As pretty much every reader of this foreword knows, the IEEE International Conference on Communications (ICC) is the worldwide largest and most relevantconference in the field of communication technologies at large - together with GLOBECOM, it is the “flagship” event of the IEEE Communications Society. ICC has come now to its 58th annual edition since its inception in 1966, and for the first time ever it comes to Italy, heralding the activity of a large and well-structured community working in the diverse fields of communication technologies in Academia, in research Institutions, in companies. ICC 2023 is held in a very special Italian city, one that holds a particular place in worldwide history, and for sure a special place in the organizers' heart: Roma.

The widespread adoption of Cooperative, Connected, and Automated Mobility (CCAM) applications requires the implementation of stringent security mechanisms to minimize the surface of cyber attacks. Authentication is an effective process for validating user identity in vehicular networks. However, authentication alone is not enough to prevent dangerous attack situations. Existing security mechanisms are not able to promptly revoke the credentials of misbehaving vehicles, thus tolerate malicious actors to remain trusted in the system for a long time. The resulting vulnerability window allows the implementation of complex attacks, thus posing a substantial impairment to the security of the vehicular ecosystem. In this paper we propose a Distributed Ledger-based Vehicular Revocation Scheme that improves the state of the art by providing a vulnerability window lower than 1 s, reducing well-behaved vehicles exposure to sophisticated and potentially dangerous attacks. The proposed scheme harnesses the advantages of the underlying Distributed Ledger Technology (DLT) to implement a privacy-aware revocation process while being fully transparent to all participating entities. Furthermore, it meets the critical message processing times defined by EU and US standards, thus closing a critical gap in the current international standards. Theoretical analysis and experimental validation demonstrate the effectiveness and efficiency of the proposed scheme, where DLT streamlines the revocation operation overhead and delivers an economically viable yet scalable solution against cyber attacks on vehicular systems.

Connected and autonomous vehicles run their control algorithms in dedicated on-board computing platforms, which will become obsolete long before the end of the life cycle of the vehicles, severely limiting the evolution of their control software and the deployment of cooperative vehicular applications. A promising solution for this problem is to delegate the most demanding computational tasks to the edge nodes the of Vehicular Ad-hoc Networks, leveraging on the Vehicular Edge Computing paradigm. This requires both low-latency, high-bandwidth communications and secure computing offloading. In this paper, we propose an architecture that ensures supporting secure computation offloading, using the IOTA-VPKI security scheme, without additional delay overhead. Furthermore, to demonstrate the applicability of the proposed scheme to a real case, we measured the time required for the execution of a maneuver plan supervised by an application, the Maneuver Control (MC), located on an edge node. Experimental results show that the described scheme is a very promising solution.

The sensing capabilities of smart vehicles are growing and enabling unprecedented functionalities that drive the future commercial adoption of autonomous vehicles. However, the real-time requirements of mission-critical vehicular applications are revealing the current limitation of Vehicular Ad-hoc Networks (VANETs). In this scenario, computing paradigms like Vehicular Edge Computing (VEC) are promising solutions to match low-latency requirements. However, there still exist security challenges that need to be addressed to assure safe and secure autonomous driving deployment. We propose in this paper a secure architecture based on the VEC cloud paradigm that enables the deployment of real-time and mission-critical autonomous driving applications at the edge of the network. We leverage the IOTA-VPKI vehicular security scheme to support the security lifecycle of vehicular applications. Furthermore, we describe a Cooperative Autonomous Driving Maneuver Control application to discuss the effectiveness of the proposed architecture in the cooperative lane change use case. We plan to extensively test this architecture in real environments located in Italy and Portugal and to contribute to the future successful deployment of autonomous vehicles.

Chemical algorithms are statistical control algorithms described and represented as chemical reaction networks. They are analytically tractable, they reinforce a deterministic state-to-dynamics relation, they have configurable stability properties, and they are directly implemented in state space using a high-level visual representation. These properties make them attractive solutions for traffic shaping and generally the control of dynamics in computer networks. In this paper, we present a framework for deploying chemical algorithms on field programmable gate arrays. Besides substantial computational acceleration, we introduce a low-overhead approach for hardware-level programmability and re-configurability of these algorithms at runtime, and without service interruption. We believe that this is a promising approach for expanding the control-plane programmability of software defined networks (SDN), to enable programmable network dynamics. To this end, the simple high-level abstractions of chemical algorithms offer an ideal northbound interface to the hardware, aligned with other programming primitives of SDN (e.g., flow rules).

In this paper, we investigate the power control problem in a cooperative network with multiple wireless transmitters, multiple amplify-and-forward relays, and one destination. The relay communication can be either full duplex or half-duplex, and all source nodes interfere with each other at every intermediate relay node, and all active nodes (transmitters and relay nodes) interfere with each other at the base station. A game-theory-based power control algorithm is devised to allocate the powers among all active nodes. The source nodes aim at maximizing their energy efficiency (in bits per Joule per Hertz), whereas the relays aim at maximizing the network sum rate. We show that the proposed game admits multiple pure/mixed-strategy Nash equilibrium points. A Q-learning-based algorithm is then formulated to let the active players converge to the best Nash equilibrium point that combines good performance in terms of both energy efficiency and overall data rate. Numerical results show that the full-duplex scheme outperforms half-duplex configuration, Nash bargaining solution, the max-min fairness, and the max-rate optimization schemes in terms of energy efficiency, and outperforms the half-duplex mode, Nash bargaining system, and the max-min fairness scheme in terms of network sum rate.

