Modified DSP with reduced complexity in adaptive equalization for DCI systems

We propose and demonstrate bi-directional (BiDi) self-homodyne coherent detection (SHCD) system experimentally, in which the multiple input multiple output free (MIMO-free) digital signal processing (DSP) is achieved with the help of BiDi-operated adaptive polarization controller (APC). Since the polarization wandering of the local oscillator (LO) is relevant to that of the counter-propagating signal within the same fiber, the APC can realize the polarization tracking and compensation for LO and the counter-propagating signal simultaneously. The power-consuming 2x2 MIMO DSP, can be eliminated in our proposed BiDi SHCD system. The frequency offset compensation (FOC), carrier phase recovery (CPR) are also omitted thanks to the SHCD. Besides, the dedicated chromatic dispersion compensation (CDC) is no longer needed due to short-reach transmission of data center interconnect (DCI). We validate in experiment the proposed system using 23GBaud DP-32QAM signal delivering 230Gb/s over 2 km of single mode fibers (SMFs) with the laser-linewidth tolerance of 1MHz. The parameters of polarization tracking algorithm are optimized to attain the polarization rotation tolerance up to 850 rad/s under the FEC threshold of 2x10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> . The proposed BiDi SHCD system achieves the coherent-lite DSP without MIMO, FOC, CPR and dedicated CDC algorithm in dynamic state of polarization (SOP) rotation while maintaining the full spectral efficiency (SE) of traditional coherent system.

We experimentally demonstrate a novel digital signal processing (DSP) structure for reduced guard-interval (RGI) OFDM coherent optical systems. The proposed concept is based on digitally slicing optical channel bandwidth into multiple spectrally disjoint sub-bands which are then processed in parallel. Each low bandwidth sub-band has a smaller delay-spread compared to a full-band signal. This enables compensation of both chromatic dispersion (CD) and polarization mode dispersion using a simple timing and one-tap-per-symbol frequency domain equalizer with a small cyclic prefix overhead. In terms of the DSP architecture, this allows for a highly efficient parallelization of DSP tasks performed over the received signal samples by deploying multiple processors running at a lower clock rate. It should be noted that this parallelization is performed in the frequency domain and it allows for flexible optical transceiver schemes. In addition, the resulting optical receiver is simplified due to the removal of the CD compensation equalizer compared to conventional RGI-OFDM systems. In this paper we experimentally demonstrate digital sub-banding of optical bandwidth. We test the system performance for different modulation formats (QPSK, 16QAM and 32QAM) over various transmission distances and optical launch powers using a 1.5% CP overhead in all scenarios. We also compare the proposed RGI-OFDM architecture performance against common single carrier modulation formats. At the same total data rate and signal bandwidth both systems have similar performance and transmission reach whereas the proposed method allows for a significant reduction of computational complexity due to removal of CD pre/post compensation equalizer.

Demonstration of C-band 112G PAM-4 transmission over 80-km stand-single mode fiber based on direct-detection

The subject addressed here is a design of an antimultipath facling used in Time-Division Multiple Access (TDMA) operation. The feature of this modem is an effective execution of fast burst acquisition and adaptive equalization by all digital signal processing. This modem mainly consists of a data memory and phase rotation circuit, an incoherent correlator, an adaptive equalizer and a frame synchronizer. Detailed acquisition algorithm using the incoherent correlator a n d the adaptive equalizer is described and perforn~ances are assessed by computer simulation and sorne hardware experimentation. This modem is capable of executing fast acquisition and stable demodulation in the presence of a large f requency offset in s e v e r e mult ipath fad ing environment.

In this paper, a new block adaptive decision feedback equalizer (DFE) implemented in the frequency domain is derived. The new algorithm is suitable for applications requiring long adaptive equalizers, as is the case in several high-speed wireless communication systems. The inherent causality problem appearing in the block adaptive formulation of the DFE equations is overcome by using tentative decisions in place of the unknown ones within each block. These tentative decisions are subsequently improved by using an efficient iterative procedure, which finally converges to the optimum decisions in a few iterations. This procedure is properly initialized by applying a minimization criterion that utilizes all the available information. The whole algorithm, including the iterative procedure, is implemented in the frequency domain and exhibits a considerable reduction in computational complexity, as compared with the conventional DFE, offering, at the same time, a noticeable increase in convergence speed. Additionally, the level of the steady-state MSE, which is achieved by the new algorithm, is practically insensitive to the block length.

