# Các mạng UTMS và công nghệ truy cập vô tuyến P2

Chia sẻ: Do Xon Xon | Ngày: | Loại File: PDF | Số trang:28

0
55
lượt xem
10

## Các mạng UTMS và công nghệ truy cập vô tuyến P2

Mô tả tài liệu

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 interworking protocols for the physical and upper layers, respectively.

Chủ đề:

Bình luận(0)

Lưu

## Nội dung Text: Các mạng UTMS và công nghệ truy cập vô tuyến P2

2. 14 The UMTS Network and Radio Access Technology 2.1.1.2 Wide-band Digital Channel Systems Some of the drawbacks and limitations in the narrow-band channel systems made room for 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 required to convey the information. These systems include primarily Spread Spectrum (SS) sys- tems, e.g. Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). In DSSS, emphasized in this book, the transmission bandwidth ex- ceeds the coherent bandwidth, i.e. the received signal after de-spreading resolves into multiple 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 greater resistance to interference effects when compared to FDMA and TDMA. The latter greatly 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 or TDMA. With DS-CDMA, e.g. adjacent micro-cells share the same frequencies, whereas interference in FDMA and TDMA does not allow this. Other benefits and features can be 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: WCDMA Figure 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 variable spreading factor and multi-code links. It supports highly variable user data rates through the 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. It realizes a chip rate of 3.84 Mcps within 5 MHz carrier bandwidth, although the actual carrier spacing can be selected on a 200 kHz grid between approximately 4.4 and 5 MHz, depending on the interference situation between the carriers. ÃArrpÃ Ã WhvhiyrÃivÃhrÃ rvprÃ QrÃ CvtuÃivÃhrÃ ##$ÃHCÃ r vprÃ UvrÃ 9vssr rÃ rhqvtÃshp Ã ÃÃ rtÃhyyvtÃ'±"'#ÃxiÃ Figure 2.1 The UTRA WCDMA or FDD mode characteristics. _______ 2 Direct Sequence Code Division Multiple Access. 3. System Analysis Fundamentals 15 The FDD has a self timing point of reference through the operation of asynchronous BSs, and it uses coherent detection in the up- and downlink based on the use of pilot reference symbols. Its architecture allows the introduction of advanced capacity and coverage enhancing CDMA receiver techniques, e.g. multi-user detection and smart adaptive antennas. In addition, it will seamlessly co-exist with GSM networks through its inter-system handover functions of WCDMA. 2.1.1.4 The UTRA TDD Mode: TD/CDMA The 2nd UTRA mode results from the combination of TDMA–FDMA and exploits spreading as part of its CDMA component. It operates in Time Division Duplexing using the same frequency channel. ÃA rrp Qr ##$ÃHC Hyvpqr Uvr Hyvy  PV IUDPH HyvpqrÃÃHyvy WhvhiyrÃTrhqvt Figure 2.2 UTRA TDD mode characteristics. In this mode, the MSs can only access a Frequency Division Multiplexing (FDM) chan- nel at specific times and only for a specific period of time. Thus, if a mobile gets one or more Time Slots (TS) allocated, it can periodically access this set of TSs throughout the duration of the frame. Spreading codes described in Chapter 4 separate user signals within one or more slots. Hence, in the TDD mode we define a physical channel by a code, one TS, and one frequency, where each TS can be assigned to either the uplink or the downlink depending on the demand. Users may obtain flexible transmission rates by occupying several TSs of a frame as illustrated in Figure 2.2, without additional proc- essing resources from the transceiver hardware. On the other hand, when more than one frequency channel gets occupied, utilization of transceiver resources will increase if the wide-band transmission cannot prevent it. We achieve variable data rates through either multi-code transmission with fixed spreading or through single code with variable spreading. In the 1st case, a single user or users may get multiple spreading codes within the same TS; while in the 2nd case, the physical channel spreading factor may vary according to the data rate.
4. 16 The UMTS Network and Radio Access Technology 2.1.2 Signal Processing Aspects In the following, we review Signal Processing characteristics for the WCDMA as well as TD/CDMA as a base to describe key functions of the UTRA FDD and TDD modes. These include spreading aspects and modulation and coding. 2.1.2.1 The Spread Spectrum Concept Digital designs of communications systems aim to maximise capacity utilization. We can for example increase channel capacity by increasing channel bandwidth, and/or transmitted power. In this context, CDMA operates at much lower S/N ratios as a result of the extra channel bandwidth used to achieve good performance at low signal-to-noise ratio. From Shannon’s channel capacity principle [22] expressed as: 6Þ & = % ORJ Î + Ï  
5. Ð 1ß à where B is the bandwidth (Hz), C is the channel capacity (bits/s), S is the signal power, and N is the noise power; we can find a simple definition of the bandwidth as: & 1 %=   
6.  6 Thus, for a particular S/N ratio, we can achieve a low information error rate by increas- ing the bandwidth used to transfer information. To expand the bandwidth here, we add the information to the spreading spectrum code before modulation. This approach ap- plies for example to the FDD mode, which uses a code sequence to determine RF bandwidth. The FDD mode has robustness to interference due to higher system process- ing gain3 Gp. The latter quantifies the degree of interference rejection and can be de- fined as the ratio of RF bandwidth to the information rate: % *S =  
7. 5 From Ref. [23] in a spread-spectrum system, thermal noise and interference determine the noise level. Hence, for a given user, the interference is processed as noise. Then, the input and output S/N ratios can relate as: Ë6Û Ë6Û Ì Ü = *S Ì Ü  
8. Í 1 ÝR Í 1 ÝL Relating the S/N ratio to the Eb/No ratio4, where Eb is the energy per bit and No is the noise power spectral density, we get: Ë6Û (E  5 (E  Ì Ü = =   
9. Í 1 ÝL 1 R  % 1 R *S From the preceding equations we can express Eb/No in terms of the S/N input and output ratios as follows: _______ 3 Reference processing gains for spread spectrum systems have been established between 20 and 50 dBs. 4 Unless otherwise specified, here we assume that N includes thermal and interference noise. o
10. System Analysis Fundamentals 17 (E Ë6Û Ë6Û = *S  Ì Ü = Ì Ü  