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1、 国脉信息学院电子信息工程多多Wideband CDMA1. IntroductionWideband Code-Division Multiple Access ( WCDMA), an International Telecommunications Union (ITU) standard derived from Code Division Multiple Access (CDMA), is officially known as IMT-2000 direct spread. WCDMA is a third-generation (3G) mobile wireless tech
2、nology that promises much higher data speeds to mobile and portable wireless devices than commonly offered in todays market.WCDMA can support mobile/portable voice, images, data, and video communications at up to 2 Mbps (local area access) or 384 Kbps (wide area access). The input signals are digiti
3、zed and transmitted in coded, spread-spectrum mode over a broad range of frequencies. A 5 MHz-wide carrier is used, compared with 200 kHz-wide carrier for narrowband CDMA.CDMA is a multiple access technology that was invented in the 1950s. Hence there are no patents covering CDMA today, as any such
4、patents would have expired long ago. CDMA was first proposed for digital cellular use at the end of the 1970s and although not widely known, spread spectrum and CDMA were also proposed as technologies for the GSM standard as early as 1986. Later, IS-95 CDMA technology was used in the 2G standard and
5、 the 3G standard WCDMA.WCDMA is a wideband spread-spectrum 3G mobile telecommunication air interface that utilizes code division multiple access. It provides simultaneous support for a wide range of services with different characteristics on a common 5 MHz carrier.The term WCDMA also refers to one o
6、f the ITUs IMT-2000 standards, a type of 3G cellular network. WCDMA is the technology behind the 3G UMTS standard and is closely allied with the 2G GSM standard. It provides new service capabilities, increases network capacity and reduces cost for voice and data services.2. BackgroundThere has been
7、a tremendous growth in wireless communication technology over the past decade. The significant increase in subscribers and traffic, new bandwidth consuming applications such as gaming, music down loading and video streaming will place new demands on capacity. The answer to the capacity demand is the
8、 provision of new spectrum and the development of a new technology Wideband CDMA or hereinafter referred to as WCDMA.WCDMA was developed in order to create a global standard for real time multimedia services that ensured international roaming. With the support of ITU a specific spectrum was allocate
9、d 2GHz for 3G telecom systems. The work was later taken over by the 3rd Generation Partnership Project (3GPP) 9 which is now the WCDMA specification body with delegates from all over the world. Ericsson has for a long time played a very active role in both ITU and 3GPP and is a major contributor to
10、WCDMA and the fulfillment of the vision of a global mobile telecommunication system.During the 1960s and 1970s, CDMA was primarily employed for military use and it was not until the late 1970s and early 1980s that CDMA was proposed as an MA technology for digital cellular mobile telephony. An advant
11、age of CDMA for cellular use is avoidance of frequency planning, because the same carrier frequencies can be used in all cells, which is typically not possible in early FDMA-based systems due to higher sensitivity to interference. However using CDMA for cellular was not without new challenges. At a
12、given base station, signals from different mobile devices could have extremely large differences in received power levels, due to radio propagation phenomena and the fact that the devices are located at different distances from the base station. Since all mobile1 国脉信息学院电子信息工程多多devices use the same c
13、arrier frequency, weaker signals would be drowned by stronger ones. Even the built-in interference robustness of CDMA would not be enough to compensate. This was known as the so-called near-far problem.While GSM took its first steps towards global success, programs to develop 3G systems began in Eur
14、ope. 3G referred to a system not only optimized for speech, but also with a high service flexibility and high throughput and capacity for high-speed data. It was anticipated that such a system would be needed at the turn of the millennium.Several potential multiple access concepts for 3G, including
15、Advanced TDMA (ATDMA, based on TDMA) and Code Division Testbed (CoDiT, based on DSCDMA), were studied in the RACE II project as part of the third EU frame program for pan-European research collaboration which began in 1992. Ericsson was heavily involved in CoDiT, which aimed to define a wideband (ar
16、ound 5 MHz per carrier) DSCDMA, It is based on radio access for 3G and to create a testbed to demonstrate its characteristics.The CoDiT testbed work, which included field demonstrations, concluded successfully in 1994 and led to the creation of the first feasible wideband CDMA concept for cellular.M
