可接收数字广播节目的CDMA移动终端的软件设计
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外文翻译第三代无线技术的需要起初，第一代无线网络与话音服务有关，允许使用者从传统的固定电话到移动电话的过渡。起初，第一代系统普遍都是模拟系统。广泛地使用移动电话使模拟技术所能提供的容量很快就被耗尽了，需要引进第二代系统。这种系统使模拟系统提供的话音服务向数字环境过渡，从而增加容量并且允许附加的服务例如:文本信息和低速数据服务。目前正在使用的是第二代网络并且快达到他所能承受的最大容量，值得注意的是快要突破移动电话。第三代系统打算发展现有的系统，进一步增加他的容量和引进多媒体通信。他们想增加功能，把视频和图象增加到话音服务中并打算改进数据网和因特网的通道。和第一代到第二代的过渡不同，第二代到第三代将更加顺利。现有的第二代网络发展到第三代网络将会以 2.5G 网络作为过渡。第三代网络的发展工作正在进行中:科技的作用是非常重要的。第二代网络面对容量增长的需求已经达到了饱和，对于 2.5G 网络来说提高数据的速率和因特网服务是非常吸引人和重要的。解决方法依赖与对现有网络技术的改进并且延长网络使用的寿命，知道第三代网络的提议被最后定下来并得到证实。限制无线网络容量的最主要因素是为这些服务提供的有效的频谱数量，选择调制方式，从根本上提高频谱的利用率，这在频谱方面是极为重要的。另外，手机内部结构来限制功率是很重要的，进一步强调调制的重要性。在第三代网络中引进多媒体服务暗示着在带宽方面的需求量将增加。为了适应容量的增长和带宽的需要，国际无线电通讯联盟的世界性管理无线探讨会为 3G 延长频谱， 2GHz 带宽左右。另外，第三代技术建议用改进的，永久的调制方案。从2G到3G来发展无线电技术1第三代技术的信道无线电技术的发展和标准将应用到可移动的开始于第一代商业移动技术网络的引进的商业服务上，大约 1946。图 2-1 展示了现有第二代到被提议的第三代的进展。第二代技术之一的 GSM 是应用的最广泛，在世界范围内超过 400 百万的使用者。从 2.5G 到 3G 的技术上最好的过渡。图 2-1 从 2G 到 3G 的无线电技术的演变 TDMA Time Division Multiple Access; TDMA-时分多址技术UWC Universal Wireless Consortium世界性的无线电联盟GSM Global System For Mobile Communications全球性的移动通信系统GPRS General Packet Radio Services全球定位系统HSCSD High Speed Circuit Switched Data， EGPRS Enhanced GPRS高速电路开关数据，增强型的全球定位系统ECSD Enhanced Circuit Switched Data增强电路开关数据PDC Pacific Digital Cellular数字蜂窝UMTS Universal Mobile Telecommunications System世界性的无线电通讯系统CDMA Code Division Multiple Access码份多址技术WCDMA Wideband Code Division Multiple Access宽带码分多址技术IMT-2000 International Mobile Telecommunications国际移动无线电通讯2CDMA蜂窝系统的优点窄带 TDMA 系统和 FDMA 系统比起来 CDMA 系统有几个优点。频谱展宽的一个重要作用就是可以防止多径衰减。一个信号的任何两个多径成分被时域分开与脉冲周期无关。多径合成接收器比这个有利事实上他可以改善信噪比。即使没有多径合成器这些不利的成分也会被接收器拒收当用户数量增多的时候CDMA 系统信噪比将会降低。由于软容量的限制不允许接线员增加原有的使用者不象其他多径技术。由于这种软容量的存在在设计一个 CDMA 布局图时平均分配容量是不可能在系统的高峰期。FDMA 和 TDMA 不允许用户超过最大容量但是他们可以为用户提供最低的服务质量然而超负荷的 CDMA 系统在它不能用之前还是可以增加他的差错率。CDMA 系统频率可再用是一个很重要的因素。相同系统中的相同蜂窝允许用户使用相同的双信道。漫游用户不需要改变信道。在两个蜂窝系统的分界线用户有可能没有信号由于 CDMA 系统存在软容量所以不存在这个问题。CDMA 是一个窄带冲突系统由于扇型天线和声音的关系增加容量是有可能的。扇型天线聚集了天线的能量因此可以减少不同扇区用户间的干扰。给每个用户提供合适的天线样式正在研究中。禁止发射机在用户不讲话时发射能量使用这种方法进一步增加容量是有可能的。 3CDMA蜂窝系统的缺点虽然CDMA允诺大的容量增益， 但是仍然有许多技术上的问题需要解决。对于 PN 序列的同步仍然是接收器急需解决的问题多普勒效应和多径效应仍然是一个复杂的问题。IS-95以提供短和长的 PN 序列来解决问题。短的 PN 序列在获得加速方面有比较短的周期。CDMA 系统一个严重的缺点是远近效应。CDMA 系统的理想性能依赖于接收器平均接收到的功率。对于功率不变的以人造卫星为基站的 CDMA 系统来说并不是一个问题。当蜂窝系统的使用者移动而且多径效应引起衰弱时无线电系统将会遭受功率衰退。在基站附近一个弱信号的使用者可能被淹没。为了保持不变的功率， 一个功率控制回路被用在移动终端或基站或两者之间。功率变化小于1-2个分贝仍然减少 15-20% 的功率。相似的功率减少也在多蜂窝干扰和有点的功率控制 的系统中出现。CDMA系统的一个潜在缺点是先前被提到的声音质量问题。在讲话停止的时候可以大幅度的改善容量。在大多数的数据传输和视频活动中这类终止是很少见的。CDMA 系统设计利用讲话，数据和视频将用户所使用的系统的容量降低。数字广播1发射机发射器给宽带CDMA系统提供上行链接。发射器为基带处理器提供数据接口。 基带处理器通过数据接口把扩展的基带传给发射器。 发射器通过无线电电波 (射频)调制基带信号。 已调射频信号被放大过滤而且通过空气链路传送到基站。为了解决远近效应发射器在链路上用一个发射功率控制器控制操作(TPC)在一个水平方向上传送功率。通过基带处理器的数字指令在水平方向上控制决定功率大小。2技术上的要求发射器的主要规格在 1.6 W+20% 传送功率上，超过-50%的发送带宽(1920-1980个百万赫兹)。基带处理器发出的数字指令将发射功率控制在 70 分贝范围内。数字指令是 7bits长。指令密码是一个二进位数字在 0000000B和1000110B之间。(或 0 到 70 小数)。0000000B代码产生最大的功率输出，1000110B代码生产的功率比最大输出功率小70分贝。控制控制周期是 0.625ms 。数据率是 128Kbps 。数据序列和传输代码一起以4。096Mcps 速率传输。调制方式是 QPSK 。基带处理器通过两个分开的信道把直接的（I）和求积的(Q) 基带信号传送到发射器。基带处理器以 32.768Msps 速率对基带信号进行抽样。抽样率是8bt。信号以8bie的数字格式发送。数模转换器 (DAC) 重发模拟信号并且这些信号以0。22的速率被过滤， 正余弦 (RRC) 滤波器。产生的模拟信号被用于发射调制器。 像本书第 2.6 节所提到那样，宽带CDMA 系统会对邻道产生干扰即邻频道干扰。邻频道信号功率与调制过的信号一起测量。邻信道输出功率谱比带内输出功率少40dBc。带宽内的功率占总功率的4.096MHz。Language TranslationThe Need for Third-Generation WirelesTechnologiesThe first generation of wireless networks was primarily concerned with the provision of voice services, allowing users to transition from conventional fixed telephony to mobiletelephony. First generation systems are commonly referred to as analog systems. Thewide acceptance of mobile telephony rapidly exhausted the capacity that could be provided with analog technologies, requiring the introduction of second-generatio systems. These systems have transitioned the voice services supported by analog networks into a digital environment, thus increasing the supported capacity and allowing for additional services such as text messaging and limited access to data services. Second generation networks (2G) are currently in use and also very near their maximum capacity, due to the remarkable penetration of mobile telephony. Third generation systems (3G) propose the evolution of existing systems, further increasing their capacity and introducing multimedia communications. They offer enhanced