On the outage behaviour of large DS-CDMA systems with LMMSE detection

The outage behaviour of the single cell DSCDMA uplink is investigated for flat Rayleigh fading channels. Goal of the work is to find an analytical way for the optimisation of the system spectral efficiency under some outage probability constraint by adjusting parameters such as modulation, spreading factor, code rate and number of codes per user. Therefore, the results of the asymptotic analysis for large CDMA systems with random spreading codes are used to obtain the signal-to-interference-and-noise ratio after the linear minimum mean-squared-error detector. This information is then deployed to characterize the system performance using bounds of the information theory. First, the ergodic behaviour is investigated with respect to the spectral efficiency for both coding at capacity and discrete modulation and coding at cut-off rate. Then, the outage probability is defined and studied for coding at capacity. The influence of the system load on the outage is determined analytically and compared to simulation results. Afterwards, the impact of modulation and coding at cut-off rate on the outage behaviour is considered. An expression is presented that allows to easily determine the additional required signal-to-noise ratio for modulation and coding at cut-off rate from the capacity results. Finally, the investigations are extended to determine the maximum system spectral efficiency under given outage constraints. The optimum system loads corresponding to the maximum efficiencies are presented and shown to vary for the different modulation schemes.