Broadband Radio Frequency Signals Research Articles

Analog spatiotemporal feature extraction for cognitive radio-frequency sensing with integrated photonics

Analog feature extraction (AFE) is an appealing strategy for low-latency and efficient cognitive sensing systems since key features are much sparser than the Nyquist-sampled data. However, applying AFE to broadband radio-frequency (RF) scenarios is challenging due to the bandwidth and programmability bottlenecks of analog electronic circuitry. Here, we introduce a photonics-based scheme that extracts spatiotemporal features from broadband RF signals in the analog domain. The feature extractor structure inspired by convolutional neural networks is implemented on integrated photonic circuits to process RF signals from multiple antennas, extracting valid features from both temporal and spatial dimensions. Because of the tunability of the photonic devices, the photonic spatiotemporal feature extractor is trainable, which enhances the validity of the extracted features. Moreover, a digital-analog-hybrid transfer learning method is proposed for the effective and low-cost training of the photonic feature extractor. To validate our scheme, we demonstrate a radar target recognition task with a 4-GHz instantaneous bandwidth. Experimental results indicate that the photonic analog feature extractor tackles broadband RF signals and reduces the sampling rate of analog-to-digital converters to 1/4 of the Nyquist sampling while maintaining a high target recognition accuracy of 97.5%. Our scheme offers a promising path for exploiting the AFE strategy in the realm of cognitive RF sensing, with the potential to contribute to the efficient signal processing involved in applications such as autonomous driving, robotics, and smart factories.

High-resolution reconfigurable RF signal spectral processor.

Recent developments in microwave photonic filters (MPFs) offer superior properties for radio frequency (RF) signal processing, such as large instantaneous bandwidth, high resolution and multifunctional shapes. However, it is quite challenging to realize two or more characteristics simultaneously to meet the diverse needs in complex electromagnetic environment. In this paper, we propose a reconfigurable RF signal spectral processor with both large instantaneous bandwidth and high resolution. In the proposed spectral processor, sufficient taps supplied by an optical frequency comb (OFC) offer a large instantaneous bandwidth to process broadband RF signals. Flexible tap coefficients can be obtained by manipulating an optical spectral shaper (OSS), which provides excellent reconfigurability. This tap-by-tap manipulation is realized with a high resolution of hundreds of megahertz, allowing precise shape configuration of the response. In the experiment, we demonstrate a flat-top response with a wide bandwidth of 7.1 GHz. Reconfigurable features such as tunable bandwidth, adjustable center frequency and diverse shapes are also shown. In particular, the measured frequency resolution of 96.5 MHz demonstrates the ability for precise configuration.

Flexible channel selection method based on optical combs for broadband signals receiving.

A flexible channel selection method based on optical combs is proposed for reconfigurable optical channels in this paper. Optical-frequency combs with a large frequency interval are used to modulate broadband radio frequency (RF) signals, and an on-chip reconfigurable optical filter [Proc. of SPIE, 11763, 1176370 (2021).10.1117/12.2587403] is used to perform periodic carrier separation of wideband and narrowband signals and channel selection. In addition, flexible channel selection is achieved by presetting the parameters of a fast-response programmable wavelength-selective optical switch and filter device. Channel selection only relies on the combs through the Vernier effect of the combs and the passbands for different periods and does not require the use an additional switch matrix. Finally, flexible switching between and selection of specific channels for 13 GHz and 19 GHz broadband RF signals are experimentally verified.

High-Efficiency Mitigation of Nonlinear Distortion in Microwave Photonics Link Assisted by Artificial Neural Network

