RF Intermodulation Interference Studies
Co-location Site RF Compatibility Analysis 

Intermodulation (IM) studies by LBA increase the potential of wireless collocation sites to support additional tenants in a compatible, non-interfering manner. These studies predict IM interference and provide important information about the isolation levels required for a compatible colocation site environment.  This will assist in site design and planning when performed in advance of construction. An IM study may also be a useful tool when investigating and seeking resolution to RF intermodulation interference problems at an existing collocation site.

Why Do An Intermodulation Study?
It is becoming routine for multiple wireless licensees to share communications sites, and cellular towers, even antennas. Shared site operations may cause harmful interference to each other through transmitter and receiver intermodulation, harmonic interference, transmitter noise, receiver desensitization (desense), and spurious interference. Since the interference situation changes whenever a radio system is added to a site, a new intermod interference analysis should be performed to assess the impact of the new system on existing users at the colocation site.

Why Intermodulation Interference Affects Wireless Collocation Sites.
The mixing of transmitter frequencies at a cellular or other wireless site produces intermodulation interference. System nonlinearities in amplifiers, antennas, and even structural components cause these frequencies mix to produce other intermod frequencies that may interfere with receivers at the colocation site.

Of these mechanisms, two are most significant. Transmitter intermodulation results when signals enter a transmitter final amplifier and mix, with the resulting intermod frequencies 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 are transmitter harmonics which 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 colocation interference analysis of a cell site. Transmitter noise is internally generated power radiated outside the assigned transmit bandwidth and causing 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 and degrading its noise floor.

How Intermodulation Studies are Performed
In preparation for a colocation intermodulation interference study, an inventory is performed to identify all frequencies, receive and transmit, in use at a site. These may include cellular, WiMax, Microwave, public safety, and AM, FM, TV broadcast transmitters. A suitable database is prepared which contains detailed parameters of each operation. At complex cell towers or rooftops, the data acquisition phase may be intense, at times requiring field audits. The quality of this step dictates the reliability of the intermodulation study outcome!

The intermodulation analysis is performed through proprietary computer programs that generate 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 which are limited by the designer to the minimum significant number. Typically, wireless practice is to evaluate five or seven “orders”, beyond which practical intermodulation interference becomes insignificant.

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

Whatever your requirements, from an IM study performed at LBA’s offices based on the data you provide, to an onsite RF interference investigation and resolution project, or some intermediate activity, call on us.  Our expert staff, backed by over 40 years of experience in identifying and resolving colocation, intermodulation, and other RF interference problems, is ready to serve you with detailed and accurately documented reports delivered to you in a timely manner.

For an intermodulation study or technical assistance, use the

or contact Cathy Scott via email or call 252-757-0279
or 800-522-4464

3400 Tupper Drive (27834), P.O. Box 8026
Greenville, North Carolina 27835
800-522-4464 / 252-757-0279 / Fax 252-752-9155
Email: Lbagrp@Lbagroup.com

Copyright 2003. LBA Group, Inc. All Rights Reserved.