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Re: [Discuss-gnuradio] rssi questions

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] rssi questions
Date: Thu, 16 Aug 2007 13:07:43 -0700
User-agent: Thunderbird (X11/20070604)

George Nychis wrote:

> I think you're right Jonathan, for performing carrier sense in the
> FPGA, we really do not need it in dB.  Calculating the average to
> determine a threshold will get the job done with less the hassle.

You may want to implement two thresholds, one for rising power and one
for falling (hysteresis), to avoid "switch bounce" when the power is
right at the threshold.

> ...and the host can decide how many RSSI samples it wants to compute
> the threshold on.

If you implement an FPGA block to calculate the trailing-"n" sample
average, you'll need to use memory to store the samples in a ring
buffer.  Much more efficient is to implement an exponential averager,
which only needs to keep track of the previous averager output. (This is
what the existing rssi.v does.)  If the time constant (~16 us) is not
correct for what you need, you can either modify the hardcoded shift, or
implement a programmable shifter.  That would at least let the user have
some selection from a small number of fixed values.

To make a completely generic one, you'd have to use two multipliers,
which the USRP1 doesn't have (but the USRP2 will.)

Johnathan Corgan
Corgan Enterprises LLC

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