Multi-antenna techniques 93 lies in how the block of frequency-domain modulation symbols are mapped to the second antenna. The benefits of SFBC compared to CDD is that SFBC pro- vides diversity on modulation-symbol level while CDD, in case of OFDM, must rely on channel coding in combination with frequency-domain interleaving to provide diversity. 6.4.2 Transmitter-side beam-forming If some knowledge of the downlink channels of the different transmit antennas, more specifically some knowledge of the relative channel phases, is available at the transmitter side, multiple transmit antennas can, in addition to diversity, also provide beam-forming, that is the shaping of the overall antenna beam in the direc- tion of a target receiver. In general, such beam-forming can increase the signal- strength at the receiver with up to a factor N T , i.e. in proportion to the number of transmit antennas. When discussing transmission schemes relying on multiple transmit antennas to provide beam-forming one can distinguish between the cases of high and low mutual antenna correlation , respectively. High mutual antenna correlation typically implies an antenna configuration