Multicarrier Waveforms Research Articles

Multicarrier SAR Image Reconstruction Using Integrated MUSIC-LSE Algorithm

In synthetic aperture radar (SAR) applications, high-resolution images and effective estimation processes are vital for the reconstruction of any targets. This can be achieved by using multicarrier waveforms such as orthogonal frequency division multiplexing (OFDM) with the help of appropriate signal processing algorithms. However, the quality of the reconstructed image degrades in low signal-to-noise ratio (SNR) environments during SAR data acquisition. In this paper, an integrated multiple signal classification (MUSIC) assisted least square estimation (LSE) algorithm (MUSIC-LSE) is proposed to enhance the quality of the reconstructed SAR image in a low-SNR environment. Simulation results measured and evaluated the quality of the reconstructed image using three performance indicators of root-mean-square-error, main lobe width and cumulative side lobe levels. These indicators are also used to investigate the effect of OFDM subcarrier selection on the reconstructed image for a different number of subcarriers. Experimental validation of the approach is carried out using two steel pipes to image and detect the curvature of the steel pipes. The results show that the proposed MUSIC-LSE approach produces better-reconstructed images compared with the existing linear frequency modulated (LFM) chirp and OFDM-LSE approaches in low-SNR (−10 dB) environments and enables the radar to distinguish and detect the curvature of the pipes even below the radar range and cross-range resolution.

Automatic Multicarrier Waveform Classification via PCA and Convolutional Neural Networks

To make up the disadvantages of OFDM-QAM applied in 4G systems, some new non-orthogonal asynchronous PHY multicarrier techniques have been proposed for multiple 5G application scenarios, such as universal-filtered multicarrier (UFMC) and filterbank-based multicarrier (FBMC). Accurate classification of multiple multicarrier waveforms is indispensable in the future wireless communication systems. Here, we propose a new multicarrier waveforms classification system, which utilizes deep convolutional neural networks to classify OFDM-QAM, UFMC, and FBMC-OQAM. Since the classification accuracy is greatly reduced in AWGN environment, principal component analysis (PCA)-based processing method is proposed in this paper to suppress the AWGN and reduce the dimensions of inputs of convolutional neural networks. Amplitude data is used in this paper as input for CNN and shows promise when compared to which uses raw I/Q data as input. The dense channel environment effects including timing errors, carrier frequency offsets, and multi-fading channels are also considered in this paper and our proposed system can obtain the high classification accuracy in noisy channels.

Experimental Testbed of Post-OFDM Waveforms Toward Future Wireless Networks

This paper details an experimental testbed conceived for studying the capability of new multicarrier waveforms to accommodate 5G requirements. Testbed experiments are done with an implementation of cyclically prefixed orthogonal frequency division multiplexing (CP-OFDM) and its most promising enhancements, i.e., weighted overlap and add based OFDM (WOLA-OFDM) and block-filtered OFDM (BF-OFDM), with configurable universal software radio peripherals (USRPs)-based software defined radio prototype. These experiments are done in a realistic laboratory-like environment, where capabilities of the selected waveforms to accommodate 5G requirements are evaluated while focusing on the optimization of the energy efficiency. On one hand, we provide details and deign guidance to improve energy efficiency and robustness of the studied waveforms through new approaches of digital predistortion (DPD) and peak-to-average power ratio (PAPR) reduction in the presence of RF power amplifier (RF PA). In particular, we focus on the mitigation of in-band and out-of-band non-linear distortions and their effects on power spectrum density (PSD) and bit error rate, respectively. It has been demonstrated that the combination of PAPR reduction and DPD allows the transmitter to significantly improve the spectrum localization without sacrificing the in-band and out-of-band waveform quality, while achieving high power efficiency, thus operating the PA very close to its saturation region, as well. On another hand, we address the impact of the lack of synchronism between transmitters on the performance of the selected waveforms, which is of special relevance for future 5G massive machine type communications applications. Experimental results show that BF-OFDM and WOLA-OFDM would permit the accommodation of 5G requirements when RF PA issues are tackled. In some specific scenarios, ideal spectrum utilization can be realized by these waveforms, using only one tone as guard band while keeping good energy efficiency.

