Radio Transmission Systems Mamoru Sawahashi Direct Sequence Code Division Multiple Access (DS-CDMA) Principles of DS-CDMA DS-CDMA is a radio-access technology that enables multiple access based on a spread spectrum system. Figure shows how DS-CDMA works [1–3]. The transmitted data sequence is spread across the spectrum after being encoded by spreading codes, each of which is assigned uniquely to each user at a higher rate than the symbol rate of the information data. [Wideband Code Division Multiple Access (W-CDMA) spreads the information data over a 5 MHz band per carrier.] The spread high-speed data sequence is referred to as chip and. | W-CDMA Mobile Communications System. Edited by Keiji Tachikawa Copyright 2002 John Wiley Sons Ltd. ISBN 0-470-84761-1 2 Radio Transmission Systems Mamoru Sawahashi Direct Sequence Code Division Multiple Access DS-CDMA Principles of DS-CDMA DS-CDMA is a radio-access technology that enables multiple access based on a spread spectrum system. Figure shows how DS-CDMA works 1-3 . The transmitted data sequence is spread across the spectrum after being encoded by spreading codes each of which is assigned uniquely to each user at a higher rate than the symbol rate of the information data. Wideband Code Division Multiple Access W-CDMA spreads the information data over a 5 MHz band per carrier. The spread high-speed data sequence is referred to as chip and the rate at which the spread data varies is called chip rate. The ratio of chip rate to symbol rate is called the Spreading Factor SF . The destination mobile phone uses the same spreading code as the one used for spreading at the transmission point to perform correlation detection a process called despreading in order to recover the transmitted data sequence. As signals received by other users carry different spreading codes the signal power is reduced evenly to 1 SF. In DS-CDMA all users share the same frequency band and time frame to communicate and each user is identified by a spreading code uniquely assigned to the user. In contrast as shown in Figure Frequency Division Multiple Access FDMA assigns to each user a different carrier frequency depending on the frequency generated in the frequency synthesizer and Time Division Multiple Access TDMA assigns to each user not only a carrier frequency but also a time slot hereinafter referred to as slot to engage in communications. At the reception point the frequency generated by the frequency synthesizer is set in such a manner that the signals in the assigned carrier frequency can be down-converted in the destination mobile phone and the transmitted data
