DFT-spread OFDM Research Articles

A Low-Complexity Widely-Linear MMSE Equalizer for DFT-Spread OFDM With Frequency-Domain Spectrum Shaping

A Low-Complexity Widely-Linear MMSE Equalizer for DFT-Spread OFDM With Frequency-Domain Spectrum Shaping

Analysis of phase noise for DFT-spread OFDM systems in coherent optical communication

Previous studies have revealed the similarity between phase noise in the symbol domain of discrete Fourier transform spread (DFT-Spread) coherent orthogonal frequency division multiplexing (CO-OFDM) systems and that in the time domain. This paper further details this characteristic through both formulas and simulation results, while also analyzing and summarizing its formation conditions and influencing factors. Based on this, a phase noise compensation (PNC) scheme based on DFT-Spread OFDM is proposed for high-order quadrature amplitude modulation (QAM) transmission with large fast Fourier transform (FFT) sizes. The proposed scheme effectively suppresses inter-carrier interference (ICI) introduced by phase noise in OFDM systems, as verified through simulations of dual-polarization (DP) 64-QAM CO-OFDM transmission over 80 km single-mode fiber (SMF). This work suggests that proposed PNC scheme enhance the tolerance for laser linewidth in OFDM systems, and may offer a possible solution for short distance high-baud-rate high-order QAM coherent transmission.

Performance of iterative decision feedback channel estimation in turbo FDE for DFT-spread OFDM

Performance of iterative decision feedback channel estimation in turbo FDE for DFT-spread OFDM

Minimizing ISL in DFT-spread OFDM Radar Using Complementary Overlaying and Greedy-MSA Optimization

Minimizing ISL in DFT-spread OFDM Radar Using Complementary Overlaying and Greedy-MSA Optimization

Orthogonal Circulant Multiplexing for Bandwidth- Limited Direct-Detected Optical Communication Systems

We propose and investigate a novel orthogonal circulant multiplexing (OCM) technology for bandwidth-limited direct detection (DD) optical systems. We analytically derive that any OCM, i.e. using any orthogonal circulant matrix for multiplexing, can redistribute the noise uniformly across the subcarriers. However, the distribution of inter-symbol interference (ISI) and inter-carrier interference (ICI) strongly depends on the design of OCM. The previously proposed orthogonal chirp division multiplexing (OCDM), although showing better ISI/ICI tolerance than discrete-Fourier-transform spread (DFT-S) orthogonal frequency division multiplexing (OFDM), is a kind of OCM with poorer performance. We propose and analytically reveal that OCM based on chirp-like polyphase sequence (CLPS), general chirp sequence (GCS), sparse CLPS (SCLPS), and sparse GCS (SGCS) can achieve flatter ISI/ICI distribution across subcarriers and so better performance than OCDM in bandwidth-limited DD systems. The optimal parameter vector of the E-parameter CLPS is a Zadoff-Chu sequence while the parameter α in GCS should be larger. The optimal performance can be achieved by either CLPS (or GCS) using a larger E (or α) or SCLPS (or SGCS) with optimized sparsity. ∼110Gbit/s experiments using Mach-Zehnder modulator or electro-absorption modulator and receiver of 10-GHz photodiode or 8-GHz avalanche photodiode are conducted. The results agree with the theory and show that the proposed OCM outperforms conventional OFDM, DFT-S OFDM, and OCDM.

Time-Frequency Domain Non-Orthogonal Multiple Access for Power Efficient Communications

We propose a power efficient non-orthogonal multiple access (NOMA) scheme suitable for Internet of Things (IoT) and device-to-device (D2D) communications. OFDM-based power-domain NOMA (PD-NOMA) is one of the promising approaches to achieve simultaneous connections of massive devices. Nevertheless, the problem of high peak-to-average power ratio (PAPR) is a major challenge, since power amplifier (PA) efficiency is a critical factor for IoT and D2D devices consisting of low-cost hardware components. This paper overcomes this problem by introducing time-frequency domain NOMA (TF-NOMA), where OFDM and DFT-spread OFDM (DFT-s-OFDM) are superposed over the time-frequency domain. By utilizing the low PAPR property achieved by DFT-precoding, TF-NOMA can efficiently reduce the PAPR of the downlink signal transmitted by the base station (BS) as well as the uplink signals of the devices located far from BS. As a result, the proposed TF-NOMA can operate with higher PA efficiency than the conventional OFDM-based PD-NOMA. Furthermore, the insertion of DFT-precoding with the aim of PAPR reduction also enhances the error propagation resistance due to the resulting Gaussian mixture signal property. Throughout the mathematical analysis and computer simulations, we demonstrate that TF-NOMA achieves better error rate performance than the conventional PD-NOMA with significant improvement of PA efficiency.

