Why do we need to minimize the TX power to require for meeting QoS? Here is the part of the paper,


  Based on these architectures, a lot of recent developments have lately appeared intending at enhancing the rate-energy performance of multiuser Multiple
  Input Single Output (MISO) SWIPT systems.These works mainly target at the optimization of TX precoding and PS operation, and can be classiﬁed into the following two categories. The ﬁrst category is based on whether RXs are required to perform both Information Decoding (ID) and Energy Harvesting(EH) (co-located ID and EH) or just act as ID or EH RXs (separated ID and EH). The second category includes performance objectives like the minimization of TX power required for meeting Quality of Service (QoS) and EH constraints, and throughput or EH maximization for a given TX power budget and QoS constraints 


Why do we need to minimize the TX power to require for meeting QoS? Does't the more powerful signal means it can be broadcasted to the farer place? 

paper link : https://www.groundai.com/project/energy-sustainable-iot-with-individual-qos-constraints-through-miso-swipt-multicasting/
I interpret that sentence the following:


  Under energy harvesting contsraints and Quality of Service constraints, the second category of works strive to minimize TX power.


So, TX power isn't minimized in order to meet QoS constraints, but as far as possible while still meeting QoS constraints.