Full-duplex Wireless Communication System Research Articles

A Low-profile Dual-band Dual-polarization Co-aperture Array Antenna with High Isolation

This paper proposes a low-profile high-isolation dual-polarization array antenna operating in the L/S band. The radiating unit of the array antenna adopts U-shaped bending dipoles to realize the staggered arrangement of high- and low-band units, and is connected to the feeding network by a coaxial cable. To achieve the non-influence of high-band and low-band feed networks, multi-layer printing technology is used and grooves are etched on the metal plate. In order to reduce the coupling of the high- and low-band dipoles at low frequency, a bandpass filter is added to the high-band feed network, which greatly improves the isolation at low-frequency. The measured results show that the impedance bandwidth of the array is 4.5% (0.971-1.015 GHz) and 7.5% (1.89-2.04 GHz), with gains of 15 and 19 dBi at 1 GHz and 2 GHz, respectively. And the isolation between the dual-polarized ports more than 50 dB and 35 dB was achieved at 1 GHz and 2 GHz, respectively. The proposed array will have practical value in L/S band full-duplex wireless communication systems.

Self-Interference Cancellation Techniques for In-Band Full-Duplex Wireless Communication Systems: A Review

In-Band Full-Duplex (IBFD) systems have the capability of simultaneously transmitting and receiving signals through the channel and require the same resources as half-duplex systems. Unfortunately, IBFD systems have self-interference (SI) issues that prevent the system from gaining double throughput with respect to half-duplex systems. Therefore, the IBFD system will be more reliable if SI is mitigated more. This contribution will look at SI cancellation in wireless radio and underwater acoustic systems. The reviewed documents cover all types of SI cancellations, including passive, analog, and digital cancellations. In a practical full-duplex system, the SI cancellation for all domains must cancel the SI below the receiver noise floor.

Channel Estimation for Reconfigurable Intelligent Surface-Assisted Full-Duplex MIMO With Hardware Impairments

We consider the problem of channel estimation in a multiple-input-multiple-output (MIMO) full-duplex (FD) wireless communication system assisted by a reconfigurable intelligent surface (RIS) with hardware impairments (HI) occurring at the transceivers and RIS elements. We propose an unbiased channel estimator that requires knowledge of only the first and second order statistics of the HI, for which we derive closed form expressions. The proposed estimator reduces to the maximum likelihood estimator (MLE) in the case of ideal hardware. We also describe FD and HD orthogonal pilot schemes that minimize the mean square error of the MLE in the case of ideal hardware. We verify the performance of the estimator under varying conditions of transceiver and RIS HI via numerical simulations.

A self-biased non-reciprocal magnetic metasurface for bidirectional phase modulation

Non-reciprocal metasurfaces can encode optical functions on forward- and backward-propagating waves, and could be used to create non-reciprocal antennas and radomes for full-duplex wireless communication and radar systems. However, such metasurfaces typically require external electric- or magnetic-field biasing or rely on non-linear effects, which makes practical implementation challenging. Here we report a self-biased non-reciprocal metasurface based on magnetic meta-atoms made from lanthanum-doped barium hexaferrite. The metasurface offers a transmittance of up to 77% and an operation angle of ±64°. We show that they can be used for on-demand bidirectional phase modulation, which provides non-reciprocal functionalities including microwave isolation, non-reciprocal beam steering, non-reciprocal focusing and non-reciprocal holography. The approach could also be potentially extended to megahertz and optical frequencies by using different self-biased magnetic materials.

Optical Multi-Tap RF Canceller for In-Band Full-Duplex Wireless Communication Systems

In-band full-duplex (IBFD) technology has become important for future wireless communications, due to its ability of increasing the radio-frequency (RF) spectrum efficiency. However, the benefits can only be realized if the self-interference (SI) is sufficiently suppressed. RF cancellation is important for IBFD systems due to its ability of SI mitigation. But it becomes more difficult for RF cancellation while the frequency and bandwidth increase. In real scenarios, the response of the multi-path SI channel is not frequency-flat, which also limits the cancellation performance. To solve these problems, we present a photonic-enabled multi-tap RF canceller in this paper. The proposed RF canceller has the ability of cancelling multi-path SI. The tap coefficients are precisely adjusted by optical spectrum processing. A prototype system with 8 taps is demonstrated in this paper. The measured results show 25 dB average cancellation depth over 1 GHz cancellation bandwidth under over-the-air conditions. To verify the signal of interest (SOI) recovery capability of the novel RF canceller, an IBFD wireless communication experiment based on 16-quadrature amplitude modulation (16-QAM) was demonstrated, the SOI was successfully recovered.

