Self-operating adaptive MLSE for digital mobile radio

A new method for adaptive MLSE is proposed to overcome the effect of error propagation on the performance of the conventional method that uses training data and decision data alternately to assist adaptive channel estimation. Simulation results demonstrate the effectiveness of the new method under severe channel environments.<>     

Adaptive equalization for TDMA digital mobile radio

Adaptive equalization for a TDMA (time-division multiple-access) digital cellular system is discussed. A survey of adaptive equalization techniques that includes their performance characteristics and limitations and their implementation complexity is presented. The design of adaptive equalization algorithms for a narrowband TDMA system is considered. It is concluded that, on the basis of implementation complexity and performance in the presence of multipath distortion and signal fading, MLSE (maximum-likelihood sequence estimation) and DFE (decision feedback equalization) are viable equalization methods for mobile radio     

A UHF channel simulator for digital mobile radio

A device to simulate the channel propagation characteristics of the ground mobile environment at UHF has been designed, built, and characterized. This device provides a flexible, easily changed set of simulated channel characteristics, which allows the performance of a mobile radio unit to be evaluated in the laboratory under controlled conditions. The channel simulator was designed to simulate mobile platform speeds up to 675 mi/h and multipath components having differential delays approaching 10 µs. The channel simulator can provide up to four easily selectable, independently fading, multipath components, having calculated time delay spreads of up to 3.5 µs. The adjustable fading bandwidths and the exceptional long delays were implemented using the relative new signal processing technologies of charged-coupled devices and surface wave devices. Envelope statistics such as fading distributions and level crossing rates produced by the channel simulator show excellent agreement with theoretical prediction and documented experimental data.     

A simple digital fading simulator for mobile radio

An instrument capable of simulating the multipath fading associated with VHF/UHF mobile radio channels is described. The implementation is simple, flexible, economical, and allows the fading waveforms to be repeated exactly. The control section, the quadrature modulator, and the method by which the simulator was tested are described. Level crossing rate and probability distribution function measurements over a wide range of Doppler frequencies indicate good agreement with theoretical results     

Microwave propagation measurements for mobile digital radio application

Measurements over a variety of urban and terrain conditions were made using a spread-spectrum waveform centered at 1370 MHz. Chip rates of 10 and 20 MHz were used, giving high time-delay resolution. The transmitter end of the measurement link was elevated and fixed while the receiver was mobile. A detailed analysis of the multipath structure was made for various terrain conditions. This included distributions of the number and spacing of individual multi-path components as a function of amplitude threshold and also distribution of total delay. Thresholds are referenced to both average signal level and the maximum level in each pulse interval. Some instances of spatial variation of the received signal are also presented.     

An adaptive MLSE receiver for TDMA digital mobile radio

The authors present a simulation study of an adaptive receiver, based on the concept of maximum-likelihood sequence estimation (MLSE), which compensates for the heavy selective distortions caused by multipath propagation. The receiver includes a matched filter and a modified Viterbi processor and is suitable for implementation in a digital form. It operates adaptively, in a training mode at the beginning of each burst, as well as in a tracking mode during message detection. This makes the receiver robust both to fast Doppler shifts and to a large frequency offset. Simulation results are presented which show the performance in different multipath environments, with echo delay in excess of 20 μs and vehicle speed up to 250 km/h     

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.     

Performance of a new adaptive diversity-equalisation for digital mobile radio

A new adaptive diversity-equalisation suitable for mobile radio signal transmission in the mobile radio fast fading environment is proposed. Adaptivity of the diversity-equalisation is enhanced by employing the recursive least-squares algorithm (RLS) with exponential weighting and two newly proposed techniques: bi-directional equalisation (BDE) and transmitter time-diversity (TDD). Computer simulation shows that QPSK transmission performance equivalent to that of four-branch diversity can be obtained with small impairment.<>     

