As an extension of the paper [1], we propose in this paper a multi-carrier CDMA modulation system for cellular mobile communication with two-way transmitter-receiver broadband far-field antenna array adaptation to the propagation path characteristics. We assume the frequency band is shared between the two directions and the same array antenna is used at each station for transmission and reception. Also assumed is error-free measurement of the transmittance from each of the transmitters to the array elements of the receivers. In the uplink, power control is employed at the side of the mobile transmitters based on the downlink reception. Based on the path model of propagation in the analysis, it is shown that the array-carrier MLE (Maximum Likelihood Estimate) reception of the code-division multiplexed symbols realizes array-enhanced bandwidth-limited Rake reception. It was confirmed by simulation, assuming independently Rayleigh-distributed path model, that the system, both in downlink and in uplink, gives much better BER versus SNR per bit performance than that of the carrier-only MLE reception employing transmitter-directed antennas. Because of the optimality of the reception, the transmitter array adaptation has nothing to do but optimally to deliver transmitted power to the paths so as the total expected arriving power at the receiver be maximum. Owing to the reciprocity of propagation by using the same antenna array for reception and transmission, the transmitter array adaptation can make use of the measurement, at the receiver, of the transmittance with omni-directional transmission to derive the optimum transmitter array weighting. The algorithm achieves two way uplink downlink optimization without interchanging any measured information.

We propose in this paper a two-way wireless OFDM multi-carrier code-division multiplexed modulation with optimum transmitter-receiver broadband far-field antenna array adaptation to the propagation path characteristics. The frequency band is shared between the two directions with TDD and we assume the same array antenna is used at each station for transmission and reception. Employing a path model in the analysis, and assuming error-free measurement of the transmittances from the center of the transmitter station to the array elements of the receiver station, it is shown that the array-carrier MLE (Maximum Likelihood Estimate) reception of the code-division multiplexed symbols discriminates the paths with delay time difference greater than the inverse of the signal bandwidth or with sufficiently different directions of arrival. And as a result bandwidth-constrained path-wise matched reception is realized. It was confirmed by simulation assuming Rayleigh (not Rice) path model that the system gives substantially higher BER versus SNR per bit performance than that of the carrier-only MLE reception employing an omni-directional antenna or an array antenna only for the purpose of directivity. Because of the optimality of the reception the transmitter array adaptation has nothing to do but optimally to deliver transmitted power to the paths so as the total expected arriving power at the receiving station be maximum. Owing to the reciprocity of propagation by using the same antenna array for reception and transmission, the transmitter array adaptation makes use of the measurement, at the receiver, of the transmittance to derive the optimum transmitter array weightings. The algorithm at each station, achieves two way optimization without interchanging any data to be obtained by the channel measurement in the pilot periods.

In this paper, an iterative detection scheme of pre-coded OFDM is proposed. The proposed scheme recovers the pre-coded data symbols correspond to the carriers whose reception powers are degraded by frequency selective fading by using the data symbols after decision and the data symbols are detected again using the recovered pre-coded data symbols. As the result of numerical analysis, it is confirmed that better performance under severe frequency selective fading channels can be achieved by iteratively repeating the proposed scheme combined with convolutional error correcting code.

In this paper, an iterative detection scheme of precoded OFDM is proposed. The proposed scheme recovers the precoded data symbols correspond to the carriers whose reception powers are degraded by frequency selective fading by using the data symbols after decision and the data symbols are detected again using the recovered precoded data symbols. As the result of numerical analysis, it is confirmed that better performance under severe frequency selective fading channels can be achieved by iteratively repeating the proposed scheme

In the communication systems using OFDM, it is necessary to compensate the influence of frequency selective fading caused by multi-path. In this paper we propose a MAP data reconstruction from the data symbols spread in the frequency domain in OFDM transmission. The performance of the proposed scheme is examined by computer simulation and good performance is confirmed.

In OFDM transmission, a loss of sub-carrier orthogonality due to Doppler-spread of the channel leads to inter-carrier interference (ICI). We have proposed a method to reduce the ICI caused by the Doppler spread of the channel. The transmission channel is modeled by a combination of multiple Doppler-shifted propagation paths, and their parameters such as attenuation, relative delay and Doppler-shift are estimated using scattered pilot symbols. The ICI is canceled by multiplying the inverse matrix of the estimated channel matrix with the received OFDM symbol vector. However, the ICI canceller causes enhancement of noise component, so it can't improve the bit error rate characteristics sufficiently. In this paper, we propose the new ICI canceller based on MMSE, which does not cause enhancement of noise component. As the result of computer simulation, it was confirmed that the ICI canceller based on MMSE could reduce the bit error rate more than ICI canceller based on zero-forcing under the Doppler-spread channel.

