Baseband Signal Research Articles

Photonics-Based Broadband Radar With Coherent Receiving for High-Resolution Detection

A microwave photonics radar with interference-suppressed high-resolution detection ability is proposed and experimentally demonstrated based on optical frequency quadrupling and coherent receiving de-chirping. Which is achieved by using a dual-parallel Mach-Zehnder modulator to perform base-band signal frequency quadrupling in transmitter, and an optical coherent receiver to realize balanced photonic in-phase/ quadrature (I/Q) de-chirping of the broadband radar echoes in receiver. Thanks to the frequency multiplication, a wideband radar transmitting signal can be generated with low speed electron devices. Meanwhile, interference-free detection by suppressing undesired common-mode interferences and image frequency could be achieved with balanced photonic I/Q de-chirping broadband radar echoes. In the experiment, a photonics-based K-band radar with a bandwidth of 8 GHz is demonstrated, corresponding to a resolution better than 2.0 cm. In addition, the balanced photonic I/Q de-chirping receiver achieves an image-rejection ratio of 40.4 dB. The proposed radar detection system is a promising candidate in interference-suppressed high-resolution radar application.

Open-access millimeter-wave software-defined radios in the PAWR COSMOS testbed: Design, deployment, and experimentation

While millimeter-wave (mmWave) wireless has recently gained tremendous attention with the transition to 5G, developing a broadly accessible experimental infrastructure will largely facilitate the research and allow the community to make significant progress in this area. In this paper, we present the design and implementation of various programmable and open-access mmWave software-defined radios (SDRs) that have been deployed in the PAWR COSMOS advanced wireless testbed, in both indoor and outdoor environments. These programmable mmWave radios are based on the IBM 28GHz 64-element dual-polarized phased array antenna module (PAAM) subsystem board and the Sivers IMA 60GHz WiGig transceiver. These front ends are integrated with USRP SDRs or Xilinx RFSoC boards, which provide baseband signal processing capabilities. Moreover, we present measurements of the TX/RX beamforming performance and example experiments (e.g., real-time channel sounding and RFNoC-based 802.11ad preamble detection), using the mmWave radios. Finally, we discuss ongoing enhancement and development efforts focusing on these radios.

Scheme to eliminate the time shift of code edges based on the optimal transmission point of a DP-MZM.

To eliminate the time shift of code edges on a single-sideband (SSB) modulation signal transmission in a radio-over-fiber (RoF) system, a new, to the best of our knowledge, SSB modulation scheme based on an optimal transmission point for a double-parallel Mach-Zehnder modulator (DP-MZM) is proposed. The scheme is based on DP-MZM to realize the separation of the carrier and the +1st-order sideband at the optimal transmission point, and the baseband signal modulates the 2.5Gb/s data signal to the +1st-order sideband of the SSB signal; then, the carrier and the +1st-order sideband are transmitted with a carrier-to-sideband ratio of 0dB. Theoretical analysis shows that compared to the traditional SSB-modulated optical millimeter-wave signal generation scheme this scheme completely solves the problem of the time shift of code edges caused by dispersion. The simulation results show that the improved SSB modulation scheme has a Q factor of 23.362, the minimum bit error rate is 4.207×10-127 at 73.453km, and the eye diagram is still very clear. Under the premise of meeting the basic requirements of communications, the maximum communications distance can reach 135km, which is 270% of the transmission distance of a traditional SSB modulation model.Thus, the system performance has been greatly improved.

Identification of Terahertz Link Modulation in Atmospheric Weather Conditions

With the rapid increase of wireless connectivity, current spectrum resources are not enough for significant requirements for large data capacity. Research interests are moving towards the high-frequency band in the terahertz range for wider bandwidth. However, multipath scattering and induced time delay, suffered by terahertz links propagating in outdoor weather, lead inevitably to increasing expenses in baseband signal processing. This trades away the advantage of low time latency and high stability, which are commonly considered as important merits of terahertz wireless communication techniques. To reduce the burden in signal processing and explore the feasibility of modulation identification in the terahertz band, a method of wireless link modulation identification is considered as a potential solution. In this work, it is investigated theoretically by employing two kinds of neural networks: the convolutional neural network (CNN) and the long short-term memory network (LSTM). Link deterioration caused by different atmospheric weather is introduced into the theoretical model, and the performance of this method is evaluated. Results show that the identification accuracy of the constructed neural networks can be up to 99%, which means such a method is efficient for identification of the modulation format of terahertz wireless links under different weather conditions. This work demonstrates the feasibility of modulation identification in outdoor terahertz communication scenarios and provides specific references.

