  In this paper, we propose an impartial simultaneous wireless information and
power transfer (SWIPT)-assisted cooperation mechanism for a non-orthogonal
multiple access (NOMA) downlink scenario. Specifically, both a cell-center user
and a cell-edge user apply the power-splitting technique and utilize the
harvested energy to forward the other user's information on the premise of
successful decoding of their own information. Both analytical and numerical
results show that the proposed impartial user cooperation mechanism (IUCM)
outperforms the traditional partial cooperation mechanism in terms of outage
probability, diversity order and diversity-multiplexing trade-off (DMT). For
comparison, we further incorporate the IUCM into an orthogonal
frequency-division multiple access (OFDMA) framework, which is shown to
preserve the same diversity order, while has a worse but more flexible DMT
performance in comparison with the IUCM in the NOMA framework. Although the
IUCM in OFDMA has a worse outage performance, it is proved that it has the same
optimal system outage probability with the IUCM in NOMA when the relaying
channel between the two users is error-free.
