# Troposcatter: really all that bad?

The previous question,

and specifically, Phil's answer raised an interesting question:

In a 50–100 km distance situation where Line-Of-Sight beams aren't an option for communicating data, can we, given the technological advances of the last decades, again consider troposcatter a sensible mode of operation for moderately high-rate bidirectional communication?

I ask because troposcatter typically has the reputation of needing pretty large TX powers, and pretty high-gain antennas. Which does sound reasonable, since, obviously, we rely on the reflection of something which isn't such a great reflector at all, plus the area we'd illuminate with an isotropic antenna would be huge (considering a sphere with r=100 km or more).

On the upside, we get great ranges if we actually do blast out a lot of power and aim large antennas at the horizon.

I'd assume you'd want to operate somewhere around 2.3 GHz. The best model description I've found (that wasn't "oh, we've always used these 2 models, and it doesn't really work all that well, but we didn't check for other models" example ) is from this nearly ancient Master's thesis:

UNDERSTANDING TROPOSCATTER PROPAGATION
by
JOSEPH HENDERSON REYNOLDS
B.S., United States Air Force Academy, 1982

available online

After inspecting the summaries on the different models, I think that for VHF, UHF frequencies, probably what the call CCIR method II (by Boithias and Battesti) is what I'd want to look at. However, best graph I could produce on short notice (p.162 from that thesis):

for other frequencies, a factor of $30\log f_\text{GHz}$ is added.

so, I'd need to add another 3 dB; OK. Now, going for a < 1% outage time, I'd have to read both the upper (year average) and lower (worst month) chart's lower two curves (99% and 99.5% temporal availability). That yields something between 212 dB and 216 dB (average) / 218 dB to 222 dB (worst month) attenuation for a 150 km link.

Now, assuming we don't want to overdo it with antennas, an antenna gain of ~ 18dB on each end sounds realistic. Swinging with -220 dB tropo path loss, we thus get an overall loss of -184 dB.

Admittedly, not very little, still much better than GPS reception, and we legally get up to 10 MHz of bandwidth in that band! (as far as I understand)

Ok, rough capacity estimate:

Rolling a few numbers in my head, assume we want to get a code bit rate of let's say 200 kb/s. We'd want to stay a solid factor of 4 below the channel capacity (please correct me!), so that requires about 46 dBm transmit power – 40 W.

I don't think the atmosphere will allow for reliable amplitude estimation, (please correct me!) so I'd roll with QPSK for an initial estimate.

That gives me the rough feeling that we have plenty of headroom (like, 99 times what we use) to add robustness – be it through spreading, channel codes, or stupid spectral repetition.

This all breaks down on one thing, though:

I could find out nil about the properties of the 2.3 GHz tropo channel w.r.t.

• Coherency Time

I might very much be coming from an indoor / close-range background, but this feels like I might be missing something important.

So

• Does anyone have info on the above properties of the wireless troposcanner channel?
• Am I looking the wrong way, using the wrong tools to understand that channel?
• What (amateur) digital modes over tropo are state of the art (rather than used because everyone uses them since the 70s)?
• Is a 40W 2.3 GHz amp suitable for PSK realistic?
• Are you asking if troposcatter is realistic? Demonstrably it is, as evidenced by commercial and military installations. Or if its practiced by hams? Very different questions, since only a fraction of the technological advances of the past few decades have found their way into common ham use! – Phil Frost - W8II Apr 18 '17 at 13:41
• In the spirit of my second (bold) question from the bottom: I was expecting that hams use troposcatter (as it's pretty cool, so are hams, and it's cold war- and WW2-era tech). So I was expecting that there's at least some places where a ham would look for info on troposcatter channel behaviour... – Marcus Müller Apr 18 '17 at 20:50
• Troposcatter and tropospheric bending are two different modes, the former being much more of a challenge (at least on 2m). And beyond-line-of-sight communications is not the restriction on VHF, because bending is very common there. Not so much in the GHZ range. – Mike Waters Apr 19 '17 at 16:36
• I like it, too. Very thought-provoking. I have to assume the military had ample power, yet still used massive antennas. I can't imagine anything that could be tower-mounted that could compete, even with 10 MHz bandwidth. I had manuals for a lot of their stuff (BMEWS, etc.), but never even thought to ask for the tropo manuals. I'd be willing to bet they are available. For our long-distance phone lines (White Alice) they used those smaller tower-mounted corner reflector antennas, and I know they had this sort of distance with those, but they were all point-to-point, not tropo. – SDsolar Apr 19 '17 at 19:32
• Elevation was key for the point-to-point systems. Towers were at least 100 meters tall, for the long hauls. But without exception, the tropo systems had huge antennas on hilltops and were pointed right at the horizon. – SDsolar Apr 19 '17 at 19:35

I don't think a better answer will come along, so I'll summarize a combination of assembled anecdotes and opinion.

As far as I can tell, your calculations are accurate. I did a little research for Can tropospheric scatter effect be used by 802.11 links? and determined the requirements aren't absurd, and at least within the grasp of a typical amateur.

Anecdotally, I've read of hams doing troposcatter on 2 meters. Many of the troposcatter studies I've seen don't consider anything below 2.4 GHz, so I guess it's possible.

Of course a 40W amplifier for 2 meters is common ham gear. 2.4 GHz isn't nearly as popular for hams, but a quick search turned up plenty of 10W amplifiers intended for 2.4 GHz WiFi.

I also was not able to find any details on the channel besides loss. However, I see many examples of military systems like these:

Some are mounted on trucks. I've read each horn has two polarities. We can infer the quad antenna diversity setup is to counter channel fading which must be significant. Although, the military certainly has higher reliability requirements than an amateur. HF channels have similar properties and that hasn't stopped anyone.

Anecdotally, most of the amateur troposcatter seems to be SSB or CW from VHF DX enthusiasts. I'm not active in this space personally, but I do know some weak-signal digital modes are gaining popularity there. WSJT is notable, and JT65 in particular which I have used personally on HF. It's very slow and not meant to exchange anything but a call sign and a signal report over a QSO that lasts several minutes.

D-STAR is a popular GMSK digital voice mode since Icom puts it in all their radios. It must be the most common digital mode on amateur VHF with a moderate bit rate. It has a data mode which enables a payload of arbitrary data instead of the usual proprietary AMBE codec for voice. I've no idea how suitable for troposcatter it would be.

• thanks! Yeah, that kind of knowledge is what I was hoping for; didn't expect someone to come up with "hey, here's extensive channel modelling", and especially the aspect of dual-pol and diversity receivers is immensely interesting! Aside from the very likely diversity gain aspect: wouldn't it also make sense to have TX separate from RX for such highly directive antennas with large TX/RX power, to minimize the need for diplexing/isolation on RX? WSJT is something I've considered,or more specifically,the JT65 mode(one version of the WSJT manual says it's designed for EME and weak troposcatter) – Marcus Müller Apr 21 '17 at 12:55

Troposcatter is the common mode for beyond line of sight on the 10 GHz ham band when the band is not otherwise open, good for 100's of miles. A radio with just a few watts output power and a moderate sized dish (60cm or more) gets enough ERP for troposcatter.

For a few random references, check out:

• wouldn't have thought frequencies this high would survive the atmospheric attenuation, but wow, this is interesting! – Marcus Müller Apr 3 at 20:28
• Hi Mike, and welcome to this site! And thanks for a great first answer! We look forward to seeing more of you here. Hope you don't mind my minor edits. :-) – Mike Waters Apr 3 at 22:41