Fig. 4: Results of the 53 km FSO Tbit/s single carrier data transmission experiments.

a Line rates (square symbols) of up to 1Tbit/s and net rates (circle symbols) as a function of symbol rate for different polarization-multiplexed (PM) QAM modulation formats. Highest data rates have been obtained with PM 126GBd-16QAM and PM 84GBd-64QAM modulation formats. One may also notice that transmission of 1 Tbit/s with 16QAM and 4QAM is a challenge as the transmitter lacks sufficient bandwidth. b Net rates as a function of 100 consecutive measurements (every 1.5 s) for PM 96GBd-16QAM and PM 124GBd-4QAM under strong-atmospheric-turbulence conditions with scintillation values SI of 1.75 and fried parameters r0 of 5.4 cm. The power fluctuations of the turbulent channel leads to a higher risk of losing data at higher modulation formats. One would rather resort to lower modulation formats such as 4QAM at highest symbol rates. c Normalized GMI (NGMI) as a function for received optical power (ROP), coupled to the fiber, for different PM QAM modulation formats at fixed line-rates of 500 Gbit/s. The dashed line indicates the threshold for a 0.75 FEC code rate. The inset shows the corresponding ROP probability distribution for each modulation format. Despite bandwidth constraints, 4QAM has still a sensitivity advantage of up to 1.7 dB and 6 dB over 16QAM and 64QAM and enables 500 Gbit/s transmission above the 0.75 FEC rate at ROP as low as −37 dBm. d NGMI as function of ROP for a single-polarization (SP) and a dual-polarization (DP) 16QAM modulation format at fixed line rates of 500 Gbit/s. The inset shows the corresponding ROP probability distribution for both measurements. Polarization-division multiplexing (PDM) is a technique that can double the spectral efficiency, allowing the same data rate to be transmitted using half the bandwidth. Theoretically, sending the information in a SP results in the same NGMI over ROP behavior. However, in practice, bandwidth limitations can limit the performance of SP transmission. For this reason, using PDM is attractive and results in a power gain of up to 5 dB