I'm transmitting QPSK through a digital pulse shaping filter (RRC, 35% excess BW, 2 samples per symbol, 64 taps) at a rate of 56Msym/sec.
Receiving it on standard lab equipment (Vector signal transceiver), where it is demodulated with RRC MF and plotting the constellation. At high symbol rates, the phenomenon below starts appearing in the constellation where the points get essentially binned into 4 different locations (looks like some exotic modulation you might call qpsk^2)
One thing of note, this effect is easily taken care of by the equalizer on the RX and reduces the constellation to a much more ideal, low EVM one. Given this, I suspect the effect is related to ISI. One possible thought I had was for example, on the I channel each symbol is perturbed by a neighbor (which could be a + or - symbol) so the effect is two offset bins. Same for Q channel, resulting in the 4 shown. Not sure why this contribution would only come from a single symbol as opposed to many though - in which case I would expect more of a cloud/awgn like effect from adding up contributions of many symbols)
For reference the basic D/A chain looks like: Generate QPSK IQ -> Filter with RRC -> Halfband Upsamplers -> DAC -> Analog Reconstruction