[USRP-users] USRP captured data information

Marcus Müller marcus.mueller at ettus.com
Wed Apr 15 05:49:17 EDT 2015

Hi Alok,

we'll, Matlab is a math tool, and thus, you'll have to find a
real-valued mathematical representation of the entities you want to plot.
Power of a digital signal is commonly defined as the square magnitude of
the samples, that is $\Re\{z\}^2+\Im\{z\}^2 = {|z|}^2$, so this might be
what you want to plot.

However, you might find yourself in a situation where you'll first want
to define more closely what you're measuring. Do you want to measure the
RX power of a single sine wave? In that case, it would be advisable to
use a very narrowband digital filter, just to get as little power as
possible into your measurement. If you transmit digital data (most
probably what you do when using benchmark_tx), you might now your TX
pulse shape, and thus you can use a matched filter on the RX side,
giving you the maximum SNR any filter could offer.

Also, the frequency range from 300MHz to 5GHz is quite large -- unless
you have an extremely broadband antenna, you will need to use different
antennas. And: every antenna has a frequency-dependent behaviour as
well, making both the geometrical properties and the maximum achievable
field strength extraction a function of the carrier frequency. You will
be fine if you use "simple" geometries like monopole antennas and for
small frequency deviations, and if you try to keep RX and TX in each
others main direction, but that's another factor to consider; so I'd
recommend doing one center frequency first, shifting your signal in
smaller steps around that, and then trying out other frequencies.

Your application really sounds exciting; subway channels really sound
like they'd actually have impressive multipath propagation, small scale
fading etc, due to being in a metal cage; this together with the doppler
of signals leaving the cabin, bouncing of the tunnel seems really
interesting! Do you have a reference channel model you're building on?
Maybe the Matlab IEEE802.11n indoor channels?


On 04/15/2015 07:17 AM, alok ranjan via USRP-users wrote:
> Dear Marcus and Raj,
> Thank you for your assistance so far.
> I am trying to do some channel modeling for wireless communication in
> subway.  To accomplish this I am using two USRP1 having DB WBX
> connected to my pc over USB 2.0.
> As I am new to both SDR and USRP , I initially used the two scripts
> i.e. benchmark_tx.py and benchmark_rx.py for transmission and
> reception.  I am sending baseband samples at different frequencies and
> modulation scheme at varying amplitude of TX. (The distance between
> the TX and RX was 5 mtr)
> I have captured the transmitted data at different frequencies at
> receiver using uhd_rx_cfile.  As, the Marcus has mentioned that these
> captured data is in terms of I/Q complex format, 16 bit for each. 
> I am trying to simply measure the power in the signal i have recorded
> at receiver using uhd_rx_cfile. I want to do this analysis at
> different frequencies say 300 MHz to 5 GHz. 
> Another things i would like to do is capture the data with the moving
> receiver while the transmitter is fixed.
> Now, I want to do some curve fitting (best fit, polynomial or linear
> regression) on the captured data therefore I converted the captured
> data into .mat format using octave. When i load the data in workspace
> in MATLAB, I am not getting how to start with the plot and do curve
> fitting for Recorded reception power level VS distance to further
> proceed for channel modeling
> Any help in this regard is appreciated.
> Thank you all for your time and endeavour  pain.
> Best Regards,
> Alok Ranjan
> Graduate student
> NIT, Rourkela
> ------------------------------------------------------------------------
> Date: Tue, 14 Apr 2015 15:32:57 -0400
> To: usrp-users at lists.ettus.com
> Subject: Re: [USRP-users] USRP captured data information
> From: usrp-users at lists.ettus.com
> Alok,
> If you tell us a bit more about what you are trying to accomplish with
> your curve fitting and plotting, maybe we can be of more assistance.
> What kind of curve fit or plot do you wish to generate? Are you trying
> to demodulate a signal? Analog or digital? Are you trying to simply
> measure the power in the signal you recorded? What center frequency
> and bandwidth are you interested in recording? What kinds of signals
> are being transmitted in that band? Are you going to move your
> receiver around and take measurements from fixed transmitters?
> -- 
> Raj Bhattacharjea
> Georgia Institute of Technology
> School of Electrical and Computer Engineering
> http://www.prism.gatech.edu/~gtg037s/
> <http://www.prism.gatech.edu/%7Egtg037s/>
> 404.894.7516
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