Physics
Encoder and Decoder
An encoder is a device that converts a physical signal into a digital signal, while a decoder performs the opposite function, converting a digital signal back into a physical signal. These devices are commonly used in communication systems to transmit and receive information.
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eBook - PDFChannel Codes
Classical and Modern
- William Ryan, Shu Lin(Authors)
- 2009(Publication Date)
- Cambridge University Press(Publisher)
Hence, this operation is often called compression. Depending on the source, the compression can be lossless (e.g., for computer data files) or lossy (e.g., for video, still images, or music, where the loss can be made to be imperceptible or acceptable). The source decoder is the encoder’s counterpart which recovers the source sequence exactly, in the case of lossless compression, or approximately, in the case of lossy compression, from the encoder’s output sequence. Channel encoder and channel decoder. The role of the channel encoder is to protect the bits to be transmitted over a channel subject to noise, distortion, and interference. It does so by converting its input into an alternate sequence possessing redundancy, whose role is to provide immunity from the various 2 Coding and Capacity Source Source encoder Channel encoder Modulator Source decoder Channel decoder Demodulator User Channel Figure 1.1 Basic digital communication- (or storage-) system block diagram due to Shannon. channel impairments. The ratio of the number of bits that enter the channel encoder to the number that depart from it is called the code rate, denoted by R, with 0 < R < 1. For example, if a 1000-bit codeword is assigned to each 500-bit information word, R = 1/2, and there are 500 redundant bits in each codeword. The function of the channel decoder is to recover from the channel output the input to the channel encoder (i.e., the compressed sequence) in spite of the presence of noise, distortion, and interference in the received word. Modulator and demodulator. The modulator converts the channel-encoder out- put bit stream into a form that is appropriate for the channel. For example, for a wireless communication channel, the bit stream must be represented by a high-frequency signal to facilitate transmission with an antenna of reason- able size. Another example is a so-called modulation code used in data storage.
eBook - PDF- Ron Roth(Author)
- 2006(Publication Date)
- Cambridge University Press(Publisher)
We demonstrate that—here as well—such information allows us to double the number of correctable errors. 1.1 Communication systems Figure 1.1 shows a communication system for transmitting information from a source to a destination through a channel . The communication can be 1 2 1. Introduction Source ✲ Source Encoder ✲ Channel Encoder ❄ Channel ✛ Channel Decoder ✛ Source Decoder ✛ Destination Figure 1.1. Communication system. either in the space domain (i.e., from one location to another) or in the time domain (i.e., by storing data at one point in time and retrieving it some time later). The role of source coding is twofold. First, it serves as a translator between the output of the source and the input to the channel. For example, the information that is transmitted from the source to the destination may consist of analog signals, while the channel may expect to receive digital input; in such a case, an analog-to-digital conversion will be required at the encoding stage, and then a back conversion is required at the decoding stage. Secondly, the source encoder may compress the output of the source for the purpose of economizing on the length of the transmission; at the other end, the source decoder decompresses the received signal or sequence. Some applications require that the decoder restore the data so that it is identical to the original, in which case we say that the compression is lossless. Other applications, such as most audio and image transmissions, allow some (controlled) difference—or distortion—between the original and the restored data, and this flexibility is exploited to achieve higher compression; the compression is then called lossy. Due to physical and engineering limitations, channels are not perfect: their output may differ from their input because of noise or manufacturing defects.
eBook - PDFThe Technology and Business of Mobile Communications
An Introduction
- Mythri Hunukumbure, Justin P. Coon, Ben Allen, Tony Vernon(Authors)
- 2021(Publication Date)
- Wiley-IEEE Press(Publisher)
The signal is then mapped back to a corresponding sequence of bits, and the transmitted message can be recovered. In general, errors can arise when decoding the transmitted message at the receiver. Typical physical processes that lead to such errors will be covered later in the chapter. At a high level, it is intuitive that one may wish to put mechanisms in place during the encoding phase of transmission that would mitigate the effects of these errors. One way of achieving Source Source Encoder Channel Encoder Modulator Sink Source Decoder Channel Decoder Demodulator and Detector Channel Figure 8.1 Basic digital communication system model. 8.2 Encoding Information 243 this aim is to append redundant bits to each word. In our example, where m bits are parsed to form a word, suppose for each bit of information (out of m bits), we introduce two more bits that are exactly the same. This is known as repetition coding , and is a simple way to improve the resilience of a communication system to possible errors that might occur dur-ing transmission of the word. The total number of bits in each codeword 1 is now 3 m . However, there are still only 2 m possible codewords since we started with m -bit sequences. The result of this encoding operation is that the receiver is able to recover the transmitted word even if some of the bits were altered during transmission (i.e. flipped from 0 to 1 or vice versa). This ability to correct errors introduced in the communication channel is what has propelled communication systems forward in the last 70 years. We will discuss this in more detail in the next section. This very simple introduction to how a digital communication system works provides a high-level view of the main processes needed to convey information digitally between two devices. Of course, specific implementations of such systems will require careful design of the different components illustrated in Figure 8.1, and these designs will differ amongst implementations.
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