# Effect of Passband Slope on QPSK Constellation

What is the effect of a passband slope on a QPSK (or any QAM) constellation? Specifically given a linear slope (in magnitude, not dB), and no phase distortion (linear phase or even zero-phase) how would the following constellation appear after being passed through such a channel?

This is a "DSP Puzzle". Please preface your answer with "spoiler" notation by typing the following two characters first ">!" (see other DSP Puzzle tagged questions for examples of what this means and does). Please do not confirm or get to your result with a simulation but post your best conjecture. An analysis is allowed and welcomed but not required. I am looking for the simplest intuitive explanation as to what the resulting shape, line or curve would be for the resulting samples in the constellation with a quantified range related to the slope in the channel ($$\alpha$$).

It is common to get a passband slope in wide-band single-carrier solutions (D/A converter Sinc with digital IF, analog filters, amplifiers and transmission lines). Recognizing this possible distortion source from the constellation alone is a useful signal forensics tool.

• The QPSK waveform is perfectly pulse shaped to have an occupied bandwidth $$B$$ with a rectangular magnitude shape in the frequency domain, and the data is independent and randomly distributed uniformly over all symbols in the constellation. The waveform as created prior to the channel distortion has zero inter-symbol interference (ISI).

• I am not looking for detailed distributions, EVM, etc, just what the final shape would be after many, many symbols.

• You need not know anything else about the receiver. In this case all pulse shaping is done for zero ISI at the transmitter to achieve the constellation as shown with no other noise added or subsequent processing.

If this helps to clarify, consider the perfect root-raised cosine waveform at the transmitter in cascade with the perfect root-raised cosine matched filter that would normally be in the receiver which together would provide the desired zero-ISI raised cosine response and for a band-limited signal. This would have the constellation as shown when that waveform is sampled at the optimum symbol sampling locations in time. In this case the pulse shaping filter is an infinite duration Sinc function with scaled complex impulses at the input representing the desired QPSK symbols for a random stream of symbols. The result would be a rectangular magnitude B Hz wide for a symbol rate of B Symbols/sec.

• This is a good exercise is some fundamental properties of the Fourier Transform, in particular decomposing the passband into even and odd functions.

• As far as the distribution; familiarity with the Random Harmonic Series is helpful, in particularly a two-sided one (which I am not sure has a formal name).

• "perfectly pulse shaped": I take this as "$B$ is half the symbol rate", right? Commented Oct 7, 2022 at 15:24
• The twitters are pointing out that for defining what the constellation at the receiver looks like, we need to define the receive "integrator" / filter + sampler. Otherwise there's an RX signal, but not an RX constellation. Commented Oct 7, 2022 at 16:26
• @MarcusMüller B is the symbol rate for an infinite duration Sinc filter as the pulse shape and the constellation will be the samples at the ideal sample clock timing location, on that same waveform. It needn’t be more complicated as having to consider a receiver etc- pretend it is at the transmitter except you used a RC filter instead of RRC and a perfect one at that (which is the Sinc impulse response with an impulse representing each symbol at its input) Commented Oct 7, 2022 at 16:43
• And didn’t realize there is a twitter back-channel! And suggesting my question may be ill-posed missing some unstated assumptions. I will try to incorporate my last comment into the assumption list. Commented Oct 7, 2022 at 16:48
• I felt bad for opening a back-channel without telling you ;) it's the utmost delightful discussion I'm having with Daniel there. Commented Oct 7, 2022 at 16:50