Why Downlink Cyclic Delay Diversity Helps Uplink Transmit Diversity

In this paper, we show that the use of cyclic delay diversity (CDD) on the downlink (DL) can improve the performance of uplink (UL) diversity methods such as antenna selection in a time-division duplexing (TDD) system. An analytical framework for showing why CDD transmission on the DL improves UL diversity transmission in TDD system is provided. The analysis is used to quantify the gain in the UL received signal-to-noise ratio (SNR) when DL CDD is used. The analytical results show that for a 2 x 2 MIMO system that uses DL CDD, UL transmit diversity based on antenna selection improves the average UL SNR by 0.41dB relative to the case when CDD is not being applied at the BS transmitter and improves the 1% CDF UL SNR by 1.58dB. Measurement-based analysis validates the analytical results using measured frequency response of a MIMO system using a wideband channel sounder.