MU-MIMO (Multi-User Multiple Input and Multiple Output) has been considered as a fundamental technology for simultaneous communications between a base station and multiple users. This is because it can generate a large virtual MIMO channel between a base station and multiple user terminals with effective utilization of wireless resources. As a method of implementing MU-MIMO downlink, Block Diagonalization (BD) was proposed in which the transmission weights are determined to cancel interference between multiple user terminals. On the other hand, Block Maximum Signal-to-Noise ratio (BMSN) was proposed which determines the transmission weights to enhance the gain for each user terminal in addition to the interference cancellation. As a feature, BMSN has a pseudo-noise for controlling the null depth to the interference. In this paper, to enhance further the BMSN performance, we propose the BMSN algorithm that has the pseudo-noise determined according to receiver SNR. As a result of computer simulation, it is confirmed that the proposed BMSN algorithm shows the significantly improved performance in evaluation of bit error rate (BER) and achievable bit rate (ABR).

A broadband microstrip-to-waveguide transition is developed in the millimeter-wave band. No additional parts and complicated structures are needed to extend the frequency bandwidth. Only the simple and novel geometrical features are added in the printed pattern on the substrate. The proposed transition operates over a quite broad frequency bandwidth due to its double resonance. The two resonant frequencies are controlled by adjusting dimensions of the structure according to the required bandwidth, the reflection level and the center frequency. Two versions of the transition are designed and reliability is confirmed by experiments in the millimeter-wave band. The design frequency is 76.5 GHz. Bandwidth 12.9 GHz (16.8%) is obtained where the reflection level is lower than -30 dB. In the other design for broadband, the bandwidth for reflection level lower than -20 dB results in 24.9 GHz (32.5%). Furthermore, it is confirmed in the experiment and simulation that the center frequency is controlled from 75.3 GHz to 78.7 GHz by changing the geometry of the printed pattern.

Many kinds of microstrip array antennas have been developed in the millimeter-wave band. In order to avoid feeding loss and the decrease of antenna gain by beam shift due to frequency changes, center-fed array antennas are advantageous. In this case, the element spacing around the feeding circuit of the transition from the waveguide to two microstrip lines is larger than one wavelength. Therefore, the sidelobe level grows significantly. In order to suppress the sidelobe level, we propose transitions with slot radiators. Moreover, any polarization angles can be achieved by changing the slot angle. A wide variety from 1.5% to 70% of slot radiator coupling powers can be achieved. To investigate the performance of the proposed transition, 10, 22 and 30-element center-fed microstrip comb-line antennas with the proposed transition were developed at 76.5 GHz, and measured performance was evaluated in the millimeter-wave band.

A novel millimeter-wave slotted waveguide array antenna is developed for automotive radar systems. An antenna structure suitable for mass-production is proposed in this paper. The waveguide is composed of two parts; an upper plate and a bottom plate. It is not necessary to contact each other closely because they are divided at the center of the broad wall of the waveguide where the electric current is small. In addition, grating lobes are suppressed by using a cylindrical cavity around each slot and by controlling the slot arrangement without using dielectric material in the waveguide. We have fabricated the proposed antenna by metal injection molding. The measured antenna efficiency results in 55%, which is quite high in comparison with any other conventional low cost millimeter-wave antenna. This efficiency is almost the same as that of the antenna fabricated by precision metal machining. In this paper, it is confirmed that the proposed antenna could be manufactured with low cost.

Resonant slots are widely used for conventional slotted waveguide array. Reflection from each slot causes a standing wave in the waveguide and beam tilting technique is essential to suppress the reflection at the antenna input port. But the slot reflection narrows the overall frequency bandwidth and the design taking it into account is complicated. This paper proposes a reflection cancelling slot pair as an array element, which consists of two slots spaced by 1/4λg. Round trip path-length difference between them is 1/2λg and reflection waves from a pair disappear and traveling-wave excitation in the waveguide is realized. The full wave analysis reveals that mutual coupling between paired slots is large and seriously reduces the radiation from a pair. Offset arrangement of slots in a pair is recommended to decrease the mutual coupling and to realize strong coupling. In practical array design, the mutual couplings from other pairs were simulated by imposing periodic boundary conditions above the aperture. To clarify the advantages of the slot pair over a conventional resonant slot, the predicted characteristics are compared. Reflection characteristics of the array using the slot pair is excellent and a boresite beam array can be realized. In addition, a slot pair can realize stronger coupling than the conventional resonant slot, while the bandwidth of the former in terms of the aperture field phase illumination is narrower than that of the latter. These suggests that the slot pair array is much more suitable for a small array than conventional one. Finally, the predicted characteristics are confirmed by experiments.

