Low TX Power when performing frequency hopping

GNU Radio
1

Dear All,

I am trying to implement some frequency hopping schemes in the ISM band
with USRP N210 with RFX 2400 board ( I also tried the same with the SBX
40MHz board). I hop to the next channel (channel seperation : 5MHz)
after
every 6ms (tuning time for RFX board is 250usec) so tuning is not an
issue.
I set max gain (20dB in case of RFX board) and 30 dB in case of (SBX
Board)
and send random modulated (BPSK) bits.

However, I get very low power at the receiver (another USRP, RX gain
20dB)
around -70 to -80 dB 1 meter away from the tx USRP. When I dont perfrom
frequency hopping I get a rx power of -10 dB. I am not able to
understand
why the tx power reduces by such an extent while perfroming fx hopping,
although I give enough settling time.

Also is there any firmware which allows to further increase the tx power
?

Any help to resolve this issue is appreciated.

Best,
Vaibhav
ETH Zurich

2

Hi Vaibhav,

tuning is an issue, here. The problem is not that you’re only retuning
the LO, the whole signal chain has to be correctly locked in again after
each tune.
If the total bandwidth you’re spanning is less than 40MHz, which is the
baseband bandwidth of the daughterboard’s you’re using, try setting your
LO frequency to the center of that band, and hop via frequency shifting
in the FPGA only[0]. To do that, you’ll need to use tune_request_t[1]
objects rather than simple using the target frequency when tuning using
set_tx_freq in your USRP sink.

Further things of interest: How do you measure the power of a 6ms burst?
How wide is your signal?

Greetings,
Marcus

[0]USRP Hardware Driver and USRP Manual: General Application Notes
[1]USRP Hardware Driver and USRP Manual: uhd::tune_request_t Struct Reference

3

Hi Vaibhav,

how do you make sure the PSD measures the power in exactly the 6ms
you’re sending in? Or do you continously measure? If the latter: do you
multiply with the duty cycle of each subchannel (ie. when you hop to 10
channels sequentially, each channel would only be used 10% of the time,
and thus, a long observation will only see 10% of power)? Are you
looking at an offline analysis, or at a live updated PSD plot, which
might actually miss a lot of bursts by only looking every
1/updates_per_second ?

I don’t know the T-Mote sky nor the Received Signal Strength Indication
algorithm it uses, so I can’t really say anything about that.

Best regards,
Marcus

4

Hi Marcus,

I run the uhd_to_file.py on the recording USRP at a sampling rate of
25Mhz
in the frequency range I am hopping in (total bandwidth 40Mhz). I set
the
center frequency as 2.43Ghz so I see band from 2.41 - 2.45 GHz. I than
plot
the recorded file (I plot the PSD and the spectrograph). I check the
peaks
I get at different frequencies which are my transmitted signals ( I make
sure I dont confuse them with other signals or noise present in the
room)
at regular intervals of time. Than i check the peak power of the signal
peak.

The Tmote sky node just reads the RSSI register values of the CC2420
radiop
at a sampling rate of 61Khz (its very low as compared to USRP) but I
just
use it to check the impact of my transmitted signals on these low power
sensor nodes and than plot the recorded values.

Best,
Vaibhav

On Mon, Mar 2, 2015 at 2:09 PM, Marcus Müller [email protected]

5

vaibhav kulkarni wrote in post #1169440:

Dear All,

I am trying to implement some frequency hopping schemes in the ISM band
with USRP N210 with RFX 2400 board ( I also tried the same with the SBX
40MHz board). I hop to the next channel (channel seperation : 5MHz)
after
every 6ms (tuning time for RFX board is 250usec) so tuning is not an
issue.
I set max gain (20dB in case of RFX board) and 30 dB in case of (SBX
Board)
and send random modulated (BPSK) bits.

However, I get very low power at the receiver (another USRP, RX gain
20dB)
around -70 to -80 dB 1 meter away from the tx USRP. When I dont perfrom
frequency hopping I get a rx power of -10 dB. I am not able to
understand
why the tx power reduces by such an extent while perfroming fx hopping,
although I give enough settling time.

Also is there any firmware which allows to further increase the tx power
?

Any help to resolve this issue is appreciated.

Best,
Vaibhav
ETH Zurich

Please @vaibhav kulkarni can you explain how you implemented your
frequency hopping using GRC.

Thanks.

6

Hi Marcus,

Thanks for your inputs. I tune the frequency as you said :

DEBUG: Received tx_freq mid-burst.
DEBUG: Received tx_freq on start of burst.
DEBUG: Received command: freq
– Tune Request: 2435.000000 MHz
– The RF LO does not support the requested frequency:
– Requested LO Frequency: 2435.500000 MHz
– RF LO Result: 2435.714286 MHz
– Attempted to use the DSP to reach the requested frequency:
– Desired DSP Frequency: -0.714286 MHz
– DSP Result: -0.714286 MHz
– Successfully tuned to 2435.000000 MHz

and the output power has considerable improved (I get around -20dB now)
my
flowgraph (I run it for 2 minutes)

[Vector Source] --> [Packet Encoder] --> [D-BPSK Modulator]–>[Frequency
Hopping Module]–>[USRP Sink]

However, the transmission takes place only for first 15 seconds and than
even If get tuning output, the USRP doesn’t transmit anymore. I have
attached the python file here, in case.
Ay hits why this may occur will be useful.

Best,
Vaibhav

On Mon, Mar 2, 2015 at 3:04 PM, vaibhav kulkarni <