Time-varying Fading Channels Research Articles

Adaptive Sensing Schedule for Dynamic Spectrum Sharing in Time-Varying Channel

Dynamic spectrum sharing is considered as one of the key features in the next-generation communications. In this correspondence, we investigate the dynamic tradeoff between the sensing performance and the achievable throughput, in the presence of time-varying fading (TVF) channels. We first establish a unified dynamic state-space model to characterize the involved dynamic behaviors. On this basis, a promising dynamic sensing schedule framework is proposed, whereby the sensing duration is adaptively adjusted based on the estimated real-time TVF channel. We formulate the sensing-throughput tradeoff problem mathematically. We then show that there exists the optimal sensing duration maximizing the throughput for the secondary user (SU), which will change dynamically with channel gains. Relying on our designed recursive sensing paradigm that is able to blindly acquire varying channel gains as well as the PU states, the sensing duration can be then adjusted in line with the evolving channel gains. Numerical simulations are provided to validate our dynamic sensing schedule algorithm, which can significantly improve the SU's throughput by reconfiguring the sensing duration according to dynamic channel conditions.

An Investigation of Two Phase Multi-Relay S-DF Cooperative Wireless Network Over Time-Variant Fading Channels With Incorrect CSI

We investigate the pairwise error probability (PEP) performance degradation of dual phase multiple relay C(0) cooperation protocol due to imperfect channel state information (CSI) and mobility of nodes. We analyze PEP performance of C (0) protocol over time selective independent, but not identically distributed MIMO channel fading links. Our analysis reveals that because of the mobility of nodes, the PEP performance is severely degraded and in some specific system conditions, encounter asymptotic floor. Moreover, when both the destination-node (DN) and the source-node (SN) are static, the PEP upper union bound performance does not experience error floor limit regardless of the fact that the relay-nodes (RNs) are mobile. In addition, except the scenario when the RNs are static, the mobility of either the DN or the SN basically influences the performance of the wireless network due to error floors limits. Simulation outcomes prove that system achieves full diversity order (DO) when all nodes are static and perfect CSI. Simulation outcomes confirm the accuracy of the derived theoretical formulations.

Iterative Joint Channel Estimation and Signal Detection for OFDM System in Double Selective Channels

Intercarrier interference caused by fast time-varying multipath fading channels degrades the system performance of high-mobility orthogonal frequency division multiplexing systems. This study consi...

Fast Time-Varying Multipath Channel Estimation and Signal Detection for of OFDM Systems with Interference Compensation

Orthogonal frequency division multiplexing (OFDM) is a widely adopted system for broadband wireless transmission, However, time-varying channels degrade the performance owing to the intercarrier interference (ICI). This study discusses the performance enhancement problems of channel estimation and signal detection in view of fast time-varying multipath fading channel characteristics of high mobility. A two-stage method is presented. In the coarse stage, initial channel estimation is performed using the signals at pilot subcarriers based on least-square (LS) estimate is performed. However, the initial estimate still needs to be improved because of the data-induced ICI. The coarse stage improves estimate accuracy by compensating the interferences. The refining stage, applies interference compensation and q-banded successive interference suppression (qSIS) methods to enhance the detection and re-estimate accuracy. Based on robustness, the detected data are called the “pseudo-pilots”, which together with the pilots are treated as a “visual-training” (VT) to improve channel re-estimation. Simulation results indicate that the proposed method is more robust than the traditional method in fast time-varying multipath fading channels with lower computational complexity. The proposed method is independent of the delay and fading properties of the channel.

A new inter‐carrier interference self‐cancellation mapping scheme over fast time‐varying fading channels

A new inter-carrier interference self-cancellation method with the data symbol mapping scheme of differential grouping weighted symmetry data-conjugate is studied and proposed to raise the bandwidth efficiency and further improve the system performance over fast time-varying fading channels. Comparing with the other existing self-cancellation methods, the proposed scheme can not only maintain the same bandwidth efficiency as the original orthogonal frequency division multiplexing system, but also show a better performance on carrier-to-interference ratio and bit error rate according to the final simulation results and performance analysis.

Optimal Sensor Data Scheduling for Remote Estimation Over a Time-Varying Channel

We consider sensor data scheduling with communication energy constraint for remote state estimation over a time-varying fading channel. Given that the fade channel state with temporal correlated variation influences the estimation accuracy, the goal is to obtain a schedule for a sensor to decide whether using high energy to send its local estimate to a remote estimator in order to minimize the expected average estimation error covariance. By exploiting the feedback information, we propose an effective closed-form dynamic stationary scheduling scheme. We prove that such simple schedule is optimal in terms of the estimation performance by comparing with a general stationary schedule under energy constraint. Furthermore, we derive the sufficient and necessary condition for estimation stability when the sensor can only use low energy at each time. Numerical examples are provided to illustrate the effectiveness of the proposed scheme.

