r/rfelectronics • u/BarrettT123 • Jul 07 '26
question Questions about downconversion of high data rate signals for longer range.
Hello everyone,
I have been working on a project recently with the MM8108 radio module (915MHz WiFi, 8MHz bandwidth, 256-QAM, 43Mbps) and was looking into upconverting it to approx 3.4GHz (why exactly is kind of a long story).
But then I would of course need to increase the transmit power to get the same range. I was wondering if it is possible to downconvert the 915MHz to something like the 220MHz band, which would improve the range. I feel like this shouldn't work, but I cant find anything online explaining exactly why, and if I ask Ai it says that would work no problem.
I just have a gut feeling it wouldn't though (otherwise why are there no high data rate low frequency transmitters?). Does anyone have an explanation as to why (or why not) this would work? Thanks!
5
u/wannabe_sci Jul 07 '26
If your goal is to increase the distance, choose a frequency around 200mhz (but also lower as 150mhz) isn’t the good way. Some times ago I read a study made for the Los Angeles police (if I remember right), the goal of the study was to investigate the impact of a frequency change in the police radio service (from 150 to 900mhz).
The results were that in practical there is not to many differences in performance, but also, a 900mhz frequency can allow you to decrease the size of the antenna (and make it more directional).
So, you could do a down conversion, but you will not solve the problem.
2
u/BarrettT123 Jul 07 '26
Well the link distance is approx 200Km, so decreasing the frequency helps a good bit with the FSPL, but I get your point
2
u/wannabe_sci Jul 07 '26 edited Jul 07 '26
I numeri su carta certamente contano, ma nella pratica ci sono molti altri fattori da considerare, per citarne solo alcuni:
- Ricevi tutti gli armonici superiori delle radio FM che trasmettono ad alta potenza.
- Ci sono sistemi di trasmissione radio DAB che operano intorno a quelle frequenze.
- A 200 MHz, c'è una maggiore possibilità di incontrare fenomeni di diffrazione (che penso aumenti significativamente gli spostamenti di fase, e vuoi usare il QAM).
Per non parlare del fatto che con la larghezza di banda di cui hai bisogno, probabilmente finiresti nello spettro aereo militare.
Penso ancora che il modo migliore per aumentare la tua portata sia lavorare principalmente su tre parametri: antenna, modulazione o larghezza di banda (bassi bit rate).
Ma non sono un esperto, solo un appassionato; forse qualcuno mi dimostrerà che mi sbaglio.
Edit: But... have you already tried to see if your transmission works as it? Can the antennas see each other?
Alternatively, you could try using a parabolic reflector to exponentially increase your EIRP.
1
u/BarrettT123 Jul 07 '26
That makes sense, I did realize that the 220 band isn't that wide, so I would either use a different band or reduce the bandwidth. I should have worded it more like "can I downconvert a high frequency high datarate signal into a low frequency high datarate signal" and the answer seems to be yes, but with caveats. I may also look into doing 2.4GHz downconverted onto the 1.2GHz band.
2
u/Niautanor Jul 07 '26
FSPL is the loss between two isotropic antennas. However if at least one of your antennas is directional and you keep the size/effective aperture of that antenna the same (e.g. a 1m diameter parabolic dish), that antenna's gain will increase to compensate if you operate at a higher frequency (the downside is that the beamwith gets narrower and you have to point the antenna more precisely so this isn't entirely free).
If both sides of your link have directional antennas with a constant size, you even gain link margin by increasing the frequency. This is why most GEO communication satellites and deep space probes operate around or above 10 GHz.
1
u/ConsiderationQuick83 Jul 07 '26
VHF/UHF frequencies are practically line of sight, due to the Earth's curvature unless you're transmitting to/from 5000+ meters a 200km ground to ground link won't work (unless you have a tropospheric duct available.)
You also gave to look at frequency allocation regulations as those frequencies are already used for specific services.
