Showing papers on "Fading published in 1992"

8-PSK trellis codes for a Rayleigh channel

Abstract: A suboptimal trellis coding approach based on the concept of combining a good convolutional code and bit interleavers is presented. The aim is to improve the reliability of digital radio communication over a fading channel. It is shown that over a Rayleigh channel and for a fixed code complexity the proposed system is superior to the baseline system. Its performance is analyzed using the generalized R/sub o/ and the upper bound on the bit error rate. The results suggest that on a Rayleigh channel, the standard trellis codes may not be the correct approach for improving the reliability of the communication channel. The discussion is restricted to a rate 2/3 coded system with 8-PSK modulation. >

Distributed cochannel interference control in cellular radio systems

Abstract: Distributed power control algorithms that use only the carrier-to-interference ratios (C/I ratios) in those links actually in use are investigated. An algorithm that successfully approximates the behavior of the best known algorithms is proposed. The algorithm involves a novel distributed C/I-balancing scheme. Numerical results show that capacity gains on the order of 3-4 times can be reached also with these distributed schemes. Further, the effects of imperfect C/I estimates due to noise vehicle mobility, and fast multipath fading are considered. Results show that the balancing procedure is very robust to measurement noise, in particular if C/I requirements are low or moderate. However, for required high C/I levels or for a rapidly changing path loss matrix, convergence may be too slow to achieve substantial capacity improvements. >

Optimum diversity combining and equalization in digital data transmission with applications to cellular mobile radio. I. Theoretical considerations

Abstract: A comprehensive theory for Nth-order space diversity reception combined with various equalization techniques in digital data transmission over frequency-selective fading channels is developed. The channels are characterized by N arbitrary impulse responses possessing random parameters as well as N additive Gaussian noise sources. Various combiner-equalizers that minimize the mean-squared error are determined. Formulas are presented for the attainable least-mean-squared errors and upper bounds on average probabilities of error. The theory is applied to optimize system parameters and to predict performance for QAM data transmission operating over a model for the mobile radio channel. For this model, estimates of average attainable error rates and outage probabilities are provided as functions of system parameters. In the channel models the uncoded data rates as well as Shannon capacity are regarded as random variables. >

Analysis of the error performance of trellis-coded modulations in Rayleigh-fading channels

Abstract: This work presents an exact expression for the pairwise error event probability of trellis-coded modulation (TCM) transmitted over Rayleigh-fading channels. It includes phase shift keying (PSK) and multilevel quadrature amplitude modulation (QAM) codes, as well as coherent and partially coherent (e.g. differential, pilot tone, etc.) detection. Due to the form of the exact pairwise error event probabilities, this calculation technique cannot be used with the transfer function technique to obtain an upper (union) bound on the overall bit error probability. For this reason, the authors estimate the bit error probability by considering only a small number of short error events. Through simulations, they found that the estimation is usually very accurate at high signal-to-noise ratios but not as accurate at lower signal-to-noise ratios. They study several coded modulation schemes this way. Among the results are the fact that TCM provides significant improvement in the error floor when detected differentially, and an asymmetry in the pairwise error event probability for 16 QAM. >

