Spreading code selection for multiple chip rate DS-CDMA systems

In this paper, we analyze the interuser interference (IUI) in multiple chip rate (MCR) direct-sequence code-division multiple-access (DS-CDMA) systems employing realistic chip waveforms. In MCR DS-CDMA systems, different users may have different chip rates, which makes IUI unavoidable even if the employed spreading codes are orthogonal. Here, we consider orthogonal variable spreading factor (OVSF) codes and random spreading codes. We find that the spreading code of the low chip rate user does not affect the IUI at all. The IUI depends only on the bit patterns of the high chip rate user corresponding to one chip duration of the low chip rate user and on the carrier frequency separation of the users. We provide a closed-form equation for the IUI power of random codes, and we show that a judicious choice of the spreading codes can reduce the IUI significantly for OVSF codes.