Unipolar Orthogonal Frequency Division Multiplexing Research Articles

Unipolar X-Transform OFDM With Index Modulation for Underwater Optical Wireless Communications

This letter presents a novel unipolar scheme called X-transform optical orthogonal frequency division multiplexing with index modulation (XO-OFDM-IM) for underwater optical wireless communication (UOWC). Unlike the conventional schemes, each modulated data in XO-OFDM-IM is transmitted via one or two subcarriers; and the information bits carried by the index of subcarriers are used to produce positive and real time-domain signal without the need for adding direct current (DC) bias or clipping processes. The performance of the proposed scheme has been evaluated via a 2-meter measured UOWC channel to validate its outperformance against its benchmarks. For instance, our scheme offers up to 7 dB improvement in term of signal to noise ratio at a target bit error rate (BER) of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> and up to 8 dB in term of peak-to-average power ratio (PAPR) compared to its counterpart asymmetrically clipped O-OFDM (ACO-OFDM) when considering the same spectral efficiency (SE).

Performance improvement of visible light communication links based on coded unipolar OFDM

Visible light communications are limited by the narrow bandwidth of their sources as the light-emitting diodes, which restricts the high data rates transmission. A potential technique such as multi-carrier modulation is considered promising for achieving a high-speed data rate and enhancing system performance. This paper proposes turbo coding with unipolar orthogonal frequency-division multiplexing (U-OFDM) for indoor environments. The system under U-OFDM considerations can reduce the inter-symbol interference caused by multiple paths and mitigate burst errors by adopting an iterative turbo decoding. The system performance is evaluated regarding the bit error rate (BER) for the range of the signal-to-noise ratio (SNR) under receiver field-of-view (FOV) restriction. The obtained results show that the performance system of the target BER (below the threshold limit of 3.8×10&lt;sup&gt;−3&lt;/sup&gt;) has a power penalty of ~9 dB for modulation order of 64 for quadrature amplitude modulation scheme, FOV of 50°, and incidence angle of half power assigned to 30°. The obtained results also showed a ~10.8 dB improvement in the SNR when comparing the present system with coded-DC-biased optical OFDM under the same considerations.

Coded DCO-OFDM Techniques in Intensity Modulation/Direct Detection (IM/DD) Systems

Optical wireless communications (OWC) are among the best alternative techniques for transmitting information-laden optical radiation across a free-space channel from one place to another. DC-biased optical OFDM (DCO-OFDM) is a technique that sacrifices the power efficiency to transmit unipolar OFDM signals. The primary drawback with DCO-OFDM is its clipping noise, which causes distortion and lowers the bit error rate (BER). Thus, in this paper, we show the performance of DCO with different coded techniques to improve the BER in additive white Gaussian noise (AWGN) for IMDD systems. The experimental results show that the coded DCO-OFDM has the best performance. Furthermore, turbo coding has the best coding technique added to the DCO-OFDM system.

Layered antisymmetry-constructed clipped optical OFDM for low-complexity VLC systems.

In this paper, antisymmetry-constructed clipped optical orthogonal frequency division multiplexing (AC-OFDM) is proposed for visible light communication (VLC) systems, in which an antisymmetry property is imposed directly in time domain. AC-OFDM has nearly the same spectral efficiency and peak-to-average power ratio (PAPR) as traditional asymmetrically clipped optical OFDM (ACO-OFDM) but is less complex to implement. Layered AC-OFDM (LAC-OFDM) is then proposed as an extension to further improve spectral efficiency, where different layers of AC-OFDM signals are added in the time domain and transmitted simultaneously. Computational complexity analysis and numerical results show that LAC-OFDM has nearly the same spectral efficiency as layered asymmetrically clipped optical OFDM (LACO-OFDM) and enhanced unipolar OFDM (eU-OFDM) but is less complex. Specifically, LAC-OFDM requires less than half the multiplication and addition operations compared to the comparable LACO-OFDM scheme. Additionally, a pairwise iterative receiver for LAC-OFDM is proposed and its computational complexity is analysed. Monte Carlo simulation results show that LAC-OFDM requires nearly the same optical signal-to-noise ratio (OSNR) to achieve the same BER performance as LACO- and eU-OFDM.