In wireless sensor and actor networks (WSANs), nodes collaborate to accomplish distributed sensing and computation. One of the most attractive features is that global macro- behaviors in the network emerge from local micro-actions in and interactions between nodes. In this paper, we review a recently proposed approach, which bases on a chemical metaphor and its related theory, to design, analyze, and implement systems with such an emerging property. As an example of distributed, emerging computation in WSNs, we extend an existing gossip-like chemistry-inspired protocol (the “Disperser”) and propose a new chemically driven solution to consensus problems that is applicable in WSNs. We analyze it with a signal processing approach and validate results through simulations. The proposed chemical algorithm is not based on the iterative packet-exchange but rather on a continuous rate-modulation. The algorithm is thought to lie at the physical layer and to directly control the basic transmission hardware of the nodes in a sort of protocol-less operation mode.

In this work, a distributed and iterative algorithm for uplink power control in the initial contention-based synchronization procedure of orthogonal frequency-division multiple access networks is derived. This is achieved by letting the mobile terminals maximize their own energy efficiency in terms of power consumption and average synchronization time exploiting a quantized feedback from the base station. The problem is formulated as a constrained finite noncooperative game in which the transmit powers are chosen from a discrete set. The theoretical solution is investigated and compared to the case of continuous powers. In addition, comparisons with existing alternatives (with or without perfect feedback) are made in terms of power expenditure, average synchronization time, and estimation accuracy.

Traffic-shaping algorithms, such as congestion avoidance or rate limiting for cloud services, control the dynamics of packet flows in a distributed setting. However, despite their significance in networking, it is still hard to design them, to predict their dynamic behavior, and to prove their robustness. With the help of a simple chemical metaphor, we recently proposed a novel engineering framework for designing, executing, and analyzing traffic-shaping algorithms: by letting packets react akin to chemical molecules, the design of (distributed) flow-control algorithms becomes drawing reaction networks. In this way, we gain in analyzability: the related fluid model, describing the emergent behavior of the overall system, can be derived from the corresponding reaction networks automatically. In this article, we describe how to fine-tune chemical algorithms. We therefore analyze their transient behavior in order to reveal the semantics and sensitivity of key parameters. Methods from different domains assist us in this endeavor: We first linearize the fluid model as proposed in Metabolic Control Analysis, we then describe it in control-theoretic terms, and finally, we characterize its sensitivity in the frequency domain. We demonstrate the feasibility and reveal the limits of our method by applying it to a chemical algorithm for distributed rate control.

Signal Time-Of-Arrival (TOA) estimation accuracy is fundamental to the functioning of Global Navigation Satellite Systems (GNSSs). This work investigates a variant of the Ziv-Zakai bound (ZZB) named modified ZZB (MZZB) as a theoretical performance limit in TOA estimation, to overcome the heavy computational effort caused by the presence of nuisance parameters (carrier amplitude/phase, channel coefficients). (M)ZZB is adopted to analyze the theoretical performance of signal delay estimators in the different phases of acquisition and tracking, and numerical results are shown for the main GNSS standard signal formats: BPSK and (filtered) Binary Offset Carriers (BOC) modulations in Additive White Gaussian Noise (AWGN) channel.

A challenging problem when engineering network and communication protocols is to predict and formally describe the dynamic behavior of a protocol. Chemical Networking Protocols (CNPs) offer the opportunity to transpose the good and well-understood analyzability of chemical reactions to networking protocols, not only at the design stage of protocols, but also for the study of their dynamic behavior. In this paper we focus on this latter aspect and we demonstrate how signal processing techniques from classical signal and control theory can be employed in the study of the average flow properties of CNPs. Our analysis methods include a model linearization as proposed in Metabolic Control Analysis, a statespace description classically used in control theory, and the system's characterization in the frequency domain, which is central to signal theory. We demonstrate the feasibility of our method by applying it to an actual congestion control protocol that models TCP's behavior. With our contribution, CNP designers can easily determine the key parameters of their protocols and understand how to calibrate them in order to obtain the desired behavior.

This paper introduces a game-theoretic formulation for the problem of resource allocation in an infrastructure CDMA wireless communication network, focusing on the issue of initial code synchronization. To capture the tradeoff between obtaining good synchronization performance and saving as much energy as possible, we propose a noncooperative game in which each transmitter-receiver pair seeks to maximize the ratio of the probability of code alignment detection to the transmitted energy per acquisition, under a constraint on the maximum probability of spurious code locks - this is achieved by properly setting the receiver threshold and the transmit power. Our framework also considers the coexistence between different types of synchronizers and different QoS requirements. The generalized Nash solution of the game is investigated and closed-form expressions are derived and compared with simulation results for an iterative decentralized algorithm.

This paper proposes a game-theoretic approach to the problem of resource allocation for the uplink of a CDMA wireless communication network, in which terminals code-locked to the access point (in-sync users) coexist with some terminals still in the code acquisition stage (out-of-sync users). To capture the tradeoff between obtaining good performance and saving as much energy as possible for both classes, we propose a noncooperative game with strict incomplete information based on the energy-efficient criterion. This formulation allows us to devise an iterative algorithm that enables each terminal to locally regulate its optimal transmit power while ignoring all others' synchronization phases. A numerical example is reported to show the improvements provided by the proposed approach.

This paper investigates the performance of distributed power control techniques in the uplink of a flat-fading CDMA wireless network, based on noncooperative game theory. Two energy-efficient criteria, tailored for data detection and code synchronization, are compared in terms of achieved utility at the Nash equilibrium, showing that the optimal power allocation devised for data detection is not the best solution during synchronization, and analogously the optimal allocation for code acquisition does not provide the best performance during data detection. Closed-form expressions for the optimal powers at the Nash equilibrium are derived, and a unified formulation is proposed to allow a more elaborated power allocation method to be adopted, in which the terminals, using the synchronization-oriented approach, can revert to the data-detection optimal criterion after synchronization is over.

