Types of Channel Coding

 Channel coding utilizes parity bits to overcome the effect of different channel faults such as interference of noise and fading in information signal. Channel coding up-scales the performance and efficiency of the transmitter as well as receiver. We have two basic ways to implement the parity for detecting and controlling the errors. These two ways are Automatic Repeat Request (ARQ) and Forward Error Control Coding (FECC).

1- Automatic Repeated Request (ARQ)

In automatic repeated request (ARQ) technique, the parity bits are added by the transmitter to information bits which will be used by the receiver for the detection of bits error. In this technique, only transmitter adds parity bits while the receiver does not add parity bits to correct the information bits. The receiver only detects the bits error and sends an alert to transmitter system for the retransmission of information bits called acknowledgment. When receiver detects the bits error, it sends an alert to transmitter for the retransmission of information bits called negative acknowledgement (NAK). When the receiver detects no bits error, it sends an alert to transmitter that the whole data is received called positive acknowledgement (PAK). Automatic repeated request (ARQ) technique is only able to detect bits error, while it has no capability to correct the bits errors which are detected. The automatic repeated request (ARQ) technique is shown in Fig. 1.

Fig.1 Automatic Repeated Request Coding Technique

2- Forward Error Control Coding (FECC) Technique

In Forward error control coding technique, the short sequence bits of information is transformed into long sequence of bits by adding enough parity bits that can protect the information bits. The parity bits will be more than enough to detect bits error as well as to correct the information bits at receiver side as shown in Fig. 2. In this technique, slow data rate and large bandwidth are not concerned just the goal is to protect the information bits. Even the speed of transmitting data will be decreased and large bandwidth is required for the transmission of longer sequence of data.

Fig. 2 Forward Error Channel Coding Technique