The time-domain signal in orthogonal frequency division multiplexing (OFDM) has a large peak-to-average power ratio (PAPR). Most existing solutions for this problem are designed for OFDM systems operating with a fully-digital MIMO architecture. However, these solutions cannot be used for an OFDM system operating with a hybrid beamforming architecture. Therefore, in this work we address PAPR reduction in OFDM with hybrid beamforming. We propose to transmit a PAPR reduction signal along with the precoded information signal. For an efficient design of the PAPR reduction signal, we propose to use extra radio frequency (RF) chains. The design of the PAPR reduction signal and the analog beamformers corresponding to the extra RF chains, is written as a constrained optimization problem. An iterative algorithm based on successive convex approximation is proposed to solve the optimization problem. We show that the proposed technique achieves a considerable end-to-end performance gain in the presence of a non-linear power amplifier.

The single carrier modulation is a potential candidate at the sub-THz band, because of the channel sparsity and the necessity for an analog front-end friendly modulation. However, the peak-to-average-power ratio (PAPR) of the single carrier modulation in a multi-user MIMO downlink scenario is still a concern. In this work, we address this problem, by transmitting a PAPR reduction signal (PRS) together with the precoded data symbols. The goal of the PRS is to reduce the PAPR of the continuous-time signal entering the power amplifier attached to each antenna. We propose two different methods to design the PRS. In the first method, we restrict the PRS to the channel null space, so that it does not cause interference with the users’ information symbols at the receiver. As such, this method has limited degrees of freedom for the PAPR reduction. Considering this, we propose a second method for the PRS design, where the PRS can interfere with the users’ information symbols in a constrained manner. This relaxation enables an increased PAPR reduction without reducing the data detection performance. The design of the PRS is formulated as an optimization problem and the performance of the proposed schemes is studied semi-analytically. Our analysis shows that the proposed scheme is able to reduce the PAPR by about 9 dB at the cost of only 0.5 dB SINR. Consequently, the bit-error-rate performance of the proposed scheme is negligibly impacted by non-linear power amplifier distortion.

The channel sparsity and the necessity for an analog front-end friendly modulation have made single carrier a considerable candidate for sub-THz communication. However, the peak-to-average-power ratio (PAPR) of the single carrier modulation in a multi-user MIMO scenario is still a concern. In this work, we reduce the PAPR of the single carrier scheme, by transmitting an additional PAPR reduction signal (PRS). The PRS is constrained to lie in the null space of the channel so that it does not cause interference with the users’ information symbols at the receiver. Moreover, the additional transmit power consumed by the PRS is studied. An optimization problem is formulated to design the PRS, and the optimal power allocation for the PRS is analyzed. Our analysis shows that by trading off only 1 % of the total transmitted power, a PAPR reduction of about 3 dB can be achieved. Further, our simulations reveal that when an optimal power is allocated to the PRS, the proposed scheme achieves a significant performance gain compared to the classical single carrier system in the presence of power amplifier distortion.

Orthogonal Chirp Division Multiplexing (OCDM), a chirp based waveform, has gathered interest because of its robustness to time and frequency selective channels. In this work, we study the phase noise effect in OCDM, which has not been investigated in the literature yet. We analytically show that phase noise causes a common phase error (CPE) rotation of data symbols and inter-symbol interference (ISI) in OCDM. We investigate the influence of OCDM system parameters on the phase noise effect. For this purpose, a system parameter, chirp frequency offset is defined and its influence on the CPE and ISI is studied. Depending on the value of this chirp frequency offset, either CPE or ISI becomes the dominant phase noise effect. Our findings also show that an OCDM system can be made robust to phase noise at the cost of spectral efficiency by reducing the number of employed chirps. Our performance analysis demonstrates that OCDM has a gain over OFDM and Single Carrier (SC) in a fading channel with phase noise because of the frequency diversity and the different distortion caused by the phase noise generated ISI terms respectively. We also investigate the performance of OCDM in the presence of narrowband interference with phase noise and observe that OCDM has a benefit over OFDM and SC at medium and low interference power.

Radar has become a key sensor in many advanced driver assistance systems (ADAS). Due to its excellent range and Doppler resolution, low cost, and robustness against environmental conditions, it is considered an attractive sensor for detecting and tracking vulnerable road users (VRUs). In this paper, we provide a theoretical analysis of the accuracy of radar based VRU detection and tracking systems, thereby focusing on a number of specific scenarios such as early detection and tracking of VRUs, a VRU crossing the road at constant speed in front of a vehicle, a VRU moving parallel to a vehicle, etc. More specifically, we derive the Cramer-Rao lower bound (CRLB) for position and velocity estimation of a moving target based on a sequence of noisy range, azimuth, and Doppler measurements taken from a moving ego-vehicle equipped with one or multiple radars. Not only does the CRLB serve as a benchmark to evaluate the performance of any practical tracking algorithm, it also allows to gain practical insights regarding the impact of the radar setup and configuration on the tracking accuracy in different realistic scenarios. Furthermore, we show that the generalized least-squares estimator (GLSE) achieves excellent performance when few measurements are available, and propose a novel active sensing (AS) application based on the CRLB where the radar configuration is optimized on the fly to improve either tracking accuracy or computational efficiency.

We consider uplink multiuser multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) communication. The transmit (Tx) side of the envisaged system consists of several single-antenna users or/and several multiple-antenna users. At the receive side, a multiple-antenna access point employs compressive sensing techniques to estimate the channel impulse response from the preamble portion of the observed packets. The traditional approach is that of orthogonal pilot allocation: during a short training period, each OFDM subcarrier is assigned exclusively to a single Tx antenna. In this case, the channel state information can conveniently be acquired on a per Tx antenna basis. To the best of our knowledge, all related research imposes that all Tx antennas are allocated the same amount of pilots (which must then be tailored for the most extreme channel conditions). However, in the considered system, Tx antennas may experience totally different channel conditions. Under these circumstances, the use of a fixed number of pilots per Tx antenna results in a lot of unnecessary overhead. To tackle this problem, our work addresses the design of efficient algorithms for adaptive orthogonal pilot allocation. The following design principles are applied: orthogonal pilot allocation, constant-modulus modulation, minimum measurement matrix mutual coherence optimization, and the condition that the number of pilot subcarriers allocated to each Tx antenna is adjusted to the channel conditions experienced by that Tx antenna. The paper tackles the problem of determining the optimal number of pilot subcarriers as well as the optimal positions of the pilots. To facilitate adaptive operation, we propose a reduced-complexity method to determine the optimal pilot positions. The performance of our algorithms is demonstrated by means of computer simulations, using both theoretical channel models and results from our own channel measurement campaign.

