Signals In Multipath Channels Research Articles

Improved Deep Neural Network for OFDM Signal Recognition Using Hybrid Grey Wolf Optimization

Orthogonal Frequency Division Multiplexing (OFDM), as the core technology in mobile communications, is a multi-carrier modulation technology with high frequency spectrum utilization, which has strong anti-multipath interference and anti-fading ability. The significant advantage of OFDM signals lies in the anti-multipath effect, so its application environment is mostly multipath fading channels. Therefore, it is of great significance to study the identification of OFDM signals in multipath channels. Deep learning, with superior big data processing and classification capabilities, is a potential solution to these problems. Based on the problem of OFDM signal recognition in complex signals in multi-path channel, an OFDM signal recognition method based on hybrid grey wolf optimization algorithm to optimize deep neural network model is proposed. Because the basic grey wolf optimization algorithm (GWO) is easy to fall into a stasis state when attacking prey, differential evolution algorithm (differential evolution algorithm) is integrated into GWO to force GWO to jump out of the stasis state with its strong search ability. The convergence speed and recognition performance of the proposed algorithm are greatly improved. The experimental results show that under the condition of low SNR, the recognition accuracy of proposed algorithm is 9.95% higher than the traditional DNN method, and nearly 4.5% higher than the other two intelligent optimization methods, and the values of Precision and Recall increase obviously, which indicates that the hybrid algorithm not only improves the accuracy of recognition, but also makes the search more complete and accurate. Compared with classical particle swarm optimization (PSO) and whale algorithm optimization algorithm (WOA), the hybrid algorithm has strong competitiveness both in recognition performance and optimization stability, which provides a new, simpler and more effective method for modulation recognition of OFDM signals in wireless communications.

A time-of-arrival estimation algorithm for OFDM signals in indoor multipath environments

Time-of-arrival (TOA) estimation of OFDM signals in multipath channels is challenging, especially when the signal bandwidth is limited, such as is the case of WIFI. In this paper, a novel low complexity TOA estimation algorithm is developed for OFDM signals that is robust to various multipath environments. We approximate the received signal as Gaussian with an autocorrelation that can be predetermined by the environment characteristics, and then derive the Maximum Likelihood estimator with an efficient implementation algorithm. The estimator performance was tested with a measurement system that was developed based on off-the-shelf WIFI 802.11g routers. The measurement system enabled the evaluation of the estimator performance in real-life indoor environments, and with real-life OFDM transmitter and receiver imperfections. The results show that the novel TOA estimator outperforms other known reference methods over various indoor environments.

The MPSK and MQAM signal modulation recognition algorithm in the multipath channel

ABSTRACTThis paper proposes a new Lempel–Ziv complexity feature recognition algorithm based on the nth-powered () spectrum to deal with modulation recognition problem regarding MPSK and MQAM signals in multipath channels. Based on differences in the nth-powered () spectrum amplitude value of MPSK and MQAM signals, the waveform characteristics of the multiplied spectrum are extracted in the multipath environment, and a total of five characteristic parameters are defined to distinguish the signals from the set {BPSK,OQPSK,QPSK,8PSK,PI/4DQPSK,16QAM,32QAM}. The simulation results show that the recognition algorithm performs well when recognising signals in a multipath situation and is less affected by the frequency deviation and shaping filter coefficient; therefore, it is very robust.

Blind synchronization of the OFDM signals in multipath channels on the basis of the time and frequency protection intervals

New methods of symbol-timing and carrier-frequency blind synchronization of an OFDM-signal receiver are developed and studied. They generalize the well-known methods which use either the protection interval in time in the cyclic prefix form or the protection interval with respect to frequency in the form of virtual subcarriers, and are based on their joint application. To reduce the computational complexity, approximate algorithms which are based on the approximation of the optimal rules, but, according to the study results, have almost the same characteristics of parameter-estimation accuracy and the reception bit-error-rate performance are proposed. It is shown that in terms of the parameter-estimation accuracy and the reception bit-error-rate performance, the proposed methods are superior to the well-known methods of synchronization by the cyclic prefix and the virtual subcarriers in the two-path Rayleigh-fading channel. For incoherent systems with the differential phase shift keying variants, using such methods makes it possible to rule out the necessity of accurate synchronization and, due to insignificant redundancy of the system band and the cyclic prefix length, closely approach the reception bit-error-rate performance for perfect synchronization.

DoA Estimation of Line of Sight Signal in Multipath Channel for GNSS Receiver

The GNSS receivers suffer from the multipath interference which is highly correlated with the line of sight (LoS) signal. Such interference results in tracking and ranging errors. In this paper, we propose a novel algorithm that can estimate the direction of arrival (DoA) of the LoS signal in the presence of highly correlated multipath interference. The proposed algorithm combines the matching pursuits algorithm for multipath estimation and the minimum norm algorithm for DoA estimation. An efficient combination of two algorithms yields reliable estimates of the DoA of LoS signal as demonstrated by computer simulations.

