Showing papers on "Fading published in 1989"

UHF fading in factories

Abstract: The authors detail the results of narrowband propagation measurements performed at five factories. The extensive empirical data indicate that path loss is dependent upon local surroundings and is log-normally distributed, temporal fading is Rician, and small-scale signal fluctuations due to receiver motion are primarily Rayleigh, although Rician and log-normal distributions fit some of the data. Shadowing effects of common factory equipment likely to obstruct indoor radio paths are also examined. >

Bandwidth efficient coding for fading channels: code construction and performance analysis

Abstract: The authors apply a general method of bounding the event error probability of TCM (trellis-coded modulation) schemes to fading channels and use the effective length and the minimum-squared-product distance to replace the minimum-free-squared-Euclidean distance as code design parameters for Rayleigh and Rician fading channels with a substantial multipath component. They present 8-PSK (phase-shift-keying) trellis codes specifically constructed for fading channels that outperform equivalent codes designed for the AWGN (additive white Gaussian noise) channel when v>or=5. For quasiregular trellis codes there exists an efficient algorithm for evaluating event error probability, and numerical results which demonstrate the importance of the effective length as a code design parameter for fading channels with or without side information have been obtained. This is consistent with the case for binary signaling, where the Hamming distance remains the best code design parameter for fading channels. The authors show that the use of Reed-Solomon block codes with expanded signal sets becomes interesting only for large value of E/sub s//N/sub 0/, where they begin to outperform trellis codes. >

TCMP-a modulation and coding strategy for Rician fading channels

Abstract: The authors describe TCMP, a novel modulation strategy for Rician fading channels that multiplexes a time domain pilot sequence with trellis-coded data to permit coherent detection. This technique is shown to provide remarkably robust performance in the presence of fading. It is also shown that, when choosing trellis codes for fading channels, time diversity is of greater important than asymptotic coding gain. The motivation for studying this strategy is to find signaling schemes for transmitting data at a 4.8 kb/s rate over a mobile satellite channel with 5-kHz channel spacing. >

Rayleigh fading compensation method for 16QAM in digital land mobile radio channels

Abstract: The authors propose a novel Rayleigh fading compensation method for 16 quadrature amplitude modulation (16QAM). The optimum parameters for 16QAM with a transmission rate of 16 k symbol/s, the BER (bit error rate) performance, and the interference performance are determined by computer simulation. It is shown that second-order interpolation is suitable for the fading compensation method. In the case of 16 k-symbol/s transmission, the optimum frame length is 16 symbols. The degradation due to the proposed fading compensation method is about 2 dB. The BER performance under Rayleigh fading environments at f/sub d/ >

ALOHA with capture over slow and fast fading radio channels with coding and diversity

Abstract: The effects of capture on the average system throughput and delay performance of slotted ALOHA were analyzed for slow and fast Rayleigh fading radio channels. A short-range multipoint-to-base station packet radio network is considered. It is shown that larger capture effects and thus improved network performance can be achieved with proper choice of modulation. It is also shown that the use of simple error-correcting codes improves capture. The use of selection diversity also improves the capture effect both for fast and slow fading. It is concluded that the inverse distance variability of the received signal is the main reason for the capture effect. The Rayleigh fading alone yields a very small contribution in terms of throughput; nonetheless, it helps to stabilize the system. Numerical results are presented for a slotted ALOHA system with 50 users. It is found that the maximum average throughput can be increased from about 36% to almost 60% by using channel coding and space diversity. >

A systematic approach to carrier recovery and detection of digitally phase modulated signals of fading channels

Abstract: The problem of optimal carrier recovery and detection of digitally phase modulated signals on fading channels by using a nonstructured approach is presented, i.e. no constraint is placed on the receiver structure. First, the optimal receiver is derived for digitally phase-modulated signals when transmitted over a frequency-nonselective fading channel with memory. The memory results from the fact that usually the coherence time of the channel is larger than the symbol period. Symbols adjacent in time cannot be detected independently and therefore the well-known quadratic receiver is not optimal in this case. A maximum a posteriori (MAP) detector is derived and explicitly utilizes the channel memory for carrier recovery. The derivation shows that the optimal carrier recovery is, under certain conditions, a Kalman filter. Some attractive properties of this carrier recovery unit (including the absence of hang up) are discussed. Then the error rate of several digital modulation schemes is calculated taking the performance of the filter into account. The differences in susceptibility of the modulation schemes to carrier phase jitter are specified. >

Adaptive synchronization and channel parameter estimation using an extended Kalman filter

