An evolutionary spectrum approach to modeling non‐stationary fading channels

To evaluate mobile communication systems, it is important to develop accurate and concise fading channel models. However, fading encountered in mobile communication is usually non‐stationary, and the existing methods can only model quasi‐stationary or piecewise‐stationary fading instead of general non‐stationary fading. To address this, this paper proposes an evolutionary spectrum (ES)‐based approach to modeling non‐stationary fading channels. Our ES approach is more general than the existing piecewise‐stationary models and is capable of characterizing a general non‐stationary fading channel that has an arbitrary ES (or time‐varying power spectral density); our ES approach is parsimonious and is also able to generate stationary fading processes. As an example, we show how to apply our ES approach to generating stationary and non‐stationary correlated Nakagami‐m fading channel processes. Simulation results show that the ES of the channel gain process produced by our ES‐based channel model agrees well with the user‐specified ES, indicating the accuracy of our ES‐based channel model. Copyright © 2011 John Wiley & Sons, Ltd.     

Stochastic Differential Equation Theory Applied to Wireless Channels

Modeling wireless channels is essential to wireless communication systems. An autoregressive (AR) process of order one for wireless channel has long been assumed, but without a rigorous mathematical/physical basis. In this paper, we derive a first-order stochastic AR model for a flat stationary wireless channel, which comes from stochastic differential equation (SDE) theory concerning the nature of multipath fading channels. The resulting AR model describes more of the origin of multipath fading channels than previous AR models, and it can efficiently model and generate Rayleigh-distributed stationary fading channels. The Markovian property of the AR model is inherited through the SDE approach.     

Effective capacity of a correlated Nakagami‐m fading channel

The grail of next‐generation wireless networks is providing real‐time services for delay‐sensitive applications, which require that the wireless networks provide QoS guarantees. The effective capacity (EC) proposed by Wu and Negi provides a powerful tool for design of QoS provisioning mechanisms. In this paper, we intend to generalize their formula for the effective capacity of a correlated Rayleigh fading channel; specifically, we derive a closed form approximate EC formula for a special correlated Nakagami‐m fading channel, for which the inverse of the correlation coefficient matrix is tridiagonal. To verify its accuracy via simulation, we develop a Green‐matrix based approach, which allows us to analytically obtain the effective capacity (given the joint probability density function of a correlated Nakagami‐m fading channel) while being able to simulate the corresponding channel gain process. Simulation results show that our EC formula is accurate. Furthermore, to facilitate the application of the EC theory to the design of practical QoS provisioning mechanisms, we propose a simple algorithm for estimating the EC of an arbitrary correlated Nakagami‐m fading channel, given channel measurements; simulation results demonstrate the accuracy of our proposed EC estimation algorithm showing its suitability in practice. Copyright © 2011 John Wiley & Sons, Ltd.     

Modeling TCP SACK performance over wireless channels with completely reliable ARQ/FEC

In this paper, we propose an analytical cross‐layer model for a Transmission Control Protocol (TCP) connection running over a covariance‐stationary wireless channel with a completely reliable Automatic Repeat reQuest scheme combined with Forward Error Correction (FEC) coding. Since backbone networks today are highly overprovisioned, we assume that the wireless channel is the only one bottleneck in the system which causes packets to be buffered at the wired/wireless interface and dropped as a result of buffer overflow. We develop the model in two steps. At the first step, we consider the service process of the wireless channel and derive the probability distribution of the time required to successfully transmit an IP packet over the wireless channel. This distribution is used at the next step of the modeling, where we derive expressions for the TCP long‐term steady‐state throughput, the mean round‐trip time, and the spurious timeout probability. The developed model allows to quantify the joint effect of many implementation‐specific parameters on the TCP performance over both correlated and non‐correlated wireless channels. We also demonstrate that TCP spurious timeouts, reported in some empirical studies, do not occur when wireless channel conditions are covariance‐stationary and their presence in those measurements should be attributed to non‐stationary behavior of the wireless channel characteristics. Copyright © 2011 John Wiley & Sons, Ltd.     

