0.5-bit/s/Hz fine-grained adaptive OFDM modulation for bandlimited underwater VLC.

In this paper, we propose and demonstrate a 0.5-bit/s/Hz fine-grained adaptive orthogonal frequency division multiplexing (OFDM) modulation scheme for bandlimited underwater visible light communication (UVLC) systems. Particularly, integer spectral efficiency is obtained by conventional OFDM with quadrature amplitude modulation (QAM) constellations, while fractional spectral efficiency is obtained by two newly proposed dual-frame OFDM designs. More specifically, OFDM with dual-frame binary phase-shift keying (DF-BPSK) is designed to achieve a spectral efficiency of 0.5 bit/s/Hz, while OFDM with dual-frame dual-mode index modulation (DF-DMIM) is designed to realize the spectral efficiencies of 0.5+n bits/s/Hz with n being a positive integer (i.e., n = 1, 2, …). The feasibility and superiority of the proposed 0.5-bit/s/Hz fine-grained adaptive OFDM modulation scheme in bandlimited UVLC systems are successfully verified by simulations and proof-of-concept experiments. Experimental results demonstrate that a significant achievable rate gain of 18.6 Mbps can be achieved by the proposed 0.5-bit/s/Hz fine-grained adaptive OFDM modulation in comparison to the traditional 1-bit/s/Hz granularity adaptive OFDM scheme, which corresponds to a rate improvement of 22.1%.

Optimized pilot structure for PS-PDM ultrahigh-order QAM coherent optical transmission.

We proposed and experimentally demonstrated a general pilot structure for probabilistic shaped (PS)-polarization division multiplexing (PDM) M-ary quadrature amplitude modulation (MQAM) coherent optical transmission, where a portion of PS-MQAM symbols is exploited as the pilot symbols with the same information entropy as the transmitted signal. The pilot symbols are simultaneously used in the entire digital signal processing (DSP) modules for polarization de-multiplexing, frequency offset estimation, carrier phase recovery, nonlinear equalization, and linear equalization. Compared to the conventional quadrature phase shift keying (QPSK) pilot structure, the proposed MQAM pilot structure can yield the nonlinear properties of the overall signal so that nonlinear equalization can effectively improve the performance of the normalized generalized mutual information. The remarkable performance has been achieved at the shaping parameters of 3.25 and 4.35 for 1024QAM and 4096QAM signals based on the proposed pilot scheme, which corresponds to the raw spectral efficiency of 16.190 and 20.381 bit/s/Hz, respectively. The pilot ratio is optimized to 5% for higher achievable information rates (AIRs).

MIMO Volterra polynomial equalizer for PDM ultrahigh-order QAM signals.

In this work, we proposed and experimentally demonstrated a multiple-input multiple-output (MIMO) Volterra polynomial equalizer (MVPE), in a probabilistic shaping (PS) polarization-division multiplexed (PDM) 4096-quadrature amplitude modulation (QAM) coherent optical transmission system. In comparison with the conventional single-input single-output (SISO) Volterra nonlinear equalizers (VNLEs), the proposed MVPE can equalize the in-phase (I) and quadrature (Q) components in both the X and Y polarizations simultaneously and effectively alleviate nonlinear distortions. In the PS-PDM-4096-QAM experiment, the normalized generalized mutual information (NGMI) reached the threshold of DVB-S2 low-density parity-check (LDPC) codes with a 20% overhead. Compared to the SISO VNLE scheme, the MVPE scheme improves receiver sensitivity by approximately 2.8 dB while maintaining nearly similar computational complexity.

Asynchronous Code Division Multiplexing-Based Visible Light Positioning and Communication Network Using Successive Interference Cancellation Decoding.

In the evolving landscape of sixth-generation wireless communication, the integration of visible light communication (VLC) and visible light positioning (VLP), known as visible light positioning and communication (VLPC), emerges as a pivotal technology. This study addresses the challenges of asynchronous code division multiplexing (ACDM) in VLPC networks, focusing on the enhancement of data transmission quality and positioning accuracy. Firstly, we propose an orthogonal pseudo-random code (OPRC) for ACDM-based VLP systems. Leveraging its excellent correlation properties, VLP signals preserve orthogonality even amidst asynchronous transmissions, achieving sub-centimeter average positioning errors. Next, by combining OPRC with successive interference cancellation decoding (SICD), we propose an enhanced ACDM-based VLPC system that utilizes OPRC for improved signal orthogonality and SICD for progressive elimination of multiple access interference (MAI) among VLPC signals. The results show substantial improvements in bit-error rate (BER) and positioning error (PE), approaching the performance levels observed in synchronized VLPC systems. Specifically, the SICD-OPRC scheme reduces average BER to 4.3 × 10-4 and average PE to 2.7 cm, demonstrating its robustness and superiority in complex asynchronous scenarios.

