Adjacent-channel interference

Origin

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 for two reasons. First, because RF filters require a roll-off, and do not eliminate a signal completely. Second, due to intermodulation in B's amplifiers, which cause the transmitted spectrum to spread beyond what was intended. 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 A's selectivity filters. Selectivity filters are designed to "select" a channel. Similarly, B's signal suffers intermodulation distortion passing through A's RF input amplifiers, leaking more power into adjacent frequencies.

Avoidance procedure

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 first-adjacent 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.

References

References

1. "Federal Standard 1037C".
2. [https://www.govinfo.gov/content/pkg/CFR-2019-title47-vol4/xml/CFR-2019-title47-vol4-sec73-807.xml "Code of Federal Regulations: Minimum distance separation between stations"]. [[Federal Communications Commission]], October 1, 2019.