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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Before establishing a communication link with the serving base station (eNodeB), user equipment (UE) operating in a long-term evolution (LTE) multi-cellular network must acquire some specific information, including the sector identity and cell group identity. For this purpose, two training sequences called primary synchronization signal (PSS) and secondary synchronization signal (SSS) are periodically transmitted in the downlink to convey such information. In this work, we present a novel maximum likelihood (ML) approach for SSS detection assuming that the PSS has been successfully identified at an earlier stage. As we shall see, the resulting scheme turns out to be too complex for practical implementation as it requires perfect knowledge of the channel covariance matrix. Therefore, we look for simpler solutions and propose two reduced-search methods that operate in a mismatched mode. The first scheme exploits channel state information emerging from both the primary and secondary synchronization signals, while the second scheme operates using only the secondary synchronization signal. Numerical analysis indicates that the proposed methods outperform existing alternatives and can be successfully applied even in a severe propagation scenario. The price for such an advantage is a certain increase of the processing requirement. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

Before establishing a communication link in a cellular network, the user terminal must activate a synchronization procedure called initial cell search in order to acquire specific information about the serving base station. To accomplish this task, the primary synchronization signal (PSS) and secondary synchronization signal (SSS) are periodically transmitted in the downlink of a long term evolution (LTE) network. Since SSS detection can be performed only after successful identification of the primary signal, in this work, we present a novel algorithm for joint PSS detection, sector index identification, and integer frequency offset (IFO) recovery in an LTE system. The proposed scheme relies on the maximum likelihood (ML) estimation criterion and exploits a suitable reduced-rank representation of the channel frequency response, which proves robust against multipath distortions and residual timing errors. We show that a number of PSS detection methods that were originally introduced through heuristic reasoning can be derived from our ML framework by simply selecting an appropriate model for the channel gains over the PSS subcarriers. Numerical simulations indicate that the proposed scheme can be effectively applied in the presence of severe multipath propagation, where existing alternatives provide unsatisfactory performance. [ABSTRACT FROM PUBLISHER]

In-phase/quadrature (I/Q) imbalances are front-end impairments which may greatly complicate the synchronization task in a low-cost direct-conversion receiver (DCR). In this paper we investigate the possibility of using the space-alternating generalized expectation-maximization (SAGE) algorithm to recover the carrier frequency offset in OFDM terminals with a DCR architecture. Our study leads to a novel scheme that operates in a recursive fashion and exploits a conventional OFDM training preamble composed by several repeated parts. At each iteration, interference arising from I/Q impairments is subtracted from the received samples before updating the frequency estimate. The performance of the new scheme is assessed in terms of estimation accuracy and processing load by considering a wireless local area network (WLAN) compliant with the IEEE 802.11a standard. The main goal is to check whether the SAGE approach exhibits some advantages compared to existing alternatives. [ABSTRACT FROM PUBLISHER]

In multi-cellular WiMAX systems based on orthogonal frequency-division multiple-access (OFDMA), the training preamble is chosen from a set of known sequences so as to univocally identify the transmitting base station. Therefore, in addition to timing and frequency synchronization, preamble index identification is another fundamental task that a mobile terminal must successfully complete before establishing a communication link with the base station. In this work we investigate the joint maximum likelihood (ML) estimation of the carrier frequency offset (CFO) and preamble index in a multicarrier system compliant with the WiMAX specifications, and derive a novel expression of the relevant Cramer-Rao bound (CRB). Since the exact ML solution is prohibitively complex in its general formulation, suboptimal algorithms are developed which can provide a reasonable trade-off between estimation accuracy and processing load. Specifically, we show that the fractional CFO can be recovered by combining the ML estimator with an existing algorithm that attains the CRB in all practical scenarios. The integral CFO and preamble index are subsequently retrieved by a suitable approximation of their joint ML estimator. Compared to existing alternatives, the resulting scheme exhibits improved accuracy and reduced sensitivity to residual timing errors. The price for these advantages is a certain increase of the system complexity. [ABSTRACT FROM PUBLISHER]

