QPSK demodulation using dsp

To demodulate QPSK requires a carrier recovery circuit.Without a recovered
and properly phased carrier it is not possible to demodulate the I and Q
channels.

In the analog world there are circuits like the squaring ckt or the Costas
Loop which recovers the carrier.

However--modern QPSK radios use DSP to demodulate a non-coherent received
signal. By non-coherent I mean that the  carrier frequency is not precisely
known  nor is its phase. Nevertheless, such a signal can be sampled via an
ADC and sent to DSP for demodulation into separate I and Q data streams.

i think polyphase filters are used--and I have a vague idea of how they
work. 

Can anyone explain, in non-methematical terms, what is being done to the
non-coherent QPSK signal  so that it gets demodulated???

I'd distinguish between two independent problems, on two different time
scales:

- carrier frequency offset: If the local oscillator frequencies of receiver
and transmitter don't match, the difference frequency appears as periodic
rotation of the complex baseband signal. 
Estimate the mismatch in terms of constant and linear term in time,
generate a rotating unity phasor that spins in the opposite direction and
multiply.

- symbol timing: You need to sample at the correct time instant, when the
trajectory is closest to the constellation points and the eye opening is
widest. 
Now this can be done with a polyphase filter, which is able to delay (but
not rotate) the baseband signal by a fraction of a sample. 
If the signal is oversampled high enough, say, 10 samples per symbol, you
can probably manage without, as 1/10 of a sample duration near the optimum
sampling instant may be close enough.

On Wed, 22 Feb 2012 14:23:12 -0600, "pgdragone"
<philip.dragonetti@n_o_s_p_a_m.L-3com.com> wrote:

>To demodulate QPSK requires a carrier recovery circuit.Without a recovered
>and properly phased carrier it is not possible to demodulate the I and Q
>channels.
>
>In the analog world there are circuits like the squaring ckt or the Costas
>Loop which recovers the carrier.
>
>However--modern QPSK radios use DSP to demodulate a non-coherent received
>signal. By non-coherent I mean that the  carrier frequency is not precisely
>known  nor is its phase. Nevertheless, such a signal can be sampled via an
>ADC and sent to DSP for demodulation into separate I and Q data streams.
>
>i think polyphase filters are used--and I have a vague idea of how they
>work. 
>
>Can anyone explain, in non-methematical terms, what is being done to the
>non-coherent QPSK signal  so that it gets demodulated???

If the signal is differentially modulated then the demodulator uses
the symbol-to-symbol phase difference rather than the absolute phase.
In this case it is not required to lock phase or frequency, but some
performance is lost as a result.

Frequency uncertainty must be low enough for this to work, though.



Eric Jacobsen
Anchor Hill Communications
www.anchorhill.com