In this letter, we consider a downlink multi-user (MU) non-orthogonal multiple access (NOMA) network. We demonstrate that utilizing knowledge of the channel gains to the users to determine the NOMA power allocation coefficients can dramatically improve throughput performance. Considering practical imperfect successive interference cancellation, expressions are derived for the optimum power allocation that ensures the minimum outage probability for the signalling scheme. The level of successive interference cancellation at each user and the decoding order are specified. It is shown that the proposed decoding order and the power allocations result in the maximum throughput. Expressions are derived for the throughput with these power allocations. Channel knowledge is exploited to determine the minimum power required for non-outage, and an expression is derived for the average value of this minimum power requirement. Computer simulations validate the derived expressions.