Coding and Modulation for Wideband CDMA

Abstract: Wideband CDMA (Code Division Multiple Access) is one of the key technologies for future mobile communication systems. This thesis studies various coding and transmission methods for wideband CDMA communications. In a cellular environment, the system performance depends on the interference from other cells. An analysis of the other-cell interference in the forward link, from the base station to the mobile terminal, is presented and compared to the other-cell interference in the reverse link, from the mobile terminal to the base station. In the forward link a coordinated transmission algorithm, based on the knowledge about the spreading sequences, channel conditions etc, can be applied to tailor the transmitted signal in order to improve the performance for the users. Coordination can be viewed as a way to transfer the complex operations to the base stations to allow lower complexity in the battery powered mobile terminals. Two coordination strategies are presented. The first is based on a fixed correlation between the users’ signals at the receivers. This strategy is analyzed by an information theoretical method, indicating a gain of up to a factor two in overall code rate over a conventional system. The second strategy is developed for a system encoded by a convolutional code. Simulations show that this method has a significant gain, in the number of active users, over a conventional system. Error correcting codes are an essential part of all digital communication systems. A low-rate turbo coding scheme based on superorthogonal convolutional codes, suitable for wideband communications is analyzed. The results, including a theoretical analysis and simulations, show that these codes can achieve low bit error rates within 0.6 dB from the Shannon limit. Impulse radio is a technology for transmission of signals of very large bandwidth. This technique does not modulate the signal by a carrier and hence, the wideband signal can be transmitted over a lower frequency band, compared to a carrier-based system. Multiple access schemes for the forward and reverse links of an impulse radio based system are presented and analyzed. It is shown that several thousands of users can co-exist in the ultrawide frequency band.