So the way you do these experiments that at the transmitter you use an arbitrary waveform generator with ~4 DAC channels which let you modulate a single wavelen

So the way you do these experiments that at the transmitter you use an arbitrary waveform generator with ~4 DAC channels which let you modulate a single wavelength channel in IQ and two polarizations (4 dimensions). These devices have typically a memory of around 500k samples and rates of up to 120 GS/s (newest one actually has 256 GS/s Google keysight AWG if you are interested). So you generate a sequence of ~120k symbols (depending on symbolrate/oversampling) with 12 bit/per symbol (assuming 64 QAM). That sequence repeats over and over. You then use the multiplexing/emulation techniques described in other posts to emulate the other channels. This is essentially due to limitations of the measurement equipment. You can't just convert a random incoming bitstream into analogue symbols (with FEC coding) in realtime. In a deployed system this would be done by specific Asics that take millions to develop and are comparatively inflexible. Thus if you want to test/research methods you use the above mentioned equipment which gives much more flexibility.