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Channel Coding Theory

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There are two classes of codes, Source coding and Channel coding: Source coding attempts to compress the data from a source in order to transmit it more efficiently. This is commonly used on the Internet. Channel coding adds extra data bits, commonly called parity bits, to make the transmission of data more robust to disturbances present on the transmission channel. Channel Coding theorem states that since any communication channel is characterized by its channel capacity C there exist a coding method that allows us to transmit reliably over the channel with a code rate R < C, i.e., with arbitrarily small error probability. It is not possible to communicate reliable over the channel with code rates R > C. Thus if the required transmission …show more content…

Principal among them is noise. For example suppose 01010101 is sent from one end. Due to noise, it may be received as 11010101, with the first digit changed by noise. Clearly if what is sent is not what is received, communication can be problematic. Error correcting codes have been developed to solve this type of problem. The aim of channel coding theory is to find codes which transmit quickly, contain many valid code words and can correct or at least detect many errors. So, different codes are optimal for different applications. The needed properties of this code mainly depend on the probability of errors happening during transmission. One simple way to detect errors …show more content…

An error detection code by itself does not control errors, but it can be used to request repeated transmission of errored codewords until they are received error-free. This technique is called Automatic Repeat Request or ARQ [30]. Every block of data received is checked using the error detection code used, and if the check fails, retransmission of the data is requested – this may be done repeatedly, until the data can be verified. ARQ is also adaptive since it only re-transmits information when errors occur. On the other hand, ARQ schemes require a feedback path which may not be available.Usually, when the transmitter does not receive the acknowledgment before the timeout occurs (i.e., within a reasonable amount of time after sending the data frame), it retransmits the frame until it is either correctly received or the error persists beyond a predetermined number of retransmissions. Three types of ARQ protocols are Stop-and-wait ARQ, Continuous ARQ (Go-Back-N ARQ, and Selective-Repeat ARQ). 4.4.1 STOP-AND-WAIT ARQ The transmitter sends a single codeword with stop-and-wait ARQ, and waits for a positive Acknowledgement (ACK) or Negative Acknowledgement (NAK) before sending any more codewords [30]. The advantage of Stop-and-Wait ARQ is that it only requires

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