Nonselective Fading Research Articles

Detection Technique for Direct-Sequence Spread-Spectrum Parallel Acquisition in a Nonselective Rician Fading Channel

This paper presents a statistical analysis of the decision threshold for direct-sequence spread-spectrum (DS-SS) parallel Pseuda-Naise (PN) cade acquisitian with a reference filter. The probabilities of detectian and false alarm are derived, and the mean acquisitian time is evaluated as a measure of the system performance in both nonfading and Rician fading channels. Fram the statistical results, it is shown that in the performance analysis of the parallel acquisition system with reference filtering, the statistical evaluatian of the decision threshold seems more appropriate than the approximation of the decision threshold adopted in other schemes [4,5]. It is also shown that the parallel acquisition scheme provides a suboptimal threshold value for a wide range of SNR such that the mean acquisition time is minimized.

Detection of narrow-band signals in spherically invariant noise

This paper deals with noncoherent discrete-time detection of a narrow-band signal subject to slow and nonselective fading and embedded in correlated non-Gaussian noise modeled as a spherically invariant random process whose modulating random variable is continuous. At first, an asymptotic sufficient statistic for an arbitrary fading law is derived; then, the asymptotically optimum detector for Rayleigh-distributed amplitude fluctuations is synthesized. The detection structure implementation requires the knowledge, but for a scale factor, of the correlation function of the noise, but is independent of the distribution function of the modulating random variable. The performance of the asymptotically optimum detector synthesized for Rayleigh fading is assessed via computer simulations. The results show that the performance degradation with respect to the fully optimum performance is scarcely significant, even for low values of the sample size. Moreover, in highly non-Gaussian noise the proposed detector largely outperforms the fully optimum detector synthesized under the correlated Gaussian noise assumption.

Techniques for the measurement and decomposition of the time varying narrow bandwidth transfer function of an HF sky wave transmission

A technique is presented for the measurement and decomposition into component propagation modes of the time varying narrow bandwidth (10 kHz) transfer function of an HF sky wave frequency modulated continuous wave (FMCW) transmission at 15.085 MHz. One event on a 5244‐km transmission path which exhibited nonselective fading within the 10‐kHz bandwidth is analyzed and found to exhibit severe phase distortion due to multipath. A component transfer function for an individual propagation mode is also obtained by two‐dimensional filtering of the signal in the Doppler frequency‐time delay domain, resulting in a significant reduction in phase distortion. The filtering technique can be used on FMCW transmissions with a repetitive waveform for the time varying identification and measurement of component multipath modes with unresolvable group time delay differences.

A stochastic importance sampling methodology for the efficient simulation of adaptive systems in frequency nonselective Rayleigh fading channels

We present an IS stochastic technique for the efficient simulation of adaptive systems which employ diversity in the presence of frequency nonselective slow Rayleigh fading and additive, white, Gaussian noise. The computational efficiency is achieved using techniques based on importance sampling (IS). We utilize a stochastic gradient descent (SGD) algorithm to determine the near-optimal IS parameters that characterize the dominant fading process. After accounting for the overhead of the optimization algorithm, average speed-up factors of up to six orders of magnitude [over conventional Monte Carlo (MC)] were attained for error probabilities as low as 10/sup -11/ for a fourth-order diversity model.

Performance of DPSK mobile receiver in impulse noise

The authors present a performance analysis of L-diversity DPSK receivers over frequency non-selective Rayleigh fading channels in Middleton's class A impulse noise. The analysis is exact and is shown by simulation to be accurate.

