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Comment by scrlk | Hacker News Reader

Radio waves travel at nearly the speed of light, whereas light in an fiber optic cable travels at ~67% of the speed of light due to the refractive index of glass.

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Ericsson blog wrote:

In a vacuum, electro-magnetic waves travel at a speed of 3.336 microseconds (μs) per kilometer (km). Through the air, that speed is a tiny fraction slower, clocking in at 3.337 μs per km, while through a fiber-optic cable it takes 4.937 μs to travel one kilometer – this means that microwave transport is actually 48% faster than fiber-optic, all other things being equal.

I worked for three years designing custom low-latency point-to-point microwave radios for HFT for this very reason. They didn't need very high bandwidths (their long-haul network was less than 200 Mbit, whereas in New York/New Jersey we had about 5 Gbps because the hops were much shorter and they had licenses for more RF bandwidth at a higher frequency).

At those time scales, the difference is so large, it was incredible what they were willing to pay to build these networks!

I somewhat regret not specialising in RF/comms in my EE degree - this side of HFT sounds like a fascinating line of work (Trading at the Speed of Light was a great read).

I doubt there's much here that's cutting edge. Any digital processing that's done in typical radio's to correct for channel impairments is avoided as it just adds latency. Meanwhile LTE is using as many digital techniques as possible to maximize bandwidth (MIMO, HARQ, OFMDA)

Haha, you got us :) - in terms of the digital side yes, kind of. We’d even try to not have any digital in the path if possible on some hops! We did have things like LDPC (and different FEC on control packets) but it was definitely not as complex as LTE or newer cellular or WiFi standards. But what was avoided digitally meant far more work going into the analogue side to improve SNR, dynamic range, NPR etc. through the signal chain.

More bluntly: light in a fibre is still bouncing around a lot.

Not in single mode fibers, which still exhibit the effect. It's just about the refractive index of glass.

Yes, even in single mode fibers light is bouncing around between the layers of glass with different diffraction indicies. The pulses just aren't dispersing while doing so.

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