Uncorrelated Channels Research Articles

Comparison on plasticity of current-driven vortex lattices in as-grown and heavy ion irradiated microbridge Bi2Sr2Ca1Cu2O8+δ single crystals

From the I–V characteristics for as-grown and irradiated Bi2Sr2CaCu2O8+δ single crystals at T=5K in a magnetic field applied parallel to the c axis, we have seen two types of vortex dynamics near the depinning threshold. For the as-grown sample, at low field, the I–V curves show steps that clearly indicate a “fingerprint phenomenon” since they reflect the current dependence of differential resistance Rd=dV/dI. This can be ascribed to vortices flow through uncorrelated channels for the highly defective lattice. As fields sufficiently increase, these peaks merge, giving broader ones, indicating a crossover from filamentary strings to braid river. In contrast, in the irradiated sample, the pinning is found to be individual at low magnetic fields and collective when the vortex–vortex interactions are involved. Our result suggests a dynamic nature of the peak effect, in agreement with recent numerical simulations and experimental works.

Capacity Limits of Multi-Antenna Multicasting Under Correlated Fading Channels

Physical layer multicasting in wireless networks has been proposed to efficiently send an identical message to multiple users simultaneously. In this paper, we consider multiple antenna multicasting where the transmitter is equipped with Mt antennas and data is transmitted to K single-antenna users. When the downlink channels are assumed to follow an uncorrelated Rayleigh distribution, the multicasting capacity scaling for the large K asymptote is known. On the other hand, the effect of channel spatial correlation on the capacity performance has not been well addressed. Therefore, we investigate the effect of correlation using the channel correlation information at the transmitter. Using extreme value theory, it is shown that signaling using uniformly allocated transmit powers on the spatial channel correlation matrix's eigenvectors with non-zero eigenvalues approaches the multicasting capacity for the large K asymptote. Compared to the performance of uncorrelated fading channels with a constraint on the trace of the spatial correlation matrix, it is shown that the channel correlation degrades the performance. Specifically, if the correlation matrix is full rank the asymptotic outage (average) capacity ratio, which is defined as the ratio of the outage (average) capacities of the correlated fading channel and the uncorrelated fading channel as K goes to infinity, is equivalent to the geometric mean of the eigenvalues of the transmit channel correlation matrix of the correlated channel. On the other hand, if the correlation matrix is not full rank, the capacity ratios become zero. To assess the value of knowledge of the channel correlation information at the transmitter, we compare the asymptotic performances with and without knowledge of the channel correlation information. Specifically, the asymptotic performance improvement due to this correlation knowledge becomes (Mt/M)1/M for the large K asymptote, where M is the rank of the transmit channel correlation matrix. In addition, we discuss the issues of nonidentical transmit channel correlations among the users and correlation between users' channels.

STBCs with orthogonal decoder for highly time-selective channels

We present bit-error-rate (BER) analyses of Orthogonal Space-Time Block Codes (O-STBCs) and their modified version (MO-STBCs) combined with the Orthogonal Decoder (OD) over uncorrelated and time-selective fading channels of generalized statistics. First, we quantify the effective diversity achieved by STBCs (O-STBCs and MO-STBCs) by resorting to a statistical characterization of the diversity gain of both schemes with OD. We, then, expand the theoretical BER expressions of O-STBCs, derived for the block fading channels, to MO-STBCs with OD in a more practical time-selective fading channel with Gray-code-mapped M-ary phase-shift-keying (PSK) modulation and M-ary quadratic-amplitude modulation (QAM). Moreover, through our BER expressions, we pinpoint the relationship between the effective diversity and BER performance of STBCs with OD, which elucidates the reason why MO-STBCs with OD architecture is very robust to the highly time-selective fading expected in 4th Generation Wireless Networks (4G). Finally, the theoretical results in conformity with the simulated ones demonstrates the accuracy of our analysis.

