CP (Cyclic Prefix)

Introduction

Cyclic prefix (CP) is a technique used in communication systems to mitigate the effects of multipath fading, which is caused by reflections of the transmitted signal arriving at the receiver with different time delays. This technique adds a guard interval (also called cyclic prefix) to the beginning of each transmitted symbol, which is a copy of the last part of the symbol. This cyclic prefix helps in reducing inter-symbol interference (ISI) and inter-carrier interference (ICI) caused by multipath fading. In this article, we will explain the concept of cyclic prefix, its significance, and its applications.

Multipath Fading and Its Effects

Multipath fading is a common phenomenon in wireless communication systems that occurs due to reflections, diffraction, and scattering of the transmitted signal by obstacles in the environment. These reflections cause the transmitted signal to reach the receiver through multiple paths, with each path having a different time delay and amplitude. As a result, the received signal is a combination of multiple copies of the transmitted signal with different time delays and amplitudes. This can cause distortion of the received signal, which can lead to errors in decoding the transmitted data.

ISI and ICI are the two main effects of multipath fading. ISI occurs when the delayed copies of the transmitted signal overlap with the subsequent symbols, causing interference between them. This results in the loss of information and errors in decoding the transmitted data. ICI, on the other hand, occurs when the frequency of the received signal changes due to the Doppler effect caused by the movement of the transmitter or receiver. This can cause interference between adjacent sub-carriers in the frequency domain, which can also result in errors in decoding the transmitted data.

Cyclic Prefix and Its Significance

Cyclic prefix is a technique used to mitigate the effects of multipath fading by adding a guard interval to the beginning of each transmitted symbol. The guard interval is a copy of the last part of the symbol and is called the cyclic prefix because it is cyclically repeated. The length of the cyclic prefix is typically chosen to be longer than the maximum delay spread of the channel, which is the time difference between the first and last arriving signals. This ensures that the delayed copies of the transmitted signal do not overlap with the subsequent symbols, thus reducing ISI.

The cyclic prefix also helps in reducing ICI by providing a smooth transition between adjacent symbols in the frequency domain. This is because the cyclic prefix is a copy of the last part of the symbol and has the same frequency content as the symbol. As a result, the frequency response of the channel is relatively constant over the length of the symbol and the cyclic prefix, which reduces the frequency selective fading caused by ICI.

Cyclic prefix also has other benefits. It allows for easier synchronization of the receiver to the transmitted signal since the cyclic prefix is a known sequence. This is because the receiver can use the cyclic prefix to estimate the delay spread of the channel and adjust its timing accordingly. Additionally, cyclic prefix can increase the spectral efficiency of the system since it allows for a higher data rate to be transmitted over the same bandwidth.

Applications of Cyclic Prefix

Cyclic prefix is used in many communication systems, including wireless communication systems such as Wi-Fi, 4G LTE, and 5G. In Wi-Fi systems, cyclic prefix is used in Orthogonal Frequency Division Multiplexing (OFDM) to mitigate the effects of multipath fading. In OFDM, the transmitted data is divided into multiple sub-carriers, and each sub-carrier is modulated using a different symbol. The cyclic prefix is added to each symbol to reduce ISI and ICI.

In 4G LTE and 5G systems, cyclic prefix is used in Orthogonal Frequency Division Multiple Access (OFDMA) to mitigate the effects of multipath fading.

OFDMA is a multi-user version of OFDM, where multiple users can transmit their data simultaneously using different sub-carriers. The cyclic prefix is added to each user's symbol to ensure that their data does not interfere with other users' data. This allows for efficient use of the available bandwidth and increases the spectral efficiency of the system.

Cyclic prefix is also used in digital television broadcasting systems such as DVB-T and ATSC. In these systems, the transmitted data is divided into multiple segments, and each segment is modulated using OFDM. The cyclic prefix is added to each segment to reduce the effects of multipath fading, which can be particularly severe in indoor environments.

Conclusion

In conclusion, cyclic prefix is an important technique used in communication systems to mitigate the effects of multipath fading. It adds a guard interval to the beginning of each transmitted symbol, which is a copy of the last part of the symbol. This helps in reducing ISI and ICI caused by multipath fading and provides a smooth transition between adjacent symbols in the frequency domain. Cyclic prefix has many applications in wireless communication systems such as Wi-Fi, 4G LTE, and 5G, as well as in digital television broadcasting systems.