How much 5GHz spectrum is enough?

Note: Post edited 2/15/13 with updated information from the NTIA report.

Recently, the chairman of the FCC proposed adding more spectrum to the 5GHz unlicensed ISM band. Currently it stretches from 5.15GHz to 5.85GHz, but with a 120Mhz gap (5.35-5.47GHz) and a 5MHz gap (5.73-5.735GHz), leaving about 575MHz of usable bandwidth (at various power levels subject to competing uses) in three parts – 5.15-5.35Ghz (200MHz), 5.47-5.73GHz (260MHz), 5.735-5.85GHz (100MHz). The FCC proposal was to fill in the 120MHz gap, as well as add 75MHz to the upper end of the band, extending it from 5.85Ghz to 5.925Ghz. However that upper 75MHz interferes with “smart car” connected vehicle program, and is opposed by AASHTO (the American Association of Highway & Transportation Officials – essentially the governing body for regulations covering roads and highways).

Filling in the first 120MHz is a good idea, and would leave us with more spectrum to add channels. It would allow for a 40% increase in the number of 80MHz channels available to broadcast on, and add a third available 160MHz channel (a 160MHz channel is made up of two 80MHz channels, its not cumulative). Currently, the specification and frequency allotment for the US only allows for five 80MHz channels and two 160MHz channels as shown below. Adding in the band between 5.35-5.47GHz would allow a contiguous span from 5.15-5.73Ghz (580MHz). The 580MHz usable span would allow for seven 80MHz channels. Those seven 80MHz channels could be paired into three 160MHz channels for even faster speed under an optional part of the 802.11ac specification.

From the NTIA report on the 5GHz reallocation

From the NTIA report on the 5GHz reallocation, via Steven Crowley’s post

The addition of the 75MHz to the upper end is more questionable, due to the conflicts AASHTO brought up (linked above). It would allow for an additional 80MHz channel (or one 160Mhz channel) in that band, but because of the drawbacks dealing with connected cars (I’d rather see connected cars drive me to work rather than faster wifi speeds at home) I’m on the fence as to whether or not this 75MHz offers as much benefit as the 120Mhz addition mentioned above.

What effects does the spectrum addition have on end users? Faster speeds, at least once the specifications are updated. Because more channels will be available for people to select, neighbors will have less collisions and more spectrum to share in a given area. WiFi access points with 2 or 4 antennas would be able to offer aggregate throughput speeds measured in gigabits to a number of devices throughout the home. Devices supporting the final 802.11ac spec should start to arrive in 2013, so its just a short wait; however the wait for this additional spectrum is likely to be a lot longer.