Demodulators – Phase shift keying or quadrature amplitude demodulator
Patent
1997-01-08
1998-05-12
Nguyen, Viet Q.
Demodulators
Phase shift keying or quadrature amplitude demodulator
329306, 329345, 329346, H03D 300, H03D 302
Patent
active
057511870
DESCRIPTION:
BRIEF SUMMARY
This application is a filing under 35 U.S.C. 371 of PCT/SE96/00664 filed on May 22, 1996.
TECHNICAL FIELD
The present invention relates to a method and a device for correcting modulation errors which have arisen in a transmitter of an information-carrying, modulated signal.
BACKGROUND OF THE INVENTION
In information transfer of digital signals over relatively long distances between a transmitting unit and a receiving unit, it is expedient to encode the digital signal in some way. The encoding is carried out in order to guarantee a reliable transfer of information despite interferences. In the case of wire transmission, one of the simplest forms of encoding can be to raise the signal levels, while a more advanced form may be some variant of phase modulation or frequency modulation. The application of optical fibres for information transfer in most cases necessitates the use of a light source modulated by the digital signals.
Transfer of information by wire or by optical fibres is not always possible because of geographical conditions, costs, or other circumstances. In such cases it may be more appropriate to use wireless transmission of the information, for example with the aid of some type of radio wave. In this case too it is expedient to use a carrier wave which is modulated by the digital information.
The three basic methods for modulating digital information are amplitude modulation, frequency modulation and phase modulation. The decision as to which method is used can be based on any one of the following demands and/or requirements: detectivity, data transfer speed, available spectrum bandwidth, hardware complexity, frequency range, cost, etc. A number of these demands/requirements are in direct conflict with each other, for which reason prioritization must be effected depending on the application. Frequency modulation and phase modulation both have a constant envelope, which makes them insensitive to amplitude nonlinearities which may arise in microwave radio links and satellite channels, for example. Consequently, frequency modulation and phase modulation are far more common than amplitude modulation.
Frequency modulation is very often the simplest to put into practice, that is to say the requisite hardware is simple, both at the transmitter end and at the receiver end, which results in low costs. Phase modulation, on the other hand, affords a better system in terms of performance, but it requires a more complicated transmitter and a more complicated receiver. Owing to these differences, types of hybrids have been developed which make use of advantages from both the modulation methods. Examples of these which may be mentioned are TFM (Tamed Frequency Modulation) and C-QPSK (Constant envelope offset Quadrature Phase Shift Key), which are the same method, but whose different names illustrate the fact that they are a hybrid solution. The modulation method consists in using a physically simple transmitter, similar to the one which is used for frequency modulation, and an advanced receiver similar to the one which is used for phase modulation. In this way a system is obtained which has advantages of frequency modulation, i.e. simpler and less expensive, and of phase modulation, i.e. better performance.
The present invention concerns the problems which arise when using such modulation methods in which the transmission signal is generated by means of frequency control by a voltage-controlled oscillator (VCO) and in which the signal is phase-demodulated in the receiver. The use of a voltage-controlled oscillator for generating a phase-modulated transmission signal may entail a simplification of the transmitter in some respects, but it at the same time imposes more stringent demands on the components included.
When different frequencies are used for phase modulation of the information-carrying transmission signal, the demands in respect of the frequencies which the voltage-controlled oscillator generates are extremely stringent if a correct phase demodulation is to take place in the receiver. The deman
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Nguyen Viet Q.
Phan Trong
Telefonaktiebolaget LM Ericsson
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