Showing papers on "Fading published in 1981"

Characteristics of a digital mobile radio channel

Abstract: A field test has been made in order to better understand the digital mobile radio channel. At the mobile receiver (450 MHz, 1200 bits/s) recordings were made of the digital signal and the field strength. These recordings were later analyzed by a computer. Some existing models for digital channels have been tested. Theoretically motivated probability density functions for the fading envelope have also been considered. A new model for the digital channel is proposed. This model is a memoryless binary symmetric channel (BSC) with field strength dependent crossover error probability. This model fits very well to the recorded data.

MMSE equalization of interference on fading diversity channels

Abstract: Adaptive equalization is used in digital transmission systems with parallel fading channels. The equalization combines the diversity channels and reduces intersymbol interference due to multipath returns. When interference is present and correlated from channel to channel, the equalizer can also reduce its effect on the quality of information transfer, important applications for interference cancellation occur in diversity troposcatter systems in the presence of jamming, diversity high frequency (HF) systems which must cope with interfering skywaves, and space diversity line-of-sight (LOS) radio systems where adjacent channel interference is a problem. In this paper we develop the general formulation for minimum mean square error (MMSE) equalization of interference in digital transmission diversity systems. The problem formulation includes the use of available receiver decisions to assist in MMSE processing. The effects of intersymhol interference are included in the analysis through a critical approximation which assumes sufficient processor capability to reduce ISI effects to levels small enough for satisfactory communication. The analysis also develops he concept of additional implicit or intrinsic diversity which results from channel multipath dispersion. It shows how the MMSE processor sacrifices diversity to suppress interference even when the interference arrives in the main beams of the receiver antenna patterns. The condition of near synchronous same-path interference is also addressed. Because the spatial angle of arrival of the interference may result in delay differences between interference signals in different antenna channels, interference delay compensation may be required. We show that this effect is compensated for with a small number of appropriately spaced equalizer taps.

A new approach to high-capacity digital mobile Radio

Abstract: Space diversity (adaptive phased-array antennas) is an effective weapon against the cochannel interference encountered in cellular mobile radio systems. High-order diversity, and hence, strong interference suppression, can be achieved with modest hardware complexity by using time-division retransmission. With this technique, which is especially well-suited to digital modulation methods, the adaptive signal processing required for space diversity can be performed at just one end of the communication link, namely, the base station. At the other end (the mobile unit) only a single-element antenna is needed. Moreover, the use of coherent phase-shift keying in such a system allows simple RF circuity, because the adaptive processing is done at baseband. In the context of cellular mobile radio, the combination of space diversity, time-division retransmission and 120-degree corner illumination of each cell can yield a reliable communication channel even in the presence of intercell interference, Ray leigh fading (both flat and frequency-selective), and shadow fading. The use of these techniques allows approximately 130 two-way channels per cell (at 32 kb/s each) to be accommodated in the 40-MHz bandwidth of the 850-MHz mobile radio band.

Effects of Multipath Propagation on Digital Radio

Abstract: This paper summarizes experimental work at Bell Laboratories in the field of multipath propagation as it affects digital radio. Field and laboratory measurements are presented on commercial-quality high-speed digital radio systems working in the 4, 6, and 11 GHz common carrier bands. Circuits and equipment used in these systems are all state of the art, but no attempt has been made to describe them in any detail. Modulation types are offset-4 PSK, conventional 8 PSK, and 16 QAM. Outages (BER > 10^{-3}) vastly exceeding accepted objectives are measured on unprotected radio hops. A very effective method for reducing outages has been the use of a space diversity combiner followed by an amplitude slope equalizer. Results from single, as well as dual-polarized digital radio relay systems, are presented. The paper also describes the laboratory measurement of an equipment "signature" as a means to compare different digital radio designs in their sensitivity to multipath fading.

More on the Multipath Fading Channel Model

Abstract: Selective fading due to muitipath on a typical narrowband line-of-sight microwave radio path is represented with a simplified three-ray model. This model provides a satisfactory description of the frequency transfer characteristics of almost all fades observed during three separate propagation experiments. Data were obtained for a 30 MHz bandwidth at 6 GHz using both a standard horn reflector and a parabolic dish, and for a 20 MHz bandwidth at 4 GHz using the horn. The statistics of the parameters of the model were found to be substantially the same for all three cases. The incidence of selective fading scales, within a factor of two, with the incidence of fading at a single frequency, for 1-month observation intervals.

