Adjacent-channel interference
Encyclopedia
Adjacent-channel interference (ACI) is interference
caused by extraneous power
from a signal in an adjacent channel
. ACI may be caused by inadequate filtering (such as incomplete filtering of unwanted modulation
products in FM
systems), improper tuning
or poor frequency control (in the reference channel, the interfering channel or both).
ACI is distinguished from crosstalk.
Broadcast regulators frequently manage the broadcast spectrum in order to minimize adjacent-channel interference. For example, in North America FM radio stations in a single region cannot be licensed on adjacent frequencies — that is, if a station is licensed on 99.5 MHz in a city, the frequencies of 99.3 MHz and 99.7 MHz cannot be used anywhere within a certain distance of that station's transmitter, and the second-adjacent frequencies of 99.1 MHz and 99.9 MHz are restricted to specialized usages such as low-power stations. Similar restrictions formerly applied to third-adjacent frequencies as well (i.e. 98.9 MHz and 100.1 MHz in the example above), but these are no longer observed.
The adjacent-channel interference which receiver A experiences from a transmitter B is the sum of the power that B emits into A's channel—known as the "unwanted emission", and represented by the ACLR (Adjacent Channel Leakage Ratio)—and the power that A picks up from B's channel, which is represented by the ACS (Adjacent Channel Selectivity). B emitting power into A's channel is called adjacent-channel leakage (unwanted emissions). It occurs because RF filters require a roll-off
, and do not eliminate a signal completely. Therefore, B emits some power in the adjacent channel which is picked up by A. A receives some emissions from B's channel due to the roll off of the selectivity filters. Selectivity filters
are designed to "select" a channel.
Interference (communication)
In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
caused by extraneous power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
from a signal in an adjacent channel
Adjacent channel
In broadcasting an adjacent channel is an AM, FM, or TV channel that is next to another channel. First-adjacent is immediately next to another channel, second-adjacent is two channels away, and so forth. Information on adjacent channels is used in keeping stations from interfering with one...
. ACI may be caused by inadequate filtering (such as incomplete filtering of unwanted modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
products in FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
systems), improper tuning
Tuner (radio)
A radio tuner is a subsystem that receives radio broadcasts and converts them into audio-frequency signals which can be fed into an amplifier driving a loudspeaker. FM tuner, AM tuner, Digital Audio Broadcasting DAB tuner, etc. are types of radio tuner dealing with transmissions using different...
or poor frequency control (in the reference channel, the interfering channel or both).
ACI is distinguished from crosstalk.
Broadcast regulators frequently manage the broadcast spectrum in order to minimize adjacent-channel interference. For example, in North America FM radio stations in a single region cannot be licensed on adjacent frequencies — that is, if a station is licensed on 99.5 MHz in a city, the frequencies of 99.3 MHz and 99.7 MHz cannot be used anywhere within a certain distance of that station's transmitter, and the second-adjacent frequencies of 99.1 MHz and 99.9 MHz are restricted to specialized usages such as low-power stations. Similar restrictions formerly applied to third-adjacent frequencies as well (i.e. 98.9 MHz and 100.1 MHz in the example above), but these are no longer observed.
The adjacent-channel interference which receiver A experiences from a transmitter B is the sum of the power that B emits into A's channel—known as the "unwanted emission", and represented by the ACLR (Adjacent Channel Leakage Ratio)—and the power that A picks up from B's channel, which is represented by the ACS (Adjacent Channel Selectivity). B emitting power into A's channel is called adjacent-channel leakage (unwanted emissions). It occurs because RF filters require a roll-off
Roll-off
Roll-off is a term commonly used to describe the steepness of a transmission function with frequency, particularly in electrical network analysis, and most especially in connection with filter circuits in the transition between a passband and a stopband...
, and do not eliminate a signal completely. Therefore, B emits some power in the adjacent channel which is picked up by A. A receives some emissions from B's channel due to the roll off of the selectivity filters. Selectivity filters
Filter (signal processing)
In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...
are designed to "select" a channel.