Fourier Transform Spread Orthogonal Frequency Research Articles

A Novel PAPR Reduction Scheme for Low-Cost Terminals in 6G

This paper presents a low peak-to-average power ratio (PAPR) modulation scheme and demodulation method for low-cost terminals in sixth-generation (6G), which is called one plus delay (1 + D) pi/2(N + 1) BPSK. Explicitly, the modulation scheme is first generated based on pi/2 BPSK, then proceeded by interpolation and (1 + D) transformation. Simulation results show that the proposed (1 + D) pi/2(N + 1) BPSK scheme has lower PAPR and smaller out-of-band leakage in discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM). Moreover, better performance can be attained by increasing the value of N. Finally, similar block error ratio (BLER) performance can be obtained at the same spectral efficiency of the conventional pi/2 BPSK scheme.

Sensing Integrated DFT-Spread OFDM Waveform and Deep Learning-Powered Receiver Design for Terahertz Integrated Sensing and Communication Systems

Terahertz (THz) communications are envisioned as a key technology of next-generation wireless systems due to its ultra-broad bandwidth. One step forward, THz integrated sensing and communication (ISAC) system can realize both unprecedented data rates and millimeter-level accurate sensing. However, THz ISAC meets stringent challenges on waveform and receiver design to fully exploit the peculiarities of THz channel and transceivers. In this work, a sensing integrated discrete Fourier transform spread orthogonal frequency division multiplexing (SI-DFT-s-OFDM) system is proposed for THz ISAC, which can provide lower peak-to-average power ratio than OFDM and is adaptive to flexible delay spread of the THz channel. Without compromising communication capabilities, the proposed SI-DFT-s-OFDM realizes millimeter-level range estimation and decimeter-per-second-level velocity estimation accuracy. In addition, the bit error rate (BER) performance is improved by 5 dB gain at the 10°3 BER level compared with OFDM. At the receiver, a deep learning based ISAC receiver with two neural networks is developed to recover transmitted data and estimate target range and velocity, while mitigating the imperfections and non-linearities of THz systems. Extensive simulation results demonstrate that the proposed deep learning methods can realize mutually enhanced performance for communication and sensing, and is robust against Doppler effects, phase noise and multi-target estimation.

Performance and complexity study of a neural network post-equalizer in a 638-nm laser transmission system through over 100-m plastic optical fiber

In recent years, plastic optical fiber (POF) has been considered as a promising cost-effective scheme for short-distance data communications, multimedia communication in cars, and in-house networks. However, due to the intrinsic nature of the relatively large numerical aperture of POF and the high attenuation rate, implementing high data rates over 100-m POF transmission length will be a significant challenge. We propose a scheme of high-speed 100-m POF transmission system based on a visible red laser and a cascaded neural network (NN) post-equalizer. To mitigate the nonlinear distortion induced by the POF, three different NNs, i.e., convolutional NN (CNN), long and short-term memory NN (LSTM), and cascaded NN structure consisting of convolutional layers and LSTM (CNN-LSTM), are employed as the post-equalizer. Experimental results show that using three different post-equalizers can significantly improve the system performance compared with the Volterra equalizer baseline. Among them, CNN-LSTM can outperform the others in terms of the bit error rate (BER) and the system Q-factor in the low nonlinear region. When the system operating in strong nonlinear region, CNN can achieve optimal performance at a lower system overhead of complexity. Finally, we successfully demonstrated a 100-m POF transmission system using 16 quadrature amplitude modulation discrete Fourier transform-spread orthogonal frequency division multiplexing modulation format at 1.8 Gbps with BER below 3.8 × 10 − 3 by utilizing CNN-LSTM.

