Herein, we focus on convergent 6G communication, localization and sensing systems by identifying key technology enablers, discussing their underlying challenges, implementation issues, and recommending potential solutions. Moreover, we discuss exciting new opportunities for integrated localization and sensing applications, which will disrupt traditional design principles and revolutionize the way we live, interact with our environment, and do business. Regarding potential enabling technologies, 6G will continue to develop towards even higher frequency ranges, wider bandwidths, and massive antenna arrays. In turn, this will enable sensing solutions with very fine range, Doppler, and angular resolutions, as well as localization to cm-level degree of accuracy. Besides, new materials, device types, and reconfigurable surfaces will allow network operators to reshape and control the electromagnetic response of the environment. At the same time, machine learning and artificial intelligence will leverage the unprecedented availability of data and computing resources to tackle the biggest and hardest problems in wireless communication systems. As a result, 6G will be truly intelligent wireless systems that will provide not only ubiquitous communication but also empower high accuracy localization and high-resolution sensing services. They will become the catalyst for this revolution by bringing about a unique new set of features and service capabilities, where localization and sensing will coexist with communication, continuously sharing the available resources in time, frequency, and space. This work concludes by highlighting foundational research challenges, as well as implications and opportunities related to privacy, security, and trust.The associate editor coordinating the review of this manuscript and approving it for publication was Ahmed Farouk .

In-band full-duplex sets challenging requirements for wireless communication radios, in particular their capability to prevent receiver sensitivity degradation due to self-interference (transmit signals leaking into its own receiver). Previously published self-interference rejection designs require bulky components and/or antenna structures. This paper addresses this form-factor issue. First, compact radio transceiver feasibility bottlenecks are identified analytically, and tradeoff equations in function of link budget parameters are presented. These derivations indicate that the main bottlenecks can be resolved by increasing the isolation in analog/RF. Therefore, two design ideas are proposed, which provide attractive analog/RF-isolation and allow integration in compact radios. The first design proposal targets compact radio devices, such as small-cell base stations and tablet computers, and combines a dual-port polarized antenna with a self-tunable cancellation circuit. The second design proposal targets even more compact radio devices such as smartphones and sensor network nodes. This design builds on a tunable electrical balance isolator/duplexer in combination with a single-port miniature antenna. The electrical balance circuit can be implemented for scaled CMOS technology, facilitating low cost and dense integration.Index Terms-In-band full-duplex, self-interference isolation, dual polarized antenna, tunable duplexer, electrical balance, transceiver macro-modeling.

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The electrical-balance (EB) duplexer concept explored in [1][2][3][4] suggests a possible integrated multiband alternative to conventional fixed-frequency surface-acoustic-wave (SAW) duplexers. The basic principle of the EB duplexer is to balance the impedances seen at the ports of a hybrid transformer to suppress signal transfer from the TX to the RX through signal cancellation ( Fig. 2.2.1). While the potential payoff is tantalizing, several challenges must still be solved before EB duplexers can become commercially viable. Specifically, the duplexer must provide high isolation and linearity in both the TX and RX bands across wide bandwidth (BW), with low insertion loss (IL), all in the presence of a real antenna whose impedance is constantly varying due to real-world user interaction. In this paper, we present a duplexer that significantly advances the state-of-the-art for two of these critical challenges: linearity and insertion loss.The single-ended duplexer topology is shown in Fig. 2.2.1. Earlier implementations of EB duplexers use a differential LNA at the RX port [1][2][3][4]. Unfortunately, even when differential-mode isolation is high, the common-mode isolation will still be unacceptable [1,4], causing the LNA to compress when the PA operates at full power [2]. A solution was proposed in [2,3], at the cost of significantly more components and increased IL. We propose the simpler approach of moving to a single-ended topology, wherein only two transfer paths exist from TX to RX. To achieve isolation between the TX and RX, a balance network impedance (Z BAL ) must be chosen so that the signals through these two transfer paths destructively interfere at the RX port. The inherent asymmetry between these two paths, arising from the hybrid transformer capacitive coupling, means that the balance network and antenna impedances will not be the same when cancellation occurs. However, this asymmetry is easily accounted for during design of Z BAL .In order to minimize insertion loss, the hybrid transformer is designed using the back-end metal stack shown in Fig. 2.2.1. It includes 3 very thick RF metals with bar-vias between all RF metals. The primary winding is stacked on top of the secondary winding to improve coupling and to shield the primary winding from the substrate. Adequate substrate shielding is particularly critical in SOI CMOS, where signals present on RF metal layers can couple into nonlinear regions at the interface between the buried-oxide layer and handle wafer to generate distortion. Thus, an orthogonally patterned grounded shield is added. A 2:1 transformation ratio is used to convert the nominal 100Ω impedance connected to the primary winding (the antenna and balance network impedance combined) to 50Ω at the single-ended RX port. The transformer center-tap (TX input) is deliberately skewed to favor TX IL over RX IL [1,4].A key challenge that motivates this proof-of-concept design is to demonstrate compliance with all linearity specifications of modern wireless standards. Most stringent among the...

This paper proposes two RF self-interference cancellation techniques. Their small form-factor enables full-duplex communication links for small-to-medium size portable devices and hence promotes the adoption of full-duplex in mass-market applications and next-generation standards, e.g. IEEE802.11 and 5G. Measured prototype implementations of an electrical balance duplexer and a dual-polarized antenna both achieve >50dB self-interference suppression at RF, operating in the ISM band at 2.45GHz.

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An electrical-balance duplexer achieving state-ofthe-art linearity and insertion loss performance is presented, enabled by partially depleted RF silicon-on-insulator (SOI) CMOS technology. A single-ended configuration avoids the common-mode isolation problem suffered by topologies with a differential low-noise amplifier (LNA). Highly-linear switched capacitors allow for impedance balancing to antennas with <1.5:1 voltage standing wave ratio (VSWR) from 1.9 to 2.2 GHz. +70 dBm input-referred 3 rd -order intercept point (IIP3) is achieved under high transmitter (TX) power (+30.5 dBm max.). TX insertion loss is <3.7 dB and receiver insertion loss is <3.9 dB.

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