We address an issue of channel sharing among users by using a random assignment/transmitter-oriented code scheme which permits the contention mode only in the transmission of a header. The reception of the data packet may be blocked due to limited number of correlators so that this effect is taken into account in our analysis. We also consider an acknowledgement scheme to notify whether the header is correctly detected, which aims at reducing the interference caused by unwanted data transmission. It is shown that a reduction in receiver complexity by about a half is allowed by choosing a proper number of codes without losing the performance quality.