Negatively Biased Solar Cell Optical Receiver for Underwater Wireless Optical Communication System With Low Peak Average Power Ratio

Charging batteries in underwater scenarios is generally very expensive and impractical, and solar cell (SC)-based underwater simultaneous lightwave information and power transfer (SLIPT) systems are a powerful solution. However, silicon SC receiver devices have limited bandwidth and are prone to deep signal-to-noise (SNR) degradation during underwater light fading effects. For these problems, this manuscript proposes a negative-biased SC optical receiver scheme to increase the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-3$</tex-math></inline-formula> dB bandwidth of silicon SC from 440 kHz to 780 kHz. For the deep fading of SNR caused by various degradation effects in the water environment, a low peak average power ratio (PAPR) discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) modulation scheme is employed to counteract the deep fading phenomenon in the system. Achieved a communication rate of 15.2 Mbps in a 60 cm underwater environment with a fading factor of 0.403 and a bit error rate (BER) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1.59\times 10^{-3}$</tex-math></inline-formula> under perturbation. Also, the performance of DFT-S-OFDM and orthogonal frequency division multiplexing (OFDM) modulation systems in water environments with different turbidity (absorption characteristics and scattering) and presence of disturbances are compared separately, and the DFT-S-OFDM system is more robust. Finally, we complete the energy harvesting during the communication process, and the experiments show that the total battery power efficiency of the energy harvesting system can be increased by 1.87 times under the white light-emitting diode (LED) continuous irradiation for three hours.

Phase Noise Resilient Three-Level Continuous-Phase Modulation for DFT-Spread OFDM

In this paper, a novel waveform with low peak-to-average power ratio (PAPR) and high robustness against phase noise (PN) is presented. It follows the discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM) signal model. This scheme, called 3MSK, is inspired by continuous-phase frequency shift keying (CPFSK), but it uses three frequencies in the baseband model – specifically, 0 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm f_{symbol}/4$ </tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{symbol}$ </tex-math></inline-formula> is the symbol rate – which effectively constrains the phase transitions between consecutive symbols to 0 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm \pi /2$ </tex-math></inline-formula> rad. Motivated by the phase controlled model of modulation, different degrees of phase continuity can be achieved, allowing to reduce the out-of-band (OOB) emissions of the transmitted signal, while supporting receiver processing with low complexity. Furthermore, the signal characteristics are improved by generating an initial time-domain constant envelope signal at higher than the symbol rate. This helps to reach smooth phase transitions between 3MSK symbols, while the information is encoded in the phase transitions. Also the possibility of using excess bandwidth is investigated by transmitting additional non-zero frequency bins outside the active frequency bins of the basic DFT-s-OFDM model, which provides the capability to greatly reduce the PAPR. The most critical tradeoffs of the oversampled schemes are that improved PAPR is achieved with the cost of somewhat reduced link performance and, in case of excess band, also the spectrum efficiency is reduced. Due to the fact that the information is encoded in the phase transitions, a receiver model that tracks the phase variations without needing reference signals is developed. To this end, it is shown that this new modulation is well-suited for non-coherent receivers, even under strong phase noise (PN) conditions, thus allowing to reduce the overhead of reference signals. Evaluations of this physical-layer modulation and waveform scheme are performed in terms of transmitter metrics such as PAPR, OOB emissions and achievable output power after the power amplifier (PA), using a practical PA model. Finally, coded radio link evaluations are also provided, demonstrating that 3MSK has a similar bit error rate (BER) performance as that of traditional quadrature phase-shift keying (QPSK), but with significantly lower PAPR, higher achievable output power, and the possibility of using non-coherent receivers.

Chirp-Convolved Data Transmission

We propose a multicarrier chirp waveform, which can be generated by a circular convolution between data modulation symbols and a transmit filter, and show that it can equivalently be represented as discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM) with frequency domain chirp filter. The chirp is defined by a complex exponential function where the phase follows a second-degree polynomial and its coefficients can be chosen to maximize the performance on a fading channel and to generate sets of orthogonal signals. The results show that the waveform outperforms OFDM and DFT-s-OFDM in terms of bit error rate (BER) and block error rate (BLER) on a time-frequency selective channel.

DFT-Spread-OFDM-Based Chirp Transmission

In this study, we propose a framework for chirp-based communications by exploiting discrete Fourier transform-spread orthogonal frequency division multiplexing (DFT-s-OFDM). We show that a well-designed frequency-domain spectral shaping (FDSS) filter for DFT-s-OFDM can convert its single-carrier nature to a linear combination of chirps circularly translated in the time domain. Also, by exploiting the properties of the Fourier series and Bessel function of the first kind, we analytically obtain the FDSS filter for an arbitrary chirp. We theoretically show that the chirps with low ripples in the frequency domain result in a lower bit-error ratio (BER) via less noise enhancement. We also address the noise enhancement by exploiting the repetitions in the frequency. The proposed framework offers a new way to efficiently synthesize chirps that can be used in Internet-of-Things (IoT), dual-function radar and communication (DFRC) or wireless sensing applications with existing DFT-s-OFDM transceivers.

