Relay Station (RS) enhance the performance of cellular system at cell edge. It also extends the transmission to areas outside the coverage. This paper studies the transmission through direct and relay link between Base Station (BS) and User Equipment (UE). This paper also proposes RS based Orthogonal Space-Time Block Codes (OSTBCs) for fourth generation cellular system (i.e. LTE-A). The received signals at UEs for different transmission scenarios are mathematically derived and analyzed. We compare the performance of LTE-A Downlink Physical Layer in the case of direct link, relay link and relay link based OSTBCs. The SER performance for LTE-A with three transmission scenarios is calculated for a certain range of SNRs. Results show that the transmission link that uses RS based OSTBCs gives better performance than direct link and relay link.