On the application of very low rate error control coding to CDMA

In code division multiple access (CDMA), the bandwidth of the transmitted signal is spread over a much larger bandwidth than that of the baseband signal yielding the so-called processing gain of the spread spectrum system. This spectrum spreading is usually performed using a pseudo-random (PN) sequence, but random coding analysis has indicated that bandwidth spreading using very low rate error correcting codes may lead to a larger system capacity over spreading based on PN sequence only. Hence a low rate convolutional code or combination of a low rate code and a spreading sequence could be used to improve the CDMA system capacity. We present an analysis of the CDMA capacity for the reverse, or uplink channel, from the mobile user to the base station. Using a constant overall bandwidth expansion, for a given target bit error probability we obtain the best sharing of the CDMA bandwidth between the error correcting code and the PN sequence. We can therefore evaluate the improvement in system capacity that can be obtained over more traditional CDMA systems where almost all the bandwidth expansion is due to the PN sequence spreading only.