Nonlinear Self-Interference Cancellation Based on CNN-BiLSTM-Attention

Full-duplex wireless communication is deemed to have remarkable application prospect for the possibility in enhancing the spectrum efficiency by 2 times. However, the strong transmitted leakage signal causes serious self-interference for the full-duplex system. To address this question, a new sectional type full-360° phase-shifter and a gm-tunable LNA are proposed for the feed-forward self-interference suppression, which provide fine phase and amplitude tuning precision smaller than 0.5° and 1.5dB respectively. Moreover, a noise attenuator factor is introduced in the LNA for noise reduction. The NF of the proposed LNA is 6.3dB and the effective signal gain is 18.3dB. Benefited from the proposed phase-shifter and LNA, the self-interference suppression depth is larger than 32dB, which demonstrate the effectiveness of the proposed circuits for self-interference suppression for RFID or other full-duplex wireless communication systems.

Nonlinear self-interference (SI) cancellation is essential for mitigating the impact of transmitter-side nonlinearity on overall SI cancellation performance in flexible duplex systems, including in-band full-duplex (IBFD) and sub-band full-duplex (SBFD). Digital SI cancellation (SIC) must address the nonlinearity in the power amplifier (PA) and the in-phase/quadrature-phase (IQ) imbalance from up/down converters at the base station (BS), in addition to analog SIC. In environments with rich signal reflection paths, however, the required number of delayed taps for time-domain nonlinear SI cancellation increases exponentially with the number of multipaths, leading to excessive complexity. This paper introduces a novel, low-complexity, frequency domain nonlinear SIC, suitable for flexible duplex systems with multiple-input and multiple-output (MIMO) configurations. The key approach involves decomposing nonlinear SI into a nonlinear basis and categorizing them based on their effectiveness across any flexible duplex setting. The proposed algorithm is founded on our analytical results of intermodulation distortion (IMD) in the frequency domain and utilizes a specialized pilot sequence. This algorithm is directly applicable to orthogonal frequency division multiplexing (OFDM) multi-carrier systems and offers lower complexity than conventional digital SIC methods. Additionally, we assess the impact of the proposed SIC on flexible duplex systems through system-level simulation (SLS) using 3D ray-tracing and proof-of-concept (PoC) measurement.

This paper describes a single channel, full-duplex wireless communication system that can receive and transmit at the same frequency band in the same time. Recent researches have proved the feasibility of full duplex wireless communication in practice, by canceling the self-interference signal to obtain the desired signal. In this paper, we propose an adaptive digital self-interference cancellation technique, which combines with existing antenna separation and balun based cancellation to achieve the quantity of self-interference cancellation needed for full-duplex system. An adaptive filter based on least mean square (LMS) algorithm is applied to track amplitude and phase variations to produce an expected cancellation signal, after the initial delay and frequency offsets are compensated. Our simulation results display that the adaptive digital interference cancellation scheme has fairly desirable self-interference signal cancellation performance.

An efficient method for simultaneous nonlinear self-interference (SI) cancellation and signal-of-interest (SOI) recovery is proposed and experimentally verified in a mmWave over fiber testbed, based on a specially designed dual-input deep neural network (DI-DNN). The mmWave band has been adopted in the U.S. to provide 5G wireless access services due to its wide-bandwidth, light-of-sight propagation, and inherent compatibility with small cell architecture. To further meet the uprising bandwidth-demanding services, the in-band full-duplex (IBFD) scheme is introduced to magnify the mmWave channel capacity for the 5G and beyond wireless communications. Fortunately, the mmWave band is more realistic to implement IBFD compared with the conventional sub-6GHz band, as it operates as a highly directional beam and shorter transmission distance resulting in a higher SOI to SI ratio. In our proposed solution, a DI-DNN is, for the first time, proposed and implemented to simultaneously realize the SI cancellation and the SOI recovery. Moreover, we firstly mitigate the nonlinearity arising from the transmitters of both the SI and SOI, as well as the nonlinear crosstalk from the SI to the SOI after detection by a nonlinear receiver. The demonstration of the DI-DNN joint with the experimental implementation in a mmWave over fiber system, promise spectral efficient, high-capacity signal transport in the 5G and beyond wireless communications.

