S2-LP - PQI stops RX timer at wrong channel

[pelgrimo]

Greetings,

I'm working on a project where S2-LP scans multiple channels and receives a packet when a valid preamble is detected. For dwell time timing I'm using the RX timer. The timer is set to 2 milliseconds and is stopped whenever a valid preamble over certain signal level is detected to allow packet reception. Whenever the timer expires or a valid packet is received the next channel is selected (cycles from 1 to 10, then the cycle repeats) 

if(chSwitch)
{
 chSwitch = false;
 newChan = S2LPRadioGetChannel() + 1;
 if(newChan > 10)
 newChan = 1;
 subGHzSetChannel(newChan);
}

chSwitch flag is set in interrupt handler whenever the RX timer expires, or a valid packet is received.

Channel switching function:

void subGHzSetChannel(uint8_t chNum)
{
 S2LPCmdStrobeCommand(CMD_SABORT);
 S2LPRadioSetChannel(chNum);
 S2LPCmdStrobeCommand(CMD_RX);
}

Radio settings:

• Base frequency: 915.5 MHz
• Data rate: 10 kb/s
• Frequency deviation: 20 kHz
• Channel filter: 65 kHz
• Channel spacing: 250 kHz
• Modulation: 2-GFSK1
• RX timer stop condition: CS AND PQI
• PQI threshold = 4 transition (PQI_TH = 1)
• RSSI threshold: -90 dBm

The receiver is a custom board. For transmitter I'm using STEVAL-FKI868V2.

The test: The receiver cycles through RX channels as described above. Transmitter transmits the following packet (transmit time = 51.2 ms) on channel 5 every 100 ms:

The preamble length is set to cover dwell time of all ten channels + 1 dwell time + channel switching time.

The problem is that the timer is occasionally stopped at adjacent channels 3, 4, 6 and 7.

What happens then:

• The timer doesn't expire
• No valid packet is detected
• The application is stuck until I set chSwitch flag manually o by a separate timeout
• When I read LINK_QUALIF2 register (QPI value), it's indeed above QPI_TH.

I also tested transmission on different channels; the timer is always stopped at -2/+2 adjacent channels to the TX channel. The problem goes away when I increase PQI_TH value, but ideally, I don't want PQI_TH higher than 1, because it forces longer dwell time and longer preamble which affects the transmitter power consumption. Also, increasing PQI_TH feels like walkaround, because why would preamble be detected (RX timer stopped) on a wrong channel in the first place?

My question is, what would cause such a behavior and how to solve the issue? Has anyone encountered a similar behavior?

Thanks,

Marcin

 

[pelgrimo]

Update: Increasing PQI_TH seems the be the only way to fix the problem.
Update 2: During transmission on channel N, the noise floor is slightly elevated on adjacent channels N - 1 and N + 1. The noise was captured by the radio and falsely interpreted as valid preamble, so to avoid that I also increased RSSI_TH together with PQI_TH. The system works now, but it costs me some link budget unfortunately.