Introduction to digital communication

Abstract: (NOTE: Most chapters begin with an Introduction and conclude with Summary, References, and Problems.) 1. Introduction to Digital Data Transmission. Components of a Digital Communication System. Communications Channel Modeling. Communication Link Power Calculations. Driving Forces in Communications. Computer Use in Communication System Analysis and Design. Preview of the Book. 2. Signals, Systems, Modulation, and Noise: Overview. Review of Signal and Linear System Theory. Basic Analog Modulation Techniques. Complex Envelope Representation of Bandpass Signals and Systems. Signal Distortion and Filtering. Practical Filter Types and Characteristics. Sampling Theory. Random Processes. Computer Generation of Random Variables. 3. Basic Digital Communication Systems. The Binary Digital Communications Problem. Signaling through Bandlimited Channels. Equalization in Digital Data Transmission. A Digital Communication System Simulation Example. Noise Effects in Pulse Code Modulation. 4. Signal-Space Methods in Digital Data Transmission. Optimum Receiver Principals in Terms of Vector Spaces. Performance Analysis of Coherent Digital Signaling Schemes. Signaling Schemes Not Requiring Coherent References at the Receiver. Comparison of Digital Modulation Systems. Comparison of M-ary Digital Modulation Schemes on Power and Bandwidth-Equivalent Bases. Some Commonly Used Modulation Schemes. Design Examples and System Tradeoffs. Multi-h Continuous Phase Modulation. Orthogonal Frequency Division Multiplexing. 5. Channel Degradations in Digital Communications. Synchronization in Communication Systems. The Effects of Slow Signal Fading in Communicative Systems. Diagnostic Tools for Communication System Design. 6. Fundamentals of Information Theory and Block Coding. Basic Concepts of Information Theory. Fundamentals of Block Coding. Coding Performance in Slow Fading Channels. 7. Fundamentals of Convolutional Coding. Basic Concepts. The Viterbi Algorithm. Good Convolutional Codes and Their Performance. Other Topics. 8. Fundamentals of Repeat Request Systems. General Considerations. Three ARQ Strategies. Codes for Error Detection. 9. Spread-Spectrum Systems. Two Communication Problems. Types of Spread-Spectrum Systems. Complex-Envelope Representation of Spread Spectrum. Generation and Properties of Pseudorandom Sequences. Synchronization of Spread-Spectrum Systems. Performance of Spread-Spectrum Systems in Jamming Environments. Performance in Multiple User Environments. Multiuser Detection. Examples of Spread-Spectrum Systems. 10. Introduction to Cellular Radio Communications. Frequency Reuse. Channel Models. Mitigation Techniques for the Multipath Fading Channel. System Design and Performance Prediction. Advanced Mobile Phone Service. Global System for Mobile Communications. Code Division Multiple Access. Recommended Further Reading. 11. Satellite Communications. Allocation of a Satellite Transmission Resource. Link Power Budget Analysis. Examples of Link Power Budget Calculations. Low- and Medium-Earth Orbit Voice Messaging Satellite Systems. Appendix A. Probability and Random Variables, Probability Theory. Random Variables, Probability Density Functions, and Averages. Characteristic Function and Probability Generating Function. Transformations of Random Variables. Central Limit Theorem. Appendix B. Characterization of Internally Generated Noise. Appendix C. Attenuation of Radio-Wave Propagation by Atmospheric Gases and Rain. Appendix D. Generation of Coherent References. Description of Phase Noise and Its Properties. Phase-Lock Loop Models and Characteristics of Operation. Frequency Synthesis. Appendix E. Gaussian Probability Function. Appendix F. Mathematical Tables. The Sinc Function. Trigonometric Identities. Indefinite Integrals. Definite Integrals. Series Expansions. Fourier Transform Theorems. Fourier Transform Pairs. Index.

Investigations into cochannel interference in microcellular mobile radio systems

Abstract: A microcell interference model termed the Nakagami m/sub x//m/sub y/ model is introduced. The desired signal and the cochannel interferers are assumed to have Nakagami statistics but with different amounts of fading. A special case of this model is obtained when the desired signal has Nakagami statistics while the cochannel interferers are subject to Rayleigh fading. The probability density function of the signal-to-interference ratio in the Nakagami model is derived. This model is also compared with a Rician/Rayleigh microcellular model. Expressions for the outage probabilities in microcell systems are derived. Numerical results show that, compared to medium/large cell systems, the microcellular systems have a lower outage probability. The impact of diversity on the microcellular system is also studied. An improvement of the outage probability due to diversity is observed. >

