S2LP Sniff mode: Packet drop issue [ S2868A2 + NUCLEO-U083RC (868 MHz) ]

[Errorr__]

Hi all,

I’m currently working on a wireless mesh network project using the S2868A2 expansion board with the NUCLEO-U083RC, operating in the 868 MHz ISM band.

This system is designed to be low-power, which is why I’ve selected the Sniff Mode (LDCR) feature of the S2-LP. My goal is to achieve a reliable connection with >200 m LoS range, but currently, I’m seeing packet drops even at short distances (50–60 cm).

I’ve been debugging this issue for the past few months without much success. I’d really appreciate it if someone could help review my firmware setup and configurations, or share best practices for using sniff mode effectively.

This post is organized as follows:

1. A brief system overview
2. S2-LP parameter configurations
3. Issues observed
4. Debugging steps and tests performed

1. Brief System Overview

•  I have used the X-CUBE-SUBG2 to communicate with S2-LP hardware.
• Each node acts as a transceiver, toggling between TX and RX.
• This setup is based on SDK_Sniff_A/B examples from ST.
• I’m using stack packet format, with Auto ACK and piggybacking disabled.
• The payload size is between 35-50 Bytes

2. S2-LP Parameter Configurations

2.1 Core Radio Parameters

• All the radio core parameters are set to default including Data Rate, Frequency Deviation and Bandwidth
  
• I have enabled FEC and using CRC_16 bits to ensure better RX

2.2  CSMA Configuration

• On the CSMA side we are again using mostly the default configuration
• Currently we are using in persistent mode
• And the CCA period is also default at 64TBIT
• The CSMA RSSI threshold is set at  -70

2.3 Sniff Mode Configurations

• The RSSI in sniff mode is set at -98 dBm and is configured in static mode
• I have configured the preamble length to be (64*4), which gives an on air time of approximately 15.5ms
• The Wake up time is around 3ms, I was being conservative and made sure the device wakes up at least 5 times
• The RX Window Timeout is kept at 600ms. Again here also I was being conservative; as per my calculations a value of 100-200 ms should have been enough, but that actually worsened reception.
• The Fast RX Timeout is set at 900 microseconds

3. Issues Observed

• Packet loss increases sharply with distance or more nodes in the network.
• With 2–3 nodes (20–30 cm): ~5–10% packet loss.
• With 5–6 nodes (70–80 cm): ~35–40% loss.
• CSMA  (along with randomized FW jitter) is enabled but doesn’t improve reliability.
• RSSI falls from ~−65 dBm (close range) to -95/−100 dBm (70–80 cm). Unsure if this is normal for S2868A2 with stock antenna.
• I understand that sniff mode will have some packet drops; that's why I have FW retransmissions enabled; but still I seem to face this issues.
• One more thing that I have observed is physically pressing the antenna or moving the device a bit suddenly increases packet reception.

4. Debugging and Tests Performed

• I have verified that from the sender device the packet is actually sent and TX DONE interrupt is generated.
• But when a packet drop occurs no RX DATA READY interrupt is generated on the other receiver device
• I have also tried playing around with some of the CSMA configurations; like setting to non-persistent mode and randomizing the BU seed for each node or setting CS period to 256 Bit but again in vain.
• Tried lower data rates (down to 9.6 kbps), smaller bandwidth and deviation, but with minimal improvement.
• Disabled fast RX termination; no significant change.
• Adjusted RX window duration; longer helps slightly. Although with my calculations 100 to 200 Ms should be enough, but that actually makes things worse.
• I couldn't enable any other interrupt other than RX_DATA_READY (Like data discarded or sniff timeout) in RX mode since then the interrupts won't be generated.

I’d appreciate any input on:

• Whether my sniff-mode configuration looks correct.
• Recommended parameter ranges for stable sniff operation (868 MHz).
• Whether the observed RSSI vs. distance is normal for this hardware.
• Any best practices or working examples for reliable S2-LP sniff mode setups.

My guess is its something with the RSSI or CSMA but I can't properly figure out what. I’m happy to provide any more information if needed.
Thanks a lot for your time and help!