Burst error performance encountered in digital land mobile radio channel

Abstract: Burst error characteristics are studied by using a Rayleigh and Nakagami-Rice fading simulator. Burst error length distribution estimated with fade duration is described. Thus burst length shortening by means of dual frequency diversity is a promising candidate in order to introduce safely forward error correction (FEC) coding into digital land mobile communication systems.

Error rate performance of noncoherent detection of duobinary coded MSK and TFM in mobile radio communication systems

Abstract: Two continuous phase constant envelope modulation schemes are considered for use in digital mobile radio communication systems. These two schemes, duobinary coded minimum shift keying (MSK) and tamed frequency modulation (TFM), use partial response signaling to achieve efficient power spectrum. Therefore, they are suitable candidates for the application of digital data transmission via mobile radio where spectrum efficiency is an important consideration. The mobile communication channel is characterized by fast Rayleigh fading and cochannel interference resulting from the reuse of the channels. The error rate performance of duobinary coded MSK and TFM has been studied under these environments with noncoherent detection. A closed form expression for the probability of error of duobinary coded MSK with discriminator detection has been derived and evaluated for different cases of fast and slow fading and cochannel interference. The probability of error of duobinary coded MSK and TFM with differential detection has been calculated by numerical integrations for different cases of slow and fast fading and cochannel interference.

Frequency selective propagation effects on spread-spectrum receiver tracking

Abstract: This paper presents results of investigations of frequency selective scintillation effects, due to transionosphetic propagation, on the performance of spread-spectrum receivers. Emphasis is placed on pseudonoise (PN) direct sequence systems. Pertinent features of the disturbed ionospheric propagation channel are described together with the characteristics of resulting satellite signal scintillations. Effects of frequency selective scintillations on PN spread-spectrum code correlation and tracking are illustrated in some detail. The design of the receiver PN code tracking loop is an important factor in determining the significance of these effects. Of the designs that have been investigated, a dedicated (non-time-shared) noncoherent code tracking loop appears to be least susceptible to losing code lock in the presence of frequency selective scintillation. Possible design modifications to further enhance the robustness of PN code tracking are discussed.

Performance of a frequency-hopped differentially modulated spread-spectrum receiver in a Rayleigh fading channel

Abstract: The performance of a spread-spectrum receiver previously described by the authors is analyzed in detail. Performance curves are given for a wide range of mobile radio channel conditions, including multipath distortion and correlated fading. The use of optimal filters to combat the former and space diversity to combat the latter are investigated. Some errors in the authors' earlier papers are corrected. The form of space diversity suggested has the unusual feature that it can be employed at the fixed station site only and provide diversity in both communication paths, i.e., to and from the mobile. Degradation due to fading and other aberrations is shown to be in the range 3-6 dB for typical mobile channel conditions. In a nonfading channel the receiver is suboptimum by about 3 dB.

Calculation of error probability for MSK and OQPSK systems operating in a fading multipath environment

Abstract: A technique is given for determining the error-rate performance of a class of digital communication systems operating with fading multipath interference. The approach uses a truncated series to represent the fading process and takes into account the effects of intersymbol interference caused by linear filtering in the receiver or channel. Application of the method to the cases of minimum shift keyed (MSK) and offset quadriphase shift key (OQPSK) systems reveals a slight performance advantage for MSK.

Block Error Probability for Noncoherent FSK with Diversity for Very Slow Rayleigh Fading in Gaussian Noise

Abstract: Expressions are derived for the average probability of block error P_{f}(M, N) , i.e., the probability of more than M errors in a block of N symbols. The average is formed over the inatantaneous receiver signal-to-noise ratio after combining. Binary frequency-shift keying with noncoherent detection is transmitted over a very slow nonselective Rayleigh fading channel and received in additive Gaussian noise. Diversity is used and the branch fading is assumed independent. Formulas for P_{f}(M, N) are derived for both ideal selection combining and maximal ratio combining.

