Analog Self-interference Cancellation Research Articles

Analog self-interference cancellation for full-duplex communication based on deep reinforcement learning

Full-duplex (FD) communication systems are expected to be extensively used in wireless communications, however, their performance is severely limited due to the self-interference (SI). Traditional analog self-interference cancellation (ASIC) methods generally do not consider estimating the delay of the SI channel, thereby requiring a large number of taps to capture channel details. In order to effectively eliminate SI in situations with limited resources or space, we propose two novel ASIC schemes based on deep reinforcement learning (DRL), named Multi-Deep Q Network (Multi-DQN) scheme and DQN-Deep Deterministic Policy Gradient (DQN-DDPG) scheme. Specifically, for the Multi-DQN scheme, we use multiple DQN units to estimate the delay and attenuation of the SI channel discretely, which can effectively reduce the number of taps required for ASIC. To overcome the loss of discretization, the DQN-DDPG scheme utilizes DQN and DDPG units to estimate the delay and continuous attenuation of the SI channel, respectively. Simulation results indicate that both proposed schemes achieve a similar performance to the multi-tap methods with fewer taps. Additionally, the effectiveness of both schemes is verified across various scenarios, encompassing system configurations, hyperparameters, and channel changes.

Dual-polarization MZM-based photonic nonlinear analog self-interference cancellation for in-band full-duplex radios

In this paper, we propose a dual-polarization Mach-Zehnder modulator-based photonic nonlinear analog self-interference cancellation (SIC) technique for in-band full duplex (IBFD) systems. By using the proposed technique, an arbitrary 4th order nonlinear transfer function can be generated, meaning the performance limitation caused by the nonlinearity of the analog SIC circuit can be overcome by imitating the nonlinear transfer function of the analog SIC circuit before cancellation. This paper also presents a performance analysis through simulations and the results of a proof-of-concept demonstration. In the experiment, the proposed nonlinear SIC technique could achieve 29 dB cancellation over 500 MHz bandwidth centered at 1.25 GHz frequency along various degree of distortion caused by nonlinearity. In addition, the performance enhancements achieved by the proposed technique are evaluated in terms of error vector magnitudes (EVMs) and constellations of the signal-of-interest (SOI) in the simulation which is based on the experimental SIC results. More than 3 dB of SOI power gain could be obtained in evaluated EVM performances.

Low-Complexity Digital-Assisted Nonlinear Analog Self-Interference Cancellation in the Optical Domain

Low-Complexity Digital-Assisted Nonlinear Analog Self-Interference Cancellation in the Optical Domain

A Channel Frequency Response-Based Secret Key Generation Scheme in In-Band Full-Duplex MIMO-OFDM Systems

Physical layer-based secret key generation (PHY-SKG) schemes have attracted significant attention in recent years due to their lightweight implementation and ability to achieve information-theoretical security. In this paper, we study a channel frequency response (CFR)-based SKG scheme for in-band full-duplex (IBFD)-multi-input and multi-output (MIMO) systems. We formulate the intrinsic practical imperfections and derive their effects on the probing errors. Then we derive closed-form expressions for the secret key capacity (SKC) in the presence of a passive eavesdropper accordingly. We analyze the asymptotic behavior of the SKC in the high-SNR regime and reveal the fundamental limits for IBFD and HD probing. Based on the asymptotic SKC, we investigate the conditions under which IBFD can outperform HD. Numerical results illustrate that effective analog self-interference cancellation (ASIC) depth is the basis for IBFD probing to gain benefits over HD. Finally, we analyze the properties of the collected samples of the CFR-based SKG scheme and propose an averaging pre-processing and a segmental quantization, which reduce the key disagreement rate and remove the effects of large-scale fading to guarantee randomness. 3GPP specification-based simulations and the National Institute of Standards and Technology (NIST) test suite verify the theoretical analysis and the effectiveness of the proposed SKG scheme.