Visible light communication (VLC) is seen as a potential access option for fifth-generation (5G) wireless communication (Wanget al., 2014) and (Ayyashet al., 2016) and beyond 5G (Strinatiet al., 2019). A reliable VLC system benefits from an accurate estimate of the receiver’s position and orientation. In many cases, the orientation of the receiver is estimated with an external orientation estimation device. However, these devices generally suffer from drift and misalignment, causing an uncertainty in the orientation presented to the receiver. Hence, the external device can only provide a probability distribution of the orientation to the position estimator, which can be used as prior information for the position estimation. Since the orientation of a receiver greatly affects the performance of a visible light system, the orientation uncertainty will degrade the performance of standard positioning algorithms, implying it should be taken into account when designing a robust positioning algorithm. In this paper, we design an received signal strength (RSS)-based hybrid position and orientation estimation algorithm using the hybrid maximum likelihood (ML)/maximuma posteriori(MAP) (HyMM) principle for a multiple LEDs - multiple photodiodes (PDs) (MLMP) system to take into account the presence of prior information on the orientation. The proposed HyMM estimator is compared with three existing estimators, i.e., the simultaneous position and orientation (SPO) estimator, the misspecified maximum likelihood (MML) estimator and the first-order-approximation-based positioning algorithm, subject to the orientation uncertainty. Further, in order to analytically assess the performance of the proposed estimator, the theoretical lower bound on the mean squared error (MSE), i.e. the hybrid Cramér-Rao bound (HCRB) for HyMM is derived. Computer simulations show an asymptotic tightness between the performance of the estimator and its associated theoretical lower bound.

Time-difference-of-arrival (TDOA) localization systems estimate the difference in propagation delay between a target transmitter and pairs of receivers in order to locate the target. One important requirement is that the receiver nodes should be accurately time-synchronized, which is hard to achieve in practice without relying on cable-connected receivers. Over-the-air synchronization algorithms often rely on periodic messages from a master node, but due to clock drift, the rate of these messages needs to be high. In this paper, we propose a new synchronization algorithm that additionally exploits the carrier frequency offset (CFO) between the target node and the receiver nodes to reduce the rate of messages from the master node while maintaining good synchronization accuracy. Simulation results show the good performance of the proposed method, and preliminary experimental results show its feasibility on a hardware platform.

In this paper, the problem of joint sub-channel assignment, antenna selection, and power control in the downlink (DL) of an orthogonal frequency division multiple access (OFDMA) femtocell network is investigated in order to maximize system throughput. This problem, due to its discrete nature and the interference, is modeled as a mixed integer non-linear programming (MINLP) optimization problem, which is complicated to solve. To deal with this complexity, the problem is decomposed into three subproblems: 1) sub-channel assignment, 2) antenna selection, and 3) power control. We then present an iterative algorithm for these subproblems resulting in an optimized network throughput. The superiority of our proposed method compared to other existing works is demonstrated through simulation results.

In ultra wideband (UWB) orthogonal frequency-division multiplexing (OFDM) systems, compressive sensing (CS) is often employed to produce a pilot-assisted estimate of the sparse channel. The corresponding estimation performance depends to a large extent on the considered pilot allocation (PA) method, i.e., the way to select which OFDM subcarriers are best used to transmit the pilot symbols. The development of good practical PA methods has recently received a lot of attention in the scientific literature. The main challenge is to provide an attractive trade-off between the complexity of the PA method and the achieved channel estimation performance (and by extension the achieved bit error rate). In this paper, we propose a novel PA method based on simulated annealing (SA). Simulations are conducted to confirm the validity of our approach. Compared to the state-of-the-art method, the proposed PA method is shown to achieve better performance with a lower complexity.

Visible light communication (VLC) is seen as a supplement for fifth-generation (5G) wireless communication in short-range high data rate communication applications [1]. A reliable VLC system relies on an accurate estimate of the position and orientation of the receiver, which corresponds to the six-dimensional positioning problem mentioned in [2]. In this paper, we investigate the simultaneous position and orientation estimation (SPO) problem using received signal strength (RSS), for a visible light system containing multiple LEDs and multiple photodiodes (PDs) (MLMP). Although in general, the position and orientation of the receiver can be represented by a vector and a rotation matrix, respectively, the constraints imposed by the rotation matrix make the numerical optimization in the estimation process cumbersome, e.g, the commonly used constrained optimization method is often very complex and non-robust. Therefore, in this paper, we design two SPO algorithms using the principle of optimization on manifolds, which alleviates the constraints from the rotation matrix. In addition, we propose an initialization algorithm, based on the direct linear transformation (DLT) principle, to obtain an initial estimate in closed-form for the iterative algorithms. To evaluate the performance of the proposed RSS-based SPO algorithms, we derive the Cramer-Rao bound (CRB). In particular, the orientation error component of the CRB corresponds to the intrinsic CRB or the CRB on manifolds, which measures the error in the estimated rotation matrix in a physically meaningful way. Finally, computer simulations show an asymptotic tightness between the performance of the proposed algorithms and the theoretical lower bound, demonstrating the effectiveness of the proposed solutions.

The identification of maximum tolerable levels for potential mismatches is critical when designing communication systems. In this paper, we derive maximum tolerable levels for time-interleaved analog-to-digital-converter (TI-ADC) gain mismatch in orthogonal frequency-division multiplexing (OFDM) systems. To this end, we first analytically evaluate the bit error rate (BER) for square quadrature amplitude modulated-OFDM systems that are impaired by: 1) the gain mismatch of a TI-ADC and 2) the channel estimation errors (CEEs) of a zero-forcing equalizer. Our analysis includes the cases of a frequency-selective Rayleigh fading channel and a wired channel. Next, built on the obtained BER expressions, a threshold is established on the gain mismatch level, at which an error floor caused by the gain mismatch is below a given BER value at high signal-to-noise ratios in the absence of CEEs. Finally, numerical results further show that if and only if we set the gain mismatch level below 0.25 of this threshold, there is essentially no BER performance degradation compared with the mismatch-free case.

