This paper introduces a learning-based approach to the near-field source localization problem adopting a hybrid analog-digital beamformer in an extremely large-scale multipleinput multiple-output (XL-MIMO) system. Hybrid analog-digital architectures gained significant attention in the literature due to the limited number of radio-frequency (RF) chains. However, the investigation of effective techniques tailored to partially connected hybrid beamformers for near-field localization is still missing in the literature. To this end, we leverage a Convolutional Neural Network (CNN)-based model to: (i) perform the analog beamformer design with proper training constraints; and (ii) estimate the single-user near-field position in a single snapshot. In the inference stage, the model is divided into two parts: the first accounts for the beamformer design, and the second acts as a localizing function. Simulation results demonstrate superior performance of the proposed method over existing solutions and robustness in multipath propagation conditions. In addition, our network is scalable and requires fewer RF chains than fullyconnected architectures.

Clock recovery and channel parameter estimation are essential functions for reliable communications over a free-space optical (FSO) link. Thanks to its large bandwidth, FSO ensures high-speed transmissions with data rates similar to those enabled by optical fiber links. In such a case, synchronization procedures that operate at symbol rate are highly desirable, since signal oversampling would require expensive and power consuming analog-to-digital converters (ADCs). In this work we investigate the problem of bit synchronization and channel estimation in an FSO link that employs on-off keying (OOK) signaling and is plagued by a mixture of thermal noise, shot noise and background radiation. In doing so we resort to the least-square (LS) estimation principle and look for some feedforward solution that can operate in a non-data-aided (NDA) fashion using symbol-spaced samples. Since the true LS estimator leads to a computationally intractable multi-dimensional optimization problem, we develop suitable approximations that can be implemented with affordable complexity. The accuracy of the proposed schemes is assessed through computer simulations and compared with the relevant modified Cramér-Rao bound (MCRB). Thanks to their ability to operate at symbol rate, they represent a promising solution for blind synchronization in high-speed FSO communications.

We consider a free-space optical (FSO) communication system employing on-off keying (OOK) signaling in the presence of background radiation and intersymbol interference (ISI) arising as a consequence of a sampling clock offset. The receiver has no prior information about the instantaneous channel coefficient, and shot noise with signal-dependent power is inserted by the photodetector device. In such a harsh scenario, suitable estimates of the timing error, channel attenuation and noise parameters are needed for reliable data detection. In this work, we employ the Expectation-Maximization (EM) algorithm for the joint estimation of all the aforementioned parameters. In order to obtain a feasible scheme, the maximization step of the EM procedure is split into a sequence of simpler optimization tasks according to the Expectation-Conditional Maximization (ECM) principle. The resulting algorithm operates in a non-data-aided fashion using symbol-spaced samples, which is particularly advantageous in high-speed FSO communications where signal oversampling may require sophisticated hardware equipment. Computer simulations indicate that the accuracy of the proposed scheme is close to the relevant modified Cramér-Rao bounds (MCRBs), which amounts to saying that no room is left for a further improvement of the system performance.

Large-scale MIMO systems with a massive number N of individually controlled antennas pose significant challenges for minimum mean square error (MMSE) channel estimation, based on uplink pilots. The major ones arise from the computational complexity, which scales with$N^{3}$, and from the need for accurate knowledge of the channel statistics. This paper aims to address both challenges by introducing reduced-complexity channel estimation methods that achieve the performance of MMSE in terms of estimation accuracy and uplink spectral efficiency while demonstrating improved robustness in practical scenarios where channel statistics must be estimated. This is achieved by exploiting the inherent structure of the spatial correlation matrix induced by the array geometry. Specifically, we use a Kronecker decomposition for uniform planar arrays and a well-suited circulant approximation for uniform linear arrays. By doing so, a significantly lower computational complexity is achieved, scaling as$N\sqrt {N}$and$N \log N$for squared planar arrays and linear arrays, respectively.

Holographic MIMO refers to a (possibly large) array with a large number of individually controlled and densely deployed antennas. The objective of this paper is to provide further insight into the use of closely spaced antennas in the uplink and downlink of a multi-user Holographic MIMO system. To this end, we utilize multiport communication theory, which ensures physically consistent uplink and downlink models. We first consider a simple uplink scenario with two side-by-side half-wavelength dipoles, two users, and single-path line-of-sight propagation, and show both analytically and numerically that the receive array gain and average spectral efficiency strongly depend on the directions from which the signals are received and on the array matching network used. The numerical results are then used to extend the analysis to more practical scenarios involving a larger number of dipoles (arranged in a uniform linear array) and a larger number of users. The case where the antennas are densely packed in a space-constrained factor form is also considered. It is found that the spectral efficiency benefits from decreasing the antenna spacing if arrays of moderate size are considered, e.g. in the order of a few wavelengths. In comparison, larger arrays with closely spaced antennas show only marginal improvements in spectral efficiency compared to half-wavelength arrays.

