Summary
Incorporation of multiple antennas in downlink nonâorthogonal multiple access (NOMA) system helps in achieving better performance by exploiting different diversity schemes. To this end, we propose a general framework of transmit antenna selection (TAS)âassisted orthogonal spaceâtime block code (OSTBC) transmission for multiple NOMA users. We derive exact expressions of probability density function of the TASâOSTBC processed signalâtoânoise ratio (SNR) at the output of maximumâratio combiner of the NOMA users. Using wellâknown moment generating function approach, we evaluate the error performance of TASâOSTBC assisted NOMA users over generalized ηâÎŒ fading channel in presence of perfect and imperfect successive interference cancelation. We investigate the proposed system in different multiâantenna scenarios and accordingly observe better error performance of the users by increasing the number of transmit and receive antennas. We perform their asymptotic analysis leading to fullâdiversity order at high SNR. For numerical evaluation, we truncate the infinite series in derived results and examine the upper bound of truncation error. We evaluate nth moment of the output SNR and obtain channel quality estimation index as a performance measure of the proposed NOMA system. Moreover, we investigate the influence of power coefficients and fading parameters, η and ÎŒ, on the error performance of TASâOSTBCâassisted NOMA users. We also compare the derived error performances with the existing schemes, wherein we observe that the proposed system shows remarkable improvement in the bit error rate (BER) of the NOMA users. For validation, we execute simulations which closely match with the derived analytical results.