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1. Optimized communication strategies with binary coherent states over phase noise channels

   https://doi.org/10.1038/s41534-019-0177-4  

   DiMario, M. T. ; Kunz, L. ; Banaszek, K. ; Becerra, F. E. ( July 2019 , npj Quantum Information)

   The achievable rate of information transfer in optical communications is determined by the physical properties of the communication channel, such as the intrinsic channel noise. Bosonic phase noise channels, a class of non-Gaussian channels, have emerged as a relevant noise model in quantum information and optical communication. However, while the fundamental limits for communication over Gaussian channels have been extensively studied, the properties of communication over Bosonic phase noise channels are not well understood. Here we propose and demonstrate experimentally the concept of optimized communication strategies for communication over phase noise channels to enhance information transfer beyond what is possible with conventional methods of modulation and detection. Two key ingredients are generalized constellations of coherent states that interpolate between standard on-off keying and binary phase-shift keying formats, and non-Gaussian measurements based on photon number resolving detection of the coherently displaced signal. For a given power constraint and channel noise strength, these novel strategies rely on joint optimization of the input alphabet and the measurement to provide enhanced communication capability over a non-Gaussian channel characterized in terms of the error rate as well as mutual information.

    

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2. High Performance Flexible Protocol for Backscattered-Based Neural Implants

   https://doi.org/10.1109/APWC.2019.8870386  

   Arjona, Laura ; Rosenthal, James ; Smith, Joshua R. ; Moritz, Chet T. ( September 2019 , 2019 IEEE-APS Topical Conference on Antennas and Propagation in Wireless Communications (APWC))

   This work presents a custom high-performance protocol for bi-directional communication with neural implants, that will eventually enable closed-loop operation. This protocol presents a flexible configuration to communicate to neural implants with different characteristics. It can support different uplink data rates, a variable number of neural channels from 2 to 16, two types of digital signal modulation (Amplitude Shift-Keying, ASK, and Binary Shift-Keying, PSK), and different RF operation frequencies (915MHz being the default). The proposed protocol is implemented in C++ (preferred to Python because it enables fast signal processing algorithms), using GNU-Radio toolkit with custom communication blocks. 

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3. On the Effect of Imperfect Reference Signal Phase Recovery on Performance of PSK System Influenced by TWDP Fading

   https://doi.org/10.3390/e25091341  

   Djordjevic, Goran T. ; Milic, Dejan N. ; Vasic, Bata ; Makal, Jarosław ; Vasic, Bane ( July 2023 , Entropy)

   We examine the effects of imperfect phase estimation of a reference signal on the bit error rate and mutual information over a communication channel influenced by fading and thermal noise. The Two-Wave Diffuse-Power (TWDP) model is utilized for statistical characterization of propagation environment where there are two dominant line-of-sight components together with diffuse ones. We derive novel analytical expression of the Fourier series for probability density function arising from the composite received signal phase. Further, the expression for the bit error rate is presented and numerically evaluated. We develop efficient analytical, numerical and simulation methods for estimating the value of the error floor and identifying the range of acceptable signal-to-noise ratio (SNR) values in cases when the floor is present during the detection of multilevel phase-shift keying (PSK) signals. In addition, we use Monte Carlo simulations in order to evaluate the mutual information for modulation orders two, four and eight, and identify its dependence on receiver hardware imperfections under the given channel conditions. Our results expose direct correspondence between bit error rate and mutual information value on one side, and the parameters of TWDP channel, SNR and phase noise standard deviation on the other side. The results illustrate that the error floor values are strongly influenced by the phase noise when signals propagate over a TWDP channel. In addition, the phase noise considerably affects the mutual information.

