This document discusses cell selection and reselection in GSM networks. It explains: 1) Cell selection is performed when a mobile first turns on to select an initial "camped-on" cell. Reselection occurs when the mobile moves to ensure it remains on the best cell. 2) C1 and C2 criteria are used for selection and reselection. C1 compares signal levels and C2 adds offsets. 3) In the scenario, the mobile selects the 900MHz cell using C1/C2 criteria. To prefer the 1800MHz cell instead, the document suggests using the C2 formula without offsets by setting the penalty time lower.

1. GSM
GSM/GPRS Radio Network Planning
Optimisation
Senario: Which type of handover (level or quality) is triggered at time moment t1 when four
last measurement results are -97, -95, -94 and -97 for the level and 0, 7, 6 and 0 for the
quality and the averaging window is 4 for level and 1 for quality? Handover thresholds are
-95 for level (px = 1, nx = 1) and 4 for quality (px = 3, nx = 4) and quality handover has
higher priority than level handover.
» level samples are averaged by using averaging window of four.
» Average of four level samples is ≤-95 dBm and only one average is needed to trigger the
level handover (px=1, nx=1).
Solution:
⇒ level handover is triggered!
⇒ quality samples are not averaged (averaging window is one) and quality handover is
triggered if three samples (px = 3) exceed the threshold four inside a last four samples (nx =
4).
⇒ Quality handover is NOT triggered because only two samples (6 and 7) exceed the
threshold and thus px = 3 never happens!
Result: level handover is triggered, because average is < −95dBm and quality is not triggered
at all!
Senario: Which cell is selected as a serving cell in the IDLE mode when the received level
from the 900 MHz cell is −80 dBm and from 1800 MHz cell −87dBm and cellReselectOffset
= 10dB, temporaryOffset = 20dB and penaltyTime = 640s are set for the 1800MHz cell.
Minumum access levels for both cells are -100 dBm.
Solution C1 and C2 criteria are used!
Cell selection and reselection
MS operates in two modes: idle mode and dedicated mode. In the idle mode, MS monitors
the broadcast channels in order to "hear" if it is being paged. It also measures other BTSs'
BCCH carrier and decides whether it should camp on another cell. This is called cell
reselection and the reselection algorithm used in GSM is detailed in [Reference: 3GPP TS
05.08: 6 Idle mode tasks]. In dedicated mode (i.e. during a call), changing cell is called a
handover (HO).
Cell selection
Cell selection is performed immediately after MS is switched on. If MS is located in the same
cell it in which it was previously was switched off, the SIM card should have the local BCCH
frequency stored in memory and MS should find network quite expeditiously. If MS has
moved to another cell since it was turned off, it enters a cell selection procedure, which we
shall skip here.
The process whereby the mobile searches for a suitable cell, this is referred to as ‘C1’ or
‘C31’ in the GSM system and ‘S’ in the UMTS system. The selected cell is then identified as
the ‘camped-on’ cell.

