Showing papers on "Fading distribution published in 1996"

Signal design for transmitter diversity wireless communication systems over Rayleigh fading channels

Abstract: Transmitter diversity wireless communication systems over Rayleigh fading channels using pilot symbol assisted modulation (PSAM) are studied. Unlike conventional transmitter diversity systems with PSAM that estimate the superimposed fading process, we are able to estimate each individual fading process corresponding to the multiple transmitters by using appropriately designed pilot symbol sequences. With such sequences, special coded modulation schemes can then be designed to access the diversity provided by the multiple transmitters without having to use an interleaver or expand the signal bandwidth. The code matrix notion is introduced for the coded modulation scheme, and its design criteria are also established. In addition to the reduction in receiver complexity, simulation results are compared to, and shown to be superior to, that of an intentional frequency offset system over a wide range of system parameters.

Good lattice constellations for both Rayleigh fading and Gaussian channels

Abstract: Recent work on lattices matched to the Rayleigh fading channel has shown how to construct good signal constellations with high spectral efficiency. We present a new family of lattice constellations, based on complex algebraic number fields, which have good performance on Rayleigh fading channels. Some of these lattices also present a reasonable packing density and thus may be used at the same time over a Gaussian channel. Conversely, we show that particular versions of the best lattice packings (D/sub 4/, E/sub 6/, E/sub 8/, K/sub 12/, /spl Lambda//sub 16/, /spl Lambda//sub 24/), constructed from totally complex algebraic cyclotomic fields, present better performance over the Rayleigh fading channel. The practical interest in such signal constellations rises from the need to transmit information at high rates over both terrestrial and satellite links. Some further results in algebraic number theory related to ideals and their factorization are presented and the decoding algorithm used with these lattice constellations are illustrated together with practical results.

Optimisation of switching levels for adaptive modulation in slow Rayleigh fading

Abstract: Multi-dimensional minimisation techniques are used to optimise, for specific applications, the upper bound bit error rate (BER) and bits per symbol (BPS) performances of adaptive modulation schemes over Rayleigh channels. This is achieved by modifying the switching criteria for selecting the appropriate modulation scheme.

Design and analysis of turbo codes on Rayleigh fading channels

Abstract: The performance of turbo codes using coherent BSPK signaling on Rayleigh fading channels is considered. In regions of low signal-to-noise, performance analysis uses simulations of typical turbo coding systems. For higher signal-to-noise regions beyond simulation capabilities, an average upper bound is used, where the average is over all possible interleaving schemes. Fully-interleaved and exponentially-correlated Rayleigh channels are explored. Furthermore, the design issues relevant to turbo codes are explored for the correlated fading channel.

Upper bound performance of adaptive modulation in a slow Rayleigh fading channel

Abstract: The upper bound bit error rate (BER) performance of a time division duplex (TDD) system, where the number of modulation levels is adapted on a very short term basis, depending upon the channels conditions, is derived and solved by numerical integration The performance shows very close correspondence with simulated results

Comment on "Estimate of channel capacity in Rayleigh fading environment

Abstract: Lee (see ibid., vol.39, p.187, no.8, 1990) proposed a very simple and elegant model for estimating the capacity of slowly fading channels. A small error in his paper, however, has led to an inexact picture. We correct that inaccuracy. As a consequence, the loss in capacity of a Rayleigh fading channel with respect to the additive white Gaussian noise (AWGN) channel turns out to be negligible for all values of the signal-to-noise ratio that are of interest. Furthermore, we extend the analytical results to the situation with M-branch diversity and Ricean fading channels. The lower moments of the associated distributions, i.e., of the /spl chi//sup 2/-distribution (Nakagami with integer m-parameter) and Rice distribution agree when the parameters of the distribution are suitably chosen. With that choice of parameters, the capacities of the M-branch diversity and of the Ricean fading channel become almost identical.

Karhunen-Loeve expansion of a fast Rayleigh fading process

Abstract: An approximate Karhunen-Loeve expansion is given for a random fading process characterised by an autocorrelation function Rḡ(τ) = J0(2πfdτ), J0(x) is the zeroth order Bessel function and fd is the process fading rate.

