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PERFORMANCE EVALUATION OF OFDM-BASED DOWNLINK SYSTEMS

Combinations of Direct-Sequence Code-Division Multiple Access (DS-CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) are considered as promising techniques to be used in the downlink of future generations of cellular systems. However, it has previously been shown that for lowcomplexity single-user receivers, there seems to be little motivation for the DS-CDMA component. This conclusion is based on systems that use convolutional codes and QPSK modulations. The objective of this work was to investigate the performance of different modulation orders in a multiple access communication system with frequency-selective fading channel and low-complexity receiver. A comparison between spread and non-spread systems was made for different system loads (i.e., different system spectral efficiencies) and for both single-cell and multiple-cell scenarios. In agreement with existing results, it is concluded that OFDMA scheme outperforms all the (TimeFrequency Localized) TFL-CDMA systems in single and multiple cell scenarios where the difference in performance increases at high system loads and high modulation orders. Due to the simplicity in the implementation of OFDMA compared to CDMA systems and since a low complexity detector is required, the simulations in this work indicate that OFDMA should be preferred for the downlink of mobile cellular systems.