TD-SCDMA

What is TD-SCDMA?

• Acronym for Time Division Synchronous Code Division Multiple Access

• Jointly developed by Siemens and the China Academy of telecommunications Technology (CATT)

• Advanced TDMA/TDD system with an adaptive CDMA component operating in a synchronous mode

• Adopted by ITU (International Telecommunication Union) and by 3GPP (Third Generation Partnership Project) as part of UMTS (Universal Mobile Telecommunications System) release 4

Main Features

• DATA RATE UP TO 2 Mbps

• FLEXIBLE UPLINK – DOWNLINK

• LARGE COVERAGE: UP TO 40 KM

• HIGH MOBILITY: AT LEAST 120 KM/H

• OPTIMUM SPECTRUM EFFICIENCY

ITU 3G standards

• IMT-2000 (International Mobile Telecommunication – 2000) – WCDMA– CDMA2000– TD-SCDMA

• 150 MHz bandwidth has been allocated to TD by the Chinese government.

TDD (Time Division Duplex)

• Uplink and downlink on the same frequency band but in different time slots

• Does not require paired bands.

• More time slots can be allocated to downlink or uplink.

TDD (cont.)

• For symmetric services used during telephone and video calls, where the same amount of data is transmitted in both directions, the time slots are split equally between the downlink and uplink.

• For asymmetric services used with Internet access, where high data volumes are transmitted from the base station to the terminal, more time slots are used for the downlink than the uplink.

Advantage over FDD

• FDD (Frequency Division Duplex) scheme – employed by conventional CDMA 3G standards – uses a pair of frequency bands for up- and downlink; with asymmetric loads, portions of the spectrum are occupied but not used, leading to an inefficient use of the spectrum.

TDMA

• TDMA uses a 5ms frame subdivided into 7 time slots, which can be flexibly assigned to either several users or to a single user who may require multiple time slots.

CDMA

• Within each time slot up to 16 CDMA codes may be transmitted. The chip rate is 1.28 Mcps. The carrier bandwidth is 1.6 MHz.

Channel structure

Joint Detection

• In CDMA other users’ signals are like noise to the desired signal that you want to receive. If there are too many users in a cell, the noise level may be too high and the quality of reception may be poor.

• In TD-SCDMA, since every time slot can have no more than 16 codes, it is possible to separate them at the receiver and retrieve the desired signal without noise.

Joint Detection (cont.)

Joint Detection (cont.)

• The implementation complexity of such an optimal multi-user receiver is an exponential function of the number of codes. Such a receiver is difficult to implement in other CDMA based 3G systems because they use a large number of codes.

• These CDMA technologies employ suboptimal detection schemes, such as the Rake receiver, which do not extract all CDMA codes in parallel.

• While the capacity of traditional CDMA systems is restricted by the interference among users, TD-SCDMA does not suffer such restrictions, and thus may have larger capacity, as its proponents claim.

Power Control• CDMA systems require sophisticated power control mechanisms• The signal received from an MS located near the base station is

much stronger than that received from an MS at the cell’s edge. Without an accurate fast power control the strong signal of an MS close to the base station could block the whole cell. Therefore the deviation of signal strengths in a cell should be within 1.5 dB (about 1.4 times).

• Such a power control mechanism is complex, difficult to implement and expensive.

• Due to the small number of codes in each time slot and the use of Joint Detection, TD-SCDMA allows the signal strength to deviate for 20 dB (100 times). Therefore it significantly reduces the complexity of the power control mechanism.

Smart Antennas

• Using omnidirectional antennas, the emitted radio power is distributed over the whole cell. As a result, intercell interference is generated in all adjacent cells using the same RF carrier.

• Smart antennas direct signals to and from the specific terminals, increasing the signal strength and reducing the inter and intracell interference.

• The number of base stations can be reduced.

Smart Antennas

A Smart Antenna

Dynamic Channel Allocation

• Takes advantage of all the available Multiple Access techniques: TDMA, FDMA, CDMA and SDMA (Space Division Multiple Access). Making an optimal use of these, TD-SCDMA provides an optimal and adaptive allocation of the radio resources according to the interference scenario.

Dynamic Channel Allocation

Terminal Synchronization

• The synchronization of the signals arriving at the base station leads to a significant improvement in multi user joint detection.

• The terminal traceability is improved and the time for position location calculations is reduced.