Starting from first principles of wave propagation, we consider a multiple-input multiple-output (MIMO) representation of a communication system between two spatially-continuous volumes. This is the concept of holographic MIMO communications. The analysis takes into account the electromagnetic interference, generated by external sources, and the constraint on the physical radiated power. The electromagnetic MIMO model is particularized for a pair of parallel line segments in line-of-sight conditions. Inspired by orthogonal-frequency division-multiplexing, we assume that the spatially-continuous transmit currents and received fields are represented using the Fourier basis functions. In doing so, a wavenumber-division multiplexing (WDM) scheme is obtained, which is not optimal but can be efficiently implemented. The interplay among the different system parameters (e.g., transmission range, wavelength, and sizes of source and receiver) in terms of number of communication modes and level of interference among them is studied with conventional tools of linear systems theory. Due to the non-finite support (in the spatial domain) of the electromagnetic channel, WDM cannot provide non-interfering communication modes. The interference decreases as the receiver size grows, and goes to zero only asymptotically. Different digital processing architectures, operating in the wavenumber domain, are thus used to deal with the interference. The simplest implementation provides the same spectral efficiency of a singular-value decomposition architecture with water-filling when the receiver size is comparable to the transmission range. The developed framework is also used to represent a communication scheme that performs only an integration over short spatial segments. This is equivalent to a classical MIMO system with uniform linear arrays made of electrically small dipoles. Numerical comparisons show that better performance than WDM can be achieved only when a higher number of radio-frequency chains is used.

Fast and reliable synchronization in free-space optical (FSO) communications is a crucial task that has received little attention so far. Since in these applications the data rate is much higher than in traditional radio-frequency (RF) systems, novel technological constraints may arise in the design of the synchronization algorithms, as for example the need to operate at symbol rate instead with an oversampled data stream. In this work, we consider an FSO link and investigate the problem of channel estimation, symbol timing recovery and frame detection using a known synch pattern. The modulation format is on-off keying (OOK) and the received signal is plagued by a mixture of thermal and shot noise. By applying the least-squares criterion, we derive a novel synchronization scheme that can jointly retrieve all the unknown parameters using symbol-spaced samples. Although designed without taking the noise statistics into account, the estimator performance is assessed in a realistic scenario where shot noise is present. Comparisons are made with the relevant Cramér-Rao bound for the joint estimation of the synchronization parameters and signal-dependent noise variances. Numerical simulations and complexity analysis indicate that the resulting scheme performs satisfactorily with an affordable processing load. Hence, it represents a promising solution for fast synchronization in high-speed FSO communications.

We consider a free-space optical (FSO) communication link for packet-based transmissions over a turbulence fading channel. The modulation format is on-off keying (OOK) and a unique-word (UW) composed by a known synch pattern is periodically inserted in the data stream to identify the start of frame. Since an avalanche photo-diode (APD) is used for direct detection of the OOK symbols, the photocurrent signal provided by the APD is plagued by a mixture of thermal and shot noise with signal-dependent power. Our goal is the detection of the UW position in the received stream, along with the estimation of the unknown channel attenuation and noise variances. The aforementioned problem has recently been studied by ignoring any information conveyed by data symbols surrounding the UW. In this work, further investigation is conducted in order to determine the maximum likelihood (ML) solution that, in addition to the UW, exploits all the information-bearing symbols belonging to the observation window. The relevant Cramér-Rao bound (CRB) is also evaluated to establish the ultimate accuracy achievable in the estimation process. Since the true ML estimator leads to a computationally intractable multi-dimensional optimization problem, we develop suitable approximations that enable accurate frame synchronization with affordable complexity. We also present an iterative solution to refine the channel and noise power estimates provided by the UW detection procedure. Numerical simulations demonstrate the superiority of the proposed synchronization and estimation schemes with respect to existing alternatives.

Sinusoidal frequency estimation in the presence of white Gaussian noise plays a major role in many engineering fields. Significant research in this area has been devoted to the fine tuning stage, where the discrete Fourier transform (DFT) coefficients of the observation data are interpolated to acquire the residual frequency error$\varepsilon $. Iterative interpolation schemes have recently been designed by employing two$q$-shifted spectral lines symmetrically placed around the DFT peak, and the impact of$q$on the estimation accuracy has been theoretically assessed. Such analysis, however, is available only for some specific algorithms and is mostly conducted under the assumption of a vanishingly small frequency error, which makes it inappropriate for the first stage of any iterative process. In this work, further investigation on DFT interpolation is carried out to examine some issues that are still open. We start by evaluating the Cramér-Rao bound (CRB) for frequency recovery by interpolation of two$q$-shifted spectral lines and assess its dependence on$\varepsilon $and$q$. Such a bound is of primary importance to check whether existing schemes can provide efficient estimates at any iteration or not. After determining the optimum value of$q$for a given$\varepsilon $, we eventually derive the maximum likelihood (ML) DFT interpolator. Since the latter exhibits the best performance at any step of the iteration process, it might attain the desired accuracy just at the end of the first iteration, which is especially advantageous in terms of computational load and processing time.

