In-band Full-duplex Operation Research Articles

Characterizing in-band full-duplex in broadcast communications: From field trials to a loopback channel model

In-band full duplex (IBFD) communications are a potential solution to spectrum scarcity. IBFD communications offer greater spectral efficiency than traditional half-duplex communications by transmitting and receiving on the same frequency channel. However, IBFD operation requires overcoming the challenge of eliminating the self-interference coupled from the transmit antenna to the receive subsystem. Knowledge of the characteristics of the loopback propagation channel makes it easier to cancel out the self-interference. Nevertheless, complex field trials are required to adequately characterize loopback channels, which is still lacking in the recent literature. This paper proposes a measurement campaign in a real and ongoing broadcast transmission center to characterize the main characteristics of loopback channels. The work proposes a set of loopback channels based on field trials and an empirical analysis of the most relevant parameters of the channel model, such as the Doppler spectrum, the delay spread, and the K-factor.

Implementation of Wireless Backhaul and Inter-Tower Communications With MIMO in ATSC 3.0

Wireless backhaul, i.e., in-band distribution links (IDL) and inter-tower communications networks (ITCN) were previously proposed as key enabling technologies for the next-generation digital broadcasting systems, e.g., the Advanced Television Systems Committee (ATSC) 3.0 system. Both ITCN/IDL can be operated in the spectrum-efficient in-band full-duplex (IBFD) mode and the same frequency band is shared between the broadcast service signal and the ITCN/IDL signals. It is desirable to integrate multi-input multi-output (MIMO) into ITCN/IDL transmission to increase the throughput and reduce the portion of spectrum occupation by ITCN/IDL. However, due to the backward compatibility constraint, conventional symmetrical MIMO techniques cannot be applied directly to the existing broadcast infrastructure without affecting legacy receivers. In this paper, the practical implementation of a non-symmetrical MIMO for ITCN/IDL while maintaining backward compatibility is considered. In addition to the existing broadcasting antenna, the non-symmetrical MIMO comprises an additional low-power RF feeder cable and one or more highly directional antennas to achieve low-cost MIMO implementation for high throughput data distribution and inter-tower communications. When the broadcast service signal and the MIMO signal are multiplexed in Time Division Multiplexing (TDM) format, the signal structure is ATSC 3.0 standard compliant. When the broadcast service signal and the MIMO signal are multiplexed in Layer Division Multiplexing (LDM) format, modification of the ATSC 3.0 standard, i.e., introducing a new pilot encoding scheme, is required. Moreover, MIMO self-interference cancellation (SIC) is investigated for IBFD operation.

Comparative analysis of antenna isolation characteristic with & without self‐interference reduction techniques towards in‐band full‐duplex operation

AbstractThe advancement of communication and radar systems has triggered the demand for in‐band full‐duplex (IBFD) operation. IBFD is regarded as one of the most outstanding technologies in terms of its ability to improve spectral efficiency and data rate. IBFD operation has the potential not only to enable communication or radar systems separately, but to help realising radar sensing and communication at the same time, known as Joint Communication and Sensing (JC&S) technology. In realisation of IBFD and JC&S systems, self‐interference (SI) is the key challenge for antenna designers. In this article, a comprehensive survey on SI reduction techniques in the antenna domain is provided and the advantages and disadvantages of each technique are studied. SI reduction techniques have been proposed separately but an elaborated review of relevant techniques with a comparative study has not been presented yet. For the first time, a systematic comparative study of different SI reduction techniques is graphically presented to cover a comparison based on isolation, gain, and impedance bandwidth in terms of before and after SI reduction techniques. Moreover, this study also explores how antenna isolation characteristics vary with respect to the thickness and the permittivity of the substrate without any SI reduction technique.

In-Band Full-Duplex Operation in High-Speed Mobile Environments: Not So Fast!

In-band full-duplex (IBFD) technology is promised to deliver us from the scarcity of spectral resources as well as create novel multifunction wireless systems <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> . This concept was initially investigated in the 1960s for continuous-wave radar applications <xref ref-type="bibr" rid="ref2" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[2]</xref> , and, in the past decade, it has undergone a shift in research toward communications <xref ref-type="bibr" rid="ref3" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[3]</xref> – <xref ref-type="bibr" rid="ref4" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"/> <xref ref-type="bibr" rid="ref5" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"/> <xref ref-type="bibr" rid="ref6" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"/> <xref ref-type="bibr" rid="ref7" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[7]</xref> and converged RF systems <xref ref-type="bibr" rid="ref8" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[8]</xref> , <xref ref-type="bibr" rid="ref9" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[9]</xref> . Since 2010, more than 600 IEEE publications alone have focused on at least one aspect of the technology, including more than 400 conference papers, 200 journal articles, and 20 magazine features <xref ref-type="bibr" rid="ref10" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[10]</xref> . After digesting these numbers, it is fair to ask: Why has IBFD functionality not been incorporated into a wireless standard yet?

