Digital Mobile Radio Channel Research Articles

A sliding-exponential window RLS adaptive filtering algorithm: Properties and applications

This paper presents a sliding-exponential window recursive least-squares (SEWRLS) adaptive filtering algorithm, which has the same fast tracking property as the sliding rectangular window RLS (SRWRLS) algorithm, but its tracking accuracy is much higher than that of SRWRLS. The initial convergence and steady-state misadjustment performances of SEWRLS, SRWRLS and exponential window RLS algorithms are analyzed. Simulation results of these algorithms are presented and compared with theoretical ones which show that the simulation results are consistent with the theoretical results. It is also illustrated that SEWRLS algorithm exhibits better numerical stability in a floating point arithmetic than a conventional SRWRLS algorithm. Applications of SEWRLS to system identification and to decision feedback equalization on digital mobile radio channels are also investigated.

Fading bit error protection for digital cellular multi-pulse speech coder

Protection of a digital multi-pulse speech coder from fading pattern bit errors common in a digital mobile radio channel is accomplished with error detection techniques which are simple to implement and require no error correcting codes. A synthetic regeneration algorithm is employed which uses only the perceptually significant bits in the transmitted frame. Separate parity checksums for line spectrum pair frequency data, pitch lag data and pulse amplitude data are added to each frame of speech coder bits in the transmitter. The bits are then transmitted through a mobile environment susceptible to fading that induces bursty error patterns in the stream. At the receiving station, the parity checksum bits and speech coder bits are used to determine if an error has occurred in a particular section of the bit stream. Detected errors are flagged and supplied to the speech decoder. The speech decoder uses the error flags to modify its output signal so as to minimize perceptual artifacts in the output speech. Separate checksums are developed for subsets of line spectrum pair (LSP) coefficients and related speech data, whereby a single subset may be error-detected and replaced, rather than an entire frame.

Estimation of unreliable packets in subband coding of speech

A robust speech communications system over a digital mobile radio channel is considered. We examine a system using 12 Kbit/s embedded subband coding of speech and rate compatible punctured convolutional (RCPC) codes with generalised Viterbi decoding for combined error detection and correction. System performance is improved by exploiting the frame-toframe redundancy in the subband energy profile used for adaptive bit allocation in the subband coder. Unreliable subband side information about the energy profile is identified through a two-stage process comprising the channel error detector and a source-based subband energy profile error detector. The subband side information deemed unreliable is replaced with the estimates based on frame-to-frame redundancy. This improved error detection capability in the subband side information allows a judicious reassignment of channel error protection bits to the subband main information, leading to an overall improvement in system performance. Using a realistic simulation model of a digital mobile radio communication system, an improvement of up to 4.4 dB in recovered speech segmental SNR is obtained at a channel SNR of 6 dB, compared with a system with only channel error protection.

Characterization of error sequences in UHF digital mobile radio channels

A statistical characterization of transmission errors for digital mobile radio links at 910 MHz in different types of urban areas is presented. Error sequences for messages of 2-s duration at a nominal rate of 20-kb/s for binary-phase-shift-keying (BPSK) demodulated transmissions have been obtained and characterized by Fritchman's channel model, including one error state and two to four error-free states. In the data analyzed, no trend has been observed in the Fritchman parameter as a function of the type of area or of vehicle speed. Typical Fritchman's parameters are established for low and intermediate error rate channels (on the order of 10/sup -3/ and 10/sup -2/ error rates) and for higher error rate channels. Results are also presented for a limited sample of quadrature-phase-shift-keying (QPSK) demodulated 40-kb/s transmissions. >

Theoretical analysis of error sequence characteristics in digital land mobile radio channels

AbstractIt is important to clarify error sequence characteristics in digital land mobile radio channels with memory for accurate performance evaluation of error control schemes such as error correction coding. This research deals with the theoretical analysis of error sequence characteristics in digital land mobile radio channels with very slow Rayleigh fading. First, the digital channel model is studied on the basis of the error generation mechanism in digital land mobile radio channels and the analytical model is simplified, assuming that the envelope fading effect is dominant and fading rate is very small. The quasi‐static approximation method is applied to simplify analysis. Numerical expressions of gap probability and joint gap probability, which are important in making the mathematical model of error process, are derived for binary PSK under the Rayleigh fading environment. The calculated result from the anlytical model and the experimental measured result from the laboratory experiment are shown to agree well when the effect of fading variation is small.

Estimation of Gilbert's and Fritchman's models parameters using the gradient method for digital mobile radio channels

A series of error sequence measurements in a typical mobile radio channel has been conducted to study the time distribution of transmission errors. E.N. Gilbert's (1960) and B.D. Fritchman's (1967) digital communication channel models are shown to describe adequately the observed error bursts occurring in the mobile radio channel. An estimation method based on the gradient algorithm is proposed for computing the parameters values of both models from the experimentally derived error distributions. A digital mobile radio channel simulator based on Gilbert's and Fritchman's models with the parameter values as estimated using the gradient technique is presented. >

Simulated throughput efficiency of TD-SAW ARQ system in digital mobile radio

Computer simulation results for the throughput efficiency of stop-and-wait automatic-repeat-request with time diversity reception (TD-SAW ARQ) are presented for digital mobile radio channels. It is shown that the efficiency η in the TD-SAW ARQ system does not decrease as much as in the SAW ARQ system, when the maximum Doppler frequency fD becomes large.

Burst error performance encountered in digital land mobile radio channel

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.

Characteristics of a digital mobile radio channel

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.

Distribution of burst error lengths in Rayleigh fading radio channels

A method for measuring burst errors which occur in v.h.f. or u.h.f. digital land mobile radio channels is described. Then it is noted that the cumulative distribution of burst error lengths in a Rayleigh fading channel is approximately governed by the log-normal distribution.