Multipath chirp signal detection based on biorthogonal fourier transform

Robust Digital Retrodirective Beamforming Technique for Multipath Channel Environment

Two subspace-based multiuser detectors are presented for a synchronous DS-CDMA system operating over a multipath fading channel with correlated noise. Based on the training sequence transmitted by the desired user and employing a noise whitening process, a linear MMSE detector is obtained which does not require knowledge of the code sequences and the propagation channel response. When the training sequence is not available, a matrix decomposition method is adopted to estimate the channel response. Through signature waveform estimation, a linear MMSE detector can be obtained blindly. Simulation results show that these two methods are robust against the near–far problem and the correlated noise.

Code division multiple access (CDMA) based on spread spectrum technique has become one of the most important multiple access technologies in contemporary communication systems. The properties of spreading codes largely govern the performance of CDMA systems. Therefore, CDMA can insure interference free operation when code design methodology takes into account of major impairing factors such as intersymbol interference (ISI), asynchronous transmission and random signs in consecutive symbols. In this study, the authors propose a design scheme for CDMA signal reception with zero ISI in the multipath environment. Cyclic shift property of maximal length sequence (m‐sequence) is utilised in the design to avoid ISI. The proposed scheme exploits inherent diversity present in the multipath channel and can be seen as antenna diversity. This scheme avoids the use of complex detectors and the performance is compared by simulations with conventional CDMA system using Walsh–Hadamard codes and m‐sequence. The impact of the Rician factor on the end‐to‐end performance is also studied in detail and suitable receiver to be used has been suggested for different fading scenarios. Simulations results presented indicate that the proposed scheme provides superior performance over traditional systems.

When a UWB signal is transmitted, we expect there to be a number of reflected multipath signals because of reflections due to objects in the environment. As a result of these reflected signals, it is impossible to define the ellipses that would be used for target detection as described by existing elliptical based localization schemes when we consider a highly multipath rich environment such as a typical UWB propagation channel unless we are able to manipulate the multipath propagation scenario and reduce it to a two-path propagation one. To tackle this problem, a pre-localization algorithm is proposed. In our proposed algorithm, we use the reflection properties of UWB signals to extract information from the reflected signals in the multipath environment and ultimately reduce the multipath propagation scenario into a two-path one based on these extracted information. Our process of extraction involves the sampling of the received signals at three receivers during regular intervals, correlating the sampled signals with a predefined database of template reflected signals; and finally using a decision engine to determine the signals that would be used for generating the ellipse for target localization. Consequently, we were able to differentiate multipath signals from one another using the relationship between individual multipath signals and their respective angle of incidences; and ultimately select a set of reflected signals that would be suitable for the localization process described by existing elliptical based localization schemes.

Orthogonal Frequency Division Multiplexing (OFDM) is employed in the current 4G cellular technologies. Because of its resistance to multipath propagation channels, OFDM is a popular multicarrier modulation technique. Although OFDM is one of the effective approaches for a 4G network, out-of-band radiation, inter-symbol interference (ISI), and inter-carrier interference make this method unsuitable for 5G. (ICI). A new approach or next-generation cellular network, i.e., very high data rate, low latency, greater throughput, more device connectivity, and improved device-to-device communication, should be used to deliver effective and dependable communication. The system introduces generalized frequency division multiplexing (GFDM). In GFDM, a single cyclic prefix is utilized throughout the frame and an adjustable pulse-shaping filter can be used to monitor the transmitted signal’s out-of-band emission. Using this ISI, the multipath channel can be controlled. In the proposed system, with large signal constellations, higher-order modulation schemes, and advanced receiver structure, a very high data rate can be achieved. By using various iterative receiver structures the performance of the GFDM system in 5G cellular networks can be analyzed.

The orthogonal frequency division multiplexing (OFDM) transceiver has enjoyed great success in many wideband communication systems. It has low complexity and robustness against multipath channels. It is also well-known that the OFDM transceiver has poor frequency characteristics. To get transceivers with better frequency characteristics, filterbank transceivers with overlapping-block transmission are often considered. However these transceivers in general suffer from severe intersymbol interference (ISI) and high complexity. Moreover costly channel dependent post processing techniques are often needed at the receiving end to mitigate ISI. We design discrete Fourier transform (DFT) modulated filterbank transceivers for multipath fading channels. The DFT modulated filterbanks are known to have the advantages of low design and implementation cost. Although the proposed transceiver belongs to the class of overlapping-block transmission, the only channel dependent part is a set of one-tap equalizers at the receiver, like the OFDM system. We show that for a fixed set of transmitting or receiving filters, the design problem of maximizing signal-to-interference ratio (SIR) can be formulated into an eigenvector problem. Experiments are carried out for transmission over random multipath channels, and the results show that satisfactory SIR performance can be obtained.

