Pulse-amplitude-modulated Discrete Multitone Research Articles

Distortion-Elimination Hybrid OFDM With Low Complexity for Optical Wireless Communications

In optical wireless communications (OWC), hybrid optical orthogonal frequency division multiplexing (O-OFDM) schemes, such as hybrid asymmetrically clipped O-OFDM (HACO-OFDM) and layered asymmetrically clipped O-OFDM (LACO-OFDM), enjoy both high power and spectral efficiency. However, due to the non-orthogonal transmission of signal components induced by the clipping distortion, successive interference cancellation (SIC) is required in these hybrid O-OFDM schemes, leading to notably increased complexity. In this paper, we conceive novel hybrid O-OFDM schemes to offer both high spectral and power efficiency, whilst possessing low complexity. By elaborately designing a time-domain (TD) distortion elimination methodology at the transmitter, a novel distortion-elimination hybrid O-OFDM (DEHO-OFDM) is first proposed, which combines asymmetrically clipped O-OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitone (PAM-DMT) in an interference-free manner. In order to further improve the spectral efficiency, an enhance DEHO-OFDM (EDEHO-OFDM) is designed by activating the remaining subcarrier resources. Both DEHO-OFDM and EDEHO-OFDM can be realized through a single-IFFT transmitter and standard OFDM receiver, leading to much lower complexity than the existing hybrid O-OFDM schemes. Simulation results have demonstrated the superiorities of the proposed schemes over HACO-OFDM and LACO-OFDM in terms of peak-to-average-power ratio (PAPR) and bit error rate (BER).

Reconstructed Hybrid Optical OFDM-NOMA for Multiuser VLC Systems

Non-orthogonal multiple access (NOMA) is deemed to be a prospective multiple access technology of the next generation. However, in visible light communication (VLC), when advanced hybrid optical orthogonal frequency division multiplexing (O-OFDM), such as hybrid asymmetrically clipped O-OFDM (HACO-OFDM), is combined with NOMA, error propagation is induced, which degrades the system performance. Therefore, a novel reconstructed hybrid O-OFDM-NOMA (RHO-OFDM-NOMA) scheme is conceived in this paper. In order to eliminate the error propagation, the users in RHO-OFDM-NOMA opt for the ACO-OFDM or clipping-free O-OFDM signals according to their channel qualities, which are subsequently superimposed on pulse-amplitude-modulated discrete multitone (PAM-DMT) to yield the spectrum-efficient hybrid O-OFDM signal. Furthermore, a reconstruction process is designed to ensure the non-negativity. Compared with HACO-OFDM, the proposed RHO-OFDM can retain the error propagation in NOMA-VLC, whilst maintaining the superiorities of high spectral and power efficiency. It is demonstrated by simulation results that RHO-OFDM-NOMA can support a notably higher data rate than the NOMA schemes using conventional O-OFDM.

A Novel OFDM Format and a Machine Learning Based Dimming Control for LiFi

This paper proposes a new hybrid orthogonal frequency division multiplexing (OFDM) form termed as DC-biased pulse amplitude modulated optical OFDM (DPO-OFDM) by combining the ideas of the existing DC-biased optical OFDM (DCO-OFDM) and pulse amplitude modulated discrete multitone (PAM-DMT). The analysis indicates that the required DC-bias for DPO-OFDM-based light fidelity (LiFi) depends on the dimming level and the components of the DPO-OFDM. The bit error rate (BER) performance and dimming flexibility of the DPO-OFDM and existing OFDM schemes are evaluated using MATLAB tools. The results show that the proposed DPO-OFDM is power efficient and has a wide dimming range. Furthermore, a switching algorithm is introduced for LiFi, where the individual components of the hybrid OFDM are switched according to a target dimming level. Next, machine learning algorithms are used for the first time to find the appropriate proportions of the hybrid OFDM components. It is shown that polynomial regression of degree 4 can reliably predict the constellation size of the DCO-OFDM component of DPO-OFDM for a given constellation size of PAM-DMT. With the component switching and the machine learning algorithms, DPO-OFDM-based LiFi is power efficient at a wide dimming range.

