The End of Spectrum Scarcity (2004) | Hacker News Reader

The End of Spectrum Scarcity (2004)

spectrum.ieee.org

28 pointsapsec11217 comments

Even if we get cognitive radio and share spectrum there are still fundamental and engineering limits that mean spectrum will remain scarce. You're either sharing one spectrum with the entire world for HF (because of ionosphere bouncing) or everything that is in line of sight for the vast majority of the spectrum. That means a lot of people are using the same 0.1-18 GHz. Once you get above 18 GHz or so then the propagation gets a lot worse, so it's less than even line of sight, but engineering problems with phase error start meaning higher order modulations are much more difficult to achieve.

Spectrum is always going to be scarce. That's why cables and fibers are always going to be important. Each one allows you to use the entire span all to yourself.

If there are ways to minimize interference, that suggests that there are fractions of the spectrum that work better than others. If that fraction is not enormous, that resource will eventually be constrained. If not everywhere, or all the time, then in certain suboptimal situations. Like a historic neighborhood, in a pocket valley, during bad weather.

Agreed. The title is disappointing, especially coming from the IEEE.

Anybody who works on radio quickly understands that the radio spectrum is the most precious most scarce resource on the planet.

Not knowing much about radio spectrum, but its a finite resource right? There's no way to expand the spectrum, or do something similar like what they did in the early days of wireless technology where they transitioned from TDMA (time division multiple access) to CDMA (code division multiple access) in order to expand the number of user you can put on a data stream.

Am I right to look at radio spectrum as a finite resource?

Those examples are ways to use the spectrum more efficiently. But the amount we have does not change.

It is finite indeed. But it’s also not something that’s consumable. Imagine it as a fixed sized room. If you move stuff out of a corner you can reuse it infinitely, but there can only be so much stuff in there on a particular moment.

With literally an unknown market price. 15 Mhz of spectrum in the United States is worth far more than 15 MHz of spectrum in Norway. As we expand from LTE to 5G to someday 6G, the price changes as well for a block of spectrum. Price is also a factor of competition in the market, and why the current situation in the United States with only 3 major wireless telecom potentially troublesome, with the only reprieve being that companies like Google and Liberty/Dish and private equity getting into the auctions.

You are definitely not going to be sharing your spectrum with 'everything that is in line of sight' by default, that only goes for substantial power levels of directed antennae. At low power levels your signal will be below the noise floor long before you get to your line-of-sight limitations, and even if it wouldn't be, someone 'sharing' that frequency with you will be locally much louder than you.

IRL the situation is much more complex than that still, but this is a useful approximation to keep in mind when thinking about these matters.

It sounds like you're talking about "IRL" but then ignoring the fact that if you're not a corporation paying the big bucks, you transmit from just above ground level, not from some tower with 30m height above terrain. Line of sight for most people is much shorter than you think because of trees, buildings, hills/valleys, etc. It's the limiting factor for VHF and up, not transmission power or antenna pattern gain.

Line of sight at groundlevel (the horizon) is 5 Km or thereabouts.

You'll run into some obstacle long before you run out of room unless you are very lucky with your sightlines. The only places where you can get those distances is when you're in relatively open country, even in an urban setting most telco infra is not more than 5-6m above the nearest high building.

I had some other trickery in mind such as phased array antenna, frequency hopping and so on.

DARPA Spectrum Collaboration Challenge[1] spent 3 years exploring this space with SDR + AI in a gamified way[2] and announced its winner last year[3].

[1] https://www.darpa.mil/program/spectrum-collaboration-challen...

[2] https://www.youtube.com/watch?v=-a-N8MiMqz0

[3] https://www.darpa.mil/news-events/2019-10-24

If there is more capacity available, people will use it.

For instance, I happily stream videos and FaceTime over cellular quite a bit now. Even though I was plenty happy without that capability and don’t really mind when the cell service isn’t good enough for it to work.

How does signal processing change when you know that part of the signal is much more likely to arrive intact than others? Do you use it for more error correction, or do you send multiple classes of traffic in each message? Like the color channels on a black and white TV signal?

I know a little bit of theory but I have a big gap in broad spectrum scenarios.

Bandwidth is physically bounded by frequency * log(noise ratio). As long as you've got a great signal, much better than what you can decode, you can increase the bandwidth.

The most common example is broadband. The 2 MHz band is split in a hundred independent bands of 20 MHz. Each one of the channel is adjusted independently to maximize bandwidth. https://en.wikipedia.org/wiki/G.992.3

We are talking about software defined radio though. How do these facts apply when there are no bands anymore? When collisions are avoided probabilistically instead of by partitioning?

No change. Just because your radio is "software defined," doesn't change physics and signal processing math. There are 'best' ways to use a given piece of spectrum, and while a SDR can make your hardware more flexible, it can't change that the most efficient way to use spectrum is usually to have a dense set of subcarriers modulating amplitude and phase. By avoiding a strict multiple-access protocol on a band, everything just gets worse; probabilistic back off is inefficient vs hard-scheduling. Every wireless system I've worked on (satellite radio, WiFi, LTE) has been very spectrum constrained, so I dont really believe in this future where spectrum is cheap.

In some sense,mesh WiFi doesn't seem spectrum constrained(considering home use) - just install enough mesh AP's.

Won't work unless the AP's scrupulously adhere to protocol to avoid interference. In practice the 2.4GHz interference has became tragical. In European high density housing, you are likely to have precipitous speed drop moving only across the room from the AP or behind one wall.

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