Digital Baseband Research Articles

Design of modulated signal waveform in fully digital programmable optical frequency comb

AbstractThis paper presents a new design method of digital baseband signal for generating fully digital programmable optical frequency comb (OFC), which solves the problem of how to optimally select the digital baseband signal when OFC is generated by binary digital signal modulation. The inverse Fourier transform is used to study the waveform of binary digital signal which is used to generate OFC. It is found that the number of “0” bits of digital signal determines the flatness of comb lines. Based on this, a tunable ultra‐fine and ultra‐flat OFC was obtained by using a digital signal containing a large number of “0” bits in the experiment. This saves substantial computing time and resources compared to the current method of the k‐means clustering algorithm.

Spectral Efficiency Maximization for Mixed-Structure Cognitive Radio Hybrid Wideband Millimeter-Wave Transceivers in Relay-Assisted Multi-User Multiple-Input Multiple-Output Systems.

This paper proposes a cognitive radio network (CRN)-based hybrid wideband precoding for maximizing spectral efficiency in millimeter-wave relay-assisted multi-user (MU) multiple-input multiple-output (MIMO) systems. The underlying problem is NP-hard and non-convex due to the joint optimization of hybrid processing components and the constant amplitude constraint imposed by the analog beamformer in the radio frequency (RF) domain. Furthermore, the analog beamforming solution common to all sub-carriers adds another layer of design complexity. Two hybrid beamforming architectures, i.e., mixed and fully connected ones, are taken into account to tackle this problem, considering the decode-and-forward (DF) relay node. To reduce the complexity of the original optimization problem, an attempt is made to decompose it into sub-problems. Leveraging this, each sub-problem is addressed by following a decoupled design methodology. The phase-only beamforming solution is derived to maximize the sum of spectral efficiency, while digital baseband processing components are designed to keep interference within a predefined limit. Computer simulations are conducted by changing system parameters under different accuracy levels of channel-state information (CSI), and the obtained results demonstrate the effectiveness of the proposed technique. Additionally, the mixed structure shows better energy efficiency performance compared to its counterparts and outperforms benchmarks.

High-Throughput Polar Code Decoders with Information Bottleneck Quantization.

In digital baseband processing, the forward error correction (FEC) unit belongs to the most demanding components in terms of computational complexity and power consumption. Hence, efficient implementation of FEC decoders is crucial for next-generation mobile broadband standards and an ongoing research topic. Quantization has a significant impact on the decoder area, power consumption and throughput. Thus, lower bit widths are preferred for efficient implementations but degrade the error correction capability. To address this issue, a non-uniform quantization based on the Information Bottleneck (IB) method is proposed that enables a low bit width while maintaining the essential information. Many investigations on the use of the IB method for Low-density parity-check code) LDPC decoders exist and have shown its advantages from an implementation perspective. However, for polar code decoder implementations, there exists only one publication that is not based on the state-of-the-art Fast Simplified Successive-Cancellation (Fast-SSC) decoding algorithm, and only synthesis implementation results without energy estimation are shown. In contrast, our paper presents several optimized Fast-SSC polar code decoder implementations using IB-based quantization with placement and routing results using advanced 12 nm FinFET technology. Gains of up to 16% in area and 13% in energy efficiency are achieved with IB-based quantization at a Frame Error Rate (FER) of 10-7 and a polar code of N=1024,R=0.5 compared to state-of-the-art decoders.

Intra-Beam Interference Mitigation for the Downlink Transmission of the RIS-Assisted Hybrid Millimeter Wave System.

