Collocation has evolved from something wireless carriers used to resist to something they do as a matter of course. The idea of collocation increasingly becomes important to independent tower companies, which rely heavily on collocation to bring in the revenues they need to sustain their business.
Cramming as many antennas as possible onto a single tower would be ideal, but the resulting interference problems have kept most antennas conservatively spaced at about 15 feet or more apart.
“Fifteen or more feet of spacing is sometimes required to prevent interference among systems at antenna sites-the trick is to know when significant spacing is necessary,” said Robert S. Mawrey, vice president of systems and technology at UniSite Inc. “Without knowledge of the wireless equipment at an antenna site, and use of sophisticated analysis techniques, it is difficult to know how much spacing is really needed.
“Fifteen or more feet of vertical spacing on a tower has become accepted because it is a safe and conservative estimate,” continued Mawrey.
However, with new software developed by UniSite, called Unistar, vertical antenna spacing could safely be reduced in some cases to only the physical spacing needed for the antennas, said the company. Each site is unique and requires its own interference analysis to determine proper spacing, said UniSite.
“One foot of vertical spacing from antenna tip to antenna tip is often sufficient to prevent interference among systems,” said Mawrey.
The implications include allowing more antennas per tower; reducing the proliferation of towers by collocating more often; increasing use of stealth sites, such as flagpoles that require close antenna spacing; and decreasing buildout costs for wireless carriers, said the company.
Radio interference at a shared site typically is caused by intermodulation, out-of-band emissions or other effects such as receiver desensitization, said the company. Unistar consists of an equipment database, a site interference analysis module and multiple graphical user interfaces to help site engineers determine the interference effects of collocating an antenna.
The site interference analysis module predicts and reports six types of shared site interference, including transmitter noise, receiver desensitization, transmitter intermodulation, receiver intermodulation, harmonics and spurious emissions. An analysis options page allows the engineer to modify various analysis parameters.
Unistar also calculates electromagnetic field levels at antenna sites to determine potential human exposure.
