Duplex Transceiver Research Articles

Joint Analog and Digital Self-Interference Cancellation for Full Duplex Transceiver With Frequency-Dependent I/Q Imbalance

An effective and practical joint analog and digital self-interference cancellation (SIC) scheme without additional signalling overhead for an I/Q imbalanced full duplex transceiver is proposed in this paper. This scheme combines an I/Q imbalanced analog least mean square (ALMS) loop at the transceiver radio frequency frontend and a two-stage digital signal processing (DSP) at the digital baseband to achieve excellent SIC performance with low complexity. The steady state weighting coefficients of the I/Q imbalanced ALMS loop with periodical transmitted signal and the loop’s convergence behaviour are firstly analysed. The residual SI is then modelled as the output of a time-varying widely linear system. With a track/hold control mechanism applied to the ALMS loop, the system model for digital SIC is further presented, followed by the DSP algorithms suitable for real-time implementation. The noise enhancement in each stage digital cancellation is also analysed and formulated. Finally, simulation results are provided to verify the theoretical analyses and demonstrate the overall SIC performance.

A 3.1-5.2GHz, Energy-Efficient Single Antenna, Cancellation-Free, Bitwise Time-Division Duplex Transceiver for High Channel Count Optogenetic Neural Interface.

We report an energy-efficient, cancellation-free, bit-wise time-division duplex (B-TDD) transceiver (TRX) for real-time closed-loop control of high channel count neural interfaces. The proposed B-TDD architecture consists of a duty-cycled ultra-wide band (UWB) transmitter (3.1-5 GHz) and a switching U-NII band (5.2 GHz) receiver. An energy-efficient duplex is realized in a single antenna without power-hungry self-interference cancellation circuits which are prevalently used in the conventional full-duplex, single antenna transceivers. To suppress the interference between up- and down-links and enhance the isolation between the two, we devised a fast-switching scheme in a low noise amplifier and used 5× oversampling with a built-in winner-take-all voting in the receiver. The B-TDD transceiver was fabricated in 65 nm CMOS RF process, achieving low energy consumption of 0.32 nJ/b at 10 Mbps in the receiver and 9.7 pJ/b at 200 Mbps in the transmitter, respectively. For validation, the B-TDD TRX has been integrated with a μLED optoelectrode and a custom analog frontend integrated circuit in a prototype wireless bidirectional neural interface system. Successful in-vivo operation for simultaneously recording broadband neural signals and optical stimulation was demonstrated in a transgenic rodent.

SICNet—Low Complexity Sample Adaptive Neural Network-Based Self-Interference Cancellation in LTE-A/5G Mobile Transceivers

The limited transmitter-to-receiver stop-band isolation of the duplexers in long term evolution (LTE) and 5G/NR frequency division duplex transceivers induces leakage signals from the transmitter(s) (Tx) into the receiver(s) (Rx). These leakage signals are the root cause of a multitude of self-interference (SI) problems in the receiver path(s) diminishing a receiver’s sensitivity. Traditionally, these effects are counteracted by the use of various different SI cancellation (SIC) architectures which typically solely target one specific problem. In this paper, we propose two novel neural networks based architectures that can handle a variety of different SI effects without the need for a different architecture for each effect. We additionally show the suitability of the proposed architecture on SI effects occurring in in-band full duplex transceivers. Further, we introduce two novel low-cost training algorithms to enable online adaptation (as opposed to offline training currently proposed in literature). The combination of these two concepts is shown to not only beat existing algorithms in their cancellation performance, but also to provide sufficiently low computational complexity allowing on-chip implementations.

A 1–2-GHz Quadrature Balanced N-Path Receiver for Frequency Division Duplex Systems

The implementation of fully integrated frequency division duplexing (FDD) transceivers imposes many challenges, such as receiver (RX) desensitization by the transceiver (TX), reciprocal mixing, and sensitivity, to antenna voltage standing wave ratio (VSWR) variations. Here, we present the quadrature balanced N-path receiver (QBNR) architecture. The QBNR is composed of a quadrature hybrid and two identical mixers, presenting a short circuit and 50- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> matching in the TX and RX bands, respectively. The TX power reflects at the mixers and reconstructs in-phase at the antenna, while the RX signal from the antenna can be reconstructed in phase at the baseband RX port. This architecture provides wideband FDD functionality while providing TX noise and local oscillator phase noise cancelation and antenna VSWR tunable compensation. Our implementation measurement results for an equalized configuration show more than 55-dB TX to RX isolation at frequencies of 1–2 GHz and for antenna VSWR of 2:1, with 3-dB TX loss and 4-dB RX loss, RX IP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1 dB</sub> of 5 dBm and TX IP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1 dB</sub> of 16 dBm, and receiver <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${B_{1\,\text {dB}}}$ </tex-math></inline-formula> of 10 dBm.

