election, are considered in LTE. 3.3.2.1 Cyclic Delay Diversity (CDD) In general, delay diversity consists of transmitting the same signal with different rela- tive delays through the different transmit antennas (Bauch and Malik 2004, 17). This is completely transparent to the mobile terminal, which only sees a single radio link with additional time dispersion and, equivalently, additional frequency selectivity. Thus, delay diversity can be seen as a means to transform antenna or space diversity into frequency diversity, which is especially desirable in scenarios with reduced mul- tipath propagation. Cyclic delay diversity is a special type of delay diversity, where cyclic shifts are applied (instead of linear delays) to the signal transmitted over the dif- ferent antennas. An example of CDD for two-antenna configuration is illustrated in Figure 3.19a, whereas extended configurations with more than two transmit antennas can be achieved by applying a different shift to each antenna. CDD operates block- wise and thus it is applicable to block-based transmission schemes such as OFDM. A cyclic shift of the time domain signal is equivalent to a frequency-dependent phase shift of the frequency domain OFDM symbol, as illustrated in Figure 3.19b. That is, complex modulation symbols, denoted as Sp are mapped onto the resources of the first antenna, whereas a phase-shifted version of the same modulation symbols is mapped onto the second antenna. The fact that CDD is transparent to the UE makes this diversity technique very suitable for being combined with other multiantenna schemes. Thus far in LTE, CDD is applied in conjunction with spatial multiplexing. 3.3.2.2 Space Frequency Block Coding (SFBC) Space frequency block coding was first proposed in Dehghani et al. (2004, 543) as an extension of traditional space-time block coding for OFDM systems. In SFBC, Frequency (OFDM subcarriers) Cyclic Sn-1 ej(n-1)0 Shift Figure 3.19 Cyclic delay diversity for two-antenna configuration. © 2009 by Taylor & Francis Group, LLC Radio-Int