We propose a polarization dependent loss (PDL) and chromatic dispersion (CD) insensitive, low-complexity adaptive equalizer (AEQ) for short-reach coherent optical transmission systems. The AEQ contains a 1-tap butterfly finite impulse response (FIR) filter and two N-tap FIR filters. It first performs polarization demultiplexing using the 1-tap filter, of which the coefficients are obtained based on Stokes space. Then it mitigates the inter-symbol interference (ISI) using the two N-tap finite impulse response (FIR) filters and adjust the filter's coefficients by utilizing constant modulus algorithm (CMA). Through theoretical and experimental analysis, we verify that this proposed AEQ can perform robust polarization demultiplexing when PDL and CD exists. Besides, our proposed AEQ has faster convergence speed compared with recently proposed AEQs. In addition, it reduces the number of multipliers and thus reduce the computational complexity of conventional butterfly filter structure AEQ. And this proposed AEQ suffers little bit error ratio loss compared with the conventional AEQ. Due to the low-complexity and robustness to PDL and CD, this proposed AEQ is well-suited for future low-cost short-reach optical communication system.

As a low-energy method to increase the data rate of optical links in data centers, we propose self-homodyne Nyquist optical time division multiplexing (OTDM). In Nyquist OTDM, spectrally efficient high-baud rate signals can be generated exceeding the limit of electronic signal processing. However, full integration of OTDM systems has not been reported, mainly because of the complicated signal detection scheme, which involves demultiplexing and clock recovery. In our proposal, the Nyquist pulse train is transmitted to the receiver as a local oscillator (LO) to leverage self-homodyne detection, which allows using large linewidth lasers and a simplified digital signal processing (DSP) algorithm. As the transmitted pulse train serves as an optical clock, demultiplexing and detection of the OTDM signal can be performed without using power-intensive high-bandwidth electronics and DSP. In this method, the LO pulse train needs to enter the coherent detector in exact synchronization with the OTDM signal for detecting the individual tributary correctly. For this purpose, we present a pulse delay control method suitable for photonic integration. A Nyquist pulse train with m carriers enables m-time multiplexing of optical signals. We explain and demonstrate the proposed concept in the case of m = 2, as it is the most feasible implementation. In the O-band where the chromatic dispersion (CD) is negligible, DSP-free operation can be achieved using the QPSK format. At the band edges where CD is non-negligible, it can be compensated by the DSP as in the conventional coherent detection. We verify this numerically and in an experiment involving the transmission of a 64-Gbaud QPSK signal at 1550 nm over a single-mode fiber. In terms of low energy, self-homodyne Nyquist OTDM is advantageous in wavelength division multiplexing (WDM). Taking it into consideration, we perform 4-channel WDM transmission of the 64-Gbaud QPSK signal over a 1-km dispersion shifted fiber without CD compensation. The results demonstrate a data rate of 512 Gb/s with a BER of <1 × 10-10.

We evaluate the performance of direct-detection 25 Gb/s Passive Optical Networks (PON) with adaptive equalization at the receiver side, comparing three transmitter schemes: two including digital signal processing (DSP), namely square-root raised-cosine pulse shaping and pre-emphasis, and the third one without any DSP pre-compensation. We show that DSP at transmitter side can provide a performance advantage only under strong bandwidth limitations and when considering feed-forward equalization (FFE) at the receiver. When including decision-feedback equalization (DFE), the use of pre-compensation at transmitter does not provide any advantage under linear transmission.

A single-chip 2400 bps modem which has been implemented in a 3.6 μm Si gate CMOS process will be described. An analog front end (AFE) and digital signal processor (DSP) are included on the IC. The transmit-side and part of the receive-side signal processing are performed in the analog domain. The DSP is dedicated to receive signal processing - timing recovery, adaptive equalization, carrier recovery/demodulation and decoding. The chip operates off a single 5 V supply, occupies 68.8 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and dissipates 120 mW. The design and performance of the IC will be presented.