Closed-loop power control in CDMA systems

Abstract: A closed-loop power control system having both fixed-step and adaptive-step power control algorithms and incorporated with multitap RAKE receivers was proposed. Simulations were performed for wide-band radio channels having path loss, slow fading, and fast fading. As the measurement of the received power and the bit energy-to-interference power spectral density ratio play an important role in a CDMA power control system, a scheme to estimate these two parameters was proposed. For a Rayleigh fading channel and a Rician fading channel having a K factor of 6 dB, a three-tap RAKE receiver is desirable. The decrease in the standard deviation of the errors in E/sub b//I/sub 0/ due to both the two-tap and three-tap RAKE receivers were larger than 1.0 dB for Rayleigh fading channels. For Rician fading channels, the improvement in performance due to a RAKE receiver was still quite insignificant for a K of 6 dB, while only marginal for a K of 12 dB. As the number of taps in RAKE receivers increases, the dynamic range of the received power variation decreases, while the power variation becomes even more random. The performance of the adaptive-step-size power control scheme considered here was only slightly better than the fixed-step-size one for less than two taps. For a mobile's speed of 96 km/h and a three-tap RAKE receiver, the standard deviation of power control errors in the bit energy-to-interference power spectral density ratio is 2.0 dB and 1.5 dB for Rayleigh and Rician fading channels having a K factor of 6 dB. The performance of the power control scheme is essentially unaffected for a control bit error rate of 10/sup -2/.

On communication over fading channels

Abstract: The channel capacity in the presence of Gaussian noise is the same whether the signal suffers fluctuations or not, so high gains may be expected in the fading case. The main tool considered towards this goal is rotation of constellations in signal space, as applied by Boutros (1996) to lattices.

Diversity transform for fading channels

Abstract: This paper investigates a transform that provides diversity to words of symbols transmitted over fading channels. This diversity transform (DRT) does not alter the distance between the input words nor the bandwidth or the information rate. The transform is based on an orthonormal transform and thus increases the channel alphabet. A considerable diversity gain is achieved over Rayleigh fading channel with perfect channel state information (CSI) and interleaving. The transform performances are evaluated both analytically and by simulation, and optimal transforms are determined for specific channels. Finally, a simple nonoptimal decoding scheme is developed, which provides a powerful and practical technique to transfer information over fading channels with or without coding.

Correlation model for shadow fading in mobile radio channels

Abstract: A novel continuous-time correlation model for shadow fading is proposed. It reveals accordance with the level crossing theory of Gaussian processes, and can be adopted to describe and simulate the shadow fading precisely. Alternatively, it can be employed to define the utilisation limits of the Markoff discrete-time exponential model.

Performance of component-interleaved signal sets for fading channels

Abstract: The authors calculate the exact (pairwise) error probability for an arbitrary component-interleaved 2-D constellation over the Rayleigh fading channel. Using this result, they improve the accuracy of the performance analysis at high error rates. For illustration, this is applied to signal sets derived from 4-PSK and 16-QAM over the Rayleigh fading channel.

Evaluation of the exact union bound for trellis-coded modulations over fading channels

Abstract: An analytical technique is presented for computing the exact union bound on the average bit error probability of trellis coded modulation schemes over Rayleigh, Rician, or shadowed Rician-fading channels. To this end, an integral expression is derived for the pairwise error event probability (PEP). Existing bounds can be obtained as special cases of this expression. It turns out that a Gauss-Chebyshev quadrature rule offers excellent accuracy for this integral. By extension, the exact union bound (i.e., the weighted sum of an exact PEPs of a code) can readily be evaluated. This method has the same complexity as the union-Chernoff bound, and a few examples are given to show its application.

