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Lora range tests  XML
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GeorgeR



Joined: 10/01/2015 02:38:33
Messages: 3
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Hi, has there been any range tests on the rfm95-98 units inLoRa modulation and a comparative test on the rfm69h w. Many thanks
stevech



Joined: 07/07/2014 18:07:27
Messages: 91
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very much dependent on your application's required data rate (bps)
GeorgeR



Joined: 10/01/2015 02:38:33
Messages: 3
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Yep, thanks for the insight, lets then clarify in a Use Case scenario to avoid the obvious.

Radios: RFM 95 W in LoRa Modulation on 433/868 or 915Mhz.
Data Rate: 1.2Kbs or approx 1200 bits per second.
What was the Line Of Sight result and what was the through dense brush/buildings result?

All I am asking is have there been range tests and under what conditions and what data rates and what frequency were the tests done at, not an unfair question, I do want to buy the product but need to know something about the unit capabilities vs the radio manufacturers suggested capabilities to see if it suits my requirement.

This message was edited 1 time. Last update was at 11/01/2015 00:44:30

stevech



Joined: 07/07/2014 18:07:27
Messages: 91
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we can do an educated guess on line of sight (LOS) range at modulation for 1200bps. It'll be better at 433MHz than 900MHz. But US 900MHz regulations are more accommodating of transmitter on-time/duty-cycle.
For non-line-of-sight (NLOS), you'll have to determine this experimentally as the blocking material-types vary widely.

I have some LoRa radios but no time now to make measurements.

Here's one quick SWAG
433MHz
Antenna gain at both ends: -3dBi (lossy, assuming crude antennas)
TX Power 20dBm (0.1Watt)
Path length: 1 mile LOS
Fade margin: +24dB (good)
Receiver sensitivity: -100dBm for a reasonable packet error rate at 1200bps, spread spectrum. You can go to -105 or -110 if you dare. Receiver sensitivity spec. has to be in context of packet SIZE and packet error rate. More bits in packet, more chance of bit error. Some protocols (like RadioHead's Reliable Datagram) have error correction via ACKs and retransmission. This effectively improves the required received signal strength OR allows larger packets, if the margin is low.

Required antenna height BAD NEWS: to clear 1st Fresnel zone at 1 mile path length: 50 ft.
Using lower antenna height still LOS, yields less fade margin. At 0.1 mile LOS, required height is 10 ft.
The Fresnel zone losses are because RF power "spreads" in space as distances increases. Visualize two antennas, your view is broadside. The Fresnel zone is a football shaped "cloud" of RF power. The Fresnel zone at the half-way point of the path, on flat terrain, touches the earth. The longer the path, leaving antenna height the same, the more occluded the "football" shape is - and that RF power is lost to absorption by the earth. The common design for point to point links is to keep the loss from the Fresnel zone to less than 30% of the power.

Also, the fatness of the shape changes with frequency; smaller (better) at, say, 2.4GHz. But at the higher freq., the path loss increases. And then.. it's much easier to get, say, 10dBi of antenna gain at each end (total 20dB benefit) at 2.4GHz than at 433MHz - due to the size of the antennas (smaller at the higher freq). A 10dBi gain at 433MHz is a large antenna, say, a 6 ft. long yagii boom. At 2.4GHz, 10dBi is like 8 inches of yagii, or a square patch antenna.

This sum of gains and losses is called a link budget. One uses a link budget with a certain fade margin according to how LOS the link is, path length, and frequency, and tolerable packet error rate. A good receiver product will spec the error rate vs. each modulation mode (bit rate) in the form of packet error rate (PER) for a 1% rate, and a certain number of bits in the packet, and for each coding (FEC) rate option. Cheap radios normally have no FEC.

Last point: The LoRa's I've seen use spread spectrum and "long" spreading codes and get their best range at speeds much lower than 1200. Spread spectrum adds a bout 6dB of benefit to the link budget. This is called post-correlation gain and it comes from the "correlation gain" of a spread spectrum receiver.

To all this, we have to add allowances for interference from other radio systems, or from other stations of our own system.

So, it's quite a balancing act.
The key issues are the packet size, use of (or not) of forward error correction bits (FEC, aka. coding), or error correction from ACKs and retransmissions.

"Wireless isn't a hundred times harder than wired, it's a million times harder". Professor Paulraj, Stanford.




This message was edited 8 times. Last update was at 11/01/2015 15:36:13

GeorgeR



Joined: 10/01/2015 02:38:33
Messages: 3
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Hi @stevech

Thanks for the detailed reply, I have looked into the RadioHead library and the reliadble datagram protocol which makes it one of the most suited to my requirement.

I have read the Semtech LoRa review and they are talking 2.5Km's in builtup (high rise) areas and +-15Km's in line of sight open area's with the SX1301 chip (data concentrator as a base station) which is impressive.

If I had an idea of the range a Anarduino LoRa board is capable of then I will know what to expect in the field give or take a few unexpected events (I would have thought tests like these would be carried out by Anarduino as it instills confidence and details the expectation to prospective clients), the frm69 boards have AES128 which provides a level of data protection which the LoRa protocol does not offer, can I do say 2.5 to 5 Km's on a RFM69 board with a small structured payload message of no longer than 100 bytes, acknowledge from the receiver required.

There are a number of shortcoming from my readings, the RFM95-98 radios dont have any level of encryption but do have hardware crc, might be part of the power savings regime, which leaves the consideration of data protection vs range, where perhaps both are required simplifies the decision, but in my case either could apply but would need to know the expected ranges.

Regards
stevech



Joined: 07/07/2014 18:07:27
Messages: 91
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I think you'll find that the Km and more ranges in cluttered urban - refers to the 100bps or so bit rates. Lots of marketing liberties taken.
Low bit rates and spread spectrum and forward error correction bits permit it to run at a -110dBm, -120dBm or so. But if there's interference from other radio systems, that'll not let the -110 or so work.
But a carefully chosen frequency in the 900MHz band might find little interference, or infrequent interference. Avoid the top of the band due to paging systems just above 928MHz that slop over.
The SCADA systems kind of dominate this band. Supervisory Control And Data. The little antennas you see on big power transformers, some traffic signal controller boxes, and others, are SCADA. The good news with SCADA is that they tansmit infrequently and briefly.

In the US 433MHz band, there are lots of odd radios. And there are stringent FCC regulations on how often the transmitter can be on and for how long. Not that the FCC will take you to jail - they respond sometimes only after repeated complaints of interference by licensed users of the same band that also permits unlicensed use.

Beware making brief measurements and basing a conclusion on that. Unlicensed bands are famous for having frequently changing conditions.
DanPea


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Joined: 01/11/2015 16:41:12
Messages: 5
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Using two RFM95 miniwireless modules I have conducted numerous long range LOS tests.

These really are great little boards.

To date the furthest I have had a two way link reliably communicating was over 20km. The Arduino code I had running indicated both packet RSSi and background noise floor.

At 20km I still had over 20dB of SNR to hand - my biggest problem was finding suitable elevated locations but this was much help ed by the use of the following web site.


http://www.solwise.co.uk/link-budget.htm

In a rural setting I have got about 800m but this was using crude untuned wire antennas at 1m elevation and non optimal modem setting.

 
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