bss parameters
TRANSCRIPT
BSS PARAMETERS
AND FUNCTIONS
2
This page has been intentionally left blank.
3
Groups of Parameters
Cell (re)selection Power Control Handover Control Radio Link Signalling timers
4
Areas of Improvement
Minimization of Interference Handover behaviour Improvement Traffic Distribution Increase Call Completion
5
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.
6
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
7
C1 = A - Max(B,0)
A = Received level Average - Rxlev_access_min.
B = MS_txpwr_max_cch - maximum output power of the MS
8
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.
9
LA1 LA2 LA1 LA2
Location Update to LA1
Location Update to LA2
10
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.
11
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.
12
DecidePower
Control
Evaluate Measurements
CalculateAveragedParameters
BSSParameters
MS BTS BSC
Um AbisInterface Interface
MSmeasurements
BTSmeasurements
PowerControlProcess
13
Before averaging
After averaging
Signal
level Time
Signal Level Averaging
14
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.
15
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%
16
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.
17
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.
18
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
19
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.
20
BSC BTS
Power Control command
New BTS power
Power Control Command
P_CON_ACK
P_CON_INTERVAL
MINIMUM TIME BETWEEN TWO COMMANDS
21
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.
22
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)
23
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
24
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.
25
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.
26
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.
27
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.
28
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.
29
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.
30
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.
31
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.
32
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.
33
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.
34
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)
35
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)
36
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