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On the spectral efficiency of large CDMA systems using higher order modulation

The spectral efficiency of large CDMA systems using higher order modulation schemes is investigated. Therefore, the asymptotic analysis for single cell systems and random spreading codes is used to obtain the signal-to-interference-and-noise ratio (SINR) after the matched filter (MF) and linear minimum mean-squared-error (LMMSE) receiver. Then the concepts of capacity and cut-off rate are applied to determine the optimum spectral efficiency with coding and modulation. It is shown that with a MF receiver higher order modulation in combination with coding has no advantage. In contrast, beyond a particular signal-to-noise ratio (SNR) threshold the spectral efficiency of a CDMA system can be improved with larger constellations when deploying an LMMSE receiver. Depending on the SNR an optimum modulation scheme exists, which was shown for a number of QAM constellations.