Realization of In-Band Full-Duplex Operation at 300 and 4.2 K Using Bilateral Single-Sideband Frequency Conversion

CMOS-integrated in-band full-duplex (IBFD) operation in wireless links and cryogenic quantum platforms was previously enabled by magnetic-free circulators using the phase non-reciprocity from spatial-temporal modulation. In this article, we present an alternative and simple integrated circuit scheme, which not only realizes non-reciprocal signal flows required for IBFD operations but also improves the isolation performance by completely eliminating any chip-level transmit (TX)-to-receive (RX) coupling. The above functions are enabled by performing a direction/frequency-independent, single-sideband down-conversion to the counter-propagating TX and RX signals, which creates opposite deviations of on-chip TX and RX frequencies with respect to the antenna (ANT) frequency. Such a principle also broadens the isolation bandwidth and enables integrated receiver down-mixing function. As a proof-of-concept, a 3.4-4.6-GHz (30% fractional bandwidth) IBFD interface is implemented using a 65-nm bulk CMOS technology. The measured TX-to-RX isolation of the circuit is 32-51 dB at 300 K, and 14-29 dB at 4.2 K. The measured TX-to-ANT and ANT-to-RX insertion losses are 3.0 and 3.2 dB at 300 K, and 1.9 and 2.0 dB at 4.2 K. At 300 K, the measured TX-to-ANT and ANT-to-RX IIP3 are 29.5 and 27.6 dBm, respectively. The IBFD core of the chip occupies an area of 0.27 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and has a dc power (nominally consumed in an on-chip modulation clock generator) of 48 mW at 300 K and 42.6 mW at 4.2 K.

RF Canceller Tuning Acceleration Using Neural Network Machine Learning for In-Band Full-Duplex Systems

The fifth-generation wireless system framework provides the option to evaluate the performance of in-band full-duplex (IBFD) operation through flexible duplexing. The resulting self-interference, however, must be mitigated within a fraction of a symbol duration for successful communication. This paper introduces the use of neural network machine learning to accelerate the tuning of multi-tap adaptive RF cancellers. Additionally, the optimal network configurations, input data structures and training dataset densities that optimize the performance of this technique are presented. The tuning results of a prototype system using a two-tap canceller were measured over 20 and 100 MHz bandwidths centered at 2.5 GHz, and demonstrated averages of 40 dB cancellation and 6 tuning iterations. These results are compared to a survey of previously-reported adaptive cancellers, and illustrate that this novel application of machine learning to RF canceller tuning provides the fastest convergence speed to date, which can enable IBFD operation in dynamic interference environments.

Multi-port monostatic antenna system with improved interport isolation for 2.4GHz full duplex MIMO applications

ABSTRACT This paper presents a five (05)-port proximity coupled monostatic patch antenna having four (04) linearly co-polarized ports (same polarization for two-transmit and two-receive ports) for 2.4 GHz in-band full duplex (IBFD) 2-order MIMO (multi-input multi-output) wireless applications. The employed differential receive mode mechanism and single-tap self-interference cancellation (SIC) circuit achieves high isolation between transmit (Tx) and receive (Rx) ports for IBFD operation. The fifth port is dual polarized with respect to the first four (04) ports to provide intrinsic isolation through polarization diversity. The implemented antenna system provides 10 dB return-loss bandwidth of more than 200 MHz for all five ports. The measured MIMO isolation (Tx-Tx ports and Rx-Rx ports) is better than 15 dB and full duplex Tx-Rx isolation ≈ 20 dB between linearly co-polarized ports for 200 MHz bandwidth. Moreover, the interport isolation between Tx ports (Rx ports) and dual polarized port (port 5) > 30 dB (25 dB) in 200 MHz bandwidth in addition to better than 60 dB (27 dB) peak isolation. To the best of our knowledge, it is the first type of such monostatic antenna with multiple co-polarized and dual polarized ports for full duplex MIMO applications.

Weather-limited in-band full-duplex transceiver model for free-space optical communication

Free-space optical (FSO) communication has gained popularity for wireless applications over legacy radio frequency for advantages such as unlicensed operation, spatial reuse, and security. Even though FSO communication can achieve high bit rates, range limitation due to strong attenuation and weather dependency has always restricted its practical applications to controlled settings such as building-to-building communication. Futuristic mobile and secure ad hoc FSO network applications such as smart cars and air-subsea links need more efficient and autonomous link acquisition capabilities, which can be enabled by in-band full-duplex (IBFD) operation. We proposed an IBFD-FSO transceiver prototype consisting of off-the-shelf components to demonstrate the concept of IBFD operation by the isolation technique. We also developed a laser-based IBFD-FSO link model by incorporating an atmospheric attenuation model and self-interference cancellation model. We found that, for clear weather, the maximum achievable link range using commercially available components can be up to 120 m. We also determined weather-dependent performance of the FSO link in terms of visibility and transmit power.

