This paper proposes a new digital beamforming adaptive array antenna (DBFAAA) that is effective in severe multipath environments in which timing and carrier synchronization circuits cannot function ideally resulting in the DBFAAA losing control. The proposed DBFAAA has two stages. In the first, the DBFAAA captures the desired signal and establishes synchronization. In the second, the DBFAAA optimizes the beam pattern of the signal. The proposed configuration employs an eigenvector beam of the maximum eigenvalue in the first stage beam-forming. In addition, a fractionally-spaced-tapped-delay-line (FS-TDL) with real tap weights, which is placed after the beam-former, is applied to achieve timing synchronization. The behavior of the proposed DBFAAA for asynchronous sampling data is investigated and the results indicate that the proposed configuration enables asynchronous sampling at the A/D converter. A prototype of the proposed DBFAAA achieving 38-Mbps real-time data communication is introduced and the transmission performance is shown.

In this paper, we propose a novel multiuser detection (MUD) method that is robust against timing offset between wireless terminals (WTs) for the multiuser multiple-input multiple-output (MU-MIMO) orthogonal frequency division multiplexing (OFDM) uplink. In the proposed method, MUD is carried out in the frequency-domain using overlapping fast Fourier transform (FFT) windows. After the inverse FFT (IFFT) operation, the samples obtained at both ends of each FFT window are discarded to suppress the effect of inter-block interference (IBI). Thus, it realizes an MUD regardless of the arrival timing differences of the signals from the WTs. The achievable bit error rate (BER) performance of the proposed MUD method is evaluated by computer simulations in a frequency selective fading channel.

In this paper, we examine techniques for improving the throughput of unlicensed radio systems such as wireless LANs (WLANs) to take advantage of multi-radio access to mobile broadband, which will be important in 5G evolution and beyond. In WLANs, throughput is reduced due to mixed standards and the degraded quality of certain frequency channels, and thus control techniques and an architecture that provide efficient control over WLANs are needed to solve the problem. We have proposed a technique to control the terminal connection dynamically by using the multi-radio of the AP. Furthermore, we have proposed a new control architecture called WiSMA for efficient control of WLANs. Experiments show that the proposed method can solve those problems and improve the WLAN throughput.

This paper proposes a novel adaptive array configuration that reduces high-power co-channel interference (CCI) by utilizing the difference in arrival times between CCI and the desired signal in asynchronous TDD systems. The proposed adaptive array extracts only the CCI and employs pre-null steering for only the CCI by utilizing the fact that only the CCI arrives during the guard time in asynchronous TDD systems. Since the proposed adaptive array enables us to apply the Minimum Mean Square Error (MMSE) algorithm through synchronization with the desired signal using the output signal obtained by pre-null steering, high quality transmission can be achieved even in the presence of high-power CCI. Moreover, based on measurements using a fading simulator and field data, an adaptive array testbed exemplifying the proposed configuration is presented to show the reduction in the high-power CCI.

To achieve a very high data rate within a limited frequency band in orthogonal frequency division multiplexing (OFDM) systems, multi-input multi-output (MIMO) techniques are very promising. Moreover, if a transmitter has the channel state information (CSI), the achievable spectrum efficiency can be maximized using the eigenbeam-space division multiplexing (E-SDM). However, this scheme demands accurate channel estimation. Therefore, in a closed-loop transmission scheme, an increase in the amount of feedback is absolutely necessary for the E-SDM. This paper describes a downlink beamforming method that significantly reduces the amount of feedback needed by using the common transmission weight vectors in all sub-carriers, compared to the amount required for E-SDM. The proposed method also applies transmission diversity to compensate for the quality. The effectiveness of the proposed method was confirmed using computer simulations in both Ricean and Rayleigh fading environments.

