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Orthogonal frequency-division multiplexing (OFDM) is the ubiquitous contemporary technology adopted for digital audio/video broadcasting, as well as wireless local and metropolitan area networks. Since the wireless multimedia services often have different quality-of- service requirements and their performance is sensitive to the channel conditions, the conventional fixed OFDM modulation scheme might not be a satisfactory solution nowadays. In this paper, we introduce a novel pilot-free adaptive modulation scheme, which is bandwidth efficient and allows variable data rates, for the future robust OFDM systems. We design a number of modulation modes in a combination of different constellation sizes and different polynomial cancellation coding (PCC) methods to combat the crucial intercarrier interference (ICI) problem. Instead of estimating the channel quality based on the overhead pilot symbols, we propose to directly estimate the signal-to-noise ratio (SNR) free of using any pilot. Moreover, our scheme offers more modulation modes than some other existing adaptive modulation methods, which are simply based on different constellation sizes. According to the Monte Carlo simulations, the empirical results show that our adaptive modulation scheme, in most channel conditions (SNR ges 13 dB), not only can satisfy the predetermined bit-error-rate (BER) requirement (BER les 10-4) but also can dynamically enhance the throughputs in the rather clean environments with high SNR values.

The intercarrier interference (ICI) due to the Doppler frequency shift, sampling clock offset, time-varying multipath fading and local oscillator frequency offset becomes the major difficulty for the data transmission via the wireless orthogonal frequency division multiplexing (OFDM) systems. The existing ICI mitigation schemes involve the frequency-domain channel estimation/equalization or the additional coding and therefore require the pilot symbols which reduce the throughput. The frequency-domain channel estimation/equalization relies on the huge matrix inversion with high computational complexity especially for the OFDM technologies possessing many subcarriers such as digital video broadcasting (DVB) systems and wireless metropolitan-area networks (WMAN). In our previous work, we proposed a semi-blind ICI equalization scheme using the joint multiple matrix diagonalization (JMMD) algorithm and empirically showed that the proposed method significantly improved the symbol error rates for QPSK- and 16QAM-OFDM systems. In this paper, we discuss the sufficient condition for the theoretical ICI equalizability and also propose an alternative semi-blind ICI equalization method based on the joint approximate diagonalization of eigen-matrices (JADE) algorithm, which is much more computationally efficient than our previous method.

Intercarrier interference is deemed as one of the crucial problems in the wireless orthogonal frequency division multiplexing (OFDM) systems. The conventional ICI mitigation schemes involve the frequency-domain channel estimation or the additional coding, both of which require the spectral overhead and hence lead to the significant throughput reduction. Besides, the OFDM receivers using the ICI estimation rely on a large-dimensional matrix inverter with high computational complexity especially for many subcarriers such as digital video broadcasting (DVB) systems and wireless metropolitan-area networks (WMAN). To the best of our knowledge, no semi-blind ICI equalization has been addressed in the existing literature. Thus, in this paper, we propose a novel semi-blind ICI equalization scheme using the joint multiple matrix diagonalization (JMMD) algorithm to greatly reduce the intercarrier interference in OFDM. However, the well-known phase and permutation indeterminacies emerge in all blind equalization schemes. Hence we also design a few OFDM pilot blocks and propose an iterative identification method to determine the corresponding phase and permutation variants in our semi-blind scheme. Our semi-blind ICI equalization algorithm integrating the JMMD with the additional pilot-based iterative identification is very promising for the future high-throughput OFDM systems. Through Monte Carlo simulations, the QPSK-OFDM system with our proposed semi-blind ICI equalizer can achieve significantly better performance with symbol error rate reduction in several orders-of-magnitude. For the 16QAM-OFDM system, our scheme can also improve the performance over the plain OFDM system to some extent.

Orthogonal frequency division multiplexing (OFDM) has been applied in the current wireless local-area networks and digital video broadcasting since it is robust against the frequency-selective channels. However, there is still a crucial intercarrier-interference (ICI) problem due to Doppler effect, local frequency drift and sampling clock offset, associated with OFDM systems. Recently ICI self-cancellation schemes have been proposed to significantly reduce the ICI and empirically they greatly outperform the convolutional coding schemes adopted by the IEEE 802.11 standard. However, all existing ICI self-cancellation receivers are still sensitive to the phase and amplitude ambiguities due to the phase offset, the local oscillator frequency drift and the multipath reflections. Therefore, in this paper, a novel receiver design integrating the ICI self-cancellation with a proposed dynamic phase and amplitude estimation mechanism is introduced, which can well solve the ambiguity problem. The Monte Carlo simulation results show that our phase and amplitude estimators can greatly decrease the error probability for the final symbol detection in the ICI self-cancellation OFDM receivers.

OFDM has the advantage over the conventional single-carrier modulation schemes in the presence of frequency-selective fadings. Nevertheless, intercarrier-interference (ICI) due to Doppler frequency drift, phase offset, local oscillator frequency drift, and sampling clock offset will be a severe problem in the wireless OFDM systems. Previous ICI self-cancellation coding schemes can greatly reduce the ICI, but they are very sensitive to the phase ambiguity, which is due to the composite effect of the phase offset, the multipath fading and the local oscillator frequency drift. In this paper, a novel receiver which combines the current ICI self-cancellation coding techniques with a new pilot-free joint phase/amplitude estimation and symbol detection scheme is proposed. Based on the energy modulation or the irregular symbol constellation, our new technique does not have any requirement of pilot symbols and it can operate on all kinds of phase error ranges. The proposed scheme is promising in comparison with other existing methods at different noise levels through OFDM simulations.

