In LTE, DL and UL transmissions are organized into radio frames of 10 ms each. Each frame is divided into ten equally sized subframes. The duration of each subframe is 1 ms. Moreover, each subframe is further divided into two equally sized time slots, that is, each slot is 0.5 ms. 3GPP defines two types of frames based on the duplexing scheme used. These are Type 1 when FDD is used and Type 2 when TDD is used. Figures 10.1 and 10.2(a) illustrate the two types, respectively.
In Type 1 frames, DL and UL transmissions use two different frequency bands. Hence, frames are not shared between the two. On the other hand, in TDD, the two transmissions share the same frequency bands but are separated in time. Hence, they share the frames. In fact, every frame is divided into two halves, one for the DL transmission while the other is for the UL transmission. Nevertheless, a Type 2 frame is similar in structure to a Type 1 frame. The only difference is the existence of one or two special subframes that help switching between UL and DL transmissions.