6
ARQ
In closed-loop systems the usage of feedback channel allows the transmission characteristics to be improved. The idea of utilizing the feedback channel is that in case of receiving a very noisy message it is possible not to make a decision, but rather ask the transmitter to repeat the transmission. In the most general cases such organization of communication allows a decision concerning transmitted message to be made only under conditions of noise absence (or very low noise level), repeating the message when the noise level is high. Surely, usage of repeated transmission decreases the overall transmission rate. However, in most cases such rate decreasing is not significant. Communication systems utilizing the feedback channel are called systems with repeat request (ARQ, Automatic Repeat reQuest) or systems with feedback [10, 16, 18].
6.1 Basic ARQ Schemes
6.1.1 Basic Concepts
Among the variety of algorithms for data transmission over the feedback channels the two basic classes of such systems may be distinguished: systems with decision feedback and systems with data feedback.
In systems with decision feedback the decision of repeated message transmission is made by the receiving side of the system as the result of analyzing the received data. In systems with data feedback the decision is made by the transmitter based on the information transmitted over the feedback channel. Systems in which ...
