Radio Frequency Signal Processing Research Articles

High-Intensity Focused Ultrasound Lesion Detection Using Adaptive Compressive Sensing Based on Empirical Mode Decomposition.

Background:The main goal of ultrasound therapy is to have clinical effects in the tissue without damage to the intervening and surrounding tissues. Treatments have been developed for both in vitro and in clinical applications. HIFU therapy is one of these. Non-invasive surgeries, such as HIFU, have been developed to treat tumors or to stop bleeding. In this approach, an adequate imaging method for monitoring and controlling the treatment is required.Methods:In this paper, an adaptive compressive sensing representation of ultrasound RF echo signals is presented based on empirical mode decomposition (EMD). According to the different numbers of intrinsic mode functions (IMFs) produced by the EMD, the ultrasound signals is adaptively compressive sampled in the source and then adaptively reconstructed in the receiver domains. In this paper, a new application of compressive sensing based on EMD (CS-EMD) in the monitoring of high-intensity focused ultrasound (HIFU) treatment is presented. Non-invasive surgeries such as HIFU have been developed for various therapeutic applications. In this technique, a suitable imaging method is necessary for monitoring of the treatment to achieve adequate treatment safety and efficacy. So far, several methods have been proposed, such as ultrasound radiofrequency (RF) signal processing techniques, and imaging methods such as X-ray, MRI, and ultrasound to monitor HIFU lesions.Results:In this paper, a CS-EMD method is used to detect the HIFU thermal lesion dimensions using different types of wavelet transform. The results of the processing on the real data demonstrate the potential for this technique in image-guided HIFU therapy.Conclusions:In this study, a new application of compressive sensing in the field of monitoring of the HIFU treatment is presented. To the best of our knowledge, so far no studies on compressive sensing have been carried out in the monitoring of the HIFU. Based on the results obtained, it was showed that the number of measurements and Intrinsic Mode Functions have the function of noise reduction. In addition, results were shown that the successful reconstruction of the compressive sensing signals can be gained using a threshold based algorithm. To this end, in this paper it was shown that by selecting an suitable number of measurements, the sparse transform, and a thresholding algorithm, we can achieve a more accurate detection of the HIFU thermal lesion size.

Analog Precoding Designs for Millimeter Wave Communication Systems

Millimeter wave (mmWave) communication is expected to be a key technology for 5G networks, because it can provide high data rates and large available bandwidth. Large antenna array is necessary to be applied to mmWave system to overcome the high path loss. Restricted by power consumption and system complexity, traditional digital precoding cannot be used for mmWave system. Therefore, analog radio frequency signal processing is introduced to simplify the architecture of precoder. In this paper, we propose dominant vector approaching (DVA) searching method for analog precoder design. Assuming that the channel state information is known at both transmitter and receiver sides, we decompose the analog precoding problem into a series of subproblems. For each subproblem, we just need to optimize one column of analog precoding matrix. Then, we can apply DVA searching method to solve these subproblems under unit modulus constraint. For the fully-connected structure (FS) and the partially-connected structure (PS), we propose DVA-FS and DVA-PS as our precoding designs, respectively. In addition, we prove that DVA searching method finds the near-optimal solutions for the nonconvex subproblems. Through simulation, we show that DVA algorithms achieve near-optimal performance with analog precoder in different scenarios. Besides, we investigate the effect of quantization and the energy efficiency of DVA algorithms, and find that they perform well with rough quantization.

Editorial Special Issue on Advances in Integrated Microwave Photonics

The field of integrated microwave photonics explores the incorporation of photonic integration technology for the generation, processing, and measurement of radio-frequency and microwave signals and is one of the fastest growing fields in signal processing. The last 10 years have seen a strong trend towards integration of microwave photonics functionalities on a single chip, with the scope to reduce size, weight, power, and cost.

Few-mode fiber true time delay lines for distributed radiofrequency signal processing.

