Open Spectrum: Free the Airwaves

akb writes: "Most of the RF spectrum in use is licensed for exclusive use. What do we get? Inefficient use through spectrum hoarding, political finagling to abuse the regulatory system to gain competitive advantage and access to the airwaves for only a few players. A good article over at CNET picks up on the example of 802.11b in using spread spectrum technology and unlicensed bands and proposes that model be applied to the rest of the spectrum. For the hardcore check out NYU law professor Yochai Benkler's writings, particularly this article (pdf) and Durga Satapathy's papers for the tech end of things."

Comments

Oversimplistic

[mesocyclone]

Current radio regulation is far from efficient, but removing the regulation entirely is foolish, and ignoring frequency sharing won't work. There are engineering realities that the writers and lawyers don't understand that limit this.

While bandwidth is an oversimplistic way of either looking at things, or regulating them, it is a fact that any communications system can be analyzed (roughly) in terms of bandwidth. And this means that any communications system can interfere with any other communications system if they share frequencies in any sense.

For a real world example of why you can't ignore bandwidth, try running WiFi in a house where you have some 2.4GHz phones. It may work. But sometimes it doesn't - the reason - radio frequency interference. They share the same bandwidth.

Ah, you say... so they don't use good enough systems... or aren't broad-band enough... or something! Not true... there are hard physical limits that no amount of scheming will get around. The rest of this post discusses that in more technical detail.

All signaling systems (INCLUDING Time-Modulated Ultra-Wide-Band)require a separation of signal from noise. Noise is either natural (thermal, atmospheric, solar, etc), incidental (power line leakage, etc) or other radio systems. Regardless of what kind of signaling system is used, it has a limit as to the amount and kind of noise that can be tolerated in any given situation. The other limits described below affect the amount of noise reduction/signal enhancement that is possible.

Limits to processing gain. WiFi and other modern technologies (CDMA cell phones) use spread spectrum to reduce the effects of interference. Unfortunately, this does not eliminate interference. In engineering terms, it is the equivalent of adding gain to the desired signal. The gain is roughly the bandwidth occupied by the transmitted signal divided by the bandwidth required to send the signal without modulation (the baseband bandwidth). This value is measured in decibels, and is typically 20-30 dB, although it can increase. But the higherhe data rate, the lower the processing gain!

The effect of distance - radio signal energy decreases by an inverse square law. This means that a nearby interference source can have a much stronger signal, proportionally, than the desired signal from a farther source. Some numerical examples:

1. A receiver at room temperature will have an inherent noise level of -174dBm (10E-20.4 Watts). This means that if you want to send a 1HZ signal, you must generate more than -174dBm in the receiver. This sounds like a tiny number, BUT...
   
2. A hand-held cell phone operates up to about 600mW which is +27dBm.
3. Now, let's transmit that signal a few miles. The antenna has roughly no gain on the handset. The receiver antenna might have a capture are of 1/4 meter. At 3 miles, the 600mW is distributed across the surface of a 3 mi radius sphere, giving a signal strength of 6.5*E-10 Watts at the receiver (-62dBm).
4. The baseband signal of this cell phone is about 2KHZ. This means that the -174dBm requirement is upped by a factor of 2000 to -140dBm. But we also need a signal to noise ratio of, say, 15dB to receive that signal well, so now we are at -135dBm. So - we have a roughly 43 dB margin.
   
5. Now add 40 dB of path loss from buildings in the way and you have a 3dB margin... your signal barely makes it adequately.
   
6. Let us fire up another cell phone on the same frequency band (I am assuming we are using spread spectrum). Let us assume a reasonable spreading gain of 1000 (30dB). Put that cell phone 100 yards away, and guess what: It gives you an effective signal of .6/1000/125000/4 = 1.2x10E6 milliwatts or -59dBm. Our desired signal is at -62dBm, so it is wiped out!
   

This illustrates that a signalling system, by itself, will not prevent interference - defeating the main argument. Specific factors are:

Imperfections in equipment. Real equipment will not reach theoretical levels of performace.

Limited dynamic range. If you have a 100,000 watt transmitter 3 feet from your receiver, there is a good chance that no matter what its technology, it will not be able to pull out the desired signal. In digital terms, this is the equivalent of running out of bits in your integer! If a number is too big, you either overflow your math, or you scale it down, losing the little bitty number you wanted.

Limited bandwidth - there is a limited amount of bandwidth, useful for a given purpose, at any place and time. This bandwidth, for many purposes, is between 1GHz and 25GHz (although for ionospheric radio, it is only 30 MHz). This means that if someone is generating a strong signal in the bandwidth you are using, there may be no other bandwidth you can jump to.

Intermodulation. Any nonlinearity in the system, including incidental nonlinearities such as a nearby rusty pipe, will cause all the RF signals impinging on them to be mixed, and the mixing products re-radiated. Receivers have inherent nonlinearity, which unfortunately gets worse as the power used by the receiver is reduced.

Leakage. You may have a great receiver, but an interfering transmitter that is close enough may leak through its plastic case and get into an intermediate stage of your receiver.

etc.

Without regulation, some other system must arise to arbitrate needs for radio spectrum, or chaos will result