5G NR Cyclic Prefix (CP) Design

Last updated on Nov 21, 2023

Cyclic Prefix (CP) is an essential component in the design of wireless communication systems, including the 5G New Radio (NR) standard. The primary purpose of the cyclic prefix is to mitigate the effects of multipath propagation, which is a phenomenon where multiple copies of the transmitted signal reach the receiver at different times due to reflections and diffractions in the wireless channel.

In the context of 5G NR, the cyclic prefix is particularly important for Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) waveforms. OFDM is a modulation technique that divides the communication channel into multiple narrowband subcarriers, each carrying a part of the data. OFDMA extends this concept to multiple users, allowing them to share the available spectrum efficiently.

Here's a technical explanation of the 5G NR cyclic prefix design:

1. Types of Cyclic Prefix in 5G NR:

Normal Cyclic Prefix (NCP): This is the standard cyclic prefix used in OFDM systems. It consists of a guard interval that is a replica of the end part of the OFDM symbol. The length of the normal cyclic prefix is chosen based on the expected delay spread in the channel.
Extended Cyclic Prefix (ECP): In some cases, an extended cyclic prefix is used to provide even more robustness against delay spread. The extended cyclic prefix is longer than the normal cyclic prefix and is beneficial in scenarios with high-mobility users or severe multipath conditions.

2. Guard Interval Length:

The length of the cyclic prefix is determined based on the delay spread of the channel. Delay spread is the time difference between the arrival of the first and last copies of a signal at the receiver. A longer cyclic prefix can accommodate longer delay spreads.

3. Implementation in Time Domain:

The cyclic prefix is inserted in the time domain at the beginning of each OFDM symbol. This guard interval helps in separating consecutive OFDM symbols and prevents inter-symbol interference (ISI) caused by multipath propagation.
In the time domain, the cyclic prefix is essentially a copy of the latter part of the OFDM symbol that is appended before the actual data transmission. This redundancy aids in dealing with the effects of channel delay spread.

4. Impact on Spectral Efficiency:

While the cyclic prefix is crucial for mitigating multipath effects, it does introduce some overhead, as a portion of the transmitted signal is used for guard intervals rather than carrying data. Therefore, there is always a trade-off between robustness and spectral efficiency.

5. Dynamic Adaptation:

In some 5G NR systems, the cyclic prefix length can be dynamically adapted based on the channel conditions. This adaptive approach optimizes the use of the cyclic prefix depending on the specific characteristics of the wireless channel at a given moment.

The 5G NR cyclic prefix design is a carefully considered element in the overall system design, balancing the need for robustness in the face of multipath propagation with the desire for spectral efficiency in the use of the available radio spectrum. The choice of cyclic prefix type and length is influenced by the specific requirements of the communication scenario and the characteristics of the wireless channel.