17、eanwhile, Ericsson had started building its own test system, the WideBand TestBed (WBTB), based largely on the CoDiT concept. A number of successful field tests in 1995 and 1996 played an important role in Ericssons growing knowledge in the field of wideband CDMA.The subsequent Future Radio wideband
18、 Multiple Access Systems (FRAMES) project began in 1995. It was developed as part of the fourth EU frame program dubbed Advanced Communication Technologies and Services and aimed to put forward a European candidate 3G proposal. Within FRAMES a number of schemes were developed and many of their funda
19、mental principles were inherited from the earlier RACE II projects, including CoDiT.In early 1997, the European Telecommunications Standards Institute (ETSI) formed five concept groups where development continued. The concepts came from FRAMES: Alpha, or WCDMA , originally from FRAMES Beta, or Ortho
20、gonal Frequency Division Multiple Access (OFDMA) Gamma, or Wideband TDM A (WTDMA), originally from FRAMES Delta, or Time Division 桟 ode Division Multiple Access (TD-CDMA), originally from FRAMES Epsilon, a concept based on Opportunity Driven Multiple Access (ODMA) , later considered a complement to
21、the othersMeanwhile in Japan, NTT DoCoMo had by 1995 developed a wideband CDMA based 3G concept including a functioning testbed. It was put forward as one of many proposals to the Japanese standardization body the Association of Radio Industries and Businesses (ARIB) and was preliminarily selected a
22、s the Japanese 3G proposal in late 1996.By 1997 the European and Japanese WCDMA concepts were already mature with respect to most of the basic building blocks of the radio part. The two concepts were then merged during 1997 and in January 1998 ETSI preliminarily selected WCDMA as its proposal for a
23、3G radio access network. Importantly, WCDMA was to use the GSM/GPRS core network, which minimized the need for new investments for the vast number of existing GSM network operators. This and the subsequent founding of the 3GPP in December 1998, which became responsible for all WCDMA and GSM standard
24、ization, paved the way for the global success of the WCDMA standard.WCDMA development is shown as Fig. 5-2.2 国脉信息学院电子信息工程多多3. Technologies in WCDMAIn this section we describe some of the important innovations that constitute the WCDMA standard, developed in response to the challenging 3G requirement
25、s placed on it. We also include descriptions of how the technologies differ from those used in the IS-95 family of standards. In particular we discuss functions related to the air interface, which is the interface between the mobile device and the network.Asynchronous network operationSeveral of the
26、 requirements for WCDMA did not exist in legacy standards, including IS-95. One such requirement is asynchronous network operation. Early on, one of the fundamental technical principles shared by the Japanese and European WCDMA concepts including CoDiT was asynchronous operation; that is, the base s
27、tations should not have to be synchronized to each other. This was an opposite requirement to that of the IS-95 which is named CDMA2000 standard later, which relies on GPS for its proper operation. The UMTS requirement for an asynchronous system was fundamentally important because it led to a standa
28、lone and easily deploy able system attractive to operators around the world. But it also gave rise to several new technological challenges for WCDMA, including cell search and handover, described below.Cell searchBefore engaging in any kind of communication with the network, a mobile device must fir
29、st synchronize to the signals sent from base stations. Mobile devices need to find out which base station code to look for and then synchronize to it. Searching for all possible base station codes at all possible times is an arduous task that could potentially affect the time from power-on to placin
30、g a call. Being asynchronous , WCDMA could not employ the principle adopted in the IS-95/CDMA2000 standards, where a common base station code with unique GPS time-shifts was used throughout the system. Clearly there was a need for innovation in this area. The early Japanese version of WCDMA included
31、 a structure based on dual synchronization channels, but it was generally considered too complex. Extensive research was conducted to find an information encoding principle that would minimize the cell search time while still keeping the complexity of the search at a negligible level. Solutions were
32、 progressively proposed to the standard and soon the goals had been achieved. One of the early major technical challenges of asynchronous CDMA-based 3G had been overcome.3 国脉信息学院电子信息工程多多Soft handoverIn most elder mobile systems, the mobile devices are connected to one base station at a time and hand
