GT (guard time)

Guard time (GT) is a concept used in communication systems, particularly in wireless communication systems, to address the problem of multipath interference. Multipath interference occurs when a radio wave that has been transmitted from a source reaches the receiver by two or more paths, and the different versions of the wave interfere with each other. This interference can cause distortion of the signal, making it difficult to accurately decode the original message.

Guard time is a period of time that is added between each transmitted symbol to mitigate the effects of multipath interference. The guard time is designed to provide a gap between symbols that is long enough to ensure that any reflections of the transmitted signal have died down before the next symbol is transmitted. The purpose of this gap is to prevent the interference caused by the reflections from affecting the subsequent symbol.

In this article, we will discuss the concept of guard time in detail, including its definition, purpose, and different types. We will also look at how guard time is implemented in various communication systems, including Wi-Fi, cellular networks, and digital TV.

Definition of Guard Time

Guard time, also known as a guard interval, is a period of time that is inserted between two symbols in a digital communication system. The purpose of guard time is to reduce the effects of multipath interference, which occurs when a signal arrives at the receiver by more than one path. Multipath interference causes the signal to be distorted, and the guard time provides a gap between symbols that is long enough to prevent the interference from affecting the subsequent symbol.

Guard time is measured in seconds or in units of the symbol period. The symbol period is the time required to transmit one symbol, which is a discrete unit of information. The length of the guard time depends on the duration of the symbols being transmitted and the characteristics of the communication channel, including the delay spread and the coherence bandwidth.

Purpose of Guard Time

The purpose of guard time is to ensure that the transmitted signal arrives at the receiver in a way that can be accurately decoded. In a wireless communication system, the signal can take many different paths from the transmitter to the receiver, due to reflection, refraction, and diffraction of the radio waves. Each of these paths can cause the signal to arrive at the receiver with a different delay, phase shift, and amplitude. The resulting interference can cause errors in the decoding of the transmitted signal, making it difficult to recover the original message.

Guard time provides a gap between symbols that is long enough to prevent the interference caused by reflections from affecting the subsequent symbol. By inserting a guard time between symbols, the receiver can distinguish between the direct path signal and the delayed signals caused by reflections. This allows the receiver to accurately decode the transmitted message, even in the presence of multipath interference.

Types of Guard Time

There are two main types of guard time used in digital communication systems: cyclic prefix and zero-padding.

Cyclic Prefix

Cyclic prefix is a type of guard time that is used in orthogonal frequency-division multiplexing (OFDM) systems, which are commonly used in wireless communication systems, including Wi-Fi and digital TV. OFDM divides the frequency spectrum into multiple subcarriers, each of which carries a part of the transmitted message. The subcarriers are orthogonal to each other, meaning that they do not interfere with each other.

In an OFDM system, cyclic prefix involves adding a copy of the end of each symbol to the beginning of the symbol, resulting in a longer symbol with a guard time at the beginning. This guard time is known as the cyclic prefix, and its length is typically equal to the channel delay spread, which is the time difference between the first and last paths that the signal takes from the transmitter to the receiver. The cyclic prefix is inserted before the symbol is transmitted, and the receiver removes the cyclic prefix before decoding the symbol.

The purpose of the cyclic prefix is to ensure that the symbol remains orthogonal to the other symbols, even in the presence of multipath interference. By adding a copy of the end of the symbol to the beginning, the interference caused by the reflections of the symbol is canceled out, allowing the receiver to accurately decode the transmitted message.

Zero-padding

Zero-padding is another type of guard time that is used in digital communication systems, particularly in time-division multiplexing (TDM) systems. TDM is a technique used to transmit multiple signals over a single communication channel by dividing the channel into time slots and allocating each signal a specific time slot.

In a TDM system, zero-padding involves adding zeros to the end of each symbol to increase its duration. The length of the zero-padding is determined by the duration of the guard time required to mitigate the effects of multipath interference. The receiver removes the zero-padding before decoding the symbol.

The purpose of zero-padding is similar to that of cyclic prefix: to provide a gap between symbols that is long enough to prevent the interference caused by reflections from affecting the subsequent symbol. However, zero-padding is less efficient than cyclic prefix, as it does not cancel out the interference caused by the reflections. Instead, it provides a gap between symbols that is long enough to ensure that the interference has died down before the subsequent symbol is transmitted.

Implementation of Guard Time

Guard time is implemented in various communication systems, including Wi-Fi, cellular networks, and digital TV. In each of these systems, the guard time is tailored to the characteristics of the communication channel, including the delay spread and the coherence bandwidth.

Wi-Fi

In Wi-Fi systems, guard time is implemented using cyclic prefix. The length of the cyclic prefix is determined by the delay spread of the wireless channel, which is the time difference between the arrival of the first and last signals. The cyclic prefix is added to each OFDM symbol before it is transmitted, and the receiver removes the cyclic prefix before decoding the symbol.

The guard time used in Wi-Fi systems is typically 800 nanoseconds for 20 MHz channels and 1.6 microseconds for 40 MHz channels. These guard times are designed to provide a gap between symbols that is long enough to prevent the interference caused by reflections from affecting the subsequent symbol.

Cellular Networks

In cellular networks, guard time is implemented using both cyclic prefix and zero-padding. The guard time is designed to provide a gap between symbols that is long enough to prevent the interference caused by reflections from affecting the subsequent symbol.

The guard time used in cellular networks varies depending on the type of system and the frequency band. For example, in Long Term Evolution (LTE) systems, the guard time is 4.7 microseconds, while in 5G systems, the guard time is 3.75 microseconds. These guard times are designed to provide a gap between symbols that is long enough to mitigate the effects of multipath interference.

Digital TV

In digital TV systems, guard time is implemented using cyclic prefix. The length of the cyclic prefix is determined by the delay spread of the transmission channel, which is the time difference between the arrival of the first and last signals.

The guard time used in digital TV systems varies depending on the type of system and the transmission standard. For example, in the European Digital Video Broadcasting (DVB) standard, the guard time is 1/32 of the symbol duration, while in the Advanced Television Systems Committee (ATSC) standard used in North America, the guard time is 1/8 of the symbol duration. These guard times are designed to provide a gap between symbols that is long enough to mitigate the effects of multipath interference.

Conclusion

Guard time is a critical concept in digital communication systems, particularly in wireless communication systems, where it is used to mitigate the effects of multipath interference caused by reflections. The two most common types of guard time are cyclic prefix and zero-padding. Cyclic prefix involves adding a copy of the end of the symbol to the beginning, while zero-padding involves adding zeros to the end of the symbol. The length of the guard time is determined by the delay spread of the communication channel.

In most digital communication systems, guard time is implemented using cyclic prefix, as it is more efficient than zero-padding. Cyclic prefix cancels out the interference caused by reflections, ensuring that the symbol remains orthogonal to the other symbols, even in the presence of multipath interference. However, zero-padding is still used in some systems, particularly in TDM systems, where it is used to provide a gap between symbols that is long enough to prevent the interference caused by reflections from affecting the subsequent symbol.