Multiplex multiple users both in time and frequency, together with pilots and control signals. The time-frequency plane is divided into chunks=minimum resource allocation unit.The traffic multiplexing is performed by allocating to each user a certain number of chunks depending on its data rate/geometry.
Physical Resource Block
In OFDMA, users are allocated a specific number of subcarriers for a predetermined amount of time. These are referred to as physical resource blocks (PRBs) in the LTE specifications. PRBs thus have both a time and frequency dimension. Allocation of PRBs is handled by a scheduling function at the 3GPP base station (eNodeB).
UPLINK:- FDD and TDD uplink transmissions have the same physical channels and signals. The main difference is that TDD frames include a special subframe and its part which can be used for SRS and PRACH for uplink transmissions.
in uplink data signals (PUSCH) LTE standard uses SC-FDMA (Single Carrier Frequency Division Multiple Access) modulation, which has a lower cost amplifiers and less power usage means lower peak-to-average power ratio.
In TDD mode occupy the same frequency spectrum but are separated in time for uplink and downlink transmissions . Uplink users transmit subframes, random-access requests (PRACH) and SRS during the UpPTS section of the special subframe.
The uplink FDD frame contains only uplink user transmissions and is same length as the downlink frame.