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Re: [Discuss-gnuradio] Low TX Power when performing frequency hopping

From: vaibhav kulkarni
Subject: Re: [Discuss-gnuradio] Low TX Power when performing frequency hopping
Date: Wed, 4 Mar 2015 12:19:49 +0100

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.


On Mon, Mar 2, 2015 at 3:04 PM, vaibhav kulkarni <address@hidden> wrote:
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. 


On Mon, Mar 2, 2015 at 2:09 PM, Marcus Müller <address@hidden> wrote:
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,

On 03/02/2015 01:41 PM, vaibhav kulkarni wrote:
Hi Marcus, 

Thanks for the quick reply. yes, that may be the issue. As I am using the frequency_hopping.py module for specifying the hopping sequence and controlling the time and connecting it to other modulation blocks. 

Signal width : 3Mhz
I record the the received signal on another USRP and plot the PSD to check power. Further I also measure the RSSI values on a T-Mote sky node and plot them to check the RSSI of the Transmitted signal. 

I start from 2.41Ghz and hop until 2.45 Ghz , separation of 5Mhz. 
Thanks a lot for your inputs. I will try them right away. 


On Mon, Mar 2, 2015 at 12:39 PM, Marcus Müller <address@hidden> wrote:
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?



On 03/02/2015 12:24 PM, vaibhav kulkarni wrote:
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.  

ETH Zurich

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