Film-Tech Forum ARCHIVE: RF Non Line of Sight Questions

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Posted by Daryl C. W. O'Shea (Member # 1303) on 12-11-2003, 10:59 AM:

Curious if any of our resident broadcast engineers are familiar with OFDM (orthogonal frequency division multiplexing), specifically whether or not I've got a chance in hell in making this work.

I want to setup a wireless link between my house and one of our factories to establish a high speed internet connection at my house. 19.2kbps just isn't cutting it.

Here's the problem... living in the middle of nowhere you tend to run in to the odd tree or two, or more precisely, a ministry forest. At a path loss of about 0.3dB/m to 1.1dB/m in a forest there's no way I'd make it through.

So, using OFDM, what's the chance of going over the forest? Would the multipath signal even go up and over and back down again, or am I going to have to jump on my bulldozer?

1.3 km (0.81 mi) house to trees
1.1 km (0.68 mi) of dense trees ~ 60-70 ft high
0.4 km (0.25 mi) trees to factory
---
2.8 km (1.74 mi) total

Starting at the right, at the house there are a few scattered trees about 500' away that I can't clear the top of, if need be, that's something Mr. Husqvarna could take care of. After that it's a big open field until I hit the ministry forest (dark green), across the 1.1km of forest and then to the factory on the left side.

Here's the biggest problem of them all. Since I live on an airport I can't go building a tower so I'm limited to attaching my antennas to the roof of one of the buildings, probably one of the hangers. That'll get me about 38 feet into the air, but it's probably another 25 to 30 feet to even clear the trees, let alone the 15 feet I should have to clear 80% of the fresnel zone.

The other end is pretty much the same situation... I can mount the antenna off the roof at about 38 feet from the ground.

Here's another thing... both roofs are steel. Am I going to need extra clearance from the roof? I was planning on using a pair of 20" mini-dish satellite dishes with biquad antenna on them, that should give me about 25dB of gain each.

Which brings me to my next question... would using a receiver that normally has diversity antenna (two antenna) and two separate dish antenna at both ends of the link improve my chances of this working. I'm thinking it would improve the situation (although not necessarily make it work). I could have up to about 100 feet of separation between antenna if it would help. If two antenna at each end is the way to go, where would the balance be between cable loss and separation 'gains' be?

Some calculations:

Line-of-sight path loss:
32.4 + 20 log 2400 + 20 log 2.8 = 108.9 dB

Link gain/loss (line-of-sight):
25dB + 25dB - 109dB = -59dB (not including connectors & cable)

Not sure if I'll be using 100mW or 32mW transceivers yet... I have no idea if I can make it over these trees with OFDM and how much loss I would incur.

I could also use 5.2GHz transceivers, instead of the 2.4GHz ones calculated above, I'd actually prefer using 5.2GHz if it would work... LoS path loss for that is 116dB though.

Also... the OFDM transceivers have a receive sensitivity of -91dBm.

Any suggestions or comments would be greatly appreciated! Thanks!

Posted by John Hawkinson (Member # 1135) on 12-11-2003, 11:23 AM:

I don't know specifically about OFDM, but I would say:

Trees can be really hard to predict. You might find something that works in winter but not summer, though (what kind of trees?). In this sort of thing, you probably have to do some testing.

Perhaps you could find some larger, higher-gain dishes (accurate pointing can be difficult, though).

The traditional solution to this problem is to add a third site and bounce through it (like that red star in the upper-left corner). Of course that may not be easy.

I'm skeptical that your diversity antenna would help you much. My gut feel is you're probably better off running 2x the power through a single antenna.

--jhawk

Posted by Daryl C. W. O'Shea (Member # 1303) on 12-11-2003, 11:49 AM:

Standard Canadian Ministry of Natural Resources forest... coniferous trees planted on a 6' by 6' grid. This particular forest is Red Pine if I remember correctly.

Trying to go through them isn't going to happen. Pine forests generally have a 1.1dB/m path loss... 1200dB.

A third site isn't an option since both sites are already the highest points in the area. The straight path is actually the clearest path, I didn't draw in the other treed areas.

