Integrated sensing and communications (ISAC), which are recognized as a key enabler for sixth generation (6G), have brought new opportunities for intelligent, sustainable, and connected wireless networks. Multiple-input–multiple-output (MIMO) transceiver technology lies at the core of this paradigm, providing the degrees of freedom required for simultaneous data transmission and accurate radar sensing. The tight integration of sensing and communication (S&C) introduces unique security vulnerabilities that extend beyond conventional physical-layer security (PLS). In particular, high-power transmissions directed at sensing targets may empower adversarial eavesdroppers, whereas passive interception of ISAC echoes can reveal sensitive information such as target locations and mobility patterns. This article presents an overview of recent advances in MIMO ISAC transceiver design, considering transmitter perspectives, receiver architectures, and full-duplex implementations. We examine MIMO transceiver designs under unique security threats specific to ISAC and highlight emerging countermeasures, including secure signaling design, interference exploitation, and transceiver optimization under adversarial conditions. Finally, we discuss challenges and research opportunities for developing secure ISAC systems in next-generation wireless networks.

In this study, we explore a terahertz (THz) wireless power transfer (WPT) system, aiming to determine the optimal transmit waveform that maximizes the rectification efficiency. Since traditional Schottky diodes may be inefficient for THz rectification, the receiver is assumed to be equipped with a resonant tunneling diode (RTD)-based rectifier. Such rectifiers are generally characterized by a non-monotonic energy harvesting (EH) behavior, necessitating the design of transmit waveforms that align with the receiver’s non-monotonic characteristics. To this end, we utilize a waveform-to-energy model and formulate an optimization problem to maximize the rectification efficiency under both average and peak power constraints. The derived closed-form optimal waveform is a pulsed tone whose adaptive duty cycle follows the rectifiers efficiency peaks via a simple, real-time implementable rule. Our results demonstrate consistent gains over multisine baselines across power regimes and clarify when peak/average power constraints change the optimal rectification efficiency.

Wireless power transfer (WPT) has emerged as a promising technology for prolonging the battery life of energy-constrained devices. Waveform optimization enhances energy harvesting efficiency, but depends on the applied energy harvesting model. Conventional models capturing rectifier nonlinearities unveiled multisines’ superiority over continuous waves, but exhibit limitations in supported waveforms and rectifier structures. A recently proposed waveform-to-energy harvesting model addresses these limitations and showed that pulsed radio frequency (RF) signals outperform multisines under ideal conditions. However, practical deployments face channel uncertainty. This paper presents a robust optimization framework for WPT systems under a bounded channel estimation error model. We develop an efficient algorithm maximizing worst-case harvested energy and propose single-and multi-frequency pulsed RF designs that maintain performance despite channel uncertainty. Numerical results demonstrate significant gains of the proposed designs over benchmarks, especially under severe channel uncertainty.

This paper examines the coverage characteristics of large inclined low Earth orbit (LEO) satellite constellations, focusing on equatorial regions. The study uses stochastic geometry to model satellites and users as uniformly distributed along their respective orbits and latitude ranges. The central angle and distance distributions between users and nth nearest satellite are derived to estimate line-of-sight (LoS) and coverage probability. Analytical models are validated with Monte Carlo simulations to provide practical insights for optimizing satellite deployments for equatorial regions. Results highlight the impact of satellite altitude and constellation density, revealing how lower inclination angles enhance satellite density over equatorial regions, thus improving coverage performance.

This paper studies a high-altitude platform (HAP) network supported by reconfigurable intelligent surfaces (RISs). The practical irregular placement of HAPs and RISs is modeled using homogeneous Poisson point processes, while buildings that cause blockages in urban areas are modeled as a Boolean scheme of rectangles. We introduce a novel approach to characterize the statistical channel based on generalized Beta prime distribution. Analytical expressions for coverage probability and ergodic capacity in an interference-limited system are derived and validated through Monte Carlo simulations. The findings show notable performance improvements and reveal the impact of various system parameters, including blockages effect which contribute in mitigating interference from the other visible HAPs. This proposed system could enhance connectivity and enable effective data offloading in urban environments.

We conduct an experimental investigation into the performance of several jamming waveforms against a common off-the-shelf (COTS) laptop. The performance metric is chosen to be bit error rate within the individual WLAN packets, as opposed to the usual system throughput. The considered jamming waveforms are noise-like waveforms generated with three methods: random QAM with OFDM symbols on varying selected subcarriers; tone signals on all the pilot subcarriers; and a triangular frequency sweep. Out of all these waveforms, those targeting the pilot subcarriers had improved effectiveness, with simple tones over the pilots giving the best results.

In this article, we review the key concepts and the progress in the design of physical-layer aspects of in-band full-duplex (IBFD) communications. One of the fundamental challenges in realizing IBFD is self-interference that can be up to 100 dB stronger than signals of interest. Thus, we start by reviewing state-of-the-art research in self-interference cancellation, addressing both model-based and emerging machine learning-based methods. Then, we turn our attention to new wireless systems with many degrees of freedom for which the traditional IBFD designs do not gracefully scale and, hence, require many innovations to enable IBFD. We provide an extensive review of basic concepts and state of the art in massive multiple-input–multiple-output IBFD. Then, we consider the mmWave band IBFD and review advanced physical-layer architectures. The above review provides the proper context to discuss IBFD innovations and new challenges for sixth-generation networks and beyond, where wireless networks are envisioned to be multifunctional, combining communications with functions such as sensing, cognitive radios, physical-layer security, and wireless power transfer. We conclude this article with a status update on the adoption of IBFD in communication standards.

This letter develops a novel transmit beamforming (BF) design for canceling self-interference (SI) in analog in-band full-duplex phased arrays. Our design maximizes transmit BF gain in a desired direction while simultaneously reducing SI power to below a specified threshold on per-antenna basis to avoid saturating receive-chain components, such as LNAs. Core to our approach is that it accounts for real-world phase shifters used in analog phased array systems, whose limited resolution imposes non-convex constraints on BF design. We overcome this by transforming these non-convex constraints into convex polygon constraints, which we then solve through semidefinite relaxation and a rank refinement procedure. Numerical results show that our proposed BF scheme reliably cancels SI to the target power threshold at each receive antenna while sacrificing little in transmit BF gain, even with modest phase shifter resolution.

In this article, we consider a multiple-input multiple-output (MIMO) transceiver performing joint communications and sensing (JCAS) using fifth-generation New Radio (5G NR) standard-compliant orthogonal frequency-division multiplexing (OFDM) waveforms. Communication links are maintained with users having multiple spatial data streams over frequency-selective non-line-of-sight channels while simultaneously transmitting separate spatial data streams to different sensing directions, where a portion of the communication data streams’ power is reallocated to the sensing data streams. The received reflections from the environment due to all transmit (TX) streams are used to obtain range–velocity and range–angle maps. Through optimizing the TX precoding and receive combining, inter-user, intra-user, and radar–communications interference are also canceled. In addition, streams transmitted in the sensing directions are optimized to minimize the lower bounds of direction-of-arrival and delay estimates jointly, and the solution is analytically derived. The simulation results illustrate that the JCAS system can reliably perform target detection while minimizing lower bounds compared with a communications-only scenario. Further, the detection probability and estimation errors of sensing can be improved while also controlling the communications capacity of the OFDM waveform, thereby indicating the need to appropriately choose the optimization parameters to obtain an optimal trade-off.

