Fading Conditions Research Articles

On the use of tracking loops for low-complexity multi-path channel estimation in OFDM systems

This paper treats pilot aided multi-path channel estimation with tracking loops for OFDM systems under slow to moderate fading conditions. Recent works have presented theoretical results for the tuning of second-order and third-order tracking loops in the particular context of Jakes׳s Doppler spectrum channel. The method for getting the loop coefficients resorted either to the use of a given constraint, which made the obtained coefficients sub-optimal, or was obtained in part by simulations. Here, we perform a global optimization of the coefficients without constraints to get the optimal coefficients, and analytical formulas are provided. One remarkable result of this optimization is that only the natural frequency depends on the transmission parameters, i.e., the channel Doppler spectrum, the power delay profile, and the noise variance. Consequently, only one parameter has to be tuned. Moreover, asymptotic performance is formulated in a more general way as a function of the 2rth moments of the Doppler spectrum (r is the loop order). Hence, all our derivations are usable for any Doppler spectrum and are not specific to Jakes׳s Doppler spectrum. A complete table sums up for the three orders of the theoretical results of the optimal coefficients together with the asymptotic performance. The performance is also compared with that of the asymptotic Kalman filter.

Ergodic Capacity of SIM-Based DF Relayed Optical Wireless Communication Systems

In this letter, we consider a dual-hop, subcarrier intensity modulation-based relayed optical wireless communication system under the combined influence of atmospheric turbulence and pointing error impairments. The turbulence-induced fading is modeled by independent but not necessarily identically distributed Gamma–Gamma fading statistics with misalignment fading. The relaying protocol followed by the relay is decode and forward. We first, derive the statistics of instantaneous signal-to-noise ratio at the destination, followed by the novel and exact analytical expressions for ergodic capacity of the system in terms of mathematically tractable Meijer’s G-function and extended generalized bivariate Meijer’s G-function.

Pilot contamination reduction by shifted frame structure in massive MIMO TDD wireless system

Pilot contamination limits the performance of massive multiple-input multiple-output (MIMO) system severely due to the degraded channel estimation. An efficient way to solve this problem in time division duplex (TDD) wireless system is shifting the location of pilots in time frames used in neighboring cells. However, the shifted frame structure has only been studied in MIMO system with the ideal independent and identically distributed (i.i.d.) channel coefficients. In this paper, the shifted frame structure is studied in a measured channel with a large number of antennas for a certain class of channel fading statistics. To deal with the high inter-cell interference caused by the shifted frame structure in such a measured channel condition, we propose a scenario with a covariance-aided channel estimator. This scenario shows that pilot contamination can be solved and the high inter-cell interference can be made to vanish asymptotically with the number of antennas. The key of the interference rejection is obtaining the precise condition on the second order statistics for the desired and interference signals. The most significant information of the second order statistics comes from two parts, one is the channel state information (CSI) among the base stations (BSs), the other comes from the estimated information exchanged by the BSs, which depicts the channel between the BSs and the users. The simulations give powerful results of the interference rejection and the achievable rate promotion.

Average Channel Capacity of Correlated Dual-Branch Maximal Ratio Combining Under Worst Case of Fading Scenario

The channel capacity and probability of outage results for optimum power and rate adaptation (OPRA) and truncated channel inversion with fixed rate (TIFR) schemes over correlated diversity branch obtained so far in literature are applicable only for m ? 1. This paper derived closed-form expressions for the average channel capacity and probability of outage of dual-branch maximal ratio combining (MRC) over correlated Nakagami-0.5 (m < 1) fading channels. This channel capacity and probability of outage are evaluated under OPRA and TIFR schemes. Since, the capacity and probability of outage expressions of dual-branch MRC under OPRA and TIFR schemes contain an infinite series; bounds on the errors resulting from truncating the infinite series have been derived for both average channel capacity and probability of outage. The corresponding expressions for Nakagami-0.5 fading are called expressions under worst fading condition with severe fading. Finally, numerical results are presented, which are then compared to the capacity and probability of outage results that previously published for OPRA and TIFR schemes. It has been observed that OPRA provides improved average channel capacity and probability of outage, as compared to TIFR under worst case of fading. It is also observed that probability of outage under TIFR scheme is not improved adequately than the probability of outage under OPRA even as employing diversity.

