GSM Optimization
Idle mode
o
PLMN Selection.
o
Cell Selection.
o
Cell Reselection.
o
Location Updating.
o
Monitor the Incoming Paging.
1) PLMN Selection
An MS will need to make a PLMN selection only incase:
a. MS is powered ON for the 1st time i.e. No PLMN was registered on before (No Information on MCC&MNC is stored on SIM)
b. Old PLMN is not available any more. PLMN Selection Criterion
- Automatic PLMN Selection Mode steps: 1) Home PLMN.
2) Each PLMN stored on the SIM card in priority order.
3) Other PLMNs have Signal Strength > -85 dBm in random order. 4) All other PLMNs in order of decreasing Signal Strength.
- Manual PLMN Selection Mode: 1) Home PLMN.
2) All other available PLMNs and give the user the choice to select.
The Cell Selection algorithm tries to find the most suitable cell in the selected PLMN and make the MS camp on.
Cell Selection is done by the MS itself.
Scanning RF Frequencies may occur in 2 ways:
1) Normal Scanning: Scan all Frequencies in the band ex:124 freq. in GSM900 Band.
MS will synchronize to the BCCH frequency and read system information (LAI,BA List,…etc)
Scan RF Frequencies one by one and calculates the Average received signal strength over 3 5 seconds Tune to the RF Frequency with the highest average received signal strength
Camp on the Cell
Check if the chosen frequency is a BCCH carrier frequency or not
Check if C1 > 0 or not Check if Cell is barred or not Check if PLMN is desired or not
Tune to the next higher frequency that wasn’t tried before
Yes Yes No Yes No Yes No No
2) Stored List Scanning: Scan the Frequencies in the Idle BA list (BCCH
Allocation) stored on the MS SIM before being switched off. (BA list can have maximum 32 frequencies)
C1 is called “Cell Selection Quantity”
It is calculated at the MS based on the below equation:
C
1= (Received SS – ACCMIN) – max (CCHPWR-P,0)
- ACCMIN Minimum allowed DL received SS at the MS in order to access
the system
- CCHPWR Maximum allowed transmitting power by the MS in the UL.
- P Maximum output power of the MS according to its class.
N.B:
1. ACCMIN and CCHPWR are cell parameters sent to the MS at the BCCH channel.
2. If CCHPWR > P then C1 will decrease and so the Received SS should be large
enough to keep C1 > 0 (May be this cell is not designed of this MS class)
3. ACCMIN, CCHPWR, P are all measured in dBm, where C1&C2 are measured in dBs
3) Cell Reselection
After a cell has been selected, the MS will start the cell reselection measurements to know if it is better to stay on the current cell or to camp on another cell.
Cell reselection measurements summary:
BSIC BCCH Data (System Information) Serving Cell - Every 30 Seconds
Six Strongest Neighbors
When will Cell Reselection occur ?!!!
1) Serving Cell became barred ( CB = YES )
2) C1serving cell falls below zero for more than 5 seconds.
3) MS tried to access the network through this cell unsuccessfully for the allowed no. of times defined by the parameter MAXRET
4) C2neighbor cell ( one of the six strongest neighbors) became greater than C2serving cell for more than 5 seconds.
5) MS detects Downlink Signaling Failure.
C2 is called “Cell Reselection Quantity”
C2 = C1 + CRO – TO * H( PT – T ) where PT ≠ 31
C2 = C1 – CRO where PT = 31 0 , X < 0
Where H(x)
1 , X ≥ 0
CRO Cell Reselection Offset, unit = 2 dB, value range = 0 to 63 TO Temporary Offset, unit = 10 dB, value range = 0 to 7 PT Penalty Time during which TO is valid
T Initiated from zero when the MS places the neighbor in the list of the Six Strongest
CRO : defines a signal strength offset to encourage or discourage MSs to reselect that cell.
TO : defines a negative temporary offset for certain time according to settings of
PT (Practically this is useful to prevent fast moving MS from camping on
microcells)
PT: If PT is set to 31, this means that a (–ve) SS offset “CRO” will be applied to this cell and it
CRH ( Cell Reselection Hysteresis )
Cell Reselection between two cells lie in two different Location Areas, will be accompanied by Location Update.
At the border between cells the Signal level may be comparable, cell reselection may occur many times accompanied by many location updating leading to huge signaling load.
To avoid this, a parameter CRH is introduced such that a cell in another location area LA2 should have C2LA2 should greater than C2LA1 of serving cell lie in LA1 by
at least CRH in order to be selected.
If C2LA1 = 5 dB, CRH = 4 dB, then C2LA2 ≥ 9 dB in order to be selected
4) Location Update
In the Idle Mode, the Network knows the location of the MS on a Location area resolution not on a cell resolution.
There are three different types of location updating defined: 1) Normal Location Updating.
2) Periodic registration.
3) IMSI attach & IMSI detach (when the MS informs the network when it enters an inactive state)
1. Normal Location Updating
2. Initiated by the MS when it enters a cell belongs to a new Location Area (LA). 3. The MS will compare the LAIold stored on the SIM with the LAInew broadcasted
from the new cell and it will found them different so it’ll perform Location Update type normal.
1. Periodic Registration
2. Regularly the MS should update the Network with its current location Area.
3. The Network will inform the MS how often it should report the location Area he is registering himself in.
4. Based on the value of the Parameter T3212 the MS will know how frequent it should make periodic registration.
5. T3212 take values from 1 (6min) to 255 (25.5 Hours), default = 40 (4 Hours)
1. IMSI Attach/Detach
2. IMSI attach/detach operation is an action taken by the MS to inform the Network either it will go to inactive state (Power off) or it returned back to idle mode. 3. ATT is a cell parameter that will inform the MS whether IMSI attach/detach is
operational or not.
4. If ATT=Yes, then before the MS will be switched off, it will send an IMSI detach request to the Network, so no paging messages will be sent to this MS while it is in this state.
5. When the MS is switched on again it will send an IMSI attach request to the Network so now paging messages can be sent normally to this MS.