Since J. Mitola's seminal work in the 90's, Software Defined Radios (SDRs) have been a hot topic in wireless communication research. Though many notable achievements were reported in the field, the scarcity of computational power on general purpose CPUs has always been a limiting factor. If conveniently applied within an SDR context, classical concepts known in computer science as space/time trade-offs can prove helpful when trying to mitigate this problem. This paper presents a novel criterion to design signal- processing software in an SDR terminal that we call Memory Acceleration (MA). The key feature of MA is making extensive use of memory resources of the SDR platform in order to accelerate the most critical signal processing functions. MA provides substantial acceleration factors when applied to conventional SDRs without reducing their peculiar flexibility and appears particularly suited to the implementation of a fully-SW SDR on a general- purpose processor (GPP). As a case study for the application of MA, results about the implementation of the ETSI DVB-T Viterbi decoder are presented. In such a case, MA provides an acceleration factor of 10.4x with respect to a standard, purely-computational implementation while having no impact on error correction performance of the decoder and by making no use of any other typical performance enhancement technique (e.g. low level programming or parallel computation).

This paper proposes a game-theoretic approach to the problem of resource allocation for a CDMA wireless communication network, focusing on code synchronization. To capture the tradeoff between obtaining good synchronization performance and saving as much energy as possible, we propose a noncooperative game in which each pair transmitter-receiver in the network seeks to maximize the ratio of the probability of detection to the transmitted energy per bit by setting its transmit power and its optimum detection threshold, under a constraint on the maximum probability of spurious code locks. This framework also considers the coexistence between different types of synchronizers in the network and different QoS requirements. The Nash solution of the game is investigated and closed-form expressions are derived and compared with simulation results for a decentralized resource control algorithm.

The Cramer-Rao bound (CRB) with its modifications is a well known lower bound on the variance of any unbiased estimator of an unknown parameter. Focusing on the estimation of the time-delay experienced by a digitally modulated signal, we first introduce an approximation of the modified Cramer-Rao bound (MCRB) as a function of the spectral properties of the modulated signal, irrespective of the specific format of the digital modulation. In particular, we show the equivalence of the MCRB for single carrier modulation and for multicarrier modulation. As a corollary, it is shown that multicarrier signals with uneven power distribution and/or non-contiguous frequency allocation bear a smaller MCRB than equally spaced and powered modulations.

A new technique to acquire pseudo-noise (PN) sequences has been recently proposed in [1] and [2]. It is based on the paradigm of iterative message passing (iMP) to be run on loopy graph. This technique approximates the maximum-likelihood (ML) estimator, providing a sub-optimal algorithm that searches all possible code phases in parallel, at low complexity and fast acquisition time. This work is addressed to the design of the architecture of an iMP detector, following the implementation methodologies typical of standard low-density parity-check (LDPC) decoders, and demonstrates its benefits in terms of acquisition time and complexity, compared with standard techniques.

This paper deals with the computation of the Cramer-Rao lower bound (CRB) for direction of arrival (DOA) estimation of ultra wideband-orthogonal frequency division multiplexing (UWB-OFDM) signals detected by antenna arrays for millimetre-wave applications. The CRB on DOA estimation of UWB-OFDM sources is evaluated by extending the mathematical model of DOA estimation of narrow band signals to wideband orthogonal multicarrier signals. Specific results are presented for the case of a uniform linear array (ULA) and line-of-sight (LOS) scenario. After a comparison of our results for OFDM with the case of wideband single carrier signals, we use the CRB for DOA to substantiate the design of medium access controllers protocols that make use of DOA information.

This paper studies the performance of partial-Rake (PRake) receivers in impulse-radio ultrawideband wireless networks when an energy-efficient power control scheme is adopted. Due to the large bandwidth of the system, the multipath channel is assumed to be frequency-selective. By making use of noncooperative game-theoretic models and large-system analysis tools, explicit expressions are derived in terms of network parameters to measure the effects of self-interference and multiple-access interference at a receiving access point. Performance of the PRake receivers is thus compared in terms of achieved utilities and loss to that of the all-Rake receiver. Simulation results are provided to validate the analysis.

Due to relative motion between the transmitter and the receiver, the carrier of the received signal in digital burst- mode transmission can be affected by Doppler distortion. This contribution deals with the problem of optimizing the burst format of packet transmission to perform enhanced-accuracy joint estimation of carrier Doppler-shift and Doppler-rate. Optimal pilot symbol distributions that minimize the Doppler- shift and the Doppler-rate Cramer-Rao bounds (CRBs) for the joint estimation of carrier phase/Doppler-shift and of Doppler-rate are obtained, and a new data aided (DA) estimation algorithm suitable for each distribution is first derived and then evaluated by analysis and by simulation, exhibiting that such low-complex estimation schemes reach their relevant CRBs even at very low values of signal-to-noise ratio.

This paper studies the impact of phased array antennas directivity onto the throughput performance of WPANs adopting the IEEE 802.15.3 MAC standard in the 60 GHz band. High gain directional transmission is desirable in the 60 GHz band to overcome the high signal attenuation typical of these frequencies and improve the wireless channel reliability in order to achieve data rates up to 5 Gbps. In our antenna system model, directional transmission at both transmitter and receiver side is accomplished through a simplified scheme of direction of arrival estimation and beamsteering. We have implemented the antenna system and the antenna control protocol within 802.15.3 MAC in ns2. Our simulation analysis shows that the accuracy of the antenna pointing direction must increase with the antenna directivity otherwise significant throughput degradation is experienced.

