High-speed Mobile Environment Research Articles

Statistical Channel Modeling for Aeronautical Cognitive Radio Communications

Due to the high speed mobile environment, the aeronautical Cognitive Radio (CR) communications base on the channel with the time-variant stochastic non-continuous spectrum. The traditional fading channel models, such as Rayleigh, Rice, Nakagami-m and multipath channel models, can not describe the whole property of the channels of CR communications. In this letter, the statistical channel modeling scheme for aeronautical CR communications is proposed with a M/M/s(1) queuing model, which properly describes the random spectrum occupation of the primary users, i.e. aircrafts in aeronautical communications. The proposed channel model can be easily utilized in the channel simulation to testify the validity and efficiency of the aeronautical CR communications.

Multiple Antennas Angle Domain Resolution Based Transceiver Algorithm for High-speed Mobile Environment

Multiple Antennas Angle Domain Resolution Based Transceiver Algorithm for High-speed Mobile Environment

고속 철도 환경에서의 디지털 역지향성 안테나 기술 성능 분석

고속 철도나 자동차와 같은 고속의 이동 환경에서 고속 데이터 전송을 위해서는 빠른 빔 추적 능력이 중요하다. 디지털 역지향성 안테나는 수신 신호의 방향에 대한 사전 정보 없이, 안테나 위상을 변화시킴으로써 신호를 수신된 방향으로 재전송을 할 수 있기 때문에 자동적으로 빔 추적이 가능하다. 또한, 디지털 역지향성 안테나는 아날로그 역지향성 안테나와 비교하여 수정과 업그레이드가 쉽다는 장점을 가진다. 본 논문에서는 AWGN과 다중 신호 환경에서 디지털 역지향성 안테나의 BER 성능을 분석하였다. 시뮬레이션 결과, 위상 검출기를 사용하여 역지향성 안테나의 정확한 위상 추적 및 위상 공액을 하는 것을 확인하였다. 두 번째 안테나의 위상 지연이 <TEX>$15^{\circ}$</TEX> 발생할 경우에 위상 공액 기법을 사용하게 되면 사용하지 않았을 경우에 비해서 전력 효율이 1 dB 향상된다. 본 논문에서는 다중 신호가 존재할 경우에 역지향성 안테나에서의 성능을 논의하였다. 다중 신호나 간섭신호가 존재할 경우, 역지향성 안테나에서 직접파의 방향에 대해서 진폭 또는 위상 오차가 발생함을 확인할 수 있다. Fast tracking is important for high-speed data transmission in high-speed mobile environment such as high speed rail and vehicular. Digital retrodirective array antenna is possible to do automatically beam tracking because it can control the phase information of the output signal toward opposite direction to input signal without no a priori knowledge of the arrival direction. Also, Digital retrodirective array antennas has merit that it is easy to upgrade and modify compare with analogue retrodirective array antennas. In this paper, we analyze the BER performance of digital retrodirective array antenna under AWGN environment and multipath signal. Simulation results show correct phase estimation and conjugation of retrodirective array antenna by using phase detector block. Also, phase conjugation technique has better BER performance about 1 dB at source than that of without phase conjugation when phase lag is <TEX>$15^{\circ}$</TEX> in AWGN environment. This paper also discusses effect of the presence of multipath signal. Phase and amplitude error about direction of direct signal occurs when retrodirective array system is affected by interference and multipath signal in the presence of multipath signal.

Channel estimation in OFDM systems operating under high mobility using Wiener filter combined basis expansion model

In this article, we first thoroughly analyze Wiener filter combined least squares based channel estimation (WF-LS) and then illustrate its limitation in high-speed mobile environments. Based on the analysis, we propose to combine WF with basis expansion model (BEM) based channel estimation to deal with channel estimation in various mobile environments, especially in high-speed cases. The expression for Wiener filter combined BEM based channel estimation (WF-BEM) is derived and the result explicitly considers the effect of intercarrier interference (ICI) that occurs in orthogonal frequency division multiplexing (OFDM) systems when operating under high mobility. The simulation results demonstrate that our proposed WF-BEM is better than WF-LS in time-varying channels, and the performance improvement is significant especially in fast time-varying channels.

Time-Frequency Training OFDM with High Spectral Efficiency and Reliable Performance in High Speed Environments

Orthogonal frequency division multiplexing (OFDM) is widely recognized as the key technology for the next generation broadband wireless communication (BWC) systems. Besides high spectral efficiency, reliable performance over fast fading channels is becoming more and more important for OFDM-based BWC systems, especially when high speed cars, trains and subways are playing an increasingly indispensable role in our daily life. The time domain synchronous OFDM (TDS-OFDM) has higher spectral efficiency than the standard cyclic prefix OFDM (CP-OFDM), but suffers from severe performance loss over high speed mobile channels since the required iterative interference cancellation between the training sequence (TS) and the OFDM data block. In this paper, a fundamentally distinct OFDM-based transmission scheme called time-frequency training OFDM (TFT-OFDM) is proposed, whereby every TFT-OFDM symbol has training information both in the time and frequency domains. Unlike TDS-OFDM or CP-OFDM where the channel estimation is solely dependent on either time-domain TS or frequency-domain pilots, the joint time-frequency channel estimation for TFT-OFDM utilizes the time-domain TS without interference cancellation to merely acquire the path delay information of the channel, while the path coefficients are estimated by using the frequency-domain grouped pilots. The redundant grouped pilots only occupy about 3% of the total subcarriers, thus TFT-OFDM still has much higher spectral efficiency than CP-OFDM by about 8.5% in typical applications. Simulation results also demonstrate that TFT-OFDM outperforms CP-OFDM and TDS-OFDM in high speed mobile environments.

