Pulse or digital communications – Receivers – Automatic frequency control
Patent
1995-12-06
1998-03-24
Chin, Stephen
Pulse or digital communications
Receivers
Automatic frequency control
375206, 4551972, H04L 2706
Patent
active
057321116
ABSTRACT:
A frequency error calibration, estimation and compensation device and method for direct sequence spread spectrum systems, such as digital cordless telephones having a handset and a base station. Initial frequency calibration occurs when the handset is physically coupled to the base station. The allowable frequency error range is divided into a plurality of frequency bins. Both the handset and base station transmit and receive messages using a predetermined frequency offset bin as an estimate of the frequency error. A signal quality value is determined and then a next bin is selected and a next signal quality value is determined. Once a signal quality value has been determined for each frequency bin, a communications link is established between the handset and the base station to allow automatic frequency control tracking to fine tune the frequency offset value. If the signal quality is not above a threshold value, the procedure is repeated. After system calibration, frequency error estimation is performed by cross product demodulation of the received in-phase and quadrature signals. The cross product output is limited and filtered. A phase rotator performs frequency compensation by multiplying the received in-phase and quadrature signals by (cos.phi.(t)+jsin.phi.(t)) where .phi.(t) is the sum of the current phase value and the limited and filtered cross product value.
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Chin Stephen
Cray William C.
O'Shaughnessy James P.
Rockwell International Corporation
Webster Bryan
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