Fading Correlation Research Articles

Bi-Directional Use of Spatial Resources and Effects of Spatial Correlation

In multiple antenna systems, spatial fading correlation causes a significant degradation in performance. In this work, a method for reducing the sensitivity to spatial fading correlation through the bi-directional use of spatial resources is investigated. Firstly, the concept of bi-directional use of spatial resources is introduced and the fading characteristics of the proposed bi-directional multiple-input multiple-output (B-MIMO) system are investigated. The achievable rates of the B-MIMO system are then investigated and compared with those of conventional MIMO systems for beamforming and full rank transmission. Based on those results, conditions needed for the B-MIMO system to demonstrate a larger average achievable rate are evaluated. B-MIMO systems are found to efficiently reduce the sensitivity to spatial fading correlation, while supporting comparable achievable rates in spatially white environments.

Performance Analysis in the Downlink of Asynchronous OFDM/FBMC Based Multi-Cellular Networks

This paper provides a theoretical performance evaluation of the downlink of asynchronous orthogonal frequency division multiplexing (OFDM) and filter bank based multicarrier (FBMC) cellular radio communication systems. An accurate derivation, for the interference caused by the timing synchronization errors in the neighboring cells, is developed. The multipath effects on the interfering and desired signal are also considered. Based on computing the moment generating functions of the interference power, exact expressions are derived for average error rates of OFDM and FBMC systems considering the frequency correlation fading in the case of block subcarrier assignment scheme.

Experiments on MIMO-OFDM system combined with adaptive beamforming based on IEEE 802.16e WMAN standard

This paper presents field experiments on a Multi-Input Multi-Output (MIMO) system that combines Adaptive Beamforming (ABF) and Spatial Multiplexing (SM) procedures. The combination of SM signal processing with ABF is applied to WiBro, the South Korean Orthogonal Frequency Division Multiplexing (OFDM) system that follows the IEEE 802.16e standard. The field experimental results show that ABF-MIMO OFDM system outperforms a simple MIMO OFDM system by 2 dB (1.5 dB) in the signal to noise ratio (SNR) for 16-QAM (64-QAM) under low correlated fading channel and 4 dB (2.5 dB) in the SNR for 16-QAM (64-QAM) under highly correlated fading channel, respectively, at the frame error rate (FER) of 1%. Details on the implementation of ABF-MIMO OFDM system is also presented in this paper. Through the system implementation and its field experimental results, we verify that the combination of MIMO OFDM system with ABF provides improved performance over a simple MIMO OFDM system in real propagation channel environment and, in particular, it is more effective in highly correlated fading channel.

Spatial Fading Correlation for Local Scattering: A Condition of Angular Distribution

Local scattering in the vicinity of the receiver or the transmitter leads to the formation of a large number of multipath components along different spatial angles. A condition of angular distribution, which is valid for only a uniform linear array, is proposed in this paper to justify whether the spatial fading correlation (SFC) remains simple as a Bessel function. If an angular distribution satisfies the condition, a class of angular distributions is revealed and results in simplifying the analysis of the SFC. To demonstrate its practical use, we apply the condition to several angular distributions that are considered in previous works. It is found that cosine and von Mises distributions follow the condition, whereas uniform, Gaussian, and Laplacian distributions do not satisfy the condition, and then, one needs to calculate the sinusoidal coefficients in the SFC computation.

Performance analysis of signal-to-interference-plus-noise ratio-based selection diversity over correlated Rayleigh fading channels

In wireless communications systems, desired signal can be exposed to fading, cochannel interference (CCI) and noise. Diversity combining, such as space diversity, is a well-known method that can be used to alleviate the effects of these degradations. L-branch selection combining (SC) receiver which uses signal-to-interference-plus-noise ratio (SINR) power algorithm over correlated Rayleigh fading channels in the presence of Rayleigh distributed CCI and Gaussian noise is considered. The outage probability as significant performance criteria is analytically derived. The main contribution of this paper is that the analysis is carried out including the noise as a stochastic process, not only its average power value. The proposed mathematical analysis is complemented by various evaluation results showing the effects of the fading correlation as well as the number of diversity branches on the system's performance. In addition, SC performance in interference-plus-noise environment is compared with performance in noise- and interference-limited environments.

