Subcarrier Grouping OFDM for Visible-Light Communication Systems

Visible light communication (VLC) system based on orthogonal frequency division multiplexing (OFDM) is promising for its ability to link a huge number of devices with high reliability and low latency. However, OFDM-based VLC systems are very sensitive to the symbol timing offsets. Thus, synchronisation algorithm is very important for OFDM-based VLC system. In this study, the authors first propose zero correlation code pair (ZCC pair) and investigate the periodical impulse-shaped correlation property of ZCC pair. Then the authors propose a novel timing offset estimation algorithm based on ZCC pair. The preamble is processed via peak-to-average power ratio (PAPR) reduction algorithms to overcome the PAPR problem in VLC systems. This algorithm achieves high accuracy and has low synchronisation error probability. The effectiveness of the authors' proposed algorithm has been demonstrated using computer simulation under non-light-of-sight VLC and Gaussian channels. The results show that the authors' algorithm outperforms the Schmidl Cox's and Park's algorithms in terms of synchronisation error probability and mean square error of timing error.

Visible light communication (VLC) is a technology that is currently being employed to achieve high data rates. Orthogonal frequency division multiplexing (OFDM) is a powerful scheme for intensity modulation and direct detection (IM-DD) that is becoming increasingly important. OFDM systems based on VLC are accomplished through the implementation of IM-DD constraints, including direct current biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), and asymmetrically clipped DC-biased (ADO-OFDM), among others. The OFDM technique based on VLC is inefficient in terms of spectral efficiency and has a high peak to average power ratio (PAPR). In this article, the symbol time compression-image adjust (STC-IMADJS) technique will be proposed for throughput maximization and PAPR reduction in the DCO-OFDM, ACO-OFDM, and ADO-OFDM systems. Furthermore, the STC technique enables the simultaneous transmission of two sub-carriers through Walsh spreading codes without inference between them. As a result, the OFDM symbol has been reduced by 50%. The IMADJS strategy reduces the high PAPR of transmitted signals by compressing large signals and expanding small signals, as long as the average power level remains constant after compression. Therefore, the proposed STC-IMADJS technique doubles the throughput as well as significantly reduces the PAPR for OFDM systems based on VLC. The simulation results are performed using Matlab-2021a for 1024 sub-carriers. As a result, the simulation results show that the proposed technique reduces the PAPR by 3.7 dB (86%), 8.16 dB (93%), and 1.81 dB (20%) for DCO-OFDM, ACO-OFDM, and ADO-OFDM systems respectively. Furthermore, the technique has a significantly lower computational complexity than conventional OFDM systems based on VLC. Finally, the measures of performance through this article are bit error rate and complementary cumulative distribution function.

In visible light communication (VLC) system, the Orthogonal Frequency Division Multiplexing (OFDM) technique has been widely used to achieve a high data rate transmission. However, OFDM will cause high peak-to-average power ratio (PAPR) in the VLC system. In this paper, we propose and experimentally demonstrate a new interleaved Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing (DFT-S OFDM) method. We compare the interleaved DFT-S technique to other PAPR reduction techniques, such as Clipping OFDM and conventional DFT-S. The experimental results show that interleaved DFT-S achieve FEC threshold for an overall 2.0Gb/s 32QAM signal over the 1.05m transmission. In addition, interleaved DFT-S presents significant PAPR reduction, low complexity and optimal BER performance, when compared to other techniques.

SUMMARY The main disadvantage of orthogonal frequency divisionmultiplexing (OFDM) is the high time domain PAPR. The larger PAPRsignal would fatallydegrade BER performance in non-linear channels. Thispaper proposes an improved DSI method, which can achieve better PAPRand BER performances in the non-linear channel with less computationcomplexity than the conventional DSI method. The feature of proposedmethod is to employ the time-frequency domain swapping algorithm inthe determination of frequency data for dummy sub-carriers. This paperpresents various computer simulation results to verify the effectiveness ofproposed DSI method. key words: OFDM, PAPR, dummy sub-carriers, non-linear amplifier 1. Introduction The OFDM technique has been received a lot of attentionespecially in the field of wireless communications becauseof its efficient usage of frequency bandwidth and robustnessto multi-path fading. One of the limitations of using OFDMtechnique is the larger peak to averaged power ratio (PAPR)of its time domain signal [1]. The larger PAPR signal wouldcause the severe degradation of bit error rate (BER) perfor-mance due to the inter-modulation noise occurring in thenon-linear amplifier.To overcome the PAPR problem in OFDM systems,various PAPR reduction methods for OFDM signal havebeen proposed including the selected mapping method(SLM) [2], partial transmit sequence method (PTS) [3] anddummy sequence insertion method (DSI) [4]. All of thesemethods improve the PAPR performance by controlling thephase of data or dummy sub-carriers. The SLM and PTSmethods control the phase of data sub-carriers and the DSImethod controls the phase of dummy sub-carriers. First twomethods are required to inform the phase information con-trolled for the data sub-carriers to the receiver as the sideinformation. Since the side information is required to trans-mit separately from the data information through the data orcontrol channels with the higher signal quality for the cor-rect demodulation of data information at the receiver, the

