The future-generation passive optical network (PON) physical layer, targeting 100 Gbps/wavelength, will have to deal with severe optoelectronics bandwidth and chromatic dispersion limitations. In this paper, largely extending our Optical Fiber Communication Conference (OFC) 2020 invited paper, we review 100 Gbps/wavelength PON downstream alternatives over standard single-mode fiber in the O- and C-bands, analyzing three modulation formats (PAM-4, partial-response PAM-4, and PAM-8), two types of direct-detection receivers (APD- and SOA
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PIN-based), and three digital reception strategies (unequalized, feed-forward equalized, and decision-feedback equalized). We evaluate by means of simulations the performance of these alternatives under different optoelectronics bandwidth and dispersion scenarios, identifying O-band feasible solutions able to reach 20 km of fiber and an optical path loss of at least 29 dB over a wide wavelength range of operation. Finally, we compare two digitally precompensated modulation schemes that are highly tolerant of chromatic dispersion, showing a possible extension to C-band operation, preserving direct-detection and linear-impairment equalization at the optical network unit side.
Next-generation high-speed passive optical network (HS-PON) transceivers supporting 25, 50 and 100 Gb/s are under the early stage of their standardization process. One key aspect of this process is the choice of the best modulation format. To this end, performance comparisons among several modulation formats against different physical constraints have been presented in literature and are still being carried out. In our contribution, we performed an exhaustive analysis on the impact of transceivers electrical frequency response shape on the performance of 2-levels pulse amplitude modulation (PAM-2), 4-levels PAM (PAM-4), electrical and optical duobinary modulation formats with adaptive equalizer at the receiver side. We show by means of numerical simulations that the specification of the typically used -3dB bandwidth is not sufficient, since also out-of-band electrical frequency response specifications (such as the -20dB bandwidth) has a huge impact on the performance of the analyzed modulation formats. The normalized graphs given at the end of the paper in terms of -3dB and -20 dB bandwidths can thus be useful for the design of the next generation of HS-PON transceivers.
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