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1. Compress-and-Forward via Multilevel Coding

   Wan, H. ; Host-Madsen, A. ; Nosratinia, A. ( July 2019 , IEEE International Symposium on Information Theory)

   We investigate the performance of discrete (coded) modulations in the full-duplex compress-forward relay channel using multilevel coding. We numerically analyze the rates assigned to component binary codes of all levels. LDPC codes are used as the component binary codes to provide error protection. The compression at the relay is done via a nested scalar quantizer whose output is mapped to a codeword through LDPC codes. A compound Tanner graphical model and information-exchange algorithm are described for joint decoding of both messages sent from the source and relay. Simulation results show that the performance of the proposed system based on multilevel coding is better than that based on BICM, and is separated from the SNR threshold of the known CF achievable rate by two factors consisting approximately of the sum of the shaping gain (due to scalar quantization) and the separation of the LDPC code implementation from AWGN capacity. 

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2. Belief propagation with quantum messages for quantum-enhanced classical communications

   https://doi.org/10.1038/s41534-021-00422-1  

   Rengaswamy, Narayanan ; Seshadreesan, Kaushik P. ; Guha, Saikat ; Pfister, Henry D. ( June 2021 , npj Quantum Information)

   For space-based laser communications, when the mean photon number per received optical pulse is much smaller than one, there is a large gap between communications capacity achievable with a receiver that performs individual pulse-by-pulse detection, and the quantum-optimal “joint-detection receiver” that acts collectively on long codeword-blocks of modulated pulses; an effect often termed “superadditive capacity”. In this paper, we consider the simplest scenario where a large superadditive capacity is known: a pure-loss channel with a coherent-state binary phase-shift keyed (BPSK) modulation. The two BPSK states can be mapped conceptually to two non-orthogonal states of a qubit, described by an inner product that is a function of the mean photon number per pulse. Using this map, we derive an explicit construction of the quantum circuit of a joint-detection receiver based on a recent idea of “belief-propagation with quantum messages” (BPQM). We quantify its performance improvement over the Dolinar receiver that performs optimal pulse-by-pulse detection, which represents the best “classical” approach. We analyze the scheme rigorously and show that it achieves the quantum limit of minimum average error probability in discriminating 8 (BPSK) codewords of a length-5 binary linear code with a tree factor graph. Our result suggests that a BPQM receiver might attain the Holevo capacity of this BPSK-modulated pure-loss channel. Moreover, our receiver circuit provides an alternative proposal for a quantum supremacy experiment, targeted at a specific application that can potentially be implemented on a small, special-purpose, photonic quantum computer capable of performing cat-basis universal qubit logic.

    

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3. Photonic Networks-on-Chip Employing Multilevel Signaling: A Cross-Layer Comparative Study

   https://doi.org/10.1145/3487365  

   Karempudi, Venkata Sai ; Sunny, Febin ; Thakkar, Ishan G. ; Chittamuru, Sai Vineel ; Nikdast, Mahdi ; Pasricha, Sudeep ( July 2022 , ACM Journal on Emerging Technologies in Computing Systems)

