Wideband Radio Frequency Signals Research Articles

Broadly tunable and ultra-highly selective detection of radio frequency signals enabled by a silicon photonic chip.

Precise and agile detection of radio frequency (RF) signals over an ultra-wide frequency range is a key functionality in modern communication, radar, and surveillance systems, as well as for radio astronomy and laboratory testing. However, current microwave solutions are inadequate for achieving the needed high performance in a chip-scale format, with the desired reduced cost, size, weight, and power. Photonics-based technologies have been identified as a potential solution but the need to compensate for the inherent noise of the involved laser sources have prevented on-chip realization of wideband RF signal detection systems. Here, we report an approach for ultra-wide range, highly-accurate detection of RF signals using a conceptually novel feed-forward laser's noise cancelling architecture integrated on chip. The technique is applied to realization of an RF scanning receiver as well as a complete radar transceiver integrated on a CMOS-compatible silicon-photonics chip, offering an unprecedented selectivity > 80 dB, spectral resolution < 1 kHz, and tunability in the full 0.5-35 GHz range. The reported work represents a significant step towards the development of integrated system-on-chip platforms for signal detection, analysis and processing in cognitive communication and radar network applications.

Multi-delay photonic correlator for wideband RF signal processing

Correlation of radio-frequency (RF) signals is a fundamental operation in many fields such as information processing, detection, and imaging techniques at large. Because of the intrinsic limitations of electronic techniques, standard digital correlators, which rely on the acquisition of signals and their processing, become very complex to implement for the real-time analysis of signals whose bandwidth exceeds a few hundred MHz. On the other hand, analog correlators are limited by the performances of RF components. In this paper, we report the proof-of-concept of a correlator architecture based on a simple photonic platform, suitable for analog wideband RF signal processing. The concept, based on multi-heterodyne interferometry, gives access in real time to the entire correlation function of two signals by computing the cross correlation coefficients for 200 values of their relative delay simultaneously. The time-delay step can be adjusted from a few ns down to a few ps, enabling us to process signals with MHz to multi-GHz bandwidth. We have applied this architecture to the localization of RF transmitters by time difference of arrival (TDoA) and obtained a precision close to 10 ps for a 100 ms integration time. This concept is expected to find practical applications in various domains, from radar and electronic warfare to telecommunications, imaging, and radio-astronomy.

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.

Algorithm for designing low-noise frequency synthesizers for remote sensing systems

Frequency synthesizer is an important component of radar tracking system for remote sensing. Synthesizers generate wideband radio frequency signals with high frequency resolution. Parameters of radar depend on the synthesizers phase noise. This paper presents algorithm for structural design of signal generators based on various synthesis methods: direct, indirect and hybrid ones. Specialized software based on presented algorithm was developed in this work. Software allows calculating frequency ratios in the structure of synthesizers, selecting optimal parameters which can provide low phase noise level. Also software allows performing mathematical modelling and analysis of phase noise power spectral density (PSD) of frequency synthesizers. Presented algorithm and software can be useful for developers of remote sensing radio systems.

Optimal Time Scheduling Scheme for Wireless Powered Ambient Backscatter Communications in IoT Networks

In this paper, we investigate optimal schemes to manage time scheduling of multiple modules, including spectrum sensing, radio frequency (RF) energy harvesting (RFH) and ambient backscatter communication (ABCom) by maximizing data transmission rate in Internet of Things networks. We first detect ambient RF signals with high signal power as the RF resource of RFH and ABCom by using spectrum sensing with energy detection techniques. Specifically, compressive sensing (CS) is adopted to detect the wideband RF signals with improving spectrum sensing efficiency at the same time. We formulate a joint optimization problem to manage time scheduling parameter and power allocation ratio. In addition, we propose to find the threshold of spectrum sensing for ABCom communications by analyzing the outage probability of backscatter communications. Numerical results demonstrate that the optimal schemes using spectrum sensing are achieved with better transmission rates. The designed time scheduling scheme with CS is confirmed to be more efficient, and the superiorities become more obvious with the increase of network operation time. Moreover, the optimal scheduling parameters and power allocation ratios are obtained. Simulations illustrate that the threshold of spectrum sensing for backscatter communications is obtained by analyzing the outage probability of backscatter communications.