A Novel Solution for OFDM Based Relay Systems

In this paper we propose a novel solution for performance enhancement of dual-hop orthogonal frequency division multiplexing (OFDM) based decode-and-forward (DF) relay system, which assumes that relay station uses more than one antenna for downlink communications and implements transmit antenna selection (TAS) on subcarrier basis jointly with ordered subcarrier mapping (SCM). Ordered SCM is technique where subcarriers from the first hop are mapped to corresponding subcarriers on the second hop in accordance to their instantaneous signal-to-noise ratios (SNRs). It is proven to be a mapping scheme that maximizes the achievable ergodic capacity in OFDM based relay systems, while enabling bit error rate (BER) improvement at the same time. In order to assess the level of BER performance improvement in the system proposed in this paper, we analytically derive closed form BER expression of binary phase shift keying modulated OFDM DF relay system with TAS and SCM, assuming a scenario with Rayleigh fading statistics on both hops. Additionally, for further BER reduction, we analyze the combination of TAS with a modified SCM scheme, in which the subcarriers having the lowest SNRs on both hops are omitted. All derived analytical results are completely verified through simulations, showing that significant BER performance improvements are achieved with the novel proposed OFDM relay system, compared to the systems implementing only SCM, or only TAS, which approves that it can be considered as an interesting solution for the next generation of mobile cellular systems.

Measurement-Based Modeling of Time-Variant Fading Statistics in Indoor Peer-to-Peer Scenarios

Investigation of small-scale fading in indoor peer-to-peer networks based on radio channel measurements shows that Rayleigh or double Rayleigh (DR) fading with or without line-of-sight (LOS) component can occur, depending on the mobility and on the scattering properties of the environment. While in double-mobile scenarios, the predominant fading mechanism is a combination of Rayleigh and DR fading, Rician fading and fading events caused by the combination of a constant term with a DR component are also occasionally observed. In single-mobile scenarios, temporal fading is Rician or Rayleigh distributed. Additionally, measurements reveal that fading statistics evolve significantly over time, even for small-scale motions of the nodes. We model the transitions between the fading states by means of a hidden Markov model, parameterized from our measurements. Eventually, we propose a complete simulation model that generates channel realizations by combining measurement-based first-order statistics with geometry-based second-order statistics for both types of node mobility.

Comparison of heterogeneous network rain fade simulation tools: global integrated network SIMulator and MultiEXCELL

Two heterogeneous network simulation tools are compared and tested against measured data from terrestrial and Earth-space microwave links. Both global integrated network SIMulator (GINSIM) and MultiEXCELL are able to produce joint distributions of rain fade time-series for arbitrary networks of microwave telecommunications links. The tools are fundamentally different in that GINSIM has, as its input, time-series of composite rain rate maps produced by networks of meteorological radars and can produce time-series of joint rain fade. MultiEXCELL uses the rain rate distribution as an input to constrain distributions of parameters of simulated rain cells. This study tests the simulators by comparing a range of annual fade statistics produced by simulation with data measured on experimental links. Link fade data were measured on two, Ka band, Earth-space links and a convergent 38 GHz terrestrial link near Chilbolton in southern England; and a further Earth-space link located in Dundee, Scotland. Both GINSIM and MultiEXCELL are shown to be able to predict joint fade distributions and diversity gain, for the satellite and terrestrial links, to useful accuracy. Differences in performance are analysed, leading to suggested development routes for both systems.

