Other Parts Discussed in Thread: CC1310,
Hi,
We've been using cc1310's for years and have a large established client base in Europe. We have now started selling in the US and have come across a few problem sites where the noise floor across the whole 915MHz spectrum is around -80dBm.
We have conducted many tests with variations in the bandwidth, frequency, packet size and baud rate. Bottom line is that on our preferred data rate (38.4kbps) we are unable to achieve more than around a 50m range with clear LOS. Inside a building, where the noise floor is attenuated, we can achieve about 150m.
The noise source is a bit of a mystery, we have tried to isolate other 915MHz protocols, but there is nothing obvious. Perhaps LoRaWAN from oil and gas drilling rigs within a few miles away, though the noise does exceed the typical channels that LoRaWAN might use. The site is very remote, 30minutes drive to the nearest town, in the middle of nowhere. At this point we're also not ruling out environmental issues (like high iron content in the ground) as phone reception is very poor despite high signal strengths.
The two main FCC $15.247 compliant strategies we have tried are:
- WB-DSSS (x8) with a deviation of 195KHz, rx windows 622KHz.
- 2-GFSK, with a deviation of 250KHz, rx window 622KHz (seems to give 500KHz bandwidth at 6dB).
However, neither achieve any more than ~50m and reception is lost when the RSSI reaches the noise floor, around the -80dBm.
Now using narrow band EU style frequencies work wonderfully across the whole 915 spectrum:
- 2-GFSK, with a deviation of 19.2KHz, rx window of 98KHz. This gives us a range of about 200m in the same noisy environment, but is of course not a legal profile.
My hypothesis is that the size of the receive window is simply so large that it's a funnel for all the local noise. I have done some testing on this, but it's a little crazy and is all to shoehorn a solution where we transmit on a wide bandwidth to comply with FCC, but then receive on a very narrow band.
One approach I have been looking at is to transmit on 2-GFSK (250KHz deviation) and receive using a narrow band OOK on one of the peaks of the 2-GFSK signal. This strategy does appear to work with some limitation:
- OOK won't go over 10kbps reliably
- OOK does not receive the data correctly if the 2-GFSK deviation is greater than about 60-70KHz.
It works surprisingly well for a bitrate of 10kbps with the GFSK deviation set to 50KHz, where the OOK listens on the GFSK centre frequency + 50KHz. However, the more the baudrate or the transmitting deviation are increased, the higher the packet error.
Two other solutions might be:
- Use OOK, with the signal shaped to transmit over a 500KHz bandwidth.
- Artificially increase the tx bandwidth for 2-GFSK without increasing the deviation beyond a narrow band.
To my knowledge, these solutions are both beyond what your API of the cc1310 radio can achieve.
So, finally, here are my questions:
- Is that a reasonable strategy? Or is there some other-way to get more reliability with a high noise floor?
- How can I get OOK working at 38.4kbps, receiving one peak of the 2-gfsk at a deviation of 250KHz? From the forum, I hear that OOK is not very well supported your end.
- Is there another way to transmit in an FCC compliant way, but receive the same signal on a narrow receive filter BW?
NB: We see no difference, using the LAUNCHXL-CC1310 or our H/W, they behave identically.
Thank you for any help or insight.
Sincerely,
- Oliver