A Markov model for block errors on fading channels

Abstract: We investigate the behavior of block errors which arise in data transmission on fading channels. Our approach is more detailed than previous studies, in that it takes into account the specific coding/modulation scheme and it tracks the fading process symbol by symbol. It is shown that a Markov approximation for the block error process (possibly degenerating into an i.i.d. process for sufficiently fast fading) is a very good model for a broad range of parameters. Also, it is observed that the relationship between the marginal error rate and the transition probability is largely insensitive to parameters such as block length, degree of forward error correction and modulation format, and only depends on an appropriately normalized version of the Doppler frequency. This observation leads to a unified approach for the channel modelling which simplifies the performance analysis of upper-layer protocols.

Single frequency estimation with non-uniform sampling

Abstract: A new frame structure is proposed for pilot symbol assisted modulation (PSAM) systems which includes a small number of known pilots at the start of each frame. This facilitates the estimation of large frequency offsets without aliasing. Cramer Rao lower bounds and the exact ML estimates are derived for AWGN and Rayleigh fading channels. Finally, simulations show that the ML estimator has much better performance than existing reduced complexity estimators when applied to this sparse observation model.

Multiple tone interference of frequency-hopped noncoherent MFSK signals transmitted over Ricean fading channels

Abstract: This paper investigates the performance degradation resulting from multitone interference of orthogonal, frequency-hopped, noncoherent M-ary frequency-shift keyed receivers (FH/MFSK) where the effect of thermal and other wideband noise is not neglected. The multiple, equal power jamming tones are assumed to correspond to some or all of the possible FH M-ary orthogonal signaling tones. Furthermore, the channel is modeled as a Ricean fading channel; and both the signaling tones and the multiple interference tones are assumed to be affected by channel fading. It is also assumed that channel fading need not necessarily affect the signaling tones and the interference tones in the same-way. When the information signal power exceeds the power of the individual interference tones, poorer overall system performance is obtained when the multiple interfering tones experience fading. This trend is accentuated as M increases. When the information signal experiences fading, the effect of fading multiple interference tones on overall system performance lessens, and for a Rayleigh-faded information signal, fading of the multiple interference tones has no effect on overall system performance regardless of M.

Method and apparatus for estimating a signal fading characteristic

Abstract: A method and apparatus for measuring a characteristic of a fading signal received using two or more branch diversity receivers (100) is presented. This fading characteristic is proportional to the speed of the user. In a first embodiment the number of times the antenna branches change in a selection diversity process is counted (160), and the result is scaled (170) for imbalance between the branches. The output of a fading quality estimator (180), indicative of the fading and generally proportional to the speed, is thereby obtained.

BER Comparison of DS-CDMA and MC-CDMAC for Frequency Selective Fading Channels

Abstract: This paper presents the advantages and disadvantages of DS-CDMA (Direct Sequence-Code Division Multiple Access) and MC-CDMA (Multi-Carrier-Code Division Multiple Access) systems in synchronous down-link mobile radio communication channels. Furthermore, the bit error rate (BER) performance is analyzed in frequency selective slow Rayleigh fading channels. We theoretically derive the BER lower bound for MC-CDMA system, and propose a simple multi-user detection method. In the BER analysis, we use the same multipath delay profiles for both DS-CDMA and MC-CDMA systems, and discuss the performance theoretically and by computer simulation. Finally, we theoretically prove that the time domain DS-CDMA Rake receiver is equivalent to the frequency domain MC-CDMA Rake receiver for the case of one user.

Double-filtering receivers for PSK signals transmitted over Rayleigh frequency-flat fading channels

Abstract: This paper describes novel detection algorithms for coded and uncoded M-ary phase shift keying (M-PSK) signals transmitted over frequency-flat fading channels. Fading distortion is estimated with per-survivor methods using the sampled outputs of two receive filters. Some of the proposed algorithms do not require information on the fading statistics and are dubbed blind algorithms. The error rate performance of blind and nonblind algorithms is assessed by simulation for uncoded and trellis-coded phase shift keying (PSK) and is compared to the performance of other detection schemes proposed in the literature.

Multisampling receivers for uncoded and coded PSK signal sequences transmitted over Rayleigh frequency-flat fading channels

Abstract: An algorithm, based on previous work [Vitetta and Taylor 1994, 1995], for the detection of coded and uncoded PSK sequences transmitted on a frequency-flat fading channel is investigated. It is based on the Viterbi algorithm and processes more than one signal sample per signaling interval. Its performance is evaluated by means of computer simulations for both uncoded and coded systems.