The paper proposes to develop the ability of planning in learning by problem-solving, providing the learners with a repertoire of problem types as constructs of plans and their expansions into sub plans as well as sample problems with solutions indexed by hierarchical use of plans and sub plans for solving the problems. The learners can construct their plans on the system-supported planning environment and retrieve solutions of sample problems by way of the structured solution plan indices. The system critiques their plans and checks, through interaction with the learners, the interim results of executing any subproblems in the solution path. A prototype system was implemented and tested in order to assess the significance of providing such an environment in developing ability of planning. The pre-post test evaluation in steps strongly suggested importance of providing sample problems with plan indices in order for the learners to be confident of applicability of newly learned plans.

In the OFDM SFN (single frequency network) system, where the received signal from the main broadcast station is rebroadcast at the relay station, the area where the delay spread of the received signal is longer than the guard interval length increases as compared to the SFN system where the signals are transmitted at the same time both from the main station and from the relay station. If the channel delay spread is longer than the guard interval length, inter-symbol interference (ISI) and inter-carrier interference (ICI) are generated at the receiver and the bit error rate (BER) characteristics are much degraded. Therefore, the service area of the OFDM SFN system where the signal is re-broadcast at the relay station is narrower that that of the OFDM system where the signals are broadcast at the same time from both stations. In order to improve this problem, an SFN system that rebroadcasts the received signal from the main station after adding an appropriate delay is proposed. We propose an array antenna receiver using a maximum ratio combining technique for the SFN system, where additive delay is inserted to the signal at the relay station. By theoretical analyses and computer simulations, better BER characteristics than the conventional system are confirmed.

In OFDM transmission, a loss of sub-carrier orthogonality due to Doppler-spread of the channel leads to intercarrier interference (ICI), especially under mobile reception environments. ICI causes significant degradation of bit error rate characteristics and this influence becomes larger as the carrier frequency or moving velocity of the receiver increases. In this paper, a method to reduce the ICI caused by Doppler-spread of the channel is proposed. In the proposed method, the transmission channel is modeled by a combination of multiple Doppler-shifted propagation paths and their parameters such as attenuation, relative delay and Doppler-shift are estimated using scattered pilot symbols. Then, ICI is canceled by multiplying the inverse matrix of the estimated channel matrix to the received OFDM symbol vector. As the result of computer simulation, it is confirmed that the proposed method can improve the bit error rate characteristics under Doppler-spread channel.

OFDM is widely known as a modulation scheme which can utilize the given frequency band very efficiently, even if the channel is distorted by multipath propagation. However, in order to improve the efficiency, a high-precision estimation technique is recommended. In this paper, we pay attention to the method which introduces a condition based on the propagation path model in order to obtain better precision. Although this method performs better than another conventional method especially under the fading environment, this method has large computational complexity. Hence, we simplify this method by using the FFT algorithm to make this method more effective. As a result, the proposed system performs better than the conventional system especially under fading environments, and the computational complexity is reduced.

As an application of NLP to computer-assisted language learning(CALL), we propose a diagnostic processing of Japanese being able to detect errors and inappropriateness of sentences composed by the students in the given situation and the context of the exercise texts. Using LTAG(Lexicalized Tree Adjoining Grammar) formalism, we have implemented a prototype of such a diagnostic parser as a component of a CALL system being developed.

An OFDM system is widely known as a modulation scheme, which can reduce the effect of frequency selective fading. However, under a large frequency selective fading environment, it is difficult to demodulate correctly even with appropriate equalization. In this paper, an OFDM system applying a frequency diversity scheme is proposed. In this system, the same symbols are transmitted on several sets of carriers, and at the receiver end, these symbols are combined so as to maximize the signal to noise ratio. As the experimental result, it is shown that as the fading level becomes larger, the proposed system provides a larger gain in the bit error rate characteristics than the conventional system.

A TDM wireless local area network system using a spread spectrum signal for its access control is analyzed. By using a spread spectrum control signal, it is possible to control the network system under severe environments where a TDM only system can't operate well. Moreover, in this system, the spread spectrum control signal is used to measure the channel characteristics between nodes and the information are used to equalize the TDM data transmission channel. This improves the performance of the adaptive equalizer and realize fast data transmission. The measured channel characteristics can be used as a criterion for routing operations.

Using the Viterbi decoding algorithm with repeat request for convolutional codes is proposed, and the resulting performance is analyzed by random coding and generating function arguments and by simulation. It is shown that the reliability function of the proposed decoding algorithm is asymptotically twice that of the Viterbi decoding algorithm without repeat request, and that in certain practical situations the proposed algorithm can save about 50 percent in constraint length over the ordinary Viterbi algorithm for a given performance.

A modified Schalkwijk-Barron transmission scheme is presented for channels with noiseless feedback. The modified scheme employs blockwise decision and a fixed length transmission in place of Viterbi's sequential decision feedback. The reliability functions of the modified scheme are derived for the additive white Gaussian noise (AWGN) channel and discrete memoryless channels (DMC's). For the AWGN channel the result obtained is asymptotically the same as in the case of the Schalkwijk-Barron scheme. On the other hand, the new result obtained for DMC's shows that the modified scheme also attains high reliability functions for DMC's.