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Time division duplex (TDD) systems employing orthogonal frequency division multiplexing (OFDM) have recently been proposed as potential candidates for next generation (4G) cellular networks. A proper multiple access scheme, however, is still to be defined and it constitutes one of the most important issues in the system design. In this paper we compare the performance of several multiple access schemes for both downlink and uplink transmissions in a multi-cell environment supporting real-time traffic. The system performance is improved using a distributed power control so as to provide the users with the required quality of service in the form of a guaranteed rate. Numerical results are shown to assess the performance of different access schemes and the relative impact of several design parameters. The obtained results may be useful in the definition of the radio access technique for 4G cellular systems.