tems: An Introduction", IEEE Commun. Mag., 37(2), 1999, 102-107. The UMTS Network and Radio Access Technology: Air Interface Techniques for Future Mobile Systems Jonathan P. Castro Copyright © 2001 John Wiley & Sons Ltd Print ISBN 0-471-81375-3 Online ISBN 0-470-84172-9 SYSTEM ANALYSIS FUNDAMENTALS FUNDAMENTALS OF SYSTEM ANALYSIS Third generation systems focus on providing a universal platform to afford multifarious communications options at all levels, i.e. the radio as well as the core network sides. This implies the application of optimum techniques in multiple access and inter- working protocols for the physical and upper layers, respectively. This chapter dis- cusses the background of the multiple access or radio part of the UMTS specification. Several sources [5-9] have already covered all types of fundamentals related to the air- interface. Thus, we focus only on the communications environment to access the radio link performance for coverage analysis and network dimensioning in forthcoming chap- ters. 2.1.1 Multiple Access Options The access technologies utilized in UTRA are unique because of the type of implemen- tation and not because they are new. The combination of CDMA and TDMA techniques in one fully compatible platform, make UTRA special. The WCDMA and hybrid TDMA/CDMA form the FDD and TDD modes to co-exist seamlessly to meet the UMTS services and performance requirements. In the sequel we cover the fundamental characteristics for each access technique which serves as a building block for the UTRA modes. 2.1.1.1 Narrow-band Digital Channel Systems The two basic narrow-band techniques include FDMA (using frequencies) and TDMA (using time slots). In the first case, frequencies are assigned to users while guard bands maintain between adjacent signal spectra to minimize interference between channels. In the second case, data from each user takes place in time intervals called slots. The ad- vantages of FDMA lie on efficient use of codes and simple technology requirements. But the drawbacks of operating