Digital Baseband Processing Research Articles

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.

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.

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 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 hardware accelerator for IEEE 802.15.4 Time-Slotted Channel Hopping transceiver

Time-Slotted Channel Hopping (TSCH), an operational mode of the IEEE 802.15.4e standard, imposes a high workload on the embedded processor, mainly due to its precise timing. The limited embedded processor of an edge device cannot satisfy the considerable memory footprint and computational complexity caused by functions that need precise timing in the protocol stack. Moving such duties to hardware seems to be a viable path towards offloading the processor and releasing its resource to be used for running firmware related to the implementation of the upper layers of the protocol stack as well as the application. In this work, a TSCH protocol hardware accelerator is designed and integrated within the digital baseband processor of an IEEE 802.15.4 transceiver. The whole system is implemented targeting the TSMC 65 nm technology. The designed TSCH accelerator has an efficient interface and is totally configurable by the software. It efficiently controls the transceiver, frees up embedded processor cycles, and reduces the interrupt callback to the processor. Therefore, the TSCH hardware accelerator makes it possible to use a cheaper embedded processor or to operate the processor on a lower clock speed which results in lower power consumption. The power analysis of the implemented system shows that, in the minimal setting of the TSCH accelerator, only 0.1% is added to the power consumption of the digital baseband processor. The power overhead increases to 5.6% for supporting three parallel slotframes which is a widely used TSCH setting in multi-hop networks. In return, the embedded processor is relaxed of processing TSCH timing related interrupts that are required for software-based TSCH implementation.

Design of a Digital Baseband Processor for UHF Tags

In this paper, we present a new digital baseband processor for UHF tags. It is a low-power and low-voltage digital circuit and adopts the Chinese military standard protocol GJB7377.1. The processor receives data or commands from the RF front-end and carries out various functions, such as receiving and writing data to memory, reading and sending memory data to the RF front-end and killing tags. The processor consists of thirteen main sub-modules: TPP decoding, clock management, random number generator, power management, memory controller, cyclic redundancy check, FM0 encoding, input data processing, output data processing, command detection module, initialization module, state machine module and controller. We use ModelSim for the TPP decoding simulation and communication simulation between tag and reader, and the simulation results meet the design requirements. The processor can be applied to UHF tags and has been taped out using a TSMC 0.18 um CMOS process.

Hybrid Beamforming for Sum Rate Maximization in Wideband Multi-User MIMO Relay Systems

Hybrid precoding has a great potential to achieve near-optimal performance with few radio-frequency (RF) chains in comparison to the number of antennas. This paper proposes a novel hybrid beamforming solution for sum rate maximization in a wideband multi-user multi-input multi-output (MU-MIMO) relay system. The problem at hand is non-convex and hence mathematically intractable due to the constant modulus constraints associated with the analog beamforming design at communicating nodes shared by all sub-carriers. Furthermore, relaying in MU-MIMO systems under frequency-selective channels is also a challenging task as it requires complicated signal processing. To address this problem, source analog beamformer, relay RF combiner and phase-only processing component at each destination are obtained by deriving the common RF processing matrix which enables beamforming gain over all sub-carriers. This process involves reconstruction loss minimization for designing the required analog beamforming vectors having ability to maximize end-to-end signal-to-interference plus noise ratio (SINR). The relay analog precoder is designed by maximizing the received power at each destination through gain maximization of the equivalent baseband channel over the allocated frequency spectrum. To reduce the computational complexity of the iterative approach used to find the relay RF precoding matrix, the closed form expression is derived for achieving the desired objective. Finally, digital baseband processing components are obtained from the equivalent baseband channels by suppressing the interference among transmitted signals indirectly. Simulation results for different system configurations reveal that the proposed algorithm can achieve near-optimal performance.

Self-interference cancelation in the presence of non-linear power amplifier and receiver IQ imbalance

