Method and apparatus for providing reference frequency aiding among peers operating in a direct communication mode
A mobile communication device ( 202 ) and peer mobile communication devices ( 204, 206 ) are capable of direct voice and data communication. The mobile communication device has a reference oscillator ( 104 ) which is used for generating operating frequencies, and is subject to frequency errors resulting from manufacturing tolerances, heat, and other error sources. The mobile communication device receives a frequency correction message ( 506 ) from a peer mobile communication device that has calibrated its reference oscillator. The frequency correction message contain offset information which is used by the mobile communication device to determine its offset ( 316 ) from a nominal frequency ( 302 ).
1. A method of acquiring a positioning signal from a global positioning satellite at a positioning receiver of an uncorrected mobile communication device, comprising:
detecting, by a communications transceiver of the uncorrected mobile communication device, a signal transmitted by a corrected mobile communication device, the signal being at an uncorrected nominal frequency of the corrected mobile communication device, the uncorrected nominal frequency being offset from a nominal frequency due to a frequency error of the corrected mobile communication device, the communications transceiver being operable to facilitate direct, peer-to-peer communication between the uncorrected mobile communication device and the corrected mobile communication;
tuning the communications transceiver of the uncorrected mobile communication device to the uncorrected nominal frequency of the corrected mobile communication device from an uncorrected nominal frequency of the uncorrected mobile communication device;
determining, by the uncorrected mobile communication device, a tuning offset between the uncorrected nominal frequency of the uncorrected mobile communication device and the uncorrected nominal frequency of the corrected mobile communication device;
receiving a frequency correction message at the uncorrected mobile communication device from the corrected mobile communication device at the uncorrected nominal frequency of the corrected mobile communication device, the frequency correction message including reference oscillator offset information of the corrected mobile communication device;
extracting the reference oscillator offset information of the corrected mobile communication device from the frequency correction message at the uncorrected mobile communication device;
comparing the reference oscillator offset information of the corrected mobile communication device and the tuning offset to determine a reference offset of a reference oscillator of the uncorrected mobile communication device from the nominal frequency;
providing the reference offset of the reference oscillator of the uncorrected mobile communication device to the positioning receiver of the uncorrected mobile communication device, the positioning receiver sharing the reference oscillator with the communications transceiver; and
adjusting a positioning receiver tuning in response to the reference offset to receive the positioning signal.
2. The method of claim 1 , further comprising:
prior to detecting the frequency correction message transmitted by the corrected mobile communication device, broadcasting a frequency assistance message from the communications transceiver of the uncorrected mobile communication device.
3. The method of claim 1 , wherein tuning the communications transceiver of the uncorrected mobile communication device to the uncorrected nominal frequency of the corrected mobile communication device comprises adjusting a frequency synthesizer of the communications transceiver so that an output frequency of the frequency synthesizer used to down-mix a signal received at the communications transceiver is equal to the uncorrected nominal frequency of the corrected mobile communication device.
4. The method of claim 3 , wherein adjusting the frequency synthesizer of the communications transceiver comprises adjusting a value of a digital control word latched by the frequency synthesizer, wherein the digital control word is used to control the output frequency of the frequency synthesizer.
5. The method of claim 4 , wherein determining the tuning offset comprises determining the difference between the digital control word latched by the frequency synthesizer when the output frequency is equal to the uncorrected nominal frequency of the corrected mobile communication device and a nominal control word corresponding to the uncorrected nominal frequency of the uncorrected mobile communication device.
6. The method of claim 1 , wherein detecting the signal transmitted by a corrected mobile communication device comprises:
tuning the communications transceiver to the uncorrected nominal frequency of the uncorrected mobile communication device;
sweeping the tuning of the communications transceiver in a band around the uncorrected nominal frequency of the uncorrected mobile communication device; and
detecting an initial signal at the uncorrected nominal frequency of the corrected mobile communication device.