PFB Filterbank & arbitrary resampler

GNU Radio
1

We are successfully using the PFB filterbank to channelize 1-6mhz into a
variety of channel widths (25K, 50K etc).

In one application, we have 25K channels which are then resampled using
PFB
resampler to 19.2K fed into gmsk_demod @ 9600bps with 2sps.

In this case, we may have 20-40 ports “active” (not null sink) with pfb
resamplers connected to each.

Is there way to perform this resampling in the filterbank to avoid the
need
for many separate resampler blocks in an effort to improve performance?

We are pushing the limits of hardware already, so any efficiencies are
helpful. We will consider a hierarchical filterbank structure if
necessary,
but our first goal is to do it with one filterbank, if possible.

2

On Sun, Sep 4, 2011 at 11:33 AM, Ryan P. [email protected] wrote:

Is there way to perform this resampling in the filterbank to avoid the
need for many separate resampler blocks in an effort to improve performance?

We are pushing the limits of hardware already, so any efficiencies are
helpful. We will consider a hierarchical filterbank structure if necessary,
but our first goal is to do it with one filterbank, if possible.

Ryan,
What you are talking about is possible, but not with the current
implementation of the GNU Radio channelizer. fred harris has recently
published a few papers on doing arbitrary channel / arbitrary bandwidth
channelizers that should meet your needs, but it would have to be
programmed
up.

Now, that having been said, there is a way to handle this. If you have a
9600 bps signal in a 25 ksps channel, you can switch to using the
gr_pfb_clock_resampler_fff block, which allows you to specify a real
number
for the number of samples per symbol (2.6042 in this case) instead of
integers. That should do the trick.

Tom

3

On Mon, Sep 5, 2011 at 11:55 AM, Tom R. [email protected]
wrote:

resamplers connected to each.
What you are talking about is possible, but not with the current

Tom,

Thank you.

To be clear I’m not looking for arbitrary channel size channelizer, when
I
said 25K/50K etc, I was just referring to the fact that we are using
channelizers of various sizes.

the gr_pfb_clock_resampler looks like exactly what I need. Will give it
a
shot.

Also, in our case where we are actively using 20% of channels from the
channelizer, it is more advantagous to provide the filterbank with the
least
computationally intensive channel filter and add a more selective filter
on
only the channels we are using (but do so with N separate filter
blocks), or
is the filterbank implementation efficient enough to worry about it and
we
should design the channel filter to best fit the ultimate need?

4

On Tue, Sep 6, 2011 at 2:25 PM, Ryan P. [email protected] wrote:

In one application, we have 25K channels which are then resampled using
but our first goal is to do it with one filterbank, if possible.
9600 bps signal in a 25 ksps channel, you can switch to using the

To be clear I’m not looking for arbitrary channel size channelizer, when I
said 25K/50K etc, I was just referring to the fact that we are using
channelizers of various sizes.

This was related to not wanting to resample after every channel, not the
different channel bandwidth.

the gr_pfb_clock_resampler looks like exactly what I need. Will give it
a

shot.

Great! It’s worked great for me in the past. Interested to hear other’s
results, though.

Also, in our case where we are actively using 20% of channels from the
channelizer, it is more advantagous to provide the filterbank with the least
computationally intensive channel filter and add a more selective filter on
only the channels we are using (but do so with N separate filter blocks), or
is the filterbank implementation efficient enough to worry about it and we
should design the channel filter to best fit the ultimate need?

For N channels, every N taps added to the prototype filter adds only 1
more
tap per channel, so it shouldn’t be too much overhead to provide a
better
filter in the channelizer. Now, the channelizer implementation uses
time-domain FIR filters internally, so if you are asking to use hundreds
of
more taps, than it might be more computationally efficient to use
external
FFT filters for the channels you need. But I doubt it, honestly. There’s
going to be some crossover somewhere that you might just have to
experiment
with to find out.

The other thing is that the PFB clock resampler is another filter. In
the
dbpsk2 and dqpsk2 demodulators that are currently in the master branch,
I
implemented the RRC filter for this, so it becomes the matched filter,
too.
You can use other filters for this stage, so you might think about how
to
pair this filtering stage with the channelizer filtering stage for the
best
computational and signal performance (choose the best point on the
Pareto
front, if you will).

Tom