Manabu Ohmiya

Superresolution techniques for time-domain measurements with a network analyzer

Superresolution techniques for time delay estimation are proposed and applied to frequency-domain data measured with a network analyzer. A MUSIC (multiple signal classification) algorithm preprocessed by spatial smoothing is used. The spatial smoothing preprocessing is performed to destroy signal coherence, and the decorrelation performance is examined in detail. The expression which gives an individual response is given. Using this expression, it is possible to eliminate unwanted signals that appear as ripples in the frequency domain. Experimental results show that the frequency bandwidth required by the MUSIC algorithm to resolve distinct time-domain responses and eliminate unwanted signals is much narrower than that required by the FFT (fast Fourier transform). Thus, the MUSIC algorithm is applicable to the time-domain measurements with the network analyzer and has much higher resolution capability than the conventional FFT techniques. The MUSIC algorithm is one of the most promising methods of enhancing the accuracy of measurement for narrowband devices such as antennas. >

An LMS Adaptive Array for Multipath Fading Reduction

Multipath fading often poses a serious hindrance in radiocommunication. The application of a least-mean-square (LMS)adaptive array to the problem of multipath fading reduction is discussed. However, it is known that multipath components are in general correlated with one another. We examine the effect of the correlation on the performance of the LMS adaptive array. When the correlation coefficient does not equal or approximate 1, the LMS adaptive array suppresses the multipath signals significantly by nulling. On the other hand, when the correlation coefficient nearly equals 1, the LMS adaptive array prevents the output signalpower from decreasing. Therefore, the LMS adaptive array mayreduce the multipath fading effectively for any correlation coefficient value. A reference signal in the LMS adaptive array is also discussed. It is shown that synchronization in the referencesignal generation must be extremely accurate. Moreover, aprocessor configuration is proposed which may generate thereference signal with the required accuracy.

An Lms Adaptive Array Using a Pilot Signal

A least mean square (LMS) adaptive array requires a reference signal. When the desired signal contains a pilot signal, it may be used as the reference signal. In this paper the steady-state performance of an LMS adaptive array in which the pilot signal is used as the reference signal is examined. It is shown that the LMS adaptive array occasionally suppresses the desired signal. The loop gain, which is an important parameter, is also considered.

Howells-Applebaum adaptive superresolution array for accelerated scanning

An approach is proposed that offers an accelerated scanning rate for a Howells-Applebaum adaptive superresolution array (H-A SRA). Analytical considerations clarify the causes of performance degradation of the H-A SRA at a high scanning rate. Then a suitable steering signal and implementation of an H-A weight control loop (H-A loop) for accelerated scanning are introduced. The weight solution determined by this method is shown to coincide approximately with the optimum Wiener one under some specific signal conditions and antenna parameters. Computer simulations show that the H-A SRA gives much better scanning performance than the conventional array. The system is readily implemented by improving the circuit inserting the steering signal in the H-A loop. >

Considerations on a scanned adaptive superresolution array

The behavior of a scanned Howells-Applebaum (H-A) adaptive superresolution array (SRA) in a steady-state after convergence is derived analytically. The steering signals are scanned (swept) with time in order to observe the signal source locations. However, the SRA performance deteriorates seriously when scanning is fast. First, we study the causes of degradation of the scanned SRA performance at a high scanning rate. Second, we derive the conditions for the low pass filter in an H-A loop that gives a better SRA performance at a certain high scanning rate. Finally, we propose a new filter configuration in the H-A loop which permits faster scanning with less degradation.

A shipborne fading reduction antenna system for maritime satellite communications

Multipath fading due to reflected waves from a sea surface is serious in maritime satellite communications. A shipborne antenna system which reduces the fading by using the cross-polarized component of the reflected waves is presented.

Upper Bound of a Loop Gain in a Power Inversion Adaptive Array

A loop gain is an important parameter in a power inversion adaptive array. The upper bound value of the loop gain is obtained which prevents the array suppressing a desired signal seriously. This result is very useful for determining the loop gain.

Time-domain measurements using a superresolution technique

The superresolution technique is applied to frequency-domain data measured with a network analyzer, and the high-resolution capability is shown by experimental results. A MUSIC (multiple signal classification) algorithm is used because it has several advantages. Experimental results demonstrate that the superresolution technique has a much better capability of resolving signals and analyzing parameters (e.g., the reflection coefficient) than the FFT and the gating technique. The superresolution technique is expected to be useful for time-domain measurements of narrowband devices such as antennas.<<ETX>>

An Analog Open-Loop Adaptive-Array Antenna System

A new open-loop adaptive-array system with excellent transient behavior is presented. The system is constructed of analog circuits and determines complex weights without using the feedback of the array output. The performance attainable with the system is described in detail. It is shown that the convergence rate of the system does not depend on a noise environment but is determined by the time constant of the low-pass filters included. Moreover, it is shown that although the steady-state performance is quite good when the interference sources differ widely in signal strength, the steady-state performance can be far below optimum when two or more interference sources are present at roughly equal power levels.