Zero-forcing multiuser detection in CDMA systems using long codes

In this article, we show that a code division multiple access (CDMA) system employing long codes can be interpreted as a multiple input multiple output system where the transmit filters are time-varying. We derive conditions for existence of zero-forcing equalizers and verify through simulations that these conditions are useful guidelines for the design of appropriate solution in the presence of noise. Compared to similar works in the literature, the derived conditions allow a reduction in the length of the equalizer filters, while imposing no constraints on the channel order.