Enhancing OFDM with index modulation using heuristic geometric constellation shaping and generalized interleaving for underwater VLC.

In this paper, we propose and demonstrate enhanced orthogonal frequency division multiplexing with index modulation (OFDM-IM) schemes for bandlimited underwater visible light communication (UVLC) systems via geometric constellation shaping (GCS) and subblock interleaving. Specifically, two heuristic GCS approaches based on particle swarm optimization (PSO) and hybrid genetic algorithm-PSO (GA-PSO) algorithms are proposed to generate IM-preferable constellations. Moreover, a generalized interleaving technique is further proposed to overcome the low-pass effect of bandlimited UVLC systems, where an optimal step size can be obtained to perform subblock interleaving. Simulation and experiments are conducted to evaluate the performance of the proposed enhanced OFDM-IM schemes in bandlimited UVLC systems, where both OFDM with single-mode index modulation (OFDM-SM) and OFDM with dual-mode index modulation (OFDM-DM) schemes are considered. The experimental results demonstrate remarkable signal-to-noise ratio (SNR) gains of 1.3 and 1.9 dB for OFDM-SM and OFDM-DM in comparison to the benchmark schemes, respectively.

Non-line-of-sight optical camera communications based on CPWM and a convolutional neural network.

Non-line-of-sight (NLOS) optical camera communications (OCC) exhibit greater link availability and mobility than line-of-sight links, which are more susceptible to blocking and shadowing. In this work, we propose an NLOS OCC system, where the data signal is mapped into color pulse width modulation (CPWM) symbols prior to transmission using a red-, green-, and blue light-emitting diode. A convolutional-neural-network-based receiver is used to demodulate the CPWM signal. Based on experimental results, the proposed scheme effectively mitigates the effects of diffuse reflection induced intersymbol interference, resulting in an increased data transmission rate to 7.2 kbps over a link span of more than 2 m, which is typical for indoor applications.

Indoor 3D NLOS VLP using a binocular camera and a single LED.

In this paper, we propose a non-line of sight (NLOS) visible light positioning (VLP) system using a binocular camera and a single light emitting diode (LED) for the realization of 3D positioning of an arbitrary posture. The proposed system overcomes the challenges of the shadowing/blocking of the line of sight (LOS) transmission paths between transmitters and receivers (Rxs) and the need for a sufficient number of LEDs that can be captured within the limited field of view of the camera-based Rx. We have developed an experimental testbed to evaluate the performance of the proposed system with results showing that the lowest average error and the root mean square error (RMSE) are 26.10 and 31.02 cm following an error compensation algorithm. In addition, a label-based enhanced VLP scheme is proposed for the first time, which has a great improvement on the system performance with the average error and RMSE values of 7.31 and 7.74 cm and a 90th percentile accuracies of < 11 cm.

Convolutional neural network-based signal demodulation method for NOMA-PON.

Non-orthogonal multiple access (NOMA) is a promising scheme for flexible passive optical networks (PONs), which provides high throughput and overall improved system performance. NOMA with the successive interference cancellation (SIC)-based receiver, which is used to detect the multiplexed signal in a sequential fashion, requires perfect channel state information and suffers from the error propagation problem. In this paper, we propose a convolutional neural network (CNN) based signal demodulation method for NOMA-PON, which performs channel estimation and signal detection in a joint manner. The CNN is first trained offline using the captured data for a given received optical power and then used to recover the data stream directly in the online mode. We show by experimental demonstration that, the proposed CNN-based receiver (Rx) outperforms the conventional SIC-based Rx and is more robust to the nonlinear distortion. We show that for the CNN-based system with 20 km optical fiber, the required received optical power levels at a bit error rate (BER) of 1×10-3 are lower by 4, 3 and 2.5 dB for power allocation ratios of 0.16, 0.25, 0.36, respectively compared with SIC-based system. In addition, the BER performance of CNN deteriorates considerably less with non-linear distortion compared with SIC.

Power-code division non-orthogonal multiple access scheme for next-generation passive optical networks.