In recent years, Orthogonal Frequency Division Multiplexing (OFDM) is preferred for transmission over a dispersive channel. It is because of its several advantages such as high spectral efficiency, low implementation complexity, less vulnerability to echoes and non–linear distortion. However, it has few limitations such as peakto-average-power ratio (PAPR) and bit error rate (BER), which determines the system’s power efficiency. High PAPR is one of the major drawbacks in OFDM systems, which causes a significant level of signal distortions, when the modulated signals are amplified through high power amplifiers (HPAs). In particularly, high PAPR increases the complexity of Analogue to Digital (A/D) and Digital to Analogue (D/A) converters and also reduces the efficiency of HPAs. A high PAPR and high noise (e.g. AWGN) may significantly distort the signal, resulting in high BER on demodulation of the received signal. Therefore, PAPR & BER reduction is very essential. For this purpose, a Zadoff-Chu matrix Transform (ZCT) based precoding and postcoding techniques are quantitatively evaluated in this paper in terms of its parameters q and r. From the analysis, it is inferred that an optimum selection of q and r can lead to a significant level of reduction in BER.

왈시-하다마다 변환(Walsh-Hadamard Transform, WHT)을 적용한 OFDM(Orthogonal Frequency Division Multiplexing) 변조방법은 시스템의 PAPR(Peak-to-Average Power Ratio)을 줄여주고, 시스템의 BER(Bit Error Rate) 성능을 향상시키는 장점이 있다. 그러나 기존의 이러한 왈시-하다마다 변환에 기반한 OFDM 시스템은 여전히 비교적으로 높은 PAPR문제가 있다. 그러므로 본 논문에서는 DGT(Data Grouping Technique)이라고 부르는 기술을 제안하고, 이 기술과 왈시-하다마다 변환 기술을 적용하여 PAPR을 더욱 줄여주는 OFDM 시스템을 설계한다. 제안한 DGT기술은 WHT-OFDM뿐만 아니라 일반 OFDM에서 PAPR을 줄여주는 방법들 중에서 큰 단점이 되는 부가정보(side information)가 성능에 대해 독립성을 가진다는 특징이 있다. 컴퓨터 시뮬레이션을 통해 제안한 시스템의 유용성을 검사하고 성능을 검증하였다. The conventional OFDM (Orthogonal Frequency Division Multiplexing) modulation can be combined with WHT (Walsh-Hadamard Transform) to reduce PAPR (Peak-to-Average Power Ratio) and improve BER (Bit Error Rate) performance. However, this WHT-based OFDM system still suffers a relatively high PAPR. Therefore, we suggest a new technique, called DGT (Data Grouping Technique) and design an OFDM system employing it and WHT to further decrease PAPR without the BER performance degradation. h salient property of DGT is the independence of the side information which is inherently a principal drawback of the well-known PAPR reduction techniques for OFDM system as well as WHT-based OFDM. The simulation programs have been also performed to verify the validity of the proposed system.

