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This work studies the spectral efficiency achievable when a very large number of terminals are connected simultaneously to a central node (uplink) through independent and identically-distributed flat-fading channels. Assuming that terminals only have statistical channel state information (CSI), the optimum random transmitted-energy allocation is formulated considering a non-orthogonal direct-sequence code-division multiple access (DS-CDMA) where all users transmit using the same modulation and error correcting code and the receiver implements successive interference cancellation (SIC). Focusing on low-power terminals, optimization is carried out by imposing constraints on both the average and peak peruser transmitted energy. Simulations have revealed that a limited number of random energy levels, whose number is determined by the channel power gain variance, is sufficient to achieve approximately the maximum spectral efficiency that would be obtained under direct optimization of the received energy profile.

Peer Reviewed