Theoretical and experimental research on diversity reception technology in NLOS UV communication system

Coverage extended MMF-based indoor OWC using overfilled launch and diversity reception

Speckle patterns generated as coherent optical beams are reflected by scattering elements. Multimode fibers (MMFs) can modify the transverse intensity distribution of speckle patterns with macro perturbations, i.e., pressures, providing a simple and low-cost way to achieve equivalent beam-steering for indoor optical wireless communications (OWCs) with divergent optical beams. However, the received optical power (ROP) variance severely limits the mobility of user terminals. In this paper, the issue is alleviated by using the overfilled launch of MMFs and the diversity gain of multi-receivers. By adjusting the axial spatial coupling distance between the MMF and the single mode fiber (SMF) emitting coherent laser, the number of excited modes of MMF can be significantly increased at 1550 nm with negligible coupling and bending losses. In addition, the signal-to-noise ratio (SNR) enhancement obtained by applying two receivers is theoretically analyzed for the case when either thermal noise or shot noise is dominant. The experimental results demonstrate that the proposed scheme can efficiently compensate for the ROP inhomogeneity, and at the same time it can extend the achievable full steering angle up to 12° at a 1.5-m free-space distance for bit error rate (BER) values of less than 3.8 × 10-3.

Hundred-meter Gb/s deep ultraviolet wireless communications using AlGaN micro-LEDs

We demonstrate the use of deep ultraviolet (DUV) micro-light-emitting diodes (LEDs) for long-distance line-of-sight optical wireless communications. With a single 285 nm-emitting micro-LED, we have respectively achieved data rates greater than 6.5 Gb/s at a distance of 10 m and 4 Gb/s at 60 m. Moreover, we obtained >1 Gb/s data rates at a distance of 116 m. To our knowledge, these results are the highest data rates at such distances thus far reported using DUV micro-LEDs and the first demonstration of Gb/s communication at >100 m using any micro-LED-based transmitter.

Bending-Insensitive Intrinsically Flexible Ultraviolet Encoding Devices Based on Piezoelectric Nanogenerator-Supplied Liquid Crystalline Polymer Fabrics

It is significantly challenging for state-of-the-art wearable electronics to stably monitor physicochemical signals under dynamic motions. Herein, a bending-insensitive, self-powered, and intrinsically flexible UV detector has been realized based on well-designed oriented composite fabrics, consisting of ionic liquid (IL)-containing liquid crystalline polymers (ILCPs) and piezoelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanogenerators. The novel composite fabrics establish effective UV illuminance-internal stress-electric signal conversion by coupling resistive and piezoelectric effects, with a fast response time of 190 ms. Particularly, benefiting from the intrinsic flexibility of composite fabrics, the ILCP/P(VDF-TrFE) device can maintain stable performance under dynamic bending even if the frequency is up to 2.5 Hz, with a bending insensitivity of less than 1% performance variation under 1.0 mW cm^-2 UV light. Combined with the Internet of Things and the American Standard Code for Information Interchange (ASCII), wearable encoding electronics have been successfully implemented with a printing speed of 3.2 s per character under dynamic bending.

Artificial neural network estimation of data and channel characteristics in free-space ultraviolet communications

In this paper, we develop an artificial neural network (ANN)-based algorithm for signal classification, i.e., data demodulation, and the estimation of channel characteristics, such as channel DC gain and turbulence distribution parameters, in free-space ultraviolet (UV) communication systems. In this scheme, the ANN-based receiver adaptively tracks the UV channel variation and is directly trained using the data generated from UV channel models. To evaluate the performance of the proposed algorithm across all turbulence regimes, log-normal, Gamma-Gamma, and negative exponentially distributed UV turbulence channels are considered. We demonstrate that the proposed algorithm is robust to perform accurate and reliable signal classification and UV channel estimation without knowing underlying UV channel models or channel state information (CSI). In addition, time complexity analysis is presented. It is shown that, even under the strong turbulence regime, the accuracy of the proposed algorithm is found to be higher than 97% in terms of correct classification. It is also demonstrated that the classification error rate performance of the proposed ANN-based detector is superior to that of a maximum-likelihood (ML)-based detector with perfect CSI.

Efficient solar-blind ultraviolet detection based on a Sn2+ ion-activated fluosilicate glass

As an effective method to improve the sensitivity of detection, spectral conversion technology has been widely used in solar-blind ultraviolet communication (UVC). However, producing a spectral converter with high efficiency, short decay lifetime, and excellent stability in the field of UVC detection remains an outstanding challenge. In this Letter, a kind of ${{\rm Sn}^{2 + }}$Sn²⁺ activated fluosilicate (SFS) glass as a spectral converter for UVC detection has been prepared, which has a short lifetime (5.65 µs) and a high quantum efficiency (85%). The SFS decorated Si-based PIN photodiode enables strong absorption and subtly UVC shifts to an emission band optimized for the detector. Moreover, the outstanding performance of detection with responsivity of 27 µA/mW@278 nm, frequency limit up to 50 KHz, and rise/fall time scale of 15.14/18.71 µs@10 KHz is achieved. The comprehensive performance shows that SFS glass has the potential for efficient UVC detection in a scalable and low-cost strategy.

