Stacked intelligent metasurfaces (SIMs) represent a key enabler for next-generation wireless networks, offering beamforming gains while significantly reducing the number of radio-frequency chains. In conventional space-only (S-only) SIM architectures, the rate of reconfigurability of the SIM is equal to the inverse of the channel coherence time. This paper investigates a novel beamforming strategy for massive downlink connectivity using a randomized space-time (ST) SIM. In addition to conventional S-only metasurface layers, the proposed design integrates an ST metasurface layer at the input stage of the SIM that introduces random variations over each channel coherence interval. These artificial time variations enable opportunistic user scheduling and exploitation of multiuser diversity under slow channel dynamics. To mitigate the prohibitive overhead associated with full channel state information at the transmitter (CSIT), we propose a partial-CSIT-based beamforming scheme that leverages randomized steering vectors and limited user-side feedback based on signal quality measurements. Numerical results demonstrate that the proposed ST-SIM architecture achieves satisfactory sum-rate performance while significantly reducing CSIT acquisition and feedback overhead, thereby enabling scalable downlink connectivity in dense networks.

Amplitude, phase, frequency, and polarization states of electromagnetic (EM) waves can be dynamically manipulated by means of artificially engineered planar materials, composed of sub-wavelength meta-atoms, which are typically referred to as metasurfaces. In wireless communications, metasurfaces can be configured to judiciously modify the EM propagation environment in such a way to achieve different network objectives in a digitally programmable way. In such a context, metasurfaces are typically referred to as reconfigurable intelligent surfaces (RISs). Until now, researchers in wireless communications have mainly focused their attention on slowly time-varying designs of RISs, where the spatial-phase gradient across the RIS is varied at the rate equal to the inverse of the channel coherence time. Additional degrees of freedom for controlling EM waves can be gained by applying a time modulation to the reflection response of RISs during the channel coherence time interval, thereby attaining rapidly time-varying RISs. In this paper, we develop a general framework where a downlink multiuser transmission over single-input single-output slow fading channels is assisted by a digitally controlled rapidly time-varying RIS. We show that reconfiguring the RIS at a rate greater than the inverse of the channel coherence time might be beneficial from a communication perspective depending on the considered network utility function and the available channel state information at the transmitter (CSIT). The conclusions of our analysis in terms of system design guidelines are as follows: (i) if the network utility function is the sum-rate time-averaged network capacity, without any constraint on fair resource allocation, and full CSIT is available, it is unnecessary to change the electronic properties of the RIS within the channel coherence time interval; (ii) if partial CSIT is assumed only, a rapidly time-varying randomized RIS allows to achieve a suitable balance between sum-rate time-averaged capacity and user fairness, especially for a sufficiently large number of users; (iii) regardless of the available amount of CSIT, the design of rapid temporal variations across the RIS is instrumental for developing scheduling algorithms aimed at maximizing the network capacity subject to fairness constraints.

Unmanned aerial vehicles (UAVs) can be integrated into wireless sensor networks (WSNs) for smart city applications in several ways. Among them, a UAV can be employed as a relay in a “store-carry and forward” fashion by uploading data from ground sensors and metering devices and, then, downloading it to a central unit. However, both the uploading and downloading phases can be prone to potential threats and attacks. As a legacy from traditional wireless networks, the jamming attack is still one of the major and serious threats to UAV-aided communications, especially when also the jammer is mobile, e.g., it is mounted on a UAV or inside a terrestrial vehicle. In this article, we investigate anti-jamming communications for UAV-aided WSNs operating over doubly selective channels in the downloading phase. In such a scenario, the signals transmitted by the UAV and the malicious mobile jammer undergo both time dispersion due to multipath propagation effects and frequency dispersion caused by their mobility. To suppress high-power jamming signals, we propose a blind physical-layer technique that jointly detects the UAV and jammer symbols through serial disturbance cancelation based on symbol-level post-sorting of the detector output. Amplitudes, phases, time delays, and Doppler shifts—required to implement the proposed detection strategy—are blindly estimated from data through the use of algorithms that exploit the almost-cyclostationarity properties of the received signal and the detailed structure of multicarrier modulation format. Simulation results corroborate the anti-jamming capabilities of the proposed method, for different mobility scenarios of the jammer.

