Simple Hybrid ARQ
The simplest version of HARQ, Type I HARQ, adds both ED and FEC information to each message prior to transmission. When the coded data block is received, the receiver first decodes the error-correction code. If the channel quality is good enough, all transmission errors should be correctable, and the receiver can obtain the correct data block. If the channel quality is bad, and not all transmission errors can be corrected, the receiver will detect this situation using the error-detection code, then the received coded data block is rejected and a retransmission is requested by the receiver, similar to ARQ (Comroe/Costello 1984, pp. 474).
In a more sophisticated form, Type II HARQ, the message originator alternates between message bits along with error detecting parity bits and only FEC parity bits. When the first transmission is received error free, the FEC parity bits are never sent. Also, two consecutive transmissions can be combined for error correction if neither is error free (Comroe/Costello 1984, pp. 474–5).
To understand the difference between Type I and Type II Hybrid ARQ, consider the size of ED and FEC added information: error detection typically only adds a couple of bytes to a message, which is only an incremental increase in length. FEC, on the other hand, can often double or triple the message length with error correction parities. In terms of throughput, standard ARQ typically expends a few percent of channel capacity for reliable protection against error, while FEC ordinarily expends half or more of all channel capacity for channel improvement.
In standard ARQ a transmission must be received error free on any given transmission for the error detection to pass. In Type II Hybrid ARQ, the first transmission contains only data and error detection (no different than standard ARQ). If received error free, it's done. If data is received in error, the second transmission will contain FEC parities and error detection. If received error free, it's done. If received in error, error correction can be attempted by combining the information received from both transmissions.
Only Type I Hybrid ARQ suffers capacity loss in strong signal conditions. Type II Hybrid ARQ does not, because FEC bits are only transmitted on subsequent retransmissions as needed. In strong signal conditions, Type II Hybrid ARQ performs with as good capacity as standard ARQ. In poor signal conditions, Type II Hybrid ARQ performs with as good sensitivity as standard FEC.
Read more about this topic: Hybrid Automatic Repeat Request
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