This paper reports the evaluation and simulated results of the nonlinear characteristics of the 4.65GHz Active Antenna System (AAS) for 5G mobile communication systems. The antenna element is composed of ±45° dual polarization shared patch antenna, and is equipped with total 64 elements with horizontal 8 × vertical 4 × 2 polarization configuration. A 32-element transceiver circuit was mounted on the back side of the antenna printed circuit board. With the above circuit configuration, a full digital beamforming method has been adopted that can realize high frequency utilization efficiency by using the Sub6GHz-band massive element AAS, and excellent spatial multiplexing performance by Massive MIMO has been pursued. However, it was found that the Downlink (DL) SINR (Signal to Interference and Noise Ratio) to each terminal deteriorated because of the nonlinear distorted radiation as the transmission output power was increased in the maximum rated direction. Therefore, it has been confirmed that the spatial multiplexing performance in the high output power region is significantly improved by installing DPD. In order to clarify the affection of nonlinear distorted radiation on spatial multiplexing performance, the radiation patterns were measured using OFDM signal (subcarrier spacing 60kHz × 1500 subcarriers in 90MHz bandwidth) in an anechoic chamber. And by the simulated analysis for the affection of nonlinear distortion on null characteristic, the accuracy of nulls generated in each user terminal direction does not depend on the degree of nonlinearity, but is affected by the residual amplitude and phase variation among all transmitters and receivers after calibration (CAL). Therefore, it was clarified that the double compensation configuration of DPD and high-precision CAL is effective for achieving excellent Massive MIMO performance. This paper is based on the IEICE Japanese Transactions on Communications (Vol.J102-B, No.11, pp.816-824, Nov. 2019).

An ultra-wideband (UWB) system usually occupies a large frequency band, which may overlap with the spectrum of a narrow band system. The latter is referred to as a victim system. To effectively use frequency, a UWB system may create a notch in its spectrum to accommodate the victim signal for interference avoidance. Parameters of the notch such as the depth and the width of a notch need to be decided in accordance to victim systems. In this paper, we investigate the effective UWB avoidance by examining the suitable notch based on experimental evaluation. In the experiments, 3GPP LTE, Mobile WiMAX, as well as an IMT Advanced Test-bed are respectively employed to represent different types of victim systems. The UWB system is set up based on WiMedia specifications and operates at the UWB low band of 3.1–4.8 GHz. A notch is fabricated by nullifying the related subcarriers of the UWB signal. In addition, a filter or a window function is formed and employed to further smooth the notch. Bit error rate (BER) or packet error rate (PER) performances of victim systems are measured and used to evaluate the UWB interference. Our results show that when a notch is properly formed, the interference level introduced by UWB can be below the permitted level by regulations.

The paper presents capability of signal detection for realizing coexistence between broadband wireless access (BWA) systems and ultra wideband (UWB) devices. The capability is experimentally evaluated for baseband signals of downlink (DL) in both mobile WiMAX and 3GPP LTE. An UWB receiver based on fast Fourier transform (FFT) compliant with MB-OFDM standard is implemented as a detector of the BWA signals. The capability is evaluated in terms of elapsed time required to achieve signal detection with probability of 99% by the implemented FFT-based UWB receiver at different conditions of the receiver. Decisions on the signal detection are made by the simplest method which is by setting a threshold which is determined by noise floor of the receiver as reference. The experiments have been conducted though baseband signals for both AWGN and multipath fading channels without any synchronization between the DL signals and UWB receiver. In AWGN environment, results show that the elapsed time depends on the duty ratio of the DL signal to be detected, however, the correlation between the required time and duty ratio is not linear since their envelopes of the DL signals are not constant. In multipath fading environments based on channel models commonly employed as mobile radio environments, the required time for the signal detection becomes as 17 times longer than that in AWGN due to its signal attenuation. For robust signal detection in multipath fading environments, it has been revealed that the number of quantization bits at ADC is crucial through the experiments.