BER enhanced receiver for PHO-OFDM-based optical wireless communications

In this paper, a soft-equalization-based single iterative receiver is proposed for the recently proposed PAM-DMT-based hybrid optical orthogonal frequency division multiplexing (PHO-OFDM) system to enhance its bit error rate (BER) performance. The interlayer interference and clipping noise of the pulse amplitude modulated discrete multitone (PAM-DMT) branch and DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) branch are first reduced by a soft-equalization method in the frequency domain. Then, the pairwise clipping (PC) and pairwise averaging (PA) are further introduced to correct the regenerated time-domain branch signals according to the corresponding branch symmetry properties. Numerical simulations demonstrate that the proposed receiver can realize improved BER performance over the conventional counterpart under both the linear and nonlinear transmission model.

Spectrally efficient revised PAM-DMT for IM/DD Optical Wireless Communication systems

The incessant demand for higher data rates has made Visible Light Communication (VLC) technology to gain enormous prevalence in contemporary communication systems. These VLC systems are lagging in terms of spectral efficiency due to the inevitable Hermitian symmetry imposed to satisfy the unipolar constraint of those systems. Therefore, in this work, a new waveform named revised Pulse Amplitude Modulated Discrete Multitone (rPAM-DMT) that provides more spectral efficiency by utilizing all subcarriers without Hermitian symmetry is pioneered for VLC systems. The generation of rPAM-DMT at transmitter and detection at the receiver are analytically explained. Simulation results for Bit Error Rate (BER) show that as spectral efficiency increases rPAM-DMT outperform PAM-DMT and Asymmetrically Clipped Optical OFDM (ACO-OFDM) under Additive White Gaussian Noise (AWGN) channel. Further, the proposed waveform is validated on a VLC testbed designed with Universal Software-defined Radio Peripheral (USRP) hardware, which is controlled by using Lab-view software.

Low-Complexity Hybrid Optical OFDM with High Spectrum Efficiency for Dimming Compatible VLC System

In visible light communications (VLC), dimming control constitutes an indispensable technique to comply with various illumination necessities and with different energy consumption constraints. Therefore, a novel dimming compatible hybrid optical orthogonal frequency division multiplexing (DCHO-OFDM) is conceived in this paper to fulfil the requirements from communications and illuminations. Explicitly, the signal branch of the unclipped asymmetrically clipped O-OFDM (ACO-OFDM) and the down/upper-clipped pulse-amplitude-modulated discrete multitone (PAM-DMT) are adaptively amalgamated in order to increase the spectrum efficiency. For the sake of precisely achieving dimming control, the chromaticity-shift-free and industry-preferred pulse width modulation (PWM) is further invoked to the hybrid signal, assisted by a time-varying biasing scheme to mitigate the non-linear distortion. As the different signal components in DCHO-OFDM are beneficially combined in an interference-orthogonal approach, the transmitted symbols are able to be readily detected upon relying on a standard OFDM receiver, as that of ACO-OFDM. Our simulations demonstrate that a high spectrum efficiency of the conceived DCHO-OFDM scheme can be achieved with less fluctuation in a wide dimming range.

VLC-based indoor positioning algorithm combined with OFDM and particle filter

Visible light communications (VLC) have recently attracted a growing interest and can be a potential solution to realize indoor positioning, however, the performance of existing indoor positioning system is limited by multipath distortion inside a room. In order to combat the effect of multipath distortion, this paper proposes an LED-based indoor positioning algorithm combined with hybrid OFDM (HOFDM), in which asymmetrically clipped optical OFDM (ACO- OFDM) is transmitted on the odd subcarriers while using pulse amplitude modulated discrete multitone (PAM-DMT) to modulate the imaginary part of each even subcarrier. In this scheme, we take a combined approach where a received-signal-strength (RSS) technique is employed to determine the location of the receiver and realize the 3-D positioning by Trust-region-based positioning. Moreover, a particle filter is used to further improve the positioning accuracy. Results confirm that this proposed positioning algorithm can achieve high accuracy even with multipath distortion, and the algorithm has better performance when combined with particle filter.

Design of PAM-DMT-Based Hybrid Optical OFDM for Visible Light Communications

In this letter, a pulse-amplitude-modulated based hybrid optical orthogonal frequency division multiplexing (PHO-OFDM) scheme is proposed. We use the high order quadrature-amplitude-modulation to replace the one-dimensional pulse-amplitude-modulated on the even subcarriers for compensating the data capacity weakness of the conventional pulse amplitude modulated discrete multitone (PAM-DMT). The combined time-domain PHO-OFDM signal has a larger amplitude range than the original PAM-DMT but with a reduced peak-to-average power ratio. Besides that, a flexible power allocation method is introduced to the proposed PHO-OFDM systems to fully exploit the whole dynamic range of LEDs. The simulation results show that, under the same average bit rate, the proposed PHO-OFDM scheme has a better bit error rate performance compared with the conventional PAM-DMT scheme, thus demonstrating its application potential in visible light communications.

