Multi-LED optical OFDM modulation for visible light communication

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.

Orthogonal frequency division multiplexing (OFDM) has been considered for\nvisible light communication (VLC) thanks to its ability to boost data rates as\nwell as its robustness against frequency-selective fading channels. A major\ndisadvantage of OFDM is the large dynamic range of its time-domain waveforms,\nmaking OFDM vulnerable to nonlinearity of light emitting diodes (LEDs). DC\nbiased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM\n(ACO-OFDM) are two popular OFDM techniques developed for the VLC. In this\npaper, we will analyze the performance of the DCO-OFDM and ACO-OFDM signals in\nterms of error vector magnitude (EVM), signal-to-distortion ratio (SDR), and\nachievable data rates under both average optical power and dynamic optical\npower constraints. EVM is a commonly used metric to characterize distortions.\nWe will describe an approach to numerically calculate the EVM for DCO-OFDM and\nACO-OFDM. We will derive the optimum biasing ratio in the sense of minimizing\nEVM for DCO-OFDM. Additionally, we will formulate the EVM minimization problem\nas a convex linear optimization problem and obtain an EVM lower bound against\nwhich to compare the DCO-OFDM and ACO-OFDM techniques. We will prove that the\nACO-OFDM can achieve the lower bound. Average optical power and dynamic optical\npower are two main constraints in VLC. We will derive the achievable data rates\nunder these two constraints for both additive white Gaussian noise (AWGN)\nchannel and frequency-selective channel. We will compare the performance of\nDCO-OFDM and ACO-OFDM under different power constraint scenarios.\n

In the recent times, Visible Light Communication (VLC) has evolved as a new communication standard due to its stunning notability to render ubiquitous connectivity by emerging as a prominent revolutionary to radio frequency (RF) based wireless communications. However, an orthogonal frequency division multiplexing (OFDM) based VLC system is susceptible to timing errors. Therefore, this work addresses the synchronization aspects in asymmetrically clipped optical OFDM (ACO-OFDM) system which is exploiting real trigonometric transform like discrete sine transform (DST). The striking notability is that without the necessity of Hermitian Symmetry criteria, this optical OFDM system takes the advantages of real signal processing transformation technique like DST. Thus, this technique reinforces the transmission of double the data symbols in comparison to conventional optical OFDM system which is based on standard Fourier signal processing. Besides, this work derives the elaborate mathematical expressions illustrating the deleterious effects of unreliable timing errors in DST-based ACO-OFDM system. In addition, this paper interprets as well as compares the performance of timing synchronization algorithms exploiting the periodicity property of the optical OFDM symbol with the training symbol based STO estimation algorithm in DST-based ACO-OFDM system. Moreover, this work validates the derived Cramer Rao Lower Bound (CRLB) and the simulated results infer that derived CRLB pertaining to the timing errors contributes the minimum mean square error (MSE) when compared with other STO estimation algorithms.

Iterative receiver for ADO-OFDM with near-optimal optical power allocation

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.

This letter proposes a novel receiver for asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) system in visible light communication to enhance its performance. In ACO-OFDM, the even subcarriers are distorted by the clipping noise and are simply discarded in the conventional receiver. In this letter we show that the signals on the even subcarriers can be viewed as data-dependent precoded OFDM. A novel receiver based on minimum mean square error criteria is then proposed to exploit the even subcarriers to collect additional diversity and coding gains, leading to improved bit error rate performance. Simulations show that the proposed receiver provides significant signal-to-noise ratio gain (up to 10 dB) over the conventional receiver, especially in non-line-of-sight channels. Moreover, the proposed receiver is also superior to some existing advanced ones.

In this paper, we study a novel optical orthogonal frequency division multiplexing (OFDM) scheme, fractional reversed polarity optical OFDM (FRPO- OFDM), which enables dimmable visible light communications. The scheme combines an asymmetrically clipped optical OFDM (ACO-OFDM) sequence with an information-carrying brightness control sequence (BCS). We show that the new scheme can create a wide range of brightness levels whilst transmitting extra information using the BCS to enhance the spectral efficiency. The detector which can recover information from both sequences is derived based on maximum likelihood (ML) detection and estimation. It is shown that the information carried by the BCS using variable on-off keying (VOOK) can be detected with extremely low bit error rate (BER) and that the resulting overall BER of FRPO-OFDM is constant over a wide dimming range.

