Mobile Fading Channels Research Articles

Local spline approximation of time-varying channelmodel

Methods of local spline approximation of random functions described by Clarke's model are investigated. Such a model is known to describe time-varying amplitudes of multipath components in mobile fading channels. Splines of zero, first and second orders with different spline coefficients are compared. The comparison shows that local linear and parabolic splines with quasi-optimal spline coefficients provide a low approximation error. This makes them attractive for estimating and modelling fast fading channels.

Serial acquisition of DS-CDMA signals in multipath fading mobile channels

This paper compares the mean acquisition time (MAT) performance of two serial search acquisition schemes over frequency-selective multipath Rayleigh fading channels. Both the conventional cell-by-cell detection and the novel joint two-cell detection are characterized. In contrast to the majority of published results considering only one correct timing state (H/sub 1/ cell) in the uncertainty region of the pseudonoise (PN) sequence, this paper analyzes the MAT performance of the serial search acquisition schemes with multiple timing states in the uncertainty region. The MAT performance comparison between the conventional cell-by-cell detection and the joint two-cell detection shows that the latter can achieve an improved acquisition performance at almost the same complexity.

An Improved Switched Diversity Combining Using Fuzzy Adaptive Control

Diversity combining techniques, due to their extreme effectiveness incounteracting fading effects in wireless environment, have been extensivelystudied in recent years. In this paper, we propose an improved switchedcombining scheme using fuzzy adaptive control, namely, fuzzy adaptive switcheddiversity (FASD), which is applicable for implementation at mobile units.Specifically, the fuzzy adaptive control dynamically adjusts the thresholdlevel, at which switching is performed, based on the present varying channelconditions. By incorporating the statistics of mobile fading channels andextensive simulations, we have achieved an effective system design of FASDwhich has a significant improvement in system performance in terms of both thediversity gain and BER (bit error rate), while maintaining reasonablesimplicity in implementation.

Design of high-speed simulation models for mobile fading channels by using table look-up techniques

This paper describes a procedure for the design of fast simulation models for Rayleigh fading channels. The presented method is based on an efficient implementation of Rice's sum of sinusoids by using table look-up techniques. The proposed channel simulator is composed of a few numbers of adders, storage elements, and simple modulo operators, whereas time-consuming operations like multiplications and trigonometric operations are not required. Such a multiplier-free simulation model is introduced as a high-speed channel simulator. It is shown that the high-speed channel simulator can be interpreted as a finite state machine which generates deterministic output envelope sequences with approximately Rayleigh distribution. The statistical properties of the designed high-speed channel simulator are investigated analytically and compared with the statistics of the underlying Rayleigh reference model. Results of experiments for measuring the speed of the presented and other types of channel simulators are also presented.

MPEG-4 image transmission using MAP source-controlled channel decoding

In this paper, we develop an error resilient variant of the MPEG-4 embedded zerotree wavelet coder, suitable for mobile fading channels. The residual redundancy in the compressed bit stream provides implicit error protection through the use of source-controlled channel decoding; no explicit channel coding is used. We propose two slight modifications to the MPEG-4 coder. The first removes the arithmetic coding in the lowest frequency subband so that hidden Markov model-based MAP estimation of both the source index and the channel state can be easily applied. The change in bit rate is negligible, while performance during severe channel fading conditions can be greatly increased. The second modification adds variable length packetization to the compressed bit stream created from the higher frequency subbands. The individual packets are independently decodable. An optional rearrangement of the bits in each packet allows MAP estimation to be applied to these subbands as well. Simulation results show a significant performance improvement for the overall system.

Robust video transmission over correlated mobile fading channels

We develop a unique set of techniques to support reliable and efficient video transmission over bandwidth-limited mobile networks. The video coder is based on the H.263 coding standard, while its transmission system design is based on a joint study of combined source and channel coding, diversity reception, and pre/postprocessing techniques. In particular, a finite-state Markov model for representing a correlated fading channel with diversity is developed. Based on this model, a recursive algorithm is further devised for an efficient estimation of block-code performance. By doing so, an important means for finding proper diversity and channel coding schemes for correlated video data in correlated fading channels is obtained. In addition, robust decoding together with a combined spatial-temporal concealment is employed to mitigate the effects of decoding mismatch caused by residual errors. Simulation results confirm that the proposed system can significantly reduce the bursty error effects on the H.263 coded video data while maintaining high transmission efficiency.

Adaptive soft‐decision algorithms for mobile fading channels

AbstractThis paper addresses the task of joint symbol‐by‐symbol decisions and channel estimation in a coded/interleaved/frequency‐selective fading channel. Based on two popular soft‐decision algorithms, namely OSA and SSA, which have an inherent trellis structure, several adaptive derivatives are proposed. In particular, by extending the concept of Conventional Adaptive MLSE (CA‐MLSE) receiver to the soft‐decision case, the CA‐OSA and CA‐SSA algorithms are derived. Moreover, using the Per‐Survivor Processing (PSP) concept, two new adaptive extensions are presented, namely PSP‐OSA and PSP‐SSA. The latter of these algorithms (PSP‐SSA) has a simplified form with complexity comparable to PSP‐MLSE. Simulation results show a significant improvement of the suggested adaptive soft‐decision algorithms over the standard configuration, which consists of a hard‐decision channel equalizer followed by a convolutional decoder.

