Amplifier linearization for broadband wireless applications.

摘要

Amplifier linearization is a novel way to obtain good amplifier efficiency performance without increasing distortion due to its nonlinear operation. The goal of this work is to develop a linearization scheme for broadband wireless systems. It was determined that digital predistortion is the most promising technique for a broadband implementation. This technique was used as a basis for the remainder of the work.; Improvements in digital predistortion look-up tables were developed. The first enhancement was in non-uniform table spacing. A sub-optimal approach was developed that provided the same performance as optimal spacing, but is simple enough to allow a realizable solution. Another advancement is the normalization of the tables at the operating point instead of the maximum gain point. This allowed the amplifier to operate over a wider dynamic range, provide more correction and improve efficiency. The final enhancement to digital predistortion was adaptive indexing. It was shown that adaptive indexing is a necessity with non-uniformly spaced tables.; Digital predistortion with an equally spaced look-up table was used in a multi-carrier spread spectrum system. It was shown that a simultaneous improvement in bit error rate could be achieved with a reduction in the Output-Back-Off, (OBO), and an increase in the amplifier efficiency. Up to 4 dB of improvement in OBO was seen with an increase in efficiency of 25 to 30 percentage points.; Selecting the appropriate model with memory was very critical to the development of the linearizer. This model was developed with a broadband measurement system and revealed how the amplifier responded differently to various data rates. The resulting amplifier model predicted these broadband responses to the input signal.; Finally a new broadband linearizer is detailed. This predistortion linearizer is composed of a nonlinear tapped delay line. Performance is quantified and compared to the standard memoryless look-up table predistorter. This new linearizer is digital, adaptive, and requires a low number of coefficients for satisfactory performance. This linearizer is the first of its kind to be a full transmitter solution that compensates for nonlinearity and memory in one topology.