# US 5,705,958 A · Apparatus for correcting quadrature error in a modulator and/or in a demodulator for a signal having a plurality of phase states, a corresponding transmitter, and a corresponding receiver

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- Number: US 5,705,958 A
- Document ID: US5705958A
- Status: Granted
- Kind: A
- Application number: 08/721,207
- Filed: 1996-09-26
- Granted: 1998-01-06
- Earliest claimed: 1995-09-27
- Assignees: Alcatel Telspace
- Inventors: Patrick Janer
- Web page: https://patentgrep.com/patent/US5705958A

## CPC

- H04L 27/3863: {Compensation for quadrature error in the received signal}
- H04L 27/364: {Arrangements for overcoming imperfections in the modulator, e.g. quadrature error or unbalanced I and Q levels}
- H04L 2027/0016: {Stabilisation of local oscillators}
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

Apparatus for correcting quadrature error in a quadrature modulator and/or in a demodulator for a signal having a plurality of phase states, or of amplitude and phase states. The correction apparatus comprises a processor (51) which, when the apparatus is used to correct quadrature error in a demodulator, transforms the levels P and Q of the digital signals output by the demodulation stage into corrected levels P' and Q' that satisfy any one of the following eight relationships: ##STR1## These corrected levels are then applied to decision element (72), optionally after their maximum levels have equalized. The apparatus combines correction accuracy with cheapness, and it enables any quadrature error to be corrected.

## Claims

### Claim 1

Apparatus for correcting quadrature error in a demodulation stage (2) supplying a digital signal constituted, at each symbol time, by a symbol of a constellation having a plurality of phase states or of amplitude and phase states, each symbol of said constellation being determined by two amplitude levels P and Q output by said demodulation stage (2);
said apparatus being characterized in that it comprises processing means (51) supplying two corrected amplitude levels P' and Q' on the basis of said amplitude levels P and Q, said corrected amplitude levels P' and Q' being respectively equal to the values in any one of the following pairs of values:
P+Q and Q-P
P+Q and P-Q
-P-Q and Q-P
-P-Q and P-Q
Q-P and P+Q
P-Q and P+Q
Q-P and -P-Q
P-Q and -P-Q.

### Claim 2 (depends on 1)

Apparatus according to claim 1, characterized in that said corrected amplitude levels are applied to decision means.

### Claim 3 (depends on 1)

Apparatus according to claim 1, characterized in that said processing means (51) are preceded by automatic gain control amplification means (49, 50) ensuring that the maximum levels of said amplitude levels P and Q are equal.

### Claim 4 (depends on 1)

Apparatus according to claim 1, characterized in that said processing means (51) are followed by automatic gain control amplification means (70, 71) ensuring that the maximum levels of said corrected amplitude levels P' and Q' are equal.

### Claim 5

Apparatus for correcting quadrature error in a modulation stage (1) to which a digital-type modulating signal is applied, the signal being constituted, at each symbol time, by a symbol of a constellation having a plurality of phase states or of amplitude and phase states, each symbol of said constellation being determined by two amplitude levels P and Q;
said apparatus being characterized in that it comprises processing means (51) supplying two corrected amplitude levels P' and Q' on the basis of said amplitude levels P and Q, said corrected amplitude levels P' and Q' being applied to said modulation stage (1) and being respectively equal to the values in any one of the following pairs of values:
P+Q and Q-P
P+Q and P-Q
-P-Q and Q-P
-P-Q and P-Q
Q-P and P+Q
P-Q and P+Q
Q-P and -P-Q
P-Q and -P-Q.

### Claim 6

A receiver for receiving signals, each of which has a plurality of phase states or of amplitude and phase states, said receiver being characterized in that it includes apparatus for correcting quadrature error in a demodulation stage supplying a digital signal constituted, at each symbol time, by a symbol of a constellation having a plurality of phase states or of amplitude and phase states, each symbol of said constellation being determined by two amplitude levels P and Q output by said demodulation stage, said apparatus being characterized in that it comprises processing means supplying two corrected amplitude levels P' and Q' on the basis of said amplitude levels P and Q, said corrected amplitude levels P' and Q' being respectively equal to the values in any one of the following pairs of values:
P+Q and Q-P
P+Q and P-Q
-P-Q and Q-P
-P-Q and P-Q
Q-P and P+Q
P-Q and P+Q
Q-P and -P-Q
P-Q and -P-Q.

### Claim 7

A transmitter for transmitting signals, each of which has a plurality of phase states or of amplitude and phase states, said transmitter being characterized in that it includes apparatus for correcting quadrature error in a demodulation stage supplying a digital signal constituted, at each symbol time, by a symbol of a constellation having a plurality of phase states or of amplitude and phase states, each symbol of said constellation being determined by two amplitude levels P and Q output by said demodulation stage, said apparatus being characterized in that it comprises processing means supplying two corrected amplitude levels P' and Q' on the basis of said amplitude levels P and Q, said corrected amplitude levels P' and Q' being respectively equal to the values in any one of the following pairs of values:
P+Q and Q-P
P-Q and P-Q
-P-Q and Q-P
-P-Q and P-Q
Q-P and P+Q
P-Q and P+Q
Q-P and -P-Q
P-Q and -P-Q.