On the statistical properties of deterministic simulation models for mobile fading channels

Rice's (9144, 1945) sum of sinusoids can be used for an efficient approximation of colored Gaussian noise processes and is therefore of great importance to the software and hardware realization of mobile fading channel models. Although several methods can be found in the literature for the computation of the parameters characterizing a proper set of sinusoids, less is reported about the statistical properties of the resulting (deterministic) simulation model. In this paper, not only is the simulation model's amplitude and phase probability density function (PDF) investigated, but also higher order statistics [e.g., level-crossing rate (LCR) and average duration of fades (ADFs)]. It is shown that due to the deterministic nature of the simulation model, analytical expressions for the PDF of the amplitude and phase, autocorrelation function (ACF), crosscorrelation function (CCF), LCR, and ADFs can be derived. We also propose a new procedure for the determination of an optimal set of sinusoids, i.e., the method results for a given number of sinusoids in an optimal approximation of Gaussian, Rayleigh, and Rice processes with given Doppler power spectral density (PSD) properties. It is shown that the new method can easily be applied to the approximation of various other kinds of distribution functions, such as the Nakagami (1960) and Weibull distributions. Moreover, a quasi-optimal parameter computation method is presented.

Performance analysis of modified/quadrature partial response‐trellis coded modulation (M/QPR‐TCM) systems

AbstractIn this paper, to improve bandwidth efficiency and error performance partial response signalling and trellis coded modulation is considered together of M‐QAM systems. The new system proposal is denoted as Modified/Quadrature Partial Response‐Trellis Coded Modulation (M/QPR‐TCM). Two new non‐catastrophic schemes M/6QPR‐TCM and M/9QPR‐TCM are investigated for 4QAM. Error event lower bounds of the considered schemes are analyzed in AWGN and colored noise. Analytical upper bounds for the mobile fading channel with ideal channel state information (CSI) and infinite interleaving/deinterleaving are derived. The interesting result is that, the bit error probability upper bounds of the proposed M/QPR‐TCM systems are better than the related QAM‐TCM schemes for SNR values greater than a threshold.

Detection technique for a serial DS-SS code synchronisationin a fading mobile channel

A performance analysis of the threshold decision technique for direct-sequence spread-spectrum (DS-SS) code synchronisation is presented. The proposed serial acquisition system is compared to the conventional serial acquisition system, and a significant improvement in performance is shown in terms of the mean acquisition time.

Performance of MHPM in Rician and Rayleigh fading mobile channels

This paper evaluates the error probability of the rectangular frequency pulse multi-h modulation (MHPM) scheme in slowly-fading, frequency-nonselective or frequency-selective Rician and Rayleigh channels. The evaluation is performed with a method combining analysis and computer simulation. Performance degradations are evaluated for various direct-to-reflected signal ratio, Doppler shifts, and relative time delays in Rician fading channels. Compared with minimum shift keying (MSK), MHPM schemes appear to have retained their coding gains.

Threshold decision technique for direct sequence code synchronisation in a fading mobile channel

The work presents the performance of a threshold decision technique for direct-sequence spread-spectrum code synchronisation. For received PN codes with different SNR, this scheme is able to detect a desired threshold value for verification in the search mode such that the detection probability of the verification mode is maximised and the false alarm probability of the verification mode is minimised. The proposed serial acquisition system is compared to the conventional serial acquisition system, and a significant improvement of performance is shown. Analysis of system performance in both the search and verification modes is presented, and key quantities such as the false alarm probability, the detection probability, and the mean acquisition time are derived in both nonfading and Rayleigh fading environments.

Cellular CDMA overlay situations

An adaptive LMS filter is applied to a cellular CDMA overlay situation in order to reject narrowband interference in a mobile fading channel, ignoring a channel white noise. Both the convergence and steady-state results of the LMS algorithm for the CDMA receiver are presented.

Adaptive fuzzy power control for CDMA mobile radio systems

Introduces a new application of fuzzy-logic control (FLC) to power control in a DS/CDMA cellular system over mobile fading radio channels. The conventional feedback power control algorithms allow the base station to send a power command to raise/lower each user transmitting signal power level by a fixed power step and then keep the received powers almost equal. The fixed-step approach is actually an integral control whose power increment is determined according to the bang-bang-like control policy. However, this control scheme suffers from several disadvantages. To tackle these difficulties, a fuzzy proportional-plus-integral (PI) control, whose input variables are the received power error and error change, is introduced to determine each user's transmitting power in order to maintain simultaneously all users' signal power received at the base station nearly equal and to achieve better system stability and control performance. The derivation of the fuzzy PI control has been carried out by analyzing both the closed-loop steady state behavior and transient response of the system with a priori knowledge of the dynamics of the CDMA mobile fading channels. In fuzzy control, linguistic descriptions of actions in controlling a process are represented as fuzzy rules. This fuzzy-rule base is used by an inference mechanism in conjunction with some knowledge of the states of process in order to determine control actions. These actions would lead to the fast rise time, minimum overshoot, and small root-mean-squared (rms) tracking error. Additional advantages of fuzzy PI control over conventional control theories are discussed.

