## 4 Experimental Setup and Results  
Figure 6 illustrates the experimental setup for the performance evaluation of the proposed CPR scheme. The transmitter is composed of an arbitrary waveform generator and an optical IQ modulator. A pseudorandom bit sequence consisting of \(2^{15}-1\) bits is generated and mapped onto symbols belonging to a C-16 QAM constellation according to the bit mapping shown in Figure 1. The I and Q output electrical signals are firstly linearly amplified and fed in the optical IQ modulators that have a 3-dB bandwidth of 25 GHz. The incoming electrical signal is modulated onto the transmitter laser having a \(\sim\)100-kHz intrinsic linewidth. A phase modulator is used to manipulate the frequency noise power spectral density of the transmitting laser. The arbitrary waveform generator is used to generate phase noise sequences corresponding to different white frequency noise levels which are linearly amplified and fed in the optical phase modulator in order to emulate the phase noise of a semiconductor laser [17; 18]. In order to avoid patterning effects and discontinuities in the phase noise sequence while it is being repeated in the AWG, the phase noise sequence needs to be large enough to ensure randomness and can be mirrored so as to match the initial and final points of the generated sequence [18]. The outgoing 28-Gbaud C-16 QAM signal is amplified using an EDFA and loaded with noise in the OSNR module. The OSNR module consists of an optical attenuator and an automatic gain control EDFA with constant output power. The signal is then directly fed into the coherent receiver and passed to the DSP module where the data was demodulated offline with different CPR algorithms in order to evaluate their performance. The Gardner algorithm was employed to achieve clock recovery, while the constant modulus algorithm followed by the multi-modulus algorithm were used for equalization [19; 20]. Differential decoding was employed in all cases to mitigate the effect of cycle slips [14]. The BER _versus_ OSNR performance of the proposed algorithm is compared with the BPSC-16 QAM and the single-stage n-PSK partitioning algorithms, as shown in Figure 7. The block  
Figure 5: Influence of the block size of each of the stages on the performance of the proposed two-stage n-PSK partitioning CPR scheme in (**a**) a C-16 QAM constellation and (**b**) a C-64 QAM constellation.  
length of each of the algorithms studied was optimized in each of the points of the curves to show the best performance. The number of test phases, \(\beta\), was set to 32 in BPS\({}_{\text{C-16 QAM}}\). The proposed two-stage n-PSK partitioning CPR scheme provides a higher performance compared to the other algorithms, and this performance gain increases with the laser linewidth. The OSNR penalty depends on the reference OSNR level to achieve a specified BER target, and it scales nonlinearly for different OSNR reference levels. Therefore, considering the extra OSNR implementation penalty in the experimental setup, the penalties observed in the experimental curves can be seen to be higher compared to those obtained in simulations where the OSNR reference was of 18.6 dB and 17.5 dB for BER targets of 3.8 \(\times\) 10\({}^{-3}\) and 1 \(\times\) 10\({}^{-2}\), respectively.