Over the last years, there has been growing interest in making user terminals more efficient in terms of cost, size, and power consumption. A step forward in this direction is represented by direct-conversion receiver (DCR) architectures, which directly convert the received waveform from the radio frequency (RF) to baseband, thereby avoiding any intermediate frequency (IF) stage. One major issue of DCR devices is the presence of RF impairments, which greatly complicate fundamental receiver functions, including the synchronization task. In this paper we propose a carrier frequency offset (CFO) recovery scheme for OFDM DCRs plagued by frequency-selective In-phase/Quadrature (I/Q) imbalances. Our approach is based on the ESPRIT algorithm and relies on the transmission of a typical OFDM training preamble having a repetitive structure in the time domain. Numerical simulations indicate that the proposed scheme outperforms existing state-of-the-art alternatives at the price of a certain increase of the processing load. [ABSTRACT FROM PUBLISHER]

The IEEE 802.16 family of standards for nomadic wireless metropolitan area networks adopts orthogonal frequency-division multiple-access as an air interface. In these systems, timing errors between the uplink signals and the base station time reference give rise to interchannel interference as well as multiple-access interference with an ensuing degradation of the error-rate performance. To mitigate this problem, users that intend to establish a communication link go through a synchronization procedure called Initial Ranging (IR) by which uplink signals can arrive at the base station synchronously and with approximately the same power level. In this work, a novel IR scheme compliant with the IEEE 802.16 specifications is presented. In contrast to existing methods, our solution operates on the basis of a generalized likelihood ratio test (GLRT) and provides improved timing and power estimates by properly taking into account the channel correlation across the signal bandwidth. In order to increase the resilience to multiple access interference, the GLRT approach is also exploited to derive a two-stage interference cancellation scheme. Numerical simulations and theoretical analysis are used to demonstrate the effectiveness of the proposed solutions and to make comparisons with existing alternatives. [ABSTRACT FROM PUBLISHER]

Spectrum sharing employs dynamic allocation of frequency resources for a more efficient use of the radio spectrum. Despite its appealing features, this technology inevitably complicates the synchronization task, which must be accomplished in an environment that may involve interference. This paper considers the uplink of an orthogonal frequency-division multiple-access (OFDMA)-based spectrum sharing system and provides solutions for estimating the frequency and timing errors of multiple unsynchronized users. In doing so, we exploit suitably designed training blocks and apply maximum likelihood methods after modeling the interference power on each subcarrier as a random variable with an inverse gamma distribution. The resulting frequency estimator turns out to be the extension to a multiuser scenario of a scheme that has previously been proposed for single-user spectrum-sharing systems. Timing recovery is more challenging and leads to a complete search over a multidimensional domain. To overcome such a difficulty, two alternative approaches are proposed. The first one relies on a simplifying assumption about the interference distribution in the frequency domain, while the second scheme operates in an iterative fashion according to the expectation-maximization algorithm. [ABSTRACT FROM AUTHOR]

In this letter, an initial ranging scheme for orthogonal frequency-division multiple-access systems is proposed by which users that intend to establish a communication link with the base station (BS) perform spreading in both the time and frequency domains and their synchronization parameters are estimated at the BS in closed-form using the ESPRIT algorithm. Compared to existing alternatives, the resulting scheme exhibits increased robustness against residual frequency offsets without involving computationally demanding peak search procedures. [ABSTRACT FROM AUTHOR]

Uplink synchronization in orthogonal frequency. division multiple-access (OFDMA) systems is a challenging task. In IEEE 802.16-based networks, users that intend to establish a communication link with the base station must go through a synchronization procedure called Initial Ranging (IR). Existing IR schemes aim at estimating the timing offsets and power levels of ranging subscriber stations (RSSs) without considering possible frequency misalignments between the received uplink signals and the base station local reference. In this work, a novel IR scheme is presented for OFDMA systems where carrier frequency offsets, timing errors and power levels are estimated for all RSSs in a decoupled fashion. The proposed frequency estimator is based on a subspace decomposition approach, while timing recovery is accomplished by measuring the phase shift between the users' channel responses over adjacent subcarriers. Computer simulations are employed to assess the effectiveness of the proposed solution and to make comparisons with existing alternatives. [ABSTRACT FROM AUTHOR]