On Optimum End-to-End Distortion in MIMO Systems

This paper presents the joint impact of the numbers of antennas, source-to-channel bandwidth ratio, and spatial correlation on the optimum expected end-to-end distortion in an outage-free MIMO system. In particular, based on an analytical expression valid for any SNR, a closed-form expression of the optimum asymptotic expected end-to-end distortion valid for high SNR is derived. It is comprised of the optimum distortion exponent and the multiplicative optimum distortion factor. Demonstrated by the simulation results, the analysis on the joint impact of the optimum distortion exponent and the optimum distortion factor explains the behavior of the optimum expected end-to-end distortion varying with the numbers of antennas, source-to-channel bandwidth ratio, and spatial correlation. It is also proved that as the correlation tends to zero, the optimum asymptotic expected end-to-end distortion in the setting of correlated channel approaches that in the setting of uncorrelated channel. The results in this paper could be performance objectives for analog-source transmission systems. To some extent, they are instructive for system design.

Filamentary flow of vortices in aBi2Sr2Ca1Cu2O8+δsingle crystal

Using micro-bridge technique, we have studied the vortex dynamics in a very low temperature region (i.e. T/Tc -> 0) of the B-T phase diagram of Bi_2Sr_2Ca_1Cu_2O_{8+\delta} single crystal. We distinguish two types of vortex dynamics near the depinning threshold depending on the magnitude of the vortex-vortex interactions. For 0.01 <= \mu_0H <= 1T, we show that current-voltage characteristics (I-V) are strongly dependent on the history of magnetic field and current cycling. The sharp peak, so called "peak effect" (PE), observed in \mu_0H-Ic curve is due to a metastable state which can be removed after current cycling. At low field, I-V curves exhibit steps which clearly enligth a "fingerprint phenomenon" as it can be seen the current dependence of the differential resistance Rd = dV/dI. We associate this to vortices flow through uncorrelated channels for the highly defective lattice. Indeed, as field sufficiently increase, these peaks merge giving broader ones indicating a crossover from filamentaty strings to braid river like in which vortex-vortex interactions becomes significant. As confirmed by the discontinuity in the critical exponent value \beta determined in the vicinity of the threshold current using the power-law scaling V (I-Ic)^\beta with a crossover from \beta = 2.2 to \beta = 1.2. The strong vortex correlation along the c-axis has been clearly demonstrated using the dc-flux-transformer geometry for transport measurements which confirms the pseudo-2D behaviour of the FLL. Our transport studies are in good agreement with recent simulations results of 2D elastic objects driven by repulsive interactions through a random pinning potential.

Precise SER Analysis and Performance Results of OSTBC MIMO-OFDM Systems over Uncorrelated Nakagami-m Fading Channels

Although the topic of multiple-input multiple-output (MIMO) based orthogonal frequency division multiplexing (OFDM) over different fading channels is well investigated, its closed form symbol error rate (SER) expressions and performance results employing orthogonal space time block codes (OSTBCs) over uncorrelated frequency-selective Nakagami-m fading channels are still not available. The closed form expressions are extremely useful for evaluating system's performance without carrying out time consuming simulations. Similarly, the performance results are also quite beneficial for determining the system's performance in the sense that many practical wireless standards extensively employ MIMO-OFDM systems in conjunction with M-ary quadrature amplitude modulation (M-QAM) constellation. This paper thus, derives exact closed form expressions for the SER of M-ary Gray-coded one and two dimensional constellations when an OSTBC is employed and Nt transmit antennas are selected for transmission over frequency-selective Nakagami-m fading channels. For this purpose, first an exact closed-form of average SER expression of OSTBC based MIMO-OFDM system for M-ary phase shift keying (M-PSK) using traditional probability density function (PDF) approach is derived. We then compute exact closed form average SER expressions for M-ary pulse amplitude modulation (M-PAM) and M-QAM schemes by utilizing this generalized result. These expressions are valid over both frequency-flat and frequency-selective Nakagami-m fading MIMO channels and can easily be evaluated without using any numerical integration methods. We also show that average SER of MIMO-OFDM system using OSTBC in case of frequency-selective Rayleigh fading channels remains independent to the number of taps, L of that fading channel and the performance of the same system for two-tap un-correlated Rayleigh and Nakagami-m fading channels is better than that of the correlated one. Moreover, Monte Carlo simulation of MIMO-OFDM system using multiple transmit and receive antennas for different modulations is presented to validate our theoretical results. Finally, due to availability of closed form expressions, we further provide the performance results of MIMO-OFDM system over frequency-selective Nakagami-m fading channels employing (M-QAM) using OSTBCs under the transmission rate equal to 1, 2 and 3 bit(s)/s/Hz, respectively.