Frequency estimation of a single complex exponential signal embedded in additive white Gaussian noise is a major topic of research in many engineering areas. This work presents further investigations on this problem with regards to the fine estimation task, which is accomplished through a suitable interpolation of the discrete Fourier transform (DFT) coefficients of the observation data. The focus is on fast real-time applications, where iterative estimation methods can hardly be applied due to their latency and complexity. After deriving the analytical expression of the Cramér-Rao bound (CRB) for general values of the system parameters, we present a new DFT interpolation scheme based on the weighted least-squares (WLS) rule, where the optimum weights are precomputed through a numerical search and stored in the receiver. In contrast to many existing alternatives, the proposed method can employ an arbitrary number of DFT samples so as to achieve a good trade-off between system performance and complexity. Simulation results and theoretical analysis indicate that, at sufficiently high signal-to-noise ratios, the estimation accuracy is close to the relevant CRB at any value of the frequency error. This provides some advantage with respect to non-iterative competing schemes, without incurring any penalty in processing requirement.

In long-term evolution (LTE) systems, there is an option to extend the size of the cyclic-prefix (CP) when the propagation environment is characterized by severe time dispersion. Such a peculiar feature makes the system more resilient against multipath distortions, but inevitably complicates the downlink synchronization task, which cannot be accomplished through conventional techniques devised for a specified CP size. In this paper, we study the problem of CP length detection in the LTE downlink in the presence of a timing uncertainty and frequency offset. In contrast to previous investigations based on heuristic arguments, our approach relies on the maximum likelihood (ML) estimation criterion. Furthermore, it accounts for the irregular LTE symbol (characterized by a different CP size), which is present in the normal CP operation mode. The resulting scheme operates in the time-domain and computes a different metric for each possible position of the irregular symbol within the observation window. Due to its flexibility, the proposed ML approach can be applied to any future multicarrier standard for the design of a CP length detection algorithm, either in the presence or in the absence of irregular symbols.

Massive multiple-input multiple-output (MIMO) systems, where base stations (BSs) are equipped with hundreds of antennas, are an attractive way to handle the rapid growth of data traffic. As the number of user equipments (UEs) increases, the initial access and handover in contemporary networks will be flooded by user collisions. In this paper, a random access protocol is proposed that resolves collisions and performs timing estimation by simply utilizing the large number of antennas envisioned in massive MIMO networks. UEs entering the network perform spreading in both time and frequency domains, and their timing offsets are estimated at the BS in closed form using a subspace decomposition approach. This information is used to compute channel estimates that are subsequently employed by the BS to communicate with the detected UEs. The favorable propagation conditions of massive MIMO suppress interference among UEs, whereas the inherent timing misalignments improve the detection capabilities of the protocol. Numerical results are used to validate the performance of the proposed procedure in massive MIMO networks, under uncorrelated and correlated fading channels. With$2.5 \times 10^{3}$UEs that may simultaneously become active with probability 1%, a total of 16 frequency–time codes, and 100 antennas, a given UE is detected with probability 75%, and the accuracy of its timing estimate is on the order of few samples.

Direct conversion receivers (DCRs) are an effective means to obtain user terminals with reduced cost, size, and power consumption. Their major drawback is the possible insertion of I/Q imbalances in the demodulated signal, which can seriously degrade the performance of conventional synchronization algorithms. In this paper, we investigate the problem of carrier frequency offset (CFO) recovery in an OFDM receiver equipped with a DCR front-end. Our approach is based on the best linear unbiased estimation (BLUE) theory and aims at jointly estimating the CFO, the useful signal component, and its mirror image. In doing so, we exploit knowledge of the pilot symbols transmitted within a conventional repeated training preamble appended in front of each data packet. Two solutions are proposed, and both of them provides the CFO in closed-form, thereby avoiding any grid-search procedure. The accuracy of the proposed methods is assessed in a scenario compliant with the 802.11a WLAN standard. Compared with existing solutions, the novel schemes achieve improved performance at the price of a marginal increase of the processing load.

This paper presents the design of a 4th-order closed-loop continuous-time filter. This filter exploits an improved analog stage that makes linearity performance constant over the entire pass-band frequencies. In other words the filter has same IIP3 at low and at high frequency. Hence the linearity performance are independent on the input tones bandwidth, whereas most of analog filters suffers from poor linearity when loop-gain reduces, i.e. close to the closed-loop poles frequency. In order to validate the hereby proposed design idea a 4th-order 25MHz-3dB bandwidth pseudo-differential filter has been designed and simulated in CMOS 28nm technology. The prototype consumes 820μW from 1V supply voltage and has 15dBm and 13dBm IIP3 at 5&6MHz and 20&21MHz input tones, respectively.

