A device and method of adjusting an equalizer in a digital communications receiver in response to relative velocity between the communications receiver and a communications transmitter. A radio frequency receiver receives a radio frequency signal from the communications transmitter through a communications channel, and an analog to digital converter digitizes a first portion of the radio frequency signal during a first time period, and generates a first data signal made up of digital samples. The analog to digital converter also digitizes a second portion of the radio frequency signal during a second time period and generates a second data signal, also made up of samples. A channel estimator estimates characteristics of the communications channel in response to a portion of the first data signal, and generates a first channel estimate in response thereto. Finally, a processor determines an estimated velocity of the communications receiver relative to the communications transmitter, weights the first channel estimate in response to the estimated velocity, and generates a revised channel estimate in response to the weighting of the first channel estimate and the second data signal.

Doppler bandwidth dependent estimation of a communications channel

Characterization of the channel, in terms that are useful for system design and performance evaluation, is addressed. An attempt is made in the current effort to link physical features of the environment to some deterministic characteristics of the channel, and to a better characterization of the statistical behavior. The predictions are then validated by an extensive measurement campaign, with an impulse channel sounder, covering many major urban areas in the US. Previous results and analysis are then revisited, to further support the findings. The delay profile is found to consist mostly of distinct peaks, shifting in delay as the mobile unit travels, and resulting from reflections from large structures. These are relatively stable compared to the leading peak, which consists of the direct path and signals from a cluster of neighboring scatterers. Signal bandwidth effects and the effectiveness of different diversity means are discussed in terms of the channel characteristics. >

Channel characteristics for land cellular radio, and their systems implications

An adaptive equalization technique using forward error control coding is described and evaluated. Computer simulation is used to compare the FEC-assisted adaptive equalization to conventional adaptive equalization. The system model employed is based on the proposed digital cellular communication standard for North America. The FEC-assisted adaptive equalization technique performs better than the conventional equalization method at low and moderate Doppler frequencies with the same level of redundancy in the transmitted data. For systems employing both FEC coding and adaptive equalization, the FEC-assisted adaptive equalization method successfully combines these two functions while using the redundancy required for only one of them. In other words, training required for adaptive equalization can be provided at no extra cost in terms of data redundancy in a system using FEC coding and vice versa. >

Forward-error-control (FEC)-assisted adaptive equalization for digital cellular mobile radio

This paper introduces a space-time decision feedback equalizer whose coefficients are recursively renewed on the basis of the modified affine projection algorithm (APA). Despite its effectiveness against a rank-deficiency problem, the complexity of the conventional APA utilizing the sample matrix inversion is considerable. Furthermore, the shift invariance property could not be exploited in the case of a space-time decision-directed equalizer (STDE) application. To overcome such challenges, this paper introduces an APA employing a hyperplane projection scheme to reduce computational complexity without any degradation in its convergence characteristic and tracking capability. The performance of the proposed equalization methodology is verified by showing its superior bit error rate behaviors, fast convergence property, and reduced computational complexity by performing adaptive STDE under the single-input-multiple-output wireless multipath fading channel.

Space–Time Adaptive Decision-Directed Equalizer Based on NLMS-Like Affine Projection Algorithm Using Iterative Hyperplane Projection

A two-stage maximum likelihood sequence estimation (MLSE) technique is developed for improving digital transmission performance over the proposed Telecommunications Industry Association (TIA) North American TDMA digital cellular radio channels The authors also investigate two simple retransmission schemes, retransmission before and after channel decoding, for possible implementation in the radio link protocol (RLP) within the proposed standard (IS-54) Simulation results indicate that the stage-one MLSE equalizer is about 3-4 dB away from the optimal performance in the wide signal-to-noise range at a vehicle speed equal to 60 mph The results also show that retransmission before coding is a more effective way to improve the system performance >

A two-stage maximum likelihood sequence estimation technique and its application to data transmission protocol in digital cellular radio

Adaptive equalization techniques designed to improve bit error rates (BERs) degraded by multipath propagation in mobile communication systems are addressed. Two kinds of equalizers are used in a simulation: the adaptive lattice decision feedback equalizer (L-DFE) and maximum-likelihood sequence estimation (MLSE). RLS (recursive-least-squares) algorithms are used to update the equalizers. The choices are based on the fact that a DFE has little noise enhancement and a RLS algorithm converges fast and has good tracking capability. The sensitivities of these equalizers to receiver velocity and signal-to-noise ratio are tested for a two-ray multipath channel where the ratio of powers of the rays can vary. Results show that these two kinds of equalizers have similar performances. >

W. Huang

Papers

A comparative study of two adaptive equalizers for mobile radio