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In this paper, we discuss how to find the optimal method for efficient integration of multicast and unicast service in wireless OFDM systems. The multicast capacity is limited by the user with the worst channel condition. In order to overcome this limitation, we propose channel structure for the multicast and unicast traffic. The multicast channel first encodes the multicast traffic and then demultiplexes it into the constituent subchannels, and the channel structure increases the multicast channel capacity with the help of the channel coding. Based on the channel structure, we develop the subchannel allocation that maximizes the total unicast throughput while guaranteeing the transmission rate requirement on the multicast traffic for all the multicast users. Since it is too complicated to solve the allocation problem directly, we devise a fitness metric that quantitatively measures how suitable the subchannel is for carrying the multicast traffic and present the fitness-based subchannel allocation (FSA) algorithm. Numerical results reveal that the FSA algorithm achieves almost the same performance as the performance upper bound in terms of average unicast throughput.