## 1 Introduction  
Wireless body area networks (WBAN) are one of the emerging technologies that has the prospect to significantly enhance healthcare monitoring and related medical procedures. WBANs are also being utilized in fields like fitness, entertainment, _etc._ Every WBAN consists of interconnected sensor nodes, which can be deployed on, near or within the human body [1]. These sensor nodes are capable of sensing human vital signs, processing the sensed data and delivering the data to the concerned medical server for treatment. It is of great interest to utilize wireless communication for remote monitoring of a human body. Such monitoring must be unobtrusive, reliable and cost effective, so that the patient can truly benefit from wireless technologies. To achieve these targets, sensors in WBANs must consist of smaller nodes with smaller batteries relative to conventional wireless sensor networks (WSN). Smaller batteries create a restriction on the energy consumed by sensors in sensing, processing, storing and delivering the data, which ultimately affect the overall energy efficiency, throughput and latency of the WBAN. WBANs are required to function accurately and independently for a long period of time without recharging the batteries. Therefore, the design of an energy-efficient and reliable communication protocol, for improving the network lifetime, is one of the major challenges in WBANs. Other factors that may greatly affect data transmission are variable path loss due to shadow or slow fading caused by physical movement of human body parts [2; 3]. Therefore, a well-designed network topology in terms of reliable data transmission is also required for WBANs, which may significantly prevent the data loss and wastage of energy. Cooperative communication is considered to be one of the best solutions to overcome the effects of fading in the wireless channel. In a conventional cooperative communication, a source transmits sensed information to a destination, not only utilizing a direct link, but also utilizing relays/cooperative nodes for transmitting the same data [4]. However, a conventional cooperative communication network makes inefficient use of the channel resources, as relays always forward the source signal to the destination irrespective of the channel conditions [5]. Although the WBAN is usually required to work as a single-hop star network, research has shown improvement in WBANs' performance by utilizing cooperative relaying schemes. Network throughput may be enhanced by utilizing the broadcast nature of wireless transmission by propagating an independent signal through different paths. The main idea behind this approach is that if a signal experiences noise on a certain path at a particular instant, then other independent paths may carry the same signal with less noise or fading. By introducing the concept of cooperative diversity, both the signal-to-noise Ratio (SNR) and packet error rate (PER) of a signal can be improved at the receiver end. Cooperative links, given a target bit error rate (BER) level, are utilized to improve network lifetime and throughput. There may be two types of cooperative communication: (i) single relay based and (ii) multiple relay based. In multiple relay-based systems, relay selection may be opportunistic or deterministic according to the needs of the network. In opportunistic selection, a node that forwards a packet is determined on-the-fly and depends on the packet receiving node. Whereas, in the deterministic approach, the node that is supposed to forward the data packet, which is predetermined. Moreover, for a specific relay selection, the relaying strategy can be fixed, selective or incremental. In fixed relaying, relays always forward the received data after processing; whereas selective relaying makes use of instantaneous channel information to decide between relay forwarding and source re-transmission. In incremental relaying, a short feedback, indicating the success or failure of sent data, from the destination is used.