Mode-Locking and Noise Characteristics of InAs/InP Quantum Dash/Dot Lasers

The mode-locking and noise characteristics of InP/InAs quantum dash (QDash) and quantum dot (QDot) multi-wavelength lasers, showing identical structural design, operating at the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> -band, are investigated and compared to each other. The QDash lasers exhibit improved repetition frequency stability with a lower threshold current and cavity loss. On the other hand, the QDot lasers show higher quality repetition frequency tunability with higher internal quantum efficiency, as well as lower average integrated relative intensity noise (RIN) and average optical linewidth. Furthermore, we demonstrate both the QDash and QDot lasers exhibit very clean constellation diagrams at 32 GBaud 16QAM base-band signal, while the QDot lasers' bit-error ratio (BER) performance outperforms the QDashes. This finding highlights the viability of InAs/InP QDash/QDot lasers to be used as a low-cost optical source for large-scale networks.

Design of a Baseband Signal for the 406 MHz Satellite Emergency Radio Transmitter Based on STM32

The baseband signal of the 406 MHz COSPAS-SARSAT emergency beacon is a 144-bit or 112-bit data with 0 and 1 code, including the information bits and error correcting check bits. In order to improve the coding efficiency of the baseband signal for the 406 MHz transmitter in the STM32 micro-controller, this paper proposes a calculation method for the first BCH error correcting code (BCH-1) and the second BCH error correcting code (BCH-2) of the COSPAS-SARAST transmitter. First, conduct the data coding process by 64-bit integral data; then, compute the BCH (127, 106) code and the BCH (63, 51) code to obtain the respective error correcting codes for improving coding rate; next, generate the baseband signal according to the GB 14391-2021 coding format; finally, modulate the baseband signal into a 406 MHz-RF signal, send it out and complete the task of distress alerting. The experimental results show that the proposed STM32 microcontroller-based baseband signal generation method is effective, and the EPIRB (Emergency Position Indicating Radio Beacon) designed based on this method has reached the industry standard.

Noise Effects on a Proposed Algorithm for Signal Reconstruction and Bandwidth Optimization

The development of wireless technology in recent years has increased the demand for channel resources within a limited spectrum. The system's performance can be improved through bandwidth optimization, as the spectrum is a scarce resource. To reconstruct the signal, given incomplete knowledge about the original signal, signal reconstruction algorithms are needed. In this paper, we propose a new scheme for reducing the effect of adding additive white Gaussian noise (AWGN) using a noise reject filter (NRF) on a previously discussed algorithm for baseband signal transmission and reconstruction that can reconstruct most of the signal’s energy without any need to send most of the signal’s concentrated power like the conventional methods, thus achieving bandwidth optimization. The proposed scheme for noise reduction was tested for a pulse signal and stream of pulses with different rates (2, 4, 6, and 8 Mbps) and showed good reconstruction performance in terms of the normalized mean squared error (NMSE) and achieved an average enhancement of around 48%. The proposed schemes for signal reconstruction and noise reduction can be applied to different applications, such as ultra-wideband (UWB) communications, radio frequency identification (RFID) systems, mobile communication networks, and radar systems.

A 28-GHz, Multi-Beam, Decentralized Relay Array

Multi-beam relays can be used to overcome the non-line-of-sight (NLOS) issues in millimeter-wave (mm-wave) communication networks that serve many users in an obstruction-rich environment. We demonstrate a modular multi-beam relay array at 28 GHz, which is scalable and fully decentralized. The individual branches of the array are independent and do not need to share a timing reference or be physically located on the same substrate. Multi-beam capability is facilitated by a multi-channel baseband signal conditioning chain that includes a reconfigurable, passive, inductorless, 3rd-order <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N$</tex-math> </inline-formula> -path filter with programmable frequency-independent phase shift. This baseband signal conditioning chain enables beams concurrently multiplexed in spatial and frequency domains. We present the theory, design, and operation of the custom RFIC, which enables independent multi-beam relaying. Multiple RFICs are used to create relay arrays. A four-element relay array demonstrates three independently steered beams that utilize the full array aperture with a total wireless throughput of 625 Mb/s.

