We devise the optimal defection scheme in multiuser multicarrier based systems. We focus on the critical uplink scenario where several multicarrier signals belonging to distinct users propagate through independent time-variant frequency selective fading channels, and experience independent time offsets and carrier frequency offsets. We consider an efficient discrete-time system implementation and we refer to it as multitone multiple access. When channel coding is deployed we propose to accomplish decoding through the turbo multiuser detection and decoding approach. A simplified detection method is also described.