4.11 AUTOMATIC-REPEAT-REQUEST (ARQ) SCHEMES

The error-control mechanisms discussed in the previous sections—collectively called forward-error correction (FEC)—allow the receiver to detect and correct errors without retransmission. FEC techniques, however, become less efficient and more complex as the carrier-to-noise ratio decreases. In many channels, particularly those with low error probability, such sophistication may be unnecessary. In these cases, a simpler automatic repeat-request (ARQ) scheme is appropriate.

ARQ provides error control by including enough redundancy for the receiver to detect errors but not to correct them. When an error is detected, the receiver requests retransmission of the affected data block. Compared with FEC, ARQ requires far less redundancy and simpler decoding hardware. However, while ARQ performs well in low-noise environments, it can yield very low information rates in noisy channels where many retransmissions are required.

Another limitation of ARQ is its need for a feedback path from receiver to transmitter (a half-duplex link is sufficient). In broadcast systems such as DVB, no return channel exists, making ARQ infeasible. ARQ is also unsuitable for delay-sensitive applications, since it requires the transmitter to await acknowledgment before proceeding. This is particularly restrictive for satellite systems, where the round-trip propagation delay can exceed 0.5 s for geostationary orbits. Consequently, ARQ is mainly confined to store-and-forward messaging or data relay systems, typically implemented on LEO networks.

Although FEC and ARQ are often treated separately, they can be combined in hybrid ARQ (HARQ) schemes that use FEC to correct the most common errors and retransmissions to recover from rarer, more severe errors. The choice between FEC, ARQ, and hybrid schemes is revisited in Section 4.14 in the context of overall system architecture.