Optical Comb Assisted OFDM RF Receiver

Orthogonal frequency-division multiplexing (OFDM) is a widely used signaling format in wireless communication and ultrawide band radio-frequency (RF) applications. An OFDM transceiver relies on a discrete Fourier transform (DFT) processor that matches both the resolution and the bandwidth of wireless link. A conventional wideband OFDM receiver necessarily relies on high-speed analog-to-digital converters (ADC) followed by a Fourier processing core. Both fundamental and practical ADC considerations that include bandwidth-resolution tradeoff impose strict limits on high precision digitization of the wideband OFDM signal. In addition to the quantization barrier, computational complexity of the high-speed DFT processor leads to high dissipation levels in conventional receivers. This report demonstrates an alternative OFDM receiver architecture that eliminates the need for high-speed ADC and digital DFT processor. The new design is based on hybrid, optoelectronic DFT processor, capable of generating DFT coefficients of wideband RF signal in real time. We demonstrate demodulation of a 4-quadratic amplitude modulation OFDM signal with 60 subcarriers positioned within 4–15.8 GHz band and separated by 200 MHz. The error vector magnitude is measured for all channels to characterize the performance of the comb-assisted OFDM RF receiver.

Broadband lower-IF RF receiver based on microwave photonic mixer and Kramers-Kronig detection.

A broadband lower- intermediate-frequency (IF) radio frequency (RF) receiver with low cost and complexity based on a microwave photonic mixer and Kramers-Kronig (KK) detection is proposed and experimentally demonstrated. The feasibility of the proposed RF receiver is verified by comparing the error vector magnitude (EVM) performance of a down-converted IF signal with and without KK detection. EVM is measured as a function of IF carrier frequency, input local oscillator (LO) power, input RF power, and the coupling ratio between RF and LO paths. With KK detection, EVM can be improved by more than 9.5% when the IF carrier frequency is close to half of the signal bandwidth. This implies that a lower IF frequency can be down-converted with good EVM and that a low-speed photodetector and a low sampling rate analog to digital converter can be used for wideband RF signal reception.

Photonics-enabled compressive sensing with spectral encoding using an incoherent broadband source.

In this Letter, we propose an approach to achieving photonics-enabled compressive sensing of sparse wideband radio frequency signals in which an incoherent broadband source is applied, and the mixing and integration functions are realized in the optical domain. A spectrum shaper is employed to slice and encode the spectrum of the broadband light according to a predetermined random sequence. Because of the dispersion-induced group delay, the mixing between the incoming signal and the random bit sequence is achieved. At the output of the spectrum shaper, an array of photodetectors is employed to realize down-sampling, and the input sparse signal can be captured in a single-shot mode. Since no pulsed laser is employed, our scheme obviates the need for time-domain synchronization between the repetitive ultra-fast pulses and the random sequence. Experimental demonstrations and numerical results are presented to verify the feasibility and potential of the approach.

Coherent photonic radio frequency channelization based on dual coherent optical frequency combs and stimulated Brillouin scattering

Coherent photonic radio frequency (RF) channelization based on dual coherent optical frequency combs (OFCs) and stimulated Brillouin scattering (SBS) is proposed and demonstrated. By using the dual OFCs to multicast a wideband RF signal and form a series of SBS gain responses simultaneously, the frequency components of the wideband RF signal are sliced into a number of parallel parts and converted into signals with the same center frequency. The bandwidth of each frequency slice is determined by the gain bandwidth of the SBS (tens of megahertz), and all the information in the RF signal is retained after the channelization. The channelization fineness can be tuned by adjusting the two OFCs’ free spectral ranges. An experiment is carried out. Photonic RF channelization with channel spacing of 80 MHz and a crosstalk suppression of more than 19.52 dB is realized.