We consider a visible light positioning system using modulated LEDs at the transmitter and photodiodes (PDs) combined with apertures at the receiver. The layout of the aperture-based receiver is designed in order to have angular diversity, implying it can detect the direction from which light is coming, by simply comparing the relative differences in received signal strength values in the different PDs. Hence, with this receiver, it is possible to extract the angle-of-arrival (AOA) of the light without needing the knowledge of the transmitted optical power. In this paper, we consider an algorithm, based on the maximum likelihood (ML) principle, to estimate the AOA, and obtain the position of the receiver in 3-D through triangulation. The ML algorithm, of which the practical implementation searches for the optimal value of the AOA starting from an initial estimate, suffers from convergence problems if the initial estimate is too far from the true AOA. Hence, we propose an initial low-complexity coarse estimation algorithm for the AOA, and make the algorithm iterative, where in each iteration, the initial estimate for the AOA is updated based on the previous position estimate. We show that the algorithm yields centimeter performance, i.e., an accuracy of 10 cm or better, using a limited number of LEDs, e.g., four LEDs for a 5 m × 5 m area.

Unique word- (UW-) orthogonal frequency division multiplexing (OFDM) is a new multicarrier technique that is shown to be superior to the cyclic prefix- (CP-) OFDM system in terms of bit-error-rate performance in fading channels. Furthermore, in the literature, it is shown by simulations that UW-OFDM has a considerably lower out-of-band (OOB) radiation compared with CP-OFDM. In this paper, we derive an analytical expression to investigate the spectral characteristics of UW-OFDM to be able to determine the effect of the different system parameters on the OOB radiation. In addition, we include in our analysis the effects of the interpolation filter and the unique word. We show that the OOB radiation in UW-OFDM is mainly determined by the number of non-modulated subcarriers reserved as the guard band, and the ratio of the UW sequence length to the number of subcarriers. By increasing either of these two parameters, we show that we can suppress the OOB radiation to any amount. This is an important advantage of UW-OFDM over classic CP-OFDM systems, and shows that UW-OFDM is an excellent candidate for cognitive radio systems.

This paper presents an efficient procedure to numerically evaluate the exact bit error rate of a rectangular quadrature amplitude modulated-orthogonal frequency division multiplexing (OFDM) system that is impaired by the gain mismatch of a time-interleaved analog-to-digital converter. As opposed to previous contributions, no approximations are involved in this procedure. The obtained results allow to accurately analyze the effect of this type of mismatch on the performance of practical high-speed OFDM systems. The accuracy and efficiency of the proposed technique are demonstrated by comparing analytical results with brute-force Monte-Carlo simulations.

Visible light positioning (VLP) for indoor applications has recently received considerable attention. In this paper, an aperture based receiver is considered and the position is estimated based on the angle-of-arrival (AOA), because the AOA can be determined by comparing the relative differences in the received signal strengths (RSS) in the different photodiodes (PDs) of the receiver. Hence, in contrast to RSS based positioning solutions, knowledge of the transmitted signal strengths is not required. The performance of the proposed approach depends on the transmitted signal strength only through the dependency on the signal-to-noise ratio. To evaluate the accuracy of the estimates of the AOA, we derive the Cramer-Rao lower bound, and show that the expected error on the AOA is very small. Applying triangulation to determine the position of the receiver, it follows that centimetre accuracy can be obtained with the aperture-based receiver.

Recently, a compact receiver structure, containing a set of apertures and photo diodes (PDs) that are arranged to offer good angular diversity, was proposed for visible light communication [1]. In [2], this receiver structure was considered for visual light positioning (VLP), where the positioning accuracy for direct estimation of the position was evaluated using the Cramer-Rao Lower bound. However, it was assumed that the receiver is parallel to the ceiling, an assumption that is not realistic in practice. Therefore, in this paper, we extend the results of [2] to include the orientation of the receiver in the performance evaluation. The results show that, provided that the receiver knows its orientation, a rotation of the receiver has essentially no effect on the performance. Further, as long as sufficient LEDs are within the field-of-view of the receiver, the performance degradation will be small. This will be the case for a wide range of tilt angles. Hence, we can conclude that centimetre accuracy can be obtained for a wide range of receiver orientations.

In this paper, we investigate the problem of indoor positioning using visible light systems. The directional detector array we use is comprised of a number of receiving elements, each consisting of an aperture and a photo diode, which are arranged to offer good angular diversity, and can be implemented within a compact receiver structure [1]. The receiving elements receive the light from a number of white LEDs, which are typically attached to the ceiling, and which act as anchors. In order to get an indication of the received signal strengths of the different LEDs, we average the received signals over time. The relative signal strengths in the different receiving elements do not only provide information on the distance between the LEDs and the detector array, but also about the angle-of-arrival of the light. By combining the information of the receiving elements, the position of the detector can be estimated. In order to assess the accuracy of positioning algorithms based on this approach, we derive the Cramer-Rao lower bound on the position accuracy. Assuming the white LEDs transmit an optical power of 1 W, and the time averaging is done over 1 millisecond, an accuracy of the order of a centimetre can be achieved.

Unique-word (UW) OFDM is a new OFDM variant that offers several advantages over cyclic prefix OFDM, such as better bit-error-rate (BER) performance in frequency selective channels and much lower out-of-band radiation. However, all results with respect to the BER performance are based on Monte-Carlo simulations only, and offer little insight in the behaviour of the system performance as function of its parameters. In this paper, we theoretically analyse the performance of the UW-OFDM system for both a deterministic channel and a fading channel, and offer a systematic way to generate the UW-OFDM signal to optimize the performance of the system. When the fading channel is known at the transmitter, we show that we can obtain a diversity order that is of the same order of magnitude as the theoretically maximum diversity order, but when the channel is not known at the transmitter, it will be very hard to achieve maximum diversity order, although the obtained diversity order is larger than one.

