Study on the Characteristic of WCDMA Uplink Complex Spreading and Modulation

In 3G standards WCDMA and CDMA2000, the complex spreading modulation technology is employed. The feature of multiple channels enabled simultaneously is one of the main characteristics of 3G systems, which maybe result in a complicated constellation and very large scope of voltage which can influence the linearity of the power amplifier and generate interference between the signals of I-path and Q-path. Taking the uplink of early versions of WCDMA as an example, the constellation is studied firstly and a result of simulation given. The simulations show that the constellation of the combination signals is QAM, not QPSK, which is mapped by the combination signals of multiple channels in I-path and Q-path. What\´s more, the feature is enhanced by complex scrambling, which make the points from constellation rotated and the vectors lengthened. Besides this, based on the research of constellation, the nonlinear orthogonal interference in power amplifier is also researched with some mathematics analyses, which explain clearly the nonlinear orthogonal interference. In order to overcome the influence of nonlinearity of power amplifier, a reform scheme of complex scrambling with phase pre-estimation, called "double complex scrambling scheme with phase pre-compensation", is proposed. A phase pre-estimator and a normal scrambling structure are employed in this scheme. Because of the simple processing of normal scrambling and the mature estimation technology, the reform scheme is feasible, which can enhance the performance of complex spreading and modulation.