In the in-band full-duplex(FD) systems, the self-interference (SI) power can be more than 100 dB higher than the power of the received data signal. In order to enable the FD transmission, several SI cancelation stages are needed in a FD transceiver. By combining the cancelation at the radio frequency (RF) with a specially designed antenna and cancelation circuitry and SI cancelation at the digital baseband, the required level of SI cancelation can be achieved even with a non-linear power amplifier. In this paper, a FD transceiver architecture is modeled with simulation tools that allow to use realistic antenna and analog transceiver models and at the same time enable algorithm studies. The analog SI cancelation at the RF is controlled by the baseband digital processing unit, and the tuning of the RF canceler is performed with an automatic gain control enhanced iterative algorithm. The combined cancelation performance of the antenna and RF canceler varies between 62 and 82 dB depending on the studied cases. The digital baseband SI cancelation is based on the Hammerstein model in order to take the power amplifier non-linearity into account. The coefficients of the Hammerstein model are estimated with a self-orthogonalizing adaptive algorithm. When realistic phase noise and IQ imbalance values are taken into account, the SI after all the cancelation stages can decrease the signal-to-interference-and-noise-ratio (SINR) by few decibels (dB). In order to further enhance the SI cancelation, the Hammerstein based SI canceler is extended to cancel also the effect of the receiver IQ imbalance. With the extended baseband canceler, the cancelation performance is mainly limited by the phase noise

Channel-bonding CMOS transceiver for 100 Gbps wireless point-to-point links

5G systems and networks are expected to provide unprecedented data-rate to final users and services, in combination with increased coverage and density. The traffic generated at the edges of the network should be hauled through high capacity data-conveyors. Extremely high data-rate links able to provide optical-fiber like performance in the order of 100 Gbps are required to reduce the cost and increase the flexibility of the network infrastructure deployment. This paper presents a full transceiver architecture based on a channel-bonding radio-frequency front-end operating at millimeter-wave frequencies and digital baseband processing units able to provide such data-rates with a feasible implementation in low-cost CMOS technologies. The baseband section of the receiver includes digital compensation algorithms that allow to cope with some of the radio front-end impairments. The main functionalities of the proposed transceiver architecture are validated in hardware.

Ultra-High-Frequency Radio-Frequency-Identification Baseband Processor Design for Bio-Signal Acquisition and Wireless Transmission in Healthcare System

Healthcare systems have been an important part of the research on consumer electronics in recent years. To be a mass-market product, a healthcare system is combined with a home control system. In this article, an ultra-high-frequency radio-frequency-identification (UHF RFID) wireless interface is built and implemented for the bio-signal monitoring of the healthcare system. This interface is composed of a digital baseband processor with an application specific integrated circuit (ASIC) for the RFID reader and tag. All functions are implemented to fit the requirement of electronic product code class 1 generation 2 protocol, such as cyclic redundancy check/verify, pulse interval encoding, and bi-phase space coding. The system platform was built to obtain bio-signals, which can demonstrate the hand-shaking stage and communication between the reader and tag at the same time. The tag was fabricated under the 180-nm standard CMOS process with the average power consumption of only 73.62 μW. The reader was implemented by a field programmable gate array in the system. The power consumption of the entire system combined with those in our previous works, which include analog front-end and radio-frequency front-end, is considered. It achieves 75 μW, which is lower than those achieved with the existing state-of-the-art wireless interface. Moreover, the wearable device with ASICs is proposed to feature on relative low power and small size. Thus, it is more suitable for consumer electronics.

Transceiver Design for Massive MIMO Two-Way Half-Duplex AF Hybrid Relay With MIMO Users

We consider a multi-pair two-way amplify-and-forward massive multi-input multi-output (MIMO) hybrid relay with MIMO user-pairs. We employ block-diagonalization-based digital baseband processing at the hybrid relay to cancel the inter user-pair interference, and equal-gain-combining-based radio-frequency (RF) processing to maximize the beamforming gain. We then construct digital relay precoder and the user precoder/decoders using generalized singular value decomposition (GSVD). We also optimize the following for the proposed GSVD transceiver. First, the network transmit power subject to per-stream quality-of-service (QoS) constraints. Second, rate of the transmit stream with the worst signal-to-noise ratio. Finally, sum-rate subject to the per-stream QoS constraints. We numerically show that the proposed hybrid relay has only marginally inferior performance than a full RF chain relay for the three optimization metrics. We also demonstrate the efficacy of the proposed GSVD design over another state-of-the-art transceiver design.

Optimization of uplink rate and fronthaul compression in cloud radio access networks

In cloud radio access networks (C-RANs), a central unit (CU) and remote radio heads (RRHs) are connected with a wired fronthaul, e.g., a common public radio interface (CPRI). Due to the limitations of the fronthaul bandwidth in 5G systems, some digital baseband processing blocks are moved from the CU to the RRHs. In this case, the uplink data and pilot symbols after digital processing are delivered from the RRHs to the CU for further processing to decode transmitted information. We consider compression of the data and pilot signals at different compression rates. In the compression of signals, as the compression rate is higher (i.e., the fronthaul rate after the compression is lower), the signal is more distorted. Moreover, compression will affect both the uplink user throughput and the fronthaul rate. The effects of the pilot and data signals on uplink throughput are formulated with an achievable rate considering channel estimation. We formulate an optimization problem to address the tradeoff, and find the optimal distortion variances for the data and pilot signals under high signal-to-noise ratio (SNR) conditions. We show that the optimal signal distortion variance is proportional to the number of data and pilot symbols. We provide numerical results that verify our analytical derivations.