A new algorithm for estimating mobile speed at the TDMA-based cellular system

Abstract: This paper presents a study on the technique of estimating mobile speed in the TDMA-based GSM network. Two algorithms are proposed. They estimate the mobile speed based on the variation of the received signal envelope cross each burst due to fast fading. Hence they are suitable for both FH (frequency hopping) and non-FH cases. Computer simulations have been done to evaluate the performance by using these two algorithms.

Multiuser detection in asynchronous CDMA frequency-selective fading channels

Abstract: Multipath fading severely limits the performances of conventional code division multiple-access (CDMA) systems. Since every signal passes through an independent frequency-selective fading channel, even modest cross-correlations among signature sequences may induce severe near-far effects in a central multiuser receiver. This paper presents a systematic approach to the detection problem in CDMA frequency-selective fading channels and proposes a low complexity linear multiuser receiver, which eliminates fading induced near-far problem. We initially analyze an optimal multiuser detector, consisting of a bank of RAKE filters followed by a dynamic programming algorithm and evaluate its performance through error probability bounds. The concepts of error sequence decomposition and asymptotic multiuser efficiency, used to characterize the optimal receiver performance, are extended to multipath fading channels. The complexity of the optimal detector motivates the work on a near-far resistant, low complexity decorrelating multiuser detector, which exploits multipath diversity by using a multipath decorrelating filter followed by maximal-ratio combining. Analytic expressions for error probability and asymptotic multiuser efficiency of the suboptimal receiver are derived that include the effects of multipath fading, multiple-access interference and signature sequences correlation on the receiver's performance. The results indicate that multiuser detectors not only alleviate the near-far problem but approach single-user RAKE performance, while preserving the multipath diversity gain. In interference-limited scenarios multiuser receivers significantly outperform the RAKE receiver.

A block memory model for correlated Rayleigh fading channels

Abstract: Channels with memory offer higher capacities provided that the system design can take advantage of this memory. In this paper, we study and characterize the dynamics of a correlated Rayleigh fading channel with known phase. The model follows a continuous-time Markov double-chain for the random fading process with the Markov states corresponding to the quantized amplitude levels. The analytical expression for the distribution of the state dwell times is derived. We also calculate the mean dwell times of the Markov states in terms of the fading properties and the quantization level. Observations from simulating a typical land-mobile channel verify that the Markovian dynamics are best represented through a finer quantization of the lower level fading amplitudes.

Optimal decision strategies for acquisition of spread spectrum signals in frequency selective fading channels

Abstract: The focus of this paper is on the initial acquisition of a direct sequence spread spectrum signal utilizing a purely parallel search strategy. A purely parallel search strategy is utilized because it reduces the acquisition time compared to either serial or partially parallel strategies. In particular, the purpose of this paper is to derive the optimal decision rule, based on the maximum-likelihood criterion, for coarse acquisition of direct sequence waveforms transmitted over frequency selective fading channels. The performance of the conventional decision rule, optimized for additive white Gaussian noise and flat fading channels, and the optimal decision rule, derived in this paper, are compared. It is shown that the performance improvement increases as the number of resolvable paths increases. Furthermore, it is shown that the optimal decision rule for Rayleigh fading channels, and a decision rule which yields approximately optimal performance for Rician fading channels, are easily implemented.

The spatial correlation of shadow fading in macrocellular mobile radio systems

Abstract: It is conventional to refer to three distance scales of channel variation in mobile radio systems, namely multipath or fast fading, shadowing or slow fading, and range variation Of these, the least studied has been the shadowing, which is the variation of the local mean signal strength (averaged over the fast fading) at a fixed range The shadowing variation is statistically characterised by its distribution (approximately lognormal) and its standard deviation (5-10 dB depending on the environment and frequency) Some studies have shown that shadowing is a deterministic effect, caused by variation in the physical environment, yet very little is known about its spatial distribution or correlations These parameters would be extremely useful for system simulation and design, leading to a better understanding of the propagation channel and to system designs of higher integrity

Letter: On the capacity of the binary input gaussian and rayleigh fading channels

Abstract: A class of binary-input discrete-time channels with symmetric distribution of the output with respect to the input is considered and the channel capacity is analytically evaluated. The Gaussian and Rayleigh fading channel belong to this class and their capacities are numerically obtained for different quantization of the output. The asymptotic capacities for low signal-to-noise ratio (SNR) are compared and it is shown that the Rayleigh fading channel loses about 1 dB.