Radio Frequency Beamforming Based on a Complex Domain Frontend

The state-of-the-art phased array-based digital beamforming systems suffer from disadvantages of high power consumption, high cost, and system complexity due to the massive use of transmitter/receiver (T/R) modules and high-speed digital and digital–analog mixed devices. In this paper, we point out that by proposing a new concept “complex domain” radio frequency (RF) frontend, the relatively slowly changed waveform delay information required to accomplish adaptive beamforming can be separated from wideband RF signals, based on which a self-contained beamforming system can be implemented with a low-speed baseband. By introducing vector RF multipliers in the proposed frontend, the amplitude and phase of RF signals can be simultaneously controlled by the real and imaginary parts of complex numbers, such that beamforming algorithms derived in complex domain can be directly applied without any form of transformation. By doing so, the massive use of conventional T/R modules and high-speed baseband devices can be avoided. Theoretical analysis and experimental demonstration based on commercial components have validated the proposed approach. Our method is able to significantly simplify the realization and decrease the cost of wideband digital beamforming systems, and can be widely used in low cost, power efficient beamforming applications.

Phase stabilized downlink transmission for wideband radio frequency signal via optical fiber link.

In this paper, we propose and demonstrate a phase stabilized wideband downlink transmission scheme, which directly transmits the received radio frequency (RF) signals from remote antennas to central station. A reference RF tone is round-trip transferred between the central station and remote end to obtain the delay variation caused by the fiber link. The delay variation is then used to alter a tunable laser. Since optical carriers with different wavelengths propagate at different velocities in fiber, a tunable optical delay line is realized to cancel the delay variation of the fiber link. The tunable delay range is in proportion to the length of the fiber link, which means a very long delivery distance can be expected. Experimentally, a RF signal at frequency of 2.50 GHz has been downlink transferred through a 45 km fiber link, with stability of 3.3 x 10⁻¹³ at 1 s and 7.5 x 10⁻¹⁷ at 10⁴s.

VLSI Design of a Monolithic Compressive-Sensing Wideband Analog-to-Information Converter

One of the key tasks in cognitive radio and communications intelligence is to detect active bands in the radio-frequency (RF) spectrum. In order to perform spectral activity detection in wideband RF signals, expensive and energy-inefficient high-rate analog-to-digital converters (ADCs) in combination with sophisticated digital detection circuitry are typically used. In many practical situations, however, the RF spectrum is sparsely populated, i.e., only a few frequency bands are active at a time. This property enables the design of so-called analog-to-information (A2I) converters, which are capable of acquiring and directly extracting the spectral activity information at low cost and low power by means of compressive sensing (CS). In this paper, we present the first very-large-scale integration (VLSI) design of a monolithic wideband CS-based A2I converter that includes a signal acquisition stage capable of acquiring RF signals having large bandwidths and a high-throughput spectral activity detection unit. Low-cost wideband signal acquisition is obtained via CS-based randomized temporal subsampling in combination with a 4-bit flash ADC. High-throughput spectrum activity detection from the coarsely quantized and compressive measurements is achieved by means of a massively-parallel VLSI design of a novel accelerated sparse spectrum dequantization (ASSD) algorithm. The resulting monolithic A2I converter is designed in 28 nm CMOS, acquires RF signals up to 6 GS/s, and the on-chip ASSD unit detects the active RF bands at a rate 30 × below real-time.

Principle and key technology of generalized high precision simulation of TT&amp;#x0026;C channel

A generalized simulation method of the tracking, telemetry and control (TT&C) channel, which is applicable to wideband and arbitrary radio frequency (RF) signal, is proposed. It can accurately simulate the dynamic transmission delay of the arbitrary RF signal in channels, especially regardless of any prior knowledge including signal form, signal parameters, and so on. The proposed method orthogonally demodulates the wideband and arbitrary RF signal to complex baseband by a known local oscillator (LO) signal. Whereafter, it takes measures to obtain the delay reconstruction signal of baseband signals based on the dynamic transmission delay between a ground station and a responder. Meanwhile, it manages to obtain the delay reconstruction signal of LO signals. The simulation output signal (the delayed RF signal) can be achieved through the synthesis of the two delay reconstruction signals mentioned above. The principle and its related key technology are described in detail, and the realizable system architecture is given.