In cooperative localization, target users take advantage of neighboring users in the network to improve their position estimates. In dense networks, the number of neighbors is high and consequently a very large amount of information is available. Using all neighbors (full cooperation) results in a considerable amount of data that is to be processed and transmitted causing high network traffic, delays and reduced battery lifetime. The goal in censoring is to limit the amount of cooperation to reduce the amount of data to be transmitted, without losing (much) in positioning accuracy compared to full cooperative localization. In this paper we propose a novel censoring technique based on the Bayesian Cramér-Rao Lower Bound (CRLB) that takes into account both the uncertainties of the neighbors and the link quality in terms of LOS/NLOS. With the use of the unscented transform and a greedy search approach, the censoring can be performed accurately and at a low computational complexity.

The current multi-camera systems have not studied the problem of person tracking under low resolution constraints. In this paper, we propose a low resolution sensor network for person tracking. The network is composed of cameras with a resolution of 30×30 pixels. The multi-camera system is used to evaluate probability occupancy mapping and maximum likelihood trackers against ground truth collected by ultra-wideband (UWB) testbed. Performance evaluation is performed on two video sequences of 30 minutes. The experimental results show that maximum likelihood estimation based tracker outperforms the state-of-the-art on low resolution cameras.

We revisit the problem of describing optimal anchor geometries that result in the minimum achievable MSE by employing the Cramer Rao Lower bound. Our main contribution is to show that this problem can be cast onto the whelm of modern Frame Theory, which not only provides new insights, but also allows the straightforward generalization of various classical results on the anchor placement problem. For example, by employing the frame potential for single-target localization we see that the directions of the anchors, as seen from the target, should optimally be as orthogonal as possible and that the existence of an optimal geometry for an arbitrary number of anchors is governed by a fundamental inequality. Furthermore, the frame-theoretic approach allows for simple derivation of some properties on optimal anchor placement that prove to be useful in a tractable approach for the more complex, multi-target anchor placement problem. In a more general sense, the paper builds a refreshing bridge between the classical problem of wireless localization and the powerful domain of Frame Theory, with far-reaching potential.

The problem of channel estimation and in-phase/quadrature-phase (IQ) imbalance is one of the main challenges that has to be faced by uplink transmission in orthogonal frequency division multiple access (OFDMA) systems. The previously reported investigations for such systems are limited to synchronous and uncoded transmission. Furthermore, in these previous works, equalization of the IQ imbalance should be performed after the estimation process in order to properly detect data symbols. In this contribution, the transmit and receive IQ imbalance for uplink OFDMA systems is investigated in the context of asynchronous and bit-interleaved coded modulation (BICM) transmission. We propose a novel data detector exploiting the unwanted IQ as a beneficial resource to achieve a diversity gain, without the necessity of an additional equalizer. In addition, we develop a novel code-aided algorithm to estimate the propagation delays and the overall channel impulse responses (CIRs), which include the physical CIRs and IQ imbalance occurring at both the transmitters and base station. Since the exact maximum-likelihood (ML) solution to this problem turns out to be too complex for practical purposes, we resort to the space alternating generalized expectation-maximization (SAGE) algorithm as a low complexity method to perform parameter estimation. The proposed estimation algorithm operates in an iterative way, in which the soft information provided by the channel decoder is utilized as a priori information to refine the estimates. The computational complexity analysis and simulation results confirm the effectiveness of the proposed estimation algorithm and the proposed detector for practical application.

In this paper, we consider the problem of pilot aided channel estimation (PACE) for CP-OFDM based systems. The positions of the pilot carriers have a strong influence on the mean squared error (MSE) of the channel estimate, and thus indirectly on the overall system performance. Hence, we are interested in the pilot carrier placement that minimizes this MSE. However, the optimization problem at hand is an integer combinatorial optimization problem and thus NP hard. In this paper, we propose two heuristic algorithms searching for a good pilot carrier placement with a low MSE. Both algorithms have comparable performance: the resulting MSE is for both algorithms similar, and in the majority of the cases the optimal pilot carrier placement is found. We compare the results from the proposed heuristic algorithms with the results for the maximum distance distribution from [1]. We are able to achieve lower channel estimate MSE than with the maximum distance distribution, although the difference becomes small for large FFT size.

Unique word (UW)-OFDM is a newly proposed multicarrier technique that has shown to outperform cyclic prefix (CP)-OFDM in fading channels. Until now, the spectrum of UW-OFDM is not thoroughly investigated. In this paper, we derive an analytical expression for the spectrum taking into account the DFT based implementation of the system. Simulations show that the proposed analytical results are very accurate. Compared to CP-OFDM, we show that UW-OFDM has much lower out-of-band (OOB) radiation, which makes it suitable for systems with strict spectral masks, as e.g. cognitive radios. Further, in this paper, we evaluate the effect of the redundant carrier placement on the spectrum.

The articles in this special issue focus on the technology and applications supported by virtual multiple antennas, or VMIMOs. The impetus for this has been spurred by the strong desire to understand VMIMO, which is a rapidly growing research area. VMIMO is believed to be a key technology for beyond 4th generation mobile communications technologies (B4G). It enables one to make use of all the neighboring terminals and amortize the cost of multiple antennas; hence, a large MIMO channel can be created to increase capacity significantly as well as improve error rate performance. Nevertheless, fundamental roadblocks need to be addressed in order to take full advantage of VMIMO.

The main drawback today for range-based indoor localization is the requirement of a sufficient amount of fixed reference nodes within radio range of the user. However, these reference nodes, called anchors, are expensive and require professional maintenance. Using ultra-wideband in an indoor environment, the number of anchors can be reduced to one when reflections are taken into account. With the help of a floorplan it is possible to obtain a set of virtual anchors that can be associated with the reflections. In this paper, a low-complex two-step algorithm is proposed that is able to accurately estimate the user positions using a single anchor. In a first step, the algorithm tries to estimate a number of rigid structures using the noisy inter-node distances and tries to fit this structure in the room by exploiting the measured reflections. It is shown that the presented algorithm can provide positioning accuracy similar to multi-anchor localization algorithms, even in scenarios with many unwanted scatterers and non-line-of-sight.

