Timing systems and methods for forward link diversity in satellite radiotelephone systems

• US 6,788,917 B1
• Filed: 01/19/2000
• Issued: 09/07/2004
• Est. Priority Date: 01/19/2000
• Status: Expired due to Term

• Current Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Karabinis, Peter D., Refai, Wail M.
• Primary Examiner(s): Maung, Nay
• Assistant Examiner(s): LELE, TANMAY S
• Application Number:
  US09/488,058
• Time in Patent Office: 1,693 Days
• Field of Search: 455/427, 455/429, 455/430, 455/12.1, 455/13.1, 455/13.2, 370/316, 370/326, 370/321
• US Class Current: 455/12.1
• CPC Class Codes: H04B 7/18534 for enhancing link reliablility, e.g. satellites diversity

Abstract

Downlink signal bursts are transmitted from a plurality of visible satellites to a plurality of user terminals in a downlink region that is serviced by the plurality of visible satellites. A first downlink signal burst is sent from a first one of the visible satellites to a first user terminal in the downlink region. A second downlink signal burst is sent from a second one of the visible satellites to the first user terminal to be received at a time that is a function of a separation distance of the first user terminal in the downlink region from the Time Alignment Center (TAC) of the downlink region. First downlink signal bursts and second downlink signal bursts also are sent to other user terminals in the downlink region. The first and second downlink signal bursts preferably are sent over respective first and second carrier frequencies. In preferred embodiments, the downlink signal bursts are transmitted in a plurality of repeating frames of downlink signal bursts of a predetermined frame repetition period. The second downlink signal burst is sent so that it will be received by the intended user terminal at a time offset from the first downlink signal burst that is one half the predetermined frame repetition period plus a delay time ΔT that is a function of a separation distance of the first user terminal in the downlink region from the TAC of the downlink region.

Claims

1. A method of transmitting downlink signal bursts from a plurality of visible satellites to a plurality of user terminals in a downlink region that is serviced by the plurality of visible satellites, the method comprising:

sending a first downlink signal burst from a first one of the visible satellites to a first user terminal in the downlink region;

sending a second downlink signal burst from a second one of the visible satellites to the first user terminal so that it will be received at a time that is a function of a separation distance of the first user terminal in the downlink region from a center of the downlink region; and

repeating the steps of sending a first downlink signal burst and sending a second downlink signal burst for other user terminals in the downlink region;

wherein the downlink signal bursts are sent in a plurality of repeating frames of downlink signal bursts having a predetermined frame repetition period; and

wherein the step of sending a second downlink signal burst comprises the step of sending a second downlink signal burst from a second one of the visible satellites to the first user terminal so that it will be received relative to the first downlink signal burst at a time offset that is one half the predetermined frame repetition period plus a delay time Δ

T that is a function of a separation distance of the first user terminal in the downlink region from the center of the downlink region.

2. A method according to claim 1:

wherein the step of sending a first downlink signal burst comprises the step of sending a first downlink signal burst over a first carrier frequency; and

wherein the step of sending a second downlink signal burst comprises the step of sending a second downlink signal burst over a second carrier frequency.

3. A method according to claim 1 wherein the delay time Δ

T corresponds to a differential propagation delay of the second downlink signal burst from the second one of the visible satellites to the first user terminal and to a user terminal that is located at the center, minus a differential propagation delay of the first downlink signal burst from the first one of the visible satellites to the first user terminal and to a user terminal that is located at the center.

4. A method according to claim 3 wherein the downlink region is sufficiently small such that the first user terminal can receive the first downlink signal burst from the first visible satellite and the second downlink signal burst from the second visible satellite when the first and second visible satellites are located on respective opposite sides of the horizon and the first user terminal is located in the downlink region farthest from the center, with sufficient time therebetween that the first user terminal can transmit an uplink signal burst between the first and second downlink signal bursts.

