In-band Full-duplex Wireless Research Articles

Analog Least Mean Square Adaptive Filtering for Self-Interference Cancellation in Full Duplex Radios

In-band full duplex (IBFD) radio represents one of the key technologies for future wireless communication and radar applications. A major challenge of this technology is to mitigate the strong self-interfer-ence (SI) so that the residual SI level falls below the receiver's noise floor. Radio frequency (RF) self-inter-ference cancellation (SIC) is essential for preventing an IBFD receiver from becoming saturated by the SI. We commence with an in-depth review of the promising analog least mean square (ALMS) adaptive filtering architecture, conceived for RF SIC in the IBFD radio RF front-end. The cancellation circuits employing this architecture can be implemented purely by analog components without any involvement of more power-thirsty digital signal processing. The behaviors, performance, and implementation of the ALMS loop are presented. Finally, their applications in various IBFD radios are discussed, and future research directions are provided.

Full-Duplex DF Relaying With Parallel Hybrid FSO/RF Transmissions

The ever-increasing demands for higher data rates in telecommunications networks, along with the significant benefits offered by wireless communications, have pushed the technological developments to new frontiers. Free-space optical (FSO) communications are among the technologies that can address these emerging demands, offering large and unregulated bandwidth. In addition, recent advances in radio frequency (RF) systems have enabled the development of in-band full-duplex (FD) radios with the employment of advanced self-interference cancellation techniques. Therefore, in this paper we study a robust FD relaying system comprised of parallel hybrid FSO/RF communication links, where the coordination of transmissions between the FSO/RF subsystems is carried out by the hard-switching protocol. The operation of the RF links is impaired by Nakagami-m fading, the residual self-interference due to the FD relaying operation, and the in-phase and quadrature-phase imbalance effect due to imperfect RF front-ends. As far as the FSO links are concerned, we consider the influence of the joint effects of atmospheric turbulence, beam wander and pointing errors. We first derive analytical closed-form expressions for the outage probability of both subsystems as well as for the overall FD relaying hybrid system. Then, asymptotic expressions are extracted for the outage performance in the high signal-to-noise ratio regime. The achievable outage diversity orders of both FSO and RF subsystems are determined, which provide significant insight into the design of such hybrid systems in a wide variety of operating conditions. Finally, the numerical results are presented using the derived outcomes, and significant conclusions are drawn about FD relaying hybrid systems.

Dual-Polarized High-Isolation Antenna Design and Beam Steering Array Enabling Full-Duplex Communications for Operation Over a Wide Frequency Range

A new dual-polarized antenna and array are presented for In-Band Full-Duplex (IBFD) applications, offering high inter-port isolation. The single-element antenna system consists of four H-shape slots, stacked patches for wide bandwidth, and two external hybrid couplers. The multilayer antenna is well matched from about 2.1 to 2.4 GHz and provides high isolation in this frequency range in excess of 60 dB. Two different prototypes are simulated and measured to highlight the design process. In addition, a new and compact hybrid coupler with excellent phase and magnitude stability is also presented for improved antenna radiation and IBFD performances. When compared to other similar types of hybrid coupler and antenna systems, the proposed configurations are simple to manufacture, provide a higher isolation bandwidth (10%), and higher gain of 7.3 dBi with cross-polarization levels of 30 dB or lower. The single-element design was also extended to a 2×2 array and studied for different beam steering and feeding scenarios. The operating bandwidths and isolation values offered by these S-band antenna and array systems can support new data link possibilities for beam steering and future low-cost IBFD wireless networks by simple antenna fabrication.

Iterative Nonlinear Self-Interference Cancellation for In-Band Full-Duplex Wireless Communications Under Mixer Imbalance and Amplifier Nonlinearity

This paper presents an iterative estimation and cancellation technique for nonlinear in-band full-duplex transceivers with IQ imbalances and amplifier nonlinearities. The estimation process of the proposed scheme consists of three stages, namely, the channel response estimation, IQ imbalance estimation, and power amplifier and low-noise amplifier (LNA) nonlinearities estimation. For the estimation of the parameters and improvement of the accuracy, distortions are compensated by cancellation or inversion with the latest estimated parameters. On the one hand, the channel response is estimated on the time domain; on the other hand, the IQ imbalance and nonlinearities are estimated on the frequency domain for a more straightforward estimation and superior accuracy. In the cancellation process of the proposed scheme, the received signal is compensated with the estimated parameters of the LNA and receiver IQ imbalance before cancellation because the desired signal is received with a high-power self-interference and is distorted by the radio-frequency receiver impairments. Simulation results show that the proposed technique can achieve higher cancellation performance compared with the Hammerstein canceller when the LNA is saturated by the self-interference. Additionally, the performance of the proposed canceller converges much faster than that of the Hammerstein canceller.