Millimeter-wave (mmWave) communication systems leverage the directional beamforming capabilities of antenna arrays equipped at the base stations (BS) to counteract the inherent high propagation path loss characteristic of mmWave channels. In downlink mmWave transmissions, i.e., from the BS to users, distinguishing users within the same beam direction poses a significant challenge. Additionally, digital baseband precoding techniques are limited in their ability to mitigate inter-user interference within identical beam directions, representing a fundamental constraint in mmWave downlink transmissions. This study introduces an innovative analog beamforming-based interference mitigation strategy for downlink transmissions in reconfigurable intelligent surface (RIS)-assisted hybrid analog-digital (HAD) mmWave systems. This is achieved through the joint design of analog beamformers and the corresponding coefficients at both the RIS and the BS. We first present derived closed-form approximation expressions for the achievable rate performance in the proposed scenario and establish a stringent upper bound on this performance in a large number of RIS elements regimes. The exclusive use of analog beamforming in the downlink phase allows our proposed transmission algorithm to function efficiently when equipped with low-resolution analog-to-digital/digital-to-analog converters (A/Ds) at the BS. The energy efficiency of the downlink transmission is evaluated through the deployment of six-bit A/Ds and six-bit pulse-amplitude modulation (PAM) signals across varying numbers of activated RIS elements. Numerical simulation results validate the effectiveness of our proposed algorithms in comparison to various benchmark schemes.

Design and Implementation of an Event-Driven Smart Sensor Node for Wireless Monitoring Systems.

In this paper, an event-driven wireless sensor node is proposed and demonstrated. The primary design objective is to devise a wireless sensor node with miniaturization, integration, and high-accuracy recognition ability. The proposed wireless sensor node integrates two vibration-threshold-triggered energy harvesters that sense and power a threshold voltage control circuit for power management, a microcontroller unit (MCU) for system control, a one-dimensional convolutional neural network (1D-CNN) environment data analysis and vibration events distribution, and a radio frequency (RF) digital baseband transmitter with IEEE 802.15.4-/.6 protocols. The dimensions of the wireless sensor node are 4 × 2 × 1 cm3. Finally, the proposed wireless sensor node was fabricated and tested. The alarming time for detecting the vibration event is less than 6 s. The measured recognition accuracy of three events (knock, shake, and heat) is over 97.5%. The experimental results showed that the proposed integrated wireless sensor node is very suitable for wireless environmental monitoring systems.

Cell-free ISAC massive MIMO systems with capacity-constrained fronthaul links

Integrated sensing and communication (ISAC), millimeter wave (mmWave) massive multiple-input multiple-output (MIMO), and cell-free networks are emerged as some of the most potential physical layer enablers for the future wireless communication networks. In this paper, we consider a cell-free ISAC mmWave massive MIMO system with hybrid analog and digital precoders while accounting for the impacts of capacity-constrained fronthaul links. In the system, the digital baseband processing is dictated at a geographical scale while the analog precoders are carried out at each transmit access point (AP). Based on the characteristics of the considered model, we formulate the optimization problem as a function of power allocation and fronthaul compression strategies. To address this problem, two block coordinate descend (BCD)-based methods are proposed to alternatively solve the three decomposed subproblems corresponding to the power allocation, the downlink and backward fronthaul compression strategies. Simulation results indicate that the proposed schemes can achieve 90% performance of the optimal full-search (FS) method with much lower computational complexity. In addition, there exists an optimal configuration of APs, UEs, and antennas for a cell-free ISAC system with capacity-constrained fronthaul links.

Practical Waveform-to-Energy Harvesting Model and Transmit Waveform Optimization for RF Wireless Power Transfer Systems

The received radio-frequency (RF) power in far-field RF wireless power transfer (WPT)—with or without simultaneous information transfer—is minuscule due to large propagation loss in wireless media. In such scenarios, adapting to the receiver characteristics by transmit waveform optimization is essential for maximizing the harvested direct current (dc) and, thus, the end-to-end efficiency of an RF WPT system. The receiver efficiency in RF WPT is governed by the RF-to-dc efficiency of the rectifier as well as the impedance mismatch at the antenna and load. In this article, we study the receiver efficiency for any fixed load and, subsequently, present a novel rectifier model that relates the average harvested dc power to the distribution, that is, the histogram, of the instantaneous power levels of the RF signal’s envelope over time. The proposed waveform-to-energy harvesting (EH) model enables us to anticipate the average harvested dc power for any waveform, including communication signals as well, given the knowledge of the power-level distribution. Consequently, we conduct rigorous waveform optimization to maximize the average harvested dc power and determine the digital baseband signal at the transmitter that does so, namely prove that a pulsed tone at appropriate frequency is optimal for RF WPT. We present a multiband test-bed for determining the receiver efficiency for any digital baseband waveform. The efficacy of the proposed model is corroborated through experiments as well as simulations, which confirm that it is operational as well as accurate in practice and that single-sine pulses yield higher efficiency than basic multisine waveforms, while a pulsed phase shift keying (PSK) is preferable for simultaneous wireless information and power transfer (SWIPT).

