LTE CDD-Cyclic Delay Diversity
This page covers LTE CDD or Cyclic Delay Diversity basics. The concept is used in MIMO based transmit systems employing more than one antenna for transmissions of same data symbol. This MIMO technique is referred as STBC(Space Time Block Coding).
Unwanted signal cancellation will result when channel is flat and same data symbols are transmitted over multiple antennas. In order to avoid this, cyclic delay is added to the signal being transmitted. This is shown with example of 2 antenna system in the figure. As shown from one antenna Tx0 is transmitted without any delay while in the other antenna cyclic delay of 'dT' is added before transmission. This concept is referred as Cyclic Delay Diversity.
This delay is about few microseconds which will introduce frequency dependent phase shift as shown. The value of this cyclic delay depends on different bandwidths and conditions. As shown in the figure, nulls and peaks are produced when both the signals are received and combined at the receiver.
LTE uses cyclic delay referred as large delay CDD. It is greater than the one used in WLAN 802.11n and WiMAX 802.16d. Large CDD help position signal on the peak of the frequency response derived from addition.
|Number of Transmit antennas||Phase Shift per subcarrier||Overall delay|
In LTE, reference signals are not applied any CDD. Table mentions delay as phase shift for adjacent carriers.