This paper presents the nonlinear effects of the analog self-interference (SI) canceller and a practical solution to suppress the nonlinear distortions in multiple-input multiple-output (MIMO) full-duplex wireless communication systems. Due to the inherent nonlinearities of the active radio frequency (RF) components used to tune the attenuations and the phase shifts in the analog SI cancellers, nonlinear distortions are introduced to the residual SI signal by the analog cancellation and enter the receive chain. In this paper, we build an extra feedback chain for each analog canceller to obtain the reference signal coupled from the transmitted RF signal and observe the characteristics of the canceller. Using the observations, we develop a nonlinear model including all the cancellers at each receive antenna with the presence of MIMO SI channel to estimate the nonlinear components. By subtracting the model estimates from the received signal instantaneously, the corresponding nonlinear distortions are effectively mitigated. After the nonlinearity mitigation, a linear digital cancellation scheme based on the Maximum Likelihood Estimation is applied to further reduce the residual SI signal caused by the MIMO SI channel to the noise floor. Experiments are performed on a 2x2 MIMO full-duplex testbed to validate the effectiveness of the proposed nonlinearity modeling and suppression method using Long Term Evolution signals of 20-MHz bandwidth.

In full-duplex wireless communication systems, knowledge about the self-interference channel must be acquired to cancel the self- interference signal. This paper presents results' analysis of indoor measurements of multi-path self-interference wireless propagation channels. Three polarimetric self-interference radio channels were concurrently measured. The measurements were performed by means of a vector network analyzer, which was configured to sweep three gigahertz of bandwidth; distributed equally over three scattered frequency bands. These bands are: 2-to-3-GHz band, 3- to-4-GHz band, and 5.9-to-6.9-GHz band. The sweeps captured scattering-parameters, which hold the frequency responses of the self-interference propagation channels. A dually-polarized open boundary quad ridged horn antenna was used to observe the polarimetric self-interference channels. Antenna's ports were connected to two test ports of the vector network analyzer. The measurement setup allowed us to capture two orthogonal co-polar self-interference channels and one cross-polar self-interference channel. The channels were measured in a modern and spacious office environment. The antenna was at two and half meter height to be sequentially positioned along the office circumference at 25 different locations - sounding the self-interference channels. The averaged power delay profiles of all three self-interference channels are presented and compared among three sounded bands. Furthermore, the time dispersive behavior of the self-interference wireless channels has been statistically analyzed.

In-band full-duplex (FD) technology has emerged as one of the most promising solutions for spectrum scarcity, allowing users to transmit and receive simultaneously at the same center frequency. However, nonlinear self-interference (SI) introduced by nonideal transceiver components is considered as the main challenge in FD systems. Traditionally, nonlinear SI is characterized by polynomial models, which are computationally expensive. In this letter, a novel neural network (NN) structure, refered to as the convolutional NN with Mish activation function based on knowledge distillation (KD-MCNN), as well as three practical methods for NN design and training are proposed to model nonlinear SI with lower computational complexity. To accommodate hardware with varying computational capabilities, nine KD-MCNN models with different scales are provided. The numerical simulations show that KD-MCNN cancellers exhibit superior performance and computational efficiency compared to both polynomial-based cancellers and existing NN-based cancellers. In particular, the nonlinear SI power can be further reduced by approximately 7.70 dB via the KD-MCNN teacher canceller, which is only 2.48 dB above the receiver's noise floor.

In this paper we experimentally demonstrate an ultra-broadband full-duplex indoor optical wireless communication system with multi-mode fiber incorporating WDM technology. Simultaneously transmission of 4 × 12.5Gbps down-link and 800Mbps up-link has been successfully achieved with error-free operation.