Analysis of suboptimal detection-discrimination algorithms of compound ultra-wideband signals in multipath channels with undefined structure

Investigation results for suboptimal processing rule of a coded ultra-wideband signal with unknown arrival time at the output of multipath communication channel with undefined structure are presented. Characteristics of signal detection-discrimination from multiple users are calculated. Statistical modeling of suboptimal signals reception is performed.

A Rapid and Reliable Two-Step Search Scheme for UWB Signal Acquisition in Multipath Channels

We propose the “Two-Step Search scheme with Linear search based Second step (TSS-LS)” by improving the conventional “Two-Step Search scheme with Bit reversal search based Second step (TSS-BS)” for reliable as well as rapid acquisition of Ultra Wide Band (UWB) signals in multipath channels. The proposed TSS-LS utilizes two different thresholds and search windows to achieve fast acquisition. Furthermore, unlike the TSS-BS in which the bit reversal algorithm is applied in both steps, the linear search is adopted for the second step in the proposed TSS-LS to correctly find the starting point in the range of effective delay spread of the multipath channels, and to obtain reliable bit error rate performance of the UWB systems.

Multiuser Receiver for DS-CDMA Signals in Multipath Channels: An Enhanced Multisurface Method

This paper deals with the problem of multiuser detection in direct-sequence code-division multiple-access (DS-CDMA) systems in multipath environments. The existing multiuser detectors can be divided into two categories: (1) low-complexity poor-performance linear detectors and (2) high-complexity good-performance nonlinear detectors. In particular, in channels where the orthogonality of the code sequences is destroyed by multipath, detectors with linear complexity perform much worse than the nonlinear detectors. In this paper, we propose an enhanced multisurface method (EMSM) for multiuser detection in multipath channels. EMSM is an intermediate piecewise linear detection scheme with a run-time complexity linear in the number of users. Its bit error rate performance is compared with existing linear detectors, a nonlinear radial basis function detector trained by the new support vector learning algorithm, and Verdu's optimal detector. Simulations in multipath channels, for both synchronous and asynchronous cases, indicate that it always outperforms all other linear detectors, performing nearly as well as nonlinear detectors.

Semi-blind space-time receiver for multiuser detection of DS∕CDMA signals in multipath channels

We consider the design of a new space-time receiver that is useful in heavily loaded DS/CDMA systems when only small data records are used in detection. This property is desirable for detection of users of high mobility in crowded systems. The receiver is based on the integration of ESPRIT-based space-time beamforming with multiuser detection based on the constant modulus (CM) property. The beamformer exploits the shift invariance property of the space-time data matrices across the antenna array, and extracts the desired signal along with a small amount of interference. The subsequent multiuser detection stage, which uses the analytical constant modulus algorithm (ACMA), then easily detects the desired signal. In order to avoid association and convergence problems, a small number of zero (off) symbols are added in each block (hence the term semi-blind) so that the ACMA yields a unique solution in a one-shot procedure. Computer simulation results demonstrate that the proposed receiver has a significant performance advantage compared to several other receivers, in the above difficult scenario. Thus the receiver may be a useful candidate for detection in dynamic and crowded systems.

On Application of MUSIC Algorithm to Time Delay Estimation in OFDM Channels

The time delays of the OFDM paths delimit the size of guard interval in OFDM symbols-important factor for system throughput. In order to apply guard interval as an adaptive parameter like in DRM system, time delays should have been evaluated in real time and in a simple way. Presented software method takes advantage of the information carried out by pilot subcarriers of the OFDM signal. It is shown that pilot subcarriers of OFDM signal in multipath channel are described by equations formally equivalent to equations of Direction-Of-Arrival (DOA) problem in antenna array processing. This analogy leads to application of the MUSIC algorithm of DOA problem to time delay estimation of the individual OFDM paths. Related condition for distribution of pilots within OFDM symbol is given. In case of time delays outside the guard interval the MUSIC algorithm is shown to produce 'shadow' and 'ghost' paths.

Code-constrained blind detection of CDMA signals in multipath channels

In this letter, a constant-modulus-algorithm-based multiuser detection scheme is proposed for a communication system under multipath propagation. To mitigate channel distortion and multiuser interference, we integrate multiple constraints into the optimization criterion. According to our analysis, the ability of the detector to remove all interference is ensured in the absence of noise when the constraints are properly preselected. However, in the presence of noise, the constraints highly affect the performance of the receiver. In order to optimally combine signals from different paths to achieve performance gains, those constraints can also be treated as variables and jointly optimized with the receiver, as verified by numerical examples.

Blind detection of multirate asynchronous CDMA signals in multipath channels

Blind detection of a desired user's signal in a multirate direct sequence code division multiple access (DS-CDMA) system [using either variable sequence length (VSL) or multicode (MC) access] is considered. A code-constrained inverse filter criterion (IFC)-based blind detector for equal-rate CDMA signals to detect a desired user's signal was presented by Tugnait and Li (2001). The IFC method exploits the higher order statistics of the data. In multirate CDMA systems, a high-rate user signal may be treated as the superposition of several virtual basic-rate signals. The code-constrained IFC-based detector may be used to detect a given basic-rate virtual signal. This, however, does not solve the problem of combining the detected virtual basic-rate signals to yield the original high-rate signal since the former may be delayed by different equalization delays, may be multiply extracted, and may be in different "order." In this paper, novel approaches combining the code-constrained IFC and a penalty function are developed to cope with this problem for VSL and MC multirate access methods. Global minima of the proposed cost functions are analyzed. Three illustrative simulation examples are presented, including an example where the proposed algorithms are compared with an existing subspace approach (and its modifications), a clairvoyant matched filter receiver, and a known channel linear minimum mean-square error (MMSE) receiver.

