Continuous Wave Signal Research Articles

Reconfigurable and Concurrent Dual-Band Doherty Power Amplifier for Multiband and Multistandard Applications

This paper proposes a design scheme of concurrent dual-band Doherty power amplifier (DPA) which can be reconfigured to two different pairs of concurrent dual bands for multiband and carrier aggregation applications. In addition to the reconfiguration of frequencies of operation, the output power back-off (OPBO) region for efficiency enhancement can also be reconfigured in this DPA. As a proof of concept, a reconfigurable and concurrent dual-band DPA is designed to operate at 1500 and 2140 MHz in the first concurrent dual band, whereas on reconfiguration it will operate concurrently at 1850 and 2350 MHz. While operating in the first concurrent dual band, the DPA has an enhanced efficiency region up to 6-dB OPBO level and, after reconfiguration to the second concurrent dual band, this region extend up to 9-dB OPBO level. The proposed DPA is tested using continuous wave (CW) signals in single mode as well as in concurrent mode. Measurement results using CW signals in single mode reveal a drain efficiency of 52%-56% at 6-dB OPBO in the first concurrent dual band. Similarly, in the second concurrent dual band, a drain efficiency of 43%-48% is achieved at around 9-dB OPBO. The peak drain efficiency at saturation is better than 60% in all the four operating bands. The measured aggregated drain efficiencies are 49% and 44% in the first and second concurrent dual bands for 6 and 9 dB OPBO level, respectively. The measured carrier to the third-order intermodulation distortion is better than 17 dBc for all of the operating bands.

MIMO High Frequency Surface Wave Radar Using Sparse Frequency FMCW Signals

The heavily congested radio frequency environment severely limits the signal bandwidth of the high frequency surface wave radar (HFSWR). Based on the concept of multiple-input multiple-output (MIMO) radar, we propose a MIMO sparse frequency HFSWR system to synthesize an equivalent large bandwidth waveform in the congested HF band. The utilized spectrum of the proposed system is discontinuous and irregularly distributed between different transmitting sensors. We investigate the sparse frequency modulated continuous wave (FMCW) signal and the corresponding deramping based receiver and signal processor specially. A general processing framework is presented for the proposed system. The crucial step is the range-azimuth processing and the sparsity of the carrier frequency causes the two-dimensional periodogram to fail when applied here. Therefore, we introduce the iterative adaptive approach (IAA) in the range-azimuth imaging. Based on the initial 1D IAA algorithm, we propose a modified 2D IAA which particularly fits the deramping processing based range-azimuth model. The proposed processing framework for MIMO sparse frequency FMCW HFSWR with the modified 2D IAA applied is shown to have a high resolution and be able to provide an accurate and clear range-azimuth image which benefits the following detection process.

Multiple-Access Interference Mitigation for Acquisition of Code-Division Multiple-Access Continuous-Wave Signals

Multiple-access interference mitigation is important in code-division multiple-access (CDMA) systems for both information recovery and signal acquisition. Therefore, in this letter, we propose an open-loop adaptive filter based on the reiterative minimum mean-square error for the acquisition of CDMA signals. The method is based on the existing multistatic adaptive pulse compression method, appropriately adapted to the characteristics of CDMA continuous-wave signals. The efficacy of the proposed method in such systems is verified by simulation, and its superiority when compared with the existing least-mean-square method in terms of the number of reiterations is established.

Advanced Fault-Monitoring Scheme for Ring-Based Long-Reach Optical Access Networks

This paper presents a novel fiber fault-monitoring system for long-reach optical access networks based on a transmission–reflection analysis approach. By launching an unmodulated continuous-wave optical signal into the feeder ring and measuring the transmitted and reflected/backscattered optical powers, the proposed monitoring scheme is able to detect, identify, and localize any major fault (covering both reflective and nonreflective events) with a good spatial accuracy (≤10 m) and a high detection speed (3 s). Both experimental and simulation results have demonstrated that the proposed system provides fiber monitoring functionality better than that obtained by the conventional methods.

