HARQ Basic Part-1

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In this section, we will learn about HARQ basic. 

What is ARQ?

ARQ stands for automatic repeat request. It refers to the re-transmission protocol in which the receiver checks for errors within the received data and if an error is detected then the receiver discards the data and request a re-transmission forms the sender. For acknowledged mode the RLC layer uses an ARQ protocol. 

What is HARQ?

HARQ stands for hybrid ARQ. It refers to a re-transmission protocol in which the receiver checks for errors in the received data and if an error is detected then the receiver buffers (soft buffers) the data and requests a retransmission from sender. HARQ receiver then is able to combine the buffered data with the re-transmitted data to channel decoding and error detection. This improves the performance of re-transmissions. HARQ re-transmissions can benefit from either chase combining or incremental redundancy.

What is the difference between ARQ and HARQ?

ARQ	HARQ
Works at RLC layer	Works at PHY layer but controlled by MAC layer
If there is an error in the received data (as detected by ARQ at RLC layer) then it is discarded, and a new retransmission is requested from the sender	If there is an error in the received data then the Receiver buffers the data (at PHY layer control by MAC) and requests a retransmission from the sender.

What is chase combining and its benefits?

Chase combining means that the physical layer applies the same puncturing pattern to both the original transmission and each retransmission. This results in re-transmissions which include the same set of physical layer bits as the original transmission. The benefits of chase combining are its lower UE memory requirement and simplicity.

What is incremental redundancy?

It means that the physical layer applies different puncturing patterns to the original transmission and retransmissions. Thus retransmission includes a different set of physical layer bits to the original transmission. The drawbacks here are increased UE memory requirement and complexity.

How LTE is HRQ is different from NR HARQ?

In LTE HARQ, downlink uses asynchronous mechanism and uplink uses synchronous mechanism. But in case of NR both downlink and uplink uses Asynchronous mechanism.

What is synchronous and asynchronous HARQ?

In synchronous HARQ retransmission for each process occur at predefined times relative to the initial transmission, thus no need to signal HARQ process number but can be inferred from transmission timing. In asynchronous HARQ, the retransmissions can occur at any time and HARQ process number (HARQ ID) is sent to correctly associate each retransmission with the corresponding initial transmission. Synchronous HARQ reduces signaling overhead while asynchronous HARQ increases flexibility in scheduling.

What is adaptive and non-adaptive HARQ?

The modulation, coding scheme, and frequency resource allocation may be changed at each retransmission in adaptive HARQ. While in non-adaptive HARQ, retransmissions are performed either by the same previous transmission attributes or by predefined rules. Adaptive HARQ brings more scheduling gain at the expense of signaling overhead.

In the next section, we will learn more about HARQ and its basic. So keep reading……. And Stay tune……………

Pinal Dobariya………..

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