Figure 4

(a1–a4) Constellations of the 20-Gbit/s QPSK signals carried by each data channel (Ch.) after MIMO-DSP. (a1) Ch. 1 - OAM^x_−1,0, (a2) Ch. 2 - OAM^y_−1,0, (a3) Ch. 3 - OAM^x_+1,0, (a4) Ch. 4 - OAM^y_+1,0. (OSNR = 25.5dB) (b) BER vs OSNR curves for all Chs. and a back to back Ch.