Channel‐estimate‐based frequency‐domain equalization (CE‐FDE) for broadband single‐carrier transmission

A channel‐estimate‐based frequency‐domain equalization (CE‐FDE) scheme for wireless broadband single‐carrier communications over time‐varying frequency‐selective fading channels is proposed. Adaptive updating of the FDE coefficients are based on the timely estimate of channel impulse response (CIR) to avoid error propagation that is a major source of performance degradation in adaptive equalizers using least mean square (LMS) or recursive least square (RLS) algorithms. Various time‐domain and frequency‐domain techniques for initial channel estimation and adaptive updating are discussed and evaluated in terms of performance and complexity. Performance of uncoded and coded systems using the proposed CE‐FDE with diversity combining in different time‐varying, multi‐path fading channels is evaluated. Analytical and simulation results show the good performance of the proposed scheme suitable for broadband wireless communications. For channels with high‐Doppler frequency, diversity combining substantially improves the system performance. For channels with sparse multi‐path propagation, a tap‐selection strategy used with the CE‐FDE systems can significantly reduce the complexity without sacrificing the performance. Copyright © 2004 John Wiley & Sons, Ltd.     

ZCZ‐CDMA and OFDMA using M‐QAM for broadband wireless communications

This paper considers direct‐sequence code‐division multiple‐access with zero‐correlation zone sequences (ZCZ‐CDMA) and orthogonal frequency‐division multiple‐access (OFDMA) schemes using M‐ary QAM signaling for broadband wireless communications. Their system structures, complexities and performances in both AWGN and multipath frequency‐selective fading channels are evaluated and compared. For ZCZ‐CDMA, joint suppression of the multipath fading interference and multiple‐access interference can be achieved with a reduced family‐size of the spreading sequences. For OFDMA, analytical and simulation results indicate that it has the same performance as ZCZ‐CDMA in fast time‐varying multipath fading channels. In time‐invariant or slowly time‐varying channels, where the channel information can be made available to transmitters, OFDMA outperforms ZCZ‐CDMA, offers a higher capacity and is more flexible for system reconfiguration with a comparable computational complexity. Copyright © 2004 John Wiley & Sons, Ltd.     

Improved Design Criterion for Space‐Frequency Trellis Codes over MIMO‐OFDM Systems

In this paper, we discuss the design problem and the robustness of space‐frequency trellis codes (SFTCs) for multiple input multiple output, orthogonal frequency division multiplexing (MIMO‐OFDM) systems. We find that the channel constructed by the consecutive subcarriers of an OFDM block is a correlated fading channel with the regular correlation function of the number and time delay of the multipaths. By introducing the first‐order auto‐regressive model, we decompose the correlated fading channel into two independent components: a slow fading channel and a fast fading channel. Therefore, the design problem of SFTCs is converted into the joint design in both slow fading and fast fading channels. We present an improved design criterion for SFTCs. We also show that the SFTCs designed according to our criterion are robust against the multipath time delays. Simulation results are provided to confirm our theoretic analysis.     

Power control and scheduling for guaranteeing quality of service in cellular networks

Providing quality of service (QoS) guarantees is important in the third generation (3G) and the fourth generation (4G) cellular networks. However, large‐scale fading and non‐stationary small‐scale fading can cause severe QoS violations. To address this issue, we design QoS provisioning schemes, which are robust against time‐varying large scale path loss, shadowing, non‐stationary small scale fading, and very low mobility. In our design, we utilize our recently developed effective capacity technique and the time‐diversity dependent power control proposed in this paper. The key elements of our QoS provisioning schemes are channel estimation, power control, dynamic channel allocation, and adaptive transmission. The advantages of our QoS provisioning schemes are (1) power efficiency, (2) simplicity in QoS provisioning, (3) robustness against large‐scale fading and non‐stationary small‐scale fading. Simulation results demonstrate that the proposed algorithms are effective in providing QoS guarantees under various channel conditions. Copyright © 2006 John Wiley & Sons, Ltd.     

