On the spectral efficiency of DS-CDMA with higher order modulation in presence of fading

The uplink spectral efficiency (SE) of single and multi cell DS-CDMA systems using higher order modulation schemes is investigated in the presence of ergodic fading channels. Therefore, the asymptotic analysis for large CDMA systems with random spreading codes is used to obtain the SINR after the single user matched filter (MF) and LMMSE receiver. Based on these results, the SE is maximized for both receivers with coding at capacity for varied load. Furthermore, the concept of cut-off rate is applied to determine the SE for realistic coding and modulation. It is shown, that with an MF receiver switching to higher order modulation has no advantage. In contrast, for SNR above a certain threshold the SE of DS-CDMA systems can be improved with larger constellations when deploying an LMMSE receiver, which is shown using a number of M-QAM constellations. In general, the behavior of multi cell systems appears to be similar to single cell systems for the deployed linear cell-array model. However, it is demonstrated that depending on the interference level from the adjacent cells the SE can be degraded significantly.