We report, for the first time to our knowledge, distributed radiofrequency signal processing built upon true time delay operation on a step-index few-mode fiber. Two 3-sample configurations with different time delay properties are implemented over the same 60-meter 4-LP-mode fiber link. The inscription of a long period grating at a specific fiber position converts part of the LP01 mode into the LP02, permitting sample time delay engineering. Delay line performance is experimentally demonstrated when applied to radiofrequency signal filtering, example of fiber-distributed processing functionality exhibiting one order or magnitude gain in terms of compactness.

Parallel radio-frequency signal-processing unit based on mode multiplexed photonic integrated circuit.

We propose and demonstrate a monolithically integrated mode division multiplexing (MDM) photonic system for parallel radio-frequency (RF) signal processing using a silicon platform. Two independent lowpass integrated microwave photonic filters (IMPFs) are successfully demonstrated using the proposed scheme. Full integration of active and passive devices ensures small footprint and good performance for microwave photonic applications. The adoption of MDM extends the parallel processing capability of a single wavelength line. The first order transverse electric (TE1) and second order transverse electric (TE2) modes are utilized for two IMPFs, with minimum bandwidth 3.7 and 3.8 GHz, respectively. Furthermore, by elaborately controlling the micro-ring based mode convertors, the continuous tuning of bandwidth is achieved from 3.7 to 8 GHz for TE1 and 3.8 to 7.6 GHz for TE2 based IMPFs.

Lithium Niobate Phononic Crystals for Tailoring Performance of RF Laterally Vibrating Devices.

This paper reports the first demonstration of phononic crystals (PnCs) in suspended lithium niobate thin films, which exhibit band gaps for tailoring the performance of laterally vibrating devices. Transmission and reflection properties of lithium niobate PnCs for both shear-horizontal (SH0) and length-extensional (S0) modes have been investigated and subsequently explored in two applications. In the first case, PnC-embedded delay lines were designed for filtering with stopbands, while in the second case, PnC-bounded resonators were implemented for spurious mode suppression. Equivalent circuit models incorporating acoustic scattering parameters of the designed PnCs and Mason's model of the transducers have been built for each application. Benchmarked to reference devices without PnCs, the measured PnCs embedded in delay lines show 20-dB attenuation in the stopbands and less than 2-dB loss in the passbands for the SH0 mode, and 30-dB attenuation in the stopbands and less than 10-dB loss in the passbands for the S0 mode. The fabricated piezoelectric PnC-bounded resonator has shown a quality factor of 434 at 142.7 MHz with undesired spurious modes significantly suppressed. These demonstrations show that lithium niobate PnCs for laterally vibrating devices can potentially lead to wideband and low-loss acoustic functions for radio frequency signal processing.

Microwave Signal Processing over Multicore Fiber

We review the introduction of the space dimension into fiber-based technologies to implement compact and versatile signal processing solutions for microwave and millimeter wave signals. Built upon multicore fiber links and devices, this approach allows the realization of fiber-distributed signal processing in the context of fiber-wireless communications, providing both radiofrequency access distribution and signal processing in the same fiber medium. We present different space-division multiplexing architectures to implement tunable true time delay lines that can be applied to a variety of microwave photonics functionalities, such as signal filtering, radio beamsteering in phased array antennas or optoelectronic oscillation. In particular, this paper gathers our latest work on the following multicore fiber technologies: dispersion-engineered heterogeneous multicore fiber links for distributed tunable true time delay line operation; multicavity devices built upon the selective inscription of gratings in homogenous multicore fibers for compact true time delay line operation; and multicavity optoelectronic oscillation over both homogeneous and heterogeneous multicore fibers.

Tunable Multiband Microwave Photonic Filters

The increasing demand for multifunctional devices, the use of cognitive wireless technology to solve the frequency resource shortage problem, as well as the capabilities and operational flexibility necessary to meet ever-changing environment result in an urgent need of multiband wireless communications. Spectral filter is an essential part of any communication systems, and in the case of multiband wireless communications, tunable multiband RF filters are required for channel selection, noise/interference removal, and RF signal processing. Unfortunately, it is difficult for RF electronics to achieve both tunable and multiband spectral filtering. Recent advancements of microwave photonics have proven itself to be a promising candidate to solve various challenges in RF electronics including spectral filtering, however, the development of multiband microwave photonic filtering still faces lots of difficulties, due to the limited scalability and tunability of existing microwave photonic schemes. In this review paper, we first discuss the challenges that were facing by multiband microwave photonic filter, then we review recent techniques that have been developed to tackle the challenge and lead to promising developments of tunable microwave photonic multiband filters. The successful design and implementation of tunable microwave photonic multiband filter facilitate the vision of dynamic multiband wireless communications and radio frequency signal processing for commercial, defense, and civilian applications.