A microwave photonics radio over fiber link can deliver the radio frequency (RF) signal to realize the antenna remote. When the signal is a broadband and multicarrier RF signal, there are some linear distortions in the link, such as the crossmodulation distortion (XMD) and third-order intermodulation distortion (IMD3). It will destroy the wide-bandwidth performance of the link. Traditional postdigital compensation methods for XMD and IMD3 mitigation extract the baseband signal and reconstruct the compensation signal with calculated compensation factor. Here, a kind of artificial neural network genetic algorithm (ANN-GA) distortion compensation technique is proposed to seek the compensation factor instead of calculations. The neural network algorithm is used to fit the mapping function between the compensation factor and the distortion and give a set of predicted data as the original individual fitness value for the genetic algorithm. Based on the original value, the minimal distortion, the corresponding optimal compensation factors γ and α are found with optimization iterations. Taking advantage of the optimal compensation factors, based on the traditional postdigital compensation method, the distortion is mitigated with a suppression ratio of -84.4 dB. In our paper, the mitigation technology of the XMD and IMD3 can be applied for any kinds of link instead of mathematically modelling the link and calculating the compensation factor. The technology can improve the intelligence and flexibility of microwave photonic link linearization design.

Acousto-optic frequency shifter-based microwave photonic channelized receiver using a single optical frequency comb.

A microwave photonic channelization receiver is a promising technology for broadband radio frequency (RF) signal monitoring and reception. In this Letter, by exploiting acousto-optic frequency shifters (AOFSs), a microwave photonic channelization receiver is proposed. The proposed microwave photonic channelized receiver can reduce dual coherent optical frequency comb (OFC) generators with detuning frequency spacing into a single OFC generator. To verify the feasibility of the proposed channelization scheme, a broadband RF signal with 3.2 GHz bandwidth is channelized into eight narrowband RF signals with 0.4 GHz bandwidth. Moreover, we investigate the effect of the tuning error imposed by AOFSs, where the error vector magnitude (EVM) of subchannels is obtained by channelizing the four 16-quadrature amplitude modulation (16-QAM) signals into four subchannels.

Broadband photonic RF channelizer with 66 channels based on acousto-optic frequency shifter

Microwave photonic channelized receiver is one of the important means to realize broadband radio frequency (RF) signal reception, and it also has great application potential in the fields of radar systems and electronic warfare. In view of the above, a microwave photonic channelizer with 66-subchannels based on acousto-optic frequency shifter (AOFS) is proposed. The 11-line optical frequency comb (OFC) used in this scheme is generated via one phase modulator. The 33 optical local oscillators can be obtained using two AOFSs to shift the frequency of 11-line OFC up and down. The 33 optical local oscillators and broadband RF signal are image-rejected and downconverted in the I / Q receiver and output by 66 subchannels with a bandwidth of 500 MHz. The simulation results verify the complete reception of 3 to 36 GHz wideband RF signal, and the spurious-free dynamic range of the system can reach 116.3 dB · Hz2/3.

Broadband radio-frequency signal synthesis by photonic-assisted channelization.

A channelized radio-frequency (RF) signal synthesis scheme is proposed to generate broadband RF signals with reconfigurable waveform, center frequency and instantaneous bandwidth. Based on dual optical frequency combs (OFCs) with different free spectrum ranges (FSRs), multiple narrowband signals are up-converted and synthesized into a broadband signal. Reconfigurable waveforms are generated in the simulation, including a symmetrical triangular linear frequency modulation continuous wave (STLFMCW) signal and a binary phase shift keying (BPSK) signal. In addition, to realize phase stability among channels, dual OFCs are differently modulated through polarization-multiplexing electro-optical modulators (EOMs). An RF signal synthesis experiment shows the relative phase fluctuation among channels is only 1.8°.

Wideband and Ambiguous-Free RF Channelizer Assisted Jointly by Spacing and Profile of Optical Frequency Comb

For civil and military applications, the ability to instantaneously capture and process wideband RF (radio-frequency) signal is of great importance. In this paper, a novel photonics-assisted RF channelized receiver aiming for instantaneous broadband RF signal reception is proposed and experimentally demonstrated. The channelizer is composed of two branches. In one branch, an optical frequency comb (OFC) is generated through a phase modulator (PM), while in the other the optical carrier is frequency-shifted via a polarization modulator (PolM) and then modulated by the captured unknown RF signal. Next, the frequency information of the unknown signal is derived from the frequency sum and the power difference of two specific beating tones, while the incorporation of the comb spacing and the amplitude profile of the OFC enable resolving the ambiguity cases. In the experiments, four receiving channels are constructed using five selected comb lines. The reception and analysis of several RF signals including single-tone, multi-tone and linear frequency modulation (LFM) ones within 1-40 GHz are experimentally validated, which can be extended up to 72 GHz in theory. The proposed channelizer is characterized with ambiguous-free capability, low measurement errors less than 2 MHz, and an SFDR of ∼92 dB·Hz2/3.

