Multiplex communications – Wide area network – Packet switching
Patent
1982-12-03
1987-02-17
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
370104, H04J 300, H04J 306
Patent
active
046445315
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a time division communication system, and more particularly to a burst signal generating system where generation of an undesired signal is suppressed, when the burst signal is generated, by switching a modulated wave under a band limitation in a baseband.
In a time division communication system, a modulated signal, such as a phase modulated or amplitude modulated signal, is transmitted in the form of a burst signal because such a signal is transmitted in an assigned time slot. Therefore, in such a time division communication system, a switch is usually provided after a modulator and a burst signal is generated by switching the modulated wave using the ON/OFF control of a rectangular signal. Such a burst signal is then inserted into the assigned time slot.
However, when such switching is carried out for the modulated wave by a switch, an undesired signal is generated because of an impulse response caused by the front and rear edges of the burst signal. The undesired signal is generated on the basis of the impulse of response of the system. Consequently, a spectrum generated by the impulse is wide, influencing other communication channels in the system. For example, a plurality of time dividing channels operating at different frequencies co-exist in a time dividing system, and in such a case, a widely spreading spectrum may adversely influence the other time dividing channels.
The impulse spectrum distribution can easily be limited by using a filter and the undesired signal can be eliminated by connecting a bandpass-filter with a narrow frequency pass band to the succeeding stage of the switch circuit.
However, this method is, of course, not capable of eliminating the undesired signal component within the pass band of the bandpass-filter and, moreover, it is difficult, as is well known, to construct a filter which provides a very narrow band limitation in an RF (radio frequency) signal.
Thus, it is necessary, in one method used, for proper band limitation, to perform such band limitation in an intermediate frequency and then convert the intermediate frequency up to the radio frequency. But this method also has a problem. Namely, when a bit rate is high as compared to the RF carrier frequency, an undesired signal can be eliminated easily because the main lobe is in the vicinity of the RF carrier frequency. Therefore, it is necessary to execute a band limitation in the baseband frequency and then convert the frequency up to the radio frequency by a heterodyne method.
Moreover, an undesired signal resulting from the switching operation can easily be eliminated by such a simple method in which the rectangular wave signal for controlling the ON/OFF of the burst signal is smoothed. However, it is difficult for this method to shape the waveform at the rising and falling portions because a diode is usually employed as the switching element and, in some cases, data is lost, level dropping occurs, and burst interference occurs at the extremely gentle rising and falling portions of the waveform.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system which can efficiently eliminate an undesired signal by making use of the amplitude characteristics of a modulated signal.
Moreover, it is another object of the present invention to provide a system which easily shapes an ON/OFF signal waveform for a switching operation and then eliminates the undesired signal produced based on the switching.
According to the present invention, generation of a momentary undesired signal based on the switching can be prevented by adding particular codes before and after the burst signal and making use of the fact that the transmitted signal, such as a phase-modulated or amplitude-modulated signal, is always accompanied by 100% amplitude modulation.
Even when this invention is applied to the multiphase modulation system having 4-phases or more by changing the offset voltage of a modulator driving circuit only before and after the burst signal, the switchin
REFERENCES:
patent: 3887873 (1975-06-01), Duncan et al.
patent: 4455651 (1984-06-01), Baran
patent: 4483000 (1984-11-01), Yamamoto et al.
Wissenschaftlichen Berichte Aeg-Telefunken, vol. 51, No. 4/5, Apr./May 1978, Berlin, S. Borner, "Modems fur Digitalubertragung im Burstbetrieb uber Nachrichtensatelliten", pp. 212-224.
Electronics and Communications in Japan, vol. 55 B, No. 6, Jun. 1972, Washington, T. Okamoto et al, "Design and Performance of the Carrier-Pulse Transmission System with the Approximate Zero-Crossing Waveform", pp. 47-55.
European Search Report, Examiner J. P. Verslype, 18-6-84, The Hague.
Chin Wellington
Fujitsu Limited
Olms Douglas W.
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