5. A method according to claim 1 wherein the downlink region is sufficiently small such that the first user terminal can receive the first downlink signal burst from the first visible satellite and the second downlink signal burst from the second visible satellite when the first and second visible satellites are located on respective opposite sides of the horizon and the first user terminal is located in the downlink region farthest from the center, with sufficient time therebetween that the first user terminal can transmit an uplink signal burst between the first and second downlink signal bursts.

6. A method according to claim 1 further comprising the step of transmitting an uplink signal burst from the first user terminal for reception by at least two of the plurality of visible satellites.

7. A method according to claim 6 wherein the transmitting step comprises the step of:

transmitting the uplink signal burst a guard time after an immediately preceding uplink signal burst from an immediately preceding one of the user terminals, the guard time being based upon a time of arrival difference for the adjacent uplink signal bursts to one of the plurality of visible satellites.

8. A method according to claim 7 wherein the transmitting step comprises the step of:

transmitting the uplink signal burst a fixed guard time after an immediately preceding uplink signal burst from an immediately preceding one of the user terminals, the fixed guard time corresponding to a maximum time of arrival difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance apart to one of the plurality of visible satellites.

9. A method according to claim 7 wherein the transmitting step comprises the step of:

transmitting the uplink signal burst a variable guard time after an immediately preceding uplink signal burst from an immediately preceding one of the user terminals, the variable guard time corresponding to a time of arrival difference between adjacent uplink signal bursts from a corresponding pair of user terminals to one of the plurality of visible satellites.

10. A method according to claim 1 wherein the first one of the visible satellites is at highest elevation angle and wherein the second one of the visible satellites is at a next highest elevation angle.

11. A method according to claim 1 further comprising the step of:

diversity combining the received first and second downlink signal bursts at each of the plurality of user terminals.

12. A satellite radiotelephone system that communicates with a plurality of user terminals in a downlink region, the satellite radiotelephone system comprising:

a first satellite that sends a first downlink signal burst to each of the plurality of user terminals in the downlink region; and

a second satellite that sends a second downlink signal burst to each of the plurality of user terminals so as to be received at a respective time that is a function of a separation distance of the respective user terminal in the downlink region from a center of the downlink region;

wherein the downlink signal bursts are sent in a plurality of repeating frames of downlink signal bursts having a predetermined frame repetition period; and

wherein the second satellite sends the second downlink signal burst to each of the plurality of user terminals so as to be received relative to the first downlink signal burst at a time that is one half the predetermined frame repetition period plus a delay time Δ

T that is a function of a separation distance of the corresponding user terminal in the downlink region from the center of the downlink region.

13. A system according to claim 12:

wherein the first downlink signal bursts are sent over a first carrier frequency; and

wherein the second downlink signal bursts are sent over a second carrier frequency.

14. A system according to claim 12 wherein the delay time Δ

T corresponds to a differential propagation delay of the second downlink signal burst from the second satellite to the corresponding user terminal and to a user terminal that is located at the center, minus a differential propagation delay of the first downlink signal burst from the first satellite to the corresponding user terminal and to a user terminal that is located at the center.

15. A system according to claim 14 wherein the downlink region is sufficiently small such that the user terminal can receive the first downlink signal burst from the first satellite and the second downlink signal burst from the second satellite when the first and second satellites are located on respective opposite sides of the horizon and the user terminal is located in the downlink region farthest from the center, with sufficient time therebetween that the user terminal can transmit an uplink signal burst between the first and second downlink signal bursts.

16. A system according to claim 12 wherein the downlink region is sufficiently small such that the user terminal can receive the first downlink signal burst from the first satellite and the second downlink signal burst from the second satellite when the first and second satellites are located on respective opposite sides of the horizon and the user terminal is located in the downlink region farthest from the center, with sufficient time therebetween that the user terminal can transmit an uplink signal burst between the first and second downlink signal bursts.

17. A system according to claim 12 wherein the first and second satellites also receive an uplink signal burst from each of the user terminals without time overlap.

18. A system according to claim 12 wherein the first satellite is at highest elevation angle and wherein the second satellite is at a next highest elevation angle.