A Distributed Link Scheduler for In-Band Full Duplex Wireless Networks

In a wireless network, a well designed link scheduler ensures links are activated frequently and experience minimal or no interference, meaning these links will have a high capacity. A challenging problem, however, is random or uncertain channel gains. Another challenge is determining whether a set of links can co-exist together, meaning they do not cause excessive interference to one another. These challenges have a direct impact on the number of activated links as well as their data rates. To this end, this paper proposes a reinforcement learning approach that enables each node to learn the best link to activate and data rate to use over random channel conditions. In addition, it allows nodes to learn the opportune time to use full-duplex or half-duplex transmissions. Our simulation results show our link scheduler results in an average throughput that is triple that of Carrier Sense Multiple Access (CSMA), and up to quadruple the average throughput of Time Division Multiple Access (TDMA). Moreover, our link scheduler remains superior when channel gains vary significantly from their average value.

A Novel Degree of Freedom (DoF) Link Scheduler for Full-Duplex Wireless Local Area Networks

We consider an Access Point (AP) equipped with multiple antenna elements and an in-band full-duplex radio. In each time slot, it needs to poll clients for uplink and/or carry out downlink transmissions, and to allocate its antenna elements for data transmissions and interference cancellation. We outline a Markov Decision Process (MDP) and use it to find the optimum polling policy that maximizes the average number of data streams. We also outline two novel Degree of Freedom (DoF)-based antenna allocation algorithms. The results show our policy is able to activate respectively up to 58% more data streams and within 83% of the theoretical upper bound of half-duplex and full-duplex polling-based methods that require perfect information of queue occupancy and interference at clients.

Cognitive Networks With In-Band Full-Duplex Radios: Jamming Attacks and Countermeasures

Although in-band full-duplex (IBFD) radios promise to double the throughput of a wireless link, they are more vulnerable to jamming attacks than their out-of-band full-duplex (OBFD) counterparts. For two communicating OBFD nodes, a jammer needs to attack both the uplink and the downlink channels to completely break the communication link. In contrast, only one common channel needs to be jammed in the case of two IBFD nodes. Even worse, a jammer with self-interference suppression (SIS) capabilities (the underlying technique of IBFD radios) can learn the transmitters' activity while injecting interference, allowing it to react instantly to the transmitter's strategies. In this work, we consider a power-constrained IBFD “reactive-sweep” jammer that sweeps through the set of channels by jamming a subset of them simultaneously. We model the interactions between the IBFD radios and the jammer as a stochastic constrained zero-sum Markov game in which nodes adopt the frequency hopping (FH) technique as their strategies to counter jamming attacks. Beside the IBFD transmission-reception (TR) mode, we introduce an additional operation mode, called transmission-detection (TD), in which an IBFD radio transmits and leverages its SIS capability to detect jammers. The aim of the TD mode is to make IBFD radios more cognitive to jamming. The nodes' optimal defense strategy that guides them when to hop and which operational mode (TD or TR) to use is then established from the equilibrium of the stochastic Markov game. We prove that this optimal policy has a threshold structure, in which IBFD nodes stay on the same channel up to a certain number of time slots before hopping. Simulation results show that our policy significantly improves the throughput of IBFD nodes under jamming attacks.

Characteristic-Mode-Based Design of Planar In-Band Full-Duplex Antennas

In-band full-duplex radios demand simultaneous transmit and receive (STAR) antennas with high isolation, compact and ultra-low profiles, and simple feed networks. Current state-of-the-art designs rarely meet all of these requirements at the same time. In this paper, we present a characteristic-mode-based design approach to achieve high isolation using compact fully-planar STAR antennas with a simple feed structure. The proposed method utilizes two characteristic modes of a conducting object as transmit and receive chains to achieve high isolation without a complicated self-interference cancellation circuit. The design example in this work is fully-planar, and it has a physical height of 1.6 mm. The measured -10 dB overlapped $|S_{11}| ~{\mathrm {and}}~ |S_{22}|$ fractional bandwidth of this STAR antenna is 2.5% and the measured isolation between the transmit and receive ports is greater than 30 dB over the entire frequency band of operation.