Absolute Value Layered ACO-OFDM for Intensity-Modulated Optical Wireless Channels

Enhanced unipolar orthogonal frequency division multiplexing (eU-OFDM) and layered asymmetrically clipped optical OFDM (LACO-OFDM) are spectrally efficient modulation techniques for intensity modulated systems which layer multiple non-negative signals. In this paper, we propose absolute value layered asymmetrically clipped optical OFDM (ALACO-OFDM), which further improves the spectral efficiency while using a smaller number of layers and no explicit direct current (DC) bias. In ALACO-OFDM, asymmetrically clipped optical OFDM (ACO-OFDM) signals are sent at the first $L$ layers and absolute value optical OFDM (AVO-OFDM) is used for the remaining subcarriers and transmitted simultaneously. Analysis indicates that ALACO- achieves higher spectral efficiency than eU- or LACO-OFDM while using a smaller number of layers. Bounds on achievable information rates of ALACO-OFDM and related layered ACO-OFDM techniques are also developed. Two optical power allocation schemes over the layers of ALACO-OFDM are developed with the objective of optimizing uncoded transmission performance and the achievable information rate respectively. Additionally, a theoretical bound on the uncoded BER of ALACO-OFDM is derived. Monte Carlo simulation results indicate ALACO-OFDM with the optimum power allocation achieves significant uncoded BER performance gains compared to its counterparts at the same spectral efficiency while having a smaller peak-to-average power ratio.

A Novel Unipolar Transmission Scheme for Visible Light Communication

This paper proposes a novel unipolar transceiver for visible light communication (VLC) by using orthogonal waveforms. The main advantage of our proposed scheme over most of the existing unipolar schemes in the literature is that the polarity of the real-valued orthogonal frequency division multiplexing (OFDM) sample determines the pulse shape of the continuous-time signal and thus, the unipolar conversion is performed directly in the analog instead of the digital domain. Therefore, our proposed scheme does not require any direct current (DC) biasing or clipping as it is the case with existing schemes in the literature. The bit error rate (BER) performance of our proposed scheme is analytically derived and its accuracy is verified by using Matlab simulations. Simulation results also substantiate the potential performance gains of our proposed scheme against the state-of-the-art OFDM-based systems in VLC; it indicates that the absence of DC shift and clipping in our scheme supports more reliable communication and outperforms the asymmetrically clipped optical-OFDM (ACO-OFDM), DC optical-OFDM (DCO-OFDM) and unipolar-OFDM (U-OFDM) schemes. For instance, our scheme outperforms ACO-OFDM by at least 3 dB (in terms of signal to noise ratio) at a target BER of 10−4, when considering the same spectral efficiency for both schemes.

Spectrally efficient optical orthogonal frequency division multiplexing.

This paper charts the development of spectrally efficient forms of optical orthogonal frequency division multiplexing (OFDM) that are suited for intensity-modulated direct detection systems, such as wireless optical communications. The journey begins with systems using a DC-bias to ensure that no parts of the signal that modulates the optical source are negative in value, as negative optical intensity is unphysical. As the DC-part of the optical signal carries no information, it is wasteful in energy; thus asymmetrically clipped optical OFDM was developed, removing any negative-going peaks below the mean. Unfortunately, the clipping causes second-order distortion and intermodulation, so some subcarriers appear to be unusable, halving spectral efficiency; this is similar for unipolar and flipped optical OFDM. Thus, a considerable effort has been made to regain spectral efficiency, using layered techniques where the clipping distortion is mostly cancelled at the receiver, from a knowledge of one unpolluted layer, enabling one or more extra ‘layers/paths/depths’ to be received on the previously unusable subcarriers. Importantly, for a given optical power and high-order modulation, layered methods offer the best spectral efficiencies and need the lowest signal-to-noise ratios, especially if diversity combining is used. Thus, they could be important for high-bandwidth optical fibre systems. Efficient methods of generating all layers simultaneously, using fast Fourier transforms with their partial calculations extracted, are discussed, as are experimental demonstrations in both wireless and short-haul communications links. A musical analogy is also provided, which may point to how orchestral and rock music is deciphered in the brain.This article is part of the theme issue ‘Optical wireless communication’.

