TABLE OF CONTENTS With CDMA the individual carriers may be present simultaneously within the same rf bandwidth, but each carrier carries a unique code waveform (in addition to the information signal) that allows it to be separated from all the others at the receiver. The carrier is modulated in the normal way by the information waveform and then is further modulated by the code waveform to spread the spectrum over the available rf bandwidth. Many of the key properties of CDMA rely on this spectrum spreading, and the systems employing CDMA are also known as spread-spectrum multiple access (SSMA). Care must be taken not to confuse the SS here with that for satellite switched (SS/TDMA) used in the Sec. 14.9.
CDMA can be used with analog and digital signals (see Dixon, 1984), but only digital systems will be described here. For illustration purposes, a polar non-return-to-zero (NRZ) waveform denoted by p( t) (see Fig. 10.2) will be used for the information signal, and BPSK modulation (see Sec. 10.6.1) will be assumed. The code waveform c( t) is also a polar NRZ signal, as sketched in Fig. 14.32. What would be called bits in an information waveform are called chips for the code waveform, and in most practical systems the chip rate is much greater than the information bit rate. The pulses (chips) in the code waveform vary randomly between + V and ? V. The randomness is an essential...