In this article, we study the advantages and disadvantages that full-duplex (FD) radio technology brings to remote-controlled drone and counter-drone systems in comparison to classical half-duplex (HD) radio technology. We consider especially the physical-layer reliability perspective that has not yet been comprehensively studied. For establishing a solid analytical background, we first derive original closed-form expressions to evaluate demodulation and detection performance of frequency-hopped and frequency-shift keyed drone remote control signals under external or self-inflicted interference. The developed analytical tools are verified by comparison to simulated results and then used to study the impact that the operation mode has on the operable area of drones and effectiveness of counter-drone systems in different scenarios, linking the physical layer performance to practical safety. Analysis of the scenarios shows that FD operation compared to HD can improve the effectiveness of a counter-drone system and that in FD mode a drone can detect the attacks from the counter-drone system from a greater distance than in HD mode. However, two-way communication between the remote controller and drone in FD mode compared to HD significantly reduces the drone’s operable area when targeted by a smart counter-drone system.

Unauthorized access to data has been a recognized risk of wireless systems for many decades. While security solutions in communications engineering have typically revolved around cryptography in the higher layers, a semi-recent development is the elevating interest into security in the physical layer, namely by utilizing jamming for protection. In this paper, we present an experimental study into a full-duplex jammer–receiver (i.e., “jamceiver”) that is able to simultaneously interfere with the same radio resources it is actively receiving from. The radio architecture is loosely based on frequency-modulated continuous-wave radars that are constant-envelope radio transceivers, which benefit from simple-but-efficient self-interference suppression in the analog baseband domain by using a passive highpass filter. Its limitation to constant-envelope transmission is not an issue for efficient jamming waveforms unlike it would be with conventional direct-conversion transceivers in full-duplex communications. To show the performance limits of a practical jamceiver, we present comprehensive measurement results from a laboratory environment as well as a jamming case study from an open park area with actual Wi-Fi signals. Especially, the experiments validate the feasibility of preventing eavesdropping with continuous low-power jamming in a large area around a full-duplex jamceiver that acts as an access point for simultaneously offering decent Wi-Fi service to an off-the-shelf laptop.

In this article, we propose multiple machine learning (ML) based physical-layer receiver solutions for demodulating orthogonal frequency-division multiplexing (OFDM) signals that are subject to high level of nonlinear distortion. Specifically, three novel deep learning based convolutional neural network receivers are devised, containing layers in time- and/or frequency-domains, allowing to demodulate and decode the transmitted bits reliably despite the high error vector magnitude (EVM) in the transmit signal. Applicable training procedures are also described, such that the learned layers in the receiver processing properly generalize over different nonlinear distortion and multipath channel characteristics. Extensive set of numerical results is provided, in the context of 5G NR uplink (UL) incorporating also measured terminal power amplifier (PA) characteristics. The obtained results show that the proposed receiver systems are able to clearly outperform the classical linear minimum mean-squared error (LMMSE) receiver as well as the existing ML receiver approaches, especially when the EVM is high compared to modulation order. This is particularly so when the devised ML receiver is of hybrid nature with layers both in time and frequency. The proposed ML receivers can thus facilitate pushing the terminal PA systems deeper into saturation, and thereon improve the terminal power-efficiency, radiated power and network coverage. Through combining the obtained radio link performance results with link budget calculations, all carried out at the 28 GHz mmWave band, it is shown that the proposed ML receivers can enhance the network coverage in terms of maximum UL link distances by close to 100%, when compared to classical LMMSE receiver based networks.

This paper investigates the ergodic capacity of reflecting intelligent surface (RIS)-aided single-input single-output communication systems with spatially correlated Rayleigh-fading channels. The ergodic capacity for such systems does not admit an exact closed-form expression. Therefore, we consider two alternative fading distributions to approximate the systems’ statistical characterization to enable the derivation of closed-form expressions for the ergodic capacity. We further simplify the ergodic capacity by proposing novel and unified approximations in the form of a weighted sum of logarithmic functions with optimized coefficients. We validate the effectiveness and the high accuracy of the adopted schemes and the proposed approximations through numerical results. Performance analysis to study the impact of several system parameters on the ergodic capacity is also conducted. Deploying an RIS to the communication system can significantly increase the ergodic capacity which increases even further with increasing the number of reflecting elements equipped on the RIS, and this effect is best seen when the direct path is weak.

We study a multifunction transceiver whose operation is based on the frequency-modulated continuous-waveform concept to facilitate self-interference suppression. This causes the received signals after downconversion to move across the spectrum according to the frequency-modulation function. It is critical to accurately compensate these frequency deviations in order to process the received information. In this paper, we introduce a method for estimating these deviations in terms of the relative time delay between the signals used for downconversion and compensation. Estimation of this delay relies on statistics of the received signal, and only requires the aid of a generic auxiliary signal. Our method is supported by analytic derivation of the estimation formula, and is validated by means of simulation results.

The dream of having ubiquitous and high-capacity connectivity is coming true by emerging low Earth orbit (LEO) Internet constellations through several commercial plans, e.g., Starlink, Telesat, and Oneweb. The analytical understanding of these networks is crucial for accurate network assessment and, consequently, acceleration in their design and development. In this paper, we derive the coverage probability and the data rate of a massive LEO network under arbitrarily distributed fading and shadowing. The conventional user association techniques, based on the shortest distance between the ground terminal and the satellite, result in a suboptimal performance of the network since the signal from the nearest server may be subject to severe shadowing due the blockage by nearby obstacles surrounding the ground terminal. Thus, we take into account the effect of shadowing on the serving satellite selection by assigning the ground terminal to the satellite which provides the highest signal-to-noise ratio at the terminal’s place, resulting in a more generalized association technique, namely the best server policy (BSP). To maintain tractability of our derivations and consider the latitude-dependent distribution of satellites, we model the satellites as a nonhomogeneous Poisson point process. The numerical results reveal that implementing the BSP for serving satellite selection leads to significantly better performance compared to the conventional nearest server policy (NSP).

Almost all current beamforming-based designs of self-interference cancellation (SIC) for digital phased array systems neglect the practical issue of the limited dynamic range of analog-to-digital converters (ADCs) and, thus, lack restriction for self-interference (SI) that is incident at each receiving antenna. Other than beamforming, these methods usually require additional SIC techniques to provide a part of isolation, which brings higher system complexity. In this letter, a SIC method is proposed to overcome these issues. By developing a transmit SIC beamformer design that minimizes the power of SI on a per-antenna basis, we can obtain more isolation before the ADCs in each receiving channel and better prevent them all from being saturated. Then, the residual SI can be further suppressed by adaptive receive beamformers to achieve high total isolation performance. The proposed method reduces the need for other SIC techniques than beamforming technology in phased array systems and, thus, facilitates lower system complexity. Numerical results demonstrate the effectiveness of the proposed method.

In this article, we study the joint communication and sensing (JCAS) paradigm in the context of millimeter-wave (mm-wave) mobile communication networks. We specifically address the JCAS challenges stemming from the full-duplex operation in monostatic orthogonal frequency-division multiplexing (OFDM) radars and from the co-existence of multiple simultaneous beams for communications and sensing purposes. To this end, we first formulate and solve beamforming optimization problems for hybrid beamforming based multiuser multiple-input and multiple-output JCAS systems. The cost function to be maximized is the beamformed power at the sensing direction while constraining the beamformed power at the communications directions, suppressing interuser interference and cancelling full-duplexing related self-interference (SI). We then also propose new transmitter and receiver beamforming solutions for purely analog beamforming based JCAS systems that maximize the beamforming gain at the sensing direction while controlling the beamformed power at the communications direction(s), cancelling the SI as well as eliminating the potential reflection from the communication direction and optimizing the combined radar pattern (CRP). Both closed-form and numerical optimization based formulations are provided. We analyze and evaluate the performance through extensive numerical experiments, and show that substantial gains and benefits in terms of radar transmit gain, CRP, and SI suppression can be achieved with the proposed beamforming methods.

