In some countries coexist several radiocellular systems such as : TACS, AMPS, NMT900 with partly common frequency bands. It can be very difficult to detect if the interfered GSM frequency is not a BCCH.
The effect is:
• intermodulation with GSM BS/MS receiver,
• spurious RACH for AMPS (AMPS Tx bands close to GSM uplink band),
• TACS : coverage hole with 600 m from TACS BTS
• AMPS 50 % reduction of range if AMPS/GSM BTS collocated.
Other RF Interferer (Radar, shop anti-theft mechanisms, medical device …) must be check case per case with a spectrum analyser, most of time on long period to detect sporadic or interference.