HARQ (Hybrid Automatic Repeat request) hybrid automatic retransmission requests

Source: Internet
Author: User
Tags ack

Data communication was originally developed on a wired network, and typically requires large bandwidth and high transmission quality. For wired connections, the reliability of data transmission is achieved through heavy transmissions. When the current attempt to transfer fails, it requires retransmission of the data packet, such a transmission mechanism is called ARQ (Automatic request retransmission). In the wireless transmission environment, the channel noise and the fading due to mobility and the interference caused by other users make the channel transmission quality very poor, so the data grouping should be protected to suppress all kinds of interference. This protection mainly uses forward error correcting codes (FEC) to transmit additional bits in the packet. However, excessive forward error-correcting coding can reduce transmission efficiency. Therefore, a hybrid scheme, HARQ, which combines ARQ and FEC, is proposed.


HSDPA technology mainly uses three kinds of incremental redundancy HARQ mechanism: type-i harq,type-ii HARQ and TYPE-III HARQ.
Type-i HARQ is also called the traditional Arq, is the most basic function. Traditional ARQ uses a CRC (cyclic redundancy check) and is encoded with FEC. Once the receiver discovers that the data package is not correctly decoded (decode) when it is received, it is discarded and requires retransmission in the uplink channel (Uplinking channel). After the sender receives the retransmission request, resend the encoded data package. Because the feedback speed is slow, the efficiency is not high, can affect the QoS.
The TYPE II Harq is an ARQ mechanism that adds redundancy (IR), the packets that receive the error are not discarded, and the retransmission data is usually not the same as the first transmission, and the two data packages are made and rounded to form a forward error-correcting code with a stronger error correction capability. IR can be divided into two categories: partial ir (H-ARQ-TYPE-LLL), full IR (H-ARQ-TYPE-11), wherein the full IR transmission is not self-decoding.
Type-ⅲharq is also called partial redundancy HARQ, combined with the advantages of Type-i HARQ and Type-ⅱharq, the packets that receive the error with TYPE-II are not discarded. Each retransmission can be self-decoding, no need to merge the previous transmission data.

Implementation mechanism

The feasibility of HARQ is limited by the buffering capability of the sending and receiving sides to the packet, so it is important to choose the appropriate HARQ protocol. The three standard HARQ protocols are the stop-wait Protocol (STOP-AND-WAIT,SAW), the fallback N-step Protocol (GO-BACK-N,GBN), and the Selective Retransmission Protocol (SELECTIVE-REPEAT,SR).
(1) Stop and wait for the agreement
Each packet sent by the sender is temporarily stopped, waiting for confirmation from the receiving end. When the packet arrives at the receiving end, it is checked for errors, and if received correctly, the acknowledgement (ACK) signal is returned, and the error returns the NACK signal. When the originator receives an ACK signal, it sends the new data, or the last transmitted packet is resent. While waiting for the confirmation message, the channel is idle and no data is sent. This method is simple to implement because the two parties operate the same packet at the same time, the corresponding signaling cost is small and the cache capacity of the receiver is low. However, because the data is not sent while waiting for the confirmation signal, too many resources are wasted, especially when the channel transmission delay is very large. Therefore, the utilization of the communication channel is low and the throughput of the system is lower due to the stop-wait protocol.
(2) Backward N-Step protocol
In the transmission system with backward N-Step Harq protocol, the sending end sends a packet of data, and does not stop to wait for confirmation information, but sends several data grouping information in succession. The receiving end sends the corresponding ACK or nack of each packet back to the sending side, and the packet number is sent back. When a nack signal is received, the sender sends back n packets that include the error data. The receiving end simply receives the packet sequentially and must discard the correct packet and resend the acknowledgement after receiving the wrong packet, even after receiving the correct packet.
(3) Select the Re-send protocol
The Windows-based Sr is a HARQ protocol used by many systems, including the RLCR99 (also known as the R3 version, the most mature and stable version of 3GPP). In order to further improve the utilization of the channel, select the retransmission protocol only retransmission error packets, but at this time the receiving end is no longer sequentially receive data packet information, then at the end of the receiver needs a considerable amount of cache space to store has been successfully decoded but not yet the sequential output of the group. At the same time the receiver must know the serial number before combining the packet, so the serial number is coded separately from the data, and the serial number needs to be more reliably encoded to overcome any errors that occur in the data at any time, thus increasing the requirement for signaling.

HARQ (Hybrid Automatic Repeat request) hybrid automatic retransmission requests

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