Phase-Shift Cyclic-Delay Diversity for MIMO OFDM Systems

Phase-shift cyclic-delay diversity (PS CDD) scheme and space-frequency-block-code (SFBC) PS CDD are developed for multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The proposed PS CDD scheme preserves the diversity advantage of traditional CDD in uncorrelated multiantenna channels, and furthermore removes frequency-selective nulling problem of the traditional CDD in correlated multiantenna channels.

Optimal detection in multiple-input multiple-output orthogonal frequency-division multiplexing systems with imperfect channel estimation

An optimal detector for orthogonal frequency-division multiplexing (OFDM) signals with pilot symbol assisted modulation in multiple-input multiple-output (MIMO) frequency selective fading channels with imperfect channel estimation is derived. This detector does not estimate the channel explicitly but maximises the likelihood function incorporating received data and pilot symbols to recover the data. The authors investigate and compare its performance with that of mismatched detectors treating maximum likelihood (ML), regularised ML or minimum mean squared error (MMSE) channel estimates as perfect channel information in spatially uncorrelated MIMO channels with binary phase shift keying (BPSK) and 16-quadrature amplitude modulation (QAM) by simulation. The authors use basis functions to model the channel frequency response and compare the performance of different basis expansion models in frequency selective fading channels with different delay spread. In single-input single-output (SISO) channels, the performance of the optimal detector is close to that of the mismatched detector with MMSE channel estimates, which provides the best performance among these mismatched detectors. However, the optimal detector significantly outperforms the mismatched detectors in MIMO channels.

Interference Coordination for E-MBMS Transmissions in LTE-Advanced

Interference coordination methods for Evolved-Multimedia Broadcast/Multicast Service (E-MBMS) in Long-Term Evolution Advanced (LTE-A) are presented. In addition, we consider signal space diversity based on Rotation Matrices (RM) known to provide good performance gains over uncorrelated Rayleigh fading channels. OFDM/OFDMA systems can make the use of RM very attractive both for single and multiple antenna transmissions. In this paper, OFDM/OFDMA signals based on LTE parameters are combined with RM, MIMO, Turbo, or LDPC codes. We have considered different types of receivers, namely, we used an MMSE (Minimum Mean Squared Error) equalizer and a Maximum Likelihood Soft Output criterion (MLSO). Frequency, signal, and space diversity gains are evaluated for different spatial channel models (SCM) based on ITU multipath propagation channels. Different adaptive frequency reuse and schedulers are considered to evaluate the E-MBMS spectral efficiency at the cell borders.

A novel orthogonal space-time-frequency block code for OFDM systems

In this letter, we propose an orthogonal spacetime- frequency block code for orthogonal frequency division multiplexing (STFBC-OFDM) systems, newly designed to be robust against both time- and frequency-selective fading. The performance of the proposed system is analyzed in spatially uncorrelated time-varying multipath Rayleigh-fading channels. The analytic results, which closely match the numerical results, show that the proposed system outperforms STBC-OFDM in time-varying channels and SFBC-OFDM in frequency-selective fading channels.

Antenna Selection with Superposition for 4 x 2 DSTTD Systems

From 1820 4 times 4 binary matrices, 253 binary preprocessing matrices are designed for double space-time transmit diversity (DSTTD) systems with two 2 times 2 space-time block code (STBC) encoders. Among them, six matrices yielding the highest average minimum-post-processing SNR are proposed from numerical experimentation under uncorrelated channel conditions. The proposed preprocessing performs a superposition of the first and second (second and first) STBC symbols and a selection of two transmit antennas. Simulation results show that the proposed method provides 1.7 dB (1.8 dB) SNR improvement in uncorrelated (correlated) channel environment at 10-3 bit error rate over the conventional antenna shuffling method.

