An RF predistorter for base station HPAs of NADC system

This paper presents the design of an effective and low cost RF predistorter for use in the base-station high power amplifiers (HPAs) of the North American Digital Cellular (NADC) system. A complex polynomial is used to model the characteristic of the HPA and the predistorter generates the inband intermodulation (IM) products required for predistortion of the HPA. The design of the predistorter has a simple structure, using Schottky diodes as the nonlinear device to generate the inband IM products. Experimental results using a practical HPA show that the predistorter can reduce the adjacent-channel-power ratio (ACPR) of the pi/4-DQPSK signal used in the NADC system by 20 dB at an HPA output power level of 30 W. Moreover, the constellation and eye diagrams of the pi/4-DQPSK signal show that the predistorter can also reduce the signal distortion caused by the HPA.