On Time-Varying Cyclic Delay Diversity

Cyclic Delay Diversity (CDD) was introduced in 2001 to obtain spatial diversity in an Orthogonal Division Multiplexing (OFDM) based transmission system. In contrast to Space-Time Codes, CDD introduces no additional effort at the receiver side, since CDD changes only the channel impulse response seen by the receiver by increasing the frequency selectivity. In this paper, we analyze CDD for an Orthogonal Frequency Division Multiple Access (OFDMA) based transmission system. To increase not only the frequency selectivity but also the time selectivity, we introduce Time-Varying Cyclic Delay Diversity (TV-CDD), which changes the cyclic shift with each OFDM symbol. Especially in systems, where each user can only allocate a few subchannels for data transmission, TV-CDD improves the performance of the OFDMA based transmission system significantly.