A compact proximity‐fed 2.4 GHz monostatic antenna with wide‐band SIC characteristics for in‐band full duplex applications

This article presents a dual polarized, proximity-fed monostatic patch antenna (single radiator for both transmit and receive modes) with improved interport isolation for 2.4 GHz in-band full duplex (IBFD) applications. The proximity-fed radiating patch offers comparatively wider impedance bandwidth for presented design. Very nice self-interference cancelation (SIC) levels for intended impedance bandwidth have been achieved through differential receive (Rx) mode configuration. The differential Rx mode based on 180° ring hybrid coupler acts as a signal inversion mechanism for effective suppression or cancelation of in-band self-interference (SI) that is, the leakage from transmit port. The implemented prototype of proposed antenna achieves ≥87 dB peak isolation for dual polarized IBFD operation. Moreover, the recorded interport isolation for validation model ≥60 dB within 10 dB-return loss bandwidth of 90 MHz (2.36-2.45 GHz). The measured radiation characteristics of implemented antenna demonstrate nice gain and low cross-polarization levels for both transmit (Tx) and receive (Rx) modes. The dimensions of implemented antenna are 70 × 75 × 4.8 mm3. The novelty of this work is wide-band SIC performance for monostatic antenna configuration with compact structure of presented design.

Bidirectional Communication Circuits for a 120-GHz PMF Data Link in 40-nm CMOS

Three different bidirectional polymer microwave fiber (PMF) links are presented and discussed. The proposed solutions exploit the following orthogonal signaling dimensions: wave direction, wave polarity, and signal polarity. First, a 120-GHz substrate integrated waveguide directional coupler is implemented and exploits the wave direction orthogonality. The 4-port on-board hybrid coupler achieves measured data rates of, respectively, 6 and 5.5 Gbps in the up- and down-link over a distance of 8 m. Second, a 120-GHz ortho-mode transducer (OMT) is micro-machined and exploits the wave polarity orthogonality. The micro-machined polarization duplexer has a measured isolation of 45 dB over the entire $F$ -band. For the horizontal and vertical paths, the insertion loss is measured with a 40-cm-long Teflon fiber and varies between −5 and −9 dB. Finally, a 120-GHz in-band full-duplex (IBFD) PMF transceiver with tunable electrical-balance duplexer (EBD) and on-chip bow-tie antenna are implemented in a 40-nm bulk CMOS technology. In this case, the duplexing principle is relying on signal polarity. The self-interference (SI) cancellation is achieved with a fully differential transformer-based EBD, resulting in a simulated SI suppression of 30 dB over a bandwidth of more than 14 GHz. The insertion loss of the duplexer is less than 11 dB from TX to fiber and less than 12 dB from fiber to RX at a frequency of 120 GHz. The balancing network provides an impedance locking range from 45 to 93 $\Omega $ and 38 to 266 fF at 120 GHz. For IBFD operation, the maximum measured data rate is 6.2 Gbps. The transceiver with integrated antenna occupies an area of 1.8 mm $\times $ 1.53 mm and has a dc power consumption of 73 mW in total.

A High-Isolation Monopole Array With Ring Hybrid Feeding Structure for In-Band Full-Duplex Systems

A compact antenna system operating in the 2.4-GHz WLAN band is presented for in-band full-duplex operation. The design includes two monopoles over a T-shaped defected ground structure fed by a 180° hybrid. The measured –10-dB bandwidths are 9% (220 MHz) and 5% (130 MHz) for the transmitting and receiving ports, respectively. Through the entire band of interest, the antenna has a measured isolation $({S_{21}}) in a reflective environment. The design also has an omnidirectional radiation pattern, reasonable gain values, and a very low envelope correlation coefficient (<0.001).

A Wideband Highly Integrated and Widely Tunable Transceiver for In-Band Full-Duplex Communication

A fully integrated wideband active duplexing transceiver with baseband noise-cancelling duplexing LNAs is presented. The circuit allows in-band full duplex operation with concurrent reception and transmission in the same band or closely-spaced channels. A passive mixer-first architecture is applied here, sharing a single passive mixer to perform simultaneous up-conversion and down-conversion. The bi-directional transparency of the passive mixer allows the duplexing function to be implemented at baseband instead of RF front end. Under the same condition as required for noise cancellation, the baseband duplexing LNAs buffer the transmitter input signals to the mixer while canceling those signals in receive path. Measurements from the transceiver implemented in 65 nm CMOS show a frequency tuning range of 0.1–1.5 GHz with ${\rm S}_{11} 20 dB, NF as low as 5.5 dB and transmitted power up to ${-}$ 7.1 dBm. A 30 dB linear isolation between receive and transmit is generally maintained across both LO frequency and in-band transmit/receive frequency separation. Significant suppression of transmit-induced noise and nonlinear intermodulation between received and transmitted signals are also achieved.

In-Band Full-Duplex Wireless: Challenges and Opportunities

In-band full-duplex (IBFD) operation has emerged as an attractive solution for increasing the throughput of wireless communication systems and networks. With IBFD, a wireless terminal is allowed to transmit and receive simultaneously in the same frequency band. This tutorial paper reviews the main concepts of IBFD wireless. One of the biggest practical impediments to IBFD operation is the presence of self-interference, i.e., the interference that the modem's transmitter causes to its own receiver. This tutorial surveys a wide range of IBFD self-interference mitigation techniques. Also discussed are numerous other research challenges and opportunities in the design and analysis of IBFD wireless systems.