Low Complexity MIMO Channel Prediction for Fast Time-Variant Vehicular Communications Channels Based on Discrete Prolate Spheroidal Sequences

Channel state information (CSI) is required at the transmitter for achieving the maximum potentials of multiple-input multiple-output (MIMO) systems. In fast time-variant vehicular communications channels high data rate feedback lines are required in a frequency division duplex (FDD) transceiver for updating the transmitter with the rapidly changing CSI. Even with high data rate feedback lines, the delay caused by channel estimation and feedback may lead to outdated CSI at the transmitter. To reduce both the feedback load and CSI delay, this paper presents a reduced rank autoregressive (AR) channel predictor based on low dimensional discrete prolate spheroidal (DPS) sequences. The new subframe-wise DPS basis expansion model (DPS-BEM) channel predictor properly exploits the channel's restriction to low dimensional subspaces for reducing the prediction error and the computational complexity. The proposed channel predictor can be applied for updating the precoding matrix in time-variant MIMO systems. Simulation results demonstrate that the proposed channel predictor outperforms the DPS based minimum energy (ME) predictor at different Doppler frequencies and has better performance than the conventional Wiener predictor for slower time-variant channels and almost similar performance for very fast time-variant channels with reduced amount of computational complexity.

A Bidirectional FSO Communication Employing Phase Modulation Scheme and Remotely Injection-Locked DFB LD

A bidirectional free-space optical (FSO) communication through a 600-m free-space transmission is built, employing a phase modulation (PM) scheme and a remotely injection-locked distributed feedback laser diode (DFB LD) for presentation. With optimum injection locking, a DFB LD is excellent for duplex transceiver operations. An injection-locked DFB LD not only operates as a PM-to-intensity modulation converter with an optical detector, but also functions as an upstream optical carrier. To be the first one of employing a remotely injection-locked DFB LD to detect a phase-modulated 25-Gb/s/25-GHz four-level pulse amplitude modulation (PAM4) passband signal, the DFB LD with remote injection locking is successfully intensity-modulated with an upstream 25-Gb/s non-return-to-zero (NRZ) signal. Good bit error rate performance and clear PAM4/NRZ eye diagrams show that this FSO communication can use a remotely injection-locked DFB LD to detect the downstream phase-modulated PAM4 signal and concurrently deliver an upstream intensity-modulated NRZ signal. This bidirectional 25-Gb/s/25-GHz (downstream)/25-Gb/s (upstream) FSO communication is prominent due to its enhancement in two-way high-speed optical wireless communications.

An Integrated, Software-Defined FDD Transceiver: Distributed Duplexing Theory and Operation

Implementing flexible, software-defined duplexing transceivers remains challenging due to the inherent self-interference from TX to RX that requires cancellation in the RF/analog domain. Previously, we presented a novel, duplexing, integrated, single antenna transceiver architecture that used an M-stage distributed amplifier combined with an artificial transmission line to achieve high transmit-receive isolation across more than an octave in center frequency. In this paper, we explain and explore the trade-offs and limits on this class of distributed, duplexing transceivers as a function of system and sub-circuit parameters.

Generalized Self-Interference Model for Full-Duplex Multicarrier Transceivers

In the modeling of full duplex (FD) transceivers using multicarrier modulations, it is usually assumed that the intercarrier interference of the coupling path is negligible. Therefore, the system can be analyzed on a subcarrier basis, because the model reduces to a set of orthogonal flat channels. However, for more general FD transceivers, or when hardware components introduce nonlinear distortion, the orthogonality is lost and a more rigorous model is needed. In this paper, we introduce a model for the coupling interference of FD systems, which allows the uplink and the downlink to have different time and frequency offsets, and different parameters such as the number of subcarriers and symbol length. In addition, we present a linear approximation for the nonlinear coupling based on a generalization of the Bussgang's theorem, for the case of transmitters with nonuniform power allocation. We validate the proposed model with measurements and show numerically that it improves the accuracy of the signal to interference plus noise ratio expression, and the formulation of optimization problems. Therefore, the proposed model paves the way for a more realistic performance analysis and the derivation of more accurate power allocation strategies than is possible with the classic model.

