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Collocation and Intermodulation Analysis

Aesthetic and cost concerns make it desirable for multiple operators to share communications sites. Unfortunately, radio systems at shared sites may cause harmful interference to each other. Potential interference mechanisms are: Transmitter Intermodulation, Receiver Intermodulation, Transmitter Noise, Receiver Desensitization, Harmonic Interference, and Spurious Interference. Whenever a radio system is added to a site, a collocation analysis should be performed to determine whether the new system will cause or suffer from harmful interference.

The principal component of a collocation analysis is an intermodulation study. Intermodulation Interference is produced by the mixing of transmitter frequencies at a site. As a result of system nonlinearities, these frequencies mix to produce other "intermodulation" frequencies that are potential sources of interference to the receivers at the site. The first step in the intermodulation analysis is the generation of an intermod "hit list". Using stored formulas, the analysis software combines the frequencies of the transmitters at the site to determine the intermod frequencies that may be generated. When an intermod frequency is sufficiently close to the frequency of one of the receivers at the site, the program reports a "hit". These intermod "hits" are then analyzed further to calculate predicted interference levels and determine whether or not harmful interference will occur.

Transmitter Intermodulation occurs when signals enter a transmitter and mix, and the resulting intermod frequencies are re-radiated by the transmitting antenna. Receiver Intermodulation occurs when signals enter a receiver and mix, and the resulting intermod products then appear at the receiver's demodulator input.

In addition, Transmitter Noise and Receiver Desensitization are interference mechanisms to be studied. Transmitter Noise is the name given to a transmitter radiating power outside its assigned channel bandwidth and causing co-channel interference to a nearby receiver. Receiver Desensitization is the name given to interference power from a nearby adjacent channel transmitter passing through a receiver's filters and degrading the noise floor. Transmitter Noise and Receiver Desensitization are usually only a problem when transmitters and receivers are operating on adjacent frequencies.

Also considered in the collocation analysis are Transmitter Harmonics which are integer multiples (2, 3, 4...) of the transmitter frequency. Produced by non-linearity of the transmitter system, primarily the RF power amplifier, these harmonics are potential sources of interference to nearby receivers.

Finally, any known spurious emissions of the transmitters at a site are analyzed as possible interference sources to the receivers sharing the site.

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