OFDM has been applied in the current wireless local-area networks and digital video broadcasting systems since it has the advantage over the conventional single-carrier modulation schemes when the frequency-selective fadings are present. Nevertheless, intercarrier-interference (ICI) due to Doppler frequency drift, phase offset, local oscillator frequency drift, and multipath fading will be a severe problem in OFDM systems. Previous ICI self-cancellation coding schemes can greatly reduce the ICI, but they are very sensitive to the phase ambiguity, which is due to the composite effect of the phase offset, the multipath fading and the local frequency drift. In this paper, the phase ambiguity and amplitude ambiguity problems in ICI self-cancellation coded OFDM receivers will be formulated and discussed. Then, a novel receiver which combines the current ICI self-cancellation coding techniques with a new expectation-maximization-based joint phase/amplitude estimation and symbol detection scheme is proposed. The outstanding performance of this proposed scheme is shown and compared with other existing methods at different noise levels through OFDM simulations.

Due to the superior spectral efficiency and robustness over the multipath channels, orthogonal frequency division multiplexing (OFDM) has served as one of the major modulation schemes for the modern communication systems. In the future, the wireless OFDM systems are expected to operate at high carrier-frequencies, high speed and high throughput mobile reception, where the fast time-varying fading channels are encountered. The channel variation destroys the orthogonality among the subcarriers and leads to the intercarrier interference (ICI). ICI poses a significant limitation to the wireless OFDM systems. In this paper, we develop a pilot-aided minimum mean square error (MMSE) ICI coefficient estimation algorithm for combating the rapid time-variant channels. Our MMSE ICI estimator utilizes the channel correlation properties. Since the channel statistics are usually unknown, we also investigate the mismatch between the underlying channel model and the actual channel statistics. Monte Carlo simulation results show that the achieved mean square ICI estimation error mostly depends on the number of pilots instead of such a mismatch. In addition, a MMSE equalizer incorporated with our ICI coefficient estimation scheme can greatly improve the symbol error rate in the rapid time-variant channels with multiple Doppler frequencies over the conventional one-tap equalizer.

Intercarrier interference is deemed as one of the crucial problems in the wireless orthogonal frequency division multiplexing (OFDM) systems. The conventional ICI mitigation schemes involve the frequency-domain channel estimation or the additional coding, both of which require the spectral overhead and hence lead to the significant throughput reduction. Besides, the OFDM receivers using the ICI estimation rely on a large- dimensional matrix inverter with high computational complexity especially for many subcarriers such as digital video broadcasting (DVB) systems and wireless metropolitan-area networks (WMAN). To the best of our knowledge, no semi-blind ICI equalization has been addressed in the existing literature. Thus, in this paper, we propose a novel semi-blind ICI equalization scheme using the joint multiple matrix diagonalization (JMMD) algorithm to greatly reduce the intercarrier interference in OFDM. However, the well-known phase and permutation indeterminacies emerge in all blind equalization schemes. Hence we also design a few OFDM pilot blocks and propose an iterative identification method to determine the corresponding phase and permutation variants in our semi-blind scheme. Our semi-blind ICI equalization algorithm integrating the JMMD with the additional pilot-based iterative identification is very promising for the future high- throughput OFDM systems. Through Monte Carlo simulations, the QPSK-OFDM system with our proposed semi-blind ICI equalizer can achieve significantly better performance with symbol error rate reduction in several orders-of-magnitude. For the 16QAM-OFDM system, our scheme can also improve the performance over the plain OFDM system to some extent.

Orthogonal frequency division multiplexing (OFDM) employs a set of subcarriers for the information symbol transmission in parallel via the multipath fading channels. Due to its spectral efficiency and robustness over multipath channels, OFDM has been adopted for broadband communications. However, for high-speed mobility, the time-varying channels pose a performance limitation to the wireless OFDM systems. In this paper, we investigate the OFDM system performance in the time-varying channels where the intercarrier interference (ICI) occurs; we study the impacts of the time-varying multipath fadings together with the multiple Doppler spreads. According to our analysis and simulation results, the maximum OFDM symbol normalized Doppler frequency must be less than 0.04 to achieve the signal-to-interference ratio (SIR) = 20 dB or larger. As the maximum OFDM symbol normalized Doppler frequency increases, the OFDM system performance degrades dramatically. Different irreducible symbol error rate (SER) floors about 10-2 for QPSK-OFDM schemes and 10-1 for 16QAM-OFDM schemes would arise in case that the maximum OFDM symbol normalized Doppler frequency is fixed at 0.06

OFDM has been widely applied in current wireless local-area networks and digital video broadcasting since it is robust in frequency-selective channels. However, there still exists a crucial intercarrier-interference (ICI) problem due to the Doppler effect, local frequency drift and multipath fading, in OFDM systems. ICI self-cancellation schemes have been proposed to significantly reduce the ICI and empirically they greatly outperform the convolutional coding schemes in the IEEE 802.11 standard. However, all current ICI self-cancellation receivers are still sensitive to the phase ambiguity due to the frequency offset, the local oscillator frequency drift and the multipath reflections. Therefore, in this paper, we propose a robust ICI self-cancellation receiver with a novel dynamic phase estimation mechanism. The proposed scheme can well solve the phase ambiguity problem and the Monte Carlo simulation results show that our phase estimator can greatly reduce the bit error rates for the final symbol detection.