Impact of clipping noise on the sum rate of NOMA with PD-DCO-OFDM and conventional DCO-OFDM

sIn visible light communication (VLC) systems, a non-orthogonal multiple access (NOMA) scheme is deemed promising technology to offer better spectral efficiency than the Orthogonal multiple access (OMA) scheme. The feasibility of power domain NOMA for VLC system has been studied with different variants of unipolar OFDM schemes. However, few research works have been presented on the impact of clipping noise on the achievable sum rate of NOMA with DC-biased optical OFDM (DCO-OFDM) and polarity divided DCO-OFDM (PD-DCO-OFDM). This paper presents the impact of clipping noise on the achievable sum rate of NOMA-DCO-OFDM and NOMA-PD-DCO-OFDM VLC systems. Moreover, NOMA-DCO-OFDM and NOMA-PD-DCO-OFDM systems are compared based on achievable sum rates for different total power constraints and signal clipping levels. For the two-user scenario, Simulation results have confirmed that NOMA-PD-DCO-OFDM can offer a better sum rate compared to NOMA-DCO-OFDM system.

BER Performance Comparison of DCO-OFDM and Convolutional Coded DCO-OFDM in IM/DD Systems

Optical wireless communication (OWC) is an alternative technology to meet the demands of the exponentially-growing high data rate applications run by broadband users. The implementation of single carrier modulation techniques in OWC is an age old technology, but for the last few years research is focused towards the multicarrier modulation techniques in OWC. In OWC, information is carried using intensity modulation and retrieved using direct detection. To perform intensity modulation, the baseband signal should be a unipolar signal. To obtain a unipolar signal, various techniques such as DC-biased orthogonal frequency division multiplexing (DCO-OFDM), Asymmetrically clipped orthogonal frequency division multiplexing (ACO-OFDM), Flip orthogonal frequency division multiplexing (Flip-OFDM) and Unipolar orthogonal frequency division multiplexing (U-OFDM) techniques are reported in the literature. Though the DCO-OFDM is spectrally efficient compared to other techniques it requires more power to achieve the targeted BER. In this article, a convolutional Coded DCO-OFDM (CDCO-OFDM) has been introduced by applying channel coding. A convolutional encoder and a hard-decision Viterbi decoder are considered in CDCO-OFDM. It has been observed that CDCO-OFDM requires less transmitted power than DCO-OFDM to attain the targeted BER. The performance of DCO-OFDM and CDCO-OFDM is evaluated for 4, 16, 64 QAM- 7dB and 13dB bias, in the presence of the additive white Gaussian noise (AWGN) channel. It is observed that CDCO requires less power to transmit than DCO-OFDM for the BER 10-4.

Clipping-Enhanced Optical OFDM for Visible Light Communication Systems

Visible light communications (VLC), a new optical wireless communication technology that uses illumination light-emitting diodes as transmitters, requires a modulation scheme that is well suited to these devices’ nonlinear response. Optical orthogonal frequency division multiplexing (OFDM) is a promising technique to provide high-speed data transmission for VLC. However, the peak transmitted power limitation and nonnegative transmitted signal constraint of the lighting sources can result in nonlinear signal distortion from clipping. In this paper, we propose a novel optical OFDM scheme for VLC systems called clipping-enhanced optical OFDM (CEO-OFDM) that transmits via extra time slots the information clipped by the peak power constraint. CEO-OFDM sacrifices bandwidth to allow a higher modulation index to improve the signal to noise ratio and reduce the clipping distortion caused by the peak power limitation. From analytical and numerical results, the proposed CEO-OFDM provides better bit error rate performance and higher data rate than DC-biased optical OFDM, unipolar OFDM, and asymmetrically clipped optical OFDM. Furthermore, CEO-OFDM can provide a better illumination performance that supports light dimming.

