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.

Comments

[brough]

A friend has just pointed out I'm being sloppy. So to be more precise:
Reflection happens when signals encounter a surface that's large compared to the signal wavelength.
Diffraction happens at the edge of a large impenetrable object (large w.r.t. the signal wavelength).
Scattering happens when a signal encounters an obstacle whose size is around that of a wavelength.

In any event, the shorter the wavelength, the more likely a signal will encounter any or all of the above. And, the shorter the wavelength, the easier it is to deploy multiple antennas and MIMO.