Chapter 6
Direct-Sequence Spreading and Slow Subcarrier-Hopping Aided Multi-user SDMA-OFDM Systems
6.1 Conventional SDMA-OFDM Systems1
In Chapters 4 and 5, Coded Modulation (CM) [314] assisted SDMA-OFDM systems invoking both Minimum Mean Square Error (MMSE) and Genetic Algorithm (GA) based Multi-User Detection (MUD) have been investigated, respectively. Specifically, in terms of the bandwidth-sharing strategy, the SDMA-OFDM systems discussed in these chapters are referred to here as the conventional SDMA-OFDM systems [3, 193], where all the users exploit the entire system bandwidth for their communications. However, this bandwidth-sharing strategy exhibits a few drawbacks.
On the one hand, the conventional SDMA-OFDM systems can exploit little frequency diversity, since each user activates all available subcarriers. This limitation can be mitigated by combining both Frequency-Hopping (FH) and SDMA-OFDM techniques, resulting in the FH/SDMA-OFDM systems. In these FH/SDMA-OFDM systems the total system bandwidth is divided into several subbands, each of which hosts a number of consecutive subcarriers, and a so-called FH pattern is used for controlling the subband allocation for the different users. Since each user activates different subbands from time to time, the achievable frequency diversity improves as the width of the subbands is reduced.
On the other hand, when the number of users becomes higher in conventional SDMA-OFDM systems, a higher Multi-User Interference (MUI) is expected ...
