## 2 Special Issue Papers  
In this Special Issue, we present nine carefully selected papers, including three review articles and six contributed papers. These papers cover advanced DSP techniques for long-distance, short-reach applications, including systems that use conventional single mode fibers (SMFs) and those based on space-division multiplexing (SDM) fibers, as well as links that use free-space wireless transmission. The following summarizes these papers:  
* Advanced DSP for Coherent Optical Fiber Communication [2] This paper provides an overview of recent progress on advanced DSP techniques for high-capacity, long-haul, coherent optical fiber transmission systems. The authors first introduce the principle and scheme of coherent detection to explain why DSP can compensate for transmission impairments. Then, the corresponding techniques for nonlinearity compensation, frequency-domain equalization (FDE), SDM, and machine learning (ML) are discussed. Relevant techniques are analyzed, and representational results and experimental verifications are demonstrated as well. This paper also provides a brief conclusion and future perspectives at the end. * Recent Advances in DSP Techniques for Mode Division Multiplexing Optical Networks with MIMO Equalization: A Review [3] This paper provides a technical review regarding the latest progress on multi-input multi-output (MIMO) DSP equalization techniques for high-capacity fiber-optic communication networks. The authors discuss state of the art of MIMO equalizers, predominantly focusing on the advantages of implementing the space-time block coding (STBC)-assisted MIMO technique. They also present a performance evaluation of different MIMO frequency-domain equalization (FDE) schemes for differential mode group delay (DMGD) and mode-dependent loss (MDL) issues in adaptive coherent receivers. Moreover, optimization of hardware complexity in MIMO-DSP is discussed, and a joint-compensation scheme is deliberated for CD and DMGD, along with a number of recent experimental demonstrations using MIMO-DSP. * Recent Advances in Equalization Technologies for Short-Reach Optical Links Based on PAM4 Modulation: A Review [4] The authors review the latest progress on DSP equalization technologies for short-reach optical links based on four-level pulse amplitude modulation (PAM4) modulation. They introduce the configuration and challenges of the transmission system, and cover the principles and performance of different equalizers and some improved methods. In addition, machine learning algorithms are discussed to mitigate nonlinear distortion for next-generation short-reach PAM4 links. A summary of various equalization technologies is illustrated, and a perspective of the future trend is given as well. * Experimental Investigation of 400 Gb/s Data Center Interconnect Using Unamplified High-Baud-Rate and High-Order QAM Single-Carrier Signal [5] The authors review the latest progress on data center interconnects (DCI). They also discuss different perspectives on the 400G pluggable module, including form factor, architecture, digital signal processing (DSP), and module power consumption, following 400G pluggable optics in DCI applications. The authors also experimentally investigate the capacity-reach matrix for high-baud-rate and high-order quadrature amplitude modulation (QAM) single-carrier signals in unamplified single-mode optical fiber (SMF) links. * Compensation of Limited Bandwidth and Nonlinearity for Coherent Transponder [6] The authors present a novel solution for optimizing the coefficients of digital filters to mitigate impairments due to limited bandwidth and nonlinearity in coherent transponders. They show that limited bandwidth is improved by the finite impulse response filter, while nonlinearity is mitigated by the memoryless Volterra filter.