This chapter considers a two-way relay network, where two terminals A and B desire to exchange information with the help of a relay R. The conventional approach based on time-division multiplexing requires a total of four time slots for messages exchange, with each terminal having two time slots individually, as illustrated in Figure 17.1(a). Network coding techniques can reduce the number of time slots required.
Network-layer network coding (NLNC): As illustrated in Figure 17.1(b), A and B take turns to send their messages to R in the first two time slots, and R transmits the XOR-ed version of the two messages in the third time slot. Each terminal can recover the message from the other terminal based on the message from the relay and its own message. The scheme applying network coding at the network layer reduces the number of required time slots from four to three by exploiting the broadcasting nature of the message from relay.
Physical-layer network coding (PLNC): As illustrated in Figure 17.1(c), in the first time slot which is known as the multiple-access (MAC) phase, both A and B transmit their own messages to R simultaneously. In the second time slot which is known as the broadcast (BC) phase, R broadcasts the XOR-ed version of the two messages. This scheme further exploits the superposition nature of wireless signals, and reduces the number of required time slots to two [131, 218, 462, 463].
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