Performance Analysis of Cognitive Radio Systems With Imperfect Channel Sensing and Estimation

In cognitive radio systems, employing sensing-based spectrum access strategies, secondary users are required to perform channel sensing to detect the activities of licensed primary users in a channel, and in realistic scenarios, channel sensing occurs with possible errors due to miss-detections and false alarms. As another challenge, time-varying fading conditions in the channel between the secondary transmitter and the secondary receiver have to be learned via channel estimation. In this paper, performance of causal channel estimation methods in correlated cognitive radio channels under imperfect channel sensing results is analyzed, and achievable rates for reliable communication under both channel and sensing uncertainty are investigated by considering the input-output mutual information. Initially, cognitive radio channel model with channel sensing error and channel estimation is described. Then, using pilot symbols, minimum mean square error (MMSE) and linear-MMSE (L-MMSE) estimation methods are employed at the secondary receiver to learn the channel fading coefficients. Expressions for the channel estimates and mean-squared errors (MSE) are determined, and their dependencies on channel sensing results, and pilot symbol period and energy are investigated. Since sensing uncertainty leads to uncertainty in the variance of the additive disturbance, channel estimation strategies and performance are interestingly shown to depend on the sensing reliability. It is further shown that the L-MMSE estimation method, which is in general suboptimal, performs very close to MMSE estimation. Furthermore, assuming the channel estimation errors and the interference introduced by the primary users as zero-mean and Gaussian distributed, achievable rate expressions of linear modulation schemes and Gaussian signaling are determined. Subsequently, the training period, and data and pilot symbol energy allocations are jointly optimized to maximize the achievable rates for both signaling schemes.

Exact Capacity of Amplify-and-Forward Relayed Optical Wireless Communication Systems

In this letter, we model a dual-hop subcarrier intensity modulated amplify-and-forward (AF) relayed optical wireless communication system with the assumption that direct communication between source and destination is not feasible. The optical channels are considered to be impaired with path loss, misalignment errors, and atmospheric turbulence, and are modeled assuming independent but not necessarily identically distributed Gamma–Gamma fading statistics. We derive exact analytical expressions for the average channel capacity of the system considering both type of AF relays, i.e., variable gain and fixed gain. Further to get valuable insight, an asymptotic analysis of channel capacity is presented at high signal-to-noise ratio to find the effect of various parameters on the performance of the system.

Fading statistics variation and optimum Mary frequency shift keying system design in sea surface fluctuation

The underwater acoustic channel can be expressed as a frequency selective fading channel due to multipath, sea surface roughness, and propagation medium change with time and space. Therefore the fading statistics varies with time, space, and frequency. Inherent factor of this fading change is related to a frequency dependent constructive or destructive interference change with time, space, and frequency. The magnitude of interference depends on closely on sea surface roughness. In this study, channel fading statistics are analyzed with delay spread and Doppler spread using pseudo noise (PN) signals. Based on this channel fading statistics, a low data rate M-ary frequency shift keying (MFSK) system of 1kbit/s, is designed for a command/control of navigation or an equipment status monitoring system. The forward error correcting code of convolution code is also applied. Experimental work is conducted in shallow water and it is found that the design parameters of MFSK system can be optimized using the measured fading statistics deduced from PN signal.

Dual-Hop Fixed Gain Relaying Transmissions with Semi-Blind in Asymmetric Multipath/Shadowing Fading Channels

In this paper, we investigate the end-to-end performance of a dual-hop fixed gain relaying system with semiblind relay under asymmetric fading environments. In such environments, the wireless links of the considered system undergo asymmetric multipath/shadowing fading conditions, where one link is subject to only the Nakagami-m fading, the other link is subject to the composite Nakagami-lognormal fading which is approximated by using mixture gamma fading model. First, the cumulative distribution function (CDF), the moment generating function (MGF) and the moments of the end-to-end signal-to-noise ratio (SNR) are derived under two asymmetric scenarios. Then, novel closed-form expressions of the outage probability, the average end-to-end SNR, the symbol error rate and the ergodic capacity for the dual-hop system are obtained based on the CDF and the MGF, respectively. Finally, some numerical and simulation results are shown and discussed to validate the accuracy of the analytical results under different scenarios, such as varying average SNR, fading parameters per hop, the choice of the semi-blind gain and the location of relaying nodes.