A Fully Integrated RFID Reader SoC.

The traditional RFID reader module relies on a discrete original design. This design integrates a microcontroller, high-frequency RFID reader IC and other multiple chips onto a PCB board, leading to bottlenecks in cost, power consumption, stability and reliability. To align with the trend towards high integration, miniaturization and low power consumption in RFID reader, this paper introduces a fully integrated RFID Reader SoC. The SoC employs the open-source Cortex-M0 core to integrate the RF transceiver, analog circuits, baseband protocol processing, memory and interface circuits into one chip. It's compatible with ISO/IEC 14443 A-type and B-type and ISO/IEC 15693 transmission protocols and rates. Manufactured using a 0.18 μm process, the chip is compatible with multiple standards. The optimized design of the digital baseband control circuit results in a chip area of only 11.95 mm2 offering clear advantages in both area and integration compared to similar work.

An End-to-End Dual ASIC OFDM Transceiver for Ultrasound In-Body Communication.

Implanted medical devices need a reliable, secure and low-energy wireless communication link. Ultrasound (US) wave propagation is promising over other techniques due to its lower body attenuation, inherent security and well-studied physiological impact. While US communication systems have been proposed, they either neglect realistic channel conditions or fail to be integrated into small-scale, energy-scarce systems. Therefore, this work proposes a custom, hardware efficient OFDM modem optimized for the diverse needs of ultrasound in-body communication channels. This custom OFDM modem is implemented in an end-to-end dual ASIC transceiver: a 180nm BCD analog front end and a digital baseband chip in 65nm CMOS technology. Furthermore, the ASIC solution provides tuning knobs to increase the analog dynamic range, to update the OFDM parameters and to fully reprogram the baseband processing, necessary to adjust for the channel variability. Ex-vivo communication experiments on a 14cm thick piece of beef achieve 470kbps with BER 3e-4, while consuming 56nJ/bit and 10.9nJ/bit Tx/Rx.

Improved TDD operation on Software-Defined Radio platforms towards future wireless standards

Software-Defined Radio (SDR) platforms are valuable for research and development activities or high-end systems that demand flexible wireless protocols. While low-latency digital baseband processing can be achieved using a dedicated processing unit, like an FPGA or hardware accelerator, its multi-purpose Radio Frequency (RF) front-end often poses a limitation. Zero Intermediate Frequency (ZIF) transceivers are favorable for SDR, however, even for Time Division Duplex (TDD) systems, these transceivers suffer from self-interference when the transmitting and receiving Local Oscillator (LO) is set to the same frequency. To achieve low self-interference, switching from receiving to transmitting mode is needed. However, the time this takes (turnaround time, TT) for configurable RF front-ends often violates the strict timing requirements of protocols like Wi-Fi and 5G, which require response times in the order of microseconds. In this work, we first evaluate the advantages and disadvantages of several methods to suppress self-interference of a ZIF transceiver. Next, a novel approach is proposed, which can reduce the TT to as low as 640ns using the widely used AD9361 configurable ZIF RF front-end, while the noise floor is at the same level as achieved by the conventional way of switching between transmit and receive mode. We have realized and validated this approach using openwifi — an open-source Wi-Fi implementation on SDR. As a result, the receiver sensitivity is improved by up to 17dB in the 2.4GHz band and 9.5dB in the 5GHz band, for over-the-air transmissions.

