Cell Selection and Re-selection in GSM

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Cell Selection Procedure

• First MS powers-on
• 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)
• Mobile compares cells which give a positive value of C1 and ‘camps-on’ to the cell with the highest C1 value.
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=neighboring cell number.

Cell Re-selection – GSM Phase 1 Mobiles

For GSM Phase 1 mobiles, cell reselection is achieved by comparing current cell C1 with neighboring 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 handovers 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)

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