Microstrip-line-fed broadband aperture antennas were developed in the millimeter-wave band. We have developed broadband microstrip-to-waveguide transitions to connect a microstrip line and a waveguide. The waveguide transmission line was replaced by a radiating waveguide with an aperture to compose a microstrip-line-fed aperture antenna. Two types of aperture antennas were developed. First, the microstrip substrate is fixed between the two metal plates of a waveguide with an aperture and a back-short waveguide. Second, both the microstrip feeding-line and the back-short waveguide are accommodated in the two-layer LTCC substrate. Broadband performance was achieved due to the potential of the transition. The characteristics of the developed antennas were evaluated by simulations and experiments in the millimeter-wave band.

In the design of a large slotted waveguide array, evaluation of mutual couplings between the slots is time consuming. This paper proposes an effective approximation analysis of the external mutual couplings using periodic boundary condition. Simple design procedure is verified for two-dimensional slot array.

New structure of broadband planar antenna, combining monopole elements with electromagnetic bandgap (EBG) structures, is proposed. The antenna has a simple single layer structure and unique beam pattern. Antenna is fabricated on a surface of a single layer dielectric substrate and back side of the substrate is covered with metal layer. At the center of the substrate, an inverted L monopole strip is fabricated and on both sides of this monopole, EBG unit cells are placed. By tuning monopole length and EBG bandgap frequency, the monopole resonates even if metal layer exists close to the monopole radiator. Three types of EBG, one dimensional (1D), square two dimensional (2D) and hexagonal 2D, are tested. By combining monopole strip with hexagonal 2D-EBG, the bandwidth of prototype antenna, whose return loss is less than 10 dB, is 840 MHz in 5 GHz band. To control beam patterns of antenna, parasitic elements are placed close to the monopole radiator and EBGs. These parasitic elements work as directors of quasi Yagi-Uda antenna and radiation gain at lower tilt angles is improved.

A new technique to estimate the Poynting vector distribution from near-magnetic-field measurement is proposed. To calculate the Poynting vector, both electric and magnetic field should be known. In the proposed method, only magnetic-field measurement of three orthogonal axes is required. Electric field is estimated from the measured magnetic field by using the Maxwell's equation. The modified Yee cell is employed to estimate electric field from the measured magnetic field. Finally, the Poynting vector is calculated from the measured magnetic field and the estimated electric field. Since the proposed method enables us to understand propagation direction of electro-magnetic energy, it can be utilized to locate an emission source and to investigate a mechanism of undesired emission. Experiments are carried out to discuss the accuracy and to validate practical usefulness.

A slotted waveguide planar array using a single-layer feed circuit is applied to high frequency and high gain use. The remarkable efficiency of 75.6% is realized for the gain of 35.9 dBi in 22 GHz band and 64% is realized for 35.1 dBi in 60 GHz band. Each antenna consists of only two components; a slotted plate and a groove base plate, and are highly mass produceable.

Narrow-wall-connected microstrip-to-waveguide transition using V-shaped patch element in millimeter-wave band is proposed. Since the microstrip line on the narrow-wall is perpendicular to the E-plane of the waveguide, waveguide field does not couple directly to the microstrip line. The current on the V-shaped patch element flows along inclined edges, then current on the V-shaped patch element couples to the microstrip line efficiently. Three types of transitions are investigated. A numerical investigation of these transitions show some relations between bandwidth and insertion loss. It is confirmed that the improved transition exhibits an insertion loss of 0.6 dB from 76 to 77 GHz, and a bandwidth of 4.1% (3.15 GHz) for the reflection coefficient below -15 dB.

