| In LINC (LInear amplification with Nonlinear Components) system, an amplitude- and phase-modulated input signal is decomposed into two constant envelope signals. Highly efficient nonlinear PAs can then be used to amplify the constant envelope signals without traditional AM/AM or AM/PM distortion taking place in the individual branches. An amplified reconstruction of the original signal is obtained by summing the amplified branch signals in a power combiner. LINC is an ideal scheme for transmitting signals with high PAPR (Peak to Average Power Ratio) in the future communication system.The inherent drawback of LINC is that, the output signal is very sensitive to the amplitude/phase imbalance in the quadrature modulators. Slight misalignments can cause remarkable degradation of the ACPR (Adjacent Channel Power Ratio). For this issue, this dissertation discusses the physical imperfections in the quadrature modulators and the effects of carrier leakage, amplitude/phase imbalance on LINC output signal. According to the AM/AM and AM/PM characteristic of I and Q channels, a polynomial model is developed, which makes preparation for the future correction of various non-ideal characteristics. The main works completed are as follows:1. Detailed analysis is made to the various non-ideal factors in the quadrature modulator, its causation and its effects on the output signal.2. The effects of carrier leakage and amplitude/phase imbalance on the LINC output signal spectrum are studied by means of mathematical derivation and simulation.3. Based on the AM/AM and AM/PM characteristic, a polynomial model is developed for the quadrature modulators. The preciseness of the model is validated through ACPR, EVM (Error Vector Magnitude) and sideband suppression.4. A method for capturing the IQ data using Spectrum Analyzer is put forward. The actual quadrature modulator is modeled. A method for compensating linear error is given, and experimental results demonstrate the effectiveness of the method. |
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