A multicarrier transmission scheme for wireless local communications

Abstract: This paper presents a novel multitone transmission scheme that employs a nonlinear binary code to specify multitone signal constellations. The scheme, which can be viewed as a generalization of M-ary frequency shift keying, is amenable to noncoherent demodulation, and it is therefore useful for communication over fading channels. We determine the performance of this modulation scheme when it is used in conjunction with a Reed-Solomon code for error control. Methods for determining and erasing unreliable multitone constellations are presented and their performance is evaluated when they are used in conjunction with errors-and-erasures decoding. The transmission scheme is evaluated with respect to complexity, bandwidth efficiency, and performance in the presence of Gaussian noise and frequency- and time-selective fading.

Comparison of counter-measures against slow Rayleigh fading for TDMA systems

Abstract: This paper compares the performance of three techniques for introducing tolerance to slow Rayleigh fading channels for TDMA fixed radio access schemes within a single cell: regular cyclic frequency hopping, adaptive modulation and coding, and frequency jumping when required. It is assumed for the channels considered that the fading rates are slow enough that simple feedback adaptive techniques are applicable. The theoretical maximum performance of each scheme is evaluated as a function of the number of conversations supportable and the average transmitted power required. (6 pages)

Performance of antenna diversity multiuser receivers in CDMA channels with imperfect fading estimation

Abstract: The performance of antenna diversity coherent and differentially coherent linear multiuser receivers is analyzed in frequency-nonselective Rayleigh fading CDMA channels with memory. The estimates of the complex fading processes are utilized for maximal-ratio combining and carrier recovery of the coherent multiuser receiver. To analyze the impact of channel estimation errors on the receiver performance, error probability is assessed directly in terms of the fading rate and the number of active users, showing the penalty imposed by imperfect channel estimation as well as the fading-induced error probability floor. The impact of fading dynamics on the differentially coherent decorrelating receiver with equal-gain combining is quantified. While performance of multiuser receivers at lower SNR is determined by both the fading dynamics and the number of active CDMA users, performance at higher SNR is given by an error probability floor which is due to fading only and has the same value as in a single-user case. The comparison of the two receiver structures indicates that the coherent decorrelating receiver with diversity reception may be preferable to the differentially coherent one in nonselective fading CDMA channels with memory.

Effects of fading and partial-band noise jamming on a fast FH/BFSK acquisition receiver with noise-normalization combination

Abstract: We present the performance analysis of a fast frequency-hopped (FH) binary orthogonal frequency-shift keying acquisition receiver for communication against adverse environments. The receiver employs noncoherent, noise-normalized, matched-filtered (MF) correlation detection for rapid acquisition in the search mode. Our analysis includes four types of communication environments, namely additive white Gaussian noise (AWGN) channel, AWGN channel with partial-band noise jamming, fading channels, and fading channels with partial-band noise jamming. The considered fading channels include Nakagami-m, Rician, and Rayleigh amplitude models. Based on Beaulieu's (see ibid., vol.38, no.9, p.1463, 1990) convergent series approach, efficient analytical formulas are developed for performance evaluation. Example performance results for various environments are presented in terms of two acquisition probabilities, namely the detection probability and the false alarm probability of the noise-normalized MF detector. It is analytically shown that with a short MF correlation length and with a sufficiently large ratio of signal power to noise power the fast FH diversity combining yields noticeable performance improvement for environments with strong fading. When the MF correlation is lengthened, this improvement tends to fade away and the diversity combining results in performance loss.