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Visible Light Communication

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Ling Cheng
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Channel impairments in powerline communication (PLC) and visible light communication (VLC) technologies are one of the causes of performance degradation in hybrid PLC and VLC (HPV) systems. In this study, we focus on one of the inherent noise types in PLC channels; impulse noise (IN), which is characterised by short bursts of rapidly modified amplitudes caused by random switching of electrical devices in a power network. Recently, a permutation coded orthogonal frequency division multiplexing (OFDM) with the $M$ -ary frequency shift keying ( $M$ FSK) scheme, in short, PC OFDM- $M$ FSK, was proposed for IN mitigation in PLC systems. It is conceived that the robustness of this scheme against IN in standalone PLC systems will enhance the error performance of the HPV system. Furthermore, since one of the disadvantages of the PC OFDM- $M$ FSK scheme compared to that using only the classical OFDM scheme, is degraded data rate, we propose a generalised PC OFDM- $M$ FSK scheme to improve the degraded data rate. This study shows that the use of OFDM- $M$ FSK improves the performance of an HPV system compared to that using only the classical OFDM modulation scheme by a signal-to-noise (SNR) ratio gain of 6 dB at a bit error rate (BER) of 10−4. Furthermore, an SNR gain of approximately 3 dB and reduction in the decoding complexity of the PC OFDM- $M$ FSK HPV system is achieved using a soft-decision Hungarian-Murty (HM) decoder. Moreover, the generalisation of the PC OFDM- $M$ FSK HPV system does not increase the decoding complexity when the SD HM decoder is used.
Mitchell Arij Cox
added a research item
Efficient utilization of the spectrum, increased resilience to inter-symbol interference (ISI) and simpler channel equalization are becoming important considerations in the design of wireless communication systems including visible light communication (VLC) systems. In this regard, orthogonal frequency division multiplexing (OFDM) has become a preferred modulation technique in wireless communication systems design. However, one of its major challenges is the high peak-to-average power ratio that leads to major inefficiencies. Symbol position permutation (SPP) is a distortion-less technique that achieves substantial PAPR reduction without BER degradation. However, the existing works focus on PAPR reduction using SPP for the radio frequency (RF) OFDM and the use of this technique in optical OFDM is not properly investigated. Therefore, in this paper, we present a new PAPR reduction technique based on lexicographical permutations called lexicographical symbol position permutation (LSPP) for PAPR reduction in direct current optical OFDM (DCO-OFDM). The proposed scheme is less complex than the conventional selective mapping (CSLM) scheme since there is no multiplication of the phase sequences with the DCO OFDM symbol to generate the candidate signals. We further introduce a new way of reducing the complexity by introducing a threshold PAPR and demonstrate that the complexity in terms of inverse fast Fourier transform (IFFT) operations can be reduced substantially depending on the selected threshold and the number of candidate signals.
Ling Cheng
added 2 research items
The high peak to average power ratio (PAPR) is the main challenge facing asymmetrically clipped optical (ACO) orthogonal frequency division multiplexing (OFDM). We propose a new hybrid technique that incorporates a modified selective mapping (SLM) and companding called low complexity hybrid selective mapping (LCHSLM) for PAPR mitigation in ACO OFDM systems. It is demonstrated through computer simulations that our proposed system significantly reduces the PAPR compared with the conventional SLM or companding alone. In addition, LCHSLM achieves a significant reduction in complexity compared with conventional SLM without bit error rate (BER) degradation.
In this paper, discrete memory channel models of a low-cost, visible light communication (VLC) and a hybrid amplify-and-forward (AF) powerline communication (PLC) and VLC channel are derived using data obtained from an underground mining testbed. The testbed is set up close to the mining face, where blasting occurs. Then, the noise distributions as a result of the connection of a blasting equipment and the mains switchboard supply on the powerline channel are modeled. The baseband signals are communicated over software-defined radios, using multicarrier modulation in the VLC and hybrid AF PLC-VLC channels. Aided by a Fritchman 3-state semi-hidden Markov model (SHMM) for channel state classification, the channels are modeled using a first-order (FO) Markov process, which considers only one previous state in the channel. In addition, the block diagonal (BD) Markov model, which groups similar successive states of the channel, is implemented in order to reduce the training complexity. The models for the error distribution are then derived for both VLC and hybrid PLC-VLC channels. Results show the probability of error obtained from the trained models closely match the probability of error obtained from the channel measurements, validating the suitability of the SHMM for modeling VLC and hybrid PLC-VLC channels. In addition, the modified BD Markov model, which groups the channel states shows better performance with reduced complexity when compared with the FO Markov model.
