## 4 Proposed Methods
### Proposed Superframe Structure  
In the previous section, how to identify the hidden devices due to collision is discussed. It is to be noted that the hidden devices are identified only when the collided frames overlap partially and the PAN coordinator recovers the sender's address correctly from those partially overlapped frames. However, it may happen that the collided frames overlap fully when two nodes can have the same backoff window, and therefore, the PAN coordinator cannot recover the sender address from those fully-overlapped frames. To overcome this problem due to simultaneous transmission, we propose here a newly-modified superframe structure. As discussed in Section 2, the superframe structure contains the beacon, CAP, CFP and the inactive part. In order to reduce the delay for retransmission and to increase the reliability, we interchange the CFP and CAP parts and modify the inactive part of the superframe as shown in Figure 6. It is proposed to use this inactive part of the superframe for failure devices to improve the reliability. We partition the inactive part into a number of retransmission opportunities, and each of these retransmission intervals is again subdivided into the notice, reserved and unreserved period. Hence, our proposed superframe structure contains the beacon, CFP, CAP, notice, reserved and unreserved periods, which are defined as follows. * Beacon period: In this period, the PAN coordinator first broadcasts the beacon in the network, and all associated devices synchronize with the PAN coordinator upon receiving the beacon successfully. * CFP period: In this period, devices allocated with GTS slots by the PAN coordinator can transmit their data without any contention. * CAP period: In this period, devices associated with the PAN coordinator compete with other devices, and the winner device can transmit its data. However, collisions may occur during this interval. * Notice period: In this period, the PAN coordinator sends the acknowledgment to the devices whose data packets are successfully received. Furthermore, it informs those devices whose  
Figure 4: Address recovery during collisions due to hidden nodes. (**a**) Address of device Ais recovered correctly; (**b**) the address of both devices \(A\) and \(B\) is recovered correctly. Figure 5: The proposed data frame structure. addresses are correctly identified during the collisions due to hidden nodes and assigns the GTS slots in the next coming reserved period. * Reserved period: During this period, the coordinator provides a retransmission opportunity to the GTS failure devices. Besides, the devices those are correctly identified during the CAP due to the hidden terminal problem are assigned new GTS slots to transmit their data without contention. * Unreserved period: In this period, the contended devices who have not received the acknowledgment or have not been identified by the PAN coordinator can compete among themselves to transmit their data. It is to be noted that both ZigBee and WiFi devices operate in the 2.4-GHz ISM band. The ZigBee devices use narrow band channels as compared to the WiFi's broadband spread spectrum channels. There are 3 WiFi and 16 ZigBee channels each of 22 MHz and 2 MHz, respectively. The guard band between any two ZigBee channel is 3 MHz. Hence, as shown in Figure 7, each WiFi device's channel overlaps with four consecutive ZigBee channels. The devices operating in those overlapping channels experience interference from the WiFi devices. The Signal-to-Noise-plus-Interference Ratio (SNIR) is a quantity used to give theoretical upper bounds on channel capacity in wireless communication. The SINR can be determined as in [27]:  
\[SINR=10 log_{10}(\frac{P_{r}}{P_{n}+P_{i}}) \tag{1}\]  
Figure 6: Proposed superframe structure. Figure 7: Channel overlapping between WiFi and ZigBee devices. where \(P_{r}\), \(P_{n}\), \(P_{i}\) are the actual power received from the desired signal, noise power and the interferer power.