me can correspond to a special sub-frame containing three particular fields: - a field for the downlink pilot time slot (DwPTS) which can contain data; - a field for the uplink pilot time slot (UpPTS) which can contain data or a preamble; - a gap period (GP), between the two preceding fields. This interval time facilitates the compensation of a time difference between different mobiles and avoids an overlap between the two transmission directions. Frame 10 ms Half-frame Half-frame Sub-frame DwPTS UpPTS DwPTS UpPTS 45678910 0.5 ms Figure 4.6. Structure of the frame in TDD mode 118 VoLTE and ViLTE The sub-frames are attributed to the data for the uplink and downlink according to diverse configurations (Table 4.2): - sub-frames 0 and 5 are always allocated to traffic in the downlink; - sub-frame 1 is always allocated to the special sub-frame containing the three particular fields; - sub-frame 2 is always allocated to traffic in the uplink; - sub-frame 6 can be allocated to the special sub-frame containing three particular fields for a periodicity of 5 ms; - sub-frames 3, 4, 7, 8, 9 are allocated to the downlink or uplink traffic according to the selected configuration. Number of the sub-frame Configuration Periodicity 012345678 DSUUUDSUUU 5 ms DSUUDDSUUD 5 ms DSUDDDSUDD 10 ms DSUUUDDDDD 10 ms DSUUDDDDDD 10 ms DSUDDDDDDD DSUUUDSUUD D (Downlink) / U (Uplink) / S (Special) Table 4.2. TDD frame configuration Radio Interface Procedures Each time slot comprises of 3 or 6 or 7 OFDM symbols (Figure 4.7). 1 slot = 15360 Ts Normal cyclic prefix Symbol 1 slot = 15360 Ts Extended cyclic prefix Symbol Figure 4.7. Time slot structure An OFDM symbol corresponds to a number of bits which depends on the modulation used: - 2 bits in the case of a quadrature phase-shift keying (QPSK) modulation whose constellation contains 4 different symbols; - 4 bits in the case of a quadrature amplitude modulation (16-QAM) whose constellation contains 16 different symbols; - 6 bits in the case of a 64-QAM modulation whose constellation contain 64 d