The Error Performance and Fairness of CUWB Correlated Channels

Abstract The symbol period becomes smaller compared to the channel delay in multiband orthogonal frequency division multiplexing (MB-OFDM) cognitive ultra wideband (CUWB) wireless communications, the transmitted signals experiences frequency-selective fading and leads to performance degradation. In this paper, a new design method for space-time trellis codes in MB-OFDM systems with correlated Rayleigh fading channels is introduced. This method converts the single output code symbol into several STTC code symbols, which are to be transmitted simultaneously from multiple transmitter-antennas. By using Viterbi optimal soft decision decoding algorithm, we investigate both quasi-static and interleaved channels and demonstrate how the spatial fading correlation affects the performance of space–time codes over these two different MB-OFDM wireless channel models. Simulation results show that the performance of space–time code is to be robust to spatial correlation. When the system bandwidth increases, the long term fairness quality will gradually become better and finally converges to 1.

Performance Analysis of OFDM Repeater Networks with Spatial Fading Correlation

This paper analyzes the performance of the OFDM diversity systems with the spatial fading correlation when the network includes an RF repeater. In case of OFDM transmission, since the combining scheme is usually employed in the frequency domain, the correlation between different subchannels should be considered. In a multipath channel, correlation varies over subchannels depending on the channel delay profile and the spatial correlation between each individual delayed paths. In this study, a multipath channel is constructed by the repeated signal instead of natural multipath due to cluttered environments and the system performance with spatial correlation per subcarrier is evaluated. Previously, it is reported that the effect of correlation can be reduced by the Time Shifted Sampling (TSS) technique which relies on the multipath diversity. Numerical results indicate that the TSS technique is also effective in reducing the effect of correlation for the multipath channels constructed by the repeaters.

Iterative Near Maximum-Likelihood Sequence Detection for MIMO Optical Wireless Systems

A major performance-limiting factor in terrestrial optical wireless (OW) systems is turbulence-induced fading. Exploiting the additional degrees of freedom in the spatial dimension, multiple laser transmitters combined with multiple receive apertures provide an effective solution for fading mitigation. Although multiple-input multiple-output (MIMO) OW systems have been extensively studied in recent years, most of these works are mainly limited to symbol-by-symbol decoding. MLSD exploits the temporal correlation of turbulence-induced fading and promises further performance gains. In this paper, we investigate MLSD for intensity-modulation/direct-detection MIMO OW systems over log-normal atmospheric turbulence channels. Even with a low-order modulation scheme such as OOK, which is typically used in OW systems, the complexity of MLSD might be prohibitive. We therefore present an iterative sequence detector based on the expectation-maximization (EM) algorithm. The complexity of the proposed algorithm is considerably less than a direct evaluation of the log-likelihood function, and it is independent of the channel's fading statistics. The Monte Carlo simulation results demonstrate that the EM-based algorithm outperforms the symbol-by-symbol decoder and achieves a performance, which lies within 0.3 dB of that of the optimal MLSD.

A low-complexity port selection and power allocation scheme in distribution MIMO systems

Information theoretical results have shown that Distributed Antenna Systems (DAS) can obtain higher capacity than Co-located Antenna Systems (CAS). In this paper, we investigate a downlink port selection and power allocation scheme in Distributed Multiple-Input Multiple-Output (D-MIMO) systems, where Distributed Antenna (DA) ports randomly locate in the cell. The contribution of this paper can be summarized as two parts. Firstly, we analyze how antenna correlation affects power allocation in D-MIMO systems. Secondly, based on large scale fading and antenna correlation, a low-complexity port selection and power allocation scheme is proposed. In the proposed scheme, we take both large scale fading and antenna correlation into consideration. Moreover, User Equipment (UE) only needs to feedback the rank of transmit antenna correlation matrix, which will not increase system complexity too much. Simulation results verify the capacity improvement based on the proposed power allocation scheme.

A Study of the Extreme Effects of Fading Correlation and the Impact of Imperfect MMSE Channel Estimation on the Performance of SIMO Zero-Forcing Receivers and on the Capacity-Maximizing Strategy in SIMO Links

In this paper, we shall consider best- and worst-case fading correlation scenarios, i.e., uncorrelated and fully correlated fading channels and will study the performance of a single-input-multiple-output (SIMO) link that uses a practical imperfect online training-based minimum mean square error (MMSE) channel estimator and a zero-forcing (ZF) receiver under these two conditions. Considering the extremes of correlation and focusing on frequency-nonselective Rayleigh block fading, we will find the probability distribution function (pdf) of the postprocessing signal-to-noise ratio (SNR) in each case and shall provide closed-form expressions for the symbol error probability (SEP) of these systems when using M-ary pulse amplitude modulation (M-ary PAM). Moreover, a lower bound will be provided for the information capacity of fully correlated SIMO links with training-based MMSE channel estimation, which will further be optimized to yield its maximum under different conditions. Monte Carlo simulations perfectly verify the analysis.

