Takeshi Onizawa’s research while affiliated with Waseda University and other places

Large capacity wireless communication services using high frequency bands are expected to become widespread. To rapidly deploy such services, high frequency band radio-over-fiber (RoF) systems are being considered that use RoF technology to separate the wireless base station function into a central station (CS) with signal processing functions and remote radio units (RRU) with antenna functions. In high frequency band RoF systems, remote beamforming, in which the CS performs beamforming control, is desirable for simplifying the RRU. We have proposed a fixed wavelength allocation beamforming scheme and a pre-beamforming (pre-BF) control method as a means of remote beamforming suitable for high frequency band RoF systems. However, when these methods are applied to RoF systems with a massive planar array antenna, the beam may not be formed properly. In this paper, the performance of applying these methods to massive planar array antennas is evaluated in simulations. Then, we propose a two-dimensional (2D) pre-BF control method that extends the pre-BF control method to two dimensions to perform high accuracy beamforming with the fixed wavelength allocation beamforming scheme even for planar array antennas. In addition, we show that the 2D pre-BF control method can conduct high accuracy beamforming by reducing the degradation from the ideal beamforming gain to about 0.2 dB with a 100-element planar array antenna.

In this study, we investigate the system performance of multiple-input multiple-output (MIMO) faster-than-Nyquist (FTN) signaling in the context of power amplifier (PA) non-linearity under multipath fading channels. We apply a more general MIMO-FTN precoded orthogonal frequency division multiplexing (OFDM) system model and provide the corresponding input-output relationship. For the PA model, the Rapp model is adopted to characterize the effect of non-linear distortion. We intend to investigate system performance through simulations under the following conditions: employing the MIMO configuration, utilizing the FTN signaling scheme, operating in a multipath environment, and accounting for PA non-linearity. This aspect has received relatively limited attention thus far. The simulation results reveal that FTN signaling still performs better than Nyquist signaling under the fixed transmission rate. For more comprehensive research, the effects of PA non-linearity are also simply demonstrated.

Radio over fiber (RoF) is attracting much attention as one way to realize high frequency bands (above about 10 GHz) systems. A fixed wavelength allocation beamforming scheme has been proposed to attain the desired link budget in such systems. However, this beamforming performance is degraded with massive antenna elements, because of increasing the phase error due to chromatic dispersion. Therefore, we propose a novel beam control method for the scheme, that implements beamforming initially with just a few antenna elements negligible beamforming deterioration. In this letter, the proposed method characteristics is evaluated, and the effects of suppressing beamforming deterioration and improving beamforming gain are shown.

In this paper, we investigate the advantages of multiple-input multiple-output (MIMO) faster-than-Nyquist (FTN) signaling under a fixed utilized bandwidth and multipath fading channels. We present derivations of the input-output relationships for MIMO-FTN systems and decompose the MIMO transmission scheme into spatial and spectral precoding procedures. An approximated frequency-domain equalizer (FDE) is introduced at the receiver as an effective solution to decode the signals. The simulation results reveal that FTN signaling outperforms conventional Nyquist signaling under the fixed-bit transmission in terms of the bit error rate (BER) and throughput. It is shown that FTN signaling has the applicability of a lower-level modulation process, which might decrease the peak-to-average power ratio (PAPR), while there exists a tradeoff between PAPR and BER/throughput. Moreover, the advantages of the approximated FDE and FTN-SCFDE systems are confirmed in the MIMO configuration.

In this study, we compare the system performance by considering faster-than-Nyquist (FTN) signaling employing single carrier frequency domain equalization (SC-FDE) and orthogonal frequency division multiplexing (OFDM) under the constraint of a fixed total utilized bandwidth. In this work, FDE also includes the appearance of colored noise, and the corresponding FDEs are formulated based on its effect to improve detection reliability. The advantages of FTN signaling and the preferable choices of compression and roll-off factors based on the bit error rate (BER) and system throughput are demonstrated under multipath fading channels via the simulation results.

This paper proposes a scheme that reduces residual self-interference significantly in the analog-circuit domain on wireless full-duplex relay systems. Full-duplex relay systems utilize the same time and frequency resources for transmission and reception at the relay node to improve spectral efficiency. Our proposed scheme measures multiple responses of the feedback path by changing the direction of the main beam of the transmitter at the relay, and then selecting the optimal direction that minimizes the residual self-interference. Analytical residual self-interference is derived as the criterion to select the optimal direction. In addition, this paper considers the target of residual self-interference power before the analog-to-digital converter (ADC) dependent on the dynamic range in the analog-circuit domain. Analytical probability that the residual interference exceeds the target is derived to help in determining the number of measured responses of the feedback path. Computer simulations validate the analytical results, and show that in particular, the proposed scheme with ten candidates improves the residual self-interference by approximately 6 dB at the probability of 0.01 that the residual self-interference exceeds target power compared with a conventional scheme with the feedback path modeled as Rayleigh fading.

This paper proposes and experimentally demonstrates a passive beamformer based remote beamforming scheme for millimeter-wave band radio-over-fiber systems. The proposed scheme can control the beam direction of remote radio units (RRUs) from the central station (CS); it yields extremely simple RRUs for flexible and cost-effective deployment. The proposed scheme sets a passive beamformer such as beamforming network, reflector, or lens in the RRU and different wavelengths are assigned to each input port of the passive beamformer. This configuration offers remote beam switching by just carrier wavelength switching at CS. Multi-beam operation can be also realized simply by using multiple wavelengths simultaneously. Experiments on a 28 GHz band reflectarray are conducted. The feasibility of remote beam switching by wavelength switching is confirmed. 5G signal transmission over 20 km of single-mode fiber and 6.3 m air link is successfully achieved with error vector magnitude values of 2.71–3.55%, which satisfies the 3GPP requirements of 8%.

This paper presents a remote beamforming scheme with fixed wavelength allocation for radio-over-fiber (RoF) systems employing single-mode fiber (SMF). In the proposed scheme, wavelengths with equal narrow spacing are fixedly assigned to each antenna element to equalize the time delay differences between adjacent wavelengths caused by chromatic dispersion. The beam direction of base station (BS) is controlled only by phase control at central station (CS). This offers efficient wavelength utilization and completely remote beam control without information of optical fiber length, and supports systems employing high carrier frequency and long optical fiber. The beamforming performance of the scheme is evaluated theoretically and experimentally. It is shown that the beam pattern depends on optical intensity modulation frequency instead of radiated frequency. Simulations indicate that the required wavelength spacing to obtain a desired beam pattern becomes narrower as more wavelengths are used or intensity modulation frequency becomes higher. Finally, to evaluate the feasibility of the scheme, experiments are conducted under the condition of intensity modulation frequency of 10 GHz, 10 km SMF, and four element array antenna. The results substantially agree with the simulation results, and the main lobe level decrease is within 0.04 dB at wavelength spacing of 50-400 GHz (0.39-3.13 nm on average in C band). The results confirm that the scheme can achieve almost ideal beam patterns and realize beamforming and beamscanning.