This chapter continues with the subject discussed in Chapter 4, expanding on it further. Whereas the previous chapter handled the spread spectrum issue from a general point of view, presenting various spread-spectrum modulation techniques and their general principles, this chapter concentrates on the schemes specified by the 3GPP documents. Both the uplink and downlink cases are studied separately, and the differences between pseudorandom codes and orthogonal codes are explained. Understanding the contents of this chapter is a prerequisite for understanding the WCDMA air interface.

Spreading codes are also known as spreading sequences. There are two types of spreading codes in the UTRAN air interface: orthogonal codes and pseudorandom codes, which are also known as pseudo-noise (PN) codes. Both are used together in the uplink and in the downlink. The same code is always used for both the spreading and despreading of a signal. This is possible because the spreading process is actually an XOR operation with the data stream and the spreading code. The reader should recall that two successive XOR operations will produce the original data.

Spreading means increasing the bandwidth of the signal. At first hearing, this may not sound like a good idea, since bandwidth is a scarce and expensive resource. There are some good reasons for doing this, however. The most important incentive for wideband spreading is the good interference resistance of a wideband signal. A wideband signal can survive in a very noisy environment. It is also difficult to jam, because its energy is...