The time-varying impulse response of rapidly fading mobile communication channels is expanded over a basis of complex exponentials that arise due to Doppler effects encountered with multipath propagation. Blind methods are reviewed for estimating the bases' parameters and the model orders. Existing second-order methods are critiqued and novel algorithms are developed for blind identification, direct, zero-forcing equalization and minimum mean square error (MMSE) equalization by combining channel diversity with temporal (fractional sampling) and/or spatial diversity which becomes available with multiple receivers. Illustrative simulations are also presented.

https://www.dtc.umn.edu/s/resources/procieee.pdf

Basis expansion models and diversity techniques for blind identification and equalization of time-varying channels

Time division multiple access (TDMA) is a classic approach to multiple access in digital cellular wireless communications systems. The authors summarize a number of frequency and time slot allocation techniques for enhancing the capacity and flexibility of TDMA-based systems. They also describe how the problems of fading, delay spread, time variability and interference affect TDMA systems, and how they may he countered and even exploited by appropriate techniques of detection, diversity, coding, adaptive equalization and slow frequency hopping (FH). It is worth emphasizing that the use of one of these techniques, slow random FH, results in a system that is in effect a hybrid of TDMA and code division multiple access (CDMA). >

/pdf/time-division-multiple-access-methods-for-wireless-personal-2wwnq0s7rs.pdf

Time division multiple access methods for wireless personal communications

Estimation and equalization of fading channels with random coefficients

The time-varying tap coefficients of frequency selective fading channels are typically modeled as random processes with low-pass power spectra. However, traditional adaptive techniques usually make no assumption on the channel's time variations and hence do not exploit this information. Kalman filtering methods are derived to track the channel by employing a multichannel autoregressive description of the time-varying taps in a decision-feedback equalization framework. Fitting a model to the variations of the channel's taps is a challenging task because the tap coefficients are not observed directly. A novel linear method is proposed in order to estimate the model parameters from input/output data. The consistency of the proposed method is shown, and some illustrative simulations are presented.

A demoodulator apparatus for a TDM radio receiver successively compares a sampled quadrature baseband signal with a known pilot symbol sequence to detect timings of correlation with a periodically received pilot symbol sequence, with a resultant correlation signal being used in generating a frame synchronizing signal as a basic timing reference signal, and with a degree of dispersion of the correlation signal being detected and used to modify generation of the frame synchronizing signal such as to minimize the effects of adverse radio signal reception conditions on that signal. In addition, demodulation of a quadrature phase-modulated carrier to obtain a quadrature baseband signal is executed by a pseudo-coherent process based on periodic estimation of a frequency offset, with a degree of compensation applied for the offset being accordingly periodically updated, making use of an automatic frequency control loop unnecessary.

Demodulator apparatus for digital radio communication receiver providing pseudo-coherent quadrature demodulation based on periodic estimation of frequency offset

It is pointed out that the future European cellular digital mobile radio system in the 900 MHz band adopts a narrowband time division multiple access (TDMA) scheme with Gaussian minimum-shift keying (GMSK) modulation and burst type transmission. Consequently, very fast adaptation methods are necessary to cope with the time- and frequency-selective distortions produced by Rayleigh and multipath fading. The authors examine a few theoretical aspects of the application of recursive least squares (RLS) adaptation algorithms to the narrowband TDMA mobile radio system and give the relevant performance results for the fast Kalman algorithm, which turns out to be suitable for the considered application. In particular, signature curves, bit error rate, speed of convergence, steady-state behavior, numerical stability, required accuracy, and hardware complexity are discussed. Linear transversal and nonlinear decision-feedback equalizers are considered. >

G. D'Aria

Papers

Fast adaptive equalizers for narrow-band TDMA mobile radio