Leakage-based precoding with power allocation for multicellular multiuser MIMO downlink

To enhance the sum rate of maximising the signal-to-leakage-and-noise ratio (Max-SLNR), a signal-to-interference-and-noise ratio (SINR) based power allocation (PA) is proposed for Max-SLNR per substream of users in multiuser MIMO systems. Simulation shows that the rate gain of SINR based PA over equal PA is about 0.5bit/s/Hz per user at low and medium signal-to-noise ratios in the presence of shadow fading and antenna correlation.

DECODE: Exploiting Shadow Fading to DEtect COMoving Wireless DEvices

We present the DECODE technique to determine whether a set of transmitters are comoving, i.e., moving together in close proximity. Comovement information can find use in applications ranging from inventory tracking to social network sensing and to optimizing mobile device localization. The positioning errors from indoor RSS-based localization systems tend to be too large, making it difficult to detect whether two devices are moving together based on the interdevice distances. DECODE achieves accurate comovement detection by exploiting the correlations in positioning errors over time. DECODE can not only be implemented in the position space but also in the signal space where a correlation in shadow fading due to objects blocking the path between the transmitter and receiver exists. This technique requires no change in or cooperation from the tracked devices other than sporadic transmission of packets. Using experiments from an office environment, we show that DECODE can achieve near-perfect comovement detection at walking speed mobility using correlation coefficients computed over approximately 60-second time intervals. We further show that DECODE is generic and could accomplish detection for mixed mobile transmitters of different technologies (IEEE 802.11b/g and IEEE 802.15.4), and our results are not very sensitive to the frequency at which transmitters communicate.

On the pilot spacing constraints for continuous time-varying fading channels

In common wireless systems pilot placing can be interpreted as a way to sample the channel with some degree of accuracy. In this letter we investigate the necessary conditions on the pilots spacing for time-varying fading to guarantee a specified average bit error rate. These constraints are evaluated in closed form for large SNR and small fading dynamics and specialized for varying fading correlation and coded/uncoded binary transmissions.

On Noncoherent Receivers for DSTM in Spatially Correlated Fading

In this letter, we derive a multiple-symbol differential detection (MSDD) and a novel MSDD-based decision-feedback differential detection (MS-DFDD) receiver for differential space-time modulation transmitted over spatially correlated multiple-input multiple-output fading channels. We show that MS-DFDD outperforms previously proposed DFDD schemes that are based on scalar and vector prediction (SP-DFDD and VP-DFDD). In addition, we prove that at high signal-to-noise ratio (SNR) VP-DFDD is equivalent to SP-DFDD and thus fails to properly exploit the spatial fading correlations.

Performance and Complexity Analysis of Eigencombining, Statistical Beamforming, and Maximal-Ratio Combining

For receive-side maximal-ratio combining (MRC) and maximum-average-SNR beamforming (BF), the wireless-channel fading correlation impacts the symbol-detection performance-decreasing correlation improves/degrades MRC/BF performance-whereas the numerical complexity of these methods is fixed-high/low for MRC/BF. Matching signal processing complexity to the actual correlation conditions and, thus, to the achievable performance is possible with a superset of MRC and BF known as maximal-ratio eigencombining (MREC). For imperfectly known and correlated fading gains, new closed-form expressions are derived for the probability density function of the MREC-output SNR, as well as for the outage probability (OP) and the average error probability. These new expressions permit seamless evaluation for any correlation value of MREC, MRC, and BF performance measures, such as the amount of fading, the deep-fade probability, diversity and array gains, and the OP. Our results confirm that, in realistic scenarios, adaptive MREC can achieve MRC-like performance for BF-like complexity.

A statistical model for indoor office wireless sensor channels

Sensor networks and ad-hoc networks, where nodes inter-communicate without fixed infrastructure, have recently attracted interest due to potential use in industrial, environmental, and safety-related applications. The fading statistics of the propagation channels between sensor nodes are essential to determine the possible data rate, outage, and latency of sensor networks. This paper presents (to the best of our knowledge) the first in-depth analysis, based on measurements, of the propagation channels between typical sensor node locations in office environments. We find that the amplitude fading distribution can be characterized as Ricean. The Rice factor is analyzed as a function of distance and it is determined that it is not a monotonically decreasing function. Even in pure line-of-sight situations, Rice factors show a random behavior and are on the order of 10 or less. We propose models for the small- and large-scale fading correlation. A simulation model based on our analysis is also provided. Our results have relevance for the analysis of bit error rates and diversity order in clustered sensor networks.

