LoRa CRC errors - Page 1
Forum section: electronics / rf-microwave

author: dpape
author_rank: Contributor
timestamp: April 15, 2020, 12:37:49 pm
content: Hello,I have set up a system which works in the 868.1MHz frequency using SX1276 radios on a NiceRF module. I have a receiver which uses a monopole whip antenna and a transmitter which uses a Johanson chip antenna. The gain of the whip antenna is about 2dBi and the gain of the chip antenna about -4dBi. I'm transmitting with +17dBm to get an ERP of about +13dBm (maximum allowed ERP in my area is +14dBm).The payload of the packets that i'm transmitting is 18 bytes and contains some sensor readings and a device address. I transmit an packet every 5 minutes.I've made sure that the settings for receiver and transmitter are equal and both use the same sync word.The system uses SF12, BW125 and CR 4/5 and I get a range of about 2 to 3km line-of-sight. The receiver is placed indoors and the transmitter outdoors.I'm noticing some things which I can't seem to explain:- With a LOS of about 100m I get RSSI of -100 and an SNR between -2 and +2. Are this acceptable values or is RSSI low.- I get occasional CRC errors even with the good SNR values. I have set up multiple receivers and some receivers don't give CRC errors while others do.It seems as if the CRC errors are grouped. The system can work good for days and than all over sudden for 1 hour i get CRC errors after which it starts working again.After reading I suspect the following possiblities:- Because of maybe bad (I'm still waiting for my VNA) antenna matching many of the RF power is reflected back to the SX1276 which causes the packet to be malformed. Is this plausible?- The NiceRF module uses a 10ppm crystal which maybe deviates to much. As both transmitter and receiver have the same crystal with initial 10ppm accuracy and a temperature accuracy of 10 ppm I can theoretically have a 40ppm deviation which is to much for SF12BW125. Is this a possibility?- The antanna system is sensitive to temperature.I'm however puzzled on how to solve this. I can live with an occasional CRC error but not with multiple consecutive CRC errors as this would let the software trigger an error and notify my backend.I'm quite new to the RF world and therefore I'm asking a bit of input to see if my assumptions make any sense.Kind regards,Daan

author: scatha
author_rank: Regular Contributor
timestamp: April 16, 2020, 07:47:00 am
content: I'm not familiar with that module and how it reports SNR/RSSI, but if the system is working out to 2km you should have plenty of link margin at 100m LOS.- A bad antenna match sounds unlikely- An oscillator frequency offset will manifest as an offset in centre frequency; according to the module datasheet it reports the frequency offset of a received frame - you should be able to check if there is any relationship between this value and the frame error rate. Again seems unlikely given the ppm values in the spec but who knows - maybe hit a transmitter with a hot-air gun and see what happens.LoRa doesn't do any clear-channel assessment before transmitting so there could be collisions at the receiver, most likely though there is some localised emitter desensitizing or otherwise interfering with the receiver. Multipath might also be a factor.