# Towards Efficient Wireless Body Area Network Using Two-Way Relay Cooperation
## 1 Introduction  
Wireless body area network (WBAN) is comprised of ultra-low-power, self-sustained, intelligent and light weight body-borne sensors, designed to work in/on and around the human body [1]. The technology designed particularly for medical purposes now outstretch to various fields like emergency management, military, sports, entertainment, consumer electronics, and is still carving its way for more [2]. The advancement in the domain has led to the development of intelligent contact lenses, retina prosthesis [3], miniature eyelid stimulator [4], non-invasiveglucometer (NGM), implantable insulin pumps, miniature leadless pacemakers [5], implantable cardiac defibrillators (ICDs), wireless capsule endoscopy (WCE), cancer detection and smart wound dressing patches [6]. Apart from medical uses, the progress in the field has resulted in smart fabrics, efficient smart homes and more. Gartner states that the worldwide wearable devices have generated a revenue of $28.7 billion in 2016, and is expected to grow from 275 million units in 2016 to 477 million units in 2020, generating a revenue of $61.7 billion [7]. The Global Mobile Data Traffic Forecast by Cisco predicts that, by 2020, the number of wearable devices will increase to 601 million globally [8]. According to a survey conducted by Ericsson Consumer Lab (Stockholm, Sweden), 60% of the participants believe that, in next five years, biomedical sensors like smart patches, indigestible pills, and other implantable chips would be commonly used [9]. WBAN consists of small-sized sensors with limited energy resources placed on the body surface or inside the body. The operation of WBAN can be compromised by dynamical channel conditions, arising from changes in temperature, pressure, muscular movement, clothing and varying body postures. Body posture changes result in channel variation and at times may cause deep fades that last longer than usual if a certain body posture sustains [10]. These channel variations can lead to retransmissions due to errors that cause an increase in power consumption and a decrease in the sensor's life time [11]. The frequent replacement of the sensor's battery is often undesirable and fosters discomfort to the person. Therefore, limited energy resources must be used effectively to combat varying channel conditions. To overcome this, channel coding has been proposed to improve the robustness and reliability of communication links. For example, temporal diversity coding (TDC) is used in space and time to enhance the link reliability, and the results have shown a 50% improvement in the throughput [12]. Iterative low-density parity-check (LDPC) decoding is employed to lower the total energy consumption by 20-25% [13]. Raptor codes are used over a frequency-shift keying (FSK) modulated link to make the WBAN more energy efficient with reduced transmitter complexity and lesser implementation cost. The raptor code showed greater efficiency over uncoded FSK and LDPC coded link [14]. The comparison result of raptor code and bose-chaudhuri-hocquenghem (BCH) code in a fading environment showed that the BCH coded data achieves higher energy efficiency for a link length of 10 m or less [15]. Although channel coding is an efficient technique, under deep fades, even the retransmissions are useless and an alternative path is required for the data to be successfully transmitted. In such scenarios, cooperative communication is a viable solution to achieve transmit diversity and improve reliability. The energy efficiency and outage probability for a single relay cooperative communication for an ultra wide band (UWB) based WBAN is discussed in [16]. The results show that in-body sensors can employ cooperation mechanism with on-body sensors to increase energy efficiency and decrease outages. An incremental relay scheme is discussed in [17] with two-stage relay cooperation. The results show an improvement in packet error rate (PER) and energy efficiency (EE) when measured against source-destination hop length.