Abstract: Unified modeling and estimation of the MD (multiplicative distortion) in finite-alphabet digital communication systems is presented. A simple form of MD is the carrier phase exp(j theta ), which has to be estimated and compensated for in a coherent receiver. A more general case with fading must, however, allow for amplitude as well as phase variations of the MD. The authors assume a state-variable model for the MD and generally obtain a nonlinear estimation problem with additional randomly varying system parameters such as received signal power, frequency offset, and Doppler spread. An extended Kalman filter is then applied as a near-optimal solution to the adaptive MD and channel parameter estimation problem. Examples are given to show the use and some advantages of this scheme. >

Mobile satellite system fade statistics for shadowing and multipath from roadside trees at UHF and L-band

Abstract: Field tests related to planned mobile satellite systems (MSS) were performed, and results that add to the existing database of propagation measurements at L-band (1.5 GHz) are described. They are considered particularly useful in that propagation effects were studied systematically with repeated and controlled runs pertaining to different path elevation angles, road types, and path geometries defining shadowing and line-of-sight modes. In addition, simultaneous L-band and UHF measurements were performed for the purpose of establishing scaling factors applicable to previous UHF (870 MHz) results. The control of the experimental parameters was made possible by using a helicopter as the source platform and a mobile van which housed the receiver. >

Direct conversion transceiver design for compact low-cost portable mobile radio terminals

Abstract: A prototype direct conversion transceiver for portable mobile radio terminals has been developed and is under evaluation. Test results are presented showing the performance of this transceiver to be equivalent to (and in some cases better than) that of existing systems. The transmitter and receiver are described, and the receiver fading performance is examined. The results presented show that a direct conversion transceiver system can be made to work at both VHF and UHF. The improved channel characteristics facilitated by digital channel filtering make this configuration particularly suited to data transmission. In addition, the integration potential of all the system components is important for the concept of a 'personal communicator'. >

Packet data transmission over mobile radio channels

Abstract: Since in mobile radio Rayleigh fading poses the main threat to accurate data transmission, a mathematical model of the dynamics of Rayleigh fading is used to explore the optimum duration of data packets. The performance criterion is the rate of information transfer through the mobile radio channel. In addition to packet size, the information rate depends on: the speed of the mobile terminal, the channel bit rate, the size of the packet header, and the fade margin of the modulation and coding techniques. In particular, attention is focused on line rates of 16 kb/s and 256 kb/s (which are representative of the rates proposed for digital mobile radio systems in North America and Europe, respectively). At 16 kb/s, the optimum packet size is approximately 17 B (8.5-ms duration). At 256 kb/s, maximum throughput occurs when the packet contains about 48 B (1.5-ms duration). The precise optimum depends on vehicle speed, header size, and fade margin. The optimum packets are considerably shorter than the 125-B packets customarily used in terrestrial and satellite systems. >

An autoregressive approach to the identification of multipath ray parameters from field measurements

Abstract: This study is concerned with using modern spectral analysis techniques to derive estimates of multipath ray parameters (delays and amplitudes) from measurements of selective fading over a wide band at microwave frequencies. The applicability of the general category of autoregressive methods is addressed, and the specific method adopted, the Prony algorithm, is described. An illustrative example in which this is applied to a typical field measurement is followed by an investigation into the capabilities and limitations of the algorithm. A statistical analysis of the method has been carried out using a database of laboratory measurements. The Prony algorithm's ability to resolve delays to better than half the Fourier limit is shown, and its tendency to yield biased estimates is studied. >

Radio communications link with diversity

Abstract: A base station 10 has two transmitting antennas 20, 22 which both transmit the same signal to a mobile station 12, the signal via antenna 22 being delayed by a time delay unit 24 to ensure de-correlation between the transmitted signals. It is unlikely that multi-path fading will occur simultaneously in both signal paths to the single antenna 24 of the mobile station wherein an equaliser 26, such as a Viterbi equaliser, decodes the originally transmitted signal from the two superimposed signals received by antenna 24. Diversity is obtained without the need for two antennas at the mobile station where space limitations would probably prevent this. The base station 10 receives the signal transmitted from the single antenna 24 at the mobile station 12 over two signal paths respectively directed to the two antennas of the base station. The signals received are de-correlated by a time delay 38 connected to one of these antennas and summed, summed 36 and passed to an equaliser 40 for decoding. A repeater station may similarly include time delays (76), (82), (Figs 7, 8), to give de-correlation between the signals it retransmits and the corresponding signals directly transmitted by the base and mobile sections.

