Abstract-The rate-t/k (t ≤ k) OFDM systems with the intercarrier interference (ICI) canceling windows are presented by generalizing the existing rate-1/k system and the capacity lower bounds of these systems for timevarying frequency selective channels are analyzed. Based on the capacity analysis, the optimal windows maximizing the capacity lower bounds are designed numerically with an assumption that the channel is flat fading. The effects of ICI canceling windows on the capacity lower bounds are examined through numerical simulations and the results indicate that considerable performance improvement can be achieved by employing the proposed windows, as compared with the conventional systems with/without windowing when the channel varies very rapidly.

A limited feedback-based transmit beamforming technique for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) is investigated in timevarying channels. The performance of the system is significantly degraded by outdated feedback information even when the channel varies slowly. To compensate for the impairment in time-varying channels, the optimal transmit beamforming vector for a future channel, which maximizes the expected effective channel gain, is derived by applying the autoregressive (AR) model to the channels. These are obtained at the receiver. Following this, schemes for the selection of beamforming vectors are proposed to reduce the feedback amount. These can effectively reduce the amount of feedback information by utilizing both the frequency and time correlation of transmit beamforming vectors. Simulation results show that the proposed techniques outperform existing schemes in terms of the bit error rate (BER) performance with the same amount of feedback.

Abstract-Rate-( ) intercarrier interference (ICI) canceling codes for orthogonal frequency division multiplexing (OFDM) systems are proposed. The capacity lower bounds of OFDM systems employing these codes are derived for time-varying frequency-selective channels. The optimal codes maximizing these bounds are designed numerically. The simulation results indicate that the optimal codes can provide both higher capacity lower bounds and lower bit error rates (BERs) than the existing ICI canceling codes.Index Terms-Capacity, intercarrier interference (ICI), orthogonal frequency division multiplexing (OFDM), time-varying channel.

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Abstract-An intercarrier interference (ICI) canceling technique for multiple-input single-output (MISO) orthogonal frequency division multiplexing (OFDM) systems in fast fading channels is proposed. The proposed scheme consists of a linear space-frequency block code (SFBC) at the transmitter and a receive frequency block code (RFBC) at the receiver. The code design is based on the upper bound of pairwise error probability (PEP) which is derived under assumptions and approximations stated. Specifically, the proposed code is designed to minimize the upper bound of the PEP. The simulation results demonstrate that the proposed technique outperforms the conventional methods in fast fading environments. I. INTRODUCTIONOrthogonal frequency division multiplexing (OFDM) is a promising technique for broadband communication systems since it converts a frequency-selective broadband channel into a series of parallel narrowband channels. Therefore, the OFDM can reduce the equalization and decoding complexity [1], [2]. In fact, many modern communication standards for high data rates adopt the OFDM as a transmission technique [3]- [5]. However, it has been recognized that the performance of the OFDM can be severely degraded if the channel varies significantly within an OFDM symbol period [6]. This degradation is due to the intercarrier interference (ICI). To compensate for the ICI, various techniques have been introduced for single-input single-output (SISO) systems, including the frequency-and time-domain equalizations (see [7] and references therein), the ICI self-cancelation [8]- [11], and the frequency-domain partial response coding (PRC) [12], [13]. Among those approaches, the equalizations are the most common, but implementation requires heavy computations. The ICI self-cancelation schemes are based on the use of a frequency-domain coding or a time-domain windowing, which has a code rate of 1/k. This method can cancel the ICI effectively with a small computational complexity at the receiver;