Hybrid DCO-OFDM, ACO-OFDM and PAM-DMT for dimmable LiFi

This paper proposes a hybrid orthogonal frequency division multiplexing (OFDM) modulation termed as hybrid DC-biased asymmetrically-clipped pulse-amplitude-modulated OFDM (HDAP-OFDM) for providing power efficient communication and dimming control for light fidelity (LiFi) schemes. The proposed HDAP-OFDM is a combination of three OFDM formats: DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM) and pulse amplitude modulated discrete multitone (PAM-DMT). In HDAP-OFDM, the lower-index subcarriers carry DCO-OFDM, while the remaining higher subcarriers carry ACO-OFDM on the odd-index subcarriers and PAM-DMT on the even-index subcarriers. Mathematical analysis shows that the required DC bias for HDAP-OFDM is a function of the required dimming levels and the root mean square of the three of its components. Furthermore, an algorithm is presented to simulate the dimming control of HDAP-OFDM. Simulations results indicate that by adjusting the proportion of subcarriers for the three OFDM/DMT elements, HDAP-OFDM can have a wide dimming range, a low peak to average power ratio, and high power efficiency. Results also show that at a bit error rate of 10−3 and a dimming range of 3%–97%, HDAP-OFDM exhibits 8 dB more optical and electrical power efficiency than existing asymmetrical hybrid optical OFDM.

A Novel Receiver Design for HACO-OFDM by Time-Domain Clipping Noise Elimination

The conventional receiver of hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) requires additional processing to regenerate and cancel the clipping noise, which increases the complexity of the receiver and makes the performance of pulse-amplitude-modulated discrete multitone (PAM-DMT) branch dependent on the accuracy of clipping noise estimation. In this work, a novel receiver of HACO-OFDM is proposed by fully exploiting the structure of time-domain signals. Through a time-domain processing, the clipping noise of the asymmetrically clipped optical OFDM (ACO-OFDM) can be removed and error propagation is avoided. Compared with the conventional receiver, the proposed receiver can achieve 58.3% complexity reduction in the case of a fast Fourier transform with size of 512. Simulation results show that the bit error rate (BER) of the proposed receiver is exactly the same as that of the conventional receiver for the ACO-OFDM branch, but when the clipping noise estimation in ACO-OFDM branch is inaccurate, the BER performance of the PAM-DMT branch is significantly improved over the conventional receiver.

PAPR Reduction for Hybrid ACO-OFDM Aided IM/DD Optical Wireless Vehicular Communications

Hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) improves the spectral efficiency compared to asymmetrically clipped optical OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitone (PAM-DMT) modulation, while retaining the advantage of high power efficiency. HACO-OFDM has found favor in numerous applications, but its peak-to-average-power ratio (PAPR) has remained a concern in optical wireless communications, since the family of existing PAPR reduction methods cannot be directly invoked for the superimposed HACO-OFDM signals. Hence, we analyze the characteristics of HACO-OFDM signals and develop a specific PAPR reduction technique relying on tone injection. Our numerical results show that the proposed method achieves significantly improved PAPR statistics compared to the conventional methods, leading to a significant bit error ratio (BER) reduction.

Enhanced subcarrier-index modulation-based asymmetrically clipped optical OFDM using even subcarriers

In this paper, we propose a subcarrier-index modulation-based asymmetrically clipped optical orthogonal frequency division multiplexing (SACO-OFDM) scheme for optical wireless communication (OWC) systems, which benefits from the subcarrier-index modulation (SIM) and asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) techniques. SACO-OFDM conveys additional information via the subcarrier indexing, and the error rate of the bit transmitted by the subcarrier indexing is much lower than that of the conventional M-ary modulation scheme. On the other hand, as the signal constellation in M-ary modulation is relieved, SACO-OFDM has simple transceiver structure and low detection complexity. Moreover, considering the spectral, an enhanced SACO-OFDM (ESACO-OFDM) using even subcarriers is proposed. In this technique, the odd subcarriers are activated for SACO-OFDM, and the imaginary part of even subcarriers are activated for pulse-amplitude-modulated discrete multitone (PAM-DMT). Clearly, ESACO-OFDM achieves better spectral efficiency than the conventional optical OFDM, since all subcarriers are used for data transmission. Simulation results verify the significant bit error rate (BER) and peak-to-average power ratio (PAPR) improvement by the proposed ESACO-OFDM, especially for the medium-to-high signal-to-noise ratio (SNR) regime.

