er, the total symbol time is the useful symbol time plus the CP. Hence, each slot can have either normal CP that is 4.7 us with seven useful symbols or extended CP that is 16.67 us with six useful symbols to transmit. One frame, Trame = 10 ms One subframe, T. subframe = 1 One slot, Tslot = 0.5 ms OFDM symbol Normal CP Tu 66.7 us Extended CP Tu = 66.7 us Figure 10-11 OFDM Frame Structure The differing length of the CP is used to address different use cases. For example, the normal CP is used in the case of small-diameter cells in urban areas where multipath occurs over relatively small distances but with high data rates. The extended CP is used in rural areas with large-diameter cells using extended delay spreads but with lower data rates. Using the CP technique makes OFDM symbols immune to time dispersion and mitigates ISI or ICI issues. However, the selection of the appropriate CP design is sensitive to timing errors in the cell, and synchronization inaccuracy can move portions of the symbol data outside the CP coverage area and cause signal degradation. LTE TDD Frame Structure In the case of TDD, the UL and DL occur on the same carrier frequency but in separate time do- mains. In each LTE frame, some subframes are allocated for uplink, and some subframes are alloc- ated for downlink. Switching between downlink and uplink is carried out using a special subframe. 3GPP defines different configuration sets; for example, one of the options is shown in Figure 10-12. 1 LTE frame, 10ms Special subframe DwPTS UpPTS Figure 10-12 Frame Structure of TDD Subframe The special subframe is split into three parts: Downlink pilot time slot (DwPTS) Guard period (GP) Uplink pilot time slot (UpPTS) The DwPTS is part of the subframe and smaller in size, but used to carry downlink transmission data, whereas the UpPTS is very small and does not carry any transmission data. The GP is a dura- tion where neither uplink nor downlink transmission occurs, and this period allows for the radio to switch between downlink and uplink. The length