DTMR: delay-aware traffic admission, mode selection, and resource allocation for D2D communication

In device-to-device (D2D) communication underlaying a cellular network, the resource control involves traffic admission, mode selection, orthogonal channel assignment, and power control. Traffic admission limits queueing delay, mode selection exploits the proximity gain, and resource allocation guarantees user performance. Jointly optimizing these factors is highly challenging due to the stochastic nature of the system and the coupled control actions. Many previous works only consider a subset of these factors. In this paper, we tackle the joint optimization problem for delay-aware D2D communication. In particular, considering both dynamic traffic arrival and time-varying channel fading, we aim to maximize the time-average sum-rate of the network subject to the time-average throughput guarantee of users and resource allocation constraints. Thus, through presenting a Lyapunov optimization framework, we design an optimal delay-aware traffic admission, mode selection, and resource allocation (DTMR) strategy with polynomial time complexity based on dual optimization and ellipsoid search. We also analytically derive lower bound of the time-average sum-rate achieved by the proposed DTMR strategy. Further, we develop a fast heuristic strategy by decoupling the binary constraints of mode selection and channel assignment. Finally, simulation results demonstrate the superiority of the DTMR strategy against alternative strategies.

Quantized H∞ filtering for discrete-time systems over fading channels

This paper addresses the problem of quantized [Formula: see text] filtering for multi-output discrete-time systems over independent identically distributed (i.i.d.) fading channels and Markov fading channels, respectively. The measurement outputs are quantized by a logarithmic quantizer, and then transmitted to the filter over fading channels. For the i.i.d. fading channels, the stochastic multiplicative noise form is used to model the unreliable communication environment. For the Markov fading channels, a set of Markov channel state processes is introduced to model time-varying fading channels, which characterizes various configurations of the physical communication environment and/or different channel fading amplitudes. The sufficient condition for stochastic stability with a prescribed [Formula: see text] performance is obtained by using a Lyapunov method and matrix decoupling technique. The corresponding filter design casts into a convex optimization problem. Finally, simulation results are provided to illustrate the effectiveness of our results.

Distributed Interference and Delay Aware Design for D2D Communication in Large Wireless Networks With Adaptive Interference Estimation

We investigate distributed flow control and power allocation strategies for delay-aware device-to-device (D2D) communication underlaying large wireless networks, where D2D pairs reuse the resource blocks of interior cellular users (CUEs). We consider a distributed D2D power allocation framework, where the D2D pairs individually attempt to maximize their own time-average throughput utility, while collectively guaranteeing the time-average coverage probability of CUEs in multiple cells. We design a novel method to compute the individual budget of interference from each D2D pair to CUEs based on stochastic geometry tools. Then, accounting for time-varying channel fading and dynamic D2D traffic arrival, we design a distributed interference-and-delay-aware (DIDA) flow control and power allocation strategy based on Lyapunov optimization and several interference estimation methods. We also analytically derive the performance bounds of D2D pairs, and prove that the coverage probability of CUEs can be guaranteed regardless of the interference estimation error at D2D receivers. Finally, simulation results suggest that adaptive interference estimation methods are preferred and demonstrate that the DIDA strategy achieves substantial performance improvement against alternative strategies.

Effect of composite channel aging on the spectral efficiency of multi‐pair massive multiple‐input multiple‐output amplify‐and‐forward relay networks

This study investigates the spectral efficiency of multi-pair massive multiple-input multiple-output amplify-and-forward relay networks by considering the composite channel aging effect. The rational of this work is that the channel aging effect caused by phase noise and node movements is an inevitable practical channel impairment in time-varying fading channels and substantially degrades the performance. The proposed model comprises multiple sources and multiple destinations, each equipped with a single antenna, which communication via a relay equipped with a very large number of antennas by employing maximal-ratio combining/maximum-ratio transmission. Based on this model, the authors first derive a closed-form lower bound expression for the achievable rate of per source–destination pair. Then, by using the derived expression, they study how the transmitted powers of each source and the relay can be reduced without compromising the spectral efficiency when the number of relay antennas approaches infinity. They also discuss the effect of channel aging coefficients, including Doppler shift and phase noise increment variance, on the asymptotic spectral efficiency under different power scaling laws. Both theoretical analysis and Monte Carlo simulations disclose that the channel aging effect does not affect the power scaling laws but degrades the spectral efficiency.

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.

