In radio meteorology, a field focused on meteorological phenomena that affect radio wave propagation, rainfall, which causes radio wave attenuation, is of significant interest. The modeling of rainfall intensity characteristics is being carried out throughout much of the world, producing a database that is used for estimating the effect of rainfall attenuation on wireless links. Since rainfall characteristics vary greatly from year to year, 10–20 years of accumulated data are required to obtain stochastically stable characteristics. In recent years, global warming has emerged as a serious issue, with global temperatures reportedly increasing by$0.73~^{\circ }$C in the past 100 years; in Japan, the increase has been$1.21~^{\circ }$C. If such significant long-term changes have occurred in rainfall characteristics, the rainfall intensity database being used for wireless system design will need to be revised. In this article, we examine rainfall statistics in Japan and assess whether they provide evidence of long-term change. The data include the maximum one-day, 1-h, and 10-min rainfall for each year over the past (approximately) 100 years, as provided by the Japan Meteorological Agency (JMA) on its website. Using a basic confidence interval framework, the statistical properties of the annual maximum rainfall are investigated to clarify, whether the data indicate a long-term trend in rainfall characteristics over the period under the investigation. As a result, this analysis confirmed the existence of a long-term tendency for the three categories of rainfall in Japan to increase by roughly 10% over the examined period of approximately 100 years. In addition, by focusing on statistics on heavy rainfall that occurs once in N years, the study also showed that effective quantitative estimation is possible by applying extreme value statistics.

This paper investigates the effect of correlation based subarray grouping in low computational massive multiple input multiple output (MIMO) multiuser downlink communication system. In previously proposed low computational MIMO design method based on block diagonalization, the performance degradation becomes large because of the subarray processing in both of transmitter and receiver sides. Particularly, in case of correlated channel, the conventional subarray method has poor characteristics, which cannot be used for the actual communication. In this work, through computer simulations, it is shown that the performance of transmit-receive subarray design can be significantly recovered to the level of practical use considering correlation based grouping.

This paper presents single carrier multiuser multiple input multiple output (MU-MIMO) downlink transmission schemes under frequency selective channels based on extension of narrow band MU-MIMO techniques. First, broadband version of channel inversion method adopting time-domain processing is proposed. Then it is further arranged to an energy saving variation using nonlinear strategy, vector perturbation (VP) method. In those methods, transmitter is equipped with spatio-temporal filters adopting tapped delay line (TDL) structure instead of simple spatial weight, and they can avoid some problems specific in frequency-domain approaches. Through computer simulations, the effectiveness and features of those approaches are shown.

Automatic modulation recognition is advantageous for wireless communication systems employing adaptive modulation, software-defined radio, and cognitive radio. In this paper, we consider a phase based maximum likelihood (ML) approach for identifying the modulation format of a linearly modulated signal. Since the optimal ML scheme is computationally intensive, we propose two approximate ML alternatives, which can offer close-to-optimal performance with reduced complexity. We then present a general performance analysis for classification of K types of modulation constellations. For K;5, we obtain a set of upper bounds on Pcc, which provide a tradeoff between accuracy and complexity in calculating the Pcc. In addition, asymptotic behavior of phase based ML classification schemes is investigated.

We consider a phase based maximum likelihood (ML) approach for identifying the modulation format of a linearly modulated signal. Since the optimal ML scheme is computationally intensive, we propose two approximate ML alternatives derived from Gauss quadrature rules. The proposed approximate ML schemes can offer virtually optimal performance with reduced complexity. We then present a general performance analysis for classification of multiple modulation constellations.

In actual cellular systems, base station (BS) may not be located in the cell center because of physical restrictions. This paper investigates, assuming such a situation, the effectiveness of BS cooperation (BSC) in MIMO (multiple input multiple output) cellular system under irregular cell geometry. The cell shapes are determined by BS position based on Voronoi diagram, namely, UTs (user terminals) communicate the nearest BS. The influence of BS displacement is evaluated for several patterns using typical spatial processing algorithms. Through computer simulations, it is demonstrated that the BSC is effective under this situation, and how the BS displacement affects the system performance.

This paper is concerned with classification of quadrature amplitude modulation (QAM) signals by using noncoherent maximum likelihood (ML) schemes, which are robust to frequency mismatch and phase shift. To reduce the high complexity involved in optimal noncoherent ML classification, we propose two suboptimal alternatives that perform well under various conditions. In particular, one suboptimal scheme, derived from high signal-to-noise ratio (SNR) approximation, is shown to provide close-to-optimal performance. We also present a general performance analysis for noncoherent ML classification of K types of QAM constellations, ending up with simplified integration for K; 5. In addition, we investigate the asymptotic behavior of noncoherent ML classification assuming the number of samples used increases to infinity. The asymptotic performance of optimal as well as suboptimal noncoherent ML classification is established.

