Unknown Fading Channels Research Articles

Event-triggered and consensus-based attitude tracking alignment for discrete-time multiple spacecraft system exploiting interference

This paper presents a discrete-time attitude control strategy with equi-global practical stabilizability for aligning the attitude of multiple spacecraft to a predesigned configuration according to a time-variant reference. By utilizing the interference of the wireless channel, the communication scheme designed in this paper can save communication resources, amount of computation, and energy proportionally to the number of spacecraft. The exact discrete-time model and approximate discrete-time model of the consensus-based spacecraft tracking system are given. Then the framework for the design of an event-triggered control scheme for the exact discrete-time system via its approximate models is developed, which avoids the periodic actuation, and Zeno behavior is proved to be excluded. Furthermore, the control scheme can handle the presence of the unknown fading channel. Finally, simulation results are presented to demonstrate the effectiveness of the control strategy.

Discrete-Time Attitude Tracking Synchronization for Swarms of Spacecraft Exploiting Interference

The attitude tracking synchronization control of an orbit-predetermined leader–follower spacecraft swarm for the space moving target is discussed in this paper. The information exchange between all spacecraft is assumed to be discrete in time and on the undirected connected graph. Moreover, due to the demand for saving communication resources, wireless interference has been utilized, which allows all the neighbors of a spacecraft to access the same channel frequency spectrum simultaneously. Then the backstepping control algorithm is designed to let the spacecraft (β,A)-practically stably synchronize their states and track a time-varying trajectory in the presence of unknown fading channels. Finally, simulation is provided to verify that using the proposed control scheme, the attitude tracking synchronization can be achieved with high precision.

Learning Tracking Over Unknown Fading Channels Based on Iterative Estimation

With fast developments in communication technologies, a large number of practical systems adopt the networked control structure. For this structure, the fading problem is an emerging issue among other network problems. It has not been extensively investigated how to guarantee superior control performance in the presence of unknown fading channels. This article presents a learning strategy for gradually improving the tracking performance. To this end, an iterative estimation mechanism is first introduced to provide necessary statistical information such that the biased signals after transmission can be corrected before being utilized. Then, learning control algorithms incorporating with a decreasing step-size sequence are designed for both output and input fading cases. The convergence in both mean-square and almost-sure senses of the proposed schemes is strictly proved under mild conditions. Illustrative simulations verify the effectiveness of the entire learning framework.

Blind Data Detection With Unknown Channel Coding

Blind data detection is widely investigated since it finds many crucial applications in the interference cancelation, cognitive radio, and military interception, etc. In this letter, we propose a blind iterative algorithm that simultaneously improves the performance of channel coding identification and data detection in the communication system with the unknown channel coding and the unknown fading channels. First, the likelihood-based channel estimator and the soft detector and regenerator are adopted in an iterative manner, which estimates the unknown parameters and detects the information bits. Then, an average log-likelihood ratio (LLR) classifier is adopted to identify the unknown channel coding and determine the information bits. Numerical results show that the proposed algorithm outperforms the existing scheme in terms of correct identification and data detection.

Exploiting Wireless Interference For Distributively Solving Linear Equations

Interference in wireless communication is generally considered as an undesired phenomenon which needs to be combatted. Unlike traditional approaches, this paper investigates the possibility of exploiting interference for the cooperative solution of a linear algebraic equation, of which each agent knows only a portion. The presented communication system, together with the designed iterative algorithm, guarantees that agents converge exponentially to a global solution (unique or non-unique) of the algebraic linear equation, notwithstanding the presence of the unknown fading wireless channel. Guaranteeing privacy is one of the benefits of this approach: the unknown fading channel prohibits the access to neighboring agents’ local equations. Such a feature is extremely useful in case local equations contain sensitive information. Randomized simulations reinforce our theoretical results.

Continuous Phase Modulation Classification via Baum-Welch Algorithm

Due to high spectral and power efficiency, continuous phase modulation (CPM) is widely adopted in wireless communications, such as satellite communications. In this letter, we investigate the classification of CPM signals in the presence of unknown fading channels. The time-varying phases of CPM are first formulated as a hidden Markov model (HMM) by observing its memorable property. Then, a likelihood-based classifier is proposed using the Baum-Welch algorithm, which is able to estimate the unknown parameters in the HMM. Simulation results show that the proposed algorithm outperforms the existing scheme using approximate entropy in terms of classification performance.

Quantised feedback stabilisation of LTI systems over partly unknown Markov fading channels

ABSTRACTThis paper is concerned with the quantised feedback stabilisation of a class of linear systems over partly unknown Markov fading channels. A set of channel state process with partly unknown transition probabilities is introduced to model time-varying fading channels, which characterises various configurations in physical communication environment and/or different channel fading amplitudes. Partly unknown Markov fading channels are more general than completely known Markov fading channels. Sufficient condition for mean square quadratic (MSQ) stabilisation over partly unknown Markov fading channels is given in the paper. Besides, necessary and sufficient conditions for MSQ stabilisation in two special cases, i.e. completely known Markov fading channels and independent identically distributed (i.i.d.) fading channels, are also given, respectively. Finally, two examples are given to demonstrate the effectiveness of the proposed results.

A Factor Graph Approach to Iterative Channel Estimation, Detection, and Decoding for Two-Path Successive Relay Networks

We consider cooperative communications between a source node and a destination node with the help of two relays. In order to overcome the half-duplex constraint, the amplify-and-forward two-path relaying protocol is used. We adopt orthogonal frequency-division multiplexing modulation to combat frequency-selective channels and asynchronous reception at the destination node. In this paper, we develop a coherent receiver architecture suitable for unknown block fading channels. A factor graph representing the joint a posteriori probability of the coded symbols and the channels in the frequency domain is introduced. Then, we derive a Bayesian inference algorithm based on message passing over the factor graph. The resulting iterative receiver maintains low-complexity, based on the interaction between an off-the-shelf soft-input soft-output decoder and a newly introduced per-subcarrier processor for two-path relaying channel estimation and symbol detection. Simulation results show that the proposed solution maintains the full diversity order, even with a limited number of training blocks.

