We discussed the basic concept of OFDM in Chapter 2, Section 2.4.2. This has become the PHY layer in the latest commercial systems based on the IEEE 802.16 series of standards for broadband wireless access networks. Since, in general, each OFDM carrier can be allocated to different users over time, these systems can be said to employ OFDMA-TDMA. In this chapter we study resource allocation problems in OFDMA-TDMA cellular systems by formulating various constrained optimization problems, the decision variables being the carriers assigned to the various users, the powers used on each carrier by each user, how much data each user sends in a slot, and so on. The constraint could be on the average total power.