In this paper, we propose self-interference (SI) cancellation in cooperative multiple-input multiple-output (Co-MIMO) for full-duplex (FD) wireless communication systems. This proposal solves SI by applying the block diagonalization used in multi-user MIMO. Therefore, there is no need for a radio frequency (RF) canceller, which has a large hardware (HW) size and is difficult to extend to MIMO. Furthermore, we can achieve higher throughput by applying Co-MIMO, in which multiple access points (APs) work together to communicate. The proposed method's implementation uses field-programmable gate array (FPGA) and software-defined radio (SDR). In the experiment, a total of 33 [dB] SI can cancel with two cooperative APs and two antennas per AP in the IEEE 802.11 system.

Nonlinear self-interference cancellation (SIC) techniques are essential for enabling full-duplex communication systems, which can offer spectral efficiencies twice that of traditional half-duplex systems. The challenge of nonlinear SIC is similar to the classic problem of system identification in adaptive filter theory, whose crux lies in constructing the optimal nonlinear basis functions of a nonlinear system. This becomes especially difficult when the system input has a non-stationary distribution, as is the case in practical wireless systems. In this paper, we propose a novel algorithm for nonlinear digital SIC that adaptively constructs orthonormal polynomial basis functions according to the non-stationary moments of the transmit signal. By combining these basis functions with the least mean squares (LMS) algorithm, we introduce a new SIC technique, called the adaptive orthonormal polynomial LMS (AOP-LMS) algorithm. To reduce computational complexity for practical systems, we augment our approach with a precomputed look-up table, which maps a given modulation and coding scheme to its corresponding basis functions. Numerical simulation indicates that our proposed method surpasses existing state-of-the-art SIC algorithms in terms of convergence speed and mean squared error when the transmit signal is non-stationary, such as with adaptive modulation and coding. Experimental evaluation with a wireless testbed further confirms that our proposed approach outperforms existing digital SIC algorithms in practical systems.

Residual self-interference (SI) cancellation in the digital baseband is an important problem in full-duplex (FD) communication systems. In this paper, we propose a new technique for estimating the SI and communication channels in a FD communication system, which is inspired from superimposed signaling. In our proposed technique, we add a constant real number to each constellation point of a conventional modulation constellation to yield asymmetric shifted modulation constellations with respect to the origin. We show mathematically that such constellations can be used for bandwidth efficient channel estimation without ambiguity. We propose an expectation maximization (EM) estimator for use with the asymmetric shifted modulation constellations. We derive a closed-form lower bound for the mean square error (MSE) of the channel estimation error, which allows us to find the minimum shift energy needed for accurate channel estimation in a given FD communication system. The simulation results show that the proposed technique outperforms the data-aided channel estimation method, under the condition that the pilots use the same extra energy as the shift, both in terms of MSE of channel estimation error and bit error rate. The proposed technique is also robust to an increasing power of the SI signal.

In this paper, we consider a decode-and-forward (DF) full-duplex relay (FDR)-aided downlink wireless communication system consisting of one base station (BS) equipped with large scale antennas and multiple MIMO FDRs, which have been recognized as essential techniques in the fifth generation (5G) wireless communications. In view of the system performance not only limited by self-interference (SI) and inter-relay interference (IRI) caused by FDRs but also by channel state information uncertainty, a distributed worst-case robust design of FDR beamforming and total power minimization for downlink transmission is proposed subject to relays' and users' target rates, a centralized solution is presented, and then its distributed implementation using alternating direction method of multipliers (ADMM) is presented as well. Finally, some simulation results are provided to demonstrate the efficacy of the proposed algorithm.

