Wireless Channel Simulation Research Articles

A complex baseband platform for spatial-temporal mobile radio channel simulations

Joint spatial-temporal signal processing has been recognized as the key to reducing the effects of the intersymbol and cochannel interference seen in very high bit-rate mobile radio communications systems. Developing hardware simulators that can simulate mobile radio propagation scenarios in time and space domains is essential for evaluating the real-time performance of spatial-temporal signal processing schemes. This paper outlines a complex baseband platform developed for spatial-temporal mobile radio channel simulations. The platform consists of a complex baseband fading/array response simulator, a digital signal processor (DSP) board, and a general-purpose parameter estimator that uses systolic array implementation of the recursive least square (RLS) algorithm. Results of experiments conducted using the developed platform are presented to confirm the proper operation of the system.

Enhanced correlated shadowing generation in channel simulation

An enhanced method of generating a shadow fading for mobile radio channel simulation is proposed. It is verified that simulated samples satisfy the properties of variance, autocorrelation and cross-correlation. The proposed channel simulation method can be utilized for evaluating the performances in mobile communication systems where the exchange of interactive information between uplink and downlink channels is required.

Modelling and Simulation of Direction-Selective and Frequency-Selective Mobile Radio Channels

Summary This paper presents a new approach to the design of simulation models for spatio-temporal mobile radio channels, which are important for the design, optimization, and test of antenna array based mobile communication systems. The proposed spatio-temporally correlated simulation model is completely deterministic and enables the emulation of the desired fading behaviour with comparably little effort. Moreover, it can be configured flexibly to simulate not only different kinds of theoretical channels with given spatial, temporal, and frequency correlation properties, but also real-world fading channels for mobile communication systems with adaptive antennas.

Performance study of an adaptive dual antenna handset for indoor communications

The focus of the paper is the design and evaluation of adaptive diversity for mobile handsets. Usually, diversity principles are optimised for base stations. However, for mobile handsets new concepts must be developed to include the size and power consumption constraints. A new modelling approach is introduced, which combines indoor radio channel simulations with circuit design parameters. This enables the inclusion of key system parameters, such as the speed of the user, the scanning speed of the antenna beams and the number of phase shifts. The radio channel simulations are based on a high-order finite-difference algorithm using the Berenger absorbing boundary condition to truncate the computational domain. The algorithm is found to be efficient and accurate enough to simulate very large structures. The analysis has resulted in an optimal design of an adaptive dual antenna handset, which combines received signals (fixed beam) while scanning the environment at the same time (scan beams). A prototype is evaluated with the numerical modelling tools and a measurement set-up. The performance is close to that of a perfect equal gain combiner.

Error floors of digital FM in simulcast and Rayleigh fading

A digital FM system in the presence of simulcast interference and/or Rayleigh fading exhibits performance floors, where increased signal strength does not reduce the error rate. Results obtained both in the lab, using a radio channel simulator, and in the field show this performance. Observation of signal waveforms indicates that the bit-error rate (BER) floor is a result of FM clicks during deep fades. We present a mathematical model that can be used to calculate the BER floors. The model is based on the calculation of the click shape as a function of amplitude imbalance, frequency offset, and postdetection filter impulse response. The Rayleigh fading is approximated with a two-signal simulcast by substituting the RMS Doppler spread for the simulcast frequency offset. A good match between calculated and measured results is obtained. It is found that the symbol errors caused by simulcast and Rayleigh fading can be treated as independent, and their effect is cumulative.

Modeling, analysis, and simulation of nonfrequency-selective mobile radio channels with asymmetrical Doppler power spectral density shapes

What terrestrial and satellite land mobile radio channels have in common is that they are usually assumed to be nonfrequency selective. For such fading channels, a highly flexible analytical model is presented. Our model takes into account short-term fading with superimposed long-term lognormal variations of the local mean value of the received signal. It is assumed that the introduced complex stochastical process, which is used for modeling the short-term fading behavior, has a Doppler power spectral density with an asymmetrical shape. Closed solutions are presented showing the influence of asymmetrical Doppler power spectral density shapes on the statistical properties of the proposed channel model. A numerical optimization procedure is shown for the optimization of the model parameters in order to fit the statistics of the analytical model [amplitude probability density function (PDF), level-crossing rate (LCR), and average duration of fades (ADF)] to measurement results of an equivalent land mobile satellite channel for light and heavy shadowing environments. Finally, from the analytical model, an efficient simulation model is derived that enables the simulation of such realistic land mobile satellite channel scenarios on a digital computer.