Unified Low Complexity Radix-2 Architectures for Time and Frequency-Domain GFDM Modem

Most of the conventional multicarrier waveforms explicitly or implicitly involve a generalized frequency division multiplexing (GFDM)-based modem as a core part of the baseband processing. Some are based on GFDM with a single prototype filter, e.g. orthogonal frequency division multiplexing (OFDM) and others employ multiple filters such as filter bank multicarrier (FBMC). Moreover, the GFDM degrees of freedom combined with multiple prototype filters design allow the development and optimization of new waveforms. Nevertheless, GFDM has been widely considered as a complex modulation because of the requirements of odd number of subcarriers or subsymbols. Accordingly, the current state of the art implementations consume high resources. One solution to reduce the complexity is utilizing radix-2 parameters. Due to the advancement in GFDM filter design, the constraint of using odd parameters has been overcome and radix-2 realization is now possible. In this paper, we propose a unified low complexity architecture that can be reconfigured to provide both time-domain and frequency-domain modulation/demodulation. The design consists of several radix-2 fast Fourier transform (FFT) and memory blocks, in addition to one complex multiplier. Moreover, we provide a unified architecture for the state of the art implementations, which is designed based on direct computation of circular convolution using parallel multiplier chains. As we demonstrate in this work, the FFTbased architecture is computationally more efficient, provides more flexibility, significantly reduces the resource consumption, and achieves similar latency for larger block size.

On MIMO‐UFMC in the Presence of Phase Noise and Antenna Mutual Coupling

AbstractThe universal filtered multicarrier (UFMC) technique has been proposed as a waveform candidate for the fifth generation (5G) communications and beyond 5G. Compared with conventional orthogonal frequency division multiplexing (OFDM), UFMC has lower out‐of‐band emission and is also compatible with the multiple‐input multiple‐output (MIMO) technique. However, like other multicarrier waveforms, it suffers from phase noise of imperfect oscillator. In contrast to the rich literature on phase noise effect on MIMO‐OFDM (where the antenna mutual coupling effect is usually omitted though), there is little work investigating the phase noise effect on MIMO‐UFMC. In this paper, we study the MIMO‐UFMC systems in the presence of phase noise and with/without mutual coupling effect. A phase noise mitigation scheme for MIMO‐UFMC systems is presented. The scheme does not require detailed knowledge of the phase noise statistics and can effectively mitigate the phase noise within each UFMC symbol. Moreover, it is shown that at small antenna separations, the performance of the MIMO‐UFMC system taking the mutual coupling effect into account is better than that when the mutual coupling effect is overlooked.

Subcarrier Assignment and Power Allocation for Device-to-Device Video Transmission in Rayleigh Fading Channels

Subcarrier assignment and power allocation for device-to-device (D2D) video transmission using a filter bank multicarrier waveform in a Rayleigh fading environment are investigated. We analyze the co-channel interference between D2D pairs, and propose a cross-layer algorithm with a subcarrier assignment outer loop and a power allocation inner loop, which aims to optimize the overall video quality. Unlike the non-convexity in physical layer power allocation for maximizing the total throughput, the cross-layer power allocation problem is convex under certain conditions, so a high quality solution for power allocation can be efficiently found. Simulation results demonstrate a higher overall video quality by the proposed cross-layer algorithm compared with baseline algorithms.

Optimal Radix-2 FFT Compatible Filters for GFDM

For a linear waveform, a finite condition number of the corresponding modulation matrix is necessary for the waveform to convey the message without ambiguity. Based on the Zak transform, this letter presents an analytical approach to compute the condition number of the modulation matrix for the multi-carrier waveform generalized frequency division multiplexing (GFDM). On top, we further propose a filter design that yields non-singular modulation matrices for an even number of subcarriers and subsymbols, which is not achievable for any previous work. Such new design has significant impact on implementation complexity, as the radix-2 FFT operations for conventional multicarrier waveforms can readily be employed for GFDM. Additionally, we analytically derive the optimal filter that minimizes the condition number.We further numerically evaluate the signal-to-interference ratio (SIR) and noise-enhancement factor (NEF) for matched filter (MF) and zero-forcing (ZF) GFDM receivers for such design, respectively.

