OFDM (Orthogonal Frequency Division Multiplexing) has been developed to combat multipath effects. Two important parameters that should be carefully considered while dealing with OFDM system are phase noise and carrier frequency because accurate predictions of the tolerable phase noise can allow the system to relax specifications. Space-Time block coding efficiently utilizes power and bandwidth by exploiting the benefits of multiple transmit antennas systems. In this book, performance analysis of Space time block coding (STBC) is done with BPSK modulation and Rayleigh fading channel. It can be observed that the BER performance for 2 transmit and 2 receive system is much better than 1 transmit 2 receive MRC case. Secondly performance of OFDM system is studied in the presence of carrier frequency offset, phase noise (with/without Rayleigh fading). Simulation results show that for a particular value of one of these parameters and increasing the other one, the SINR value decreases continuously. Finally, STBC-OFDM is studied and comparison is made for OFDM system with/without STBC. Results show that OFDM with STBC gives lesser bit error rate and hence gives better performance.