DFT-s-OFDM With Enhanced Time-Domain Spreading for B-IFDMA

Enhanced time-domain spreading is proposed for block-interleaved frequency division multiple access (B-IFDMA) with discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM). By repeating each consecutive modulation symbol prior to the spreading, we show that the DFT of the spreading sequence becomes a window function, which compared to the existing method of repeating the whole block of modulation symbols, offers a signal with both lower cubic metric (CM) and block error rate (BLER). Modulation symbol-specific spreading sequences are further shown to improve the BLER. The new transmission scheme is supported by both time- and frequency domain despreaders, as well as by two channel estimation methods for B-IFDMA.

RETRACTED ARTICLE: Peak to average power ratio reduction of ZT DFT-s-OFDM signals using improved monarch butterfly optimization-PTS scheme

For traditional orthogonal multiplexing frequency division (OFDM) systems, a novel Zero Tail Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing Division (ZT DFT-s-OFDM) waveform architecture has recently been proposed as a problem solution of low peak to average power ratio (PAPR) efficiency. The ZT DFT-s-OFDM system allows the delay in the transmission of the channel with multipath to be dynamically copied, thus the limitations to be rectified with the hard-coded Cyclic Prefix (CP). However it is affected by few roadblocks during the transmission of data, and the primary one involves the high peak-to-average power ratio (PAPR), which results in saturation observed in the power amplifier, production of more amount of interference and decreased resolution in elements such as digital/analog converters, which were considered as nonlinear. Partial transmit sequences (PTSs) is a promising plan and direct technique, ready to accomplish a viable PAPR decrease execution, yet it requires a thorough hunt to locate the ideal stage factors, which causes high computational multifaceted nature expanded with the quantity of sub-blocks. Right now, the author proposed a reduced computational complexity PTS scheme, in view of a new swarm knowledge algorithm, which is termed as Improved Monarch Butterfly Optimization (IMBO) for PAPR decrease with the ZT DFT-s-OFDM framework. The IMBO is a new swarm intelligence algorithm, with the capability of achieving an efficient optimization search that implements phase weighting process with less complexity for selecting optimum phase factors. Also, the proposed technique is quite efficient in looking for a fusion with optimal character of phase rotation factors for minimizng the computational difficulty involved. The outcomes of simulation indicate that IMBO-based PTS algorithm can substantially reduce PAPR employing an easy network structure in comparison with classical algorithms such as PTS scheme for conventional OFDM and PTS with ZT-DFT-s-OFDM.

Bit‐level implementation for polynomial cancellation coded DFT‐s‐OFDM transmitters

Polynomial cancellation coded discrete Fourier transform-spread orthogonal frequency division multiplexing (PCC-DFT-s-OFDM) is famous for the flexibility of DFT-s-OFDM on the trade-off between peak-to-average power ratio performance and spectral efficiency. Deploying it on numerous user equipments (UEs) as enhanced uplink signalling becomes a practical issue. In this Letter, a bit-level implementation of PCC mapping is proposed, so that the compatibility of implementation of corresponding circuits on UE transmitters would make the deployment more easily. By taking Long Term Evolution and New Radio standards as examples, the equivalency of bit/symbol-level implementations is validated in terms of bit error rate performance.

Convolution neural network-based time-domain equalizer for DFT-Spread OFDM VLC system

Abstract This paper presents a novel time-domain equalizer for visible light communication (VLC) system using machine learning (ML) method. In this work, we employ discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) as modem scheme and convolution neural network (CNN) as kernel processing unit of equalizer. After estimating channel state information (CSI) from training sequence, the proposed equalizer recovers transmitted symbols according to the estimated CSI. Numerical simulations indicate that the equalizer can significantly enhance bit error rate (BER) performance. For example, when signal-to-noise ratio (SNR) is 20 dB and 16/32/64-quadrature amplitude modulation (QAM) is exploited, original BER is about 0.5 while the BER after recovery achieves 1 0 − 5 , which is much lower than forward error correction (FEC) limit 3 . 8 × 1 0 − 3 . This work promotes the application of ML in VLC domain. To the best of our knowledge, this is the first time a CNN-based equalizer has been explored.