features, adding videoand images to the voice services and allowing improved access to data networks and to the Internet. Unlike the transition from first to second generation, the migration from 2G to 3G will occur smoothly. Existing 2G networks will evolve to 3G, with transitional solutions wn as 2.5G bridging the gap between them. The development work on 3G is still underway; the technological challenges it presents are extraordinary. The increasing demand for capacity in the already saturated 2G networks, as well as for enhanced data and Internet services, have made 2.5G solutions very appealing and important. These solutions rely on technology improvements to existing networks and allow for an extension of their “lifespan”, until the 3G proposals are finalized and validated. The primary factor limiting the capacity of wireless networks is the amount of spectrum available for these services, making the choice of modulation schemes and, ultimately, spectral efficiency, of paramount importance in the resulting capacity. In addition, power limitations imposed by the intrinsic nature of the handsets further accentuate the importance of the modulation and its characteristics. The introduction of multimedia services in third generation networks implies an increase in the bandwidth requirements. In order to accommodate the growth in capacity and bandwidth needs, the World Administrative Radio Conference (WARC) of the ITU (International Telecommunications Union) has identified extended spectrum for 3G, around the 2GHz band. Additionally, the third generation technology proposals, known within the ITU as IMT-2000, use improved, more sophisticated modulation schemes, so as to maximize the new spectrum allocation.Evolution of Wireless Technologies from 2G to 3G1.The Path to Third Generation (3G)The evolution of wireless technologies and standards applied to commercial mobile services began with the introduction of the first generation of commercial mobile telephony networks, circa 1946Rap96. Figure 2-1 shows the technology evolution from the current second generation to the proposed 3G solutions. Among the secondgeneration technologies GSM (Global System for Mobile communications) is the most widespread, with over 400 million users worldwide And01. It is also the best-positioned technology to provide a 2.5G transitional solution to 3G.Figure 2-1 - Evolution of Wireless Technologies from 2G to 3G. TDMA Time Division Multiple Access; UWC Universal Wireless Consortium; GSM Global System For MobileCommunications;GPRS General Packet Radio Services; HSCSD High Speed Circuit SwitchedData, EGPRS Enhanced GPRS;ECSD Enhanced Circuit Switched Data; PDC Pacific Digital Cellular; UMTS Universal Mobile Telecommunications System; CDMA Code Division Multiple Access; WCDMA Wideband Code Division Multiple Access;IMT-2000 International Mobile Telecommunications2.Advantages of CDMA in CellularA CDMA system has several advantages over narrowband TDMA or FDMA systems. One important feature of spread spectrum is the inherent resistance to multipath fading.Any two multipath components of a signal separated by a time greater than a pulse duration are highly uncorrelated and resolvable. Multipath combining receivers may take advantage of this and actually improve the signal to noise ratio. Even without multipath combining, these interfering components will be rejected by the receiver because of the cross-correlation properties of CDMA. CDMA systems also undergo a gradual degradation in signal to noise ratio as the number of users is increased. This soft capacity limit allows operators to push the number of active users beyond the design value, unlike other multiple access techniques. In designing a CDMA layout, it is possible to assume an average load and still handle peak tra_c times because of this soft capacity. FDMA and TDMA do not allow more users 3 beyond a _xed limit but they can guarantee a minimum quality of service for the active users, whereas an overloaded CDMA system will see gradually increasing bit error rates before it begins dropping calls. In CDMA, the frequency reuse factor is one. The same duplex channel is allocated to all users in every cell in the system. Switching channels is not necessary for the roaming user and