Joint Direct Estimation of Emitter Location and Antenna Beam Direction Based on Baseband Reconstruction

This paper addresses the problem of locating a stationary noncooperative emitter using multiple moving receivers. A signal baseband reconstruction based algorithm is proposed to locate the emitter directly from the received signals, without assuming static delay and Doppler shift in any observing interval. Besides the information of time-differences-of arrival (TDOA) and frequency-differences-of-arrival (FDOA), the received signal strength (RSS) information is further utilized when the source emits signals in free space with an omni antenna. In a more complex scenario where the emitter adopts a directional antenna, an iterative algorithm is proposed to jointly estimate the emitter location and its antenna beam direction, where a traditional direct position determination (DPD) method is used to initiate the iteration. The Cramér-Rao lower bounds (CRLBs) of the emitter position and the beam direction are derived. Three numerical cases are included to investigate the performance of the proposed algorithms. The signal baseband reconstruction based DPD algorithm shows improved applicability when the assumption of static time delay and Doppler shift is inappropriate and performs similarly with the classical DPD methods when the assumption holds. In case of free space propagation, it shows that the localization accuracy can be improved when RSS is further utilized. When the transmitting beam is directional, however, simply ignoring the directivity characteristics would lead to nonnegligible localization biases. By jointly estimating the emitter location and antenna beam direction, the RSS information can be effectively utilized to improve the localization accuracy, and a much more accurate estimation of beam direction can be obtained simultaneously.

Utilizing same WDM wavelengths for delivering both baseband and FSO signals in ring-type access network

Utilizing same WDM wavelengths for delivering both baseband and FSO signals in ring-type access network

Dual-use baseband signal design for RadCom with position index and phase modulation

This paper deals with the synthesis of dual-use signal to simultaneously achieve detection and communication for a dual-function radar communication (DFRC) system. A new position index and phase (PIP) modulation is proposed for information embedding, where position index modulation is realized by selecting the information embedding subpulses (InSp) positions. Moreover, differential phase modulation is utilized to distinguish InSp from other available subpulses. As for radar detectability, peak sidelobes level (PSL) of signal auto-correlation function is minimized. Meanwhile, constant-envelope and phases constraints are forced to ensure compatibility with the current hardware technique and modulation demand, respectively. To tackle the resulting nonconvex and NP-hard optimization problem, a spectral majorization-minimization (SM) dual-use baseband signal design method is proposed. In each iteration, the original problem is approximated by a tractable problem with a closed-form solution. Finally, simulation and semi-physical experimental results demonstrate the designed dual-use signal is capable of realizing high data rate communication and ensuring acceptable radar performance.

RF Coverage Design for the Implementation of a Broadband Monitoring Service in the Context of 5G-Enabled Smart Cities

As the recent COVID-19 pandemic has aptly demonstrated, emergency scenarios concerning public health and safety may require citizens to remain at home even as patients, potentially in the context of a municipal or national lockdown. Homestay patients may require real-time monitoring, which will offer not only communication but also empirical data and will contribute to developing their personalized electronic health record in knowledge bases. Our paper features an extensive RF coverage design for such a municipally deployed and administered 5G-enabled smart city network, supporting a broadband monitoring service. The antenna deployment for the outdoor urban topology is analytically described (for the downlink channel), and the intrinsic indoor propagation characteristics are considered for the uplink channel. A digital baseband signaling scheme is assumed on the basis of a user-customized health-related monitoring service. Path loss and fading calculations consider the potential worst-case propagation conditions so that the RF coverage will be reliable, leading to a resilient city-wide municipal network.

Signal Reconstruction Deep Residual Neural Network-Based Bandwidth Augmented Methods for DPD Linearization

In this letter, a signal reconstruction deep residual neural network (SRDRNN)-based bandwidth augmented method for digital predistortion (DPD) linearization of a radio frequency (RF) power amplifier (PA) is proposed. The SRDRNN is utilized to interpolate the samples from the analog-to-digital (ADC) converter in the DPD feedback path of the RF PA so as to increase its bandwidth. The experimental results illustrate that the SRDRNN can accurately produce the out-of-band spectrum regrowth of the equivalent baseband signal for a PA. It can also be directly applied to another PA with the same type. Finally, the effectiveness of SRDRNN-based bandwidth augmented method is verified with DPD linearization experiment. Both of the feedback signals reconstructed by the SRDRNN for the training PA and the test PA exhibit an excellent linearization performance.