Hybrid Multibeamforming Receiver With High-Precision Beam Steering for Low Earth Orbit Satellite Communication

A hybrid multibeamforming receiver for low Earth orbit (LEO) satellite communication in the Ku-band is designed and tested. The proposed receiver is implemented with eight channels to enable multibeam control. The proposed receiver, including both analog and digital beamforming structures, is capable of multibeam reception using both analog and digital beamforming techniques simultaneously or selectively. Analog beamforming is implemented by employing a local oscillator phase shifter using direct digital synthesis and phase-locked loop (PLL) structures. In digital beamforming, a software-defined radio can control the phase of the signal in the digital baseband. The multibeam received using the proposed hybrid multibeamforming receiver was measured at 11.7 GHz and 12.7 GHz in an anechoic chamber. The phase-control resolutions of analog and digital beamforming were 0.022° and 0.72°, respectively. Analog beam-switching speed is slower than digital beamforming because it depends on the settling time of the PLL circuit, but it has a high phase-control resolution. Therefore, by using analog and digital beamforming when precise beam tilting and fast beam switching are required, respectively, the proposed hybrid multibeamforming is shown to be suitable in accordance with the communication situation.

Regularized Zero-Forcing Aided Hybrid Beamforming for Millimeter-Wave Multiuser MIMO Systems

This paper considers hybrid beamforming consisting of analog beamforming (ABF) coupled with digital baseband beamforming (DBF) which is designed for multi-user (MU) multiple input multiple output (MIMO) millimeter-wave (mmWave) communications. ABF uses a limited number of radio frequency (RF) chains and finite-resolution phase-shifters to alleviate the power consumption at the base station (BS), while DBF uses either zero-forcing beamforming (ZFB) or regularized zero forcing beamforming (RZFB) to restrain MU interference. The joint design of ABF and DBF constitutes a computationally challenging mixed discrete continuous optimization problem. The paper develops efficient algorithms for its solution, which iterate scalable-complex expressions. Furthermore, we conceive a new class of MU RZFB for attaining higher rates. Simulations are provided to demonstrate the viability of the proposed algorithms and the advantages of the conceived RZFB.

Optimised design of next‐generation multiplexing schemes for Global Navigation Satellite Systems

AbstractMultilevel and multicarrier component signals are now common in many Global Navigation Satellite Systems challenging the employed multiplexing method that needs to be more flexible and powerful. In this work, the authors demonstrate how, by acting on two parameters of the digital baseband representation of component signals (the sampling frequency and the central frequency of the baseband complex envelope), it is possible to optimise the performance of the multiplexer, while still obtaining a composite signal that fulfils the required system constraints.

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.

Hybrid Beamforming and Relay Selection for End-to-End SNR Maximization in Single-User Multi-Relay MIMO Systems.

This paper proposes a novel hybrid beamforming and relay selection scheme for spectral efficiency maximization in a non-regenerative multi-relay multi-input multi-output (MIMO) system. The analog beamforming component in the radio-frequency (RF) domain must follow an element-wise constant modulus constraint, which makes the underlying design problem mathematically intractable and therefore, it is quite challenging to obtain the global optimal solution. To address this problem, phase-only precoding/combining matrices are derived by maximizing the end-to-end received signal-to-noise ratio (SNR) under transmit power constraint at the source and each relay node. This task is achieved by decomposing the original complicated optimization problem into two independent components. The first component designs the RF precoder/combiner at source and relay nodes by maximizing the received SNR at relay nodes. While the second component attempts to derive the analog precoder/combiner at relay nodes and destination by maximizing the received SNR at the destination. Digital baseband processing matrices are obtained by deriving the closed-form expression, which minimizes interference among different sub-channels. Finally, the relay selection is made by maximizing the overall SNR from the source to the destination. Computer simulations reveal that the performance of the proposed algorithm is close to its fully digital counterpart and approximately 6% higher than the specified relay-assisted hybrid beamforming techniques. Moreover, the proposed method achieves more than 15% higher performance in a sparse scattering environment when compared with the given relay selection techniques.