The use of antenna arrays in wireless communications makes it possible to estimate the directions of arrival (DOAs) of impinging waveforms. The latter can be exploited to enhance channel estimation accuracy or as an input for advanced mobile positioning systems. In this paper, we consider the uplink of a multicarrier code-division multiple-access (MC-CDMA) network in which the base station is endowed with multiple receiving antennas arranged in a uniform linear array. Transmission takes place over a multipath channel, and the goal is the joint estimation of the channel responses and DOAs of the uplink signals. In doing so, we follow a maximum-likelihood (ML) approach and assume that users transmit orthogonal training sequences to facilitate the task of separating the signals. This way, all unknown parameters are estimated independently of each other with affordable complexity. Theoretical analysis and computer simulations are used to assess the performance of the proposed scheme and make comparisons with existing alternatives. [ABSTRACT FROM AUTHOR]

We consider a unified framework comprising both multicarrier code-division multiple-access (MC-CDMA) and orthogonal frequency-division multiple-access (OFDMA), and discuss nonlinear prefiltering for downlink transmissions based on Tomlinson—Harashima precoding. The base station (BS) is equipped with multiple transmitting antennas and channel state information is assumed to be available at the transmit side. We design the prefiltering matrices so as to minimize the sum of the mean square errors at all mobile terminals when a conventional single-user data detector is employed at the receiver side. In this way, most of the computational burden is moved to the BS, where power consumption and computational resources are not critical requirements. Computer simulations are used to assess the performance of the proposed scheme under different operating scenarios. It turns out that OFDMA outperforms MC-CDMA when the system resources (subcarriers and/or spreading codes) are optimally assigned to the active users according to the channel quality. [ABSTRACT FROM AUTHOR]

Orthogonal frequency division multiple access (OFDMA) has recently attracted vast research attention from both academia and industry and has become part of new emerging standards for broadband wireless access. Even though the OFDMA concept is simple in its basic principle, the design of a practical OFDMA system is far from being a trivial task. Synchronization represents one of the most challenging issues and plays a major role in the physical layer design. The goal of this paper is to provide a comprehensive survey of the latest results in the field of synchronization for OFDMA systems, with tutorial objectives foremost. After quantifying the effects of synchronization errors on the system performance, we review some common methods to achieve timing and frequency alignment in a downlink transmission. We then consider the uplink case, where synchronization is made particularly difficult by the fact that each user's signal is characterized by different timing and frequency errors, and the base station has thus to estimate a relatively large number of unknown parameters. A second difficulty is related to how the estimated parameters must be employed to correct the uplink timing and frequency errors. The paper concludes with a comparison of the reviewed synchronization schemes in an OFDMA scenario inspired by the IEEE 802.16 standard for wireless metropolitan area networks. [ABSTRACT FROM AUTHOR]

Combined equalization has recently been proposed to enhance the error rate performance of conventional multicarrier code-division multiple-access (MC-CDMA) systems. This technique applies pre-equalization at the transmitter in conjunction with post-equalization at the receiver, thereby splitting the overall equalization process into two separate parts. In this way, efficient power allocation over the available subcarriers is possible at the transmitter, while leaving the interference cancellation task at the receiver. In this paper, we consider the uplink of an MC-CDMA system employing combined equalization. As the users transmit from different locations, the uplink signals arrive at the base station after passing through different multipath channels and the goal is to estimate the pre-equalized channel frequency response of each user. This is pursued following two different approaches. The first operates in the frequency-domain and treats the channel gains over adjacent subcarriers as independent unknown parameters. The second operates in the time-domain and achieves better performance by reducing the number of unknown parameters. Both schemes are based on maximum-likelihood reasoning and require knowledge of the transmitted symbols. Numerical examples are given to highlight the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

