How can you handle noise in multi-level pulse amplitude modulation?
So you have PAM.

We don't know how precisely your levels are transmitted, or how precisely the receiver slices between levels, or where-in-time the decisions are made.

So, assume you want the PAM to settle to 1% accuracy at decision time, or 40dB. 

Given a NEPER is 8.7dB, and 40/8.7 is about 4.5 tau, 
if your bit times are 45 us, you need a 10us time constant RC LPF on your datalink.