ELEC5508_Wireless Engineering_2013 Semester 2_Hybrid ARQ

Automatic Repeat reQuest(ARQ), also known as Automatic Repeat Query, is an error-control method for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a data frame or packet) and timeouts (specified periods of time allowed to elapse before an acknowledgment is to be received) to achieve reliable data transmission over an unreliable service. If the sender does not receive an acknowledgment before the timeout, it usually retransmits the frame/packet until the sender receives an acknowledgment or exceeds a predefined number of re-transmissions.Hybrid automatic repeat request (hybrid ARQ or HARQ) is a combination of high-rate forward error-correcting coding and ARQ error-control. In standard ARQ, redundant bits are added to data to be transmitted using an error-detecting (ED) code such as a cyclic redundancy check (CRC). Receivers detecting a corrupted message will request a new message from the sender. In Hybrid ARQ, the original data is encoded with a forward error correction (FEC) code, and the parity bits are either immediately sent along with the message or only transmitted upon request when a receiver detects an erroneous message. The ED code may be omitted when a code is used that can perform both forward error correction (FEC) in addition to error detection, such as a Reed-Solomon code. The FEC code is chosen to correct an expected subset of all errors that may occur, while the ARQ method is used as a fall-back to correct errors that are uncorrectable using only the redundancy sent in the initial transmission. As a result, hybrid ARQ performs better than ordinary ARQ in poor signal conditions, but in its simplest form this comes at the expense of significantly lower throughput in good signal conditions. There is typically a signal quality cross-over point below which simple hybrid ARQ is better, and above which basic ARQ is better.Simple Hybrid ARQThe 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 ARQIn 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.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. Ifreceived 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.Hybrid ARQ with soft combiningThe hybrid ARQ operation described above discards erroneously received packets and requests retransmission. However, despite that the packet was not possible to decode, the received signal still contains information, which is lost by discarding erroneously received packets. This shortcoming is addressed by hybrid ARQ with soft combining. In hybrid ARQ with soft combining, the erroneously received packet is stored in a buffer memory and later combined with the retransmission to obtain a single, combined packet which is more reliable than its constituents.Decoding of the error-correcting code operates on the combined signal. If the decoding fails (typically a CRC code is used to detect this event), a retransmission is requested.Retransmission in any hybrid ARQ scheme must, by definition, represent the same set of information bits as the original transmission. However, the set of coded bits transmitted in each retransmission may be selected differently as long as they represent the same set of information bits. Hybrid ARQ with soft combining is therefore usually categorized into Chase combining and Incremental Redundancy, depending on whether the retransmitted bits are required to be identical to theoriginal transmission or not.With Chase combining retransmissions consists of the same set of coded bits as the original transmission. After each retransmission, the receiver uses maximum-ratio combining to combine each received channel bit with any previous transmissions of the same bit and the combined signal is fed to the decoder. As each retransmission is an identical copy of the original transmission, retransmissions with Chase combining can be seen as additional repetition coding.Therefore, as no new redundancy is transmitted, Chase combining does not give any additional coding gain but only increases the accumulated received Eb/N0 for each retransmission (Figure 1).Figure 1 Chase combiningSeveral variants of Chase combining exist. For example, only a subset of the bits transmitted in the original transmission might be retransmitted, so-called partial Chase combining. Furthermore, although combining is often done after demodulation but before channel decoding, combining can also be done at the modulation symbol level before demodulation, as long as the modulation scheme is unchanged between transmission and retransmission.Figure 2 Incremental RedundancyWith Incremental Redundancy (IR), each retransmission does not have to be identical to the original transmission. Instead, multiple sets of coded bits are generated, each of them representing the same set of information bits. Whenever a retransmission is required, the retransmission typically uses a different set of coded bits than the previous transmission. The receiver combines the retransmission with previous transmission attempts of the same packet. As the retransmission may contain additional parity bits, not included in the previous transmission attempts, the resulting code rate is generally lowered by a retransmission. Furthermore, each retransmission does not necessarily have to consist of the same number of coded bits as the original and, in general, also the modulation scheme can be different for different retransmissions. Hence, incremental redundancy can be seen as a generalization of Chase combining or, stated differently, Chase combining is a special case of incremental redundancy.Typically, incremental redundancy is based on a low-rate code and the different redundancy versions are generated by puncturing the output of the encoder. In the first transmission only a limited number of the coded bits are transmitted, effectively leading to a high-rate code. In the retransmissions, additional coded bits are transmitted. As an example, assume a basic rate-1/4 code. In the first transmission, only every third coded bit is transmitted, effectively giving a rate-3/4 code as illustrated in Figure 2. In case of a decoding error and a subsequent request for a retransmission, additional bits are transmitted, effectively leading to a rate-3/8 code. After a second retransmission the code rate is1/4. In case of more than two retransmissions, already transmitted coded bits would be repeated.In addition to a gain in accumulated received Eb/N0, incremental redundancy also results in a coding gain for each retransmission. The gain with IR compared to Chase is larger for high initial code rates while at lower initial coding rates, Chase combining is almost as good as IR.。