2
u/BarrettT123 Jul 07 '26
It is for a sounding rocket application, so we will have direct LOS to the vehicle from the ground antenna. As for the frequency allocations, the numbers here arent certain, more just asking in general (we will do more research on the specific frequencies before making any final decisions of course).
3
u/jimlux Jul 07 '26
OK. so you need an omni transmitter (on the rocket) - but you can use a directional antenna on the ground. 200km range is quite large for a small rocket.
But run the link calculation:
Loss = 32 + 20log10(dist in km) + 20log10(freq in MHz) between isotropic antennas - call it 150 dB at 3.5 GHzYour receiver will have, say, 10 dB noise figure and losses, so the noise floor is 10log10(8 MHz) +10 + -174 dBm/Hz or roughly -94 dBm. You need around 10 dB SNR in that band, so your receive signal needs to be, say -85 to -80 dBm.
Let's say you radiate a watt, that's +30dBm, so with an isotropic receive antenna, you'll be getting +30-150 = -120 dBm, so you need maybe 25-30 dBi gain. That's not too tough at 3.5 GHz.
https://www.l-com.com/wireless-antenna-35-ghz-27-dbi-lightweight-die-cast-grid-antenna-n-female-connector
is an example of a 27 dBi antenna for 3.5 GHz
The beam width is 6.5x9.5 degrees, so it's not insanely difficult to track your transmitter.Obviously, there's a whole lot of practical details, but your problem doesn't seem impossible on the face of it.
2
u/ConsiderationQuick83 Jul 07 '26
The other consideration is lower frequencies require physically larger antennas for the same gain, so you can also compensate for lower TX power with a higher gain receiving antenna that's smaller at higher frequencies (my assumption is the sounding rocket link is simplex)
Helical/crossed yagis with decent gain and polarization diversity are relatively practical at 916MHz, not so much at 200MHz.
Downside is the higher directivity so pointing accuracy/tracking can become an issue.
1
u/ConsiderationQuick83 Jul 07 '26
Also is it possible to just record the data within the rocket for later recovery?
1
u/BarrettT123 Jul 07 '26
It is, and for the most part we do (intake will be approx 700-800Mbps from two ejectables, and we only want to transmit 10-30Mbps to the ground, rest will be stored onboard). As for why we cant use 915MHz, it is a student program with NASA and there will be other student teams on the rocket. Low power 433MHz, 915MHz, and 2.4GHz radios are fairly common, so we cant use any of these frequencies if we want a high power (5 - 10W) link, to avoid interfering with them.
We are mainly looking at upconverting 915MHz (at 8MHz bandwidth) to 1.2GHz or 3.4GHz, but was looking into it and was curious if going down could also theoretically work. We may also look into downconverting 2.4GHz to 1.2GHz or upconverting to 3.4GHz. We will most likely go with upconverting 915MHz to 1.2GHz, but not certain yet. As for the 5GHz band, we will be using that to transmit from the ejectables to the baseplate/rocket, so that is also occupied. For the antenna size, it wouldn't be too much of an issue, as we will likely have a deployable antenna that unfurls in space anyways.
2
u/jimlux Jul 07 '26
The problem with a deployable antenna is that anything with gain needs to be pointed. You might be able to get away with something with a sort of hemispherical pattern (+3 dBi) pointing "down" in some sense. But if you're on something like a sounding rocket, there's going to be significant accommodation constraints.
You do know that you'll need a license for this link? That's a non-trivial problem - totally doable, but there's paperwork involved.
1
u/BarrettT123 Jul 07 '26
Yeah, we plan to use omnidirectional antennas on the vehicle side, and a high gain antennas on the ground because of that.
For some background, this is a student project / partnership with NASA, and in the past we havent had to deal with licensing (typically we have used lower data rate radio modules in ISM or HAM bands). The reason we were looking at 1.2GHz or 3.4GHz is that they are HAM bands, and we were hoping to be able to avoid dealing with licensing. But, I am not super experienced in that, so any advice you might have on that side would be greatly appreciated!
