Recently, QAM (Quadrature Amplitude Modulation) is paid to attention in digital modulation method, and it has been used for wireless LAN and digital broadcasting. QAM is a modulation method that puts information on carrier amplitude and phase.So QAM can achieve high-capacity data transmission that the use efficiency of frequency band is good. But QAM has a bad characteristic that is weak to the noise and the interference because distance between each symbol is short. On the other hand, CDMA (Code Division Multiple Access) can transmit two or more signals to the same frequency band, and this can be achieved by spread spectrum (SS). Moreover, because this is good also in the characteristic to noise, this is a technique widely used.Then we propose the method that not transmission data but CDMA signal, obtained after transmission data is modulated by spread code and multiplexed, is given to each symbol of QAM. Figure 1 shows Block diagram of CDMA-QAM method. According to this figure, in transmitter, the transmission data of the serial is converted into parallel data of the number of multiple-access. After that, this data is modulated by spread codes. And, when the number of multiple-access is 63, because 64 values from 0 to 63 are taken as amplitude of CDMA since each channel is added, this is allocated in each symbol of 64QAM. In receiver, received signal is first Fig. 1. Block diagram of CDMA-QAM method demodulated by QAM, this is demodulated by spread codes afterwards. Here, because not transmission data but amplitude of CDMA is allocated in each symbol of 64QAM in this method, if it is possible to recover by demodulation using spread codes even if there is an error in the symbol of QAM, it is possible to transmit correctly without mistaking the transmission data. In addition, when a highorder bit is allocated in I channel and the low-order bit is allocated in Q channel, because the influence that I channel has on BER characteristic is large, the constellation map of QAM is made the one that amplitude of I channel was enlarged more than that of Q channel.Basing on above, we studied about railway signal system on track circuit as a sample to apply this method, we named it CDMA-QAM rail transmission system. Figure 2 shows BER characteristic as the result of computer simulations. According to this figure, we verified that BER characteristic is able to be improved as the amplitude of I channel enlarged more compared with that of Q channel.In addition,we studied about the method of a synchronous acquisition. As the result of computer simulations,we established a good method by giving the same data for a synchronous acquisition to some channels.
In recent years, the environment of railways and the systems such as CBTC (communication based train control) have been changing. To respond the changes and the needs of customers, a UTCS (unified train control system) has been developed to realize a system that evolves with customers. Previous type systems consist of independent components such as ATC (Automatic train control) system, electronic interlocking system, and facility monitoring system, and there are a complicated overlap of system configurations and functions and difference in concept between the systems. On the other hand, the integrated train control system consists of horizontal layers such as function layer, network layer, and terminal layer. Therefore, the system has been developed to make it simple with no unnecessary redundancy and evolving to meet the needs of customers. In this paper, we explain a method that realizes the interlocking function for CBTC system in the function layer based on the concept of "securing a train travelling path" including path blocking and routing, and evaluate the safety of the method using STAMP/STPA.
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