Orthogonal Frequency Division Multiplexing Techniques Comparison for Underwater Optical Wireless Communication Systems.

Optical wireless communication is an energy-efficient and cost-effective solution for high-speed and highly-secure wireless connections. In this paper, we compare, discuss, and analyze three popular optical orthogonal frequency division multiplexing (OFDM) techniques, such as DC-biased optical OFDM (DCO-OFDM), asymmetrically-clipped optical OFDM (ACO-OFDM), and unipolar OFDM (U-OFDM), for underwater optical wireless communication systems. The peak power constraint, bandwidth limit of the light source, turbulence fading underwater channel, and the channel estimation error are taken into account. To maximize the achievable data propagation distance, we propose to optimize the modulation index that controls the signal magnitude, and a bitloading algorithm is applied. This optimization process trades off the clipping distortion caused by the peak power constraint and the signal to noise ratio (SNR). The SNR and clipping effects of the three compared OFDM techniques are modeled in this paper. From the numerical results, DCO-OFDM outperforms ACO- and U-OFDM when the transmitted bit rate is high compared to the channel bandwidth. Otherwise, U-OFDM can provide a longer propagation distance or requires less transmitted power.

Variable Pulse Width Unipolar Orthogonal Frequency Division Multiplexing for Visible Light Communication Systems

Visible light communication (VLC) systems working as high-speed and short-range wireless connection methods have attracted more and more attention. This paper proposes a variable pulse width unipolar optical orthogonal frequency-division multiplexing (VPW-OFDM) scheme for indoor VLC systems. Similar to the polar-based optical OFDM, Hermitian symmetric data are not required to generate real signals. Instead, the magnitude and phase components of the complex-valued OFDM signal are transmitted successively. Unlike the polar-based optical OFDM, the proposed VPW-OFDM allows using variant pulse widths to reduce the effects of the noise added on the phase component. By adaptively optimizing the pulse widths, the proposed VPW-OFDM outperforms the polar-based OFDM and other state-of-the-art optical OFDM techniques, such as dc-biased optical OFDM, asymmetrically clipped optical OFDM, and unipolar-OFDM. For indoor VLC systems, illumination requirement is crucial and needs to be considered. In this paper, we analyze the effects of the illumination requirements on system performance. Bit error rate and transmitted bit rate are used as criteria to evaluate the communication performances for the techniques tested. In addition, the system channel with bandwidth limitation is taken into account. To enhance the data throughput, a single-tap equalizer at the receiver and the bit loading algorithm are applied. From the numerical results, the proposed VPW-OFDM outperforms DCO-, ACO-, U-, and polar-based OFDM for a wide channel bandwidth.

A Comparative Study of Unipolar OFDM Schemes in Gaussian Optical Intensity Channel

We study the information rates of unipolar orthogonal frequency division multiplexing (OFDM) in discrete-time optical intensity channels (OIC) with Gaussian noise under average optical power constraint. Several single-, double-, and multi-component unipolar OFDM schemes are considered under the assumption that independent and identically distributed. Gaussian or complex Gaussian codebook ensemble and nearest neighbor decoding (minimum Euclidean distance decoding) are used. We obtain an array of information rate result. These results validate existing signal-to-noise-and-distortion-ratio-based rate analysis, establish the equivalence of information rates of certain schemes, and demonstrate the evident benefits of using component-multiplexing at high signal-to-noise-ratio (SNR). For double- and multi-component schemes, the component power allocation strategies that maximize the information rates are investigated. In particular, by utilizing a power allocation strategy, we prove that several multi-component schemes approach the high SNR capacity of the discrete-time Gaussian OIC under average power constraint to within 0.07 bits.

