Ken Hiraga

On the Transmission Method for Short-Range MIMO Communication

This paper investigates a transmission scheme that is suitable for short-range multiple-input-multiple-output (MIMO) transmission. Since the distance between two array antennas that face each other is comparable with the size of the array antenna aperture in short-range MIMO, the propagation characteristics are greatly different from those in conventional MIMO. Unlike conventional MIMO, the optimal element spacing, which maximizes channel capacity, exists in short-range MIMO. Moreover, the channel capacity with optimal antenna spacing exceeds the ergodic capacity of independent identically distributed (i.i.d.) channels since optimal eigenvalue distribution, which can maximize channel capacity, is obtained in the short-range MIMO. In this paper, we focus on the actual transmission methods, because complex transmission schemes such as eigenmode transmission or maximum-likelihood detection are required to obtain ideal channel capacity. We clarify that the channel capacity obtained by zero forcing (ZF) at the receiver without beamforming at the transmitter is almost the same as that using eigenmode transmission when considering the optimal element spacing. The effectiveness of short-range MIMO communication is also clarified using a 4 × 4 MIMO testbed with actual signals based on the IEEE 802.11n standard. Simulated and measured results show that optimal element spacing is a key parameter in the short-range MIMO communication. We found that designing antenna arrays with optimal element spacing is a very effective approach to achieving a simple hardware configuration.

Development of class-F load rectennas

We designed and fabricated the rectennas which use class-F load as an output filter in order to develop highly efficient rectennas at 24GHz and 60GHz. We also fabricated conventional rectennas which use a capacitor as an output filter, and compared the efficiency of class-F load rectennas with that of capacitor rectennas. We selected the diode which can produce high efficiency even at high frequency, and we parallelized the diode to improve the efficiency. The experimental result of efficiency of class-F load rectennas was 65.6%, on the other hand, that of capacitor rectennas was 52.1%.

Development of 24 GHz rectennas for Fixed Wireless Access

We need electricity to use wireless information. If we reduce amount of batteries or electrical wires with a wireless power transmission technology via microwave (MPT), it is a green communication system. We Kyoto University propose a Fixed Wireless Access (FWA) system with the MPT with NTT, Japan. In this paper, we show mainly development results of 24GHz rectennas, rectifying antenna, for FWA. We developed some types of the rectennas. Finally we achieve 65% of RF-DC conversion efficiency with output filter of harmonic balance.

Experimental evaluation of high speed parallel data transmission technology for wireless repeater system

System studies are being conducted at millimeter-wave frequency for high-speed wireless communications such as several Gbit/s. However, the highly rectilinear propagation characteristics of these systems, created by the millimeter-wave frequencies, place significant limitations on the applications and the usage environments. To address this problem, we already proposed the adoption of a high data-rate relay system using short range multi input multi output (MIMO) transmission technology. This paper reports the results of experiments conducted to verify the simulation method. We evaluate 4x4 MIMO transmission, using the signal specified by IEEE 802.11n, and confirm that the optimal array spacing. The measured and simulated frequency utilization rates of 4×4 MIMO transmission are in good agreement. Frequency utilization of over 20 bits/s/Hz can be achieved: 800Mbps/1.6Gbps transmission can be realized when the bandwidth of 40/80 MHz is assumed. Moreover, the measured and simulated results indicate higher frequency utilization rates than found in the i.i.d. channel.

Broadband and compact 3-dB MMIC directional coupler with lumped element

This paper proposed a broadband and compact 3-dB MMIC edge-coupled directional coupler. The proposed coupler consists of eight stepped-impedance coupled sections and a tiny capacitor. The capacitor is constructed by using the MIM capacitor process and is located on the center of the coupler. The capacitor and stepped structure effectively realize 3-dB coupling and enhanced amplitude/phase characteristics. A millimeter-wave prototype MMIC coupler is fabricated on the 100-µm thick GaAs substrate by using the GaAs pHEMT process. The total coupler length is only 440 µm. The fabricated coupler achieves a coupling loss of −4dB±0.5dB from 30 GHz to 69.3 GHz. The output amplitude and phase imbalances are less than 1.4 dB and 90 degrees ± 5 degrees at the same frequency range, respectively. The proposed MMIC coupler promises very compact and broadband MMIC.