    

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4. Utilizing adaptive optics to mitigate intra-modal-group power coupling of graded-index few-mode fiber in a 200-Gbit/s mode-division-multiplexed link

   https://doi.org/10.1364/OL.394307  

   Zhang, Runzhou ; Song, Hao ; Song, Haoqian ; Zhao, Zhe ; Milione, Giovanni ; Pang, Kai ; Du, Jing ; Li, Long ; Zou, Kaiheng ; Zhou, Huibin ; et al ( June 2020 , Optics Letters)

   We experimentally demonstrate the utilization of adaptive optics (AO) to mitigate intra-group power coupling among linearly polarized (LP) modes in a graded-index few-mode fiber (GI FMF). Generally, in this fiber, the coupling between degenerate modes inside a modal group tends to be stronger than between modes belonging to different groups. In our approach, the coupling inside the${\mathrm{L}\mathrm{P}}_{11}$group can be represented by a combination of orbital-angular-momentum (OAM) modes, such that reducing power coupling in OAM set tends to indicate the capability to reduce the coupling inside the${\mathrm{L}\mathrm{P}}_{11}$group. We employ two output OAM modes$l=+1$and$l=-<\#comment/>1$as resultant linear combinations of degenerate${\mathrm{L}\mathrm{P}}_{11\mathrm{a}}$and${\mathrm{L}\mathrm{P}}_{11\mathrm{b}}$modes inside the${\mathrm{L}\mathrm{P}}_{11}$group of a$\sim <\#comment/>0.6\text{-}\mathrm{k}\mathrm{m}$GI FMF. The power coupling is mitigated by shaping the amplitude and phase of the distorted OAM modes. Each OAM mode carries an independent 20-, 40-, or 100-Gbit/s quadrature-phase-shift-keying data stream. We measure the transmission matrix (TM) in the OAM basis within${\mathrm{L}\mathrm{P}}_{11}$group, which is a subset of the full LP TM of the FMF-based system. An inverse TM is subsequently implemented before the receiver by a spatial light modulator to mitigate the intra-modal-group power coupling. With AO mitigation, the experimental results for$l=+1$and$l=-<\#comment/>1$modes show, respectively, that (i) intra-modal-group crosstalk is reduced by$><\#comment/>5.8\mathrm{d}\mathrm{B}$and$><\#comment/>5.6\mathrm{d}\mathrm{B}$and (ii) near-error-free bit-error-rate performance is achieved with a penalty of$\sim <\#comment/>0.6\mathrm{d}\mathrm{B}$and$\sim <\#comment/>3.8\mathrm{d}\mathrm{B}$, respectively.

    

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5. “Hiding” a low-intensity 50  Gbit/s QPSK free-space OAM beam using an orthogonal coaxial high-intensity 50  Gbit/s QPSK beam

   https://doi.org/10.1364/AO.396386  

   Song, Haoqian ; Almaiman, Ahmed ; Song, Hao ; Zhao, Zhe ; Zhang, Runzhou ; Pang, Kai ; Liu, Cong ; Li, Long ; Manukyan, Karapet ; Zach, Shlomo ; et al ( August 2020 , Applied Optics)

   In this paper, we experimentally demonstrate an approach that “hides” a low-intensity 50 Gbit/s quadrature-phase-keying (QPSK) free-space optical beam when it coaxially propagates on the same wavelength with an orthogonal high-intensity 50 Gbit/s QPSK optical beam. Our approach is to coaxially transmit the strong and weak beams carrying different orthogonal spatial modes within a modal basis set, e.g., orbital angular momentum (OAM) modes. Although the weak beam has much lower power than that of the strong beam, and the beams are in the same frequency band and on the same polarization, the two beams can still be effectively demultiplexed with little inherent crosstalk at the intended receiver due to their spatial orthogonality. However, an eavesdropper may not readily identify the weak beam when simply analyzing the spatial intensity profile. The correlation coefficient between the intensity profiles of the strong beam and the combined strong and weak beams is measured to characterize the potential for “hiding” a weak beam when measuring intensity profiles. Such a correlation coefficient is demonstrated to be higher than 0.997 when the power difference between the strong fundamental Gaussian beam and the weak OAM beam is$\sim <\#comment/>8,\sim <\#comment/>10$, and$\sim <\#comment/>10\mathrm{d}\mathrm{B}$for the weak OAM$-<\#comment/>1,-<\#comment/>2$, and$-<\#comment/>3$beams, respectively. Moreover, a 50 Gbit/s QPSK data link having its$Q$factor above the 7% forward error correction limit is realized when the power of the weak OAM$-<\#comment/>3$beam is 30 dB lower than that of the strong fundamental Gaussian beam.

    

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