EP0113246A2 — Radio communication system

Claim 1

Independent Claim 252 words

A radio communication system operated under a quadrature amplitude modulation (QAM) method, including: a transmitter unit being operative to modulate an in-phase-channel (I-ch) data signal and quadrature-channel (Q-ch) data signal with two carriers CR having a phase difference of 2 with each other, to add thus modulated signals to each other, and to transmit, a transmission signal to a transmission line; and a receiver unit being operative to receive said transmission signal via said transmission line, to separate said transmission signal into both a first receiving signal representing said I-ch data signal and a second receiving signal representing said Q-ch data signal, to demodulate the first and second receiving signals with two carriers having a phase difference of 2 with each other, which carriers are output from a carrier recovery circuit, and to reproduce original data signals from the thus demodulated signals, characterized in that in the transmitter unit, a direct current (DC) offset source is introduced therein, which adds a DC offset to either one of said I-ch and Q-ch data signals, in the receiver unit, the demodulation operation is achieved through synchronous detections for said first and second receiving signals by utilizing a leakage carrier which is created due to said DC offset and included in said transmission signal, the leakage carrier is phase controlled and transformed into two leakage carriers having a phase difference of 2 with each other in said carrier recovery circuit and then applied to said first and second receiving signals, respectively, for said synchronous detections.

Claim 2

Dependent Claim 252 words

A radio communication system operated under a quadrature amplitude modulation (QAM) method, including: a transmitter unit being operative to modulate an in-phase-channel (I-ch) data signal and quadrature-channel (Q-ch) data signal with two carriers CR having a phase difference of 2 with each other, to add thus modulated signals to each other, and to transmit, a transmission signal to a transmission line; and a receiver unit being operative to receive said transmission signal via said transmission line, to separate said transmission signal into both a first receiving signal representing said I-ch data signal and a second receiving signal representing said Q-ch data signal, to demodulate the first and second receiving signals with two carriers having a phase difference of 2 with each other, which carriers are output from a carrier recovery circuit, and to reproduce original data signals from the thus demodulated signals, characterized in that in the transmitter unit, a direct current (DC) offset source is introduced therein, which adds a DC offset to either one of said I-ch and Q-ch data signals, in the receiver unit, the demodulation operation is achieved through synchronous detections for said first and second receiving signals by utilizing a leakage carrier which is created due to said DC offset and included in said transmission signal, the leakage carrier is phase controlled and transformed into two leakage carriers having a phase difference of 2 with each other in said carrier recovery circuit and then applied to said first and second receiving signals, respectively, for said synchronous detections.

Claim 3

Dependent Claim 21 words

A system as set forth in claim 1, wherein addition of said DC offset is performed by means of an adder.

Claim 4

Dependent Claim 100 words

A system as set forth in claim 2, wherein said transmitter unit includes a first high-pass filter and a second high-pass filter therein, both allotted to said I-ch and Q-ch data signals, and is operative to form a V-shaped recess, in the vicinity of said leakage carrier component, in the spectrum of said transmission signal component; and said receiver unit includes a first low-pass filter and a second low-pass filter therein, both allotted to said I-ch and Q-ch data signals and is operative to recover the filtered signal components by said first and second high-pass filters in the transmitter unit.

Claim 5

Dependent Claim 33 words

A system as set forth in claim 3, wherein, in the receiver unit, said leakage carrier to be utilized for the synchronous detections is extracted from said transmission signal by a filter means.

Claim 6

Dependent Claim 131 words

A system as set forth in claim 4, wherein said carrier recovery circuit comprises: a band-pass filter, as said filter means, for extracting said leakage carrier; a variable phase shifter receiving, at its input, the output from the band-pass filter and providing its output, on one hand, via a ; phase shifter, to a first mixer for demodulating the I-ch receiving signal and, on the other hand, to a second mixer for demodulating the Q-ch receiving signal in case said DC offset is added to said I-ch data signal; a low pass filter for filtering the output from the second mixer; and a DC amplifier for DC amplifying the output from the low-pass filter, the phase shift operation in the variable phase shifter-being feedback-controlled with the output from said DC amplfiier.

