[IEEE 2020 IEEE International Conf on Natural and Engineering Sciences for Sahel's Sustainable Development - Impact of Big Data Application on Society and Environment (IBASE-BF) - Ouagadougou, Burkina Faso (2020.2.4-2020.2.6)] 2020 IEEE International Conf on Natural and Engineering Sciences for Sahel's Sustainable Development - Impact of Big Data Application on Society and Environment (IBASE-BF) - Efficiency in RF energy harvesting systems: A comprehensive review