Calculating the power to ensure a Given reception quality in a pd-noma System with two users and qam signals

This paper proposes a method for determining the minimum required power for the weak user in a two-user power-domain non-orthogonal multiple access (PD-NOMA) system employing quadrature amplitude modulation (QAM), such that a given symbol error rate (SER) constraint is satisfied under known channel conditions. Most existing methods rely on the Shannon model with idealized signals and a number of simplifying assumptions. Such models make it possible to assess the theoretical potential of the system; however, the corresponding methods are inapplicable under practical transmission conditions. For models with QAM constellations, an analytical solution to the problem is not available due to the complex structure of signal superposition and the dependence of the error probability on inter-user interference; therefore, the suboptimal and computationally expensive exhaustive search method is widely used. In contrast to such approaches, this paper formulates an equation based on an analytical expression for SER, whose solution is obtained using the Newton method. The proposed approach provides fast convergence and significantly reduces the computational complexity. It constitutes a part of the power allocation procedure on the “weak” user side and specifies a lower bound on the required power. Simulation results confirm the correctness and robustness of the method over a wide range of PD-NOMA parameters.