In cellular system, user terminals (UTs) are sometimes suffered from the attenuation of target signal from base stations (BSs) and severe interferences from adjacent cells, and their influence is serious particularly in the cell edge. This paper considers the performance improvement of multi-antenna cellular system under such a bad situation utilizing cooperative works of BSs and relay stations (RSs). We deal with downlink case assuming perfect channel state information (CSI) and amplify and forward (AF) relay transmission. Following SINR (signal to interference plus noise ratio) based analysis of the system, several cooperation schemes are investigated and their performances are compared through computer simulations. The advantage of BS and RS cooperation under sufficient relay SINR, particularly in case of BS-RS data sharing is shown.

This paper describes design methods of multiuser MIMO (multiple input multiple output) AF (amplify and forward) relay system. Assuming the application to multi-cellular systems, the cancellation of interuser interference is considered both in the first and second hops under perfect knowledge of CSI (channel state information). First, a maximal SINR (signal to interference plus noise ratio) approach using iterative optimization is given. Then a partial ZF (zero forcing) based method which cancels all the interferences not only in one side of transmitter or receiver, but by cooperative work of source-relay and/or relay-destination is considered both for nonseparable and separable relay weight matrices. By this operation, the restriction on the antenna number required for ZF could be released. Through computer simulations, the effectiveness of the proposed method is demonstrated.

We propose to use frequency-domain oversampling (FDO) to efficiently exploit diversity inherent in a multipath channel. Both linear minimum mean square error (LMMSE) and nonlinear Vertical-Bell Laboratories-Layered-Space-Time (VBLAST) receivers are considered. Numerical results show that compared with conventional counterparts without FDO, the proposed receivers yield significantly better uncoded as well as coded symbol error rate (SER) performance. In addition, we find through simulations that an minnimum FDO exists, which is determined by the number of subcarriers and the number of multipath components.

This paper is concerned with amplitude-based maximum likelihood classification for quadrature amplitude modulation formats in the presence of frequency offset. We first propose a simplified classification algorithm that incurs a small loss in performance. A general performance analysis is then elaborated, which can achieve a higher computational efficiency, compared with the existing one. By integrating blind time synchronization and signal-to-noise (SNR) estimation, we further develop a robust classifier that is able to deal with some commonly unknown system parameters including frequency offset, phase shift, time delay, and SNR.

This paper presents a subband adaptive array (SBAA) with reduced pilot signal based on maximal ratio combining (MRC) scheme. By the subband processing, MRC design based on eigenfiltering which is widely used in frequency flat environments could be applied to frequency selective channels, but the efficient synthesis of output signal through the inverse DFT (discrete Fourier transform) is disturbed by the nonuniqueness of eigenvectors. In the proposed method, a two-step approach is adopted to solve this problem: in the first step, we use simple compensation of weight uncertainty with short (or possibly no) pilot signal and derive a hard-decision temporary output. Then in the second step, the output data is used as a training sequence to determine optimum compensation. It is shown through computer simulations that the proposed scheme improves the transmission rate of SBAA scheme keeping good performance, and some fundamental features are also investigated.

We consider modulation classification for quadrature amplitude modulation (QAM) formats. The received signal is assumed to be unsynchronized in both time and frequency, since in practice the receiver has little prior knowledge about the transmitted signal. To tackle this challenging problem, we propose a classifier that is based on a combination of blind time synchronization, differential processing, and maximum likelihood (ML) detection. A computationally efficient scheme is then developed. Numerical results are provided to justify our approach.

This paper gives an analysis method of i.i.d. (independent and identically distributed) double fading channels including LOS (line of sight) environment which are typically found in keyhole MIMO (multiple input multiple output) models. To investigate the potential of LOS double fading (double Rician fading) MIMO channels, the density function of SNR (Signal to Noise Ratio) which corresponds to the only one non-zero eigenvalue of channel correlation matrix is considered. After the description of the exact expression with an infinite series form, an approximation formula of SNR density with a simple monomial form is given utilizing Nakagami m approximation of Rician distribution. Through computer simulations, the effectiveness of the proposed formulae is verified, and the advantage of the approximated formula particular in case of large array size and/or large Rician factor is demonstrated.

We propose an implementation of the tapped delay line adaptive array (TDLAA) at the base station for improving the BER performance of asynchronous multi-user in mobile communication over fast fading channels using multiple antennas. The data in each user at the mobile station, which applies two transmit antennas, are encoded by space time block code (STBC). The proposed scheme transmits the pilot signal and information data in alternate time slots. We derive performance criteria for designing such scheme under the assumption that fading is classified into fast fading condition. We show that our scheme can suppress co-channel interference (CCI) and defeat Doppler spread effectively.