PAPR analysis for OFDM visible light communication.

Orthogonal frequency-division multiplexing (OFDM) is a practical technology in visible light communication (VLC) for high-speed transmissions. However, one of its operational limitations is the peak-to-average power ratio (PAPR) of the transmitted signal. In this paper, we analyze the PAPR distributions of four VLC OFDM schemes, namely DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), pulse amplitude modulated discrete multitone (PAM-DMT), and Flip-OFDM. Both lower and upper clippings are considered. We analytically derive the complementary cumulative distribution functions (CCDFs) of the PAPRs of the clipped VLC OFDM signals, and investigate the impact of lower and upper clippings on PAPR distributions. Our analytical results, as verified by numerical simulations, provide useful insights and guidelines for VLC OFDM system designs.

Asymmetrically Reconstructed Optical OFDM for Visible Light Communications

In conventional optical orthogonal frequency-division multiplexing (OOFDM) systems, such as pulse-amplitude-modulated discrete multitone (PAM-DMT) and asymmetrically clipped optical OFDM (ACO-OFDM), half of the time-domain transmitted signals carry meaningful information, while the other half are set at zero after asymmetric clipping. By specifically exploiting the asymmetric structure of OOFDM such as PAM-DMT and ACO-OFDM, we present a new framework for implementing OOFDM with enhanced performance in terms of both bit error rate (BER) and peak-to-average power ratio (PAPR). The proposed scheme introduces asymmetric signal reconstruction, instead of direct clipping, in the time domain before transmission. The designed asymmetric reconstruction refines the time-domain statistics of the OOFDM signals by constructively exploiting zero-padded signal positions in conventional OOFDMs. This way, the PAPR of the reconstructed OOFDM signals is effectively reduced. For receivers, to recover the received signals precisely, we further develop an optimal maximum a posteriori detection method, along with an efficient simplified method. Simulation results validate that the proposed scheme enhances the BER performance compared with conventional OOFDMs under both an ideal and a dispersive visible light communication (VLC) channel via our prototype. Specifically, under an ideal additive white Gaussian noise channel, an approximately 4-dB gain is achieved at the BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , along with a 2.5-dB lower PAPR constraint. While under the real dispersive channel, our test shows about 6-dB gain in BER performance.

A Novel Receiver Design for PAM-DMT in Optical Wireless Communication Systems

Pulse-amplitude-modulated discrete multitone (PAM-DMT) modulation is an optical orthogonal frequency division multiplexing scheme, which is compatible with intensity modulation and direct detection channel. In the conventional receiver for PAM-DMT, only the imaginary parts of subcarriers are detected, while the real parts are considered as clipping noise and simply discarded, causing insufficient usage of the received power. This letter proposes a new receiver characterizing temporal and spectral diversity combining for PAM-DMT, which extracts the effective information in clipping noise to recover the data. The proposed receiver achieves a better performance than the conventional receiver with a gain of at least 2.3 dB in E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> at the system bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> in additive white Gaussian noise channel by simulation when using 4-PAM modulation.

Asymmetrical Hybrid Optical OFDM for Visible Light Communications With Dimming Control

This letter proposes an asymmetrical hybrid optical orthogonal frequency division multiplexing (AHO-OFDM) scheme for dimmable visible light communication systems. In the proposed scheme, either asymmetrically clipped optical OFDM (ACO-OFDM) or pulse-amplitude-modulated discrete multitone (PAM-DMT) signal is inverted and then both the signals are combined for transmission, where pulsewidth modulation is no longer required for dimming control. The power of ACO-OFDM and PAM-DMT signals is adjusted so that the amplitude of the combined AHO-OFDM signal is asymmetrical, which could utilize all the available subcarriers as well as the entire dynamic range of light-emitting diodes with various dimming levels. Simulation results show that the proposed scheme could achieve a wide dimming range with a small throughput fluctuation.