BER Performance of Stratified ACO-OFDM for Optical Wireless Communications over Multipath Channel

In intensity modulation/direct detection- (IM/DD-) based optical OFDM systems, the requirement of the input signal to be real and positive unipolar imposes a reduction of system performances. Among previously proposed unipolar optical OFDM schemes for optical wireless communications (OWC), asymmetrically clipped optical OFDM (ACO-OFDM) and direct current biased optical OFDM (DCO-OFDM) are the most accepted ones. But those proposed schemes experience either spectral efficiency loss or energy efficiency loss which is a big challenge to realize high speed OWC. To improve the spectral and energy efficiencies, we previously proposed a multistratum-based stratified asymmetrically clipped optical OFDM (STACO-OFDM), and its performance was analyzed for AWGN channel. STACO-OFDM utilizes even subcarriers on the first stratum and odd subcarriers on the rest of strata to transmit multiple ACO-OFDM frames simultaneously. STACO-OFDM provides equal spectral efficiency as DCO-OFDM and better spectral efficiency compared to ACO-OFDM. In this paper, we analyze the BER performance of STACO-OFDM under the effect of multipath fading. The theoretical bit error rate (BER) bound is derived and compared with the simulation results, and good agreement is achieved. Moreover, STACO-OFDM shows better BER performance compared to ACO-OFDM and DCO-OFDM.

Performance analysis and optimization of unipolar OFDM based relay-assisted visible light communications☆

In this paper, we consider a relay-assisted visible light communication (VLC) system where an intermediate light source cooperates with the main light source. Following the IEEE 802.15.7r1 VLC reference channel model, we assume the presence of two different light sources in an office space. The first one is the ceiling light that serves as the source terminal while the second one is the desk lamp that serves as the relay terminal. Our system builds upon unipolar optical orthogonal frequency division multiplexing (U-OFDM) where the desk light performs amplify-and-forward relaying to assist the ceiling light and operates in a half-duplex mode. In addition, we consider the use of enhanced U-OFDM (eU-OFDM) which doubles the spectral efficiency of U-OFDM at the expense of additional computational complexity. We analyze the bit error rate performance of the relay-assisted VLC system building on these two OFDM types and quantify performance improvements over point-to-point (i.e., no relaying) VLC systems. In an effort to further improve system performance, we investigate optimal power allocation between source and relay terminals.

Optical dual-mode index modulation aided OFDM for visible light communications

Dual-mode index modulation aided orthogonal frequency multiplexing (DM-OFDM) is recently proposed, where the spectral efficiency is enhanced compared with conventional OFDM schemes. In this paper, we propose two different optical DM-OFDM schemes for visible light communications, dual-mode index modulation aided DC-biased optical OFDM (DM-DCO-OFDM) and dual-mode index modulation aided unipolar OFDM (DM-U-OFDM). In the optical DM-OFDM schemes, subcarriers are partitioned into OFDM subblocks, divided into two groups within each subblock, and modulated by two different modes of constellations. Additional information bits can be transmitted implicitly by the indices of subcarriers modulated by the same constellation alphabet. In order to generate non-negative signals, the real-valued time-domain signals are DC-biased in DM-DCO-OFDM, whilst positive and negative signals are transmitted separately in DM-U-OFDM. At the receiver, both a maximum-likelihood (ML) detector and a log-likelihood ratio (LLR) detector are employed for signal demodulation. Besides, simulation results demonstrate that the proposed optical DM-OFDM schemes are capable of enhancing the spectral efficiency compared with other existing optical OFDM schemes, and DM-DCO-OFDM as well as DM-U-OFDM can achieve significant performance gains over their conventional counterparts at the same spectral efficiency, when the nonlinear transfer characteristic of light-emitting diodes (LEDs) is considered.

