Question regarding frequency offset correction

GNU Radio
1

Hi all,

In many applications very good synchronization of carrier and sampling
frequencies is required.
Available sources of signal not always have good clock reference. One of
the examples is cheap RTL SDR receiver based on a DVB-T dongle.
Without any additional effort to correct frequency offset it is
impossible to decode GPS or GSM transmissions with such receivers.

The frequency offset cannot be calibrated once as it changes with time
and temperature. Good way to fight with it is to implement some
correction algorithm that continuously computes frequency offset
estimates and applies correction by:

  • performing frequency shifting and re-sampling in software,
  • or changing some hardware parameter that enables tuning of the
    frequency of an internal oscillator (like ‘ppm’ option in RTL SDR
    source).

My question: is it possible to build working frequency correction with
available GNU Radio blocks? Can you point some successful example? Or if
not - can you share some ideas how it can be done? I’m especially
interested in situations where frequency offset correction and
estimation are in separate blocks i.e:

                    ______freq. offset_________
                    |                         |
                    v                         |

|sig.source|–>|freq.offset|–>(processing)–>|freq. offset|
|correction | |estimation |

–
Best Regards,
Piotr K.

2

On Mon, Jul 14, 2014 at 12:14 PM, Perper [email protected] wrote:

and temperature. Good way to fight with it is to implement some
estimation are in separate blocks i.e:
Piotr K.

​Hello Piotr,

You could take a look at the OFDM RX example. Look at the Schmidl-Cox
​block. It performs timing as well as coarse-grained frequency offset
estimation. Once this estimation is done, correction is a simple matter
of
derotation. The OFDM RX example has all of this.

Good luck.

best,
aditya

3

On 07/14/2014 01:01 PM, Aditya D. wrote:

One of
frequency of an internal oscillator (like 'ppm' option in RTL SDR
                        |                         |

​Hello Piotr,

Also offset artefacts vary by modulation in use. For example in WBFM,
you can use residual DC-offset in the demodulated audio as a proxy
for frequency offset and use that to drive correction machinery.

But with other modulations, the approach is different.

4

Hi Piotr,
there’s various control loop based offset correction, look for the PLL
and FLL examples.
There’s a feedback loop base class in GNU Radio for monolithic
correction. For the frequency correction estimate feedback, you could
easily use a message bus, that will occasionally inform the upstream
frequency correction block.

Greetings,
Marcus