Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
1998-04-29
2001-10-02
Marcelo, Melvin (Department: 2733)
Multiplex communications
Communication over free space
Repeater
C370S350000, C370S508000
Reexamination Certificate
active
06298048
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to timing tracking in a communications receiver, and more particularly to timing tracking in the communications receiver relative to multiple communications transmitters. Even more particularly, the present invention relates to timing tracking in an earth-based communications receiver relative to multiple earth-orbit satellite transmitters.
Time division multiple access (TDMA) communications systems share bandwidth between multiple users by dividing a physical communications channel into discrete time slots. Transmission and reception are effected in bursts within a time slot structure. A TDMA timer is used to track system timing and generate control gates at proper times for the transmit and receive bursts within the time slot structure.
In a TDMA system where multiple transmitters, such as multiple earth-orbit satellite transceivers, are used for communication simultaneously, timing with each transmitter must be maintained.
In the multiple satellite transceiver environment, multiple TDMA timers account for different propagation delays and Doppler effects affecting the respective communications channels between an earth-based communications device and each satellite transceiver, as well as independently changing propagation delays for each satellite transceiver. Timing errors attributable to the TDMA timers themselves must also be accounted for.
Thus, prior art approaches dictate the use of a separate TDMA timer for each satellite transceiver from which a signal is to be tracked. Each TDMA timer, each of which may be, for example, a counter, may, in accordance with prior approaches, “roll over” on frame boundaries and thus provide an indication of relative timing within each frame. Transmit and receive bursts are defined relative to the frame boundaries for each frame. A roll over period of each TDMA timer may be adjusted, i.e., advanced or retarded to accommodate timing changes in the timing between the earth-based communications transceiver and the respective satellite transceiver. In accordance with the prior art systems, each TDMA timer tracks a different satellite transceiver, and independently adjusts its roll over to its particular satellite transceiver. Each TDMA timer also includes registers that define where transmit and receive bursts start and stop within the frame boundaries established for the respective satellite transceiver.
A generic TDMA timer tracks frame boundaries in conventional systems. Burst timing is modified, in the case of drift or changes in propagation delay, by modifying these frame boundaries. As a result, each frame is tied to these frame boundaries. If a single generic TDMA timer is used in a multi-satellite communications network, timing must be reset for each successive frame, including resetting the frame boundaries and resetting start and stop times for the transmit and receive bursts. Unfortunately, both time and power are expended in resetting timing on a frame by frame basis and thus such an approach is generally considered undesirable.
Thus, as outlined above, in conventional systems, individual TDMA timers are dedicated to each satellite transceiver from which communications are to be received and to which communications are to be transmitted. These single TDMA timers roll over on frame boundaries, i.e., reset to zero on frame boundaries, in order to remain synchronized with their respective satellite transceivers and keep track of frame time, accounting for timing errors, by adjusting this roll over. Since different satellites with which an earth-based transceiver may need to communicate have different timing, each of the single timers maintains and adjusts a different roll over. Each single TDMA timer also has registers that define where the transmit and receive bursts start and stop within the frames being tracked. Unfortunately, this approach requires several TDMA timers in each earth-based transceiver, which increases cost, power consumption and complexity.
The present invention advantageously addresses the above and other needs.
SUMMARY OF THE INVENTION
The present invention advantageously addresses the needs above as well as other needs by providing an approach to timing correction in a communications receiver, and more particularly by providing an approach to timing correction in the communications receiver relative to multiple communications transmitters.
In one embodiment, the present invention can be characterized as a method for timing correction in a communications receiver. The method includes the steps of (a) setting a timer to a known value; (b) defining a first window by defining a first roll over value; (c) defining a second window by defining a second roll over value; (d) defining a first receive gate transition relative to the first roll over value; (e) defining a second receive gate transition relative to the second roll over value; (f) starting the timer; (g) receiving a first receive burst in the first window; (h) receiving a second receive burst in the second window; (i) in the event a first receive burst is received after the first receive gate transition, increasing the second rollover value, and decreasing the first rollover value by an equal amount; (j) in the event the first receive burst is received before the first receive gate transition, decreasing the second rollover value, and increasing the first rollover value by an equal amount; (k) in the event the second receive burst is received after the second receive gate transition, increasing the first rollover value, and decreasing the second rollover value by an equal amount; (1) in the event the second receive burst is received before the second receive gate transition, decreasing the first rollover value, and increasing the second rollover value by an equal amount; (m) in the event the first receive burst is received after the first receive gate transition, and the second receive burst is receiver after the second receive gate transition by a similar amount, increasing one of the first rollover value and the second rollover value for at least one frame and then restoring the one of the first rollover value and the second rollover value; (n) in the event the first receive burst is received before the first receive gate transition, and the second receive burst is received before the second receive gate transition by a similar amount, decreasing one of the first rollover value and the second rollover value for at least one frame and then restoring the one of the first rollover value and the second rollover value; (o) in the event the first receive burst is received time-shifted relative to the first receive gate transition, and the second receive burst is received time-shifted relative to the second receive gate transition by a different amount, adjusting the second rollover value including: in the event the second receive burst is received after the second receive gate transition, increasing the first rollover value, and decreasing the second rollover value by an equal amount; and in the event the second receive burst is received before the second receive gate transition, decreasing the first rollover value, and increasing the second rollover value by an equal amount; and then adjusting the first rollover value including: in the event a first receive burst is received after the first receive gate transition, increasing the second rollover value, and decreasing the first rollover value by an equal amount; and in the event the first receive burst is received before the first receive gate transition, decreasing the second rollover value, and increasing the first rollover value by an equal amount; and then adjusting the first rollover value to match an error between the second receive burst and the second receive gate transition.
REFERENCES:
patent: 3562432 (1971-02-01), Gabbard
patent: 3730998 (1973-05-01), Schmidt et al.
patent: 4472802 (1984-09-01), Pin et al.
patent: 4642806 (1987-02-01), Hewitt et al.
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patent: 507244
Blakely Michael
Killen Katherine A.
Lamkin Allan B.
Nouri Richard
Hughes Electronics Corporation
Lee Chiho Andrew
Marcelo Melvin
Sales Michael W.
Whelan John T.
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