Optical Camera Communications: Principles, Modulations, Potential and Challenges

Optical wireless communications (OWC) are emerging as cost-effective and practical solutions to the congested radio frequency-based wireless technologies. As part of OWC, optical camera communications (OCC) have become very attractive, considering recent developments in cameras and the use of fitted cameras in smart devices. OCC together with visible light communications (VLC) is considered within the framework of the IEEE 802.15.7m standardization. OCCs based on both organic and inorganic light sources as well as cameras are being considered for low-rate transmissions and localization in indoor as well as outdoor short-range applications and within the framework of the IEEE 802.15.7m standardization together with VLC. This paper introduces the underlying principles of OCC and gives a comprehensive overview of this emerging technology with recent standardization activities in OCC. It also outlines the key technical issues such as mobility, coverage, interference, performance enhancement, etc. Future research directions and open issues are also presented

Analysis and Demonstration of Quasi Trace Orthogonal Space Time Block Coding for Visible Light Communications

In the VLC context, pulse position modulation (PPM) and similar modulations are typically used when the overall complexity of the transmitter is required to be low or the system needs to support dimming, i.e., dynamically control the illumination level of the transmitter. However, despite the power efficiency of PPM, it is known to be bandwidth inefficient. Having known the trade-off between reliability and spectral efficiency, in this paper we propose a PPM-based space-time block coding (STBC) technique named as quasi-trace-orthogonal (QTO), derived from trace-orthogonal, to increase the spectral efficiency of PPM VLC's by limiting the reliability loss. We provide Monte-Carlo simulations for a 4×4 MIMO-VLC system and validate the results experimentally, and show that for a given signal-to-noise-ratio, the QTO-STBC based 4-PPM VLC system offers higher spectral efficiency at a cost of higher symbol error rate compared to trace-orthogonal STBC

A Simplified Model for the Rolling Shutter Based Camera in Optical Camera Communications

A simplified model of the camera for optical camera communications (OCC) based on the rolling shutter effect is proposed and experimentally verified. In OCC, the key parameters are the exposure time, which is proportional to the DC gain and inversely proportional to the bandwidth, and the rolling shutter delay or the sampling period. We demonstrate a good agreement between experimental and numerically simulated results for the proposed receiver model

Data rate enhancement in optical camera communications using an artificial neural network equaliser

In optical camera communication (OCC) systems leverage on the use of commercial off-the-shelf image sensors to perceive the spatial and temporal variation of light intensity to enable data transmission. However, the transmission data rate is mainly limited by the exposure time and the frame rate of the camera. In addition, the camera’s sampling will introduce intersymbol interference (ISI), which will degrade the system performance. In this paper, an artificial neural network (ANN)-based equaliser with the adaptive algorithm is employed for the first time in the field of OCC to mitigate ISI and therefore increase the data rate. Unlike other communication systems, training of the ANN network in OCC is done only once in a lifetime for a range of different exposure time and the network can be stored with a look-up table. The proposed system is theoretically investigated and experimentally evaluated. The results record the highest bit rate for OCC using a single LED source and the Manchester line code (MLC) non-return to zero (NRZ) encoded signal. It also demonstrates 2 to 9 times improved bandwidth depending on the exposure times where the system’s bit error rate is below the forward error correction limit.publishe

The Utilization of Artificial Neural Network Equalizer in Optical Camera Communications

In this paper, we propose and validate an artificial neural network-based equalizer for the constant power 4-level pulse amplitude modulation in an optical camera communications system. We introduce new terminology to measure the quality of the communications link in terms of the number of row pixels per symbol , which allows a fair comparison considering the progress made in the development of the current image sensors in terms of the frame rates and the resolutions of each frame. Using the proposed equalizer, we experimentally demonstrate a non-flickering system using a single light-emitting diode (LED) with of 20 and 30 pixels/symbol for the unequalized and equalized systems, respectively. Potential transmission rates of up to 18.6 and 24.4 kbps are achieved with and without the equalization, respectively. The quality of the received signal is assessed using the eye-diagram opening and its linearity and the bit error rate performance. An acceptable bit error rate (below the forward error correction limit) and an improvement of ~66 in the eye linearity are achieved using a single LED and a typical commercial camera with equalization

Integration of an LED/SPAD optical wireless transceiver with CubeSat on-board systems

We demonstrate the integration of Cube-Sat on-board equipment with a field programmable array gate-based light-emitting diode/single-photon avalanche photodiode transceiver using an inter-integrated circuit protocol

MIMO Visible Light Communications with Camera-Based Receiver for Intelligent Transport Systems

