Intercarrier Interference Components Research Articles

Iterative MMSE-DFE and Error Transfer for OFDM in Doubly Selective Channels

An iterative design of minimum mean square error decision feedback equalizer (MMSE-DFE) to suppress the intercarrier interference (ICI) of orthogonal frequency division multiplexing systems in the time-variant channels is analyzed in this paper. The channel is assumed to vary in a linear fashion within a frame. The error power transfer is investigated in parallel with the equalizing process, showing the gain in iterations and aiding the MMSE decision. Iteratively, the ICI components are estimated and canceled from the receive symbol, providing an MMSE estimate of the source symbol with an increasing reliability. It is shown that based on the error power transfer analyzed in this paper, ICI components can be significantly removed via the iterative MMSE-DFE, leaving the error composed of noise and negligible residue of ICI. Simulations have verified the accuracy of the error power estimation.

FADAC-OFDM: Frequency-Asynchronous Distributed Alamouti-Coded OFDM

We propose frequency-asynchronous distributed Alamouti-coded orthogonal frequency-division multiplexing (FADAC-OFDM). The proposed scheme effectively mitigates the intercarrier interference (ICI) due to frequency offset (FO) between two distributed antennas. The transmitter side of the proposed scheme transmits each of the two subslots in Alamouti code through two remote subcarriers symmetric to the center frequency and is referred to as space-frequency reversal schemes by Wang et al. or Choi. The receiver side of the proposed scheme is significantly different from the conventional scheme or the scheme proposed by Wang et al. in that it performs two discrete Fourier transform (DFT), each of which is synchronized to each transmit antenna's carrier frequency. The decision variables are generated by performing a simple linear combining with the same complexity as that of the conventional Alamouti code. The derivation shows that in flat-fading channels, the dominant ICI components due to FO cancel each other during the combining process. The proposed scheme achieves almost the same performance as the ideal Alamouti scheme, even with large FO. To use this ICI self-cancellation property for selective fading channels or in cases with timing offset (TO) between two transmit antennas, the total subcarriers are divided into several subblocks, and the proposed scheme is applied to each subblock. For mildly selective channels or cases with practically small TO, the proposed scheme achieves significantly improved performance compared with the conventional space-frequency Alamouti-coded OFDM.

Performance enhancement of the DSRC system using frequency-domain equalization for 5.9 GHz DSRC applications

This paper proposes the use of equalization concepts in frequency domain that exploit the frequency domain channel matrix to combat inter-carrier interference (ICI) instead of inter-symbol interference (ISI) in 5.9 GHz Dedicated Short Range Communications (DSRC) systems. The physical layer (PHY) of DSRC is currently being developed by work group of IEEE 802.11p. The conventional system currently assumes static channel characteristics. Channel tracking schemes were investigated and the Viterbi-aided channel estimation scheme was proposed for DSRC systems that did not explicitly exploit the ICI components caused by the time-varying channels. The performance of the DSRC system is investigated for a time-varying channel using a conventional DSRC model, a decision-directed (Viterbi-aided) channel estimation model, and the frequency-domain equalization design. It is shown that the DSRC system with the frequency-domain equalization achieves a considerable performance enhancement compared to both the conventional and the Viterbi-aided channel estimation schemes in terms of both packet error rate (PER) and bit error rate (BER) at relatively high and low velocities.

위상 잡음과 직교 불균형이 있는 OFDM 수신 신호의 보상

OFDM(Orthogonal Frequency Division Multiplexing) 시스템에서 직교 불균형 문제는 송수신기의 front-end에서 발생하며, 성상도에 영향을 주게 되어 BER(Bit Error Rate)을 증가시킨다. 또한, 위상 잡음은 송수신시 국부 발진기에서 발생되는 잡음으로 각 부반송파의 직교성을 깨뜨림으로써 시스템 성능을 크게 저하시킨다. 기존 방식인 PNS(Phase Noise Suppression) 알고리즘은 이러한 위상 잡음을 효과적으로 제거하는 방법이지만 직교 불균형 이동시에 적용되면 오히려 성능이 감소된다. 본 논문에서는 OFDM 시스템의 수신기에서 하향 변환 시 발생하는 직교 불균형과 위상 잡음의 영향을 분석하고, 수신기 FFT(Fast Fourier Transform) 후단에서 파일럿 심볼을 사용하여 CPE를 먼저 제거하고 직교 불균형과 위상 잡음의 성분을 검출하여 등화기의 판정 기준으로 사용하여 보상하는 방법을 제시하였다. 또, 다른 기존 방식들은 FFT 후단에서 추정하고 피드백 시키거나 프리엠블과 같은 시퀀스를 사용하는 방식이지만, 본 논문에서는 FFT 후단에서 MMSE 등화기만을 사용하여 제거하므로 기존의 방법보다 복잡도가 줄어든다. 기존의 위상 잡음 제거 방식에 ICI(Inter Carrier Interference) 제거 기능을 추가하고 직교 불균형 성분을 추출하여 MMSE(Minimum Mean Square Error) 과정 중에 적응 forgetting factor를 적용하면 성능 개선과 직교 불균형 성분의 영향이 줄어들며 성능이 개선됨을 보인다. In the OFDM system, IQ imbalance problem happens at the RF front-end of transceiver, which degrades the BER(bit error rate) performance because it affects the constellation in the received signal. Also, phase noise is generated in the local oscillator of transceivers and it destroys the orthogonality between the subcarriers. Conventional PNS algorithm is effective for phase noise suppression, but it is not useful anymore when there are jointly IQ(In-phase and Quadrature) imbalance and phase noise. Therefore, in this paper, we analyze the effect of IQ imbalance and phase noise generated in the down-conversion of the receiver. Then, we estimate and compensate the IQ imbalance and phase noise at the same time. Compared with the conventional method that IQ imbalance after IFFT is estimated and compensated in front of FFT via the feedback, this proposed method extracts and compensates effect of IQ imbalance after FFT stage. In case IQ imbalance and phase noise exist at the same time, we can decrease complexity because it is needless to use elimination of IQ imbalance in time domain and training sequences and preambles. Also, this method shows that it reduces the ICI and CPE component using adaptive forgetting factor of MMSE after FFT.

Reduction of Doppler effects in OFDM systems

This paper observes the inter-carrier interference (ICI) by Doppler effect in time domain in orthogonal frequency division multiplexing systems. This observation allows us to propose a new time domain two-stage equalization scheme that iteratively cancels the ICl. The proposed scheme consists of the following two stages: First, we apply low-complexity linear minimum mean squared error (MMSE) employing partial rank of time impulse response matrix to shorten the ICI components into diagonal region. Second, we propose new parallel interference cancellation with hard decision feedback to remove the residual ICI. Performance evaluations and complexity analyses show that the bit error rate and the signal-to-interference-plus-noise ratio performances of the proposed scheme are very close to those of the classical frequency domain linear MMSE equalization scheme with much reduced complexity