Monostatic Antenna In-Band Full Duplex Radio: Performance Limits and Characterization

This paper presents an in-band full duplex (IBFD) architecture, comprised of a monostatic antenna and software defined radio (SDR) implementing digital self-interference cancellation (DSIC). For this architecture, three alternative monostatic antennas with high passive suppression are integrated with a least squares linear DSIC algorithm implemented on the WARP v3 orthogonal frequency-division multiplexing based IEEE 802.11 SDR. Via detailed laboratory tests, the performance of the proposed IBFD radio architecture is investigated, considering various indoor positions and orientations for each antenna. The measurement results have been utilized to devise an analytical model for characterizing the power of the residual self-interference in the monostatic antenna IBFD radio as a function of transmit power levels. It is shown that the monostatic IBFD architecture can achieve up to almost 99 dB total cancellation, enabling medium range full duplex communication with moderate transmit power levels. Bidirectional communication is demonstrated between the two IBFD nodes with the proposed architecture, and error vector magnitude is observed as a function of increasing transmit power, i.e., SI. The throughput gain of full-duplex communication over half-duplex communication is also calculated as a function of payload size and the gain is found to be close to two for the settings used in the experiments.

Photonic-Enabled RF Canceller for Wideband In-Band Full-Duplex Wireless Systems

In-band full-duplex (IBFD) wireless systems offer the ability to revolutionize frequency spectrum utilization for future networks. This is only possible if the self-interference generated by each wireless node is sufficiently mitigated, and becomes more challenging as the bandwidth increases. RF cancellation is one of the critical components that enables this interference reduction, but has previously been limited to narrowband operation or restricted to distinctive environments. This paper presents a novel RF canceller that utilizes photonic components to create a wideband vector modulator architecture with tunable time-delay taps. A prototype system with 20 canceller taps has been demonstrated to provide 25 and 20 dB of cancellation over 500-MHz and 1-GHz instantaneous bandwidths, respectively, and is tunable between 0.5 and 5.5 GHz. This unique photonic-enabled RF canceller design provides a path forward to achieve the wideband operation and high tap counts that will be required to successfully deploy future wireless systems with IBFD technology.

A Novel Frequency Allocation Scheme for In Band Full Duplex Systems in 5G Networks

In band full duplex (IBFD) is an emerging transceiver technology that facilitates simultaneous transmission and reception on the same frequency to double spectral efficiency. Self-interference (SI) from its own transmitter is the biggest challenge in IBFD radios causing significant degradation to its receiver performance. SI cancellation (SIC) techniques proposed in literature involve considerable hardware and software complexities for practical realization of IBFD radios. In this letter, a hybrid cellular architecture is proposed composing of IBFD base station and legacy half duplex user equipments (UEs), thus avoiding complex SIC requirement at UE. A novel frequency allocation scheme is proposed for this hybrid architecture that allows sharing of carrier frequencies among UEs based on distance criteria. As the number of UEs increases, chances of finding UEs that satisfy the distance criteria also increase steadily. Simulation results reveal that the probability of 100% frequency reuse exceeds 0.9 for as few as 8 UEs itself, demonstrating the potential of the proposed idea.

Queueing and channel access delay analysis in in-band full-duplex wireless networks

In-band full-duplex wireless communication, which supports a node to transmit and receive simultaneously in the same frequency band, is receiving growing interest. The latency of packets in an in-b...

Performance Analysis and Enhancements for In-Band Full-Duplex Wireless Local Area Networks

This paper presents an analytical model of power consumption for In-Band Full-Duplex (IBFD) Wireless Local-Area Networks (WLANs). Energy-efficiency is compared for both Half-Duplex (HD) and IBFD networks. The presented analytical model closely matches the results generated by simulation. For a given traffic scenario, IBFD systems exhibit higher power consumption, however at improved energy efficiency when compared to equivalent HD WLANs.