A method is presented for increasing wireless LAN (WLAN) capacity in high-density environments with IEEE 802.11ax systems. We propose using coordinated scheduling of trigger frames based on our mobile cooperative control concept. High-density WLAN systems are managed by a management server, which gathers wireless environmental information from user equipment through cellular access. Hierarchical clustering of basic service sets is used to form synchronized clusters to reduce interference and increase throughput of high-density WLAN systems based on mobile cooperative control. This method increases uplink capacity by up to 19.4% and by up to 11.3% in total when WLAN access points are deployed close together. This control method is potentially effective for IEEE 802.11ax WLAN systems utilized as 5G mobile network components.

This paper proposes a hardware configuration using only single pole dual throw (SPDT) switches to realize the previously proposed automatic calibration method using transmitting signals (ACT) for the adaptive array in TDD communication systems. The proposed configuration obtains the same calibration values as the conventional ACT does while reducing the number of switch branches. The transmission pattern using the proposed calibration method is also presented based on an experimental adaptive array testbed in an actual microcell environment. The experimental results show that the ideal radiation pattern formation is achieved by employing the proposed calibration method in an environment with a moving terminal station and where arriving co-channel interference exists.

This paper proposes a channel capacity maximization method for Multiple-Input Multiple-Output (MIMO) antennas with parasitic elements. Reactive terminations are connected to the parasitic elements, and the reactance values are determined to achieve stochastically high channel capacity for the environment targeted. This method treats the S-parameter and propagation channel of the antenna, including the parasitic elements, as a combined circuit. The idea of the 'parasitic channel,' which is observed at the parasitic antenna, is introduced to simplify the optimization procedure. This method can significantly reduce the number of necessary measurements of the channel for designing the antenna. As a design example, a bidirectional Yagi-Uda array, which has two driven antennas at both ends of the linear array, is measured in an indoor environment. The resulting design offers enhanced channel capacity mainly due to its improved signal-to-noise ratio compared to the antenna without the parasitic antennas.

Path loss in high frequency bands above 6GHz is the most fundamental and significant propagation characteristic of IMT-2020. To develop and evaluate such high frequency bands, ITU-R SG5 WP5D recently released channel models applicable up to 100GHz. The channel models include path loss models applicable to 0.5-100GHz. A path loss model is used for cell design and the evaluation of the radio technologies, which is the main purpose of WP5D. Prediction accuracy in various locations, Tx positions, frequency bands, and other parameters are significant in cell design. This article presents the prediction accuracy of UMa path loss models which are detailed in Report ITU-R M.2412 for IMT-2020. We also propose UMa_A' as an extension model of UMa_A. While UMa_A applies different equations to the bands below and above 6GHz to predict path loss, UMa_A' covers all bands by using the equations of UMa_A below 6GHz. By using the UMa_A' model, we can predict path loss by taking various parameters (such as BS antenna height) into account over a wide frequency range (0.5-100GHz). This is useful for considering the deployment of BS antennas at various positions with a wide frequency band. We verify model accuracy by extensive measurements in the frequency bands from 2 to 66GHz, distances up to 1600 m, and an UMa environment with three Tx antenna heights. The UMa_A' extension model can predict path loss with the low RMSE of about 7dB at 2-26.4GHz, which is more accurate than the UMa_A and UMa_B models. Although the applicability of the UMa_A' model at 66GHz is unclear and needs further verification, the evaluation results for 66GHz demonstrate that the antenna height may affect the prediction accuracy at 66GHz.

Multiuser multiple input multiple output (MU-MIMO) systems are attracting attention due to their frequency efficiency. However, in uplink MU-MIMO systems, different frequency offsets among multiple mobile stations (MSs) significantly degrade the transmission quality, especially when orthogonal frequency division multiplexing (OFDM) is used. In this paper, the influence of these frequency offsets is first analyzed in a frequency selective fading environment. Numerical analysis shows that an error floor occurs in the bit error rate and the influence of the frequency offset becomes larger in short delay spread environments. To overcome this problem, a new beamforming method is proposed to compensate for the frequency offset by introducing an auto frequency controller after frequency-space equalization in each data stream. The effect of the proposed method is evaluated in a frequency selective fading environment by computer simulations and measured results.