On the performance of DQPSK communication systems impaired by timing error, mixer imbalance, and frequency nonselective slow Rayleigh fading

In this paper, the error performance of the differential detection scheme is assessed for differentially encoded quaternary-phase-shift-keying (DQPSK) and /spl pi//4-shifted-DQPSK signals, It is assumed that the receiver suffers from mixer imbalance and timing error impairments. Expressions for the system bit-error rate (BER) are obtained when the channel is free of fading and when the channel suffers from frequency nonselective (flat) slow Rayleigh fading in the face of additive white Gaussian noise. To arrive at the desired expressions, the probability density function (pdf) of the phase angle between a pair of Gaussian random vectors is used to obtain the probability of bit error conditioned on the channel fading and timing error. The resulting expressions are then averaged over the nonnegligible timing error and the channel fading to arrive at the desired expressions for the system BER. Finally, with the aid of numerical analysis and in the presence of the aforementioned impairments, a number of conclusions are drawn and the performance of differential receivers for DQPSK and /spl pi//4-shifted-DQPSK modulation schemes are compared. It is shown here that the performance of a DQPSK receiver in the absence of fading and timing error is substantially impaired by mixer imbalance for BERs less than 10/sup -4/. In the presence of fading, the performance is noticeably degraded due to mixer imbalance when a nonzero timing error is present.

A multiuser approach to narrowband cellular communications

We compare three receivers for coded narrowband transmission affected by fading and co-channel interference. The baseline receiver is based on conventional diversity reception with maximal-ratio combining. A multiuser approach allows us to derive a maximum-likelihood multiuser receiver and its reduced-complexity suboptimal version. Finally, a decorrelating diversity receiver, which seeks a tradeoff between performance and complexity, is studied. Closed-form performance parameters are derived for all the proposed receivers in the case of coded coherent PSK and independent frequency nonselective Rayleigh fading.

Efficient signal processing techniques for exploiting transmit antenna diversity on fading channels

A class of powerful and computationally efficient strategies for exploiting transmit antenna diversity on fading channels is developed. These strategies, which require simple linear processing at the transmitter and receiver, have attractive asymptotic characteristics. In particular, given a sufficient number of transmit antennas, these techniques effectively transform a nonselective Rayleigh fading channel into a nonfading, simple white marginally Gaussian noise channel with no intersymbol interference. These strategies, which we refer to as linear antenna precoding, can be efficiently combined with trellis coding and other popular error-correcting codes for bandwidth-constrained Gaussian channels. Linear antenna precoding requires no additional power or bandwidth and is attractive in terms of robustness and delay considerations. The resulting schemes have powerful and convenient interpretations in terms of transforming nonselective fading channels into frequency- and time-selective ones.

Bit-error-probability for noncoherent orthogonal signals in fading with optimum combining for correlated branch diversity

Due to the interest in wireless personal communications, there has been a lot of research on the performance of receivers with diversity. Most analyses assume the diversity branches are independent. This paper presents an analysis of the bit-error probability for receivers in which the diversity branches are correlated. Noncoherent orthogonal digital modulation (NCODM) with Rician and Rayleigh slow, nonselective fading models are assumed. Through the use of the diagonalization of quadratic forms, most of the calculations of the bit-error probability can be reduced to a two-dimensional numerical integration. For some cases for dual diversity, a closed-form expression for the error probability is given. A number of diversity combining laws, including square law and maximum likelihood, are considered. We find that Rician fading can be worse than Rayleigh fading in correlated diversity environments, a situation quite different from the independent diversity case. Also, for the Rayleigh fading model with correlated branch diversity, we find that an equal-weight, square-law combiner usually has the same error performance as the more complex maximum-likelihood combiner. However, this is not the case for a Rician fading model with the same correlation environment. Simple diagonalization methods that compensate for the lossy effect of correlation are specified and found to be effective when the dominant noise and interference have almost the same correlation distribution as the fading signals.

Asymptotically optimum diversity detection in correlated non-Gaussian noise

The letter deals with noncoherent diversity detection of bandpass signals subject to slow and nonselective fading and embedded in correlated non-Gaussian noise modeled as a spherically invariant random process. At first, an asymptotic sufficient statistic for arbitrary fading laws on the diversity branches is derived; then, the asymptotically optimum detector for Rayleigh-distributed amplitude fluctuations is synthesized and its performance is assessed.

