How much spacing to you really need between antennas at radio sites?
After many years of resistance by wireless licensees, collocation or sharing of antenna sites is now a common network build-out strategy. Despite the proliferation of site sharing led by industry leaders American Tower Corporation and Crown Castle, many collocation opportunities are still being missed due to a common industry misconception that fifteen or more feet of vertical spacing is always required between antennas arrays on tower sites. The missed collocation opportunities result in a higher density of towers and more costly network build-outs.
Fifteen or more feet of spacing is sometimes required to prevent interference among systems at antenna sites – the key is to know when significant spacing is necessary. Without knowledge of the wireless equipment at an antenna site, and use of sophisticated analysis techniques, it is difficult to determine the necessary spacing requirement. Fifteen or more feet of vertical spacing on a tower is accepted by many operators because it is a safe and conservative estimate.
Actual antenna spacing requirements can be estimated using comprehensive interference analysis techniques. The interference analysis should include investigation of
Most interference studies performed by radio engineers only consider intermodulation and harmonics and ignore other effects. Typical intermodulation analyses only involve investigating potential combinations of frequencies, an analysis that can be performed using simple software programs. Unfortunately, the results of simple intermodulation analyses do not provide much information – they only indicate whether combinations of frequencies are likely, and provide little or no information on the potential strength of various combinations. A simple intermodulation study, for example, provides insufficient information to determine required antenna separation. In many instances, the vertical spacing among antenna arrays is limited only by physical spacing needed for the antennas. One foot of vertical spacing from antenna tip to antenna tip is often sufficient to prevent interference among systems. Comprehensive interference analysis makes it possible to estimate when this is possible.
Interference analysis makes it possible to colocate antennas more closely together, which make it possible to:
Collocating radio equipment at shared antenna sites creates the possibility of RF interference between tenants at the same site. Factors driving interference include the number of active channels at the site, the relative placement of the antennas, the frequency bands used, and the characteristics of the technology and base station equipment.
Many of the factors affecting the potential for interference between wireless operators have been accelerated by industry trends, including the auctioning of frequency bands by the FCC and the development of new broadband and multi-channel technologies. The probability of interference effects, such as intermodulation, is highly dependent on the number of channels at each site; and, cellular and PCS operators typically transmit many more individual signals per antenna than land-mobile and broadcast operators. The increased density of operators at shared sites is driven by the demand for sites, resulting in reduced antenna separation between tenants, lower isolation between antennas, and increased probability of shared site interference. Much of the newer technology and equipment is also still under development and has not been extensively tested for compatibility in a shared site environment.
These concerns and a lack of understanding of how to address potential interference issues have led to excessive antenna spacing guidelines being applied to all antenna collocations by wireless licensees.
In general terms, interference may be defined as follows:
The effect of unwanted energy due to one or a combination of emissions, radiation, or induction upon the reception of a radio system, manifested by the serious degradation, obstruction, or repeated interruption in communication.
Interference may be generated by sources at a shared site as well as by signals located some distance away from a shared site. Site owners, site managers, and wireless operators typically have little control over interfering signals generated away from the shared site and are therefore limited to concentrating on interference generated by sources at the shared site. Strong signals generated away from the shared site should, however, also be considered where possible.
Radio frequency interference at a shared site is typically caused by one of the following mechanisms that have their origin in some form of equipment or other non-linearity:
Many other mechanisms are also possible and are described in detail elsewhere. One of the most common shared site interference mechanisms is transmitter intermodulation. Signals coupled into the output stage of a non-linear base station transmitter can result in intermodulation products being generated that interfere with other receivers at the site or with mobile receivers near the site. Intermodulation products are generated at frequencies described by the following expression:
fintermodulation = mf1 + nf2
where m and n are integers and (|m| + |n|) is the order of the intermodulation product. Part of the output spectrum of a non linear device excited by two signals f1 and f2 is shown in the following figure.
The most effective way to combat shared site radio frequency interference is to isolate non-linear devices from strong signal sources. As illustrated in the following figure this is typically achieved using a combination of antenna isolation, filtering, and the selective use of more linear devices and ferrite isolators.
Passive intermodulation can be reduced by minimizing the number of loose metallic joints within system components such as antennas, cables and connectors, and in the external environment on towers and wire fences.