Performance Analysis of IRS-Assisted Full-Duplex Wireless Communication Systems With Interference

This letter considers a full-duplex wireless communication system, where two users communicate to each other through an intelligent reflecting surface (IRS). Each user is assumed to be equipped with two antennas, one each for reception and transmission. The system suffers from loop-interference (LI) between the transmit and receive antennas, self-interference (SI) through the IRS, and external interferers (EI) available at the IRS and users. The fading channels are modelled by Weibull distribution as it is a generalized and accurate model for both indoor and outdoor communications. By taking into account the IRS random reflection coefficient (RRC), and by applying the Laguerre expansion approximation, we derive closed-form expressions for outage probability and ergodic capacity. Numerical and simulation results show that LI, SI, EI and RRC have sever impacts on the system performance. The results also show the robustness of the Laguerre expansion approximation compared to non-central Chi-squared distribution, in particular, for a small number of IRS elements.

A Lazy Learning-Based Self-Interference Cancellation Approach for In-Band Full-Duplex Wireless Communication Systems

We propose a new lazy learning-based cancellation approach to improve spectral efficiency for current wireless communication systems, suppress self-interference (SI) sent from base stations, and enable in-band full-duplex (IBFD) transmissions in cellular networks. Our proposed approach consists of two phases based on traditional IBFD systems: an offline phase for database generation and an online phase for data transmission. In the offline phase, the output before a 0/1 decision is premeasured without the desired signal input and recorded in a database with self-defined feature vectors (FVs). In the online phase, a suitable result is sought from the generated database with the help of a learning method and FV for the same system architecture with the desired signal input. The result is then assigned an SI cancellation value. Regular and eager learning-based cancellation approaches are employed to evaluate the proposed method and simulate the transmission output. Computer simulation results indicated that the proposed cancellation methods could achieve about 134 dB SI suppression and achieve nearly the same transmission levels as methods with no SI effect, enabling the IBFD operations in wireless communication systems better than the regular and eager learning-based techniques.

A Compact Dual-Band Dual-Circularly Polarized SIW Cavity-Backed Antenna Array for Millimeter-Wave Applications

A compact millimeter-wave (mm-Wave) dual-band dual-circularly polarized (CP) antenna based on substrate integrated waveguide (SIW) cavity-backed structure is proposed in this letter. By introducing the square aperture with two symmetric perturbation notches on the SIW cavity, right-hand CP (RHCP) can be generated at the low frequency band. On this basis, the dumbbell-shaped patch is placed inside the square aperture to generate left-hand CP (LHCP) at the high frequency band. To optimize the impedance matching and adjust resonant frequency, stacked parasitic patches are adopted. In addition, the two shorted vias are introduced to realize the independent tuning of high frequency band. The measured results show that the array achieves the impedance bandwidths of 7.3% and 7.5%, AR bandwidths of 2.8% and 2.6%, with gains of 18.2 and 18.5 dBic at 28 and 38 GHz, respectively. The proposed array will be a potential candidate component for 5G mm-Wave full-duplex wireless communication systems.

Antenna/Propagation Domain Self-Interference Cancellation (SIC) for In-Band Full-Duplex Wireless Communication Systems

In-band full duplex (IBFD) is regarded as one of the most significant technologies for addressing the issue of spectrum scarcity in 5G and beyond systems. In the realization of practical IBFD systems, self-interference, i.e., the interference that the transmitter causes to the collocated receiver, poses a major challenge to antenna designers; it is a prerequisite for applying other self-interference cancellation (SIC) techniques in the analog and digital domains. In this paper, a comprehensive survey on SIC techniques in the antenna/propagation (AP) domain is provided and the pros and cons of each technique are studied. Opportunities and challenges of employing IBFD antennas in future wireless communications networks are discussed.

Impacts of Residual Self-Interference, Hardware Impairment and Cascade Rayleigh Fading on the Performance of Full-Duplex Vehicle-to-Vehicle Relay Systems.