Beamforming Design for In-Band Full-Duplex Multi-Cell Multi-User MIMO Networks With Global and Local CSI

This paper investigates beamforming techniques for in-band full-duplex multi-cell multi-user (IBFD-MCMU) wireless networks considering hardware impairments (HWIs), channel uncertainty, and limited channel state information (CSI). For the global CSI scenario, we first enhance zero-forcing (ZF), maximum ratio transmission and combining (MRTC), and minimum mean-squared error (MMSE) beamforming to be compatible with multi-antenna users and IBFD base stations. Then, we investigate beamforming schemes for the local CSI assumption where only intra-cell channel knowledge is fully available at the base stations, and inter-cell channels are known statistically due to the limited training resources. With these limitations, two MMSE-based methods are proposed, which regard unknown CSI as random instances (i.e., eMMSE-RI) and noise (i.e., eMMSE-N). We explore the self-interference cancellation (SIC) capability of beamformers and evaluate the performance of these methods in 3GPP scenarios. Numerical results reveal that the enhanced MMSE beamforming can achieve the desired IBFD with low analog SIC depth, inspiring a low-cost IBFD transceiver design. The practical imperfections (e.g., transceiver HWIs, channel uncertainty, limited CSI) decrease the achievable sum rate but do not reduce the IBFD gain. With CSI limitations, eMMSE-RI outperforms eMMSE-N for microcells, where the probability of the presence of LOS is high, whereas eMMSE-N performs better when cell size is enlarged.

Analog Self-Interference Cancellation With Practical RF Components for Full-Duplex Radios

Analog Self-Interference Cancellation With Practical RF Components for Full-Duplex Radios

Evaluating Analog Self-Interference Cancellation for In-Band Full-Duplex Wireless Communication

In-band full-duplex (IBFD) technology allows simultaneous transmission and reception in the same frequency band to boost spectral efficiency. However, selfinterference, introduced by transmitter-to-receiver leakage, poses the biggest obstacle to the realization of IBFD. Altogether IBFD radio performance and effective network throughput are directly correlated with the amount of energy suppressed by self-interference cancellation (SIC). In this work a model of an IBFD transceiver, featuring an adaptive, analog-domain, tapped-branch self-interference canceller, was developed to evaluate the inimical effects attributable to the various noise and nonlinear distortions of the simulated transceiver components. Results of the simulation show that noise incursions and nonlinearity in the transmission channel produced a negligible effect on the overall cancellation performance. However, the effectiveness of the cancellation hardware is highly dependent on the level of intrinsic nonlinearity within the canceller’s own components. Consideration of these non-ideal characteristics should be an essential part of analog SIC design to prevent any intrinsic limitations on the cancellation performance.

Quantization Noise Reduction by Digital Signal Processing-Assisted Analog-to-Digital Converter for In-Band Full-Duplex Systems

In-band full-duplex (IBFD) systems are attracting attention as a key technology for ideally doubling channel capacity for beyond 5G and 6G. First, we propose an analytical model of the effect of <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> -bit analog-to-digital converter (A/D) quantization noise on channel capacity quasi-upper limit of the desired signal at the base station of the IBFD systems. We derive averaged and peak powers of input signal to <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> -bit A/D and quantization noise power of <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> -bit A/D to formulate uplink (UL) channel capacity quasi-upper limit. Numerical analysis results by the proposed analytical model show that the UL channel capacity quasi-upper limit degrades by 16% and 80% in the 12- and 8-bit A/D, respectively. Second, we propose digital signal-processing (DSP)-assisted dual- and quad-A/Ds to reduce the quantization noise effect in IBFD systems. When A/D resolution bit is 8-bit, the proposed DSP-assisted dual- and quad-A/Ds improve the UL channel capacity quasi-upper limit by up to 92.3 and 192%, respectively, compared to conventional A/D for peak-to-average power ratio condition of 15 dB and analog self-interference (SI) cancellation capability of 30 dB. The proposed dual- and quad-A/Ds can reduce the burden on the analog SI cancellation circuitry by 6 and 12 dB, respectively.

Analog self-interference cancellation based on zero-forcing sampling for in band full duplex radios

A uniformly zero-forcing sampling method applied in the base-band analog domain is proposed for Self-Interference Cancellation (SIC) in Band Full-Duplex (IBFD) radios. In order to releasing the ADC dynamic range to enable the desired signal for further processing in the digital domain, the SIC is performed when ADC samples the received signal. Specifically, first, an auxiliary signal is designed to rectify the zero-crossings of the self-interference (SI) to be consistent with the ADC sampling points. Second, to achieve the auxiliary signal, a zero-forcing function is established by leveraging one-to-one mapping from the complex domain to the complex exponential domain. Meanwhile, a closed-form expression of the zero-forcing function is derived according to the Z transformation. Additionally, an orthogonal pilot structure is proposed to improve the SIC capability, and the computational complexity is investigated. Finally, simulation and experimental results show the effectiveness of the proposed method.

