Kazuo Nagatani

A highly efficient adaptive digital predistortion amplifier for IMT-2000 base stations

We have developed the worlds most compact, energy-efficient transmitter amplifier for IMT-2000 base station systems that uses an adaptive digital predistorter (DPD). The power efficiency is improved by reducing the output back-off (the margin from the saturation point) of power amplifiers and by suppressing the resulting increase in nonlinear distortion with the DPD. This method uses a predistortion technique that estimates the distortion characteristics of amplifiers by using the least mean square (LMS) algorithm, and it adds a complementary signal to the digital baseband signal with a reverse-characteristic of the distortion. The power amplifiers we have developed achieve twice the power efficiency of conventional types. In this paper, we describe the DPD equipment and the experimental results of a DPD transmitter prototype. Currently, IMT-2000 base station systems equipped with the DPD are being shipped worldwide.

A novel adaptive digital predistortion for wideband power amplifiers with memory effects

In order to provide broadband communication, the signal bandwidth has continuously increased in existing 4G and will keep increasing in upcoming 5G communication systems. Therefore, as the bandwidth of the transmitting signal increases, memory effects arise in high power amplifiers (HPAs). A GaN Doherty HPA is known as the device that can amplify the RF signals with high efficiency. However, the performances of the linearizer are severely degraded due to memory effects in GaN active devices as the bandwidth of the input signal increases. This paper proposes a novel scheme that improves the linearization performance of the digital predistortion (DPD) for wideband transmitting signals with 100 MHz continuous bandwidth and 3.5 GHz GaN Doherty HPA. It is shown that the knowledge of the phase trajectory between adjacent transmitting samples gives an extra degree of freedom for DPD coefficient selection compared with traditional approaches and results in enhancement of the linearization performance. Thus, the proposed DPD based on the memory polynomial is able to compensate for strong nonlinearity with memory in GaN Doherty HPA by considering the phase shift between adjacent transmitting samples. The experimental results obtained for a transmitting signal with 100 MHz continuous bandwidth show that the proposed approach achieves an 8 dB adjacent channel leakage ratio (ACLR) performance improvement over standard memory polynomial DPD.

Joint Linearization for Radio-over-Fiber Links Equipped with High Power Amplifiers

In this study, we experimentally investigated the adjacent channel leakage ratio (ACLR) performance degradation for the output signal of the RF HPA equipped with the adaptive linearization caused by RoF links placed in both direct and feedback paths of the transmitting system. We show that ACLR exceeding -57 dBc @ 5 MHz offset, which completely satisfy the requirement defined in the 3GPP technical specification, can be achieved for a 20 W class Doherty power amplifier linearized through 1 km fiber with commercial RoF links. With the experimental results, we show that the achieved ACLR strongly depends on the RoF link noise figure; meanwhile most of the nonlinear distortions caused by RoF can be successively suppressed with the proposed joint linearization approach.

Adaptive Peak-to-Average Power Reduction Architecture for Multicarrier Signals with Mixed Modulations

One of the major drawbacks of multicarrier transmission is the high peak-to-average power ratio (PAPR) of the transmit signal. In this paper, we propose a PAPR reduction technique for mixed-modulation multicarrier transmission into subcarriers. We show that the knowledge of the allocation of modulation schemes among the subcarriers gives an extra degree of freedom compared with traditional approaches, and results in the simultaneous enhancement of the PAPR reduction level and achievement of the required error vector magnitude (EVM) performances. We demonstrate that PAPR in the transmitting signal is significantly reduced when the EVM distortion for each modulation schema is allowed to increase up to its upper specification limit. We also show that the PAPR regrowth is prevented when the spectrum shaping filters passband is allowed to increase.

Impact of radio over fiber links in feedback on linearization performances of RF power amplifiers

Radio over fiber (RoF) advanced technology is already integrated into existing 3G and 4G radio access networks where the digital unit and remote radio head equipped with the nonlinear high-power amplifiers are connected through RoF-based fronthaul links. In this study, we experimentally investigated the adaptive predistorter adjacent channel leakage (ACLR) ratio performance degradation caused by RoF links placed in both direct and feedback paths of the 3G transmitting system. We focused our investigation on the adaptive predistorter performance degradation caused by an RoF link placed in the feedback path because the distortions that originated in the feedback path define the total system distortion. With the experimental results, we show that the achieved ACLR strongly depends on RoF driving conditions. We find that there is an optimum signal level for feedback RoF that maximizes ACLR in high-power amplifier output.

Linearization through Narrowband Feedback: Problems and Solutions.

An adaptive predistorter is an effective technique to compensate for the nonlinear distortion of power amplifiers. To enable broadband communications, the bandwidth of communication systems has been continuously increased. With these increases in bandwidth, frequency distortions due to limitations in the bandwidth of the predistorters building blocks can no longer be ignored. Such distortions are important factors that deteriorate a predistorters linearization performance. Two main problems directly related with linearization through the narrow feedback path have been the subject of investigation in this study: predistorters performances degradation due to feedback signal bandwidth restrictions and selection of appropriate predistorter architecture suitable for the linearization through narrowband feedback path. Also, in this study we propose a new predistorter architecture based on the inverse modeling for the power amplifier model that is robust against feedback signal bandwidth restrictions. Simulation and experimental results show that with the proposed predistorter architecture, the required feedback signal bandwidth can be reduced approximately to 1.15 times of the transmitting signal bandwidth with minimal degradations in the linearization performances.