Online Phase-Noise Estimation in FMCW Radar Transceivers Using an Artificial On-Chip Target

The presence of phase noise (PN) severely deteriorates the sensitivity and range of frequency-modulated continuous wave radar systems. Thus, characterization and continuous monitoring of the PN is indispensable, and its estimation is often employed directly on chip. Although many contributions investigate methods for PN estimation, almost all of them consider a continuous-wave (CW) input signal. Differently, in this paper, we aim to estimate the PN from a linear frequency modulated continuous wave (FMCW) signal. For that, we propose two methods utilizing a so-called artificial on-chip target and further digital signal processing. These methods are evaluated in real-time and are the first-known solutions to determine PN estimates during the operation of an FMCW radar transceiver. We prove our methods with both simulation and measurement results from a hardware prototype. Further, we present techniques to efficiently realize the concepts in digital hardware. Finally, we compare and trade off the proposed methods against computational complexity and performance. The novel techniques may be applied to arbitrary input frequencies and bandwidths, and do not require a reference-clock input.

A Highly Efficient and Linear Power Amplifier for 28-GHz 5G Phased Array Radios in 28-nm CMOS

This paper presents the first linear bulk CMOS power amplifier (PA) targeting low-power fifth-generation (5G) mobile user equipment integrated phased array transceivers. The output stage of the PA is first optimized for power-added efficiency (PAE) at a desired error vector magnitude (EVM) and range given a challenging 5G uplink use case scenario. Then, inductive source degeneration in the optimized output stage is shown to enable its embedding into a two-stage transformer-coupled PA; by broadening interstage impedance matching bandwidth and helping to reduce distortion. Designed and fabricated in 1P7M 28 nm bulk CMOS and using a 1 V supply, the PA achieves +4.2 dBm/9% measured $P_{\text {out}}$ /PAE at −25 dBc EVM for a 250 MHz-wide 64-quadrature amplitude modulation orthogonal frequency division multiplexing signal with 9.6 dB peak-to-average power ratio. The PA also achieves 35.5%/10% PAE for continuous wave signals at saturation/9.6 dB back-off from saturation. To the best of the authors’ knowledge, these are the highest measured PAE values among published ${K}$ - and Ka -band CMOS PAs.

Toward a More Generalized Doherty Power Amplifier Design for Broadband Operation

The conventional Doherty power amplifier (DPA) theory is limited to single carrier operations, leading to a nongeneric structure. This paper presents a new analysis that generalizes the conventional DPA (CDPA) theory for increased efficiency and bandwidth. We demonstrate that by introducing a theoretical parameter $\alpha$ at the output combiner, we can redefine the relationships among the output combiner elements for a greater level of design flexibility than it was possible in the CDPA. We also show that previously published works in this area can be considered as special cases of the proposed general theory. As a demonstrator, a specific design, named reduced- $\alpha$ DPA, realized using GaN HEMTs is provided to illustrate the robustness of the approach. This design proves effective for further improving the performance of the previously published 2.14-/2.655-GHz dual-band parallel DPA. A maximum drain efficiency of 84% and 67% at an average of 43-dBm peak and 6-dB back-off power levels, respectively, was measured with continuous wave signals. To quantify the linearity performance, the proposed DPA was tested using wideband CDMA and long-term evolution signals where the adjacent channel leakage ratio was recorded at −25 dBc with an average output power of 38.7 and 36.5 dBm at 2.14 and 2.655 GHz, respectively.

Perfect Isolation: Dealing with Self-Jamming in Passive RFID Systems

Passive RF identification (RFID) readers transmit a continuous wave (CW) powering signal to remotely power up batteryless transponders, while simultaneously receiving a backscattered signal at the same frequency [1]. To separate the transmitter and receiver paths, readers use either a bistatic or monostatic antenna configuration. The first configuration employs two separate antennas, one to transmit and one to receive; in the second, the transmitter and receiver share a common antenna by means of a coupler or circulator. Both cases, however, suffer from poor transmitter-to-receiver isolation: strong CW signals can leak into the receiver, degrading its sensitivity to the weak transponder backscattered signals. Receiver sensitivity degradation occurs due to the leaked local oscillator (LO) phase noise [1] and also to the saturation of the front end caused by strong CW leakage. This requires high-dynamic-range capability in the RF stage [the low-noise amplifier (LNA) and mixer] as well as in the baseband stage [the amplifiers and analog-to-digital converters (ADCs)].

