3G科普之TD-SCDMA

TD-SCDMA
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时分同步码分多址(Time Division - Synchronized Code Division Multiple Access, 缩写为：TD-SCDMA），是ITU批准的三个3G标准中的一个，相对于另两个主要3G标准(CDMA) 或 (UMTS)它的起步较晚。

该标准是由中国大陆独自制定的3G标准，1999年6月29日，中国原邮电部电信科学技术研究院(大唐电信)向ITU提出。该标准将智能无线、同步CDMA和软件无线电(SDR)等技术融于其中。另外，由于中国的庞大的市场，该标准受到各大主要电信设备厂商的重视，全球一半以上的设备厂商都宣布可以支持TD-SCDMA标准。

TD-SCDMA在频谱利用率、对业务支持具有灵活性、频率灵活性及成本等方面的独特优势。

TD-SCDMA由于采用时分双工，上行和下行信道特性基本一致，因此，基站根据接收信号估计上行和下行信道特性比较容易。因此，TD-SCDMA使用智能天线技术有先天的优势，而智能天线技术的使用又引入了SDMA的优点，可以减少用户间干扰，从而提高频谱利用率。

TD-SCDMA还具有TDMA的优点，可以灵活设置上行和下行时隙的比例而调整上行和下行的数据速率的比例，特别适合因特网业务中上行数据少而下行数据多的场合。但是这种上行下行转换点的可变性给同频组网增加了一定的复杂性。

TD-SCDMA是时分双工，不需要成对的频带。因此，和另外两种频分双工的3G标准相比，在频率资源的划分上更加灵活。

一般认为，TD-SCDMA由于智能天线和同步CDMA技术的采用，可以大大简化系统的复杂性，适合采用软件无线电技术，因此，设备造价可望更低。

但是，由于时分双工体制自身的缺点，TD-SCDMA被认为在终端允许移动速度和小区覆盖半径等方面落后于频分双工体制。

TD-SCDMA产业发展的核心组织被称为TD-SCDMA产业联盟（TDIA：TD-SCDMA Industry Alliance），成立于2002年10月30日，由大唐科技产业集团，南方高科，华立，华为，中兴，普天，中国电子信息产业集团，联想等8家企业发起。目前TDIA已经有7家理事成员，22家非理事成员。

TD-SCDMA
From Wikipedia, the free encyclopedia

TD-SCDMA (Time Division-Synchronous Code Division Multiple Access) is a 3G mobile telecommunications standard, being pursued in the People's Republic of China by the Chinese Academy of Telecommunications Technology (CATT), Datang and Siemens AG, in an attempt to develop home-grown technology and not be "dependent on Western technology" [1]. This is likely primarily for practical reasons, both current 3G formats may require the payment of patent fees to Qualcomm and UMTS requires license fees from the primarily European-based 3G consortium. It is based on spread spectrum CDMA technology. The launch of an operational system was initially projected by 2005 but is now projected by 2006.

On January 20, 2006, Ministry of Information Industry of the People's Republic of China formally announced that TD-SCDMA is the country's standard of 3G mobile telecommunication. On February 15th, 2006, a timeline for deployment of the network in China was announced, stating pre-commercial trials would take place starting after completion of a number of test networks in select cities. These trials are expected to run from March to June, 2006. TD-SCDMA enabled handsets are also set to start testing around the same time, and are expected to be available in Q2 or Q3 of 2006. TD-SCDMA 3G phones are expected to become available at the end of 2006 and other 3G networks will be delayed until TD-SCDMA is ready.

Technical highlights

TD-SCDMA uses TDD, in contrast to the FDD scheme used by W-CDMA. By dynamically adjusting the number of timeslots used for downlink and uplink, the system can more easily accommodate asymmetric traffic with different data rate requirements on downlink and uplink than FDD schemes. Since it does not require paired spectrum for downlink and uplink, spectrum allocation flexibility is also increased. Also, using the same carrier frequency for uplink and downlink means that the channel condition is the same on both directions, and the base station can deduce the downlink channel information from uplink channel estimates, which is helpful to the application of beamforming techniques.

TD-SCDMA also uses TDMA in addition to the CDMA used in WCDMA. This reduces the number of users in each timeslot, which reduces the implementation complexity of multiuser detection and beamforming schemes, but the non-continuous transmission also reduces coverage (because of the higher peak power needed), mobility (because of lower power control frequency) and complicates radio resource management algorithms.

The "S" in TD-SCDMA stands for "synchronous", which means that uplink signals are synchronized at the base station receiver, achieved by continuous timing adjustments. This reduces the interference between users of the same timeslot using different codes by improving the orthogonality between the codes, therefore increasing system capacity, at the cost of some hardware complexity in achieving uplink synchronization.
The standard has been adopted by 3GPP since Rel-4, known as "UTRA TDD 1.28Mcps Option".