Boon Chong Ng

Sensor-array calibration using a maximum-likelihood approach

High-resolution array processing algorithms for source localization are known to be sensitive to errors in the model for the sensor-array spatial response. In particular, unknown gain, phase, and mutual coupling as well as errors in the sensor positions can seriously degrade the performances of array-processing algorithms. This paper describes a calibration algorithm that estimates the calibration matrix consisting of the unknown gain, phase, and mutual-coupling coefficients as well as the sensor positions using a set of calibration sources in known locations. The estimation of the various parameters is based on a maximum likelihood approach. Cramer-Rao lower-bound (CRB) expressions for the sensor positions and the calibration matrix parameters are also derived. Numerical results are shown to illustrate the potential usefulness of the proposed calibration algorithm toward better accuracy and resolution in parametric array-processing algorithms.

Space-time modems for wireless personal communications

This article reviews space-time modem technology for mobile radio applications. We begin with motivations for the use of space-time modems and then briefly discuss the challenges posed by wireless propagation. Next, we develop a signal model for the wireless environment. Channel estimation, equalization, and filtering techniques for space-time modems in the forward and reverse links are then discussed. Finally, we review applications of space-time modems to cellular systems and discuss industry trends.

Empirical Near Ground Path Loss Modeling in a Forest at VHF and UHF Bands

Near ground radio wave propagation is examined in a tropical plantation experimentally at VHF and UHF bands. The propagation loss with forest depth is empirically analyzed using an integrated model combining both the foliage induced effect and the ground effect. Several well-known empirical foliage models are compared and analyzed. It is observed that the fitted ITU-R model taking into account the ground reflection, can provide a close approximation to the path loss in a tropical palm plantation. However, the accuracy of this model becomes poor when lateral wave dominates in the VHF band. A modification to the ITU-R model is then proposed taking into consideration the lateral wave effect. The modified model is verified using measured and published data, and found to have higher accuracy for large foliage depth in the VHF band as compared to the existing empirical foliage models.

STUDY OF PROPAGATION LOSS PREDICTION IN FOREST ENVIRONMENT

A comprehensive review of radio wave attenuation in forest environments is presented in this paper. The classic analytical methods of propagation loss modeling and prediction are described flrst. This provides information on the physical processes that the radio waves undergo while propagating through a forest. The focus of this paper is on the review and summary of the experimental work done in this area and the development of empirical propagation loss prediction models. The propagation loss variation due to external factors such as antenna height-gain, depolarization, humidity efiect etc. are examined and discussed individually. In view of current research work done in this area, some possible future work is proposed to improve the performance of radio links in forest environment.

A structured channel estimator for maximum-likelihood sequence detection

This letter describes a channel estimator using known prior information about the transmit and receive filters. It is shown that the composite channel lies in the subspace of the transmit and receive filter impulse responses. A structured linear channel model is then developed that is linearly parametrized by an unknown vector. To illustrate the potential usefulness of such an approach, the estimated structured channel is used in a multisensor and oversampled maximum-likelihood sequence estimation (MLSE) receiver.

Joint angle and delay estimation (JADE) for signals in multipath environments

We propose a subspace-based approach to estimate the angles-of-arrival (AOAs) and delays of multipath signals from one or more digitally modulated sources arriving at an antenna array. Our approach uses a collection of estimates of a consistent space-time vector channel. The channel estimates from different time slots in TDMA systems have constant path AOA and delay over the observed periods, but variable fade amplitudes between the slots. With this approach, we are able to estimate more parameters than the number of antennas. The Cramer-Rao bound for the parameter estimates is also provided.

The Effects of Tropical Weather on Radio-Wave Propagation Over Foliage Channel

This paper investigates the dynamic property of a tropical forested channel due to the weather effect on very high frequency (VHF) and ultrahigh frequency (UHF) radio-wave propagation. In this paper, continuous-wave (CW) envelope fading waveforms are recorded over a period of 50 s with static antennas. This paper focuses on the analysis of the combined effect of wind and rain, which is often encountered in a tropical forest. The induced temporal effects are discussed and compared with theoretical models. It is found that the distribution of temporal fading components resembles a Rician distribution function. Its Rician K factor gradually decreases as the strength of either wind or rain increases due to the movement of the forest components.

PATH LOSS MODELING FOR NEAR-GROUND VHF RADIO-WAVE PROPAGATION THROUGH FORESTS WITH TREE-CANOPY REFLECTION EFFECT

This paper presents a new methodology to model the near- ground short-range propagation loss in forested areas at the VHF and the UHF band. The path loss modeling is performed by an integration of the foliage induced efiect and the efiect from the radio- wave re∞ection/re∞ections. The analysis shows that the re∞ection from the dense tree canopy is important for short-range propagation at the VHF band and therefore, cannot be ignored. When taking into consideration the contribution from the possible tree-canopy re∞ection, the modeled path loss is reduced by more than 15dB in the VHF band. A good agreement of the modeled path loss with the measured loss in tropical forested areas is achieved.

Space-time processing for fast fading channels with co-channel interferences

In this paper, we propose the use of space-time (ST) adaptive algorithms for fast fading TDMA channels and co-channel interference reduction in the uplink of a wireless mobile radio system. The ST structure consists of an antenna array at the base station followed by a fractionally spaced equalizer per channel. In particular, we consider the joint ST adaptive equalization and co-channel interference reduction on the IS-54 channel. Two adaptive algorithms in updating the ST weights are considered: the RLS algorithm and the Gerchberg sliding window algorithm. Numerical results under urban and hilly environments show significant improvement of the BER over conventional single channel differential demodulator.

Correlation and Set Size Bounds of Complementary Sequences with Low Correlation Zone

The correlation lower bounds, as well as the set size upper bounds, of low-correlation-zone complementary sequences are presented in this paper. They can be treated as an extension of Tang-Fan-Matsufuji bounds in and , which considered only the conventional (non-complementary) low-correlation-zone sequence sets.