This paper presents approximation formulae of statistical distributions of the largest eigenvalue of i.i.d. (independent and identically distributed) MIMO (multiple input multiple output) channel correlation matrices under Rician (Nakagami- Rice) fading environment. The proposed formulae are based on space diversity theory adopting MRC (maximal ratio combining) scheme under Nakagami m fading channel which is known as a good approximation of Nakagami-Rice fading, hence it well approximates the largest eigenvalue distribution in MIMO Rician channels. The derived marginal distributions have a simple monomial form which is suitable for calculations of various performance indices of MIMO systems. Computer simulations show that the proposed approximation formulae are effective and have a better precision than conventional one.

The present study investigates an inter-vehicle communication (IVC) scheme for safety driving support systems, such as collision avoidance warning systems at intersections. In order to realize high communication reliability, we apply a multiple-input multiple-output (MIMO) with space-time block coding (STBC) method in the IVC system. We investigate the transmission quality of the method in collisions when making a right turn, as an example of a dangerous situation. A key transmission feature of the MIMO system is determined by radio wave propagation. Therefore, the radio wave propagation at an intersection in this dangerous situation is analyzed by computer simulation using the ray-tracing method and by radio wave measurements in a field experiment. In addition, we propose propagation channel models in a shadowing environment generated by a large vehicle. Finally, we clarify the effectiveness of the proposed MIMO-STBC scheme by computer simulations.

This paper presents an adaptive array (AA) antenna using processing of subsets based on combined recursion structures (CORPS) for the wireless communication under frequency selective fading (FSF). Four types of CORPS algorithms considered here attempt the reduction of computational cost of AA with tapped delay line (TDLAA), and enable flexible choice of performance and computational efficiency taking into account their trade-off. The construction and design strategy of AAs using CORPS are described centering on CORPS in time domain (CORPS-T). The performance of the proposed algorithms is investigated through computer simulations, and it is shown that CORPS has better performance than SBAA (subband adaptive array) and reduced operation number compared to TDLAA. Some properties of CORPS are also presented.

This paper presents a design method of multiple input multiple output (MIMO) communication system using spatio-temporal processing with the reduced computational cost working under frequency selective fading (FSF) environment. Instead of conventional simultaneous design of all the weights in the system, weights in transmitter and receiver are divided into several groups, and the optimization is carried out group by group as a reduced scale problem. The grouping scheme gives us the flexibility of the design regarding both the performance and computational cost. The computer simulations are carried out to investigate the performance of the proposed method and to provide useful information for the actual design

This paper proposes an inter-vehicle communication (IVC) scheme for safety driving support systems that transmits collision avoidance warnings at intersections. In order to realize high communication reliability, a transmission scheme based on multiple-input multiple-output (MIMO) with space-time block coding (STBC) is proposed. A key transmission feature of MIMO systems is determined by radio wave propagation. Therefore, the radio wave propagation at an intersection is analyzed by computer simulations using the ray-tracing method. In addition, this paper verifies the effectiveness of the proposed scheme using the propagation characteristics in computer simulations

In this paper, we describe about the statistical distribution of eigenvalues of correlation matrices in independent and identically distributed (i.i.d.) MIMO Rayleigh fading channels. First, the statistical eigenanalysis of MIMO channel correlation matrices is carried out, and the exact expression of the marginal distribution of eigenvalues is presented. Then, a monomial approximation of above distribution with a simple form convenient for the calculation of MINIO performance is proposed based on the theory of space diversity in SIMO systems adopting maximal ratio combining. Through computer simulations, the effectiveness of the proposed approximation method is demonstrated. Inidex Terms-MIMO, eigenvalue, Wishart matrix, space diversity, maximal ratio combining.

The paper presents a simple method of the maximum SINR (signal to interference plus noise ratio) design of MIMO (multiple input multiple output) communication systems using a TDL (tapped delay line) structure. The transmitter and receiver weights are determined alternately, optimizing one side by fixing the other, and this operation is repeated until the SINR converges. The performance of MIMO systems using the proposed approach is investigated, through computer simulations, and it is demonstrated that, though it requires high computational cost, the TDL structure brings high ability to mitigate the influence of frequency selective fading, particularly when the duration of the delay profile is long. Moreover, experimental results show that the equable distribution of the resources (weights and delay units) to both arrays is a better choice than the concentration of them to one side of the transmitter or receiver.

The subband adaptive array (SBAA) is a method for the parallel implementation of the tapped delay line adaptive array (TDLAA) which possesses high ability to mitigate the degradation of the quality of wireless communications under the environment where plural coherent propagation waves are arriving from different directions (multipath fading environment). This paper presents a unified analytical overview and an evaluation criterion for SBAA accompanied with sampling rate conversion. Here the signal-to-interference-plus-noise ratio (SINR) is adopted as quality factor and the performance of the SBAA under multipath fading is investigated theoretically based on this criterion. As an improvement for the SBAA, the cyclic prefix (CP) which is used in orthogonal frequency division multiplexing (OFDM) communication systems is introduced and experimentally verified to be effective through computer simulations. This paper demonstrates the advantage of this approach from the analytical viewpoint with respect to the correlation matrix in each band.

