Pulse or digital communications – Receivers – Automatic gain control
Reexamination Certificate
1999-06-03
2001-10-23
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Automatic gain control
C455S234100
Reexamination Certificate
active
06307902
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to improvements in digital telephones and, more particularly, pertains to new and improved gain imbalance compensation for quadrature receivers in cordless direct sequence spread spectrum telephones.
2. Description of Related Art
The cordless telephone products that exist today for the most part use analog frequency modulation in the 46-49 MHz band, and provide good speech quality when the handset is at close proximity to the base station. Although these analog cordless telephones are inexpensive and offer advantages to the customer which have made the products a successful replacement for corded telephones, these analog phones still leave much room for improvement. For example, it would be desirable to have a longer handset-to-base station range without dropouts, more channels, and greater security.
Use of digital modulation and digital coding techniques offers more robust voice communication over a radio channel, although requiring greater channel bandwidth. Digital modulation also has a capture effect that greatly surpasses co-channel and adjacent channel interference, thereby providing a more noise-free conversation. Use of digital modulation encoding also allows for the addition of effective scrambling codes to thereby greatly improve telephone security. In addition, by utilizing the industrial, medical, and scientific (ISM) band for radio transmission and reception (902-920 MHz), increased power levels above those in the 46-49-MHz band are permitted, thus increasing the operating range. A requirement for highest power in the ISM band is that direct sequence spread spectrum (DSSS) or FHSS modulation be utilized.
Gain imbalance exists on radio systems that are quadrature receivers mixed to basebands with analog techniques. Radio systems generally have gain imbalance between I and Q receiver paths due to the difficulty in matching gains in the in-phase and quadrature paths. This gain imbalance offsets the accuracy of demodulation and consequently degrades the performance of the quadrature receiver. Typical direct sequence spread spectrum systems are incapable of digitally correcting for the errors caused by the gain imbalance because of the low A/D dynamic ranges used.
SUMMARY OF THE INVENTION
The quadrature receiver of the present invention is able to estimate the imbalance error during a calibration cycle and remove the error if gain error is actually present during operation. The gain imbalance estimation is performed during a calibration cycle when the handset is parked in the cradle of the base station. A frequency error is introduced. The difference |I|−|Q| is obtained from the A/D converters of the receiver as an indication of the gain error present. This gain error is measured, and the gain error compensation is increased or decreased until the desired gain balance is achieved. Gain error compensation is provided by D/A converters driving the receiver's A/D converter.
REFERENCES:
patent: 3878468 (1975-04-01), Falconer et al.
Conexant Systems Inc.
Corrielus Jean B
Pham Chi
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