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

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.

Probability of error and outage in a Rice-lognormal channel for terrestrial and satellite personal communications

This letter addresses performance evaluation in a nonselective fading channel modeled by a combination of Rice and lognormal (RLN) statistics. The RLN model is valid under widely different environmental conditions, both for terrestrial cellular and for satellite personal communication systems. The letter provides semianalytical expressions for the average error probability in the RLN channel for coherent M-ary phase shift keying (PSK) and noncoherent M-ary orthogonal transmissions, then it highlights the relationship between outage probability and cell coverage for macro and microcellular systems, and finally it provides some error probability results for nongeostationary (non-GEO) satellite systems.

PSK symbol synchronization performance of ML-oriented data-aided algorithms for nonselective fading channels

Two maximum-likelihood (ML)-oriented data-aided (DA) symbol synchronization algorithms are investigated in this paper. They operate on a noisy M-PSK signal (M>2) transmitted over a slowly time-varying nonselective Rician fading channel. The first algorithm is channel-aided (CA): it makes use of an estimate or the instantaneous channel gain. The second algorithm is nonchannel-aided (NCA): timing is acquired independently of the channel gain estimation. Both algorithms operate either with a known training sequence or in a decision-directed mode, and are suitable for fully digital implementation. We find that the CA algorithm yields a smaller tracking error variance than the NCA algorithm, but the difference in performance decreases with increasing carrier-to-multipath (C/M) ratio. >

Symbol-aided channel estimation with nonselective Rayleigh fading channels

New algorithms are proposed to implement coherent detection of PSK signals transmitted over frequency-flat Rayleigh channels. Channel gain estimates are derived both from known symbols multiplexed with the data stream and from tentative data decisions. The performance of these algorithms is assessed by computer simulation with uncoded and trellis-coded modulation. Comparisons are also made with differentially coherent detection (DPSK) and with perfect coherent detection (corresponding to ideal knowledge of the fading process). Simulation results demonstrate that these algorithms can track adequately the severe fading fluctuations encountered on a Rayleigh channel and that their superiority over DPSK becomes impressive with fast fading.

Two-stage carrier synchronization techniques for nonselective fading

Transmitted reference carrier synchronization techniques can potentially provide near-coherent performance in the presence of time-varying fading. Such techniques are spectrally not as efficient as nonreference-based architectures and can have performance several dB worse than ideal coherent in higher order modulations (e.g., 64-QAM). This paper presents a two-stage demodulator that facilitates improved spectral efficiency or provides better performance (or sometimes both). The two-stage architecture results from examining the optimal detector for M-QAM. The first stage produces tentative decisions and removes the modulation from the received signal. The second stage implements a smoothing filter for the resulting signal, providing an improved carrier estimate. Monte Carlo simulation results demonstrate the improved performance For 4-QAM, 16-QAM, and 64-QAM.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Identification of frequency non-selective fading channels using decision feedback and adaptive linear prediction

In this paper we propose an algorithm for tracking the phase and amplitude of frequency nonselective fading channels. In contrast to Irvine-McLane's (see IEEE Journal on Selected Areas in Communications, vol.10, no.8, p.1289-1299, 1992) symbol-aided plus decision-directed (SADD) tracker which mainly uses training symbols for channel estimation, the new algorithm exploits data-bearing-bearing symbol correlation of fading channels. This algorithm combines decision feedback and adaptive linear prediction (DFALP) by using tentative coherent detection and the least mean square (LMS) algorithm. Both simulation results and theoretical analysis demonstrate that the performance of this algorithm compares favorably with the SADD algorithm in terms of decision delay, mean-square error and bit-error-rate. The algorithm is also applied to a system similar to EIA/TIA recommendation IS-54 and has achieved 3 dB gain using the same channel coding and similar interleaving.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Tracking performance of ML-oriented NDA symbol synchronizers for nonselective fading channels