The introduction of turbo and low-density parity-check (LDPC) codes with iterative decoding that almost attain Shannon capacity challenges the synchronization subsystems of a data modem. Fast and accurate signal synchronization has to be performed at a much lower value of signal-to-noise ratio (SNR) than in previous less efficiently coded systems. The solution to this issue is developing specific synchronization techniques that take advantage of the presence of the channel code and of the iterative nature of decoding: the so-calledturbo-synchronizationalgorithms. The aim of this paper within this special issue devoted to the turbo principle is twofold: on the one hand, it shows how the many turbo-synchronization algorithms that have already appeared in the literature can be cast into a simple and rigorous theoretical framework. On the other hand, it shows the application of such techniques in a few simple cases, and evaluates improvement that can be obtained from them, especially in the low-SNR regime.

Digital burst-mode transmission is ordinary in a few wireless communications systems such as satellite time-division multiple access (TDMA) and terrestrial mobile cellular radio. In those situations the received carrier may be affected by non-negligible Doppler shift and Doppler rate due to relative motion between transmitter and receiver. This contribution is concerned with the introduction of new enhanced-accuracy and low-complexity carrier Doppler shift and Doppler rate estimators which exploit the presence of (known) pilot symbols in the burst. In particular, we consider a special burst format encompassing two groups of consecutive pilot symbols placed at the beginning (preamble) and at the end (postamble) of each packet. The performance of the proposed estimation schemes is assessed by analysis and by simulation, and is compared to the relevant Cramer-Rao bounds. The particular pilot arrangement is shown to provide enhanced estimation accuracy at low signal to noise ratio as compared to conventional (preamble-only) estimation.

In I/Q receivers characterized by direct conversion or low-IF operations, phase imbalance between the I and Q branches stands for a substantial issue which may degrade significantly the overall link performance. An alternative solution to high cost analog components with small tolerances which make the I/Q imbalance effect negligible is to estimate and compensate it digitally. In this paper we present a novel data aided (DA) joint estimator of the carrier phase offset and the I/Q phase imbalance derived following the maximum likelihood (ML) criterion. The performance accuracy analysis proves that the proposed estimator is efficient and achieves its vector Cramer-Rao lower bound (VCRLB). Simulation results in terms of BER performance of uncoded and turbo coded systems show that the proposed imbalance compensation method, at the cost of a low additional implementation complexity of the digital receiver, allows negligible performance degradation in the context of multi-level modulation formats, such as 16-QAM and 64-QAM.

Filterbank multicarrier modulation (FBMCM) is an attractive technology for high-speed twisted-pair transmission and for broadband wireless communications, as well. In wireline applications, signal transmission takes place at baseband, so the issue of carrier acquisition and tracking for coherent demodulation does not apply. In wireless communications, on the contrary, carrier-frequency recovery reveals the Achille's heel of multicarrier modulation, so that robust signal processing algorithms are needed in this respect. In this paper, we derive a nondata-aided carrier-frequency offset recovery method for wireless FBMCM modems. In particular, we illustrate how to derive a low-complexity closed-loop tracker starting from a maximum-likelihood approach. We then show that the proposed simplifications do not entail large performance losses. In this respect, we derive the standard performance metrics of a closed-loop tracker (S-curve, root mean square estimation error, acquisition time) both on the additive white Gaussian noise channel and on a typical static frequency-selective wireless channel. We also demonstrate by simulation good robustness of the frequency tracker with respect to FBMCM symbol-timing errors.

Filter bank (FB) multicarrier modulation exhibits several properties which make it an attractive approach not only for high speed copper wire communications networks but also for broadband wireless applications. In this paper, we derive a nondata aided carrier frequency offset recovery algorithm for wireless FB-based systems. The estimator is based on a maximum likelihood approach and has a feedback loop structure. A number of proper simplifications make the problem tractable and allow to reduce the computational complexity. The algorithm accuracy is examined by computer simulation for AWGN and frequency selective multipath channel showing an considerable robustness against timing errors.

In this paper, we introduce a low-complexity carrier phase estimation algorithm to be integrated into the data decoder of a turbo-coded modem employing a linear modulation. The estimator is based on a pseudo-maximum-likelihood approach and makes iterative use of soft decisions provided by the soft-in/soft-out decoders within the overall turbo-decoding scheme. In doing so, iterative decoding and carrier phase recovery go together iteration after iteration in a "soft decision-directed" mode. This allows performing reliable blind phase estimation and almost ideal coherent detection for values of the signal-to-noise ratio down to a few decibels only, and without the need to resort to narrowband phase-locked loops with large acquisition time. Performance in terms of mean estimated value, root mean-squared estimation error, and overall decoder bit-error rate as derived by simulation are also reported.

Many frequency-recovery algorithms for orthogonal frequency-division multiplexing systems are geared to recover offsets up to half the distance of the subcarriers. If the error is larger, an ambiguity of an integer number of subcarrier spacings is left. This paper develops a blind algorithm for ambiguity resolution that works effectively over frequency-selective radio channels. Simulation results show a sizable improvement over existing techniques, expecially at high signal-to-noise ratios.

In this letter, we show how different signal processing techniques can be combined to optimize the performance of a typical wideband nonlinear satellite link with spectrally efficient high-level modulation techniques. In particular, we follow two concurrent approaches: on one side, we pursue signal optimization in the form of special amplitude and phase shift keying (APSK) constellations to reduce the effect of nonlinear distortions, while on the other, we analyze the feasibility of adaptive data predistortion (DP) at the transmitter and adaptive nonlinear equalization (NLE) at the receiver. We demonstrate that the use of such optimized constellations relieves the complexity of the nonlinearity compensation techniques, and we also show that by clever adoption of these techniques the sensitivity to nonlinear distortion of both uncoded and turbo coded quadrature amplitude modulation and APSK constellations is greatly reduced.