33、over is the process of seamlessly shifting a mobile device connection from one base station to another. In soft handover however, mobile devices communicate the same information through two or more base stations simultaneously ( see Fig. 5- 3 ). In practice, soft handover is a prerequisite for any h
34、igh-capacity DS-CDMA-based system operating with the same carrier frequency inneighboring cells. As early as 1988, Jan etal. , of Ericsson, pioneered the fundamental principles of soft handover in some of Ericssons early patents in the digital cellular field.In soft handover, the signals transmitted
35、 from different base stations need to arrive at the device at the same time. This posed some interesting design challenges, because the relative timing of the base stations was not known beforehand due to the asynchronous operation requirement in WCDMA. To solve this problem,WCDMA enbracesa kind of
36、per-call synchronization principle, earlier adopted in CoDiT. Prior to connecting to a new base station, mobile devices measure a time difference related to the new base station and the already connected base stations. This measurement is reported to the network controller, which adjusts the transmi
37、ssion timing of the new base station accordingly.Compressed modeAlready from the beginning, virtually all WCDMA mobile devices were expected to be dual-mode to support GSM as the initial WCDMA coverage would probably not be as wide as that of GSM. Therefore , handover of ongoing connections between
38、WCDMA and GSM was crucial for a smooth early rollout of WCDMA. For a secure handover from WCDMA to GSM, the mobile device needs to perform signal monitoring on potential GSM target base stations in the area, even during an active call on the WCDMA system. The problem is to find time for this, becaus
39、e the mobile device is busy all the time with sending and receiving on a given WCDMA carrier frequency. A technology denoted 44 Compressed Mode, originally developed in CoDiT, was adopted in WCDMA to solve this problem.Time for monitoring is made available to the mobile device by, for example changi
40、ng the so-called Spreading Factor (SF) and hence compressing the information to be received and transmitted into a shorter time span, leaving idle time for measurements on other frequencies, including other systems. These measurements are then reported to the network so a decision on the WCDMA to GS
41、M handover can be made. Compressed mode is also used for handover within the WCDMA system from one carrier frequency to another (see Fig. 5-4).4 国脉信息学院电子信息工程多多Uplink power controlThe near-far problem resulting from using DS-CDMA for cellular was already known in the late 1970s and one envisaged solu
42、tion was to use some form of power control in the uplink, which is the transmission from the mobile device to the network. Open-loop essentially means that the mobile device sets its transmission power based on measurements of the received signal power, while closed-loop means the mobile device sets
43、 its transmission power based on explicit commands received from the network (see Fig. 5-5). Such commands are in turn based on measurements of the received signal made at the network side.When IS-95 was defined, a combination of open-loop and closed-loop uplink power control was chosen and standard
44、ized as a solution to the near-far problem. This meant that the mobile device set its power based on both the received signal power as well as on commands received from the network. This scheme was later inherited by CDMA2000, a modification of the IS-95 standard.However, when the uplink power contr
45、ol solution was standardized for WCDMA, no particular benefits of such a dual-loop solution could be identified and instead a pure closed-loop system was selected, but with an update rate twice as high (1500Hz versus 800Hz).Random accessRandom access, realized in WCDMA through the Random Access Chan
46、nel (RACH) , is the mechanism through which the mobile terminal performs the initial access to the system. As the terminal is not yet connected to the system, no uplink codes have been allocated, nor is there any control of the uplink power. Thus, mechanisms for RACH code allocation and power settin
47、g were needed. To minimize signaling and required power, a solution was adopted in WCDMA in which no explicit signaling of codes is needed. All RACH-related codes are derived according to given rules using only the downlink , which is the transmission from the network, scrambling code and a signatur
48、e that is randomly selected by the mobile device.As illustrated in Fig. 5-6, in WCDMA preambles with randomized signatures are sent ( uplink) with increasing power in a ramping procedure. When the power is sufficient, the network transmits an acquisition indicator to request transmission of the actual message from the terminal. This ramping procedure is very fast and together with the signature randomization leads to an efficient, high-capacity RACH in WCDMA.In contrast, the IS-95/CDMA2000 stan