On the BER Performance of Constant Envelope OFDM in Frequency Selective MIMO Channels With ML Detection

In this contribution, we investigate the issue of constant envelope orthogonal frequency-division multiplexing (CE-OFDM) transmission in multipath fading channels, by quantifying its bit-error rate (BER) performance with the aid of maximum likelihood (ML) detection in multiple-input multipleoutput (MIMO) configurations. Through theoretical derivation, we demonstrate the advantage of CE-OFDM over its traditional counterparts as original OFDM and discrete Fourier transformspread-OFDM (DFT-s-OFDM) in terms of BER performance, which is ultimately supported by the simulation results.

Performance comparison of single-carrier and multi-carrier waveforms over terahertz wireless channels

Terahertz (THz) wireless communication has been recognized as a powerful technology to meet the ever-increasing demand of ultra-high rate services. In order to achieve efficient and reliable wireless communications over THz bands, it is extremely necessary to find an appropriate waveform for THz communications. In this paper, performance comparison of various single-carrier and multi-carrier waveforms over THz channels will be provided. Specifically, first, a system model for terahertz communication is briefly described, which includes amplifier nonlinearity, propagation characteristic, phase noise, etc. Then, the transceiver architectures related to both single-carrier and multi-carrier waveforms are presented, as well as their corresponding signal processing techniques. To evaluate the suitability of the waveforms, key performance metrics concerning power efficiency, transmission performance, and computational complexity are provided. Simulation results are provided to compare and validate the performance of different waveforms, which demonstrate the outstanding performance of Discrete-Fourier-Transform spread Orthogonal Frequency Division Multiplexing (DFT-s-OFDM) to THz communications when compared to Cyclic Prefix-OFDM (CP-OFDM) and other single-carrier waveforms.

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.

FDSS-Based DFT-s-OFDM for 6G Wireless Sensing.

Integrated sensing and communications (ISAC) is emerging as a key technology of 6G. Owing to the low peak-to-average power ratio (PAPR) property, discrete Fourier transform spread orthogonal frequency-division multiplexing (DFT-s-OFDM) is helpful to improve the sensing range and suitable for high-frequency transmission. However, compared to orthogonal frequency-division multiplexing (OFDM), the sensing accuracy of DFT-s-OFDM is relatively poor. In this paper, frequency-domain spectral shaping (FDSS) is adopted to enhance the performances of DFT-s-OFDM including sensing accuracy and PAPR by adjusting the correlation of signals. Specifically, we first establish a signal model for the ISAC system, followed by the description of performance indicators. Then, we analyze the influence of amplitude fluctuation of frequency domain signals on sensing performance, which shows the design idea of FDSS-enhanced DFT-s-OFDM. Further, a FDSS-enhanced DFT-s-OFDM framework is introduced for ISAC, where two types of FDSS filters including a pre-equalization filter and an isotropic orthogonal transform algorithm (IOTA) filter are designed. The simulation results show that the proposed scheme can obtain about 4 dB performance gain in terms of sensing accuracy over DFT-s-OFDM. In addition, FDSS-enhanced DFT-s-OFDM can significantly reduce PAPR and improve the power amplifier efficiency.

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.

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.

Physical Layer Performance Modeling of Modern Multicarrier Modulation Techniques

The fifth-generation (5G) and beyond standards are being challenged by the diverse requirements of modern use cases. Multicarrier modulation techniques are one of the key components of the physical layer (PHY) design, which has immense potential to improve efficiency and reliability. In current state-of-the-art wireless technologies (i.e., NR and IEEE 802.11ax) orthogonal frequency division multiplexing (OFDM) is used which has many disadvantages such as peak-to-average power ratio (PAPR), out-of-band emission (OOBE), and sensitivity to carrier frequency offset (CFO). To overcome these drawbacks several alternate multicarrier modulation techniques are being considered, such as discrete Fourier transform-spread-OFDM (DFT-s-OFDM), generalized frequency division multiplexing (GFDM), and orthogonal time-frequency space (OTFS). In this paper, we develop the physical layer abstraction (PLA) of these candidate multicarrier techniques to evaluate their performance under various use cases and scenarios. The PLA is a commonly used technique to avoid time-consuming PHY simulations in system-level simulators. To improve the accuracy of PLA in different fading conditions, we derive a fitting parameter as a function of the received signal-to-interference-plus-noise ratio (SINR) variance. The validation results show that performance can be accurately estimated through the proposed multicarrier PLA. Moreover, PLA techniques are at least thousands of times faster compared to PHY simulations.