An energy‐efficient anti‐jam cognitive system for wireless OFDM communication

In this paper, anti‐jamming capabilities are proposed for a generic OFDM‐based wireless communication system. The performance of a wireless system may severely degrade in presence of jamming or intentional interferences. A malicious entity can send strong interference or noisy signals in the same transmit frequency band thus preventing the intended receiver to correctly decode the transmitted data. As a countermeasure, our work proposes algorithms and techniques to restore the system performance in presence of such malicious entities. Proposed anti‐jamming system consists of an adaptive framework that adapts itself to varying jammer behavior and wireless environment, and then, it chooses an optimal strategy to effectively cancel out the effect of jamming and channel fading. Optimization problem is formulated as a multi‐objective criterion for maximization of system throughput and minimization of energy consumption. Simulation results computed in Rayleigh fading channels with different wideband jamming modes and powers show that the proposed system effectively cancels out the jamming and channel fading effects thus maximizing the system performance and minimizing cost of energy consumption. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of low noise amplifier non‐linearities on blind adaptive beamforming performance in CDMA wireless systems

We report a new approach to analyse the effects of low noise amplifier (LNA) non‐linear distortions in the code division multiple access (CDMA) wireless communication systems using spatio‐temporal analysis and Volterra series theory. For this purpose, the bit error rate (BER) performance of three blind algorithms is studied based on post correlated model of received signal, and a time‐varying multiple vector channel model which is an extended form of the Gaussian wide sense stationary uncorrelated scattering (GWSSUS) channel. By using the Volterra series theory, an analytical expression for amplitude modulation to phase modulation (AM–PM) conversion is determined as a phase statement of LNA compression. In this approach, by combining the analytical expression for AM–PM conversion and CDMA blind beamforming techniques, we evaluate the AM–PM distortion effects on BER performance of a CDMA system originated from multiple non‐linear LNA blocks. Simulation results show that conditions are found to minimize AM–PM conversion introduced by multiple non‐linear blocks in the system leading to low BER. Copyright © 2006 John Wiley & Sons, Ltd.     

Cross‐layer design for energy efficient communication in wireless sensor networks

There is a plethora of recent research on high performance wireless communications using a cross‐layer approach in that adaptive modulation and coding (AMC) schemes at wireless physical layer are used for combating time varying channel fading and enhance link throughput. However, in a wireless sensor network, transmitting packets over deep fading channel can incur excessive energy consumption due to the usage of stronger forwarding error code (FEC) or more robust modulation mode. To avoid such energy inefficient transmission, a straightforward approach is to temporarily buffer packets when the channel is in deep fading, until the channel quality recovers. Unfortunately, packet buffering may lead to communication latency and buffer overflow, which, in turn, can result in severe degradation in communication performance. Specifically, to improve the buffering approach, we need to address two challenging issues: (1) how long should we buffer the packets? and (2) how to choose the optimum channel transmission threshold above which to transmit the buffered packets? In this paper, by using discrete‐time queuing model, we analyze the effects of Rayleigh fading over AMC‐based communications in a wireless sensor network. We then analytically derive the packet delivery rate and average delay. Guided by these numerical results, we can determine the most energy‐efficient operation modes under different transmission environments. Extensive simulation results have validated the analytical results, and indicates that under these modes, we can achieve as much as 40% reduction in energy dissipation. Copyright © 2008 John Wiley & Sons, Ltd.     

On selecting the best transmission mode for WiFi devices

The design of efficient IEEE 802.11 physical (PHY) rate adaptation algorithms is a challenging research topic and usually the issues surrounding their implementations on real 802.11 devices are not disclosed. In this paper, we identify and evaluate the key parameters to design such algorithms. We then present a survey on existing PHY rate adaptation mechanisms and discuss their advantages and drawbacks. We also propose three new 802.11 PHY rate adaptation mechanisms, named adaptive auto rate fallback (AARF), closed loop adaptive rate allocation (CLARA), and adaptive multi‐rate retry (AMRR). AARF, proposed for low‐latency systems, has low complexity and obtains similar performance than RBAR in stationary and non‐fading wireless channels. CLARA is a culmination of the best attributes of the transmitter‐based ARF and RBAR control mechanisms with additional practical features such as adaptive fragmentation to improve multipath‐fading channel sensing and to provide feedback control signaling. AMRR is designed for high‐latency systems; it has been implemented and evaluated on an AR5212‐based device. Experimentation results show more than 20% performance improvement in throughput over the default algorithm implemented in the AR5212 MADWIFI driver. Copyright © 2008 John Wiley & Sons, Ltd.     

