RF Intermodulation (Interference) Analysis

It is becoming increasingly more common for multiple wireless licensees to share communication sites, cellular tower, and even antennas. Shared site operations may cause harmful interference due to the following:

 

  • Transmitter and receiver intermodulation
  • Harmonic interference
  • Transmitter noise
  • Receiver desensitization (“desense”)
  • Spurious interference

 

The situation changes, however, whenever a radio
system is added to the site. An RF intermodulation analysis increases the potential of wireless sites to support additional tenants in a compatible, non-interfering manner, thereby improving a site’s marketability and value. These studies predict intermodulation interference and provide important information about the isolation levels required for a compatible co-location site environment.

An intermodulation study can assist in site design and planning when performed in advance of construction, however the study may also be a useful tool when investigating and seeking resolution to RF intermodulation interference problems at an existing co-location site.

 

What are the effects of intermodulation on Wireless Co-location Sites?

The mixing of transmitter frequencies at a cellular or other wireless site produces intermodulation interference. System non-linearities in amplifiers, antennas, and structural components cause transmitter frequencies to interfere with receivers at the co-location site.

Of these mechanisms, there are two that are most significant. Transmitter intermodulation results when signals enter a transmitter final amplifier and mix. The resulting intermod frequencies are then reradiated by the transmitter antenna. Receiver intermodulation results when signals enter and mix in a receiver front end, and the resulting intermod products are detected at the receiver’s demodulator. Related to intermodulation, transmitter harmonics are integer multiples of the transmitter frequency and spurious emissions, both produced by non-linearity of the transmitting system.

Transmitter noise and receiver desensitization are two other interference mechanisms often studied as part of a co-location interference analysis of a cell site. Transmitter noise is internally generated power radiated outside the assigned transmitter bandwidth. This radiated power can cause interference to a nearby receiver assigned channel. Receiver desensitization, often called “desense,” refers to overwhelming power coupled through a receiver’s filters from a nearby transmitter that degrade its noise floor.

How an Intermodulation Interference Analysis is Preformed

The intermodulation analysis is performed through Waterford Consultants’ software which generates the thousands of potential interactions that result from even a modestly congested cell site. An intermod / interference analysis generates an infinite number of “orders” of interaction. The designer limits these “orders” to the minimum significant number. Typically, wireless practice concentrates on evaluating five or seven “orders.” Any number greater than seven is a state beyond which practical intermodulation interference becomes insignificant. 

The intermodulation co-location software then screens the potential intermod interference threats against preset system intermod interference thresholds. Further evaluation of these results identifies predicted co-location intermodulation interference levels, noise floor degradation, and other data sufficient for the system designer to make frequency allocation and hardware choices.

When completed, Waterford will deliver a comprehensive, certified report that includes:

  • Report Summary
  • Action Statement identifying any potential interference problem(s)
  • Results of the interference analysis
  • Recommended engineering solutions for alleviating any predicted problem(s)
  • Appendices that contain all input parameters • PE certification

To request an RF Intermodulation Analysis, please email a current RF Data Sheet (RFDS) to sales@waterfordconsultants.com. If you do not have an RFDS you may download our IMOD Data Form, fill in the applicable information and send via email or contact your Waterford representative.