Unique-word (UW) OFDM is a new multicarrier technique that was recently proposed in [1]. To construct the UW-OFDM signal, some carriers must be sacrificed to transmit redundant information -- these carriers are called the redundant carriers. It turns out that the average redundant energy needed strongly depends on the positions of the redundant carriers. In this paper, we look for the redundant carrier placement that minimizes the average redundant energy -- this needs an exhaustive search over all possible redundant carrier placements. Hence, the computational complexity to find the optimal redundant carrier distribution is very high. Therefore, we introduce a suboptimal redundant carrier placement -- the quasi-uniform (QU) distribution, that is based on the results of the exhaustive search, and determines the positions of the redundant carriers at no computational cost. We show in this paper that the redundant energy needed for the proposed QU distribution is very close to the theoretical minimum average redundant energy.

Unique word (UW) OFDM is a new multicarrier technique that was first introduced in [1]. In this technique, the guard interval is a part of the inverse FFT (IFFT) output, and is filled with known symbols. To be able to construct - by using an IFFT - a time domain signal containing a block of known samples, it is necessary to sacrifice a number of carriers. These carriers are called redundant carriers and the symbols modulated on these redundant carriers depend on the data to be transmitted. In this paper, we evaluate the distribution of the redundant carriers over the bandwidth. It turns out that when the positions of the redundant carriers are not carefully selected, the extra energy needed for the redundant carriers can become very large. We propose a novel algorithm for the placement of the redundant carriers that is based on a simple analytical expression, such that extensive simulations to find the optimal distribution is avoided. The proposed distribution yields essentially minimal average redundant energy.

To obtain good position accuracy with state-of-the-art indoor localization algorithms, multiple anchors must be within radio range of the user. However, anchor placement and maintenance is expensive. By reducing the number of anchors per room, overall costs are reduced. In [1], an algorithm that can estimate the position of a user using only one anchor and a priori floor plan information. However, performance for static localization was poor due to the presence of ambiguities. This paper describes a cooperative positioning algorithm that utilizes a single anchor and provides both the position and the position uncertainty for multiple users, by letting the users exchange their position information. The problem is represented with a factor graph, and belief propagation (BP) is used to extract the positions and their accuracy. The proposed cooperative algorithm leads to a significant improvement of the positioning accuracy compared to the non-cooperative method from [1].

In this paper, we consider unique-word (UW) OFDM, where the guard interval, filled with known samples, is a part of the inverse FFT output. In order to construct the known samples in the UW-OFDM signal, a number of carriers needs to be reserved as redundant carriers. The information modulated on these redundant carriers depends on the transmitted data. However, the energy transmitted on the redundant carriers strongly depends on the positions of these redundant carriers. In this paper, we consider the distribution of the redundant carriers from that is suboptimal but yields an average redundant energy that is close to the minimum, and analytically evaluate the redundant energy as a function of the system parameters, i.e. the FFT length, the number of redundant carriers and the length of the unique word. Based on our results, a simple rule of thumb can be obtained for the selection of the system parameters, which is helpful in the design of UW-OFDM systems.

We propose an iterative decision-directed joint frequency offset (FO) and channel estimation algorithm in a known symbol padding (KSP) orthogonal frequency division multiplexing (OFDM) system, where the guard interval is filled with pilot symbols. Besides those time domain pilot symbols, some additional pilot symbols are transmitted on the pilot carriers. The decision-directed algorithm is initialized by pilot-aided FO estimation without channel knowledge. We propose a possible initialization algorithm that operates in the frequency domain (FD). After the initialization phase, the iterative decision-directed estimation algorithm is applied. For the channel estimation step, an existing pilot-aided channel estimation algorithm is extended to a decision-directed algorithm which uses the Fast Fourier Transform outputs at both the pilot and data carrier positions. For the uncoded case, the proposed iterative decision-directed joint FO and channel estimation algorithm reaches the bit error rate performance of a receiver with perfect synchronization and perfect channel knowledge. For a coded system, there is small loss in performance of less than 1 dB when our proposed algorithm is applied compared to a receiver with perfect knowledge about the FO and the channel.

In this paper we present a time delay estimation algorithm for known symbol padding (KSP) orthogonal frequency division multiplexing (OFDM) systems. The estimator exploits both the pilot symbols in the guard interval and on the pilot carriers, and takes the frequency selectivity of the fading channel into account. Our simulations indicate that the time delay estimator gives rise to only a small degradation of the bit error rate (BER) performance, as compared to a receiver with perfect synchronization.

As a multicarrier signal can exhibit large peaks in the time domain, the amplifier used to transmit the multicarrier signal must be highly linear, and thus expensive, to avoid nonlinear signal distortion. To reduce the high peak-to-average power ratio, and hence to allow cheaper amplifiers, several techniques are described in the literature. However, the drawback of these techniques is that they cause non-linear in-band distortion and out-of-band radiation, reduce the system throughput or require side-information. To avoid these drawbacks, we propose a PAPR reduction technique that has none of the above mentioned disadvantages. In the proposed technique, we replace some of the transmitted data symbols by nulls, i.e we introduce errors in the transmitted signal. To counteract the effect of the introduced symbol errors, the transmitted information is encoded. At the receiver, an iterative decoder is used to correct the transmitter and channel errors. The performance of the proposed technique is compared with the clipping technique. Although the clipping technique slightly outperforms the proposed technique with respect to the obtainable PAPR reduction and corresponding BER degradation, the proposed technique does not suffer from non-linear in-band distortion and out-of-band radiation.

In this paper, we propose an iterative joint DA/DD channel estimation algorithm for known symbol padding (KSP) OFDM. The pilot symbols used to estimate the channel are not only located in the guard interval, but also on some of the OFDM carriers. Initially, the channel is estimated using the pilot symbols only. Next, a decision is made with respect to the transmitted data symbols. We consider both hard and soft decision of the data symbols. The decisions on the data symbols are then used to update the channel estimate in a joint DA/DD estimation algorithm. The algorithm iterates between data detection and channel estimation until convergence is reached. At high SNR, the MSE performance of the iterative estimator converges to the MSE performance of the case where all data symbols are prior known at the receiver, i.e. the all pilot DA estimator. It turns out that the MSE performance of hard decision of the data symbols reaches the MSE of the all pilot DA estimator at lower SNR than that of soft decision of the data symbols; this is caused by some approximations needed to simplify the estimation algorithm.