Joint Device Positioning and Clock Synchronization in 5G Ultra-Dense Networks

In this paper, we address the prospects and key enabling technologies for highly efficient and accurate device positioning and tracking in fifth generation (5G) radio access networks. Building on the premises of ultra-dense networks as well as on the adoption of multicarrier waveforms and antenna arrays in the access nodes (ANs), we first formulate extended Kalman filter (EKF)-based solutions for computationally efficient joint estimation and tracking of the time of arrival (ToA) and direction of arrival (DoA) of the user nodes (UNs) using uplink reference signals. Then, a second EKF stage is proposed in order to fuse the individual DoA and ToA estimates from one or several ANs into a UN position estimate. Since all the processing takes place at the network side, the computing complexity and energy consumption at the UN side are kept to a minimum. The cascaded EKFs proposed in this article also take into account the unavoidable relative clock offsets between UNs and ANs, such that reliable clock synchronization of the access-link is obtained as a valuable by-product. The proposed cascaded EKF scheme is then revised and extended to more general and challenging scenarios where not only the UNs have clock offsets against the network time, but also the ANs themselves are not mutually synchronized in time. Finally, comprehensive performance evaluations of the proposed solutions on a realistic 5G network setup, building on the METIS project based outdoor Madrid map model together with complete ray tracing based propagation modeling, are provided. The obtained results clearly demonstrate that by using the developed methods, sub-meter scale positioning and tracking accuracy of moving devices is indeed technically feasible in future 5G radio access networks operating at sub-6 GHz frequencies, despite the realistic assumptions related to clock offsets and potentially even under unsynchronized network elements.

Multi‐carrier radar waveforms for communications and detection

The use of multi-carrier waveforms, such as Orthogonal Frequency Division Multiplexing (OFDM) as used in radio communication, for radar operations has gained strong interest recently. While the authors have completed research on using multi-carrier waveforms for simultaneous radar and communications operations, this study focuses on the recent research on using these waveforms for communications. Namely, they demonstrate how to employ OFDM to modulate Multi-Frequency Complementary Phase Coded sequences for wireless communications.

Performance of emerging multi-carrier waveforms for 5G asynchronous communications

This paper presents an extensive and fair comparison among the most promising waveform contenders for the 5G air interface. The considered waveform contenders, namely filter-bank multi-carrier (FBMC), universal-filtered multi-carrier (UFMC), generalized frequency-division multiplexing (GFDM) and resource-block filtered orthogonal frequency-division multiplexing (RB-F-OFDM) are compared to OFDM used in 4G in terms of spectral efficiency, numerical complexity, robustness towards multi-user interference (MUI) and resilience to power amplifier non-linearity. FBMC shows the best spectral containment and reveals to be almost insensitive to multi-user interference. It however suffers from its bad spectral efficiency for short bursts and from its poor multiple input multiple output (MIMO) compatibility. GFDM reveals to be the most promising contender, with the best spectral efficiency and the smallest complexity overhead compared to OFDM. It is also the most resilient to multi-user interference after FBMC and is MIMO compatible as soon as the interference can be managed. UFMC and RB-F-OFDM are finally the closest to OFDM and benefit therefore from a better compatibility with existing systems, even if their performance is generally lower than FBMC and GFDM.