   Photonic network-on-chip (PNoC) architectures employ photonic links with dense wavelength-division multiplexing (DWDM) to enable high throughput on-chip transfers. Unfortunately, increasing the DWDM degree (i.e., using a larger number of wavelengths) to achieve a higher aggregated data rate in photonic links and, hence, higher throughput in PNoCs, requires sophisticated and costly laser sources along with extra photonic hardware. This extra hardware can introduce undesired noise to the photonic link and increase the bit error rate (BER), power, and area consumption of PNoCs. To mitigate these issues, the use of 4-pulse amplitude modulation (4-PAM) signaling, instead of the conventional on-off keying (OOK) signaling, can halve the wavelength signals utilized in photonic links for achieving the target aggregate data rate while reducing the overhead of crosstalk noise, BER, and photonic hardware. There are various designs of 4-PAM modulators reported in the literature. For example, the signal superposition (SS)–, electrical digital-to-analog converter (EDAC)–, and optical digital-to-analog converter (ODAC)–based designs of 4-PAM modulators have been reported. However, it is yet to be explored how these SS-, EDAC-, and ODAC-based 4-PAM modulators can be utilized to design DWDM-based photonic links and PNoC architectures. In this article, we provide a systematic analysis of the SS, EDAC, and ODAC types of 4-PAM modulators from prior work with regards to their applicability and utilization overheads. We then present a heuristic-based search method to employ these 4-PAM modulators for designing DWDM-based SS, EDAC, and ODAC types of 4-PAM photonic links with two different design goals: (i) to attain the desired BER of 10 -9 at the expense of higher optical power and lower aggregate data rate and (ii) to attain maximum aggregate data rate with the desired BER of 10 -9 at the expense of longer packet transfer latency. We then employ our designed 4-PAM SS–, 4-PAM EDAC–, 4-PAM ODAC–, and conventional OOK modulator–based photonic links to constitute corresponding variants of the well-known CLOS and SWIFT PNoC architectures. We eventually compare our designed SS-, EDAC-, and ODAC-based variants of 4-PAM links and PNoCs with the conventional OOK links and PNoCs in terms of performance and energy efficiency in the presence of inter-channel crosstalk. From our link-level and PNoC-level evaluation, we have observed that the 4-PAM EDAC–based variants of photonic links and PNoCs exhibit better performance and energy efficiency compared with the OOK-, 4-PAM SS–, and 4-PAM ODAC–based links and PNoCs. 

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4. 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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5. Triply resonant coupled-cavity electro-optic modulators for RF to optical signal conversion

   https://doi.org/10.1364/OE.385856  

   Gevorgyan, Hayk ; Khilo, Anatol ; Ehrlichman, Yossef ; Popović, Miloš A. ( January 2020 , Optics Express)

   We propose an on-chip triply resonant electro-optic modulator architecture for RF-to-optical signal conversion and provide a detailed theoretical analysis of the optimal “circuit-level” device geometries and their performance limits. The designs maximize the RF-optical conversion efficiency through simultaneous resonant enhancement of the RF drive signal, a continuous-wave (CW) optical pump, and the generated optical sideband. The optical pump and sideband are resonantly enhanced in respective supermodes of a two-coupled-cavity optical resonator system, while the RF signal can be enhanced in addition by an LC circuit formed by capacitances of the optical resonator active regions and (integrated) matching inductors. We show that such designs can offer 15-50 dB improvement in conversion efficiency over conventional microring modulators. In the proposed configurations, the photon lifetime (resonance linewidth) limits the instantaneous RF bandwidth of the electro-optic response but does not limit its central RF frequency. The latter is set by the coupling strength between the two coupled cavities and is not subject to the photon lifetime constraint inherent to conventional singly resonant microring modulators. This feature enables efficient operation at high RF carrier frequencies without a reduction in efficiency commonly associated with the photon lifetime limit and accounts for 10-30 dB of the total improvement. Two optical configurations of the modulator are proposed: a “basic” configuration with equal Q-factors in both supermodes, most suitable for narrowband RF signals, and a “generalized” configuration with independently tailored supermode Q-factors that supports a wider instantaneous bandwidth. A second significant 5-20 dB gain in modulation efficiency is expected from RF drive signal enhancement by integrated LC resonant matching, leading to the total expected improvement of 15-50 dB. Previously studied triply-resonant modulators, with coupled longitudinal (across the free spectral range (FSR)) modes, have large resonant mode volume for typical RF frequencies, which limits the interaction between the optical and RF fields. In contrast, the proposed modulators support maximally tightly confined resonant modes, with strong coupling between the mode fields, which increases and maintains high device efficiency across a range of RF frequencies. The proposed modulator architecture is compact, efficient, capable of modulation at high RF carrier frequencies and can be applied to any cavity design or modulation mechanism. It is also well suited to moderate Q, including silicon, implementations, and may be enabling for future CMOS RF-electronic-photonic systems on chip.

    

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