Hybrid PAPR reduction schemes for different OFDM-based VLC systems

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 paper fundamentally investigates the peak-to-average power ratio (PAPR) theory in all optical orthogonal frequency division multiplexing (OFDM) systems which employ intensity modulation-direct detection (IM-DD) scheme. We propose a low-complexity PAPR reduction scheme based on phase pre-emphasis. Simulations show that the proposed scheme brings about a 3.74 dB PAPR reduction and better nonlinear impairment tolerance in a 16x10Gb/s IM-DD all optical OFDM system.

Orthogonal frequency division multiplexing (OFDM) system has been widely used for the high speed digital communication area and it is very promising for the next generation communication standard because of high data rate capability. However, OFDM system has inherent serious problem of high peak-to-average power ratio (PAPR) that makes the OFDM signal be distorted in the nonlinear HPA (high power amplifier). In this paper, CI (carrier interferometry)-OFDM system is considered to overcome this problem and is newly analyzed in the selective fading and narrow-band interference channel. CI-OFDM system distributes each parallel data to all sub-carriers in differently phase-rotated form. So, PAPR becomes lowered and it shows the frequency diversity effects. BER (bit error rate) performance is analyzed in the selective fading and narrow-band interference environment. From the simulation results, it is confirmed that CI-OFDM system calms down the high PAPR problem and CI-OFDM system is more robust than the ordinary OFDM system in the selective fading and narrow-band interference channel.

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 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 this paper, we propose a low-complexity peak-to-average power ratio (PAPR) reduction scheme that combines both multi-band (MB)-Hadamard precoding and clipping for the optical orthogonal frequency division multiplexing (OFDM) systems. Approximations of PAPR distribution for baseband OFDM signals are analyzed and the effective signal-to-noise ratio (SNR) of the whole transmission link considering both the clipping and quantization noise are presented. After that, the MB-Hadamard precoding is adopted to compresses the peak signals, minimizing the contaminating influence of signal distortions in subsequent clipping operations. In addition, the received SNRs and bit error rate (BER) are calculated theoretically for each split sub-band. The 50-m step-index polymer optical fiber (POF) transmission is adopted as a special case to both evaluate the system performance and then compare the proposed scheme with other well-known PAPR reduction techniques. With this scheme, the PAPR is reduced effectively and the system’s BER performance is improved significantly. The results show that the proposed scheme with appropriate number of sub-bands precoding provides favorable trade-offs among PAPR reduction, power spectral density, transmission rate, BER, and computational complexity, which demonstrates its feasibility and validity.

A three-dimensional (3D) orthogonal frequency division multiplexing (OFDM) system with peak-to-average power ratio (PAPR) reduction method for the wireless sensor networks (WSNs) is presented. The transmit power of wireless nodes in an ad hoc network is strictly limited. Thus, high PAPR of the 3D OFDM system is a possible drawback when it is considered as a physical layer transmission scheme for the WSNs. Here, we propose an improved partial transmit sequence (PTS) technique to reduce PAPR of the OFDM system with 3D modulation formats. In the proposed algorithm, the components of 3D signals are assigned to different subblocks using a diagonal rule (DR), increasing randomness of the signals in the disjoint subblocks forcibly. As a result, the proposed method reduces PAPR of the 3D OFDM system significantly without increase in computational complexity. Hence, the proposed algorithm makes the 3D OFDM system be a possible candidate for a physical layer transmission scheme in future WSNs.

The limited modulation bandwidth and nonlinearity of light-emitting diodes (LEDs) are the important factors that constrain the high-speed transmission of visible light communication (VLC). As a LED lamp includes multiple LED chips and the superposition characteristics of light is approximately linear, a multi-LED optical orthogonal frequency division multiplexing (OFDM) system is proposed. The multiple LED chips are used to transmit multiple asymmetrically clipped optical OFDM (ACO-OFDM) and unipolar OFDM (U-OFDM) signals in parallel, while the signals can be recovered at the receiver. When adopting the same M-ary quadrature amplitude modulation (M-QAM), its spectral efficiency is higher than the conventional ACO-OFDM system. The power of the multi-LED transmission system is split equally to each branch, and the amplitude range of the signal is smaller, thus it has better anti-nonlinearity performance. At last, its advantages are verified by the simulation comparisons with other optical OFDM systems.

