Hsueh-Jyh Li

A Generalized Interpretation and Prediction in Microwave Imaging Involving Frequency and Angular Diversity

The Fourier transform and back-projection methods are shown to be equivalent in both the monostatic and the bistatic cases. However, the back-projection method provides more physical insight into the image formation process. To predict the microwave image of an object given an imaging scheme and specified frequency and angular window, one can first predict the range profile of each aspect and then back-project the range prifle along the corresponding directions into the image plane graphically. The results obtained are illustrated by examples.<<ETX>>

Applicability of ray-tracing technique for the prediction of outdoor channel characteristics

When the ray-tracing technique is employed to calculate outdoor channel characteristics, the input parameters, such as the geometrical layout of streets, buildings, and surroundings, are usually much simplified. In this paper, whether the ray-tracing technique with simplified input parameters can be applied to predict outdoor channel properties is examined and discussed. The delay profiles, the average propagation losses, and the root mean square delay spreads obtained by simulations and by wide-band measurements are compared and analyzed. It is found that the respective delay profiles can be very different. This indicates that the scattering mechanisms in the real environment are different from those considered in simulations. However, the respective average propagation losses and root mean square delay spread can match very well. Nevertheless, the neglect of the distant buildings and incorrect input of the layout geometry can result in a large difference in the calculated statistical parameters.

ANTENNA MEASUREMENTS IN THE PRESENCE OF MULTIPATH WAVES

In this paper antenna measurements in the presence of mul- tipath waves are discussed. Methods are proposed to diagnose the degradation sources of a compact range measurement system operat- ing at a frequency much lower than the designed frequency range. A range-gating technique is employed to improve the capability of the compact range measurement system. With this technique, the field re- sponses over a bandwidth for each rotation angle are coherently mea- sured, and the fast Fourier transform is applied to obtain the range profile. A suitable window function is applied to extract the desired path and eliminate all other undesired paths. The antenna pattern is plotted according to the filtered response of the desired path. We have expressed the receiving voltage of a testing antenna in terms of its gain pattern and input impedance of the testing antenna operating in mul- tipath environment. If the bandwidth of the testing antenna is very narrow and the mismatch problem is very serious over the required bandwidth, an algorithm is proposed to compensate the mismatch ef- fect so that the obtained radiation pattern and the antenna gain are more accurate. Numerical and experimental results have verified the effectiveness of our method.

Joint Beamforming and Power Allocation for MIMO Relay Broadcast Channel With Individual SINR Constraints

In this paper, system design for a multiple-input-multiple-output (MIMO) relay broadcast channel with individual signal-to-interference-plus-noise ratio (SINR) constraints at mobile stations (MSs) is considered. By exploring the structure of downlink-uplink duality at either the base station (BS) or the relay station (RS), we propose two schemes of joint power allocation and beamforming design at the BS and the RS. The problem of the existence of feasible solutions under practical power constraints at the BS and the RS with given SINR targets is considered first. Then, the problem of sum power minimization is considered. Each design problem can be efficiently solved using optimal joint power allocation and beamforming under the framework of convex optimization. We also show that with subchannel pairing at the RS, the transmission power can be further reduced. Finally, a generalization to the multihop scenario is provided to further improve power efficiency.

Device-to-Device assisted downlink broadcast channel in cellular networks

In this paper, two protocols of device-to-device (D2D) communication are proposed to assist multi-input multi-output downlink broadcast channel in cellular networks. Devices in a D2D group are arranged to extract their own data originated from a base station (BS) in an order according to their decoding performance. At each intermediate hop, a best-decoding node is selected and responsible for the next hop transmission. In the first protocol, the selected user forwards its own data stream after decoding, then a proper interference cancellation is performed at each receiving node to eliminate this interference from the buffered reception of the broadcast channel in the first hop from the BS. A closed form representation of the outage probability of each node is derived. In the second protocol, the selected user re-constructs its own signal and cancels this signal from its received vector and forwards the resultant vector. This forwarded signal contains less interference and preserves the data information for the users who have not yet decoded their data. The receiving nodes decode their data streams by jointly considering the received vectors from all the previous hops. The comparison of the two proposed protocols in terms of computational complexity, achieved diversity order and performance are provided. We show that the proposed protocols provide promising diversity gain and avoid error propagations as compared to traditional schemes.

