM

No Handover

AV_RXLEV_NCELL (i) >RXLEV_MIN(i) +

2 x Max(0,P-MS_TXPWR_MAX(i))& Traffic load Criteria

Inter Cell Handover

Select the best fromneighbouring cells fulfillingthe following condition

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Handover Parameters

L_RXLEV_XL_H– The higher it is set, the more

handover attempts are made.

L_RXQUAL_XL_H– The lower it is set, the more

handover attempts are made. RXLEV_XL_IH

– The lower it is set, the more intra-cell handover attempts are made.

Free factor_X– Classifies the traffic load to

different bands depending upon no. of free traffic channels.

FREELEVEL_X– Analyzes the number of free

traffic channels in a cell to adjust the cell order.

Link factor– Defines the order preference of

the concerned adjacent cell.

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Radio link

Access Grant Blocks– On 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 retransmissions– MAX_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.

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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 frames– BS_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 <= 0 cell reselection is performed.

– This parameter will also determine the DRX (Discontinuous reception). DRX is implemented in order to save battery power.

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Radio Link Failure The 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 Out– If 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.

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Air Timers - Um

T3101 - Immediate assignment timers– This 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 Timer– This 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.

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Air Timers - contd.

T3109 - CHANNEL RELEASE TIMER– This 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 TIMER– This 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.

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Other Timers

T10 - BSS MAP TIMER– Time 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.

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Paging Load CCCH Load Indication Period

Used for the load indication of CCHs. This parameter specifies the periodicity of

the message load indication CCH which is sent from the BTS to the BSC. The

load indication is calculated with the parameters RACH_MEAS_PERIOD and

RACH_BUSY_THRES. If the received signal level is greater than or equal to the

value of RACH_BUSY_THRES then the RACH burst in question will be

indicated as busy (one or more MSs have tried to access the network).

CCCH_LOAD_IND_PERIOD >= RACH_MEAS_PERIOD

CCCH Load Indication Threshold

Paging load threshold. Defines the number of busy slots to start/stop sending messages between BTS and BCE.

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RACH slots measurement

RACH_Busy _ThresholdDetermines the threshold of the RACH receive level in the BTS. The level of each

RACH timeslot is evaluated. All the RACH timeslots that are above the defined

level are used for the load measurements, and are forwarded to the BSC.

RACH_MEAS_PERIOD

This parameter specifies the period of RACH measurements. It is used to calculate

the total number of RACH bursts for which a measurement is performed and

compared to RACH_BUSY_THRES.

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Location Area Code - LAC

Used 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.

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Parameter values

Parameter Escotel Value Recommended valueRXLEV_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)

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Parameter values - contd.

Parameter Escotel Value RecommendedL_RXLEV_XL_P - 90 dBm - 90U_RXLEV_XL_P - 86 dBm - 80

L_RXQUAL_XL_P 3 3U_RXQUAL_XL_P 1 1L_RXQUAL_XL_H 4 4L_RXLEV_XL_H - 102 dBm - 102 dBmRXLEV_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)

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Parameter values - contd.

Parameter Escotel value Recommended valueMAX_RETRANS 4 4

TX-INTEGER 20 TDMA frames 10 TDMA framesBS_AG_BLKS_RES 0 0

BS_PA_MFRMS 7 4T3212 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 sT 3107 23 s 23 sT 3109 12 s 12 sT 3111 1 s 1 s