RFLTE Cyclic Delay Diversity (CDD) Explained
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 the same data symbol. This MIMO technique is referred to as STBC (Space Time Block Coding).
Unwanted signal cancellation will result when the channel is flat and the same data symbols are transmitted over multiple antennas. In order to avoid this, a cyclic delay is added to the signal being transmitted. This is shown with an example of a 2-antenna system in the figure above.
As shown, from one antenna (Tx0) the signal is transmitted without any delay, while in the other antenna, a cyclic delay of ‘dT’ is added before transmission. This concept is referred to as Cyclic Delay Diversity. This delay is about a few microseconds, which will introduce a 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 to as large delay CDD. It is greater than the one used in WLAN 802.11n and WiMAX 802.16d. Large CDD helps position the signal on the peak of the frequency response derived from the addition.
CDD Delay and Phase Shift
The following table outlines the relationship between the number of transmit antennas, the phase shift per subcarrier, and the overall delay.
| Number of Transmit antennas | Phase Shift per subcarrier | Overall delay |
|---|---|---|
| 2 | 180 Degree | tsym /2 |
| 3 | 120 Degree | tsym /3 |
| 4 | 90 Degree | tsym /4 |
In LTE, reference signals are not subjected to any CDD. The table mentions delay as phase shift for adjacent carriers.
Conclusion
Cyclic Delay Diversity (CDD) in LTE is an effective technique for improving signal diversity, enhancing coverage, and increasing system performance, especially in MIMO (Multiple Input Multiple Output) environments. By introducing controlled delays across multiple antennas, CDD helps combat fading and improves the reliability of transmitted signals. It is particularly beneficial for scenarios with low mobility, where it enhances spectral efficiency without requiring additional bandwidth or power. However, its effectiveness depends on channel conditions and the receiver’s ability to process diversity schemes efficiently. Overall, CDD contributes to LTE’s robust transmission mechanisms, making it a valuable feature for modern wireless communication networks.
