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A novel transmitter code optimisation technique based on adaptive code-to-user allocation is presented for interference exploitation on the time division duplex downlink of binary phase shift keying-based multiple carrier code division multiple access systems. The principle of the proposed technique is to exploit the dependency of multiple access interference on the instantaneous symbol values of the active users. The objective is to adaptively allocate the available spreading sequences to users on a symbol-by-symbol basis to enhance the ratio between constructive and destructive interference and thus optimise the decision variables at the downlink receivers. The resulting signal-to-interference plus noise improvement happens by making use of the energy inherent in the system so the performance benefit is attained with no additional power-per-user investment. It will be shown that when this optimisation technique is incorporated with common pre- and post-equalisation schemes, a significant bit error rate performance improvement is achieved while the associated adaptation overhead is kept less than 6% of the available bandwidth.