The Linc Transmitter Linearity And Slip Imbalance Compensation

The basic principle of the LINC technique is separating the envelope modulated bandpass waveform into two constant-envelope phase-modulated signals that can be efficiently processed with nonlinear PAs without traditional AM-AM or AM-PM distortion taking place in the individual branches. With this approch, the efficiency and linearity of the transmitter can be improved greatly. And it is also of extremely important significance for the application of high spectrum efficiency modulation techniques. Although this technique has the great advantage, it hasn't been widely used in communication market. Its major drawback is the extremely high request on the matching of the two amplifier paths to achieve acceptably small out-of-band rejection. In practical applications, due to impairment of the device and the circuit design structure, it is difficulty to achieve the accurate gain and phase matching required between the two paths.In this thesis, the path mismatch of the practical LINC system has been analyzed. After deeply analyzing several major imbalance sources that contribute to the linearity degradation, two effectual calibration schemes and algorithms have been introduced and researched. The main work and contributions of this paper are as follows:1. This thesis discusses the LINC system linearity performance. Signal distortion resulting from the mismatch between the two amplifier branches are categorized into the path imbalance, the quadrature error, and the SCS quantization error. These sources of distortion are analyzed successively. At the same time, the effects of the nonlinear distortion of the Chireix-outphasing combiner are also briefly discussed.2. Two simple and effective calibration schemes are researched for practical LINC system, which are used to characterize the path imbalance and correct the error effects at the baseband level. The detailed anlysis, simulation results, and a few practical implementation are presented. The first scheme is a direct path mismatch detection method with a closed-form expression, through a test vector signal set with three phase differences and one amplitude difference being used, the amplitude and phase mismatches value can be detected without any iterations. However, this method only considers the amplitude and phase imbalances parameters, so in practical applications it will be restricted. The other scheme solves this problem, and through the ACPR evulation module being introduced in the feedback channel, an direct searching algorithm is used to adaptively search the best ACPR value of the output signal. In the end the design and realization of ACPR evalution module in FPGA has been researched. The simulation and experiment results show that these two method can effectively improve the linearity of the LINC transmitter.3. This thesis focuses on the linearity performance of the LINC transmitter, and restrains the degradation of linearity resulted from path imbalance after introducing gain and phase precompensation in one of the paths, therefore it improves the linearity of the convential LINC transmitter while still maintaining its high efficiency. At the same time, it also provides theoretical support for the practical application of the LINC transmitter.