This paper proposes a new iterative channel estimation algorithm for known symbol padding (KSP) orthogonal frequency division multiplexing (OFDM) based on the Expectation Maximization (EM) algorithm . The guard interval is filled with pilot symbols and the remaining part of the pilot symbols is put on some of the OFDM carriers. To start up the EM algorithm an initial channel estimate is obtained by using only the pilot symbols. Then the EM algorithm is applied until convergence is reached. The performance of this estimator is compared with the performance of the iterative joint data-aided (DA) / decision-directed (DD) estimator proposed in [1]. The EM algorithm converges to the performance of the all pilot estimator for lower SNR than the iterative joint DA/DD estimator, but the gain in performance results in a higher computational complexity.

Spectrum-overlay scenarios for wide-band multi-carrier (MC) systems bring new technical challenges along that must be considered during the system design. In such scenarios, the receiver has to perform actions, such as channel estimation and synchronization, in the presence of possibly strong in- band interference. In this paper, we investigate the impact of digitally modulated narrowband interference (NBI) on the iterative algorithm for joint channel estimation and frame synchronization as proposed in (Guenach et al., 2008) for uplink MC-CDMA. Moreover, we modify the algorithm to have better performance in the presence of NBI. The performance of the modified algorithm taking into account the statistics of NBI is verified through computer simulations and compared with the results from (Guenach et al., 2008).

The estimation and tracking of the fractional carrier frequency offset (CFO) is a crucial issue in the implementation of orthogonal frequency division multiplexing (OFDM) systems. In this contribution, we present a novel code-aided fractional CFO estimation algorithm based on the expectation-maximization (EM) algorithm. The proposed algorithm exchanges soft information from the channel decoder, in the form of a posteriori probabilities of the coded symbols, between the demapper, the decoder, and the CFO estimator in an iterative way. The proposed estimation scheme can work with any detector as long as the detector is able to compute the a posteriori probabilities (APPs) of the data symbols.

In this contribution, we propose a novel code-aided channel tracking and carrier frequency offset (CFO) and/or phase noise (PN) estimation algorithm for orthogonal frequency-division multiplexing (OFDM) systems. Starting from the maximum-likelihood (ML) principle, we derive a low complexity estimator based on the expectation-maximization (EM) algorithm. It turns out that the estimator receives soft information from the channel decoder to refine the CFO, PN, and channel estimates. Simulation results revealed that the proposed algorithm outperforms the conventional techniques in terms of bit-error rate (BER).

In this contribution, we propose a new code-aided synchronization and channel estimation algorithm for uplink MC-CDMA. The space alternating generalized expectation- maximization (SAGE) algorithm is used to estimate the channel impulse responses, propagation delays and carrier frequency offsets of the different users. The estimator, multi-user detector, equalizer, demapper and channel decoder exchange soft information in an iterative way. The performance of the proposed algorithm is evaluated through Monte Carlo simulations. Impressive performance gains are visible as compared to a conventional data-aided estimation scheme.

In this paper, we derive the Cramer-Rao bound (CRB) for data-aided channel estimation for OFDM with known symbol padding (KSP-OFDM). The pilot symbols used to estimate the channel are distributed over the guard interval and OFDM carriers, in order to keep the guard interval length as small as possible. An analytical expression for the CRB is obtained by performing a proper linear transformation on the observed samples. At low SNR, the CRB corresponds to the low SNR limit of the CRB obtained in [1], where it is assumed that the influence of the data symbols on the channel estimation can be neglected. At high SNR, the CRB is determined by the observations that are independent of the data symbols; the observations that are affected by data symbols are neglected. The CRB depends on the number of pilots and slightly increases with increasing guard interval length, but is essentially independent of the FFT size and the used pilot sequence. Further, a low complexity ML channel estimation technique is derived based on the linear transformation. Although in this estimation technique only a part of the observation is used, the mean squared error (MSE) performance of this estimate reaches the CRB for a large range of SNR, but a high SNR, the MSE reaches an error floor caused by the approximations made in the derivation.

This paper compares parametric and nonparametric channel estimation methods in a multipath environment. For both methods, the MSE on the received symbol pulse is shown to consist of a modeling error term and an estimation error term. We lower bound the latter term by computing the associated Cramer-Rao bounds. We point out that the MSE can be minimized by a proper selection of the number of parameters to be estimated. In addition, we present simulation results from practical estimators that confirm our theoretical analysis. Our numerical results indicate that the parametric method outperforms the nonparametric approach.

Spectrum-overlay scenarios for wideband multi-carrier (MC) systems bring new technical challenges that must be considered during the system design. In such a scenario, the receiver has to perform actions, such as channel estimation and synchronization, in the presence of possibly strong in-band interference. In this paper, we modify the iterative maximum likelihood joint frame synchronization and channel estimation algorithm for downlink MC-CDMA presented in (M. Guenach et al., 2006) to have better performance in the presence of narrowband interference. The performance of the modified algorithm is verified through computer simulations and compared with the original one.

In this paper, we consider an overlay system where narrowband AM signals interfere with a broadband multicarrier system (M. Schnell et al., 2004). To reduce the effect of the AM narrowband interference on the multicarrier system, we propose a low complexity algorithm to estimate AM narrowband interference. We derive analytical expressions for the performance of the estimator and verify the results with simulations. The proposed estimator is able to produce accurate estimates of the frequencies, and track the time-varying amplitudes of the AM signals. The estimator can reduce the power of the AM signal to a level that is approximately 20 dB lower than the multicarrier power, independent of the AM signal power.

