# Reliability Improved Cooperative Communication over Wireless Sensor Networks
## 2 Related Work  
Many research works focus on improving the reliability of WSNs. In recent years, several methods and schemes with great data delivery rates have been proposed. 1. Retransmission-based mechanism. Automatic Repeat-Request can improve reliability of data transmission compared with traditions scheme [32]. The main strategy of an ARQ protocol is as follows: once node transmits a data packet, it waits for an acknowledgement (ACK, a message sent by the receiver) indicating that its data packet was correctly received. The sender does not send any additional packets until it receives an ACK signal or a timeout. In contrast, with GBN and SR, the nodes transmit packets continuously without the need to wait for individual ACKs from the receiver; only the ACK for the last packet is required [32]. In WSNs, an ACK could possibly be lost, which results in the transmitter resending a packet that was already successfully transmitted but an ACK was not received correctly or in time. Thus, in [33], we proposed a scheme to reduce the retransmission of packets by increasing the transmissions times of ACK when the receiver manages to receive a packet. Additional ACK transmissions will not cause significant increases in energy consumption but can further improve the lifetime of the WSN [33]. The advantage of the retransmission-based mechanism is that it improves the network reliability at a low energy cost, so it is quite suitable for WSNs whose energy is extremely limited [4; 32]. However, the unfavorable is that the network delay is usually long due to the repeat transmissions of data packets, especially in a link with high packet error rate [32] because the retransmission-based mechanism sends several data packets to the sink until the nodes receive ACK signals in response. If the ACK is dropped, nodes cannot receive the ACK and will send data packets to the sink, thereby increasing the transmission delay. 2. Forward error correction (FEC) [34; 35]. In the FEC scheme, when source node sends one data packet to the sink, the data packet is encoded through errors-correcting code. If the error ratio of data packet is in a certain value, the sink will correct the error of data packet using errors-correcting code. But the transmission bandwidth is higher, and the cost is higher [34; 35]. The reliability of data packet in the FEC is improved, however, in this scheme, the cost for encoding the message in a redundant way is larger; thus, the network lifetime is shorter. 3. Cooperative Communication (CC). [PERSON] et al. [36], after an in-depth analysis on cooperative ARQ (automatic re-request), found that the cooperative ARQ protocols ensure the reliability of data packet better [33; 36]. [PERSON] et al. [33], for the first time, determined the most appropriate value of the bit error ratio (BER) along with the number of cooperative nodes that reduce energy consumption for transmitting one bit of data through theoretical analysis. Then, they adjusted the transmission BER according to the energy left of nodes. As a result, not only could the energy of nodes in the clusters far from the sink node be fully used, but the energy cost of nodes in the nearer cluster could also be reduced, so the network lifetime was prolonged. Figure 1: Network model. Network coding techniques. In the network coding techniques, the source node encodes the packets at the application layer with some level of redundancy. Because the data transmitted by the source node have some redundancy, this approach will not affect the reception of the correct packets by the destination node. Network coding techniques have many advantages, such as small data transmission delay, while their main disadvantage is the relatively large energy consumption due to the complex algorithm. Other studies can be found in [37]. 5. Network-coding-based cooperative communications (NCCC). Reference [31] proposed a communication scheme whose main idea is the combination of coding techniques and cooperative communications.