Saturday, October 18, 2014

FCC Looks at 24 GHz for Mobile Communications

The Federal Communications Commission is looking at whether 24 GHz spectrum can be released for mobile communications applications.

The Notice of Inquiry occurs at the same time that Google has asked for permission to test communications across different high-frequency spectrum bands, including millimeter-wave systems operating in the 71 GHz to 76 GHz band and the 81 GHz to 86 GHz range.

“Years ago, engineers and policymakers debated the feasibility and practicality of using spectrum above 2 GHz for mobile wireless services,” FCC Chairman Tom Wheeler noted.

More recently, 3 GHz has been seen as the highest frequency that could be used to support mobile operations.

The difference now is signal processing that allows practical communications at frequencies traditionally unusable. But cheaper signal processing now means it is possible to overcome propagation issues that have prevented use of millimeter waves for mobile or fixed communications apps.

So there now is optimism that frequencies above 24 GHz could be used to support mobile service, a previously-unthinkable option.

This matters for obvious reasons. More spectrum is needed. Also, the basic trade off--capacity and distance are inversely related--means very-high capacity is possible at millimeter wave frequencies, even if distance is limited.

Physics dictates the higher bandwidth possible at millimeter wave frequencies, even if coverage is more limited than at frequencies below 2GHz. Despite digital coding, potential bandwidth still is dictated by the number of oscillations a radio wave makes in a single unit of time.

In other words, at the peak of the cycle, coders might represent a positive bit, at the trough, a negative bit. So the total number of possible symbols depends on the frequency, or number of instances in a given unit of time that the waveform crosses between high and low states.

As the name implies, higher frequency signals have many more oscillations than lower frequency signals. Hence, more potential bandwidth, using any particular coding and modulation scheme.

The trade off is the effective distances at which such waves are useful for mobile or fixed communications, as millimeter waves are attenuated by water and, in some cases, oxygen. The trick is to use frequencies where attenuation is relatively lower, as is the case for optical communications as well.

Still, it seems highly probable that new frequencies, best suited for use in dense population areas, will be released for service, at some point.

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