Nobukazu Fudaba

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.

Investigation of a Femto-DAS Hybrid System for In-Building Wireless Solutions

A hybrid scheme of femtocell and distributed antenna system (DAS) is proposed that utilizes a subband dedicatedly assigned to each of multiple femtocells for system capacity gain and a subband shared for DAS simulcasting for cell-edge interference reduction. This paper presents a fundamental downlink performance evaluation and analysis of the in-building wireless system. Proportional fair (PF) scheduling is used for the user assignment, and the throughput performance is evaluated under various conditions. Simulation results show that the system can provide remarkable gains in cell-edge throughput while maintaining high average throughput, particularly in situations where high signal quality is available at the subband shared for the DAS and the system is of high upper bound of spectrum efficiency.