On Spectral Coexistence of CP-OFDM and FB-MC Waveforms in 5G Networks

Future 5G networks will serve a variety of applications that will coexist on the same spectral band and geographical area in an uncoordinated and asynchronous manner. It is widely accepted that using cyclic prefix-orthogonal frequency division multiplexing (CP-OFDM), the waveform used by most current communication systems, will make it difficult to achieve this paradigm. Especially, CP-OFDM is not adapted for spectral coexistence because of its poor spectral localization. Therefore, it has been widely suggested to use filter bank-based multicarrier (FB-MC) waveforms with enhanced spectral localization to replace CP-OFDM. Especially, FB-MC waveforms are expected to facilitate coexistence with legacy CP-OFDM-based systems. However, this idea is based on the observation of the power spectral density of FB-MC waveforms only. In this paper, we demonstrate that this approach is flawed and show that interference between FB-MC and CP-OFDM systems should be rated on precise estimation of the error vector magnitude. Our analysis, which is confirmed through simulations on both flat and frequency selective channels and software radio implementation, shows that the interference caused by FB-MC waveforms on CP-OFDM receivers is multiple orders of magnitude higher than expected in the literature. Finally, our results show that using FB-MC waveforms does not facilitate coexistence with CP-OFDM-based systems to a high extent.

Multicarrier Waveform Harmonization and Complexity Analysis for an Efficient 5G Air Interface Implementation

The coexistence of multiple air interface variants in the upcoming fifth generation (5G) wireless technology remains a matter of ongoing discussion. This paper focuses on the physical layer of the 5G air interface and provides a harmonization solution for the joint implementation of several multicarrier waveform candidates. Waveforms based either on cyclic prefix-orthogonal frequency division multiplexing (CP-OFDM) or on filter bank multicarrier (FBMC) are first presented through a harmonized system model. Complexity comparisons among five different waveforms are provided. Then, the complexity of a proposed configurable hardware implementation setup for waveform transmission and reception is evaluated. As a result, the harmonized transmitter and receiver exhibit 25–40% and 15–25% less complexity in floating-point operations, respectively, in comparison to two standalone implementations of the most complex waveform instances of the CP-OFDM and FBMC families. This highlights the similarities between both families and illustrates the component reuse advantages associated with the proposed harmonized solution.

Adaptive filterbank-based multi-carrier waveform design for flexible data rates

Filterbank-based multicarrier (FBMC) scheme is one of the candidate waveforms to handle possible problems in next-generation communication systems, such as high out-of-band (OOB) emission and large peak-to-average power ratio (PAPR). In LTE (Long Term Evaluation), the frequency spectrum is shared by users with pre-defined bandwidths (i.e., 1,4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz), which is not a flexible solution for users having different data rate demands. Similarly, in conventional FBMC systems, bandwidth is divided into equal subbands. Recent studies on FBMC modulation have not provided an effective remedy for such users. Motivated by this requirement, we propose a novel FBMC waveform design that allows an adaptive bandwidth allocation scheme. Accordingly, users will not be restricted to the specified bandwidth schedules. In the paper, subband-based and subcarrier based filtering models are constructed with polyphase filter structures. The proposed structures are realized analytically and tested through computer simulations for different scenarios.

Low probability of intercept-based adaptive radar waveform optimization in signal-dependent clutter for joint radar and cellular communication systems.

In this paper, we investigate the problem of low probability of intercept (LPI)-based adaptive radar waveform optimization in signal-dependent clutter for joint radar and cellular communication systems, where the radar system optimizes the transmitted waveform such that the interference caused to the cellular communication systems is strictly controlled. Assuming that the precise knowledge of the target spectra, the power spectral densities (PSDs) of signal-dependent clutters, the propagation losses of corresponding channels and the communication signals is known by the radar, three different LPI based criteria for radar waveform optimization are proposed to minimize the total transmitted power of the radar system by optimizing the multicarrier radar waveform with a predefined signal-to-interference-plus-noise ratio (SINR) constraint and a minimum required capacity for the cellular communication systems. These criteria differ in the way the communication signals scattered off the target are considered in the radar waveform design: (1) as useful energy, (2) as interference or (3) ignored altogether. The resulting problems are solved analytically and their solutions represent the optimum power allocation for each subcarrier in the multicarrier radar waveform. We show with numerical results that the LPI performance of the radar system can be significantly improved by exploiting the scattered echoes off the target due to cellular communication signals received at the radar receiver.

