GI (Guard interval)
Introduction:
In digital communication, the transmitted signal can be corrupted by several factors such as multipath fading, inter-symbol interference, and noise. These factors can lead to errors in the received signal, which may result in degraded performance. To mitigate the effects of these factors, a guard interval (GI) is introduced in the transmitted signal.
In this article, we will discuss the concept of guard interval, its types, and how it helps in mitigating the effects of multipath fading.
What is a Guard Interval (GI)?
A guard interval is a time interval inserted between successive symbols in a transmitted signal. The purpose of the guard interval is to mitigate the effects of multipath fading. Multipath fading occurs when the transmitted signal reaches the receiver through different paths, resulting in signal components arriving at different times. This leads to inter-symbol interference (ISI), which can cause errors in the received signal.
The guard interval is inserted to provide a time gap between successive symbols, allowing the delayed signal components to arrive and settle before the next symbol is transmitted. This helps to reduce the effect of ISI, resulting in a better quality of the received signal.
Types of Guard Intervals:
There are two types of guard intervals used in digital communication:
- Cyclic Prefix (CP)
- Zero Padding (ZP)
Cyclic Prefix (CP):
The cyclic prefix is the most commonly used guard interval in digital communication systems. In the cyclic prefix technique, a copy of the last part of the symbol is appended to the beginning of the symbol. The length of the cyclic prefix is usually a small fraction of the symbol length.
The cyclic prefix technique is widely used in Orthogonal Frequency Division Multiplexing (OFDM) systems. In OFDM, the transmitted signal is divided into several subcarriers, and each subcarrier is modulated independently. The cyclic prefix is inserted between the subcarriers to mitigate the effects of multipath fading.
The cyclic prefix technique is also used in single-carrier systems such as Digital Video Broadcasting-Terrestrial (DVB-T) and Long-Term Evolution (LTE) systems.
Zero Padding (ZP):
The zero padding technique is less commonly used than the cyclic prefix technique. In the zero-padding technique, zeros are inserted between successive symbols to create the guard interval. The length of the guard interval is usually equal to the symbol duration.
The zero-padding technique is used in systems where the length of the symbol is fixed and cannot be extended. The zero-padding technique is also used in systems where the cyclic prefix technique is not suitable, such as frequency division multiplexing (FDM) systems.
How Guard Interval helps in mitigating Multipath Fading:
The guard interval helps in mitigating the effects of multipath fading by providing a time gap between successive symbols. The time gap allows the delayed signal components to arrive and settle before the next symbol is transmitted. This helps to reduce the effect of inter-symbol interference, resulting in a better quality of the received signal.
Let's consider an example of an OFDM system with the cyclic prefix guard interval. In an OFDM system, the transmitted signal is divided into several subcarriers, and each subcarrier is modulated independently. Each subcarrier can experience different channel conditions due to multipath fading.
When the received signal is passed through a channel with multipath fading, the signal components arrive at different times. Without the cyclic prefix guard interval, the delayed signal components can cause inter-symbol interference, leading to errors in the received signal.
The cyclic prefix guard interval is inserted between the subcarriers to mitigate the effects of multipath fading. The cyclic prefix guard interval allows the delayed signal components to arrive and settle before the next subcarrier is transmitted. This helps to reduce the effect of inter-symbol interference, resulting in a better quality of the received signal.
In addition to mitigating the effects of multipath fading, the guard interval also helps in reducing the effect of noise. The time gap provided by the guard interval allows the noise in the received signal to settle before the next symbol is transmitted. This helps to reduce the effect of noise, resulting in a better quality of the received signal.
Conclusion:
In summary, the guard interval is an important technique used in digital communication to mitigate the effects of multipath fading and noise. The guard interval provides a time gap between successive symbols, allowing the delayed signal components to arrive and settle before the next symbol is transmitted. This helps to reduce the effect of inter-symbol interference and noise, resulting in a better quality of the received signal.
There are two types of guard intervals used in digital communication: cyclic prefix (CP) and zero padding (ZP). The cyclic prefix technique is the most commonly used guard interval in digital communication systems, while the zero-padding technique is less commonly used.
The guard interval is widely used in communication systems such as OFDM, DVB-T, and LTE, where multipath fading is a common problem. The guard interval helps to improve the performance of these systems by mitigating the effects of multipath fading and noise.
Overall, the guard interval is an important technique used in digital communication to improve the quality of the received signal, and it is essential to understand its concept and application in modern communication systems.