Tower siting gets more difficult
After years of struggling with a spectrum shortage in the U.S., the wireless industry appears to be on the cusp of getting a great deal of relief from upcoming FCC auctions of airwaves in the 1.7 GHz and 2.1 GHz bands, as well as the possible availability of 700 MHz spectrum in a few years.
All of these new frequencies are relatively close to the 1.9 GHz and 800 MHz bands that host most existing commercial wireless voice services today. In many cases, this means the same towers from an existing network can host transceivers operating in the new spectrum bands because they have similar propagation characteristics as airwaves used for existing services.
However, the proliferation of wireless antennas hanging from a given tower can create co-location interference — an issue that likely will become more significant with the introduction of services on the new spectrum, said David Kiesling, director of marketing and technical services for Radio Frequency Systems. None of the intermodulation problems are expected to be insurmountable, but network operators must consider them and factor the cost of solutions into their plans, he said.
“It may impact a carrier’s deployment strategy,” Kiesling said. “You may just think that you can use all your existing towers, but it may be more complicated than that.”
It’s a sign of the times, as the simplistic “might-is-right” days for tower siting — which was predicated almost solely on height — are long gone, said Ted Abrams, chief technology officer for SpectraSite, which recently agreed to merge with American Tower.
“It is going to be more complicated,” Abrams said. “The siting business is not as simple as it used to be.”
Being able to use existing towers for new networks is economically significant throughout the wireless industry. For carriers and other system operators — including public safety — it means relatively quick deployments without the expenses associated with the politics and delays often inherent in any attempt to build a new base station. For tower owners, it means the potential to realize additional revenue from infrastructure — in this case, towers — for which the most significant costs already have been paid.
Indeed, a typical tower can host antennas from five to eight different operators, said Woodie Williams, district manager in south Florida for tower company Crown Castle.
“It depends on the tower itself … and how willing you are to perform modifications [to reinforce the tower],” Williams said.
These towers are expected to become even more popular as more spectrum becomes available. During the next year, the FCC is expected to auction airwaves at 1.7 GHz and 2.1 GHz, with existing commercial wireless operators expected to be the primary bidders. Meanwhile, government officials hope to clear television broadcasters from the 700 MHz band by the end of the decade and see that prime spectrum used by new wireless players wanting to offer WiMAX and mobile video applications (see news story on page 6).
The potential for using tower sites that currently host 1.9 GHz antennas will be attractive to bidders wanting spectrum at 1.7 GHz and 2.1 GHz, just as 800 MHz sites will be candidates to host 700 MHz antennas if the broadcasting bands are cleared.
But such co-locations can create intermodulation issues, Kiesling said. In particular, spurious emissions — particularly third-order intermodulations (see graphic on page 32) — from antennas operating at the same tower location on close frequencies may interfere with signals from other antennas. In most circumstances, these problems can be anticipated via a mathematical formula and resolved with appropriate co-location filters.
“Filtering is not that hard to do, but you have to plan ahead for it,” RFS’ Kiesling said. “As more and more systems come into play, there will be more of a requirement for filters.”
In addition to filtering, other methods to alleviate co-location interference at tower sites include reducing signal power and altering antenna positions on the tower, which can rectify the problem because spurious emissions are a factor only at very close range, Kiesling said. Focusing antenna energies in a specific direction is another common strategy to combat unwanted intermodulation.
“Rather than using an antenna with an omnidirectional pattern, most operators choose to use antenna elements that are more precisely controlled and directed,” Abrams said.
When RFS carrier clients have been told about the potential impact of spurious emissions, Kiesling said, it surprises many of them.
Indeed, co-location interference at tower sites generally is not a problem today, Crown Castle’s Williams said.
“Right now, we haven’t seen too much of that, except with Nextel and [emergency-services networks],” he said. “In my five years with the company, we’ve only had two occasions, but we were able to remedy those quickly … in one case, with a simple filter. In the other case, we had to change some equipment.”
Although not much of an issue today, Williams acknowledged that co-location interference could increase significantly as more services are provided from a tower at new frequencies.
“I would say that is something we would have to look at,” Williams said. “But we probably won’t be able to address it until we see the applications and how they work with each other on the tower.”
In addition to spurious emissions, subharmonics and cross products are other forms of co-location intermodulation distortion that tower owners and wireless operators need to address, Abrams said. While computers are employed to crunch all the relevant mathematical formulas used in an effort to anticipate interference, there are times it is not found until after various networks are deployed, Kiesling said, noting that activities in unlicensed bands could present some unique problems in the future.
“How does WiMAX at 2.4 [GHz] mix with 1.9 [GHz] and 2.1 [GHz]? … What’s going to happen at 2.4 [GHz]?” Kiesling asked. “We all have a lot of ideas, but the fact is, until you build it, I don’t think anybody really knows.”
Abrams agreed but expressed confidence that the tower industry will be able to work with wireless operators to find solutions that prevent co-location interference from becoming a significant problem.
“It’s a doable thing — the technology challenges are not show-stoppers,” Abrams said, noting that new tower sites inevitably will have to be part of the long-term plan. “It will not be done without effort and a need to communicate well … but the costs will be reasonable.”
Indeed, for all the potential co-location interference issues that the new spectrum may create, Kiesling is quick to note that the circumstances will not negatively impact the value of the spectrum the FCC plans to auction.
“It’s going to clutter up the landscape … but the value of the spectrum far outweighs the risk this brings,” he said.