Future evolution of public safety communications in the 5G Era

Public Safety (PS) or Professional Mobile Radio (PMR) communications technologies are undergoing significant transformation towards broadband. The Terrestrial Trunked Radio (TETRA) as one of the earliest critical communication systems has evolved from narrowband data in its first release to the current wideband data Tetra Enhanced Data Service (TEDS). TEDS gives higher data rates with up to several hundred kbit/s and very well fits into the existing PMR frequency landscape. The APCO25 suite of standards in North America had a similar evolution. While still providing essential PMR services like voice, group calls and messaging, the evolution of Public Safety communication must include data-rich broadband services to support a wide range of applications to enhance the operational capability of PMR users. Examples of services and applications include remote healthcare, video-streaming and health monitoring of the first responders. In comparison to mobile applications in the consumer market, PMR applications have severe requirements in terms of signal quality, security, reliability, and coverage. In addition, the usage patterns can be significantly different from the consumer market, as public safety officers must often face unexpected situations (e.g. emergency crisis) and still be able to rely on full traffic capacity and high Quality of Service (QoS) from the networks. The design of future PMR communications must take into consideration these requirements and needs. Significant advances have been achieved in the last years and are still under development focusing on the use of advanced signal processing techniques using flexible mul-ticarrier waveforms in PMR bands to satisfy emerging new data service needs in cohabitation with existing networks in the same frequency bands, to facilitate a smooth migration towards broadband systems and to increase spectrum efficiency. One example is Spectrum sharing, which is expected to bring substantial benefits for a better use of the radio spectrum, helping to mitigate the severe spectrum scarcity problem (mainly in the EU PS band). However, there are several issues that put a question mark on the deployment of PMR systems and their integration with fifth generation (5G) communication technologies and vision. First of all, the integration of PS system in LTE-Advanced (LTE-A) is still not optimized for voice calls (and especially not for the group calls), meaning that it will have to coexist for some time with the PMR conventional networks (e.g. TETRA) for basic voice services. Moreover, LTE-A is still not designed as a mission critical system. High availability and security features are not inherent in all the aspects – so some add-ons to the LTE-A standard or implementation scenarios have to be made, especially in the case of future interoperability and usage of commercial 5G networks for PS scenarios. In addition, standards for direct communications, like LTE Device to Device (D2D) or ad-hoc networks, must be still validated on the basis of the requirements of security and reliability. With proper implementation of these concepts, PMR users may rely on future 5G systems in the same manner they rely on the current narrowband PMR systems. Meanwhile, the design and integration of parallel or complementary usage of traditional PMR systems for voice communication and next generation PMR users in 5G systems for mission critical broadband data transfer still remain as open challenges. In particular, the research and standardization for internetworking of heterogeneous networks for PMR is still not completed. This special issue aims to investigate all aspects relevant to the development of the PMR communications systems and networks and their future evolvement in the fifth generation (5G) communication environment. We received over this special issue ten submissions altogether; out of which, eight were accepted through a rigorous peer review process. Note that in the following sections, the terms PMR and PS are used with the same meaning. In this context, the work presented in the paper Analysis of strategies for progressive 5G emergency network deployment by Vilhar et al. provides an analysis on network deployment strategies. It is evident that the temporary communication systems for emergency situations have traditionally been provided by a single communication technology. They are typically based on the 2G technologies for mobile radio that cannot meet the data rate requirements of contemporary applications. Emergency systems based on the 3GPP LTE technology are presently under investigation, and there is an increasing interest in including new 5G concepts. 5G may enable the employment of multiple heterogeneous communication systems such as fast deployable areal platforms, ad-hoc portable terrestrial stations and satellite communications, all of which integrated with the preserved part of the existing wireless infrastructure. Such an integrated approach is studied and evaluated in this work, based on the simultaneous use of the remaining terrestrial systems and additional LAPs (Low Amplitude Platforms). Simulations of the system were performed using the GRASS-RaPlaT tool, augmented