Optical transmitters for four-level pulse amplitude modulation (PAM-4) have attracted a significant amount of research in recent years, in large part due to the standardization of the format for the 200 and 400 Gigabit Ethernet optical interconnects in data centers. However, combining low-power and linear operation of the electro-optical frontend with sufficiently large bandwidths has proven challenging, especially for the 100 Gb/s/lambda links (i.e., employing 50 Gbaud PAM-4). The most straightforward solution has been to deal with the non-idealities of the modulator in the electrical domain: predistorting the signal levels and/or equalizing the frequency response with the help of digital signal processing (DSP). However, this typically requires fast digital-to-analog converters (DACs), either capable of delivering large swings (&gt;1 Vpp) or supplemented with an additional linear amplifier to drive the optical modulator. Both options substantially increase the power consumption and the complexity of the transceiver. Rather than allocating effort to linearize the electrical to optical conversion of a single modulator, we propose a topology that performs the DAC operation in the optical domain. Two compact electro-absorption modulators in an interferometer layout are driven with NRZ data to generate the four-level signal in the optical domain. Using this topology, we demonstrate the first real-time 128 Gb/s PAM-4 transmission with a silicon photonic transmitter in a chip-to-chip link. In a back-to-back setup, we obtained a bit-error ratio (BER) of 4 x 10(-10) without requiring any DAC, DSP, or modulators with large traveling wave structures. Over 1 km of standard single mode fiber a BER of 8 x 10(-6) is recorded, still well below the KP4 forward error-coding limit. These results correspond to the lowest BERs reported for any real-time PAM-4 link at 100 Gb/s or higher, illustrating the benefit of performing the DAC operation in the optical domain.

We experimentally demonstrate a seamlessly integrated fiber-wireless system that delivers a 108 Gb/s signal through 80 km fiber and 1 m wireless transport over free space at 100 GHz adopting polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) modulation and heterodyning coherent detection. The X- and Y-polarization components of the optical PDM-QPSK baseband signal are simultaneously upconverted to 100 GHz wireless carrier by optical polarization-diversity heterodyne beating, and then independently transmitted and received by two pairs of transmitter and receiver antennas, which form a 2×2 multiple-input multiple-output wireless link. At the wireless receiver, two-stage downconversion is performed firstly in the analog domain based on balanced mixer and sinusoidal radio frequency signal, and then in the digital domain based on digital signal processing (DSP). Polarization demultiplexing is realized by the constant modulus algorithm in the DSP part at the receiver. The bit-error ratio for the 108 Gb/s PDM-QPSK signal is less than the pre-forward-error-correction threshold of 3.8×10(-3) after both 1 m wireless delivery at 100 GHz and 80 km single-mode fiber-28 transmission. To our knowledge, this is the first demonstration to realize 100 Gb/s signal delivery through both fiber and wireless links at 100 GHz.

In order to enhance the performance of 112 Gb/s polarization-multiplexed quadrature phase-shift keying (PM-QPSK) coherent optical receivers, a novel digital signal processing (DSP) framework is presented in this study. The suggested method combines cutting-edge signal processing techniques to address important constraints in long-distance, high data rate coherent systems. The framework uses overlap frequency domain equalization (OFDE) for chromatic dispersion (CD) compensation, which offers a cheaper computational cost and higher dispersion control precision than traditional time-domain equalization. An adaptive carrier phase recovery (CPR) technique based on mean-squared differential phase (MSDP) estimation is incorporated to manage phase noise induced by cross-phase modulation (XPM), providing dependable correction under a variety of operating situations. When combined, these techniques significantly increase Q factor performance, and optimum systems can handle transmission distances of up to 2400 km. The suggested DSP approach improves phase stability and dispersion tolerance even in the presence of nonlinear impairments, making it a viable and effective choice for contemporary coherent optical networks. The framework’s competitiveness was evaluated by comparing it against the most recent, cutting-edge DSP methods that were released after 2021. These included CPR systems that were based on kernels, transformers, and machine learning. The findings show that although AI-driven approaches had the highest absolute Q factors, they also required a large amount of computing power. On the other hand, the suggested OFDE in conjunction with adaptive CPR achieved Q factors of up to 11.7 dB over extended distances with a significantly reduced DSP effort, striking a good balance between performance and complexity. Its appropriateness for scalable, long-haul 112 Gb/s PM-QPSK systems is confirmed by a complexity versus performance trade-off analysis, providing a workable and efficient substitute for more resource-intensive alternatives.