I have considered a passive re-transmitter though. If I can build one cheap enough for it to be destroyed and or stolen I could climb some trees. Considering the Midland Point is surrounded by water on four sides by a Great Lake, I don't think mounting a passive transmitter in a tree would work to well... it gets a little windy.

If OFDM will work, given enough power, increasing the effective power output is certainly possible (I could use a C-BAND dish at the house). However, the whole point of OFDM is to take advantage of the strongest multi-path signal, so I figure separating the antenna might increase the chance that one of the multi-path signals makes it. Although given the separation relative to the distance, it might not help much at all.

I'm pretty sure that there's not a snowballs chance in hell that any other modulation method will work since the signal would be useless due to multi-path issues.

Posted by Ian Price (Member # 14) on 12-11-2003, 12:00 PM:

I'm sorry, I stopped listening after you said that you live on an airport.

How cool is that?

First Dirk Pitt and now Daryl C. W. O'Shea.

Posted by John Pytlak (Member # 331) on 12-11-2003, 01:07 PM:

Is there a tall building that you could turn into a reflector, or mount a reflector, like at the corner of Sandy Bay Road and Fuller Avenue? What's along McDonald Road? Is Hilltop Drive behind you really on a hill? How high?

Posted by Paul Mayer (Member # 355) on 12-11-2003, 01:22 PM:

'Tis very cool to live on an airport--here's the one I've had staked out ever since Orville signed me off: Pine Mountain Lake, Groveland, CA

Regarding Daryl's project, I can't add any useful info but I have some questions and a comment. First, what polarization scheme are you planning? Circular, or linear at some angle other than vertical?

Second, OFDM should indeed give you good multipath performance assuming you can get enough signal into your receivers. A version of OFDM called COFDM (Coded) is what most of the world went with for ATV/ATSC terrestrial broadcasts. Field tests have shown very solid COFDM performance in highly multipath-prone areas (the US FCC chose to go with 8-VSB which doesn't do multipath so well). Is this a homebrew project or is the RF gear to implement this readily available?

[ 05-10-2007, 01:34 AM: Message edited by: Paul Mayer ]

Posted by Daryl C. W. O'Shea (Member # 1303) on 12-11-2003, 02:00 PM:

Absolutely nothing to bounce off of. [Frown]

I'm planning on using some D-Link Access Points with bridge capabilities. I've had luck with using consumer access points for similar distance line-of-sight links in the past... if it weren't for that damned forest.

Unfortunately D-Link's 200mW transceivers don't support OFDM. The OFDM transceivers are only 32mW. [Frown]

I'll have to crack one open to see if they're combinable. [evil]

Linksys sells an inexpensive amplifier which might be handy too.

If I have to I might buy some Orinco transceivers. I sure as heck can't afford a 3Com or Cisco one though.

For antenna... I'm going to build my own. I figured I'd use horizontal polarization on my directional parabolic dishes, unless someone has a better idea to improve the chances of me clearing the obstruction.

Now I'm just hoping somebody will have some thoughts on the chance of the signal going up and over this forest before I go spending a bunch of cash on equipment.

Posted by Randy Stankey (Member # 64) on 12-11-2003, 03:49 PM:

I've had some luck getting my Linksys wireless router to go through cement block walls using a homemade can-tenna. I beam the signal out of my office (drywall/stud construction) through the upstage wall of the auditorium (blocks) and to the back row of the balcony. That's over a 200 foot throw. Not bad, I think.

I have read about some people using can-tennas and old satellite dishes to extend their wireless signal up to several miles. There's no harm in trying if you're willing to experiment.

Did I hear somebody mention living at the airport?

Erie International/Tom Ridge Field

Posted by Daryl C. W. O'Shea (Member # 1303) on 12-12-2003, 12:31 PM:

I'm not worried about a consumer access point not working. I've used them before in long distance LoS applications. I really just need to come up with a ballpark path loss for my non-LoS link so that I can determine how much antenna gain I need.

I recall reading that L = 32.4 + 40log(f) + 40log(d) might be a good approximation for transmitting through a treed area. I have no idea if that's really valid though. Does it sound familiar to anyone?

Hmm... maybe I should just rent a ground-cable layer and stitch 8000 feet of wire into the ground and use a pair of xDSL modems.