Analysis of maximum-likelihood sequence estimation performance for quadrature amplitude modulation

Abstract: This paper considers maximum-likelihood sequence estimation (MLSE) for quadrature amplitude modulation (QAM) signaling at rates approaching several baud/Hz. In this regime, intersymbol interference and possibly cross channel coupling are the dominant transmission impairments. We derive the structure of a detector that optimally accommodates both impairments. A bit error rate performance bound is found, and the concept of an error state transition matrix is introduced to facilitate the analysis. We explore a modulation scheme wherein cross-channel coupling is intentionally introduced, and find that it improves detection efficiency. The use of MLSE may be an important consideration for power and spectrally efficient digital radio systems, either terrestrial or satellite, since rates approaching the Shannon limit may be attainable without channel coding, and frequency selective fading is handled in an optimum manner.

Microwave Radio obstruction fading

Abstract: The occurrence of obstruction fading on line-of-sight microwave radio paths can require tower heights that are substantially larger than those needed for transmission in a normal atmosphere In this paper, we show that the occurrence probability of obstruction fading can be estimated from the probability distribution of positive vertical refractivity gradients The occurrence of positive refractivity gradients is related to general meteorological variables in a companion paper The work summarized is part of a recent effort that has resulted in a new tower-height design method where tower-height requirements are quantitatively determined from transmission performance requirements

Digital Modulation in Rayleigh Fading in the Presence of Cochannel Interference and Noise

Abstract: The error rate performance of several digital modulation methods in a fading environment and in the presence of one cochannel interference and AWG noise is analyzed. A unique formula emerges for different PSK systems. It is shown how this information can be used in determining acceptable degradation caused by cochannel interference in the case of a digital mobile communication system.

Diversity reception system

Abstract: To reduce fading in a diversity receiver, the signals received on two spaced-apart antennas are added together without phase shift while amplitude of the sum is above a certain threshold. When the signal falls below the threshold, the signal from a squelch detector causes the sum amplitude to be stored and than a 180° time delay to be inserted between one of the antennas and the adding circuit, after which time, the new sum amplitude is compared with the stored signal amplitude. If the new signal has increased or remained the same, the 180° phase shift remains in place but, if it has deteriorated, it is removed and the circuit reset.

Space-diversity broad-band digital radio receiver

Abstract: In order to reduce bit errors resulting in a space-diversity broad-band digital radio receiver from selective fading, an amplitude dispersion detector detects the amplitude dispersion caused, if any, by the selective fading in each of IF signals produced by the space-diversity reception and produces a detector output having an amplitude variable with the detected amplitude dispersion and used in combining the IF signals. The detector may comprise three band-pass filters having different narrow passbands in the IF band to produce filter outputs with amplitudes dependent on first-order and second-order dispersion components of the amplitude dispersion. Alternatively, the detector may comprise a band-pass filter having a narrow passband repeatedly variable throughout the IF band at a lower frequency to produce a filter output with an amplitude variable with the first-order and the second-order dispersion components and, above all, with a notch-shaped dispersion component.

Modeling multlipath fading responses using multitone probing signals and polynomial approximation

Abstract: We show in quite a general way that highly accurate modeling of multipath fading responses is possible using low-order complex polynormals. This applies to all terrestrial radio systems in the channelized common carrier bands below 15 GHz, where channel widths are 40 MHz or less. The context of the study is a new multipath experiment being conducted in New Jersey over a 23-mile path at 11 GHz. The transmitted signal consists of up to nine tones in a 40-MHz bandwidth. These tones are coherently processed, sampled, and digitized in the receiver and recorded, during fading events, for later off-line reductions. Simple routines can be used to determine polynomial coefficients from these recorded data. This paper describes the signal processing and data reduction methods and analyzes them to assess the accuracy of polynomial fitting. The analysis uses a mean-square error measure and assumes a representative form for the underlying response function. Our results predict that the vast majority of multipath fading responses can be accurately approximated over bandwidths of 40 (62) MHz using first- (second-) order complex polynomials.

Spectrally Efficient Communication via Fading Channels Using Coded Multilevel DPSK

Abstract: This paper investigates the use of multilevel differential phase-shift keying with forward error-correcting coding as a technique for achieving limited bandwidth, yet reliable communication over a class of fading channels. Error rate bounds based on the cutoff rate are computed to show that multilevel modulation can be used to allow significant coding gains, while maintaining or reducing the bandwidth of the transmitted signal.

