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10 October 2016

AirGig is less radical than it seemed, but a useful addition to 5G roadmap

AT&T’s futuristic AirGig is not quite what it initially seemed. Subsequent clarifications from AT&T have confirmed that no powerline communications are used, as initially reported. The basis of AirGig is that it places cheap antennas onto US powerlines, but does not communicate across them.

The key element is that this is a plentiful form of backhaul. The new plastic antenna devices which AT&T is proposing will sit on very tall medium voltage power poles, not because they are medium voltage, but because they are high up. There may not be any need to have power either. It seems the devices take their energy off the powerlines using near field induction or something similar, as there don’t appear to be any power lines going down to the ground in AT&T’s YouTube video about AirGig.

If one of these is planted on a power pole every 100 yards or so, AT&T could connect them by sending wireless signals from one antenna to the next in millimeter wave bands – this could be LMDS, 57 GHz to 64 GHz, which is unlicensed, or 70 GHz to 90 GHz, which have been allocated as ‘lightly licensed’ bands for multi-gigabit wireless communications. They all need unobstructed line of sight communications. The problem with these is that while they can send a mile or so, it’s not advisable if you have any chance of any interference, and this type of spectrum hates rain, or fog and if there are trees or water around, hates the reflections that this causes.

The presence of medium voltage power cables could presumably also affect such signals. The upsides here are that complex new antenna arrays mean that the millimeter waves do not have to be professionally lined up like microwave point-to-point communication, and it can get past some obstructions. It can effectively be point-to-multipoint, working as a dumb, but very fast, repeater.

In every other respect this is very like the way some European operators have put broadband cables over the same poles as power. These lines are self-powering from the center and powered by the home router over the last lap of G.fast. Because of the wireless energy being used by AT&T, AirGig needs some power and this is an ideal solution.

There are still political problems in the way, but AT&T may be far closer to cracking this technology than we thought. Tiny companies like Starry (led by Chet Kanojia, the founder of Aereo) are using an active phased array antenna to achieve something similar, in 37 GHz to 39 GHz (which AT&T is more likely to reserve for the actual 5G signals). If Starry can think of this, then Google can also think of it, and so can every backhaul supplier that has used millimeter wave spectrum.

While AT&T will obviously have the large pockets for a move like this, the power companies will rapidly sense their strength and make the best deals they can.

AT&T, and other networks, could use this as the distribution network for G.fast (and later XG.fast), where it currently does not have fiber or the right economic terms for installing it – so think rural. It could easily be simply the quick way to get to fiber speeds, and then in-fill later over a longer economic cycle. And at the same time it can use this as backhaul for 4G and 5G wireless.

The onramp may also be another G.fast or XG.Fast line using the same antennas to feed the ones on the power lines.

The completed architecture will need quite robust management tools, because you don’t want to be climbing those powerlines more than once, and forward error correction systems. And hurricanes would almost certainly bring down a lot more broadband capacity, but it could be the difference between 5G speeds being 20 years away, and being seven years away. Unobstructed millimeter wave might run every bit as fast as PON, which reaches around 10Gbps, but this architecture is unlikely to be robust enough to quite reach these speeds.