Distributed MIMO CW Radar for Locating Multiple People and Detecting Their Vital Signs

The distributed-multiple-input multiple-output (MIMO) continuous-wave (CW) radar with low-intermediate frequency (low-IF) receiver that is proposed in this work uses uniform linear array (ULA) antennas to estimate the direction of arrival (DOA) of subjects and recover Doppler signals by the digital beamforming (DBF) technique. Without using any modulation bandwidth, the location of multiple subjects can be determined from the known antenna positions and the estimated DOAs. This work proves that the Doppler-weighted method improves the extraction of the initial phase of the baseband signal. With respect to the recovery of each subject’s vital signs, the modified-DBF is utilized to separate individual Doppler signals while nullifying interference. The frequency estimation algorithm (FEA) is used to improve the Doppler frequency and angular resolution by 50% and 44%, respectively, in the detection of two closely spaced moving metal plates. Finally, the presented system is experimentally validated by determining the locations and vital signs of people sitting side-by-side. The estimated DOA and position errors were less than 3.3° and 10 cm, respectively. With a 25 s sensing period, the errors in the measured respiration rates (RRs) and heart rates (HRs) were less than 1.2 breath/min and 3 beats/min, respectively.

Real-Time Demonstration of 100 GbE THz-Wireless and Fiber Seamless Integration Networks

We propose and experimentally demonstrate a real-time fiber-THz-fiber 2 × 2 multiple-input multiple-output (MIMO) seamless integration system at 340–510 GHz using commercial digital coherent optics (DCO) modules for baseband signals processing, which realizes a record net rate of 103.125 Gb/s dual polarization quadrature-phase-shift-keying (DP-QPSK) signals delivery over two spans of 20 km wireline single-mode fiber-link and 3 m wireless 2 × 2 MIMO link without using THz power amplifier under 15% soft-decision forward-error-correction threshold (SD-FEC) of 1.56 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> . Two kinds of THz-to-optical conversion schemes based on an integrated dual-polarization Mach-Zehnder modulator (DP-MZM) and two discrete intensity-modulators (IMs) are extensively investigated and compared. To the best of our knowledge, this is the first real-time demonstration of THz-wireless and 100 GbE fiber-optic networks seamless integration transmission, which is compatible with IEEE 802.3 and ITU-T G.798 specifications. It is a promising scheme to meet the demands of future fiber-wireless-integrations communication for low power consumption, low cost and miniaturization.

Robust Design for IRS-Aided C-RAN With Constrained Wireless Fronthaul Links

In this paper, an intelligent reflecting surface (IRS)-aided downlink cloud radio access network (C-RAN) is studied, wherein the baseband unit (BBU) pool communicates with the users through multiple remote radio heads (RRH). Specifically, the IRSs are deployed to assist both the wireless fronthaul link and the user access link. Due to the limitation of the fronthaul capacity, the BBU pool deals with the baseband signals by point-to-point compression or multivariate compression and sends the quantization bits to the RRHs via the fronthaul link. Considering the imperfect channel state information (CSI) for both the conventional direct channel and the IRS cascaded channel, we investigate the robust design by jointly optimizing the transmit beamformers for BBU pool and RRHs, the passive beamformer for IRSs and the fronthaul compression under point-to-point compression and multivariate compression, respectively. Specifically, we derive the closed-form user downlink rate and the achievable rate for the wireless fronthaul under imperfect CSI. To tackle the non-convex problem, we adopt the successive convex approximation (SCA) approach and transform it into a tractable form. Then utilizing the alternating optimization (AO) approach, we divide the transformed problem into three sub-problems which can be efficiently solved. Furthermore, since the fronthaul link and access link work on time division duplex (TDD) mode, we also discuss the joint optimization taking the time allocation into consideration. Finally, via numerical results, the effectiveness of the proposed robust design is verified.

Accounting for Acceleration—Signal Parameters Estimation Performance Limits in High Dynamics Applications

The derivation of estimation lower bounds is paramount to designing and assessing the performance of new estimators. A lot of effort has been devoted to the range-velocity estimation problem, a fundamental stage on several applications, but very few works deal with acceleration, being a key aspect in high dynamics applications. Considering a generic band-limited signal formulation, we derive a new general compact form Cramér–Rao lower bound (CRB) expression for joint time-delay, Doppler stretch, and acceleration estimation. This generalizes and expands upon known delay/Doppler estimation CRB results for both wideband and narrowband signals. This new formulation, especially easy to use, is created based on baseband signal samples, making it valid for a variety of remote sensors. The new CRB expressions are illustrated and validated with representative GPS L1 C/A and linear frequency modulated chirp band-limited signals. The mean-square error of a misspecified estimator (conventional delay/Doppler) is compared with the derived bound. The comparison indicates that for some acceleration ranges the misspecified estimator outperforms a well-specified estimator that accounts for acceleration.

