A transmitter-based linear precoding technique is considered for downlink multicarrier code division multiple access systems. The key idea is to shift the required signal processing to the base station, where power and computational resources are plentiful, thus simplifying the receiver complexity and extending the battery life of mobile terminals. The multiple access interference (MAI) resulting from the frequency-selective channel inevitably limits the system performance. The minimum mean square error criterion is employed to optimise the linear transformation at the transmitter subject minimising the total transmitted energy in order to presuppress the MAI before the transmission. Two types of transmitter precoders are investigated in uncorrelated and correlated frequency-selective Rayleigh fading channels; one is combined with equal gain combining (PRE-EGC) and the other with maximum ratio combining (PRE-MRC) at the receiver. Numerical results show that the PRE-EGC can provide the same performance to all users and outperform the PRE-MRC. More importantly, the performance of the former is comparable to, or superior to, receiver-based techniques, such as the decorrelating detector using EGC (DEC-EGC) and MRC (DEC-MRC), provided the channel state information of all users is slowly time-varying and known to the transmitter.