About
TD-SCDMA
Time Division Synchronous CDMA (TD-SCDMA) was proposed by China Wireless Telecommunication Standards group (CWTS) and approved by the ITU in 1999 and technology is being developed by the Chinese Academy of Telecommunications Technology and Siemens. TD-SCDMA uses the Time Division Duplex (TDD) mode, which transmits uplink traffic (traffic from the mobile terminal to the base station) and downlink traffic (traffic from the base station to the terminal) in the same frame in different time slots. That means that the uplink and downlink spectrum is assigned flexibly, dependent on the type of information being transmitted. When asymmetrical data like e-mail and internet are transmitted from the base station, more time slots are used for downlink than for uplink. A symmetrical split in the uplink and downlink takes place with symmetrical services like voice service.
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.
The standard has been adopted by 3GPP since Rel-4, known as "UTRA TDD 1.28Mcps Option". This, and TD-CDMA (an independently developed TDD CDMA system more closely related to W-CDMA), are offered as air interfaces for the UMTS-TDD system, a version of UMTS used largely to provide Internet access. The use of TDD is more efficient than FDD at dynamically providing asymmetric data rates, which are typical in ordinary Internet use.
On March 28, 2008, China Mobile Group announced TD-SCDMA will start operating "commercial trials" for 60,000 test users in eight cities from April 1.
TD-SCDMA Technical Summary
Frequency band: 1880 MHz - 1920 MHz, 2010 MHz - 2025 MHz, 2300 MHz - 2400 MHz( in China )
Minimum frequency band required: 1.6MHz
Frequency re-use: 1
Chip rate: 1.28 Mcps
Frame length: 10ms
Number of slots: 7
Modulation: QPSK or 8-PSK
Voice data rate: 12.2kbit/s
Circuit switched services: 12.2 kbits/s, 64 kbits/s, 144 kbits/s, 384 kbits/s, 2048 kbits/s
Packet data: 64kbits/s, 144kbits/s, 384kbits/s, 2048kbits/s
Receiver: Joint Detection, Multi-Cell Interference Cancellation
Power control period: 200 Hz
Number of slots / frame: 7
Sub-Frame length: 5ms
Multi carrier: option
Handovers: Hard or Baton handover
Smart antennas: option
Uplink synchronisation: yes
Physical layer spreading factors: 1, 2, 4, 8, 16
TD-SCDMA Technical highlights
TD-SCDMA uses TDD, in contrast to the FDD scheme. 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. 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, 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.
