the mobile chooses the cell with the highest C1
C1 = (RXLEV – RXLEV_ACCESS_MIN) – MAX(MS_TXPWR_MAX_CCH – P, 0)
with P = maximum power of MS
For more details, it’s all detailed in these 2 3GPP documents :
43.022-Functions related to Mobile Station (MS) in idle mode and group receive mode.doc
45.008-Radio subsystem link control.doc