In practice, self-interference (SI) in full-duplex (FD) wireless communication systems cannot be completely eliminated due to imperfections in different factors, such as the SI channel estimation and hardware circuits. Therefore, residual SI (RSI) always exists in FD systems. In addition, hardware impairments (HIs) cannot be avoided in FD systems due to the non-ideal characteristics of electronic components. These issues motivate us to consider an FD-HI system with a decode-and-forward (DF) relay that is applied for vehicle-to-vehicle (V2V) communication. Unlike previous works, the performance of the proposed FD-HI-V2V system is evaluated over cascaded Rayleigh fading channels (CRFCs). We mathematically obtain the exact closed-form expressions of the outage probability (OP), system throughput (ST), and ergodic capacity (EC) of the proposed FD-HI-V2V system under the joint and crossed effects of the RSI, HIs, and CRFCs. We validate all derived expressions via Monte-Carlo simulations. Based on these expressions, the OP, ST, and EC of the proposed FD-HI-V2V system are investigated and compared with other related systems, such as ideal hardware (ID) and half-duplex (HD) systems, as well as a system over traditional Rayleigh fading channels (RFCs), to clearly show the impacts of negative factors.

On Performance of Two-Way Full-Duplex Communication System With Reconfigurable Intelligent Surface

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.

Blind Source Separation For Full-Duplex Systems: Potential and Challenges

Full-duplex communications systems that transmit and receive simultaneously suffer self-interference due to the mixing of the transmitted signal and the weaker received signal at the same node. The problem becomes compounded in Multi-Input Multi-Output (MIMO) systems, where considerable overhead is dedicated to training. In this article, we discuss using blind source separation techniques, namely Independent Component Analysis (ICA) to reduce training overhead in MIMO in-band full-duplex wireless communication systems. Practical limitations are discussed and experimental results that compare ICA to traditional Least Square approaches are presented, showing the superiority of ICA, especially in low SNR regimes.

Cooperative Communications for Improving the Performance of Bidirectional Full-Duplex System With Multiple Reconfigurable Intelligent Surfaces

In this paper, we exploit the benefits of full-duplex (FD) transmission and reconfigurable intelligent surfaces (RISs) by considering a bidirectional full-duplex wireless communication system with the support of RISs. Specifically, cooperative communications are applied at two FD terminals in a RISs-assisted bidirectional full-duplex (RIS-FD) system. In this context, we successfully derive the mathematical expressions of the outage probability (OP) and the ergodic capacity (EC) of the RIS-FD system with a direct link between two terminals and multiple reflecting paths created by RISs over the Rayleigh fading channel. Numerical results show that the performance of the considered RIS-FD system depends significantly on the total number of reflecting elements in the RISs. Particularly, when the total number of reflecting elements is small, the OP and EC saturate at a high signal-to-noise ratio (SNR). However, when the total number of reflecting elements is large enough, OP decreases greatly while EC enhances significantly. In addition, the effect of residual self-interference (SI) caused by FD transmission on OP and EC is remarkable, even the residual SI level is very small. On the other hand, the influence of other system parameters such as the number of RISs ( $N$ ) and the number of reflecting elements ( $L$ ) are also investigated to fully consider the performance of the RIS-FD system. An important observation is that when current techniques cannot completely eliminate SI in FD transmission, the use of multiple RISs with larger $L$ can indirectly reduce the impact of SI and significantly improve the performance of the proposed RIS-FD system. Finally, the obtained expressions are validated via Monte-Carlo simulations.

Intelligent Reflecting Surfaces to Achieve the Full-Duplex Wireless Communication

The full-duplex (FD) radio-frequency wireless communication theoretically doubles the data rates compared to the half-duplex counterpart. However, its performance depends critically on the self-interference (SI) that exists between transmit and receive antennas. The intelligent reflecting surfaces (IRSs) which consist of several reflecting elements (known as meta-atoms) are used to engineer the wireless environment and thus to achieve improved performance through constructively focussing the reflected signal. In this letter, the IRS technology is explored for lowering down the effect of SI in an FD wireless communication system. The utility of IRSs in FD technology is investigated in terms of outage and error probabilities. Specifically, the closed-form expressions for outage and average symbol error probabilities are obtained. All the channel coefficients are assumed to be distributed as the generalized Nakagami-m random variables. The analytical results are corroborated through Monte-Carlo simulations.

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.

