Thursday, March 13, 2014

RF Propagation Modeling & a Field Test

One of the biggest risks of this project hinges on the fact that the remote weather station needs to be able to connect back to my base station, or it has no chance of success. Given that, I decided to undertake some RF Propagation Modeling to boost my confidence in the final system as well as determine such things as antenna placement (elevation angle/direction) and antenna type.

Due to the miracles of the internet (and procrastination), the blog is slightly behind the reality of where my project is, so I captured below the RF Modeling that took place (spoiler alert) and the field test to check the RF Modeling. As you might guess, the field test only only took place since there was at least a  glimmer of hope (Read: ~10dbm wiggle room) that the radios would connect.

After getting the DNT900 and RPi's working together nicely, it seemed a prudent time to do some more research before investing in all the other ancillary hardware to pull the remote weather station off.  Having some notion of the concept of RF Propagation Modeling, I began to search online for some tools that might help with this. (Perhaps if I had been a classically trained engineer I could have determined my link budget with pen and paper, but that is not my case, and I generally like pretty pictures and well made software.)

A search online yielded an helpful presentation(PDF) from John Hopkins ARL on Modeling and Simulation for RF Propagation. For those looking for a technical review on the subject, check it out, good read.  I happened to also use it for some ideas for possible tools to do the modeling.  Page 16 of the presentation has all sorts of suggestions. My initial criteria was simple: free (or free during trial) & decent usability (or good reference materials). Luckily I didn't have to explore for too long before finding a great solution: SPLAT!  The balance of this post includes output from SPLAT! & Google Earth.

I will also mention I tried  a software package called TAP (Softwright Terrain Analysis Package) but found it too difficult to ramp-up on (perhaps it is meant for the a very patient and forgiving  professional), I only gave it an hour of my time after install before bailing with nothing to show for it.) Perhaps not fair to TAP, but I was looking for a fairly simple answer/analysis, and the work to get that from the tool was too painful.

Some inputs to SPLAT! Included knowing the location of my antennas, EIRP/ERP, local terrain data (SPLAT! provides instructions to import); also referenced dB/Watt conversion  and Free Space Loss for sanity checks.

Going in I saw the DNT900P data sheet made claims about 40+  Line-of-Sight (LOS) coverage, an item that I needed to check for my setup. The data sheet also contains helpful nuggets about receiver sensitivities (the later being something closely looked at during this analysis). I first used Google Maps to determine the Lat/Long of my Base and Remote antennas. I also found this Elevation Profile view helpful to casually observe possible obstacles and get a feel for the elevation that would play into my ability to (not) achieve LOS.

So in summary so far:
Base: Elevation 289ft  (actually ~ +40 in attic  ) ~329
Remote: Elevation:   328 (if I can get +10 feet off the ground)
Distance 1.64 Miles

Using Google Maps Ruler tools I also was able to find the heading needed for each Antenna. (Nice to have in hand & to check against SPLAT! values, which concurred.)

Base to remote: ~192 degrees.
Remote to base: ~12 degrees

Getting going onto SPLAT! just required a few files, of which I provide generic examples of here. The actual SPLAT! site details these files in greater detail but feel free experiment with these if it can get you using the tool any quicker. I chose to Model Omni Directional antennas for simplicity.Here are some of the SPLAT! outputs.

Below we see the anticipated receive dBm of the remote, as related to the base:

Below we see the anticipated receive dBm of the base, as related to the remote:

Pictures are valuable, but SPLAT! also provides a detailed report, here is a sample from a similar model to shown above.

From the prior data I get a feel for the impact of not achieving LOS and anticipated receiver strengths for the sights, all just barely within spec of DNT900 which called for -98 at a 200 kb/s data rate. The input values could be tweaked to achieve the desired result (aka adjusting elevation and antenna gain where necessary).

SPLAT! also puts out some data in a convenient KML format which actually does a nice job visualizing the LOS obstacles. (Base to remote LOS obstacles, followed by remote to base below.)

