Transform Spread Orthogonal Frequency-division Multiplexing Research Articles

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.

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.

PAPR reduction using twin symbol hybrid optimization-based PTS and multi-chaotic-DFT sequence-based encryption in CP-OFDM system

Orthogonal frequency division multiplexing (OFDM) is considered as one of the most significant transmission methodologies of the recent past. Moreover, it permits easy demodulation and modulation. To find the new OFDM-based waveform to be used in fifth generation which is one of the foremost open issues for wireless networks of the next generation. In addition, the OFDM is affected by the maximum Peak-to-Average Power Ratio (PAPR). In order to minimize these problems, this paper proposed a Twin Symbol Hybrid Optimization used as a basis of the Partial Transmit Sequence (TSHO-PTS) method of Cyclic Prefix-OFDM (CP-OFDM). This CP-OFDM achieves the requirements of 5G telecommunication standards. Moreover, the exhaustive searching for optimal phase factors might increase the computational cost of PTS. To beat this problem, a hybrid version of slap swarm optimization (SSO) and Bald Eagle Search (BES) algorithm is introduced to investigate the phase factor optimally by the PTS method. Digital chaotic sequences are used to ensure the physical layer security during the data transmission scheme for the Discrete Fourier Transform Spread OFDM (DFT-S-OFDM) subcarrier allocation. The simulation takes place in the MATLAB platform, and the performances are evaluated by several performance metrics like Complementary cumulative distribution function (CCDF), Bit Error Rate (BER), and computational complexity. The performance of the proposed model is compared with various existing approaches and previous works. From the implemented results, the proposed strategy achieved less (5 dB) PAPR, minimum (10–8) BER, less processing time (0.18 s) than the existing schemes, and hence the complexity also very low (7%) than others.

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.

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.

Discrete Fresnel Transform Spread OFDM for Coherent Optical Fiber Communication

In the polarization-division multiplexed coherent optical fiber communication, conventional orthogonal frequency-division multiplexing (OFDM) and discrete Fourier transform spread OFDM (DFT-S-OFDM) sacrifice the baud rate with either zero-padded subcarriers in the high-frequency region to mitigate the frequency fading, or with a large cyclic prefix (CP) to eliminate the residual inter-symbol interference (ISI). In this letter, we propose a discrete Fresnel transform spread OFDM (DFnT-S-OFDM) scheme, which spreads information over both temporal and frequency dimensions. A low-complexity encoder was also developed for the proposed precoding scheme. Simulations show that DFnT-S-OFDM is more resilient against the frequency fading than OFDM and against the residual ISI than DFT-S-OFDM. Therefore, it maximizes the net data rate by reducing the CP length or increasing the useful signal bandwidth.

An Uplink Transmission Scheme for Pattern Division Multiple Access Based on DFT Spread Generalized Multi-Carrier Modulation

In order to meet the requirements of high system throughput, massive connectivity, and asynchronous transmission for 5G and beyond, non-orthogonal transmission techniques, including modulation techniques and multiple access techniques, have attracted a great deal of attention in both academia and industry. Traditional non-orthogonal multiple access (NOMA) schemes are investigated mainly based on the orthogonal waveform, i.e., orthogonal frequency-division multiplexing (OFDM). However, OFDM is not the most suitable waveform for the foreseeable mMTC scenarios because it is vulnerable to carrier frequency offset (CFO). To employ the advantages of both NOMA and non-orthogonal waveforms, in this paper, we focus on NOMA transmissions with the non-orthogonal waveform modulation. Specifically, an uplink transmission scheme for pattern-division multiple access (PDMA) based on discrete Fourier transform spread generalized multi-carrier (DFT-S-GMC) modulation, DFT-S-GMC-PDMA for short, is studied. First, implementation schemes of the proposed DFT-S-GMC-PDMA in the time domain and frequency domain are presented, respectively. Second, the equivalent channel response matrix and noise formulas of both implementation schemes are derived. Furthermore, simulation results are given to show that the DFT-S-GMC-PDMA can achieve a comparable performance to PDMA based on discrete Fourier transform spread OFDM (DFT-S-OFDM-PDMA), while the improvement of complexity is less than 3%. system performance under different equalizers and different PDMA patterns are also evaluated where neglectable performance loss is observed. Thanks to the robustness against CFO, the multiple-access interference performance of the DFT-S-GMC-PDMA scheme is about 0.5 dB superior to that of the corresponding the DFT-S-OFDM-PDMA scheme. It indicates by simulation that the proposed DFT-S-GMC-PDMA is robust to CFO at the cost of neglectable performance loss, and thus it is a more attractive candidate for the uplink transmission of mMTC.

Experimental research of adaptive OFDM and OCT precoding with a high SE for VLLC system

Abstract In this paper, an adaptive orthogonal frequency division multiplexing (OFDM) modulation scheme with 128/64/32/16-quadrature amplitude modulation (QAM) and orthogonal circulant matrix transform (OCT) precoding is proposed and experimentally demonstrated for a visible laser light communication (VLLC) system with a cost-effective 450-nm blue-light laser diode (LD). The performance of OCT precoding is compared with conventional the adaptive Discrete Fourier Transform-spread (DFT-spread) OFDM scheme, 32 QAM OCT precoding OFDM scheme, 64 QAM OCT precoding OFDM scheme and adaptive OCT precoding OFDM scheme. The experimental results show that OCT precoding can achieve a relatively flat signal-to-noise ratio (SNR) curve, and it can provide performance improvement in bit error rate (BER). Furthermore, the BER of the proposed OFDM signal with a raw bit rate 5.04 Gb/s after 5-m free space transmission is less than 20% of soft-decision forward error correlation (SD-FEC) threshold of 2.4 × 10 −2 , and the spectral efficiency (SE) of 4.2 bit/s/Hz can be successfully achieved.

Secure Transmission of Optical DFT-S-OFDM Data Encrypted by Digital Chaos

This paper proposes and demonstrates a physical layer security-enhanced transmission scheme for discrete Fourier transform spread orthogonal frequency-division multiplexing (DFT-S-OFDM) signals in a passive optical network. During the DFT-S-OFDM transmission, the reconfigurable DFT matrix, the subcarrier allocation, and the training sequence for OFDM symbol synchronization are well encrypted by digital chaotic sequences generated via a 4-D hyperdigital chaos, which provides a huge key space of $\sim\!\! 10^{45}$ . Secure transmission of 13.3-Gb/s encrypted DFT-S-OFDM signals is demonstrated over a 20-km standard single-mode fiber, which proves the excellent confidentiality of the proposed secure DFT-S-OFDM transmission. Moreover, the receiver sensitivity is improved ∼2 dB (BER@10−3), which is due to the effective reduction of peak-to-average power ratio via DFT precoding. In addition, this scheme has the advantages of low computational complexity and has no requirement of redundant sideband information.

Analysis of power efficiencies of polar transmitter and HPA in OFDM communication system

Polar transmitter is known as good candidates for high data rate systems such as OFDM (orthogonal frequency division multiplexing) because they can obtain high-efficiency switched-mode radio frequency (RF) power amplifiers. But it is important to consider the nonlinear effects and power efficiency of the amplifier in the OFDM communication systems. High PAPR (peak-to-average power ratio) causes performance degradation by non-linear distortion in OFDM communication system. So to avoid non-linear distortion, back-off is needed. In addition, output of amplifier is lower and the power efficiency is reduced because of back-off. In this article, we compare PAE (power added efficiency) of polar transmitter in OFDM system and PAE of HPA (high power amplifier) in OFDM system. Also, we analyse PAE according to back-off in DFT-spread OFDM (discrete Fourier transform-spread OFDM), which is PAPR reduction method and we compare with PAE of polar transmitter in OFDM system. And we analyse output spectrum of OFDM and DFT-spread OFDM system. Through spectrum analysis, we analyse PAE of OFDM and DFT-spread OFDM, which is satisfied with output spectrum mask of 3rd generation long term evolution (3G LTE) and also compare with PAE polar transmitter of OFDM system, which is satisfied with spectrum mask. Finally, we analyse PAE according to modulation method and analyse PAE of OFDM and DFT-spread OFDM at same bit error rate.

DFT 확산 방식의 OFDM 통신 시스템에서 위상잡음과 직교 불균형 보상

DFT(Discrete Fourier Transform) 확산 방식의 OFDM(Orthogonal Frequency Division Multiplexing) 통신 시스템은 PAPR(Peak-to-Average Power Ratio) 저감에 매우 효과적인 시스템이며, 3GPP LTE(<TEX>$3^{rd}$</TEX> Generation Partnership Project Long Term Evolution)의 상향 링크에 SC-FDMA(Single Carrier-Frequency Division Multiple Access)에 사용된다. SC-FDMA는 일반적으로 OFDM 시스템보다 위상잡음과 직교 불균형으로 인한 ICI(Inter-Carrier Interference)에서 더 성능 열화가 발생하며, 등화기에 심각한 영향을 미친다. 그러므로 본 논문에서는 상향 링크에서 신호 전송 시 발생하는 위상잡음과 직교 불균형(IQ Imbalance: In-phase/Quadrature Imbalance), 그리고 전력 증폭기(HPA: High Power Amplifier)의 백-오프 특성에 따른 영향을 분석하고, ICI 성분을 제거할 수 있는 효과적인 등화 알고리듬을 제시한다. 제안된 등화기는 FDE(Frequency Domain Equalizer) 방식을 기반으로 설계하였으며, 기존의 PNS(Phase Noise Suppression) 알고리듬을 FDE에 사용될 수 있도록 수정하고 개선하여 위상 잡음과 직교 불균형으로 인한 ICI를 보상한다. 시뮬레이션 결과를 통하여 back-off 5.5 dB 상태에서 위상 잡음 <TEX>$0.06\;rad^2$</TEX>, 위상 에러 5도, 진폭 에러 0.005인 경우, 위상 잡음과 직교 불균형을 보상하여 SNR=14 dB 정도에서 <TEX>$BER=10^{-4}$</TEX>의 성능을 만족할 수 있다. DFT-spread OFDM(Discrete Fourier Transform-Spread Orthogonal Frequency Division Multiplexing) is very effective for solving the PAPR(Peak-to-Average Power Ratio) problem. Therefore, the SC-FDMA(Single Carrier-Frequency Division Multiple Access) which is basically same to the DFT spread OFDM was adopted as the uplink standard of the 3GPP LTE (<TEX>$3^{rd}$</TEX> Generation Partnership Project Long Term Evolution). Unlike the ordinary OFDM system, the SC-FDMA using DFT spreading method is vulnerable to the ICI(Inter-Carrier Interference) problem caused by the phase noise and IQ(In-phase/Quadrature) imbalance and effected FDE(Frequency Domain Equalizer). In this paper, the ICI effects from the phase noise and IQ imbalance which can be problems in uplink transmission are analyzed according the back-off level of HPA. Next, we propose the equalizer algorithm to remove the ICI effects. This proposed equalizer based on the FDE can be considered as up-graded and improved version of PNS(Phase Noise Suppression) algorithm. This proposed equalizer effectively compensates the ICI resulting from the phase noise and IQ imbalance. Finally, through the computer simulation, it can be shown that about SNR=14 dB is required for the <TEX>$BER=10^{-4}$</TEX> after ICI compensation when the back-off is 4.5 dB, <TEX>$\varepsilon=0.005$</TEX>, <TEX>$\phi=5^{\circ}$</TEX>, and <TEX>$pn=0.06\;rad^2$</TEX>.