Kohji Itoh

Viterbi decoding algorithm for convolutional codes with repeat request

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.

New estimation and equalization approach for OFDM under Doppler-spread channel

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.

A Study on Iterative Impulse Noise Reduction in OFDM Signal by Recovering Time Domain Samples

In OFDM transmission, performance is degraded by the existence of time domain impulse noise because its influence spreads over the whole carriers by the DFT operation in the receiver. Especially influence of the impulse noise whose amplitude is large is very severe. In this paper, the scheme of improving influence of impulse noise in OFDM transmission is proposed. In the proposed scheme, the symbols after decision are remodulated to generate a time domain signal to replace the samples that are degraded by the impulse noise in the time domain. The time domain signal where the influence of the impulse noise is reduced is demodulated again. This results in the improvement of bit error rate. Moreover, the performance of the proposed scheme can be more improved by iteratively applying the proposed scheme

An analysis of incident information transmission performance using MCS/CDMA scheme

Incident information transmission using an IVC (inter-vehicle communication) system is one of the most important factors in ITS in order to increase safety and efficiency of road traffic. In IVC systems that use a spread spectrum communication (SS) scheme, when communication traffic goes into crowded conditions due to an increase of density of vehicles and IVC traffics, communication resources beyond the capacity of the system are sometimes required and this leads to unstable communication. Under this situation, the incident information that must be transmitted immediately is afraid to be jammed by excessive IVC traffics and relay transmission of the incident information may be intercepted. In this paper, an IVC system based on slotted MCS/CDMA scheme is proposed in order to avoid such a situation. The proposed system prepares an exclusive time slot for incident information in order to prioritize incident information from the general IVC packets that are less important than the incident information packets. As the result, it is considered that the possibility of interception of relay transmission can be decreased. Since the proposed system prepares an exclusive slot of incident information, there is a disadvantage where IVC throughput is reduced by an addition of an exclusive slot. In this paper, the performance of the proposed system is examined by computer simulation and optimization of parameters is discussed.

A study on interference detection using AIC for UWB MB-OFDM systems

In this paper, we propose a new method to detect the presence of other wireless systems on the UWB band. In our proposed algorithm, the fittest interference model is selected from the assumed several models based on the akaike information criterion (AIC) which is the estimate of a measure of fit of the model. Using the AIC, we can improve the goodness of fit of the model by the number of free parameters in comparison with the maximum likelihood. Moreover, the expectation maximization (EM) algorithm is introduced into the proposed algorithm to estimate the transmitted signals and the interference signals simultaneously. By computer simulations, we show that the proposed algorithm provides valid detection techniques.

Improving influence of impulse noise to OFDM signal by recovering time domain samples

In OFDM transmission, performance is degraded by the existence of time domain impulse noise because its influence spreads over the whole carriers by the DFT operation in the receiver. Especially influence of the impulse noise whose amplitude is large. In this paper, the scheme of improving influence of impulse noise is proposed. In the proposed scheme, the symbols after decision are remodulated to generate a time domain signal to replace the samples that are degraded by the impulse noise in the time domain. The time domain signal where the influence of the impulse noise is reduced is demodulated again. This results in the improvement of bit error rate. Moreover, the performance of the proposed scheme can be more improved by iteratively applying the proposed scheme.

A Study on the Detection Scheme of WiMAX signal for DAA Operation in MB-OFDM

In this paper, the detection scheme of the WiMAX signal using the MB-OFDM receiver is studied in order to realize DAA (Detect and Avoid) operation. By comparing the estimated power of the WiMAX signal obtained from the FFT outputs corresponding to the WiMAX band with the background noise power, detection is performed. And, UWB has wideband than WiMAX, therefore the carrier elements of WiMAX are grouped in the FFT outputs. It is assumed that the FFT outputs correspond to the WiMAX signal is approximated by mutually independent Gaussian random variables using the central limit theorem, and the Power detection scheme is analyzed. Moreover, the scheme that detects correlation of the guard interval of WiMAX signal is proposed. Performances of these schemes are compared and it was confirmed that it is possible detect the WiMAX signal whose level is low by detecting its guard interval.

An analysis of incident information transmission performance using an IVC system that assigns PN codes to the locations on the road

Incident information transmission using an IVC (Inter-Vehicle Communication) system is one of the most important features in ITS in order to increase safety and efficiency of road traffic. In IVC systems that use a spread spectrum communication (SS) scheme, it is necessary for each vehicle to know the PN code used to receive information and more than two equivalent PN codes must not be used in one communication area to avoid interference. Therefore, an appropriate scheme to assign a PN code to each communication under limited number of available PN codes. Especially it is necessary to assign PN codes efficiently to transmit incident information because it is very important for safety and many information transmissions are expected under an incident situation. Performance of incident information transmission based on the IVC system that assigns a PN code used by each vehicle to the absolute location on the road is analyzed. As the result of computer simulations, good performance is confirmed.

Improving Communication Performance of DS/SS Inter-Vehicle Communication System using MMSE Adaptive Array Antenna for Transmission and Reception

Recently, many studies have been being made on ITS (intelligent transport system) and IVC (inter vehicle communication) is one of the most important research topics. Improvement of communication is essential for IVC system which communicates incident information and accident information. In IVC, many studies are focused on DS/SS CDMA (direct sequence / spread spectrum, code division multiple access) scheme. In this paper, an application using the DS/SS CDMA scheme is studied. In DS/SS CDMA scheme, the packet error rate characteristic sometimes degrades under the influence of the near-far problem situation and the increase of PN code interference due to the number of vehicles. In order to improve this degradation, an IVC system that divides the receiving direction is proposed and its performance is evaluated by computer simulation. As the results of simulation good performance was confirmed.

A Study on Improving Performance of Pre-Post-RAKE Combining in UWB-IR System

The purpose of this study is to analyze and improve performance of pre-post-RAKE combining in UWB-IR system. In UWB-IR system, it is easy and effective to improve communication quality by means of the path-diversity techniques such as RAKE combining. However, combining many paths requires large amount of calculations and this leads to the increase of the hardware complexity. This complexity may spoil the advantages of UWB-IR system in hardware implementation. Therefore, this research has focused on the division of RAKE combining to the transmitter and the receiver using Pre-Post-RAKE to improve this complexity problem and to maintain high quality communication. As seen in the result, there are some good pre-post combinations based on system parameters and an accuracy of channel estimation. Especially it is shown that the case of using channel estimation over 10 symbols is enough to achieve the good BER performance. Moreover, the results show that IPI must be taken care. The results also show that complex channel estimation scheme and other complex combining scheme that make the UWB-IR system complex are not always necessary