These are amplify-and-forward, decode-and-forward, and compress-and-forward. Amplify and forward (AF) is the simplest and fastest of the three. It works like an analog repeater and was introduced in 2004. Whenever this type of relay receives a signal, all it does is amplify the signal received from the source and transmit it to the destination. The target receives two independently faded versions of the information so it can make better decisions. Obviously errors will occur using this strategy. No coding or decoding takes place here and the noise captured by the signal from the source to the relay will also be amplified! The decode and forward (DF, can also be described as digital regenerative repeaters) strategy decodes the message received at the relay. Here it works very well, but when it fails to decode correctly, an error propagation phenomenon is observed. To combat this problem, methods have been introduced in which the relay detects and forwards source information only in case of high instantaneous signal-to-noise ratio of the source-relay link