Mean square detectability of LTI systems over finite-state digital block-fading channels

This paper studies the mean square quadratic (MSQ) detectability for multi-output networked systems over finite-state digital block-fading channels. The packet-loss rate of each digital fading channel depends on the channel power gain, as well as packet length and power level used for transmission. A finite-state random process is introduced to model time-varying fading channels, which characterizes various configurations of physical communication environment and/or different channel fading amplitudes. Necessary and sufficient conditions for MSQ detectability over finite-state Markov digital block-fading channels are given in the form of algebraic Riccati equations or linear matrix inequalities (LMIs). The estimation gain is given as a function of estimated/observed channel state. In addition, explicit conditions on network for MSQ detectability over finite-state independent identically distributed (i.i.d.) digital block-fading channels are presented in terms of the unstable poles of the multi-output plant. Finally, an application to Gilbert-Elliott channels (GECs) is provided to demonstrate the derived results.

Time domain equalization method for TS-OFDM signal under higher mobile environments

In higher time-varying fading channels, the signal quality of orthogonal frequency division multiplexing (OFDM) technique would be degraded relatively due to the occurrence of inter-carrier interference (ICI). To solve this problem, this paper firstly proposes a new design of time domain training sequence (TS) in the estimation of channel impulse response (CIR) for the TS-OFDM signal which can reduce the leakage of power spectrum density (PSD) at the outside of OFDM allocated frequency bandwidth with keeping higher CIR estimation accuracy. Secondly, this paper proposes a time domain equalization (TDE) method which can achieve better bit error rate (BER) performance with keeping lower computation complexity even in higher time-varying fading channels. The salient feature of the proposed TDE method is to employ a partial differentiation for the time domain CIR matrix for solving the maximum likelihood (ML) equation in which the time domain CIR matrix becomes a symmetric banded matrix. From this feature, a low-complexity parallel block inverse matrix algorithm can be employed in the calculation of inverse matrix in keeping the same accuracy as that of the direct inverse matrix calculation. This paper presents various computer simulation results to demonstrate the effectiveness of the proposed TDE method for the TS-OFDM signal as compared with conventional frequency domain equalization (FDE) methods under higher mobile environments.

Spectrum Sensing for Cognitive Radios With Unknown Noise Variance and Time-variant Fading Channels

The unknown noise variance and time-variant fading channels make the spectrum sensing design a challenging task for cognitive radios. Most existing sensing methods suffer from the information uncertainty and can hardly acquire promising performances in the adverse situations. To address this challenge, in this paper, we first formulate a dynamic state-space model for spectrum sensing, in which the unknown noise variance and time-variant flat fading channels are all taken into considerations. The dynamic behaviors of both primary user states and fading channels are characterized by two discrete state Markov chains. Based on this model, a novel spectrum sensing scheme is designed to recursively estimate the occupancy state of primary users, by estimating the time-variant fading channel gain and noise parameters jointly. The joint estimation is primarily premised on a maximum a posteriori probability criterion and the marginal particle filtering schemes. Simulation results are provided to demonstrate the advantages of our proposed method, which can significantly improve the sensing performance over time-variant flat fading channels, even with unknown noise variance.

快时变衰落信道下OFDM系统的SAGE检测算法研究

OFDM技术可以有效地抑制多径效应的影响，从而被广泛地应用到无线通信系统中。快时变衰落信道下的OFDM系统将产生严重的子载波间干扰(ICI)，从而影响系统性能。本文对快时变OFDM系统，提出一种基于空间交替广义期望最大化算法(SAGE)的信号检测算法。理论分析的实验仿真结果表明，所提SAGE算法是有效的。 Orthogonal frequency division multiplexing (OFDM) technology can effectively suppress the im-pact of multipath effects, which is widely used in wireless communication systems. However, OFDM systems will lead to severe inter carrier interference (ICI) under fast time-varying fading channels, which will determine system performance. In this paper, an efficient data detection al-gorithm is proposed based on the space alternating generalized expectation maximization (SAGE) technique for OFDM systems under fast fading channels. Simulation results show that the pro-posed algorithm is effective for improving the performance of OFDM systems under fast time- varying fading channels.