2.4 GHz dual polarised monostatic antenna with simple two‐tap RF self‐interference cancellation (RF‐SIC) circuitry

This Letter presents a dual polarised monostatic patch antenna with a simple two-tap RF/analogue domain-based self-interference cancellation (SIC) (RF-SIC) circuitry to achieve high interport isolation in a comparatively wider bandwidth for a 2.4 GHz in-band full duplex wireless transceiver. A brief mathematical description for the deployed two-tap RF-SIC is also presented to provide insight into self-interference (SI) suppression in a comparatively wider bandwidth. The implemented antenna with a two-tap RF-SIC circuitry achieves around 80 dB T-x-R-x isolation for a 20 MHz bandwidth in addition to more than 90 dB peak isolation for measurements performed in a lab environment where SI may result from the nearby placed objects. The deployed two-tap RF-SIC circuit can be tuned to effectively cancel out SI caused by reflections of the radio signal from the surroundings. Moreover, an 80 dB isolation bandwidth can be tuned within a 10 dB return loss bandwidth of the implemented dual polarised antenna.

Single Layer, Differentially Driven, LHCP Antenna With Improved Isolation for Full Duplex Wireless Applications

This paper presents a unidirectional, left handed circularly polarized (LHCP), single layered antenna array with improved interport isolation for simultaneous transmit and receive (STAR) or in-band full duplex (IBFD) wireless applications. The proposed IBFD antenna is comprised of three identical and sequentially rotated LHCP radiating elements where two Rx patches are symmetrically placed with respect to a single Tx patch. The symmetry of proposed antenna structure results in same amount of coupling or self interference (SI) from Tx element and differentially driven Rx patches achieve effective suppression of resulting SI to obtain improved Tx-Rx interport isolation required for STAR applications. The deployed feed network for differential-driven Rx operation is composed of a simple 3dB/180° rat-race coupler (ring hybrid coupler) with nice in-band amplitude and out-of-phase balance characteristics. The implemented single layer, compact (antenna elements and feeding network etched on single-layered PCB) antenna array achieves 10dB return-loss bandwidth ≥ 75MHz for both Tx and Rx ports. The prototype achieves ≥ 47dB interport isolation in 75MHz bandwidth and 3 dB axial ratio beam-width is ~ 60° for implemented antenna.

Geometry-Based Statistical Channel Models of Reflected-Path Self-Interference in Full-Duplex Wireless

Self-interference (SI) channel characteristics are crucial for SI cancellation (SIC) techniques of in-band full-duplex (IBFD) wireless communications. This paper develops two geometry-based statistical channel models-geometry-based concentric ellipses model (GBCEM) and geometry-based concentric circles model (GBCCM)-for full-duplex systems in micro- and macrocell environments, respectively, to analyze the characteristics of reflected-path SI (RSI) compared with the desired signal (DS). The probability density functions of incident angle and path length are derived to identify the fading characteristics of RSI and DS. The simulations based on GBCEM and GBCCM intuitively present the signal characteristics by the signal envelope in the time domain, the statistics of signal envelope, the autocorrelation properties, and the power spectral density. We find that the fading characteristics of RSI and DS can be modeled as Rayleigh when reflectors sit far away from the receiver. We also find that RSI is still likely to be Rayleigh fading, even if DS is not when reflectors scatter in a specific region restricted by the range of incident angle. Our works on the characteristics of RSI and DS are generally helpful to explore a more efficient SIC mechanism for IBFD systems, and the developed models are useful for both simulation and analysis purposes.

Distribution of the Residual Self-Interference Power in In-Band Full-Duplex Wireless Systems

This paper derives the distribution of the residual self-interference (SI) power in an analog post-mixer canceler adopted in a wireless in-band full-duplex communication system. We focus on the amount of uncanceled SI power due to SI channel estimation errors. Closed form expressions are provided for the distribution of the residual SI power when Rician and Rayleigh fading SI channels are considered. Moreover, the distribution of the residual SI power is derived for low and high channel gain dynamics, by considering the cases when the SI channel gain is time-invariant and time-variant. While for time-invariant channels the residual SI power is exponentially distributed, for time-variant channels the exponential distribution is not a valid assumption. Instead, the distribution of the residual SI power can be approximated by a product distribution. Several Monte Carlo simulation results show the influence of the channel dynamics on the distribution of the residual SI power. Finally, the accuracy of the theoretical approach is assessed through the comparison of numerical and simulated results, which confirm its effectiveness.

