Modeling burst channels using partitioned Fritchman's Markov models

Discrete models based on functions of Markov chains (also referred to as hidden Markov models or finite-state channel (FSC) models) have been used to characterize the error process in communication channels with memory. One important property of these models is that the probability of any observed sequence can be expressed as a linear combination of the probability of a finite set of sequences of finite length, the so-called basis sequences. In this paper, we express the parameters of a class of FSC models as a simple function of the probability of the basis sequences. Based on this approach, we propose a new method for the parameterization of the Fritchman (1967) channel with single-error state as well as the interesting cases of Fritchman channels with more than one error state and the Gilbert-Elliott channel ((GEC) nonrenewal models). To illustrate the method, FSC models for the nonfrequency-selective Rician fading channel are presented. The number of states and the probability of state transitions are estimated for a given set of fading parameters     

Estimation of time-varying digital radio channels

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     

Performance of digital code tracking loops for direct-sequencespread-spectrum signals in mobile radio channels

The performance of first- and second-order non-coherent digital delay lock loops (DDLL) for direct-sequence spread-spectrum (DS-SS) signals is investigated in the mobile radio environment. The mobile radio channel is first characterized by Rayleigh fading and Doppler shift. A closed-form expression for the timing error transition probability density function of the Chapman-Kolmogorov (C-K) equation is proposed. The probability density function of the steady-state timing error for the first- and second-order DDLL is obtained by solving the C-K equation numerically, and the results are confirmed by computer simulations. Furthermore, the mean time to lose lock (MTLL) of the first-order loop is evaluated, and some numerical results and simulation results are reported. Finally, the steady-state timing error and MTLL of the first-order loop for DS-SS signals in the log-normal fading environment are also presented, and the results are compared with those of Rayleigh and AWGN channels     

High-speed digital transmission experiments in 920 MHz urban and suburban mobile radio channels

Field experiments have been conducted to clarify the feasibility of introducing a newly developed Gaussian filtered minimum shift keying (GMSK) modulation method into 920 MHz mobile radio systems, and have been performed in the Tokyo urban, and the Yokosuka suburban areas.     

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     

Equalization of digital radio channels with large multipath delayfor cellular land mobile applications

A new maximum a posteriori (MAP) equalizer is proposed for digital radio links affected by large multipath delays. The “sparse” nature of the channel, where a few nonzero powerful taps are spaced by many negligible taps, is exploited to achieve a complexity proportional to the number of nonzero taps. When the channel is time-varying, an efficient nonlinear Kalman like channel estimator is employed to track only the nonzero taps     

Error-rate performance of digital FM with differential detection in land mobile radio channels

Error-rate performance of a digital FM with differential detection in the presence of both thermal Gaussian noise and cochannel interference is theoretically analyzed in the fast Rayleigh fading environment encountered in the typical UHF or microwave land mobile radio channels. The temporal correlation of the fades is included in the performance analysis. The error probability is presented by a simple closed form for the important situations where both effects of Gaussian noise and cochannel interference predominate in causing errors. Finally, a comparison with the other detection schemes, e.g., discriminator and coherent detections, is given.     

ARQ error control for fading mobile radio channels

In this paper, we study the correlation properties of the fading mobile radio channel. Based on these studies, we model the channel as a one-step Markov process whose transition probabilities are a function of the channel characteristics. Then we present the throughput performance of the Go-Back-N and selective-repeat automatic repeat request (ARQ) protocols with timer control, using the Markov model for both forward and feedback channels. This approximation is found to be very good, as confirmed by simulation results     

Capture models for mobile packet radio networks

The probability q_i of successful reception in a nonfading mobile radio channel with i contending mobiles transmitting to a central base station is studied for a number of different capture and spatial distribution models. It is shown that a generalized capture model can be used to estimate q_i's for a simplified example system which uses noncoherent frequency shift keying modulation. This model can be applied to other systems as well. An example of the use of the q_i 's in the throughput evaluation of a finite population slotted ALOHA system is given. In most practical systems, the mobiles cannot get arbitrarily close to the base station. The effect of this constraint on q_i is examined. Finally, the dependence of the capture probability for a test mobile on its distance from the base station is obtained     

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.     

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     

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.     

Statistical characterization of non-WSSUS mobile radio channels

In this paper, statistical characterizations of mobile radio channels that do not satisfy the assumption of wide-sense stationary uncorrelated scattering (WSSUS) are discussed, most importantly the local scattering function. The framework presented is particularly suited for doubly underspread non-WSSUS channels. Application examples and measurement results illustrate its practical usefulness.     

A flexibly configurable spatial model for mobile radio channels

The application of multiple directive antennas, i.e., directional diversity, may lead to significant capacity benefits in cellular mobile radio systems. A flexibly configurable statistical channel model for mobile radio systems using directional diversity is presented. The parameters of this model, which is available as a FORTRAN77 program, can be easily adjusted to various propagation areas such as, for example, rural, urban, microcellular, and picocellular environments. Therefore, the model is well suited to perform simulations, evaluations, and comparisons of mobile radio systems. Simulation results concerning a code-division multiple-access (CDMA) mobile radio system which uses multiple directive base station (BS) antennas in combination with joint detection illustrate the application of the presented channel model     

Block-error probabilities for mobile radio channels derived fromchaos equations

A novel approach to model mobile radio channels (in particular, DECT ones) is introduced, using nonlinear equations proposed by chaos theory. A simple procedure to extract meaningful statistics from strange attractors' trajectories is presented, and some results well matched with those obtained by standard generative models are shown. Although introducing a quasi-deterministic behavior, the chaotic equations seem to be sufficient to mimic the apparently random error patterns exhibited by mobile radio channels. An initial attempt is made to sketch guidelines for selecting the chaotic parameters     

Correlation model for shadow fading in mobile radio channels

A novel continuous-time correlation model for shadow fading is proposed. It reveals accordance with the level crossing theory of Gaussian processes, and can be adopted to describe and simulate the shadow fading precisely. Alternatively, it can be employed to define the utilisation limits of the Markov discrete-time exponential model     

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.     

Packet data transmission over mobile radio channels

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