BER analysis of convolution coded QDPSK, in digital mobile radio

The bit error rate (BER) performance of convolutional coded quaternary differential phase-shift keying (QDPSK) with Viterbi decoding is theoretically investigated in Rayleigh fading environments. The probability density functions of the path and branch metric values of Viterbi decoding are derived. The BERs after decoding due to additive white Gaussian noise and cochannel interference are theoretically analyzed. Rate 1/2 codes and their symbol punctured high-rate codes are considered, and the symbol positions for deletion to minimize the BER after decoding are presented for the codes with a constraint length K=3-7. It is shown that Viterbi decoding considerably reduces the desired signal-to-interference power ratio as well as the signal energy per information bit-to-noise power spectrum density ratio necessary to achieve a certain BER. The spectrum efficiency of the cellular mobile radio system, achievable by the use of the symbol punctured codes, is also evaluated     

Channel estimation and compensation for preamble-assisted DAPSKtransmission in digital mobile radio system

This paper presents a novel channel estimation and compensation technique, combining fading estimation and frequency offset estimation, for preamble-assisted differentially encoded amplitude- and phase-shift keying (DAPSK) transmission in a digital mobile radio system. The received preamble symbols are used to estimate the combined distortions caused by multipath fading and frequency offset. The detected data symbols as well as preamble symbols are used to enhance the accuracy of the estimation process. The combined distortions can be compensated in the data symbols at the same time. The storage-delay time of the estimation process is only several symbols long. A series of computer simulations have been carried out to investigate the bit error rate performance of the preamble-assisted 16-DAPSK signals with uncertain frequency offset in the frequency-nonselective and frequency-selective Rayleigh fading channels     

Matched filter performance bounds for diversity combining receiversin digital mobile radio

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 new modulation scheme for multitransmitter simulcast digital mobile radio communication

Many mobile radio communication systems require digital signaling simultaneously from multiple transmitters from the viewpoints of efficient frequency utilization and simplified receiver construction. A new modulation scheme intended to realize simulcast digital transmission is described. In this method, a modulation signal is made by superimposing a specific wave on an ordinary rectangular digital wave. The two conditions necessary for obtaining a diversity effect equivalent to the maximal-ratio combining are mathematically derived. The specific waves for the various transmitters may be sinusoidal waves having a frequency equal to the original bit rate, and phases which differ from one transmitter to another. In order to clarify the feasibility of this technique, experimental simulation tests have been performed in the 900-MHz band in a two-transmitter configuration. It is proved that this modulation scheme, in comparison with the carrier-frequency offset strategy, not only gives the same amount of improvement in digital signaling in an overlapped area, but also makes the allowable carrier frequency drift more than ten times larger.     

Burst error correction in high-speed digital mobile radio communications

In high-speed digital mobile radio communication, transmission performance is severely degraded by frequency-selective fading caused by the delay time spread of multipath propagation. Forward error correction (FEC) is one of the most effective techniques to improve transmission quality. The authors evaluate the effect of FEC on burst errors under frequency-selective fading. The FEC effect with diversity reception and interleaving is also investigated.<>     

Optimum combining in digital mobile radio with cochannel interference

This paper studies optimum signal combining for space diversity reception in cellular mobile radio systems. With optimum combining, the signals received by the antennas are weighted and combined to maximize the output signal-to-interference-plus-noise ratio. Thus, with cochannel interference, space diversity is used not only to combat Rayleigh fading of the desired signal (as with maximal ratio combining) but also to reduce the power of interfering signals at the receiver. We use analytical and computer simulation techniques to determine the performance of optimum combining when the received desired and interfering signals are subject to Rayleigh fading. Results show that optimum combining is significantly better than maximal ratio combining even when the number of interferers is greater than the number of antennas. Results for typical cellular mobile radio systems show that optimum combining increases the output signal-to-interference ratio at the receiver by several decibels. Thus, systems can require fewer base station antennas and/or achieve increased channel capacity through greater frequency reuse. We also describe techniques for implementing optimum combining with least mean square (LMS) adaptive arrays.     