Ultra-Dense 5G Small Cell Deployment for Fiber and Wireless Backhaul-Aware Infrastructures

In this paper, we study the cell planning problem for a two-tier cellular network containing two types of base stations (BSs), i.e., with fiber backhaul, referred to as wired BSs (W-BSs), and BSs with wireless backhaul, referred to as unwired-BSs (U-BSs). In-band full-duplex wireless communications is used to connect U-BSs and W-BSs. We propose an algorithm to determine the minimum number of W-BSs and U-BSs to satisfy given cell and capacity coverage constraints. Furthermore, we apply our proposed non-dominated sorting genetic algorithm II (NSGA-II) to solve both cell planning and joint cell and backhaul planning problem to minimize the cost of planning, while maximizing the coverage simultaneously. Additionally, the considered cell planning program is developed into an optimization by including the problem of minimizing the cost of fiber backhaul deployment. In order to analyze the performance of the proposed algorithm, we study three different deployment scenarios based on different spatial distributions of users and coverage areas. The results show the superiority of our proposed NSGA-II algorithm for both cell planning and joint cell and backhaul planning to other well-known optimization algorithms. The results also reveal that there is a tradeoff between cell deployment costs and SINR/rate coverage, and W-BSs are placed in congested areas to consume less resources for wireless backhauls. Similarly, a tradeoff between cell and fiber deployment costs and SINR/rate coverage is observed in planning. We show that for realistic scenarios desirable solutions can be selected from the Pareto front of the introduced multi-objective problem based on given cellular operator policies.

Asym-FDMAC: In-band full-duplex medium access control protocol for asymmetric traffic in wireless LAN

Recent advancements in antenna design and signal processing have made it feasible to transmit and receive simultaneously by using the same carrier frequency, which is defined as in-band full-duplex (IBFD) wireless communications. Along with physical layer advancements, efficient medium access control (MAC) design is required to derive optimal benefit from this latest technology. In this paper, an IBFD MAC protocol named Asym-FDMAC, is proposed for infrastructure based WLAN to support asymmetric lengths of traffic for uplink and downlink. This MAC protocol enables multiple users to transmit data to the access point (AP) during the transmission of a single downlink frame from the AP. In Asym-FDMAC, the AP always initiates the transmission. Therefore, there is no contention period and thus, there is no collision among the user terminals and the AP for channel access. A mathematical analysis of Asym-FDMAC is presented together with an evaluation of its performance, which involved a comparison with traditional IBFD and half-duplex (HD) communications. On average, the highest throughput gain achieved by Asym-FDMAC was approximately 54% and 94% compared with traditional IBFD and HD communications respectively, although the lowest average throughput gain was observed as about 50% and 89%, respectively.

Full-Duplex MIMO Radios: A Greener Networking Solution

Relative to half-duplex (HD) radios, in-band full-duplex (FD) radios have the potential to double a link’s capacity . However, such gain may not necessarily extend to the network-wide throughput , which may actually degrade under FD radios due to excessive network interference. This paper identifies the unique advantages of FD radios and leverages multi-input multioutput (MIMO) communications to translate the FD spectral efficiency gain at the PHY level to the throughput and power efficiency gain at the network layer. We first derive sufficient conditions under which FD-MIMO radios can asymptotically double the throughput of the same network of HD-MIMO ones. Specifically, if a network of $2{N}$ HD radios ( ${N}$ links) can achieve a total throughput of dN bps (i.e., ${d}$ bps per link), then an FD-capable network with the same number of links and network/channel realization can achieve $2{N}$ ( $\textit{d}-1$ ) bps [i.e., ( ${d} -1$ ) bps per direction of a bidirectional link]. To leverage this theoretical gain, we exploit the “spatial signature” readily captured in the network interference to design an MAC protocol that allows multiple FD links to concurrently communicate while adapting their radiation patterns to minimize network interference. The protocol does not require any feedback nor coordination among nodes. Extensive simulations show that the proposed MAC design dramatically outperforms traditional CSMA-based and the non-orthogonal multiple access protocols with either HD or FD radios with respect to both throughput and energy efficiency. Note that in the literature, network interference is often treated as colored noise that then gets whiten during the signal detection process. However, through our MAC protocol, we emphasize that, unlike random noise, network interference has its own structure that can be “mined” for “intelligence” to better align the transceiver’s signal.