The transport capacity of wireless networks over fading channels

We consider networks consisting of nodes with radios, and without any wired infrastructure, thus necessitating all communication to take place only over the shared wireless medium. The main focus of this paper is on the effect of fading in such wireless networks. We examine the attenuation regime where either the medium is absorptive, a situation which generally prevails, or the path loss exponent is greater than 3. We study the transport capacity, defined as the supremum over the set of feasible rate vectors of the distance weighted sum of rates. We consider two assumption sets. Under the first assumption set, which essentially requires only a mild time average type of bound on the fading process, we show that the transport capacity can grow no faster than O(n), where n denotes the number of nodes, even when the channel state information (CSI) is available noncausally at both the transmitters and the receivers. This assumption includes common models of stationary ergodic channels; constant, frequency-selective channels; flat, rapidly varying channels; and flat slowly varying channels. In the second assumption set, which essentially features an independence, time average of expectation, and nonzeroness condition on the fading process, we constructively show how to achieve transport capacity of /spl Omega/(n) even when the CSI is unknown to both the transmitters and the receivers, provided that every node has an appropriately nearby node. This assumption set includes common models of independent and identically distributed (i.i.d.) channels; constant, flat channels; and constant, frequency-selective channels. The transport capacity is achieved by nodes communicating only with neighbors, and using only point-to-point coding. The thrust of these results is that the multihop strategy, toward which much protocol development activity is currently targeted, is appropriate for fading environments. The low attenuation regime is open.     

Cross‐layer bandwidth and power allocation for a two‐hop link in wireless mesh network

In this paper, a cross‐layer analytical framework is proposed to analyze the throughput and packet delay of a two‐hop wireless link in wireless mesh network (WMN). It considers the adaptive modulation and coding (AMC) process in physical layer and the traffic queuing process in upper layers, taking into account the traffic distribution changes at the output node of each link due to the AMC process therein. Firstly, we model the wireless fading channel and the corresponding AMC process as a finite state Markov chain (FSMC) serving system. Then, a method is proposed to calculate the steady‐state output traffic of each node. Based on this, we derive a modified queuing FSMC model for the relay to gateway link, which consists of a relayed non‐Poisson traffic and an originated Poisson traffic, thus to evaluate the throughput at the mesh gateway. This analytical framework is verified by numerical simulations, and is easy to extend to multi‐hop links. Furthermore, based on the above proposed cross‐layer framework, we consider the problem of optimal power and bandwidth allocation for QoS‐guaranteed services in a two‐hop wireless link, where the total power and bandwidth resources are both sum‐constrained. Secondly, the practical optimal power allocation algorithm and optimal bandwidth allocation algorithm are presented separately. Then, the problem of joint power and bandwidth allocation is analyzed and an iterative algorithm is proposed to solve the problem in a simple way. Finally, numerical simulations are given to evaluate their performances. Copyright © 2008 John Wiley & Sons, Ltd.     

Performance analysis of secure communications over correlated slow‐fading additive white Gaussian noise channels

The broadcast nature of communications in wireless communication networks makes it vulnerable to some attacks, particularly eavesdrop attack. Hence, information security can have a key role to protect privacy and avoid identity theft in these networks, especially in distributed networks. In the wireless systems, the signal propagation is affected by path loss, slow fading (shadowing), and fast fading (multi‐path fading). As we know, there is a correlation between communication channels in the real radio environments. This correlation is defined by the correlation between their shadowing and/or multipath fading factors. So when there are several channels in the wireless systems, there is certainly a correlation between the channels. In this paper, we assume that the transmitter knows the full channel state information (CSI), it means the transmitter knows both the channel gains of the illegitimate (ie, eavesdropper) and the legitimate receivers and study the performance of secure communications of single‐input single‐output (SISO) systems consisting of single antenna devices, in the presence of a single antenna passive eavesdropper over correlated slow fading channels, where the main (transmitter to legitimate receiver) and eavesdropper (transmitter to illegitimate receiver) channels are correlated. Finally, we present numerical results and verify the accuracy of our analysis by Monte‐Carlo simulations.     

