Time to tune into the Weather Channel: Meteorological and other natural conditions are likely to dampen the spirits of WiMax network engineers that plan to deploy microwave backhaul links.

That's because common weather conditions, such as rain, can affect the performance of the high-speed wireless connections.

Sprint Corp. (NYSE: S) and Clearwire LLC (Nasdaq: CLWR) have been vocal recently about the cost and bandwidth benefits of using high-speed microwave links as part of their planned mobile WiMax deployment. Backhaul vendor Proxim Wireless Corp. says the pair aren't the only operators interested in microwave. (See Clearwire's Backhaul Bet.)

"There's a huge amount of drive and interest in microwave," says Geoff Smith, VP of marketing and business development at Proxim. "Operators just need more bandwidth with the increased downloads over 3G networks -- it's only going to get worse with 4G."

Smith says Proxim's Tsunami "wireless Ethernet" GX bridges are designed to deliver up to 200 Mbit/s over a 20 mile range. "You can get them up to 30 miles if you get them up high," he adds.

Sprint is working with FiberTower Corp. to build its microwave links. Clearwire is said to be working with a variety of suppliers but will get access to Sprint's cell towers as the current deployment efforts get rolled into the "new" Clearwire venture by the end of 2008.

But like WiMax, Microwave backhaul has a couple of issues with which veteran wireless hands are very familiar, the most crucial being that microwave links have metro-area line-of-sight range constraints.

And those line-of-sight issues must be addressed at the network design stage. "For any microwave solution, if you're shooting it through the trees you need to design that with summer-time in mind so you allow for maximum leaf-coverage," comments Smith.

In addition, microwave links can also be adversely affected when it rains, a condition known in the industry as "rain fade." At higher frequencies, the radio signal can get progressively attenuated by fog, rain, ice, or snow in the air. Operators can get around this to a degree by automatically increasing system gain at the site.

That's an issue that hampered many of the free space optics "wireless laser" startups that emerged around the turn of the century: To an even greater extent than the microwave firms, they struggled with the issue of rain interfering with their wireless signals. (See The Infrared Solution.)

Nonetheless, microwave technology represents one of the only serious options for this new age of broadband wireless backhaul, unless operators invest in lots of fiber. As Clearwire CTO John Saw told Unstrung last week, "A couple of T1s just isn't good enough anymore."

— Dan Jones, Site Editor, Unstrung