Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
1997-10-17
2001-08-28
Hsu, Alpus H. (Department: 2662)
Multiplex communications
Communication over free space
Repeater
C370S342000, C375S211000, C342S353000, C342S354000, C455S013100
Reexamination Certificate
active
06282179
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of telecommunications. More particularly, the present invention relates to a method and a system for eliminating multipath fading caused by multiple uplink beams coupling in a bent-pipe satellite communications system.
2. Description of the Related Art
In a bent-pipe satellite communications system that combines four spatially adjacent uplink beams, with each beam using spread-spectrum techniques for beam separation, multipath fading can occur if two or more of the adjacent uplink beams are routed on a satellite to the same destination earth station receiver, such as an earth station for a terrestrial gateway communications system. While the spread-spectrum code set used with a particular uplink beam would be different from the code set used on adjacent uplink beams, an uplink signal transmitted from the edge of one beam couples equally well to an adjacent beam. If the two adjacent beams carrying the uplink signal are routed to the same gateway receiver, a multipath condition can exist because the same signal has been effectively transmitted on the two adjacent beams and combined prior to being received at the gateway receiver. That is, since RF phasing of the transponders of the two adjacent uplink beams can vary, destructive interference may occur between the two RF paths, resulting in a substantial loss of signal power at the gateway receiver. In effect, the gateway receiver receives the same signal from two different sources by the intercoupling of uplink signals between adjacent uplink beams.
FIG. 1
shows an exemplary multi-beam bent-pipe satellite communications system
10
that illustrates multipath fading caused by an uplink beam signal being coupled into two adjacent uplink beams. Satellite communications system
10
uses four spread-spectrum code sets based, for example, on synchronous Walsh codes, in a well-known manner for separating uplink beams
12
,
13
,
17
and
18
, which each accept transmitted signals using uplink frequency F
1
. The exemplary code sets are referred to herein as W
1
-W
4
. A ground station
11
transmits an uplink communications signal using uplink beam
12
. While only ground station
11
is shown within the geographic region covered by beam
12
, there are a plurality of ground stations within the geographic region covered by beam
12
, but are not shown. Ground station
11
is geographically located near the edge of beam
12
such that the transmitted signal is also coupled into adjacent uplink beam
13
. Beams
12
and
13
are received by satellite
14
combined (with themselves and beams
17
and
18
) and transmitted over a downlink beam
15
to a common earth station receiver
16
,. such as a terrestrial gateway. System
10
includes a plurality of gateway receiver stations, of which only gateway receiver
16
is shown. Multipath fading occurs at gateway receiver
16
since multiple versions of the signal from ground station
11
are received (effectively) from two different paths and are potentially destructively combined prior to being received by gateway receiver
16
.
FIGS. 2 and 3
respectively show conventional uplink beam tiling patterns for a reuse of four which can be used in satellite communications systems. Each hexagon shown in
FIGS. 2 and 3
represents a different geographic region covered by a corresponding uplink beam. The numbers within the hexagons represent a partitioning of resources (such as frequencies, polarizations or code sets) between groups of geographic regions. All the users in geographic regions labeled
1
use the same resources, which are different from the resources used in regions
2
,
3
and
4
. A similar statement can be made for regions
2
,
3
, and
4
. The patterns shown in
FIG. 2 and 3
are representative, and other tiling patterns for sharing resources between groups of 3, 7, 8, etc., regions also exist.
Also shown in an outlined area of the Figures is a grouping of hexagons referred to herein as a “Beam Group” (BG). Each member of the group is assigned a different set of resources. By replicating the beam group multiple times, the full tiling pattern can be generated. It is assumed for the tiling patterns of
FIGS. 2-7
, that for a prior art design, each beam group is serviced by a different gateway receiver.
FIG. 4
shows the tiling pattern of
FIG. 2
applied to a conventional Frequency Division Multiple Access (FDMA) based system utilizing four different uplink frequency groups F
1
, F
2
, F
3
and F
4
. Again, the outlined section indicates one beam group.
FIG. 5
shows the tiling pattern of
FIG. 2
applied to an FDMA-based system utilizing different combinations of uplink frequency groups F
1
and F
2
and two polarizations P
1
and P
2
.
FIG. 6
shows a conventional uplink beam frequency tiling pattern for a Code Division Multiple Access (CDMA) based system. Isolation between users in different beams for such a system is provided by proper allocation of code sets between the beams.
FIG. 7
shows the tiling pattern of
FIG. 2
applied to codes for a representative CDMA-based system utilizing code sets W
1
-W
4
for isolation between the beams.
Each of the tiling patterns of
FIGS. 2-7
suffer from multipath fading that occurs when two adjacent uplink beams are downlink-routed to the same terrestrial gateway receiver. In an FDMA system, this multipath condition is easily remedied through the application of appropriate filtering onboard the satellite. However, for CDMA systems where adjacent beams use the same frequency and polarization, the filtering techniques that are applied in an FDMA system cannot be used. Consequently, what is needed is a method and a system that eliminates the multipath fading effect caused by an uplink transmission being coupled into two adjacent uplink beams utilizing the same frequency and polarization, and downlink routed to the same terrestrial gateway receiver.
SUMMARY OF THE INVENTION
The present invention eliminates the multipath fading effect caused by an uplink transmission being coupled into two adjacent uplink beams and downlink routed to the same terrestrial gateway receiver. The advantages of the present invention are provided by a method and system of forming communication links in a satellite communications system. The satellite communications system includes a plurality of Transmitter Groups (TGs), a satellite and a plurality of downlink receivers. Each transmitter is associated with only one Transmitter Group. A Transmitter Group is defined herein according to a satellite uplink communications beam assignment. All member of a particular TG are assigned the same beam. Each TG is assigned a different beam and corresponding uplink resources. Each transmitter of a transmitter group transmits an uplink communications signal that is a CDMA signal having a predetermined code that is selected from a code set that is assigned to the TG with which the transmitter is associated. Each respective code set is different from other code sets associated with selected groups of transmitters and forms a code set pattern that is repeated within each BG. The satellite receives the uplink communications signals transmitted by the transmitters and transmits each of the received uplink communications signals as a downlink communications signal directed to a different receiver of the plurality of downlink receivers, such that each downlink communications signal includes no more than one downlink communications signal having a predetermined code.
REFERENCES:
patent: 4259741 (1981-03-01), Kawai
patent: 5394560 (1995-02-01), Kane
patent: 5473601 (1995-12-01), Rosen et al.
patent: 5555257 (1996-09-01), Dent
patent: 5825325 (1998-10-01), O'Donovan et al.
patent: 5848060 (1998-12-01), Dent
AT&T Corp.
Hsu Alpus H.
Qureshi Afsar M.
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