移动通信中无线信道特性的研究

从等效低通信道的时变冲激响应的相关函数和功率密度谱函数出发,研究了一些描述广义平稳非相关散射(WSSUS)的无线信道特性的统计参数,并对无线移动衰落信道的特性进行分类。分析了3种常见的小尺度多径衰落信道的统计模型,给出了在平坦衰落信道中系统传输比特误码率的一般分析方法。最后,利用这些模型和方法,给出了一些常见数字调制信号的传输误码率的数值结果。结果表明：衰落对无线移动通信系统传输性能的影响是很大的,在实际中应设法消除它。     

Channel‐power profile estimation for orthogonal frequency‐division multiplexing systems

In this work, a channel‐power profile estimation for orthogonal frequency‐division multiplexing systems, based on the cyclic prefix (CP), is introduced. By knowing the delay of each path, the time‐dispersion information can be derived. The proposed method, considering long intersymbol interference (ISI) fading channels, requires only the coarse symbol timing information. More specifically, quasi‐stationary fading channels are considered. The basic contribution is to obtain the maximum‐likelihood estimation of the correlation coefficient based on the CP. Subsequently, the relationship between the correlation coefficient and the channel‐tap powers is explored. With the estimate of correlation coefficient, the least‐square solution of the channel‐tap powers can be determined. The proposed method is suitable for both short and long ISI channels. Furthermore, the Cramér–Rao lower bound of the channel‐power profile estimation is analyzed, and simulations confirm the advantages of the proposed estimator. Copyright © 2011 John Wiley & Sons, Ltd.     

Concatenation of space‐time block codes and turbo trellis coded modulation (ST‐TTCM) over Rician fading channels with imperfect phase

In this paper, the performance of space time‐turbo trellis coded modulation (ST‐TTCM) is evaluated over Rician and Rayleigh fading channels with imperfect phase. We modify Baum–Welch (BW) algorithm to estimate the fading and phase jitter parameters for multi‐antenna configurations. Thus, we assume that the channel parameters change slower than carrier frequency. We know that, at high data rate transmissions over wireless fading channels, space–time block codes (STBC) provide the maximal possible diversity advantage. Here, the combined effects of the amplitude and the phase of the received signal are considered, each one modelled by Rician and Tikhonov distributions, respectively. We investigate space time‐turbo trellis coded modulation (ST‐TTCM) for 8‐PSK for several Rician factor K and phase distortion factor η. Thus, our results reflect the degradations both due to the effects of the fading on the amplitude and phase noise of the received signal while the channel parameters are estimated by BW algorithm. Copyright © 2004 John Wiley & Sons, Ltd.     

Effective SNR for space-time modulation over a time-varying Rician channel

Rapid temporal variations in wireless channels pose a significant challenge for space-time modulation and coding algorithms. This letter examines the performance degradation that results when time-varying flat fading is encountered when using trained and unitary space-time modulation. Performance is characterized for a channel having a constant specular component plus a time-varying diffuse component. A first-order autoregressive (AR) model is used to characterize diffuse channel coefficients that vary from symbol to symbol, and is shown to lead to an effective signal-to-noise ratio (SNR) that decreases with time. Differential modulation is shown to have an advantage in effective SNR over trained unitary modulation at high power. Simulation results are provided to support our analysis.     

Performance analysis of two‐way MAC layer network coding under finite relay buffer and non‐negligible signalling overhead

Two‐way exclusive OR (XOR) relay can enable hidden nodes to exchange data with low delays and high data rate, while keeping signal processing simple. In this paper, we analyse practical two‐way XOR relaying systems, where finite relay buffer, non‐negligible signalling overhead, and lossy wireless channels are all captured. A two‐layer model is developed to characterise such practical two‐way relay systems, which is then reformulated into a Markov process after we project and combine inter‐layer state transitions of the two‐layer model. Using Markov techniques, we evaluate the steady state probabilities of the Markov process and, in turn, the key performance measures of two‐way XOR relaying, such as throughput, delay, and packet loss. The accuracy of our model is validated by simulations. Our model can also be used as an online tool to configure the buffer resources, adapting to wireless channel conditions and signalling requirements. Copyright © 2016 John Wiley & Sons, Ltd.