The direct conversion receiver (DCR) architecture has received much attention in the last few years as an effective means to obtain user terminals with reduced cost, size, and power consumption. A major drawback of a DCR device is the possible insertion of in-phase/quadrature imbalances in the demodulated signal, which can seriously degrade the performance of conventional synchronization algorithms. In this paper, we investigate the problem of carrier frequency offset (CFO) recovery in an orthogonal frequency-division multiplexing receiver equipped with a DCR front-end. Our approach is based on maximum likelihood (ML) arguments and aims at jointly estimating the CFO, the useful signal component, and its mirror image. In doing so, we exploit knowledge of the pilot symbols transmitted within a conventional repeated training preamble appended in front of each data packet. Since the exact ML solution turns out to be too complex for practical purposes, we propose two alternative schemes which can provide nearly optimal performance with substantial computational saving. One of them provides the CFO in closed-form, thereby avoiding any grid-search procedure. The accuracy of the proposed methods is assessed in a scenario compliant with the 802.11a WLAN standard. Compared to existing solutions, the novel schemes achieve improved performance at the price of a tolerable increase of the processing load.

Massive MIMO systems, where base stations are equipped with hundreds of antennas, are an attractive way to handle the rapid growth of data traffic. As the number of users increases, the initial access and handover in contemporary networks will be flooded by user collisions. In this work, we propose a random access procedure that resolves collisions and also performs timing, channel, and power estimation by simply utilizing the large number of antennas envisioned in massive MIMO systems and the inherent timing misalignments of uplink signals during network access and handover. Numerical results are used to validate the performance of the proposed solution under different settings. It turns out that the proposed solution can detect the collisions with a probability higher than 90%, while providing reliable timing and channel estimates at the same time. Moreover, numerical results demonstrate that it is robust to overloaded situations.

This paper investigates the problem of carrier frequency offset (CFO) recovery in an OFDM receiver affected by frequency-selective in-phase/quadrature (I/Q) imbalances. The analysis is based on maximum-likelihood (ML) methods and relies on the transmission of a training preamble with a repetitive structure in the time domain. After assessing the accuracy of the conventional ML (CML) scheme in a scenario characterized by I/Q impairments, we review the joint ML (JML) estimator of all unknown parameters and evaluate its theoretical performance. In order to improve the estimation accuracy, we also present a novel CFO recovery method that exploits some side-information about the signal-to-interference ratio. It turns out that both CML and JML can be derived from this scheme by properly adjusting the value of a design parameter. The accuracy of the investigated methods are compared with the relevant Cramer-Rao bound. Our results can be used to check whether conventional CFO recovery algorithms can work properly or not in the presence of I/Q imbalances and also to evaluate the potential gain attainable by more sophisticated schemes.

Distance bounding protocols make it possible to determine a trusted upper bound on the distance between two devices. Their key property is to resist reduction attacks, i.e., attacks aimed at reducing the distance measured by the protocol. Recently, researchers have also focused on enlargement attacks, aimed at enlarging the measured distance. Providing security against such attacks is important for secure positioning techniques. The contribution of this paper is to provide a probabilistic model for the success of an enlargement attack against a distance bounding protocol realized with the IEEE 802.15.4a ultra-wideband standard. The model captures several variables, such as the propagation environment, the signal-to-noise ratio, and the time-of-arrival estimation algorithm. We focus on non-coherent receivers, which can be used in low-cost low-power applications. We validate our model by comparison with physical-layer simulations and goodness-of-fit tests. The results show that our probabilistic model is sufficiently realistic to replace physical-layer simulations. Our model can be used to evaluate the security of the ranging/positioning solutions that can be subject to enlargement attacks. We expect that it will significantly facilitate future research on secure ranging and secure positioning.

Wireless sensor networks enable a wealth of new applications in areas such as military, medical, environmental, transportation, smart city, and so on. In many of these scenarios, we need to measure in a secure way the positions of the sensors. Existing range-based techniques for secure positioning require a burdensome infrastructure, with many fixed anchors. Reducing the infrastructure would reduce deployment cost and foster the adoption of secure positioning solutions in wireless sensor networks. In this article, we propose SPEM, a secure positioning system based on multilateration and ultra-wideband (UWB) distance bounding protocols. The key idea behind SPEM is to leverage the low probability that an adversary has of controlling enlargement attacks against UWB. We estimate such a probability by a thorough study and signal-level simulations of the UWB physical layer. We test SPEM both in a simulated environment and in a real indoor environment using real UWB transceivers. We show that SPEM needs far less infrastructure than state-of-the-art solutions ( − 22% to − 93%, depending on the anchor deployment method), while achieving high levels of security against smart and determined adversaries.

This paper presents a 4th-order low-pass continuous-time filter with 21.4 MHz bandwidth. Using a positivefeedback in combination with resistors, this architecture synthesizes two complex conjugate poles. The filter is designed and optimized to feature high-linearity along all filter passband, and, in particular, at the cut-off frequency edge. The filter consumes only 100 μA with 1.8V voltage supply, and achieves 45μW/pole power consumption, 300 μVRMSinput-referred noise, 21.9 dBm IIP3 with 2–3 MHz input tones and 12dBm IIP3 near the cut frequency using 19–20 MHz tones.