Combined effects of phase sweeping transmitter diversity and channel coding

Abstract: A novel transmitter diversity scheme that generates forced fading to improve the performance of channel coding is proposed and investigated. Since the required phase sweeping frequency is much smaller than the transmission bit rate, bandwidth expansion is negligible. A sinusoidal phase sweeping function Delta Theta sin(2 pi f/sub H/t) is employed in laboratory experiments using 32 kbit/s quarternary differential phase shift keying (QDPSK) with differential detection and BCH(23,12) code. It is shown that Delta Theta =200 degrees and f/sub H/=67 Hz can be used when m (interleaving depth)=10 b. Hence, excessively long interleaving is not required by the use of transmitter diversity. Under very slow Rayleigh fading (f/sub D/=1 Hz), a measured improvement of 4.8 dB is obtained at a word error rate of 10/sup -2/ without receiver diversity. Applications include paging systems that require very simple receivers. >

Channel modeling and simulation in satellite mobile communication systems

Abstract: An analog model describing signal amplitude and phase variations on shadowed satellite mobile channels is proposed. A linear combination of log-normal, Rayleigh, and Rice models is used to describe signal variations over an area with constant environment attributes while an M-state Markov chain is applied to represent environment parameter variations. Channel parameters are evaluated from the experimental data and utilized to verify a simulation model. Results, presented in the form of signal waveforms, probability density functions, fade durations, and average bit and block error rates, show close agreement with measurements. >

Repeater diversity spread spectrum communication system

Abstract: A repeater diversity spread spectrum communication system provides substantially fade free communications between a transmitter (1) and a receiver (7). A transmitted signal is relayed through a plurality of linear communications repeaters (3-6) that produce copies of the transmitted signal, the copies each arriving through an independently fading signal path. The transmitter and/or the receiver may be mounted upon a mobile platform, and the plurality of repeaters may be terrestrial, or may be provided in satellites in low earth orbit or in geosynchronous earth orbit. The receiver processes the received signal copies to equalize them to one another in delay, frequency, and phase, and then combines the multiple received and equalized signal copies to produce a composite signal having a greatly reduced fading depth.

Qam system in which the constellation is modified in accordance with channel quality

Abstract: A radio system is so designed as to allow each of a pair of transceivers to assess the quality of the link, and to modify the signal constellation accordingly. Each transceiver monitors the quality of the channel by assessing the strength of the received signal, or the bit error rate, or both. The system is specifically adapted to time division duplex (TDD) transmission over fast fading channel. In each block of data, the first bit is a QPSK (4QAM) signal, which indicates which constellation is to be used.

Error performance of multiple-symbol differential detection of PSK signals transmitted over correlated Rayleigh fading channels

Abstract: The error performance of multiple-symbol differential detection of uncoded QPSK signals transmitted over correlated Rayleigh fading channels is studied. The optimal detector is presented, along with an exact expression for the corresponding pairwise error event probability. It is shown that multiple-symbol differential detection is a very effective strategy for eliminating the irreducible error floor associated with a conventional differential detector. In all of the cases investigated, a detector with an observation interval as small as two symbols is sufficient for this purpose. It is also found that the error performance of a multiple-symbol differential detector is not sensitive to the mismatch between the decoding metric and the channel fading statistics. >

Performance evaluation for cellular CDMA

Abstract: The authors consider the performance of a cellular radio, direct-sequence code-division multiple access, (CDMA) system. The base-to-mobile link is modeled as a flat Rayleigh fading channel, with all signals transmitted from a given base station fading in unison. For the mobile-to-base link, the authors use a similar model, except that the waveforms from all users are assumed to experience independent fading. The effects of imperfect power control are shown. >

