WiMax Interference on the RadarWiMax Interference on the Radar

Even before many planned wireless broadband networks have been deployed, the challenge of overcoming radio interference in WiMax systems has already become obvious to vendors and operators.

Irish antenna vendor Stella Doradus says it has already seen interference between cellsites -- known as "cell-to-cell suppression" in pre-WiMax wireless broadband sites. "We first came across it in Mexico City," says the firm's CEO James Browne.

To manage interference issues, operators need to be able to reposition antennas at cellsites. Justin Collery, VP of sales and marketing at Stella Doradus, describes the process as "pulling in the footprint of the antennas."

"Tuning" cellsites in this manner can be time-consuming, and pricey. Browne says that it can cost between $1,000 and $3,000 for a single tower-climb in the U.S. An average city WiMax deployment will likely require around 1,000 basestations to provide decent coverage, according to Stella Doradus calculations, so costs can quickly mount up.

The Irish supplier is attempting to reduce or eliminate these costs by using what it calls a "Netamorphic" antenna, with steerable radio beams that improve coverage and reduce cell-to-cell interference. A network engineer can control the direction of radio signals via a computer console. The firm estimates that the reduction in tower climbing could save operators around $5 million for every 500 basestations deployed.

Browne says in the future Stella Doradus is looking at further automating the process of beam-forming.

Already familiar to vendors in the wireless LAN market, beam-forming is used by vendors like Xirrus Inc. and the now-defunct Vivato, an early pioneer of WiFi switching, with varying degrees of success. (See Xirrus & the Big Box.)

Phil Marshall, analyst at the Yankee Group Research Inc. , doesn't expect beam-forming to completely solve interference problems in WiMax deployments: "It's part of the answer, particularly if you're using your beam for coverage, but if you're using it for capacity -- with lots of sites working together -– it gets pretty hard to steer the beam around."

Other antenna technologies, such as multiple input, multiple output (MIMO) arrays could help expand WiMax capacity while managing RF conflicts. (See Nortel Takes WiMax MIMO.)

— Dan Jones, Site Editor, Unstrung