This paper is devoted to turbo synchronization, that is to say the use of soft information to estimate parameters like carrier phase, frequency offset or timing within a turbo receiver. It is shown how maximum-likelihood estimation of those synchronization parameters can be implemented by means of the iterative expectation-maximization (EM) algorithm [A.P. Dempster, et al., 1977]. Then we show that the EM algorithm iterations can be combined with those of a turbo receiver. This leads to a general theoretical framework for turbo synchronization. The soft decision-directed ad-hoc algorithm proposed in V. Lottici and M. Luise, [2002] for carrier phase recovery turns out to be a particular instance of this implementation. The proposed mathematical framework is illustrated by simulations reported for the particular case of carrier phase estimation combined with iterative demodulation and decoding [S. ten Brink, et al., 1998].

In this paper, we present a low-complexity carrier phase estimation algorithm suited for turbo-coded 16-QAM modems. The estimator is based on a maximum likelihood (ML) approach, and unlike previously published work, makes iterative use of soft decisions provided by the soft-in soft-out decoders within the overall iterative turbo decoding scheme, yielding negligible degradation with respect to ideal phase synchronization. Performance in terms of mean estimated value, mean-squared estimation error, and overall decoder bit error rate (BER) as derived by simulation are also reported.

This paper introduces a blind (i.e., data independent) algorithm for carrier frequency offset recovery in an orthogonal frequency-division multiplexing (OFDM) receiver operating over frequency-selective fading channels. The main idea behind this algorithm is to exploit the time-frequency-domain exchange inherent to the modulation scheme. Due to this feature, a carrier frequency offset has a similar impact on OFDM as a clock timing offset has in a quadrature amplitude modulation (QAM) system. The scheme we propose is a variant of Oerder-Meyr's (1988) feedforward clock recovery. Its performance is assessed by simulation, and the results are compared to those obtained from Van de Beek-Sandell-Borjesson's (1997) frequency synchronizer, which bears comparable complexity. The new scheme is shown to outperform the latter over frequency-selective fading channels, notably at medium to high signal-to-noise ratios. We also evaluated the efficiency of two different (time domain and frequency domain) offset correction strategies embedded in a particular OFDM receiver.

This paper presents the design and the main performance results of a single-ASIC implementation of the recently proposed extended complex-valued blind anchored interference-mitigating detector (EC-BAID) for code division multiple access (CDMA) transmission. Such a detector, which exhibits a remarkable robustness to multiple access interference, operates in blind mode, i.e., it only requires knowledge of the timing of the wanted user's signature code, and it is therefore very well-suited for integration into handheld single-user terminal demodulators. The implementation of the interference-mitigating detector is based on a patented optimized architecture which leads, in 0.25-/spl mu/m CMOS technology, to a roughly 25 Kgate plus 23-Kbit RAM single-chip ASIC supporting chip rates up to 4 Mchip/s with a maximum internal clock frequency of 32.768 MHz. The main design drivers are thoroughly discussed, and the relevant performance results are compared to the theoretical behavior. A possible extension to multirate CDMA systems adopting orthogonal variable spreading factor (OVSF) sequences is also addressed.

One key requirement for radio access in advanced, third generation (3G) mobile communication systems is the ability to accommodate a variety of services via a flexible and efficient medium access control (MAC) protocol. The paper presents and evaluates a new multiple access protocol termed CRDA (collision resolution and dynamic allocation), which has the potential to meet the above requirement. CRDA is basically a slotted packet-reservation multiple access technique with dedicated reservation slots, which allows the main shortcoming of previous PRMA schemes, i.e., contention, to be overcome through the integration in the MAC protocol of a code-division multiple-access (CDMA) transmission mode used to access the reservation slots. This prevents collisions during the reservation phase and enhances channel throughput, notably in the case of mixed voice/data traffic. Our simulations of the CRDA MAC integrate voice channels with data sources, generating what we call advanced data traffic, which has a very similar shape to the actual traffic generated by World Wide Web (WWW) applications. The standard assumption of Poissonian data traffic is also considered. Our conclusion is that the CRDA MAC protocol satisfactorily accommodates both types of traffic.

This paper deals with symbol timing recovery for multirate transmission systems employing wavelets as signaling pulses either at baseband or with linear bandpass modulation. Our analysis is twofold. First, we investigate the impact of clock errors on the bit-error rate of the optimum receiver for a wavelet-based multirate signal. Second, we consider a nondata-aided maximum-likelihood clock synchronization scheme implemented as a multiple delay-locked loop. We derive a linearized loop model and assess its performance in terms of tracking jitter variance. The analytical results are contrasted with those obtained by simulation and with the relevant modified Cramer-Rao bound.

This paper presents an enhanced version of the previously proposed blind anchored interference-mitigating detector (BAID) for code-division multiple-access transmission. Such a detector, named extended complex BAID (EC-BAID), features invariance to a phase error on the useful channel's carrier and reveals resistance to a large frequency shift (e.g., Doppler shift) on interfering carriers. The EC-BAID is also shown to bear a sensitivity to residual carrier frequency errors on the desired channel which is lower by three orders of magnitude with respect to the case of a data-aided minimum mean-square-error receiver. The performance of the EC-BAID is computed theoretically, and is validated by computer simulations under a variety of system configurations.

More and more interference-mitigating algorithms are being investigated in an attempt to increase the capacity of code-division multiple-access radio networks. At the moment, the main question about interference-resilient receivers is: do they really bring forth the capacity increase they promise on theoretical grounds? The aim of this study is to give a preliminary answer to such a question, through the capacity analysis of the forward link of a multisatellite, multibeam, radio communication network. Specifically, starting from simplified yet realistic assumptions on the beam layout, the antenna footprint shape, and the coding/multiplexing/modulation and power control strategies, it is shown that the blind, adaptive, interference-mitigation receiver, selected as an appealing representative of the lot, can indeed boost the system quality of service in terms of outage probability. The results are derived after a mix of theoretical analysis (as far as the detector performance evaluation is concerned) and simulation (to examine a number of different random system configurations) to circumvent the inherent complexity of the issue.

