Multiplex communications – Diagnostic testing – Determination of communication parameters
Reexamination Certificate
1998-08-31
2003-04-01
Nguyen, Chau (Department: 2663)
Multiplex communications
Diagnostic testing
Determination of communication parameters
C370S335000, C455S238100
Reexamination Certificate
active
06542471
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) radio communication system for mobile objects, in particular, a system suitable for high-speed mobile objects, such as a vehicle.
2. Description of the Related Art
Heretofore, digital mobile communication systems have been used for telephones and data transmission. In such conventional mobile communication systems, a TDMA (Time Division Multiple Access) system or an FDMA (Frequency Division Multiple Access) system is used. Thus, multiple access is realized by physically dividing a time period and a frequency.
Lately, the CDMA system has been proposed and its development is in progress. In the CDMA system, a plurality of mobile stations simultaneously use the same frequency band. Further, multiple access is realized by using codes based on spread spectrum communication technology. For example, in DS (direct sequence) type spectrum diffusion, the diffusion modulation and demodulation of communication signals is performed using PN (pseudorandom noise) codes. By allocating a different PN code to each user, it is possible for a plurality of users to perform their respective communication using frequencies in the same band.
Adoption of the CDMA system can lead to sharp improvement in frequency efficiency compared with the conventional systems. In other words, the number of mobile stations capable of being simultaneously connected with the base stations can be increased. Therefore, the CDMA system is expected to be a system for coping with the increase of subscribers which is a type of problem in mobile communication systems. Moreover, the CDMA system has come to public notice due to its advantages that it has high confidentiality, soft hand-off between base stations is possible, and the like.
In the CDMA communication, a communication speed capable of securing stable communication is limited depending on the traveling speed of a mobile object. Therefore, when a mobile object which travels at a high speed, such as a vehicle, was provided with a mobile station, a communication speed had to be set at a sufficiently low value. This made it difficult to meet the needs for high speed communication. This problem will be subsequently explained exemplifying a vehicle as a mobile object.
In the CDMA system like other communication technologies, it is desired to make the communication speed as high as possible. The CDMA system is applied to mobile computing and data transmission is carried out between a terminal device on the mobile station side and a base station. Due to simultaneous data transmission of dynamic images, static images, voice, and the like, an amount of data to be transmitted tends to be increasing. Thus, it is required to perform high speed and accurate transmission for the following reasons: there is a need to reduce communication expenses by shortening a data transmission time; in the case of mobile objects, it may be limited in a time period which is possible to maintain continuous connection.
As is the case with a portable terminal for pedestrians, if it is assumed that communication would be performed in a stationary state or a state of traveling at a low speed, the communication speed of the CDMA system can be set at a considerably high speed. Even in the case of a vehicle, if it is at a standstill or traveling at a low speed, similar high speed communication can be performed. However, if the traveling speed is accelerated during high speed communication, the communication cannot be normally performed. This is because, as will be explained later, Doppler shift exerts more influence on the communication, for example. Therefore, if it is assumed that the terminal would be used in a vehicle, the communication speed will have to be set at a low speed.
For reference, in the conventional systems using the TDMA and FDMA systems, it is planned to transmit data at a high speed by simultaneously allocating a plurality of data channels to each user. For example, in the case of PDC (Personal Digital Cellular) in Japan, a transmission speed of 9.6 Kbps is accelerated to 28.8 Kbps in digital mobile communication.
Further, also in the conventional systems, a maximum data transmission speed can be obtained in mobile computing performed on the assumption that data is transmitted to a portable terminal carried by an ordinary pedestrian or a portable terminal in a stationary state. However, if the traveling speed of a terminal is 10 Km/h or more during communication, as is the case with vehicles, it will be difficult to realize a transmission speed similar to the speed at the time of standstill or traveling at a low speed. Therefore, this will cause such a problem that it is difficult to meet the need of restricting the transmission time.
Japanese Patent Laid-Open Publication No. Hei 8-149543 relates to a PHS (Personal Handyphone System). As is well known, in the PHS, the TDMA or FDMA system is used. According to the Publication mentioned above, a traveling speed of a mobile object is detected based on an input signal transmitted from a GPS (Global Positioning System) satellite. When the traveling speed reaches a certain value or more, an alarm is given to a user of the mobile terminal to inform that it will be difficult to perform communication. In this case, the user cannot continue the communication without decreasing the traveling speed. In other words, the user cannot continue the communication while traveling at a high speed.
The subject is now returned to the CDMA communication system for mobile objects. In the CDMA system, the communication speed is restricted depending on the traveling speed of a mobile object as described above. Specially in a wide band CDMA (W-CDMA or Wide-CDMA) system, between a terminal device for pedestrians used only at a low traveling speed and a terminal device for mobile objects, such as a vehicle, there is a big difference in communication speed which can be set. The communication speed of the former is several times as high as that of the latter. Therefore, there is a complete difference in environment of mobile computing during the CDMA communication between these two types of terminal devices. In particular, as is the case with ITS (Intelligent Transport Systems) technology, data communication in vehicles tends to be becoming more and more important. Thus, it is desired to make a data communication speed as high as possible.
The Doppler shift is one of the reasons why the communication speed is restricted depending on the traveling speed. When the traveling speed is high, the Doppler shift exerts an influence on inverse diffusion in diffusion demodulation. In particular, if the traveling speed is not constant and always changes, like the case with vehicles, it will be difficult to follow the Doppler shift as will be described hereunder.
If the DS system is adopted, as described above, the diffusion modulation and the demodulation is performed using PN codes. In the case of vehicles, the PN codes are rendered correlative by a diffusion demodulation process (inverse diffusion). The PN codes have waveforms in which normally pseudo +1 or −1 appears at random, as shown in
FIG. 8. A
waveform corresponding to one +1 or −1 is referred to as a chip. A time period of maintaining one chip is referred to as a chip time. The number of chips per unit time is referred to as a chip rate. The chip rate and the bandwidth of communication signal correspond to each other. Thus, if the chip rate becomes great, the bandwidth will become wide.
Here, with the increase of the communication speed, the chip rate and the bandwidth become great. For example, the communication speed is faster when the chip rate is 4.096 Mbit/s than when it is 1.024 Mbit/s. If the chip rate is high, the chip time (time length per chip) will become short. On the other hand, if the range of fluctuation of traveling speed is large like vehicles, fluctuation of the Doppler shift will be large. If the communication speed is set at a high speed, fluctuatio
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Nguyen Chau
Toyota Jidosha & Kabushiki Kaisha
Trinh D.
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