On the symbol error probability of maximum-selection diversity reception schemes over a Rayleigh fading channel

Abstract: The symbol error probability of two selection schemes, namely, maximum signal-to-noise ratio (M gamma ) selection and maximum output (MO) selection, for M-ary multidiversity reception over a Rayleigh fading channel are discussed. The symbol error probability of the MO scheme is lower than that of the M gamma scheme. The more diversity receptions that are used, the larger is the difference. A simple expression of crossover average signal-to-noise ratio (per bit) is presented as a guideline for increasing the number of diversity receptions. >

Estimation of multipath channel response in frequency selective channels

Abstract: A novel discrete-time method is proposed for estimating the impulse response of a frequency-selective digitally modulated communication channel. The received signal is first demodulated and sampled and then the fourth-order cumulants of the resulting discrete-time sequence are estimated. The method estimates the channel impulse response from the complex cepstrum of the aforementioned fourth-order cumulants (i.e. tricepstrum). The method depends only on the second- and fourth-order statistics of the transmitted sequence and is capable of reconstructing nonminimum-phase impulse responses. Monte Carlo simulation results demonstrate the effectiveness of the method, its low sensitivity to observation noise, and its improved performance in terms of probability of error or the reconstructed transmitted sequence. >

Direct sequence spread spectrum parallel acquisition in a fading mobile channel

Abstract: The acquisition performance of a direct-sequence spread-spectrum system is investigated in a fading mobile environment. A parallel acquisition scheme is applied to make up for the effect of the fading environment on key parameters such as the probability of detection, the probability of false alarm, and the mean acquisition time. An expression for the mean acquisition time is derived in terms of detection, false alarm, and missing probabilities. These probability expressions are derived in both a nonfading and fast-fading AWGN (additive white Gaussian noise) channels, and the mean acquisition time is shown for nonfading, slow-fading, and fast-fading channels. It is shown that in a nonfading channel, the designer should choose the MF (matched filter) length as large as practically possible and then cover the rest of the uncertainty region with parallel MFs. The same applies for the slow-fading channel. In fast-fading channels the contrary is the right choice; for better performance the designer needs to increase parallelism at the cost of MF length. >

Performance of interleaved trellis-coded differential 8-PSK modulation over fading channels

Abstract: The performance of trellis-coded differential octal phase-shift keying (coded 8-DPSK) with differentially coherent detection and soft-decision Viterbi decoding is investigated. A suitable receiver is presented whose signal processing is based on Nyquist signaling, requiring only one complex sample per modulation interval. Symbol synchronization and automatic frequency control are performed in a decision-directed way. Bit-error-rate (BER) performance over Gaussian, Rayleigh, and Rician channels is determined by means of computer simulations. The performance of coded 8-DPSK on the Gaussian channel is shown for a four-state convolutional trellis code. The unquantized outputs of up to three symbol detectors with delays of 1, 2, and 3 symbol periods are used for metric computation. The coding gain which includes losses due to timing and frequency synchronization errors is found to be 2.5 dB at BER=10/sup -5/ with respect to uncoded 4-DPSK. Much larger gains are achieved for fading channels if interleaving is applied. Using an eight-state trellis code the performance is determined on Rayleigh and Rician channels for various Doppler spreads and interleaver sizes. >

Coded modulation for mobile radio

Abstract: Interleaved block-coded modulation with built-in time diversity is used for fading channel applications. Various modulated block codes of various dimensionalities are disclosed, each built up from an M-DPSK constellation. The signal points making up each codeword are re-ordered by an interleaver in a way which matches the interleaving to the block code, thereby increasing the effective size of the interleaver.

On the modeling of fading multipath indoor radio channels

Abstract: Multipath profiles obtained from propagation measurements done on several manufacturing floors and college campus laboratories at 910 MHz are analyzed to model the arrival of the paths, the respective amplitudes of the paths, and the received power. The discrepancies between the empirical distribution of the arriving paths and Poisson arrivals are discovered. The modified Poisson process is shown to fit the arriving paths closely. Rayleigh, Weibull, Nakagami, log-normal, and Suzuki distributions are considered as potential models for the amplitude of the arriving paths. The path amplitudes are shown to closely follow a log-normal rather than a Rayleigh distribution. The delay power spectrum is shown to fit an exponential function closely. The statistics of RMS multipath delay spread and the values of the distance/power law gradient are also computed and compared for these experimental sites. The interarrival times of the signals were modeled by the Weibull distribution. A model for the distribution of the number of signals was presented using the modified Beta distribution. Finally, for the data with no threshold at the receiver a suitable model for the distribution of the gain coefficients is found to be the log-normal distribution. >

Reduction of synchronous fading effects by time hopping of user slots in TDMA frames

Abstract: A method for the reduction of synchronous fading effects in a time-diversity multiple-access communications system, in which the data to/from a particular user is to appear in a particular assigned slot, varies the position of the time slot assigned to that particular user, so that the slot has a different position in each successive frame in which that assigned slot appears. The position may be set relative to a frame feature, such as its start time and the like. Selected-schedule patterns may be used, with the pattern either being preset at the various stations or being sent to the stations prior to use, as part of a preamble, in-band supervisory signaling and the like.