Providing truly ubiquitous connectivity requires development of low Earth orbit (LEO) satellite Internet, whose theoretical study is lagging behind network-specific simulations. In this paper, we derive analytical expressions for the downlink coverage probability and average data rate of a massive inclined LEO constellation in terms of total interference power’s Laplace transform in the presence of fading and shadowing, ergo presenting a stochastic geometry-based analysis. We assume the desired link to experience Nakagami-$m$fading, which serves to represent different fading scenarios by varying integer$m$, while the interfering channels can follow any fading model without an effect on analytical tractability. To take into account the inherent non-uniform distribution of satellites across different latitudes, we model the LEO network as a nonhomogeneous Poisson point process with its intensity being a function of satellites’ actual distribution in terms of constellation size, the altitude of the constellation, and the inclination of orbital planes. From the numerical results, we observe optimum points for both the constellation altitude and the number of orthogonal frequency channels; interestingly, the optimum user’s latitude is greater than the inclination angle due to the presence of fewer interfering satellites. Overall, the presented study facilitates general stochastic evaluation and planning of satellite Internet constellations without specific orbital simulations or tracking data on satellites’ exact positions in space.

We present tight yet tractable approximations and bounds for the ergodic capacity of any communication system in the form of a weighted sum of logarithmic functions, with the focus on the Nakagami and lognormal distributions that represent key building blocks for more complicated systems. A minimax optimization technique is developed to derive their coefficients resulting in uniform absolute or relative error. These approximations and bounds constitute a powerful tool for the statistical performance analysis as they enable the evaluation of the ergodic capacity of various communication systems that experience small-scale fading together with the lognormal shadowing effect and allow for simplifying the complicated integrals encountered when evaluating the ergodic capacity in different communication scenarios. Simple and tight closed-form solutions for the ergodic capacity of many classic and timely application examples are derived using the logarithmic approximations. The high accuracy of the proposed approximations is verified by numerical comparisons with existing approximations and with those obtained directly from numerical integration methods.

This paper introduces and justifies a novel system concept that consists of full-duplex transceivers and uses a multifunction signal for simultaneous two-way communication, jamming and sensing tasks. The proposed device structure and wave-form enable simple-yet-effective interference suppression at the cost of being limited to constant-envelope transmission— this is a weakness only for the communication functionality that becomes limited to frequency-shift keying (FSK) while frequency-modulated continuous wave (FMCW) waveforms are effective for jamming and sensing purposes. We show how the transmission and reception as well as different interference and distortion compensation procedures are implemented in such multifunction transceivers. The system could be also applied for simultaneous spectrum monitoring with the above functions. Finally, we showcase the expected performance of such a system through numerical results.

In this paper, we propose a machine learning (ML) based physical layer receiver solution for demodulating OFDM signals that are subject to a high level of nonlinear distortion. Specifically, a novel deep learning based convolutional neural network receiver is devised, containing layers in both time- and frequency domains, allowing to demodulate and decode the transmitted bits reliably despite the high error vector magnitude (EVM) in the transmit signal. Extensive set of numerical results is provided, in the context of 5G NR uplink incorporating also measured terminal power amplifier characteristics. The obtained results show that the proposed receiver system is able to clearly outperform classical linear receivers as well as existing ML receiver approaches, especially when the EVM is high in comparison with modulation order. The proposed ML receiver can thus facilitate pushing the terminal power amplifier (PA) systems deeper into saturation, and thereon improve the terminal power-efficiency, radiated power and network coverage.

In this paper, we consider a full duplex (FD) system using spatial modulation (SM) technique for simultaneous transmission. Despite the recent studies on the combination of these two techniques, nonlinear effects of devices have been ignored. This paper demonstrates, for the first time, the nonlinear power amplifier (PA) effects on a full duplex spatial modulation (FDSM) system. A digital canceller is adopted to deal with the strong self-interference (SI) in presence of PA nonlinearities. The results are verifying that the considered canceller can suppress SI close to the thermal noise level and has high reliability to ensure the performance of the system.

This article contributes to the experimentation of joint vehicular communications and radio-based sensing using a fifth-generation (5G) New Radio (NR) waveform. Firstly, simulations are carried out to observe the effect of using default (communication-purpose) 5G NR waveforms for sensing, and they indicate high side-lobes in the range profile due to the existence of unused communication subcarriers within the frames of the 5G NR waveform. These can be filled with optimized frequency-domain symbols to minimize the side-lobes. As the main result, these observations are validated through over-the-air measurements with practical 5G NR waveforms, operating at the mm-wave frequency of 27.7 GHz. For this, an outdoor environment is mapped with both the default 5G NR waveform and the optimized waveform, and the latter showcases considerable improvement in the mapping image due to side-lobe suppression.

Requirements and technological advancements towards 6th generation (6G) wireless networks lead to enabling and development of massive low Earth orbit (LEO) satellite constellations to provide ubiquitous and high-capacity connectivity, particularly for maritime and airborne platforms. Consequently, new methodologies to study the performance of LEO networks are of great importance. In this paper, we derive both downlink and uplink analytical expressions for coverage probability and data rate of a massive inclined LEO constellation under general shadowing and fading. We model the LEO satellite network as a nonhomogeneous Poisson point process with general intensity in order to take into account uneven distribution of satellites along the latitudes. The results provided in this study facilitate the stochastic evaluation and design of the future massive LEO networks, regardless of satellites’ exact trajectories in orbits.

In this paper, we present simple and tight approximations for the integer powers of the Gaussian Q-function, in the form of exponential sums. They are based on optimizing the corresponding coefficients in the minimax sense using the Remez exchange algorithm. In particular, the best exponential approximation is characterized by the alternation of its absolute error function, which results in extrema that alternate in sign and have the same magnitude of error. The extrema are described by a system of nonlinear equations that are solved using Newton- Raphson method in every iteration of the Remez algorithm, which eventually leads to a uniform error function. This approximation can be employed in the evaluation of average symbol error probability (ASEP) under additive white Gaussian noise and various fading models. Especially, we present several application examples on evaluating ASEP in closed forms with Nakagami-m, Fisher-Snedecor F, η - μ, and κ - μ channels. The numerical results show that our approximations outperform the existing ones with the same form in terms of the global error. In addition, they achieve high accuracy for the whole range of the argument with and without fading, and it can even be improved further by increasing the number of exponential terms.

In this paper, we present a comprehensive overview of (perhaps) all possible approximations resulting from applying the most common numerical integration techniques on the Gaussian Q-function. We also present a unified method to optimize the coefficients of the resulting exponential approximation for any number of exponentials and using any numerical quadrature rule to produce tighter approximations. Two new tight approximations are provided as examples by implementing the Legendre numerical rule with Quasi-Newton method for two and three exponential terms. The performance of the different numerical integration techniques is evaluated and compared, and the accuracy of the optimized ones is verified for the whole argument-range of interest and in terms of the chosen optimization criterion.

Joint communication and sensing (JCAS) is an emerging technology for managing efficiently the scarce radio frequency (RF) spectrum, and is expected to be a key ingredient in beyond fifth-generation (5G) networks. We consider a JCAS system, where the full-duplex radar transceiver and the communication transmitter are the same device, and pursue orthogonal frequency-division multiplexing (OFDM) waveform optimization by jointly minimizing the lower bounds of delay and Doppler estimation. This is attained by filling the empty subcarriers within the OFDM frame with optimized samples while reallocating a proportion of the communication subcarriers’ power, which essentially controls the fairness between the two functionalities. Both communication and filledradar subcarriersare used for radar processing. The optimized sample values are found analytically, and a computationally feasible algorithm is presented for this task. We also address how the peak-to-average power ratio of the waveform can be controlled and minimized along the optimization process. The results are then numerically evaluated in 5G New Radio (NR) network context, which indicate a trade-off between the minimization of the lower bounds. The main-lobe width and the peak side-lobe level (PSL) of the range and velocity profiles of the radar image are also analyzed. An inverse relation between the lower bounds and the PSLs is observed, while the main-lobe width can be minimized simultaneously. The trade-off between communication and sensing is investigated, which indicates that the lower bounds can be improved at the cost of the communication capacity. Moreover, over-the-air RF measurements are carried out with unoptimized and optimized 5G NR waveforms at the 28 GHz mm-wave band, to validate the range profile’s PSL improvement in an outdoor mapping scenario, depicting considerable performance gain.

