Performance of current digital pulse modulation schemes for optical wireless communications

Guest Editorial

The shadowing effect is one of the most critical concerns in optical wireless communication (OWC) systems, causing the system performance to degrade drastically as the signal path is blocked by an obstruction. In this paper, we introduce a probabilistic hierarchical model of shadowing that relies on prior knowledge of object uncertainties such as its dimensions, as well as practical uncertainties like mobility and object detection error. Numerical results presented for both the line of sight (LOS) and non-line sight (NLOS) parts of the received OWC signal in indoor environments consider a pedestrian as the main moving object. The result shows a maximum standard deviation of 10 nW from the randomness in the object’s reflection, using the range of human body dimensions, when the location of the object is detected accurately; the variability can increase by orders of magnitude considering mobility uncertainty as well as object detection error. The model presented also enables the network to predict LOS blockage based on perfect knowledge of the object location.

The combination of orthogonal frequency division multiplexing (OFDM) and spatial modulation (SM) can enhance the capacity of optical wireless communication (OWC) systems with low complexity. In this paper, we propose a novel SM scheme for intensity modulation/direct detection (IM/DD) OWC systems by employing discrete Hartley transform based OFDM (DHT-OFDM). Due to the use of DHT with one-dimensional constellations, the Hermitian symmetry constraint, which is generally imposed in conventional discrete Fourier transform based OFDM (DFT-OFDM) to obtain a real-valued output signal, is not required in DHT-OFDM. As a result, DHT-OFDM based SM can achieve much higher spectral efficiency than that of DFT-OFDM based SM in OWC systems. Simulation results show that, for an indoor 4×4 SM-OWC system with a spectral efficiency of 6 bits/s/Hz, DHT-OFDM achieves a remarkable 4.5-dB transmit signal-to-noise ratio reduction for an overall bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> in comparison to conventional DFT-OFDM.

Beam-steering-based optical wireless technologies are being widely investigated due to the capability of providing high-speed wireless connectivity in indoor applications. However, high-speed indoor optical wireless systems are traditionally realized with discrete bulky components, significantly limiting their practical applications. In this Letter, we demonstrate an infrared optical wireless communication system employing a miniaturized silicon integrated photonic circuit for beam steering for the first time. Experimental results show that up to 12.5Gb/s optical wireless communication can be achieved with error-free performance over a free-space range of 140cm, and limited mobility of users can be realized. The experimental results of this Letter open the way for realizing integrated high-speed optical wireless communications.

In optical wireless communication (OWC) systems, channel estimation and detection of the transmitted symbols have been conventionally performed using analytical methods assuming that the optical channel follows a certain model, e.g., free-space model, input-dependent noise model, or Poisson model. In practical OWC systems, channels do not necessarily follow a specific model. Hence, it is difficult, if not impossible, to derive analytical models that provide optimal performance in realistic optical channels. Motivated by the success of neural networks in estimation and classification in various fields, we propose a neural network-based methodology for detection and estimation for OWC that does not rely on a channel model. Simulation results show that the proposed learning-based estimation and detection schemes achieve the optimal performance of the maximum likelihood detector under different channel state information assumptions.

We propose and experimentally demonstrate a full-duplex integrated passive optical network (PON) and indoor optical wireless communication (OWC) system. For the PON part, an external cavity laser (ECL) and a directly modulated laser (DML) are used as optical sources for downlink and uplink, respectively. The OWC subsystem utilizes a visible light-emitting diode (LED) for downlink and an infrared LED (IR-LED) for uplink transmission. To enable high spectral efficiency (SE), high-speed transmission, and flexible multiple access with simplified optical network unit (ONU)-side digital signal processing (DSP), multilevel, multiband quadrature amplitude modulation (QAM) and hybrid 128/64/16 Nyquist single-carrier frequency domain equalization (N-SC-FDE) modulation are implemented in this paper. In terms of 62.5 MHz OWC N-SC-FDE signals, the system successfully delivered a further 140 cm indoor distance after transmitting over a span of 25 km standard single mode fiber (SSMF) together with three subbands of 250 MHz PON signals. As a proof of concept, an integrated system of symmetrical full-duplex 5 Gb/s PON and 250 Mb/s OWC at each direction for three wired users and one wireless user is successfully achieved. The measured bit error ratios (BERs) for each user are all smaller than the 7% pre-forward-error-correction (pre-FEC) limit of 3.8 × 10−3, which shows the promising potential and feasibility of this proposal to extend multiple services from metro areas to suburban areas.

This paper is an overview of indoor OFDM (orthogonal frequency division multiplexing)/DMT (discrete multitone) optical wireless (OW) communication systems. Indoor OW OFDM/DMT systems can be classified into two groups. One group produces half-wave symmetry time signal at the output of the OFDM modulator by special assignment of subcarriers. Thus, allowing signal clipping at the zero level and avoiding the need of DC bias at the expense of data rate reduction. ACOOFDM (asymmetrically clipped OFDM system) and PAM (pulse amplitude modulation)-DMT are two techniques from the first group. The second group assigns data to all possible subcarriers to increase the data rate. However, half-wave symmetry signals cannot be achieved and DC bias is needed to convert the bipolar signal to a unipolar signal before modulating the LED (light emitting diode) intensity. DC-biased OFDM and a novel technique, proposed in this paper, called orthogonal PAM-DMT (OPAM-DMT) that is an extension of the proposed PAM-DMT by using discrete sine transform and discrete cosine transform to transmit two orthogonal signals at the same time, are two techniques from the second group. This paper considerers a practical LED model and studies the performance of all these systems in terms of average electrical OFDM signal power versus bit-error-ratio (BER) in the presence of additive white Gaussian noise channel (AWGN). It is shown that LED clipping has significant impact on the performance of all these systems and the performance of these systems substantially depends on the considered modulation order.