TCM on Frequency Selective Land-Mobile Fading Channels

Abstract: A power and bandwidth eecient concatenated coding system for reliable data transmission on time-and frequency{selective mobile fading channels is considered. Multi{tone signaling with spectrally overlapping pulses is achieved by orthogonal frequency{division multiplexing (OFDM). The inner coding/modulation system is trellis{coded modulation (TCM) consisting of two binary convolutional codes, outputting 16QAM symbols. This class of codes was found to be optimum in sense of complexity and performance on Rayleigh/Rician fading channels. A soft{output (Viterbi) decoding algorithm accepts channel state information, and even delivers reliability information to an outer decoder. The outer code is a rate{compatible punctured convolutional (RCPC) code providing unequal error protection (UEP). Concatenated coding in conjunction with double in-terleaving and slow frequency hopping (SFH) provides eecient dual diversity. A two{ dimensional pilot sequence multiplexed into the time and frequency sequence according to the sampling theorem allows a robust, truly coherent carrier synchronization, which can be realised, e.g., by a two{dimensional Wiener lter. Emphasis here is on sophisticated, yet reduced complexity channel estimation (CE) making use of the multi{tone structure and a{priori channel knowledge. The use of a guard interval circumvents the implementation of explicit equalization even under severe multipath distortions. Results (BER und power/bandwidth diagram) are presented on basis of a hilly{terrain land{mobile fading channel with f Dmax =200 Hz one{sided maximum Doppler frequency, max 64 s maximum echo delay, and system parameters (2 128 kbit/s data rate, bit sensitivity classes ranging from 5 10 ?7 to 5 10 ?2 , 500 ms overall delay) according to the European digital audio broadcasting (DAB) system. The required average E b =N 0 for 8{state inner TCM coding, 64{state outer coding, and low{complexity channel estimation is about 10 dB at a bandwidth eeciency of about 1 bit/s/Hz, which is an improvement of several dB's in power and/or bandwidth compared to the current diierentially coherent DAB proposal. We gratefully acknowledge the DAB project support by the Robert Bosch GmbH, Hildesheim.

Exact analysis of the outage probability in multiple-user mobile radio

Abstract: A novel mathematical method is presented to express the outage probability for a desired radio signal received from a mobile transmitter in the presence of multiple interfering signals with combined log-normal and Rayleigh fading. In contrast to previously reported analyses, this exact method avoids approximation of the PDF of the received powers of the various signals. This is useful in determining the spectrum efficiency and performance of (interference-limited) radio networks for high-capacity cellular telephony, two-way paging, packet radio and other mobile data networks. >

Characteristics and reduction of coherent fading noise in Rayleigh backscattering measurement for optical fibers and components

Abstract: The characteristics of fading noise in Rayleigh backscattering measurements made with coherent lightwaves such as in coherent-OTDR (optical time-domain reflectometry) and coherent-OFDR (optical frequency-domain reflectometry) are studied. The effects of frequency shift averaging on fading noise reduction are clarified theoretically, and the relationships between measurement accuracy and other parameters, such as spatial resolution and frequency variation range are derived. The calculated results of loss measurement accuracy are in good agreement with experimental data. The formula can also be applied to low-coherence interferometric OTDR. >

Outage probabilities of diversity cellular systems with cochannel interference in Nakagami fading

Abstract: Analytical, closed-form expressions for cellular outage probabilities in generalized Nakagami fading are derived for three practical diversity combining schemes. The outage is defined as the probability that the signal-to-interference power ratio (SIR) is less than a power protection ratio. The analysis considers L-branch equal gain (EG), selection (SC), and switched (SW) diversity combining schemes. The analyses are not limited to a single interferer, but rather assume the presence of multiple independent cochannel interferers. Previous results have used some approximations to study the performance of the EG combiner. A precise method is used to analyze the performance of an L-branch EG combiner. Selection diversity combining using the total power algorithm, the desired power algorithm, and the signal-to-interference power algorithm is analyzed. The effects of diversity on the reuse factor and on the spectrum efficiency of cellular mobile radio systems are considered in detail. The results for the Rayleigh fading channel are obtained and presented as a special case of the generalized Nakagami fading model. >

Path loss and fading models for microcells at 900 MHz

Abstract: Narrowband measurements have been performed at 870 MHz in outdoor urban microcells using low antenna heights. Typical path loss is shown, and an empirical path loss model is proposed. The slow fading is investigated, and a fading model for simulation purposes is suggested. The main characteristic of the fast fading is described, and a narrowband fast fading model is proposed. >

Matched filter performance bounds for diversity combining receivers in digital mobile radio

