Purpose of Upsampling in QPSK Tx
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I am working through a QPSK transmitter example here:
And can follow each of the elements, but I am confused by the rasied cosine filter. Presumably this is there to reduce intersymbol interference caused by a bandwidth limited channel, but why is it upsampled by 2?
Pratik Chavan on 11 Jun 2020
Usually in a transmitter you need good control of the shape of the transmitted spectrum, including out-of-band emissions, adjacent energy suppression. Pulse-shaping for matched filtering is also done for some of the same reasons but also for maximization of SNR at the receiver. All of these tasks are often difficult or impossible with only one sample per symbol in order to meet transmit mask requirements (i.e., the out-of-band emission specs).
The performance after getting upsampled by two must be optimum for them to choose it