RACH busy threshold, defines a threshold for the signal level on the RACH. The general purpose of this parameter is to define a minimum level criterion a received RACH signal must fulfill to be regarded as a real RACH access.
BTS continuously observes the signals received on the RACH slots. As even without any MS RACH access there are always at least some ‘noise’ signals on the RACH slots. The layer 1 subsystem is to evaluate the received signal with respect to specific criteria, e.g. it measures the receive level and checks if it exceeds defined minimum.
The value entered for parameter RACHBT is not only relevant for the CHANNEL REQUEST message on the RACH but also for the HANDOVER ACCESS message on the FACCH! The level evaluation
of RACHBT against the receive level of the handover access burst on the FACCH is exactly the same as for the RACH. Thus, the setting of RACHBT also has an influence on the handover success rate.
From my experiences, value of RACHBT has big impact to the SDCCH Drop rate.