Re: [Discuss-gnuradio] in-band signaling & dependent packets (i.e., ACK generation)

[Brian Padalino]

On Dec 4, 2007 8:43 PM, George Nychis <address@hidden> wrote:
> So there is no way of getting a generalized matched filter on the USRP?
>    Is there anything that can be done to get around the hardware
> multipliers?  If there is absolutely no way, limiting to GMSK, PSK, and
> QAM is not that bad.  What makes OFDM need different processing?  I'm
> trying to read up on matched filters now.

There's always a way to do something, but you are resource limited.
You can write something that will use 1 complex multiplier and can do
a 64 tap matched filter at a symbol rate of 1Msps.  Or you can use 8
complex multipliers to do the same thing since the minimum decimation
of the USRP is 8.  You can slice it any way you want and do TDM on the
multipliers you infer or directly instantiate.  You can possibly even
generalize the RTL to be parameterized so anyone can re-build it and
re-program the FPGA with ease using their own matched filter.

A matched filter is just a FIR filter that does correlation because
the coefficients are a specific sequence instead of a frequency
response.  This is how CDMA works.  You "spread" your one bit out over
this PN sequence and then use a matched filter to correlate against
the expected PN sequence.  You can either get a 1 or a -1 depending if
you sent a 0 or a 1.

Wikipedia has good entries for both a matched filter and cross correlation.

    http://en.wikipedia.org/wiki/Cross-correlation
    http://en.wikipedia.org/wiki/Matched_filter

The reason OFDM is different is because the symbols are actually
presented in the frequency domain, so you have to perform an FFT on a
window of samples and read which tones are present.  Because there is
that extra step of taking the FFT first, the time domain samples are
pretty much useless by themselves.

Brian