Các mạng UTMS và công nghệ truy cập vô tuyến P2

SYSTEM ANALYSIS FUNDAMENTALSFUNDAMENTALS OF SYSTEM ANALYSISThird 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 interworking protocols for the physical and upper layers, respectively.
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Các mạng UTMS và công nghệ truy cập vô tuyến P2 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 FUNDAMENTALS2.1 FUNDAMENTALS OF SYSTEM ANALYSISThird generation systems focus on providing a universal platform to afford multifariouscommunications 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 radiolink performance for coverage analysis and network dimensioning in forthcoming chap-ters.2.1.1 Multiple Access OptionsThe 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 techniquesin one fully compatible platform, make UTRA special. The WCDMA and hybridTDMA/CDMA form the FDD and TDD modes to co-exist seamlessly to meet theUMTS services and performance requirements. In the sequel we cover the fundamentalcharacteristics for each access technique which serves as a building block for the UTRAmodes.2.1.1.1 Narrow-band Digital Channel SystemsThe 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 bandsmaintain between adjacent signal spectra to minimize interference between channels. Inthe 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 at a reduced signal/interference ratio and the inhibitingflexibility1 of bit rate capabilities outweigh the benefits. TDMA allows flexible rates inmultiples of basic single channels and sub-multiples for low-bit rate broadcast transmis-sion. It offers frame-by-frame signal management with efficient guard band arrange-ments to control signal events. However, it requires substantial amounts of signal proc-essing resources to cope with matched filtering and synchronization needs._______1 The maximum bit per channel remains fixed and low.14 The UMTS Network and Radio Access Technology2.1.1.2 Wide-band Digital Channel SystemsSome of the drawbacks and limitations in the narrow-band channel systems made roomfor wide-band channel system designs. In wide-band systems the entire bandwidth re-mains available to each user, even if it is many times larger than the bandwidth requiredto convey the information. These systems include primarily Spread Spectrum (SS) sys-tems, e.g. Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping SpreadSpectrum (FHSS). In DSSS, emphasized in this book, the transmission bandwidth ex-ceeds the coherent bandwidth, i.e. the received signal after de-spreading resolves intomultiple time-varying delay signals that a RAKE receiver can exploit to provide an in-herent time diversity receiver in a fading environment. In addition, DSSS has greaterresistance to interference effects when compared to FDMA and TDMA. The lattergreatly simplifies frequency band assignment and adjacent cell interference. In addition,capacity improvements with DSSS or more commonly referred to as DS-CDMA2, re-sulting from the voice activity factor, which we cannot apply effectively to FDMA orTDMA. With DS-CDMA, e.g. adjacent micro-cells share the same frequencies, whereasinterference in FDMA and TDMA does not allow this. Other benefits and features canbe found in [10–12]. Here we focus on the WCDMA or FDD mode and TDMA/CDMA or TDD mode of the UTRA solution.2.1.1.3 The UTRA FDD Mode: WCDMAFigure 2.1 illustrates some of the UTRA Frequency Division Duplexing (FDD) charac-teristics. This mode uses Wide-band Direct-Sequence Code Division Multiple Access(DS-CDMA), denoted WCDMA. To support bit rates up to 2 Mbps, it utilizes a variablespreading factor and multi-code links. It supports highly variable user data rates throughthe allocation of 10 ms frames, during which the user data rate remains constant, al-though the latter may change from frame to frame depending on the network control. Itrealizes a chip rate of 3.84 Mcps within 5 MHz carrier bandwidth, although the actualcarrier spacing can be selected on a 200 kHz grid between approximately 4.4 and5 MHz, depending on the interference situation between the carriers. ÃA…r„ˆrp’à à Wh…vhiyrÃiv‡Ã…h‡rà †r…‰vpr†Ã Q‚r…à CvtuÃiv‡Ã…h‡rà ##$ÃHC“à †r…‰vpr†Ã Uv€rà 9vssr…r‡Ã†ƒ…rhqvtÃshp‡‚…†Ã À†Ã rtÃhyy‚vtñ#Ãxƒi†Ã Figure 2.1 The UTRA WCDMA or FDD mode characteristics._______2 Direct Sequence Code Division Multiple Access.System Analysis Fundamentals ...