Reference-Less Time-Division Duplex Transceiver IC for a Renal Denervation System

This paper presents the first time-division duplex transceiver integrated circuit (IC) for a 5 Fr (1.6 mm), six-electrode renal denervation (RDN) catheter for treating resistant hypertension. The diameter of the resulting RDN catheter, with each IC encapsulated in a platinum electrode, is 37.5% smaller than those of conventional catheters, enabling minimally invasive surgery with reduced complications. The six electrodes sharing only four electrical wires perform packet communication with an external power control unit for independent operation and deliver the designated radio-frequency (RF) energy to the renal artery wall while measuring the local temperature and impedance. The measured 3 $\sigma $ inaccuracy of a bipolar junction transistor (BJT)-based on-chip temperature sensor from 65 °C to 75 °C is ±0.5 °C after one-point trimming. Two 500-kHz, 21-Vrms ac signals with ±35-V dc potentials are used as power supplies to transfer up to 6-W RF ablation signals with 120-Vpp swing while keeping the maximum dc supply voltages of the ICs less than the breakdown voltage of 70 V. The proposed RDN IC fabricated using a 0.18- $\mu \text{m}$ high-voltage bipolar, CMOS, LDMOS (HV BCDMOS) occupies an area of 2.1 mm2.

A Low Power Receiver Front-End Design With Tunable Notch Filter for TX Leakage and Blocker Suppression

An active transmitter self-leakage suppressor for frequency-division duplex transceivers is presented. A wideband active notch filter is implemented at the mixer IF ports to suppress out the transmitter leakage. Since the active notch filter is operating at IF frequencies, its noise, power, and area consumption penalty for the overall transceiver is negligible, and it does not require any complicated amplitude and phase calibration circuitry that is necessary for the cancelers which operate at RF frequencies. Due to the transfer function seen by the active notch-filter, its noise and distortion contribution is shaped and reduced further. The proposed design is implemented in a 65 nm standard CMOS process for an LTE receiver and provides minimum of 5 dB attenuation at duplex distance for E-UTRA band 20. For the same frequency band, it enhances the IIP2 and IIP3 of the receiver with a minimum of 12 and 6 dB, respectively. The active filter consumes only 1.1 mA/channel from a 2.5 V supply and occupies 0.078 mm2/channel. Its noise contribution is within the NF measurement error, thus negligible. The design is intended and tested for transmitter self-interference and wideband ACS blockers.

Multi-User Full Duplex Transceiver Design for mmWave Systems Using Learning-Aided Channel Prediction

Millimeter Wave (mmWave) technology coupled with full duplex (FD) communication has the potential of increasing the spectral efficiency. However, the self-interference (SI) encountered in the FD mode and the ubiquitous multi-user interference (MI) contaminates the signal. Furthermore, the system performance may also be limited by channel aging that arises because of the time-varying nature of the channel. Therefore, in this paper, we conceive FD hybrid beamforming (HBF) for K -user multiple-input multiple-output (MIMO)-aided orthogonal frequency division multiplexing (OFDM) using learning-aided channel prediction. We first derive a joint precoder and combiner design for full duplex K -user MIMO-OFDM interference channels, where we aim for minimizing both the residual SI and the MI, followed by an iterative hybrid decomposition technique developed for OFDM systems. Then, we propose a learning-aided channel prediction technique for systems suffering from channel aging relying on a radial basis neural network, where we show by simulation that upon using sufficient training, learning-assisted channel prediction can faithfully estimate the current channel. Furthermore, we demonstrate by simulations that our proposed joint hybrid precoder and combiner design outperforms the popular Eigen beamforming (EBF) technique by about 5 dB for a 128×32 -element MIMO aided OFDM system having 32 sub-carriers.

