Warren L. Stutzman

Spatial, polarization, and pattern diversity for wireless handheld terminals

This paper examines the antenna diversity configurations that improve the performance in handheld radios. Experiments using spatial, polarization, and pattern diversity were conducted for both line-of-sight (LOS) and obstructed outdoor and indoor multipath channels that experienced Ricean fading. Antenna separation, polarization, and pattern were varied independently to the extent possible. Envelope correlation, power imbalance, and diversity gain were calculated from the measurements. Diversity performance is measured by diversity gain, which is the difference in signal-to-noise ratio (SNR) between the output of a diversity combiner and the signal on a single branch, measured at a given probability level. Diversity gain increases with decreasing envelope correlation between the antenna diversity branches. However, diversity gain decreases as the power imbalance between diversity branches increases because a branch that has a weak signal has only a small contribution to the combined signal. Diversity gain values of 7-9 dB at the 99% reliability level were achieved in non-line-of-sight (NLOS) channels for all diversity configurations even with very small antenna spacings. The use of polarization diversity reduced polarization mismatches, improving SNR by up to 12 dB even in LOS channels.

A new ultrawideband printed monopole antenna: the planar inverted cone antenna (PICA)

A new antenna, the planar inverted cone antenna (PICA), provides ultrawideband (UWB) performance with a radiation pattern similar to monopole disk antennas , but is smaller in size. Extensive simulations and experiments demonstrate that the PICA antenna provides more than a 10:1 impedance bandwidth (for VSWR<2) and supports a monopole type omnidirectional pattern over 4:1 bandwidth. A second version of the PICA with two circular holes changes the current flow on the metal disk and extends the high end of the operating frequency range, improving the pattern bandwidth to 7:1.

Array antenna pattern modeling methods that include mutual coupling effects

Results from an investigation into methods of modeling the radiation patterns of phased arrays that include the effects of radiative mutual coupling are presented. The approaches are based on either the principle of pattern multiplication or the use of active element patterns. Theoretical derivations of the various active element pattern methods are presented. A new method, the hybrid active element pattern method, is introduced. It accurately predicts the patterns of small and medium-size arrays of equally spaced elements. Example arrays of center-fed dipoles are analyzed to verify and illustrate the representations. The results are general and can be applied to arrays of any type of element. The array patterns computed using both the classical pattern multiplication approach and the methods based on active element patterns are compared to those computed using accurate numerical codes based on the method of moments. >

Smart antennas in wireless communications: base-station diversity and handset beamforming

We review diversity and smart antenna research applied to both base-stations and terminals. To illustrate the performance gains possible, the paper describes research being conducted by the Smart Antenna Group at Virginia Tech, in both smart base-stations and smart handheld terminals.

Modeling and simulation of mobile satellite propagation

Mobile satellite systems are subject to severe fading due to blockage of the line-of-sight (LOS) path by roadside vegetation. A thorough understanding of the fading effects is necessary for the design of a reliable land-mobile satellite system. Analytical and empirical models are presented for predicting fade statistics for vegetative shadowing of mobile satellite terminals. A software simulator for generating simulated fade data is also presented. A physical model relating physical path parameters to propagation model parameters is presented, and results using the model are shown. >

Estimating directivity and gain of antennas

Gain is the most important performance parameter of an antenna. However, in many practical situations it is not possible to measure or calculate the gain of an antenna. Also, the interest in wireless applications has increased the need of system engineers to accurately estimate antenna gain. Many simple formulas are available for estimating gain. But, each formula has a range of applicability, and inappropriate use of these formulas will result in inaccurate gain values. This paper explains the process of gain evaluation, and gives several simple gain-estimation formulas, together with recommendations for their use.

Synthesis of shaped-beam radiation patterns using the iterative sampling method

The iterative sampling method is introduced for the synthesis of shaped-beam radiation patterns using either line sources or uniformly spaced arrays. Given an original pattern which is some approximation to the desired pattern, a series of correction patterns is added to it. Successive iterations are applied in this manner until the desired performance is achieved. The current distribution is found by a corresponding series of corrections. Several examples show that patterns with either low main-beam ripple and/or low sidelobe level or sharp cutoff from the main beam can be obtained. Unless there is significant ripple or sidelobe improvement, the complexity of the required source current is usually lower than that of the original pattern. For a ten-element array example with \pm 1 -percent error in the current amplitudes and phases about -40 dB, sidelobe and ripple performance was achieved. Furthermore, the iterative sampling method is simple to apply and converges rapidly.

Analysis of a two-branch maximal ratio and selection diversity system with unequal SNRs and correlated inputs for a Rayleigh fading channel

An analytical expression for the probability density function of the signal-to-noise ratio (SNR) at the output of a two-branch maximal ratio and selection diversity system is given. The two branches are assumed to be Rayleigh fading, correlated, as well as of unequal average SNRs. Measurements of the cumulative distribution functions after selection and maximal ratio combining were made in Rayleigh fading channels and compared with the analytical results. Also presented are the exact analytical average probabilities of symbol error for coherent binary phase-shift keying and coherent quaternary phase-shift keying before and after two-branch maximal ratio combining for a slow and flat fading correlated Rayleigh channel.

A UWB antenna with a stop-band notch in the 5-GHz WLAN band

A new UWB antenna, the sail-boat antenna, is proposed that provides a stop-band notch in the 5-GHz WLAN band. The CPW-fed sail-boat antenna offers a compact planar structure. Measured results for input impedance, VSWR, patterns, and gain demonstrate that the antenna provides the stop-band notch of 11/spl sim/13.5 dB in the WLAN band.

Cellular-Phone and Hearing-Aid Interaction: An Antenna Solution

With the introduction of digital cellular phones, hearing-aid users have experienced a severe buzzing noise caused by the interaction between digital cellular phones and hearing aids. The cellular-phone industry, the hearing-aid industry, and consumers have been seeking a solution for the interference issue. Efforts reported in the literature have focused on measurements, modeling, and evaluation of interference and RF emission, but not on methods to solve the problem. In this paper, we focus on the causes of the interference and an understanding of the problem. We also present a method to reduce near-field electromagnetic energy around a cellular phone, mitigating the interference between cellular phones and hearing aids. The theoretical investigation of both the radiation mechanisms and fundamental limits on antennas suggested that a low-g antenna, such as an ultra-wideband antenna, could reduce the near-field intensity. Simulations and measurements were performed at 900 and 1880 MHz, using both low- and high-Q test antennas mounted on a mock cellular phone. The results showed that the peak electric and magnetic near-field strengths of the low-g test antenna were lower than those of a high-Q test antenna by at least 5 dBV/m and 4 dBA/m, respectively. The improvement in the near-field performance for the low-g antenna was without any sacrifice in far-field performance. Furthermore, in the presence of a human head, the simulation results showed that the radiation efficiency of the low-Q test antenna was better than that of the high-Q test antenna.