Low complexity full duplex MIMO systems: Analog canceler architectures, beamforming design, and future directions

The hardware complexity of the analog Self-Interference (SI) canceler in conventional full duplex Multiple Input Multiple Output (MIMO) designs mostly scales with the number of transmit and receive antennas, thus exploiting the benefits of analog cancellation becomes impractical for full duplex MIMO transceivers, even for a moderate number of antennas. In this paper, we provide an overview of two recent hardware architectures for the analog canceler comprising of reduced number of cancellation elements, compared to the state of the art, and simple multiplexers for efficient signal routing among the transceiver radio-frequency chains. The one architecture is based on analog taps and the other on AUXiliary (AUX) Transmitters (TXs). In contrast to the available analog cancellation architectures, the values for each tap or each AUX TX and the configuration of the multiplexers are jointly designed with the digital transceiver beamforming filters according to desired performance objectives. We present a general optimization framework for the joint design of analog SI cancellation and digital beamforming, and detail an example algorithmic solution for the sum-rate optimization objective. Our representative computer simulation results demonstrate the superiority, both in terms of hardware complexity and achievable performance, of the presented low complexity full duplex MIMO schemes over the relative available ones in the literature. We conclude the paper with a discussion on recent simultaneous transmit and receive operations capitalizing on the presented architectures, and provide a list of open challenges and research directions for future FD MIMO communication systems, as well as their promising applications.

Photonics-assisted wideband RF self-interference cancellation with digital domain amplitude and delay pre-matching

A photonics-based digital and analog self-interference cancellation approach for in-band full-duplex communication systems and frequency-modulated continuous-wave radar systems is reported. One dual-drive Mach-Zehnder modulator is used to implement the analog self-interference cancellation by pre-adjusting the delay and amplitude of the reference signal applied to the dual-drive Mach-Zehnder modulator in the digital domain. The amplitude is determined via the received signal power, while the delay is searched by the cross-correlation and bisection methods. Furthermore, recursive least square or normalized least mean square algorithms are used to suppress the residual self-interference in the digital domain. Quadrature phase-shift keying modulated signals and linearly frequency-modulated signals are used to experimentally verify the proposed method. The analog cancellation depth is around 20 dB, and the total cancellation depth is more than 36 dB for the 2-Gbaud quadrature phase-shift keying modulated signals. For the linearly frequency-modulated signals, the analog and total cancellation depths are around 19 dB and 34 dB, respectively.

On the performance of active analog self-interference cancellation techniques for beyond 5G systems

The in-band full-duplex (IBFD) mechanism is of interest in beyond 5G systems due to its potential to enhance spectral efficiency and reduce delay. To achieve the maximum gain of IBFD systems, the significant self-interference (SI) must be efficiently suppressed. The challenges of wideband self-interference cancellation (SIC) lie in the radio frequency (RF) domain, where the performance will be limited by the hardware. This paper reviews current RF cancellation mechanisms and investigates an efficient mechanism for future wideband systems with minimum complexity. The working principle and implementation details of multi-tap cancellers are first introduced, then an optical domain-based RF canceller is reviewed, and a novel low-cost design is proposed. To minimize the cost and complexity of the canceller, the minimum required number of taps are analyzed. Simulation results show that with the commonly used 12-bits analog-to-digital converter (ADC) at the receiver, the novel optical domain-based canceller can enable efficient SIC in the 3GPP LTE specifications compatible system within 400 MHz bandwidth.

Analog Self-Interference Cancellation in Dual-Polarization Full-Duplex MIMO Systems

Full-duplex (FD) technology combined with dual-polarization (DP) multiple-input multiple-output (MIMO) systems is attractive to improve spectral efficiency and to enhance link capacity. Cancelling self-interference (SI) in such DPFD MIMO systems using beamforming techniques is very challenging due to a significant difference of the co-polarization and cross-polarization SI channels. In this letter, an analog adaptive filter structure is proposed to mitigate both co-polarization and cross-polarization SIs in DPFD MIMO systems. Stationary analysis is applied to evaluate the performance of the proposed structure. Simulation results show that about 45 dB to 55 dB of SI cancellation can be achieved regardless of the isolation differences between cross-polarization and co-polarization channels.

