Chorng-Ren Sheu

A Low-Complexity Concatenated ICI Cancellation Scheme for High-Mobility OFDM Systems

In this paper, a novel low-complexity scheme is proposed for current OFDM systems to reduce inter-carrier interference (ICI) caused by time-varying channel. Under the assumption that channel delay spread is smaller than the guard interval period of an OFDM symbol, the suitably selected inter-symbol interference (ISI) free received signals are exploited through compensating the corresponding linear phases and taking the average in the frequency domain, and in consequence some ICI terms can be automatically cancelled. However, the performance of this self-ICI cancellation scheme is restricted by the inadequate ISI-free region in current OFDM systems. Therefore, a concatenated scheme, which consists of the self-ICI cancellation scheme and a residual ICI reconstruction scheme, is proposed to further improve the BER performance. The simulation results are provided to support that the ICI effect can be significantly removed by the proposed low-complexity concatenated scheme for current high-mobility OFDM systems.

Low-Complex ICI Reduction Method by Applying Franks Window Coefficients in Linear Time-Varying Channel

In this paper, we consider the ICI effect induced by time-varying channel. A simple and low-complex method is proposed to suppress ICI by windowing the received signal using Franks window coefficients in time domain and combining the extended signal interval which was not interfered by multipath to the corresponding information part of the cyclic-prefix (cp) based OFDM system. The ICI are suppressed by equivalently reducing the channel time-variation according to the useful extended signal interval. If the channel time-variation is linear in one OFDM symbol and the useful extended signal interval period as long as the information part period, the equivalent channel time-variation of information part will turn into a constant which means no ICI occurs. And then, a N-point fast Fourier-transform (FFT) and one-tap equalizer is used for demodulation and channel equalization. The complexity of the proposed method is only N log N. The analysis of Franks window coefficients for equivalent channel mitigation in linear time-varying channel are investigated and simulation results are provided to validate the performance of the proposed method.

A Low Complexity Interference Suppression Scheme for High Mobility STBC-OFDM Systems

A novel low complexity scheme based on received data symbol pair reordering and optimal selection, is proposed for space time block coded (STBC) OFDM systems to suppress co-channel interference (CCI) and inter-carrier interference (ICI) caused by time-varying channels. The proposed scheme reorders the conventional received data symbol pairs to produce extra received data symbol pairs. Then, the STBC decoding is performed on all the received data symbol pairs, and the STBC decoding output signals are used to obtain multiple combined signals. Finally, the optimal combined signal with minimum interference is selected to suppress CCI and ICI, and the system performance will be improved as long as the channels are mutually uncorrelated. Our computer simulation results verify that the proposed low complexity scheme achieves good performance for high mobility 2×2 STBC-OFDM systems in the ITU Vehicular A (VA) channels.

A Differential Sliding Correlation Scheme for Symbol Timing Detection in Time Domain Synchronous OFDM Systems

In this paper, a new symbol timing detection scheme robust to carrier frequency offset is proposed for time domain synchronous (TDS) OFDM systems. The proposed method computes the sliding correlation from both the differential received signal and the differential PN sequence. Rapid symbol timing detection can be obtained based on the correlation peaks. As differential operation is adopted before sliding correlation in the proposed method, the time-dependent linear phase effect caused by the carrier frequency offset will not influence the correlation peaks. Nevertheless, the incorrect correlation peaks caused by the cross terms of the multipath may be detected. An ambiguity protection module is added to validate the correlation peak detection results and further improve the system performance. Our simulation results show that the proposed scheme for symbol timing detection is accurate and simultaneously robust to carrier frequency offset for TDS-OFDM systems in both a time-varying TU6 channel and a fixed two-path channel.

Correlated Scrambling Scheme for Time-Frequency Diversity in OFDM Single-Frequency-Network Systems

For orthogonal frequency division multiplexing- based single frequency network systems, such as the terrestrial digital broadcasting systems, the performance of a static/quasi- static receiver at cell edge can be severely degraded by the slow and flat fading effect due to a destructive combination of signals from adjacent transmitters. Recently, the cyclic delay diversity (CDD) and group-wise scrambling diversity (GSD) schemes have been presented to address this issue. However, both the CDD and GSD schemes provide no time diversity against the slow fading nature. Moreover, discontinuities of composite channel responses inherent in the GSD scheme make channel estimation even more difficult. In this paper, a new correlated scrambling diversity (CSD) scheme is proposed that introduces both time and frequency diversities via the time-frequency scrambling independently among transmitters. Continuity of composite channel responses resulting from utilization of correlated scrambling patterns further makes channel estimation easy for receivers.

Time-frequency scrambling scheme for improving cell-edge performance in multi-Cell multimedia broadcast multicast service

In this paper, we propose a novel transmission structure with the time-frequency scrambling scheme. The major concept of this technique is coming from combining the Toshibas approach and our proposed time-dimension group scrambling scheme. In this technique, several OFDM symbols and several sub- carriers can be grouped into a grid structure and then scrambled by a scrambling pattern, the time- frequency dimension diversity gains are both increased after channel decoder. The time-frequency scrambling scheme uses the random behaviors of transmitted signals from multiple cells to create the plentiful time and frequency channel diversity, indeed, control the coherent time and the interleaver length. Hence, the burst error due to the flat fading and the long-term shadowing fading can be mitigated and channel coding will work more efficiently. The simulation results show that the destructive interference can be improved at the cell-edge by the proposed scheme. The SNR gains can be derived by the proposed scheme for mutli-cell MBMS environments (5 dB and 1.5 dB for two equal-gain and TU6 channels, respectively).

A transmission architecture with scramble selection for improving cell-edge performance and reducing PAPR in multi-cell MBMS

To solve the cell-edge problem (deep and flat fading for long time) of multi-cell MBMS in SFN and reduce PAPR in OFDM systems simultaneously, a novel transmission architecture is proposed. Grid scrambling diversity scheme is adopted in the architecture to increase time and frequency diversity. Several grid scramblers are used and the time domain signal with smallest PAPR is selected for transmission. Some simulation results are provided to demonstrate the efficacy of the proposed architecture.

A Novel Low-Complexity Self-ICI Cancellation Scheme for High-Mobility OFDM Systems

In this paper, a novel low-complexity scheme is proposed for cyclic-prefix based OFDM systems to reduce inter-carrier interference (ICI) caused by time-varying channel. Under the assumption that channel delay spread is smaller than the guard interval (GI) of an OFDM symbol, the suitably selected inter-symbol interference (ISI) free received signals are exploited through compensating the corresponding linear phases and taking the average in the frequency domain. Therefore, some ICI terms can be automatically cancelled with less number of FFT operations. Moreover, the theoretical results of bit error rate performance are derived. The simulation results are shown that the ICI effect can be significantly removed by the proposed scheme for high-mobility OFDM systems when the ISI-free region q is adequate. Meanwhile, the proposed scheme has the advantages of increasing the mobile speed up to N/(N-</) times of that of the OFDM systems with N sub-carriers. For example, the proposed scheme can give two times mobile speed improvement when q =N/2 and GI=N/4, the extended length of GI is only required extra N/4.