Spectrum is weak link in AT&T's copper retirement plan

15 November 2016 by Steve Blum
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Download while you can, that line of sight won’t go so far in spring.

Slowly, we’re learning more about AT&T’s plans to abandon wireline broadband service in rural areas of California, and replace it with what it calls wireless local loop. AT&T has been presenting its WLL roadshow to boards of supervisors around northern California, but the content is misleading – skipping the part about ending copper service, for example – and at times, completely false – federal Connect America Fund subsidies are not limited to wireless service, and may just as easily be spent on fiber.

Rob Osborne, a senior analyst with the California Public Utilities Commission, has been taking a look at AT&T’s plans in his excellent blog. One of the aspects he focuses on is AT&T’s choice of 2.3 GHz spectrum for WLL service, and reserving more robust frequencies, such as the 700 MHz band, for mobile purposes

There are solid engineering arguments why 700 MHz is more valuable than 2,300 MHz for delivering wireless broadband to homes. I’ll address only the line of sight argument. The larger-wavelength 700 MHz frequency penetrates buildings and foliage better than the shorter-wavelength 2,300 MHz frequency. For 2,300 MHz service to reliably deliver broadband, there needs to be line of sight, meaning you can physically see, without obstruction, between cell tower antenna and customer antenna. Without line of sight, service will be less reliable than with 700 MHz. Many parts of rural California are mountainous and have trees, which makes line of sight broadband difficult and expensive to deploy.

In terms of performance, 2.3 GHz is pretty much the same as the 2.4 GHz band used for WiFi. AT&T plans to install outdoor antennas and receivers at customers’ homes, which are absolutely necessary in order to get through walls that block those frequencies. But foliage, particularly when it’s as dense as it is in the mountain areas of California, can stop it just as cold.

In theory, to qualify for the federal money, AT&T has to reliably serve virtually all the homes in the subsidised areas with 10 Mbps download and 1 Mbps upload speeds. One thing to watch for: the time of year AT&T does its testing. What’s fine at 2.3 GHz in the dead, grey winter won’t necessarily work when summer comes and everything turns to green.