放大器预失真技术分析与实现

【作者】 梁伦敏；

【导师】 彭启琮；

【作者基本信息】 电子科技大学 ， 通信与信息系统， 2007， 硕士

【摘要】 绝大多数无线通信系统都要求对相邻频段的用户产生最小的干扰,也就是必须在所规定的频段范围内传送信号。但是,由于非线性器件的失真,必然会对相邻信道产生不同程度的干扰。如果采用恒包络调制(MSK、FM等),谐波干扰可通过滤波器滤除。为了利用有限的带宽,容纳更多的信道,现代通信系统多采用频谱利用率较高的线性调制和宽带通信传输技术。典型的线性调制技术,如QAM(Quadrature Amplitude Modulation)和QPSK,它们的包络上下波动起伏。这种信号通过非线性功率放大器时,工作在饱和区域附近的器件会产生交调失真,也就是由功率放大器的AM-AM、PM-PM特性所带来的失真。这种失真表现在频谱上,则是其谱发散到邻近信道,从而对其它用户产生干扰。现代无线通信系统一般都要求邻近信道干扰的水平比带内低40-60dB。所以,线性调制信号由于其非恒包络的特性,在带来优越的带宽利用效率的同时,也对系统的非线性失真非常敏感。为了获得更大的输出功率,放大器不得不工作在饱和状态,此时非线性失真也随着功率的增高变得更为严重。既要提高频谱利用率,又要保证信号不失真,解决这一问题的办法,便是使功率放大器线性化。在GSM ( global system for mobile )、IS-95系列, CDMA2000 ,WCDMA(wide-band code division multiple access)等现代无线通信系统中,人们已经对射频功率放大器的线性化技术做了广泛而深入的研究。本文研究了前人所提出的各种功放线性化技术,如功率回退法、正负反馈法、预失真和非线性器件法等等,采用射频技术与数字技术相结合,组成自适应预失真系统。论文的特色在于,以数字信号处理技术为核心的数字电路与射频模拟电路相结合,有效实现自适应算法,进一步提高功率放大器的线性化程度;将数字信号处理器与FPGA相结合,使系统既有灵活的可操作性,又具有快速处理数字信号的能力,缩短了自适应搜索的时间,提高了系统的整体性能和稳定性。最后,通过实际测试,指出了存在的问题和不足,并提出了可能的改进方案。更多还原

【Abstract】 Most of the wireless communication systems are required to achieve least adjacent channel emissions and high bandwidth efficiency, that is to say signals must be transmitted in a limited band. However, distortion caused by nonlinear devices certainly will disturb other channels. In order to take good advantage of the limited band and contain more channels, modern communication systems often use linear modulation and wideband transmission techniques. Typical modulation techniques, such as QAM (Quadrature Amplitude Modulation) and QPSK, have varying envelopes. Devices working near saturation will produce inter-modulation distortion while those kind of signal flowing through nonlinear power amplifier, specifically the distortion brought by the AM-AM and PM-PM character of the amplifier which will disturb other users. In modern mobile communication systems, the adjacent channel interference requirements range from -45 to -60 dBc relative to the in-band carrier level. The linear modulation signal which has the advantage of high band efficiency is sensitive to the nonlinear distortion of the system because of its nonconstant envelope. In order to obtain high output power, amplifier has to work at saturation situation, while the nonlinear distortion becomes severer as the power is increasing.People have done widely research on the linearization technique of RF power amplifier in modern wireless communication systems, such as GSM, IS-95 serial, CDMA2000 and WCDMA.This theme adequately analyses several main types of linearization techniques people proposed: feed-forward, feedback, and predistortion. The application of the digital signal processing combined with the RF circuit makes the predistortion algorithm more efficient and the system more flexible, stable and better performance.At last, based on the actual testing results, the existing problems are pointed out, as well as the possible method for improvement.更多还原