Wi-Fi on Crutches


The FCC recently announced that this month (September 2010) it will address the next step in its plans for unlicensed use of the TV whitespace (the portions of the TV band that are not used in a particular location to carry TV signals). Proponents of this policy claim it will bring WiFi on steroids. (A Google search for “WiFi on steroids” gave 117,000 hits with the first hit listed being Google’s own policy blog.)

I doubt that we will see anything like WiFi on steroids in the TV white space. First, modern WiFi (802.11n) is already WiFi on steroids! Computer networking in the TV whitespace is more likely to be WiFi on crutches. (TV white space might be good for some dynamite, low-cost cordless phones.)

Why do I say that performance will be so poor? Well, it’s a combination of physics, FCC regulations, and economics. Because the TV band is divided into 6 MHz channels and is heavily occupied in some urban areas, a white space device will never be guaranteed more than 6 MHz of available spectrum. So, it will be natural to build TV white space devices to operate in 6 MHz channels. Indeed, I read the current rules as limiting operation to 6 MHz channels. (See 47 CFR 15.709(a) which refers to ‘the TV channel of operation’. Given the need to protect adjacent channels, the real useful bandwidth will be more like 5 MHz.

WiFi operates today in the 2.4 and 5-GHz bands—using bandwidths three to six times greater than the 6 MHz of TV whitespace devices. Wi-Fi traditionally used a 20-MHz channel and now has an option for using a 40-MHz channel. At short ranges, such as inside the house, signals do not attenuate much between the transmitter and the receiver, and the received power is relatively high. In such high power situations, the capacity of a wireless link is determined more by bandwidth than by power. Thus, a 20 MHz channel yields 3 times the capacity of a 6 MHz channel.

There are some propagation advantages in the TV band compared to the current WiFi bands—it is probably a naïve consideration of those advantages that led some to speak of “WiFi on steroids.” However when operating at short ranges (as LANs usually do) the additional bandwidth at 2.4 GHz more than makes up for the propagation advantages in the TV band. Moreover, multiple-input, multiple-output (MIMO) technology works better 2.4 GHz and 5 GHz than at the TV band. Better MIMO performance probably gives WiFi operation in the existing bands another factor of two capacity advantage over WiFi operation in the TV white space. The general proposition is undeniable—at short ranges, WiFi operation in the existing WiFi bands permits far higher data rates than are possible in the TV white space under the current FCC rules.

Given that WiFi in the TV white space can only have a third to a tenth of the data rate of today’s WiFi in most applications, I doubt if it will sell very well.

Now there may be a variety of other uses—cordless phones, baby monitors, security cameras, TV remote controls, video game controllers—for which the TV white space will work well. But, they sure aren’t WiFi on steroids.

The TV white space is well suited for long-range services, such as wireless Internet access. However, licensed operation in the white space would be more likely than unlicensed operation to best meet this need. On balance, our nation’s TV white space policy is likely to lead to decades of waste of a valuable resource—with the TV white space delivering far fewer benefits to consumers that it could. It’s a shame.

Disclosure. I have authored or coauthored several studies on the TV white space that were supported by QUALCOMM, a manufacturer of radio equipment for use in both licensed and unlicensed bands. Because they were written in a universe with the same FCC rules and laws of physics, those studies came to the same conclusion as this blog post.