IEEE Transactions on Wireless Communications
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A method called open loop cyclic delay diversity (CDD) precoding has recently become part of the standard for 3GPP Long-Term Evolution (LTE) cellular communications systems. CDD had previously been proposed as single stream diversity method for OFDM systems. The proposal in LTE is an extension to spatial multiplexing using so called large delay CDD. The purpose of this paper is twofold: First, we give two different interpretations of the CDD precoding scheme in LTE as beam switching and transformation of spatial diversity to frequency diversity. We then discuss the impact of the delay parameter in CDD precoding on the achievable rates and BER performance. It turns out that while the choice of large delay causes a capacity degradation in single stream CDD, the achievable rates in CDD based spatial multiplexing are robust to variations of the delay parameter. This is even true in correlated scenarios and when only a resource block of a few subcarriers is allocated to a specific user. However, in case of separate encoding of the spatial streams, large delay CDD precoding shows a BER advantage over small delay CDD precoding since the spatial diversity is transformed into frequency diversity among adjacent subcarriers and, hence, is easily picked up by the decoder. Finally, we give a performance comparison of open loop CDD based precoding and closed loop precoding in LTE. It is shown that for 2 transmit antenna systems, only little gain is achieved by the additional effort of closed loop precoding. Closed loop precoding shows substantial benefits over CDD based precoding only for a higher number of transmit antennas.