Capacity of wideband TDMA digital mobile radio cellular schemes

Wideband TDMA digital cellular mobile radio schemes can offer a capacity similar to that of analogue modulation schemes. To find the optimum coding to be used a balance must be made between bandwidth, power and number of channels. A comparison is made for an idealised system using QPSK transmission and biorthogonal coding. Coding with a bandwidth expansion of 2.7 is shown to provide a good capacity at low carrier/noise ratio and an acceptable error rate.     

A periodic switching diversity technique for a digital FM land mobile radio

A simple and efficient diversity technique is proposed for use in a digital FM land mobile radio communication system. This technique receives two RF signals periodically by switching two antenna branches at a rate moderately higher than the bit rate. The improved bit error rate (BER) performance resulting from the use of diversity is shown to be the effect of transforming the probability density function of the signal energy per bit to noise power density ratio to a sharper distribution. Laboratory simulation test results show that in a Manchester-coded frequency-shift keying (FSK) system with a bit rate of 600 bit/s and a frequency deviation of ±5 kHz, the diversity gain at an average BER of 1 × 10^-3is about 10 dB for an optimum switching rate of about 2 kHz. This diversity improvement is also verified by the field test performed in a suburban area.     

Evaluation of 16 kbit/s digital voice transmission for mobile radio

In this paper 16 kbit/s digital voice transmission with conventional channel spacing of 25 kHz, employing a 16 kbit/s adaptive delta modulation (ADM) coder-decoder (CODEC) is evaluated. The main characteristics of narrow-band digital FM modulation schemes, such as tamed FM, Gaussian filtered minimum shift keying (GMSK), four-level FM and phase locked loop-quaternary phase shift keying (PLL-QPSK), are compared by laboratory tests. Digitized voice quality in a digital channel incorporating a 16 kbit/s ADM CODEC and GMSK coherent detection was compared with voice quality of a conventional analog FM channel. Bit error ratio (BER) performance is shown to depend primarily on demodulation schemes. Digital voice quality is inferior to that of analog voice with an opinion score difference of about 0.5 in fading environments. This kind of digital voice transmission will be applicable for those systems that require high security at an expense of speech quality.     

Error rates for narrow-band digital FM with discriminator detectionin mobile radio systems

Error rates for a narrowband digital FM system with discriminator detection in a land mobile radio channel characterized by fading and Doppler frequency shift are obtained using a new analytical method. The fading rate is assumed to be much slower than the bit rate so that signal distortion through a narrow bandpass filter can be analyzed via a quasistatic approach. An experiment was conducted using a Rayleigh fading simulator, and the error rate measurements are shown to agree well with the calculated results, except at high SNR (signal-to-noise ratio) where an irreducible error rate was observed     

Postdetection selection diversity effects on digital FM land mobile radio

The postdetection selection diversity effects on a binary digital FM system are theoretically analyzed in the fast Rayleigh fading signal environment encountered in the typical UHF or microwave land mobile radio channels. Both differential and discriminator detections are considered for demodulation of digital FM signal. The average error rate is presented by a simple closed form including both effects of Rayleigh envelope fading and random FM noise. A few examples of numerical results for minimum shift keying (MSK) are graphically presented.     

The performance of rate-compatible punctured convolutional codesfor digital mobile radio

The unequal error protection capabilities of convolutional codes belonging to the family of rate-compatible punctured convolutional codes (RCPC codes) are studied. The performance of these codes is analyzed and simulated for the first fading Rice and Rayleigh channels with differentially coherent four-phase modulation (4-DPSK). To mitigate the effect of fading, interleavers are designed for these unequal error protection codes, with the interleaving performed over one or two blocks of 256 channel bits. These codes are decoded by means of the Viterbi algorithm using both soft symbol decisions and channel state information. For reference, the performance of these codes on a Gaussian channel with coherent binary phase-shift keying (2-CPSK) is presented. A number of examples are provided to show that it is possible to accommodate widely different error protection levels within short information blocks. Unequal error protection codes for a subband speech coder are studied in detail. A detailed study of the effect of the code and channel parameters such as the encoder memory, the code rate, interleaver depth, fading bandwidth, and the contrasting performance of hard and soft decisions on the received symbols is provided