Frequency-domain multipacket detection: a high throughput technique for SC-FDE systems

The traditional approach to cope with collisions is to discard the packets involved in it and to ask for their retransmission. However, since the signal associated to a collision has important information concerning the packets involved, we can efficiently resolve collisions with proper retransmissions. In this paper we consider severely time-dispersive channels and we propose a technique that allows efficient packet separation. We employ SC-FDE schemes (single-carrier with frequency-domain equalization) and we propose an iterative frequency-domain multi-packet detection. Since our technique requires uncorrelated channels for different retransmissions, we also propose an SP technique (Shifted Packets) for retransmissions in fixed channels. Since the total number of transmissions is equal to the number of packets involved in the collision (even when the channel remains fixed for the retransmissions), our technique allows high throughputs. The analysis of our detection technique combined with the NMDA (network-assisted diversity multiple access) MAC (medium access control) protocol shows significant throughput and delay improvements for low E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> values compared to a TDMA (time division multiple access) approach, where collisions are avoided. This technique is particularly appealing for the uplink of broadband wireless systems, since we consider SC-FDE schemes and the complexity is concentrated in the receiver. By employing the SP scheme we can use the same channel for the retransmissions, with only a small performance degradation.

Decentralized Communication-Aware Motion Planning in Mobile Networks: An Information-Gain Approach

In this paper we consider decentralized motion-planning in mobile cooperative networks in the presence of realistic stochastic communication links, including path-loss, fading and shadowing effects. We propose a communication-aware motion-planning strategy, where each node considers the information gained through both its sensing and communication when deciding on its next move. More specifically, we show how each node can predict the information gained through its communications, by online learning of link quality measures such as received Signal to Noise Ratio (SNR) and correlation characteristics, and combine it with the information gained through its sensing in order to build objective functions for motion planning. We show that in the presence of path loss, our proposed strategy can improve the performance drastically. We furthermore show that while uncorrelated low-SNR fading channels can ruin the overall performance, the natural randomization of uncorrelated channels can potentially help the nodes leave deep fade spots with small movements. We finally show that highly correlated deep fades, on the other hand, can degrade the performance drastically for a long period of time. We then propose a randomizing motion-planning strategy that can help the nodes leave highly correlated deep fades.

Transmit Antenna Selection with Optimal Power Allocation and Channel Estimation Error

In this paper, we consider the capacity of transmit antenna selection with optimal power allocation and channel estimation error over uncorrelated Rayleigh fading channels. We assume that one transmit antenna is chosen for transmission and the maximal ratio combining (MRC) is applied at the receiver. We derive the exact capacity expression. Numerical result to validate our analysis is also presented.

Adaptive spatial multiplexing for time‐varying correlated MIMO channels

AbstractThis work presents an adaptive spatial multiplexing scheme for time‐varying correlated multiple‐input multiple‐output (MIMO) channels. Unlike the conventional spatially uncorrelated block fading channel model, both temporal channel variation in each data block and spatial correlation are considered in the design. The proposed system continuously transmits packets with identical structure. Each packet consists of a training phase followed by a data transmission phase. The training signal is used to estimate instantaneous channel state at the receiver. In each symbol period of data transmission phase, the transmitter simultaneously transmits multiple data streams, which are decoded at the receiver based on the estimated channel state. The transmitter is assumed to know only the channel statistics, with which the power and rate for each data stream as well as the number of data streams are adjusted in each symbol period to achieve a target bit error rate (BER). The results reveal that the throughput‐maximising transmission strategy makes a judicious diversity‐multiplexing tradeoff by allocating power to an optimum number of data streams. Moreover, the packet length is also optimised based on the temporal channel variation rate. The simulation results demonstrate that the throughput can be significantly enhanced by the optimum packet length. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Joint receive antenna selection and symbol detection for MIMO systems: a heterogeneous genetic approach

This letter presents a joint scheme of receive antenna selection and symbol detection for multiple-input,multiple output (MIMO) systems based on the maximum likelihood (ML) criterion. To mitigate the computational complexity, a novel variant of the genetic algorithm (GA) is addressed to solve the nonlinear optimization involved, where a new heterogenous crossover and a new heterogenous mutation are conducted in the chromosome evolution process. Furnished simulations show that the new approach provides superior performance with substantially lower computational load compared with previous works in both correlated and uncorrelated channels.