algorithms for supporting handos at the base station are simpli_ed. Users at the boundary of two cells will have the probability of being dropped signi cantly reduced because the system can aord to maintain a users radio link with in two dierent cells temporarily through a procedure called soft hando_. CDMA is an interference limited system so through the use of multisectored antennas and voice activity detection it is possible to increase the capacity. Multisectored antennas focus the antenna energy, spatially reducing the interference for users in other sectors. Adaptive antenna patterns where individual spot beams are given for each user have also been studied Lib94. Further capacity gains are possible by suppressing the transmitter during quiet periods of speech. Voice inactivity is exploited greatly in IS-95 by permitting a transmitter to turn o_ during inactivity or reduce the transmission power level.3.Disadvantages of CDMA in CellularWhile CDMA promises great capacity gains, there are still many technical issues that need resolving. Synchronization to PN sequences is still a di_cult task for a receiverwith Doppler and multipath e_ects further complicating the issue. IS-95s solution to the problem is to provide short and long PN sequences. The short PN sequence has a signi_cantly shorter repeat period, speeding up acquisition. A serious disadvantage in CDMA, is the near-far e_ect. Ideal performance of a CDMA system depends on the received powers having equal strengths at the receiver. This has not been a problem in satellite based CDMA systems where the powers are fairly constant. Wireless system will undergo power fades as the user moves about the cell and as multipath components induce deep fades. A weak user might be drowned out by users who are near the base station tower. To maintain uniform power levels, a power control loop is used by either the mobile, base station, or both. A power variation of as little 4 as 1-2 dB may still reduce capacity by 15-20% Cam92. Similar capacity reduction were also demonstrated in the presence of multicellular interference and imperfect power control Mil92. Another potential disadvantage of CDMA is a quality that was mentioned previously, voice activity detection. This greatly improves capacity because of the pauses naturally occurring in speech. There are few such pauses in most data transfer and video applications. In a CDMA system designed to take advantage of voice activity detection, data and video users will force the system to a lower capacity.Digital radio1.TransmitterThe transmitter supports the uplink of the W-CDMA system. It provides a digital interface for the baseband processor. The baseband processor sends the spread baseband signal through the digital interface to the transmitter. The transmitter modulates the baseband signals on a radio frequency (RF) carrier. The modulated RF signal is then amplified, filtered and transmitted to the base station through the air link. To combat the near-far problem, the transmitter operates in conjunction with a transmit power control (TPC) to maintain the transmit power at an appropriate level. The control determines the power level based on the digital command from the baseband processor. Figure 10 is a block diagram of the transmitter.2.Technical SpecificationsThe key specification for the transmitter is to deliver transmit power at 1.6 W +20%, -50% over the transmitting band (1920 1980 MHz). A digital command from the baseband processor can control the transmit power over a 70dB range. The digital command is 7-bits long. The command code is a binary number between 0000000B and 1000110B (or 0 to 70 decimal). The code 0000000B produces the maximum power output, while the code 1000110B produces 70dB less than the maximum output. The power control cycle time is 0.625ms. The data rate is 128Kbps. The data sequence is spread with the spreading codes at 4.096Mcps chip rate. The modulation type is QPSK. The baseband processor sends the direct (I) and quadrature (Q) baseband signals to the transmitter in two separate channels.The baseband processor samples the baseband signals at 32.768Msps. The sample rate is eight times the chip rate. The signals are sent in 8-bit digita
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