Decision feedback detection of minimum shift keying

A new decision feedback (DF) receiver for minimum shift keying (MSK) is derived, which uses the previously detected symbols to estimate the initial phase in the current symbol for optimum coherent detection. Some of theoretical and simulated bit error rate (BER) performance results are presented. The results show that the DF detection provides good bit error performance in the AWGN channel, and is also an efficient detection method for MSK in a fast Rayleigh fading mobile channel.

Adaptive fuzzy proportional integral power control for a cellular CDMA system with time delay

We investigate the use of fuzzy logic control techniques for adaptive power control in a direct-sequence code-division multiple-access (DS/CDMA) cellular system over the mobile fading radio channels with propagation time delay. A fuzzy PI (proportional-plus-integral) control whose input variables are the received power error and error change is introduced to determine each user transmitting power in order to simultaneously equalize all users' signal powers received at the base station and achieve better system stability and control performance. This control strategy can ensure long loop transmission delays without causing the system to become unstable. According to the well-known phase-plane method, the derivation of the fuzzy PI control has been carried out by analyzing the response areas, cross-over, and extreme points of the system step response with apriori knowledge of the dynamics of the CDMA mobile fading channels. For a long time-delay fading process, a methodology is developed to modify the rule base to contain the delay information for reducing the deadtime effects of the process. Moreover, the additional advantages of fuzzy PI control over conventional control theories are: increased robustness in spite of interference and the ability to handle the time-delay process whose parameters are not accurately known. Simulation results show that a good performance can be achieved both in RMS tracking error and traffic capacity by use of fuzzy PI power control, especially in poor interuser interference and long time delay conditions.

Performance of coherent and differential detection for trellis‐coded 8‐PSK over satellite flat rician fading channels

AbstractTrellis‐coded 8‐PSK (TC‐8‐PSK) modulation is a power and bandwidth efficient modulation scheme which is finding increasing applications in terrestrial and satellite systems. In particular, TC‐8‐PSK is demonstrating attractive performances over die satellite mobile fading channel where it provides considerable coding gain with respect to uncoded QPSK with no bandwidth efficiency loss. In this paper die capabilities of a decision‐directed maximum‐likelihood coherent phase tracker are investigated for flat Rician fading channel. Open loop and steady‐state performances of die digital phase tracker are analytically determined and verified by means of computer simulations. By optimizing phase loop parameters in presence of Rician fading with respect to the mean time to lose lock, it is shown that, for typical satellite fading channels, die coherent decision‐directed phase tracker performs poorly. This is also true for feed forward decision‐feedback phase estimator. In these cases, the additional power loss required by differential detection of TC‐8‐PSK respect to coherent detection has been evaluated by means of computer simulations.

Differentially coherent detection technique for direct-sequence code acquisition in a Rayleigh fading mobile channel

A novel differentially coherent (DC) detection scheme for rapid code acquisition is proposed for a direct sequence (DS) spread-spectrum system being used to communicate over a fast Rayleigh fading mobile radio channel. The scheme processes the received signal at baseband using a complex differential detector with one-chip time delay and then performs a coherent partial correlation of the detector output with the product of a local DS code and its one-chip delayed phase to form a test sample. While still preserving the pseudo-noise attribute at its output, the differential detector is able to effectively suppress slowly fluctuating phase components due to fading and carrier frequency offset, thereby enhancing the in-sync correlation at the following correlator. The performance of this detection scheme is analysed based on a central-limit-theorem argument, and verified by simulation. Both serial and parallel acquisition systems employing the proposed DC detector are compared with the conventional parallel I-Q system by mean acquisition time performance. It is shown that the serial DC acquisition system significantly outperforms the parallel I-Q system in very fast Rayleigh fading environments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Construction of optimal frequency hopping sequences for minimising bit errors in selective fading channels characteristic to digital cellular systems

The author discusses the problem of constructing frequency hopping patterns for use in TDMA-FDMA cellular systems that employ frequency diversity to minimise bursty errors occurring due to the fading channel. Conditions under which two frequency and time channels fade independently in mobile fading channels are reviewed. The construction of a family of effective frequency hop sequences for achieving maximum diversity is given. The construction is based on the theory of linear congruences, and the number of sequences for every integer is established. It is shown that when sequences are constructed using prime numbers they are also orthogonal. Examples of constructed sequences applicable to the GSM cellular system are given.

Performance off 8-PSK trellis codes over nonlinear fading mobile satellite channels

In this paper a new analytical bound for large effective code length codes (ECL) is derived. The accuracy of the analytical bounds is validated by simulation. Based on the new bound a trellis code design procedure for fading channels is defined. A quasi-analytical technique for BER estimation on nonlinear channels is proposed. This method is used to estimate the performance degradation of nonlinear communication systems with the pilot tone.