The Hybrid ARQ (HARQ) protocol in MAC layer operates such that the protocol uses multiple stop-and-wait hybrid ARQ processes. The motivation for this type of protocol is to allow continuous transmission, which cannot be achieved with a single stop-and-wait scheme, while at the same time having some of the simplicity of a stop-and-wait protocol. The functionality and performance is similar to that of a window based selective repeat protocol but only single-bit HARQ feedback is required.
Number of HARQ Processes
The protocol is modeled as a number of parallel HARQ processes, where each process uses a simple stop-and-wait protocol. By using N HARQ parallel HARQ processes, where N HARQ > Round trip time / sub-frame length, a continuous transmission is achieved.
The maximum UE processing time before sending a HARQ feedback has been specified such that 8 HARQ processes are needed for continuous transmission in FDD with a typical eNodeB implementation.
Types of HARQThere are two types of HARQ procedures.
In 3GPP, the current working assumption is to use a synchronous HARQ for the uplink and an asynchronous HARQ for the downlink. That is, for the uplink, the sub-frame when the re-transmission occurs is known at the receiver, while for the downlink the scheduler has the freedom to choose the sub-frame for the retransmission dynamically. For both up and downlink a synchronous, single-bit HARQ feedback (ACK/NACK) is sent providing feedback about the success of the previous transmission. The HARQ protocol is adaptive in both uplink and downlink, meaning that the scheduler can decide to use a different resource for a retransmission compared to the previous (re)transmission.
The HARQ procedure is explained pictorially below.
Redundancy and Retransmission
The redundancy version of a (re)transmission needs to be known by the receiver. Thus, the redundancy version and an indication whether the transmission contains a first transmission or a retransmission is indicated on the PDCCH.
In case the data is a retransmission of previously stored data, the received data is soft combined with the data stored in the soft buffer. In case the received data is not a retransmission or a retransmission of data that has not been stored, the soft buffer is cleared and only the latest received data is placed in the buffer.