This paper is concerned with the estimation of the rate-of-change of the instantaneous frequency of an unmodulated carrier. Such an issue arises, for instance, in satellite communication systems based on a low-Earth-orbit constellation, wherein the speed of each satellite relative to earth is rapidly time-varying, and consequently, the received carrier is affected by nonnegligible Doppler shift and Doppler rate. First, we present simple yet efficient estimators of the Doppler rate derived from maximum-likelihood estimation theory. Next, we investigate their performance in terms of bias and estimation error variance. Finally, we compare our variance results with the relevant modified Cramer-Rao bound.

A novel detection algorithm for orthogonal frequency division multiplexing (OFDM) signals transmitted over frequency-selective channels is introduced and analyzed. Simulation results show that the proposed detection strategy, implemented via a standard Viterbi algorithm, provides improved performance over differential detection with a moderate increase in receiver complexity, and without requiring the periodic transmission of a training block.

The widespread application of direct-sequence spread-spectrum code division multiple access (DS/SS-CDMA) to wireless communication systems asks for ever faster and more reliable real-time signal processing operations to be performed by highly integrated and low-power consumption digital receivers. One of the most critical signal processing tasks to be performed by the DS/SS-CDMA receiver is signal presence detection and code epoch estimation. This paper deals with the design and realization of an application-specific integrated-circuit (ASIC) for fast signal recognition and code acquisition (SR/CA) in packet DS/SS-CDMA receivers operating in a satellite or terrestrial radio network. In particular, we show how a parallel acquisition circuit can be effectively implemented on a single-chip with a 1.0-/spl mu/m CMOS technology according to the specifications of the ARCANET Ku-band CDMA VSAT satellite network sponsored by the European Space Agency (ESA). It is shown that the ASIC performance closely follows analytical predictions.

A novel equalization/detection algorithm for orthogonal frequency division multiplexing (OFDM) signals transmitted over frequency-selective channels is introduced and investigated. The algorithm stems from the recognition that the Fourier transform processing inherent in OFDM turns a single wideband frequency-selective channel into a set of correlated narrowband frequency-flat fading channels. This suggests that sequence detection techniques, such as those discussed by Vitetta et al. (see IEEE Trans. Commun., vol.43, p.2750-8, 1995, IEEE Trans. Commun., vol.43, pt.II, p.1256-9, 1995, and Proc. IEEE Commun. Theory Mini-Conf (Globecom '96), London, UK, p.153-7, 1996), for time-selective flat-fading channels, can be also profitably utilized for joint equalization and decoding of OFDM signals in the frequency domain. Simulation results show that the proposed detection strategy, implemented via a standard Viterbi algorithm, provides improved performance over differential detection, with a moderate increase in receiver complexity and without requiring the periodic transmission of training blocks.

Code-division multiple-access (CDMA) with direct-sequence/spread-spectrum (DS/SS) modulated signals has emerged as a strong candidate for the air interface of the universal wireless personal communication network planned for the end of the century. In this paper, we take into consideration a low-complexity blind adaptive interference-mitigating detector (BAID) scheme that minimizes the detrimental effect of the multiple-access interference (MAI) on the bit-error-rate (BER) performance of a CDMA data demodulator. We describe a few modifications to the original algorithm that make it more suitable to a practical implementation with dual-binary phase-shift keying (D-BPSK)-modulated DS/SS signals. Specifically, we show how to make the BAID invariant to a possible carrier phase offset introduced in the front-end of the data demodulator, and we also suggest how to increase the robustness of such detector to asynchronous MAI. The uncoded BER performance of the resulting "extended" detector is then evaluated theoretically and by computer simulation in the case of coherent and differential signal detection.

The context of this paper is parameter estimation for linearly modulated digital data signals observed on a frequency-flat time-selective fading channel affected by additive white Gaussian noise. The aim is the derivation of Cramer-Rao lower bounds for the joint estimation of all those channel parameters that impact signal detection, namely, carrier phase, carrier frequency offset (Doppler shift), frequency rate of change (Doppler rate), signal amplitude, fading power, and Gaussian noise power. Time-selective frequency-flat fading is modeled as a low-pass autoregressive multiplicative distortion process. In particular, the important case of "slow" fading, with the multiplicative process remaining constant over the whole data burst, is specifically discussed. Asymptotic expressions of the bounds, valid for a large observed sample or for high signal-to-noise ratio (SNR), are also derived in closed form. A few charts with numerical results are finally reported to highlight the dependence of the bounds on channel status (SNR, fading bandwidth, etc.).

Interference-resilient receivers for CDMA transmissions promise a noticeable capacity increase on theoretical grounds. Do they really bring forth such performance boost in a realistic scenario? The aim of this study is to give a preliminary answer to such a question, through the capacity analysis of the forward link of a multi-satellite, multi-beam, radio communication network at S-band. Specifically, starting from simplified yet realistic assumptions on the beam layout, the antenna footprint shape, and the coding/multiplexing/modulation strategies, it is shown that the blind, adaptive, interference-mitigation receiver, selected as representative of the lot, can indeed boost the system quality of service in terms of outage probability. The results are derived after a mix of theoretical analysis (as far as the detector performance evaluation is concerned) and simulation (to examine a number of different random system configurations) to circumvent the inherent complexity of the issue.

