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<p class="MsoNormal" style="text-align: left; margin: 0cm 0cm 0pt; layout-grid-mode: char;" align="left"><span class="text"><span style="font-family: ";Arial";,";sans-serif";; font-size: 9pt;">This paper analyzes the influence of time-varying cyclic delay diversity (TV-CDD) on the channel fading correlation properties in orthogonal frequency division multiplexing (OFDM) based systems. The underlying transmit diversity technique CDD only increases the frequency diversity at the receiver. In contrast, TV-CDD introduces additionally time diversity which can be exploited without the need of additional complexity at the receiver. This paper gives investigations regarding the resulting channel characteristics from TV-CDD and the impact on the system performance. Due to the increased frequency and time selectivity, an unintended higher channel estimation effort is possible. Therefore, we analyze the impact of choice of the maximum cyclic delay. We show that the resulting channel for TV-CDD can be seen as an uncorrelated Rayleigh fading channel (except for the first sub-carrier) for a large maximum cyclic delay. Furthermore, analysis and simulation results demonstrate a feasible choice of small time-varying cyclic delays for guaranteeing the standard conformability of the TV-CDD technique at the receiver without significant performance degradations.</span></span><span style="font-family: ";Arial";,";sans-serif";; font-size: 9pt;"></span></p>

SUMMARYCyclic delay diversity (CDD) was introduced in 2001 to obtain spatial diversity in an orthogonal frequency 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. Furthermore, CDD has the advantage that there is no rate loss even for more than two transmit antennas. In this paper, we analyse 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 subcarriers for data transmission, TV-CDD improves the performance of the OFDMA-based transmission system significantly.