A 1.34-Gb/s Real-Time Li-Fi Transceiver With DFT-Spread-Based PAPR Mitigation

This letter presents the design of a Gbit/s real-time light-fidelity transceiver based on field programmable gate array (FPGA). We exploit discrete Fourier transform spread orthogonal frequency division multiplexing as a modem scheme, while wavelength division multiplexing technology is employed to enhance the data rate by commercial red-green-blue light emitting diodes. The transceiver achieves a peak rate of 1.34 Gbit/s over 3.5 m of free space below the forward error correction limit of $3.8 \times 10^{-3}$ . This letter also considers the requirement of mapping the proposed FPGA design into very large-scale integration.

Adjustable PAPR reduction for DFT‐s‐OFDM via improved general precoding scheme

An improved and specification-compatible general precoding scheme (GPC) for adjustable peak-to-average power ratio (PAPR) reduction in discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM) is proposed. Through the rearrangement of periodic padding for precoded data symbols in the GPC precoding matrix, the proposed algorithm does not increase any computational complexity compared to the original GPC. The LTE-based simulation shows the proposed algorithm has a better improvement of PAPR reduction especially in low-order modulations and the performances can be scaled to the other transmission bandwidth. These works contribute to the flexibility of DFT-s-OFDM for supporting 5G transmission over miscellaneous power amplifiers.

Advanced modulation formats for underwater visible light communications [Invited

In this paper, we present a detailed comparison of applying three advanced modulation formats including carrierless amplitude and phase modulation (CAP), orthogonal frequency division multiplexing (OFDM), and discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S OFDM) in underwater visible light communication (UVLC) systems. Cascaded post-equalization schemes are suggested to compensate the system impairments. For the first time, a two-level post-equalizer is presented to mitigate the nonlinear effect and improve the system performance of UVLC. The first post-equalization is based on a novel recursive least square Volterra. These modulation formats are all experimentally demonstrated with corresponding digital signal processing (DSP) algorithms. The experimental results show that single carrier modulations including CAP and DFT-S OFDM can outperform OFDM. Our experiment results show that up to 3 Gb/s over a 1.2 m underwater visible light transmission can be achieved by using DFT-S OFDM 64QAM and CAP-64. The measured bit error rate is well under the hard decision-forward error correction (HD-FEC) threshold of 3.8×10−3.

Comparison of 100G PAM-8, CAP-64 and DFT-S OFDM with a bandwidth-limited direct-detection receiver

We present a detailed comparison of applying three advanced modulation formats including pulse amplitude modulation-8 (PAM-8), carrier-less amplitude and phase modulation-64 QAM (CAP-64), and discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S OFDM) with a bandwidth-limited direct-detection receiver for 100 Gb/s/λ optical transmission systems. These modulation formats are all experimentally demonstrated with corresponding digital signal processing (DSP) algorithms. The comparison is carried out to evaluate the performance of each modulation format in terms of nonlinear equalization, received optical power and optical signal to noise ratio (OSNR). Our experimental results show that only 112 Gbit/s DFT-S OFDM is successfully achieved over 50 km of SSMF under the hard decision-forward error correction (HD-FEC) threshold of 3.8 × 10−3.

Transmission Performance Comparison for 100-Gb/s PAM-4, CAP-16, and DFT-S OFDM With Direct Detection

Metro network, as a medium distance transmission system, poses a special challenge of transmission capacity and cost. In this paper, we present a detailed comparison of applying three advanced modulation formats including pulse amplitude modulation-4 (PAM-4), carrierless amplitude and phase modulation-16 QAM (CAP-16), and discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S OFDM) with low-cost direct detection for 100-Gb/s/λ metro optical transmission systems. These modulation formats are all experimentally demonstrated with similar digital signal processing algorithms. Three kinds of chromatic dispersion (CD) compensation ways named CD precompensation method, single sideband (SSB), and dispersion compensating fiber (DCF) are also compared. Two types of modulators including an IQ modulator and a dual-drive Mach–Zehnder modulator are employed to generate SSB or CD precompensated signals in this experiment and their transmission performance is also evaluated. To the best of our knowledge, this is the first time that the performance of single-wavelength 100-G signal transmission based on direct detection is evaluated over 480 km of standard single mode fiber with PAM-4, CAP-16, and DFT-S OFDM modulation formats.