Detection of digital signals transmitted over a known time-varying channel

Abstract: A technique has recently been developed whereby it is possible, without undue complexity and without any adaptive linear prefiltering, to achieve near-maximum-likelihood detection of a sampled digital signal, where there is intersymbol interference extending over several samples of the signal. It is, however, important here that the channel impulse response does not undergo large changes with time, since, when it does, a considerable increase in the complexity of the system may be required to maintain correct operation. The paper describes a development of the detection process whereby correct operation is achieved with a relatively simple system, even when the channel introduces severe frequency-selective fading of the type sometimes experienced over HF radio links. Results of computer-simulation tests are presented, showing the tolerance of a synchronous serial data-transmission system to additive white Gaussian noise, when a 4-point quadrature amplitude modulated signal is transmitted at 2400 bit/s over a model of an HF radio link, with two independent Rayleigh fading sky waves and frequency spreads of 0.5, 1 and 2Hz, and when the novel detection process is used at the receiver. Correct estimation of the channel is assumed throughout.

Single sideband receiver with intersyllabic gain correction limit control

Abstract: In a single sideband radio receiver utilizing a limited, pilot-controlled, gain correction arrangement (38) to compensate for radio channel fading, a signal (RECT CAP) representing receiver audio output level is utilized (97) during intersyllabic quiet intervals to increase the limiting threshold on the gain correction signal to suppress noise and interference.

A stochastical model of the urban UHF radio channel

Abstract: A random model for urban ultrahigh frequency (UHF)- multipath propagation is proposed. The model uses an optical analogy, which can be well justified at these high frequencies. Several results are obtained. The transfer function and its autocorrelation properties are investigated. Also the time domain behavior is studied. The model is finally tested against experimental data from a field trial made in the town of Linkoping, Sweden.

Site diversity measurements of low-angle fading and comparison with a theoretical model

Abstract: Since 1974, several experiments have been conducted at Eureka, Canada (80° N, 86° W), to investigate the properties of lowangle fading. For a variety of site diversity configurations, it was found that neither a horizontal spacing of 500 m nor a vertical spacing of 20 m provided good diversity performance. For a vertical spacing of 180 m, however, fading at the two sites was essentially uncorrelated. Lowangle fading was more prevalent and severe in the summer and absent when the atmosphere was wellmixed or during radiative temperature inversions. A model of the refractive index structure of the atmosphere has been developed which predicts these characteristics and shows that deep fades need not result from interference fading.

Comparison of Performance of 16-ary QASK and MSK Over a Frequency Selective Rician Fading Channel

Abstract: The probability of error of both a 16-ary QASK system and an MSK system, each operating over a Rician fading channel, is considered. The channel model has two key features: short term variations are modeled by making the channel frequency selective, and long term variations are accounted for by allowing the specular component of the received signal to fade according to a Rayleigh density. The improvement gained by using maximal-ratio diversity combining is derived, as well as the degrading effects of an adjacent channel. Finally, a comparison between MSK and 16-ary QASK operating over the same channel is presented, and it is shown that MSK results in a smaller probability of error.

Modulation techniques for land mobile radio

Abstract: PM systems with 12.5 kHz and 25 kHz channel spacing have been compared with 12.5kHz AM and 5kHz SSB systems, and the effect of applying companding techniques to SSB as well as to PM has been investigated. The systems have been compared with respect to audio signal quality, speech intelligibility, spectrum utilization and applicational flexibility. The traditional - uncontrollable - field trials have been replaced by laboratory tests using advanced propagation simulation techniques, and the analysis has included pulse noise susceptibility and performance with multipath fading. The overall conclusion is that 12.5 kHz PM provides the best balance between transmission quality, spectral economy and applicational flexibility.

Probability distributions for ratios of fading signal envelopes and their generalization

Abstract: This paper describes statistical characteristics of the ratios of fading signal envelopes. The Rayleigh fading is contained as a special case. We derive the ratios of the fading signals following the m distribution, generalized Nakagami-Rice distribution and generalized Nakagami-Hoyt distribution. An extension to the case of multiple variable distribution is attempted under some conditions. The results in this paper are given in simple functional forms appropriate for theoretical analysis. The distributions of the ratios of fading signal envelopes can be applied to a wide range of problems of radio wave interference such as evaluation of the ratio of desired signal to unwanted interference signal (DU ratio) and of degradation of cross-polarization discrimination (XPD) in a microwave transmission channel, determination of frequency reuse distance and of modulation types for mobile communications, and analysis of ratio detection in radar technology.