Adaptive Virtual Waveform Design for Millimeter-Wave Joint Communication–Radar

Joint communication and radar (JCR) waveforms with fully digital baseband generation and processing can now be realized at the millimeter-wave (mmWave) band. Prior work has proposed a mmWave wireless local area network (WLAN)-based JCR that exploits the WLAN preamble for radars. The performance of target velocity estimation, however, was limited. In this paper, we propose a virtual waveform design for an adaptive mmWave JCR. The proposed system transmits a few non-uniformly placed preambles to construct several receive virtual preambles for enhancing velocity estimation accuracy, at the cost of only a small reduction in the communication data rate. We evaluate JCR performance trade-offs using the Cramer-Rao Bound (CRB) metric for radar estimation and a novel distortion minimum mean square error (MMSE) metric for data communication. Additionally, we develop three different MMSE-based optimization problems for the adaptive JCR waveform design. Simulations show that an optimal virtual (non-uniform) waveform achieves a significant performance improvement as compared to a uniform waveform. For a radar CRB constrained optimization, the optimal radar range of operation and the optimal communication distortion MMSE (DMMSE) are improved. For a communication DMMSE constrained optimization with a high DMMSE constraint, the optimal radar CRB is enhanced. For a weighted MMSE average optimization, the advantage of the virtual waveform over the uniform waveform is increased with decreased communication weighting. Comparison of MMSE-based optimization with traditional virtual preamble count-based optimization indicated that the conventional solution converges to the MMSE-based one only for a small number of targets and a high signal-to-noise ratio.

Encrypted Tag Design for RFID Systems

The wireless implantable RFID tag combines wireless communication technology, medical sensor technology, and data conversion technology to achieve continuous detection of body indicators. The implantable tag system includes a small signal detection circuit, a readout circuit to complete sensor data acquisition and conversion, and a digital baseband and RF front end to perform communication with an external reader. The digital baseband processor handles communication protocols, controls communication flow, configures the RF front end, and controls the sampling of human body data by the ADC, which is a key component of the implantable tag system.Based on the 13.56MHz IS015693 radio frequency identification protocol, this paper designs and implements an encrypted tag baseband. Through the analysis of the protocol and the application requirements of the tag, the system architecture of the digital baseband is formulated. Subsequently, the functional logic was verified using Cadence’s Incisive EDA tool and synthesized using Synopsys’ EDA synthesis tool Design Compiler. The simulation results show that the logic function is correct. The synthesis tool report shows that under the 0.13um process, timing closures. The setup time and hold time meet the requirements.

Progressive Hybrid Precoding and Combining for Massive MIMO ARQ Systems

In this paper, we consider a progressive transceiver design for point-to-point massive multiple input multiple output (MIMO) with automatic repeat request (ARQ). As an alternative to address the difficulties such as prohibitive hardware complexity, cost, and power consumption encountered by a fully digital implementation of massive MIMO, a hybrid architecture, which introduces RF analog processing together with baseband digital processing, is employed. Based on the perfect channel state information, the hybrid RF-baseband transceiver is derived, aiming at increasing the achievable rate through exploiting the time diversity inherent in ARQ packet retransmissions. Due to the intractability of joint optimization of the hybrid precoder and combiner, we heuristically assume that the fully digital linear minimum-mean-square-error (MMSE) filter is perfectly realizable by its hybrid counterpart at the receiver, and decouple the precoding from the combining design, where the previous failed retransmissions are sequentially incorporated into the joint RF-baseband precoding/combining optimization. Specifically, for each ARQ round, we construct the RF precoder either from the transmit array response vectors or from a discrete Fourier transform-based codebook. In consideration of the RF precoding effect, the optimal baseband precoder is analytically derived. To minimize the performance loss incurred by the decoupled precoder/combiner optimization, the proposed two-step design technique is further applied to derive the hybrid combiner as an approximation of the optimal linear MMSE solution in terms of error performance. The efficacy of the proposed progressive hybrid solutions is numerically evaluated by performance comparison with various baselines. The effect of RF precoding/combining with finite-resolution elements is also examined.