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Abstract
A radio frequency (RF) carrier can be used to mitigate the phase noise impact in n-level PSK and QAM systems. The systems performance is influenced by the use of an RF pilot carrier to accomplish phase noise compensation through complex multiplication in combination with discrete filters to compensate for the chromatic dispersion (CD). We perform a detailed study comparing two filters for the CD compensation namely the fixed frequency domain equalizer (FDE) filter and the adaptive least-mean-square (LMS) filter. The study provides important novel physical insight into the equalization enhanced phase noise (EEPN) influence on the system bit-error-rate (BER) versus optical signal-to-noise-ratio (OSNR) performance. Important results of the analysis are that the FDE filter position relative to the RF carrier phase noise compensation module provides a possibility for choosing whether the EEPN from the Tx or the LO laser influences the system quality. The LMS filter works very inefficiently when placed prior to the RF phase noise compensation stage of the Rx whereas it works much more efficiently and gives almost the same performance as the FDE filter when placed after the RF phase noise compensation stage.
Highlights
Fiber impairments, such as chromatic dispersion (CD) and phase noise, severely impact the performance of high speed optical fiber transmission systems [1,2]
In this type of Rx, the radio frequency (RF) pilot carrier can be used to mitigate the phase noise influence in n-level PSK and QAM systems, and a discrete type filter can be used to compensate for the chromatic dispersion influence
The system phase noise performance – for instance specified in terms of bit-error-rate (BER) versus optical signal-to-noise-ratio (OSNR) - is influenced by the implementation of a RF pilot carrier to accomplish phase noise compensation through complex multiplication in combination with discrete filters to compensate for the chromatic dispersion
Summary
Fiber impairments, such as chromatic dispersion (CD) and phase noise, severely impact the performance of high speed optical fiber transmission systems [1,2]. Digital coherent receivers allow complete equalization of chromatic dispersion (a linear transmission impairment) in the electrical domain by using discrete signal processing (DSP) techniques. Some earlier works for PSK/QAM systems have considered an RF pilot tone transmitted in the orthogonal polarization relative to the signal [24,25] This removes any limitations for the RF bandwidth and significantly simplifies the receiver implementation which would be more difficult to perform when including filtering to recover the RF pilot tone. This represents a limiting (best possible) situation for the elimination of phase noise and is the RF tone implementation considered in this paper. We utilize the software tool from VPI [26] for the system simulations, and we evaluate the bit-error-rate versus optical signal-to-noise ratio (OSNR)
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