rssi-1 measurements DECT NR+

[nrfuser2137]

Hello,

I am scheduling an RSSI measurement operation using DECT PHY firmware v1.0.0.
The parameters used for scheduling the operation are:
- start_time = 0
- carrier = 1677
- duration = 48 (in subslots)
- reporting_interval = NRF_MODEM_DECT_PHY_RSSI_INTERVAL_24_SLOTS

The operation scheduling is successful, nrf_modem_dect_phy_rssi() returns 0.

In the callback to receive RSSI measurements, void (*rssi)(const uint64_t *time, const struct nrf_modem_dect_phy_rssi_meas *meas)
I am printing the *time and the meas->meas_start_time.

My expectation was that the difference between *time and meas->meas_start_time for each operation would be roughly 10ms (in modem time units) as the rssi measurement duration is 48 subslots, and reporting interval is also the same. 

However, the output printed in rssi_cb is as follows.

[00:00:04.137,207] <inf> phycb: cbtime 249018865 and meas->meas_start_time 249010078
[00:00:04.147,735] <inf> phycb: cbtime 249746536 and meas->meas_start_time 250428979
[00:00:04.158,264] <inf> phycb: cbtime 250473924 and meas->meas_start_time 251847555
[00:00:04.168,792] <inf> phycb: cbtime 251201314 and meas->meas_start_time 253266159
[00:00:04.179,321] <inf> phycb: cbtime 251928707 and meas->meas_start_time 254684742

So, my question is, what exactly does meas->meas_start_time refer to?

[Charlie]

Hi,

Try to catch up with your calculation.

The meas_start_time interval is around 1418900 modem time unites. Considering the following definition in nrfxlib\nrf_modem\include\nrf_modem_dect_phy.h. The actual meas_start_time interval is around 1418900/69120=20.5ms.

#define NRF_MODEM_DECT_MODEM_TIME_TICK_RATE_KHZ 69120

The subslot duration is defined in ETSI TS 103 636-3 [2], clause 4.4. a slot duration is 0,41667ms. There are 48 subslots measurement duration and 24 slots reporting interval, but I am not sure if you will get a report if the measurement is not finished. You can also easily verify with a test like 72 subslots measurement duration. If this is true, 48*0,41667ms=20ms, still have 0.5ms mismatch. 

I will check with our modem developer anyway.

Best regards,

Charlie