Current measurements

felixdonkers

Today we’ve been doing some current measurements using an SODAQ NB-IoT board with an uBlox SARA-N211-02B (with v6.57 firmware). We notice some ‘strange’ behavior that might be related to the network. See the two pictures below. The first one is when transmitting 32 bytes, this is what I observe:

sleep mode (~3 seconds)
connection mode (~200 msec)
transmit mode (~200 msec)
receive mode (~22 seconds)
idle mode (~1 second)
eDRX mode (~27 seconds)

Now with 512 bytes I see a lot more, which a cannot explain:

connection mode ? (from t=~3 until t=~23 seconds)
transmit mode ? (from t=~24 until t=~35 seconds), i.e. 11 seconds for 512 bytes?
a second transmit mode ? (from t=~56 until t=~58 seconds), why ?

Can this 512 byte transfer behavior be explained ?

Maad

I rise similar question.
I had cases were it took up to 2 minutes to transmit 32 bytes and actually the data size did not matter all the time, I got a delay in receiving the data for up to 2 min!.
what could it be ?and why?

Andre Rodenburg

@felixdonkers Dear Felix,

How did you meassure the current? Did you use a shunt resistor or another method?

techniek

@felixdonkers hi Felix,

The network experts tel me that NB-Iot has a devides a packet into variable length segments, dempending on the quality of the transmission channel…

3gpp specs that might shed some light… for the deternmined…
http://www.3gpp.org/ftp/Specs/archive/45_series/45.820/45820-d10.zip

felixdonkers

@andre-rodenburg We used the SODAQ board, cut one of the traces to the uBlox modem and put a 1 ohm resistor in series to measure the current consumption. Since the SIM is also powered by the modem, that current is also included.

Andre Rodenburg

Heb zelf ook een stroommeting gedaan:

alt text

Er gebeurt dit: (denk ik)

Modem is in PSM (+NPSMR = 1)
Ik stuur +NMGS=3,“AABBAA”
Modem wordt uit zichzelf wakker (+NPSMR = 0)
Verstuurd het bericht (TX piek)
Luisterd naar inkomend verkeer? (RX)
Nog een paar RX- pieken?
PSM is weer actief (+NPSMR = 1)

Op de spectrum analyzer is het volgende te zien, het NB-IoT signaal (QPSK modulatie!) met een beetje spiegeling door de lokale oscillator:
alt text

Wat interessant is is dat elke +NMGS TX- piek wordt opgevolgd met een tweede TX- piek met een lagere intensiteit 20 seconden later:
alt text
Iemand enig idee wat dit is?

techniek

Ik vermoed dat de 3 piekjes momenten zijn waarop het device nog luistert naar een paging request.