Abstract: By employing the technique known as the matched filter bound, the authors derive analytical expressions for the distribution and average of the bit-error-rate in an ideal space diversity mobile radio receiver. Each diversity branch receives from a frequency-selective Rayleigh fading channel of arbitrary delay profile, and is subjected to additive Gaussian noise of arbitrary spectral shape. Numerical results calculated from the analytical expressions give insight into the relative benefits of antenna diversity and wideband transmission over the mobile radio channel. >

A decision feedback equalizer with time-reversal structure

Abstract: This work describes the use of a receiver with a time-reversal structure for low-complexity decision feedback equalization of slowly fading dispersive indoor radio channels. Time-reversal is done by storing each block of received signal samples in a buffer and reversing the sequential order of the signal samples in time prior to equalization. As a result, the equivalent channel impulse response as seen by the equalizer is a time-reverse of the actual channel impulse response. Selective time-reversal operation, therefore, allows a decision feedback equalizer (DFE) with a small number of forward filter taps to perform equally well for both minimum-phase and maximum-phase channel characteristics. The author evaluates the theoretical performance bounds for such a receiver and quantifies the possible performance improvement for discrete multipath channels with Rayleigh fading statistics. Two extreme cases of DFE examples are considered: an infinite-length DFE; and a DFE with a single forward filter tap. Optimum burst and symbol timing recovery is addressed and several practical schemes are suggested. Simulation results are presented. The combined use of equalization and diversity reception is considered. >

Fading noise reduction in coherent OTDR

Abstract: The authors describe a reduction in fading noise which causes amplitude fluctuation on a backscattered trace in coherent OTDR (optical time-domain reflectometry). In order to reduce the fading noise, the optical frequency of the DFB-LD (distributed feedback laser diode) is changed by changing the LD temperature during the integrations of the backscattered signals. At the same time, the state of polarization of the launched signal pulses is changed. The amplitude fluctuation has been reduced to as low as 0.06 dB. Measurements were performed on a 10 km length of conventional single-mode fiber. >

Uncoded and coded performance of MFSK and DPSK in Nakagami fading channels

Abstract: The author presents uncoded and coded performance results for noncoherent M-ary frequency-shift keying (MFSK) and differentially coherent binary phase-shift keying (DPSK) in a slow nonselective Nakagami-m (1960) fading channel. He gives simple expressions for the asymptotic slopes of probability of bit error for large signal-to-noise ratio and shows that the effective order of diversity compared to an uncoded Rayleigh channel is the product of two parameters, one for the channel and one for the code. He also compares the uncoded Nakagami-m results to those of the Rician channel in order to show performance differences between these two generalized fading channel models. >

Analysis of 16-APSK modulation in AWGN and Rayleigh fading channel

Abstract: The error probability for fully differential 16-APSK in a Rayleigh fading channel has been evaluated theoretically and through the use of extensive computer simulation. The optimum ring ratio and detection thresholds have been determined and the results indicate that 16-APSK can be considered as a potential candidate for spectrally efficient digital transmission over practical mobile fading channels.

Symbol-aided plus decision-directed reception for PSK/TCM modulation on shadowed mobile satellite fading channels

Abstract: The symbol-aided (SA) synchronization concept developed by Moher and Lodge (1989) is applied to the MSAT channel modeled with a shadowed Rician process. Simulation data demonstrate that it can track the severe phase jitter encountered on the fading channel free of the false lock which plagues conventional techniques. The algorithm multiplexes known symbols into the data stream, establishing an absolute reference free of decision errors that is used to estimate the fading phase. An improvement to the SA algorithm which extracts phase information from the data-bearing symbols is proposed. It is found that the new technique is more effective for larger K. The improved algorithm is referred to as symbol-aided plus decision-directed (SADD) phase estimation. A system employing SADD phase estimation, trellis-coded modulation, interleaving, and amplitude weighting within the Viterbi decoder yielded the best BER performance on the shadowed MSAT channel considered. >

