Hsin-Piao Lin

Experimental evaluation of smart antenna system performance for wireless communications

In wireless communications, smart antenna systems (or antenna arrays) can be used to suppress multipath fading with antenna diversity and to increase the system capacity by supporting multiple co-channel users in reception and transmission. This paper presents experimental results of diversity gain, interference cancellation, and mitigation of multipath fading obtained by using a smart antenna system in typical wireless scenarios. Also given are experimental results for the signal-to-interference ratio (SIR) of two moving users, comparing different beamforming algorithms in typical wireless scenarios. All of the experiments were performed using the 900-MHz smart antenna testbed at The University of Texas at Austin.

Experimental studies of spatial signature variation at 900 MHz for smart antenna systems

A spatial signature is the response vector of a base-station antenna array to a mobile unit at a certain location. Mobile subscribers at different locations exhibit different spatial signatures. The exploitation of spatial diversity (or the difference of spatial signatures) is the basic idea behind the so-called space-division multiple-access (SDMA) scheme, which can be used to significantly increase the channel capacity and quality of a wireless communication system. Although SDMA schemes have been studied by a number of researchers, most of these studies are based on theoretical analyses and computer simulations with ideal assumptions. Not much experimental study, has been reported on spatial signature variation due to nonideal perturbations in a real wireless communication environment. The purpose of this paper is to present, for the first time, extensive experimental results of spatial signature variation using a smart antenna testbed. The results presented include the spatial signature variation with time, frequency, small displacement, multipath angle spread and beamforming performance. The experimental results show the rich spatial diversity and potential benefits of using an antenna array for wireless communication applications.

Experimental studies of space-division-multiple-access schemes for spectral efficient wireless communications

The paper presents some preliminary results of experimental studies of space-division-multiple-access (SDMA) systems for wireless communications. Although SDMA schemes and its potential capability of increasing channel capacity have been studied by a number of researchers, most of these studies are based on theoretical analyses and computer simulations. Very few real RF or microwave experiments have been conducted to study the channel propagation associated with multiple antennas and to validate various theoretical results and system designs about SDMA. The purpose of the paper is to present preliminary experimental results, based on which the authors discuss the feasibility of proposed SDMA systems and propose some new and feasible SDMA implementation schemes to increase channel capacity and improve system performance.<<ETX>>

Interference management of femtocell in macro-cellular networks

This paper investigates the impact of femtocell interference on existing macrocells with fractional frequency reuse (FFR). Instead of using complicated transmission techniques by coordinating multiple base stations (BSs), this paper adaptively configures FFR pattern to avoids interference caused by femtocells, depending on density and locations of femtocells. Simulation results show that the proposed femtocell BS deployment can effectively reduce downlink interference to macro-cellular networks.

Hybrid SADOA/TDOA mobile positioning for cellular networks

A signal attenuation difference of arrival (SADOA) scheme is proposed to combine with the time difference of arrival (TDOA) method for mobile location estimation. On the basis of ratio of distances between the mobile and base stations derived from differences of signal attenuations, each SADOA measurement yields a circle on which the mobile may lie. Meanwhile, each TDOA measurement defines a hyperbola on which the mobile may reside. The proposed hybrid SADOA/TDOA scheme uses Taylor-series expansion to linearise the circles and hyperbolas and iteratively computes the mobile position based on least-squares estimation. Without perfect path loss modelling and hardware modification, the proposed scheme reduces location errors compared with either technique separately. Simulations demonstrate encouraging performance with 50% improvement over the conventional TDOA method in shadowing and non-line-of-sight propagation environments.

Mobile location estimation based on differences of signal attenuations for GSM systems

Radio location by time advance for GSM systems had been published. However, the resolution of time advance in GSM systems is too coarse to locate a mobiles position. The paper proposes a mobile location estimation based on the differences of downlink signal attenuations. This provides a possible mobile location method if the relationship between distance and signal attenuation is determined. Then, the mobile location can be estimated from those possible locations. The error of the proposed method is much smaller than the error of the cell-ID method in the practical microcell system. The greatest advantage of this method is the non-necessity of a known and accurate path loss model and the reduction of the shadowing effect.

Multipath direction finding with subspace smoothing

Antenna arrays can be employed in mobile communications to increase channel capacity as well as communication quality via spatially selective reception/transmission at base stations. In most wireless communications systems, directions of arrival of multipath signals need to be found for spatial selective transmission. Unfortunately, due to the coherent nature of multipath signals, it is quite difficult to find their directions of arrival. In this paper, we will present a subspace smoothing algorithm for finding the directions of arrival of multipath signals based on the mobile terminal signals received at different time instances. More importantly, we will present our experimental results to demonstrate that the spatial diversity is present for slight movements of a mobile terminal and that the subspace smoothing approach is effective in real wireless scenarios. All of the experiments were performed using the smart antenna testbed at the University of Texas at Austin.

Experimental studies of SDMA schemes for wireless communications

This paper presents some preliminary results of experimental studies of space-division-multiple-access (SDMA) systems for wireless communications to expand capacity, increase coverage, and improve quality. Although SDMA schemes have been studied by a number of researchers, most of these studies are based on theoretical analyses and computer simulations. Very few real RF or microwave experiments have been conducted to validate the feasibility of various signal processing algorithms, such as direction finding and signal copy techniques. Also, no extensive experiments have been conducted to study the channel propagation associated with multiple antennas. The purpose of this paper is to present our preliminary experimental results using our recently developed antenna array testbed. We will also discuss the implications of these results on various array signal processing algorithms.

Mobile location estimation and tracking for GSM systems

This paper proposes a mobile location estimation and tracking technique for wireless communication systems. The location estimation is based on the differences of downlink signal attenuations, which are used to determine circles composed by possible mobile locations. Then the actual location is given by the intersection of the circles. The great advantages of this method are the non-necessity of a known and accurate path loss modelling and the reduction of shadowing effect. Furthermore, a mobile tracking technique via piecewise linear optimization using a simple genetic algorithm is applied to improve the locations estimation. As the results are shown, the estimation errors are much smaller than the errors from cell-ID method in a real GSM system.

An Equivalent Channel Time Variation Mitigation Schefme for ICI Reduction in High-Mobility OFDM Systems

In time-varying channel, the Doppler effect destroys the subcarrier orthogonality of orthogonal frequency-division multiplexing (OFDM) system which leads inter-carrier interference (ICI). This paper presents a time domain ICI self-cancellation method from the view of equivalent channel time variation mitigation. By utilizing the assumption of linear time-varying channel model, a set of windowing coefficients is derived to mitigate the equivalent channel time variation of the corresponding part into flat. The ICI is therefore reduced by lessening the equivalent channel time variation. The proposed window shape is found as the Franks window. The theoretical ICI power and bit error rate (BER) performance for using the proposed window shape are investigated and validated by simulation results. Comparing to Changs equivalent windowing method and the 4-step window shape for low-complex windowing method, the proposed method can provide better ICI cancellation performance given with the same criterion.