## 1 Introduction
Preambles cost quite a lot of the energy of the energy-constrained sensors. [PERSON] _et al._ proposed a star topology TDMA-based MAC protocol for remote monitoring of physiological signals [21]. The protocol takes advantage of the static nature of the BSNs to implement an effective TDMA scheme with little overhead. It uses a broadcast network control packet to assign time slots to sensors. The sensors only wake up in their own slots to finish data transmission. This protocol achieves a low duty cycle therefore this reduces the energy consumption. [PERSON] _et al._ proposed BodyMAC which used flexible bandwidth allocation to improve the energy efficiency [22]. Contention free slots are used in the uplink scheme. Different types of bandwidth allocation mechanisms are proposed. [PERSON] _et al._ proposed a traffic-adaptive MAC protocol, named TaMAC [23]. It dynamically adjusts the duty cycle of the sensor nodes according to their traffic patterns. If a sensor node has no data to send/receive, it will not receive frequent synchronization and control packets. [PERSON] _et al._ integrate a fuzzy-logic system in each body sensor to deal with multiple cross-layer input variables of diverse nature in an independent manner [24]. By being autonomously aware of their current condition, body sensors are able to demand a \"collision-free\" time slot, whenever they consider it strictly required. Similarly, they may refuse to transmit, if there is a bad channel link, thus permitting another body sensor to do so. The above four proposed protocols do not consider the mobile scenarios. [PERSON] _et al._ proposed a \"pulling\" MAC protocol [25]. The coordinator transmits command to ask the data from the sensors. The sensors then send the corresponding data to the base station (BS) in a passive way. There is no need for synchronization. The schedules are controlled by the coordinator. However, the protocol requires sensor nodes to listen all the time without sleep. This will consume quite a lot of energy. In [26], all the activities are also initiated by the master node, but after the link establishment between master node and sensor node, the master node schedules the sleep duration for sensor nodes. Once the sensor wakes up, it listens for the command from the master node. The disadvantage is that there is still a lot of idle listening due to the passive data transfer. An on-demand wakeup radio is proposed to use for BSNs [27]. It allows a device to sleep and be woken up by suitable transmission from another device. This requires a special wakeup circuit in the sensor nodes which increases the complexity. [PERSON] _et al_. proposed an energy efficient multi-hop communication in body area network [28]. A minimum spanning tree (MST) routing is adopted for the multi-hop network. The coordinator constructs the MST using the battery status of sensor nodes as well as their distances. This proposal does not mention adaptive transmit power which could make the communication more energy efficient and achieve a longer network lifetime. Also due to this, the protocol could not deal well with mobile scenarios. [PERSON] _et al_. investigated the benefits and limitations of adaptive transmit power control [29]. The adjustment of parameters can achieve different trade-offs between energy saving and reliability, making them suitable across diverse applications under different operation conditions. [PERSON] _et al_. proposed a heartbeat driven MAC protocol for BSNs [30]. The heartbeat rhythm information, which is inherent in the human body, is used instead of periodic beacons. Biosensors can extract the heartbeat rhythm from sensing data by detecting waveform peaks. There are two big challenges in terms of energy saving for the protocol implementation. First, the sensors must have extra heartbeat sensors to receive the heartbeat signals. Whether this is really energy and resource efficient is a question. Second, making the rhythm signal acquisition system small and accurate is not easy.