Earth-space propagation studies in Japan were originally concerned with measuring atmospheric absorption and rain attenuation by means of a suntracker and a radiometer, following which a number of long-term experiments were performed using various experimental and commercial satellites. The first experiments involving Ka-band Earth-space propagation measurements in Japan were carried out in 1977 using the Engineering Test Satellite Type II, and since 1978, these experiments have been conducted using Japan's domestic communication satellite. Based on these studies to date, the understanding of Earth-space propagation characteristics at higher frequency bands, modeling of propagation channels, and development of countermeasure technologies have shown dramatic progress. In Japan at present, advanced propagation studies are under way in wideband digital-transmission satellite systems for the forthcoming multimedia era. This paper summarizes recent study activities concerning wave propagation in Ka-band Earth-space paths and related topics, such as radiometeorology in Japan. Last, future plans for conducting Ka-band and millimeter-wave propagation experiments by using the Japanese experimental satellite "COMETS" and rain observations in tropical regions by "TRMM" are introduced briefly.

The mechanism and fundamental nature of the "cycle slip phenomenon" in clock recovery, which triggers the occurrence of burst-type irreducible errors in frequency-selective fading conditions, are theoretically clarified for PSK transmission systems with a coherent detection scheme. The appropriateness of the theory is identified through measurement by means of a fading simulator and a 64 kb/s QPSK modem. Moreover, a simple formula for predicting the frequency of cycle slip occurrence in Rayleigh-fading environments is developed, the accuracy of which is confirmed by means of computer simulation.

An adaptive array antenna is effective for correcting degradation due to multipath interference in mobile communication systems. In a beam space adaptive array antenna, the weights of the component beams are optimized. Thus the main beam directions and the optimum weights of the component beams are known factors in transmitting. We propose a method for null-beam forming at the transmitting frequency that uses these parameters, which makes it possible to carry out adaptive null-beam forming both in receiving and transmitting.>

A digital signal processor (DSP) for a digital beam forming antenna (DBF antenna) in the L-band has been implemented using a field programmable gate array (FPGA). The DSP has 16 channel processors, where each processor calculates both I and Q data in one channel. The DSP provides multi-beam synthesis and beam selection for the DBF antenna, which has 16 element antennas.>

In CDMA mobile radio communication systems, degradation of instantaneous signal to interference power ratio (C/I), which causes impairment of frequency utilization factor, will be significant in multipath fading environments. In this paper, a hybrid diversity scheme for fading reduction combining effects of space diversity and path diversity (represented by the RAKE method) is investigated. A quantitative evaluation of fading reduction effects of the hybrid diversity is performed, comparing them with those of space diversity only and path diversity only. The hybrid diversity scheme is promising in environments where the delay spread of a transmission path is 1 /spl mu/s or less.>

Based on the theoretical method developed by Clarke for the analysis of correlation characteristics in Rayleigh fading conditions, we develop a general model for envelope correlation statistics of line-of-sight fading known as Nakagami-Rice fading. The model developed here has a considerable degree of universality and can be applied to the analysis of correlation statistics in frequency and spatial domains for microcellular mobile radio, maritime and aeronautical mobile-satellite communications. Using this model, we examine the frequency correlation statistics of two typical Nakagami-Rice fading environments.

By examining measured 1-min rain rate data presently available and 1-h rain rate data obtained by AMeDAS (Automated Meteorological Data Acquisition System) in Japan, a simple conversion method of 1-h rain rate data to 1-min rain rate distribution is presented. The prediction accuracy of the method is evaluated, and good accuracy is confirmed. Maps are given of the mean and standard deviation (normalized by the mean value) of 1-min rain rate for 0.01% of time based on AMeDAS data in Japan. Moreover, the effect of a record-breaking heavy rain even on long-term statistics is discussed. Since the method has been successfully applied to various locations in Japan, it can be expected to be applicable to regions of the world where 1-min rain rate data have not yet been available.>

Quantitative analysis of the coherent and incoherent power of sea-reflected waves at 1.5 GHz was performed using multipath-fading data obtained by field experiments. The experiments were carried out for shore-to-shore, satellite-to-shore, and satellite-to-ship paths, and antennas with gains from 13 dBi to 21 dBi were used for signal reception. Results indicate that in rough sea conditions where the incoherent component is dominant, the power of the incoherent component can reach the maximum power of the coherent component (generated in calm sea) in most cases, except for a few cases such as when measurements using narrow-beam antennas or elevation angles below 7 degrees are made. On the results obtained, a simple prediction method for multipath fading caused by sea reflection is presented.>