Agile photonic fractional Fourier transformation of optical and RF signals

The last 20 years have seen the spectacular emergence of optically assisted solutions for the analog generation and processing of radio-frequency (RF) signals [J. Lightwave Technol.27, 314 (2009)JLTEDG0733-872410.1109/JLT.2008.2009551]. Among these, real-time Fourier transformation (FT) is of particular importance for signal processing and filtering, but is of limited use for analyzing signals with time-dependent spectrum, especially chirped RF waveforms. On the contrary, fractional Fourier transformation (FrFT), which decomposes a waveform onto a continuous basis of linearly chirped functions, provides a generalization of FT and constitutes a valuable tool for analyzing signals with time-dependent spectrum [Signal Process.91, 1351 (2011)10.1016/j.sigpro.2010.10.008]. Here we prove a new and simple concept, enabling agile computation of the FrFT of both optical and RF signals in real time, with minimum latency time and a frequency resolution in the tens of kHz range. We demonstrate two practical applications of the technique: the absolute measurement of RF/optical chirp rates and the detection of weak chirped RF signals buried under noise. The introduced concept should be of practical interest for RF signal filtering and radar signal processing.

An All-Passive Negative Feedback Network for Broadband and Wide Field-of-View Self-Steering Beam-Forming With Zero DC Power Consumption

This paper presents an all-passive negative feedback network that performs autonomous radio-frequency (RF) front-end beam-forming and dynamic beam-tracking toward the direction of the incident RF signal. The proposed feedback network consists of a passive RF signal processing network, voltage rectifiers, and voltage-controlled delay-line phased shifters, all of which are passive-only circuits. The negative feedback loop is realized by passive phase detection, phase-to-voltage conversion, and voltage-controlled phase shifting, achieving a large loop-gain and autonomous operation with zero DC power consumption. The nonlinear behavior of the loop is exploited to substantially expand the array field of view (FoV). A proof-of-concept broadband four-element all-passive self-steering beam-former at 5 GHz with a wide FoV is implemented in a standard 130 nm CMOS process. A high-quality four-element synthesized array factor is measured for the input progressive phase shift $\phi _{\mathrm {in}}$ from −180° to 180°. When the proposed negative feedback loop is enabled, the normalized array factor is −2.87/−2.8 dB at $\phi _{\mathrm {in}}= + 90^{\circ }$ /−90° with an input RF power $\text{P}_{\mathrm {in}}$ of −17 dBm/element at 5 GHz, achieving >25 dB array factor improvement over the open-loop operation. Moreover, the nonlinear feedback loop allows for significant array factor improvement even at $\phi _{\mathrm {in}} = +180^{\circ }$ /−180°. The proposed beam-former also achieves high-quality self-steering beam-forming from 4 to 5.68 GHz with 34.7% fractional bandwidth. Therefore, the proof-of-concept all-passive self-steering beam-former outperforms the state-of-the-art active designs in terms of beam-forming quality, FoV, and fractional bandwidth. To the best of the authors’ knowledge, this is the first demonstration of an all-passive negative feedback network for a broadband and wide FoV self-steering beam-forming with zero DC power consumption.

Dispersion-engineered multicore fibers for distributed radiofrequency signal processing.