The integration of satellite-terrestrial networks is beneficial in terms of the increase of the network capacity and coverage. In such a heterogeneous network, highly efficient spectrum utilization is extremely important. This could be achieved by the single frequency reuse which allows increasing the capacity at the cost of increased interference. Interference is one of the main parameters that limits the link-level performance in such a network. In this paper, we examine the frequency reuse scenario by analyzing the impact of terrestrial interference to the uplink of a low Earth orbiting (LEO) satellite constellation in the high International Mobile Telecommunications (IMT) frequency bands. To this end, we propose a novel stochastic geometry based analytical framework that is able to accommodate various aspects of realistic satellite networks. The accuracy of the analysis is verified by using advanced simulation tools.

The large available bandwidths at millimeter-wave (mmW) frequencies enable very high data rates and reduced latencies while can also facilitate high-resolution radio-based sensing. In this paper, we address the problem of providing the communications and sensing functionalities simultaneously at the same frequencies, with specific emphasis on the emerging 5G New Radio (NR) networks. To this end, a novel RF beamforming design and optimization approach is proposed, for dual-functional joint radar-communication systems, providing multiple simultaneous transmit beams to support efficient beamformed communications while an additional beam simultaneously senses the environment around the base-station. The proposed beamforming approach jointly optimizes the transmitter and receiver beamforming weights in order to maximize the sensing performance and mitigate the possible interference stemming from the communication beam, while guaranteeing also the target beamforming gain for the communications link. The performance of the proposed approach is assessed through comprehensive numerical evaluations, demonstrating that substantial gains and benefits can be achieved compared to more ordinary beamforming approaches.

This paper discusses waveform design for a joint radar and communication system, where the radar transceiver and the communication transmitter are considered to be the same full-duplex base station. The downlink orthogonal frequency-division multiplexing (OFDM) waveform of the communication system is used also for sensing. Thus, unused subcarriers within the OFDM symbols are exploited for radar purposes and filled up with optimized complex-valued data, so as to minimize the lower bounds of the variances of the delay and Doppler estimates of radar target parameters, while maintaining an acceptable level of performance for the communication system. The results indicate that significant improvements can be made for the radar system, but in compensation, the power allocated for the communication subcarriers needs to be reduced. Thus, a trade-off between the converged systems allows both to operate together with reduced performance degradation.

Cheap off-the-self drones and other radio control (RC) equipment are readily available for customers and are becoming increasingly popular, thus also increasing the adversary or illegal activities with the RC devices. Generic RC systems can be used, e.g., in multicopter drones or roadside bombs. This raises a demand for defense and security authorities to restrict the use of RC receivers by blocking their usage. This paper studies what is the most effective waveform for jamming a typical off-the-shelf RC receiver. As a case study, the targeted system is FlySky Automatic Frequency Hopping Digital System (AFHDS), which utilizes Gaussian frequency-shift keying (GFSK) control signal with frequency-hopping spread spectrum (FHSS). The frame error rate (FER) of the receiver is measured with varying jamming power levels. The successful identification of the most efficient waveform enables the jamming of the RC receiver with the least amount of power or at the longest distance possible.

As emerging massive constellations are intended to provide seamless connectivity for remote areas using hundreds of small low Earth orbit (LEO) satellites, new methodologies have great importance to study the performance of these networks. In this paper, we derive both downlink and uplink analytical expressions for coverage probability and data rate of an inclined LEO constellation under general fading, regardless of exact satellites' positions. Our solution involves two phases as we, first, abstract the network into a uniformly distributed network. Secondly, we obtain a new parameter, effective number of satellites, for every user's latitude which compensates for the performance mismatch between the actual and uniform constellations. In addition to exact derivation of the network performance metrics, this study provides insight into selecting the constellation parameters, e.g., the total number of satellites, altitude, and inclination angle.

The emerging full-duplex (FD) radio concept is set to double the spectral efficiency of commercial wireless networks, but it also has potential applications in the defense and security domains. In the form of multifunction military full-duplex radios (MFDRs), the FD capability could enable armed forces to conduct simultaneous electronic attacks, electronic support measures, and tactical communications. This paper demonstrates the feasibility of simultaneous jamming and reconnaissance of drones' remote control (RC) systems using a prototype MFDR. Alongside, we apply deep learning in the form of a convolutional neural network (CNN) for classifying the RC signals and analyze the effect of FD operation on the classification performance.

Inband full-duplex (IBFD) technology enables radios to simultaneously transmit and receive (STAR) on the same frequencies with the benefit of, e.g., enhanced spectral efficiency in non-military communications. In addition, there is significant potential in the IBFD concept in military applications as currently conventional time-or frequency-division half-duplex radios are used in all military applications. A military full-duplex radio (MFDR) would be capable of simultaneous integrated tactical communication and electronic warfare operations. This paper presents an application where an MFDR enables the user to successfully detect an electronic attack, i.e., jamming from an adversary, while simultaneously transmitting tactical transmissions to an ally on the same frequency channel. Successful detection enables the MFDR to gather intelligence and take countermeasures against the jamming, e.g., switching to a different carrier frequency. The experimental results reported herein prove that the radio is able to reliably detect the presence of jamming for received jamming signal powers down to -95 dBm while simultaneously transmitting to an ally at 10-dBm power level. Therefore, the full-duplex radio can give armed forces a significant technical lead over an enemy by detecting enemy jamming even when the adversary only transmits jamming during friendly transmissions.

As low Earth orbit (LEO) satellite communication systems are gaining increasing popularity, new theoretical methodologies are required to investigate such networks' performance at large. This is because deterministic and location-based models that have previously been applied to analyze satellite systems are typically restricted to support simulations only. In this paper, we derive analytical expressions for the downlink coverage probability and average data rate of generic LEO networks, regardless of the actual satellites' locality and their service area geometry. Our solution stems from stochastic geometry, which abstracts the generic networks into uniform binomial point processes. Applying the proposed model, we then study the performance of the networks as a function of key constellation design parameters. Finally, to fit the theoretical modeling more precisely to real deterministic constellations, we introduce the effective number of satellites as a parameter to compensate for the practical uneven distribution of satellites on different latitudes. In addition to deriving exact network performance metrics, the study reveals several guidelines for selecting the design parameters for future massive LEO constellations, e.g., the number of frequency channels and altitude.

This paper presents a novel systematic methodology to obtain new simple and tight approximations, lower bounds, and upper bounds for the Gaussian Q-function, and functions thereof, in the form of a weighted sum of exponential functions. They are based on minimizing the maximum absolute or relative error, resulting in globally uniform error functions with equalized extrema. In particular, we construct sets of equations that describe the behaviour of the targeted error functions and solve them numerically in order to find the optimized sets of coefficients for the sum of exponentials. This also allows for establishing a trade-off between absolute and relative error by controlling weights assigned to the error functions' extrema. We further extend the proposed procedure to derive approximations and bounds for any polynomial of the Q-function, which in turn allows approximating and bounding many functions of the Q-function that meet the Taylor series conditions, and consider the integer powers of the Q-function as a special case. In the numerical results, other known approximations of the same and different forms as well as those obtained directly from quadrature rules are compared with the proposed approximations and bounds to demonstrate that they achieve increasingly better accuracy in terms of the global error, thus requiring significantly lower number of sum terms to achieve the same level of accuracy than any reference approach of the same form.