Iterative Self-Interference Mitigation in Full-Duplex Wireless Communications

This paper considers a full-duplex wireless communication system in which detection of the desired signal is hindered not only by the self-interference (SI), but also phase noise, in-phase and quadrature-phase imbalance and power amplifier’s nonlinearity distortion. An iterative algorithm is proposed in which the processes of SI cancellation and detection of the desired signal aid each other in each iteration. In each iteration, the SI cancellation process performs widely linear estimation of the SI channel and compensates for physical impairments to improve the detection performance of the desired signal. The detected desired signal is in turn removed from the received signal to improve SI channel estimation and SI cancellation in the next iteration. Simulation results show that the proposed algorithm significantly outperforms existing algorithms in SI cancellation and detection of the desired signal.

USRP 장치를 이용한 동일대역 전이중 무선통신 연구

The implementation of an in-band full-duplex wireless communication system is demonstrated in this study. In the analog/RF domain, the self-interference(SI) signal is reduced using a separate antenna for the transmitter and receiver paths, and most of the SI signal is canceled in the digital domain. A software defined radio(SDR) is used to implement the in-band full-duplex wireless communication system. The USRP X310 device uses transmitting and receiving antennas. By adjusting the gain of the transmitting and receiving ends of the SDR device, the magnitude of the SI signal entering the receiving antenna, and the size of the received signal from the outside, are both set to −64 dB. To verify the in-band full-duplex wireless communication performance, the source data is image and orthogonal frequency-division multiplexing is used for modulation. A WiFi standard frame with a carrier frequency of 2.67 GHz and bandwidth of 20 MHz is used. In the received signal, the SI signal is canceled by digital signal processing and the SI signal is attenuated by up to 34 dB. OFDM demodulation was impossible when the SI signal was not removed. However, the bit error rate is reduced to 2.63×10−5 when the SI signal is attenuated by 34 dB, and no error is detected in the 100 Mbit data output as a result of passing through the Viterbi decoder.

Interference Exploitation in Full-Duplex Communications: Trading Interference Power for Both Uplink and Downlink Power Savings

This paper considers a multiuser full-duplex (FD) wireless communication system, where an FD radio base station (BS) serves multiple single-antenna half-duplex uplink and downlink users simultaneously. Unlike conventional interference mitigation approaches, we propose using knowledge of the data symbols and the channel state information (CSI) at the FD radio BS to exploit the multi-user interference constructively rather than to suppress it. We propose a multi-objective optimization problem (MOOP) via the weighted Tchebycheff method to study the tradeoff between the two desirable system design objectives, namely the total downlink transmit power and the total uplink transmit power, at the same time ensuring the required quality-of-service (QoS) for all users. In the proposed MOOP, we adapt the QoS constraints for the downlink users to accommodate constructive interference for both generic phase shift keying modulated signals and for quadrature amplitude modulated signals. We also extended our work to a robust design to study the system with imperfect uplink and downlink CSI. The simulation results and analysis show that significant power savings can be obtained. More importantly, however, the MOOP approach here allows for the power saved to be traded off for both uplink and downlink power savings, leading to an overall energy efficiency improvement in the wireless link.

Transceiver Design for MIMO Full-Duplex Two-Way Relay Network

Mitigation of loopback self-interference (LSI) is one of the main challenges in full-duplex (FD) wireless communication systems. In this letter, we present the design of optimal transceiver and relay processing algorithms for an FD two-way amplify-and-forward (AF) multiple-input multiple-output relaying system. We consider a realistic scenario where the available channel state information of the LSI channels is imperfect. The resultant residual LSI is suppressed by the suitable design of precoders and receive filters. All the precoders and receive filters are updated in each time slot in order to combat the cumulative interference effect caused by AF operation of the FD relay. We derive a simple recursive equation to keep track of this cumulative interference. The performance of the proposed design is demonstrated via the simulation results.

Joint Digital Self-interference Cancellation in Full-duplex Radios under IQ Imbalance and Transmitter Non-linearity

RF imperfections can significantly degrade the performance of full-duplex wireless communication system by introducing non-idealities and random effects, which make it difficult to cancel the self-interference completely. In this paper, we first address the adverse benefits of both the transmitter non-linearity and the IQ imbalance. Then on the basis of these, a joint digital self-interference cancellation scheme is proposed, in which not only the effect of IQ imbalance and power amplifier non-linearity individually, but also the comprehensive function of them are taken into account. Furthermore, the simulation is implemented in the MATLAB platform using standard WiFi 802.11ac PHYs. The results show that the proposed canceller can eliminate more compared with other cancellation schemes, and the overall self-interference attenuation can attain 108dB, which makes the residual self-interference closer to the noise floor.