CHD (Channel Decoder)

Last updated on Mar 14, 2023

Channel decoding is a process used in communication systems to correct errors that may occur during the transmission of data over a noisy communication channel. The channel decoder, or CHD, is an essential component of this process. In this article, we will explain what a channel decoder is, how it works, and its applications in communication systems.

What is a Channel Decoder?

A channel decoder is a component of a communication system that is used to correct errors that occur during the transmission of data over a noisy channel. A channel can be considered as a medium through which data is transmitted from a transmitter to a receiver. A channel can be noisy due to various factors, such as electromagnetic interference, signal attenuation, and thermal noise.

A channel encoder is used to add redundancy to the data to be transmitted. This redundancy is used by the channel decoder to correct errors that may occur during transmission. A channel decoder decodes the received signal to reconstruct the original data sent by the transmitter.

The process of channel decoding involves the use of a decoding algorithm that is designed to correct the errors introduced during the transmission of data. The decoding algorithm takes the received signal as input and produces an estimate of the original data.

How does a Channel Decoder work?

A channel decoder works by comparing the received signal with all possible transmitted signals. The channel decoder computes the likelihood of each transmitted signal based on the received signal and the known properties of the communication channel. The likelihood of each transmitted signal is used to determine the most likely transmitted signal.

The decoding algorithm used by the channel decoder can be categorized into two types: maximum likelihood decoding and soft-decision decoding. In maximum likelihood decoding, the decoder computes the likelihood of each possible transmitted signal and selects the signal with the highest likelihood. In soft-decision decoding, the decoder computes the likelihood of each possible transmitted signal and uses this information to make a decision based on a threshold.

Maximum Likelihood Decoding

Maximum likelihood decoding is the most commonly used decoding algorithm in communication systems. In maximum likelihood decoding, the decoder computes the likelihood of each possible transmitted signal and selects the signal with the highest likelihood.

The likelihood of a transmitted signal is computed using the received signal and the known properties of the communication channel. The channel is characterized by a channel matrix, which is a mathematical representation of the channel.

The likelihood of a transmitted signal given the received signal and the channel matrix can be computed using Bayes' theorem:

P(x|r,H) = P(r|x,H)P(x|H) / P(r|H)

where x is the transmitted signal, r is the received signal, H is the channel matrix, P(x|r,H) is the likelihood of the transmitted signal given the received signal and the channel matrix, P(r|x,H) is the likelihood of the received signal given the transmitted signal and the channel matrix, P(x|H) is the prior probability of the transmitted signal, and P(r|H) is the likelihood of the received signal given the channel matrix.

The decoder computes the likelihood of each possible transmitted signal and selects the signal with the highest likelihood. The decoded signal is then compared with the original signal to determine the number of errors that have been corrected.

Soft-Decision Decoding

Soft-decision decoding is an alternative to maximum likelihood decoding that is used when the noise level in the channel is high. In soft-decision decoding, the decoder computes the likelihood of each possible transmitted signal and uses this information to make a decision based on a threshold.

The threshold used in soft-decision decoding is called the decision threshold. The decision threshold is a function of the noise level in the channel and the properties of the communication system. The decision threshold is used to determine whether a bit in the received signal should be interpreted as a 0 or a 1.

In soft-decision decoding, the likelihood of a transmitted signal given the received signal and the channel matrix is computed using the soft-decision metric. The soft-decision metric is a function of the received signal, the transmitted signal, and the noise level in the channel.

The soft-decision metric is used to make a decision based on a threshold. If the soft-decision metric is greater than the threshold, the bit is interpreted as a 1. If the soft-decision metric is less than the threshold, the bit is interpreted as a 0.

Applications of Channel Decoders

Channel decoders are used in a variety of communication systems, including wireless communication systems, satellite communication systems, and wired communication systems.

Wireless Communication Systems

Wireless communication systems are used to transmit data over radio waves. These systems are used in mobile communication systems, wireless LANs, and satellite communication systems.

Wireless communication systems are subject to interference from other wireless devices, such as microwave ovens, and environmental factors, such as buildings and trees. Channel decoders are used in wireless communication systems to correct errors introduced by these factors.

Satellite Communication Systems

Satellite communication systems are used to transmit data over long distances. These systems are used in television broadcasting, internet communication, and military communication.

Satellite communication systems are subject to interference from solar radiation, atmospheric conditions, and other factors. Channel decoders are used in satellite communication systems to correct errors introduced by these factors.

Wired Communication Systems

Wired communication systems are used to transmit data over wires, such as telephone lines and fiber-optic cables. These systems are used in telephone communication, internet communication, and cable television.

Wired communication systems are subject to interference from electrical noise and other factors. Channel decoders are used in wired communication systems to correct errors introduced by these factors.

Conclusion

In conclusion, channel decoding is a process used in communication systems to correct errors that may occur during the transmission of data over a noisy communication channel. The channel decoder is an essential component of this process.

The channel decoder works by comparing the received signal with all possible transmitted signals and computing the likelihood of each transmitted signal based on the received signal and the known properties of the communication channel.

The decoding algorithm used by the channel decoder can be categorized into two types: maximum likelihood decoding and soft-decision decoding. Maximum likelihood decoding is used when the noise level in the channel is low, and soft-decision decoding is used when the noise level in the channel is high.

Channel decoders are used in a variety of communication systems, including wireless communication systems, satellite communication systems, and wired communication systems. They are used to correct errors introduced by noise and other factors in the communication channel, which is critical for ensuring reliable communication.