In millimeter-wave (MMW) multiple-input multiple-output (MIMO) communications, users and their corresponding base station (BS) have to align their beam during both initial access and data transmissions to compensate for the high propagation loss. The beam alignment (BA) procedure specified for 5th Generation (5G) New Radio (NR) has been designed to be fast and precise in the presence of non-malicious interference and noise. A smart jammer might exploit this weakness and may launch an attack during the BA phase in order to degrade the accuracy of beam selection and, thus, adversely impacting the end-to-end performance and quality-of-service experienced by the users. In this paper, we study the effects of a jamming attack at MMW frequencies during the BA procedure used to perform initial access for idle users and adaptation/recovery for connected users. We show that the BA procedure adopted in 5G NR is extremely vulnerable to a smart jamming attack and, consequently, we propose a countermeasure based on the idea of randomized probing, which consists of randomly corrupting the probing sequence transmitted by the BS in order to reject the jamming signal at the UE via a subspace-based technique based on orthogonal projections and jamming cancellation. Numerical results corroborate our theoretical findings and show the very satisfactory accuracy of the proposed anti-jamming approach.

Backscattering communications have been recently proposed as an effective enabling technology for massive Internet of Things (IoT) development. A novel application of backscattering, called ambient backscattering (AmBC), has been gaining much attention, wherein backscattering communications exploit existing RF signals without the need for a dedicated transmitter. In such a system, data demodulation process is strongly complicated by the random nature of the illuminating signal, as well as by the presence of the direct-link interference (DLI) from the legacy system. To overcome these shortcomings, one can resort to noncoherent detection strategies, aimed at reducing or even nullifying the amount of a priori information needed to reliably perform signal demodulation. This paper deals with the problem of noncoherent maximum-likelihood (ML) signal detection for backscatter communications over ambient OFDM. The performance of the proposed detector is corroborated through Monte Carlo simulations.

In multiple antenna systems employing time-division duplexing, spatial precoder design at the base station (BS) leverages channel state information acquired through uplink pilot transmission, under the assumption of channel reciprocity. Malicious eavesdroppers can start pilot spoofing attacks to alter such design, in order to improve their eavesdropping performance in downlink. The aim of this paper is to study the effects of pilot spoofing attacks on uplink channel estimation, by assuming that the BS knows the angle of arrivals (AoAs) of the legitimate channels. Specifically, after assessing the performance of the simple least squares estimator (LSE), we consider more sophisticated estimators, such as the maximum likelihood estimator (MLE) and different versions of the minimum mean square error estimator (MMSEE), involving different degrees of a priori information about the pilot spoofing attacks. Theoretical analysis and numerical simulations are used to compare the performance of such estimators. In particular, we analytically demonstrate that the spoofing effects in the high signal-to-noise ratio regime can be completely suppressed, under certain conditions involving the AoAs of the legitimate and spoofing channels. Moreover, we show that even an imperfect knowledge of the AoAs and of the average transmission power of the spoofing signals allows the MLE and MMSEE to achieve significant performance gains over the LSE.

In this paper, we analyze the performance of in-band full-duplex (IBFD) relay systems that use the decode-and-forward (DF) protocol at the relay under the impact of imperfect self-interference cancellation and hardware impairments. Three practical relay scenarios are considered in our analysis: (i) there is no direct link from the source to the destination; (ii) there is a direct link, but the signal from the source is considered interference; and (iii) there is a direct link, and the signal from the source is cooperatively combined with that from the relaying path. Specifically, we derive exact and asymptotic expressions for the outage probability (OP) of the IBFD system. Based on the OP, the exact expression for symbol error probability (SEP) is also derived. Moreover, in order to cope with the effect of imperfect self-interference cancellation (SIC) due to the full-duplex mode, we propose optimal and suboptimal power calculation methods for the relay to minimize the OP and SEP. A performance evaluation shows that the IBFD relay system is significantly affected by both imperfect SIC and hardware impairments. However, the optimal power values can help to improve the system performance, significantly.

