Re: [Discuss-gnuradio] Square Wave Pulse Shapping Filter

[Andy Walls]

> Date: Sat, 19 May 2018 11:29:14 -0300
> From: Jose Marcelo Lima Duarte <address@hidden>

> Hello everyone,
> 
> I was giving the task of quickly develop a receiver for an old
> satellite system that use a pulse shaping filter with square wave
> impulse response and QPSK constelation.
> 
> Does GNU Radio has a prompt solution for this? By a prompt solution a
> mean a single block in GNU Radio Companion that handles carrier phase
> synch, symbol time synch and sampling. From its documentation its
> seams to me that it lacks of prompt solution for old fashion square
> wave pulse shaped signals. Am I right?

You are correct in that there is no single block in GNURadio that
handles all that.


> I have seen that GNU Radio has a prompt solution for demodulating
> QPSK signals with Squared RRC filter. And I known that it can be used
> for square wave signals too. However, I want to avoid the performance
> degradation from not using a matching filter at the receiver end.

Actually the one in GNURadio specifically can't be used for square
pulses, if it uses the Polyphase Clock Sync block in the underlying
implementation, as that block can't make a proper derivative pulse
filter for a square wave.

> Any suggestion for a quick solution for my problem?

For coarse carrier frequency synchronization, if you don't have high
doppler, the "FLL band-edge" block will keep RRC pulse filtered PSK
signals roughly on frequency.  You can probably abuse it for PSK with
square pulses too.

For matched filtering of a square pulse, just use a "Decimating FIR
Filter" block with a decimation of 1 (no decimation) and taps of
[1.0/N]*N  where N is the number of samples in th unit pulse width.
This will introduce receiver side ISI, since your square pulses are not
Nyquist pulses, but you should get nice triangular pulse peaks.

For symbol syncronization, use a "Symbol Sync" block with the signal
times slope ML approximation TED.  Also select the 8 tap MMSE
interpolation filter. 

For fine carrier frequency and phase synchronization use the "Costas
Loop" block.

Regards,
Andy 

>