We analyze a cellular asynchronous code-division multiple access system operating in the 63-64 GHz band for short-range communications. A simple propagation model for the 63-64 GHz band is first introduced and discussed. The system capacity, expressed as the maximum number of users per cell, is then analytically derived and favorably compared with other CDMA schemes operating in conventional frequency bands (i.e. below 2 GHz). Also, simple expressions for the system bit error rate are analytically derived for different cases and validated through computer simulations. Finally, power requirements are derived and the impact of atmospheric conditions is evaluated. It turns out that the use of 63-64 GHz band significantly increases the capacity of A-CDMA, and proves to be suitable for cellular systems and personal communication networks (PCNs) provided that adequate power margins are considered.

This paper is concerned with the design and performance evaluation of a direct-sequence spread-spectrum transmission and signal-detection technique for application to code-division multiple-access mobile radio networks. Unlike the conventional differential encoding/detection of data symbols, such a technique envisages differential encoding/detection of the spreading code chips to counteract the fast time-selective fading encountered in mobile radio transmissions. Binary PSK modulation on a Rician frequency-flat-fading channel is assumed, with Gaussian and Rayleigh channels as particular cases. The bit-error rate performance of the receiver is analytically evaluated as a function of the relevant system parameters, and the impact on the receiver performance of a carrier frequency offset is also investigated.

An ubiquitous network for multimedia personal communications (Personal Communications Network (PCN)) with small, individual low-cost terminals is one of the most ambitious worldwide projects for the 21st century that are being pursued nowadays. In the development of such PCN, Geostationary, Medium, and Low Earth Orbiting (GEO, MEO, LEO) satellite constellations will play a fundamental role to provide worldwide coverage for most services required by the end-user. The characteristics of efficiency and flexibility inherently required by that scenario suggest, amidst other possibilities, to take into special consideration a radio interface based on code division multiple access (CDMA) to ensure, in addition to the features mentioned above, a sufficient grade of power and spectral efficiency of the relevant satellite radio link. The aim of this paper is a review of the current status of those issues in the field of satellite CDMA transmission systems design that, in our opinion, appear fundamental to the successful operation of an efficient PCN. In particular, we survey the techniques for multiplexing, coding and transmission of direct-sequence spread spectrum (DS/SS) signals, and we touch upon the techniques for the minimization of the self-noise effect, and the related topics of power-control and multiuser detection. We also shortly address in this respect some technological aspects related to an efficient modem design via digital signal processing techniques. The final part of the paper deals more specifically with some typical issues of satellite transmission, namely the minimization of the detrimental effects of the nonlinear satellite transponder and of multipath propagation; the applicability of diversity reception to a multisatellite network is also addressed as possible means of performance boost.

This paper describes an effective technique for the optimization of the clock recovery circuit in an all-digital modem for linearly modulated signals. Starting from the concept of prefiltering of the data signal (already pursued by these authors in the context of analog data receivers), it is shown how to design an optimum digital prefilter for the minimization of jitter due to both Gaussian and pattern noise in the closed-loop clock recovery scheme by Gardner (1986). The numerical results of the theoretical analysis, obtained after iterative resolution of a constrained-minimum problem via the Lagrange multiplier method, are checked by simulation and can be nicely justified by the consideration of the frequency response of the optimum prefilter. The key outcome of such an approach is the demonstration of a substantial performance improvement in terms of steady-state clock jitter, even with remarkably simple FIR prefilters with a small number of taps.

We present and analyze a technique for fast acquisition and accurate tracking of the carrier frequency in orthogonal frequency division multiplexing (OFDM) receivers. The scheme is based on a data-aided frequency estimation algorithm. The presence of known symbol sequences periodically inserted in the OFDM frame allows the data demodulator to rapidly lock onto the carrier frequency during the acquisition phase, even in the presence of frequency offsets up to a few tenths of the overall signaling rate. Once acquisition is over, the circuit switches to a decision-directed mode to perform fine frequency tracking for reliable data demodulation. The algorithm performance is analyzed in terms of width of the lock-in frequency range and of lock-in probability in the acquisition mode, and of mean-square frequency estimation error in the tracking mode. Since OFDM is known to be extremely sensitive to carrier frequency errors, the impact of the carrier frequency synchronizer on the receiver error rate is also investigated.

In recent years, code division multiple-access (CDMA) techniques have received a great deal of attention for mobile terrestrial/satellite communication systems. Primarily considered for the noteworthy features of low power flux density emission and robustness to interference and multipath, CDMA is known to bear reduced bandwidth and power efficiency when compared to traditional TDMA and FDMA due to the intrinsic cochannel self-noise. Early attempts to increase the capacity of CDMA-based systems for commercial applications relied on voice activation and frequency reuse. More recently, practical solutions to implement (synchronous) orthogonal CDMA signaling are being developed independently in Europe and in the USA. This paper is focused on the comparative performance analysis of those two orthogonal CDMA schemes in the operating renditions of a mobile satellite communications system. In particular, the two CDMA systems are compared in the presence of that and frequency-selective multipath fading and a typical satellite transponder nonlinearity. Most numerical results are derived through a time-domain system simulation that confirms and integrates the theoretical findings.>

Although providing high spectral efficiency, sixteen-level quadrature amplitude modulation (16-QAM) is customarily considered a power-inefficient transmission technique for satellite communication systems, due to the considerable output back-off (OBO) of the optimum operating point of the nonlinear amplifier. The authors present a technique for the compensation of nonlinear distortions in the received signal constellation in an all-digital demodulator for rate-3/4 16-state trellis coded (TC) 16-QAM. They show that, by simple modifications to the metric computer in the Viterbi decoder, the optimum operating point of the amplifier can be moved much closer to saturation than in the conventional case, without any need for on-board amplifier linearization. The simulation results demonstrate a BER of 10/sup -5/ with a power saving of roughly 3.5 dB in a typical nonlinear channel, when compared to conventional uncoded 8-PSK. This figure includes all of the impairments produced by increased satellite amplifier OBO and by the presence of the subsystems for symbol timing recovery, carrier phase estimation and automatic gain control.>