An S-band solid-state radio frequency power amplifier used at Shanghai soft X-ray FEL facility

In this paper, we present the general design methods and parameter measurements of a 1-kW solid-state radio frequency (RF) power amplifier at 2856 MHz, for the soft X-ray free electron laser facility. Three-stage amplification with a 4-way combination is used. An RF switch module is integrated with the solid-state RF power amplifier to convert the continuous wave (CW) signal into pulse signal, with adjustable pulse width. The power gain is measured at 57.7 dB at 60 dBm output. The RF phase noise, which is measured by the low-level RF system, is <0.015 degree (RMS), while the pulse frontier jitter is <5 ns.

Kramers–Kronig coherent receiver

The interest for short-reach links of the kind needed for inter-data-center communications has fueled in recent years the search for transmission schemes that are simultaneously highly performing and cost effective. In this work we propose a direct-detection coherent receiver that combines the advantages of coherent transmission and the cost-effectiveness of direct detection. The working principle of the proposed receiver is based on the famous Kramers–Kronig (KK) relations, and its implementation requires transmitting a continuous-wave signal at one edge of the information-carrying signal spectrum. The KK receiver scheme allows digital postcompensation of linear propagation impairments and, as compared to other existing solutions, is more efficient in terms of spectral occupancy and energy consumption.

A 77-GHz 2T6R Transceiver With Injection-Lock Frequency Sextupler Using 65-nm CMOS for Automotive Radar System Application

In this paper, a CMOS multichannel transceiver (TRX) is proposed for angular identification in automotive car radar applications. Two transmitters (TXs) and six receivers (RXs) are placed on the same die using 65-nm CMOS technology with chip size $3.5 \times 3$ mm2. To generate a millimeter-wave (MMW) frequency-modulated continuous-wave signal, an injection-lock frequency sextupler cascaded a 1-to-8 Wilkinson power dividing network with wideband isolation is designed as the LO-chain to convert external source from 12.5–13.7 to 75–82.2 GHz for both the TXs and RXs. Each TX achieves above 11-dBm output power and the RXs achieve 30-dB conversion gain from 75 to 82 GHz, and the total power dissipation of whole chip is 1.43 W. Compared with other published multichannel TRXs in silicon germanium (SiGe) process, this paper demonstrates compatible performances and the potential of multichannel TRX using CMOS for MMW automotive car radar application.

A cascadable all-optical half-subtracter based on cross-modulation effects in a single highly nonlinear fiber (HNLF)

An all-optical half-subtracter is proposed based on harnessing of 3rd order (χ(3)) non-linear effects such as cross-gain modulation (XGM) and cross-phase modulation (XPM) inside a single highly non-linear finer (HNLF), which is capable of operating at 100 Gbps operating bit rate. The XGM effect occurrence in HNLF has been exploited for the implementation of both borrow logics \(\left( {A.\bar{B}\,\& \,\bar{A}.B} \right)\). Whereas, the difference output of proposed half-subtracter represented by \(A \oplus B\) logic is obtained from XPM process taking place on low-energy continuous-wave (CW) probe signal which is co-propagating with two high energy pump signals. The proposed half-subtracter can exhibit cascadability, owing to single wavelength centric nature of all the three logic functions generated such as \(A.\bar{B}\), \(\bar{A}.B\,\&\,A \oplus B\). The proposed half-subtracter functionality is verified through inspection of temporal waveforms and eye diagrams of achieved output logic functions. Further, dependence of output function’s Q-factor and Extinction Ratio (dB) values on following parameters: pump/data signal powers (PA\(\&\)PB), CW probe power (PCW) and choice of CW probe operating wavelength (λCW) is also investigated and discussed. In the final stages, an investigation has been carried out by implementing secondary logic functions \(\left( {\left( {A.\bar{B}} \right).\bar{C},\,\left( {\overline{{A.\bar{B}}} } \right).{\text{C}}\text{,}\,\left( {\bar{A}.B} \right).\bar{C},\,\left( {\overline{{\bar{A}.B}} } \right).C,\,\left( {\overline{A \oplus B} } \right).C\,\& \,\left( {A \oplus B} \right).\bar{C}} \right)\) to examine the cascade-ability characteristics of the proposed half-subtracter. It has been shown by calculating temporal waveforms and eye diagrams of secondary logic functions such as \(\left( {A.\bar{B}} \right).\bar{C},\,\left( {\overline{{A.\bar{B}}} } \right).{\text{C,}}\,\left( {\bar{A}.B} \right).\bar{C},\,\left( {\overline{{\bar{A}.B}} } \right).C, \left( {\overline{A \oplus B} } \right).C\,\& \,\left( {A \oplus B} \right).\bar{C}\) that proposed cross-modulation effects in HNLF based half-subtracter is indeed cascadable.

