# Mutual-Information-Based Incremental Relay Communications for Wireless Biomedical Implant Systems
## 1 Introduction  
Wireless body area networks (WBANs) are becoming increasingly popular recently due to their suitability and flexibility in medical and nonmedical applications. WBANs deploy low-power biosensors and devices inside, on, or around the human body [1; 2]. The biosensors are able to sense and transmit real-time physiological and contextual information profiling human body activities such as rehabilitation monitoring, blood pressure, and digital medical treatment. Even more fascinating and promising is the near-future healthcare revolution where biomedical devices implanted inside the human body are wirelessly connected and organized for early disease detection or organ transplantation monitoring over an extended period [3]. Data transmission can be conducted between implanted devices and an external coordinator, which is furnished with high computational capacity, sufficient memory space, and an appropriate power source. Often in modern healthcare, the coordination function may be performed by a contemporary smartphone. Intra-body implanted sensor networking is expected to bring revolutionary health applications and services. However, there exist several technical constraints in wireless in-body sensor networks. Analysis of intra-body communication channel characteristics is a crucial research challenge due to the complex in-bodytransmission environment. Radio frequency signals suffer significant energy attenuation when transmitting from the intra-body area to the on-body region [3,4]. Moreover, owing to the strict technical constraints of implanted device battery design, the limited energy supply is also a major bottleneck for a wireless in-body sensor network [1,5]. Since the network energy consumption is directly related to the overall communication distance, relay-based routing protocol techniques are considered to reduce the transmission distance [6]. Relay-based energy efficient protocol routing enables the selection of the best possible data transmission route, which significantly decreases the transmission length. Moreover, cooperative communications have been applied to the protocol design to address research difficulties related to prolonging the network lifetime and improving the energy efficiency [7]. We consider using implantable devices employed with biosensors that transmit their sensed physiological data to a coordinator through wearable devices which perform as on-body relays. By forwarding the sensed data to the relays, the complexity and power consumption are transferred from the implanted sensor to the relay node, which may easily be recharged. According to [8,9], for continuous healthcare monitoring services, not all sensed data needs to be transmitted to the external coordinator. For example, when the current scanned information is similar to the previously sent message where both of them are within the normal range, transmission would be highly redundant. Therefore, the motivation to improve the lifetime of all implanted sensors is to reduce the amount of redundant normal data transmission. In addition, data transmission from different implanted sensors is affected by noise and channel fading, which result in data packet loss and increased path loss. The coordinator can gather the received information packets from relay nodes and implanted sensors using numerous data combining techniques, which are energy efficient schemes to enhance the network reliability in the presence of channel fading. Moreover, this paper is also motived by the tradeoff between the transmitting power and various quality of service (QoS) metrics for implanted sensors, considering the power constraints and the essential network reliability regarding the outage probability (OP). In this paper, we present an MI-based incremental cooperative routing protocol for wireless in-body sensor networks. The primary target of the proposed protocol is to maximize the network lifetime. Firstly, MI criteria are considered to prevent redundant information transmission.