Performance measurement of broadband simple decoding in short-range MIMO

In a short-range multiple-input and multiple-output (SR-MIMO) transmission system that relies on line-of-sight (LOS) propagation, the channel matrix is determined by the antenna geometry, hence radio frequency (RF) signal processing by using an analog matrix circuit is a valid means of separating spatially-multiplexed multiple signal streams. To date several simple decoding methods utilizing this feature for SR-MIMO transmission have been proposed and demonstrated at one frequency point. This paper presents a demonstration of a simple decoding method for two-branch SR-MIMO transmission in a broadband with eight percent relative bandwidth, which corresponds to two-channel usage in the 60-GHz band. An experimental evaluation of the method was performed after prototyping the array antenna and weight matrix circuit. The evaluation results showed signal separation performance of 15 dB was obtained through the frequency range of 24.0-26.0 GHz. Bit error rate (BER) was simulated using measured transmission characteristics and we determined that 16 QAM signals at this band can be transmitted in the prototype system.

A Novel ZF-Based Fast Beamforming Method for Short-Range MIMO Transmission

Millimeter-wave short-range multiple-input multiple-output (SR-MIMO) transmission employing zero-forcing beamforming (ZF-BF) at the information kiosk side can improve transmission rate without increasing mobile terminal power consumption for MIMO signal processing. However, processing delay for ZF weight generation is large, so the effects of time-varying channels due to the users hand jiggling, which cause performance degradation, cannot be ignored. In this letter, a novel fast beamforming method is described with which ZF-based weight can be generated with low delay by utilizing the relationship of channel variation and ZF weight variation. Simulation results show that the method can achieve better performance than ZF-BF.

An SD Method Utilizing Null Dependency on Transmission Distance Due to Two-Ray Fading

A spatial division (SD) transmission method utilizing the characteristics of two-ray fading due to ground reflection with linear antenna arrays horizontal to the ground is proposed. Formulations of the optimum array arrangements as functions of the transmission distance and achievable channel capacity are clarified. For two- and three-element arrays, channel capacity is respectively doubled and tripled over that of free-space propagation. The proposed method provides increased capacity without the extra signal processing cost incurred when using conventional multiple-input-multiple-output (MIMO) transmission.

Study on short-range MIMO transmission using interference cancellation by antenna directivities

In this paper, a simple method for canceling interference by using antenna directivities is proposed for short-range transmission systems. For higher data transmission systems the millimeter-wave frequency bands are useful because of their wide bandwidths. Also, Multiple-Input Multiple-Output (MIMO) technology can be applied to these bands because the application enables channel capacity to be increased by using multiple antennas at the transmitter and receiver without expanding the frequency bandwidth. However, since MIMO transmission schemes are complicated we consider parallel transmission, a simple method for transmitting multiple data streams that is suitable for short-range MIMO transmission. We propose a simple method for canceling interference by using antenna directivities and improving channel capacity in parallel transmission. Numerical analysis shows that the method maximizes channel capacity at the optimal spacing Lopt = 2λ0. It is also found that the channel capacity of the method is 14 % higher than that of Eigenmode beamforming (EM-BF) for two transmission streams and 12 % higher for four streams.

An SDM Method Utilizing Height Pattern Due to Two-Ray Fading Characteristics

A spatial division multiplexing (SDM) transmission method utilizing the characteristics of two-ray fading without relying on narrow beam of the antennas is introduced. This letter formulates the optimized array antenna arrangements and channel capacity as functions of the transmission distance and shows achievable channel capacity for two- and three-element antenna arrays. The letter also describes bandwidth dependency up to 15% bandwidth based on an assumed application to 60-GHz-band gigabit wireless systems. The proposed method provides increased capacity comparable to multiple-input–multiple-output (MIMO) transmission without the extra signal processing cost incurred when using conventional MIMO.