Talking about 3G(1)

3G (the third generation) is a specification developed by the International Telecommunication Union for the third generation of mobile communications technology, (analog cellular was the first generation, digital PCS the second). 3G digital cellular technology promises increased bandwidth: up to 384K bit/sec when a device is stationary or moving at pedestrian speed, 128K bit/sec in a car, and 2M bit/sec in fixed applications.

GSM (Global System for Mobile Communications) is an open, nonproprietary system that is the dominant cellular technology throughout the world today. GSM uses a variation of the Time Division Multiple Access (TDMA) protocol. Data is digitized and compressed, then sent down a channel with two other streams of user data, each in its own time slot. It operates at either the 900-MHz or 1800-MHz and 1900-MHz frequency band.

Another so-called second-generation wireless air interface is CDMA (Code Division Multiple Access). CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in cellular telephone systems in the 800-MHz and 1.9-GHz bands.

Current circuit-switched GSM networks transmit data at 9.6K bit/sec or up to 43.2K bit/sec using multitime slot high-speed circuit switched data (HSCSD) upgrades. GSM’s 3G counterpart，Wideband Code Division Multiple Access (WCDMA), offers much higher data speeds. WCDMA can support mobile voice, images, data and video communications at up to 2M bit/sec (local-area access) or 384K bit/sec (wide-area access). The input signals are digitized and transmitted in a coded, spread-spectrum mode over a range of frequencies. A 5 MHz-wide carrier is used, compared with 200 KHz-wide carrier for narrowband CDMA.

In fact, 3G technology applies equally to all standards that are being considered, including CDMA2000 in the United States. CDMA2000 can support mobile data communications at speeds ranging from 144K bit/sec to 2M bit/sec. Deployment is in the planning stages.

GSM-based packet radio services (GPRS), already extremely popular in Europe, are being rolled out now in the United States. With GPRS, datastreams are broken up into packets of data rather than the continuous stream of GSM circuit-switched networks. Packetized GPRS offers “always on”connectivity versus GSM, where users pay if they occupy a channel and don’t transmit data or voice. GPRS is central to the promise of mobile Internet capability, delivering voice, video and data to subscribers at up to 115K bit/sec. In Europe, GPRS-equipped handsets are projected to be available for the mass market by early 2002.

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谈谈3G(1)

3G(第三代)是国际电信联盟为第三代移动通信技术开发的一种规范（模拟蜂窝电话是第一代，数字个人通信服务为第二代）。3G数字蜂窝电话技术承诺增加带宽：当设备是静止的或者以步行速度行进时为384Kbps，在（行进的）汽车中为128Kbps，在固定的应用中为2Mbps。

GSM（全球通）是非专有的开放系统，它是目前遍布全球的主导的蜂窝电话技术。GSM采用时分多址（TDMA）协议的一个变种。数据经数字化和压缩后，连同其他 两个用户数据流一起在一个信道上送出，每个都有自己的时间隙。它工作在900MHz或者1800MHz和1900MHz频段。

另一个所谓的第二代无线天波接口是CDMA（码分多址接入）。它采用一种多路复用的形式，允许很多信号只占一个传输信道，优化了可用带宽的使用。在800MHz和1900MHz频段上的蜂窝电话系统采用此技术。

目前的电路交换GSM网以9.6Kbps的速率传送数据，利用多时间隙高速电路交换数据（HSCSD）的升级，速率可达43.2Kbps。GSM的3G对手——宽带码分多接入（WCDMA），能提供更高的数据速率。WCDMA能以高达2Mbps（本地接入）和384Kbps的速率（广域接入），支持移动的话音、图像、数据和视频通信。输入信号在一个频率范围内以编码的扩频方式，实现信号的数字化和传输。与窄带CDMA的200kHz宽的载波相比，它采用了5MHz宽的载波。

事实上，3G技术能等同地应用到现在正考虑采用的所有标准，包括美国的CDMA2000。CDMA2000能支持速度范围为144Kbps到2Mbps的移动数据通信。它现处于计划阶段。

在欧洲已非常流行的GPRS（基于GSM的分组无线电服务），目前在美国也正在推出。利用GPRS，数据流被分成数据分组，而不是GSM电路交换网络的连续流。分组化的GPRS提供“永远在线”的连接，而对于GSM，只要你占了信道不管你是否传送了数据和话音都要付钱。GPRS是承诺移动因特网功能的核心，以最高达115Kbps的速率向用户提供话音、视频和数据。在欧洲，预计到2002年初，具备GPRS功能的手机将进入大众市场。

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