PAPR reduction of asymmetrically clipped optical OFDM signals: Optimizing PTS technique using improved flower pollination algorithm

In this letter, we present a novel hybrid intensity modulation and direct detection communication system, which integrates asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and on-off keying (OOK) modulation schemes. First, the negative ACO-OFDM is proposed based on the conventional ACO-OFDM system to accommodate the on case with direct current in the OOK modulation, while the ACO-OFDM can match the off case in the OOK modulation. Therefore, the novel hybrid system combines the ACO-OFDM and OOK modulation schemes, while the signals can be recovered at the receiver to support the different qualities of service with high spectral efficiency and can be adapted to various receivers with different complexities. Simulation results are reported for the visible light channel and show that both the ACO-OFDM and OOK signals can be well recovered.

A number of unipolar optical orthogonal-frequency-division-multiplexing (OFDM) schemes have been proposed as a solution to the high energy consumption in the widely adopted direct-current-biased optical OFDM (DCO-OFDM) modulation scheme. These schemes have a reduced spectral efficiency due to the restrictions imposed on their frame structure. The enhanced unipolar OFDM (eU-OFDM) modulation scheme was recently introduced to compensate for the reduced spectral efficiency in unipolar OFDM (U-OFDM). The concept exploits the frame structure of U-OFDM and allows for multiple U-OFDM information streams to be combined, thus increasing the overall spectral efficiency of the communication system. In this paper, the concept of the enhanced U-OFDM scheme is generalized for arbitrary combinations of U-OFDM data streams with various constellation sizes and various power allocations. A closed-form theoretical bound on the bit error rate (BER) performance of the GeneRalizEd EnhaNcEd unipolaR OFDM (GREENER-OFDM) is derived and verified by comparison with the results of Monte Carlo simulations. The proposed scheme has an improved power efficiency compared with a spectrally equivalent DCO-OFDM. The GREENER-OFDM allows the gap in spectral efficiency between DCO-OFDM and the inherently unipolar optical OFDM schemes to be completely closed.

Visible light communication (VLC) has emerged as a viable complement to traditional radio frequency (RF) based systems and as an enabler for high data rate communications for beyond-5G (B5G) indoor communication systems. In particular, the emergence of new B5G-based applications with quality of service (QoS) requirements and massive connectivity has recently led to research on the required service-levels and the development of improved physical (PHY) layer methods. As part of recent VLC standards development activities, the IEEE has formed the 802.11bb “Light Communications (LC) for Wireless Local Area Networking” standardization group. This paper investigates the network requirements of 5G indoor services such as virtual reality (VR) and high-definition (HD) video for residential environments using VLC. In this paper, we consider such typical VLC scenarios with additional impairments such as light-emitting diode (LED) nonlinearity and imperfect channel feedback, and propose hyperparameter-free mitigation techniques using Reproducing Kernel Hilbert Space (RKHS) methods. In this context, we also propose using a direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM)-based adaptive VLC transmission method that uses precomputed bit error rate (BER) expressions for these RKHS-based detection methods and performs adaptive BER-based modulation-order switching. Simulations of channel impulse responses (CIRs) show that the adaptive transmission method provides significantly improved error rate performance, which makes it promising for high data rate VLC-based 5G indoor services.

Data transmission between the downhole and surface called the downhole telemetry system is a critical part of monitoring and extraction processes in the oil and gas industry. Numerous real-time data transmission techniques for downhole telemetry systems have been implemented, however, growing demands of higher bandwidth and data rate for downhole communications made it necessary to find a new, reliable, and low-cost communication system. In this area, visible light communications (VLC) with a huge available spectrum providing high bandwidth and data rate are indispensable opportunities for downhole telemetry systems. This paper addresses the issues related to the VLC usage in downhole gas pipeline telemetry/monitoring systems. To emulate real field conditions, a carbon steel coated and carbon dioxide-filled gas pipeline is created with a circulating pump and a pressure gauge, and the VLC setup is established. The performance of a VLC system employing an asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) modulation technique with 128-FFT and guard band is practically tested by taking channel estimation and synchronization into account. Moreover, the channel impulse response of the carbon dioxide-filled gas pipeline is presented along with a proposed path-loss model. The BER against SNR performance of the proposed downhole telemetry system is investigated for carbon dioxide gas at atmospheric pressure. Furthermore, the effect of the circulating pump and different colors of led on system performance is presented. Measurement results of the free-space air medium are also included as a benchmark.

