Channel Code-Aided Synchronization of Code-Spread CDMA Systems

Direct-sequence code-division multiple-access (DS-CDMA) signals are often regarded as a result of the multiplication of the data bits by a spreading sequence, which is clocked at a higher rate than the data sequence (where the ratio of the rates is the spreading factor), cf. figure 1 a. A conceptually different, but equivalent, description is that the symbol rate is raised by up-sampling (repetition encoding) of the data bits by a factor equal to the spreading factor, followed by a multiplication of a scrambling sequence, cf. figure 1 b. Systems, where the spreading is accomplished by the use of a powerful forward error-correcting code instead of a repetition code, are referred to as code-spread CDMA (CS-CDMA) systems. In [1, 2] it is shown that maximum throughput is achieved by employing low-rate channel codes alone for bandwidth expansion.