Authentication is the process by which it is verified that someone is who they claim they are. Authentication is a security service that checks the identity of the party at the other end of the line. Most of the times, security is provided above physical layer, but in this paper we are concentrating on authentication at the physical layer in wired and wireless communication systems. The authentication signal is generated from modulated signal using shared secret key by transmitter and receiver and these two signals are superimposed as in the spread spectrum technique. This authentication signal is transmitted concurrently with data in physical medium without using extra bandwidth. For recovering the authentication signal at receiver side MMSE equalizer is used to reduce intersymbol interference (ISI) and noise. This authentication is designed to be stealthy and secured. The simulation results have shown the proof of concept in this paper using Matlab7.6.