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.

Fairness-aware hybrid NOMA/OFDMA for bandlimited multi-user VLC systems

In this paper, we for the first time propose and experimentally demonstrate a fairness-aware hybrid non-orthogonal multiple access/orthogonal frequency division multiple access (NOMA/OFDMA) scheme for practical bandlimited multi-user visible light communication (VLC) systems. The proposed fairness-aware hybrid NOMA/OFDMA scheme using discrete Fourier transform-spread-orthogonal frequency division multiplexing (DFT-S-OFDM) modulation can efficiently guarantee both the fairness of users within each user pair and the fairness of different user pairs. Specifically, an interleaved subcarrier multiplexing approach is proposed to mitigate user pair unfairness caused by the low-pass frequency response of bandlimited VLC systems, and a two-dimension (2D) power allocation strategy is further proposed to ensure both the fairness of users within each user pair and the fairness of user pairs with different channel conditions. Experimental results show that, in a bandlimited four-user VLC system applying the proposed fairness-aware hybrid NOMA/OFDMA scheme, the transmission distance at a target bit error rate (BER) of 3.8 × 10−3 can be extended by 12.3% in comparison to that employing the conventional hybrid NOMA/OFDMA scheme.

Novel Tone Reservation Method for DFT-s-OFDM

In this letter, a novel guard-tone reservation (GTR) method is proposed to reduce the peak-to-average power ratio (PAPR) of discrete Fourier transform-spread-orthogonal frequency-division multiplexing (DFT-s-OFDM) waveform. Unlike existing PAPR reduction methods, the proposed GTR operates directly in the data symbol domain, estimating the peaks through the relations between the data symbols, while the corresponding peak-cancellation signal (PCS) is efficiently embedded into the waveform processing. Furthermore, the PCS is generated by exploiting only the guard-band tones, leaving thus the inband data tones undistorted. The performance of the method is evaluated in 5G New Radio (NR) uplink context including also realistic measured power amplifier (PA) characteristics. The obtained results show that significant PAPR reduction and corresponding PA output power gains can be obtained while the computational complexity is shown to be low.

DFT-spread OFDM with quadrature index modulation for practical VLC systems.

In this paper, we for the first time propose and investigate a novel discrete Fourier transform-spread-orthogonal frequency division multiplexing with quadrature index modulation (DFT-S-OFDM-QIM) scheme for practical visible light communication (VLC) systems. By performing subcarrier index modulation on both the in-phase and quadrature components of each subcarrier, OFDM-QIM achieves a higher spectral efficiency than conventional OFDM with index modulation (OFDM-IM). Moreover, the peak-to-average power ratio (PAPR) of OFDM-QIM can be substantially reduced by applying DFT spreading, and hence DFT-S-OFDM-QIM exhibits high tolerance against light-emitting diode (LED) nonlinearity. The superiority of the proposed DFT-S-OFDM-QIM scheme over other benchmark schemes has been successfully verified by both simulation and experimental results.

A new PAPR and BER enhancement technique based on lifting wavelet transform and selected mapping method for the next generation waveforms

Orthogonal frequency division multiplexing (OFDM) is widely used in many digital communication systems because of its immunity to inter-symbol interference, high spectral efficiency and capability of handling multipath. Walsh-Hadamard transform (WHT) and discrete wavelet transform (DWT) have also been suggested in order to enhance the performances of multi-carrier wireless communication systems. In general, high peak-to-average power ratio (PAPR) is one of the biggest disadvantages of these systems and OFDM also has this disadvantage. Many techniques such as amplitude clipping and filtering, encoding, interleaving, tone reservation (TR), tone addition (TI), partial transmission sequence (PTS), and selected mapping (SLM) techniques have been proposed to reduce the high PAPR. Without any modification these techniques are useless due to the related symbol addition process. For this reason, we developed a new technique composed of Lifting wavelet transform (LWT) and SLM method in order to decrease the PAPR in multi-carrier systems. The performance of this new LWT-SLM scheme is measured by applying it to zero tail (ZT) discrete Fourier transform-spread OFDM (ZT DFT-s OFDM) and unique word OFDM (UW OFDM) waveforms, which are promising next generation waveforms. Simulation results show that, the ZT DFT-s OFDM system obtains the 3.3 dB PAPR gain and 0.8 dB better bit error rate (BER) performance compared to classical OFDM with proposed scheme without sacrificing BER performance.

Retracted: Peak to average power ratio reduction of ZT DFT‐S‐OFDM signals using hybrid improved monarch butterfly optimization‐PTS scheme with TR‐clipping scheme

SummaryCurrent research on zero‐tail discrete Fourier transform spread OFDM, an add on to conventional discrete Fourier transform DFT‐OFDM modulation without cyclic prefix (CP). Its low power tail is configured for coping up with channel's delay spread. Advantageous feature of ZT DFT‐S‐OFDM has drawn researchers for deploying it as a potential waveform candidate to fifth generation (5G). However, the intrinsic shortcoming of high peak to average power ratio (PAPR) should be alleviated. In order to improve that work, we propose a hybrid PAPR scheme that implements optimized partial transmit sequence (PTS) with tone reservation (TR) with clipping technique. Its earlier work stresses for reducing PTS scheme's computational complexity with enhanced version of monarch butterfly optimization (MBO) for PAPR reduction in ZT DFT‐S‐OFDM system. The proposed model reduced the correlation among the candidate signals. In this paper, we further reduce PAPR by deploying TR clipping pair, which reduces the influence of clipping noise and decreases ZT DFT‐S‐OFDM signal's PAPR at same time. The proposed hybrid technique produces excellent PAPR reduction while improving receiver sensitivity and effective throughout. The performance comparison is done using the parameters like PAPR and BER to confirm that this introduced design exceeds current waveforms designs with more economical design structure.

Generation of faster-than-Nyquist coherent optical DFT-spread OFDM signals with high-baud and high-order modulations

In this paper, we successfully demonstrate the generation and reception of faster-than-Nyquist (FTN) orthogonal frequency-division multiplexing (OFDM) signals with coherent detection. We can improve the spectrum efficiency by using higher order QAMs. However, we believe that the realization of super-Nyquist channel has the following advantages to improve spectral efficiency: Firstly, when we use higher order modulation, the requirements for equipment will be increased, such as the linearity of the modulator. Secondly, flexibility. The signal compression ratio can be arbitrarily set within the maximum compression ratio in our experiment. However, if we increase the modulation order, it can only be an integral multiple of 2, such as 4QAM to 8QAM, which can’t fully and effectively use the channel resources. In the FTN OFDM transmission system, frequency shaping is realized by a delay-and-add filter (DAF) at the transmitter to ensure the energy of signal is concentrated at low frequencies, which will make the OFDM much more robust to strong filtering effect to some extent. We generate a quadrature phase-shift keying OFDM (QPSK-OFDM) signal with equivalent baud rate (In this paper, the generation of FTN signal is in the frequency domain since we used M-point DFT and N-point IDFT while M is greater than N. In other words, the high-frequency parts are discarded to increase spectrum efficiency. Hence, we use “equivalent baud rate” instead of a simple “baud rate”) of 100–120 Gbaud, which is much higher than the sampling rate (80 GSa/s) of digital-to-analog converter (DAC). With this FTN Discrete-Fourier transform spread (DFT-spread) OFDM signal generation technique, we achieve the highest equivalent baud rate (120 GBaud) QPSK-OFDM signal generation and transmission over 80 km single mode fiber (SMF) with bit error rate (BER) under soft-decision forward-error-correction (SD-FEC) limitation of 2.4 × 10−2. We also investigate the generation and reception of 32 Gbaud FTN 16-ary quadrature-amplitude modulation OFDM (16QAM-OFDM) with coherent detection to achieve higher spectrum efficiency in this paper. The experimental results show that there is negligible optical signal to noise ratio (OSNR) penalty when 32 Gbaud FTN DFT-spread 16QAM-OFDM is compressed and transmitted within 28 GHz bandwidth with the BER at 2.4 × 10−2 in optical back to back (OBTB).