TD-SCDMA overview

- Time Division Synchronous Code Division Multiple Access (TD-SCDMA), the 3G cellular system from China

TD-SCDMA is a third cellular system that has been developed and adopted as a 3G standard by the International Telecommunications Union (ITU). Originating in China, TD-SCDMA, uses a combination of synchronous CDMA and Time Division Duplex (TDD) techniques. Much of the initial work for the system was undertaken by the China Academy of Telecommunications Technology (CATT)

TD-SCDMA offers a number of significant advantages for operators because it does not require paired frequencies for the uplink and downlink as a single channel is time multiplexed. The time division duplex approach means that it is easy to support asymmetric up/downlink purely by altering the times allowed for uplink and downlink transmissions. In addition to this TD-SCDMA is able to support IP services, and it has been designed to incorporate new technologies such as joint detection, adaptive antennas, and dynamic channel allocation.

As a further advantage, TD-SCDMA uses the same RAN as that used for UMTS. In this way it is possible to run TD-SCDMA alongside UMTS, and thereby simplifying multi-system designs.

Although UMTS (W-CDMA) and cdma2000 are widely recognized as 3G cellular standards, TD-SCDMA is equally valid. In fact it has been adopted as the low chip rate (LCR) version of the 3GPP TDD standard.

Basic features
The TD-SCDMA standard provides many advantages. As already mentioned it has many similarities to W-CDMA, although a summary of the basic features and specification is given below:

Characteristic	Figure
Bandwidth	1.6 MHz
Chip rate per carrier	1.28 Mcps
Spectrum spreading mode	DS SF=1/2/4/8/16
Modulation	QPSK
Channel coding	Convolutional codes: R=1/2,1/3 Turbo implemented
Interleaving	10/20/40/80 ms
Frame structure	Super frame 720ms,Radio frame 10ms Subframe 5 ms
Uplink synchronisation	1/2 chip
Number of voice channels per carrier	48
Spectrum Efficiency	25Erl./MHz
Total transmission rate provided by each carrier	1.971Mbps

Summary
While TD-SCDMA is not a contender to compete with W-CDMA in many areas, it has many advantages and is likely to be rolled out to provide service, particularly within China. It is also likely to be employed in other countries around the globe for specific applications.