Design of downlink broadcast channel via device-to-device communications

In this paper, an application of Device-to-Device (D2D) communication as a means of supporting multi-input multi-output (MIMO) downlink broadcast channel in cellular networks is proposed. Devices in the D2D group are arranged to extract their own data originated from a base station (BS) in an order according to a well-scheduled hopping sequence. At each forwarding stage, a best-decode node is selected to decode its own data stream, and then the decoded stream is cancelled from the received vector of the selected node before next transmission. Our design scheme not only provides distributed diversity for the selected nodes but also preserves data information and reduces interference terms for not yet decoded users. Besides, with the cascade of receive vectors in multi-hop transmissions, spatial diversity increases at each incremental hop. Simulation results show that our scheme provides promising diversity gain and therefore also avoids the effect of error propagations compared to traditional schemes.

Distributed SINR Balancing Beamformer Design for Coordinated Muti-Cell Systems

This paper considers the beamformer design problem of coordinated multi-antenna base stations (BSs), where the BSs jointly optimize their respective beamformers to improve the overall system performance. Previous research aimed at minimizing the sum transmitted power of all BSs under per-user signal to interference plus noise ratio (SINR) constraints. However, feasibility of the SINR constraints cannot be determined in advance. In this paper, we focus on the design criteria of maximal achievable worst-case users SINR for a particular quality of service (QoS) requirement under multiple sets of power constraints. The main contribution of the paper is an efficient algorithm for finding the globally optimal beamformers across all BSs. The proposed algorithm is based on a generalization of the uplink-downlink duality to the multi-cell setting along with an application of subgradient methods. An important feature is that it leads to a distributed implementation in the coordinated multi-cell system.

MIMO relay design for multiple link pairs based on SINR constraints

We address the problem of multiple-input multiple-output (MIMO) relay design for multiple distributed source and destination pairs with signal-to-interference-plus-noise-ratio (SINR) constraints. Multiple link pairs communicating through a relay can be beneficial in the wireless digital home network (DHN) scenario where various equipments exchange or share digital media contents. In the first hop from the source nodes to the relay, cooperative transmit-receive beamforming design is performed to maximize SINR at the relay. Then joint power allocation and transmit-receive beamforming is conducted in the second hop from the relay to the destination nodes based on the SINR constraints for each link pair. The beamforming filter is a group maximum SINR filter which exploits intra-group cooperation. The group-based uplink-downlink duality property in downlink channel is adopted and modified for amplify-and-forward (AF) relaying. An iterative algorithm is then proposed to solve the problem. Simulation results show the superiority of the proposed algorithm over other methods.

Evaluation of Bistatic Far-Field Quantities From Near-Field Measurements - Abstract

In this paper the relationship between the range-corrected far fields and the near field measurements is established. We propose methods to obtain far field quantities, including the bistatic RCS, bistatic range profiles, and bistatic images from near field measurements. We show how to obtain the Fourier space data in the Ewald sphere when the object is normally or obliquely illuminated and its scattered fields are received by a probe scanning over a planar aperture. We carry out a simulation of a near-field configuration and calculate near field data. From these data, we derive bistatic range profiles and reconstruct three-dimensional (3-D) bistatic images. We also implement a near-field measurement system and propose a calibration procedure so that the Fourier space data can be accurately obtained. A focused projective image has been reconstructed from near field data measured along one-dimensional scanning. Simulation and experimental results have verified the effectiveness of our algorithms.

Determination of Propagation Mechanisms Using Wideband Measurement Techniques - Abstract

In this paper we propose using wideband measurement techniques to determine propagation mechanisms. The factors that can affect the accuracy of the measurements are analyzed. The delay profiles can be obtained by applying inverse fast Fourier transforming (IFFT) on the measured frequency response. To avoid range ambiguity in the delay profile, the frequency response should be finely sampled. The peak values sampled in the delay profiles can be used to represent certain scattering mechanisms such as reflection coefficients or edge diffraction coefficients. To obtain accurate peak value enough zeros will be padded to the frequency domain measurement data before applying IFFT. In this paper we also propose a calibration procedure so that the ordinate value in the delay profile can represent the propagation loss relative to that at the reference position. Wideband measurement techniques will be employed to determine wall reflection coefficients and edge diffraction coefficients. Consistency between the experimental and simulation results shows the effectiveness of the proposed methods.