# Cross-Layer Methods for Ad Hoc Networks--Review and Classification
## 3 Cross-Layer Method Publications Review
### One Layer
#### 3.1.2 Channel Layer
_Channel layer (cooperative transmission)_  
In cooperative transmission-oriented works (Table 5), the channel layer interacts with the network and physical layer. Cooperative transmission is based on the fact that when the sending node sends a data packet, the packet is received not only by the receiving node but also by several other nodes (other nodes are usually called auxiliary nodes). The receiving node sends an acknowledgment packet. If the acknowledgment packet has not been sent, one of the nodes that received the data packet and whose communication channel is less attenuated sends the packet to the recipient. The difficulty of cooperative transmission is the correct formation of cooperative transmission groups and the choice of when cooperative transmission is more efficient and when cooperative transmission causes interference and is less reliable than ordinary transmission. Interaction with the physical layer in the cooperative transmission is used to obtain information about channel fading and to set signal-code structures for packet transmission from the sending node and auxiliary node. The network layer provides information about the direction of data flows. With this information, groups of nodes can be effectively selected for cooperative transmission. \begin{table}
\begin{tabular}{c c c c c} \hline
**Publication** & **Year** & **Citations** & **Layers** & **Goal** \\ \hline
[53] & 2003 & 157 & 1, 2, 3 & line throughput, energy consumption \\
[54] & 2005 & 121 & 1, 2 & line throughput, energy consumption \\ \hline \end{tabular}
\end{table} 
Table 4: Cross-layer methods concentrated on channel layer random access. [PERSON] et al. [55] propose the interaction of channel and physical layer in cooperative transmission. In this work, nodes are expected to possess orthogonal communication channels (frequency, time slot, etc). When a packet is delivered, it is received by the recipient node and nodes that can participate in the cooperative transmission. The authors suggested the use of automatic packet forwarding by auxiliary nodes if they did not receive a confirmation packet after some time. Only nodes with the same communication channels as the recipient node are used for cooperative transmission forwarding. [PERSON] et al. [56] propose a protocol where, at the beginning, the sender node codes the packet encoded with error-correcting code so that half of the packet bits can be repaired. The sender node then sends half of the packet to the helper node and the other half to the recipient node. The helper node restores the second half of the packet, encodes again and sends it to the recipient node. By splitting the packets into two halves, the packet halves can be transmitted at different speeds depending on the channel state. For cooperative transmission, the channel layer uses physical layer information about the channel state and available signal-code structures for packet transmission. [PERSON] et al. [57] propose cooperative beamforming (virtual antenna with the help of multiple nodes). For cooperative beamforming, the physical and channel layers of the nodes interact. Nodes in the neighborhood use time division multiple access (TDMA) to broadcast data packets. Then, nodes that participate in the cooperative beamforming transmit the packets to recipients. To form a virtual antenna, node coordinates and time synchronization are required. [PERSON] et al. [58] make a channel layer to use information about signal strength from the physical layer to choose whether to use cooperative transmission or not. Cooperative transmission groups are formed based on route date from the network layer. [PERSON] et al. [59] use network layer data about routes to form cooperative transmission groups. At the same time, at the physical layer, the control of transmission power is optimized based on data from the channel layer about cooperative transmission groups. [PERSON] et al. [60] propose a cooperative transmission using network packet coding.