A fully integrated high IP1dB CMOS SPDT switch using stacked transistors for 2.4 GHz TDD transceiver applications

A transmit/receive (T/R) switch is an essential module of every modern time division duplex (TDD) transceiver circuit. A T/R switch with high power handling capacity in CMOS process is difficult to design due to capacitive coupling of radio frequency signals to the substrate. This paper proposes a single-pole-double-throw (SPDT) T/R switch designed in a standard Silterra 130 nm CMOS process for high-power applications like RFID readers. The results reveal that, in 2.4 GHz ISM band, the proposed switch exhibits a very high input P1dB of 39 dBm with insertion loss of only 0.34 dB and isolation of 40 dB in transmit mode but 1.08 dB insertion loss and 30 dB isolation in receive mode. Stacked thick-oxide triple-well transistors, resistive body floating and negative control voltages are used to achieve such lucrative performance. Moreover, the chip size of the designed switch is only 0.034 mm2 as bulky inductors and capacitors are avoided. The Monte-Carlo and corner analyses confirm that the performance of the switch is also quite stable and reliable.

Digital Suppression of Transmitter Leakage in FDD RF Transceivers: Aliasing Elimination and Model Selection

The transmitter (TX)-induced interference due to power amplifier nonlinearities poses severe desensitization problems to the receiver chain in frequency-division duplexing transceivers. Due to nonlinear signal process involved, a high sampling rate is normally required in the existing digital suppression approaches, which can result in high cost and high power consumption in wideband systems. In this paper, a new digital suppression model is proposed to cancel the TX leakage at baseband with a low sampling rate. The cancellation model is based on the modified decomposed vector rotation model. With the addition of cross-term products, the enhanced model is capable of eliminating the aliasing effect arising from the reduced sampling rate. Theoretical analysis of aliasing elimination is presented, and the algorithm is subsequently verified by both simulation and experiment results, confirming the effectiveness and feasibility of the proposed cancellation technique for TX leakage suppression. Compared with conventional solutions, the new approach uses much less hardware resource and consumes much lower power while achieving comparable performance.

Wideband RF Self-Interference Cancellation Circuit for Phased Array Simultaneous Transmit and Receive Systems

With the growing demand for spectrum utilization, the concept of full duplex transceivers has become attractive. These simultaneous transmit and receive systems (STAR) are attractive as they double bandwidth utilization. To realize STAR, we must suppress self-interference (SI) between transmit and receive antennas. Doing this across a wide bandwidth presents an even greater challenge. Further, for antenna arrays, SI can be higher due to mutual coupling among neighboring elements. In this paper, we achieve 100 dB isolation for practical STAR, across a large 500 MHz bandwidth. Specifically, four stages of the self-interference cancellation are proposed. In addition to high isolation cross-polarized array elements, we employ multi-tap filters at the transceiver RF front-end for interference cancellation. These filters emulate direct SI coupling using frequency-domain optimization techniques. Measured results show an average of 25 dB filter cancellation is possible across 500 MHz.

Novel Active Bandpass Switchplexer for RF Transceiver Front-End Applications

In this paper, the design of RF transceiver front-end using a novel multifunction microwave circuit is proposed. By designing one arm of the proposed active bandpass switchplexer as a bandpass power amplifier (PA) and the other arm as a single-to-balanced bandpass low-noise amplifier (LNA), the resulted multifunctional microwave filtering device thus effectively integrates the functions of single-pole-double-throw RF switch, bandpass filter, balun, PA, and LNA. In this way, all the function blocks between the antenna and up/down converters of a single-band time-division duplex transceiver front-end can be integrated into a single circuit. Compared to the previous works on multifunction microwave circuit designs, the proposed active bandpass switchplexer features the highest number of integrated functions. A printed circuit board-based design example of the proposed active bandpass switchplexer is presented to validate the proposed design concept and to demonstrate the performance achieved.

Radio Frequency Self-Interference Cancellation With Analog Least Mean-Square Loop

A multitap adaptive filter with analog least mean-square (ALMS) loop is proposed in this paper for effective and low complexity self-interference cancellation implemented as part of the radio frequency frontend in a full duplex transceiver. Comprehensive analyses of the ALMS loop’s behaviors at both micro and macroscales are presented for a wireless communication system with single carrier signaling. It is revealed that there is always an irreducible residual interference due to the cyclostationary property of the transmitted signal. The interference suppression ratio (ISR) lower bound is derived accordingly, which can be used as a design rule for determining the ALMS loop parameter. Stationary analysis shows that the convergence speed and achievable ISR of the ALMS loop are determined by the loop gain and the autocorrelation function of the transmitted signal. The interference channel modeling error with the adaptive filter also accounts for part of the residual interference power. These theoretical findings are verified by simulation and experimental results.