Diversity selection using coding in a portable radio communications channel with frequency-selective fading

Abstract: It is shown that the degradation in the performance of wideband digital transmission systems due to intersymbol interference can be reduced by utilizing diversity selection using coding A BCH (31, 21, 5) double-error-correcting code and QPSK (quadrature-phase-shift-keying) modulation with coherent detection are used for this study The probability of word error is taken to be the measure of system performance Analytic expressions are derived for the word error probability; these expressions simplify the simulation processes It is found that a diversity selection algorithm using coding can reduce the floor of word error probability, and hence increase the dynamic range of received power before the word error probability reaches that floor >

Error rate performance of digital FM mobile radio with postdetection diversity

Abstract: An analysis of bit error rate (BER) in a binary digital FM system with postdetection diversity is presented. Expressions for the average BER due to additive white Gaussian noise (AWGN), random FM noise and delay-spread in the multipath channel are derived for reception using differential demodulation (DD), and frequency demodulation (FD) assuming independent fading signals. Calculated results for MSK show that the BER performance is strongly dependent on the RMS-delay/bit-duration ratio and that the delay-spectrum shape is of no importance when the receiver predetection filter product is optimized for the effect of AWGN. The effect of fading correlation on the diversity improvement is also analyzed for a two-branch case with multiplicative Rayleigh fading signals. Expressions for the average BER due to AWGN and random FM noise are derived. Calculated results are shown for the average BER due to random FM noise assuming a horizontally spaced antenna system at a mobile station. It is shown that the use of small antenna spacings leads to a diversity improvement greater than that obtainable for the case of independent AWGN. >

An efficient modulation/coding scheme for MFSK systems on bandwidth constrained channels

Abstract: The performance of a bandwidth-efficient multiple-tone modulation scheme for M-ary frequency-shift keying (MFSK) is presented. The use of balanced incomplete block (BIB) designs is proposed to form the signaling frames. On each symbol interval the modulator selects a group of elements from a BIB design and divides its energy into the orthogonal waveforms corresponding to these elements. The multiple-tone FSK scheme based on these block designs is shown to increase greatly the bandwidth efficiency of a conventional M-ary FSK system. An implicit diversity is incorporated in the modulation scheme. Thus, a performance improvement comparable to that obtained by using time or frequency diversity is shown on a Rayleigh fading channel and also on an interference channel with partial-band Gaussian noise. The multiple-tone scheme based on this design is compared to a multiple-tone scheme based on Hadamard matrices suggested by J.F. Pieper et al. (1978). It is shown that similar performance is achieved on a fading channel, while an advantage close to 4 dB is obtained for the proposed scheme on an AWGN (additive white Gaussian noise) channel. >

Clipped diversity combining for channels with partial-band interference. II. Ratio-statistic combining

Abstract: For pt.I see ibid., vol.COM-3, no.12, p.1320 (1987). Ratio-statistic combining is proposed for mitigating partial-band interference in systems with diversity transmission and frequency-hop signaling. Systems with noncoherent demodulation and binary orthogonal signaling are covered. The partial-band interference is Gaussian, and Gaussian quiescent noise is included in the analysis to account for wideband noise sources. The exact probability of error is found for a receiver using ratio-statistic combining, and this is compared to the exact error probabilities for receivers with optimum combining with perfect side information, clipped-linear combining, the ratio-threshold test with majority-logic decoding, and self-normalization diversity combining. Numerical results are also given for a frequency-hop system which uses ratio-statistic combining for channels with Rayleigh fading and partial-band interference. It is determined that ratio-statistic combining is an excellent diversity combining scheme for systems with partial-band interference and fading. >

Dual polarization demodulation with cross polarization cancellation

Abstract: In a digital demodulation system (FIGS. 2, 3, 4, 7, 8) in a dual polarization radio system, having a first demodulator (4) and a first A/D converter (14) for a main signal, a second demodulator (5) and a second A/D converter (15c) for an interference signal (cross polarization signal), a transversal filter (18) for providing a compensation signal for cancelling an interference component in the main signal depending upon the demodulated interference component; the second A/D converter (15c) for providing the interference component in digital form is operated with a clock signal (102) regenerated in the main signal branch (4, 14) so that system operates correctly even when the main signal and cross polarization signal are in an asynchronous condition. The A/D converter (14, 15c, 105) is provided with an attenuator (104) for compressing amplitude of an input signal to avoid saturation of the circuit (105) in case of fading and/or distortion, and a ROM table (107) for correcting digitalized output after equalization and/or cancellation. Tap coefficients of the transversal filter (18) are obtained by a control (19) through a ZF method when inter-symbol interference is small, and a MLE method when inter-symbol interference is large. Further, an integration constant for the coefficients is adaptive depending upon inter-symbol interference.

