# Time and Energy Efficient Relay Transmission for Multi-Hop Wireless Sensor Networks
## 2 Background  
### Overview of IEEE 802.15.4 Standard  
The IEEE 802.15.4 protocol is the international standard for a wireless sensor network and defines the PHY/MAC layer for the wireless sensor network and the interface with the network layer for the wireless sensor network applications. The IEEE 802.15.4 standard supports the offset quadrature phase-shift keying (O-QPSK) PHY, which employs a 16-ary quasi-orthogonal modulation technique. The data rate of the O-QPSK PHY shall be 250 kbps when operating in the 2.4 GHz band. When the PAN coordinator builds a WPAN, it periodically broadcasts its beacon frame after it selects a channel for its operation. Then, when a sensor device receives the beacon frame, it transmits an association frame to the PAN coordinator to join the WPAN. Figure 1 shows an example of a sensor network with a cluster tree topology.  
In Figure 1, all devices in the wireless sensor network can connect to the Internet through a gateway. Also, PANs 1 and 2 can extend to the cluster tree topology. Namely, there can exist multiple PANs in the same wireless channel environment. The PAN coordinator connected to the gateway manages its WPAN and can be connected to multiple devices and coordinators. Also, all coordinators can be connected to multiple devices and other coordinators.  
Figure 2 shows the structure of a superframe defined in the IEEE 802.15.4 standard.  
In Figure 2, the superframe of the IEEE 802.15.4 standard is divided into three parts, a beacon period, a superframe duration (SD), and an inactive period. In the beacon period, the PAN coordinator broadcasts its beacon frame, and all devices in the WPAN communicate with each other in the SD. Also, in the inactive period, all devices in WPAN enter into a sleep mode or a standby mode to reduce an energy consumption. The beacon period is located at the beginning of the superframe, and a beacon interval (BI) is equal to the length of one superframe.  
The SD of the superframe of IEEE 802.15.4 standard is divided into a contention access period (CAP) and a contention free period (CFP). In the CAP, all devices in the WPAN can communicate with each other anytime. However, if multiple devices transmit frames at the same time, collisions occur in the corresponding time slot. Contrary to the CAP, in the CFP, only allowed devices can transmit data frames in the time slot which is allocated by the PAN coordinator. Thus, collisions among devices do not occur in the CFP.  
As mentioned above, the PAN coordinator can communicate with multiple coordinators on the same channel. Each coordinator can broadcast its beacon frame independently and configure its superframe. To put this more concretely, the SD of each coordinator has to locate in the inactive period of the PAN coordinator. Otherwise, there can occur collisions between member nodes of the PAN coordinator and member nodes of the coordinator.  
Figure 1: An example of a sensor network with a cluster tree topology.  
Figure 2: The structure of the superframe of the IEEE 802.15.4 standard.  
In Figure 1, coordinators can work as relay nodes between the PAN coordinator and sensor devices in the inactive period of the PAN coordinator. However, when the number of coordinators increases, the available resources in the superframe decrease. Furthermore, when the coordinator works as the relay device, the available SD of the PAN coordinator is reduced by half. This feature of IEEE 802.15.4 standard may limit the number of member devices in the WPAN and obstruct the extension of the network coverage. Therefore, in this paper, we propose new relay transmission scheme to expand wireless network coverage and to optimize the wireless resources of the wireless sensor network. To address these issues, we propose a new superframe structure with variable CAP and CFP.