Joint users selection and beamforming in downlink millimetre-wave NOMA based on users positioning

In this study, joint users selection and beamforming based on users positioning are proposed for enabling nonorthogonal multiple access (NOMA) in the millimetre-wave (mmWave) transmissions. In the proposed scheme, the mmWave users are arranged in descending order based on their estimated distances from the mmWave base station (BS), which guarantees the users ordering based on their line-of-sight availability, as well. Out of the ordered users, the two users having the minimum pairing angle and expected to lie within the specified mmWave beamwidth are selected as the two NOMA users. In this study, the users' pairing angle is defined as the maximum expected difference angle between two users considering their location-uncertainty areas. Consequently, the BS antenna beam is formed towards the two selected NOMA users by adjusting its boresight angle to lie in between their estimated location angles. Then, the fixed power allocation scheme is used to adjust the power allocation coefficients of the selected NOMA users. Mathematical analysis is performed to bound the performance of the proposed mmWave NOMA scheme. Also, numerical simulations are conducted to prove the mathematical findings and to show the superior performance of the proposed scheme over the well-known technique of using random beamforming.