Hardware-efficient blind frequency offset estimation for digital subcarrier multiplexing signals

Abstract

Since the frequency offset estimation (FOE) must be implemented before the subcarrier de-multiplexing and chromatic dispersion compensation (CDC) for digital subcarrier multiplexing (DSM) signals, traditional FOE algorithms for single carrier transmission is no longer suitable. Here, we propose a hardware-efficient blind FOE solution for the DSM signals by monitoring spectral dips in the frequency domain. With the use of a smoothing filter, the estimation accuracy of FOE can be significantly increased. Moreover, we identify that the proposed FOE method is robust to various transmission impairments, including amplified spontaneous emission (ASE) noise, optical filtering, and fiber nonlinearity. The effective function of the proposed FOE method is numerically and experimentally verified under scenarios of both back-to-back (B2B) and the 2560 km standard single-mode fiber (SSMF) transmission, leading to a FOE error less than 100 MHz with a FFT size of 1024.

A Chromatic Dispersion-Tolerant Frequency Offset Estimation Algorithm Based on Pilot Tone for Digital Subcarrier Multiplexing Systems

“…In this section, the computational complexity of the proposed FOE scheme is analyzed. The complexity of two reported blind FOEs for DSCM systems from [13,15] are also referred to as comparisons.…”

“…These lead to the FOE algorithms for DSCM systems should be tolerant to CD. However, the mainstream blind FOE algorithms designed for SC systems, such as fast Fourier transform (FFT)-based FOE and differential-based FOE, would fail to work when CD is not compensated [13]. A FOE algorithm that estimates FO by searching the SCM spectrum dip bottom between subcarriers is proposed to address this challenge.…”

“…A FOE algorithm that estimates FO by searching the SCM spectrum dip bottom between subcarriers is proposed to address this challenge. This algorithm estimates FO by comparing the dip bottom frequency of the original SCM spectrum with that of the received SCM spectrum [13]. However, it has been found that this algorithm cannot work with the DSCM systems using a small root-raised-cosine (RRC) roll-off factor.…”

“…In addition, the two-stage FOE structure for reducing computational complexity is described, and the interaction between the pilot tone-to-signal power ratio (PSR) and the Q-factor is discussed in this section. Then, the complexity of the proposed algorithm and two reported FOE algorithms from [13,15] are analyzed and compared under the assumption of the same estimation accuracy in Section 3. In Sections 4 and 5, simulations and offline experiments verify that the proposed algorithm is tolerant to CD and feasible for DSCM systems.…”

Digital subcarrier multiplexing-enabled carrier-free phase-retrieval receiver

“…The DSM technique is based on dividing a high baud-rate single-carrier signal into multiple lower baud-rate subcarrier signals, which has been extensively explored in coherent fiber optical transmission systems 17 – 22 For the generation of conventional single-carrier signals, a sequence of QAM symbols is first generated after the bit-to-symbol mapping, then up-sampled to two samples per symbol (Sps) and spectrally shaped by a root-raised cosine (RRC) filter in the frequency domain with a roll-off factor of α. As for the generation of DSM signals shown in Fig.…”