Gallager's exponent for MIMO channels: a reliability-rate tradeoff

In this paper, we derive Gallager's random coding error exponent for multiple-input multiple-output (MIMO) Rayleigh block-fading channels, assuming no channel-state information (CSI) at the transmitter and perfect CSI at the receiver. This measure gives insight into a fundamental tradeoff between the communication reliability and information rate of MIMO channels, enabling to determine the required codeword length to achieve a prescribed error probability at a given rate below the channel capacity. We quantify the effects of the number of antennas, channel coherence time, and spatial fading correlation on the MIMO exponent. In addition, the general formulae for the ergodic capacity and the cutoff rate in the presence of spatial correlation are deduced from the exponent expressions. These formulae are applicable to arbitrary structures of transmit and receive correlation, encompassing all the previously known results as special cases of our expressions.

Spatial Fading Correlation model using mixtures of Von Mises Fisher distributions

In this paper new expressions for the Spatial Fading Correlation (SFC) functions of Antenna Arrays (AA) in a 3-dimensional (3D) multipath channel are derived. In particular the Uniform Circular Array (UCA) antenna topology is considered. The derivation of the novel SFC function uses a Probability Density Function (PDF) originating from the field of directional statistics, the Von Mises Fisher (VMF) PDF. In particular the novel SFC function is based on the concept of mixture modeling and hence uses a mixture of VMF distributions. Since the SFC function is dependent on the Angle of Arrival (AoA) as well as the power of each cluster, the more appropriate power azimuth colatitude spectrum term has been used. The choice of distribution is validated with the use of Multiple Input Multiple Output (MIMO) experimental data that was obtained in an outdoor drive test campaign in Germany. A mixture can be composed of any number of clusters and this is mainly dependent on the clutter type encountered in the propagation environment. The parameters of the individual clusters within the mixture are derived and an estimation of those parameters is achieved using the soft-Expectation Maximization (EM) algorithm. The results indicate that the proposed model fits well with the MIMO data.

Geomorphological correlation of the tectonically displaced Tejo River terraces (Gavião–Chamusca area, central Portugal) supported by luminescence dating

A suite of fluvial terraces (T1–T6, from top to bottom of the staircase) occur along a ∼70 km stretch of the Tejo River in central Portugal, between the small towns of Gavião and Chamusca. Terrace correlation was based upon the following: (a) aerial photograph analysis, geomorphological mapping and field topographic survey; (b) sedimentology of the terrace deposits, namely the maximum particle size, clast composition and lithofacies identification; and (c) K-feldspar infrared stimulated luminescence (IRSL) dating of the three lower terraces levels, as quartz optically stimulated (OSL) signal of the samples from these terraces was too close to saturation for all but two samples. The two upper terraces (T1 and T2) lack suitable materials for luminescence dating (e.g. sands/silts), but also their probable ages are beyond the upper range of the dating method. Faults affecting terraces and older deposits have been reported. The luminescence dating results suggest that some assignments of local terrace remnants were incorrect because of fault-related vertical displacements. The luminescence dating procedure also included a correction for anomalous fading in order to obtain more reliable estimates of the burial ages. The fading rate was identical for all samples, so for correlation of the terraces anomalous fading of the feldspar IRSL signal is considered not to be a significant problem. The T5 terrace has corrected ages of ∼42–99 ka, the T4 from ∼107 to 222 ka and the T3 terrace has a minimal age of ∼300 ka. Fluvial incision appears to have been principally controlled by regional uplift but also by localised movements along fault structures. Using the corrected ages of the T4 surface a time-averaged incision rate can be quantified as follows: (1) ∼0.29 m/ka for reach III, (2) ∼0.53 m/ka in the Chamusca area (east side on reach IV) and (3) ∼0.13 m/ka in the Mato Miranda area (west side on reach IV, the less uplifted block according to the geomorphic framework).

Code Acquisition Performance in Correlated MIMO Channel

In this paper, the impacts of using multiple transmit antennas under doubly correlated MIMO channels on CDMA uplink code acquisition is studied. The performance of a MIMO code acquisition system is analyzed by considering spatial fading correlations, which depend on antenna spacing and azimuth spread at both MS and BS. The detection performance and mean acquisition time in the presence of spatially correlated MIMO channel are presented on a frequency selective fading channel and compared with the cases of spatial fading decorrelation via numerical evaluation. It is observed that the acquisition performance relies on the degree of spatial fading correlations. In addition, it is surprisingly seen that a MIMO code acquisition system provides worse performance than SIMO.