erface Physical Layer Frequency (OFDM subcarriers) OFDM Signal Generation Freq. domain OFDM symbol -Sn+1 OFDM Signal Generation Figure 3.20 Space frequency block coding (SFBC) for two-antenna configuration. the basic idea of the Alamouti scheme (1998, 1451) is applied in frequency domain instead of time domain. Figure 3.20 illustrates the SFBC operation for the particu- lar two-antenna configuration. SFBC operation is performed on pairs of complex valued modulation symbols. Hence, each pair of symbols has an associated pair of frequency resources (i.e., a pair of OFDM subcarriers). Modulation symbols are mapped directly onto the available resources of the first antenna. However, map- ping of each pair on the second antenna is reversely ordered, sign reversed, and complex conjugated, as illustrated in Figure 3.20. For a proper reception of the transmitted modulation symbols, the mobile ter- minal must be informed about SFBC transmission and a simple linear operation (Dehghani et al. 2004, 543) has to be applied to the received signal. Contrarily to cyclic delay diversity, SFBC provides diversity on the level of modulation symbols. 3.3.3 Spatial Multiplexing The objective of spatial multiplexing is to increase data rates by transmitting differ- ent data streams over the different parallel channels provided by the multiple trans- mit and receive antennas. Let us assume a system with M transmit and N receive antennas, as depicted in Figure 3.21. Then the MIMO radio channel consists of M X N ideally uncorrelated paths. This configuration theoretically provides L = min (M, NV parallel channels that allow simultaneously transmission of L data streams. Signals from transmit antennas cause interferences to each other at the receiver, and hence spatially multiplexed streams are overlapped due to propaga- tion through the MIMO channel, as seen in Figure 3.21. Therefore, a MIMO processing stage is needed at the receiver in order to suppress interfering signals. This processing stage can be seen as a detection of the original