TDS-OFDMA: a novel multiple access system based on TDS-OFDM

As the key technology of Chinese national digital television terrestrial broadcasting (DTTB) standard, time domain synchronous OFDM (TDS-OFDM) utilizes known training sequence instead of cyclic prefix (CP) as the guard interval between adjacent inverse discrete Fourier transform (IDFT) blocks to achieve better system performance. Until now, TDS-OFDM was primarily used in downlink broadcasting. When TDS-OFDM is directly used in the uplink, it is difficult to eliminate the superposed interferences caused by different training sequences and IDFT blocks from multiple users. To solve this problem, a novel scheme called time domain synchronous orthogonal frequency division multiple access (TDS-OFDMA) is proposed in this paper. The novel space-time frame structure of the TDS-OFDMA scheme is proposed to achieve joint cyclicity reconstruction as well as joint channel estimation at the receiver. Simulation results show that the proposed scheme can achieve better bit error rate (BER) performance than conventional OFDMA system under highspeed mobile environments.

Carrier aggregation for LTE-advanced mobile communication systems

In order to achieve up to 1 Gb/s peak data rate in future IMT-Advanced mobile systems, carrier aggregation technology is introduced by the 3GPP to support very-high-data-rate transmissions over wide frequency bandwidths (e.g., up to 100 MHz) in its new LTE-Advanced standards. This article first gives a brief review of continuous and non-continuous CA techniques, followed by two data aggregation schemes in physical and medium access control layers. Some technical challenges for implementing CA technique in LTE-Advanced systems, with the requirements of backward compatibility to LTE systems, are highlighted and discussed. Possible technical solutions for the asymmetric CA problem, control signaling design, handover control, and guard band setting are reviewed. Simulation results show Doppler frequency shift has only limited impact on data transmission performance over wide frequency bands in a high-speed mobile environment when the component carriers are time synchronized. The frequency aliasing will generate much more interference between adjacent component carriers and therefore greatly degrades the bit error rate performance of downlink data transmissions.

고속 모바일 네트워크 환경에서의 모바일 콘텐츠 발전방향에 관한 연구

This study focused on deriving successful strategies as well as presenting the developing type of mobile contents under the coming high-speed network environment by analyzing changing environment, using type and influencing factors including the evolution process and technology evolution of mobile contents. Mobile contents classifying systems are variously presented because of huge amount of mobile contents, variety of classifying method and viewpoint of research company. However, presented contents classifying systems so far looked variously, almost all of systems have classified based on 3 attributes-Communication, Informaton and Entertainment. As research showed that personalized entertainment contents and communication contents are mainly used and users want to use life centric service such as mobile commerce and location based contents, the need of customers is changing into the suitable contents of mobile environment. The influencing factors for the developing direction of mobile contents are the development of mobile technology and the function of mobile phone, the cost of service and the change of socio-cultural environment. Mobile contents are activating and growing by entertainment pursuit, the real time information acquiring of user, the evolving mobile phone as a high effectiveness multimedia instrument, increasing data speed and commercializing of two way streaming technology based on the development of network technology. As mobile contents is changing as suitable for mobile environment and converged type of various contents, this study reconstructed mobile contents classifying system such as communication-centric, information-centric, entertainment-centric and economy-centric. It constructed and proposed mobile contents type which is useful for high-speed mobile network environment as messenger integrating service type, multimedia integrating service type, LBS integrating service type, mobile commerce integrating service type and portal service type based on the analyzed result of mobile using analysis and influencing factors. It also proposed following considering items for developing mobile contents. First, mobile contents are suitable for the none user of mobile contents and middle aged user. Second, the planning and producing of useful and differentiated contents through understanding customers' requirement are needed. Third, comparative contents which have creativeness and differentiation are considered. Fourth, easy interface for user is presented. This study implies that the producing of useful and comparative mobile contents would be possible through amalgamating industrial, technical and user focused approach. It will be expected to use and enjoy better contents under high speed mobile network environment, if the results of study are referred to establish and examine policies for activating data service.

Adaptive CI-OSDM in Time-Frequency Selective Fading Channel

Orthogonal Signal Division Multiplexing (OSDM), also known as SD-OFDM, has been proposed for information transmission with high spectrum efficiency. In this paper, a new signal construction method named Adaptive Carrier Interferometry OSDM (ACI-OSDM) is proposed for time-frequency selective fading channel. Particularly, the Adaptive CI codes originated from CI-OFDM are employed in the frequency domain of OSDM signal. Compared with traditional OFDM, the ACI-OSDM improves the performance considerably of broadband transmission, i.e., spectrum efficiency, Peak-to-Average Power Ratio (PAPR) mitigation and interference cancelation in the high speed mobile environment with multipath emission, e.g. super express train with speed more than 250km/h.

Doppler Diversity in MC-CDMA Systems with T-ZCZ Sequences for Doppler Spread Cancelation

Doppler diversity has been proven effective to combat time variation caused by Doppler spread in single carrier systems. However, it is not efficient to directly apply Doppler diversity into Multi-Carrier Code Division Multiple Access (MC-CDMA) systems because Inter-Carrier-Interference (ICI) increases with the artificial frequency shifts in diversity branches. In this paper, a novel Doppler diversity scheme in MC-CDMA with Three Zero Correlation Zones (T-ZCZ) sequences is proposed to further improve the performance of Doppler diversity. Particularly, zero correlation zones are employed in frequency domain for ICI cancelation caused by Doppler spread, which confirms the validity of the contribution to the wideband wireless communications in high speed mobile environment.