## 5 Bit- and Power-Loaded DMT for Highest Bit Rates
### Maximizing the Capacity of the RSOA-FCL  
In our experiments, we performed bit- and power-loading based on [PERSON] algorithm [38]. [PERSON]'s algorithm can briefly be described as follows. It takes in the SNR characterization of a transmitter at the subcarrier frequencies of the DMT signal. It also uses the SNR requirement of all advanced modulation formats (see Figure 3) to achieve a predetermined BER. Then, it disregards subcarriers with the lowest SNR that cannot support transmission at the predetermined BER for even the simplest format. The power of such unusable subcarriers is then transferred to usable subcarriers. For subcarrier with non-integer bit per symbol, the bit number is rounded off to the nearest integer, which requires the power to be re-adapted accordingly. This power adaptation step (power-loading) helps to achieve similar BER for all subcarriers.  
First, we performed bit- and power-loaded DMT transmission for the RSOA-FCL with 15 m, 115 m and 1 km cavities. The signal was optimized for a BtB experiment and for transmission over 25 km NZDSF. Figure 10a-c show the SNR, and the bit- as well as the power-loading patterns of the DMT and the measured per-subcarrier BER for 15 m, 115 m and 1 km long RSOA-FCLs, respectively. The 15 and 115 m cavities can support up to 5 bit/symbol (32 QAM) for low frequency subcarriers. In case of the 1 km RSOA-FCL, the highest order modulation format is 16 QAM, with 4 bits/symbol. The power-loading pattern is arranged to guarantee a minimum SNR difference for all subcarriers with the same modulation format, so that a similar BER performance can be obtained. The solid lines in the BER plots indicate the overall BER, i.e., the total BER across all subcarriers. It can be seen that the overall BER is below 9.29 \(\times\)\(10^{-3}\) in all instances, which is the limit for an FEC (forward error correction) with 12.5% overhead [54].  
Figure 10: Measured SNR, bit- and power-loading pattern, and bit error ratio (BER) versus sub-carrier frequencies and the achieved total line rate for different RSOA-FCL cavity lengths: Plots (**a–c**) show the results for 15, 115 m and 1 km long cavities, respectively. The red and orange colors indicate BtB (back-to-back) and 25 km NZDSF transmission measurements. In all cases, the overall BER (solid lines in the BER plots) are kept below the FEC limit of 9.29 \(\times\)\(10^{-3}\)[53]; (**d**) Plots the measured total line rate for 15, 115 m and 1 km long cavities, both for BtB and after 25 km transmission. In a BtB measurement, the RSOA-FCL offers up to 20 Gbit/s line rates. The line-rate is still 10 Gbit/s when transmitting signals over 25 km.  
Figure 10d shows the maximum line rates obtained with RSOA-FCL cavity lengths of 15 m, 115 m and 1 km length for a BtB and a transmission over 25 km NZDSF. Maximum capacities are obtained for short FCL cavities of 15 and 115 m length. The capacities are between 20 and 10 Gbit/s depending on the transmission distance. For longer cavity lengths the capacity dropped to 12.4 Gbit/s due to a reduced SNR and a limited bandwidth for the BtB experiment and 7 Gbit/s could at best be transmitted over 25 km.