This paper analyzes the performance of indoor orthogonal frequency division multiplexing (OFDM) optical wireless communication systems in the presence of light emitting diode (LED) nonlinear distortions. There are several forms of optical OFDM using intensity modulation [7th Int. Symp. on Communication Systems Networks and Digital Signal Processing (CSNDSP), 2010, pp. 566-570]. In this paper, DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are considered. ACO-OFDM produces a 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 for DC bias at the expense of data rate reduction. DCO-OFDM assigns data to all possible subcarriers to increase the data rate. However, half-wave symmetry signals cannot be achieved and a high DC bias is needed to convert the bipolar signal to a unipolar signal before modulating the LED intensity. This paper considers a practical LED model and studies the performance of both systems in terms of average electrical OFDM signal power versus bit error ratio in the presence of an additive white Gaussian noise (AWGN) channel. In addition, DC power consumption and the transmitted optical power for the two systems are compared. The analytical results are validated through Monte Carlo simulations and the obtained results demonstrate close match. It is shown that LED clipping has significant impact on the performance of both systems and an optimum system design should take into account the OFDM signal power, DC-bias point, and LED dynamic range.

Orthogonal frequency division multiplexing(OFDM) is a multicarrier modulation(MCM) system that enables high-speed communications using multiple carriers and has advantages of power and spectral efficiency. Therefore, this study aims to complement the existing shortcomings and to design an efficient MCM system. The proposed system uses the inverse discrete wavelet transform(IDWT) operation instead of the inverse fast Fourier transform(IFFT) operation. The bit error rate(BER), spectral efficiency, and peak-to-average power ratio(PAPR) performance were compared with the conventional OFDM system through the OFDM system design based on wavelet transform. Our results showed that the conventional OFDM and Wavelet-OFDM exhibited the same BER performance, and that the Wavelet-OFDM using the discrete Meyer wavelet had the same spectral efficiency as the conventional OFDM. In addition, all systems of Wavelet-OFDM based on various wavelets confirm a PAPR performance lower than that of conventional OFDM.

Based on superposition of signals in the time domain, we propose a new low-complexity scheme for the reduction of the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. The proposed scheme first generates signal candidates by combining an OFDM signal and its cyclically delayed versions with different delays and phases, and then selects the candidate signal with minimum PAPR. It is shown that the complexity of the proposed scheme is significantly lower than that of the selective mapping (SLM) at the cost of some degradation in the bit error rate (BER) performance and, at similar PAPR and coded BER performance, is lower than that of the low-complexity SLM proposed recently.

A new precoding scheme for spectral efficient optical OFDM systems

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.

In this paper, we consider out-of-band radiations caused by high sidelobes in orthogonal frequency division multiplexing (OFDM) system. Suppression of sidelobes in OFDM system allows higher spectral efficiency and less interference with existing legacy systems in OFDM based overlay system. We combine different sidelobe suppression techniques to suppress the sidelobes in OFDM system. The multiple-choice sequence (MCS) is combined with cancellation carriers (CCs) and also MCS is combined with subcarrier weighting (SW) technique. MCS is applied prior to CCs or SW techniques. Simulation results show that combining MCS with CCs and MCS with SW techniques the sidelobes in OFDM system can be significantly reduced, which allow a successful co-existance between OFDM overlay system and an existing legacy system.

In this paper, a novel scheme of the weighted distortion-to-signal ratio (WDSR) based partial transmit sequence (PTS) is proposed to enhance the bit error rate (BER) performance in nonlinear distorted orthogonal frequency-division multiplexing (OFDM) systems, and its key idea is to employ the WDSR metric to predict the nonlinear distortion caused by high power amplifier (HPA). Furthermore, several alternative signals can be generated by the PTS technique, and the signal with the minimum WDSR is selected as the transmitted OFDM signal. Since the WDSR metric predicts the nonlinear distortion of OFDM systems, the BER performance of nonlinear distorted OFDM systems can be significantly improved. Simulation results show that the proposed WDSR-PTS scheme offers better BER performance than the conventional peak-to-average power ratio (PAPR) based PTS scheme.