Signal to interference ratio (sir) analysis and interference cancellation in mimo ofdm systems with carrier frequency and timing offsets

Abstract

The orthogonal frequency-division-multiplexing(OFDM) scheme has been adopted in many applications such as the digital broadcast television, the wireless communications, and the high-bit-rate communications over the existing copper networks. OFDM has found the applications in several mobile radio communication systems, such as the European DAB (digital audio broadcasting) system and the DVB (digital video broadcasting) system. The basic concept of OFDM is to sub-divide the information bit stream into a large number of bit streams, each with low individual bit rates, which are then carried on individual orthogonal subcarriers. This transmission technique is especially suited for mitigating the effect of the multipath fading channel. Multicarrier systems such as OFDM are well known to allow a bandwidth efficient transmission over strongly frequency selective channels at a moderate implementation effort. OFDM link the subcarriers are perfectly orthogonal only if transmitter and receiver use exactly the same frequencies. The overall performance of OFDM systems is very sensitive to time and frequency synchronization offsets. It is therefore important to design stable synchronization blocks in order to guarantee the proper orientation of the receiver. The OFDM systems however are highly sensitive to the frequency offset. Therefore, an accurate estimation of the frequency offset is critical. A false estimate leads to ISI which may disturb the orthogonality of the system and cause essential degradation due to ICI.OFDM is sensitivity to frequency offset in the channel, immediately results in inter carrier interference(ICI). One of the drawbacks of OFDM has been proved to be its high sensitivity to frequency offset. This frequency offset is usually divided into an integral part, which is a multiple of the subcarrier spacing, and a fractional part which is less than one half of the subcarrier spacing, 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. The comparisons of these schemes in terms of various parameters will be useful in determining the choice of ICI mitigation techniques for different applications and mobile environments. 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). 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. Later we proposed MISO diversity technique such as SFBC OFDM to enhance the performance of the system by reducing the BER for different values of signal to noise ratio (SNR). Specifically, we propose and evaluate the performance of an interference cancelling receiver for SFBC-OFDM which effectively alleviates the effects of ISI and ICI.