Architecture and implementation of an efficient and robust TDMA frame structure for digital portable communications

Abstract: The architecture and implementation of a TDMA (time-division multiple access) digital portable communication system using short bursts are considered. The authors describe an efficient and robust TDMA frame structure that requires very low overhead for burst demodulation and synchronization while being robust against both fading and carrier frequency offset. Based on this frame structure, an experimental radio link was implemented, using a hardware architecture that is suitable for digital signal processing and VLSI implementation. Performance results are presented. >

Probability of error for selection diversity as a function of dwell time

Abstract: An expression is derived for the probability of error of an Nth-order selection diversity system for the case where the receiver is forced to swell on one channel for several symbols before being allowed to make a decision regarding the best channel. It is found that the time-varying nature of a fading channel causes significant degradation of the probability of error when the dwell time becomes longer than about 10% of the inverse of the fading bandwidth of the channel. The onset of degradation is a function of the signal-to-noise ratio and of the order of diversity. Specific probabilities of error are calculated for differential phase-shift keyed modulation (DPSK). However, the calculations can be done for any other modulation technique. >

The effects of delay spread on 2-PSK, 4-PSK, 8-PSK and 16-QAM in a portable radio environment

Abstract: Computer simulation results are presented of a study of delay spread on digital modulations with different constellations in a quasi-static multipath radio channel. Unfiltered 2-, 4-, and 8-PSK and 16-QAM with a rectangular signaling pulse are compared first, followed by 4- and 16-QAM with a raised-cosine Nyquist pulse. The bit error rate performances averaged over fading samples under the influence of the intersymbol interference caused by delay spread are compared for modulations of different levels. It is found that 4-level modulation is the most desired method for both performance and implementation in a quasistatic, frequency-selective fading radio channel. Both the spectral and the power efficiencies can be enhanced using Nyquist signaling pulses. >

Optimal decoding in fading channels: a combined envelope, multiple differential and coherent detection approach

Abstract: A maximum likelihood sequential decoder for the reception of digitally modulated signals with single or multiamplitude constellations transmitted over a multiplicative, nonselective fading channel is derived. It is shown that its structure consists of a combination of envelope, multiple differential, and coherent detectors. The outputs of each of these detectors are jointly processed by means of an algorithm. This algorithm is presented in a recursive form. The derivation of the new receiver is general enough to accommodate uncoded as well as coded (e.g. trellis-coded) schemes. Furthermore, reduced complexity, near-optimal versions of the algorithm are also presented. Performance evaluation results for a reduced-complexity trellis-coded QPSK (quadrature phase shift keying) system have demonstrated that the proposed receiver dramatically reduces the error floors caused by fading. At E/sub b//N/sub 0/=20 dB the new receiver structure results in bit error rate reductions of more than three orders of magnitude compared to a conventional Viterbi receiver, while being reasonably simple to implement. >

Multi-level quadrature amplitude modulator system with fading compensation means

Abstract: A multi-level quadrature amplitude modulator system that can be used in communication channels in which there is intense fluctuation has a transmitter section that is provided with a frame synchronizer that inserts a reference symbol into each group of message symbols, and a receiving section that is provided with an estimator for the transmission path distortion in the message symbols on the basis of distortion found in the reference symbols. The receiving section is also provided with a compensator for the distortion thus described.

Fast carrier-tracking coherent detection with dual-mode carrier recovery circuit for digital land mobile radio transmission

Abstract: A new type of carrier recovery circuit suitable for coherent detection in digital land-mobile radio transmission is studied experimentally. A dual-mode carrier recovery (DCR) circuit is proposed that adaptively selects one of two phase locked-loop operation modes: the conventional mode or the adaptive carrier-tracking (ACT) mode. The ACT mode digitally controls the reference phase, and enables instantaneous phase tracking in burst signal operation and/or in a fast fading environment. The DCR coherent demodulator performance is identical to that of conventional coherent detection in the low-CNR region, and to that of differential detection in the high CNR region where the ACT mode is selected. >