FPGA Central

Sorry if this is a naieve question.


Typically in a QPSK output when a data stream is being passed through
a root cos filter before being used to modulate a carrier, what
rate will the filter be clocked relative to the character rate?

Ie if I've got data at 10 KHz, what rate should I be clocking it
through the filter?

Alternatively, how many symbols would the filter typically span?


Thanks for any pointers,


Nial

Nial Stewart wrote:
> Sorry if this is a naieve question.
>
>
> Typically in a QPSK output when a data stream is being passed through
> a root cos filter before being used to modulate a carrier, what
> rate will the filter be clocked relative to the character rate?
>
> Ie if I've got data at 10 KHz, what rate should I be clocking it
> through the filter?
>
> Alternatively, how many symbols would the filter typically span?
>
>

This depends entirely on your sample rate. You might have 10 samples
per bit of data per channel (I and Q), or you could have 10000 samples
per bit per channel. You'll use your data stream to generate your
modulating waveform. This waveform is what you pass through your
root-cosine pulse-shaping filter before using it to modulate the I and
Q carriers.

Sorry if this is a double post; I'm having issues trying to submit the
message.

Jason

Nial Stewart wrote:
> Sorry if this is a naieve question.
>
>
> Typically in a QPSK output when a data stream is being passed through
> a root cos filter before being used to modulate a carrier, what
> rate will the filter be clocked relative to the character rate?
>
> Ie if I've got data at 10 KHz, what rate should I be clocking it
> through the filter?
>
> Alternatively, how many symbols would the filter typically span?
>
>
> Thanks for any pointers,
>
>
> Nial

It depends on your requirements. Nyquist will tell you that just a few
samples/baud is good enough. Sometimes the samples/baud is interpolated
up to a higher value in order to push the DAC images out so they can be
dealt with using a cheap LPF. As for the RRC length, it will impact the
transmit PSD, which you probably care about or else you wouldn't be
pulse shaping in the first place. Ideally the RRC goes on forever.

John

Nial Stewart wrote:

> Typically in a QPSK output when a data stream is being passed through
> a root cos filter before being used to modulate a carrier, what
> rate will the filter be clocked relative to the character rate?

It all depends on your accuracy requirements. Typically, the sample rate
is from x4 to x8 baud rate.

>
> Ie if I've got data at 10 KHz, what rate should I be clocking it
> through the filter?
>
> Alternatively, how many symbols would the filter typically span?

This is a different question, and again, it depends.
Typically the filter span is from 6 to 8 symbols.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

On Fri, 1 Dec 2006 16:56:05 -0000, "Nial Stewart"
<[email protected]> wrote:

>Sorry if this is a naieve question.
>
>Typically in a QPSK output when a data stream is being passed through
>a root cos filter before being used to modulate a carrier, what
>rate will the filter be clocked relative to the character rate?
>
>Ie if I've got data at 10 KHz, what rate should I be clocking it
>through the filter?
>
>Alternatively, how many symbols would the filter typically span?
>
>Thanks for any pointers,
>
>Nial

I think the real answer is that it only needs to be oversampled just
enough to get the transmit mask you're looking for. In practice I
think this often works out to about 4 samples/symbol or more, but it
will depend on the rest of the processing chain (i.e., the rf, etc.).

Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org