Cyclic Prefix (CP) in OFDM: Advantages and Disadvantages

This page explores the pros and cons of using a Cyclic Prefix (CP) in Orthogonal Frequency Division Multiplexing (OFDM) systems. We’ll break down the benefits and drawbacks to help you understand its role in modern wireless communication.

What is Cyclic Prefix?

Introduction: OFDM stands for Orthogonal Frequency Division Multiplexing. Unlike FDM, OFDM uses densely packed subcarriers that are orthogonal to each other. The peak of one subcarrier aligns with the null of others. This makes OFDM more bandwidth-efficient than FDM and is a cornerstone of technologies like WLAN, WiMAX, and LTE.

OFDM boosts bandwidth efficiency by using multiple orthogonal subcarriers. Each of these subcarriers can carry multiple bits (n), which provides higher bit rates based on modulation schemes such as QPSK (n=2), 16QAM (n=4), 64QAM (n=6), 256QAM (n=8), and so on.

OFDM effectively mitigates frequency-selective fading in multipath channel environments. However, under channel fading conditions, OFDM can suffer from Intersymbol Interference (ISI) and Intercarrier Interference (ICI) due to delay spread.

To combat this, a Cyclic Prefix (CP) is added. The last few samples of an OFDM symbol are extracted and prefixed to the beginning of the transmission. This repeated section is known as the Cyclic Prefix, and the time it occupies is called the “guard interval (Tg)”. The total OFDM symbol duration is designated as “Ts”. Depending on the delay spread characteristics, CP ratios like 1/4, 1/8, 1/16, and 1/32 are used. LTE, for example, utilizes both normal and extended-length CPs.

Here’s a breakdown of the Cyclic Prefix addition process:

• K = Length of FFT/IFFT

• Kcp = Number of samples in cyclic prefix

• OFDM vector = [ X0, X1, X2, … X K-cp ,…, X K-2 , X K-1 , X K ]

• CP vector = [X K-cp ,…, X K-2 , X K-1 , X K ]

• OFDM with CP vector = [X K-cp ,…, X K-2 , X K-1 , X K , X0, X1, X2, … X K-cp ,…,X K-2 ,X K-1 ,X K ]

The following diagram illustrates where the Cyclic Prefix is added in the OFDM transmitter and removed in the receiver. It’s inserted after the IFFT in the transmitter and removed before the FFT in the receiver.

Example #1: Consider an OFDM symbol with an FFT size of 256 samples, as used in WiMAX, with a Cyclic Prefix ratio of 1/16.

• N FFT = 256
• CP = 1/16 = 16 samples

Example #2: For an OFDM symbol with an FFT size of 64 samples, commonly used in the WLAN 802.11a standard, and a CP of 1/4:

• N FFT = 64 samples
• CP = 1/4 = 16 samples

The CP duration is carefully selected based on the channel’s delay spread. Ideally, the CP should be longer than the delay spread to effectively avoid ISI.

Benefits or Advantages of Cyclic Prefix (CP)

Here are the main benefits of using a Cyclic Prefix in OFDM:

• Robustness: The retransmitted data within the CP enhances the robustness of the OFDM signal against multipath delay spread.
• ISI Reduction: The guard interval effectively reduces the impact of Intersymbol Interference (ISI).
• ACI Suppression: By inserting the cyclic prefix within the guard interval, it helps suppress Adjacent Channel Interference (ACI) and preserves the orthogonality of the OFDM subcarriers.
• Time Synchronization: It aids in packet time synchronization at the receiver using correlation techniques.
• Ease of Implementation: The Cyclic Prefix is relatively straightforward to implement using various programming languages like MATLAB and C.

Drawbacks or Disadvantages of Cyclic Prefix (CP)

Despite its advantages, the Cyclic Prefix also has some drawbacks:

• Reduced System Capacity: The redundant data used for the CP reduces the overall data rate and system capacity.
• Increased Power Consumption: Transmitting the additional data of the CP increases the power required for transmission.
• Increased Latency: Adding the CP increases the transmission time, which can lead to higher latency for the useful payload carried by the OFDM subcarriers.