Pulse or digital communications – Receivers – Angle modulation
Patent
1993-05-20
1995-05-16
Chin, Stephen
Pulse or digital communications
Receivers
Angle modulation
375344, 331 1A, 329325, 329336, H04L 2714, H04L 2716, H04L 2722
Patent
active
054168031
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a digital data transmission system, and more particularly to a telecommunications system by wave or by digital radio beam, said system including a receiver that performs coherent demodulation directly at microwave frequency.
The radio beam is a transmission channel for the data contained in the digital stream. The system includes at least one transmitter and at least one receiver. The modulation generally used is modulation having a plurality of amplitude and phase states, thereby defining a "constellation" having an even number of states in the complex plane.
DESCRIPTION OF THE RELATED ART
A conventional digital transmission system of this type is shown in block diagram form in accompanying FIGS. 1 and 2.
The system uses coherent demodulation, i.e. demodulation that is synchronous with the transmission, said demodulation being performed directly at microwave frequency, i.e. without transposition to an intermediate frequency. FIG. 1 is a diagram showing a portion of the transmitter and FIG. 2 is a diagram showing a portion of the receiver.
The transmitter as shown in FIG. 1 comprises in succession from an upstream end to a downstream end:
A microwave oscillator 1 which may be synthesized or otherwise, and which delivers the transmission carrier frequency on an output terminal 2.
A microwave modulator 3 which receives said carrier frequency from 2 and which also receives in conventional manner data trains P and Q to be transmitted which are delivered in baseband on two modulation inputs 4 and 5.
An amplifier 6 for amplifying the modulated signal obtained at output 7 from the modulator 3, the output signal 8 from said amplifier 6 being applied to an antenna via a transmission filter (not shown) for the purpose of being transmitted towards the receiver.
The receiver as shown in FIG. 2 comprises, in turn:
A low noise amplifier 9 (LNA) having an input 10 that receives the radiofrequency signal from a reception antenna and via a reception filter (not shown).
A microwave demodulator 11 which has an input 12 on which it receives the signal amplified by the amplifier 9.
A voltage controlled oscillator 13 (VCO) for use in performing microwave demodulation, the oscillator having an output 14 connected to the demodulator 11. Since this oscillator is required to have good spectrum purity and low frequency drift, it is generally made using dielectric resonators having a high Q-factor.
A portion 15 performs signal processing in baseband. This portion 15 conventionally includes the following, for example: a low noise amplifier, a lowpass filter, a group propagation delay corrector, and an automatic gain control amplifier (AGC). It processes the two received and demodulated data trains P and Q which are applied thereto via respective input terminals 16 and 17 connected to the demodulator 11, and after processing it provides processed versions of these two data trains P and Q in baseband on output terminals 18 and 19 of said receiver.
A phase estimator 20 operating in baseband receives the two data trains P and Q at the output from the processor device 15 via inputs 21 and 22, and it delivers a phase error signal via an output 23 in the form of a DC voltage which is applied to the control input of the oscillator 13, thereby forming a phase locked loop which tends to cancel any phase difference between the received signal and the reconstituted carrier. In other words, the phase estimator 20 generates a voltage which is a function of the phase error between the recovered carrier and the received carrier. This voltage output on 23 and the phase estimator enables the reception oscillator 13 to be servo-controlled in frequency and in phase, thereby enabling "coherent" demodulation of the received signal.
A conventional installation as described above suffers from the following drawbacks:
The receiver has a narrow passband:
Since the demodulator oscillator 13 must have good spectrum purity and low frequency drift, it cannot have high frequency agility, such that in practice this kind of receive
REFERENCES:
patent: 4879728 (1989-11-01), Tarallo
patent: 5140284 (1992-08-01), Petersson et al.
patent: 5150384 (1992-09-01), Cahill
patent: 5283780 (1994-02-01), Shuchman et al.
Microwave Journal, vol. 33, No. 4, Apr. 1990, Dedham US, pp. 255-264, Surinder Kumar: "Directly modulated VSAT transmitters".
Alcatel Telspace
Chin Stephen
Vo Don N.
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