We report a trench-assisted heterogeneous multicore fiber optimized in terms of higher-order dispersion and crosstalk for radiofrequency true time delay operation. The analysis of the influence of the core refractive index profile on the dispersion slope and effective index reveals a tradeoff between the behavior of the crosstalk against fiber curvatures and the linearity of the propagation group delay. We investigate the optimization of the multicore fiber in the framework of this tradeoff and present a design that features a group delay relative error below 5% for an optical wavelength range up to 100 nm and a crosstalk level below -80 dB for bending radii larger than 103 mm. The performance of the true time delay line is validated in the context of microwave signal filtering and optical beamforming for phased array antennas. This work opens the way towards the development of compact fiber-integrated solutions that enable the implementation of a variety of distributed signal processing functionalities that will be key in future fiber-wireless communications networks and systems.

Channel Estimation and Equalization in Multiuser Uplink OFDMA and SC-FDMA Systems Under Transmitter RF Impairments

Single-carrier frequency-division multiple access (SC-FDMA), which is a modified form of orthogonal frequency-division multiple access (OFDMA), has been adopted as the uplink physical-layer radio access technique for the Third-Generation Partnership Project Long-Term Evolution (3GPP-LTE) and LTE-Advanced. Radio transceiver implementations for such OFDM-based systems with the direct-conversion architecture are desirable to enable small-size, low-cost, and low-power-consumption terminals. However, the associated circuit impairments stemming from the processing of analog radio frequency (RF) signals, such as in-phase and quadrature-phase (I/Q) imbalance and carrier frequency offset errors, can severely degrade the obtainable link performance. In this paper, we analyze the effects of these radio impairments in a multiuser SC-FDMA uplink system and present digital-signal-processing-based methods for the joint estimation and equalization of impairments and channel distortions on the receiver side with an arbitrary number of receiver antennas. For the equalization, linear equalizers such as the zero-forcing (ZF) and the minimum mean square error (MMSE) equalizers that utilize pairs of mirror subcarriers are formulated, and the MMSE equalizer is developed to effectively handle mirror subband users with different power levels. Furthermore, for reduced computational complexity, the joint channel and impairment filter responses are efficiently approximated with polynomial-based basis function models. The parameters of the basis functions are then estimated by exploiting the time-multiplexed reference symbols in the LTE uplink subframe structure. The performance of the proposed estimation and equalization methods is assessed with extensive multiuser link simulations, with both single-antenna and dual-antenna base-station receivers, and the results show that the proposed algorithms are able to significantly reduce the impact of channel distortions and radio impairments. The resulting receiver implementation with the proposed techniques enables improved uplink link performance, even when the mobile terminals fulfill their emission requirements, in terms of I/Q images, with no changes in the LTE standard's frame and pilot structures.

Optical real-time Fourier transformation with kilohertz resolutions

Real-time Fourier transformation (RTFT) of signals is a fundamental concept that enables Fourier analysis at speeds beyond the limitations of conventional digital signal processing engines. In the optical domain, RTFT is commonly performed by inducing large amounts of group-velocity dispersion on the signal (time-varying light wave) of interest to map the signal’s frequency spectrum along the time domain. However, the optical frequency resolution of this method is typically restricted above the gigahertz range, which represents a critical limitation for applications in real-time spectroscopy, ultrafast detection, imaging and sensing, and, more particularly, for photonic-assisted generation and processing of radio-frequency (RF) signals. Here we propose a new concept for realization of RTFT that involves superposition of multiple signal replicas that are shifted simultaneously along the temporal and frequency domains, leading to an output temporal waveform that effectively maps the input optical spectrum. This configuration overcomes the frequency-resolution limitations of dispersion-based RTFT schemes, while providing the desired signal’s spectrum with minimal latency, equal to the inverse of the frequency resolution. We experimentally demonstrate a practical implementation of the concept on optical signals using a frequency shifted feedback (FSF) laser, achieving a frequency resolution of ≃30 kHz and a time–bandwidth product exceeding 400, while the predicted linearity of the frequency-to-time mapping process is shown over a 20 GHz bandwidth. The introduced concept should be of general interest for high-speed, real-time Fourier analysis, beyond the optical-domain implementation reported here; moreover, this work also paves the way for novel applications of FSF lasers in several areas, including high-precision metrology and optical or RF waveform synthesis and processing.