Gbit/s ultraviolet-C diffuse-line-of-sight communication based on probabilistically shaped DMT and diversity reception

We demonstrated a high-speed 1×2 single-input and multiple-output (SIMO) diffuse-line-of-sight (diffuse-LOS) ultraviolet-C (UVC) solar-blind communication link over a distance of 5 meters. To approach the Shannon limit and improve the spectral efficiency, we implemented probabilistically shaped discrete multitone modulation. As compared to a single-input and single-output (SISO) counterpart, we observed significant improvement in the SIMO link in terms of the angle of view of the receiver and the immunity to emulated weather condition. A wide angle of view of ± 9^° is achieved in the SIMO system, with up to a 1.09-Gbit/s achievable information rate (AIR) and a minimum value of 0.24 Gbit/s. Moreover, the bit error rate of the SIMO link in emulated foggy conditions is lowered significantly when compared to that of the SISO link. This work highlights the practicality of UVC communication over realistic distances and in turbulent environments to fill the research gap in high-speed, solar-blind communication.

Spatially correlated MIMO for exploiting the capacity of NLOS ultraviolet turbulent channels

In order to break through the inherent limitation of channel capacity, we employ a correlated multiple input multiple output (MIMO) framework over non-line-of-sight (NLOS) ultraviolet (UV) turbulent channels. Subsequently, a method of segmented correlation is proposed to define the spatial correlation of UV MIMO structures. Furthermore, the correlation and channel capacity are studied with varying elevation angle, turbulence intensity, signal-to-noise ratio (SNR) and number of transceiver in pairs (NTPs). The results show that this correlated MIMO framework can obtain higher channel capacity by reducing the elevation angle of the transceivers or increasing turbulence intensity or NTPs. Moreover, channel capacity increases with the SNR. But at low SNR, a larger gain of channel capacity can be provided. Therefore, this paper develops the capacity of UV turbulent channels and provides a guide for optimal parameter configuration of correlated UV MIMO.

71-Mbit/s ultraviolet-B LED communication link based on 8-QAM-OFDM modulation

A demonstration of ultraviolet-B (UVB) communication link is implemented utilizing quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM). The demonstration is based on a 294-nm UVB-light-emitting-diode (UVB-LED) with a full-width at half-maximum (FWHM) of 9 nm and light output power of 190 μW, at 7 V, with a special silica gel lens on top of it. A -3-dB bandwidth of 29 MHz was measured and a high-speed near-solar-blind communication link with a data rate of 71 Mbit/s was achieved using 8-QAM-OFDM at perfect alignment. 23.6 Mbit/s using 2-QAM-OFDM when the angle subtended by the pointing directions of the UVB-LED and photodetector (PD) is 12 degrees, thus establishing a diffuse-line-of-sight (LOS) link. The measured bit-error rate (BER) of 2.8 ×10^-4 and 2.4 ×10^-4, respectively, are well below the forward error correction (FEC) criterion of 3.8 ×10^-3. The demonstrated high data-rate OFDM-based UVB communication link paves the way for realizing high-speed non-line-of-sight free-space optical communications.

Experimental research on the MRC diversity reception algorithm for UV communication

Ultraviolet (UV) communication is an emerging communication method with non-line-of-sight, anti-interference, and anti-interception capabilities, along with high flexibility and reliability. Herein, the maximum ratio combining (MRC) diversity reception algorithm for a UV communication system is studied. Simulation and experimental results indicate that single and multiple outputs are useful and achievable, with an obvious diversity gain, and the MRC diversity reception algorithm can reduce the system bit error rate more effectively than the equal-gain combining method. The simulation and experimental results are analyzed, and the differences between them are discussed. These results provide guidelines for UV communication system design and implementation.

Research on multiple-scattering channel with Monte Carlo model in UV atmosphere communication

A non-line-of-sight (NLOS) ultraviolet (UV) communication channel model is established by using a Monte Carlo simulation method based on photon tracing. This method considers the multiple-scattering effects of UV signal propagation in the atmosphere, and simulates the condition of dual receivers for diversity reception. The channel characteristics of the UV communication are obtained by simulating the photon arrival probabilities. The model is employed to study the characteristics of NLOS UV scattering channels for a variety of scattering conditions, including the separation distance between transmitter and receiver, transmit/receive elevation angle, beam divergence, and field of view. The model has advantages in reliable prediction of UV communication for the dual-receiver condition, as validated by outdoor experiments at fixed elevation angles.