Communications

TV White Spaces - Fundamentals misunderstood

The FCC's recent decision to allow secondary use of the TV bands by license-exempt devices is a political breakthrough we can all rejoice in, but the widely claimed technology benefits are at least partly bogus.

Here's a typical claim:

This is the same spectrum we have referred to as "prime beachfront property."  < ... >  These lower frequency signals lose less power as they travel, hence requiring fewer base stations to cover an area. They also feature far better penetration characteristics, which is key to improving indoor operation.

This is true of deployed systems today, but for engineering reasons, not due to the physics of electromagnetic waves.

Today, wireless technology is advancing at such a pace that higher frequencies, for example at 5 GHz, will be more useful than TV White Space, perhaps within the time it takes TVWS equipment to get widely deployed -- certainly within 5-10 years.  What's happening is MIMO and advanced MIMO driven by emerging WiFi (802.11n), WiMAX and LTE systems.  As these systems get developed and deployed, signals at 5 GHz will achieve similar or greater range than TVWS, similar or better building penetration, lower costs and substantially higher carrying capacity.

The reason buildings appear to attenuate high frequency signals (like 5 GHz) more than those in the TVWS bands is, as radio signals hit the different materials that make up walls, windows, furniture, and other building elements, they scatter (due to diffraction and reflection).  With conventional receivers, only the primary signal is detected while scattered signals appear as noise, degrading the received signal.  Shorter wavelength signals are scattered by smaller objects and thus by more of the building's elements.  The higher the frequency the shorter the wavelength, so it's higher frequencies that experience more scattering.  But if you have multiple antennas and multiple receiver channels (MIMO), you overcome scattering.

Indeed with MIMO, the tables are turned.  MIMO systems use multiple antennas, typically separated by several wavelengths, or beamforming with many antenna elements each separated by 1/2 wavelength, but that's a problem for TVWS sysems.  At 5 GHz, a wavelength is 6 cm or just over 2 inches, but TVWS wavelengths range from 1.4 meters (55 inches) to 5.6 meters (over 18 feet).  Separating multiple antennas by several wavelengths is a problem at TVWS frequencies.  It might be feasible at a fixed base station but it's too bulky for a residential device and completely nuts for a laptop computer or a mobile phone.  So expect MIMO to be widely deployed at 5 GHz, but not so widely or not at all in the TVWS bands.

There are many other reasons why physics favors 5 GHz over the TVWS bands, but MIMO is the one place where engineering is catching up with physics as we speak.

Vladimir Kotelnikov – Father of Sampling Theory

Actually one of several "fathers."  Caution: the following is for the DSP engineers among you.  :-)

When I learned communications theory it was attributed to Claude Shannon (his 1948 paper) and sampling theory to Nyquist.  I do remember my father once telling me that E. T. Whittaker had published the relevant mathematics long before Shannon, but I never looked up the history.  And I never asked my father to elaborate as he was pushing his copy of Whittaker and Watson at the time, together with the idea I should be a mathematician or a physicist – not an engineer!

But on the plane back from Paris I read the June issue of Communications Engineer from The Institution of Engineering and Technology which included an interesting article on V. A. Kotelnikov by Professor Chris Bissell. 

... in the late 1940s <Shannon> wrote that the sampling theorem was “common knowledge in the communication art, but in spite of its evident importance it seems not to have appeared explicitly in the literature of communication theory.”  But Shannon was only partly correct. Ideas about sampling were indeed common knowledge in the late 1940s, and the theorem in various forms had appeared in the mathematical literature. But the theorem had also been published in the ‘literature of communication theory’ as early as 1933. Trouble was, it was published in the proceedings of a conference in Stalinist Russia – and in Russian.

The article goes on to give Kotelnikov credit for putting the problem of sampling a continuous, band-limited signal into an engineering context. Now that I'm back home and on-line again, I've been able to track down more of the story and access an English translation of Kotelnikov's seminal 1933 paper, On the capacity of the 'ether' and cables in electrical communication.  Not surprisingly, Wikipedia has an even more complete historical background on sampling theory in communications.

Suffice it to say, "Whittaker-Kotelnikov-Raabe-Shannon-Someya sampling theorem" would be a better name, as many people reached similar conclusions with varying degrees of parallelism.  Kotelnikov suffered from writing in Russian under Stalin and before the west started tracking Russian science.

Even with the English translation of Kotelnikov now available, Shannon's 1948 paper remains the best and most complete source for me.  I think it's only available in hardcopy forms, but other URLs of possible interest are here.