What is GSM Cell Reselection and How it works.

When mobile has been selected successfully the mobile now will start reselection tasks.

Perform Cell reselection measurement first

Mobile will continuously make measurements on its neighboring cells (as  indicated by the BA list) to initiate cell reselection if necessary. At least 5 measurement sample per neighboring cell is needed. A running average of the received signal level will be maintained for each carrier in the BA list.

All system information messages sent on the current BCCH on the serving cell must be read by the mobile every 30 seconds to monitor changes in cell parameters (ex: MsTxPwrMax).

The mobile also has to read the 6 strongest BCCH every 5 minutes to receive its cell parameters (ex: MsTxPwrMax). The 6 strongest can be seen from the BA list which has the updated measurement of the 32 BCCH carrier.

The neighboring list for the best 6 neighbors is updated every 60 seconds, which means the mobile has to measure each neighbor by 10 seconds.

The mobile also has to read the BSIC of the 6 strongest BCCH every 30 seconds to confirm that it is still monitoring the same cells. If a new BSIC is detected, then the BCCH of this BSIC will be read to receive the cell parameters.

Cell reselection Criteria :

The mobile will reselect and camp on another cell if any of the following criteria is satisfied:

  1. The serving cell is barred.
  2. C1 value in the current cell is below 0 for 5 seconds which indicates that the path loss is high and the mobile needs to change cell.
  3. The Mobile has unsuccessfully tried to access the network as defined by the MAXRET parameter or MaxNumberRetransmissions.

MAXRET is the maximum number of retransmission a mobile can do when it is accessing the system It is defined per cell.

Assuming that one of the criteria above was satisfied then the mobile will select a cell with a better C1. However if the cell belongs to a different location area then the C1 for that cell has to exceed a reselection hysterisis parameter called CRH or CellReselectHyseteris for the reselection to happen !!

If the mobile is moving in a border area between location areas, it might repeatedly change between location areas. Each change requires location updating and cause heavy signaling load and risk paging message being lost. To prevent this, a cell reselect hysteresis parameter CRH is used.

The cell in a different location area will only be selected if the C1 of that cell is higher than the C1 of the current serving cell by the value of the Reselect Hysteresis. Since the Value of CRH may be different for each cell, the CRH used for comparison will be the one broadcast by the serving cell.

If the value is set very low then the mobile will Ping-Pong between location areas which will increase signaling load. If the value is set very high the mobile may camp in the wrong cell too long.

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