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07.08.2013
Originally featured on BroadcastEngineering.com
New technology could redefine the term “beachfront spectrum”
Multiple Input/Multiple Output antennas represent the new frontier for expanding capacity.

Assessing the value of spectrum usually revolves around how well signals propagate — that is, how easily they pass through walls and how far they travel to be reliably received. Engineers and government regulators pay a lot of attention to the notion of “beachfront spectrum,” the high-value spectrum that lives in the low-frequency band.

While this low-frequency spectrum is considered ideal today, new technology will soon broaden what we describe as beachfront spectrum, according to an industry expert who posted an item on GigaOm, the technology website.

Smart antennas, particularly Multiple Input/Multiple Output (MIMO) versions, are the new frontier for expanding radio capacity, throughput, coverage and cell-edge performance, reports Peter Rysavy, president of Rysavy Research, a wireless technology specialist. With MIMO, signals can propagate through the environment along multiple paths, effectively creating simultaneous parallel transmissions.

Most currently deployed LTE networks employ 2 x 2 MIMO systems, which have two transmit antennas at the base station and two receive antennas within the mobile device. However, 2 x 2 is entry-level in the MIMO world. Operators, Rysavy said, are already gearing up for 4 × 2 MIMO deployments, with four base-station antennas. And in the future are 8 × 2 and even 8 × 4 configurations.

However, according to Rysavy, that’s just the beginning. The future, he said, belongs to massive MIMO, which involves dozens and, ultimately, hundreds of antenna elements at the base station. Large MIMO systems, he said, are much more practical at higher frequencies — 2GHz and higher. That’s because antenna elements must be spaced based on wavelength; since low frequencies have larger wavelengths, they result in unwieldy and impractical antennas for physical towers.

Based on Rysavy’s analysis, massive MIMO systems could, within five years, deliver double the capacity for the same amount of spectrum in systems operating above 2GHz compared with systems operating below 1GHz.

There’s a reason Clearwire gleefully touts the vast amount of 2.5GHz spectrums it owns: MIMO developments will make 2.5GHz even more valuable in the future.

It’s not the distance, said Rysavy, but the volume that matters most.

Another aspect of the spectrum discussion suffering from oversimplification is propagation, Rysavy said.

“Yes, lower frequencies propagate farther; an operator can cover an area with fewer sites at 700MHz vs. 1.7GHz," he said. "Most cell sites, however, are deployed for capacity, not coverage: Twice as many cells in a coverage area translates to twice as much capacity.”

That means the cells must be spaced more tightly, and thus an operator can use either low or high bands since propagation is no longer a concern, he said. Deploying more towers does cost more, but operators have no choice if they want to deliver adequate bandwidth to all of their customers.

In fact, the number of non-urban cell sites driven by coverage-only considerations is diminishing as operators are forced to keep adding sites to address capacity, even in smaller towns and along highways. Video streaming clearly does not stop at the city line.

As wireless technology continues to improve, the higher frequency bands will provide the most capacity, Rysavy said. The idea of beachfront spectrum is overblown.

“Radio technology is on the move, and constraints that exist today will no longer apply tomorrow,” he said. “Shape government spectrum policy based only on how technology functions now as opposed to its future inherent capabilities, and we run the risk of future unintended, and adverse, consequences.”



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