Concatenated Iterative ICI Cancellation with BICM-OFDM system over Rayleigh fading channels

Abstract

In orthogonal frequency division multiplexing (OFDM) communication systems, the orthogonality is maintained over the symbol period because inter-symbol interference (ISI) and inter-carrier interference (ICI) can be entirely eliminated by guard interval in multipath fading channels. In a case, however, that a specific subcarrier is heavily attenuated with low received SNR, bit error ratio of the system greatly increases. To prevent the performance degradation caused by this problem, forward error correction (FEC) code has been generally applied to the OFDM system. There are several FEC codes which can be applied to OFDM system such as convolutional code, trellis code and concatenated code. Especially, the concatenated code composed of inner coder and outer coder has been employed to obtain high order of coding gain. By the same token, bit interleaved coded modulation (BICM) which can be considered as a serially concatenated coding scheme that the linear inner decoder is replaced by a soft demapper has been combined with the OFDM system. The BICM-OFDM system, however, is affected by the ICI generated by the frequency offset, as frequency offsets in mobile radio channel especially distort the orthogonality between subcarriers in general OFDM systems. Since the frequency offset affects all the subcarriers through the FFT block, the ratio that the desired data is precisely sent to the demodulation scheme and decoder tend to be lower. This paper introduces a new system that the ICI cancellation scheme is directly concatenated to the BICM-OFDM system with iterative decoding. In the proposed ICI cancellation procedure, ICI cancellation function which is determined by the estimated carrier to interference ratio (CIR) and an index of the desired subcarrier and the other undesired subcarriers’ indices is successively multiplied to channel output data. Since the CIR level is closely continuous with the value of frequency offset, then the cancellation function can take the information about frequency offset. The cancellation function is an exponential form to the frequency offset and difference between the subcarrier indices. The ICI self cancellation has been usually used to reduce the ICI caused by frequency offset in frequency domain, however, this scheme causes a reduction in bandwidth efficiency by adapting additional modulation or demodulation processes. In the BICM system, however, the modulation scheme is combined with coding scheme; the bandwidth efficiency can be maintained through the system. Without loss of bandwidth efficiency, the proposed scheme concatenated with BICM-OFDM system can be used effectively. The proposed cancellation function is applied to the channel data in every time when the channel data is entering the demodulation process after acquiring the information of coded data. The proposed scheme with BICM-OFDM system shows dramatically improved performance especially when the generated frequency offset value is large and the Eb/N0 is increased because the cancellation function includes the information of frequency offset compared with conventional system.