This paper presents an FSK modulator suitable for a transmitter chain within the new USB Power Delivery standard. The proposed solution performs a binary FSK modulation using a relaxation voltage-controlled oscillator with a voltage averaging feedback. A reference proportional to supply is used to compensate real-time frequency variations, whereas the frequency spread due to the process is corrected by an 8 bit trimming system. The considered input signal bit rate is 300 kbps and the carrier frequency is 23.2 MHz with ±0.5 MHz frequency deviation. The proposed modulator grants ±0.9% accuracy against PVT spread by consuming 1.2 mW and phase noise at 20kHz from the carrier lower than -56 dBc/Hz.

In this paper we propose an algorithm for joint carrier phase and timing estimation with OQPSK modulations. The derivation is based on the maximum-likelihood criterion, and exploits a very efficient algorithm for the detection of differentially encoded M-PSK symbols already described in literature. Though we are mainly interested in measuring the phase and clock parameters, estimates of the transmitted symbols are also obtained as by-products. The resulting scheme has a feedforward structure and provides phase and timing information in a fixed time, differently from closed-loop architectures. It can be implemented in digital form and is particularly suitable for burst mode transmissions. Its performance is investigated by simulation and the results are compared with Cramér-Rao bounds. It turns out that the estimation accuracy is very close to the theoretical limits, even with short observation intervals and small values of the excess bandwidth. In such conditions, the proposed estimators largely outperform other schemes already known in literature. Their superiority becomes less significant as the signal bandwidth increases.

In this work, we focus on separable convex optimization problems with linear and box constraints and compute the solution in closed-form as a function of some Lagrange multipliers that can be easily computed in a finite number of iterations. This allows us to bridge the gap between a wide family of power allocation problems of practical interest in signal processing and communications and their efficient implementation in practice.

In this letter, we consider Cramér-Rao bounds (CRBs) on the variance of unbiased vector parameter estimators. It is well known that the CRB for the estimation of a parameter α, assuming that a second parameter β is unknown, is not smaller than the CRB computed assuming β known. The performance loss can be measured by the ratio between these two bounds. In this work, we derive a “reciprocity” property of the CRB for vector parameter estimation, which indicates that the performance loss in estimating α when β is unknown equals the performance loss in estimating β with α unknown. Though this property is of mainly theoretical interest, some examples are given which show that it can be useful also in practical applications.

The paper is concerned with the impact of clock frequency offsets on the accuracy of ranging systems based on time of arrival (TOA) measurements. It is shown that large TOA errors are incurred if the transmitter and receiver clocks are mistuned by more than just one part per million (ppm). This represents a serious obstacle to the use of commercial low-cost quartz oscillators, as they exhibit frequency drifts in the range of ± 10 ppm and more. A solution is to estimate first the transmitter clock frequency relative to the receiver's and then compensate for the difference by acting on the receiver clock tuning. An algorithm is proposed that estimates the transmitter clock frequency with an accuracy better than 0.1 ppm. Computer simulations indicate that its use in ranging systems makes TOA measurements as good as those obtained with perfectly synchronous clocks.

The remarkable promise of multiple-input multiple-output (MIMO) wireless channels has motivated an intense research activity to characterize the theoretical and practical issues associated with the design of transmit (source) and receive (destination) processing matrices under different operating conditions. This activity was primarily focused on point-to-point (single-hop) communications but more recently there has been an extensive work on two-hop or multi-hop settings in which single or multiple relays are used to deliver the information from the source to the destination. The aim of this tutorial is to provide an up-to-date overview of the fundamental results and practical implementation issues in designing amplify-and-forward MIMO relay systems.

This paper deals with impulse-radio ultra wide-band systems with multiple receive antennas. For binary PPM modulations, we propose various combining/detection schemes derived by the generalized likelihood ratio criterion. These schemes first combine delayed versions of the waveforms received at the different antennas and then compute the energies of the resulting signal over intervals smaller than half the symbol period. The decision statistic is finally obtained by a linear combination of these energies. The proposed algorithms can be implemented using currently available technology. They outperform a detection scheme that simply combines the decision statistics obtained (at each antenna) by means of a conventional energy-based detector. Their advantage increases with the number of antennas.

Theoretical limits on time-of-arrival (equivalently, range) estimation are derived for multicarrier systems in the presence of interference. Specifically, closed-form expressions are obtained for Cramer-Rao bounds (CRBs) in various scenarios. In addition, based on CRB expressions, an optimal power allocation (or, spectrum shaping) strategy is proposed. This strategy considers the constraints not only from the sensed interference level but also from the regulatory emission mask. Numerical results are presented to illustrate the improvements achievable with the optimal power allocation scheme, and a maximum likelihood time-of-arrival estimation algorithm is studied to assess the effects of the proposed approach in practical estimators.

