## 2 Related Work
A two-hop communication system with energy-harvesting nodes was presented in [29] where both source and relay were able to harvest energy from environment during communication. The works in [13; 14] also aimed at harvesting energy from ambient resources apart from conventional objectives such as energy efficiency, reliability, throughput maximization etc. A comprehensive taxonomy of the various energy-harvesting sources in WSN was presented by the authors in [30]. In [12; 31], authors included energy-harvesting process as an input criterion to develop optimal transmission policy as well. In [12], authors devised a transmission strategy by taking into account energy level of nodes, event generation, battery recharge and packet error probability. Here two transmission modes are assumed to be available for the sensors allowing tradeoff between energy consumption and packet error probability. Decision policies are formulated here to obtain the transmission mode to be used at a given instant of time to maximize the quality of coverage. The problem was structured exploiting MDP framework and an upper bound in the performance of arbitrary policies was figured out. However, in [31], authors considered energy replenishment process and battery capacity to find optimal transmission policies for rechargeable nodes. Here optimal solutions have been identified for two related problems; first the transmission policy that maximizes the short-term throughput in terms of the amount of data transmitted in a finite time horizon is obtained and next minimization of the transmission completion time for a given amount of data is addressed accordingly. In this work, a model with discrete packets of energy arrival has been considered for battery replenishment. However, the recharging or replacement of batteries of body-sensor nodes is not always feasible in the case of WBAN particularly in case of implanted nodes. A cross layer design was proposed in [7] to address the pivotal issues of WBAN communication such as transmission reliability, energy efficiency, lifetime grounded on transmission power control, relay decision and packet selection within WBAN. The work adopted cross layer design involving physical, MAC as well as network layer. The work is found to focus on choosing optimal transmission power by maximizing energy efficiency over a single link and after that optimal relay is decided through the tradeoff between maximization of energy and minimization of energy consumption speed. Next, remaining energy of leaf nodes of WBAN topology is exploited to enhance transmission reliability without any loss of lifetime. At the end, the optimized packet size has been selected for optimizing energy efficiency. Thus, the existing works opt for developing transmission strategies for energy-constrained networks (i.e., WSN and WBAN) particularly focusing on finding optimal transmission power subject to different input conditions such as current energy level, event generation, data importance etc. Thus, prime issues related to communication (such as energy efficiency, reliability, throughput etc.) along with energy harvesting (to cope up with scarcity of energy) can be addressed. Initially, research was carried out focusing on one or two input conditions, but more dimensions were gradually added for better analysis. In addition, previously most of the approaches were designed to be used for single-hop data transmission but later, the trend moved towards multi-hop scenario. Still there is room for further exploration of modeling multi-hop strategy. Besides, the transmission power has severe impact on heat generation in WBAN which is hardly investigated in state-of-the-art literature. The transmission policies framed for WSN cannot be applied directly in WBAN due to its inherent challenges as well. Furthermore, use of mathematical model with intricate numerical formulations to predict optimal strategy could intensify the complexity of resource-constrained network.