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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.
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.
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.
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.
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.