Abstract-Automatic repeat request (ARQ) protocols deal with the situations where the selected data rate is not supported by the current channel realization. The inappropriate selection is motivated by the limited channel state information at the transmitter (CSIT). This work looks into how the ARQ protocols are applied to the half-duplex relay-assisted transmission with decode-and-forward relays by exploiting the ACK feedback channel of the terminals. Two implementations are tackled: 1) only the source or 2) source and relay manage the retransmissions under the pure-ARQ protocol. This work presents an analytical framework for the comparison of both options. The study allows the optimization of the bitrate and amount of resource allocated for the relay transmission phases.
Index Terms-ARQ, relay, cooperative system
I. INTRODUCTIONRelay-assisted has become an effective tool for combating the effects of propagation channel in wireless networks thanks to the relay terminal. Originally proposed in [1] with capacity results proved in [2], it has regained attention due to the diversity gain obtained in wireless channels, [3], [4], [5]. This work assumes half-duplex relays under decode-and-forward (DF), [4]. Hence, the transmission is carried out through two orthogonal phases: relay-receive phase (or phase I) and relaytransmit phase (or phase II).Among the many existing relay protocols [6], we have selected the relay-assisted transmission described by Fig. 1. In phase I the source transmits to the relay and destination. Afterwards, only the relay transmits to the destination. This is the same procedure used in [4] and named protocol I in [6]. The relay may transmit under repetition coding, RC, (using the same codeword as the source) or unconstrained coding, UC, (employing a different codebook from the source), [8].The exploitation of the ACK feedback channels from the terminals allows implementing the relay-assisted transmission procedure in different ways. For instance, if the source is in charge of the Automatic Repeat reQuest (ARQ) procedure and the source uses only the requests from destination, the retransmission is repeated along phase I and phase II. If the relay feedbacks to the source its success (ACK/NACK) in decoding the message, then the source might transmit during phase II, remaining the relay silent. Similarly, an ARQ protocol proposed in [9] enforces the use of the relay only when it receives successfully in phase I of the first transmission, also with ACKs from relay and destination. That analysis is carried out for the pure-ARQ protocol [10] with adaptive modulation. Moreover, the optimal bitrate and resource allocation is addressed in [7], where the source manages the ARQ with ACKs only from destination. When source and relay control the ARQ procedure, the source receives the feedback from the relay and destination while the relay listens the feedback from destination (ACKs from relay and destination). In such a case, the source transmits till either the relay or destination decodes the message. Once ...
