Study And FPGA-based Implementation On Digital Baseband Predistortion

Nonlinear distortion caused by power amplifiers exists widely in modern digital communication. In order to solve it, linearization approaches have been presented, which include Power Back-off, Feedback, Forward feedback, Envelop Elimination and Restoration, Linear Amplification using Nonliear Components and Predistortion. Among all the above mentioned methods, Digital Baseband Predistortion is a promising linearization technique with good performance, flexible implementation and low cost.The implementation of baseband predistortion mostly bases on the polynomial or look-up table. Since the polynomial-based method has high complexity, difficult implementation and low precision etc., the look-up table based method is generally used for the hardware implementation of current predistorters. However, one of the existing design methods requires the processor, the speed of which becomes the bottleneck of the whole system. The other one needs the operation of complex multiplications. A complex multiplication equals to four real multiplications and two additions or subtractions, which increases the complexity of the system dramatically.Through analysis and comparison of several implementation methods for look-up-table-based predistortion, an implementation scheme of the polar coordinates based predistortion technique is proposed in this paper. Moreover, for the compromise of area and speed of IC design, a coordinate conversion circuit and a look-up-table-based predistorter are designed and implemented in the FPGA-based platform. One advantage of this design is that the processor is not required and all of the computing modules consist of the basic circuits in order to meet the requirement of high speed. Furthermore, all calculations are done in the integer domain rather than complex computation in order to meet the requirement of low complexity. The simulation results show that the proposed design can effectively compensate the nonlinear distortion caused by power amplifiers. Compared with other implementation methods, it also has some advantages such as high efficiency of designs, high operation speed, low complexity, less hardware resources and so on.