Sub-Nyquist Ultra-Wideband Sparse Signal Reception via Variable Frequency Comb

We demonstrate a new class of ultra-wideband (UWB) sparse radio frequency (RF) signal receiver relying on direct sub-sampling in frequency domain and compressive sensing (CS) techniques. The sampling process is realized with a hybrid RF-photonic architecture for discrete Fourier transform (DFT) on broadband RF signals using a variable-pitch frequency comb. Sub-sampled DFT coefficients are used to recover the input RF signal which is sparse in a chosen domain. Instead of digitizing signals with a full-rate analog to digital converter (ADC), the new approach requires sub-rate quantization, reducing hardware complexity via sub-sampling in frequency domain. Both simulated and experimental verifications were achieved by high-fidelity reception of various sparse signals within 3 GHz to 7.9 GHz band.

Implementation of a Highly-Sensitive and Wide-Range Frequency Measurement Using a Si3N4 MDR-Based Optoelectronic Oscillator

Microwave photonic technologies have been introduced for achieving broadband radio-frequency signal measurement. However, few of the proposed schemes mention the low-power radio-frequency signal detection, which stringently limits their practical applications in certain areas. In this paper, we designed and demonstrated a wideband low-power radio-frequency signal measurement system with optoelectronic oscillator. Here, the unknown radio-frequency signal matched the potential oscillation mode is allowable to be detected, amplified and estimated. The key component in the tunable optoelectronic oscillator is a silicon nitride micro-disk resonator with a very high Q-factor, which is utilized to achieve frequency selection as a microwave filter. A frequency measurement system range from 1 ∼ 20 GHz with radio frequency power as low as −105 dBm, measurement errors of ±375 MHz and the maximum gain of 61.7 dB were realized experimentally

A coherent photonic RF scanning receiver based on a flat optical frequency comb

A coherent photonic-assisted radio-frequency (RF) receiver is proposed and experimentally demonstrated. In this receiver, a laser light is modulated by an input broadband RF signal through an intensity modulator (IM) in one optical branch; simultaneously, it is used as a seed to generate an optical frequency comb (OFC) in another optical branch. The OFC generator consists of two cascaded IMs, and can produce 5 lines with a spectral flatness less than 1.1 dB over a broad bandwidth of 3–12 GHz. By scanning the frequencies of the OFC, the input broadband RF tone can be directly down-converted to a given intermediate frequency (IF) band. We experimentally demonstrate the down-converting scheme for the input RF tones from 2 GHz to 12 GHz. The RF signals are successfully down-converted to several IF bands, respectively.

Chirped-pulse-based broadband RF channelization implemented by a mode-locked laser and dispersion.

Based on chirped pulses, a wideband radio frequency (RF) channelized receiver that can easily support hundreds of channels is proposed. The mixing chirped pulses and its own delayed copy produces an equivalent RF local oscillation (LO). The LO frequency can be changed by simply setting the delay between the two paths. Channelized receiving of broadband RF signals can be realized by parallel delay line arrays. Meanwhile, the use of in-phase/quadrature demodulation avoids the extra optical or electrical filtering as well as image interference. A receiver with channel spacing of 100MHz, covering the spectrum from DC to 18.4GHz is experimentally demonstrated. The performances, including signal-to-noise ratio, frequency response, spurious-free dynamic range, and image rejection, are analyzed.