In the modeling of full duplex (FD) transceivers using multicarrier modulations, it is usually assumed that the intercarrier interference of the coupling path is negligible. Therefore, the system can be analyzed on a subcarrier basis, because the model reduces to a set of orthogonal flat channels. However, for more general FD transceivers, or when hardware components introduce nonlinear distortion, the orthogonality is lost and a more rigorous model is needed. In this paper, we introduce a model for the coupling interference of FD systems, which allows the uplink and the downlink to have different time and frequency offsets, and different parameters such as the number of subcarriers and symbol length. In addition, we present a linear approximation for the nonlinear coupling based on a generalization of the Bussgang's theorem, for the case of transmitters with nonuniform power allocation. We validate the proposed model with measurements and show numerically that it improves the accuracy of the signal to interference plus noise ratio expression, and the formulation of optimization problems. Therefore, the proposed model paves the way for a more realistic performance analysis and the derivation of more accurate power allocation strategies than is possible with the classic model.

A key feature of the non-orthogonal multiple access (NOMA) technique is that users with better channel conditions have prior information about the messages of other users. The technique to exploit the prior knowledge of strong users in order to improve the performance of weak users is known as cooperative NOMA. In this paper, we study the physical layer security in such a cooperative NOMA system. In order to reduce the complexity, the considered system in this paper has two users. Through the cooperative NOMA concept, the performance of the weak user is enhanced by the strong user. Given that there is an eavesdropper in the system that can hear all transmissions, we study the secrecy rate of the strong and the weak users. More specifically, we make an attempt to derive the secrecy outage probability (SOP) of both the users. Due to the intractable nature of the exact analysis for the weak user, we provide the closed form expression for the SOP of this user in high SNR regime while keeping the exactness for the strong user. Through numerical simulations, we verify the correctness of our analytical derivations under different scenarios. Besides, we provide the insights of achieving optimal secrecy performance in such a system.

We analyze an inband full-duplex access node that is serving mobile users while simultaneously connecting to a core network over a wireless backhaul link, utilizing the same frequency band for all communication tasks. Such wireless self-backhauling is an intriguing option for the next generation wireless systems since a wired backhaul connection might not be economically viable if the access nodes are deployed densely. In particular, we derive the optimal transmit power allocation for such a system in closed form under quality-of-service (QoS) requirements, which are defined in terms of the minimum data rates for each mobile user. For comparison, the optimal transmit power allocation is solved also for two reference scenarios: a purely half-duplex access node, and a relay-type full-duplex access node. Based on the obtained expressions for the optimal transmit powers, we then show that the systems utilizing a full-duplex capable access node have a fundamental feasibility boundary, meaning that there are circumstances under which the QoS requirements cannot be fulfilled using finite transmit powers. This fundamental feasibility boundary is also derived in closed form. The feasibility boundaries and optimal transmit powers are then numerically evaluated in order to compare the different communication schemes. In general, utilizing the purely full-duplex access node results in the lowest transmit powers for all the communicating parties, although there are some network geometries under which such a system is not capable of reaching the required minimum data rates. In addition, the numerical results indicate that a full-duplex capable access node is best suited for relatively small cells.

Genuine full-duplex operation requires effective mitigation of self-interference (SI) due to simultaneous transmission and reception at the same frequency band. In addition to its well-known harmful effect on signals of high peak-to-average power ratio, nonlinear behavior of a power amplifier (PA) complicates SI cancellation and induces spectral regrowth. We introduce a digital predistortion architecture that linearizes the response of the PA in a full-duplex transceiver. Specifically, we propose a two-step procedure to estimate the predistorter parameters and the SI canceller coefficients. Different from a direct application of conventional predistorters, the proposed architecture does not need an extra RF chain to estimate the PA response and exploits the inherent SI signal instead. Finally, simulation results show that the proposed scheme is able to increase significantly the signal-to-interference-plus-noise ratio at the transceiver output and to reduce out-of-band emissions when compared to linear and nonlinear cancellation without predistortion.

In this paper, a self-backhauling radio access system is studied and analyzed. In particular, we consider a scenario where a full-duplex access node is serving mobile users simultaneously in uplink and downlink, while also maintaining a wireless backhaul connection. The full-duplex capability of the access node, together with large antenna arrays, allows it to do all of this using the same center frequency. The minimum transmit powers for such a system are solved in a closed form under the condition that certain Quality of Service (QoS) requirements, defined in terms of minimum uplink and downlink data rates, are fulfilled. It is demonstrated with numerical results that, by using the derived expressions for the optimal transmit powers, the probability of fulfilling the QoS requirements is greatly increased, while simultaneously the overall transmit power usage of the system is significantly reduced when compared to a benchmark scheme.

We study the physical-layer performance of a wideband multihop system in which a long chain of amplify-and-forward relays is used for routing orthogonal frequency division multiplexing (OFDM) symbols from a source to a destination. Especially, the symbols are modulated in the source and demodulated in the destination while the relays simply forward respective waveforms with scalar amplification. We first conclude that the power-delay profile (PDP) of the effective end-to-end channel in such a system converges to a specific bell-shaped function when each per-hop channel in the link is modelled with a uniform or exponential PDP. We then derive new closed-form expressions for evaluating the signal-to-interference-and-noise ratio (SINR) of the system in the case of an asymptotically large hop count.

We consider a system where a source node communicates with a destination node with the assistance of a wireless energy-powered full-duplex relay node. The relay node splits its received signal into two components for energy harvesting and information decoding, respectively, and forwards the decoded information using a portion of the harvested energy. To maximize the end-to-end transmission rate, the power splitting factor and energy consumption proportion are jointly optimized for the relay with a single transmit antenna in the presence of self-interference. Furthermore, for the relay with multiple transmit antennas, a suboptimal relay beamformer is first designed and the power splitting factor and energy consumption proportion are then jointly optimized. Finally, the asymptotic transmission rate is analyzed with a large number of transmit antennas. Simulation results are provided to demonstrate that the proposed schemes offer significant rate gain compared with some typical reference schemes, irrespective of the residual self-interference level. Especially, by employing a large number of source or relay transmit antennas, the wireless energy-powered relay system is capable of cutting its energy consumption significantly.

Channel estimation is a major overhead factor in large-scale multiple-input multiple-output (MIMO) systems, especially under the high-speed railway scenarios. This paper proposes a position-aided channel estimation scheme for high-speed railway communication systems, where both the transmitter and the receiver are equipped with large-scale antenna linear arrays. By joint spatio-temporal correlation, the pilot overhead can be significantly reduced. Furthermore, the optimal design of transmit power and time interval partition between the training and data phases as well as the antenna size are presented accordingly. Both analytical and simulation results show that the system throughput with position-aided channel estimation does not deteriorate significantly as the mobility increases, which is sharply in contrast with the conventional one that intends to re- estimate the entire channel matrix each block.