Soft-Decision-Aided, Smoothness-Constrained Channel Estimation over Time-Varying Fading Channels With No Channel Model Information

We consider frequency-flat time-varying fading channels with no channel model information (CMI). By introducing the smoothness function to measure the extent of channel fluctuation, we derive a robust soft-decision-aided (SDA) channel estimator based on Pareto optimality of the double-objective optimization of the likelihood function of the received signal sequence and the smoothness constraint of the channel estimates. Compared with the conventional maximum-likelihood-based channel estimators derived under the block-fading assumption, the newly derived SDA-Pareto estimator gives more freedom to the channel estimation process, allowing channel estimates to have controlled variations to track the time-varying channel more closely. Compared with estimators derived based on the maximum a posteriori probability or the minimum mean-square error criterion which require explicit acquisition of the CMI, the SDA-Pareto estimator significantly simplifies the channel measurement process by requiring only a suitable regularization parameter to balance the trade-off between the likelihood function and the smoothness condition. An adaptive algorithm is proposed to adjust the regularization parameter adaptively, enabling an efficient and effective implementation of the SDA-Pareto estimator in practical applications. Simulation studies are provided to demonstrate the advantage of the SDA-Pareto estimator over the conventional estimators in both channel estimation accuracy and error-rate performance.

Performance Analysis of an Adaptive Incremental Cooperative Relaying Scheme for Wireless Relay Networks

In wireless relaying networks, the traditional incremental cooperative relaying scheme (IR) could improve the system throughput enormously over fading channels by exploiting relay nodes to retransmit the source data packet to the destination. In order to enhance the system throughput over time-varying fading channels, this paper proposes an adaptive incremental cooperative re-laying scheme (AIR) based on adaptive modulation and coding, which implements adaptive rate transmission for the source and relay nodes according to channel state information. We derive expressions for the AIR system throughput, and then give a gradient-based search algorithm to find the optimized adaptive solution for the AIR system by maximizing throughput subject to the constraint of packet error rate at the data link layer. The results indicate that throughput of AIR system outperforms that of traditional IR system greatly for any SNR value.

Performance Analysis of STBC/SFBC System in Time-varying and Multi-path Channels

This study analyzed STBC and SFBC-OFDM systems having two or four transmission antennas in the time-varying and multi path fading channels environment. According to the performance analysis result, performance degradation was confirmed in this study, as channel variation period becomes shorter, when orthogonal or quasi-orthogonal STBC/SFBC technique was applied to the system having two or four transmission antennas in the time-varying and multi path fading channels. Also, the detection method using synchronization was applied for performance improvement. As a result of performance analysis on the STBC system in a time-varying channel and on the SFBC-OFDM system in the multi path channel, performance fell, as maximum time delay increased due to coherence time decline arising from the decrease of channel correlation coefficient, respectively, and the increase of the number of taps. However, BER (bit error rate) performance at SNR 25dB or higher was ascertained to improve up to 10 -3 ~10 -4 . Compared to the detection technique using ZF (zero forcing), the MMSE (minimum mean square error) detection technique showed good performance by acquiring 1~5dB or higher SNR gain. But, the ZF detection technique had a merit of being simpler from the hardware complexity aspect, compared to the MMSE detection technique. This study showed a possibility of high speed data transmission without additional bandwidth expansion by increasing the number of transmission antennas as a result of a performance analysis in the time-varying and multi path fading generated according to user mobility in the mobile communications environment.

Relay selection in cooperative communication systems over continuous time-varying fading channel

In this paper, we study relay selection under outdated channel state information (CSI) in a decode-and-forward (DF) cooperative system. Unlike previous researches on cooperative communication under outdated CSI, we consider that the channel varies continuously over time, i.e., the channel not only changes between relay selection and data transmission but also changes during data transmission. Thus the level of accuracy of the CSI used in relay selection degrades with data transmission. We first evaluate the packet error rate (PER) of the cooperative system under continuous time-varying fading channel, and find that the PER performance deteriorates more seriously under continuous time-varying fading channel than when the channel is assumed to be constant during data transmission. Then, we propose a repeated relay selection (RRS) strategy to improve the PER performance, in which the forwarded data is divided into multiple segments and relay is reselected before the transmission of each segment based on the updated CSI. Finally, we propose a combined relay selection (CRS) strategy which takes advantage of three different relay selection strategies to further mitigate the impact of outdated CSI.

Pilot Assisted Channel Estimation in MIMO-STBC Systems over Time-Varying Fading Channels- A Review

this paper, challenge about the condition of channel state info (CSI) in multiple-input multiple-output (MIMO) systems supported space time block codes (STBC) over slow time- varying Rayleigh attenuation channels are addressed. We develop a novel MIMO control assessment algorithmic rule that adopts a pilot symbol assisted modulation (PSAM) which has been proved to be effective for attenuation channels. In this approach, pilot symbols are periodically inserted into the information stream that's sent through the orthogonal STBC encoder. At the receiver, we propose a simple MIMO channel estimation methodology before getting used by STBC decoder. Simulation results indicate that the proposed pilot- assisted MIMO conception provides accurate channel estimates. The impact of Doppler frequency on performance scheme is also investigated by simulation. Keywords-input- multiple-output (MIMO), pilot assisted systems, rate distortion. Channel Estimation.