Radio Interference

Question

Hi,

We are using nRF52832 with a custom protocol communication (not BLE, not Nordic). The communication is simple, one device is sending message and the other is receiving it on the predefined frequency. There is no channel changing. The issue we experience is related to possible interference between channels. The scenario is following:

1. Device "A" and device "B" are communicating on NRF_RADIO->FREQUENCY = 65UL; every 1-5 minute.

2. The distance between "A" and "B" is 15 meters with two walls between them. This positions are kept the same during all described tests.

3. This communication is working just fine with no data loss, once the device "C" described below is off.

4. Additional device "C" is working on different frequency, and sending a data packet every 1-50ms (we tested two options NRF_RADIO->FREQUENCY = 60UL; NRF_RADIO->FREQUENCY = 10UL; and got the same results described below)

5. In case that "C" is sending data every 1 ms, the communication between "A" and "B" is lost and no data is transferred between them. Note that once we put "A" and "B" in 2-3m range communication is fine again with no data loss.

6. In case that "C" is sending data every 50 ms, the communication between "A" and "B" is working but with some data lost (I suppose once it match the moment when "C" is sending). Note that once we put "A" and "B" in 2-3m range communication is fine again with no data loss.

Can the interference from device "C" be so significant even it works on totally different channel/frequency?

Do you have any recommendations for channel/frequency selection, since we did not expect that device on one channel/frequency can influence that much on the device working on totally different one?

We would appreciate any advice or comment that could lead us to solving this issue.

Note - some additional settings that are the same on all devices "A", "B" and "C"

- NRF_RADIO->TXPOWER = (RADIO_TXPOWER_TXPOWER_Pos4dBm << RADIO_TXPOWER_TXPOWER_Pos);

- NRF_RADIO->MODE = (RADIO_MODE_MODE_Ble_1Mbit << RADIO_MODE_MODE_Pos);

Peter_53 · Accepted Answer

Hi Andreas, 
 
 C is indeed close to A. It is on 10cm in one case and 1 - 1.5m in the other. We did not identify any big improvement in case we moved C on 1.5m from A. 
 Thank you for the quick reply. Just to be sure that I do understand the Rx selectivity table right. Rx selectivity tells us how many dBs higher can a blocker signal (C in our case) be comparing to out signal (A and B), right? So for example:

C/I 1MBLE,-2MHz 
 
 1 Msps BLE mode, Adjacent (-2 MHz) interference

-22 
 
 dB

- In this case C signal must be 22dB lower than our signal, right? 
 - Is this the case when C signal frequency is 2MHz lower than A and B? 
 - How can I know which line in this table is referring to our case (what does -2 MHz, +2MHz, >3MHz means)? 
 
 Is the following conclusion correct: If the C is much closer to A, even though they are working on different frequencies, C will block the signal of B trying to send a message to A. 
 
 Thanks!

Andreas · Answer

Hi, 
 
 Your understanding is correct. When C is transmitting at 2MHz lower than B to A, it will block the signal from B to A if it is more than 22 dB stronger than the signal from B to A. Even if moving it 1.5m away from A, it will still be quite strong an potentially block a weak incoming signal. Your application has 5 MHz spacing, so you should refer to the line >3MHz

C/I 1MBLE,>3MHz 
 
 1 Mbps BLE mode, Adjacent (≥3 MHz) interference

-50 
 
 dB

This is a typical spec for all channels. Based on your description A to B is quite 'far' away, the received signal strength is likely quite low, meaning the higher power from the C will be blocking, your conclusion is correct. You either need to make C keep quiet for longer, or even the signal strengths. You could also probably land somewhere in the middle by experimenting with acks and retransmissions. 
 
 Best regards, 
 Andreas

Radio Interference

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