This letter deals with second-order statistics (SOS) of continuous-phase modulated (CPM) signals. To overcome some mathematical inconsistencies emerging from the idealized assumption that the CPM signal evolves from t = -∞, we consider a one-sided model for the signal, which starts from t = 0, noting also that such a model emerges naturally when building practical SOS estimators. On the basis of such a model, we first evaluate the SOS of the pseudosymbols, which arise when expressing a CPM signal in terms of its Laurent representation, as well as closed-form expressions of the cyclic autocorrelation and conjugate correlation functions of one-sided CPM signals.

Ambient backscatter is an intriguing wireless communication paradigm that allows small devices to compute and communicate by using only the power they harvest from far-field radio-frequency (RF) signals in the air. Ambient backscattering devices reflect RF signals emitted by existing or legacy communications systems, such as digital TV broadcasting, cellular, or Wi-Fi ones, which are designed for transporting information and are not intended for RF energy transfer. This paper deals with mathematical modeling and performance analysis of wireless broadband networks operating over fading channels with ambient backscatter devices. After introducing a detailed signal model of the relevant communication links, we study the influence of physical parameters on the capacity of both legacy and backscatter channels, by considering different receiver architectures. We analytically show that, under reasonable operative conditions, a legacy system-employing an orthogonal frequency-division multiplexing (OFDM) modulation scheme-can turn the RF interference arising from the backscatter process into a form of multipath diversity that can be exploited to increase its performance. Moreover, our analysis proves that a backscatter system-transmitting one symbol per OFDM symbol of the legacy system-can achieve satisfactory data rates over relatively short distances, especially when the intended recipient of the backscatter signal is co-located with the legacy transmitter, i.e., they are on the same device.

In this paper, we consider a broadband cognitive radio network, where the primary and secondary systems employ a multicarrier modulation scheme. The proposed spectrum-sharing scheme is based on additive superposition coding: specifically, the secondary transmitter acts as an amplify-and-forward relay, forwarding a linearly weighted combination of the received multicarrier primary signal and its own block of multiple symbols. Such a superposition is performed by the secondary transmitter on the fly, in a single multicarrier symbol of the primary system. Moreover, the proposed scheme does not require cooperation between the primary and secondary systems. An information-theoretic performance analysis of the proposed scheme is developed, whose findings are corroborated by means of numerical Monte Carlo simulations.

In this paper, we assess the information-theoretic performance of a point-to-point link that exploits ambient backscattering to support green Internet-of-Thing (IoT) communications. In this framework, an IoT passive device transmits its information by reusing ambient radio-frequency signals emitted by an existing or legacy multicarrier communication system. After introducing the signal model of the relevant communication links, the information-theoretic capacity of both the legacy and backscatter systems is derived. It is found that, under reasonable operative conditions, the legacy system can turn the RF interference arising from backscattering into a form of multipath diversity, which can be exploited to increase its own performance. Moreover, it is shown that, even when it employs simple single-carrier modulation techniques, the backscatter system attains significant data rates over relatively short distances.

Recently, we proposed a spectrum-sharing paradigm for single-carrier cognitive radio (CR) networks, where a secondary user (SU) is able to maintain or even improve the performance of a primary user (PU) transmission, while also obtaining a low-data rate channel for its own communication. According to such a scheme, a simple multiplication is used to superimpose one SU symbol on a block of multiple PU symbols.The scope of this paper is to extend such a paradigm to a multicarrier CR network, where the PU employs an orthogonal frequency-division multiplexing (OFDM) modulation scheme. To improve its achievable data rate, besides transmitting over the subcarriers unused by the PU, the SU is also allowed to transmit multiple block-precoded symbols in parallel over the OFDM subcarriers used by the primary system. Specifically, the SU convolves its block-precoded symbols with the received PU data in the time-domain, which gives rise to the term convolutive superposition. An information-theoretic analysis of the proposed scheme is developed, which considers different amounts of network state information at the secondary transmitter, as well as different precoding strategies for the SU. Extensive simulations illustrate the merits of our analysis and designs, in comparison with conventional CR schemes, by considering as performance indicators the ergodic capacity of the considered systems.

