  Recently, coherent digital subcarrier multiplexing (DSCM) technology has
become an attractive solution for next-generation ultra-high-speed datacenter
interconnects (DCIs). To meet the requirements of low-cost and low-power
consumption in DCI applications, a comprehensive simplification of the coherent
DSCM system has been investigated. The pilot-tone-based polarization
demultiplexing (PT-PDM) technique, known for its low-power consumption and
ultra-fast polarization tracking capabilities, has emerged as a compelling
alternative to the power-hungry N-tap adaptive multi-input multiple-output
(MIMO) equalizer. However, the effectiveness of this PT-PDM technique is
extremely vulnerable to the receiver-side XY-skew (Rx-XY-skew), which is
revealed in this paper for the first time. Then, a pilot-tone-enabled modified
Godard phase detector (PT-MGPD) scheme is proposed to realize Rx-XY-skew
estimation, serving as the prerequisite for the successful implementation of
the PT-PDM and simplification of the adaptive equalizer. Both the simulation
and experiment are conducted to evaluate the accuracy of the proposed PT-MGPD
scheme. The results prove it can achieve accurate estimation with an error of
less than 0.3ps. Besides, a low-complexity, high-spectral-efficiency, and
ultra-fast polarization demultiplexing method based on a single pilot tone
(SPT) is proposed for the DSCM system in this work. Based on the proposed
PT-MGPD and SPT schemes, the conventional N-tap MIMO equalizer served for each
subcarrier can be successfully pruned into two polarization-independent
single-input single-output equalizers, and there is no performance penalty even
if the polarization rotation speed reaches 10Mrad/s. According to the results,
the proposed schemes provide a hardware-efficient and reliable coherent DSCM
solution for next-generation ultra-high-speed DCIs.
