BER Performance of Code-Spread CDMA Systems with Delay Estimation

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, clocked at a higher rate than the data sequence (where the factor 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. The scrambling sequence is actually the same sequence as the spreading sequence, but the different names indicates that the purpose is different in the two descriptions. The spreading sequence is used both for increasing the bandwidth of the signal as well as for making the interfering signal more random, whereas the purpose of the scrambling code only is the latter. When looking at CDMA this way, it seems, that using a forward error correcting code (FEC) that is better than the repetition code, the performance of the system could be increased. Systems where the spreading is accomplished by the use of a powerful FEC 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. One difficulty with CS-CDMA is to find good codes at the very low rate in the order of 1/100 that often is needed in a CDMA system. In [2] orthogonal convolutional codes were proposed, but one drawback of these is that the available code rates are coupled to the constraint lengths. Encoder and decoder complexity might also be a problem for CS-CDMA systems with high spreading factors (corresponding to low code rates). As an example, using a Viterbi decoder to decode a rate 1/100