Advanced modulation and multiple access schemes with high spectral efficiencies are desirable to overcome the bandwidth limitation in low-cost optical and electrical devices to fulfill the high-data rate requirements in passive optical networks (PONs). We propose a non-orthogonal multiple access (NOMA) scheme, known as power-code division NOMA (PCD-NOMA), for the next-generation PON, where the optical network units (ONUs) are divided into several groups with similar path losses. The ONUs per groups are allocated in the same power domain multiplexing layer with different codebooks. We show by experimental demonstration that the proposed PCD-NOMA with a high spectral efficiency offers improved overall system performance and reduced required transmission power in the next-generation PON, particularly in flexible PON where ONUs have different path losses. For PON with a power difference loss of 14 dB, the reduced required transmission powers are 5 and 11 dB for downstream and upstream, respectively, compared with orthogonal frequency division multiple access.

Experimental demonstration of bidirectional NOMA-OFDMA visible light communications.

We propose a non-orthogonal multiple access (NOMA) scheme combined with orthogonal frequency division multiplexing access (OFDMA) for visible light communications (VLC), which offers a high throughput, flexible bandwidth allocation and a higher system capacity for a larger number of users. Bidirectional NOMA-OFDMA VLC is experimentally demonstrated. The effects of power allocation and channel estimation on the bit error rate performance are investigated. The experiment results indicate that accurate channel estimation can eliminate the inter-user interference more effectively. The optimum power allocation ratios for uplink and downlink are both about 0.25 in the case of two users.

Two-Dimensional Metal-Organic Layers as a Bright and Processable Phosphor for Fast White-Light Communication.

A metal-organic layer (MOL) is a new type of 2D material that is derived from metal-organic frameworks (MOFs) by reducing one dimension to a single layer or a few layers. Tetraphenylethylene-based tetracarboxylate ligands (TCBPE), with aggregation-induced emission properties, were assembled into the first luminescent MOL by linking with Zr6 O4 (OH)6 (H2 O)2 (HCO2 )6 clusters. The emissive MOL can replace the lanthanide phosphors in white light emitting diodes (WLEDs) with remarkable processability, color rendering, and brightness. Importantly, the MOL-WLED exhibited a physical switching speed three times that of commercial WLEDs, which is crucial for visible-light communication (VLC), an alternative wireless communication technology to Wi-Fi and Bluetooth, by using room lighting to carry transmitted signals. The short fluorescence lifetime (2.6 ns) together with high quantum yield (50 %) of the MOL affords fast switching of the assembled WLEDs for efficient information encoding and transmission.

Upstream WDM-PON transmission scheme based on PDM-OOK modulation and digital coherent detection with dual-modulus algorithm.

Wavelength-division-multiplexing passive optical network (WDM-PON) is a promising architecture for next-generation access networks because of its large bandwidth, protocol transparency and scalability. In this paper, we propose a cost-effective, high-speed upstream WDM-PON scheme adopting polarization division multiplexed (PDM) on-off keying (OOK) modulation at the optical network unit (ONU) and digital coherent/self-coherent detection with a novel blind dual-modulus equalization algorithm at the optical line terminal (OLT). As such, the upstream capacity can be directly enhanced at low ONU expenditure, and receiver sensitivity as well as power budget can be also improved. Enabled by the scheme, 40-Gb/s upstream transmission in 80-km WDM-PON is experimentally demonstrated.

Warm-White-Light-Emitting Diode Based on a Dye-Loaded Metal-Organic Framework for Fast White-Light Communication.

A dye@metal-organic framework (MOF) hybrid was used as a fluorophore in a white-light-emitting diode (WLED) for fast visible-light communication (VLC). The white light was generated from a combination of blue emission of the 9,10-dibenzoate anthracene (DBA) linkers and yellow emission of the encapsulated Rhodamine B molecules. The MOF structure not only prevents dye molecules from aggregation-induced quenching but also efficiently transfers energy to the dye for dual emission. This light-emitting material shows emission lifetimes of 1.8 and 5.3 ns for the blue and yellow components, respectively, which are significantly shorter than the 200 ns lifetime of Y3Al5O12:Ce3+ in commercial WLEDs. The MOF-WLED device exhibited a modulating frequency of 3.6 MHz for VLC, six times that of commercial WLEDs.