Ling Cheng
added a research item
A multiple access (MA) soft-decision (SD) decoder based on the cost rankings of the channel output matrix, is proposed and applied in an optical wireless communication system in which permutation-based optical codes (POCs) are used. The proposed decoder determines the possible transmitted MA codewords by implementing the Hungarian and Murty’s algorithms to solve and rank the costs of all the possible codewords of the received signal. The performance of the proposed Hungarian-Murty (HM) decoder is evaluated in comparison with two decoders based on the Exhaustive Search (ES) technique, in terms of computational complexity and block error rate (BER). It is found that when POCs are employed, the proposed HM decoder achieves better BER performance with a low decoding complexity in comparison to the ES based decoders.
Ling Cheng
added a research item
Errors in realistic channels contain not only substitution errors, but synchronisation errors as well. Moreover, these errors are rarely statistically independent in nature. By extending on the idea of the Fritchman channel model, a novel error category-based methodology for determining channel characteristics is described for memory channels that contain insertion, deletion, and substitution errors. The practicality of such a methodology is reinforced by making use of real communication data from a visible light communication system. Simulation results show that the error-free and error runs using this new method of defining the channel clearly deviates from the Davey-Mackay synchronisation model which is memoryless in nature. This further emphasises the inherent memory in these synchronisation channels which we are now able to characterise. Additionally, a new method to determine the parameters of a synchronisation memory channel using the Levenshtein distance metric is detailed. This method of channel modelling allows for more realistic communication models to be simulated and can easily extend to other areas of research such as DNA barcoding in the medical domain.
Ling Cheng
added 3 research items
Visible light communication (VLC) has the potential to supplement the growing demand for wireless connectivity. In order to realise the full potential of VLC, channel models are required Discrete channel models based on semi-hidden Markov models (Fritchman model) for indoor VLC using low data rate LEDs are presented. Each channel considered includes background noise and differing types of interference from fluorescent lights and pulse-width modulated (PWM) LEDs, which could be part of an indoor smart lighting system. Models were developed based on experimental error sequences from a VLC system using an on-off keying (OOK) modulation scheme. The error sequences were input into the Baum-Welch algorithm to determine the model parameters by expectation maximisation. Simulated error sequences generated by the models are compared to and, in most cases, perform better than simpler models with a single bit error rate. The models closely approximate the experimental errors sequences in terms of error distribution. The models performed better in channels where there is less interference. It was also found that periodic errors were introduced as a results of the PWM modulated smart lighting LEDs. These models have use for designing error control codes and simulating indoor VLC environments with different types of interference.
Visible light communication (VLC) has the potential to supplement the growing demand for wireless connectivity. In order to achieve the full potential of VLC, channel models are required. Discrete channel models based on semi-hidden Markov models (Fritchman model) for indoor VLC using low data rate LEDs are presented. Each channel considered includes background noise and differing types of interference from fluorescent lights and pulse-width modulated (PWM) LEDs, which could be part of an indoor smart lighting system. Models were developed based on experimental error sequences from a VLC system using an on-off keying (OOK) modulation scheme. The error sequences were input into the Baum-Welch algorithm to determine the model parameters by expectation maximisation. Simulated error sequences generated by the models are compared to and, in most cases, perform better than simpler models with a single bit error rate. The models closely approximate the experimental errors sequences in terms of error distribution. The models performed better in channels where there is less interference. It was also found that periodic errors were introduced as a results of the PWM modulated smart lighting LEDs. These models have use for designing error control codes and simulating indoor VLC environments with different types of interference.