In wireless communications, the channel can often be modelled as a time-selective frequency-selective fading channel with line-of-sight (LOS) component. Although orthogonal frequency division multiplexing (OFDM) systems are developed to cope with the frequency selectivity of the channel, they will suffer from interference caused by the time-varying character of the channel, present in particular in the LOS component, when the number of carriers increases. A proper selection of the system parameters, such as the number of carriers and the length of the cyclic prefix, can alleviate the effect of the doubly-selective channel on the system performance. In this paper, we investigate analytically the effect of the aforementioned system parameters on the performance, and determine the optimum values of the system parameters. Further, we study the effect of deviations from the optimum parameters on the system performance

This paper investigates the effectiveness of a (non-)code-aided ML-based FB phase synchronizer at the low operating signal-to-noise ratio of capacity-approaching codes. We show that the performance of the code-aided synchronizer is very close to that of a data-aided synchronizer that knows all data symbols in advance. This illustrates the optimality of the code-aided synchronizer. For the non-code-aided and the data-aided synchronizer, the linearized mean square phase error (MSPE) is evaluated analytically in the case of a first order loop. We demonstrate that, the MSPE of the non-code-aided synchronizer equals that of the data-aided synchronizer when the carrier phase is essentially constant and the loop filter gain is the same for both synchronizers, but that the non-code-aided synchronizer (as compared to the data-aided synchronizer) yields a larger MSPE due to phase fluctuations. This proves that code-aided FB phase estimation outperforms non-code-aided FB phase estimation when that the phase to be estimated is time-varying.

In orthogonal frequency division multiplexed (OFDM) systems affected by carrier frequency offsets, frequency ambiguity resolution, i.e. the estimation of the part of the frequency offset corresponding to an integer times the carrier spacing, is a crucial issue. The proper action of frequency ambiguity resolution algorithms can be strongly affected by the presence of disturbances, like narrowband interference (NBI). In this paper, the susceptibility of the blind and data aided ML frequency ambiguity estimators to NBI signals is investigated in an analytical way. The analytical results are verified by means of simulations. Although the estimators turn out to be essentially independent of the bandwidth of the interferers and the number of interferers, the performance of the estimators is very sensitive to the positions of the interferers.

In this paper, we present a novel code-aided joint synchronization and channel estimation algorithm for downlink multicarrier code-division multiple access. The expectation-maximization algorithm is used to locate the maximum-likelihood estimate of the channel impulse response, propagation delay, and carrier frequency offset. The estimator accepts soft information from the decoder in the form of a posteriori probabilities of the coded symbols, and can be interpreted as performing joint estimation and data detection. The performance of the proposed algorithm is verified through computer simulations. Impressive performance gains are visible as compared with a conventional data-aided estimation scheme.

This contribution deals with the problem of combining (non-binary) error-correcting codes with higher-order modulation schemes on AWGN and flat fading channels through interleaved coded modulation. We extend the idea of bit-interleaved coded modulation (BICM) to a more general form. With the aid of factor graph representations, we show how higher-order modulation can be combined with non-binary codes so that noniterative detection becomes optimal. This is in contrast with conventional BICM, for which non-iterative detection can be far from optimal. Through computer simulations, we compare an optimal non-iterative scheme with conventional BICM. It turns out that the gain due to optimal detection is outweighed by the loss in time-diversity as compared to BICM-ID.

This contribution deals with the comparison of the sum-product algorithm (SPA) and its log-domain version (log-SPA) for decoding LDPC (low density parity check) codes over general binary extension fields. For both algorithms, we determine their computational complexity based on the number of real-valued operations and investigate their sensitivity to quantization effects. Whereas the log-SPA yields the shorter decoding time in the case of binary LDPC codes, we point out that increasing the field size tends to favor the SPA, especially when a multiplication takes only slightly more time than an addition. Further, we show that log-SPA requires fewer quantization levels and suffers less from a quantization induced error-floor.

This contribution considers the joint estimation of the carrier phase and the frequency offset from a noisy linearly modulated burst signal containing random data symbols (DS) as well as known pilot symbols (PS). The corresponding Cramer-Rao lower bound (CRB) is derived. This bound indicates that it is potentially more accurate to estimate carrier phase and frequency from such a 'hybrid' burst than from a burst without PS or from the limited number of PS only. The new bound is then compared with the performance of new and existing carrier synchronizers. We present the iterative Soft-Decision-Directed estimator with combined Data-Aided/Nondata-Aided initialization, which performs closely to the CRB, and provides a large improvement over the classical Nondata-Aided estimator at low and moderate Signal-to-Noise Ratio.

This paper introduces a log-domain decoding scheme for LDPC codes over GF(q). While this scheme is mathematically equivalent to the conventional sum-product decoder, log-domain decoding has advantages in terms of implementation, computational complexity and numerical stability. Further, a suboptimal variant of the log-domain decoding algorithm is proposed, yielding a lower computational complexity. The proposed algorithms and the sum-product algorithm are compared both in terms of simulated BER performance and computational complexity.

This paper considers the Cramer-Rao bound (CRB) related to estimating the carrier frequency of a noisy phase-shift keying signal. The following scenarios are discussed: 1) carrier frequency estimation irrespective of the carrier phase, based on either known or random data and 2) joint carrier phase and frequency estimation, based on either known or random data. Ideal symbol timing is assumed. We compare the results obtained from a (commonly used) simplified observation model against those resulting from the correct model. Because of the presence of nuisance parameters (random data and/or random carrier phase), the analytical computation of the corresponding CRBs is often not feasible. Here we present results that are based upon a combined analytical/numerical approach. Our results show that the choice of the observation model has essentially no effect on the CRBs at moderate and high signal-to-noise ratios. We also show that of the two scenarios considered, joint frequency and phase estimation yields the smaller CRB; the penalty resulting from frequency estimation, irrespective of the carrier phase, decreases with increasing observation interval.

This paper derives the Cramer-Rao bound (CRB) related to the estimation of the time delay of a linearly modulated bandpass signal with unknown carrier phase and frequency. We consider the following two scenarios: joint estimation of the time delay, the carrier phase, and the carrier frequency; and joint estimation of the time delay and the carrier frequency irrespective of the carrier phase. The transmit pulse is a bandlimited square-root Nyquist pulse. For each scenario, the transmitted symbols constitute either an a priori known training sequence or an unknown random data sequence. In spite of the presence of random data symbols and/or a random carrier phase, we obtain a relatively simple expression of the CRB, from which the effect of the constellation and the transmit pulse are easily derived. We show that the penalty resulting from estimating the time delay irrespective of the carrier phase decreases with increasing observation interval. However, the penalty, caused by not knowing the data symbols a priori, cannot be reduced by increasing the observation interval. Comparison of the true CRB to existing symbol synchronizer performance reveals that decision-directed timing recovery is close to optimum for moderate-to-large signal-to-noise ratios.