In this paper, we consider the architecture of an all-digital modem for band-limited spread-spectrum radio communication networks employing code-division multiple access with generalized MSK modulated signals. After a general description of the modem scheme, we focus on the problem of spreading code synchronization, presenting three different chip timing recovery loops and analyzing their relative performance in terms of RMS tracking error.>

Reliable data detection in time division multiple access (TDMA) communication systems strictly depends on the availability of accurate estimates of the synchronization parameters of the received signal, i.e., carrier frequency/phase and symbol timing, which must be derived from the burst preamble. The authors focus on the carrier frequency estimation aspect, and present a fast, open-loop, all-digital frequency offset estimation technique, whose performance is assessed in two different communication scenarios: a TDMA satellite link employing standard modulation and burst formats, and a mobile cellular terrestrial radio system with signal and channel characteristics obeying the pan-European Group Special Mobile (GSM) recommendations. The use of the algorithm as a frequency error detector (discriminator) in a recursive ("closed-loop") frequency offset estimator is also discussed, and some results concerning both the transient and the steady-state behavior of such a scheme are presented. Finally, the impact of the algorithm on the receiver BER is briefly analyzed.>

Performance of the digital clock recovery scheme proposed by F.M. Gardner (1986), when applied to quadrature-amplitude-modulation (QAM) microwave radio communication systems is investigated. In particular, it is shown how to improve such a scheme by means of appropriate prefiltering of the incoming signal. The impulse response of the shaping filter is designed to achieve pattern-jitter-free clock recovery in the case of an ideal transmission channel. The impact of multipath fading on synchronizer performance is evaluated by means of computer simulation.>

Migration towards a full-digital implementation of modems is currently one of the main trends in transmission systems design. The authors describe a noncoherent all-digital delay lock loop (DDLL) suited for chip timing synchronization in band-limited direct sequence spread spectrum (DS/SS) systems, and they thoroughly analyze its performance. The key features of this novel scheme are represented by its low-complexity processing section together with its good tracking capability. Analytical expressions for the DDLL S-curve and steady-state timing jitter are derived and confirmed by a time-domain computer simulation. Furthermore, the Mean Time to Lose Lock (MTLL) of the loop is evaluated and some numerical results are reported. The proposed chip timing synchronization scheme reveals also an improved tracking performance when compared to the traditional analog DLL for rectangular chip DS/SS signals.>

The problem of combined coding and modulation is addressed for M-ary continuous-phase frequency-shift-keying (CPFSK) with a redundant symbol alphabet. It is shown that in many cases of interest, the use of a nonuniform mapping between coded bits and phase increments at the CPFSK modulator allows for a significant increase of free Euclidean distance in the coder/modulator state trellis as compared to the uniform alphabet case. A systematic search for optimum encoder/mapper/modulator structures is carried out for short constraint length rate 1/2 and 2/3 codes combined with 4-ary and 8-ary CPFSK, respectively, and for a relevant set of rational modulation indexes. The power-bandwidth efficiency of the optimum schemes is discussed and compared with that exhibited by the best coder/modulator structures previously proposed in the literature.>

The key synchronization aspects in the system design of a QQPSK (quadrature-quadrature phase-shift-keying) modem are addressed. The sensitivity of the data demodulator to synchronization errors is discussed, and contextually the performances of some IF and baseband carrier phase and symbol timing recovery schemes are evaluated both theoretically and by computer simulations. In particular, a fourth-power IF carrier/clock regenerator and two baseband clock recovery schemes, with and without the aid of data decisions, respectively, are taken into account. The analysis shows on the one hand the substantial robustness of QQPSK to carrier phase errors and the adequacy of the examined carrier extraction scheme. On the other hand, the remarkable sensitivity of QQPSK to symbol timing inaccuracy is stressed and the need to resort to the newly proposed decision-aided baseband clock recovery scheme is pointed out.>

The authors present a nonconventional joint data demodulation-pseudo-noise (PN) code tracking scheme for direct sequence (DS) spread-spectrum (SS) signals which solves problems of component imbalance and sensitivity with hardware simplicity and no performance degradation. An integrate-and-dump Costas loop is used for carrier recovery and data demodulation of the SS signal. Both data and carrier are then used to derive the baseband error signal of the code tracking loop. Moreover, a single passband correlator is used to perform the early-late correlation, leading to a hardware complexity equivalent to that of the tau-dither scheme, but without its loss in performance. Results of a thorough theoretical analysis of the system in an additive Gaussian noise (AWGN) environment are reported. They provide performance curves in terms of steady-state jitter and mean time to first lock loss. A superior jitter performance for low values of E/sub b//N/sub 0/ with respect to a traditional noncoherent delay lock loop (DLL) is shown, along with the potential gain of Manchester coding upon the more usual NRZ format.>

The minimum-mean-square-error approximation to a generalized minimum-shift-keying (MSK) signal using an offset quadrature phase-shift keying waveform with an elementary pulse, g(t), is considered. It is shown that the optimum shape of G(t), gamma (t), coincides with the average pulse of the generalized MSK signal. Therefore, gamma (-t) is the impulse response of the corresponding average matched fiber.>

A theoretical evaluation is presented of the mean time to first slip for some closed-loop synchronizer schemes to be used in a coherent serial receiver for offset binary modulations. Various modulation formats belonging to this class are considered, and the mean time to first slip is computed for each modulation type for different values of loop bandwidth and signal-to-noise ratio. Theoretical results are confirmed by extensive computer simulations.>