TV White Space & The Future of Wireless Broadband

DeviceGuru writes "The unoccupied radio spectrum between broadcast TV channels may soon become a source of low-cost, ubiquitous broadband connectivity. Earlier this month, the U.S. Federal Communications Commission began Phase II testing of 'white space device' prototypes, to determine whether WSDs can operate without interfering with the other wireless devices commonly used in homes, offices, and public locations. A key advantage of white space wireless technology, compared to the combination of WiFI and WiMAX, is its TV-like ability to cover broad areas and penetrate walls and trees, using relatively low power levels."

Re:Practical value?

[Anonymous Coward]

Huh? After the switch in 2009, you'll just need a fancy new tuner to get your broadcast. No cable or satellite needed.

Re:Why are we running out?

[TimeTraveler1884]

There is actually more bandwidth between 3GHz and 4Ghz than 0Ghz and 1Ghz. Say you were to pick 100mhz wide channel right in between those ranges. 3.5ghz has 7 times the amount of cycles/sec than 500mhz. So assuming PCM and no losses, that's 7 times the bandwidth.

Re:Why are we running out?

[link5280]

Higher frequencies get attenuated by walls, rain, trees, etc... more easily than the lower ones. Yes the higher frequencies have more available bandwidth but there are disadvantages. TFA mentions the pros of using the UHF and VHF spectrum.

Re:Does "white space" last forever?

[Ephemeriis]

The problem with this idea is that it assumes that TV broadcasting will always be done the way it is today with unused space between the channels. If "white space" equipment gets deployed it is going to create a massive problem for any attempts to change use of the existing TV spectrum. Any future users of this spectrum are going to have to work around the applications now running in what used to be the "white space".

If I understand correctly, part of what this device does is detect television signals and avoid spectrum that is actually being used. That is part of what is being tested. The idea is that these devices would be allowed to transmit over fairly large chunks of the spectrum, but that they would automatically detect what is actually being used for television and only transmit in the gaps between channels. That is why there's so much concern about these devices causing interference - nobody is sure how well this detection/avoidance mechanism is going to work.

If it does actually work like it is supposed to it won't matter if the white space between channels moves or vanishes - the device will stop using that chunk of spectrum and move to another. The only real problem you'd have is if you completely saturated the spectrum with television, which could happen. But in that case the devices would simply be unable to find any white space and would not be able to transmit - it wouldn't actually interfere with the television broadcast.

That's how it's supposed to work, at least.

Re:Practical value?

[GreggBz]

Currently, DOCSIS cable modems use a TDMA [wikipedia.org] scheme for return traffic. (Basically, divide the wavelengths up into "time slots" and each modem gets a certain time slot. The signal is generally modulated using 16-QAM. [wikipedia.org] Think of two waves offset by 90 degrees, one vertical and one horizontal. Add to that 2 levels of amplitude for each wave. So, you can have 16 combinations. Or 4 bits for each wavelength of signal. You can increase the numbers of levels of different amplitude to increase your possible bit combinations per wavelength, but this is risky and few cable companies actually do it.

With this system running in the typical 25-35Khz frequency, you can have enough time slots for 400-600 modems, and about 7Mb of shared traffic.

To me, it's amazing they can build silicon to distinguish 500 different cable modems within microseconds. Kind of like what really happens in a Pentium IV.

But, this stuff is susceptible to noise and it can become hard to tell what bits are meant to be on the line if there's enough noise.

It's a good model, but I have no idea how you do it over the air, unless you increase the signal so high that the noise floor drowns out. But then jets would fall from the sky and we'd all get throbbing headaches.. or something.

Re:Why are we running out?

[infinity314159]

No, information carrying capcity, (also referred to as "bandwidth" confusingly), is proportional to the analog frequency range, (which was the original "bandwidth" concept). Check the wikipedia article. There are physics-related reasons for this, but I'm not that much of a physicist: http://en.wikipedia.org/wiki/Bandwidth [wikipedia.org]

Re:Why are we running out?

