matched filters

[hatemt]

I am a little confused about the concept of matched filtering: Does it apply to baseband signals or to passband as well? The reason I am asking is that the tutorial and text books talk about matched filters or correlation receivers when dealing with pass band BPSK or QPSK. For example correlating the received signals with coswt and sinwt where w is the carrier frequency.

[Charan Langton]

Matched filters are applicable at baseband only. They recover the transmitted information signal. You can have a transmitted shape that is a piece of a sine or a cosine wave, BUT, the purpose of the whole thing is to decode the transmitted bit. Passband matched filtering does not make much sense because of the presence of the carrier as you rightly mention.

At passband, the purpose of filtering is to get rid of out of band noise and so filters such as Butterworth, Elliptic are used. These are not doing matched filtering.

Correlation receiver is exactly the same thing as matched filtering and it too occurs at baseband after the signal has been low pass filtered usually by a Butterworth filter and then by a matched filter.

You can safely ignore association of the passband with matched filtering.

Charan

[Charan Langton]

I think I see your confusion about correlating with sin and cosine. This is for carrier recovery purposes and to down-convert the signal to baseband and is not part of matched filtering. After this process you have the baseband signal left with added noise. Now you do matched filtering on this signal to recover the bits.