In this paper, we apply a reconfigurable intelligent surface (RIS) to a two-way (TW) full-duplex (FD) wireless communication system, where a RIS has N reflecting elements and operates as a relay station forwarding signals between two FD terminals. To demonstrate the advantages of RIS compared with traditional relay, we also present the system and signal models of the TW-FD amplify-and-forward relay (AFR) system. We mathematically analyze the performance of the TW-FD-RIS system by deriving the exact closed-form expressions of the average outage probability (AOP) and the average symbol error rate (ASER) in the case N=1, and approximate expressions of the AOP and the ASER in the case N ≥ 2. Besides, we provide the exact closed-form expression of the AOP and approximate expression of the ASER of the TW-FD-AFR system. Monte-Carlo simulations validate all derived mathematical expressions. Numerical results demonstrate that using the RIS greatly improves the TW-FD communication system's performance compared with using amplify-and-forward (AF) relay. Specifically, even the RIS with one element ( N=1), the AOP and ASER of the TW-FD-RIS system are still lower than those of the TW-FD-AFR system in the low signal-to-noise ratio (SNR) regime. However, in the high SNR regime, the AOP and ASER of the TW-FD-RIS system are higher. In the case N ≥ 2, the AOP and ASER of the TW-FD-RIS system are significantly reduced. On the other hand, the impact of residual self-interference (SI) due to FD transmission mode on the performance of TW-FD-RIS and TW-FD-AFR systems is remarkable, making the AOP and ASER of these two systems reach the error floor in the high SNR regime. Fortunately, the usage of RIS with N ≥ 2 can greatly reduce this impact and avoid the error floor for the TW-FD-RIS system.

A broadband common-aperture, and dual-polarized sub-array of Cavity-Backed Slot Antenna (CBSA) with two ports having a very high level of isolation is presented in this paper. The antenna is designed for wideband full-duplex wireless communication and FMCW radar systems. The antenna is composed of a thin rectangular cavity on which four cross-slots are cut out for radiation. The radiating cross-slots are fed by two concentric orthogonal slots on the back wall of the cavity. One polarization is excited by an end-launch coaxial to waveguide transition which is used to excite the cavity through a feeding slot. A two-pronged microstrip line symmetrically crossing over another feeding slot, oriented orthogonal to the feeding slot, is used to excite the other polarization. The microstrip line is designed such that each prong would couple to the waveguide feed out-of-phase and as a result, a very high isolation level is achieved between channels over a very wide bandwidth. Designed to be compatible with metal 3D printing, the devised antenna is fabricated out of aluminum using Direct Metal Laser Sintering (DMLS) technology. This sub-array slot antenna with dimensions smaller than 1. 28 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> ×1.28 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> ×0. 27 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> (where λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> is the wavelength at the lowest frequency), provides a minimum gain of 10 dB for both channels and exhibits a polarization isolation better than 20 dB within the main beam and a channel isolation above 54 dB over 40% fractional bandwidth.

We provide the design of a co-polarized simultaneous transmit and receive (STAR) array antenna, aiming to address the issue of undesired isolation in the antenna domain of the millimeter-wave (mm-Wave) full-duplex co-polarized communication system. As the cornerstone for the design and development of the antenna, the guided-wave structures of the virtual-electric-wall (VEW)-based ridge substrate-integrated waveguide (RSIW) and ridge parallel plate waveguide (RPPW) are proposed and applied to the full-duplex antenna design. Compared with conventional RSIW, VEW-based RSIW has the merits of lower ohmic loss and releasing the fabrication tolerance in the array antenna. Additionally, on the basis of parallel plate waveguide (PPW), the proposed RPPW exhibits a Chebyshev filtering response. The radiating part of the proposed array employs a slot-pair array excited along orthogonal directions by using the preceding guided-wave structures. Among them, the RSIW array serves as the Tx antenna, and the RPPW array is regarded as the Rx antenna. To achieve co-polarized radiation patterns in the boresight direction, the substrate-integrated waveguide (SIW) feed network is integrated into the reverse side of the antenna, providing differential and common-mode excitations for the RSIW-excited and RPPW-fed arrays. Furthermore, the inter-port isolation is effectively ameliorated by inserting the mushroom-type metamaterial between linear arrays. As a proof-of-concept, a shared-aperture full-duplex array operating in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> -band is developed and fabricated to demonstrate the above concept by using standard printed circuited board technology. The measured overlapped impedance bandwidth covers from 38.18 GHz to 39.13 GHz. The measured Tx-to-Rx isolation is greater than 25.5dB in the Q-band. The measured peak gains in Tx- and Rx-mode are 18.4 dBi and 17.8 dBi. The simulation and measurement results show good consistency, verifying the effectiveness of the proposed array in inter-port isolation, array size, planarization, and lightweight, making it suitable for mm-Wave shared-aperture full-duplex communication systems.