In visible light communications (VLCs) relying on intensity-modulation and direct detection (IM/DD), the conversion from electrical signals to optical signals and the limited dynamic range of the light-emitting diodes (LEDs) constitute the fundamental impediments in the way of high-integrity communications, especially when orthogonal frequency-division multiplexing (OFDM) is employed. In IM/DD VLCs, only real-valued positive signals are used for signal transmission. However, the Fourier transform of OFDM systems is operated in the complex domain. In order to meet the requirements of the IM/DD VLCs, the complex-to-real conversion is achieved at the cost of reducing the bandwidth efficiency. Moreover, OFDM signals experience a high peak-to-average power ratio; hence, typically clipping is used for confining the positive-valued signals within the LED’s dynamic range. However, hard clipping leads to the loss of orthogonality for optical OFDM (O-OFDM) signals, generating inter-carrier interference. As a result, the performance of the clipping-based O-OFDM systems may be severely degraded. In this paper, the concept of piecewise companding transform (CT) is introduced into the O-OFDM system advocated, forming the CTO-OFDM arrangement. We first investigate the general principles and design criteria of the piecewise CTO-OFDM. Based on our studies, three types of piecewise companders, namely, the constant probability sub-distribution function, linear PsDF (LPsDF), and the non-LPsDF-based CT, are designed. Furthermore, we investigate the nonlinear effect of hard clipping and of our CT on O-OFDM systems in the context of different scenarios by both analytical and simulation techniques. Our investigations show that the CTO-OFDM constitutes a promising signaling scheme conceived for VLCs, which exhibits a high bandwidth efficiency, high flexibility, high reliability, as well as a high data-rate, despite experiencing nonlinear distortions.

In this paper, fractional reverse polarity optical orthogonal frequency division multiplexing (FRPO-OFDM) is studied to enable dimming compatible visible light communications. The scheme combines a layered asymmetrically clipped optical OFDM (ACO-OFDM) sequence with an information-carrying brightness control sequence (BCS) in the form of $M$ -ary pulse position modulation. We derive the expressions of the FRPO-OFDM signal and its achievable brightness level, and develop an effective detector which can recover information from both sequences based on maximum likelihood detection. We show that when the detector is to be implemented, the use of multi-layer ACO-OFDM imposes strong periodicity on the BCS, which leads to a trade-off between spectral efficiency and brightness resolution for dimming control. It is shown that high spectral efficiency can be achieved with practical dimming requirements. Simulation results show that the extra information carried by the BCS can be decoded with extremely low bit error rate and thus has negligible impacts on the demodulation of the ACO-OFDM signal, when the system nonlinearity is not dominating.

In this paper, we investigate and compare the performance of single- and multi-carrier modulation schemes for indoor visible light communication (VLC). Particularly, the performances of single carrier frequency domain equalization (SCFDE), orthogonal frequency division multiplexing (OFDM) and on-off keying (OOK) with minimum mean square error equalization (MMSE) are analyzed in order to mitigate the effect of multipath distortion of the indoor optical channel where nonlinearity distortion of light emitting diode (LED) transfer function is taken into account. Our results indicate that SCFDE system, in contrast to OFDM system, does not suffer from high peak to average power ratio (PAPR) and can outperform OFDM and OOK systems. We further investigate the impact of LED bias point on the performance of OFDM systems and show that biasing LED with the optimum value can significantly enhance the performance of the system. Bit-interleaved coded modulation (BICM) is also considered for OFDM and SCFDE systems to further compensate signal degradation due to inter-symbol interference (ISI) and LED nonlinearity.