Time-selective Fading Channels Research Articles

Double-filter differential detection of PSK signals transmitted over linearly time-selective Rayleigh fading channels

Linearly time-varying fading models are used to investigate differential detection of PSK signals transmitted over frequency-flat fading channels. The structure of the optimal differential PSK detector is derived and an analytical technique is developed to compute its error performance. Results obtained by computer simulation are in excellent agreement with the analytical predictions.

Analysis and optimization of the performance of OFDM on frequency-selective time-selective fading channels

In mobile radio communication, the fading channels generally exhibit both time-selectivity and frequency-selectivity. Orthogonal frequency-division multiplexing has been proposed to combat the frequency-selectivity, but its performance is also affected by the time-selectivity. We investigate how various parameters, such as the number of carriers, the guard time length, and the sampling offset between receiver and transmitter, affect the system performance. Further, we determine the optimum values of the above parameters, which minimize the degradation of the signal to-noise ratio at the input of the decision device.

Excess bandwidth is necessary for ISI-free double-filter receivers

Previous work has indicated that highly effective double-filter receivers can be implemented for phase-shift keying (PSK) transmissions over linearly time-selective fading channels provided that the transmitter and receiver filters have root-raised-cosine transfer functions with unity rolloff factor. Revisiting the detection problem within the frame of maximum-likelihood (ML) sequence estimation we prove that this filter design is unique within the class of signaling pulses which are band limited to the symbol rate. It is concluded that some waste of bandwidth is inevitable for achieving ISI-free operation with double-filter receivers.

Cramer-Rao bounds in the parametric estimation of fading radiotransmission channels

The context of this paper is parameter estimation for linearly modulated digital data signals observed on a frequency-flat time-selective fading channel affected by additive white Gaussian noise. The aim is the derivation of Cramer-Rao lower bounds for the joint estimation of all those channel parameters that impact signal detection, namely, carrier phase, carrier frequency offset (Doppler shift), frequency rate of change (Doppler rate), signal amplitude, fading power, and Gaussian noise power. Time-selective frequency-flat fading is modeled as a low-pass autoregressive multiplicative distortion process. In particular, the important case of slow fading, with the multiplicative process remaining constant over the whole data burst, is specifically discussed. Asymptotic expressions of the bounds, valid for a large observed sample or for high signal-to-noise ratio (SNR), are also derived in closed form. A few charts with numerical results are finally reported to highlight the dependence of the bounds on channel status (SNR, fading bandwidth, etc.).

Optimal noncoherent detection of FSK signals transmitted over linearly time-selective Rayleigh fading channels

Linearly time-varying fading models are used to investigate noncoherent detection of frequency shift keying (FSK) signals transmitted over frequency-flat fading channels. The structure of the optimal noncoherent FSK detector is derived and a novel analytical technique is proposed to compute the error performance of noncoherent FSK detectors in fast fading. Error performance results obtained by computer simulation are in excellent agreement with the analytical predictions.

Trellis coded modulation on time-selective fading channels

A general analysis of trellis coded modulation (TCM) on time-selective fading channels with correlated signal samples is presented. Gaussian quadratic forms are used to evaluate the pairwise sequence error probability. The effect of interleaving is studied and it is shown that moderate interleaving with respect to the doppler frequency of the channel suffices to achieve good performance. The analysis is then extended to treat non-ideal estimation of the complex channel gain in the coherent case using a general linear channel estimator. It is shown that differential phase shift keying (DPSK) and differential trellis coded modulation (DTCM) on correlated fading channels call be treated analytically and that the error probability of DTCM, like of coherent TCM, depends only on the Euclidean distance of the error sequences. Results show that DTCM suffers a degradation of up to 3 dB with respect to coherent TCM on slow fading channels but performs better on fast fading channels. Error floor curves are presented for DTCM for different interleaving depths. The event error probability P/sub e/ and the bit error probability P/sub b/ of TCM, approximated by truncated averaged union bounds, are calculated for the codes used. A comparison of the approximations of P/sub b/ with simulation results show that the union bound is relatively loose for fading channels. >

Multi‐Level Block Coded Modulations with Unequal Error Protection for the Rayleigh Fading Channel

AbstractBlock coded 8‐DPSK modulations are presented for the time selective Rayleigh fading channel. These coded modulations are based on multi‐level code constructions and utilize short binary block codes of length 2 to 8 (binary symbols) as building blocks to construct 8‐DPSK codes of length 2 to 8 symbols, respectively. All the constructed block coded modulations have a built in time diversity of at least 2 to combat Rayleigh fading. Unequal error protection (UEP) is obtained by providing higher time diversity for the important data. Further UEP can be obtained by using a non‐uniform signal constellation that provides higher product distance (through increased component Euclidean distance) for the more important data. Low transmission delay is obtained by matching the code rate and interleaver to the channel conditions (Doppler spread). Block coded 8‐DPSK schemes with rate 1.5‐2.25 information bits per symbol and code length 2 to 8 symbols have been designed and simulated for various values of Doppler spread. Gains of more than 15 dB can be obtained at a bit error rate of 10−3 for a fraction of the data that is deemed to be most important. By using different binary codes in the multilevel scheme and by using different nonuniform signal constellations, the unequal error probability profile and the data rates can be controlled in a flexible manner.

Analysis of Direct-Sequence Spread-Spectrum Multiple-Access Communication Over Rician Fading Channels

This paper is concerned with the performance of biphase direct-sequence spread-spectrum multiple-access communication for a general class of fading channels. The channels considered are those for which the channel output consists of a strong stable specular signal plus a faded version of this signal. Such channels, which are referred to as Rician fading (or Rice fading) or specular-plus-Rayleigh fading, are the result of a transmission medium which gives rise to a major stable communication path and a number of additional weaker communication paths. The fading channel is modeled as a general wide-sense-stationary uncorrelatedscattering (WSSUS) channel-a model which is general enough to exhibit both time and frequency selectivity and to impose no restrictions on the fading rate. For the general WSSUS model, results are obtained on the average signal-to-noise ratio at the receiver output in terms of the spread-spectrum signature sequences and the covariance function for the fading process. These results are then specialized to each of two important classes of WSSUS channels: time-selective fading channels and frequency-selective fading channels. Numerical evaluations are presented for specific examples of each of these two types of channels. Analytical expressions are derived for a spread-spectrum multiple-access system with random signature sequences, and the use of these expressions in preliminary system design is discussed.