5 ms·
It's interesting to see the huge channel bandwidths they're using to attain this. Advances like this are largely driven by higher sample rate ADCs and DACs beco
by wrigby 10y ago
It's interesting to see the huge channel bandwidths they're using to attain this. Advances like this are largely driven by higher sample rate ADCs and DACs becoming more viable in recent years.
Edit for clarity: channel bandwidths in the datasheet are up to 2 GHz. Need to close the link at 32 QAM to hit 10 gb before error correction overhead at that bandwidth, which is certainly doable. Also, it's interesting to note that they quote 7 gbps throughput @ 1 GHz bandwidth, and 10 gbps at 2 GHz. It implies that they can run at 256 QAM at 1 GHz, but only 32 QAM at 2 GHz, which also makes sense.
Having worked in the satellite communications industry, where channel sizes used to be limited to 72 MHz (because of hardware limitations of the spacecraft), getting modems designed to operate on larger channels is no small task. I would love to learn more about the internal architecture of these radios to understand exactly what's going on - are they using interleaved ADCs? Is the modulation/demodulation being done in an FPGA, or an ASIC?
If there are terrestrial microwave engineers on here, I'd love to hear your thoughts on this!
- walrus01 10y agoterrestrial microwave engineer here: "80 GHz" is actually 71-86 GHz FDD. The original FCC band plan allowed for 5000 MHz wide channels each direction and basically OOK or BPSK level modulation. Newer radios use 250, 500 or 1000 MHZ wide FDD channels and QPSK or better. Incredibly wide channels can be used because it falls off in the atmosphere so rapidly after a few km, and the antennas are all very narrow parabolic reflectors with less than 1 degre beamwidth. Basically no colocation interference issues are possible unless two companies try to shoot from the same rooftop, to also the same rooftop, using the same channel AND the same linear polarity.
- wrigby 10y agoThanks for jumping in! Sounds like it's really nice for urban small-cell deployments, campus networks, etc. Do you experience a lot of issues with keeping links aligned at those tiny beamwidths?
- walrus01 10y agoNot really, 80 GHz is definitely difficult to aim, the very center of the beam is quite small. The mounts for the most popular antennas accommodate this and have very fine adjustments for the azimuth and elevation. Keeping them aligned long term? Not a problem as long as the mounts the radios are on (whether non penetrating or bolted to a wall/structure on a building roof) are done properly.
- nrs 10y agoI'm wondering how the natural motion of some taller buildings affects this kind of deployment. Lots of buildings swing and twist in the normal course of the day. We found this out with some of the free space optical solutions available.
- walrus01 10y agoIt can have some effect but at the distances aimed (max 3km for a high reliability, high Tx power 80 GHz link) it doesn't affect much. A link that is a nominal -34 RSL on a clear sunny day might drop to -36.5 as a result of building sway (such as if one end is on top of a 60 floor tower). Weather is the main issue not heat expansion/contraction or sway.
- e2phd 10y agoYou can pack many bits into a constellation point. Think 1024 QAM.
- walrus01 10y agoMost new licensed band 6, 11, 18, 23 GHz radios these days are 1024QAM capable. For 80 GHz the hot new thing is radios capable of 16/64/256QAM at varying code rates.
- Dwolb 10y agoWhat's the limiting factor when designing to a constellation? Otherwise, what's improved about the chip or firmware to be able to move from 256QAM to 512QAM?
- walrus01 10y agosignal/noise ratio is a big one and RSL level. There are even 4096QAM radios now but they need a signal like -51 to operate at their full modulation. Otherwise they ACM down to 1024 (around -62), 256 and then 64QAM.
- Dwolb 10y agoWhat's the limiting factor when designing to a constellation? Otherwise, what's improved about the chip or firmware to be able to move from 256QAM to 512QAM?
- ArkyBeagle 10y agoThat's a pretty narrow ledge to stand on @ 800 GHz. Wonder what the FEC regime is?
- cmiller1 10y ago>higher sample rate ADCs and DACs becoming more viable Question, on optical networks doesn't this make dispersion more of a problem?
- ArkyBeagle 10y ago80 GHz isn't quite optical yet. It's a couple octaves down from infrared. SFAIK, it's still very much radio.
- walrus01 10y agoa great deal lower, the optical ITU DWDM grid is around 191.7 THz to 196.1 THz
- ArkyBeagle 10y agoThe more general dividing line between microwave and infrared ( outside comms ) is 300 GHz, which was what I meant.
- walrus01 10y agoThat is true. One of the fun things illustrating first-hand how microwave/millimeter wave and fiber are the same thing is if you ever get to handle a 120 GHz band waveguide/feed. Some radar and military stuff operates in the 120 band. It's so incredibly tiny and narrow compared to a 11 or 18 GHz microwave waveguide. Then imagine continuing to make it narrower while increasing the frequency, follow that to its logical conclusion and you have imagined a 9/125 fiber.
- ArkyBeagle 10y agoBut it's something of an apparently completely different... phenotype from a small waveguide. The seemingly arbitrary 300Ghz line was probably more cultural than essential; the instrumentation was just very different. What's the analog for a prism for microwave?
- cm2187 10y agoWhat sort of latency would you expect with a system like this (I mean the link in this submission, not satellites)?
- amazon_not 10y agoPoint to point wireless links like these usually have low latency, some under a millisecond.
- jlgaddis 10y agoFWIW, I've got several similar point-to-point microwave links in my network, including the following setup: h1 - sw1 - ptp1a -/- ptp1z - sw2 - ptp2a -/- ptp2z - sw3 - r1 - h2 h1 : Sun Fire server sw1 : Cisco 3560G switch ptp1a : 6 GHz point-to-point microwave transceiver ("A" end) ptp1z : 6 GHz point-to-point microwave transceiver ("Z" end) sw2 : Cisco 3560G switch ptp2a : 6 GHz point-to-point microwave transceiver ("A" end) ptp2z : 6 GHz point-to-point microwave transceiver ("Z" end) sw3 : Cisco 3560G switch r1 : Cisco 3945 h2 : Sun Fire server All connections are 1 GbE. ptp1a to ptp1z : 12.80 miles (20.60 km) ptp2a to ptp2z : 05.01 miles (08.06 km) From h1 to h2: --- x.x.x.x ping statistics --- 20 packets transmitted, 20 received, 0% packet loss, time 19305ms rtt min/avg/max/mdev = 0.955/0.995/1.157/0.049 ms Each of the two microwave links are advertised as being capable of 761 Mbps full-duplex.
- tzs 10y ago> Advances like this are largely driven by higher sample rate ADCs and DACs becoming more viable in recent years. I'm looking forward to what these will do for SDR. I salivate over the thought of an SDR using something like the TI ADC12J4000 [1] which has a 4 GSPS sampling rate. [1] http://www.ti.com/product/ADC12J4000/description http://www.ti.com/product/ADC12J4000/description
- rfdave 10y agoThe digital hardware to handle the output of a 4 GSPS ADC is beefy as hell. This part has integrated DDC's if you're happy with downsampling, but if you want full nyquist data coming out of this, you need to handle 6 Gbyte/Sec.
- curiousfab 10y agoThe specs sound very similar to what the BCM8500 can do. Add an analog frontend and an antenna, and that's basically it. https://www.broadcom.com/products/Microwave-%26-Mobile-Backhaul/Baseband/BCM85100 https://www.broadcom.com/products/Microwave-%26-Mobile-Backh...