Adaptive MAPSD algorithms for symbol and timing recovery of mobile radio TDMA signals

Dual-mode adaptive algorithms with rapid convergence properties are presented for the equalization of frequency selective fading channels and the recovery of time-division multiple access (TDMA) mobile radio signals. The dual-mode structure consists of an auxiliary adaptive filter that estimates the channel during the training cycle. The converged filter weights are used to initialize a parallel bank of filters that are adapted blindly during the data cycle. When the symbol timing is known, this filter bank generates error residuals that are used to perform approximate maximum a posteriori symbol detection (MAPSD) and provide reliable decisions of the transmitted signal. For channels with timing jitter, joint estimation of the channel parameters and the symbol timing using an extended Kalman filter algorithm is proposed. Various methods are described to reduce the computational complexity of the MAP detector, usually at the cost of some performance degradation. Also, a blind MAPSD algorithm for combining signals from spatially diverse receivers is derived. This diversity MAPSD (DMAPSD) algorithm, which can be easily modified for the dual-mode TDMA application, maintains a global set of MAP metrics even while blindly tracking the individual spatial channels using local error estimates. The performance of these single-channel and diversity MAPSD dual-mode algorithms are studied via computer simulations for various channel models, including a mobile radio channel simulator for the IS-54 digital cellular TDMA standard.

Computer simulation of radio channels using a harmonic decomposition technique

Based on a harmonic decomposition of stochastic processes, a new approach for approximating the time variant radio channel impulse response, given an arbitrary Doppler power density spectrum is presented. The basic idea is to use a Fourier expansion with uncorrelated coefficients to approximate the actual radio fading process. An upper bound on the mean square error between the actual and approximated fading as a function of the number of terms in the expansion is also given. The proposed method is particularly adapted for simulating broadband radio channels. >

A novel approach to modeling and efficient simulation of frequency-selective fading radio channels

A multipath radio channel concept based on channel orthogonalization techniques in two inner product spaces is presented and compared to the conventional approach of modeling each individual Rayleigh or Rice fading multipath ray individually. The proposed simulator can be applied to linear amplitude/phase modulation and linear fading channels including Nyquist filtering. It is shown to be a good approximation to the conventional model in the case of tight rolloff factors. Channels having quasi-or truly continuous delay profiles can now be properly represented at significantly reduced computational complexity. The proposed simulation concept leads to the lowest level of complexity being achieved for the prevailing channel and noise conditions on a particular channel. >

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. >

An investigation of delta modulation of speech for mobile radio

A characteristic of a mobile radio channel is the occurrence of correlated signal fading that results in burst errors. The use of adaptive delta modulation (ADM) based on explicit transmission of the quantizer step size was proposed earlier for speech communication over such a channel. Two other variable step-size delta modulation (VSDM) schemes are presented, and their performance in a mobile radio environment is discussed. One of them is the constant factor delta modulation that uses one-bit memory and produces fast and instantaneous step-size changes. The other is the digitally controlled delta modulation (DCDM) that incorporates a new step-size adaptation strategy using seven bits of memory. In some cases, bit scrambling has been used. This is equivalent to scrambling (spreading out in time) the, clustered errors. Computer simulations providing values of coder parameters for satisfactory signal-to-noise ratios for band-limited speech signals and Gaussian noise are described. New hardware realizations are given that allow those parameters to vary smoothly for a wide range of sampling frequencies. Results of informal listening tests obtained with a mobile radio channel simulator are included. It is shown that for mobile radio, DCDM, as expected, is the better of the two coders. This is because it does not sacrifice its overload performance for the sake of error resistance.