5 ms·

Bold plan: opening 1,000 MHz of federal spectrum to WiFi-style sharing

91 points by boh 14y ago

th0ma5 14y ago
Beyond merely the frequencies themselves, there's a ton of logistics involved in things like harmonic or other interference and propagation, all of this related to power. However, it is increasingly such that a lot of devices are software-based radios, so changes to a lot of existing technology could be just a firmware upgrade, if possible.
- pbsurf 14y ago
  Unfortunately, we are not yet at the point where a range of frequencies anywhere near as wide as discussed in the article can be supported by a simple firmware upgrade. RF receivers typically include a very narrow bandpass filter as the first component after the antenna. As spectrum becomes more crowded, these filters are even more critical to prevent saturation of the low-noise amplifier and unwanted products from the downconverter/mixer.
  - th0ma5 14y ago
    Yes, yes! Sorry to over simplify it. I guess my main point was mostly the first one, there's a lot to it, heh.
  - aaronblohowiak 14y ago
    and the bp filter isn't tunable?
    - patrickyeon 14y ago
      A cheap one isn't. An expensive one may be, but you could trade away a lot of performance to get that feature.
  - CamperBob2 14y ago
    RF receivers typically include a very narrow bandpass filter as the first component after the antenna Historically true, but as ADCs get better (and more important, cheaper), that will be less and less common. Software-defined radios with broadband front ends don't play by the same IMD rules as conventional radios, anyway.
    - patrickyeon 14y ago
      In my experience, the ADC is far from the limiting factor for linearity in an SDR receiver. The analog components up front (LNA and mixer, especially) are the limiting components. Broadband front ends make it even worse, because it's more likely you'll have an interferer passed through the filtering, even if it's nowhere near (in frequency) your signal.
      - Nrsolis 14y ago
        Don't forget the antenna. In many cases, the antennas themselves have specific polarization and beamwidth/radiation patterns along with SWR that is directly tied to the frequency range of the antenna itself. SDR doesn't change any of this.
w-ll 14y ago
I worked on the first USA test of White Space WiFi up till last year. Pretty cool stuff, smart engineers. Right now it's being utilized for public services (cops, firemen, highway cams... a few other things that were a little more interesting but... NDA'd) It's great to see more initiative on rolling out this technology.
dusing 14y ago
Anyone know what kind of speeds could go over these frequencies?
- a1k0n 14y ago
  The Shannon limit would be at most 2Gbps...
  - juiceandjuice 14y ago
    Based on what?! OP's question wasn't very well defined and your answer is completely arbitrary.
    - gringomorcego 14y ago
      2.2GBPS
  - pbsurf 14y ago
    The channel capacity depends on the amount of noise present, not just the bandwidth. Specifically for a AWGN channel, bits per second <= W*log_2(1+SNR), where W is the bandwidth in Hz and SNR is the signal-to-noise ratio. For example, 5 Gbps has been demonstrated using only 100 MHz of bandwidth: http://en.wikipedia.org/wiki/LTE_Advanced http://en.wikipedia.org/wiki/LTE_Advanced
  - claudeshannon 14y ago
    That's not right. it should be 2 Giga-samples-per-second. You can pack a large number of bits into each symbol.
jakeonthemove 14y ago
That's a pretty good idea: it's like public roads: normally, they're used by the citizens, but if there's a VIP guest or a military situation, the government can clear and use them for themselves only. Makes sense to do the same with frequencies (although it will be a bit harder to manage - like the EBS test in 1971 has shown)...

91 points by boh 14y ago

Beyond merely the frequencies themselves, there's a ton of logistics involved in things like harmonic or other interference and propagation, all of this related to power. However, it is increasingly such that a lot of devices are software-based radios, so changes to a lot of existing technology could be just a firmware upgrade, if possible.

pbsurf 14y ago
Unfortunately, we are not yet at the point where a range of frequencies anywhere near as wide as discussed in the article can be supported by a simple firmware upgrade. RF receivers typically include a very narrow bandpass filter as the first component after the antenna. As spectrum becomes more crowded, these filters are even more critical to prevent saturation of the low-noise amplifier and unwanted products from the downconverter/mixer.
- th0ma5 14y ago
  Yes, yes! Sorry to over simplify it. I guess my main point was mostly the first one, there's a lot to it, heh.
- aaronblohowiak 14y ago
  and the bp filter isn't tunable?
  - patrickyeon 14y ago
    A cheap one isn't. An expensive one may be, but you could trade away a lot of performance to get that feature.
- CamperBob2 14y ago
  RF receivers typically include a very narrow bandpass filter as the first component after the antenna Historically true, but as ADCs get better (and more important, cheaper), that will be less and less common. Software-defined radios with broadband front ends don't play by the same IMD rules as conventional radios, anyway.
  - patrickyeon 14y ago
    In my experience, the ADC is far from the limiting factor for linearity in an SDR receiver. The analog components up front (LNA and mixer, especially) are the limiting components. Broadband front ends make it even worse, because it's more likely you'll have an interferer passed through the filtering, even if it's nowhere near (in frequency) your signal.
    - Nrsolis 14y ago
      Don't forget the antenna. In many cases, the antennas themselves have specific polarization and beamwidth/radiation patterns along with SWR that is directly tied to the frequency range of the antenna itself. SDR doesn't change any of this.

w-ll 14y ago

I worked on the first USA test of White Space WiFi up till last year. Pretty cool stuff, smart engineers. Right now it's being utilized for public services (cops, firemen, highway cams... a few other things that were a little more interesting but... NDA'd) It's great to see more initiative on rolling out this technology.

dusing 14y ago

Anyone know what kind of speeds could go over these frequencies?

a1k0n 14y ago
The Shannon limit would be at most 2Gbps...
- juiceandjuice 14y ago
  Based on what?! OP's question wasn't very well defined and your answer is completely arbitrary.
  - gringomorcego 14y ago
    2.2GBPS
- pbsurf 14y ago
  The channel capacity depends on the amount of noise present, not just the bandwidth. Specifically for a AWGN channel, bits per second <= W*log_2(1+SNR), where W is the bandwidth in Hz and SNR is the signal-to-noise ratio. For example, 5 Gbps has been demonstrated using only 100 MHz of bandwidth: http://en.wikipedia.org/wiki/LTE_Advanced http://en.wikipedia.org/wiki/LTE_Advanced
- claudeshannon 14y ago
  That's not right. it should be 2 Giga-samples-per-second. You can pack a large number of bits into each symbol.

jakeonthemove 14y ago

That's a pretty good idea: it's like public roads: normally, they're used by the citizens, but if there's a VIP guest or a military situation, the government can clear and use them for themselves only. Makes sense to do the same with frequencies (although it will be a bit harder to manage - like the EBS test in 1971 has shown)...