Asymptotic Deployment Gain: A Simple Approach to Characterize the SINR Distribution in General Cellular Networks

In cellular network models, the base stations are usually assumed to form a lattice or a Poisson point process (PPP). In reality, however, they are deployed neither fully regularly nor completely randomly. Accordingly, in this paper, we consider the very general class of motion-invariant models and analyze the behavior of the outage probability (the probability that the signal-to-interference-plus-noise-ratio (SINR) is smaller than a threshold) as the threshold goes to zero. We show that, remarkably, the slope of the outage probability (in dB) as a function of the threshold (also in dB) is the same for essentially all motion-invariant point processes. The slope merely depends on the fading statistics. Using this result, we introduce the notion of the asymptotic deployment gain (ADG), which characterizes the horizontal gap between the success probabilities of the PPP and another point process in the high-reliability regime (where the success probability is near 1). To demonstrate the usefulness of the ADG for the characterization of the SINR distribution, we investigate the outage probabilities and the ADGs for different point processes and fading statistics by simulations.

Wireless Compressive Sensing Over Fading Channels With Distributed Sparse Random Projections

We address the problem of recovering a sparse signal observed by a resource constrained wireless sensor network with fading channels. Sparse random matrices are exploited to reduce the communication cost in forwarding information to a fusion center. The presence of channel fading leads to inhomogeneity and non-Gaussian statistics in the effective measurement matrix that relates the measurements collected at the fusion center and the sparse signal being observed. We analyze the impact of channel fading on recovery of a given sparse signal by leveraging the properties of heavy-tailed random matrices. We quantify the additional number of measurements required to ensure reliable signal recovery in the presence of nonidentical fading channels compared to that is required with identical Gaussian channels. Our analysis provides insights into how to control the probability of sensor transmissions at each node based on the channel fading statistics to minimize the number of measurements collected at the fusion center for reliable sparse signal recovery. We further discuss recovery guarantees of a given sparse signal with any random projection matrix where the elements are subexponential with a given subexponential norm. Numerical results are provided to corroborate the theoretical findings.

The Sum-Capacity of the Ergodic Fading Gaussian Cognitive Interference Channel

This paper characterizes the sum-capacity of the ergodic fading Gaussian overlay cognitive interference channel (EGCIFC), a time-varying channel with two source-destination pairs in which a primary/licensed transmitter and a secondary/cognitive transmitter share the same spectrum and where the cognitive transmitter has noncausal knowledge of the primary user's message. The throughput/sum-capacity is characterized under the assumption of perfect knowledge of the instantaneous fading states at all terminals, which are assumed to form an ergodic process. A genie-aided outer bound on the sum-capacity is developed and then matched with an achievable scheme, thereby completely characterizing the sum-capacity of the EGCIFC. The power allocation policy that maximizes the sum-capacity is derived. It is shown that the sum-capacity achieving scheme for an EGCIFC is “separable” in all regimes (i.e., coding across fading states is not necessary), as opposed to the classical interference channel. Extensions to the whole capacity region are discussed. As a capacity achieving scheme for the EGCIFC under certain channel gain conditions, and as a topic of independent interest, the ergodic capacity of a point-to-point multiple-input-single-output channel with per-antenna power constraints and with perfect channel state information at all terminals is also derived.

A Unified Approach to Performance Analysis of Multihop Relay Fading Channels Using Generalized Gamma Model

AbstractIn this paper analysis of various fading statistics of generic fading distribution, termed as

Attitude Estimation for In-Service Base Station Antenna Using Downlink Channel Fading Statistics

A maximum-likelihood-estimation method is proposed for extracting the attitude of a sectoring base station (BS) antenna by using the received signal strengths observed by multiple user equipments (UEs) in this contribution. This method calculates the likelihood function of the antenna attitude derived by taking into account the multiscale fading statistics, that is, path loss, shadowing, and multipath fading. Depending on whether a calibration result of these fading statistics is available or not, the proposed method can be utilized in either calibration-based estimation (CBE) or calibration-free estimation (CFE) approaches. The performance of both methods is evaluated by Monte-Carlo simulations and real experiments. The results obtained demonstrate that the estimation accuracy of both CBE and CFE approaches increases when the percentage of UEs in the line-of-sight (LoS) condition among all available UEs increases and, moreover, the total number of UEs has no significant impact on the estimation accuracy. Furthermore, the CFE exhibits more robust performance than the CBE particularly in the case where the calibration results involve uncertainties.

