Incorrect RSSI samples in IEEE 802.15.4 RADIO mode

Question

Hi,

we're developing a firmware for the nRF52811 that makes exhaustive use of RSSI sampling during packet reception. While this gives reliable results for Nordic proprietary and BLE RADIO modes (both for 1 MBit/s and 2 MBit/s), we encountered incorrect RSSI samples in IEEE 802.15.4 RADIO mode during specific parts of the packet.

We could reproduce this behaviour using the minimal working example specified below. The used HAL is not from Nordic but it should be self-descriptive anyway. We measured this for two different distances between the receiving and the transmitting antenna. As you can see from the diagram below, the sampled values around bits ~10 to ~15 and ~30 to ~55 are plausible. Indeed they match measurements done in other RADIO modes for the corresponding antenna distances. However, at the beginning, somewhere in the middle and at the end of the packet we get obviously incorrect RSSI samples. Furthermore, the signal level around bits ~10 to ~15 differs from that one around ~30 to ~55.

The only part of the manual with information about RSSI samples related to IEEE 802.15.4 mode is this in chapter 6.14.13.7:

When receiving a frame the RSSI (reported as negative dB) will be measured at three points during the reception. These three values will be sorted and the middle one selected (median 3) to be remapped within the LQI range.

Does this automatic RSSI sampling corrupt the RSSI sample values manually read from RSSISAMPLE register? Is there any chance to workaround this behaviour? What about the different signal levels? Many questions...

Thanks in advance and

best regards,

Gerrit Maus.

#include <libopencm3/nrf5/radio.h>
#include <libopencm3/nrf5/clock.h>
#include <libopencm3/cm3/nvic.h>

volatile uint8_t RSS[1000];
volatile uint16_t RSSC = 0;
volatile bool goOn = true;
uint8_t packet[255];

int main(){
	
	CLOCK_TASKS_HFCLKSTART = 1;
	while(!CLOCK_EVENTS_HFCLKSTARTED);
	
	RADIO_SFD = 0xcb; // Campus network's SFD
	RADIO_MODE = RADIO_MODE_MODE_IEEE802154_250KBIT;
	RADIO_CRCCNF = RADIO_CRCCNF_LEN_VAL(2)
	             | RADIO_CRCCNF_SKIPADDR_IEEE802154;
	RADIO_CRCPOLY = UINT32_C(0x11021);
	RADIO_CRCINIT = 0;
	RADIO_PCNF0 = RADIO_PCNF0_LFLEN_VAL(8)
	            | RADIO_PCNF0_PLEN_32BITZERO
	            | RADIO_PCNF0_CRCINC_INCLUDE;
	RADIO_PCNF1 = RADIO_PCNF1_MAXLEN_VAL(255);
	RADIO_FREQUENCY = 45; // 2445 MHz
	RADIO_PACKETPTR = (uint32_t) packet;
	
	nvic_enable_irq(NVIC_RADIO_IRQ);
	
	RADIO_SHORTS = RADIO_SHORTS_READY_START_ENABLED
	             | RADIO_SHORTS_PHYEND_DISABLE_ENABLED;
	RADIO_INTENSET = RADIO_INTENSET_RSSIEND_SET
	               | RADIO_INTENSET_FRAMESTART_SET;
	RADIO_TASKS_RXEN = 1;
	
	while(!RADIO_EVENTS_PHYEND){
		
		if(goOn){
			// Start another RSSI sampling process once
			// the previous one was processed by the ISR
			goOn = false;
			RADIO_TASKS_RSSISTART = 1;
		}
	}
	
	while(1);
	return 0;
}

void radio_isr(){

	if(RADIO_EVENTS_RSSIEND){
		RADIO_EVENTS_RSSIEND = 0;
		
		if(!RADIO_EVENTS_END && !RADIO_EVENTS_CRCOK){
			// Only process if packet is still on air!
			RSS[RSSC++] = RADIO_RSSISAMPLE;
			goOn = true;
		}
		
		while(RADIO_EVENTS_RSSIEND);
	}
	if(RADIO_EVENTS_FRAMESTART){
		RADIO_EVENTS_FRAMESTART = 0;
		RADIO_TASKS_RSSISTART = 1;
		while(RADIO_EVENTS_FRAMESTART);
	}
}