At this point, I consulted a few co-workers who encouraged me that lack of LOS was not a show-stopper, but given the narrow margin of theoretical receive dBm (on both ends), as compared to DNT900 radio sensitivity it was a sketchy area (that could be impacted/fail on loss from poor cabling/connector choices, vegetation, precipitation, or un-describable obstructions*.) The model takes most of that into account, but it is a difficult to know how accurate is compared to the real world.  *Due to HOA rules my home-base antenna location is less than ideal. It is inside my attic, so I have the unknown variable of plywood and tar shingles in the mix. I think I ball-parked  an additional -20dBm for this in my model.

Being 'forever the optimist', I felt moderately confident to purchase my antennas and get ready for a field test. Installing my home-base station (RPi + RPi900 + DNT900) was abit cumbersome in the attic, requiring power and network to be run. Network simplified by using an old SlingNet Ethernet-over-power device. I wasn't too worried about the bandwidth limitations of such an install, as my bottle neck for data is really between my remote and base station.

With my base station in place, I prepped my remote station for a field trip to the garden. The setup was crude, but effective. I mounted the antenna, battery-pack and  RPi remote system (RPi + RPi900 + DNT900) to a good 'ol Hula Hoa (which from a gardening perspective, I also highly recommend.) The remote station-on-a-stick setup was to facilitate testing different locations and elevations within the garden.

As I ventured off to the garden, I had my wife monitoring a computer at home logged into my base station. I already configured my RPi900's to establish a PPP network, so she was monitoring a ping I had going to the remote unit I carried. I was keeping an eye on the RPi900 indicator lights to know when to ask her to pay closer attention. Here is an one of the first examples of the system establishing a PPP network connection, over the 1.64 mile RF connection:

From basepi (192.168.1.###) icmp_seq=1988 Destination Host Unreachable...........
64 bytes from remotepi (192.168.1.###): icmp_seq=2008 ttl=64 time=49.3 ms
64 bytes from remotepi (192.168.1.###): icmp_seq=2010 ttl=64 time=60.4 ms

When confident the DNT900 radio's were linked (steady green light) the dnt900 line discipline was used to check the receive rssi (signal strength) at the base, of the remote RPi I had in the field (garden).

[@basepi ~]# cat /sys/class/dnt900/ttyAMA0/0x00####/rssi
[@basepi ~]# cat /sys/class/dnt900/ttyAMA0/0x00####/rssi

A shot of the field test setup above when working (perched about 10 feet above my head) on a convenient pole on the perimeter of the garden.  From the field test I learned the very center of the garden was pretty much an RF nightmare. All sorts of metal caging around the individual plots, and the large deer fence around the parameter of the garden acting like a large Faraday cage (I believed.) My success was on the perimeter of the garden - getting in-front of, and above, those immediate impediments. (The SPLAT! modeling did in-fact predict the need for greater antenna height at the remote sight then I orginally planned.)

I am going to claim success on this field test, and endorse SPLAT! since it was extremely close in it's prediction (of between -80 to -90 dBm receive strength), and pretty easy to use.  I was able to find a few potential locations for the remote weather station at the garden, all establishing network connections at the 2nd lowest data rate (with approximately 7 dBm to spare.)

I also still have the option to drop down to an even lower data rate (38.4 kb/s and gain an additional 10 dBm of operability), since the radios are sensitive to -108 dBm, which seems believable given the performance at 200 kb/s.  I think I can also eek out a few dBm in both RPi900/antenna setups by ordering high-quality (lower loss) cable, and limiting some adapters in the mix.

 I didn't  explore the DNT900 claims of 40+ miles of LOS operation from a real world test, or even bother modeling it. I'd imagine it could be possible  in perfect/ideal conditions, or maybe in airborne/ground station setups. Either way, I'd suggest everyone should model their respective communications TX/RX setups (even on an existing functional system), I found it a helpful and informative exercise.

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