Time Division Synchronous Code Division Multiple Access (TD-SCDMA) or UTRA/UMTS-TDD 1.28 Mcps Low Chip Rate (LCR), is an air interface found in UMTS mobile telecommunications networks in China as an alternative to W-CDMA. Together with TD-CDMA, it is also known as UMTS-TDD or IMT 2000 Time-Division (IMT-TD).
TD-SCDMA was developed in the People's Republic of China by the Chinese Academy of Telecommunications Technology (CATT), Datang Telecom, and Siemens AG in an attempt to avoid dependence on Western technology. This is likely primarily for practical reasons, since other 3G formats require the payment of patent fees to a large number of Western patent holders.
TD-SCDMA proponents also claim it is better suited for densely populated areas. Further, it is supposed to cover all usage scenarios, whereas W-CDMA is optimised for symmetric traffic and macro cells, while TD-CDMA is best used in low mobility scenarios within micro or pico cells.
TD-SCDMA is based on spread spectrum technology which makes it unlikely that it will be able to completely escape the payment of license fees to western patent holders. The launch of a national TD-SCDMA network was initially projected by 2005 but only reached large scale commercial trials with 60,000 users across eight cities in 2008.
On September 21, 2009, China Mobile officially announced that it had 1,327,000 TD-SCDMA subscribers as of the end of August, 2009.
While TD is primarily a China-only system, it may well be exported to developing countries. It is likely to be replaced with a newer TD-LTE system over the next 5 years.
Deployment and usage
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 15, 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 ran from March to October, 2006, but the results were apparently unsatisfactory. In early 2007, the Chinese government instructed the dominant cellular carrier, China Mobile, to build commercial trial networks in eight cities, and the two fixed-line carriers, China Telecom and China Netcom, to build one each in two other cities. Construction of these trial networks was scheduled to finish during the fourth quarter of 2007, but delays meant that construction was not complete until early 2008.
On March 28, 2008, China Mobile Group announced TD-SCDMA "commercial trials" for 60,000 test users in eight cities from April 1, 2008. Networks using other 3G standards (WCDMA and CDMA2000 EV/DO) had still not been launched in China, as these were delayed until TD-SCDMA was ready for commercial launch.
In January 2009 the Ministry of Industry and Information Technology (MIIT) in China took the unusual step of assigning licences for 3 different third-generation mobile phone standards to three carriers in a long-awaited step that is expected to prompt $41 billion in spending on new equipment. The Chinese-developed standard, TD-SCDMA, was assigned to China Mobile, the world's biggest phone carrier by subscribers. That appeared to be an effort to make sure the new system has the financial and technical backing to succeed. Licences for two existing 3G standards, W-CDMA and CDMA2000 1xEV-DO, were assigned to China Unicom and China Telecom, respectively. Third-generation, or 3G, technology supports Web surfing, wireless video and other services and the start of service is expected to spur new revenue growth.
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. Using the same carrier frequency for uplink and downlink also 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.
- Siemens (2004-06-10). "TD-SCDMA Whitepaper: the Solution for TDD bands" (pdf). TD Forum. pp. 6–9. Retrieved 2009-06-15.
- ITU-D Study Group 2. "Guidelines on the smooth transition of existing mobile networks to IMT-2000 for developing countries (GST); Report on Question 18/2". pp. 4, 25–28. Retrieved 2009-06-15.
- Forkel et al. (2002). "Performance Comparison Between UTRA-TDD High Chip Rate And Low Chip Rate Operation". Retrieved 2009-02-16.
- 3G Licensing introduces the W-CDMA Patent Licensing Programme at the "Low Cost 3G Devices" conference, London, March 13, 2007
- 3G in China still held up, EE Times Asia, Global Sources
- China Mobile to Test Td-scdma on 60,000 Phones from April 1,, Cellular News
- China issues 3G licences to main carriers The Reuters UK
- TS 25.201 Physical Layer – General Description – Describes basic differences between FDD and TDD.
- TS 25.221 Physical channels and mapping of transport channels onto physical channels (TDD)
- TS 25.222 Multiplexing and channel coding (TDD)
- TS 25.223 Spreading and modulation (TDD)
- TS 25.224 Physical layer procedures (TDD)
- TS 25.225 Physical layer – Measurements (TDD)
- TD-SCDMA Forum
- TD-SCDMA Industry Alliance
- China will not interfere in 3G standard selection
- China will provide 3G Service in cities where Olympic Games 2008 are held