Visible light communication (VLC) is a promising and viable optical wireless communication (OWC) system that utilizes light-emitting diodes (LEDs) to achieve high data rates, energy efficiency, and resistance to electromagnetic interference (EMI). Direct-current biased optical orthogonal frequency division multiplexing (DCO-OFDM) is a widely used scheme in VLC due to its high spectral efficiency. However, it suffers from significant challenges related to its elevated peak-to-average power ratio (PAPR), which causes nonlinear distortion and signal clipping due to LED limitations. To address this issue, this paper proposes a combination of Vandermonde-like matrix (VLM) precoding and nonlinear companding transform (NCT) techniques (specifically A-law and μ-law) over DCO-OFDM-based VLC. The VLM precoding minimizes the autocorrelation of the OFDM signal, hence reducing its PAPR, and NCT compresses the dynamic range of the signal. Simulation results indicated that the hybrid techniques achieved a PAPR reduction of around 3.5 dB (by 76 %); this is in the best case reduction compared to the conventional DCO-OFDM while maintaining an acceptable bit error rate (BER), moderately higher signal-to-noise ratio (SNR) is required.

Encrypted image transmission in OFDM-based VLC systems using symbol scrambling and chaotic DFT precoding

This paper investigates optical orthogonal frequency division multiplexing (O-OFDM) systems with index modulation (IM) based on discrete Hartley transform (DHT) for visible light communications (VLC). An interesting trade-off between spectral efficiency (SE), energy efficiency, peak-to-average power ratio (PAPR) and bit error rate (BER) performance can be made by using the IM technique. And the DHT-based O-OFDM-IM systems can achieve higher SE compared with the DFT (discrete Fourier transform)-based counterparts. Employing DHT instead of DFT enables the removal of the Hermitian symmetry requirement, which contributes to transmitting more index bits and results in SE improvement. To acquire the same SE, the proposed systems can significantly lower the constellation order leading to better BER performance by contrast with the DFT-based counterparts. We employ DC (direct current) biasing and AC (Asymmetrically clipped) techniques to investigate the O-OFDM-IM systems in this paper. Simulation results indicate that SNR gains can be obtained by the DHT-based DCO/ACO-IM in comparison to the DFT-based counterparts. We also propose a novel reduced-complexity ML (maximum likelihood) detector specifically applicable to VLC. The new detector exhibits the same BER performance as the ML detector and performs better than the LLR (log-likelihood ratio) detector with reduced computational complexity.

This chapter discusses techniques on multiple-input multiple-output (MIMO) visible light communications (VLC) system which improved the speed of data communications. It discusses the use of pilot signal in reducing the peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM)-based intensity-modulated optical wireless communication system. The introduction of solid-state light-emitting diode (LED) lighting has attracted the attention of communications engineers worldwide, enabling the achievement of the dual functionality of room illumination while simultaneously transmitting wireless data via VLC. A Q factor analysis of the signal between single transmitter and single receiver as function of horizontal displacement beneath the center point of a transmitter was carried out, to investigate at what distance and how fast the quality of the signal degrades. The OFDM technique is attracting a lot of interest in optical wireless communications because of its resilience to multipath propagation-induced intersymbol interference (ISI), high spectral efficiency, and immunity to fluorescent light noise near digital converter (DC) region.

Knowing the water level of the Akosombo Dam would help Ghanaian since we depend heavily on hydroelectric power.When the future of the water level is known, society would be able to plan on the usage of electricity for the industries, society, individuals who use some of the water storage for irrigation, water supply purposes.The study employed rainfall from the 12 catchment areas to the River Volta and the daily water level of the dam for a period of 78-years.Principal Component Regression was applied to the input variables for the reduction of its large size to a few principal components to explain the variations in the original dataset.The outcome of the PCR extraction was two principal components.Time Series using Seasonal Autoregressive Integrated Moving Average was used to model the data.The appropriate model that fit the data well was ARIMA (2,1,2) (1,0,0) [12] after comparing other models AICs.The model with the smallest AIC and the least number of parameters was selected as the best model.

High peak to average power ratio (PAPR) is a serious drawback for orthogonal frequency division multiplexing (OFDM) system as it reduces the efficiency of high-power amplifiers commonly used in the transmitter and causes spectral spillage. OFDM transmissions are also sensitive to frequency selective fading channels which can severely attenuate certain parts of the signal making it much more vulnerable to additive noise. This paper proposes an effective precoding technique with the dual ability to both reduce PAPR and condition the OFDM signal against frequency selective fading channels. It will be demonstrated that by advisedly creating diversity among the subcarriers of one OFDM symbol, and exploiting it with a dirty- paper-coding (DPC) approach, excellent bit-error-rate (BER) performance is achieved without reducing transmission rate. Low PAPR factor can also be obtained through a proper selection of diversity matrix that distributes the power of each modulated symbol over the OFDM block. Simulation results show that the proposed technique has not only better BER performance than the ordinary channel inversion but also similar PAPR reduction ability to the selective mapping (SLM) technique.

DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) has been proposed in visible light communication (VLC) to overcome the limited modulation bandwidth of light emitting diode (LED). Due to the implementation of the inverse fast Fourier transform at the DCO-OFDM transmitter, DCO-OFDM suffers from its high peak-to-average power ratio (PAPR), which restricts its use in some VLC applications, especially where the optical power efficiency is a crucial requirement. That is because the LEDs used in VLC systems have a limited optical power-current linear range. To this end, a novel discrete Fourier transform spread-optical pulse amplitude modulation (DFTS-OPAM) signal scheme based on the single carrier-interleaved frequency division multiple access (SC-IFDMA) signal is introduced in this paper to address the high PAPR issue of OFDM. DFTS-OPAM is achieved by considering a PAM as an SC-IFDMA data symbol and duplicate the output vector of the fast Fourier transform at the SC-IFDMA transmitter side. Simulation results show that the PAPR of the proposed scheme is 7 dB lower than that of DCO-OFDM. Furthermore, this significant PAPR improvement is experimentally investigated where the practical results show that the proposed scheme can provide more 2.5 dB reduction in the average transmitted power requirement compared to DCO-OFDM and can subsequently increase the maximum achieved distance between the transmitter and the receiver up to 44%.

We propose a CCP (companding combined with predistorter) scheme that maintains BER (bit error rate) performance, while effectively reducing PAPR (peak-to-average power ratio) of OFDM (orthogonal frequency division multiplexing) signals. The proposed CCP scheme combines the conventional companding scheme proposed by the authors with the predistorter, in order to prevent degradation of BER performance caused by signal distortion from nonlinear property of the HPA (high power amplifier) and the compression property of the companding scheme. Hence, the proposed scheme effectively reduces the PAPR and simultaneously maintains the BER performance independent of nonlinear property of the HPAs. Simulation results show that the proposed scheme provides the lower total degradation for all the modulations, when compared with the conventional companding scheme proposed by Wang and the clipping scheme.

The aim of the experiment was to assess the effects of supplemental lighting of cucumber grown in greenhouse using lamps with differentiated light spectra: sodium lamps – high-pressure sodium (HPS) and light emitting diodes (LEDs). Plants (cucumber ‘Pacto’ F 1 ) were grown in two greenhouse compartments with five light treatments: I – HPS + LED (top light with HPS LEDs as interlighting), II – only HPS as top light, III – LED R (LED chips on board (COB) type with an increased level of red band), IV – LED W (LED COB type, white), V – LED B (LED COB type with an increased level of blue spectrum). Light treatments: HPS + LEDs and HPS were grown in one greenhouse compartment and the other three light treatments: LED R (red light supplementation), LED W (without additional supplementation) and LED B (red spectrum supplementation) in the second compartment in analogous climatic conditions. The LED lamps using COB technology are known to be a very efficient source of light. Plants were cultivated from December 2018 to March 2019 in mineral wool slabs Grotop Master (100 × 15 × 10) with four plants on one mat. They were illuminated for 18 h (from 5 am to 11 pm), setting the threshold value (on and off) at 130 W. The plants were drip-irrigated with a complete nutrient solution. The irrigation was controlled based on a weighting system. The assessment of the effect of lighting on early yield and quality of cucumbers was completed after 8 weeks of cropping. It was shown that it was possible to obtain 3.59 kg from one plant during the 8-week period of evaluation by illuminating plants with sodium lamps (HPS), while using HPS and LEDs as additional illuminated inter-rows 3.89 kg. The yield of plants illuminated by LED lamps varied depending on the variant of the spectrum used and was respective for LED R, LED W and LED B, 3.30 kg, 3.90 kg and 3.25 kg. The obtained results indicated that the yield of cucumber ‘Pacto’ F 1 grown with HPS lamps at top lighting and at the same time using interlighting with LED lamps was similar to LED W lamps (i.e. without additional supplementation in the range of red (LED R) and blue (LED B) light). Due to good results of LED lamps (type COB for top lighting or as interlighting) used for the cucumber supplemental lighting and high energy efficiency of LEDs, the promising future for that type of lamps compared with traditionally used HPS during winter cultivation in a greenhouse was demonstrated.