ARQ versus HARQ
This page on ARQ versus HARQ describes ARQ and HARQ techniques and provides difference between ARQ and HARQ techniques.
ARQ stands for Automatic Repeat Request. This is the protocol used at data link layer. It uses CRC to determine whether the packet received is correct or not. If the packet is received correctly receiver send ACK to the transmitter, but if the packet is received incorrectly receiver send NACK to the transmitter. After receiving NACK transmitter re-transmits the same packet again. The retransmission is done for set number of times; if it is not successful for few tries than the system will come out of the loop. Above mentioned type is simple stop and wait ARQ technique.
The most common ARQ technique in use is go-back-N ARQ technique.
Go-back-N ARQ - This ARQ technique is based on sliding window protocol. Let us see how this works. When source/transmitting station transmits PDUs numbered sequentially from PDU-1 to say PDU-n.
In case of no errors and receiver/destination station is satisfied with the packet (PDU) received, it sends positive acknowledgement (ACK) to the transmitter.
In case of errors in any one of the PDU that is received, receiver sends negative acknowledgement (NACK) to the transmitter for that PDU. Upon receiving NACK transmitter suspends current PDU and all the future PDUs under transmission and will retransmit erroneous PDU and PDUs sent after that erroneous PDU.
HARQ does not retransmit packet/PDU as it is; as done by ARQ technique. HARQ modifies certain physical parameters before retransmission. Various HARQ techniques are available such as Chase Combining HARQ, Incremental redundancy (IR) HARQ type II and IR HARQ type III.
In Chase Combining type of HARQ, power or energy or Eb/No will increase at the receiver due to summation operation performed by MRC (Maximum Ratio Combining) technique. Here each retransmission helps increase energy of the received packet.
In Incremental redundancy type of HARQ, redundant information is changed/retransmitted rather than the entire packet based on channel conditions by varying puncturing configuration used at physical layer of the transmitter.
In poor channel conditions HARQ performance is better than ARQ while in good channel conditions ARQ seems to be better than HARQ as HARQ performance decreases here.
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