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.

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.

Ambient backscatter (AmBC) communications cap-italize on ambient radio-frequency (RF) signals to enable communications among ultra-low-power devices, thus representing a promising cost-effective solution for wireless sensor networks in the Internet of Things. In this paper, we study the scenario where single-antenna AmBC sensors are employed to perform decision fusion over multiple-access fading channels. Specifically, AmBC sensors detect the presence/absence of a phenomenon of interest and transmit their decisions to a multiple-antenna fusion center reader (FCR), by reflecting part of an incident RF ambient signal. In this scenario, we derive fusion rules at the FCR by considering both the cases of instantaneous and statistical channel state information, as well as their corresponding low-complexity alternatives. Numerical simulation results are provided to compare the proposed fusion rules and highlight the relevant trends.

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.

In recent studies (Verde et al .), capitalising on randomised space–time block coding (STBC), a decentralised coding method has been developed for amplify‐and‐forward (A&F) relays, which can closely achieve the performance of a centralised A&F linear dispersion coding (LDC) scheme and notebly outperform the decentralised A&F LDC counterpart, by requiring a reduced amount of signalling and processing overhead. However, A&F relaying process introduces correlation among the noise samples at the destination which, along with the non‐Gaussian nature of the relaying channel and the fact that the relays are located in different positions, significantly complicates the performance analysis of the system. In this study, a theoretical performance analysis of the randomised STBC A&F rule is carried out in terms of average pairwise error probability to evaluate its attainable diversity order. Numerical results are provided to corroborate the analytical findings.

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.

Relay-assisted cooperative wireless communication has been shown to have significant performance gains over the legacy direct transmission scheme. Compared with single relay based cooperation schemes, utilizing multiple relays further improves the reliability and rate of transmissions. Distributed space-time coding (DSTC), as one of the schemes to utilize multiple relays, requires tight coordination between relays and does not perform well in a distributed environment with mobility. In this paper, a cooperative medium access control (MAC) layer protocol, called STiCMAC, is designed to allow multiple relays to transmit at the same time in an IEEE 802.11 network. The transmission is based on a novel DSTC scheme called randomized distributed space-time coding (R-DSTC), which requires minimum coordination. Unlike conventional cooperation schemes that pick nodes with good links, STiCMAC picks a transmission mode that could most improve the end-to-end data rate. Any station that correctly receives from the source can act as a relay and participate in forwarding. The MAC protocol is implemented in a fully decentralized manner and is able to opportunistically recruit relays on the fly, thus making it robust to channel variations and user mobility. Simulation results show that the network capacity and delay performance are greatly improved, especially in a mobile environment.

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.

Cooperation diversity schemes employing space-time block coding (STBC) techniques have been proposed for wireless networks to increase network capacity and coverage even when each node is equipped with a single antenna. Such schemes allow several relay stations distributed in space to assist the transmission between a given source-destination pair. A key design problem in cooperative networks is to take advantage from spatial diversity by reducing the amount of signaling and processing overhead as far as possible. In this paper, capitalizing on randomized STBC (RSTBC), a coding method which has been recently developed for decode-and-forward (D&F) relay nodes, a totally decentralized cooperative communication scheme is proposed for amplify-and-forward (A&F) relays, where each relay is unaware of both the effective STBC being employed by the other nodes and the number of cooperating stations. Numerical results are provided to highlight the effectiveness of the proposed scheme in comparison to its D&F counterpart.

Physical (PHY) layer cooperation in a wireless network allows neighboring nodes to share their communication resources in order to create a virtual antenna array by means of distributed transmission and signal processing. A novel medium access control (MAC) protocol, called CoopMAC, has been recently proposed to integrate cooperation at the PHY layer with the MAC sublayer, thereby achieving substantial throughput and delay performance improvements. CoopMAC capitalizes on the broadcast nature of the wireless channel and rate adaptation, recruiting a single relay on the fly to support the communication of a particular source-destination pair. In this paper, we propose a cross-layer rate-adaptive design that opportunistically combines the recruitment of multiple cooperative nodes and carrier sensing multiple access with collision avoidance. We focus on a single-source single-destination setup, and develop a randomized cooperative framework, which is referred to as randomized CoopMAC (RCoopMAC). Thanks to the randomization of the coding rule, the RCoopMAC approach enables the blind participation of multiple relays at unison relying only on the mean channel state information (CSI) of the potential cooperating nodes, without introducing additional signaling overhead to coordinate the relaying process. The proposed RCoopMAC scheme is not only beneficial in substantially improving the link quality and therefore the sustainable data rates but, thanks to the decentralized and agnostic coding rule, it also allows to effectively recruit multiple relays in a robust fashion, i.e., even when the required mean CSI is partially outdated.

