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In this letter, based on the NOMA concept, a qualityof-service (QoS) optimization problem for 2-user multiple-inputsingle-output (MISO) broadcast systems is considered, given a pair of target interference levels. The minimal power and the optimal precoding vectors are obtained by considering its Lagrange dual problem and via Newton's iterative algorithm, respectively. Moreover, closed-form expressions of the minimal transmission power for some special cases are also derived. One of these cases is termed as quasi-degraded, which is the key point and will be discussed in detail in this letter. Our analysis further figures out that the proposed NOMA scheme can approach nearly the same performance as optimal dirty paper coding (DPC), as verified by computer simulations.

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This paper investigates the optimal power allocation scheme for sum throughput maximization of non-orthogonal multiple access (NOMA) system with α-fairness. In contrast to the existing fairness NOMA models, α-fairness can only utilize a single scalar to achieve different user fairness levels. Two different channel state information at the transmitter (CSIT) assumptions are considered, namely, statistical and perfect CSIT. For statistical CSIT, fixed target data rates are predefined, and the power allocation problem is solved for sum throughput maximization with α-fairness, through characterizing several properties of the optimal power allocation solution. For perfect CSIT, the optimal power allocation is determined to maximize the instantaneous sum rate with α-fairness, where user rates are adapted according to the instantaneous channel state information (CSI). In particular, a simple alternate optimization (AO) algorithm is proposed, which is demonstrated to yield the optimal solution. Numerical results reveal that, at the same fairness level, NOMA significantly outperforms the conventional orthogonal multiple access (MA) for both the scenarios with statistical and perfect CSIT.Index Terms-Non-orthogonal multiple access, α-fairness, outage probability, ergodic rate, power allocation.

In this paper, a new evaluation criterion is developed to investigate the performance of nonorthogonal multiple access (NOMA) from an information theoretic point of view. In particular, the relations among the capacity region of the broadcast channel and two rate regions achieved by NOMA and timedivision multiple access (TDMA) are first illustrated. Based on these relations, a new evaluation criterion is proposed for NOMA in wireless fading scenarios, where the key idea is to compare NOMA with TDMA statistically in terms of not only the sum rate but also the individual rates. In a wireless downlink scenario with user pairing, the developed analytical results show that NOMA can outperform TDMA not only in terms of the sum rate but also in terms of each user's individual rate, particularly when the difference between the users' channels is large. The optimal power allocation for a special case of NOMA user pairing is also established.INDEX TERMS NOMA, capacity region of the broadcast channel, sum rates, individual rates.

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In this paper, the outage performance of downlink non-orthogonal multiple access (NOMA) is investigated for the case where each user feeds back only one bit of its channel state information (CSI) to the base station. Conventionally, opportunistic one-bit feedback has been used in fading broadcast channels to select only one user for transmission. In contrast, the considered NOMA scheme adopts superposition coding to serve all users simultaneously in order to improve user fairness. A closed-form expression for the common outage probability (COP) is derived, along with the optimal diversity gains under two types of power constraints. Particularly, it is demonstrated that the diversity gain under a long-term power constraint is twice as large as that under a short-term power constraint. Furthermore, we study dynamic power allocation optimization for minimizing the COP, based on one-bit CSI feedback.This problem is challenging since the objective function is non-convex; however, under the short-term power constraint, we demonstrate that the original problem can be transformed into a set of convex problems. Under the long-term power constraint, an asymptotically optimal solution is obtained for high signal-to-noise ratio. Index TermsNon-orthogonal multiple access, downlink transmission, common outage probability, one-bit feedback, power allocation.