The large-scale multiple-input multiple-output (MIMO) system with full-duplex relay is a promising candidate for future mobile communication systems, where accurate channel estimation is very challenging due to interference. Here, we address two channel estimation problems for such systems: individual estimation, where both the base station (BS) and the relay estimate their respective channels, and cascaded estimation, where only the BS estimates the cascaded two-hop channel. For the BS, we propose an estimator that exploits the sparsity and slowly varying nature of the channel in the beam domain. Then, we analyze the probability of correctly distinguishing the desired and interfering direct-link channels. For the relay, we present an estimator that simultaneously estimates both the source-to-relay and self-interference channels based on the expectation–maximization algorithm. The performance of this estimator is also analyzed. Numerical results demonstrate the excellent performance of the proposed channel estimators and corroborate the analysis results

In this paper, we consider the problem of channel and phase noise estimation for an orthogonal frequency division multiplexing (OFDM) radio link. We solve this problem by first investigating the subspace in which the phase noise spectral vector lies and then exploiting this information during estimation. Building upon earlier works, the phase noise spectral estimate is obtained by minimizing a homogeneous quadratic cost function and the channel estimate in turns depends on the obtained phase noise estimate. We show that, at infinite signal-to-noise ratio, the true phase noise spectral estimate lies in the null space of the matrix associated with the cost function. We utilize this knowledge by imposing constraints that adhere to this null space when minimizing the cost function. In addition, we also propose constraints based on knowledge of the covariance matrix of the phase noise process. Through simulations, we demonstrate lower phase noise mean-square error (MSE) and consequently lower channel MSE when incorporating the subspace information.

Hardware non-idealities in wireless transmitter electronics cause distortion that is not captured by conventional linear channel models; in fact, error-vector magnitude (EVM) measurements in conformance testing conceptually reduce their collective effect to an additive noise component at each subcarrier. Motivated by the EVM, the present paper considers a `binoisy' multiple-input multiple-output (MIMO) channel model where the additional non-idealities manifest themselves as an additive distortion noise term at the transmit side. Through this extended MIMO relation, the effects of hardware impairments on the achievable rates of different digital modulation schemes are studied via large system analysis. The numerical results illustrate how tolerable EVM levels depend non-trivially on various factors, including the signal-to-noise ratio, modulation order and the level of asymmetry in antenna array configurations.

This paper investigates the achievable throughput of a multi-antenna two-hop relay link under hybrid full/half-duplex operation. The analysis is facilitated by realistic waveform simulations, which explicitly model all the essential circuit impairments occurring in the relay transceiver together with degrading channel estimation and self-interference cancellation. The obtained results indicate that pure full-duplex operation does not ensure optimal performance but additional half-duplex transmission periods are usually needed to maximize the end-to-end throughput. Especially, it is shown that the estimation of the self-interference channel within the relay should be performed when the source is not transmitting anything while also the source should be allowed to transmit alone to avoid making the first hop a bottleneck. These findings form a solid basis for optimizing the full-duplex MIMO relay deployments in future mobile networks.

Hardware impairments in radio-frequency components of a wireless system cause unavoidable distortions to transmission that are not captured by the conventional linear channel model. In this paper, a “binoisy” single-user multiple-input multiple-output (SU-MIMO) relation is considered where the additional distortions are modeled via an additive noise term at the transmit side. Through this extended SU-MIMO channel model, the effects of transceiver hardware impairments on the achievable rate of multi-antenna point-to-point systems are studied. Channel input distributions encompassing practical discrete modulation schemes, such as, QAM and PSK, as well as Gaussian signaling are covered. In addition, the impact of mismatched detection and decoding when the receiver has insufficient information about the non-idealities is investigated. The numerical results show that for realistic system parameters, the effects of transmit-side noise and mismatched decoding become significant only at high modulation orders.

Despite the intensive recent research on wireless single-channel full-duplex communications, relatively little is known about the transceiver chain nonidealities of full-duplex devices. In this paper, the effect of nonlinear distortion occurring in the transmitter power amplifier (PA) and the receiver chain is analyzed, beside the dynamic range requirements of analog-to-digital converters (ADCs). This is done with detailed system calculations, which combine the properties of the individual electronics components to jointly model the complete transceiver chain, including self-interference cancellation. They also quantify the decrease in the dynamic range for the signal of interest caused by self-interference at the analog-to-digital interface. Using these system calculations, we provide comprehensive numerical results for typical transceiver parameters. The analytical results are also confirmed with full waveform simulations. We observe that the nonlinear distortion produced by the transmitter PA is a significant issue in a full-duplex transceiver and, when using cheaper and less linear components, also the receiver chain nonlinearities become considerable. It is also shown that, with digitally intensive self-interference cancellation, the quantization noise of the ADCs is another significant problem.

This paper addresses the analysis of oscillator phase-noise effects on the self-interference cancellation capability of full-duplex direct-conversion radio transceivers. Closed-form solutions are derived for the power of the residual self-interference stemming from phase noise in two alternative cases of having either independent oscillators or the same oscillator at the transmitter and receiver chains of the full-duplex transceiver. The results show that phase noise has a severe effect on self-interference cancellation in both of the considered cases, and that by using the common oscillator in upconversion and downconversion results in clearly lower residual self-interference levels. The results also show that it is in general vital to use high quality oscillators in full-duplex transceivers, or have some means for phase noise estimation and mitigation in order to suppress its effects. One of the main findings is that in practical scenarios the subcarrier-wise phase-noise spread of the multipath components of the self-interference channel causes most of the residual phase-noise effect when high amounts of self-interference cancellation is desired.

Simultaneous reception and transmission in the same frequency, so-called full-duplex operation, causes an infinite feedback loop in an amplify-and-forward relay. The unwanted echoes may result in oscillation at the relay, making it unstable, and distorting the spectrum. This paper presents an adaptive MIMO filtering method for full-duplex amplify-and-forward relays that aims at solving the joint problem of self-interference mitigation and equalization of the source-relay channel. The scheme exploits the knowledge of the autocorrelation of the transmitted signal as the only side information while allowing the relay, in the best case, to implement precoding as if there was not any self-interference or frequency selectivity in the source-relay channel. Finally, the proposed adaptation algorithm is investigated by determining its stationary points and by performing simulations in a MIMO-OFDM framework.

Bidirectional wireless communication between two multi-antenna terminals that operate in a full-duplex mode is considered. The effects of loopback self-interference and transmit-side noise due to radio front-end imperfections are analyzed in the asymptotic regime where the number of antennas at both terminals grows large. The study compares two transparent mitigation schemes by which conventional spatial multiplexing can operate without being aware of the self-interference issue: spatial-domain null-space projection and time-domain subtractive cancellation. Both schemes eliminate the information-bearing part of the self-interference perfectly, but due to transmitter imperfections, the cancellation approach suffers from residual self-interference. The receive-side null-space projection, on the other hand, suppresses also the transmit-side distortion, but at the cost of reduction in the degrees of freedom available for data reception. The results indicate that suppression becomes preferred when the level of transmit-side noise increases and the receiver does not have instantaneous knowledge about the spatial structure of the self-interference. Exhaustive Monte Carlo simulations also show that the asymptotic analytical results are useful for evaluating the performance of practical not-so-large systems.

A new approach to relay and user selection is proposed using threshold-based transmission to reduce outage probability and save transmit power. By exploiting properties of dual-hop transmission using amplify-and-forward (AF) relaying, new threshold-based relay and user (TRU) selection protocols that employ one-bit channel state information (CSI) feedback are proposed for multiple-relay, multiple-user systems. These new protocols have the advantages of (1) economical feedback, and (2) optional adaptive power-saving transmission. Centralized selection (CS) and distributed selection (DS) policies as well as lower-complexity suboptimal selection schemes are investigated for downlink transmission. Exact expressions for outage probability, outage diversity and feedback requirement are provided to quantify the complexity/performance trade-offs. The analysis shows that selection threshold(s) can be chosen such that transmission is interrupted when system outage is certain, which allows power saving without impairing outage performance. These protocols are compared in terms of performance, energy-efficiency and system requirement to others that trade outage performance for additional energy saving and/or reduced complexity.

