## 3 RTS/CTS Scheme for IEEE 802.15.6
### Proposed RTS/CTS Scheme  
There are several methods to solve the above-mentioned problems in the existing literature, such as increasing transmission power levels of nodes to increase the wireless range of a network, using omni-directional antennas, moving the hidden nodes, and removing obstacles. As discussed above, most researchers have focused on adapting the RTS/CTS scheme to enhance the performance of the CSMA/CA protocol by resolving hidden and exposed node problems. The CSMA/CA defined in the  
Figure 1: Hidden node problem in a single-hop IEEE 802.15.6 Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) based networks. IEEE 802.15.6 is different from the conventional CSMA/CA protocol because it adjusts the backoff process according to eight different user priorities. Unlike previous CSMA/CA, where the contention window size is doubled for each failure, the IEEE 802.15.6 CSMA/CA doubles the contention window size for an even number of failed attempts, as shown in Figure 3.  
Figure 3: Backoff process in IEEE 802.15.6. Figure 2: Exposed node problem in multiple IEEE 802.15.6 CSMA/CA based networks. In the proposed scheme, when the data frame is available for transmission, the node senses the channel to check ongoing activity or transmission by other nodes. When the channel is free for the duration of Short Interframe Spacing (called pSIFS in the standard), the node chooses a random number called a backoff counter in an interval (1, \(CW\)), where \(CW\) is the contention window. As shown in Algorithm 1, the \(CW=CW_{i,min}\) in the first stage, where \(CW_{i,min}\) indicates the minimum contention window for nodes in priority class \(i\). The counter represented by \(C\) is decremented when the channel is free for the duration of a CSMA slot. When the counter reaches 0, the node transmits an RTS frame to the destination. When the CTS is received by the transmitter, the data frame is sent. Other nodes hold their transmission for the total transmission duration, or Network Allocation Vector (NAV). Figure 4 shows a general description of the RTS/CTS scheme between two nodes. If the frame collides in the channel due to simultaneous node transmission, the \(CW\) is doubled for an even number of failures. The value of \(CW\) may not exceed the maximum contention window represented by \(CW_{i,max}\). The value of \(CW\) is not altered when the failure occurs for odd number of times. The backoff counter is frozen when the channel is busy or when the current time is outside the access phase lengths or when the there is not enough time left in the superframe to accommodate frame transmission. When the node does not receive the CTS, it assumes that the RTS packet may have been lost, and therefore increases the transmission power level to the next level. This is shown in Algorithm 1, which is based on our previous work in [30]. The transmission power level represented by \(P_{L}\) is incremented when the CTS is not received. We consider discrete power levels predefined in the transceiver. The discrete power levels will not exceed the maximum power level represented by\(P_{Lmax}\). This technique extends the wireless range of nodes. It may help to solve the exposed node problem by including exposed nodes in the wireless range of the transmitter. It may be noted that increasing the transmission power of nodes affects overall power consumption, and therefore the power levels are increased for high priority nodes intending to send medical data. As shown in Algorithm 1, we consider priority class 5 where nodes send medical data to the hub. We assume that for patients with critical health status, reporting emergency events is most crucial, and this is achieved through an increase in the transmission power levels. This, however, does not largely affect the power consumption of nodes compared to the overall throughput obtained through such a power increase. The values \(N\), \(T_{sf}\), and \(T_{f}\) represent the backoff stage, superframe duration, and frame duration, respectively.