Integrated full duplex radios

Full duplex wireless has drawn significant interest in the recent past due to the potential for doubling network capacity in the physical layer and offering numerous other benefits at higher layers. However, the implementation of integrated full duplex radios is fraught with several fundamental challenges. Achieving the levels of self-interference cancellation required over the wide bandwidths mandated by emerging wireless standards is challenging in an integrated circuit implementation. The dynamic range limitations of integrated electronics restrict the transmitter power levels and receiver noise floor levels that can be supported in integrated full duplex radios. Advances in compact antenna interfaces for full duplex are also required. Finally, networks employing full duplex nodes will require a complete rethinking of the medium access control layer as well as cross-layer interaction and co-design. This article describes recent research results that address these challenges. Several generations of full duplex transceiver ICs are described that feature novel RF self-interference cancellation circuits, antenna cancellation techniques, and a non-magnetic CMOS circulator. Resource allocation algorithms and rate gain/improvement characterizations are also discussed for full duplex configurations involving IC-based nodes.

Impact of Full Duplex Scheduling on End-to-End Throughput in Multi-Hop Wireless Networks

There have been some rapid advances on the design of full duplex (FD) transceivers in recent years. Although the benefits of FD have been studied for single-hop wireless communications, its potential on throughput performance in a multi-hop wireless network remains unclear. As for multi-hop networks, a fundamental problem is to compute the achievable end-to-end throughput for one or multiple communication sessions. The goal of this paper is to offer some fundamental understanding on end-to-end throughput performance limits of FD in a multi-hop wireless network. We show that through a rigorous mathematical formulation, we can cast the multi-hop throughput performance problem into a formal optimization problem. Through numerical results, we show that in many cases, the end-to-end session throughput in a FD network can exceed 2x of that in a half duplex (HD) network. Our finding can be explained by the much larger design space for scheduling that is offered by removing HD constraints in throughput maximization problem. The results in this paper offer some new understandings on the potential benefits of FD for end-to-end session throughput in a multi-hop wireless network.

Low-Complexity Subband Digital Predistortion for Spurious Emission Suppression in Noncontiguous Spectrum Access

Noncontiguous transmission schemes combined with high power-efficiency requirements pose big challenges for radio transmitter and power amplifier (PA) design and implementation. Due to the nonlinear nature of the PA, severe unwanted emissions can occur, which can potentially interfere with neighboring channel signals or even desensitize the own receiver in frequency division duplexing (FDD) transceivers. In this article, to suppress such unwanted emissions, a low-complexity sub-band DPD solution, specifically tailored for spectrally noncontiguous transmission schemes in low-cost devices, is proposed. The proposed technique aims at mitigating only the selected spurious intermodulation distortion components at the PA output, hence allowing for substantially reduced processing complexity compared to classical linearization solutions. Furthermore, novel decorrelation based parameter learning solutions are also proposed and formulated, which offer reduced computing complexity in parameter estimation as well as the ability to track time-varying features adaptively. Comprehensive simulation and RF measurement results are provided, using a commercial LTE-Advanced mobile PA, to evaluate and validate the effectiveness of the proposed solution in real world scenarios. The obtained results demonstrate that highly efficient spurious component suppression can be obtained using the proposed solutions.

Wavelength-Multiplexed Duplex Transceiver Based on III-V/Si Hybrid Integration for Off-Chip and On-Chip Optical Interconnects

A six-channel wavelength-division-multiplexed optical transceiver with a compact footprint of 1.5 $\times$ 0.65 mm2 for off-chip and on-chip interconnects is demonstrated on a single silicon-on-insulator chip. An arrayed waveguide grating is used as the (de)multiplexer, and III-V electroabsorption sections fabricated by hybrid integration technology are used as both modulators and detectors, which also enable duplex links. The 30-Gb/s capacity for each of the six wavelength channels for the off-chip transceiver is demonstrated. For the on-chip interconnect, an electrical-to-electrical 3-dB bandwidth of 13 GHz and a data rate of 30 Gb/s per wavelength are achieved.