Achievable rate of MIMO systems with multi-user diversity and delayed feedback

The effects are investigated of delayed feedback on the system achievable rate (AR) of a multi-user diversity multiple-input multiple-output (MIMO) system over uncorrelated Rayleigh fading channels in a general framework allowing for arbitrary numbers of transmit/receive antennas and various spatial combining schemes. Exact AR expressions are derived for different MIMO schemes, including the selective combining, maximum ratio combining and space-time block codes. Numerical results are presented to examine the validity of the theory.

EXIT-Chart Optimized Block Codes for Wireless Video Telephony

In this paper we investigate a turbo detection scheme designed for robust video transmission over an uncorrelated Rayleigh fading channel. We first introduce a novel block error-correcting code, referred to as an extrinsic information transfer (EXIT)-chart optimized block code (ECOBC), which benefits from soft decision decoding. The ECOBC exploits both the residual redundancy which manifests itself in terms of the non-uniform probability distribution of K-bit strings inherent in the video encoded bit-stream, as well as the intentional redundancy imposed by the ECOBC for mitigating the effects of transmission errors. As our first novel contribution, we address the design of ECOBCs by formulating the necessary and sufficient condition, which ensures that the EXIT function of the softbit ECOBC decoder reaches the (1,1) point in the EXIT chart. Secondly, we propose a novel real-valued free-distance metric for characterizing the ECOBC's performance. Furthermore, the attainable performance and the computational complexity imposed by the ECOBC are investigated. The effects of the interleaver delay on the ECOBC's performance are also quantified. As an additional result, we demonstrate that at a fixed system bandwidth, the turbo detection scheme using the H.264 video codec with the error concealment tools disabled but amalgamated with the proposed ECOBC as well as an additional generalized low-density parity-check (GLDPC) code may lead to an approximately 2.5 dB signal-to-noise ratio gain in comparison to the benchmarker having the same total bit rate, which operated the H.264 scheme with the error concealment tools enabled hence resulting in a higher video rate and amalgamated with the same-rate GLDPC code, but dispensing with our novel ECOBC.

Power allocation for two different traffics in layered MIMO systems

In multiple input multiple output (MIMO) relay networks, an MIMO node can relay different traffics from multiple MIMO source nodes. Thus, an MIMO relay node has to allocate the power across multiple channels to relay different traffics. In this paper, we consider the power allocation across multiple layers of MIMO channel when two different traffics, real-time and best-effort traffics, are presented. Generally, realtime traffics have stringent delay constraints and their data streams could not be multiplexed with the data streams from best-effort traffics due to different quality of service (QoS). Since MIMO channels can provide multiple parallel channels, we are able to transmit both different traffics separately (and simultaneously). For efficient power use, we investigate the power allocation across multiple parallel channels in presence of two different traffics with different performance index for each traffic. Performance analysis is carried out for uncorrelated Rayleigh-fading MIMO channels.

Space–time code selection for OFDM–MISO systems

In this paper, a space–time code selection technique for multiple transmit antenna OFDM systems is proposed, and a constant-rate transmission mode for the space–time code selection technique is introduced. Subcarriers are divided into subcarrier groups and a comparison of the equivalent SISO channel envelopes with a set of predetermined threshold levels for a central subcarrier of each group is performed at the receiver, which decides on the subcarriers to be used at each antenna and sends this information to the transmitter. The transmitter adjusts the space–time code to the number of selected antennas at each subcarriers group using four bits for feedback information per group. Simulation results show that the system performs well for a range of Doppler frequencies for correlated and uncorrelated channels, outperforming transmit antenna selection techniques and adaptive transmit antenna diversity systems for both variable-rate and constant-rate transmission modes.