The position and velocity information of high-speed trains (HSTs) are essential to passenger safety, operational efficiency, and maintenance, for which an accurate navigation system is required. In this paper, we propose a two-stage federated Kalman filter (TS-FKF) for an HST navigation system that uses multi-sensors, such as tachometer, inertial navigation system, differential GPS, and RFID, with a feedback scheme. However, the FKF with a feedback scheme often shows severe performance degradation in the presence of undetected large sensor errors. Tachometers often have large slip or slide errors during the train's acceleration, deceleration, and moving along a curved railway, and there are significant performance differences between different sensors. To make the proposed system robust to these errors, we propose a slip and slide detection algorithm for the tachometer and an adaptive information-sharing algorithm to deal with a large tachometer error and performance difference between sensors. We provide theoretical analysis and simulation results to demonstrate the performance of the proposed navigation system with the proposed algorithms.

Inter-carrier interference (ICI) and multiuser interference (MUI) occur in an uplink orthogonal frequency division multiple access system if a channel between a mobile user and a base station is rapidly time-varying. To reduce the ICI and MUI, this letter proposes a subcarrier allocation method. The key idea is to allocate a high-speed user's subcarrier near a low-speed user's one. Then, a wide Doppler spread of a high-speed user can be spilled over a low-speed user's subcarrier band with insignificant influence, and the signal-to-interference-plus-noise ratio (SINR) at each high-speed user's subcarrier can be maximized. This can provide a significant performance improvement of multiuser detection in an uplink-OFDMA.

As channel time-variation increases, orthogonality among subcarriers in an orthogonal frequency division multiplexing (OFDM) symbol can be destroyed because of the relatively long symbol period, whereupon intercarrier interference (ICI) appears, and hence an irreducible error floor occurs. It was shown recently that grouping pilot tones into a number of equally spaced clusters can yield better channel estimation against the doubly selective channel than placing each pilot tone in an equally spaced manner. However, a random pilot sequence was used in the literature, and an optimal sequence has not yet been studied. This paper presents how to optimize the clustered pilot sequence. First, this paper (a) proves the existence of an optimum pilot sequence and (b) suggests a guideline for finding an optimum sequence. Second, this paper shows that an optimum sequence is independent of the signal-to-noise ratio and Doppler rate, and the sequence is generally optimum for any channels. Third, this paper verifies through analysis and simulation that the coded-OFDM system with an optimized cluster pilot sequence can yield a smaller mean square channel estimation error and lower bit error rate than the system with an equidistance pilot or a random cluster pilot under a rapidly time-varying channel.

An inter-carrier interference (ICI) occurs in an orthogonal frequency division multiplexing (OFDM) system when a channel is rapidly time-varying. The proposed scheme employs a polynomial cancellation code (PCC) to suppress this ICI, and uses a linear complex field (LCF) code as a transmit-diversity technique to make the transmission rate compatible up to {nT} / {(nT+1)}, where nTis the number of transmit antennas used. The conventional orthogonal code (OC), such as the Alamouti code, with accompanying PCC cannot achieve this rate. Both analysis and simulation results verify that the proposed scheme can achieve higher channel capacity as well as lower bit error rate (BER) than the other schemes in rapidly time-varying channels, even with a linear receiver and inaccurate channel estimation.

This paper proposes a new inter-carrier interference (ICI) cancelling scheme for multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) in time and frequency selective fading channels. Based on the ICI distributions, we try to extend the conventional MIMO detection schemes. Nevertheless this extension increases the computational complexity significantly whereas the performance gain is not satisfactory. To improve the capability of ICI cancellation, we employ successive interference cancellation (SIC) method not in MIMO detection but in ICI cancellation. However, the processing delay time due to the SIC in OFDM will grow according to the increase of the number of subcarriers. To overcome this effect, we propose a modified SIC (MSIC) that can reduce the processing delay dramatically at the expense of some performance degradation. In simulation, we show the expense is so small that it can be ignored in severe time-selective channels.

This paper describes parameters design of multiuser OFDM on doubly selective fading channels. It is well-known that OFDM is vulnerable to time selectivity due to the extension of symbol period, generating inter-carrier interference (ICI). In this paper, we first derive the closed form of signal to interference plus noise ratio (SINR) for single user OFDM with a linear time-varying model. In multiuser case, we propose several design criteria depending on the channel environments. We also discuss an effect of subcarrier allocation on the parameters optimization in multiuser access systems.

OFDM can significantly reduce the receiver complexity by using one-tap equalizer in frequency selective channels, and it can be enhanced the capacity and diversity by using cooperative scheme. However, high data rate transmission with OFDM requires the channel state information (CSI), and therefore OFDM channel estimation for cooperative communication can be a challenging problem. In this paper, we investigate the OFDM channel estimation suitable for the amply-and-forward (AF) cooperative channel. Cramer-Rao lower bound (CRB) of the estimation is derived, and optimal preamble conditions that minimize the CRB with several power constraints are proposed. Moreover, minimum variance unbiased (MVU) estimator that achieves the CRB is introduced for the cooperative channel. Simulations with analytical results confirm the performance of our methods.

Orthogonal frequency division multiplexing (OFDM) can significantly reduce a receiver complexity using one-tap frequency domain equalizer in a frequency-selective fading channel. However, the channel variation due to high mobility gives a time-selectivity in one OFDM symbol. As a result, each OFDM subcarrier experiences inter-carrier interference (ICI) that makes the equalization process very complicate. We propose a low complexity ICI cancellation method that takes advantage of the ICI power distribution. Instead of computing the inversion of the whole channel frequency response (CFR) matrix, we split the matrix into several small sub-matrices that contain significant channel information and perform a linear minimum mean squared error (LMMSE) equalization. We also propose an efficient successive interference cancellation (SIC) method that can effectively utilize a time diversity resulting from the mobility. Simulation results show that the performance improvements is remarkable even though the sub-matrix size is much smaller than that of the whole CFR matrix while taking high computational efficiency.