Retransmission is an important part of LTE to ensure transmission quality. There are two components of the retransmission. Hybrid ARQ and Radio Link Control.

A ) Hybrid ARQ (HARQ): In this the retransmissions occur quickly. The principle of HARQ is the same for uplink and downlink:

• Transmission: The Transmitter sends a transport block.
• Decoding: The Receiver tries to decode. If successful, it sends an ACK. In case of spatial multiplexing, two transport blocks can exist, so that the ACK has to have 2 bits.
• New Transmission/Retransmission: If the previous transmission was successful, the TX sends a new packet (and indicates this fact in the “new data” field); if the previous transmission was unsuccessful, the transport block is retransmitted (the “new data” field showing that it is a retransmission, and the “redundancy version” field indicating which type of retransmission is used).

The key difference between uplink and downlink is that the retransmission in the downlink might occur using arbitrary RBs while in the uplink it is fixed. It is always eighth subframes after the first transmission attempt, and the RBs used for retransmission are the same as for the first transmission. Since there is a delay of eight frames between retransmissions, up to eight parallel HARQ processes must be active such that payload data can be transmitted during every frame.

B) Radio Link Control (RLC): It is a higher layer retransmission protocol that arranges for the retransmissions of all the data blocks that fail even after HARQ. This mechanism is quite a bit slower, but will be invoked only rarely if HARQ is configured the right way. Being a “fallback” solution, it provides the extra reliability required for some applications (e.g., file transfer). The retransmission function can be switched off whenever it is not necessary/helpful (e.g., for voice calls, when the delay introduced by the protocol would be larger than what can be tolerated in phone conversations ).