In this paper, we consider the effect of timing jitter on the performance of a multicarrier direct-sequence code-division multiple-access system for both uplink and downlink transmission, assuming orthogonal spreading sequences. Theoretical expressions are derived for the performance degradation caused by the timing jitter, in the presence of a multipath channel. Assuming an additive white Gaussian noise channel, perfect power control, and full load, it is shown that the performance degradation for the downlink transmission is independent of the number of subcarriers, of the spreading factor, and of the spectral contents of the jitter at the receiving mobile station, but only depends on the jitter variance. Under the same assumptions, we point out that, if the jitter spectra of all transmitting mobile stations are the same, the degradation on the uplink is the same as the degradation on the downlink.

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].

This contribution considers the Cramer-Rao bound (CRB) related to the joint estimation of the carrier phase and frequency of a noisy linearly modulated signal with random data symbols, using the correct continuous-time model of the received signal. We compare our results with the existing results obtained from a (common used) simplified discrete-time model of the matched filter output samples, that ignores useful signal reduction and ISI caused by nonzero frequency offset. We show that both models yield the same CRB for phase estimation. As far as frequency estimation is concerned, we point out that: (i) the correct observation models yield the smaller CRB, and (ii) the difference between the CRBs resulting from the two models is apparent only at very small SNR. This indicates that at practical SNR values, the frequency offset affects the likelihood function mainly through the signal rotation (rather than signal magnitude) at the matched filter output.

In this paper, we consider the effect of a carrier phase offset on the performance of a low-density parity-check (LDPC) coded QAM modulated system. We investigate an ML-based carrier phase synchronization algorithm that makes use of the posterior probabilities of the data symbols, provided by the iterative decoder. The resulting carrier phase synchronizer is an extension, to LDPC coded systems, of the iterative phase estimator for turbo coded systems, presented in V.Lottici and M.Louse [2002], and has negligible BER performance degradation as compared to the ideally synchronized system.

This contribution considers the Cramer-Rao bound (CRB) related to phase-independent carrier frequency estimation from a noisy PSK signal. Instead of estimating the frequency jointly with the carrier phase, we treat the phase as a nuisance parameter. Ideal symbol timing is assumed. Both cases of known data (training sequence) and random data are considered. We show that frequency estimation irrespective of the carrier phase yields a larger CRB than does joint frequency and phase estimation; the penalty resulting from the former strategy vanishes with increasing observation interval.

In this contribution we consider the Cramer-Rao bound (CRB) for the estimation of the time delay of a noisy linearly modulated signal with random data symbols. In spite of the presence of the nuisance parameters (i.e., the random data symbols), we obtain a closed-form expression of this CRB for arbitrary PSK, QAM or PAM constellations and a bandlimited square-root Nyquist transmit pulse.

We study the effect of carrier phase jitter on the performance of both uplink and downlink multicarrier direct-sequence CDMA (MC-DS-CDMA). We determine the resulting performance degradation in terms of the various systems parameters. Assuming an AWGN channel, perfect power control and full load, it is shown that this degradation is independent of the number of carriers, of the spreading factor, and of the jitter spectrum shape, but only depends on the jitter variance.

In this paper, we study the sensitivity of uplink and downlink MC-DS-CDMA to carrier frequency offsets, assuming orthogonal spreading sequences. For both uplink and downlink MC-DS-CDMA, we show that the performance rapidly degrades for an increasing ratio of maximum frequency offset to carrier spacing. We point out that the degradation in the uplink is larger than in the downlink because only the former is affected by multiuser interference. For a given (small) ratio of maximum frequency offset to carrier spacing, enlarging the spreading factor in a fully loaded system does not affect the downlink degradation but strongly increases the uplink degradation. Finally, we show that the downlink degradations of MC-DS-CDMA and fully loaded MC-CDMA are the same, provided that for both systems the ratio of frequency offset to carrier spacing is the same.

We investigate the sensitivity of MC-CDMA systems to synchronisation errors. We show that the signal-to-noise ratio (SNR) of the MC-CDMA system at the input of the decision device in the presence of a carrier frequency offset or a clock frequency offset is a rapidly decreasing function of the number of carriers. For a maximal load, carrier phase jitter and timing jitter give rise to a degradation that is independent of the spectral content of the jitter; moreover, the degradation caused by carrier phase jitter is independent of the number of carriers. A constant carrier phase offset and a constant timing offset cause no degradation of the MC-CDMA system performance.

We investigate the influence of the carrier phase jitter of a phase-locked local oscillator on the performance of a multicarrier code-division multiple-access (MC-CDMA) system in a nondispersive channel. This carrier phase jitter degrades the signal-to-noise ratio at the input of the decision device. When all users have the same power level and phase jitter spectrum, it is shown that for the highest load, the degradation only depends on the jitter variance but not on the specific shape of the jitter spectrum nor on the number of carriers.

In mobile radio communication, the fading channels generally exhibit both time-selectivity and frequency-selectivity. Orthogonal frequency-division multiplexing has been proposed to combat the frequency-selectivity, but its performance is also affected by the time-selectivity. We investigate how various parameters, such as the number of carriers, the guard time length, and the sampling offset between receiver and transmitter, affect the system performance. Further, we determine the optimum values of the above parameters, which minimize the degradation of the signal to-noise ratio at the input of the decision device.

This paper deals with the derivation and comparison of some promising TDMA and OFDM/CDMA receiver structures for communication over the return path channel of the cable TV (CATV) network. Regarding TDMA, it is shown that the performance substantially improves by using equalization. Given the slowly varying, frequency selective nature of the ingress noise, OFDM seems an interesting possible access technique to employ. Here it is considered in combination with CDMA. With respect to the performance in terms of the SNR at the input of the decision device, the TDMA structure gives better results than the OFDM/CDMA schemes. Also with respect to the design complexity TDMA seems more interesting.

We investigate the sensitivity to carrier phase jitter of an orthogonal frequency-division multiple-access (OFDMA) system. When all OFDMA carriers have the same power level and jitter spectrum, the degradation caused by the jitter is shown to be equal to the degradation of an OFDM system. Also, traditional FDMA is found to be slightly more robust than OFDMA.