In this paper, we study the joint design of uplink and downlink resources in OFDMA-based systems. We first analyze the interactions between uplink and downlink, due essentially to the usage of Time Division Duplexing (TDD) and the TCP ACK/NACKs. This analysis is based on analytical models for interference and throughputs that we develop for both directions. We next study the capacity of the system using a Markovian analysis and matrix geometric solutions. Our performance analysis study allows us to determine the optimal proportion of resources that has to be allocated to the uplink; In addition to that, we show that an admission control policy reserving some uplink resources to TCP ACKs may improve the overall capacity of the system