All-optical processing to optical and radio frequency (RF) signals

Signal processing is always the heart of the overall information technology and industry, providing enabling solutions for the processing, understanding, learning, retrieval, mining, and extraction of information from different signals. Regarding the all-optical signal processing, it dates back to the 1980s when the electronic bottleneck was considered as a big obstacle to the bandwidth and capacity of optical communications. Fortunately, all-optical signal processing provides intrinsic wide instantaneous bandwidth and large time-bandwidth product (TBP), powered by optics or photonics technology. Although we are happy to see various breakthrough achievements in electronic signal processing over past decades, all-optical signal processing are still highly required today along with the exponential growth of data capacity, the ubiquitous access, and the realtime interactions. There are some increasing motivations listed as follows, all-optical storage and switching to for large-capacity and energy-efficient networks, broadband and complex microwave signal processing for 5G and beyond systems and defense scenarios, and innovative materials and devices for quantum information processing. In particular, all-optical signal processing is of great significance in both optical and radio frequency (RF) domains, providing intrinsic advantages on large bandwidth to circumvent the electronic bottleneck. Here, latest and state-of-art advances on all-optical processing to both optical and RF signals are comprehensively reviewed, with warped transform and compression, integrator and differentiator, optical memory, fully photonics-based radar, and nonlinear photonic RF signal processing being highlighted in details. Moreover, advances on photonic integrated circuits (PICs) for all-optical processing are demonstrated, especially the CMOS-compatible PICs and high-density nanophotonic integration. Optical signal processing refers to a wide variety of techniques and applications, including compression, transform, analog/logic operations, arbitrary waveform processing, compensation and equalization, wavelength and format conversion, all-optical storage/switching/ memory [1–10]. Here, we will concentrate on the latest advances on warped transforms and compression, differentiation and integration, and optical memory. First of all, a concept of the photonic hardware accelerator is proposed to process wideband waveforms [1]. In principle, nonlinear group delay dispersions (sub-linear or super-linear ones) are introduced to perform warped transform or anamorphic stretch transform, such that wideband waveforms are transformed and processed according to the non-uniform entropy of their spectra. Potential applications include non-uniform Fourier domain sampling, data compression, Big Data predicament in biological cell screening, signal-to-noise ratio (SNR) enhancement, spectrotemporal encoding, and so on. For example, a compression in the TBP of a signal is achieved to circumvent the Big Data problem in high-throughput measurements and ultrafast imaging [1, 2], without sacrificing the spectral resolution or bandwidth. The authors demonstrated a modulation bandwidth compressed by 500 times, while the input optical signal had a 1,000-GHz modulation bandwidth and a 50-ps duration. Consequently, the TBP and then the digital data size were reduced by 2.73 times. These requirements are already available in optics X. Zou (&) W. Pan L. Yan Center for Information Photonics and Communications, Southwest Jiaotong University, Chengdu 610031, China e-mail: zouxihua@swjtu.edu.cn

Wafer-level hysteresis-free resonant carbon nanotube transistors.

We report wafer-level fabrication of resonant-body carbon nanotube (CNT) field-effect transistors (FETs) in a dual-gate configuration. An integration density of >10(6) CNTFETs/cm(2), an assembly yield of >80%, and nanoprecision have been simultaneously obtained. Through combined chemical and thermal treatments, hysteresis-free (in vacuum) suspended-body CNTFETs have been demonstrated. Electrostatic actuation by lateral gate and FET-based readout of mechanical resonance have been achieved at room temperature. Both upward and downward in situ frequency tuning has been experimentally demonstrated in the dual-gate architecture. The minuscule mass, high resonance frequency, and in situ tunability of the resonant CNTFETs offer promising features for applications in radio frequency signal processing and ultrasensitive sensing.