A joint TOA/AOA estimator is proposed for UWB indoor ranging under LOS operating conditions. The estimator employs an array of antennas, each feeding a demodulator consisting in a squarer and a low-pass filter. Signal samples taken at Nyquist rate at the filter outputs are processed to produce TOA and AOA estimates. Performance is assessed with transmitted pulses with a bandwidth of either 1.5 GHz (type-1 pulses) or 0.5 GHz (type-2 pulses), which correspond to sampling rates of 3 GHz and 1 GHz, respectively. As expected, the estimation accuracy decreases with the pulse bandwidth. Ranging errors of about 10 cm and angular errors of about 1° are achieved at SNR of practical interest with type-1 pulses and two antennas at a distance of 50 cm. With type-2 pulses the errors increase to 35 cm and 3°. Comparisons are made with other schemes discussed in literature.

Recent studies have investigated alternative methods to the realization of transmitted-reference ultra-wideband communication systems. In addition to the conventional scheme, in which data and reference are separated in time, two other methods have been proposed in which the separation is achieved through a frequency shift or via code multiplexing. In a single-user scenario the alternative methods have advantages in terms of error rate and receiver implementation. In this paper the three schemes are compared in a multi-user environment. It is shown that even in the presence of multiple access interference the unconventional solutions are preferable and, in particular, the code multiplexing method has the best performance and is the simplest to implement.

This paper discusses a ranging algorithm appropriate for impulse radio ultra-wideband signals compliant with the IEEE 802.15.4a standard. Three steps are indicated to locate the so-called ranging marker in the received signal. The first is to find the highest peaks in the channel responses to the transmitted pulses. The second consists in measuring the distance of each peak to the starting time of the response. The last one involves the identification of the first symbol of the physical-layer header. Algorithms to accomplish all these tasks are indicated. They are implemented in digital form and are in keeping with a non-coherent receiver structure. The achievable accuracy depends on the sampling rate. For example, at SNR = 20 dB and with a rate of 1 GHz the ranging errors are on the order of 20 cm.

Recent studies have investigated alternative methods to the realization of the transmitted reference scheme in UWB communications. In addition to the conventional system, in which data and reference are separated by a time shift, two other methods have been proposed in which the separation is achieved through a frequency offset (FO) or code multiplexing (CM). Their advantage over the traditional system is both in performance and receiver implementation. In this paper they are compared in a multiple access environment. It is shown that the CM-based scheme outperforms its FO-based counterpart and is simpler to implement.

An efficient scheme for limiting the inherent high out-of-band radiation of the orthogonal frequency-division multiplexing signal has been studied by Brandes et al.. The scheme achieves sidelobe suppression by using few subcarriers, referred to as cancellation carriers (CCs), positioned at the edges of the spectrum and modulated by data-dependent complex-valued symbols that are designed so as to minimize the out-of-band radiation. Unfortunately, its performance has been investigated at the input of the power amplifier (PA) without considering the spectral re-growth induced by PA's non-linear region. Motivated by the above consideration, in this work we return to the aforementioned scheme and investigate its performance at the output of the PA. In addition, since reducing the time-domain signal peaks may alleviate the problem of spectral re-growth, we also adopt two well-known tone reservation (TR)-based peak reduction algorithms. Computer simulations are used to assess the performance of the investigated schemes under different operating conditions. It turns out that the CC-based solution may provide some performance gain at the PA output only for input power backoffs larger than 6 dB. Additional benefits can be achieved when used in conjunction with TR and some linearization of the PA characteristic.

In traditional transmitted reference (TR) ultrawideband systems the reference component is time-shifted and orthogonal relative to the data-bearing signal. This paves the way to a correlation receiver in which the local template is derived from the incoming waveform using a delay line. As analog delay lines are difficult to implement with current technology, an alternative TR system has recently been proposed in which reference and data components are made orthogonal by a frequency shift rather than a time shift. The resulting receiver has no delay lines and has better performance compared to the traditional scheme. In the present paper we discuss a third way to achieve orthogonality, i.e., by modulating reference and data components with two distinct code sequences. Even in this case the receiver has no delay lines. However, it is simpler to implement and has better performance than the frequency-shift based receiver.

This paper investigates the joint Tx-Rx design of a multicarrier multiple-input multiple-output (MIMO) system employing Tomlinson-Harashima precoding (THP). The optimality criterion is the minimization of the mean-square-error (MSE) at the receiver side under a constraint on the overall transmitted power. Theoretical analysis and computer simulations are used to assess the performance of the proposed scheme in terms of bit-error-rate (BER) and minimum achievable MSE. It is shown that non-linear precoding may provide much better results than linear pre-filtering, especially when the number of parallel data streams is close to the number of transmitting and receiving antennas. Compared with other existing methods based on THP, our scheme is different in that the preceded symbols are linearly prefiltered before being launched over the MIMO channel. This results into a power loading operation over the channel eigenmodes, which leads to a remarkable improvement of the error-rate performance.

This paper investigates time of arrival estimation via impulse radio ultra-wideband technology. A dense multipath channel is assumed and frequency-selective propagation effects are taken into account that make the individual received pulses different in shape from the transmitted monocycles. In these conditions a conventional correlation-based estimator cannot be used and other schemes must be resorted to. Assuming only an approximate knowledge of the received pulses' duration, a TOA estimator is derived making use of least mean square techniques. The algorithm operates on energy measurements made on a folded version of the received signal. Its performance is investigated by simulation under implementation conditions of different complexity. In particular, sub-Nyquist sampling rates are considered and analog-to-digital conversion with limited resolution is explored. Comparisons are made with other available schemes.

