Effect Of TO & CFO on OFDM and SIR Analysis and Interference Cancellation in MIMO - OFDM

OFDM is a multicarrier modulation technique in which a high rate bit stream is split into N parallel bit-streams of lower rate and each of these are modulated using one of N orthogonal sub-carriers. In a basic communication system, the data is modulated onto a single carrier frequency. OFDM is a promising candidate for achieving high data rates in mobile environment because of its multicarrier modulation technique. The available bandwidth is then totally occupied by each symbol. The variations in Time Offset (TO) can lead to intersymbol-interference (ISI) in case of frequency selective channel. A well known problem of OFDM is its sensitivity to frequency offset between the transmitted and received signals, which may be caused by Doppler shift in the channel, or by the difference between the transmitter and receiver local oscillator frequencies. This carrier frequency offset(CFO) causes loss of orthogonality between sub-carriers and the signals transmitted on each carrier are not independent of each other, which results in inter-carrier interference (ICI).The undesired ICI degrades the performance of the system. ICI mitigation techniques are essential in improving the performance of an OFDM system in an environment which induces frequency offset error in the transmitted signal. In this paper , the focus is on the problem of ICI. We proposed ICI reduction using self cancellation scheme and compared with standard OFDM system. . The simulation of OFDM was done with different digital modulation schemes such as BPSK and QPSK modulation techniques . the performance of the designed OFDM system by finding their bit error rate (BER) for different values of signal to noise ratio (SNR). Later we proposed MIMO diversity technique such as STBC OFDM to enhance the performance of the system by reducing the BER for different values of signal to noise ratio (SNR). BER Analysis for BPSK in Rayleigh channel With two transmit and one receive antenna as well as two transmit and two receive antennas for Alamouti STBC case shows higher performance, which effectively alleviates the effects of ISI and ICI.