Receiver Design for PAM-DMT in Indoor Optical Wireless Links

In the conventional receiver for pulse-amplitude-modulated discrete multitone (PAM-DMT), the real parts of the subcarriers are distorted by the clipping noise and ignored in detection. In this letter, the signal analysis shows that the real parts of the subcarriers also contain the information of the transmitted signal. An iterative receiver is then proposed to improve the transmission performance of PAM-DMT by exploiting both the real and imaginary parts of the subcarriers. Simulation results show that the proposed iterative receiver can significantly improve the transmission performance of PAM-DMT systems.

Iterative Receiver for Hybrid Asymmetrically Clipped Optical OFDM

This paper proposes an iterative receiver to enhance the performance of hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) in optical wireless communication systems. In HACO-OFDM scheme, asymmetrically clipped optical OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitone (PAM-DMT) signals are transmitted simultaneously, which is more spectrally efficient compared with ACO-OFDM and PAM-DMT. However, the existing HACO-OFDM receiver directly recovers the signals in the frequency domain, which could not eliminate the interference thoroughly between ACO-OFDM and PAM-DMT signals and limits its performance. In our proposed receiver, the ACO-OFDM and PAM-DMT signals are detected in the frequency domain and regenerated in the time domain. After that, they are subtracted from the received signals iteratively. Thus, ACO-OFDM and PAM-DMT signals can be distinguished. By taking advantage of the signal symmetry properties of ACO-OFDM and PAM-DMT in the time domain, pairwise clipping is utilized to further reduce the effect of noise and estimation error, resulting in improved performance. In addition, unequal power allocation is proposed to guarantee that ACO-OFDM and PAM-DMT signals have similar performance in HACO-OFDM systems. Simulation results show that the proposed method provides significant signal-to-noise ratio gain over the conventional receiver for both equal and unequal power allocations at the cost of slightly increased complexity.

Performance analysis of precoding-based asymmetrically clipped optical orthogonal frequency division multiplexing wireless system in additive white Gaussian noise and indoor multipath channel

We have compared the bit error rate (BER) performance of precoding-based asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and pulse amplitude modulated discrete multitone (PAM-DMT) optical wireless (OW) systems in additive white Gaussian noise (AWGN) and indoor multipath frequency selective channel. Simulation and analytical results show that precoding schemes such as discrete Fourier transform, discrete cosine transform, and Zadoff-Chu sequences do not affect the performance of the OW systems in the AWGN channel while they do reduce the peak-to-average power ratio (PAPR) of the OFDM output signal. However, in a multipath indoor channel, using zero forcing frequency domain equalization precoding-based systems give better BER performance than their conventional counterparts. With additional clipping to further reduce the PAPR, precoding-based systems also show better BER performance compared to nonprecoded systems when clipped relative to the peak of nonprecoded systems. Therefore, precoding-based ACO-OFDM and PAM-DMT systems offer better BER performance, zero signaling overhead, and low PAPR compared to conventional systems.

Hybrid Asymmetrically Clipped OFDM-Based IM/DD Optical Wireless System

In this paper, we present a hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (OFDM) system. This system uses a combination of asymmetrically clipped optical OFDM (ACO-OFDM) and pulse amplitude modulated discrete multitone (PAM-DMT) techniques, which can be used in intensity modulated direct detection (IM/DD) optical wireless (OW) systems. In this hybrid scheme, ACO-OFDM and PAM-DMT signals are transmitted together. Clipping noise is estimated at the receiver and canceled to recover the PAM-DMT symbols. This scheme does not require any DC bias for transmission, which makes the transmitter less complex and very power efficient. With this system, we can increase the data rate of the ACO-OFDM system by almost twice. In addition, no bandwidth penalty is incurred. Extensive computer simulations show that the bit-error rate (BER) performance of ACO-OFDM in the additive white Gaussian noise environment is not affected by any kind of interference, but only due to half of the available transmit power, we see a 3&#xA0;dB degradation. However, the BER performance of PAM-DMT shows some degradation at low signal-to-noise ratio (SNR) but is identical to the conventional scheme at higher SNR. We also see a slight improvement in peak-to-average power ratio (PAPR) of the output combined signal. Therefore, advantages such as increased data rate, DC-bias elimination, no bandwidth, and PAPR penalty make this scheme very attractive for OW systems using IM/DD.