A Quasi-Doherty SOI CMOS Power Amplifier With Folded Combining Transformer

A fully integrated transformer-based quasi-Doherty power amplifier (DPA) with an adaptive power divider (APD) is presented in this paper. A novel folded combining transformer is designed for power combining, which has smaller insertion loss than a conventional one. An APD adaptively controls the power delivered to carrier and peaking amplifiers by altering the input impedance of the peaking amplifier, which has a variable resonance frequency that changes according to the input power. Most of the power is delivered to the carrier amplifier at low incoming power, and it is divided between the carrier and the peaking amplifiers at high incoming power. With continuous wave signal at 1850 MHz, the quasi-DPA implemented with an SOI CMOS process achieves 39.8% and 44.4% power-added efficiencies (PAEs) at the first and the second peak, respectively. With wideband code division multiple access signal, it has 29.2-dBm average linear output power and a 40.47% PAE with a −33-dBc adjacent channel leakage ratio 1. With long-term evolution (LTE) signal, it delivers 27.2-dBm average linear output power and a 37.7% PAE, satisfying linearity requirements for the LTE.

An Effective Nonlinear Phase Compensation Method for FMCW Terahertz Radar

The nonlinearity of signal usually leads to the range profile blurring and defocusing in the terahertz (THz) radar ranging, which should be compensated to achieve a well-focused range profile. In this letter, an effective nonlinear phase compensation method is proposed to compensate for the signal distortion caused by the radar waveform nonlinearities. It comprises two processing steps, namely, coarse phase compensation and precise phase compensation, which are based on the time-frequency distribution and the phase gradient autofocus algorithm, respectively. The proposed method is a nonparametric method and can eliminate the errors caused by phase discontinuity of the dechirped frequency-modulated continuous-wave (FMCW) signal. Furthermore, the compensation method can be used in the processing of multi-period real data that are obtained from the FMCW THz radar.

A Simplified Technique for Distance and Velocity Measurements of Multiple Moving Objects Using a Linear Frequency Modulated Signal

In this paper, we propose a simplified algorithm for the unbiased estimation of range and the radial velocity of moving objects observed by a sensor with a frequency-modulated continuous wave signal. Linear frequency modulated signals and their different modifications are widely used in various microwave sensors for short- to medium-range measurements in automotive, safety, industrial, and other applications. Since the range (distance) to a detected object is measured as a beat frequency at the homodyne receiver output, the Doppler shift results in a biased measurement of the range. This phenomenon has been well known for a long time. The situation becomes more uncertain with an increase in the number of objects within the radar’s field of view. There are many proposed techniques to solve this problem with one main technique adding a number of equations according to the number of detected objects in the hope of finding a single solution. We propose a simplifed system of those equations excluding all uncertainties from the hypothesis tree at the beginning of the procedure. When the performance of the signal processing hardware is limited, the simplicity of the algorithm increases in importance. The proposed technique is simple enough to be used on an embedded microprocessor. We performed an experiment involving a real microwave sensor to confirm a validity of the proposed algorithm.

Analysis and Validation of Active Eavesdropping Attacks in Passive FHSS RFID Systems

In this paper, we present a generalized framework for active eavesdropping in a frequency hopping spread spectrum passive radio frequency identification system. In our model, there exists an adversarial reader who is able to transmit its own continuous wave signal outside the frequency band of the legitimate reader. Due to the fact that under backscatter modulation, the tag cannot distinguish different frequencies and simply sets the impedance in its circuitry to either low or high to reflect a bit of 1 or 0, and the adversarial reader’s received signal is a weighted sum of the response to both its own signal and the legitimate reader’s signal. Using this model, we provide a theoretical analysis of the capability of the adversarial reader in terms of the decoding error probability for slow frequency and fast frequency hopping systems. We derive analytic formulas and conduct experiments using software defined radios that act as the legitimate reader, the adversarial reader, and Intel Wireless Identification Sensing Platform tags with parameters as specified in EPC Gen2. Simulations are also used to validate our findings. We find from the theoretical analysis as well the experimental results that the active eavesdropper can achieve a better decoding error rate than a conventional passive eavesdropper, even in the case that the eavesdropper’s signal is a low power signal.

