Regardless of what we’re doing, we all want to receive information quickly and free of error. Obtaining all of the information we need the first time around would be our first choice, of course. But with complex mobile radio systems and an increasingly noisy environment, this isn’t always possible. So if we don’t get the data the first time, we still expect it to arrive intact eventually and with minimal delay.

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Network operators must support these demands while using minimal resources. For example, the 3GPP Long Term Evolution (LTE) standard takes advantage of adaptive coding and modulation (AMC) in addition to the hybrid automatic repeat request (HARQ) process. This is critical to minimize the turnaround time and maximize the data throughput of the system.

The telecommunications industry has used the Automatic Repeat Request (ARQ) layer 2 protocol for many years to ensure that data is sent reliably from one node to another. In regular ARQ, error-detecting (ED) codes such as cyclical redundancy checking (CRC) and a sliding window are used to identify when an error has occurred in a transmission. If errors are detected, the destination requests a retransmission from the source.

During good radio conditions, ARQ can be considered very efficient, as no additional forward error correction (FEC) bits are added to the basic data to be transmitted. Yet bandwidth efficiency will suffer significantly in poor channel conditions due to excessive retransmissions. Hybrid ARQ performs better than regular 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 crossover point below which simple hybrid ARQ is most efficient and above which basic ARQ is the best solution.

The simplest version of HARQ, Type I HARQ, in addition to ED adds FEC information to each message prior to transmission. If channel quality is sufficiently good, all transmission errors should be correctable and the receiver can decode the data block correctly. If the channel quality is poor and not all transmission errors can be corrected, the data block will be discarded and the receiver (similar to ARQ) will request a retransmission. Although ED adds only a few bits to each transmission, the additional FEC bits will add significant overhead to each transmission. In periods of good channel quality, it will significantly reduce the user data rate and therefore the bandwidth efficiency.

Type II HARQ is a significantly more sophisticated solution that transmits a subset of the data, ED, and FEC bits on a given transmission. Successive transmissions include a different subset of these bits. In Type II HARQ, the first transmission contains enough data, ED, and FEC bits to decode the transmission in good channel conditions, but not in poor conditions.

If this first transmission is received error-free, the transmitting node will prepare and transmit the next block of data. If the data from the first transmission is received in error, though, the second transmission will contain a different set of data, ED, and FEC bits. If received error-free, the transmitting node will prepare and transmit the next block of data. Error correction can be attempted by combining the information received from both the first and second transmissions in a process known as incremental redundancy (IR). Each subsequent transmission is combined with earlier transmissions until the packet is received correctly.

Only Type I HARQ suffers the capacity loss when channel quality is good. Type II hybrid does not, because the code rate is iteratively reduced on subsequent retransmissions only if they are required. When channel quality is good, Type II HARQ obtains similar channel capacity as standard ARQ, eliminating unnecessary bandwidth inefficiencies.

Type II HARQ uses a mother code that can be punctured to achieve the desired code rate. For LTE, this mother code is a rate 1/3 turbo code. This code contains systematic bits, which means the input and ED bits are present verbatim in the output. The first transmission of the packet would send mostly these systematic bits by puncturing out most of the FEC bits. Subsequent retransmissions would send fewer of the systematic bits and more of the FEC bits.

The different transmitted versions of the packet containing different combinations of systematic and FEC bits are called redundancy versions (RVs). LTE uses four RVs that are repeatedly sequenced through until the packet is received correctly or until a maximum number of retransmissions have been sent, at which time HARQ declares a failure and leaves it up to ARQ running in radio link control (RLC) to try again.