Performing a comprehensive interference analysis at shared antenna sites requires good information on the radio equipment and site characteristics as well as use of sophisticated analysis techniques. It is typically not practical to perform a comprehensive interference analysis without using shared site RF interference analysis software.
One of the earliest references to shared site RF interference analysis software was the CO-Site Analysis Model (COSAM) published by the ITT Research Institute, Electromagnetic Compatibility Analysis Center (ECAC), in 1970. This software, developed primarily for naval shipboard applications, was followed in 1978 by the development of two software modules called DEsign Communication ALgorithm (DECAL), and Performance Evaluation (Communication) ALgorithm (PECAL). Other software developed during this period included Shipboard ElectroMagnetic Compatibility Analysis (SEMCA), developed by General Electric and Atlantic Research Corporation, and Interference Prediction Model (IPM), developed by Litton.
In 1989, the Telecommunications and Information Sciences Laboratory at the University of Kansas published a paper on a Communications Engineering Design System (COEDS). COEDS was a software shell written around the ECAC software that consisted of a graphical user interface, database managers and a performance evaluation post processor. The Mitre Corporation published a paper in 1990 on the Cosite Analysis Platform Simulation program (CAPS). In the late 1990s while working for UNIsite, Inc. the authors developed a software package called UNIstar.
All the software mentioned so far was developed for the military and is either not available or is not directly suitable for commercial applications. Prior to the development of the UNIstar software no comparable software known to the authors has been developed for commercial applications, apart from a program called ComSitePlus developed by Douglas Integrated Software.
The answer to the question of how much spacing one really needs between antennas at radio sites depends on the configuration of each individual site. Analysis and practice have shown that the fifteen or more feet of vertical antenna spacing, used as a general guideline by wireless operators, is not always required. In many cases only the physical dimensions of the antennas dictate the antenna spacing requirements. However, without detailed information on the site, and use of sophisticated analysis software, it is difficult to know when significant spacing is needed, and even more difficult to know how what the minimum spacing requirements are, particularly on antenna sites with many collocated systems. Use of sophisticated software tools such as those used by AntennaWise makes it possible to analyze whether interference is likely on shared antenna sites, and also makes it possible to optimize the antenna placement to accommodate far more antenna systems than would be possible using generally adopted antenna spacing guidelines. Minimizing antenna spacing provides a number of benefits including: collocating more systems on the same tower; using stealth solutions such as a flagpole that require close antenna spacing; reducing the proliferation of towers by collocating more often; and saving build-out capital for wireless licensees by making cost effective colocation possible.
P. L. Lui, “Passive intermodulation interference in communication systems,” Electronics and Communication Engineering Journal, pp. 109-118, June 1990.
M. N. Lustgarten, “COSAM (Co-site Analysis Model),” IEEE Electromagnetic Compatibility Symposium Record, Anaheim, California, pp. 394-406, July 1970.
J. W. Rockway, and S.T. Li, “Design Communication Algorithm (DECAL),” IEEE International Symposium on Electromagnetic Compatibility, Atlanta, GA, pp. 288-292, June 1978.
L. C. Minor, F. M. Koziuk, J. W. Rockway, and S.T. Li, “PECAL: A New Computer Program for the EMC Performance Evaluation of Communication Systems in a Cosite Configuration,” IEEE International Symposium on Electromagnetic Compatibility, Atlanta, GA, pp. 295-301, June 1978.
R. P Bouchard, A. J. Heidrich, R. R. Raschke, and J. T. Sterling, “Shipboard Electromagnetic Compatibility Analysis, SEMCA (V) User’s Reference Manual,” vol. 10A, General Electric Co. December 1973.
J. C. McEachen, “Topside EM Environment Analysis in Designing the DD-963 Class Ship,” IEEE International Electromagnetic Compatibility Symposium Record, pp. 155-162, July 1972.
P. Alexander, P. Magis, J. Holtzman, S. Roy, “A methodology for interoperability analysis,”
IEEE Military Communications Conference (MILCOM 89), Boston, MA, pp. 905-910, Oct. 1989.
J. Low and A. S. Wong, “Systematic approach to cosite analysis and mitigation techniques,” Proceedings of the Tactical Communications Conference, vol. 1, pp. 555-567, April 1990.
ComSitePlus User Manual, Douglas Integrated Software, Tallahassee, FL, 1995.
