I've heard about shaping the symbol with a rect filter at the transmitter, but why? Isn't this just implicit from the fact that it's going to be transmitted as a symbol truncated to the symbol period regardless? Or is that simply what they're referring to by shaping it with a rect filter i.e. isolating the symbol to that window. Am I right in thinking it's just the default. Also does the receiver extend the signal as if it were infinite when performing the fourier transform and therefore the rect pulse shaping at the receiver is simply the receiver not doing that?
In most waveforms for data communication we do NOT want to transmit a rect for each pulse given the excessive bandwidth required (since a rect is a Sinc function in frequency which is relatively very wide-band). This is the motivation for pulse shaping (such as the commonly implemented root-raised cosine pulse shape): to limit the transmitted spectrum and improve the overall spectral efficiency of the waveform.
This has been resolved. It was due to a misunderstanding of what a pulse filter was. It's used on the ZOH output of the DAC to convolve it, which is a multiplication in the frequency domain, i.e. it's a frequency domain filter of a certain shape to remove excess images by shaping the overall freuency domain. This is filtering, whereas what I was referring to was windowing of an OFDM symbol, which is by default a rect shape and is a convolution of the frequency domain with orthogonal sincs at the subcarriers