Third-order intermodulation distortion in graphene resonant channel transistors

Third-order intermodulation distortion (IM3) is an important metric for electromechanical resonators used in radio frequency signal processing applications since it characterizes the nonlinearity of the device, and the amount of in-band interference it generates when subject to unwanted, out-of-band signals. In this letter, we measure and model IM3 in a strain-engineered graphene mechanical resonator operated as a graphene resonant channel transistor (G-RCT). The device analyzed in this work has a voltage third-order intercept point (VIIP3) of 69.5 dBm V at a gate-to-source DC bias (Vgs) of 2.5 V, which drops to 52.1 dBm V at Vgs = 4.5 V when driven with two out-of-band input tones spaced 5 and 10 MHz from the resonant frequency. The decrease in the VIIP3 with Vgs coincides with an increase in the transmission response (S21) of the device, illustrating a trade-off between transduction efficiency and linearity. In addition, we find that conventional micro-electro-mechanical systems theory for IM3 calculation does not accurately describe our measurement data. To resolve this discrepancy, we develop a model for IM3 in G-RCTs that takes into account all of the output current terms present in the embedded transistor structure, as well as an effective Duffing parameter (αeff).

Low-power, chip-based stimulated Brillouin scattering microwave photonic filter with ultrahigh selectivity

Highly selective and reconfigurable microwave filters are of great importance in radio-frequency signal processing. Microwave photonic (MWP) filters are of particular interest, as they offer flexible reconfiguration and an order of magnitude higher frequency tuning range than electronic filters. However, all MWP filters to date have been limited by trade-offs between key parameters such as tuning range, resolution, and suppression. This problem is exacerbated in the case of integrated MWP filters, blocking the path to compact, high-performance filters. Here we show the first chip-based MWP bandstop filter with ultrahigh suppression, high resolution in the megahertz range, and 0–30 GHz frequency tuning. This record performance was achieved using an ultralow Brillouin gain from a compact photonic chip and a novel approach of optical resonance-assisted RF signal cancellation. The results point to new ways of creating energy-efficient and reconfigurable integrated MWP signal processors for wireless communications and defence applications.

Ultra-high suppression microwave photonic bandstop filters

A bandstop radio frequency (RF) filter can enhance the capabilities of RF communication systems by removing unwanted signal components such as distortion, or interference. In modern cognitive radio systems, frequency agile filters are desired to block interferers with dynamically changing frequencies and powers. These filters should be tunable over a broad frequency range, and they should be capable of suppressing very strong signals with a high resolution. To simultaneously meet these requirements with traditional RF filters are highly challenging. Microwave photonic (MWP), a technology that primary deals with the generation, distribution, and processing of high speed RF signals using photonic techniques and components, is promising for creating frequency agile RF filters. However, traditional MWP filters are lacking the high resolution and peak suppression exhibited by state-of-the-art RF electrical filters. We recently introduced a new class of MWP notch filter which is free from this limitation. This scheme allowed the creation of anomalously high suppression MWP notch filter from virtually any kind of optical resonance, irrespective of its type (gain or absorption), or its magnitude. This enabled, for the first time, simultaneous optimization of the MWP filter resolution, peak attenuation, and frequency tuning range. In this paper, we present the analysis of notch filter response creation using the novel sideband processing technique. We then show the applicability of this technique to a wide range of optical filters. We compare simulated and experimental results of the notch filter response created using three types of optical filter commonly used in MWP signal processing, namely stimulated Brillouin scattering (SBS), an integrated optical ring resonator (ORR), and a phase-shifted fiber Bragg-grating (FBG).

Multi-Notch Filter (MNF) Algorithm for Automotive Radio-Frequency (RF) Signal Processing and Applications

Multi-Notch Filter (MNF) Algorithm for Automotive Radio-Frequency (RF) Signal Processing and Applications

Time reversal of light by linear dispersive filtering near atomic resonance

Based on the similarity of paraxial diffraction and dispersion mathematical descriptions, the temporal imaging of optical pulses combines linear dispersive filters and quadratic phase modulations operating as time lenses. We consider programming a dispersive filter near atomic resonance in rare earth ion-doped crystals, which leads to unprecedented high values of dispersive power. This filter is used in an approximate imaging scheme, combining a single time lens and a single dispersive section and operating as a time-reversing device, with potential applications in radio-frequency signal processing. This scheme is closely related to a three-pulse photon echo with chirped pulses, but the connection with temporal imaging and dispersive filtering emphasizes new features.