Frequency-Oriented Subsampling by Photonic Fourier Transform and I/Q Demodulation

Subsampling can directly acquire a passband within a broad radio frequency (RF) range, avoiding down-conversion and low-phase-noise tunable local oscillation. However, subsampling suffers from band folding and self-image interference. In this paper, we propose a frequency-oriented subsampling to solve those two problems. With ultrashort optical pulse and a pair of chromatic dispersions, the broadband RF signal is first short-time Fourier-transformed to a spectrum-spread pulse. Then, a time slot, corresponding to the target spectrum slice, is coherently optical-sampled by in-phase/quadrature (I/Q) demodulation. We demonstrate the novel bandpass sampling by a numerical example, which shows the desired uneven intensity response, i.e., prefiltering, to avoid the band folding. We show in theory that appropriate time-stretch capacity from dispersion can result in prefiltering bandwidth less than sampling rate. Image rejection due to I/Q sampling is also analyzed. A proof-of-concept experiment, which is based on a time-lens sampling source and chirped fiber Bragg gratings, shows the center-frequency-tunable prefiltered subsampling with bandwidth of 6 GHz around, as well as imaging rejection larger than 26 dB. Our technique may benefit future broadband RF receivers for frequency-agile Radar or channelization.

Image-Free Microwave Photonic Down-Conversion Approach for Fiber-Optic Antenna Remoting

For commercial and military applications, it is highly desired to acquire broadband radio frequency (RF) signals from remotely located receiving antennas to the central office (CO) for centralized signal processing such as down-conversion. In this paper, a photonic approach for image-free microwave frequency down-conversion is proposed for antenna remoting scenarios. The RF signal is captured by the phase modulator (PM) at remote antenna unit and transmitted over a long fiber link to the CO for performing frequency down-conversion. In analog, with the Hartley architecture in the electronic domain, two in-phase (I) and quadrature (Q) intermediate frequency (IF) components are generated by using an electro-optic polarization modulator (PolM) and two polarizers. To achieve large image rejection ratio (IRR), the digital post-processing technique is introduced to accurately compensate the amplitude and phase imbalances between the I and Q IF signals. In the experiments, a 2-km single-mode fiber link is inserted between the PM and PolM. As the local oscillator signals are set as 35 and 3 GHz, two target sinusoidal signals at 35.5 and 3.1 GHz are applied with two image signals, a sinusoidal signal at 34.5 GHz and a broadband RF signal centered at 2.9 GHz, respectively. Then, both real-time analog and off-line digital processing methods are used to process the generated I and Q IF signals for image rejection, yielding an IRR over 45 and 60 dB respectively, when the sinusoidal image signal is applied. The distortions from the broadband 2.9-GHz image RF signal are also effectively suppressed by using the two processing methods. The proposed approach is capable of covering a wide frequency range from 5 to 40 GHz.

Broadband Photonic Radio-Frequency Channelization Based on a 39-GHz Optical Frequency Comb

A novel photonic channelization scheme for a wideband radio-frequency (RF) signal based on an optical frequency comb (OFC), a comb filter, and an optical de-mux is proposed and demonstrated. In the proposed channelizer, the input broadband RF signal is multicast by the OFC, spectrally sliced by a Fabry-Perot filter (FPF) and then channelized by a regular optical de-mux. Compared to previous proposals, the OFC can provide uniform and low-noise channelization and simplify the spectral alignment to the FPF. We demonstrate our scheme experimentally based on a 39-GHz-spaced OFC. As an application, instantaneous multiple-frequency measurement is demonstrated by directly detecting the channelized RF signal.

Wideband radio frequency spectrum analyzer: improved design and experimental results

We report on significant progress accomplished in spectral-hole-burning-based instantaneous spectral analysis of optically carried broadband radio frequency signals. This concept aims at becoming the next generation spectrum analyzer for broadband radar and astronomical applications. Since the initial demonstration (Opt. Lett. 26 (2001) 1245) the spectral range has been broadened by two orders of magnitude and the channel number has been improved by nearly one order of magnitude. We discuss the next steps to be considered, namely 100% duty cycle and 1000-channel operation. On the way to 100% duty cycle operation, we demonstrate frequency offset-locking of a frequency-chirped laser to a fixed frequency laser. We examine two-dimensional angular addressing as a technique to increase the spectral channel number.