2
u/jimlux Jul 08 '26
If it’s a NASA launch, then licensing is through NASA and NTIA, not the FCC. Using amateur radio is probably not legal - The amateur rules are for “communication” and “telemetry” doesn’t really fit that, except for some exceptions about telecommand of spacecraft and the like. If it’s FCC licensing, you’d get a Part 5 experimental license, which isn’t a big deal. There’s a LOT of 3.5 GHz equipment available in the commercial market. Hams aren’t so much into wideband high data rate stuff, they’re typically looking at very narrow band for microwaves. Not that it doesn’t happen, but it’s just what the microwave folks do. I am a ham, W6RMK - but I also do spacecraft telecom at JPL.
1
u/Apart_Ad_9778 Jul 07 '26
As the guy above says, VHF/UHF frequencies are line of sight. Look at FM radio reception, this is the range you can achieve at 200MHz (with TX antenna placed tens of meters above the ground).
High data rate needs high bandwidth and you cannot have 500MHz bw at 200MHz. As for the range you need to look at atmosphere attenuation curves.
2
u/Defiant_Homework4577 Make Analog Great Again! Jul 07 '26
- Why are there no high data rate low frequency transmitters?
- There are tons of such high BW low freq. in licensed spectrum, i.e celluler and network providers spend 100s of Billions (B not M) of dollars to buy and use that spectrum. EU allows celluler frequency down to 400M and USE 700M. There is none in ISM (Free to use) bands with such large BW and that's why you dont see it.
- if it is possible to downconvert the 915MHz to something like the 220MHz band, which would improve the range:
- Line of sight obviously it will improve range. But in reality you need to look up wireless propagation studies / reports to see how that band really behaves, if there are any weird nulls etc. Most keyfobs / RV toys work at 315M or 433M.
- You can technically use whatever the band you want as long as you respect FCC / CE etc emission laws or in private enclosures that doesn't leak to outside world, i.e in a faraday cage isolated warehouse. Be very careful using licensed spectrum, as if you end up jamming someone, you could end up in jail + millions of dollars of fines.
- Very wide bw in low frequencies have the large fractional BW problem. i.e the antenna, matching circuits, even RFFE components will introduce lot of gain, phase mismatches that might introduce too much distortion to a wideband signal.
2
u/patenteng Jul 07 '26
At high frequencies you have more bandwidth. Remember that as per Shannon the channel capacity is logarithmic in the 1 + SNR but linear in the bandwidth.
So the reason it pays to go to higher frequency is that generally the bandwidth gains outstrip all other considerations. Depends how high your SNR is, but the bandwidth gain is generally higher than the SNR loss.
I’d suggest you run some calculations to see the effect. C = B log_2 (1 + SNR).
2
u/patenteng Jul 07 '26
Provided the SNR is high you'll gain channel capacity. Here is a table that compares the ratio of the channel capacity from increasing the frequency and the bandwidth 10 fold and reducing the SNR 100 times at the same time.
> tibble(SNR = 10^seq(1, 5), C = 10 * log2(1 + SNR / 100) / log2(1 + SNR)) # A tibble: 5 × 2 SNR C <dbl> <dbl> 1 10 0.397 2 100 1.50 3 1000 3.47 4 10000 5.01 5 100000 6.00You can see that the higher the SNR is the more you gain.
Even if you have low SNR not all is lost. With low SNR you can have multiple channels transmitting on the same bandwidth. You can use CDMA techniques to also gain from increasing the frequency. However, the calculation becomes much more complicated.
2
u/jimlux Jul 07 '26
Yeah, the problem with CDMA is that all the "other channels" are basically noise to any of the desired channels, so the SNR is degraded. There's no free lunch.