Posted by John Pytlak (Member # 331) on 12-12-2003, 12:57 PM:

quote:
Hmm... maybe I should just rent a ground-cable layer and stitch 8000 feet of wire into the ground and use a pair of xDSL modems.

If you're laying cable, why not fiber? [Smile]

Still bet you could be creative in finding a place for a reflector. Or find a cell phone company who wants to build a tower on your property and has the clout to get it approved, so you can mount your dish on it, or even provide your link as part of the lease agreement.

Posted by Paul Trimboli (Member # 1509) on 12-13-2003, 01:48 AM:

Well In Perth (Australia) it is a hobby of people to set up wireless networks and we have losts of them running all set up by hobbiests. Things they have built themselves, and losts of people are now connected to the servers and play games and share files all without the net! a good site for this stuff is http://www.e3.com.au/

Posted by Daryl C. W. O'Shea (Member # 1303) on 02-24-2004, 08:05 PM:

In case anybody was interested... here's some pictures of the site.

Point 1 is on the left, point 2 is on the right.

Point 1... that water tower in to the north east of point 1. While knocking snow off of the plane's skiis with the treetop's I noticed you can just see the top of the tower. I don't like the bounce angle at all... but it might be something to try.

Point 2. Point 1 is to the west (right).

Those trees inline with the hanger and the water tower get pretty high. The house might just miss them.

Posted by Gary Crawford (Member # 2038) on 02-25-2004, 01:53 PM:

Daryl, this might be "out there" a little but maybe worth looking into. You might look to area amateur radio operators to help. In my area of the world hams are always looking for places they can set up on a reasonably permanent basis. A lot of these guys are shooting birds (satellites) so maybe, in exchange for a place to do it, you could operate a package on a satellite, you get a slice of a transponder in exchange for you providing a place they can establish an uplink. In the US the ARRL has dedicated satellite space they can access so maybe you can team up with them somehow.

Otherwise LOS is going to give you fits unless you can fire over the obstacles somehow.

Posted by Daryl C. W. O'Shea (Member # 1303) on 02-26-2004, 05:19 AM:

There's not exactly a lot of HAM radio people around... in fact I can count them all (that I know of) on one hand.

One has a fairly large tower at his farm. Another has a tower at a government funded wildlife preserve / public education centre, and another had a huge antenna array / tower that bounces signals off of the moon.

John, I must have replied to your first post before you finished editing it back in December... McDonald Road doesn't actually exist, Hilltop Drive is indeed at the top of a hill, but that hill is about 75 ft lower than the ground the two points are at. Fuller Avenue & Sandy Bay Road (and that entire area) is just more trees (at least until the land drops away on the other side).

I came across this "multi polarized" antenna. I'm pretty skeptical about their claims, but maybe it's something to try. It'll have to wait though... $600 CDN for a pair of antenna isn't going to happen anytime before the summer (damn tuition).

Posted by William Hooper (Member # 71) on 02-26-2004, 06:51 AM:

quote:
I came across this "multi polarized" antenna.

Who wants to wear those stupid glasses while they listen to the radio?

Posted by System Notices (Member # 2357) on 04-17-2007, 12:34 PM:

It has been 1146 days since the last post.

Posted by Daryl C. W. O'Shea (Member # 1303) on 04-17-2007, 12:34 PM:

I finally got around to setting this wireless link up. Even after moving the factory down the street to get fewer trees in the way (ambitious eh?) I still couldn't get high enough to get a link running between the house and factory.

So... plan B... give up on free internet access (well access you're already paying for) and point somewhere else (and pay for their access).

I've now got a link running over a few miles between the red dots using a pair of Linksys WRT54GL wireless access points running OpenWRT and a pair of Pacific Wireless 24 dBi grid antennas (mounted for vertical polarization simply because they look nicer that way). EIRP is a little under 4 watts. SNR is around 30 dB.

The same gear did a ~15 mile link for fun between the blue dots.

Posted by Paul Mayer (Member # 355) on 04-17-2007, 10:56 PM:

Kewl! The antenna with the windsock almost looks like a do-it-yourself glideslope transmission array... [Smile]

Posted by Daryl C. W. O'Shea (Member # 1303) on 04-17-2007, 11:20 PM:

Hmm... it is conveniently pointed in the right direction.