ACI

Adjacent channel interference occurs when a radio transmitter operating on one frequency leaks enough power into neighboring frequency channels to degrade or block reception there. It is fundamentally a problem of imperfect filtering and spectral spillover. A transmitter sending on 460 MHz, for instance, does not confine all its power to exactly 460 MHz; it spreads across a bandwidth determined by its modulation scheme and amplifier characteristics. If a receiver on 461 MHz (or 459 MHz) is sensitive enough, it will pick up this spillover and the wanted signal becomes corrupted by unwanted RF energy from the adjacent channel.

The severity of ACI depends on three factors: the transmitter's out-of-band emission levels, the receiver's selectivity (how sharply it rejects unwanted frequencies), and the physical separation between channels. Professional land mobile radio systems operating in the 450-470 MHz and 800 MHz bands are particularly vulnerable because channels are spaced closely, often only 25 kHz or 12.5 kHz apart. A transmitter meeting FCC Part 90 emission masks can still cause ACI to a poorly designed or aging receiver in the adjacent slot.

Receivers defend against ACI through better front-end filtering and higher selectivity in intermediate frequency stages. Dual-conversion receiver architectures with steep RF filters help, but cost rises sharply. Transmitters can reduce ACI by using cleaner amplifiers, narrower modulation bandwidth, and sharp output filtering. In modern digital systems, ACI has become less dominant than co-channel interference because tighter channel spacing and better frequency stability make adjacent channels less of a problem relative to other users on the same frequency.

ACI is distinct from co-channel interference (two transmitters on the same frequency) and blocking, where a strong out-of-band signal desensitizes a receiver across multiple channels. Engineers often tackle ACI through network planning, ensuring transmitters in a coverage area do not use immediately adjacent frequencies if they will interfere with each other's receivers. In dense urban deployments with multiple base stations, this constraint forces operators to use interleaved channel schemes or wider geographic separation between co-frequency sites.