OFDM Cyclic Prefix, CP

- the cyclic prefix used in Orthogonal Frequency Division Multiplexing provides an essential element of the overall signal acting as a guard band between each OFDM symbol.

Use of cyclic prefix is a key element of enabling the OFDM signal to operate reliably.

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.

Cyclic prefix basics

The basic concept behind the OFDM cyclic prefix is quite straightforward.

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.

Diagram showing how the cyclic prefix enables multipath delays to be accommodated
OFDM signal with cyclic prefix

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.

By Ian Poole


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