Investigating the Use of Message Reneging in Multi-hop 802.11p VANETs

Vehicular ad hoc networks (VANETs) are becoming an important technology to enable enhanced road safety. Based on the 802.11p standard model, VANETs are known to suffer from channel congestion when the number of vehicles increases. Also, problems of fading and channel loss conditions on the road, make the effective delivery rate in VANETs very low beyond distances that are half of the transmission range. In this paper, we are interested in multi-hop message delivery in VANETs to extend the delivery range of safety messages beyond a one-hop range. We investigate the use of priority to favor on-hop messages over relayed ones according to the distance from the event originator. We also investigate the effect of using a limited delay in messages queues versus message reneging with such a scheme. An analytic and simulation study show interesting results under various constraints.

Performance of Open-Loop Transmit Diversity with Intra-Subframe Frequency Hopping and Iterative Decision-Feedback Channel Estimation for DFT-Precoded OFDMA

Open-loop (OL) transmit diversity is more subject to the influence of channel estimation error than closed-loop (CL) transmit diversity, although it has the merit of providing better performance in fast Doppler frequency environments because it doesn't require a feedback signal. This paper proposes an OL transmit diversity scheme combined with intra-subframe frequency hopping (FH) and iterative decision-feedback channel estimation (DFCE) in a shared channel for discrete Fourier transform (DFT)-precoded orthogonal frequency division multiple access (OFDMA). We apply intra-subframe FH to OL transmit diversity to mitigate the reduction in the diversity gain under high fading correlation conditions among antennas and iterative DFCE to improve the channel estimation accuracy. Computer simulation results show that the required average received signal-to-noise power ratio at the average block error rate (BLER) of 10-2 of the space-time block code (STBC) with intra-subframe FH is reduced to within approximately 0.8dB compared to codebook-based CL transmit diversity when using iterative DFCE at the maximum Doppler frequency of fD =5.55Hz. Moreover, it is shown that STBC with intra-subframe FH and iterative DFCE achieves much better BLER performance compared to CL transmit diversity when fD is higher than approximately 30Hz since the tracking ability of the latter degrades due to the fast fading variation in its feedback loop.

2P6-1 Fading Statistics Characterization of Shallow water Acoustic Communication Channel

2P6-1 Fading Statistics Characterization of Shallow water Acoustic Communication Channel

Human Body Affected Small-Scale Fading for Indoor UWB Channel

This paper deals with the small-scale fading distribution for UWB channels in the absence and presence of human bodies in indoor line-of-sight (LOS) environments and performance analysis of UWB systems considering the small-scale fading distribution. To obtain small-scale fading statistics, the channel measurements are performed in five representative environments that have different structure and size while locating the receiver (Rx) antenna on 49 (7×7 grid) local points with a fixed transmitter (Tx) antenna in each environment. The measured channel data are processed by a vector network analyzer and the target frequency bands range from 3 to 4.6GHz. From the measured data, we find the best fitted channel model among several typical theoretical distribution models such as Lognormal, Nakagami, and Weibull distributions, showing good agreement with the empirical channel data. We analyze the amplitude variation of the small-scale fading distribution in the absence and presence of human bodies. The results show that the small-scale fading statistics are best described by Weibull distribution and the two parameters of the distribution that determine the shape and the scale of the distribution depend on whether or not human bodies exist. We modeled and analyzed two parameters at different excess delays for all environments. Based on the measured small-scale fading distribution, this paper deals with the performance of UWB system using Rake receivers and also compares the performance with the existing channel model. The results suggest that the small-scale fading distribution in the absence and the presence of human bodies in indoor LOS environments should be considered when assessing the performance of UWB systems.