Distributed multiple-input multiple-output (MIMO) system is a new architecture for next generation wireless communication systems with distributed multiple antennas. In this paper, an uplink model for multiuser distributed MIMO systems with wideband code-division multiple-access (CDMA) technology is presented. We focus on the case where there is no channel state information at the transmitter, but channel state information is assumed at the receiver. The spectral efficiency of the uplink system with single-user detectors at the base stations under multipath fading channels is investigated carefully. Two cases with different assumptions of interference knowledge are studied in detail. The spreading sequences of the CDMA systems are assumed to employ loosely synchronized (LS) spreading sequences and random spreading sequences respectively. LS spreading sequences exhibit a so-called interference free window (IFW), where both the auto-correlation and cross-correlation of the codes become zero. Therefore LS sequences have the promise of mitigating the effects of inter symbol interference (ISI) and multiple access interference (MAI) in multipath fading channels. It is shown by numerical results that the uplink spectral efficiency of the distributed MIMO-CDMA system using LS sequences can significantly outperform that of the system using random spreading sequences. Copyright © 2005 John Wiley & Sons, Ltd.

GNSS remains one of the key building blocks in mass-market positioning applications, many of which require a high level of accuracy, integrity and availability. Conventionally, the GNSS receiver and antenna are a part of a multisensor integrated solution with an inertial measurement unit (IMU) at the core of the navigation system. One of the multisensor fusion challenges is to continuously adjust the Kalman filter stochastic model to reflect the environment of operation. Apart from poor satellite geometry, the reception of multipath-contaminated signals is the main factor contributing to GNSS performance degradation in urban areas. Signal quality monitoring (SQM) techniques are implemented to first detect and then exclude, de-weight or correct the multipath-contaminated GNSS measurements to minimize the impact of multipath-induced errors on the multisensor data fusion filter performance. The implementation of such an approach for kinematic scenarios in deep urban canyons with mass-market hardware suffers from high rates of false-positive and false-negative multipath detection due to frequent cycle slips, discontinuous satellite tracking, and a complex multipath environment. The alternative approach for the IMU/GNSS integration filter stochastic model tuning is to extract the a priori statistics characterizing the probability of the multipath-contaminated signal reception from a GNSS multipath environment map. The map is generated with collectively recorded carrier-to-noise-density ratio (C/N0) readings streamed from the connected vehicles operating in a given urban area and assigned to a space-time cube. While improving positioning accuracy, the application of the concept is constrained by the GNSS multipath environment map availability only to the areas directly surveyed by the connected vehicles. The novel contributions of this paper are as follows. To extend availability of the GNSS multipath environment map, a random forest machine-learning model for predicting the spatial pattern of the map is developed. The model is trained with a real-world GNSS multipath environment map covering the area of ten square kilometres including downtown Montreal. A LiDAR elevation profile, 2D building polygons, street polygons, street types and foliage polygons are used as feature data. An 89% map prediction accuracy is reached. Further, the Extended Kalman Filter (EKF) stochastic model adjustment architecture combining the SQM multipath detection and the GNSS multipath environment map-aided multipath prediction is developed. The architecture aims to address the limitations of each method and allows for continuous multipath monitoring increasing the resilience of the multisensor data fusion. The method is tested in several use cases with low-cost hardware: loosely-coupled and tightly-coupled IMU/GNSS integration. The evaluation of the proposed method shows 20% positioning accuracy improvement compared to standard Kalman filter performance. The results of this work are expected to facilitate future improved integration of GNSS in multisensor platforms operating in challenging urban areas.

The paper studies the capacities of time hopping ultra wideband (TH-UWB) systems by adopting orthogonal pulse-based modulation schemes in a multipath environment with the consideration of multiple access interference (MAI) and inter symbol interference (ISI). Recently, orthogonal pulse-based modulation schemes such as pulse shape modulation (PSM), biorthogonal PSM (BPSM) and the combination of orthogonal pulse position modulation (OPPM) and BPSM have been proposed for UWB system to reduce ISI and MAI. In this paper, we first investigate the influences of ISI and MAI on the capacities of TH-UWB systems in terms of signal-to-interference and noise ratio (SINR) for high-order modulation schemes based on various sets of orthogonal pulses. How mutual information varies with the number of multipath components (MPCs) is then analyzed. The complete mathematical analysis and simulation results are provided in detail. When maximizing the mutual information, the Gaussian distribution approximate is assumed to describe the characteristics of MAI and ISI. Simulation results are provided to verify the theoretical analysis. Some conclusions are drawn thereafter, such as that capacity depends on the pulse patterns adopted and mutual information varies with inversely proportional to the number of MPCs. We also observed that the capacities of UWB systems for OPPM-BPSM schemes are larger than those of PSM and BPSM schemes.