In this paper we investigate the tracking performance or two maximum-likelihood (ML)-oriented nondecision-aided (NDA) symbol synchronization algorithms, operating on a noisy M-PSK signal (M>2) transmitted over a time-varying nonselective Rician fading channel. The first algorithm is nonchannel-aided (CA): timing is acquired independently of the channel gain estimation. The second algorithm is channel-aided (CA): it makes use of (an estimate of) the instantaneous channel gain. As the channel gain estimator requires timing information, use of the CA algorithm implies that the timing and the channel gain must be acquired jointly. Both algorithms are suitable for fully digital implementation, and have a similar complexity. For E/sub 3//N/sub 0/ values of practical interest, we show that the NCA algorithm is to he preferred over the CA algorithm. >

Further results on the bit error probabilities of MDPSK over the nonselective Rayleigh fading channel with diversity reception

This paper presents a fundamental approach for deriving the bit error probability of BDPSK and QDPSK over the nonselective Rayleigh fading channel for a receiver with an arbitrary IF filter, and for a fading process with an arbitrary Doppler spectrum with arbitrary Doppler bandwidth. The results generalize those published earlier which were restricted to matched filter reception and to a fading process with a small Doppler bandwidth compared to the symbol rate. This allows the error probability to be studied in the presence of varying degrees of ISI due to the bandlimitation of the received signal by the IF filter, and in the presence of fading fluctuations of various rates. The analytical approach presented is simple, and yet powerful in that it can handle the case of diversity reception. This is a great advantage over the alternative approach of using the distribution of the differential phase of the received signal over a symbol interval. The bit error probability results apply to both conventional BDPSK and QDPSK, as well as /spl pi//2-2DPSK and /spl pi//4-4DPSK, and allow the irreducible bit error probability as well as the SNR at which this irreducible value sets in to be studied as a function of the Doppler bandwidth and IF filter bandwidth. The computed results are applicable to the design of digital cellular mobile communication systems.

Performance of sequence estimation scheme of narrowband digital FM signals with limiter-discriminator detection

A communication scheme using binary FM with noncoherent limiter-discriminator detection has been well known. Up to now, the improvement of bit error rate at the receiver side has been carried out through the bandwidth optimization of the IF filter, the decision feedback equalization (DFE), or simple two-state maximum likelihood sequence estimator (MLSE). This channel is inherently the intersymbol interference (ISI) channel due to the premodulation baseband filtering as well as the narrowband IF filtering. So the sequence estimation scheme using the Viterbi algorithm can be applied successfully, although the channel is not additive white Gaussian and maximum likelihood in the strict sense. In this paper, through computer simulations, we examine the actual BER improvement of the sequence estimation scheme with multiple-state trellis especially for MSK and GMSK signals. We mainly consider static AWGN and frequency nonselective Rician fading channels. Consequently, by adjusting the IF filter bandwidth, very large estimation gains are obtained compared to the conventional DFE or MLSE detector for AWGN and Rician fading channels. This scheme does not produce large demodulation delay and is implemented only by adding the signal processing part to the final stage of the receiver. This scheme seems to be very useful for any applications including satellite mobile channels. >

Performance of a coded multi-carrier DS-CDMA system in multi-path fading channels

Multi-carrier DS-CDMA has been considered as an effective scheme for reducing multiple access interference in quasi-synchronous transmission. The scheme allows the reduction of multiple access interference by transferring the orthogonality property of the signals into the frequency domain where the orthogonality property is robust to relative chip offsets between the spreading codes of the various users. However in multi-path channels, the multi-carrier technique results in frequency non-selective fading in the sub-channels, due to the narrower bandwidth, hence a reduction of the capability of the spread spectrum signal to mitigate the effect of multi-path propagation. In this paper, we consider the use of a Reed-Muller code with soft decision decoding to regain the corresponding loss in performance, and compare the resulting system with a single carrier DS-CDMA system. The effect of system parameters such as the number of sub-channels is investigated through numerical calculation and simulation, from which a number of system design criteria are arrived at.