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1. A 27 Gb/s 5.39 pJ/bit 8-ary Modulated Wireline Transceiver Using Pulse Width and Amplitude Modulation Achieving 9.5 dB SNR Improvement over PAM-8

   https://doi.org/https://doi.org/10.23919/VLSICircuits52068.2021.9492426  

   Megahed, M; Chun, Y; Wang, Z; and Anand, T (June 2021, Symposium on VLSI Circuits)

   This paper presents a novel 8-ary modulation technique with higher SNR compared to the PAM-8. The proposed modulation (SNR-Enhanced), modulates the pulse width and amplitude to achieve an average SNR improvement of 9.5 dB over PAM-8 in the near-end eye at the cost of 8.2% reduction in the horizontal eye margin. Using 3-tap FFE and CTLE, the proposed transceiver achieves 1×10 -7 BER at 9 dB channel loss with an efficiency of 5.39 pJ/bit in the 65 nm CMOS process. 

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2. Joint Probabilistic-Nyquist Pulse Shaping for LDPC-coded 8-PAM Signal in DWDM Data Center Communications

   Han, Xiao; Yang, Migwei; Djordjevic, Ivan B; Yue, Yang; Wang, Qiang; Qu, Zhen; Anderson, Jon (November 2019, Applied sciences)

   M-ary pulse-amplitude modulation (PAM) meets the requirements of data center communication because of its simplicity, but coarse entropy granularity cannot meet the dynamic bandwidth demands, and there is a large capacity gap between uniform formats and the Shannon limit. The dense wavelength division multiplexing (DWDM) system is widely used to increase the channel capacity, but low spectral efficiency of the intensity modulation/direct detection (IM/DD) solution restricts the throughput of the modern DWDM data center networks. Probabilistic shaping distribution is a good candidate to offer us a fine entropy granularity and efficiently reduce the gap to the Shannon limit, and Nyquist pulse shaping is widely used to increase the spectral efficiency. We aim toward the joint usage of probabilistic shaping and Nyquist pulse shaping with low-density parity-check (LDPC) coding to improve the bit error rate (BER) performance of 8-PAM signal transmission. We optimized the code rate of the LDPC code and compared different Nyquist pulse shaping parameters using simulations and experiments. We achieved a 0.43 dB gain using Nyquist pulse shaping, and a 1.1 dB gain using probabilistic shaping, while the joint use of probabilistic shaping and Nyquist pulse shaping achieved a 1.27 dB gain, which offers an excellent improvement without upgrading the transceivers. 

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3. High-Speed CMOS Silicon Photonic PAM4 Transceiver Front-End Circuits

   Palermo, S; Kumar, A; Liu, R; Yuan, Y; Peng, Y; Hong, C; Huang, Z; Kim, I; Fan, Y; Zhu, Y; et al (April 2025, 2025 International VLSI Symposium on Technology, Systems and Applications)

   Growing interconnect bandwidth demand in large datacenters requires energy-efficient optical transceivers that operate with four-level pulse amplitude modulation (PAM4) to enable high per-wavelength data rates. Further increases in bandwidth density is possible by leveraging wavelength-division multiplexing (WDM), which optical link architectures based on silicon photonic microring modulators (MRMs) and drop filters inherently enable. This paper presents high-speed PAM4 transmitter and receiver front-ends implemented in a 28nm CMOS process that are co-designed with these silicon photonic optical devices to enable energy-efficient operation. The transmitter utilizes an optical digital-to-analog converter (DAC) approach with two PAM2 AC-coupled pulsed-cascode high-swing voltage-mode output stages to drive the MRM MSB/LSB segments. A 3.42Vppd output swing is achieved when operating at 80Gb/s PAM4 with an energy efficiency of 3.66pJ/bit. The receiver front-end interfaces with a silicon-germanium avalanche photodiode (APD) and utilizes a low-bandwidth input transimpedance amplifier followed by continuous-time linear equalizer and variable-gain amplifier stages. Biasing the APD to realize a gain of 2 allows for -7dBm optical modulation amplitude (OMA) sensitivity at 56Gb/s PAM4 with a BER=10-4 and an energy efficiency of 1.61pJ/bit. Experimental verification of the full PAM4 transceiver at 50Gb/s operation shows -4.66dBm OMA sensitivity at a BER~4x10-4. 

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4. Orthogonal time-frequency space modulation for underwater mobile acoustic communications

   https://doi.org/10.1121/10.0035938  

   Xue, Yukang; Zhu, Xiyuan; Zheng, Y Rosa (February 2025, The Journal of the Acoustical Society of America)

   This paper presents a new turbo decision feedback equalizer and decoder (TDFED) for the orthogonal time-frequency space (OTFS) system of underwater mobile acoustic communications where the communication channel suffers from severe multipath and Doppler effects simultaneously. The proposed TDFED employs a set of feedforward and feedback filters in the time domain instead of the common approach that employs a normalized least mean square equalizer in the delay-Doppler domain. The receiver also utilizes low-complexity improved proportionate normalized least mean square channel estimation in the delay-Doppler domain. Practical OTFS modulation schemes are designed for acoustic transmission at a center frequency of 115 kHz and a symbol rate of 11.5 ksps (kilo-symbols-per-second). Several lake experiments in mobile communication scenarios are conducted to evaluate the proposed OTFS in comparison to the single-carrier coherent modulation (SCCM) and the orthogonal frequency division modulation (OFDM) schemes. The experimental results demonstrate that the proposed OTFS receiver effectively reduces the accuracy requirements of the Doppler compensation algorithm compared to the SCCM and OFDM schemes. The proposed TDFED algorithm achieves a much better bit error rate against long-multipath fading and severe Doppler shift than the existing delay-Doppler domain equalizers. 

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5. Independent component analysis for impairment mitigation in direct-detected Stokes vector modulation

   https://doi.org/10.1364/OE.474094  

   Bnyamin, Mario_V; Jiang, Xin; Feuer, Mark_D (November 2022, Optics Express)

   In this paper we theoretically and experimentally demonstrate a novel adaptation of independent component analysis (ICA) for compensation of both cross-polarization and inter-symbol interference in a direct-detection link using Stokes vector modulation (SVM). SVM systems suffer from multiple simultaneous impairments that can be difficult to resolve with conventional optical channel DSP techniques. The proposed method is based on a six-dimensional adaptation of ICA that simultaneously de-rotates the SVM constellation, corrects distortion of constellation shape, and mitigates inter-symbol interference (ISI) at high symbol rates. Experimental results at 7.5 Gb/s and 15Gb/s show that the newly developed ICA-based equalizer achieves power penalties below ∼1 dB, compared to the ideal theoretical bit-error rate (BER) curves. At 30-Gb/s, where ISI is more severe, ICA still enables polarization de-rotation and BER < 10⁻⁵before error correction. 

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