CP-OFDM Cyclic Prefix OFDM Explained

Understand what CP-OGFDM, Cyclic Prefix OFDM is and how it is used in a number of wireless communications systems to improve the intersymbol interference performance.

OFDM Orthogonal Frequency Division Multiplexing Includes:
OFDM basics OFDM synchronisation CP-OFDM DFT-S-OFDM

See also: Multicarrier modulation basics What is FBMC

CP-OFDM Cyclic-Prefix, Orthogonal Frequency Division Multiplexing is a form of OFDM that uses a cyclic prefixelement within the OFDM waveform to improve the reliable operation of the signal in the presence of multiple reflections etc.

The cyclic prefix acts as a buffer region or guard interval to protect the OFDM signals from intersymbol interference. This can be an issue in some circumstances even with the much lower data rates that are transmitted in the multicarrier OFDM signal.

By reducing the effects of ISI, Intersymbol Interference This provides for simplified signal processing at the receiver.

What is a cyclic prefix

The concept of using a cyclic prefix with OFDM is straightforward in its concept, but requires additional complexity in the transmitter and receiver processing.

The Cyclic Prefix is achieved by placing last few samples of the OFDM symbol before the start in addition to retaining them at the end.

The cyclic prefix symbols are placed in such a way that they act as guard interval between any two different OFDM symbols.

This guard interval helps to reduce or even eliminate the effects arising from ISI from the previous symbol. Since the prefixed symbol is cyclical in nature i.e. the CP symbol repeats the end of the OFDM data, the resulting CP-OFDM data is periodic or cyclic, unlike the case of standard OFDM.

In summary, the cyclic prefix performs two main functions.

The cyclic prefix provides a guard interval to eliminate intersymbol interference from the previous symbol.
It repeats the end of the symbol so the linear convolution of a frequency-selective multipath channel can be modeled as circular convolution, which in turn may transform to the frequency domain via a discrete Fourier transform. This approach accommodates simple frequency domain processing, such as channel estimation and equalization.

The cyclic prefix is created so that each OFDM symbol is preceded by a copy of the end part of that same symbol.

Different OFDM cyclic prefix lengths are available in various systems. For example within LTE a normal length and an extended length are available and after Release 8 a third extended length is also included, although not normally used.

Cyclic prefix advantages and disadvantages

There are several advantages and disadvantages attached to the use for the cyclic prefix within OFDM.

Advantages

Provides robustness: The addition of the cyclic prefix adds robustness to the OFDM signal. The data that is retransmitted can be used if required.
Reduces inter-symbol interference: The guard interval introduced by the cyclic prefix enables the effects of inter-symbol interference to be reduced.

Disadvantages

Reduces data capacity: As the cyclic prefix re-transmits data that is already being transmitted, it takes up system capacity and reduces the overall data rate.

The use of a cyclic prefix is standard within OFDM and it enables the performance to be maintaiend even under conditions when levels of reflections and multipath propagation are high.