3G TD-SCDMA Tutorial
- overview or tutorial of the basics of TD-SCDMA, the 3G UMTS TDD system that has been widely deployed in China.
TD-SCDMA is an additional TDD version of UMTS. Devised in China, the system provides a number of advantages in several applications. TD-SCDMA has been adopted as a 3G standard by the International Telecommunications Union (ITU), and it is part of the 3GPP UMTS system being defined in the 3GPP standards.
Much of the initial work for the system was undertaken by the China Academy of Telecommunications Technology (CATT). Apart from the advantages of the basoc TDD approach, 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
While similar in many was to UMTS TDD, TD-SCDMA is has a number of differences and handsets for the two systems would not be compatible unless the capability for both systems was specifically built in to them.
One of the key elements of TD-SCDMA is the fact that it uses a TDD, Time Division Duplex approach. As seen with UMTS TDD this has advantages in a number of areas, enabling the balance to be changed between uplink and downlink to accommodate the different levels of data transfer. It also has advantages in terms of using unpaired spectrum, spectrum efficiency for certain loads and it does not require expensive diplexers in the handsets to enable simultaneous transmission on the uplink and downlink, although transmit / receive switching times must be accommodated and can reduce the efficiency of the system.
Note on TDD and FDD duplex schemes:
In order for radio communications systems to be able to communicate in both directions it is necessary to have what is termed a duplex scheme. A duplex scheme provides a way of organizing the transmitter and receiver so that they can transmit and receive. There are several methods that can be adopted. For applications including wireless and cellular telecommunications, where it is required that the transmitter and receiver are able to operate simultaneously, two schemes are in use. One known as FDD or frequency division duplex uses two channels, one for transmit and the other for receiver. Another scheme known as TDD, time division duplex uses one frequency, but allocates different time slots for transmission and reception.
Click on the link for more information on TDD FDD duplex schemes
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.
TD-SDCMA specification overview
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:
|Chip rate per carrier||1.28 Mcps|
|Spectrum spreading mode||DS SF=1/2/4/8/16|
|Modulation||QPSK / 8PSK / 16QAM|
|Channel coding||Convolutional codes: R=1/2,1/3 Turbo implemented|
|Frame structure|| Super frame 720ms,Radio frame 10ms
Subframe 5 ms
|Uplink synchronisation||1/2 chip|
|Number of voice channels per carrier||48|
|Total transmission rate provided by each carrier||1.971Mbps|
The UMTS TD-SCDMA system has adopted a number of advanced techniques and technologies to optimise the operation. These are often above and beyond those that have been catered for in the more widely used standard forms of FDD and TDD UMTS. Some of these result from the fact that TD-SCDMA uses the same frequency for both uplink and downlink, and as a result of the higher processing levels now available.
- Smart antennas: Smart antenna technology is incorporated into the base station. This enables beams to be formed and this is able to reduce interference between terminals and concentrate transmitted power at active terminals. This technique is implemented using smart antenna arrays that incorporate advanced DSP algorithms. The base station is able to locate the mobile terminals and to steer transmit beams to specific terminals. In this way spatial beamforming is able to reduce interference within a given channel with a resulting improvement in the downlink capacity.
- Joint detection technology: Within CDMA, multiple users all occupy the same frequency band, accessing he base station using different codes. In this way, multiple-access interference results and this is a major problem in CDMA-based systems. A technique referred to as joint detection technology treats signals from all users as useful and processes them in parallel. As the maximum number of users in any time slot is 16, the processing complexity to separate users is kept within manageable limits.
- User terminals and base station synchronisation: The synchronisation of the network enables precise adjustment of the timing advances for transmission from terminals so that signals from different users arrive at the base station together, and not overlapping in time into the transmit time frames making detection much simpler. This synchronisation enables faster search for neighbouring cells during handover and it also removes the need for soft handover.
By Ian Poole
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