We derive the average capacity for an OFDM radio link that is impaired by phase noise and carrier frequency offset at both transmitter and receiver side. It is well known that, for OFDM, phase noise and frequency offset cause intercarrier interference (ICI). We first notice the dependency of the instantaneous capacity on the ICI power and on the fading channel. Using a Taylor series approximation, we show that the ICI power can be well represented in a quadratic form whose probability density function we derive. Using the derived distribution and assuming a Rayleigh-fading channel, we derive analytical expressions of average capacity. Different test cases are presented to study the sensitivity of the capacity on phase noise and frequency offset. The main observations suggest that, for reasonable phase noise levels, the capacity is practically insensitive to the frequency offset.

We propose a discrete-time model for phase noise processes occurring in phase locked loop (PLL) based oscillators. By using linear time-invariant analysis in the phase domain, we arrive at a parallel auto-regressive moving-average phase noise model that captures the major noise sources and corresponding loop transfer functions of the PLL circuitry. The acquired model allows us to derive analytically the distribution of the inter-carrier interference (ICI) power in the receiver, shown to be a sum of correlated gamma random variables with a model-specific covariance matrix. Finally, we utilize the derived ICI distribution and proceed to analyze the effects of phase noise and multipath fading on the performance of OFDM radio links. For example, we evaluate the rate loss without ICI compensation or knowledge at the receiver, and illustrate the trade-off introduced by phase noise between the OFDM symbol length and the overhead caused by the cyclic prefix. Throughout the paper, simulations confirm the accuracy of the analytical expressions.

Predistortion techniques are implemented in OFDM systems to improve power amplifier efficiency while keeping in-band and out- of-band distortion at acceptable levels. Predistorter (PD) design for broadband MIMO-OFDM systems introduces several implementation challenges. Coupling effects cannot be avoided in MIMO transceivers where the paths are implemented on the same chipset. Moreover, baseband PDs are affected by imbalances in the IQ modulator. This paper proposes a MIMO-PD that linearizes the power amplifier (PA) response and compensates for both crosstalk and IQ imbalance. Error vector magnitude (EVM) and adjacent channel power ratio (ACPR) results show a significant improvement compared with conventional PD systems.

In this paper, we study the performance of a LTE-A relay-aided system with limited buffer size at the relay node. In presence of a link quality mismatch between the backhaul and access links of the relay node, we analyze the impact of the buffer size on the occurrence of undesirable events such as buffer overflows or underflows. Based on simulations, we find out that the size of the buffer at the RN can positively influence the throughput performance of the served users. Finally, we observe that in different relaying conditions, the buffer size setting plays an important role on the overall system performance, especially in terms of underflow and overflow occurrence.

We analyze the effect of Wiener phase noise on the capacity and signal-to-interference-plus-noise (SINR) ratio. Our analysis includes phase noise at the transmitter and receiver end of an OFDM communication link. We see that the capacity and SINR are random variables whose distribution depends on the phase noise and the fading channel. Using a Taylor series approximation, we show that the random variable, characterizing the phase noise, in the performance metrics can be expressed as a sum of correlated gamma variables with rank-deficient square-root normalized covariance matrix. The approximation holds well when the ratio between the subcarrier spacing and the 3dB bandwidth of the oscillator power spectral density is at least one order of magnitude, which for most practical oscillators is the case. In earlier literature, the probability density function of a sum of correlated gamma variables with full-rank square-root normalized covariance matrix was derived. We extend these results to the rank-deficient case and apply them to the random variable of our case. With the probability density functions characterizing the phase noise and the fading channel at hand, we proceed to obtain closed-form statistical measures of capacity and SINR. The simulations show the good agreement with our analytical expressions.

Focusing on two-antenna infrastructure relays employed for coverage extension, we develop hybrid techniques that switch opportunistically between full-duplex and half-duplex relaying modes. To rationalize the system design, the classic three-node full-duplex relay link is first amended by explicitly modeling residual relay self-interference, i.e., a loopback signal from the transmit antenna to the receive antenna remaining after cancellation. The motivation for opportunistic mode selection stems then from the fundamental trade-off determining the spectral efficiency: The half-duplex mode avoids inherently the self-interference at the cost of halving the end-to-end symbol rate while the full-duplex mode achieves full symbol rate but, in practice, suffers from residual interference even after cancellation. We propose the combination of opportunistic mode selection and transmit power adaptation for maximizing instantaneous and average spectral efficiency after noting that the trade-off favors alternately the modes during operation. The analysis covers both common relaying protocols (amplify-and-forward and decode-and-forward) as well as reflects the difference of downlink and uplink systems. The results show that opportunistic mode selection offers significant performance gain over system design that is confined to either mode without rationalization.

We study the fundamental rate-interference trade-off between full-duplex and half-duplex transmission modes in a two-hop infrastructure-based decode-and-forward relay link. First, we derive closed-form expressions for average end-to-end capacities. Then, we show that it may be better to tolerate some loop interference in the full-duplex mode than to halve the end-to-end symbol rate by allocating two orthogonal time slots in the half-duplex mode. Furthermore, we evaluate the loop interference power levels below which the full-duplex mode achieves better capacity than the half-duplex mode. Our results indicate that the full-duplex mode is preferable in terms of capacity for many practical signal-to-noise ratios, e.g., above 3-10dB depending on the setup. Finally, we show that full-duplex relays can further improve the end-to-end capacity by adjusting transmit power.

Direct-conversion OFDM transceivers are seriously affected by front-end distortions like IQ imbalance and phase noise. Moreover, OFDM systems are sensitive to nonlinear distortions in power amplifiers and A/D converters, and to carrier frequency offset (CFO). We present a novel baseband compensation technique that compensates these main impairments in OFDM systems. Unlike compensation scheme that target a subset of impairments, our method jointly mitigates the effects of IQ imbalance, phase noise, carrier frequency offset and nonlinear distortion. Simulations of the proposed scheme with large levels of CFO, IQ imbalance and phase noise show an excellent performance in terms of bit error rate.

This paper proposes new selection schemes based on one-bit feedback for a system with multiple amplify-and-forward relays and users over Rayleigh fading channels. We introduce new distributed selection (DS), simplified DS (DSS), and centralized selection (CS) schemes that reduce outage and save transmit power using one-bit feedback. The CS scheme is optimal by selecting only the dual-hop link with equivalent end-to-end SNR above the outage threshold. DS and DSS schemes, on the other hand, require less feedback at the BS and employ a decentralized selection policy. The feedback bit in those schemes render the comparison result between the first- and second- hop SNR and an appropriate threshold. The chosen selection threshold in DS and DSS schemes allows the dual-hop links for which the system outage is certain to be discarded. In the three selection schemes, transmission is switched off to save power when there is no dual-hop link meeting the selection requirement. Closed-form expressions for the DS, DSS, and CS outage probabilities are derived. Numerical results reveal that, even though DSS is a simplified version of the DS scheme, they have similar outage performance. Additionally, DS and DSS are quite competitive with CS for a small number of relays relative to the number of users.

We investigate the impact of fixed relay station deployment in a single frequency network (SFN) using orthogonal frequency-division multiplexing (OFDM). We provide semi-analytical methods for relay-based SFN performance evaluation. Monte Carlo simulations are performed to provide numerical calculation of multi-cell network performance. The performance metric is the cumulative distribution function of the signal-to-interference-plus-noise-ratio (SINR) for different user positions in the network. Common relaying methods for two-hop amplify-and-forward (A&F) relay networks are evaluated. The impact on the SINR is compared for different relaying gains, locations and densities of relays, as well as different transmission protocols, such as full-duplex (FD) and half-duplex (HD). Overall, taking into account their different rates and physical layer performances, FD appears to outperform HD. In addition to showing the benefit of relay deployment for enhancing network performance, our simulations show that the fixed and variable gains perform equally well.