200-Gbps DFT-S OFDM Using DD-MZM-Based Twin-SSB With a MIMO-Volterra Equalizer

We experimentally demonstrate 208-Gb/s/λ discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S OFDM) transmission over a 40-km standard single-mode fiber without chromatic dispersion compensation using a low-cost direct-detection architecture based on the dual-driver Mach-Zehnder modulator (MZM). One twin-single sideband (SSB) signal is generated by driving the upper and lower arms of a dual-driver MZM with a real signal and its respective Hilbert pair. A MIMO-Volterra equalizer is proposed to mitigate the interference and nonlinearity penalty of the twinSSB signal. Using the Twin-SSB without the MIMO algorithm can only reach 165 Gb/s/λ, and applying a conventional SSB can only achieve 164 Gb/s/λ with a bit error ratio less than 5×10 -3 . However, we achieve a 240-Gb/s/λ twin-SSB DFT-S OFDM signal in the back-to-back (BTB) case using the twin-SSB signal and MIMO-Volterra Equalization method, which implies that double spectral efficiency and more than 45% data rate improvement are obtained.

Improving performance of channel equalization in RSOA-based WDM-PON by QR decomposition.

In reflective semiconductor optical amplifier (RSOA)-based wavelength division multiplexed passive optical network (WDM-PON), the bit rate is limited by low modulation bandwidth of RSOAs. To overcome the limitation, we apply QR decomposition in channel equalizer (QR-CE) to achieve successive interference cancellation (SIC) for discrete Fourier transform spreading orthogonal frequency division multiplexing (DFT-S OFDM) signal. Using an RSOA with a 3-dB modulation bandwidth of only ~800 MHz, we experimentally demonstrate a 15.5-Gb/s over 20-km SSMF DFT-S OFDM transmission with QR-CE. The experimental results show that DFTS-OFDM with QR-CE attains much better BER performance than DFTS-OFDM and OFDM with conventional channel equalizers. The impacts of several parameters on QR-CE are investigated. It is found that 2 sub-bands in one OFDM symbol and 1 pilot in each sub-band are sufficient to achieve optimal performance and maintain the high spectral efficiency.

Comparison of DSP schemes with frequency domain equalization for passive optical networks

In recent years, digital signal processing (DSP) has been widely investigated for the applications in future next generation passive optical networks (PONs). In this paper, we compare four transmission technologies based on DSP with frequency domain equalization (FDE) for PON transmission with double-side band (DSB) intensity modulation and direct detection. These schemes include orthogonal frequency division multiplexing (OFDM), single-carrier frequency domain equalization (SCFDE), discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) and interleaved frequency division multiplexing (IFDM). We analyze their computational complexity and flexibility in PON applications, and compare their transmission performance by experiments. Based on above work, we propose and experimentally demonstrate a hybrid DSP-enhanced PON architecture with downstream OFDM modulation and upstream SCFDE modulation.

English

The 3GPP long term evolution (3GPP-LTE) has adopted the Discrete Fourier Transform Spreading Orthogonal Frequency Division Multiple Access (DFTS-OFDMA) technique to be used in the uplink air interface for the next generation cellular systems. The DFTS-OFDMA is also a candidate of radio interface technologies for the IMT-Advanced standards in ITU-R. This technique has been adopted for uplink transmission due to its attractive feature of lower PAPR archived by precoding data symbols using a DFT operation at the transmitter. In this paper, we analyse the performance of DFT-Spreading OFDMA technique in PAPR reduction by using the IFDMA and LFDMA subcarriers mapping modes. We also analyse the performance of the DFTS OFDMA technique for PAPR reduction as the number of subcarriers allocated to each user varies, specifically with the LFDMA subcarriers mapping mode. Simulation results show that, the IFDMA improves PAPR performance significantly than LFDMA with pulse shaping and wihout pulse shaping but at a cost of increased bandwidth.Moreover, the performance of DFT-Spreading with LFDMA improves when the number of subcarriers allocated to users becomes small. Although, in-terms of PAPR power efficiency, the IFDMA performs better than LFDMA allocation scheme, but LFDMA is very superior in terms of throughput when channel dependent scheduling is utilized and has been widely implemented in LTE.