r detection code Modulation Error correction code Rate adaptation Mapping on spatial layers Relactor Precoding Physical channel Mapping on resource elements SC-FDMA signal Figure 4.4. Transmission chain: uplink 116 VoLTE and ViLTE 4.1.4.2. Frequency-division multiplexing Support for both transmission directions uses two bandwidths matched in the frequency division duplex (FDD) mode or a single bandwidth in the time-division duplex (TDD) mode. For the FDD mode, each transmission direction operates simultaneously in the assigned bandwidth. For the TDD mode, both directions of transmission operate alternately in the same bandwidth, meaning each direction is assigned a portion of time. The bandwidth of the radio channel is flexible and can take several values: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. Carrier aggregation (CA) involves combining the use of several component carriers (CC) or radio channels to increase the cell bit rate. The aggregation can be performed on five radio channels, bringing the maximum bandwidth to 100 MHz. The radio channel is formed in the frequency domain of an orthogonal frequency-division multiplexing (OFDM) with sub-carrier spacing of 15 kHz or 7.5 kHz. 4.1.4.3. Time-division multiplexing Two structures of time frames are defined according to the FDD or TDD mode. The type-1 structure defined for the FDD mode lasts 10 ms and contains 10 sub-frames (Figure 4.5). Each sub-frame is made up of two time slots. Frame 10 ms Sub-frame 123456789 01234567891 0.5 ms Figure 4.5. Structure of the frame in FDD mode Radio Interface Procedures For the uplink, the signals transmitted by different mobiles must be temporally aligned, to the reception by the eNB entity. The mobiles must, therefore, be synchronized temporally by the eNB entity which conveys the timing advance (TA) to them, to be applied for the uplink. The type-2 structure defined for the TDD mode also lasts 10 ms and contains two half-frames of 5 ms each (Figure 4.6). Each half-frame comprises five sub-frames and the second sub-fra