Self-resonant helical antenna and capacitor-loaded helical antenna of the same dimension for coupled-resonant wireless power transfer is discussed. At first, fundamental difference of the self-resonant and the capacitor-loaded antenna is demonstrated by calculating electric- and magnetic-coupling coefficient. Next, performance of the helical antennas are discussed from viewpoints of 1) transmission efficiency, 2) undesired emission, 3) near-field leakage, 4) effect of human body and 5) effect of conductivity. We have found that the self-resonant helical antenna has an advantage in low transmission loss due to a conductivity of wire. On the other hand, the capacitor-loaded antenna has an advantage in low emission, long transfer distance, and low influence of resonant frequency from human body. This is because both electric-field coupling and magnetic-field coupling are dominant for the self-resonant antenna while only magnetic-field coupling is dominant in the capacitor-loaded antenna.

A broadband microstrip comb-line antenna using a corporate feeding system was developed. The antenna was composed of four colinearly-arranged comb-line antennas with traveling-wave excitation fed by a parallel-feeding circuit of tournament configuration. The total phase deviation due to frequency change became one fourth of the ordinary series feeding from the end of the antenna. Furthermore, the terminations of the inner two comb-lines were connected at the overall center of the developed antenna. Therefore, the narrowband matching elements are not necessary and the amplitude deviation of the aperture distribution for input from one side due to frequency change is compensated by deviation for input from the other side. Broad bandwidth can be expected by using the proposed configuration. The proposed antenna was designed at 76.5 GHz. The effect of the proposed feeding-circuit for broadband operation was confirmed by comparing the measured performances of the antennas fed by other feeding circuits; the end feeding, the center feeding and the ordinary corporate feeding. The bandwidth of the proposed corporate feeding antenna with the center connecting was approximately 14% and 7% wider than the antennas of the center feeding and of the ordinary corporate feeding, respectively.

A one-input four-way power divider is developed for feeding circuit of a microstrip comb-line antenna. The four-way power divider is composed of two-stage tournament-configuration of three Y-junctions. To control the sidelobe level and beam width in radiation pattern of the array, the power dividing ratio is assigned for the four-way power divider. Required dividing ratio is designed by changing the line width to control the impedance. To take impedance matching at the connection parts of the Y-junctions, 1/4-wavelength impedance-transformers are applied to the divider and taper structure is supplied at discontinuities. Four-comb-line antennas are designed and fabricated at 76.5 GHz. We evaluated sidelobe level and beam width by experiment to confirm the performance of the power divider.

A novel method for automatically creating an optimum direction-of-arrival (DOA) estimation algorithm for a given radio environment using a genetic algorithm (GA) is proposed. DOA estimation algorithms are generally described by parameters and operators. The performance of a DOA estimation algorithm is evaluated using root mean square error (RMSE) through computer simulations. A GA searches for the combination of parameters and operators that gives the lowest RMSE. Because a GA can treat only bit strings, Polish notation is used to convert bit strings into a DOA estimation algorithm. A computer simulation showed that the proposed method can create a new angle spectrum function. The created angle spectrum function has higher resolution than the Capon method.

We developed a slotted waveguide planer array antenna with partially parallel feeding in millimeter-wave band. Travelling-wave excitation is more effective for low loss feeding of array antennas than parallel feeding systems. However, array antenna with travelling-wave excitation essentially possesses a significant problem of long line effect which degrades gain due to beam shift by frequency change when the array antenna is fed from the edge of the radiating waveguide. We propose the way to reduce the gain degradation due to frequency change, thus, partially parallel feeding system is developed. Measured performance of the developed antenna is evaluated in this paper.

A new scheme to avoid null zone for HF-band RFID without expanding antenna size is proposed. At first, we demonstrate by FDTD simulation that the null zone occurs because of cancellation of magnetic fields over the loop surface. To prevent cancellation of magnetic fields, the loop antenna is split into four parts, which work as a planar array antenna. The outputs of antennas are gathered by using combining circuit. We have validated by FDTD simulation that the proposed scheme enlarges the worst received power by 13.1 dB.

A new structure to improve channel capacity of short-range MIMO is proposed. The proposed structure consists of back reflector and side reflector. FDTD simulation demonstrates a role of back reflector and side reflector. The back reflector increases all eigen values. The side reflector equalizes eigen value distribution. Consequently, the proposed structure enhances the channel capacity.