Software-Defined Radio-Based 5G Physical Layer Experimental Platform for Highly Mobile Environments

In this study, we developed a 5th generation mobile communication (5G) physical layer (PHY) experimental platform based on software-defined radio (SDR), which makes it easy to customize the transmission and reception process. The developed platform consists of an SDR-based transmitter and a receiver. Digital baseband processing is performed using internal software-based programs. Therefore, it can be used to evaluate the wireless communication performance of 5G (or user-defined modified/customized 5G) not only in a laboratory but also in any field. In addition, as an example of the use of this platform, we proposed signal processing that can receive signals at speeds of 500 km/h or higher and performed an experimental evaluation by running it on the developed platform. Through laboratory experiments in a multipath fading environment using a fading emulator, performance of the developed platform was validated. Moreover, it was confirmed that this platform can satisfy the required block error rate of 0.1 even in a multipath environment at a velocity of approximately 630 km/h, which exceeds the requirements of 5G. Therefore, it has sufficient resilience to be used in high-moving-speed environments. This platform is expected to enable simple testing and evaluation of various new PHY technologies.

Design of Digital Frequency Synthesizer for 5G SDR Systems

The previous frequency synthesizer techniques for scalable SDR are not compatible with high end applications due to its complex computations and the intolerance over increased path interference rate which leads to an unsatisfied performance with improved user rate in real time environment. Designing an efficient frequency synthesizer framework in the SDR system is essential for 5G wireless communication systems with improved Quality of service (QoS). Consequently, this research has been performed based on the merits of fully digitalized frequency synthesizer and its explosion in wide range of frequency band generations. In this paper hardware optimized reconfigurable digital base band processing and frequency synthesizer model is proposed without making any design complexity trade-off to deal with the multiple standards. Here fully digitalized frequency synthesizer is introduced using simplified delay units to reduce the design complexity. Experimental results and comparative analyzes are carried out to validate the performance metrics and exhaustive test bench simulation is also carried out to verify the functionality.

A Sub-GHz multimode digital polar transmitter for 802.11ah and NB-IoT applications

This paper presents a sub-GHz multimode efficiency-enhanced digital polar transmitter for IoT applications. The Class-G technique and wideband matching network are introduced in the switched-capacitor digital power amplifier (DPA) to enhance the 6 dB back-off efficiency and achieve 550–950 MHz wide frequency coverage. The digital baseband processing module is flexibly reconfigurable to support both 802.11ah and NB-IoT protocols. Moreover, the canonic signed digit (CSD) coding and first-order hold (FOH) interpolation techniques are employed to reduce power consumption. Implemented in 55 nm CMOS technology, this digital transmitter (DTX) achieves 23.7dBm peak power, 44.6% peak PAE and 36.1% PAE at 6 dB power back-off at 800 MHz. The DTX obtains 16.7dBm average output power, 33.9% average PAE with −28.8 dB EVM for the 2 MHz 64QAM 802.11ah signal, and 18.6dBm average output power, 37.5% average PAE with −19.8 dB EVM for the 12-subcarrier 180 kHz NB-IoT signal.

Towards 100 Gbps over 100 km: System Design and Demonstration of E-Band Millimeter Wave Communication

Long-range E-band communication with fiber-equivalent speed is emerging extensively as a critical technology in the next-generation communication. This paper firstly reviews the relevant progress in recent research. A brief survey is presented on high-speed, long-range E-band communication systems and their relevant techniques that are essential to the link design, including antenna, power amplifier (PA), channel, and digital baseband processing. In the second part, we review our recent field trial of a long-range air-to-ground E-band link, which maintains steady transmission from a slow-moving helium balloon to the ground station with a vertical dimension of 20 km. The improvement directions and future research topics are then discussed.

A Coprime Array-Based Technique for Spoofing Detection and DOA Estimation in GNSS

In this article, we propose a coprime array-based technique that enables global navigation satellite system (GNSS) receivers to reliably defend against spoofing attacks with a small time offset. The technique can not only detect spoofing, but also provide the direction of arrival (DOA) of the spoofing source. Our approach is implemented on the raw digital baseband signals, hence it does not need to perform the complex despreading and acquisition stage of the GNSS receiver. Crucially, the detector is still available when the number of signals (including spoofing and satellite signals) is more than the number of array elements. The simulation results demonstrate the effectiveness of the proposed technique in terms of DOA estimation and spoofing detection.