## 2 Related Work  
Given that WBAN was spotlighted, much research has been conducted to address this limited environment. IEEE 802.15.6 [2] standard technology, which was published for communications of WBAN environment in 2012, illustrated an energy-efficient scheme on both Physical (PHY) and MAC layers. This standard technology allowed only the star-topology; therefore, all nodes deployed on the human body should be supervised by a coordinator named as hub, and the scheme on the MAC layer of the standard employed combined approach between slot-assigned and random-accessed methods. The superframe is classified into several time periods and a beacon frame determines the length of each frame duration. In this protocol, all nodes can transmit their reserved data through the slot-assigned period without any contention, denoted as Type-I/II Access Phase. Furthermore, this technology employed Exclusive Access Phase (EAP) and Random Access Phase (RAP) for non-reserved data transmissions. Nodes can use these periods to transmit data when they have emergency data or remaining data in only EAP and RAP with the approach of Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) or Slotted-Aloha. The main challenge of IEEE 802.15.6 compared to other standard technologies (e.g., 802.15.4 [3]) is the consideration for emergency data transmissions. However, non-reserved emergency data transmission is allowed in only EAP and RAP, so this can be limited point of this protocol. Besides IEEE 802.15.6, there are many types of research for enhancing and developing the communication method, and H-MAC [4] was a representative novelty mechanism that employs Time-Division Multiple Access (TDMA) approaches for body sensor networks. The general TDMA-based MAC protocols should require base time tick for synchronization such as beacon,However, H-MAC did simply address this problem by using the heartbeat vibration measured from the physiological signal with some sensors as a synchronization tick for TDMA. Hence, all nodes could transmit data without any contention under the assumption of that all nodes already had the sequence for slot assigned ownership. Nevertheless, they did not receive any beacon frame from the coordinator. Some research [5; 6] had been proposed to facilitate WBAN use-cases with developing previous IEEE 802.15.4 standard which was widely used for wireless sensor networks. Dynamic Time-Division Multiple Access (DTDMA) [7] designed a reservation-based dynamic TDMA MAC protocol for evolving energy efficiency. The overall superframe of this protocol was composed of the beacon, Contention Free Period (CFP), Contention Access Period (CAP), and an inactive period. But the frame sequence of DTDMA structure was contrary to IEEE 802.15.4. This protocol prioritized CFP to minimize the latency for emergency data transmissions, and then assigned CAP for periodic or remaining data transmissions. The rest of the technical parts including the beacon interval and the length of the inactive period complied with IEEE 802.15.4 protocol. Another representative scheme of the 802.15.4-based protocol is BodyMAC [8]. In this protocol, device roles were classified into coordinator and implant, and these would be matched to the hub and node in IEEE 802.15.6. This protocol had cut off the inactive period from IEEE 802.15.4 and newly assigned downlink subframe instead of CFP in the superframe. All implants are guaranteed to be assigned to respective downlink slots to conserve energy from idle. Furthermore, this protocol allowed that some implants do not receive a beacon from the coordinator. The coordinator will perform wake-up signal transmission process to activate a corresponding implant if it had data to be sent. After the sleeping implant was activated with this process, it synchronizes to the wake-up signal and receives data from the coordinator after finding the next beacon frame. More recent research for WBAN has been also conducted with the same goal. [PERSON] et al.