Delay spread measurements for the digital cellular channel in Toronto

Abstract: This paper describes a set of measurements performed in five existing cells in the Toronto area, in order to assess the impact of multipath propagation on the performance of the CTIA IS-54 digital standard. A sweeping correlator apparatus with a 0.1 mu s resolution and 910 MHz carrier frequency was used. Generally, the measurements exhibit multipath propagation with considerably smaller excess delays than some reported recently but are consistent with earlier results obtained in the U.S. and Europe. In view of the results, it appears that the long delay (high selectivity) problem has been somewhat over-emphasized in the past, at least for areas like Toronto. Problems associated with short delays (flat fading), on the contrary, appear to have been underestimated, although these would be typical in any dense urban environment. >

Single-block filter for antenna duplexing and antenna-switched diversity

Abstract: A single-block ceramic filter (102) is coupled to two antennas (142 and 144) for providing both antenna duplexing and antenna-switched diversity in a duplex radio transceiver (100). One antenna (142) is coupled by the filter (102) to a transmitter (132), and both antennas (142 and 144) are switchably coupled by the filter (102) to a receiver (130) by diversity control circuitry (101) in response to a diversity control signal (137). A microcomputer (134) in the transceiver (100) is coupled to the receiver (130) for monitoring the received signal strength (135). When the received signal strength (135) drops in level indicating that the signal being received on one of the antennas (142 or 144) has become degraded due to fading or other interference, the microcomputer (134) changes the binary state of the diversity control signal (137) for switching the receiver (130) to the other one of the antennas (142 or 144 ).

Direct-sequence spread-spectrum parallel acquisition in nonselective and frequency-selective Rician fading channels

Abstract: This work presents the performance of the direct-sequence spread-spectrum (DS-SS) parallel acquisition system, previously proposed by the authors (1989, 1990), for nonselective and frequency-selective Rician (i.e. specular plus Rayleigh) fading channels. The acquisition system utilizes a bank of parallel I-Q noncoherent matched filters for the search mode, and a coincidence detector for the verification mode. The probabilities of detection and false alarm are derived and the mean and variance of the acquisition time are evaluated as a measure of the system performance. The nonselective channel is a Rayleigh fast fading channel, while the frequency-selective channel model is the so-called wide sense stationary uncorrelated scattering (WSSUS), selective only on frequency. These channels are typical for aircraft-satellite and line-of-sight (LOS) communications. >

Modeling and simulation of mobile satellite propagation

Abstract: Mobile satellite systems are subject to severe fading due to blockage of the line-of-sight (LOS) path by roadside vegetation. A thorough understanding of the fading effects is necessary for the design of a reliable land-mobile satellite system. Analytical and empirical models are presented for predicting fade statistics for vegetative shadowing of mobile satellite terminals. A software simulator for generating simulated fade data is also presented. A physical model relating physical path parameters to propagation model parameters is presented, and results using the model are shown. >

Estimation of time-varying digital radio channels

Abstract: In the design of block-oriented digital communication systems that must operate over time-dispersive channels, it is usually assumed that the channel is constant over the duration of a data block, even if the channel fades. The authors study the degradation in receiver performance caused by actual interblock channel variation. For tractability, attention is restricted to the case in which the channel variations are not tracked using decision-directed adaptation. The results suggest that unless the system parameters are carefully chosen, the constant-channel assumption is far from accurate. The quantitative results also offer a measure of the rate of channel time-variations, and give guidelines for deciding when those variations can be considered slow. These guidelines can be used as first-order evaluators of important system design decisions, such as total block size, training overhead, and data rate, for particular channel conditions. >

The capacity of wireless communication systems can be substantially increased by the use of antenna diversity

Abstract: For a broad class of interference-dominated wireless systems including mobile, personal communications, and wireless PBX/LAN networks, the authors show that a significant increase in system capacity can be achieved by the use of spatial diversity (multiple antennas), and optimum combining. This is explained by the following observation: for independent flat-Rayleigh fading wireless systems with N mutually interfering users, it is demonstrated that with K+N antennas, N-1 interferers can be nulled out and K+1 path diversity improvement can be achieved by each of the N users. Monte Carlo evaluations show that these results also hold with frequency selective fading when optimum equalization is used at the receiver. Thus an N-fold increase in user capacity can be achieved, allowing for modular growth and improved performance by increasing the number of antennas. >