Telecommunications – Carrier wave repeater or relay system – Portable or mobile repeater
Reexamination Certificate
1999-01-13
2001-05-01
Maung, Nay (Department: 2681)
Telecommunications
Carrier wave repeater or relay system
Portable or mobile repeater
C455S013200, C455S502000
Reexamination Certificate
active
06226492
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a mobile satellite communication method and a mobile satellite communication system for carrying out communications via a communications satellite between an earth station and a mobile station with an automatic tracking type antenna.
In a satellite communications system, multiplex techniques called multiple accesses are used in order to effectively utilize assigned frequencies and to decrease transmission power up to now. Among the multiple accesses, a frequency division multiple access (FDMA) system, which assigns frequencies for a satellite transponder, is widely used in the satellite communications system presently. This is because the FDMA system can apply technique established in a ground communication, has a low initial cost, and can be relatively easily realized.
However, the FDMA system is disadvantageous in that throughput drastically decrease when the number of channel connection increases. More specifically, when the number of channel connection increases, transmission power in the satellite transponder increases and an amplifier in the transponder may operate at an operating point near to a saturation point thereof. Therefore, intermodulation accompanied with waveform distortion occurs and interference between channels occurs. As a result, quality of a communication path decreases, it is difficult to use the communication path, and throughput drastically decreases. Accordingly, in the satellite communication system of the FDMA system, use is made of carrier activation in order to avoid these problems.
For example, in a case of a satellite communications system offering a transmission service of speech, carrier activation is described by G. Maral, et al. in a book under the title of “SATELLITE COMMUNICATIONS SYSTEM 2nd Edition”, John Wiley & Sons, Inc. 1993 or is described by Vijay K. Bhargava, et al., in a book under the title of “DIGITAL COMMUNICATIONS BY SATELLITE”, John Wiley & Sons, Inc. 1981. According to these books, as the carrier activation, voice activation is carried out which detects a speech and transmits a carrier signal during a time duration when the speech is detected in a telephone call. In the voice activation, inasmuch as the carrier signal is not transmitted during a pause time duration in the telephone call when the voice is not detect, it is possible to decrease an average transmission power in the satellite and to ensure a channel capacity with intermodulation prevented.
FIGS. 6A and 6B
are time charts for showing signals in a case of carrying out the voice activation.
FIG. 6A
shows a speech signal while
FIG. 6B
shows a transmission signal. During a time duration Ti when the speech signal is interrupted, sending of the transmission signal (a carrier signal) is basically stopped.
Such carrier activation is applied not only to a case of transmitting a speech signal but also to a case of transmitting a data signal for use in a facsimile equipment or a computer and transmission of the carrier signal is stopped when the data signal to be transmitted is not supplied.
In addition, in the satellite communications system carrying out the carrier activation, as shown in
FIG. 6B
, a burst carrier signal
102
is periodically transmitted at predetermined time intervals in order to maintain a channel connection state between an earth station and a mobile station when the time duration of a transmission stop is not less than a predetermined time duration with no information signal (no speech signal or no data signal to be transmitted) supplied. Such a burst carrier signal is call a maintaining burst signal.
On the other hand, when the mobile station uses an automatic tracking type antenna as a mobile antenna, the mobile station normally receives a signal from the satellite and carries out a tracking control on the basis of a level of the received signal so as to turn a direction of the mobile antenna toward the satellite. Accordingly, such a mobile station must carry out the tracking control using only the maintaining burst signal during a time duration when the carrier signal from the earth station is interrupted due to the carrier activation. It is difficult to control accurately when a bad condition is piled up.
Particularly in a case of a mobile satellite communications system, the received signal may be not received in stable due to a shadowing which occurs because variation of a transmission condition peculiar to a mobile satellite communication. It is increasingly difficult to carry out the tracking control in such a state. In addition, it is supposed in a mobile station loaded into a vehicle loaded with the automatic tracking type antenna that the mobile station rapidly turns in its moving direction. Under the circumstances, it is impossible to carry out the tracking control using only reception of the maintaining burst signal during operation of the carrier activation. This may cause a case where it is difficult only to maintain a received synchronization.
Therefore, in the mobile satellite communications system, it results in remarkably injuring reliability in a communication path in a case of carrying out the carrier activation.
As a method of avoid this problem, there is a method of always transmitting a tracking exclusive signal from the satellite, and of monitoring a signal level of the tracking exclusive signal in the mobile station. However, inasmuch as this method requires hardware for receiving the tracking exclusive signal and has a large scale of hardware as a whole, this method is unsuitable for the mobile station which requires high mobility.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a mobile satellite communication method and a mobile satellite communications system which are capable of carrying out carrier activation with reliability in a communication path secured.
Other objects of this invention will become clear as the description proceeds.
A mobile satellite communication method to which the present invention is applicable carries out communications via a communications satellite between an earth station and a mobile station with an automatic tracking type antenna. The earth station transmits, to the mobile station, a modulated signal into which a carrier signal is modulated by an information signal supplied thereto. The mobile satellite communication method carries out carrier activation where the earth station periodically transmits a burst carrier signal having a burst transmission duration to the mobile station when the information signal is not supplied thereto. According to an aspect of this invention, the earth station extends the burst transmission duration for the burst carrier signal when the earth station receives, from the mobile station, a carrier signal having a carrier frequency which varies with variation exceeding a reference value.
A mobile satellite communications system to which the present invention is applicable carries out communications via a communication satellite between an earth station and a mobile station with an automatic tracking type antenna. The earth station transmits, to the mobile station, a modulated signal into which a carrier signal is modulated by an information signal supplied thereto. The mobile satellite communications system carries out carrier activation where the earth station periodically transmits a burst carrier signal having a burst transmission duration to the mobile station when the information signal is not supplied thereto. According to an aspect of this invention, the earth station comprises a carrier frequency variation detecting arrangement for detecting variation in a carrier frequency of a received carrier signal and a carrier signal controlling arrangement for carrying out control of a carrier transmission for a signal to be transmitted to the mobile station. When the carrier frequency of the received carrier signal varies with variation exceeding a reference value, the carrier frequency variation detecting arrangement produces a carrier frequency variation detecte
Maung Nay
NEC Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
Trinh Sonny
LandOfFree
Mobile satellite communication method and system capable of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Mobile satellite communication method and system capable of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Mobile satellite communication method and system capable of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2450938