Watts Up, Doc! FCC Grants Huge Power Increase for Some (But Not All) Unlicensed Use at 57-64 GHz

This band needs high power to overcome poor propagation through air.

If you work in this business long enough, radio bands start taking on individual personalities. The 57-64 GHz band is the cantankerous child genius: underdeveloped, enormously promising, and hard to work with.

This band, which stretches across 7 GHz, has the widest swath of spectrum anywhere in the FCC rules. The cell carriers fight over 10 MHz at a time; this band is 700 times bigger. Back in 2008, carriers paid $19 billion at auction for a mere 52 MHz of the 700 MHz band; at those same prices, the 57-64 GHz band would go for 2.6 quadrillion dollars – about 30 times the total economy of the world.

But operations in the band are unlicensed, so the spectrum is free – and that looks like a bargain. But the band has its downsides. All frequencies in these upper reaches of the spectrum propagate poorly. The 57-64 GHz region is worse than most, due to the pesky laws of physics. Much like a playground swing goes back and forth at a steady rate, several times a minute, oxygen molecules in the atmosphere vibrate at their own steady rate, about 60 billion times each second. Much like the playground swing, which absorbs energy from the parent pushing at the high point of each swing, oxygen molecules absorb energy from passing radio waves that happen to hit them at the right frequency: about 60 GHz.

Transmitted energy that goes into pushing oxygen molecules never gets to the receiver. That’s why, at these frequencies, it takes (relatively) a lot of power to move a signal any reasonable distance.

Previously, the FCC’s rules governing unlicensed operations in this band allowed about 10 watts of power. That’s high, by unlicensed standards, but in most applications would reach only several tens of meters. You could link nearby buildings on a campus, or closely neighboring cell towers, but not much else.

An industry group asked for a power increase to help combat the oxygen fog. That was in 2004. The FCC thought about it for three years and issued a Notice of Proposed Rulemaking in 2007. Now, six years later still, it has finally acted on the request.

The FCC has now raised the power limits in this band by a factor of almost 16,000. This is possibly the largest single power increase in any band in FCC history. The new limit is equivalent to 158,489 watts average power – with no license. Peak power can be twice this amount.

But there is a catch.

Only extremely directional outdoor antennas – those having beamwidths of about half a degree or less – are allowed this much power. Less directional antennas have to dial the power back.

Operators hope the new rules will allow links in this band using suitable antennas to cover a mile or more.

Indoor antennas are stuck with the old power limits. So are transmitters installed indoors and aimed out a window. The same order also made changes to the procedures for assessing technical compliance, and eliminated a requirement for station ID.

Other services that want a 16,000-fold power increase? Maybe a 64 kilowatt CB radio? Just ask the FCC. You never know.

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