This paper investigates time of arrival estimation via impulse radio ultra-wideband technology. A dense multipath channel is assumed and frequency-selective propagation effects are taken into account that make the individual received pulses different in shape from the transmitted monocycles. Assuming only an approximate knowledge of the received pulses' duration, a time of arrival estimator is derived making use of least mean square techniques. The algorithm operates on energy measurements made on a folded version of the received signal. The estimator performance is investigated by simulation under implementation conditions of different complexity. Comparisons are made with other available schemes.

This paper deals with a novel energy-detection receiver for ultra-wideband systems operating in dense multi-path environments. For binary PPM modulation we propose a detection scheme that operates on signal energy measurements taken on small fractions (bins) of the symbol period. Assuming that the fractional energies of the channel response over those bins can be estimated in some way, we look for the decision strategy that minimizes the error probability. This leads us to a receiver structure that generalizes the conventional energy- detection scheme. The new strategy turns out to be superior to the conventional strategy and its performance is found to improve as the bin size decreases. A simple method is proposed to estimate the fractional energies of the channel response exploiting a training sequence. The impact of the estimation errors on the receiver performance is shown to be marginal.

This paper investigates timing recovery for differential transmitted reference (DTR) ultra-wideband receivers. In DTR systems the symbol energy is distributed over several time slots (frames) and a single pulse is transmitted per frame. The information is conveyed by differentially encoding the pulse polarities. Equal polarities correspond to a bit +1 while alternate polarities correspond to a bit -1. The detector computes the decision statistic by correlating pulses from adjacent frames and summing the contributions from all the frames of a symbol. Synchronization consists in providing accurate timing for the correlation computations. A timing algorithm is proposed requiring a limited amount of circuitry in addition to that needed for detection purposes. No training sequences are needed as the synchronizer operates in a blind mode. Two possible receiver architectures are envisioned, analog or digital. The effects of the timing errors on the receiver performance are investigated by simulation. In general, they are found to be negligible, except when the implementation is digital and the analog-to-digital converter resolution is limited to 1 bit.

This paper deals with energy detection receivers for ultra-wideband systems operating in a dense multipath environment. For binary PPM modulation we propose a novel detection scheme that exploits some information on the channel energy-delay profile. We show that the BER performance of this detector is superior to that of a conventional scheme operating with no knowledge of the channel response. A simple method for estimating the channel delay profile is discussed and it is shown that the impact of the estimation errors on the receiver performance is negligible.

This paper investigates timing recovery for differential transmitted reference ultra-wideband receivers. The detector computes the decision statistic by correlating pulses from adjacent frames and summing the contributions from all the frames of a symbol. Synchronization consists of providing accurate timing for the correlation computations. A solution to this problem is proposed that requires a limited amount of extra circuitry. No training sequences are needed as the synchronizer operates in a blind mode. Two possible receiver architectures are envisioned, analog and digital. The effects of the timing errors on the receiver performance are investigated by simulation.

This paper investigates the estimation of the frame frequency in ultrawideband (UWB) communication systems. An estimation method is proposed that exploits the transmission of a periodic pulse sequence at the frame frequency. The samples of the received waveform are used to compute a cost function that depends on a trial value of the incoming pulse frequency. The location of the maximum provides an estimate of the transmitted frequency. The performance of the estimator is assessed theoretically and is compared to the Cramer-Rao lower bound. It is shown that in certain conditions the estimator achieves the bound at high SNR values. Simulations validate the theory and show the degradations in the estimation performance caused by multiple-access interference. They also give an idea of the estimation accuracy needed in a correlation receiver.

We consider a UWB communication system operating in a typical indoor environment and we propose a novel differential receiver in which encoding is performed symbol-by-symbol rather than frame-by-frame, as is normally done. The detector structure is derived in a single-user scenario using a generalized likelihood ratio test approach. Its performance is assessed by simulation in the presence of multiple-access interference. The proposed receiver has a simple structure and outperforms other differential schemes of comparable complexity

This paper deals with channel acquisition and tracking for the uplink of direct-sequence code-division multiple-access systems. The transmission medium is characterized by multipath propagation, and the goal is to estimate the time-varying channel impulse response (CIR) for a new user entering the system. Channel acquisition is pursued through maximum-likelihood techniques. The resulting scheme may be too complex in some applications, as it requires the online inversion of a large matrix. Therefore, we also consider a simpler solution based on the least-squares (LS) criterion. Channel tracking is performed through weighted LS methods. At each signaling interval, the CIR estimate is updated using data decisions and exploiting the inverse of the interference covariance matrix to mitigate the near-far problem. Performance is assessed by simulation in a scenario inspired by the frequency-division duplexing component of the universal mobile telecommunications system. The acquisition/tracking algorithms are found to be resistant to multiuser interference and suitable for transmissions over fast-fading channels.

