On the Influence of Cyclic Delay Diversity and Doppler Diversity on the Channel Characteristics in OFDM Systems

In this paper, we analyze the influence of the transmit antenna diversity techniques cyclic delay diversity (CDD) and discontinuous Doppler diversity (DDoD) on the channel observation at the receiver side for wireless communications systems based on orthogonal frequency division multiplexing (OFDM). These antenna diversity techniques influence the channel fading characteristic, and thus, are described as a modified channel. We analyze this modified channel in terms of its channel transfer function and fading correlation in time and frequency direction. We will see, that the fading correlation factorizes into an antenna related part and a propagation channel part. This is, the transmitter does not require any information about the channel itself in order to design roots in the correlation function. This analysis yields a guidance for the choice of the antenna configuration parameters, which is the goal of this paper. The proper choice of these parameters (cyclic delays, Doppler shifts) allows for reducing respectively removing fading correlation in time and/or frequency direction, and therefore, optimizing diversity gains.