When the optical signal is transmitted in atmospheric channel in the wireless optical communication (WOC) systems, the inter-symbol interference (ISI) will be caused by the atmospheric motion and the fading characteristics of the atmospheric channel. In addition, device noises such as photodetectors noises can also affect the signal in the real communication systems. It is difficult to eliminate both types of interference by the single equalization algorithm at the same time, therefore, a cascaded equalization algorithm is proposed. First of all, we use the MATLAB to simulate the cascaded equalization algorithm, the convergence performance of the algorithm is analyzed, the bit error rate (BER) under different signal-to-noise ratios (SNR) are also compared. Then, the algorithm is verified under laboratory conditions, we calculate the BER of the receiving signal, which is processed by the cascade equalization algorithm. The simulation analysis and experiments results show that, the convergence performance of the cascaded equalization algorithm is improved effectively, the ISI is decrease, the BER can reduce to about 10-8 , decreased by 4 orders of magnitude than without equalization. It can improve the communication performance of the WOC systems, the validation of the cascaded equalization algorithm is verified.

In this paper we experimentally investigate the impact of polarization state on our recently proposed indoor gigabit optical wireless communication system. Our findings indicate that ~0.7 dB variation in the receiver sensitivity will be introduced.

The metasurface based beam-steering devices with the advantages of large steering angles, arbitrary channels and ultra-compactness have played an important role for data allocation and exchange in the optical wireless communication. However, the current metasurface based optical wireless communication systems are mainly on intensity modulation and direct detection (IM/DD), which shows a relative lower transmission capacity, lower received optical signal-to-noise ratio (OSNR) and complexity of system. In this study, a bidirectional multichannel optical wireless system enabled by a polarization independent metasurface with coherent modulation and reception is designed and experimentally demonstrated, which exhibits exclusive 100 Gbps coherent optical signals to multiusers with their own wavelengths, 2 m free space distance and field of view of 20° × 20°. In addition, the proposed system can support optical broadcasting system with capacity of 900 Gbps. The demonstrated metasurface assisted optical wireless communication system merges the optical coherent communication techniques and emerging concept of metasurface, which reduces the complexity and cost of the system while contributing a high transmission capacity, opening a new avenue for high performance optical wireless communications.

Optical Wireless Communication Systems (OWCSs) are becoming more popular each day, especially after numerous mobile applications are being employed within the concept of Internet of Things (IoT). OWCSs are largely used in both terrestrial and non-terrestrial environments, like underwater, air, and space scenarios. Due to the large applicability of OWCS, it represents one of the main candidate technologies for the future 6G wireless communication systems. Naturally, this market trend forces the system designers to reach the best performance in their designs, as well as optimize the cost. In this survey paper, we intend to provide information to the researchers working in this field on the statistical models adopted in OWCS, the methods and techniques used to improve their performances, mainly in outdoor environment like air, space, and underwater. In this respect, the background on theoretical aspects of OWCS, together with their benefits, limitations and challenges are presented. Performance improvement techniques employed in OWCSs, such as power increase, partial coherence, beamforming, aperture averaging, spatial diversity, and intelligent reflecting surfaces, are also introduced. Finally, we discuss the open challenges that researchers are still facing, together with future directions on next steps for a large-scale adoption of OWCS.

An indoor optical wireless communications system with mobility feature providing both high bandwidth and ubiquitous coverage in personal area networks is proposed. We demonstrate the scheme in a proof-of-concept experiment with results indicating its feasibility.

In this paper we experimentally demonstrate an ultra-broadband full-duplex indoor optical wireless communication system with multi-mode fiber incorporating WDM technology. Simultaneously transmission of 4 × 12.5Gbps down-link and 800Mbps up-link has been successfully achieved with error-free operation.

Optical wireless communication (OWC) using line-of-sight connections has great application potential for indoor scenes due to its advantages of high data transmission speed and privacy. In our proposed system, we use infrared tunable vertical-cavity surface-emitting laser (VCSEL) as light source, array waveguide gratings (AWG) combined with 6 × 6 integrated optical fiber arrays as a router to realize ultrafast beam-steering and high capacity point-to-point data transmission, which makes the indoor OWC system compact, low-cost, and easy to install. The high tuning rate of VCSEL enables the channel switching to be completed within 1.7 µs. Based on the modulation format of non-return-to-zero on-off keying (NRZ-OOK), a data rate of 12 Gbit/s per channel can be realized with high sensitivity through 3.1 m free space when the detected optical power is low. The system is proved to be a flexible link with high-speed communication for mobile terminals in a limited space.

The shortage on radio spectrum forced to a high sophistication in spectrum efficiency. Optical wireless communication (OWC), rather than RF communication, may be a game changer, as the available optical spectrum is sheer unlimited. In addition, light can more easily be directed to the desired user (only). Narrowing the emitted light beams allows denser reuse, even within one room, and enables an increase in throughput. The authors report their experience from creating indoor OWC systems and verify these insights against throughput models. The trade-off between high throughput in only a narrow beam versus offering a wide coverage area is discussed. LEDs and free-form optics allow simple ways to direct a beam, which is more attractive than a phased-array as used in RF. The suitability of a Lambertian radiation patterns is challenged and compared to an optical system that is designed to provide constant irradiance. An example of a sectorized system comprising four segments with free-form optics is presented and its performance and characteristics are discussed, for a Lambertian and a directional detector