This paper deals with channel acquisition and tracking in the uplink of a multicarrier code division multiple access (MC-CDMA) system. The transmission medium is characterized by multipath propagation, and the goal is to estimate the channel frequency response and the noise-power of each active user. Channel acquisition is performed jointly with the noise-power estimation following two different approaches. The first assumes independently faded subcarriers, while the second exploits the fading correlation across the signal bandwidth to improve the system performance. Both schemes are based on the maximum likelihood (ML) criterion and exploit some training blocks carrying known symbols. Channel tracking is pursued through least mean square (LMS) techniques using data decisions provided by a partial parallel interference cancellation (PPIC) receiver. Numerical results are given to highlight the effectiveness of the proposed methods and to present comparisons with other existing solutions. [ABSTRACT FROM AUTHOR]

Maximum-likelihood estimation of the carrier frequency offset (CFO), timing error, and channel response of each active user in the uplink of an orthogonal frequency-division multiple-access system is investigated in this study, assuming that a training sequence is available. The exact solution to this problem turns out to be too complex for practical purposes as it involves a search over a multidimensional domain. However, making use of the alternating projection method, we replace the above search with a sequence of mono-dimensional searches. This results in an estimation algorithm of a reasonable complexity which is suit- able for practical applications. As compared with other existing semi-blind methods, the proposed algorithm requires increased overhead but has more flexibility as it can be used with any sub- carrier assignment scheme. Simulations indicate that the accuracy of the CFO estimates asymptotically achieves the Cramer-Rao bound. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The time-division duplex (TDD) component of the universal mobile telecommunications system (UMTS) employs synchronous code-division multiple-access techniques with orthogonal spreading codes to provide protection against cochannel interference. In the presence of multipath propagation, however, the code orthogonality is lost and multiaccess interference is generated at the receiver. In such conditions, an estimate of the channel impulse response is required for reliable detection. In this paper, we propose and compare two pilot-assisted schemes for channel estimation in the downlink of the UMTS-TDD system. Both algorithms also provide estimates of the users' energies, which are needed to perform multiuser detection. Theoretical analysis and computer simulations are used to assess the channel estimation performance in terms of mean-squared errors and bit-error rate. It is shown that the accuracy of the proposed estimators attains the Cramer-Rao bound at intermediate/high signal-to-noise ratios. [ABSTRACT FROM AUTHOR]

This paper deals with timing and frequency recovery for the uplink of an orthogonal frequency-division multiple access (OFDMA) system. The frequency estimator is derived from ad hoc reasoning, whereas the timing estimator is based on the maximum-likelihood criterion. Both schemes rely on the repetition of a fixed pilot symbol. Their main feature is that they provide feed- forward estimates and allow synchronization in only two OFDM blocks. In contrast to other existing methods, they do not require that the subcaniers of a given user occupy adjacent positions in the signal bandwidth. This makes it possible to interleave subcarriers of different users so as to optimally exploit the frequency diversity of the channel. Theoretical analysis and computer simulations are used to assess the performance of the proposed synchronizers. It is found that the degradations due to residual frequency and timing errors are negligible at signal-to-noise ratios of practical interest. [ABSTRACT FROM AUTHOR]

Presents information on a study which proposed an algorithm for Doppler rate estimation in burst-mode phase shift keying transmissions. Description of the estimation algorithm; Sensitivity of proposed algorithm to Doppler shift; Simulation results; Conclusions.

Presents a study which dealt with the estimation of the channel impulse response in orthogonal frequency division multiplexed systems. Description of the signal model; Performance analysis; Simulation results.

Deals with carrier-frequency estimation for burst transmissions over frequency-selective channels. Maximum-likelihood (ML) frequency estimation; ML estimation with periodic training sequences; Ad hoc frequency estimation; Simulation results.

Presents information on a study which investigated a novel nondata-aided method for jointly estimating timing and carrier frequency offset in MSK-type modulation. Signal model; Timing and frequency estimation; Conclusions.