 Packet 2 Packet 3 Packet 4 Packet 2 Packet 3 Packet 4 Packet 5 Packet 6 Packet 7 Packet 8 Packet 1 Packet 2 Packet 3 Packet 4 Packet 2 Packet 3 Packet 4 Packet 5 Packet 6 Packet 7 Receiver Error detected Figure 3.4-17 Go-back-N ARQ. 3.4.5.1.3 Selective-repeat ARQ received. In this way, the buffer size of the receiver can Selective-repeat ARQ is similar to go-back ARQ. The be limited to u(N - 1) + 1. difference is that when an NACK for codeword Ci is received, only a is retransmitted before the transmission 3.4.5.2 Hybrid ARQ protocols proceeds where it stopped, as in Figure 3.4-18. In addition to the N-codeword buffer at the transmitter, FEC and ARQ can be combined to provide for channels a buffer is needed at the receiver SO that the decoded with high error rates better reliability than FEC alone and codewords can be delivered in the correct order. larger throughput than ARQ alone. This buffer must be large enough to avoid overflow. Selective-repeat ARQ with a finite-size buffer is 3.4.5.2.1 Type- hybrid ARQ protocols presented in [29]. An alternative is to combine selective- In a type-I hybrid ARQ system, each information block is repeat ARQ with go-back-N ARQ [29]. Here the encoded with a channel code with error detecting and transmitter switches from selective-repeat ARQ to go- error correcting capabilities. This can be a single linear back-N ARQ whenever u retransmissions of a codeword code (see Section 3.4.3.1) or a concatenation of an error have been done without receiving an ACK. It switches detection code as an outer code and an error correction back to selective-repeat ARQ as soon as an ACK is code as an inner code. If the received word can be Retransmission Retransmission of Packet 2 of Packet 6 Transmitter Packet 1 Packet 2 Packet 3 Packet 4 Packet 2 Packet 5 Packet 6 Packet 7 Packet 8 Packet 6 Packet 9 Packet 1 Packet 2 Packet 3 Packet 4 Packet 2 Packet 5 Packet 6 Packet 7 Packet 8 Packet 6 Receiver Error detected Error detected Figure 3.4-18 Selective-repeat ARQ. Channel protection fundamentals CHA