The most commonly used relay strategies in a cooperative communication or relay system are decode-and-forward (DF) and amplify-and-forward (AF). The performance of the two strategies has been intensively studied and compared. For a DF system, the relay nodes decode the transmitted packet first, and then forward the re-encoded
packet to the destination if decoding is successfully. There is another DF strategy called adaptive DF. Adaptive DF in which the source uses either source-relay channel state information (CSI) or feedback from the relay to decide between retransmitting the message or permitting the relay to forward the message does achieve second-order diversity in the high SNR region. In AF mode, the relay nodes may simply amplify their received packets and forward to the destinations. No demodulation or decoding of the received signal is performed in the case. No matter what strategies have been shown to improve the overall rate or diversity in wireless network.
In [15], it points out the location is the key to achieve a good cooperative diversity rather than the specific cooperative strategy. When inter-user channel tends to be a much better channel than either of the user channels, DF is a better option due to its higher received signal-to-noise ratio (SNR). On the other hand, when a relay node is located nearer to the destination than the source, AF may be a better relay strategy because the probability of decode error on the relay node is higher. A mixed DF-AF scheme that can switch to AF mode when relay nodes decode incorrectly is not practical though it can provide a better performance. [15] showed that there is certainly no practical benefit in considering a mixed-mode system since the gain is not worth the trouble.
Time and frequency domain are frequently used resources in wireless communication, and full-duplex and half-duplex are most popular communication protocol. In relay net-works, many previous works assume relay nodes can transmit and receive simultaneously in the same frequency band, i.e., full-duplex. However, since many limitations in prac-tical implementation preclude the terminals from full-duplex, half-duplex gathers more attention. While in half-duplex protocol, the relay node can not receive and transmit simultaneously by using the same communication resource. There are mainly two ac-cess schemes of half-duplex protocol. One is time division duplex (TDD) and the other is frequency division duplex (FDD). The relay node receives and transmits packets at
different time slots when using TDD. Similarly, the relay node receives and transmits packets on different frequency band when using FDD. Most cooperative protocols are consider in time-division multiple-access (TDMA).
In [8], the relay node assists in communication with the destination by either AF or DF the received signal. It proposes three different time-division multiple-access-based relay protocols that vary the degree of broadcasting and receive collision, and only half-duplex considered here. In Protocol I, the source communicates with the relay node and destination during the first time slot. In the second time slot, both the relay and source communicate with the destination terminal. This protocol realizes maximum degrees of broadcasting and receive collision. In Protocol II, the source communicates with the relay and destination over the first time slot. In the second time slot, only the relay node communicates with destination. This protocol realizes a maximum degree of broadcasting but realizes receive collision. The third Protocol is identical to Protocol I apart from the fact that the destination chooses not to receive the direct S −→ D signal during the first time slot. This protocol does not implement broadcasting but realizes receive collision. Note that while the signal conveyed to the relay and destination over the two time slots is the same under Protocol II, Protocol I and III can potentially convey different signals to the relay and destination .
Additional comments on the three protocols described above are in order. This conditions and setup for Protocol I are self-evident. Protocol II is logical in a scenario where the source engages in data reception from another nodes in the network over the second time slot thereby rendering it unable to transmit. Similarly, for Protocol III the destination may be engaged in data transmission to another nodes during the first time slot. Hence, the transmitted signal is received only at the relay nodes and buffered for subsequent forwarding. The different protocols convert the spatially distributed antenna system into effective single-input-multiple-output (SIMO) (with Ptorocol II), multiple- input-single-output (MISO) (with Protocol III), and
input-output (MIMO) (with Protocol I) channels allowing the fundamental gains of multiple-antenna systems such as diversity gain, array gain and interference cancelling gain to be exploited in a distributed fashion. There is still a simple protocol that [8] does not propose. The source communicates with the relay node during the first time slot, and the relay forwards to the destination in the second time slot. This protocol is also considered in many previous works due to its simplicity on the destination.