Multiuser detection for multirate CDMA communications

We study ways of devising multirate channels for wireless CDMA communications with a focus on asynchronous uplink reception. Methods based on parallel and serial transmission are reviewed. We consider a serial transmission scheme in which the data rate is increased by varying the processing gain. In this transmission concept the processing gain is changed for multirate transmission while maintaining a fixed chip rate and symbol alphabet. A spatio-temporal multistage multiuser receiver is introduced for providing acceptable service quality in a cell where multiple users with different data rates coexist. First, it is argued that multiuser detection is imperative, when the information capacity is maximized for mixed services. Second, it is shown that the proposed multirate multiuser receiver is considerably less complex than a low rate system with comparable information capacity. The simulations carried out in the pico and micro cellular channel models developed in the European CODIT project verify the performance of the considered concept. Simulation results are provided for antenna diversity reception with closed loop power control in a 5.12 MHz band with data rates ranging from 165 kbit/s to 2.62 Mbit/s.