Claim 7

Dependent Claim 67 words

A system as set forth in claim 5, wherein said first low-pass filter and said second low-pass filter, both in the receiver unit, have inverse characteristics of transfer functions relative to those of said first high-pass filter and said second high-pass filter, both in the transmitter unit, the outputs from the first and second low-pass filters being then amplified by first and second base band amplifiers, respectively.

Claim 8

Dependent Claim 55 words

A system as set forth in claim 6, wherein, in the receiver unit, said first low-pass filter is not independent but is introduced into said first base band amplifier, at its feedback path, and also said second low-pass filter is not independent but is introduced into said second base band amplifier, at its feedback path.

Claim 9

Dependent Claim 79 words

A system as set forth in claim 7, wherein, in the receiver unit, an automatic level control (ALC) circuit is employed at its input stage for receiving said transmission signal, the ALC circuit being operative to detect the level of a cosine wave component appearing at the output of said first mixer in case said DC offset is added to said I-ch data signal and achieving the ALC operation in accordance with the thus detected cosine wave component level.

Claim 10

Dependent Claim 102 words

A system as set forth in claim 8, wherein said ALC circuit comprises: a low-pass filter for extracting said cosine wave component from the output of said first mixer; a DC amplifier amplifying the output from this low-pass filter; a variable gain amplifier being gain- controlled in accordance with the output from this DC amplifier; and a subtractor producing a voltage difference between a predetermined reference gain control voltage inherent to said variable gain amplifier and the output voltage of said DC amplifier and achieving the gain control operation for said variable gain amplifier in accordance with the thus produced voltage difference.

Claim 11

Dependent Claim 53 words

A system as set forth in claim 3, wherein, in the receiver unit, said leakage carrier component to be utilized for the synchronous detections is used as a feedback signal for constructing said carrier recovery circuit as a phase lock looped (PLL) circuit, without positively extracting the leakage carrier from said transmission signal.

Claim 12

Dependent Claim 104 words

A system as set forth in claim 10, wherein said PLL circuit comprises: a low-pass filter for extracting the leakage carrier component from the output from a second mixer for demodulating the Q-ch receiving signal in case said DC offset is added to said I-ch data signal; an amplifier for amplifying the output from this low-pass filter; and a voltage-controlled oscillator whose oscillation frequency is controlled by the output from this amplifier, the oscillation output being applied, on one hand, via a 2 phase shifter, to said second mixer, and, on the other hand, to a first mixer for demodulating the I-ch receiving signal.

Claim 13

Dependent Claim 67 words

A system as set forth in claim 11, wherein said first low-pass filter and said second low-pass filter, both in the receiver unit, have inverse characteristics of transfer functions relative to those of said first high-pass filter and said second high-pass filter, both in the transmitter unit, the outputs from the first and second low-pass filters being then amplified by first and second base band amplifiers, respectively.

Claim 14

Dependent Claim 54 words

A system as set forth in claim 12, wherein, in the receiver unit, said first low-pass filter is not independent but introduced into said first base band amplifier, at its feedback path, and also said second low-pass filter is not independent but is introduced into said second base band amplifier, at its feedback path.

Claim 15

Dependent Claim 79 words

A system as set forth in claim 13, wherein, in the receiver unit, an automatic level control (ALC) circuit is employed at its input stage for receiving said transmission signal, the ALC circuit being operative to detect the level of a cosine wave cmponent appearing at the output of said first mixer in case said DC offset is added to said I-ch data signal and achieving the ALC operation in accordance with the thus detected cosine wave component level.

Claim 16

Dependent Claim 102 words

A system as set forth in claim 14, wherein said ALC circuit comprises: a low-pass filter for extracting said cosine wave component from the output of said first mixer; a DC amplifier amplifying the output from this low-pass filter; a variable gain amplifier being gain controlled in accordance with the output from this DC amplifier; and a subtractor producing a voltage difference between a predetermined reference gain control voltage inherent to said variable gain amplifier and the output voltage of said DC amplifier and achieving the gain control operation for said variable gain amplifier in accordance with the thus produced voltage difference.

Radio communication system

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