This paper considers the problem of jointly compensating carrier frequency offsets (CFOs) and in-phase/quadrature-phase (I/Q) imbalance effects in orthogonal frequency-division multiple-access (OFDMA) or single-carrier frequency-division multiple-access (SC-FDMA) systems. Specifically, we propose a widely-linear time-varying minimum mean-output energy compensation scheme, by deriving its data-dependent frequency-shift implementation. The performance of the proposed receiver is assessed by Monte Carlo computer simulations.

In this letter, we consider the problem of equalizing finite-impulse response (FIR) channels in orthogonal frequency-division multiplexing (OFDM) systems, which employ at the receiver a blanking nonlinearity to mitigate impulsive noise (IN). By exploiting the frequency redundancy associated with the presence of OFDM virtual carriers, we design a frequency-domain linear FIR equalizer that compensates for the intercarrier interference (ICI) generated by nonlinear preprocessing. Monte Carlo computer simulations, carried out assuming a Middleton Class A model for the IN, allows one to assess the error probability performance of the proposed equalizer.

In this paper, we propose a cognitive radio scheme that allows a secondary user (SU) to transmit over the same time-frequency slot of a primary user (PU), even when the PU is active. In our scheme, the SU amplifies and forwards the signal of the PU, by using as scaling factor the value of its information symbol to be transmitted towards the secondary receiver. The information-theoretic limits of the proposed protocol are investigated in terms of ergodic channel capacities of both the PU and SU links. It is shown that: 1) under certain operating conditions, the SU can superimpose its information symbols on the PU signal, without violating the cognitive radio principle of protecting the PU transmission; and 2) when the primary link is busy, the SU offers the PU its own transmitting power in exchange for a low-capacity communication channel, which improves the packet delay performance of the SU. In this barter, the tempting incentive for the PU consists of a noticeable improvement of its achievable rate at the price of a slight increase in the computational complexity of the primary receiver.

This paper deals with the synthesis of a new reception scheme for the uplink of a single-carrier interleaved frequency-division multiple-access (SC-IFDMA) wireless network. In such networks, the in-phase/quadrature-phase (I/Q) imbalance, introduced at each user transmitter and at the base station receiver, and the carrier frequency offsets (CFOs) between the transmitters and the receiver are sources of severe performance degradation. The proposed receiver, based on the minimum mean-output energy (MMOE) criterion, jointly mitigates CFO and I/Q imbalance effects by processing the received signal and its complex conjugate version. Monte Carlo computer simulations are carried out to assess the effectiveness of the MMOE solution.

In this paper, we propose an opportunistic amplify-and-forward relaying scheme for a cognitive radio network, which is aimed at allowing a secondary user (SU) to transmit over the same time-frequency slot of a primary user (PU). In our scheme, the SU amplifies and transmits the PU signal it receives, by using as relaying gain the information symbols that the SU wishes to transmit towards its own secondary receiver. The information theoretic limits of the proposed protocol are investigated by showing that, in some operative conditions of practical interest, the SU can embed its information symbols in the PU signal, without violating the cognitive radio principle of protecting the PU transmission and, at the same time, by attaining low transmission rates.

This paper proposes a blind channel shortening algorithm for uplink reception of a single-carrier interleaved frequency-division multiple-access (SC-IFDMA) system, affected by timing offsets (TOs) and carrier frequency offsets (CFOs), and transmitting over a highly dispersive channel. When the cyclic prefix (CP) cannot completely compensate for channel dispersion and TOs, a convenient strategy is to shorten the channel by timedomain equalization (TEQ) in order to ease signal reception. The proposed receiver exhibits a three-stage structure: the first stage performs blind channel shortening without requiring a priori knowledge of the channels or preliminary compensation of the CFOs; the second stage performs multiuser CFO compensation; finally, to alleviate noise amplification effects possibly arising from CFO compensation, the third stage implements blind peruser signal-to-noise ratio (SNR) maximization. The performance of the proposed method are assessed and compared with those of existing algorithms by Monte Carlo computer simulations.