Ling Cheng
added 2 research items
In this paper, the energy efficiency and the spectral efficiency for orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) schemes are studied, which is crucial for practical application with limited energy resources. The conventional schemes including asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone (PAM-DMT), and direct current biased optical OFDM (DCO-OFDM) are compared in terms of energy efficiency and spectral efficiency relationship. The influence of power allocation for asymmetrically clipped DC biased optical OFDM (ADO-OFDM) and hybrid ACO-OFDM (HACO-OFDM) is also investigated. The energy efficiency and spectral efficiency of layered ACO-OFDM (LACO-OFDM) with variable layer number are calculated and their relationship is also formulated. These conventional and hybrid modulation schemes are analyzed and compared through computer simulations, which should be considered in practice according to the requirements of illumination and transmission.
Visible Light Communications (VLC) as well as Mode Division Multiplexing is a research area that is currently receiving significant interest which is beginning to migrate from proof-of-principle demonstrations towards optimized engineering. The ability to easily transmit and receive arbitrarily encoded and modulated optical signals is critical to prototyping in a lab environment. Software defined radios enable cost-effective development and testing of almost any coding scheme and modulation format with a significant bandwidth, if required. We present our experiences with using software defined radios and other platforms to drive LEDs and laser diodes and subsequently receive the transmitted signals.
Ling Cheng
added 4 research items
The integration of power line communication (PLC) and visible light communication (VLC) is increasingly receiving a lot of research interest with the advent of (IEEE 1901, ITUT G.9960/61) and IEEE 802.15.7 standards for PLC and VLC respectively. In particular, there is an underlying gain that could be achieved by leveraging the existing ubiquitous power line network infrastructure to render connectivity, while we also exploit the illumination system of power-saving Light Emitting Diodes (LEDs) for wireless data communication. The ubiquitous nature of these two systems makes us belief that VLC can offer a good complementary wireless data transmission technology to the existing In-House PLC in a similar manner broad-band Ethernet connections enjoys the support of Wi-Fi. This paper thus reports an implementation of a low complexity FSK-OOK In-House PLC and VLC Integration, as well as it's Second-Order Semi-Markov Model. The resulting statistical models facilitates the design and evaluation of forward error correcting codes to mitigate burst error occurrences, as well as optimizing the performance of the overall system.
Telecommunication engineering may substantially benefit from the integration of power line communications (PLC) and visible light communications (VLC) technologies. This integration is firstly enabled by the ubiquitous nature of PLC and VLC infrastructures, and secondly by the recent advances made in both technologies, and will be effective with the knowledge of the cascaded PLC-VLC channel. This paper reports on the spectral analysis and estimation of the dual channel for hybrid systems involving PLC and VLC technologies. The channels are statistically analysed using the non-parametric windowed Welch method. Experimental results giving varieties of simulated PLC-VLC channel are presented. They show the influence of the number of taps (PLC side), the line-of-sight (LOS) and the non-LOS (NLOS) gains (VLC side) on the PLC-VLC frequency response. It also indicates that the cascaded channel is influenced by the propagation distance and the room area. We also show the influence of PLC and VLC channel parameters on the gain and notches.
Visible light communication (VLC) is regarded as one of the promising candidates to complement the conventional radio frequency communication as it can potentially perform communication and illumination simultaneously. Recently, VLC is combined with generalized spatial modulation (GenSM) technology to improve spectral and energy efficiencies. In order to provide high speed communication and flexible dimming control simultaneously, in this paper, a spectral-efficient GenSM based hybrid dimming scheme with layered asymmetrically clipped optical OFDM (LACO-OFDM) is proposed for VLC, which combines the spatial-domain and time-domain dimming strategies. The channel capacities of the proposed scheme are analyzed and closed-form expressions of the channel capacity are derived in both the rich scattering scenario and highly correlated scenario. According to the required illumination level, the spatial-domain and time-domain strategies are realized based on the maximization of the channel capacity. Simulation results substantiate the superior performance of the proposed dimming scheme over state-of-art dimming schemes.