The surge in interest regarding the next generation of optical fiber transmission has stimulated the development of digital signal processing (DSP) schemes that are highly cost-effective with both high performance and low complexity. As benchmarks for nonlinear compensation methods, however, traditional DSP designed with block-by-block modules for linear compensations, could exhibit residual linear effects after compensation, limiting the nonlinear compensation performance. Here we propose a high-efficient design thought for DSP based on the learnable perspectivity, called learnable DSP (LDSP). LDSP reuses the traditional DSP modules, regarding the whole DSP as a deep learning framework and optimizing the DSP parameters adaptively based on backpropagation algorithm from a global scale. This method not only establishes new standards in linear DSP performance but also serves as a critical benchmark for nonlinear DSP designs. In comparison to traditional DSP with hyperparameter optimization, a notable enhancement of approximately 1.21 dB in the Q factor for 400 Gb/s signal after 1600 km fiber transmission is experimentally demonstrated by combining LDSP and perturbation-based nonlinear compensation algorithm. Benefiting from the learnable model, LDSP can learn the best configuration adaptively with low complexity, reducing dependence on initial parameters. The proposed approach implements a symbol-rate DSP with a small bit error rate (BER) cost in exchange for a 48% complexity reduction compared to the conventional 2 samples/symbol processing. We believe that LDSP represents a new and highly efficient paradigm for DSP design, which is poised to attract considerable attention across various domains of optical communications.

Digital coherent receivers are essential to the attainment of high spectral efficiency in high capacity fiber-optic transmission. Coherent receivers detect both amplitude and phase of the optical field, which permits the use of advanced modulation formats by the transmitter. A real-time coherent optical receiver using Digital Signal Processing (DSP) enables the transmission system to perform dispersion compensation, carrier phase estimation, polarization de-multiplexing, polarization mode dispersion compensation and data recovery for an advanced modulated format. DSP methods that have been suggested to de-multiplex data from mixed polarizations fall into two classes. First class includes the Least Mean Square algorithm and the decision-directed algorithm require training sequences. The second class that includes the Constant Modulus Algorithms (CMA) and Independent Component Analysis (ICA) is preferable because they work blind and do not need training sequences. The blind source separation methods are preferable since they estimate the source signals directly from the observed signals and, in addition, do not limiting the spectral efficiency. Although, ICA solves the convergence-to-same-source problem of CMA while possessing similar polarization tracking capability, it inherently suffers from a drawback, which is called permutation ambiguity.

A polarisation-division-multiplexing (PDM)-based four-level pulse amplitude modulation (PAM4) fibre-free-space optical (FSO) convergent system with optical band-pass filters (OBPFs) for polarisation de-multiplexing is feasibly demonstrated for the first time. In a PDM scenario with PAM4 modulation, the transmission capacity of fibre-FSO convergent systems is enhanced four times with an aggregate channel capacity of 128 Gb/s (64 Gb/s PAM4/polarisation × 2 polarisations). With an OBPF, polarisation-tracking free de-multiplexing is attained by eliminating other optical carrier with orthogonal polarisation. An OBPF is a simple polarisation de-multiplexing scheme in which the polarisation-orthogonal carrier can be effectively de-multiplexed and the cross-polarisation interference can be nearly eliminated. Compared with traditional PDM-based fibre-FSO convergent systems with sophisticated polarisation-tracking mechanism and elaborate digital signal processing (DSP) approach, it reveals a noteworthy one with the advantage of simplicity. Through 25 km single-mode fibre transport and 500 m FSO link, sufficiently low bit error rate and qualified PAM4 eye diagrams are attained. This proposed polarisation-tracking free PDM-based fibre-FSO convergent system is notable because it not only incorporates the fibre backbone and optical wireless feeder, but it also simplifies the framework since complicated polarisation-tracking mechanism and DSP approach are not involved.