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.

In this letter, the constant-modulus (CM) cost function is analyzed under the general assumptions that improper modulation schemes of practical interest are employed and the baseband equivalent of the channel impulse response is complex-valued. This study allows one to determine a broad family of undesired minima of the CM cost function, which do not lead to perfect symbol recovery in the absence of noise. The results developed herein generalize and subsume as a particular case existing studies of the CM cost function, which exclusively consider real-valued binary modulations.

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

We consider in this letter the problem of perfectly equalizing finite-impulse response (FIR) channels in digital communication systems employing nonredundant precoding at the transmitter. In this case, as it is well known, there is no linear FIR zero-forcing (ZF) block equalizer that is capable of perfectly recovering information symbols in the absence of noise. We show that, when the symbol constellation is improper, it is possible to design widely linear FIR-ZF block equalizers that, unlike their linear counterparts, process both the received signal and its complex-conjugate version. In addition, we propose a simple procedure to synthesize universal modulation-induced cyclostationary (MIC) precoders that assures perfect symbol recovery, regardless of the channel-zero locations.

A subspace-based linear minimum mean-square error (MMSE) multiuser detector is proposed for quasi-synchronous multicarrier code-division multiple-access (MC-CDMA) systems. The proposed receiver is totally blind, i.e., it is both channel- and delay-independent and, unlike many subspace-based algorithms, requires only an upper bound (rather than the exact knowledge) of the channel order, without needing explicit delay estimation. The effectiveness of the proposed blind approach is tested by computer simulation results, which show that it assures satisfactory performances in severe near-far scenarios, while exhibiting a lower computational complexity in comparison with existing subspace-based methods.

The paper deals with the problem of blind mitigation of intersymbol interference (ISI) as well as multiple-access interference (MAI) in asynchronous high data-rate direct-sequence code-division multiple-access systems. A blind adaptive multiuser receiver based on the constant-modulus algorithm (CMA) is proposed, which demodulates each desired user by exploiting only the knowledge of its spreading code, without requiring estimation of the users's channels and timings. In order to overcome the CMA interference capture problem, which arises in a multiuser scenario, a two-stage adaptive receiver is adopted: In the first stage, partial MAI and ISI suppression is blindly achieved by exploiting the desired user signature structure properties; in the second stage, the residual MAI and the ISI are removed by using the CMA, and the information symbols of the desired user are reliably recovered. Theoretical analysis and simulation results show that the first stage is an effective blind adaptive strategy which allows the CMA detector in the second stage to lock on the desired-user symbol, at a particular delay. The proposed blind receiver achieves a significant performance gain in comparison with existing blind methods.

We address the problem of synthesizing blind channel identification and equalization methods for digital communications systems, aimed at counteracting the presence of cochannel or adjacent-channel interference. Owing to the presence of the interfering signal, the minimum mean-square error equalizer turns out to be linear periodically time-varying, which is implemented by resorting to its Fourier series representation. Moreover, by exploiting the cyclic conjugate second-order statistics of the channel output, we propose a new weighted subspace-based channel identification method, which is asymptotically immune to the presence of high-level interference. Computer simulation results confirm the effectiveness of the proposed identification/equalization technique.

The problem of jointly equalizing a digital communication signal distorted by a linear time-invariant channel and rejecting co-channel or adjacent-channel digital interference is tackled. Owing to the presence of the interfering signal, the proposed optimum linear MMSE equalizer turns out to be periodically time-varying (LPTV). Moreover, new simple and effective blind channel identification procedures are presented, which can be applied as long as the desired and interfering signal exhibit different circularity and/or cyclo-stationarity properties. Simulation results confirm the effectiveness of the proposed techniques.