ALMS Loop Analyses With Higher-Order Statistics and Strategies for Joint Analog and Digital Self-Interference Cancellation

Joint analog and digital self-interference cancellation (SIC) is essential for enabling in-band full duplex (IBFD) communications. Analog least mean square (ALMS) loop is a promising low-complexity high-performance analog SIC technique with multi-tap adaptive filtering capability, but its properties on the tap coefficient variation have not been fully understood. In this paper, analysis based on higher-order statistics of the transmitted signal is performed to solve the problem of evaluating the variance of the ALMS loop’s weighting coefficient error, which reveals two additional types of irreducible residual self-interference (SI) produced by an ALMS loop if it runs freely. The residual SI channel impulse response in digital baseband is also analysed and its unique properties are investigated. By introducing a simple track and hold control to the ALMS loop’s tap coefficients, a joint analog and digital SIC scheme is proposed to stop the tap coefficient variation and achieve very low residual SI close to the IBFD receiver’s noise floor. In a coordinated application scenario, the noise figure of the digital SIC algorithm is proved to be only 1.76 dB at most. Simulation results are provided to verify the theoretical analyses.

MIMO In-Band-Full-Duplex PLC: Design, Analysis and First Hardware Realization of the Analog Self-Interference Cancellation Stage

In-band full-duplex (IBFD) is an attractive solution for increasing the throughput of Power Line Communication (PLC) systems. In IBFD, each network node is allowed to transmit and receive simultaneously in the same frequency band. This paper discusses the importance of defining figures of merit to analyze IBFD performance, which is often forgotten by the literature that addresses IBFD from a pure system level and signal processing perspective. This is because in IBFD hardware-related aspects are of great importance. The focus is then given to the first self-interference (SI) cancellation stage, namely the analog coupling and self-interference cancellation stage (ASICS). Two architectures are considered. A detailed analysis is offered by taking into account circuit-level aspects. A broadband multiple conductor PLC scenario is assumed to enable multiple-input multiple-output (MIMO) IBFD communication. The first considered architecture performs SI subtraction exploiting operational amplifiers. The second proposed architecture exploits the characteristics of a three ports magnetic circuit together with a signal generator to remove the SI. Design, analysis, and hardware realization of these circuits have been done to compare and show the practical feasibility of the ASICS for PLC, well known to be challenged by its line impedance matching problems and severe frequency selective channel characteristics. This paper is the first documented contribution of the IBFD ASICS for 2×2 MIMO broadband PLC physically realized and tested in the field.

Hybrid wideband multipath self-interference cancellation with an LMS pre-adaptive filter for in-band full-duplex OFDM signal transmission.

A wideband multipath self-interference cancellation (SIC) system employing both dual-drive Mach-Zehnder modulator-based analog SIC and least mean square (LMS) algorithm-based pre-adaptive filter digital SIC is proposed and demonstrated for the cancellation of multipath self-interference (SI) and facilitation of in-band full-duplex (IBFD) orthogonal frequency-division multiplexing (OFDM) signal transmission. The multipath effect is an unavoidable challenge in SIC due to the dynamic and unpredictable properties in each path, as well as the need for separate matching components for compensating for each path. In this Letter, an LMS algorithm-based adaptive filter is used as a pre-equalizer to adapt and generate the matching signal to the closest approximate of the multipath SI signal. The adaptation is based on the minimization of the error signal generated from the matching signal and multipath SI signal in the LMS algorithm. With the introduction of the LMS adaptive filter to the analog SIC, an additional 9 dB cancellation improvement is obtained, resulting in a total of 32 dB cancellation depth over a cancellation bandwidth of 2.7 GHz at a center frequency of 1.65 GHz. To the best of our knowledge, the achieved performance is by far the widest cancellation bandwidth in a multipath SIC system, which is essential in a large bandwidth and high data rate transmission system. With the help of the proposed LMS adaptive filter digital SIC assisted analog SIC located at the remote node, power-efficient IBFD transmission of an OFDM signal through a 25 km fiber is experimentally demonstrated with a 6 dB bit error rate and 8% error vector magnitude improvements.