Two signaling formats for ultrawide bandwidth indoor communications are compared in terms of bit error rate performance in the presence of multiple access interference. The channel exhibits multipath propagation and its impulse response is either assumed known or is estimated with least squares methods. Computer simulations show that, even with perfect channel knowledge, time-hopping pulse-amplitude modulation is superior to time-hopping pulse-position modulation. The superiority increases with the number of users and becomes substantial in the presence of channel estimation errors. An intuitive explanation of this fact is provided.

We consider the uplink of a direct-sequence code-division multiple-access system operating over a multipath fading channel, and we aim at estimating the propagation delay and the frequency offset of a new user entering the network. The joint estimation of these parameters is investigated using a weighted-least-squares approach. The exact solution turns out to be too complex for practical purposes, as it involves a numerical search over a bidimensional domain. As an alternative, we propose a suboptimal procedure, in which the propagation delay is first computed with a monodimensional search and then is exploited to derive the frequency offset in closed form. Both data-aided and nondata-aided estimation methods are considered. The proposed synchronizers have a reasonable complexity and are suited for third-generation cellular systems. Their performance is assessed by simulation in a scenario inspired by the specifications of the frequency-division duplexing component of the Universal Mobile Telecommunications Systems standard. It is found that they provide accurate estimates and are useful even in applications over rapidly varying channels.

We consider the uplink of a direct-sequence code-division multiple-access system and we assume that the base station is endowed with a linear antenna array. Transmission takes place over a multipath channel and the goal is to estimate the channel parameters (path gains and delays) and the directions of arrival of the signal echoes from a user entering the network. We propose an estimator that operates in an iterative fashion and exploits knowledge of the transmitted symbols (training sequence). Compared to other existing schemes, it is simpler to implement as it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional searches. Computer simulations indicate that the proposed scheme is useful even in applications over rapidly varying channels where the training sequence must be short compared with the channel decorrelation time.

This paper investigates channel acquisition and tracking for the uplink of a DS-CDMA system. The transmission medium is characterized by multipath propagation and the goal is the estimate the time-varying channel impulse response (CIR) of a new user entering the system. Channel acquisition is pursued through maximum likelihood techniques while channel tracking is performed through weighted least squares methods. At each signaling interval the CIR estimate the inverse of the interference covariance matrix to mitigate the near-far problem. Performance is assessed by simulation in a scenario inspired by the UMTS system.

We consider the uplink of a DS-CDMA system and we assume that the base station is endowed with a linear antenna array. Transmission takes place over a multipath channel and the goal is to estimate the channel parameters and the directions of arrival of the signals from a user entering the network. Maximum likelihood estimation of all these parameters is not feasible as it involves a search over a multidimensional domain. Through suitable approximations we replace the above search by a sequence of mono-dimensional searches. This results in an estimation algorithm of reasonable complexity of third-generation cellular applications. The performance of the algorithm is assessed by simulation in a scenario inspired by the specification of the FDD component of the UMTS standard. It is found that the channel parameters and the directions of arrival can be estimated with accuracy close to the Cramer-Rao bound.

This paper deals with channel estimation in the uplink of a direct-sequence code-division multiple-access system operating in a multipath environment. The parameters of interest are the delays and the complex attenuations incurred by the signal echoes along the propagation paths. We propose an iterative approach for estimating the channel parameters of a new user entering the system. The method is based on the space-alternating generalized expectation-maximization algorithm and exploits a training sequence. In comparison to other estimation algorithms, it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional problems. In addition, it can be effectively used in applications over fast-fading channels. Computer simulations are employed to assess the performance of the proposed scheme. It is found that it is resistant to multiuser interference and has accuracy close to the Cramer-Rao lower bound even with very short training sequences.

This paper investigates channel estimation in the uplink of a DS-CDMA system. The transmission medium is characterized by the presence of multipath components and the parameters of interest are the delays and complex gains of the propagation paths. We propose a new method for estimating the channel parameters of a user entering the system. The method is based on the maximum likelihood criterion and exploits a training sequence. In comparison with other estimation algorithms it has two advantages. First, it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional problems. Secondly, it has better capability to resolve closely-spaced multipath components. Computer simulations are used to assess its performance. It is found that it is resistant to multiuser interference and can be effectively used in third generation mobile cellular networks.

This paper deals with the analysis and design of efficient non-data-aided clock and carrier (frequency/phase) synchronization algorithms intended for use in satellite digital video broadcasting systems employing turbo-coding techniques to enhance the power efficiency. The above issue appears quite challenging in view of both the extremely low signal-to-noise ratio, typical of turbo-codes operation, and the very short time allocated to the acquisition of synchronization parameters. These constraints rule out most conventional clock/carrier recovery schemes and demand a careful search for specific, highly efficient algorithms. We propose and analyze a clock/carrier synchronization scheme capable of operating at values of E/sub b//N/sub o/ as low as 1 dB with lock-in delay not exceeding 50 ms.