ABSTRACT
Code division multiple access (CDMA) is a key radio access technology used in the development of third-generation wireless systems in response to a worldwide growth and demand in mobile communications. This technology, based on spread spectrum communication techniques, inherently fulfills most of the requirements for future generation wireless systems and achieves better bandwidth efficiency. Since CDMA signals occupy larger bandwidth, they are resilient to interference and show smooth capacity degradation as more users are accepted and served. These attractive features of CDMA enable higher bit rates over the air interface, offer better coverage, and quality of service to more users. This chapter describes the CDMA concept and design aspects and highlights the benefits and characteristics of the CDMA technique. The chapter then focuses on CDMA as used in UMTS by emphasizing code construction and describing reception using Rake receiver structures. The chapter serves also as background material for Chapters 3 and 4 addressing the UMTS network and the HSDPA technique used for enhancing UMTS bit rates, radio quality, and capacity.
2.1 CDMA Principle The multiple-access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA) are the keys of the actual cellular systems, 2 and 3G. In fact, cellular systems are conceived as a whole of resources,
time-frequency-power, to be shared by a given number of users. FDMA and TDMA systems are considered as a degree of limited freedom since the frequency and the time are multiplexed among users. CDMA systems are based on a completely different approach: All resources are allocated to all simultaneous users, and the transmission power is controlled to maintain a given value of signal to noise ratio (SNR) with the minimum required power. This use of all time-frequency degrees of freedom is possible by using the direct sequence spread spectrum technique.
