Cell Selection and Reselection

On switch-on, an MS periodically measures the received power level on each of the BCCH
frequencies of all cells within range. From these periodic measurements the MS calculates the
mean received level value from each cell, stored in the parameter RXLEV(n) where
n=neighbouring cell number.
Based on these calculated values, the MS selects which cell to connect to. This connection
process is referred top as ‘Camping-on’ to that cell.
An MS in idle mode must periodically measure the received power level on each of the BCCH
frequencies of neighbouring cells. From these periodic measurements the MS calculates the
mean received level value from each cell, stored in the parameter RXLEV(n) where
n=neighbouring cell number. Based on these calculated values, the MS selects which cell to
camp-on to.
Once a mobile has camped on to a cell, it will continue to measure neighbouring BCCH
carriers, looking for a better cell.
The OFFSET (hysteresis) term prevents unnecessary re-selection on a location area boundary
which would require extra signalling to perform the location update.

Cell Selection Procedure

• MS powers-up
• MS starts measuring received power level from all cells in
range
• MS calculates average power level received from each cell
• Stored in RXLEV(n) parameter
• MS calculates C1 parameter for each cell based on RXLEV(n)
and vendor-specific parameters
• Mobile compares cells which give a positive value of C1 and
‘camps-on’ to the cell with the highest C1 value

Cell Re-selection – GSM Phase 1 Mobiles

For GSM Phase 1 mobiles, cell reselection is achieved by comparing current cell C1
with neighbouring C1 cell measurements:
• Between cells within a Location Area:
C1 (new) > C1 (old)
(for more than 5 seconds)
• Between cells on a Location Area boundary:
C1 (new) > C1 (old) + OFFSET
(for more than 5 seconds)

Cell Re-selection – GSM Phase 2 Mobiles

• GSM Phase 2 introduced a separate cell re-selection
parameter, C2
• Intended to:
• Prevent multiple reselections for fast-moving mobiles
• Ensure MS camps on to cell with greatest chance of successful
communications
• The C2 calculated is:
C2 = C1 + OFFSET – (TEMPORARY_OFFSET x H(PENALTY_TIME –T)

In order to optimize cell reselection, additional cell reselection parameters can be broadcast on
the BCCH of each cell. The cell reselection process employs a parameter C2 which depends on
these parameters.
The parameters used to calculate C2 are as follows:
C2 = C1 + CELL_RESELECT_OFFSET - TEMPORARY OFFSET * H(PENALTY_TIME
- T)
for PENALTY_TIME <> 11111
C2 = C1 - CELL_RESELECT_OFFSET
for PENALTY_TIME = 11111
Where: H(x) = 0 for x < 0 {x = PENALTY_TIME-T}
H(x) = 1 for x >= 0
CELL_RESELECT_OFFSET
This optional parameter is a a positive or negative offset applied to each cell to
encourage or discourage MSs to reselect that cell.
PENALTY_TIME
When the MS places the cell on the list of the strongest carriers (Neighbour list), it
starts a timer which expires after the PENALTY_TIME. This timer will be reset when
the cell is taken off the list. For the duration of this timer, C2 is given a negative
offset. This will tend to prevent fast moving MSs from selecting the cell.
TEMPORARY_OFFSET
This is the amount of the negative offset described in (2) above. An infinite value can
be applied, but a number of finite values are also possible.