Microwave Link DesignTrends in 2012 and 2013

The use of microwave for mobile backhaul has so far been limited in North America, although it is popular in Europe and other international markets. Microwave is expected to play a larger role as LTE backhaul technology, as next-generation networks continue to evolve and carriers begin deploying large numbers of small cells to form heterogeneous networks (HetNets).
Stuart Little, director of product marketing for microwave backhaul provider Aviat Networks, spoke with RCR Wireless about the company’s perspective on trends in microwave link design.

While many private and governmental entities make use of microwave networks — in particular, railroad companies — the technology has historically seen scant use in telecom networks in North America. In the days when copper lines had sufficient capacity to support mobile networks, Little said, deploying microwave made little sense to U.S. wireless operators. However, LTE backhaul capacity requirements are such that operators are taking another look at microwave.
“Fiber is extremely expensive to deploy in most cases,” Little said. “It’s obviously the desired goal to run fiber everywhere … but the reality is that all the carriers have increasingly begun to rely on microwave to replace their leased-line connections.”
Aviat’s customers include Verizon Wireless and AT&T.
Little said that the use of microwave for base station connections in North America several years ago was only between five to 10 percent of cell sites; now he estimates that number is around 10 to 15 percent. Little expects that figure to rise to 20 to 25 percent, and perhaps as high as 30 percent, over the coming years. However, there are challenges to be solved along the way.
Microwave Spectrum: More Needed
The biggest challenge for microwave link design, Little said, is the fact that there is “a lot less spectrum available for microwave” than in other parts of the world. However, he added, the Federal Communications Commission is trying to work toward addressing that issue.
The bands most commonly used for microwave backhaul in Europe, Little said, are all privately owned in the U.S., and much of the spectrum is currently dormant. Getting that spectrum back into the marketplace would be a “big opportunity for [the spectrum owners] to finally start earning real returns on that investment of years and years ago,” he added.
Microwave Link Design: Cost Factors
Little said that microwave link design itself is becoming easier, although it still takes careful calculations to make sure the link is robust.
“You have to take into consideration the weather, the geographical region; the obstructions on the path, and environmental issues,” he said.
However, the element of cost plays a major role in microwave link design, he added. Because wireless operators have sold off their tower assets, they must lease space for network antennas – typically by the foot.
According to Little, the typical cost to leave space for a microwave antenna is about $400 per month, plus a fee of $100 per linear foot of antenna. So a link between two four-foot microwave antennas would cost an operator about $1,600 per month. An eight-foot antenna, then, would cost about $1,200 per site.
“When it comes to mobile carriers, that is the biggest, number one criteria – how do you make that antenna smaller?” Little said. “The op-ex [operating expenditures] for five to 10 years for microwave completely dwarfs the actual cost of the radio itself.”
However, he added, “If you can save a foot an antenna, or even two feet, you can save yourself tens of thousands of dollars per link.”
The use of adaptive modulation and higher power in the microwave link design can help reduce the size of the antenna, Little said. However, carriers are also sensitive to the cost of power use.
In North America, Little noted, carriers typically prefer their equipment installed within a shelter instead of on a tower, so that it can be accessed on the ground versus requiring a worker to scale a tower. The microwave link design must take into account that difference in physical infrastructure.
Capacity vs. Cost in microwave link design
Capacity and cost must constantly be balanced, Little said – and more capacity drives up cost.
Because the microwave spectrum is limited, he said, it becomes more complicated to squeeze as much data through the available spectrum as possible. Typically, that means more power is necessary – which can be accomplished with a larger (and more costly) antenna. Link aggregation using Ethernet networking techniques is another way to increase capacity, he added.
“It’s a constant push and pull between higher capacity and higher costs,” Little said.
With that in mind, however, he said that carriers tend to overestimate capacity needs and that clever network and microwave link design can dramatically cut costs while still meeting LTE’s backhaul capacity needs.
“There is a lot of talk about technology and how the carriers are going to support LTE,” he said. “We think microwave is more than up to the task.”