Doppler correction and path loss compensation for airborne cellular system

• US 6,675,013 B1
• Filed: 06/26/2000
• Issued: 01/06/2004
• Est. Priority Date: 06/26/2000
• Status: Expired due to Fees

• Current Assignee: Torsal Technology Group Limited LLC (Intellectual Ventures LLC)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Emmons, Thomas Peter Jr., Gross, Jonathan H., White, Scott C., Hanson, Duke E.
• Primary Examiner(s): Tran, Sinh
• Assistant Examiner(s): BEAMER, TEMICA M
• Application Number:
  US09/605,144
• Time in Patent Office: 1,289 Days
• Field of Search: 455/13.1, 455/12.1, 455/11.1, 455/516, 455/431, 340/827, 342/175, 342/352, 370/315, 370/316
• US Class Current: 455/431
• CPC Class Codes: H04B 7/18504 Aircraft used as relay or high altitude atmospheric platform

Abstract

The present invention corrects for Doppler shift in both forward and reverse links in a cellular communications system including an airborne repeater. A reverse link pilot reference signal in a band similar to a communications signal band is received at a reverse link processor, and the Doppler shift in the reverse feeder link is corrected based on the reverse link pilot reference signal. The Doppler shift in the forward feeder link is also corrected based on the reverse link pilot reference signal prior to the forward feeder link being affected by the Doppler shift. The present invention also compensates for signal strength variations due to changing flight pattern positions of the repeater. Pre-compensation for forward feeder link path losses due to movement of the airplane is performed to cause communications signals transmitted to and from the cellular communications system repeater to have identical strength before the signals are transmitted to the system user cell phones within the area of coverage.

Claims

1. A method of factoring out signal errors in both forward and reverse links, each including a feeder link and a user link, in a cellular communications system including an airborne repeater, comprising:

receiving a reverse link pilot reference signal in a band similar to a communications signal band;

correcting for Doppler shift in the reverse link based on the reverse link pilot reference signal;

correcting for the Doppler shift in the forward link based on the reverse link pilot reference signal prior to the forward link being affected by the Doppler shift by pre-compensating for forward link path losses due to movement of the airborne repeater to cause communications signals transmitted from the airborne repeater via the forward link to have substantially uniform strength; and

wherein the correcting for the Doppler shift in the forward link comprises shifting the forward link prior to transmission thereof to provide coverage to a predetermined geographic area.

2. The method of claim 1, wherein the correcting for Doppler shift in the reverse link based on the reverse link pilot reference signal compensates for movement of the airborne repeater relative to terrestrial cellular telephone communicating through the repeater via the reverse link.

3. The method of claim 1, wherein the correcting for the Doppler shift in the forward link based on the reverse link pilot reference signal provides forward-looking compensation for movement of the airborne repeater.

4. The method of claim 1, wherein the reverse link and forward link are C-band links.

5. The method of claim 4, wherein the reverse link pilot reference signal is one of a C-band signal and a guard band signal adjacent C-band communications signals transmitted over the reverse link.

6. The method of claim 1, wherein the correcting for the Doppler shift in the reverse link and the correcting for the Doppler shift in the forward link comprise:

measuring a frequency difference in the reverse link pilot reference signal when received and the reverse link pilot reference signal when transmitted to determine an error correction value;

correcting for the Doppler shift in the reverse link based on the error correction value; and

correcting for the Doppler shift in the forward link communication signal based on the error correction value.

7. The method of claim 6 wherein the measuring of a frequency difference in the reverse link pilot reference signal when received and the reverse link pilot reference signal when transmitted to determine an error correction value is based on GPS measurements.

8. A cellular communications system, comprising:

a terrestrial base transceiving station including a telephony switch;

an airborne repeater for providing both a forward link between the base transceiving station and system users within a predetermined geographic area of coverage, and a reverse link between the system users within the predetermined geographic area of coverage and the base transmitting station;

a forward link ground converter for stepping up signals transmitted from the base transmitting station to the airborne repeater;

a reverse link ground converter for stepping down signals transmitted from the system users to the airborne repeater;

a frequency shift correction device for determining and compensating for a Doppler frequency shift in tile reverse link by calculating an error correction value and feeding the error correction value to the reverse link ground converter via a closed feedback loop; and

wherein the frequency shift correction device comprises all airplane telemetry link between the airborne repeater and the base transceiving station that facilitates calculation of an airplane speed value used as the error correction value; and

wherein the frequency shift correction device is configured to correct for the Doppler shift in the forward link based on the reverse link pilot reference signal prior to the forward link being affected by the Doppler shift by pre-compensating for forward link path losses due to movement of the airborne repeater to cause communications signals transmitted from the airborne repeater via the forward link to have substantially uniform strength.

9. The cellular communications system of claim 8, wherein the frequency shift correction device is also for compensating for the Doppler frequency shift in the forward link by feeding the error correction value to the forward link ground converter to factor out the Doppler frequency shift in the forward link in a forward-looking manner.

10. The cellular communications system of claim 8, wherein the frequency shift correction device comprises:

a pilot frequency generator for generating a pilot frequency signal for use in determining the Doppler frequency shift in the forward and reverse links;

a pilot frequency measurement device for measuring a frequency of the pilot frequency signal received thereat;

a Doppler shift calculator for calculating the error correction value by determining a frequency difference in the pilot frequency signal received at the pilot frequency measurement device and the pilot frequency signal generated at the pilot frequency generator, and for subsequently transmitting the error correction value to the forward link ground converter via the closed feedback loop.

11. The cellular communication system of claim 8, wherein the frequency shift correction device is an airplane-based device.

12. The cellular communication system of claim 8, wherein the frequency shift correction device is a terrestrial device.

13. A method of compensating for signal strength variations due to changing flight pattern positions of an airborne cellular communications system repeater, comprising:

detecting a flight pattern position of an airplane carrying the cellular communications system repeater;

pre-compensating for forward link path losses due to movement of the airplane carrying the cellular communications system repeater to cause communications signals transmitted from the cellular communications system repeater via the forward link to have uniform strength; and

transmitting the uniform strength communications signals to the system user cell phones within the area of coverage after the pre-compensating for forward link path losses.

14. The method of claim 13, wherein the detecting of a flight pattern position of an airplane carrying the cellular communications system repeater is performed using at least one of global positioning satellite information and telemetry link information.

15. The method of claim 13, wherein the pre-compensating for forward link path losses is further for increasing or decreasing gain of the forward link to maintain a required minimum link margin.

16. The method of claim 13 wherein the pre-compensating for forward link path losses is one of an airborne operation and a terrestrial-based operation.

17. The method of claim 13, wherein the pre-compensating for forward link path loses comprises pre-compensating for forward feeder link path losses.