Level allocation of four-level pulse-amplitude modulation signal in optically pre-amplified receiver systems

This study numerically investigates the symbol-level allocation of four-level pulse-amplitude modulation (PAM4) signals for optically pre-amplified receiver systems. Three level-allocation schemes are examined: intensity-equispaced, amplitude-equispaced, and numerically optimized. Numerical simulations are conducted to comprehensively com-pare the receiver sensitivities for these level-allocation schemes under various system conditions. The results show that the superiority or inferiority between the level allocations is significantly dependent on the system conditions of the bandwidth of amplified spontaneous emission light, modulation bandwidth, and signal extinction ratio (ER). The mechanisms underlying these dependencies are also discussed.