Should Analogue Pre-equalisers be Avoided in VLC Systems?

Visible light communication (VLC) systems are highly constrained by the limited 3-dB bandwidth of light-emitting diodes (LEDs). Analogue pre-equalisers have been proposed to extend the LED's bandwidth at the cost of reduced signal-to-noise ratio (SNR). Compared with the pre-equaliser, the multi-carrier modulation with bit-loading can efficiently use the spectrum beyond the LED's raw 3-dB bandwidth without incuring SNR penalties by employing multiple narrow quasi-flat sub-bands to eliminate the need for equalisation. In this work we show by means of experimental investigation that VLC with multi-band carrierless amplitude and phase modulation with bit-loading can outperform VLC with analogue pre-equalisers

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

Investigation into Using Compensation for the Nonlinear Effects of the Output of LEDs in Visible Light Communication Systems

This paper investigate the effects of the nonlinear output power to the input current transfer function of a light emitting diode (LED) used as the transmitter in visible light communication systems, and proposes a compensation technique to mitigate the non-linearity. Using an off the shelf red, amber, blue, green (RAGB) LED the nonlinear transfer function is measured and then compensated for. The results show that, for the green LED, which has the highest degree of nonlinearity, an improvement in the received error vector magnitude (EVM)of almost 7 dB can be achieved. We also show that, the improvement in EVM over the uncompensated scheme decreases with increased LED linearity

Experimental Demonstration of a 40 Mb/s VLC System Using a Large Off-the-Shelf LED Panel

This paper experimentally demonstrates a VLC sys-tem using a large off-the-shelf LED panel with dimensions of 60 × 60 cm2 and achieves a data rate of 40 Mb/s based on multi-band carrier-less amplitude and phase (CAP) modulation. The proposed system demonstrates the potential of utilising the existing LED-based lighting infrastructure for simultaneous indoor illumination and data communication

A 40 Mb/s VLC System Reusing an Existing Large LED Panel in an Indoor Office Environment

With advances in solid-state lighting, visible light communication (VLC) has emerged as a promising technology to enhance existing light-emitting diode (LED)-based lighting infrastructure by adding data communication capabilities to the illumination functionality. The last decade has witnessed the evolution of the VLC concept through global standardisation and product launches. Deploying VLC systems typically requires replacing existing light sources with new luminaires that are equipped with data communication functionality. To save the investment, it is clearly desirable to make the most of the existing illumination systems. This paper investigates the feasibility of adding data communication functionality to the existing lighting infrastructure. We do this by designing an experimental system in an indoor environment based on an off-the-shelf LED panel typically used in office environments, with the dimensions of 60 × 60 cm2. With minor modifications, the VLC function is implemented, and all of the modules of the LED panel are fully reused. A data rate of 40 Mb/s is supported at a distance of up to 2 m while using the multi-band carrierless amplitude and phase (CAP) modulation. Two main limiting factors for achieving higher data rates are observed. The first factor is the limited bandwidth of the LED string inside the panel. The second is the flicker due to the residual ripple of the bias current that is generated by the panel’s driver. Flicker is introduced by the low-cost driver, which provides bias currents that fluctuate in the low frequency range (less than several kilohertz). This significantly reduces the transmitter’s modulation depth. Concurrently, the driver can also introduce an effect that is similar to baseline wander at the receiver if the flicker is not completely filtered out. We also proposed a solution based on digital signal processing (DSP) to mitigate the flicker issue at the receiver side and its effectiveness has been confirmed

Using equalisers to increase the bandwidth of VLC systems

This thesis investigates and proposes the equalisation techniques for visible light communications (VLC) to increase a VLC system’s bandwidth and achievable data rates from a practical standpoint. The equalisation techniques are mainly classified into two categories. The first category is the analogue equaliser, which is implemented at either transmitter or receiver side using analogue devices with a reverse frequency response of the raw system to extend the overall system bandwidth. The second category is the digital equaliser, which is highly based on advanced digital signal processing (DSP) algorithms. The digital approach provides significant flexibility and adaptivity to combat the frequency response variance in VLC wireless channels and analogue components compared with the analogue method. On the other hand, the analogue equalisers can be easily designed and implemented cost-effectively, demonstrating fast improvement in baseband modulation schemes like on-off keying (OOK). Although data transmission of Gbit/s OOK has been demonstrated by simply using analogue pre-equalisers, the cost of such a pre-analogue equaliser is avoidable massive power attenuation, which is limiting its use in practical applications. In contrast, the digital post-equaliser does not have such a power loss issue and shows its advantages over the pre-analogue equaliser. Therefore, this thesis firstly studies the fundamentals of the analogue pre-equaliser and proposes a universal analogue equaliser design method to validate that such a pre equaliser actually decreases system capacity because of the SNR penalty introduced by the analogue pre-equaliser. The analogue pre-equaliser should be avoided if the system is aiming at the highest data rates. The better solutions can be single-carrier modulation with complex digital post equalisers or multi-carrier modulation with simpler digital post-equalisers. Then this work investigates how the post-equaliser can be implemented in a practical single-carrier CAP receiver. The CAP modulation scheme was selected because it has been one of the most popular schemes proposed for VLC for its low implementation cost. However, real-time oriented CAP receiver has been a topic rarely touched. For these reasons, this work proposes a full-digital CAP receiver architecture with both equalisation and synchronisation included. The inclusion of synchronisation adds more problems because the coupling between synchronisation and equalisation creates a “chick-and-egg” problem. It is the use of the blind equalisation algorithm, i.e., constant modulus algorithm (CMA), that decouples the equalisation and synchronisation and makes a full-digital receiver possible. The thesis finally investigates the possibility of applying multi-carrier CAP modulation with low-complexity post equalisation in a real-world office environment by upgrading a large white LED panel light with the VLC function. Benefited by not losing any power, the multi-carrier CAP modulation enables the light panel with a raw bandwidth of only 1 MHz to deliver up to 40 Mbit/s at a standard illuminance height

Security Performance of LDPC and Polar Codes in UV Wireless Communications

The growing demand for access to high-capacity wireless communication systems opens up the need for the alternative and complementary technology of optical wireless communications including ultraviolet (UV). In modern wireless communications networks the users and data security, which is extremely important in order to ensure users’ data protection and confidentiality, has become a hot topic. In this paper, we investigate security in a UV communications system by adopting two well-known coding schemes of low-density parity-check (LDPC) and polar codes (PCs). We show that the UV system with coding offers enhanced security performance compared to the un-coded system with the PCs offering a higher security level and a longer transmission span compared to the LDPC

Equivalent Circuit Model of High Power LEDs for VLC Systems

The equivalent circuit model for two commercial high power LEDs for illuminance is proposed and extensive measurements have been carried out to verify the accuracy of the model. In addition, the frequency response of two LEDs in the optical domain can be estimated using the transfer function of electrical equivalent circuit. It is shown that, the electrical parasitics in the chip and packaging introduce significant attenuation beyond a resonance frequcency. Care should be taken when designing high-speed driver and/or equalization circuits for VLC systems

An Equivalent Circuit Model of a Commercial LED With an ESD Protection Component for VLC

International audienceAn equivalent circuit model for a typical commercial light-emitting diode (LED) used in visible light communication (VLC) applications is proposed. The parameters of the elements in the model are extracted from the measured impedance using a vector network analyser. The model shows high accuracy in fitting the measured results and shows the effect of bonding/packaging parasitics and the electrostatic discharge (ESD) component on the LED's impedance and bandwidth. This study aims to raise the designer's awareness when designing and fabricating high-speed LED/µLED devices to achieve the optimal modulation bandwidth in the emerging VLC applications