Wavelength-compensated photonic multibeam-forming system for two-dimensional wideband radio-frequency phased-array antennas

Traditional lens-based photonic Fourier beam-forming systems can be used to steer multiple beams for narrowband radio-frequency (RF) phased-array antennas. For wideband RF phased-array antennas, such Fourier beam-forming systems suffer from frequency-dependent beam steering, known as beam squint. We present a novel squint-free Fourier-based photonic multibeam-forming system for wideband two-dimensional RF phased-array antennas using a lens and frequency-mapped modulation. In this new beam-forming system, we modulate the receiving wideband RF signals onto a broadband light source in a frequency-mapped manner by a traveling-wave tunable filter at each antenna element. These modulated signals are launched in a miniaturized topology of the RF antenna array, and the wavelength-scaling factor in the lens Fourier transform exactly compensates the frequency dependence of beam steering. Heterodyne detection at the Fourier plane between the focused modulated multicolor spots and the broadband laser reference beams from the same light source recovers the received RF signals. An analysis with numerical simulations and then demonstrated with preliminary experimental results of this beam-forming system is presented.

Fiber-lattice accumulator design considerations for optical ΣΔ analog-to-digital converters

Integrated optical sigma-delta (??) analog-to-digital converters (ADCs) use a pulsed laser to oversample an input signal at two Mach-Zehnder interferometers. A fiber-lattice accumulator is embedded within a feedback loop around a single-bit quantizer to spectrally shape the quantization noise to fall outside the signal band of interest. Decimation filtering is applied to the quantizer output to construct the input signal with high resolution. Applications of integrated optical ?? ADCs include digitizing wideband radio-frequency signals directly at an antenna (digital antenna). In this paper, a novel fiber-lattice accumulator design is presented, and a coherent simulation of an integrated optical first-order, single-bit ?? ADC is reported. The accumulator leakage resulting from a mismatch in the optical circuit parameters is quantified. A time-domain analysis is presented, and the simulation results from an all-electronic ?? ADC are presented for comparison. A frequency-domain analysis of a ten times oversampling (n = 4 bits) simulation is used to compare the dynamic performance parameters, including the spurious-free dynamic range, signal-to-noise-plus-distortion ratio, and effective number of bits. The formation of image frequencies when the accumulator is overloaded (i.e., the optical amplifier gain is too large) is also investigated.

Photonic security system using spatial codes and remote coded coherent optical communications

A novel photonic security system is described using 2-D spatial codes based on both optical phase and amplitude information. This security system consists of an optical interferometric encoding subsystem that rapidly reads and encodes the 2-D complex-valued spatial code, forming a wideband frequency modulated optical beam and a colinear optical reference beam. After appropriate coherence coding of this beam pair, the light is launched into a low probability of intercept communication channel such as an optical fiber or a narrow beamwidth free-space optical link. At the remote code receiving and data processing site, the received light beam pair is first coherently decoded. Then, a high speed photodetector via optical heterodyne detection generates an encoded wideband radio frequency signal that contains the original 2-D code. Decoding is implemented in parallel via two independent systems. One decoder uses a Fourier transforming lens to reconstruct an electronic image interferogram of the complex-valued user code. This image interferogram is sent to a high speed electronic image processor for verification purposes. The other decoder is a high speed coherent acousto-optic time integrating correlator that optically determines matchmismatch between the received encoded signal and the code signal generated by the electronic database. Improved security to the overall communication network is added by using various keycodes such as a time varying keycode that determines the exact spatial beam scanning sequence required for both proper encoding and decoding of the 2-D code information. This paper describes preliminary experiments using a simple 1-D amplitude modulated spatial code.

Analytic representation of wide-band radio frequency signals

A comparison is made of the “analytic signal” defined by Gabor with the more conventional “exponential” representation for wide-band radio frequency signals of finite duration. It is shown that the rms difference between them, with respect to both modulus and phase, is bounded by the fractional spectral energy of the “exponential” representation in the left half frequency plane. The difference in each case is shown to be small in most practical applications for fractional bandwidths up to one half.