Author Topic: How to manage low power, low speed long range RF comms?  (Read 6580 times)

0 Members and 1 Guest are viewing this topic.

Offline RerouterTopic starter

  • Super Contributor
  • ***
  • Posts: 4694
  • Country: au
  • Question Everything... Except This Statement
Re: How to manage low power, low speed long range RF comms?
« Reply #25 on: March 26, 2018, 01:36:04 am »
Wow!, That is a lot of input, Thank you all!  ^-^

Ok starting from the top:

you said 220m? sensors could be in 400m range? line of sight?

It would be 80 at most, and the general use case would be within 100m, however I am hoping to design in some margin, they will be in a flat field, with some burned shrubs

Pinging the nodes to request data may be problematic, Can you have a mains powered aggregator/gateway that would collect somewhat randomly timed node transmissions at a site, and then do a drive-by-query of that at a higher bandwidth?

Can you add a solar panel to each node, and only try to collect data when it is sunny?

I can definatly have an agregator node, even using much larger batteries (say 3x 18650 cells), however not mains powered, Solar panel is unknown, as the area may have dust storms, still the sensors would need to talk to this aggregator.

The nodes have a unique serial number programmed for identification.

What is the application?

reasonable to assume that you can transfer 2kBy in 8.6 seconds of TX time or less which would be an effective bit rate of ~2kbits/second net.  It only gets better from there if you can transmit and shut up even faster.

if they detect the collection unit's transmission would they transition into a mode where they communicate their data with the collector over the next few seconds to allow all of the nodes time to transfer their data.

The application is ground water, salinity and composition sensors that are driven in to the soil. and are built to be both long lasting and fairly cheap.

If I go with the above suggestion of an aggregator, then a longer receive window should be possible, they are running off watch crystals, so accurately timed receive windows for the agregator should be possible.

To the others, I have internal timers that I can use for arbitration should 2 conflict, however if there is an existing protocol I would probably trust that more than my own dreams,

As for with no arbritation node, My previous experience with multidrop RS485 was if the Id packet was garbled, the base node sent back an error, and the effected nodes xor'ed there timer value by there ID and waited that long. and tried again.
 

Offline ogden

  • Super Contributor
  • ***
  • Posts: 3731
  • Country: lv
Re: How to manage low power, low speed long range RF comms?
« Reply #26 on: March 26, 2018, 02:08:26 am »
If I go with the above suggestion of an aggregator, then a longer receive window should be possible, they are running off watch crystals, so accurately timed receive windows for the agregator should be possible.

In case you have aggregator node which can continuosly run receiver @20mA+processing_overhead in receive+ACK mode, it can solve a lot of sensor communication difficulties. Just don't pick too narrowband channel for sensor comms because such requires high precision TCXO crystals which are not cheap.
 

Online Marco

  • Super Contributor
  • ***
  • Posts: 6693
  • Country: nl
Re: How to manage low power, low speed long range RF comms?
« Reply #27 on: March 26, 2018, 05:45:28 am »
however if there is an existing protocol I would probably trust that more than my own dreams
You'll probably still want to roll your own protocol because your use case is unique. Open sores code for configuration/interrupt/packethandling can be found for some modules.

Just don't pick too narrowband channel for sensor comms because such requires high precision TCXO crystals which are not cheap.

Semtech has a suggestion to work around this ... the low power node simply walks the frequencies to lock in.



PS. how do I link a YT video as a normal link instead of embedding the video?
« Last Edit: March 26, 2018, 05:48:24 am by Marco »
 

Offline ogden

  • Super Contributor
  • ***
  • Posts: 3731
  • Country: lv
Re: How to manage low power, low speed long range RF comms?
« Reply #28 on: March 26, 2018, 10:42:00 am »
Just don't pick too narrowband channel for sensor comms because such requires high precision TCXO crystals which are not cheap.

Semtech has a suggestion to work around this ... the low power node simply walks the frequencies to lock in.

You mean SiLabs? Well... They are supposedly solving problem on 169MHz which is rarely used. Better they tell how to use cheap crystals @200Hz bandwidth on 866MHz :)

Let's check: 11 channels * 180Hz spacing = 1980Hz. Worst case allowable drift for two crystals 1980/2=990Hz. This is ~6ppm for 169MHz. I would say it's not "standard crystal" at all :)

Spectrum analyzer settings and screen (video time 3:10)  is especially interesting: central frequency 3.4GHz, span 100KHz RBW 910Hz. WTF?!

Perhaps RX synthesizer spectrum picture was so bad that they had to fake it
[edit] They are showing RX VCO frequency, not LO. Then SA settings makes sense indeed.

« Last Edit: March 26, 2018, 11:00:53 am by ogden »
 

Offline Koen

  • Frequent Contributor
  • **
  • Posts: 502
Re: How to manage low power, low speed long range RF comms?
« Reply #29 on: March 26, 2018, 10:54:38 am »
If you wish to get going, grab some SX127x or some HopeRF modules including it and go. You'll be able to try LoRa or revert to FSK, GFSK, MSK, GMSK and OOK. LoRa is quite tolerant of beginner's mistakes and cheap XOs. It would be hard not to achieve 200m.

Answers above have LoRa (modulation) confused with LoraWAN (subscription-based networks) which you don't need.
 

Offline rhb

  • Super Contributor
  • ***
  • Posts: 3476
  • Country: us
Re: How to manage low power, low speed long range RF comms?
« Reply #30 on: March 26, 2018, 01:21:39 pm »
A friend of mine designed such a device as a retrofit for water meters.  He managed to achieve a 15 year design life for the battery.  Not easy.  He was rather proud of it when he showed it to me. He used an MSP430 for the MCU.  I don't know what he did for the RF part.  The whole thing was the size of a silver dollar.
 

Online Marco

  • Super Contributor
  • ***
  • Posts: 6693
  • Country: nl
Re: How to manage low power, low speed long range RF comms?
« Reply #31 on: March 26, 2018, 01:51:53 pm »
RFM95 915MHz uses a rebadged SX1276 BTW.
« Last Edit: March 26, 2018, 01:53:34 pm by Marco »
 

Offline Lord of nothing

  • Super Contributor
  • ***
  • Posts: 1581
  • Country: at
Re: How to manage low power, low speed long range RF comms?
« Reply #32 on: January 18, 2019, 12:43:51 am »
does anyone here use 169 MHz?
That was the only tread I could found who is talking about.
Duty cycle limit of 0.1 %. -->  :=\
Made in Japan, destroyed in Sulz im Wienerwald.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf