OFDM Waveform Research Articles

Cross-Domain Dual-Functional OFDM Waveform Design for Accurate Sensing/Positioning

Cross-Domain Dual-Functional OFDM Waveform Design for Accurate Sensing/Positioning

Coprime-Based Frame-Level Sensing OFDM Waveform for 5G NR Integrated Radar Sensing and Communication via Downlink Sensing

Coprime-Based Frame-Level Sensing OFDM Waveform for 5G NR Integrated Radar Sensing and Communication via Downlink Sensing

PAPR Reduction of CC-OFDM MIMO Radar Waveform using Golay Codes

The development of modern radar technology demands multi-functional features that lead to the importance of designing a flexible, software-controlled digital waveform generator. The CC-OFDM MIMO radar waveforms allow flexibility in subcarrier management; hence it is possible to generate multiple narrow beams which transmitted simultaneously for providing all time coverage of the area.However, the main issue of OFDM is the high level of its PAPR value. This will cause non-linear distortion that potentially ruin the orthogonality of the OFDM subcarriers and in the end gives error in the radar detection results. In this paper, PAPR reduction using phase coding is done, by comparing the performance of complementary Hadamard, complementary Golay codes and combining it with Selective Mapping (SLM). The PAPR results show that complementary Golay codes without SLM are superior to complementary Hadamard with or without SLM. The maximum PAPR value of 65 beams transmitted simultaneously are 4.9 dB and 11.1 dB for CC OFDM waveform using complementary Golay codes and Hadamard respectively.

An Improved PAPR Reduction Scheme using Green OFDM

OFDM has become popular method to be applied in data communication as it has high data rate. Selective Mapping OFDM (SLM OFDM) is introduced to over-come the OFDM’s disadvantage which is high PAPR. SLM OFDM offers PAPR reduction by selecting OFDM waveform with the lowest PAPR among many copies of waveform candidates, U. As the world become more digitalized and demanding, Green OFDM shows up with more candidates, U^2/4, without making computation more complex. More candidates will promote more choices with lower PAPR values compare to the SLM OFDM. In the recent years, re-searchers have come out with a new improved Green OFDM version 2 with more waveform candidates to be chosen, U^2. The improved Green OFDM ver-sion 2 scheme will produce lower PAPR values compare to the SLM OFDM and the original Green OFDM. Thus, technologies with higher data transmission are able to be created.

Demonstration of MIMO-SAR Echo Separation Scheme for Improved OFDM Waveforms With Airborne X-Band DBF-SAR

Demonstration of MIMO-SAR Echo Separation Scheme for Improved OFDM Waveforms With Airborne X-Band DBF-SAR

OFDM Waveform Design with Subcarrier Interval Constraint for Narrowband Interference Suppression in ISAC Systems

OFDM Waveform Design with Subcarrier Interval Constraint for Narrowband Interference Suppression in ISAC Systems

Multi-User Downlink Beamforming Using Uplink Downlink Duality With CEQs for Frequency Selective Channels

High-resolution fully digital transceivers are infeasible at millimeter-wave (mmWave) due to their increased power consumption, cost, and hardware complexity. The use of low-resolution converters is one possible solution to realize fully digital architectures at mmWave. In this paper, we consider a setting in which a fully digital base station with constant envelope quantized (CEQ) digital-to-analog converters on each radio frequency chain communicates with multiple single antenna users with individual signal-to-quantization-plus-interference-plus-noise ratio (SQINR) constraints over frequency selective channels. We first establish uplink downlink duality for the system with CEQ hardware constraints and OFDM-based transmission considered in this paper. Based on the uplink downlink duality principle, we present a solution to the multi-user multi-carrier beamforming and power allocation problem that maximizes the minimum SQINR over all users and sub-carriers. We then present a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">per sub-carrier</i> version of the originally proposed solution that decouples all sub-carriers of the OFDM waveform resulting in smaller sub-problems that can be solved in a parallel manner. Our numerical results based on 3GPP channel models generated from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Quadriga</i> demonstrate improvements in terms of ergodic sum rate and ergodic minimum rate over state-of-the-art linear solutions. We also show improved performance over non-linear solutions in terms of the coded bit error rate with the increased flexibility of assigning individual user SQINRs built into the proposed framework.

Generation of OFDM Waveform with 4.8 GHz Bandwidth Passing Through Terahertz and Millimeter-Wave Bands in Beyond 5G System

Generation of OFDM Waveform with 4.8 GHz Bandwidth Passing Through Terahertz and Millimeter-Wave Bands in Beyond 5G System

Adapt and Aggregate: Adaptive OFDM Numerology and Carrier Aggregation for High Data Rate Terahertz Communications

We propose a communication framework suitable for data rate maximization in the (THz) bands using adaptive (OFDM) numerology and carrier aggregation. OFDM is a widely adopted waveform due to the simplicity of its implementation and its effectiveness in combating frequency selectivity when the numerology is carefully chosen. However, it suffers from a multitude of limitations, including phase noise due to local oscillator inaccuracies, high peak-to-average power ratio, and is particularly sensitive to time-frequency synchronization errors, which can considerably impact its performance. This is especially relevant at THZ frequencies where larger-than-usual bandwidth is available, and the choice of the numerology should be carefully made given the intrinsic transceiver constraints. Moreover, the abundance of frequency resources in the THZ band imposes new design challenges that should be addressed, especially since the bandwidth usability at these frequencies depends on the communication distance. Hence, we propose a dynamic OFDM numerology adaptation mechanism, where the bandwidth of a CC covered by a single OFDM waveform is changed. For each (CC), the Component Carrier Data Rate (CCDR) is evaluated while considering the effect of both hardware impairments and the wireless channel statistics. We further propose the adoption of a dynamic distance-aware CC allocation such that the available frequency resources are fully utilized, and maximize an (ADR) through the aggregation of several CCs. Simulation results show that the proposed approach yields the highest ADR out of all possible setups.

Chip-to-Chip Interfaces for Large-Scale Highly Configurable mmWave Phased Arrays

This article presents a chip-to-chip (C2C) interface for constructing reconfigurable phased arrays to be used in fifth-generation (5G)/sixth-generation (6G) wireless systems. The C2C interface further facilitates building phased array panels by allowing the use of grid-based PCB routing, thus providing flexibility in the system design. An eight-element RFIC capable of handling two independent data-streams is fabricated using 45-nm CMOS technology. The RFIC incorporates four C2C interfaces operating at 27 GHz, two C2C interfaces operating at 9 GHz, and a complex baseband (BB) with single-sided bandwidth in excess of 400 MHz. The architecture is tested by flip-chip bonding two fabricated RFICs on an eight-layer Megtron 7 PCB. In this article, only the receiver path of the RFIC and the phased array is described. Performance of both the single RFIC and the combination using the 27-GHz C2C interface is demonstrated using conductive and over-the-air (OTA) measurements. OTA measurements are conducted using 5GNR FR2 OFDM waveforms with a signal bandwidth of up to 800 MHz. The measured RF to BB conversion gain for a single element is larger than 23 dB and the minimum measured noise figure (NF) is 6.2 dB. The nominal dc power consumed by the receiver per element per stream is 116.5 mW. The RFIC occupies a normalized area per element per data stream of 2.7 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^2$</tex-math> </inline-formula> . The RFIC is capable of supporting dual-polarized antennas or in a large-scale panel utilizing the same antenna elements to two independently weighted data streams as part of the hybrid beamforming architecture.

Dual-carrier multiplexed laser-based hybrid transmitter for high data-rate indoor optical wireless communication

In this paper we experimentally demonstrate dual-carrier modulation scheme to achieve spectrally efficient OFDM transmission in an optical wireless communication link using a hybrid laser-LED transmitter. The hybrid transmitter used in this work consists of two near infrared laser diodes at 785 nm and 808 nm used to separately modulate the real and imaginary components of the complex OFDM waveforms, collinearly combined and subsequently mixed with white LED array at a diffuser surface for indoor illumination. At the receiver end, two separate high-speed photodetectors with spectral filters are used to detect the real and imaginary components and subsequently combined to generate the complex OFDM waveforms. The optical system performance is characterized to quantify the beam spread, spot-size and spectral content of the laser beams and white LED array. The communication performance is also characterized using 16, 32, and 64-QAM/OFDM with 1, 1.25 and 1.5 Giga-samples/second sampling rates. The best communication performance with a net data-rate of 4.2 Gbps for 64-QAM/OFDM at an optimum sampling rate of 1.5 G-Samples/second is demonstrated. The dual-carrier modulation scheme studied in this work helps achieve improved spectral efficiency for OFDM transmission in an OWC link using dual transmitter-receiver pair, while at the same time requiring simplified electronic front-ends and encoding/decoding algorithms.

Sensing Using Coded Communications Signals

A key challenge for common waveforms for Integrated Sensing and Communications – widely regarded as a resource-efficient way to achieve high performance for both functionalities – lies in leveraging information-bearing channel-coded communications signal(s) (c.c.s) for sensing. In this paper, we investigate the range-Doppler sensing performance of c.c.s in multi-user interference-limited scenarios, and show that it is affected by sidelobes whose form depends on whether the c.c.s modulates a single-carrier or OFDM waveform. While uncoded signals give rise to asymptotically zero sidelobes due to the law of large numbers, it is not obvious that the same holds for c.c.s, as structured codes (e.g., linear block codes) induce dependence across codeword symbols. In this paper, we show that c.c.s also give rise to asymptotically zero sidelobes – for both single-carrier and OFDM waveforms – by deriving upper bounds for the tail probabilities of the sidelobe magnitudes that decay as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\exp ( - O(\text {code rate} \times \text {block length}))$ </tex-math></inline-formula> . Consequently, for any code rate, c.c.s are effective sensing signals that are robust to multi-user interference at sufficiently large block lengths, with negligible difference in performance based on whether they modulate a single-carrier or OFDM waveform. We verify the latter implication through simulations, where we observe the sensing performance (i.e., the detection and false-alarm probabilities) of a QPSK-modulated c.c.s (code rate = 120/1024, block length = 1024 symbols) to match that of a comparable interference-free FMCW waveform even at high interference levels (SIR of −11dB), for both single-carrier and OFDM waveforms.

Low-PAPR OFDM Waveform Design for Radar and Communication Systems

In this correspondence we address the problem of designing OFDM waveforms with low peak-to-average power ratio (PAPR) for joint radar and communication (JRC) applications. The design problem is formulated as the minimization of PAPR with the constraint that the integrated periodic autocorrelation sidelobe level is zero. We propose an iterative algorithm based on the majorization-minimization (MM) technique to solve this design problem. The proposed algorithm yields OFDM waveforms with PAPR values close to one. The algorithm can also be used to design waveforms with spectral nulls. In extensive numerical simulations we compare the performance of our algorithm with that of a recently proposed benchmark.

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.

LED non-linearity mitigation in a NOMA-OFDM VLC system using a union of precoder and companding

The power domain non-orthogonal multiple access (NOMA) has been considered as an outstanding candidate for 5G visible light communication networks. Due to the various salient features like robustness to multi-path fading, low latency, high compatibility with multiple input multiple output systems, etc. orthogonal frequency division multiplexing access (OFDM) is foreseen to exist in 5G systems. NOMA-based optical OFDM (O-OFDM) is expected to be a promising candidate for supporting high rate internet connectivity, especially for green indoor mobile networks. The OFDM waveform suffers from high PAPR issue. The high PAPR degrades the BER and also enhances the non-linearity in an O-OFDM system. In this paper, a hybrid technique using a blend of precoder with compander is implemented to reduce the PAPR of NOMA based DC-biased O-OFDM (DCO-OFDM) system. The proposed NOMA DCO-OFDM system exhibits a low PAPR of only 4.3 dB. Also, for a reference bit error rate of 10\(^{-4}\), the proposed technique displays an error vector magnitude of 13.2 dB.

A Transmitter Design for the Multi-beam CC-OFDM Azimuth Scanning MIMO Radar

A conventional phased array radar provides a high gain by transmitting coherent signals over a large number of antenna elements. However, it does not provide a full time-on-target because it produces beams that scan the whole angular view of the radar. This paper describes an implementation of circulating codes (CC) in azimuth scanning MIMO radar that adopts OFDM waveform. The CC MIMO technique can produce a beam toward a certain angle by applying an appropriate delay difference between the MIMO antennas that transmit the same waveform across the azimuth. The adoption of OFDM waveform in the CC MIMO radar, gives the possibility to apply the phase difference for beamforming in the frequency domain of the OFDM signals. CC-OFDM MIMO with spectrum division into sub-bands and use of orthogonal codes, allow for simultaneous transmission of orthogonal OFDM symbols that generate orthogonal beams toward different angles. This strategy makes it possible for the radar system to have multiple and simultaneous beamforming to provide a full-time coverage. Furthermore, the transmission of a series of orthogonal OFDM symbols in each beam enables the detection of long-range targets. By taking as an example the design of long-range surveillance radar, the paper discusses further issues pertinent to the implementation of the CC OFDM MIMO radar, including the beam squinting problem and its remedy, arrangement of the multiple beams, the radar system structure, use of Golay codes for orthogonalization and PAPR mitigation, use of software-defined radar that eliminates the need of amplitude and phase control in each transmitter, transmit scheduling for multi-beam long-range target detection, and multidimensional ambiguity function analysis. The latter indicates that the CC OFDM MIMO radar can achieve a high resolution in angle, range, and velocity, with beam isolation of -30 dB for neighboring beams and co-channel beams.

Polarization Combining and Equalization in 5G Mobile-to-Mobile Systems

This article develops maximum likelihood (ML) detection for post-FFT processing of dual-polarized antenna outputs with a cyclic-prefix OFDM (CP-OFDM) waveform for a frequency selective multipath fading in a 5G mobile-to-mobile setting. The suggested maximum likelihood detector (MLD) comprises a combiner applied to the channel matched filter outputs (designated MLC) followed by a decision rule based on correlation and signal energy. When MLC is coupled with a frequency-domain equalizer, this structure is called MLC+FDE. The designed MLD and MLC+FDE are compared to the traditional combining techniques: maximum ratio combining, equal gain combining, and selection combining. Computer simulations performed over a stochastic channel model with polarization state information. The simulation results show that the difference between MLD and maximum ratio combining (the best performing method), MLC+FDE (almost the best performing method) and selection diversity with frequency-domain equalization (the worst performing method) is 2 dB. This diversity improvement is limited by the correlation between the two channels that characterize the two polarization states. This correlation makes it difficult to achieve reliable transmission through combining alone; some form of error control coding is also needed. When channel estimates are used in place of perfect channel knowledge, an error floor is observed.

5G F-OFDM Waveform Based Software-Defined Radio Technology

Filtered-orthogonal frequency division multiplexing (F-OFDM) is a quasi-orthogonal waveform candidate for the applications of the fifth generation (5G) communication system. In this study, an F-OFDM waveform with unequal sub-band sizes is proposed to improve the spectrum efficiency (SE) of the 5G system. The proposed waveform is modeled with the Blackman window-sinc filter and is developed based on the software-defined radio (SDR) technology for practical implementation. The result shows that the F-OFDM performance of the simulation and hardware implementation is approximately the same. The SE using the proposed F-OFDM waveform is 6% and 5.8% higher than the SE using the conventional OFDM waveform under the simulation in the LabVIEW NXG simulator and under the practical use in the universal software radio peripheral (USRP) platform, respectively.

Towards an OFDM radar waveform for detection of far located targets with relatively low radar cross sections

The joint use of the OFDM waveform for radar as well as for communications has gained high attention recently. One problem that arises from using this waveform is the high spur level that appears on the range-velocity profile where it may eclipse an existing target of relatively small radar cross-section (RCS). This spur is due to nulling some subcarriers such as the DC-subcarrier. Another problem that results from using this waveform is the small maximum range which is far less than the unambiguous range. In our paper, we propose two diverse solutions to overcome the aforementioned problems. The first is based on using the Autoregressive (AR) model to address the problem of the high spur level. The second relies on repeating the same QAM modulated symbol through some subcarriers of the same OFDM symbol to address the problem of the relatively small maximum range. Our analysis and simulations show that applying our proposed methodology contributes to efficiently reducing the spur level from one side, and extending the maximum range to the limit of the unambiguous range from the other side. Furthermore, a trade-off is offered between the data rate of the communication system and the maximum range of the radar.

Novel filter bank-based cooperative spectrum sensing under practical challenges for beyond 5G cognitive radios

Cognitive radio (CR) technology with dynamic spectrum management capabilities is widely advocated for utilizing effectively the unused spectrum resources. The main idea behind CR technology is to trigger secondary communications to utilize the unused spectral resources. However, CR technology heavily relies on spectrum sensing techniques which are applied to estimate the presence of primary user (PU) signals. This paper firstly focuses on novel analysis filter bank (AFB) and FFT-based cooperative spectrum sensing (CSS) techniques as conceptually and computationally simplified CSS methods based on subband energies to detect the spectral holes in the interesting part of the radio spectrum. To counteract the practical wireless channel effects, collaborative subband-based approaches of PU signal sensing are studied. CSS has the capability to relax the problems of both hidden nodes and fading multipath channels. FFT- and AFB-based receiver side sensing methods are applied for OFDM waveform and filter bank-based multicarrier (FBMC) waveform, respectively, the latter one as a candidate beyond-OFDM/beyond-5G scheme. Subband energies are then applied for enhanced energy detection (ED)-based CSS methods that are proposed in the context of wideband, multimode sensing. Our first case study focuses on sensing potential spectral gaps close to relatively strong primary users, considering also the effects of spectral regrowth due to power amplifier nonlinearities. The study shows that AFB-based CSS with FBMC waveform is able to improve the performance significantly. Our second case study considers a novel maximum–minimum energy detector (Max–Min ED)-based CSS. The proposed method is expected to effectively overcome the issue of noise uncertainty (NU) with remarkably lower implementation complexity compared to the existing methods. The developed algorithm with reduced complexity, enhanced detection performance, and improved reliability is presented as an attractive solution to counteract the practical wireless channel effects under low SNR. Closed-form analytic expressions are derived for the threshold and false alarm and detection probabilities considering frequency selective scenarios under NU. The validity of the novel expressions is justified through comparisons with respective results from computer simulations.