Digital Techniques for Compensation of Radio Frequency Impairments in Mobile Communication Terminals

Abstract: The 3G and 4G systems make use of spectrum efficient modulation techniques which has variable amplitude. The variable amplitude methods usually use carrier‘s amplitude and phase to carry the message signal. As the amplitude of the carrier signal is varied continuously, they are sensitive to the disturbances affecting the information signal by introducing nonlinearities. Nonlinearities not only introduce errors in the data but also lead to spreading of signal spectrum which in turn leads to the adjacent channel interference. In transmitters, the power amplifier (PA) is the main source for introducing nonlinearities in the system, further to this, analog implementation of Quadrature modulator suffers from many distortions, at the same time receiver also suffers from Quadrature demodulator impairments, in particular, gain and phase imbalances and dc-offset from local oscillator, which all together degrades the performance of mobile communication systems. The baseband digital predistortion technique is used for compensation of the Radio Frequency (RF) impairments in transceivers as it provides significant accuracy and flexibility. The Thesis work is organized in two phases: in the first phase, a bibliography on available references is documented and later a simulation chain for compensation of RF impairments in mobile terminal is developed using Matlab software. By using the loop back features of Lime LMS 6002D architecture, it is possible to separate the problems of the Quadrature modulator (QM) and Quadrature demodulator (QDM) errors from the rest of the RF impairments. However in the Thesis work Lime LMS 6002D chip wasn‘t used, as the work was optional. So, in the Thesis work, algorithm is developed in Matlab software by assuming the LMS 6002D architecture. The idea is performed by sequential compensation of all the RF impairments. At first the QM and QDM errors are compensated and later PA nonlinearities are compensated. The QM and QDM errors are compensated in a sequential way. At first the QM errors are compensated and later QDM errors are compensated. The QM errors are corrected adaptively by using a block called as Quadrature modulator correction by assuming an ideal QDM. Later, the QDM errors are compensated by using Hilbert filter with the pass band interval of 0.2 to 0.5. Later, the PAs nonlinearities are compensated adaptively by using a digital predistorter block. For finding the coefficients of predistorter, normalized least mean square algorithm is used. Improvement in adjacent channel power ratio (ACPR) of 13dB is achieved and signal is converging after 15k samples.