Nonlinear Optical Angle Modulation for Suppression of RF Interference

A new optical technique for suppression of electronic interference is detailed theoretically and experimentally. The method is based on operating an angle-modulated photonic link in its nonlinear regime, where Bessel functions govern the response. This novel signal-processing technique is analyzed for optical intensity and phase modulation. Experimental results demonstrate suppression of continuous-wave, pulsed, and chirped signals at levels ranging from at least 30 dB to upwards of 70 dB relative to small-signal conditions.

Hidden Markov model tracking of continuous gravitational waves from a neutron star with wandering spin

Gravitational wave searches for continuous-wave signals from neutron stars are especially challenging when the star's spin frequency is unknown a priori from electromagnetic observations and wanders stochastically under the action of internal (e.g. superfluid or magnetospheric) or external (e.g. accretion) torques. It is shown that frequency tracking by hidden Markov model (HMM) methods can be combined with existing maximum likelihood coherent matched filters like the F-statistic to surmount some of the challenges raised by spin wandering. Specifically it is found that, for an isolated, biaxial rotor whose spin frequency walks randomly, HMM tracking of the F-statistic output from coherent segments with duration T_drift = 10d over a total observation time of T_obs = 1yr can detect signals with wave strains h0 > 2e-26 at a noise level characteristic of the Advanced Laser Interferometer Gravitational Wave Observatory (Advanced LIGO). For a biaxial rotor with randomly walking spin in a binary orbit, whose orbital period and semi-major axis are known approximately from electromagnetic observations, HMM tracking of the Bessel-weighted F-statistic output can detect signals with h0 > 8e-26. An efficient, recursive, HMM solver based on the Viterbi algorithm is demonstrated, which requires ~10^3 CPU-hours for a typical, broadband (0.5-kHz) search for the low-mass X-ray binary Scorpius X-1, including generation of the relevant F-statistic input. In a "realistic" observational scenario, Viterbi tracking successfully detects 41 out of 50 synthetic signals without spin wandering in Stage I of the Scorpius X-1 Mock Data Challenge convened by the LIGO Scientific Collaboration down to a wave strain of h0 = 1.1e-25, recovering the frequency with a root-mean-square accuracy of <= 4.3e-3 Hz.

DOA estimation exploiting a uniform linear array with multiple co-prime frequencies

The co-prime array, which utilizes a co-prime pair of uniform linear sub-arrays, provides a systematical means for sparse array construction. By choosing two co-prime integers M and N, O(MN) co-array elements can be formed from only O(M+N) physical sensors. As such, a higher number of degrees-of-freedom (DOFs) is achieved, enabling direction-of-arrival (DOA) estimation of more targets than the number of physical sensors. In this paper, we propose an alternative structure to implement co-prime arrays. A single sparse uniform linear array is used to exploit two or more continuous-wave signals whose frequencies satisfy a co-prime relationship. This extends the co-prime array and filtering to a joint spatio-spectral domain, thereby achieving high flexibility in array structure design to meet system complexity constraints. The DOA estimation is obtained using group sparsity-based compressive sensing techniques. In particular, we use the recently developed complex multitask Bayesian compressive sensing for group sparse signal reconstruction. The achievable number of DOFs is derived for the two-frequency case, and an upper bound of the available DOFs is provided for multi-frequency scenarios. Simulation results demonstrate the effectiveness of the proposed technique and verify the analysis results.

RF Energy Harvesting From Multi-Tone and Digitally Modulated Signals

This paper presents the design of an RF energy-harvesting circuit when excited by signals with a time-varying envelope such as multi-tone signals or digitally modulated signals with random modulation. The input matching network and the output load of a rectifier circuit are simultaneously optimized using harmonic balance in order to maximize its RF–dc conversion efficiency. This paper focuses on identifying the optimum load value, which corresponds to maximum efficiency for different types of input signals. The efficiency curves versus the load value show a single optimum efficiency point, which is a different for signals with a time-varying envelope and continuous wave (CW) signals. Specifically, for the series diode rectifier that was considered, the optimal load shifts to larger values as the signal peak-to-average-power-ratio (PAPR) increases compared to a CW signal with the same average power. As a result, for certain load values a signal with a time-varying envelope can result in a larger efficiency value than a CW signal. The peak efficiency value does not necessarily improve by using a signal with a time-varying envelope. A UHF rectifier prototype is built and its performance is evaluated experimentally showing good agreement with simulation.