2. Cell reselection
Phase 1: Cell reselection is performed as MS traverses through a network in idle mode. MS
continuously keeps list of the six strongest BCCH carriers. From the radio propagation point
of view it is desirable that MS camps to a cell with the lowest path loss. The most favorable
cell is indicated by the so called C1 parameter for a MS of phase 1, or by C2 for a MS of
phase 2 capabilities. The parameter C2 is essentially an improved version of C1. C1 is
evaluated separately for each cell and it is defined according to the criterion [Reference:
3GPP TS 05.08: 6.4 Criteria for cell selection and reselection]
When camped on a cell, the mobile shall regularly search for a better cell according to the
cell reselection criteria. If a better cell is found, that cell is selected. This process is identified
as the ‘C2’ or ‘C32’ in the GSM system and ‘R’ in the UMTS system.
C1 is a cell selection algorithm employed in GSM and GPRS. The algorithm uses the power
received from cells plus additional parameters in order to assess the cell that will provide the
best radio connection for the MS (Mobile Station).
C2 is the GSM cell reselection algorithm. Once the MS (Mobile Station) has camped onto a
cell it will continue to assess the surrounding cells to ensure it is monitoring the cell that will
offer the best radio connection. As the user moves the camped on cell may become
unsuitable. This situation will generate a cell reselection.
C31 is a criteria based algorithm used as part of the GPRS cell reselection process. It is used
to assess if prospective cells qualify as reselection candidates prior to applying the C32 GPRS
cell reselection algorithm.
C32 is the GPRS cell reselection algorithm. Once the MS (Mobile Station) has camped onto a
cell it will continue to assess the surrounding cells to ensure it is monitoring the cell that will
offer the best radio connection. As the user moves the camped on cell may become
unsuitable. This situation will generate a cell reselection.
What is Camp On: After conducting the cell selection process a MS (Mobile Station) or UE
(User Equipment) will camp onto the cell that offers the best perceived radio connection with
the network.
C1 = (A−max(B,0)) =>>> EQUATION 1
where A = Received average level − RX_ACCESS_MIN (in dBm)
B = MS_TXPWR_MAX_CCH − P (in dBm).
The received average level (AV_RXLEV) is found by averaging RXLEV samples over a
period of 3 to 5 seconds [SBS95]. RX_ACCESS_MIN is a cell dependent parameter dictating
the minimum allowed RXLEV for an MS to access that cell. MS_TXPWR_MAX_CCH is
the maximum TX power an MS may use when accessing the system (using RACH). P is the
maximum RF output power of the MS, usually 33dBm for a handheld GSM900 and 30dBm
for a handheld GSM1800 MS. Often the latter term in C1 equals 0 and equation (1) can be
simplified to;
C1 = A = AV_RXLEV − RX_ACCESS_MIN =>>> EQUATION 2
For example, if the minimum allowed level to gain access to a cell is −100dBm and the
received average level at the cell’s BCCH frequency is +80 dBm, MS calculates C1 as +20

3. for that particular cell. MS camps to the cell with the highest C1 value.
There is an exception to the standard procedure described above. When MS evaluates C1
values for cells belonging to a different Location Area (LA), it subtracts a parameter called
CELL_RESELECT_HYSTERESIS from the C1 value, which means that those cells are
given a negative offset. The reason for this is that changing LA requires a Location Update
(LU) procedure that consumes network signaling capacity. Thus, by assigning a negative
offset to C1, unnecessary LUs caused by slow fading can be reduced. MS receives
information of the cell dependent CELL_RESELECT_HYSTERESIS values through BCCH.
Phase 2: Cell reselection criterion C2 is defined as
C2 = C1 + CELL_RESELECT_OFFSET − TEMPORARY_OFFSET =>>> EQUATION 3
when timer T < PENALTY_TIME and C2 = C1 + CELL_RESELECT_OFFSET =>>>
EQUATION 4
otherwise.
The timer T is started separately for each cell in the list of the six strongest cells immediately
after it is placed on the list. This is illustrated in the following picture. MS camps in the cell
with the highest C2 value.
The criterion C2 is applied in hierarchical cell structures to keep fast moving MS in an upper
layer and slow moving MS in micro cells. It is assumed that a fast moving MS passes through
the micro cell before PENALTY_TIME is reached. This efficiently prevents unnecessary
LUs and thus saves network signaling capacity.
As in the case of C1, MS receives the C2 parameter information through BCCH. A parameter
called CELL_RESELECT_PARAM_IND informs MS about which reselection criterion (C1
or C2) is used in the cell. It is broadcast on the BCCH.
⇒C2 criteria has two alternatives depending on the value of penalty time.
⇒ Penalty time is set 640 and thus C2 = C1 − cellReselectOffset
⇒ C1 is used for 900MHz cell and C1 = −80 − (−100) = 20dB.
⇒ C2 is used for 1800MHz cell and C1 = −87 − (−100) = 13dB
=> C2 = 13 − 10 = 3 dB
=> 20 (900MHz) > 3 (1800MHz)
=> 900MHz cell is selected!
Senario: What could be done in order to prefer 1800MHz cell?
Solution: ⇒ in order to prefer 1800MHz cell the C2 criteria C2 = C1 + cellReselectOffset,
should be used!
penalty time can be set < 640 and temporaryOffset = 0 dB for 1800MHz cell.