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

[Marcus Müller]

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

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. 

Best, 

Vaibhav

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?

Greetings,
Marcus

[0]http://files.ettus.com/manual/page_general.html#general_tuning_process
[1]http://files.ettus.com/manual/structuhd_1_1tune__request__t.html

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.  

Best, 

Vaibhav

ETH Zurich

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