Blind detection of asynchronous CDMA signals in multipath channels using code-constrained inverse filter criterion

A code-constrained inverse filter criterion based approach is presented for blind detection of asynchronous short-code direct sequence code division multiple access (DS-CDMA) signals in multipath channels. Only the spreading code of the desired user is assumed to be known; its transmission delay may be unknown. We focus on maximization of the normalized fourth cumulant of inverse filtered (equalized) data with respect to (w.r.t.) the equalizer coefficients subject to the equalizer lying in a subspace associated with the desired user's code sequence. Constrained maximization leads to extraction of the desired user's signal, whereas unconstrained maximization leads to the extraction of any one of the active users. Illustrative simulation examples are provided.

A multistep linear prediction approach to blind asynchronous CDMA channel estimation and equalization

A multistep linear prediction approach is presented for blind channel estimation, multiuser interference (MUI) suppression, and detection of asynchronous short-code direct sequence code division multiple access signals in multipath channels. Only the spreading code of the desired user is assumed to be known; its transmission delay may be unknown. We exploit the previously proposed multistep linear prediction approach for blind multiple-input multiple-output channel estimation in conjunction with the structure imposed by the desired user's spreading code sequence. With the knowledge of the desired user's code sequence, only the second-order statistics of the data are needed under certain sufficient conditions on the underlying multiuser MIMO transfer function. Based on the desired user's channel estimate, a linear minimum mean square error filter is designed for simultaneous equalization and MUI suppression. Three illustrative simulation examples are presented.

Adaptive minimum-BER linear multiuser detectionfor CDMA signals in multipath channels with 4-QAM constellation

An adaptive minimum-BER linear multiuser detector called the least BER (LBER) algorithm, originally developed for BPSK modulation, is extended to 4-QAM modulation.

Support vector machine multiuser receiver for DS-CDMA signals in multipath channels

The problem of constructing an adaptive multiuser detector (MUD) is considered for direct sequence code division multiple access (DS-CDMA) signals transmitted through multipath channels. The emerging learning technique, called support vector machines (SVM), is proposed as a method of obtaining a nonlinear MUD from a relatively small training data block. Computer simulation is used to study this SVM MUD, and the results show that it can closely match the performance of the optimal Bayesian one-shot detector. Comparisons with an adaptive radial basis function (RBF) MUD trained by an unsupervised clustering algorithm are discussed.

Linear space-time multiuser detection for multipath CDMA channels

We consider the problem of detecting synchronous code division multiple access (CDMA) signals in multipath channels that result in multiple access interference (MAI). It is well known that such challenging conditions may create severe near-far situations in which the standard techniques of combined power control and temporal single-user RAKE receivers provide poor performance. To address the shortcomings of the RAKE receiver, multiple antenna receivers combining space-time processing with multiuser detection have been proposed in the literature. Specifically, a space-time detector based on minimizing the mean-squared output between the data stream and the linear combiner output has shown great potential in achieving good near-far performance with much less complexity than the optimum space-time multiuser detector. Moreover, this space-time minimum mean-squared error (ST-MMSE) multiuser detector has the additional advantage of being well suited for adaptive implementation. We propose novel trained and blind adaptive algorithms based on stochastic gradient techniques, which are shown to approximate the ST-MMSE solution without requiring knowledge of the channel. We show that these linear space-time detectors can potentially provide significant capacity enhancements (up to one order of magnitude) over the conventional temporal single-user RAKE receiver.

Blind adaptive reduced-rank detection for DS-CDMA signals in multipath channels

Blind adaptive multiuser detection for direct sequence code division multiple access (DS-CDMA) signals over static and time-varying intersymbol interference (ISI) limited channels is considered. Blind adaptive detectors must be robustified for ISI channels, when there is significant mismatch between the received signature vector and the transmitted code (assumed known at the receiver). A new low-complexity detector is presented that improves on some previously proposed methods without explicit estimation of the ISI channel. The key innovation is a reduced-rank detector architecture combined with an efficient subspace tracker that yields direct accurate estimation of the desired user's received signature. Several representative simulation examples of detector output signal-to-noise-and-interference ratio (SINR) for fading channels are provided in support of our claims of improved efficacy of the method.

The multipath coherence function for uncorrelated underwater channels

A new measure is introduced for determining the coherence of acoustic signals in multipath channels which have random fluctuations that are uncorrelated between them. The analysis is based upon a formulation of the spectral coherence function in terms of the random multipath channel transfer functions. The derived coherence function allows each channel to be analyzed individually, and separates the effects of random fluctuations from the effects of deterministic multipath interface (frequency selective fading). Examples of applications are given to determining the effects of independent sensor motion on coherence, and the effects of internal waves on signal coherence between widely spaced receivers.