Uplink non-orthogonal multiple access in heterogeneous networks: A review of recent advances and open research challenges. - Rehman, Bilal Ur; Babar, Mohammad Inayatullah; Ahmad, Arbab Waheed; Amir, Mohammad; Habib, Wasim; Farooq, Muhammad; Abdel Azim, Gamil (International Journal of Distributed Sensor Networks, 2022)