Design of a wideband CTDMA modem for broadband satellite communications

Code division multiple access (CDMA) promises to become the enabling technology for the future generation of satellite communications. For instance, CDMA is being used in Globalstar, Skybridge and is also proposed for the satellite component of UMTS. The evolution of satellite systems towards multimedia communications, necessitates not only wideband CDMA but also requires a hybrid code and time division multiple access (CTDMA) technique because of the packet nature of the data. In this paper, we give an overview of the challenges in designing a wideband CTDMA modem. As a first challenge, the wideband nature of the CDMA modem dictates that the tracking loops of the modem can no longer be executed in software but must be fully integrated in VLSI. Second, the packet based nature of the communication in a CTDMA modem requires ultra fast acquisition for each packet. The extremely low signal-to-noise ratio in satellite communications largely complicates this fast acquisition task