## 4 Proposed Methods
### Proposed Data Transmission Frame Structure  
In this section, the structure of a new data frame is proposed, which can detect the hidden devices attached to the coordinator in a star topology. The main concept of the hidden terminal problem is that the carrier sensing range of a node is not large enough to detect the ongoing transmission. As per our assumption, the carrier sensing range of one device is equal to its transmission range, and a device cannot detect the ongoing transmissions by other devices due to the presence of obstacles. However, in a star topology, all devices are present within the communication range of the PAN coordinator. Hence, collision can occur only at the PAN coordinator for which the PAN coordinator does not transmit the acknowledgment to the devices.  
In WSN, when a packet is partially received, the information in the header can often be recovered and is used to identify the transmitter of that corrupted packet through the capture effects [26]. Even if the strength of two signals is the same, the radios of the sensor devices can detect and decode the undamaged signals through this capture effect. This can be only possible if the header of the colliding packet is not overlapped. Suppose two devices \(A\) and \(B\) are hidden from each other and device \(B\) transmits its data when the transmission of device \(A\) is going on. Hence, collision occurs at the PAN coordinator as their transmissions overlap fully or partially as shown in Figure 3. According to the IEEE 802.15.4e standard, the data transmission frame contains the MAC header (MHR), MAC payloadand MAC footer (FHR). As shown in Figure 3a, partial overlapping occurs when device \(B\) transmits data after the MHR part of device \(A\). The data frames overlap fully, when both devices \(A\) and \(B\) transmit simultaneously as shown in Figure 3b. The PAN coordinator can recover the sender's address correctly from the partial frame; those are not overlapping with the subsequent frames after collision occurs due to such hidden devices.  
For example, as shown in Figure 4a, the address of device \(A\) is recovered correctly [26] after the collision with hidden device \(B\). Then, the PAN coordinator allocates one GTS slot to the device \(A\) in order to avoid subsequent hidden device collisions. However, in order to identify the other device associated with the collision (here, device \(B\)), we propose to add a replica of the sender device address in the rear part of the data frame as shown in Figure 5. This enables the PAN coordinator to correctly recover the address of the second device associated with the collision (here, device \(B\)), as shown in Figure 4b. In the sparse network, the probability that the second device has a collision with another third device is less. However, if multiple collisions occur due to hidden nodes, then the first and last device associated with the collision are correctly identified, and both devices are allotted GTS slots for data transmission. This proposed method reduces the subsequent collisions in the network by restricting the collided devices to go for further contention.  
Figure 4: _Cont._  
Figure 3: Overlapping of the data transmission frame due to the hidden terminal problem. (**a**) Partial overlapping of the data transmission frame; (**b**) full overlapping of the data transmission frame.