  The application of network non-orthogonal multiple access (N-NOMA) technique
to coordinated multi-point (CoMP) systems has attracted significant attention
due to its superior capability to improve connectivity and maintain reliable
transmission for CoMP users simultaneously. Based on the concept of
quasi-degraded channel for N-NOMA, this paper studies the precoding design for
downlink N-NOMA scenarios with two base stations (BSs) equipped with multiple
antennas. In specific, under quasi-degraded channels, simple linear precoding
based N-NOMA can achieve the same minimal total transmission power as
theoretically optimal but complicated dirty paper coding (DPC) scheme, when the
users' target rates and minimal transmission power of each BS are given. In
this paper, the channel quasi-degradation (QD) condition is first rigorously
derived for the scenario with single CoMP user and two NOMA users. The
closed-form optimal precoders for N-NOMA under quasi-degraded channels are also
provided. Then, based on QD condition, a novel hybrid N-NOMA (H-N-NOMA) scheme
is proposed, which is a mixture of N-NOMA and conventional zero-forcing
beamforming (ZFBF) scheme. Further, for the scenarios with more users, a
low-complexity QD based user pairing (QDUP) algorithm is proposed. Numerical
results are presented to reveal the impact factors of QD channels, and also
demonstrate the superior performance of the proposed H-N-NOMA/QDUP scheme. It
is shown that the proposed H-N-NOMA/QDUP scheme can effectively exploit the
benefit of multi user diversity.