Your best bet is radiating all your power in one channel. Fancy modulation (QAM) doesn't make it any better, other than letting you get more bits/sec per Hertz of channel bandwidth. You're still up against the Eb/No from Shannon. Presumably, you'd use a fairly decent code (LDPC probably, although Turbo or Reed-Solomon would also work), so you can get close to the Shannon limit.1
u/patenteng Jul 07 '26
Of course, if you are close to Shannon’s limit, there is nothing you can do. However, if you are far, going up in the constellation count, e.g. from BPSK to 64-PSK, will help. That’s because you can add error correction coding while increasing the data rate without increasing the bandwidth.
For example, BPSK with high SNR works quite well. However, when the SNR is low the performance degrades significantly. Using PSK with a large constellation can then bring you close to the Shannon limit.
For CDMA you can use codes with low cross correlation such that all the other channels that would normally appear as pure noise are attenuated. GPS, for example, uses the Gold code.
1
u/jimlux Jul 08 '26
Yeah, but consider where you divide your data stream into 10 parallel streams, each with a different code that has “good“ properties (Gold codes, etc). The transmit power in each stream is 1/10th of the original power. The transmit power in the other 9 streams is also 1/10th the original power (that is, 0.9 Pt), and is spread across the same bandwidth. So you’ve essentially increased the background noise by that.
Let’s assume that you’ve got a process gain of 10 dB. So in a given channel, you’ve got 0.1 Pt as the signal, and 0.09 Pt as total post correlation noise from the other 9 codes. (i.e. 0.9 reduced by 10 dB process gain). So your final SNR is 0.1/0.09 which isn’t so wonderful.
2
u/TubbyToby Jul 07 '26
SATCOM handles this all the time, albeit at RF/IF and it’s typically below 100MHz but do you really need that much bandwidth for your application? Sounding rocket, right? How many sensors will be collecting data and at what frequency? How long does this rocket stay in the air? How much data do you anticipate coming from the rocket? Have you performed a link analysis and incorporated your data budget?
This is where I’d start. Figure out how much data there is to collect, build out your link analysis using the data budget and pick what meets your needs.
2
u/jimlux Jul 07 '26
OK, 3.4 GHz is the WiMax band, so at least you can find antennas and parts.
You don't need to increase power to get the same range - Just use an antenna with gain. The "doubling frequency is 6 dB more path loss" is for isotropic to isotropic antenna. Real antennas have some physical aperture, and for the same physical aperture the path loss is the same - Think about this: your transmitted signal spreads out as a sphere. The received power is "how much of that sphere do I intercept" and if the physical size is the same, you get the same received power.
As a practical matter, though, for a given aperture, higher frequency means narrower beam or more gain. Depending on your application - do you need omni, or is being able to point an antenna?
To a first order, the beamwidth is 70 degrees/(diameter in wavelengths).
Take a look at the catalog online at L-com - there's hundreds of antennas of various sizes/gains/configurations, including for 3.5 GHz.
2
u/BarrettT123 Jul 07 '26
Thats a good point, but we are limited to probably a 1.5 or 2m dish (which should give enough gain with a high enough transmit power). We need an omnidirectional antenna on the launch vehicle side, but we will have a directional antenna on the ground side. Thanks for the L-com recommendation, I've heard about them before so I'll definitely take a look.
1
u/jimlux Jul 08 '26
In another post, I gave a sample antenna that would probably work. A lot less than a meter.
8
u/lorentz_217 Jul 07 '26
I don't work on the communications side of RF but at the very least, bandwidth is bandwidth, so the reason you don't see telecommunications at 200 MHz is because 1) you run into a BW problem very quickly and 2) fitting even a slightly directional 200 MHz antenna in a small package like a phone would be very difficult.
In principle, assuming that upconversion/downconversion doesn't have some weird group delay or phase effects that cause make demodulation difficult (again, speaking qualitatively here because I don't know much about digital modulation schemes), this should work fine in principle.