Transmitter-Based Pre-Processing for OFDM-CDMA Forward-Link Systems with Two-Dimensional Spreading

Transmitter-based signal pre-processing is investigated for the two-dimensional (2-D) OFDM-CDMA (orthogonal frequency division multiplexing, code division multiple access) forward link systems, aiming to increase the system sum-capacity and shift the signal processing complexity of a mobile unit to the base station. For the non-selective fading channels, the optimum pre-processing that achieves the maximum ergodic sum-capacity is obtained by applying the principle of multi-user water filling under the condition of zero multiple access interference (MAI). For the selective fading channels, on the other hand, three sub-optimum methods including ZF (zero-forcing) with power normalization, MMSE (minimum mean square error), and the newly proposed ZF with multi-user water filling are investigated under the constraint of total transmit power. Numerical results show that: (i) The new pre-processing outperforms the traditional other two in terms of achievable sum-capacity, (ii) 2-D spreading pattern should be adapted to the channel conditions so as to minimize MAI and thus maximize the sum-capacity. In this regard, 2-D spreading has advantage over the 1-D one. (iii) The fewer the users scheduled at a time, the higher the system capacity; a result obtained from exploiting the multi-user diversity.