Generalized Multicarrier Radar: Models and Performance

Multicarrier waveforms bring several major advantages over single carrier waveforms in radar systems: frequency diversity, waveform diversity, short time on target and the possibility to optimize the transmitted waveforms, to mention a few. Interesting waveform designs utilizing diversity have been proposed in the literature already. In this paper, we develop a generalized model that can accommodate a wide variety of design options, both existing and novel ones, in an easy and intuitive way. The developed matrix equations for transmitter and receiver allow for implementing different waveforms simply by filling in the elements to corresponding matrices accordingly. Moreover, intuitive agile generation of waveforms in simulation environments and in practice is facilitated. Waveform optimization examples are provided using the derived model. A Mutual Information based criterion is employed to formulate the optimization problems which are solved analytically. Novel multicarrier spread spectrum waveforms are proposed and generated using the derived model. The radar performance of one of these waveforms is investigated through simulations. It is demonstrated that it can outperform well-known existing multicarrier waveforms. It is also shown that such waveform can lower the peak-to-average-power ratio due to the spreading operation, which is a benefit for the front end designs of the transmitter and receiver.

Joint optimization of peak-to-average power ratio and spectral leakage in 5G multicarrier waveforms

High peak-to-average-power ratio (PAPR) and spectral leakage are two main drawbacks of multicarrier systems, such as in orthogonal frequency division multiplexing (OFDM), in future 5G wireless communications. For generating optimized waveforms for 5G communications, this paper proposes an iterative scheme combining time-domain N-continuous OFDM (TD-NC-OFDM) and serial peak cancellation (SPC). Based on the theory of projection onto convex sets (POCS), the effectiveness of the proposed scheme is proved, and the optimized time-frequency domain multicarrier waveforms are analyzed in terms of a balanced tradeoff between out-of-band spectral leakage and PAPR. Both theoretical analysis and simulation results show that the proposed scheme can jointly optimize both the PAPR and out-of-band radiation, with moderate computational complexity.

MULTILEVEL INVERTER WITH LEVEL SHIFTING SPWM TECHNIQUE USING FEWER NUMBER OF SWITCHES FOR SOLAR APPLICATIONS

A multilevel inverter (MLI) is a popular inverter for solar based high power applications. The drawback of conventional H-bridge inverter is non-sinusoidal output voltage, which reduces the output quality of inverter. Later, the drawback of conventional Hbridge was overcome by conventional MLI. But, conventional MLI needs maximum number of diodes and switches. In order to overcome this drawback proposed MLI topology with level shifting sinusoidal pulse width modulation (SPWM) technique can be employed. Proposed MLI contain fewer number of switches and diodes, which helps in optimizing the circuit layout, reducing gate driver circuit for those switches. SPWM technique uses multicarrier waveforms with level shifting ensuring the reduction in total harmonics distortion (THD). In this Paper level shifting SPWM technique has been incorporated in which 5 kHz carrier wave is compare with 50Hz of sinusoidal wave with a modulation index of 0.8. THD of proposed 9-level inverter is 17.27% without filter and 4.29% with LC filter. Simulation of proposed inverter is carried out in MATLAB/SIMULINK.

Frequency-Shift Offset-QAM for GFDM

This paper presents a novel perspective to apply the offset quadrature amplitude modulation (OQAM) scheme on top of the multicarrier waveform termed Generalized Frequency Division Multiplexing (GFDM). The conventional time-shift OQAM is described for GFDM and, with the introducing of the general use of unitary transform, an interesting counterpart, i.e., frequency-shift OQAM, is proposed. The conventional long prototype pulse with time-shift of one half subsymbol becomes a short prototype pulse with frequency-shift of one half subcarrier. The frequency-shift OQAM scheme offers advantages such as low out-of-band emission and low implementation complexity. The concept can be applied to the broader scope of filtered OFDM without penalties in terms of performance in time variant frequency-selective channels.

Waveform design solutions for EHF broadband satellite communications

The problematic RF environment experienced by broadband satellite communications at EHF frequency bands, in particular Q/W bands, call for the use of novel waveforms. This paper presents a detailed comparison of several waveforms in presence of nonlinear distortions and typical values of phase noise introduced at Q/W band. Two main types of waveforms have been compared: Constant Envelope multicarrier waveforms (CE-OFDM and CE-SCFDMA) and single carrier impulse-based waveforms (TH-UWB, DS-UWB and PSWF-based PSM). This comparison will allow to draw some practical guidelines for the waveforms design of EHF broadband satellite communications.