This paper proposes a blind channel shortening algorithm for uplink reception of a single-carrier interleaved frequency-division multiple-access (SC-IFDMA) system transmitting over a highly-dispersive channel, which is affected by both timing offsets (TOs) and frequency offsets (CFOs). When the length of the cyclic prefix (CP) is insufficient to compensate for channel dispersion and TOs, a common strategy is to shorten the channel by means of time-domain equalization, in order to restore CP properties and ease signal reception. The proposed receiver exhibits a three-stage structure: the first stage performs blind shortening of all the user channel impulse responses (CIRs), by adopting the minimum mean-output energy criterion, without requiring neither a priori knowledge of the CIRs to be shortened, nor preliminary compensation of the CFOs; the second stage performs joint compensation of the CFOs; finally, to alleviate noise amplification effects, possibly arising from CFO compensation, the third stage implements per-user signal-to-noise ratio (SNR) maximization, without requiring knowledge of the shortened CIRs. A theoretical analysis is carried out to assess the effectiveness of the proposed shortening algorithm in the high-SNR regime; moreover, the performances of the overall receiver are validated and compared with those of existing methods by extensive Monte Carlo computer simulations.

We investigate the impact of cooperative relaying on uplink multi-user (MU) wireless video transmissions. We analyze and simplify a MU Markov decision process (MDP), whose objective is to maximize the long-term sum of utilities across the video terminals in a decentralized fashion, by jointly optimizing the packet scheduling and physical layer, under the assumption that some nodes are willing to act as cooperative relays. The resulting MU-MDP is a pricing-based distributed resource allocation algorithm, where the price reflects the expected future congestion in the network. Compared to a non-cooperative setting, we observe that the resource price increases in networks supporting low transmission rates and decreases for high transmission rates. Additionally, cooperation allows users with feeble direct signals to significantly improve their video quality, with a moderate increase in total network energy consumption that is far less than the energy these nodes would require to achieve the same video quality without cooperation.

This paper deals with multiple-input multiple-output (MIMO) broadband wireless communication systems, employing orthogonal frequency-division multiplexing (OFDM) with cyclic prefix (CP) in combination with space-time-frequency block coding (STFBC). In order to exploit the benefits of OFDM and STFBC in highly frequency-selective channels, without incurring in a significant increase of receiver complexity, a channel shortening prefilter is inserted at the receiver, aimed at reducing the length of the MIMO channel impulse response before CP removal. Two MIMO channel shorteners are proposed, both relying on linearly-constrained minimization of the mean-output-energy of the signal at the output of the channel shortener, where the linear constraints are optimally chosen so as to maximize the signal-to-noise ratio either at the output of the channel shortener or at the input of the STFBC maximum-likelihood decoder. Unlike other solutions proposed in the literature, these shortening designs are blind ones, i.e., a priori knowledge of the MIMO channel impulse response to be shortened is not required, and can be carried out in closed-form, i.e., iterative computation of the prefilter weights is not necessary. Numerical simulations show that, without a substantial increase in computational complexity, receivers equipped with the proposed blind channel shorteners pay only a negligible performance penalty with respect to non-blind channel shorteners, while being significantly more robust to finite sample-size effects.

We investigate the impact of cooperative relaying on uplink multi-user (MU) wireless video transmission. We formulate the problem as an MU Markov decision process (MDP) that explicitly considers the cooperation at the physical layer and the medium access control sublayer, the video users' heterogeneous traffic characteristics, and the dynamically varying network conditions. Although MDPs notoriously suffer from the curse of dimensionality, our study shows that the complexity of the MU-MDP can be mitigated. Our simulation results show that cooperation allows users with feeble direct signals to achieve improvements in video quality on the order of 5-10 dB peak signal-to-noise ratio, with less than 0.8 dB quality loss by users with strong direct signals.