This paper studies the performance of a two-hop infrastructure-based relay link. The relay operates in a half-duplex mode using an amplify-and-forward protocol. Considering transmission of OFDM signals with nonlinear power amplifiers (PAs), we determine exact closed-form expressions for end-to-end bit error probabilities (BEPs). The expressions allow us to analyze the effect of nonlinear distortion as well as the special case of linear PAs. Furthermore, we derive asymptotic BEP floor values which illustrate how the end-to-end performance is limited by both the nonlinear PA distortion and the weakest hop. Our discussion indicates that the influence of the nonlinear distortion can be mitigated by properly adjusting the PA back-offs.

We study the physical-layer performance of a wideband multihop system in which a chain of amplify-and-forward relays is deployed for routing data from a source to a destination. The novelty of the system model w.r.t. prior work is in assuming frequency-selective multipath channels for all hops. We propose a new approach in which the channels are specified in terms of clustered exponentially decaying power-delay profiles (PDPs). Focusing on relays that apply time-domain signal processing, the distinctive form of the end-to-end source-destination PDP motivates us to introduce a new hypoexponential PDP and study it in detail. Then, we analyze the performance of orthogonal frequency division multiplexing (OFDM) transmission over the multihop relay link. The type of the adopted analysis is suitable mainly for investigating the typical time-domain issues of OFDM transmission such as excessive multipath delay spread, non-ideal time syncronization, and interference due to insufficient cyclic prefix duration. In particular, we derive new closed-form expressions for evaluating the signal-to-interference and noise ratio. Our analysis confirms that multihopping increases considerably the delay spread of the end-to-end PDP and makes the channel more frequency selective. This imposes significant effect on the design of receiver time synchronization and pilot structures, and on the choice of OFDM physical layer parameters.

This paper studies the resource allocation for orthogonal frequency division multiple access (OFDMA) in a fixed amplify-and-forward (AF) relay link. We express closed-form average capacity results for the system when frequency allocation is based on limited and imperfect feedback information and multiple antennae are assumed at the relay node (RN) and the user equipment (UE). Feedback strategies based on the best-M method and resource block (RB)-wise one bit quantization are analyzed and compared. Capacity expressions are presented for space-time block coding (STBC) and transmit antenna selection (AS) schemes. The capacity results with imperfect feedback path are of particular interest. The results illustrate that promising tradeoff between the system performance, feedback overhead, and robustness against feedback errors can be achieved by using AS and limited feedback in frequency allocation.

We consider a downlink OFDMA transmission system in which an infrastructure-based relay node is deployed for extending the coverage of a base station. Focusing on decode-and-forward (DF) operation, we study how different relay functionalities affect the system performance. The functionalities include fixed or adaptive subcarrier pairing, information redistribution, buffering, and adjustment of the time shares allocated for the two hops. In particular, it is of great interest for system design to be able to evaluate the performance gains due to each functionality, because they come at the expense of increased complexity and cost of the relay architecture. The main contribution of the paper is to analyze the performance of the proposed protocols by deriving new closed-form expressions for the average end-to-end capacity. The calculated expressions facilitate the performance comparison between the different functionalities.

We study the performance of a two-hop full-duplex OFDM relay link, where the relay, or in-band repeater, amplifies and forwards its input signal on the same channel as the main transmitter. We derive new closed-form expressions for the end-to-end signal-to-interference and noise ratio (SINR) when the multipath channels are modeled with clustered exponential power-delay profiles. Our system model incorporates the deleterious effect of loop interference associated with full-duplex relays due to signal leakage from transmission to reception. In particular, our analysis shows that the relay gain is a central parameter in the mitigation of residual loop interference and for the optimization of the end-to-end performance.

Current broadband wireless networks are characterized by large cell sizes. Yet, even in advanced networks, users on the cell edge will face relatively low Signal-to-Interference-plus-Noise-Ratio (SINR). An attractive solution for this problem is provided by multi-hop technologies. In this paper, we consider the performance of full duplex Amplify-and-Forward (AF) and half duplex Decode-and-Forward (DF) Relay Nodes (RNs) from 3G LTE-Advanced perspective. The comparison between AF and DF relaying is important because both approaches are currently under consideration in LTE-Advanced study item in 3GPP. Performance evaluation considers AF RN loop back signal interference and concurrent DF RN transmissions on the access link. Results show that the concurrent transmissions improve the spectral efficiency for DF RN over performing AF RN.

This paper studies receiver techniques for nonlinear amplifier distortion compensation in an OFDM relay-assisted cooperative communication system. The system model includes a nonlinear amplifier at the amplify-and-forward relay, modeled as a solid state power amplifier (SSPA). A maximum ratio combiner (MRC) is introduced that includes the effects of the amplifier distortions. The MRC is obtained by a proper modeling of the nonlinear distortion noise. Furthermore, we introduce a power amplifier nonlinearity cancellation (PANC) technique that is suitable for cooperative systems. Our simulation results confirm that the new MRC and PANC techniques offer substantial performance improvements if the relayed signal is subject to nonlinear distortion.

We study the fundamental capacity trade-off between full-duplex and half-duplex transmission modes in a two- hop communication system with a fixed infrastructure-based amplify-and-forward relay. First, we derive closed-form expressions for the average end-to-end capacity in the relay link. We show that it may be better to tolerate some loop interference with the full-duplex mode than to consume channel resources by allocating two orthogonal channels with the half-duplex mode. Furthermore, we evaluate the maximum loop interference power levels that still allow the full-duplex mode to achieve the same capacity as the half-duplex mode. Our results indicate that with practical signal-to-noise ratio values, the full-duplex mode is preferable in terms of capacity.

This paper considers usage of a single-frequency mode for cell coverage extension in infrastructure-based relay links. We provide new closed-form expressions for outage probability by taking into account practical constraints such as interference due to frequency reuse and signal leakage between the relay transmitter and receiver. The analysis covers both decode-and-forward and amplify-and-forward protocols both in downlink and in uplink. For amplify-and-forward relaying, variable gain and fixed gain methods for transmit power normalization are discussed. Simulations show excellent agreement with theory and confirm that the single-frequency mode can be applied at the cost of tolerable signal degradation.

This letter derives new gain control schemes for an amplify-and-forward single-frequency relay link in which loop interference from the relay transmit antenna to the relay receive antenna has to be tolerated. The proposed gain control schemes take into account the effect of residual loop interference that remains after imperfect loop interference cancellation. As a result of our gain control strategy, the signal-to-interference and noise ratio can be maximized while, at the same time, transmit power is decreased. Finally, we evaluate system performance by deriving closed-form outage probability expressions for the gain control schemes.

We study a wireless two-hop relaying network where a destination node receives a superposition of direct link and relayed signals, because the same physical channel is employed for both transmissions. We assume fixed-gain amplify-and-forward operation in the relay and develop a power allocation technique that requires only slowly varying channel state information. The optimal power allocation maximizes the end-to-end signal- to-noise ratio under total transmit power constraint. Furthermore, we propose a nearly optimal power allocation that can be determined using closed-form expressions.

We consider a Gaussian parallel relay network where multiple wireless relays forward cooperatively so that a destination receives a superposition of signals propagated through multiple distinct relay paths. Previous research often assumes that the relayed signals arrive synchronously in the receiver. This is not feasible in practical wireless networks, because spatial separation causes different propagation delays for the signal components and the destination observes a multipath channel. We model the effect of signal delays by introducing spatial separation to the network geometry and present trade-off analysis between diversity, signal bandwidth, relay density, and the number of relays for both narrowband and wideband reception at the destination