SPREADING in WCDMA

WCDMA applies a two-layered code structure consisting of a orthogonal spreading codes and pseudo-random scrambling codes. Spreading is performed using channelization codes, which transforms every data symbol into a number of chips, thus increasing the bandwidth of the signal. Orthogonality between the different spreading

factors can be achieved by the tree-structured orthogonal codes. Scrambling is used for cell separation in the downlink and user separation in the uplink.

Uplink Spreading

In the uplink, either short or long spreading (scrambling) codes are used. The short codes are used to ease the implementation of advanced multiuser receiver techniques; otherwise, long spreading codes can be used. Short codes are S(2) codes of length 256 and long codes are Gold sequences of length 241, but the latter are truncated to form a cycle of a 10-ms frame. IQ/code multiplexing used in the uplink leads to parallel transmission of two channels, and therefore, attention must be paid to modulated signal constellation and

related peak-to-average power ratio (crest factor). the transmitter power amplifier efficiency remains the

same as for QPSK transmission in general.

Downlink Spreading

In the downlink, the same orthogonal channelization codes are used as in the uplink. For scrambling, Gold codes of length 218 are used, but they are truncated to form a cycle of a 10-ms frame (i.e., 384,000 chips). To form a complex-valued code, the same truncated code is used with different time shifts in I and Q channels. It is possible to generate 218-1 scrambling codes, but only 8191 of them are used. Each cell is allocated one primary scrambling code. In order to reduce the cell search time, the primary scrambling codes are divided into 512 sets. Thus, the mobile station needs to search  at maximum 512 10-ms-long codes. In addition to primary scrambling codes, there are 15 secondary scrambling code sets. Secondary scrambling codes are used when one set of orthogonal channelization codes is not enough. This can be the case when adaptive antennas are used in the downlink. It should be noted that use of the secondary scrambling code destroys the orthogonality between code channels.