通信原理英文版教学课件：Chapter 10 Channel Coding and Error Control.ppt

Chapter 10 of the Communication Principles course delves into the essential concepts of Channel Coding and Error Control, which play a crucial role in ensuring reliable signal transmission in computer and internet communications. Channel coding, as the name suggests, is implemented primarily to enhance the dependability of data transfer by adding redundant bits. These extra bits are designed to help identify or correct errors that might occur during transmission due to various factors such as noise, interference, or channel impairments. Error control encompasses all techniques aimed at correcting errors, which can be broadly classified into three categories: 1. **Error Detection and Retransmission**: This method allows the detection of erroneous symbols but does not pinpoint their positions. It requires bidirectional communication channels for the receiver to request a retransmission of the data when errors are detected. However, this approach has its drawbacks, including the necessity for a return channel and potentially lower transmission efficiency. 2. **Forward Error Correction (FEC)**: FEC methods go beyond mere error detection by incorporating error control symbols that both discover and correct errors. This eliminates the need for retransmissions and is particularly useful in scenarios where bidirectional channels are not feasible or in real-time communication systems. 3. **Feedback Check**: In this technique, the received symbols are sent back to the transmitter for comparison with the originally transmitted symbols. Feedback checks provide error detection but require a bidirectional channel and may result in reduced transmission efficiency. Key parameters in a code sequence include: - **n**: The total number of symbols in the code sequence. - **k**: The number of information symbols, representing the original data without redundancy. - **r**: The number of error control symbols added for redundancy. - **Code Rate (k/n)**: This represents the proportion of information-bearing symbols to the total symbols in the sequence. - **Redundancy Ratio ((n - k) / k = r / k)**: This ratio indicates the amount of additional data introduced solely for error detection and correction. To illustrate the basic principles of error control coding, consider block codes, a common type of channel coding. Suppose we have a code consisting of 3 binary symbols, resulting in 8 possible codewords. By selecting a subset of these codewords as "permission" or allowed codewords, the receiver can detect errors. For instance, if we allow only 4 out of 8 codewords for transmitting weather conditions, the receiver can detect one error symbol but not correct it. By reducing the number of permission codewords further, like in the case of just two allowed codewords, the code becomes more capable of detecting or correcting errors, albeit with a higher potential for undetected errors if the actual transmitted codeword lies outside the allowed set. Error control coding forms the backbone of modern communication systems, ensuring the integrity of data transmission in applications ranging from internet browsing to satellite communications. Advanced error control techniques, such as convolutional codes, Reed-Solomon codes, and turbo codes, have been developed to provide even greater protection against errors, enabling reliable communication in challenging environments. Understanding these concepts is vital for professionals working in the fields of telecommunications, networking, and data processing, as they directly impact system performance and user experience.