Ming-Chien Tseng

An Equivalent Channel Time Variation Mitigation Schefme for ICI Reduction in High-Mobility OFDM Systems

In time-varying channel, the Doppler effect destroys the subcarrier orthogonality of orthogonal frequency-division multiplexing (OFDM) system which leads inter-carrier interference (ICI). This paper presents a time domain ICI self-cancellation method from the view of equivalent channel time variation mitigation. By utilizing the assumption of linear time-varying channel model, a set of windowing coefficients is derived to mitigate the equivalent channel time variation of the corresponding part into flat. The ICI is therefore reduced by lessening the equivalent channel time variation. The proposed window shape is found as the Franks window. The theoretical ICI power and bit error rate (BER) performance for using the proposed window shape are investigated and validated by simulation results. Comparing to Changs equivalent windowing method and the 4-step window shape for low-complex windowing method, the proposed method can provide better ICI cancellation performance given with the same criterion.

Field trial results for integrated WiMAX and radio-over-fiber systems on high speed rail

As high speed rails become more and more popular, providing broadband internet access on high speed rails is not widespread due to the large inter-carrier interference, terrestrial obstacles and inherent carriage shielding. In this paper an integrated Worldwide Interoperability for Microwave Access-Radio over Fiber (WiMAX-RoF) system is proposed and deployed along Taiwan High Speed Rail (THSR) rail tracks. Field trial and laboratory experiment results are presented. According to the field trial results, excessive delay induced by unequal fiber length deteriorates the throughput performance and can be overcome by equalizing the fiber length. On the other hand, rate adaptation for maximum throughput is challenging at high mobility. Careful calibration is needed to tradeoff the throughput gain against robustness to signal fluctuations. Laboratory experiments distinguish the impact of echo channel and remote antenna unit (RAU) distance from others. The results give insights in the analysis of field trial results.

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.

Performance analysis of M-ary PSK adaptive modulation system over Rayleigh-lognormal fading channel

An analytic study applying an M-ary PSK adaptive modulation scheme in a Rayleigh-lognormal fading channel is proposed. The closed-form expressions of the average BER and the average data throughput (BPS) of the proposed adaptive modulation system with constant transmitted power over the fading channel are presented. The effect of shadowing on BER and throughput in the adaptive modulation system is also investigated. The results show analytical expressions for the average BER and data throughput derived for transmission over a Rayleigh-lognormal fading channel. Our analyzed results show good agreement with Monte Carlo simulation ones. The proposed method is useful in enhancing the data throughput of a communication system, such as a third generation wireless communication system, where a huge amount of bandwidth is needed for high-speed multimedia services.

Link adaptation of MIMO-OFDM systems using hidden Markov model for high speed railway

A link adaptation scheme for MIMO-OFDM systems over high speed rail environment is addressed in this paper. In this study, a selection method of MIMO transmission mode and the modulation and coding scheme (MCS) according to the Rician channel K-factor from our previous work is considered. A hidden Markov model of time-varying Rician channel K-factor is constructed by using the real measured channel data on the high speed rail train. With the constructed hidden Markov model, the time correlation of Rician channel K-factor can be obtained and the time-varying K-factor value can be generated. By using the hidden Markov model, the K-factor value can be predicted to facilitate link adaptation since the K-factor may vary during feedback, especially in high mobility scenario. Simulation results show an average throughput improvement of 5Mbps.

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.

Interference Suppression for Railway Radio-over-Fiber Communication Systems

Radio-over-fiber (RoF) is a promising technology for railway communication systems. The base station (BS) transmits to and receives from the mobile station (MS) through geographically distributed remote antenna units (RAU), which are individually connected to the BS via fiber of different length. Unequal fiber length may cause severe inter-symbol interference (ISI) beyond the system capability and cause disconnection. We propose an RAU power switching method, which adjusts the RAU power according to MSs position on the rail to suppress the ISI. We further consider two MSs crossing each other and extend the RAU power switching method for this case. The effectiveness of the proposed method is verified by simulation results conducted via practical BS/MS and channel emulator.

Adaptive coding and modulation scheme for satellite‐UMTS TDD systems based on a photogrammetric channel estimation method

The conventional wireless communication systems are designed to overcome the worst-case channel, using the huge amount of redundant bits to assure communications performance and quality of services. Those systems cannot achieve the optimum spectrum and power efficiency. This paper presents an adaptive coding and modulation scheme used in the user terminals of the third-generation satellite communication system. A three-state photogrammetric channel estimation method is introduced for tracing the variations of large-scale environments. The mobile user terminal dynamically switches the suitable coding and modulation schemes according to the result of photogrammetric channel estimator in order to maximize the power efficiency and data throughput. The real measurement data were used to validate our proposed method. The results show that the proposed method not only reduces the system complexity, but also mitigates the power control requirements and increases the data throughput for the land mobile satellite personal communication systems. Copyright

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).