Re: [Discuss-gnuradio] High bandwidth channel estimation using the USRP

[Johnathan Corgan]

On Mon, Dec 15, 2008 at 8:28 AM, Emanuel Birge <address@hidden> wrote:

> I'm interested in calculating an estimate of the impulse response of a
> speaker cable, using pseudo-white noise and bypassing all forms of
> interpolation/decimation/modulation/demodulation-blocks, for as wide a
> bandwidth as possible. However, and please correct me if I'm wrong, the
> USB-interface on the USRP is too slow (8 MHz) to, so to speak, do this
> "online". Say I want to use 10,000 pseudo-white samples with at least 8bit
> resolution. Is this possible without rewriting very much of the
> Verilog-code?

The BasicRX and TX do not pass DC; they are transformer coupled and
have a ~100 KHz highpass response at the low end.  The alternative is
the LFRX and LFTX cards, which do pass DC, but have lowpass filtering
at ~30 MHz.  So you will only be able to calculate the impulse
response convolved with that of one of these filters.

There is a GNU Radio component called gr-sounder, which implements a
custom FPGA build for the USRP.  The transmitter will generate an
m-sequence PN code of a configurable length and chipping rate for
channel sounding purposes; this is likely suitable for what you need.
The receiver will correlate the received signal at all possible PN
code phases and produce a complex vector of the correlation results.

Also, your results will be affected by the coupling of the USRP
daughterboards to the speaker cable.  They are designed for a
broadband match of 50 ohms across their frequency range.  Speaker
cable is not typically designed with this in mind and the mismatch
will like cause a frequency-dependent amplitude response.  Maybe that
is what you trying to measure, however.

-Johnathan