  In this letter, the design of non-orthogonal multiple access (NOMA)
beamforming is investigated in a spatial division multiple access (SDMA) legacy
system. In particular, two popular beamforming strategies in the NOMA
literature, one to use existing SDMA beams and the other to form new beams, are
adopted and compared. The studies carried out in the letter show that the two
strategies realize different tradeoffs between system performance and
complexity. For example, riding on existing beams offers a significant
reduction in computational complexity, at the price of a slight performance
loss. Furthermore, this simple strategy can realize the optimal performance
when the users' channels are structured.
