In a multicode DS-CDMA transmission, high-speed data rate can be achieved by using orthogonal code multiplexing while retaining the multi access capability. For further increase of the transmission data rate, the use of multi-input multi-output (MIMO) multiplexing is very effective. However, channels become severely frequency-selective, as a result, the performance is severely degraded in a frequency-selective fading channel due to orthogonality distortion among spreading codes. Recently, we proposed an iterative parallel interference cancellation (PIC) for SC-MIMO multiplexing in a frequency-selective fading channel. In this paper, we apply the frequency-domain iterative PIC to multicode DS-CDMA MIMO multiplexing in a frequencyselective fading channel and investigate, by the computer simulation, the impact of spreading factor and channel frequency-selectivity on the achievable bit error rate (BER) performance. Packet access will be the core technology of the next generation mobile communications systems. As an efficient error control technique, rate compatible punctured turbo coded hybrid automatic repeat request (RCPT-HARQ) is well known. We also evaluate, by computer simulation, the throughput performance of RCPT-HARQ in a frequency-selective Rayleigh fading channel.