With sophisticated signal and information processing algorithms, air interfaces with space-time (ST) coding and multiple reception antennas substantially improve the reliability of wireless links. This paper proposes a new receiver algorithm for differential ST coded transmissions over the finite-impulse-response (FIR) rich multipath fading channels. The symbol detection introduced in this paper is a deterministic subspace-based approach in a multiple-input and multiple-output (MIMO) system framework. The receiver (i) operates in a blind fashion without estimating the channel or its inverse and (ii) is able to work with a small number of signal samples and hence can be applied in the quasistatic channels. The proposed scheme employs multiple antennas at both sides of the transceiver and exploits both the antenna diversity and the multiple constant modulus (MCM) characteristics of the signaling. The receiver is able to blindly mitigate the intersymbol interference (ISI) in a rich multipath propagation environment, and this has been verified through the extensive Monte Carlo simulations.

The capacity of direct-sequence code-division multiple-access systems is severely limited by multiple access interference produced by the other co-channel users. To improve the capacity, joint-detection receivers and multi-access interference cancellation techniques have been proposed. Frequency-selective multi-path fading, however, has a significant effect on the performance of these types of receiver. This paper presents an adaptive neural-network-based receiver that has the capability of combating the 'near-far' problem and rejecting the multi-user interference in a multipath channel. A comparative performance analysis of the conventional matched-filter, a RAKE receiver and the adaptive neural network receiver is carried our using a Monte Carlo simulation. It is found that the new receiver performs extremely well as a suboptimal receiver in multipath and multiple access channels with a significant 'near-far' effect.

In this paper, we propose a generic receiver based on optimum maximum likelihood sequence estimation (MLSE) employing a generalized Ungerboeck metric with significantly reduced complexity for the general class of M-ary quasi orthogonal signaling (MOS) in severe multipath fading channels. First, by using the factor graph (FG) of the obtained generalized Ungerboeck metric for MOS and sum-product algorithm (SPA) framework, a highly efficient reduced state sequence estimation (RSSE) algorithm is proposed that operates on forward and backward directions and is directly applied to unwhitened channel matched filter (CMF) and code matched filter outputs at symbol rate. The proposed structure, generating the a posteriori probabilities (APP) by bidirectional RSSE recursions, are substantiated with symbol rate bidirectional decision feedback (BDF) based on surviving paths in order to eliminate the post- and pre-cursor inter-symbol interference (ISI) as well as multi code interference (MCI) due to the non-ideal properties of the signaling waveforms and multipath channel. It appears as the unification of Ungerboeck type reduced trellis equalization framework for MOS and achieves near-optimum performance especially for channels with large dispersion. Second, we identify a bias term through an error probability analysis which helps us improve the proposed receiver's performance. Furthermore, a tight approximation for the bit error rate (BER) of the proposed receiver is derived. These analyses lead to significant insight on the selection of system parameters and clearly demonstrate the high performance and efficiency of the proposed scheme by a proper choice of the signaling parameters.

Chirp signal can be modulated to transmit digital data. We investigated quadrature phase shift keying (QPSK) modulation for chirp signals and analyzed the bit error rate (BER) performance in multipath fading channel. In receiver, synchronous demodulation is used to demodulate received chirp signal, and the output of multipath signal in matched filter has two factors to interfere with the decision vector of direct signal. When the initial frequency equals to the frequency deviation during a half symbol duration, and the chirp rate is negative slope, the output of multipath signal in matched filter has the same phase or the reverse phase as that of direct signal, the impact of multipath signal on BER performance of demodulation is alleviated. Finally, numerical simulation results show the effectiveness of the scheme for mitigating multipath interference.

In MIMO detection, the signals transmitted by multiple antennas can be detected jointly based on the ML principle for optimal performance. However, its complexity is prohibitively high for a large number of transmit antennas and a higher order modulation method. There are suboptimal approaches of low complexity. Unfortunately, they cannot exploit a full receive diversity gain, which results in the performance gap between the ML and suboptimal approaches (e.g., linear MMSE detection). This gap usually increases with SNR. Therefore, it is desirable to find some new detection methods that can provide near ML performance with low complexity which is comparable to that of linear detectors. In this chapter, we introduce two key ingredients: list decoding and lattice (basis) reduction, which can be used to develop different computationally efficient algorithms with reasonably good performance.

In this paper, we propose symbol-based receivers for orthogonal frequency division multiplexing OFDM code-division multiple-access CDMA multiple-input-multiple-output MIMO communications in multipath fading channels. For multiuser and multipath fading environments, both intersymbol interference and multiple-access interference must be considered. We propose narrowband and wideband antennas and Wiener code filter for MIMO OFDM-CDMA systems. The proposed receivers are updated symbol-by-symbol to achieve low computational complexity. Simulation results show that the proposed Wiener code filter can improve the system performance for the proposed adaptive antennas. The wideband antenna can achieve better error-rate performance than that of the narrowband antenna when multipath effect exists. The convergence rate of the recursive least squares antennas is faster than that of the least mean square antennas. Copyright © 2013 John Wiley & Sons, Ltd.