chanism provokes a de-sequencing of the blocks received by the destination. The RLC layer assures the re-sequencing of different blocks received. For the downlink, the HARQ mechanism is adaptive because the retransmission can modify the transmission parameters. For the downlink, the HARQ mechanism is asynchronous because a HARQ process transmission is not constrained by an imposed period. For the uplink, the HARQ mechanism is adaptive or non-adaptive. It is non-adaptive if the retransmission must use the same initial transmission parameters. For the uplink, the HARQ mechanism is synchronous because the HARQ process transmission must be done according to a defined timing. 4.2.2.4.1. Data transfer for uplink The number of HARQ entities for transmission mode 2 (mode with MIMO) is double that of transmission mode 1 (mode without MIMO), because one HARQ entity is allocated to each transport block: - one transport block is used for transmission mode 1; - two transport blocks are used for transmission mode 2. In case of the carrier aggregation (CA), a HARQ entity is developed for each radio channel. For the FDD mode, by the HARQ entity, the HARQ process number has a fixed value: - value equal to 8 for transmission mode 1; - value equal to 16 for transmission mode 2. Figure 4.12 described the HARQ mechanism for the transmission mode 1 in the FDD mode. The transmission moments for each HARQ process is synchronous, with an 8 ms periodicity. 138 VoLTE and ViLTE The PHICH physical channel transports the HARQ indicator (ACK or NACK bits) for each transport block, 4 milliseconds after the transmission of the data in the PUSCH physical channel. Under a HARQ process, if a transport block is acknowledged, a new transport block can be transmitted 8 milliseconds later. If a transport block is not acknowledged, the RV1 redundancy version is transmitted 8 milliseconds later. The HARQ processes used depend on the transmission moment of transport blocks. The 8 HARQ processes are systematically used if the eNB entity allocates every mil