Maximum Likelihood Synchronization for DVB-T2 in Unknown Fading Channels

In this paper, we consider the timing and carrier frequency offset (CFO) estimation problem for digital video broadcasting-second generation terrestrial (DVB-T2) transmissions by utilizing the P1 symbol. Unlike previous studies, an unknown fading channel scenario is considered. Two maximum likelihood (ML) methods are derived by modeling the transmitted signal as a stochastic and, respectively, a deterministic unknown waveform. The proposed stochastic ML (SML) algorithm can jointly estimate the received signal power, time delay, and CFO. By ignoring the presence of null carriers in the P1 symbol, the SML method can be simplified with a lower complexity. Meanwhile, the proposed deterministic ML method is more flexible and computationally simpler compared with the SML method. Numerical results are presented to illustrate the performances of the proposed estimators along with several existing solutions for DVB-T2 signal synchronization. In addition, Cramer–Rao lower bounds are evaluated for the considered estimation problem.

A Simple and Effective NDA Carrier Frequency Estimation Method for MPSK Signals in Unknown Fading Channel

The problem of the frequency estimation of MPSK signal in unknown fading channel is too complicated and there is no research result for this problem at present, while we often meet these problems in practice. The method mentioned in this paper compensates the channel effects before the estimation the carrier frequency. This method is more simple and efficient than the method which uses frequency and channel impulse response for the joint estimation, and it is more convenient to remove modulation for MPSK signal. Compared with the frequency estimation research of non-fading channel, the estimation performance falls only about 3dB. So, this method can satisfy most application needs.

Spectrum sensing and data transmission trade-off in cognitive radio under outage constraints

For a cognitive radio, there is a trade-off between time used for spectrum sensing and the achievable throughput. For given constraints on the probability of detection, finding the optimum sensing time that maximises secondary users' throughput is of great importance. This issue has been addressed previously but the effect of the unknown fading channel was not taken into account. When the fading channel is random and unknown, the constraints on the probability of detection cannot be met for all instantaneous signal-to-noise ratios. Here, a more realistic approach is adopted, which allows the constraint on the probability of detection to be in outage with a specified percentage. The spectrum sensing time/secondary user's achievable throughput trade-off is then formulated and optimised accordingly.

Distributed Estimation over Unknown Fading Channels

In this paper, we consider the data detection problem involved in distributed estimation over unknown non-orthogonal fading channel. In general, when studying the distributed estimation problem, the impairments induced by communication channels are restricted to additive noise, quantization or packet loss. In addition, communication protocols are often of TDMA or FDMA type. Herein, by modulating the local data with doubly spread waveforms, we show that although each node receives a mixture of data transmitted by its neighbors, these data exhibit a trilinear structure, which can be used for separating the neighbors contributions. We state identifiability conditions and study the embedding of the data detection steps in a distributed estimation problem.

OPTIMUM RECEIVER UNDER CLASS-A IMPULSIVE NOISE FOR FLAT FADING CHANNELS

The paper presents an optimum receiver for digitally modulated signals in additive class-A impulsive noise for flat fading channels. The receiver is based on maximum likelihood (ML) approach. Two algorithms are presented to estimate the unknown fading channel. The convergence properties of the estimation algorithms are investigated and compared. The performance of the receiver is evaluated by simulation. The performance is measured by the bit error probability. Computer simulations are illustrated to validate the theoretical developments. It is shown that the performance of the developed receiver is sensitive to the impulsive index of the noise. Decreasing the value of the impulsive index degrades the performance of the receiver and reduces the receiver resistivity against frequency and time offset.

Fast convergent monte carlo receiver for OFDM systems

The paper investigates the problem of the design of an optimal Orthogonal Frequency Division Multiplexing (OFDM) receiver against unknown frequency selective fading. A fast convergent Monte Carlo receiver is proposed. In the proposed method, the Markov Chain Monte Carlo (MCMC) methods are employed for the blind Bayesian detection without channel estimation. Meanwhile, with the exploitation of the characteristics of OFDM systems, two methods are employed to improve the convergence rate and enhance the efficiency of MCMC algorithms. One is the integration of the posterior distribution function with respect to the associated channel parameters, which is involved in the derivation of the objective distribution function; the other is the intra-symbol differential coding for the elimination of the bimodality problem resulting from the presence of unknown fading channels. Moreover, no matrix inversion is needed with the use of the orthogonality property of OFDM modulation and hence the computational load is significantly reduced. Computer simulation results show the effectiveness of the fast convergent Monte Carlo receiver.

Joint parameter estimation of multiuser asynchronous DS CDMA signals in unknown fading channel in DS CDMA system with multiple antennas at the base-station and single antenna at the mobile

We present MUSIC type algorithm for joint parameter estimation in asynchronous DS CDMA system with multiple antenna system at base station and single antenna system at the mobile users. The proposed algorithm is applied in downlink processing and it enables the joint estimation of time delay frequency shift as well as spatial parameters (direction of departure - DOD of transmitted signal) of asynchronous DS CDMA signals using single antenna system at the mobile. The proposed algorithm is well suited to unknown fading channel, frequency selective or dispersive, and it can be primarily used as a method of multipath channel identification. It seems to the authors that it can be used as a basis for the further improvement of process of detection at the mobile receiver. The proposed algorithm is near-far resistant, requires no preamble, and can be applied when the user sequences are not perfectly orthogonal.