The number of vehicle accidents in the world each year is considerable which yield to an approximately 1.2 million deaths. Besides, the car manufacturers are now investing significantly on autonomous vehicles, which highlights the need for communication between vehicles and the surrounding environment. Intelligent Transportation Systems (ITS) enables the vehicles to realize this need. In ITS, vehicles and infrastructure can communicated directly without the need of cellular networks. Currently, the developed technology for ITS is based on radio frequency (RF) under the name of dedicated short-ranged communication (DSRC), which shares the frequency spectrum with other RF applications such as fixed satellite and wireless services, mobile services, radiolocation, etc. Therefore, DSRC can be a potential interfering source for other communication services. Besides, the licence to use this spectrum makes this service costly in the frequency range of 5.855-5.925 GHz and in heavy traffics may cause severe packet collision. Alternatively, visible light communication (VLC) can be used to release the pressure on RF. The equipment required for VLC, such as light emitting diodes (LEDs), camera, and computer, is already available on most of the vehicles nowadays. Therefore, VLC can be considered as a less expensive option for ITS. Most of the works done in camera-based VLC, also known as optical camera communication (OCC), consider a line-of-sight (LOS) link from the transmitter (Tx) to the receiver (Rx). However, in some scenarios, the LOS link might not be available. In this thesis, a non-LOS (NLOS) link is considered to decrease the probability of outage. Accordingly, this thesis highlights another advantage of using camera compared to photodiodes, i.e., the Rx can extract the data information from the off-axis projection of the Tx on the road surface when the LOS link is blocked or is not available. An end-to-end NLOS OCC system is proposed which employs differential signalling and frame subtraction. Throughout the thesis, different detection techniques are proposed based on spatial division multiplexing (SDM) and time division multiplexing (TDM), hybrid selection/equal gain combining (HS/EGC), hybrid selection/maximal ratio combining (HS/MRC), and zero forcing (ZF) equalization, respectively. The thesis deals with different types of the road in terms of the density of the light sources. An experimental investigation of the proposed system shows that, using HS/EGC and simple detection zero forcing equalization (SDZFE), the system can achieve the 7% overhead forward error correction limit of 3.8×10-3 at a very low transmit power of 9 dBm over a link span of 5 m at a data rate of 30 bps, when the Tx is 2 m above the floor surface. It is also shown that by doubling the ISO level, exposure time and aperture size, the performance of the system improves by ~3 dB. In addition, by increasing the link span from 5 to 10 m, the power penalty is ~3 dB. This is because at very low light environment, the intensity of received light is close to the nonlinear region of the camera and since the footprint shrinks as the link span increases, the nonlinearity affects the signal less in longer link spans. The thesis states that the focal length and focusing distance of the camera does not make a significant impact on the performance of the system. Therefore, the camera can have a large field of view with a wide-open aperture. In addition, the thesis demonstrates that the ZF-based detection schemes outperform the HS/EGC schemes under severe level of interference. It is shown that, in ZF-based cases the normalized height of the eye diagram in is 70% higher compared to HS/EGC when the spacing between Txs is double the height of them

A new concept of multi-band carrier-less amplitude and phase modulation for bandlimited visible light communications

Abstract: In this paper, we propose a novel method of multi-band carrier-less amplitude and phase (m-CAP) modulation for optical wireless communication (OWC) systems. In conventional m-CAP systems, the total signal bandwidth is divided into m equally distributed subcarriers. In this work, for the first time, we set the subcarrier bandwidths such that the first subcarrier is the same as the LED bandwidth, and subsequently distribute the remaining bandwidth equally between m - 1 subcarriers. We show that using m = 4, 6, 8 and 10 subcarriers and 16-QAM, the first subcarrier is able to achieve a bit error rate (BER) target of 10-4 (i.e., which is below the 7% forward error correction (FEC) BER limits of 3.8 × 10-3)at Eb/N0 = 15.5 dB. This is a power penalty of -1.5 dB to achieve the same average performance as the first five, four and three subcarriers of the conventional 10, 8 and 6-CAP and a power penalty of ~1 dB for new 4-CAP to achieve the average performance of the first two subcarriers of the conventional 4-CAP. Consequently, using the proposed m-CAP concept we show a reduction in the complexity by reducing the number of finite impulse response (FIR) filters by 80%, 75%, -67% and 50% in contrast to the conventional 10, 8, 6, and 4-CAP, respectively

Comparison of LED illumination patterns for camera based car to car communications

In this paper a comparison between a spiral and a strip shaped LED is presented in terms of the maximum link distance achievable in visible light based car to car communications (VLC-C2C). The transmitted data is recovered from the frame brightness of the video signal. The intensity modulated LED is captured from two scenarios, using a focused and unfocused camera. A data rate of 180 bps with the bit error rate performance below the FEC limit of 10-3 at a distance of 100 cm is successfully achieved, sufficient for transmitting road safety messages for VLC-C2C. It is shown that under the same conditions, the strip LED can effectively recover the data from a greater distance than its spiral counterpart

ROI Assisted Digital Signal Processing for Rolling Shutter Optical Camera Communications

In this paper we propose a novel region of interest (ROI) detection based digital signal processing scheme for optical camera communications (OCC): it provides significant improvement in recovering the source shape induced signal deformations in a rolling shutter optical camera communication (RS-OCC) system over a link of 100 cm. We propose also a novel approach to address the packet losses due to the RS-OCC bursty channel, which relies on repeated packet transmission. We show simulation results for two different packet repetition schemes and compared them with the link with no repetition in RS-OCC