A Fully-Integrated Wideband Digital Polar Transmitter With 11-bit Digital-to-Phase Converter in 40nm CMOS

This work presents a wideband digital polar transmitter (DPTX) with an integrated capacitor-digital-to-analog converter (C-DAC)-based digital-to-phase converter (DPC). A normalized CORDIC algorithm is introduced to convert the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I/Q$ </tex-math></inline-formula> baseband signals to a constant envelope to overcome the PM bandwidth expansion issue and avoid the amplitude modulation (AM)–phase modulation (PM) nonlinearity. In the DPC, the switched-capacitor DAC topology is employed for good linearity, and the eight-phase cell-reused technique is proposed to reduce the power consumption and increase the phase amplitude. Besides, the harmonic rejection technique is introduced to remove the third-/fifth-order and higher order local oscillator (LO) harmonics to maintain phase linearity. A broadband switched-transformer digital power amplifier (DPA) is adopted in this proposed DPTX for 0–18-dB deep power back-off efficiency enhancement. Implemented in 40nm CMOS, the DPTX is powered by only one 1.1-V supply and occupies a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.4 \times 1.4$ </tex-math></inline-formula> mm2 die area. The DPC achieves the maximum integral nonlinearity (INL)/differential nonlinearity (DNL) of ±1°/±0.8° and 12.7-mW power consumption at 1.5 GHz. The DPTX realizes wideband frequency coverage of 1.2–2.5 GHz with only 0.7-dB power variations. The DPTX achieves 20.1-dBm peak output power with 23.7% system efficiency at 1.5 GHz. With a 10-MHz 64-quadrature amplitude modulation (QAM) LTE signal, the DPTX achieves 15.0 dBm average output power, 15.2% system efficiency, and an error vector magnitude (EVM) of −28.6 dB at 1.5 GHz.

A Distortion-Free All-Digital Transmitter Based on Noise-Shaped PWM

A novel pulse-width modulation (PWM) architecture for all-digital transmitters (ADT) that achieves good in-band and out-band performance with a very low ratio between the modulated signal frequency and the carrier frequency is presented in this paper. The proposed scheme reduces the harmonics leaving only those corresponding to multiples of the carrier frequency, overcoming the spectral limitations of other PWM implementations and relaxing the filter requirements. The binary signal is obtained using a table-based approach with very low computational complexity, mapping the complex I/Q baseband signal to duty cycles of a square wave with low distortion. In addition, a new parallelized noise shaping circuit to reduce the distortion caused by the discretization of the duty-time intervals is presented. The digital radio frequency signal is generated using a high-speed serializer and a standard pulse former. The ADT is simulated under different scenarios to evaluate the influence of time resolution in terms of Adjacent Channel Power Ratio (ACPR) and Error Vector Magnitude (EVM). The transmitter performance is verified by an FPGA implementation using 64-QAM modulated signals, achieving values of ACPR1, ACPR2, and EVM better than −47 dBc, −53 dBc, and −35 dB, respectively, using 64 quantization levels for the PWM signal.

Photonic Generation of Dual-Band Microwave Waveforms with Simultaneous and Diverse Modulation Formats

A photonic method of generating dual-band microwave waveforms with simultaneous and different formats is proposed and experimentally verified using a dual-polarization dual-parallel Mach-Zehnder Modulator (DP-DPMZM). The four sub-MZMs of DP-DPMZM are driven by two independent radio frequency (RF) signals and two independent baseband signals. After photo-detection, two baseband signals are up-converted into two different higher frequency bands. The two RF signals and two baseband signals are utilized to adjust the central frequencies and modulation formats of two frequency bands, respectively. Dual-band phase-coded microwave signal with different bit rates and coding patterns at two different frequency bands, dual-band dual-chirp microwave signal with different chirp rates, and one-band dual-chirp and one-band phase-coded microwave signal are successfully generated in our experiments. The central frequencies of two frequency bands are independently tunable, and the modulation formats of waveforms at two frequency bands can also be diverse, which brings more convenience and feasibility into real-world applications. The flexible tunability in the central frequencies and modulation formats of different frequency bands can meet different requirements for radar and communication systems.