ifferent symbols. The introduction of a guard time between the symbols facilitates the removal of inter-symbol interference. The guard time of a time slot is found at the beginning of the symbol. It contains the copy of the end of the symbol in order to avoid a very high dynamic for the amplifiers. This copy is called the cyclic prefix (CP). The normal cyclic prefix is used in case the delay between the different reflected signals is low, which is the case in cells with a smaller diameter. 120 VoLTE and ViLTE The extended cyclic prefix is used in case the delay between the different reflected signals is significant, which is the case in cells of a large diameter. 4.1.4.4. Spatial multiplexing Four modes characterize the transmission system on the radio channel (Figure 4.8). It should be noted that the term input is applied to the input of the radio channel and the term output to the output of the same channel. Figure 4.8. Transmission modes The single input single output (SISO) mode is the basic signal propagation mode in which a transmitting and a receiving antenna are used. The single input multiple output (SIMO) mode is characterized by the use of a single transmitting antenna and multiple receiving antennas. The SIMO mode is often referred to as receive diversity. The transmitted bit rate is identical to the SISO mode. On the other side, the selection of the received signal allows the receiver to protect against fading of the radio signal. The multiple input single output (MISO) mode features multiple transmit antennas and a single reception antenna. The same signal is transmitted on the transmit antennas. The MISO mode is often referred to as transmit Radio Interface Procedures diversity. As for the SIMO mode, the MISO mode allows the receiver to protect against radio signal fading. The MISO mode is also used for beamforming, directed towards the mobile, by controlling different phases of the emitted signals. The multiple input multiple output (MIMO) uses multiple antennas for transmission and reception. It 