915 MHz Ambient Signals: Possibility of Interference

[john k2ox]

Ever wonder why your Moteino sometimes needs more than one try to communicate?  It just could be that it's being interfered with. 

Way back when, the FCC said you could only transmit once for a few milliseconds, every 20 seconds.  The idea being: a smaller chance of different devices jamming each other.

The current rules are that you have to be at least 500 KHz wide or use a pseudo random frequency hopping scheme for unlicensed operation.  In the case of wide-band FSK the TX power is spread out so that there is very little power at any one frequency.  For randomly hopping signals the chance of them being at the same place(freq) in time is very small.  And, much, much smaller that they collide more than once.

Attached is a picture of a 10 minute capture of the 915 MHz signals at my house.  There are a lot more signals out there than I imagined. Notice how many are above -80dbm.

The second picture shows 20 seconds of capture.  Just below 915 MHz are some fast hopping radios that are continuously talking back and forth.  They never go away.  I set my Motenio's to use the quieter 915.05 MHz frequency.  That's it in the center of the first picture.

The third is about 10 or so seconds with the white trace showing the latest.

The last picture shows a 5mS capture.  In this screen shot you can see how it is possible that all the radios in my area are able to get along.  BTW, 5ms is about the shortest transmission a Moteino will make with Felix's default settings.  Now you know why he worked so hard on the 'send with retries' code.

Regards,
john





Moteino 905mhz to 925mhz 10min A by Felix Rusu, on Flickr

[Felix]

Very insightful. Now that they installed smart meters on my home that work in the 915mhz band im sure that spectrum has more traffic. I wonder how the 2.4ghz bands compare to this, i'm sure a lot heavier traffic.
Any comments on how well Moteinos at 915mhz "comply"?

[john k2ox]

I'll take a look.  If they don't it's a simple fix.  More deviation.

Keep up the good work!

[hneiraf]

The FCC apply the same rule to 2.4Ghz?

(have to be at least 500 KHz wide or use a pseudo random frequency hopping scheme)

Best,

HN

[john k2ox]

I don't know the rules for 2.5 GHz. I do know that the path loss is much greater. So for the same modulation and power you get less range. The plus is smaller resonant antennas and 2.5 is opened for unlicensed operation world wide.

In the USA it is a freq shared with the government and you have to shut down automatically if they decide to use the frequency.