In this paper, the problem of synthesizing a blind channel shortening algorithm for orthogonal frequency-division multiplexing (OFDM) systems is addressed. In particular, a commonly adopted assumption in the channel shortening framework is discussed, showing that, when it is violated, the data statistics usually needed for the synthesis of the shortening algorithms turn out to be periodically time varying (PTV) rather than time-invariant. Elaborating on this point, and considering in particular the recently proposed minimum-mean output-energy (MMOE) blind channel shortening algorithm, it is shown how its synthesis must be modified in order to account for the PTV nature of the data statistics. Numerical results assessing the performance of the proposed blind PTV-MMOE channel shortening algorithm are reported.

Randomized space-time block coding (RSTBC) is a decentralized cooperative technique that ensures diversity gains through the recruitment of multiple uncoordinated relays, with virtually no signaling overhead. In this paper, RSTBC is applied to two-way relaying wireless networks which, when two terminals want to send a message to each other, can potentially improve the network throughput by allowing them to exchange data over two or three time slots via bidirectional relay communications. Specifically, two decode-and-forward relaying strategies are considered which take up only two time slots. In the first slot the two sources transmit simultaneously. In the former scheme which we refer to as decode and forward both (DFB) RSTBC, only relays which can reliably decode both source blocks via joint maximum likelihood decoding cooperate, and do so by modulating the bit-level XOR of the decoded data through a single RSTBC. In the latter scheme called decode and forward any (DFA) RSTBC, the relays cooperate in the second slot also when they can decode only one of the two source data. In this case each source data that is decoded is mapped into an independent RSTBC. If the relay decoded reliably both sources, after cancellation of the strong interference, then it sends the two RSTBCs encoding the symbol vectors from each of the sources. A randomized forwarding scheme is also proposed for three-time-slot relaying, which is also a DFA strategy, although without joint decoding or interference cancellation after the first slot. The diversity orders achievable through the three proposed schemes are calculated and the obtained theoretical results are validated by means of Monte Carlo numerical simulations.

The problem of channel shortening equalization (CSE) for multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems is addressed. Relying on constrained minimization of the mean-output-energy (MOE) at the CSE output, we propose a blind channel shortening technique, which does not require a priori knowledge or estimation of the MIMO channel impulse responses to be shortened. Numerical results showing the attainable performance gain of the proposed method are reported.

Recently, much progress has been made on the cross-layer optimization of video streams in multiple access networks. The key idea is to use the granular data structure of compressed video to trade quality with bits, and optimally prioritize transmissions given the available bandwidth, in order to obtain proportionally optimal video quality across video streams. Herein, we discuss the effect and potential benefit of using a cooperative-relay strategy in a wireless network, where proportionally optimal video schedules are computed via a multi-user Markov decision process. The idea is that feeble signals of nodes that are located far away from the destination can be enhanced via the cooperation of intermediate nodes, acting as cooperative relays. Our contribution is to indicate a possible solution that would require relatively modest changes to the multi-user optimization framework, while warranting a uniformly better experience to the video users thanks to cooperative coding.

Recently, we proposed a successive narrowband interference (NBI) cancellation method for orthogonal frequency-division multiplexing (OFDM) systems, which empowers the conventional one-tap frequency-domain equalizer to gain in robustness against NBI. In this letter, we generalize such an approach by using soft (instead of hard) feedback from the decoding unit, when the digitally-modulated NBI is (possibly) improper. It is shown that, with respect to our previous work, the devised equalizers allow one to achieve a significant performance gain, with a moderate increase in computational complexity.

Constrained optimization techniques, based on the maximum signal-to-noise-plus-interference (SINR) criterion,, are considered for joint equalization and narrowband interference (NBI) suppression in orthogonal frequency-division multiplexing (OFDM) systems. Specifically, we show that a recently proposed linear receiver (D. Darsena et al., 2005), which mitigates, in the minimum mean-output-energy sense, the NBI contribution at the receiver output, can be regarded as the solution of a constrained maximum-SINR optimization criterion and, moreover, admits an interesting three-stage decomposition. This decomposition improves the receiver robustness against errors in the estimated statistics of the received data and, most important, allows one to greatly simplify the channel estimation problem. Computer simulations are earned out to illustrate the performance improvements achievable by adopting the constrained maximum-SINR equalizer, in comparison with its unconstrained counterpart