[Anonymous Coward]

That is usually not a limitation because it's based on the carrier frequency the data is being placed on along with the modulation techniques in use. Using some cool modulation schemes I can put 2Gbps of data onto a 1 GHz carrier and the cycle rate is not a factor. Remember there is a difference between the data rate and the symbol rate (actually changes to the sine wave). A 2 Gbps digital signal can equate to a much smaller symbol rate based on the modulation in use. Many cable modems use either a 16 QAM or 32 QAM, so 8 or 16 bits are represented in a single phase shift for any given sine wave. For example, a 2 Gbps data rate using 16 QAM only uses 250 MHz of spectrum. I say "only", 250 Mhz is a huge amount of the spectrum, but you get the idea. Additional note, the limitations on the various frequency spectrums is not technology limited but regulation limited to prevent interference. We have to transmit in perfect harmony :) http://en.wikipedia.org/wiki/Symbol_rate [wikipedia.org] http://en.wikipedia.org/wiki/Gross_bit_rate [wikipedia.org] http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation [wikipedia.org]

Superhet

[tepples]

Your neighbor's device has no need to transmit anything.

But it does anyway. Some methods of tuning into radio-frequency transmissions, such as superhet [wikipedia.org], create weak incidental emissions at any of several intermediate frequencies [wikipedia.org]. In countries with a TV licence [wikipedia.org], those responsible for enforcing licence compliance drive vans [bbc.co.uk] carrying equipment to detect these emissions.

Re:Great, another choice for those who have lots

[Forseti]

Why would you need transmitters to provide whitespace? Whitespace is a range of frequencies where nobody transmits. Just because that whitespace is there even when there's a transmitter doesn't mean that it's provided by said transmitter! You do need a separate transmitter to provide this new service in those whitespaces, but that has nothing to do with the presence or absence of existing transmitters. Plus, at those frequencies, a transmitter could more easily provide service to rural areas, so it might indeed help people who have no other broadband options.

And no, you can't count on a wireless technology to help people living in a radio quiet zone. That's the kind of thing someone could (should) find out about and consider before buying property there. In any case, a quick search on the NRQZ seems to indicate that it is not, in fact, a deadzone. There are merely more requirements and restrictions for anyone wanting to place a transmitter there...

Re:Great, another choice for those who have lots

[bmwm3nut]

I have satellite (Base package from WildBlue), so here are some numbers. Latency (measured by pings to google.com) measures about 1200-1600ms. Throughput (measured by downloading an Ubuntu .iso from mirrors.kernel.org) is about 60kbps. It's not bad, but it's not great either. My only other option where I live is $300/mo for a T1, which I think I'll be doing once my satellite contract runs out (and my wife lets me).

Re:Why are we running out?

[frieko]

Nope. You're counting everything from 0 to 3.5gHz as your "3.5gHz band". That's a 3.5 gHz channel, not a 100mHz channel.

When you want to send a signal on a 100 mHz wide channel, you would first construct a signal that uses frequencies between 0 and 100 mhz. Then you can shift it up by 450 mHz and get a 100mHz channel centered at 500 mHz, or shift it up by 100 gHz to each frequency and get a 100 mhz channel centered at 100.05 gigahertz. But it's still the same bandwidth and capacity.

Re:Does "white space" last forever?

[Anonymous Coward]

Between broadcast channels is a padding space meant to shield adjacent channels from sub-standard broadcasting equipment that might cause a frequency drift.
This is called whitespace, unused frequencies between channels to minimize collisions. Like airplanes keeping a couple mile buffer zone between each other, to prevent a bad pilot from accidentally colliding with another plane.

So even if every channel is used, there will still be some space left to squeeze a datastream into. And if this thing works as intended, shifting from bad broadcasters will be accommodated. If a broadcast starts drifting into other frequencies, this would back out of those and jump into the newly opened space on the other side of the broadcast.