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One of the major challenges for FSO is that it requires direct line of sight (LoS). One reason that electromagnetic radiation in the RF range is more desirable
by zw123456 10y ago
One of the major challenges for FSO is that it requires direct line of sight (LoS). One reason that electromagnetic radiation in the RF range is more desirable for communications is because it can be non-Los (think about your cell phone, you do not have to see the tower for it to work, you can be indoors for example).
The second issue is the free space pathloss (see https://en.wikipedia.org/wiki/Free-space_path_loss https://en.wikipedia.org/wiki/Free-space_path_loss ) you can see by the formula that the loss goes up exponentially by not only the distance but also the frequency, which is why the wireless carriers are so interested in the 600Mhz spectrum, you need to build fewer towers using 600Mhz than say 3.5Ghz.
The sudden interest in mm-wave (e.g. 28Ghz) for 5G is because the spectrum is much wider, that is the channels can be very wide allowing more data to be encoded, the trade off is that you start losing the NLoS capability, or it becomes much more difficult.
With light, all those problems are orders of magnitude more difficult to overcome. Also, there are regulations around the level of emissions of light for safety reasons, if a laser is turned up too high it can harm a persons eyes if they happen to look at it, so although the spectrum itself is not regulated, the amount of power you can emit is. That makes it very difficult to implement an FSO system that can safely transmit more than one or two kilometers, but if you get fog, rain or snow, that distance quickly becomes 100's of meters at best.
They are correct that the best way to overcome this issue is by developing sensors with much better sensitivity, by making them larger you can theoretically collect more photons, like a telescope. Since it is unlikely you can overcome the safety issue of increasing the transmission power level, it is smart to focus on the receive end and improve the sensitivity. But that does not help the LoS problem.
But a very interesting development nevertheless.
- tlb 10y agoLoss goes up quadratically with distance, not exponentially. The graphs showing the quadratic dependence of loss vs. frequency assume a half-wave antenna, meaning the antenna size decreases inversely to wavelength, so most of the effect is from having a smaller antenna. In any case, path loss doesn't usually limit network capacity. Densely deployed systems are limited by interference, and the interference experiences the same path loss as the signal.
- zw123456 10y agoYes, by the square, this is HN I cannot speak so loosely:) For Multiple access RF systems yes, but got point to point systems, that is line of sight, noise is not an issue, free space loss, also impairments from fog, trees and other things are dominant.
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- njoubert 10y agoI think you are hugely overestimating the importance of non-line-of-sight communication. Your example is cell phone communication. That is not the intended use case of this technology. Facebook's laser comms is part of their Aquila project, attempting to build a network of drones to supply internet. This application works just fine with a line-of-sight constraint. At 50,000ft just about everything is line of sight. Anyone building backhand RF links also hugely care about line-of-sight. Microwave towers carrying multi-gigabit speeds are carefully arranged for line of sight: this much improves reliability and throughput.
- vvanders 10y agoYeah, was going to say something similar. UHF/VHF don't have "line of sight" but if I've got a hill between me and someone else I'm sure as hell going to have a hard time hearing him. They put repeaters up on tall areas for that reason. Same with cell towers, yes you can get through non-dense material but there's still a signal loss associated with it.
- niftich 10y agoThe effect detailed in this comment and its parent is the Fresnel Zone [1], which isn't quite direct, human intuition 'line-of-sight', but that concept applied to RF waves. Essentially, any object intruding into a blimp-shaped region between the receiver and the transmitter has the potential to mess things up, and is corrected by having a payload in the signal that's tolerant or self-correcting of errors. [1] https://en.wikipedia.org/wiki/Fresnel_zone https://en.wikipedia.org/wiki/Fresnel_zone
- zw123456 10y agoYes, if they are thinking about deploying vertically to a LEO satellite then perhaps LoS is not an issue unless you are indoors. But if you want to require the user install an antenna or receiver outdoors then it can work, but again, the atmospheric loss can be an issue, for example if it is cloudy outdoors then light will have difficulty getting through that. Some wavelengths may be better than others but anyone who has had satellite TV knows what happens when it rains. But I do think what they are doing is interesting and has a long term promise for a lot of use cases.
- walrus01 10y agoAll of these challenges you've described are why Facebook's drone group also has a team working on 60/70/80 GHz PTP millimeter wave radio tech. Not very dissimilar from existing high-capacity 1GbE 60/70/80 GHz bridge radio products on the market now, from a radio perspective (example, one polarity in a 1500 MHz wide FDD channel in the 71-86 GHz bandplan, 256QAM modulation with ATPC+ACM), but self-aiming and motorized with stepper motors/servo mechanisms and service-channel data transfer for GPS-assisted aiming at a moving platform.
- zw123456 10y agoYes, the higher you go in the frequency the more it is like light. But even at mm-wave such as say for example 28Ghz you still have penetration through walls, but the loss is very high, for 28Ghz it is about 19db for a according to IWPC for a typical house wood wall. For light, the loss is basically infinity. Building a user device, like a phone or tablet with stepper motors or servo mechanisms is probably not very realistic I don't think.
- walrus01 10y agoI'm not talking about anything other than going through free space air with clear Fresnel zone, this is for backhaul/trunk links... A drone might be a flying series of sector antennas/phased array in 5.x GHz serving CPE radios, but it also needs an uplink.
- robryk 10y agoI don't really get why the range is this low. I can see flashes of light from a signal mirror from a place more than 10km away (and there were signal mirror links of ~100km in very good weather). In these systems, the noise should be way lower (there are many sources of relatively-low-frequency visual noise that I know of and I know of none that would generate at least hundreds of MHz frequency noise) and attenuation should be similar. Signal mirrors are eyesafe, so similar signal levels should be possible. What am I wrong about or missing?
- zw123456 10y agoGood question, you can see a light flash on and off but to encode large amounts of information onto a channel is determined by the signal to noise ratio per Shannon's law https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore... the information rate is proportional to the SNR. So you may be able to see a light flash on and one once per second but to put 1Gbps on it you need a pretty high SINR. Things like dispersion come into play.
- robryk 10y agoDo you mean dispersion as in "a pulse gets longer in time due to different length of paths taken by different photons due to scattering"? If not, what do you mean? My point about seeing a flash was precisely that SNR is high, because I can distinguish the flash from noise. EDIT: SNR depends on frequency only inasmuch noise level depends on frequency.