Collaborative spectrum sensing for cognitive visible light communications.

Cognitive visible light communication (VLC) has attracted increasing attention. By sharing underutilized VLC spectrum resources of primary users (PUs) with secondary users (SUs) opportunistically, improved spectrum utilization can be achieved without interfering with PUs. As an essential component in cognitive VLC, reliable spectrum sensing is crucial to ensure accurate cognition of PU's signal. However, due to limiting factors such as low signal-to-noise ratio (SNR) and link blocking in VLC systems, it would be difficult for a single SU to identify the status of PUs accurately and rapidly. To tackle this issue, we propose a new collaborative sensing (CS) scheme which can enhance sensing accuracy effectively by coordinating multiple SUs to participate in spectrum sensing. To evaluate the performance of the proposed CS scheme, we first develop an analytical model for the scenario of a single SU, subject to various factors such as indoor reflections and signal sampling size. Next, based on the single-SU evaluation, we further analyze the performance of the CS scheme by extending the single-SU analytical models to the multi-SU scenario. It is found that the analytical models can accurately predict the performance of the proposed CS scheme and match the results obtained by simulations. Moreover, the proposed CS scheme is effective in improving the sensing accuracy by about 40% and 10% compared with the local-sensing and the conventional CS schemes, respectively.

Time domain reshuffling for OFDM based indoor visible light communication systems.

For orthogonal frequency division multiplexing (OFDM) based indoor visible light communication (VLC) systems, partial non-ideal transmission conditions such as insufficient guard intervals and a dispersive channel can result in severe inter-symbol crosstalk (ISC). By deriving from the inverse Fourier transform, we present a novel time domain reshuffling (TDR) concept for both DC-biased optical (DCO-) and asymmetrically clipped optical (ACO-) OFDM VLC systems. By using only simple operations in the frequency domain, potential high peaks can be relocated within each OFDM symbol to alleviate ISC. To simplify the system, we also propose an effective unified design of the TDR schemes for both DCO- and ACO-OFDM. Based on Monte-Carlo simulations, we demonstrate the statistical distribution of the signal high peak values and the complementary cumulative distribution function of the peak-to-average power ratio under different cases for comparison. Simulation results indicate improved bit error rate (BER) performance by adopting TDR to counteract ISC deterioration. For example, for binary phase shift keying at a BER of 10-3, the signal to noise ratio gains are ~1.6 dB and ~6.6 dB for DCO- and ACO-OFDM, respectively, with ISC of 1/64. We also show a reliable transmission by adopting TDR for rectangle 8-quadrature amplitude modulation with ISC of < 1/64.

Design, synthesis and SAR of piperidyl-oxadiazoles as 11ß-hydroxysteroid dehydrogenase 1 inhibitors.

The potential roles of 11ß-HSD1 inhibitors in metabolic syndrome, T2D and obesity were well established and currently several classes of 11ß-HSD1 inhibitors have been developed as promising agents against metabolic diseases. To find potent compounds with good pharmacokinetics, we used the bioisosterism approach, and designed the compound 2 and 3 bearing an 1,2,4-oxadiazole ring to replace the amide group in compound 1. Guided by docking study, we then transformed compound 3 into a potent lead compound 4a by changing sulfonamide group to amide. To elaborate this series of piperidyl-oxadiazole derivatives as human 11ß-HSD1 inhibitors, we explored the structure-activity relationship of several parts of the lead compound. Based on their potency toward human 11ß-HSD1 two compounds 4h and 4q were advanced to pharmacokinetic study. It was found that 4h and 4q are potent and selective human 11ß-HSD1 inhibitors with better pharmacokinetic properties than those of the original piperidine-3-carboxamide compound 1, and suitable for further in vivo preclinical study in primate model.