Comment on Why don’t wireless connections (WiFi, Bluetooth, etc.) use anything between 2.4Ghz and 5Ghz?
litchralee@sh.itjust.works 4 weeks ago
Also, what if things that require very little data transmission used something lower than 2.4Ghz for longer range? (1Ghz or something?)
No one seemed to touch upon this part, so I’ll chime in. The range and throughput of a transmission depends on a lot of factors, but the most prominent are: peak and avg output power, modulation (the pattern of radio waves sent) and frequency, background noise, and bandwidth (in Hz; how much spectrum width the transmission will occupy), in no particular order.
If all else were equal, changing the frequency to a lower band wouldn’t impact range or throughput. But that’s hardly ever the case, since reducing the frequency imposes limitations to the usable modulations, which means trying to send the same payload either takes longer or uses more spectral bandwidth. Those two approaches have the side-effect that slower transmissions are more easily recovered from farther away, and using more bandwidth means partial interference has a lesser impact, as well as lower risk of interception. So in practice, a lower frequency could improve range, but the other factors would have to take up the slack to keep the same throughput.
Indeed, actual radio systems manipulate some or all of those factors when longer distance reception is the goal. Some systems are clever with their modulation, such as FT8 used by amateur radio operators, in order to use low-power transmitters in noisy radio bands. On the flip side, sometimes raw power can overcome all obstacles. Or maybe just send very infrequent, impeccably narrow messages, using an atomic clock for frequency accuracy.
To answer the question concretely though, there are LoRa devices which prefer to use the ISM band centered on 915 MHz in The Americas, as the objective is indeed long range and small payload, and that means the comparatively wider (and noisier) 2.4 GHz band is unneeded and unwanted. But this is just one example, and LoRa has many implementations that change the base parameters. Like how MeshCore and Meshtastic might use the same physical radios but the former implements actual mesh routing, while the latter floods to all nodes.
But some systems like WiFi or GSM can be tuned for longer range while still using their customary frequencies, by turning those other aforementioned knobs. Custom networks could indeed be dedicated to only sending very small amounts of data, like for telemetry. That said, GSM does have a hard cap of 35 km, for reasons having to do with how it handles multiple devices at once.
skillissuer@discuss.tchncs.de 4 weeks ago
i’d also note that antennas, amplifiers and so on have bandwidth that is some % of carrier frequency, depending on design, so just going up in frequency makes bandwidth bigger. getting higher % of bandwidth requires more sophisticated, more expensive, heavier designs. LoRa is much slower, caused by narrowed bandwidth but also because it’s more noise-resistant
litchralee@sh.itjust.works 4 weeks ago
In my limited ham radio experience, I’ve not seen any antennas nor amplifiers which specify their bandwidth as a percentage of “carrier frequency”, and I think that term wouldn’t make any sense for antennas and (analog) amplifiers, since the carrier is a property of the modulation; an antenna doesn’t care about modulation, which is why “HDTV antennas” circa 2000s in the USA were merely a marketing term.
The only antennas and amplifiers I’ve seen have given their bandwidth as fixed ranges, often accompanied with a plot of the varying gain/output across that range.
Yes, but also no. If a 200 kHz FM commercial radio station’s signal were shifted from its customary 88-108 MHz band up to the Terahertz range of the electromagnetic spectrum (where infrared and visible light are), the bandwidth would still remain 200 kHz. Indeed, this shifting is actually done, albeit for cable television, where those signals are modulated onto fibre optic cables.
What is definitely true is that way up in the electromagnetic spectrum, there is simply more Hertz to utilize. If we include all radio/microwave bands, that would be the approximate frequencies from 30 kHz to 300 GHz. So basically 300 GHz of bandwidth. But for C band fibre optic cable, their usable band is from 1530-1565 nm, which would translate to 191-195 THz, with 4 THz of bandwidth. That’s over eight times larger!
For less industrial use-cases, we can look to 60 GHz technology, which is used for so-called “Wireless HDMI” devices, because the 7 GHz bandwidth of the 60 GHz band enables huge data rates.
To actually compare the modulation of different technologies irrespective of their radio band, we often look to special efficiency, which is how much data (bits/sec) can be sent over a given bandwidth (in Hz). Higher bits/sec/Hz means more efficient use of the radio waves, up to the Shannon-Hartley theoretical limits.
Again, yes but also no. If a receiver need only receive a narrow band, then the most straightforward design is to shift the operating frequency down to something more manageable. This is the basis of superheterodyne FM radio receivers, from the era when a few MHz were considered to be very fast waves.
We can and do have examples of this design for higher microwave frequency operation, such as shifting broadcast satellite signals down to normal television bands, suitable for reusing conventional TV coax, which can only carry signals in the 1-2 GHz band at best.
The real challenge is when a massive chunk of bandwidth is of interest, then careful analog design is required. Well, maybe only for precision work. Software defined radio (SDR) is one realm that needs the analog firehose, since “tuning” into a specific band or transmission is done later in software. A cheap RTL-SDR can view a 2.4 MHz slice of bandwidth, which is suitable for plenty of things except broadcast TV, which needs 5-6 MHz.
I feel like this states the cause-and-effect in the wrong order. The designers of LoRa knew they wanted a narrow-band, low-symbol rate air interface, in order to be long range, and thus were prepared to trade away a faster throughput to achieve that objective. I won’t say that slowness is a “feature” of LoRa, but given the same objectives and the limitations that this universe imposes, no one has produced a competitor with blisteringly fast data rate. So slowness is simply expected under these circumstances.
In the final edit of my original comment, I added this:
skillissuer@discuss.tchncs.de 4 weeks ago
sorry for being unclear, i forgor a word. what i meant that certain antenna designs would have specific fractional bandwidth, so that just scaling that design to higher frequency makes usable bandwidth wider in kHz terms. in order to get higher fractional bandwidth more complex or bulkier designs would be required, like thicker conductors, added parasitics, something LPDA-shaped, or maybe elaborate matching circuit, all of which cost money. i guess that while resonant amplifiers are a thing, probably bigger limitation would be bandwidth of mixer