Multiuser detectors in CDMA systems are proposed to combat multi-access interference. We present system design analysis of an RSFQ multiuser detector considering the realistic parameters of a long-code asynchronous CDMA system with frequency-selective fading paths, 100 voice equivalent channels per sector and 10 resolvable multipath components. We compared different architectures via both theoretical analysis and numerical simulations, optimizing bit error rate performance and targeting a double increase of the uplink capacity. The advantages and constraints of block-wise or simple Gauss-Seidel algorithm for practical implementation were examined in order to minimize the hardware requirements. The influence of quantization process on the system performance was studied.