A Simple OFDM Scheme for VLC Systems Based on u-Law Mapping

A simple orthogonal frequency division multiplexing (OFDM) scheme termed ${\mu }$ -OFDM is proposed for visible light communication (VLC). In this scheme, the peak-to-average power ratio (PAPR) reduction technique, ${\mu }$ -law companding, is first introduced in VLC systems to achieve the required signal format for VLC transmission. Due to the inherent characteristics of the companding operation, low PAPR signals are obtained in the scheme. Compared with most of the unipolar optical OFDM schemes, such as asymmetrically clipped optical OFDM scheme, ${\mu }$ -OFDM doubles the spectral efficiency. Furthermore, this scheme shows significant energy advantages over direct-current biased optical OFDM scheme and polar OFDM scheme. To the best of our knowledge, this is the first design of a unipolar optical OFDM scheme, which is able to achieve both high spectrum and energy efficiency under a constraint of the dynamic range of the light-emitting diodes.

Color Multiplexing Based Unipolar OFDM for Indoor RGB LED Visible Light Communication

In this paper, a novel unipolar orthogonal frequency-division multiplexing (OFDM) scheme is proposed for indoor intensity modulation/direct detection (IM/DD) visible light communication (VLC) using red-green-blue (RGB) light emitting diode (LED). The positive and negative parts of a bipolar OFDM signal are transmitted separately on two colors of a RGB LED through color multiplexing. Compared with conventional DC biased optical OFDM (DCO-OFDM) or asymmetrically clipped optical OFDM (ACO-OFDM), the proposed color multiplexed optical OFDM (CMO-OFDM) can greatly improve the overall power efficiency of indoor VLC systems without reducing the total transmission capacity. Two types of receivers are designed and their performances are examined for CMO-OFDM based indoor VLC systems. By utilizing a practical RGB LED model, performance comparisons between CMO-OFDM and conventional unipolar OFDM schemes have also been conducted. The obtained results show that, to achieve the same bit rate, CMO-OFDM based indoor VLC system requires the least average optical power.

Improving the power efficiency of enhanced unipolar OFDM for optical wireless communication

Enhanced unipolar orthogonal frequency division multiplexing (eU-OFDM) is a novel unipolar multicarrier modulation scheme used in intensity modulation and direct detection systems for optical wireless communication. It improves the spectral efficiency of unipolar OFDM at the expense of lower power efficiency. A novel symbol detection method exploiting the intrinsic signal structure of the received signal is proposed. Analysis and simulations show that the power efficiency of eU-OFDM could be improved up to 3 dB in the high signal-to-noise ratio regime using the proposed method.

Unlocking Spectral Efficiency in Intensity Modulation and Direct Detection Systems

A number of inherently unipolar orthogonal frequency division multiplexing (OFDM) modulation schemes have been introduced recently in an attempt to improve the energy efficiency of OFDM-based intensity modulation and direct detection (IM/DD) systems. All such algorithms, including asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) and unipolar orthogonal frequency division multiplexing (U-OFDM), experience an inherent loss in spectral efficiency caused by the restrictions imposed on the OFDM frame structure required for the generation of a unipolar signal. The current paper presents a modified modulation approach, termed enhanced U-OFDM (eU-OFDM), which compensates the spectral efficiency loss in U-OFDM. At the same time, it still allows for the generation of an inherently unipolar modulation signal that achieves better performance in terms of both electrical power and optical power dissipation compared to the conventional state-of-the-art technique direct current (DC)-biased optical OFDM (DCO-OFDM). To the best of the authors' knowledge, the current work also presents the first experimental proof-of-concept demonstration of both U-OFDM and eU-OFDM, and clearly demonstrates the significant energy advantages that these two schemes can introduce in an optical wireless communications (OWC) system.