In this paper, the Bit Error Rate (BER) performance of cooperative downlink Multi-Carrier (MC) - interleave division multiple access scheme is investigated. This uses Transmitter Preprocessing technique, where a single relay supports all the users. Multiple Access Interference (MAI) and Inter Relay Interference happen in relay aided cooperative communication. To alleviate these interferences, we use the Zero Forcing (ZF) approach Multi-User Transmitter Processing technique (MUTP) at the base station and the relay station. The BER performance is particularly evaluated between the base station to relay and relay to mobile station. This is done considering the SUI-1 channel model for MUTP assisted downlink co-operative aided IDMA system. The simulations reveal that a better BER performance is obtained from the ZF based transmitter preprocessing technique for MC-IDMA accessing with relay aided co-operative DL transmission. The system performance can be further enhanced by increasing the number of relays between BS and MS for MUTP.