Hi Johnathan,

Â

Thank you so much for your reply.Â I would like to make sure I don’t

misunderstand what you mentioned. Do you mean that I can chooseÂ a

decimation factor less than 320 (64M/200k) (without considering the 6dB

droop at the passband edges at this moment)? Then, I can tune the

decimation factorÂ lower to see whatÂ the differences are between them.

Thank you,

Jane

From: Johnathan C. removed_email_address@domain.invalid

To: Jane C. removed_email_address@domain.invalid

Cc: removed_email_address@domain.invalid

Sent: Wednesday, February 4, 2009 6:45:26 PM

Subject: Re: [Discuss-gnuradio] decimation rate of GSM

2009/2/4 Jane C. removed_email_address@domain.invalid:

I am confused. Could anyone please help me?

Since the USRP performs quadrature downconversion to complex baseband

samples, the Nyquist limit is *equal* to the maximum passband

bandwidth.Â So a 200 KHz wide signal would need a minimum of 200K

(complex) samples per second to faithfully represent its spectral

content.Â This is different from dealing with real-valued signals,

which do require a sample rate of at least twice the frequency

content.

However, other factors come in to play.Â You will want to allow for

the fact that the USRP’s downsampler has a significant (6dB) droop at

the passband edges, and this would affect your signal fidelity.Â This

would call for having the baseband sample rate be something higher.

In addition, if you are going to actually start demodulating the

signal, you will need at some point in your signal processing chain to

resample to a sample rate that is related to GSM symbol rate.Â There

are a variety of choices that trade off CPU usage vs. complexity, and

one of the variables is the USRP decimation rate you start with.

Johnathan