Analog Self-Interference Cancellation Using Auxiliary Transmitter Considering IQ Imbalance and Amplifier Nonlinearity

In-band full-duplex communication, which transmits and receives simultaneously on the same frequency, causes self-interference (SI). In this paper, to cancel SI present in the radio frequency (RF) domain, we propose a novel nonlinear SI cancellation approach using an auxiliary transmitter which is effective in the presence of IQ imbalance and nonlinear distortion. The proposed approach estimates the local transceiver channel by using a time-domain least squares method and creates a signal for SI cancellation based on estimation results and a finite impulse response filter, whose coefficients are derived in this paper. Additionally, we theoretically calculate the SI cancellation limit of the proposed approach. Information about the SI cancellation limit due to phase noise is important for meeting SI cancellation requirements and being able to compare the effects of RF impairments such as IQ imbalance and nonlinear distortion. From simulation results, we show that the proposed approach outperforms the conventional approach and the case of using a general adaptive algorithm for the proposed approach. Furthermore, the SI cancellation limit is improved by adjusting the propagation delay of the SI signal and the canceling signal in addition to sharing one local oscillator in the local transceiver.

Implementation of time-window based analog self-interference cancellation for full-duplex radios

Multi-tap delay line circuits are widely used to model the analog self-interference multipath channel in full-duplex wireless communication systems. However, the complexity of these circuits is a considerable problem. To overcome this issue, this paper investigates a simple dual-channel analog self-interference cancellation circuit. Instead of modeling the multi-tap for multipath reflections, the key sight is that the light-of-sight self-interference is estimated by designing a dual-channel with the fixed delay and tunable attenuator circuit. Feasibility of only canceling the light-of-sight self-interference is explored first. Then, according to the distance of transceiver antennas, delays of the dual-channel are calculated which acts as a time-window to let the light-of-sight self-interference pass. Additionally, due to the error caused by the non-light-of-sight self-interference, attenuators of the dual-channel are designed be tuned adaptively based on the minimum residual mean square error. Finally, the experimental results show that the proposed method can provide up to 50 dB cancellation over different transmission signal bandwidths.

Beam-Based Analog Self-Interference Cancellation in Full-Duplex MIMO Systems

Self-interference (SI) cancellation for full-duplex (FD) multiple input multiple output (MIMO) systems is challenging due to both hardware and signal processing complexity. In this paper, a beam-based adaptive filter structure with analog least mean square (ALMS) loops is proposed to significantly reduce the complexity of SI cancellation for FD MIMO systems. With this structure, the number of adaptive filters required for SI cancellation scales linearly with the number of transmit beams rather than quadratically with the number of antennas. Furthermore, to avoid additional transmit chains used to up-convert the beam signals to generate reference signals for the ALMS loops, a novel method is proposed to select the optimized reference signals from all transmitted signals. In addition, our stationary analysis shows that the proposed structure for FD MIMO systems outperforms the ALMS loop employed for an FD single input single output system. Simulations are conducted to confirm the theoretical analyses.

1-bit Phase Shifters for Large-Antenna Full-Duplex mmWave Communications

Millimeter-wave using large-antenna arrays is a key technological component for the future cellular systems, where it is expected that hybrid beamforming along with quantized phase shifters will be used due to their implementation and cost efficiency. In this paper, we investigate the efficacy of full-duplex mmWave communication with hybrid beamforming using low-resolution phase shifters. We assume that the self-interference can be sufficiently cancelled by a combination of propagation domain and digital self-interference techniques, without any analog self-interference cancellation. We formulate the problem of joint self-interference suppression and downlink beamforming as a mixed-integer nonconvex joint optimization problem. We propose LowRes, a near-to-optimal solution using penalty dual decomposition. Numerical results indicate that LowRes using low-resolution phase shifters perform within 3% of the optimal solution that uses infinite phase shifter resolution. Moreover, even a single quantization bit outperforms half-duplex transmissions, respectively by 29% and 10% for both low and high residual self-interference scenarios, and for a wide range of practical antenna to radio-chain ratios. Thus, we conclude that 1-bit phase shifters suffice for full-duplex millimeter-wave communications, without requiring any additional new analog hardware.