Method and system for reducing interbeam interference and multipath fading in bent-pipe satellite communications systems

A method and system of forming and processing communication links in a satellite communications system. The satellite communications system includes a plurality of groups of transmitters, a satellite and a plurality of destination receivers. Each transmitter is associated with only one transmitter group and transmits a communications signal. Each respective communications signal is a CDMA signal having a predetermined code that is associated with a transmitter. Each respective predetermined code is selected from a code set assigned to the group of transmitters with which the transmitter is grouped. Each code set is different from code sets associated with the transmitters of other selected transmitter groups so that the code sets are arranged in a code set pattern that is repeated for selected groups of transmitters. The satellite receives the communications signals and groups together each received communications signal having a same predetermined code set. The code sets of selected uplink communications signals in a group of received communications signals are fully or partially despread, filtered, and respread so that the group of communications signals contains communications signals from only one code set. Each group of communications signals is then transmitted to a different destination receiver, or groups on different code sets to the same destination receiver.

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 interbeam interference and 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-board 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. In addition, second tier interference may 
be introduced by transmitters in other beams utilizing the same code sets. 
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 and second tier 
interference. Satellite communications system 10 uses four spread-spectrum 
code sets, based, for example, on synchronous Walsh codes, for separating 
uplink beams 12, 13, 17 and 18 in a well-known manner, each of which 
accept transmitted signals using uplink frequency F1. The exemplary code 
sets are referred to herein as W1-W4. 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 each other (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 because 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. Other beams, for instance, beam 19, which utilizes the same 
code set as beam 12 (and potentially the same code as ground station 11), 
couple into the beams adjacent to beam 12, such as beams 13 and 18, 
causing second tier interference when combined by the satellite and 
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, 
polarities, or code sets) between groups of geographic regions. All the 
users in geographic regions labeled 1 use the same resources, which are 
different from those used in regions 2, 3 and 4. A similar statement can 
be made for each 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 regions 3, 7, 8, etc., also exist. Also shown in an 
outlined area of these Figures is a grouping of hexagons referred to 
herein as a "Beam Group". Each number of a beam group is assigned a 
different set of resources. By replicating a beam group multiple times, a 
full tiling pattern can be generated. It is assumed for this example, that 
each beam group is serviced by a different gateway receiver, although this 
is not required by the present invention. For example, see application 
Ser. No. 08/953,020. 
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 F1, F2, F3 and F4. Again, the outlined 
section indicates a beam group. FIG. 5 shows the tiling pattern of FIG. 2 
applied to an FDMA-based system utilizing different combinations of uplink 
frequency groups F1 and F2 and two polarizations P1 and P2. 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 W1-W4 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. Also, they suffer from second tier 
interference through the introduction of signals that are twice removed 
from a desired beam, using the same resources, and are coupled via 
adjacent beams. In more advanced tiling schemes (such as described in 
application Ser. No. 08/953,020 and shown in FIG. 8) this "second tier" 
interference may still exist, although not strictly originating from 
second tier beams. In an FDMA system, this multipath condition and 
interference is easily remedied through the application of appropriate 
filtering on-board 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 and 
interference effects caused by an uplink transmission being coupled into 
adjacent uplink beams utilizing the same frequency and polarization, and 
downlink-routed to the same terrestrial gateway receiver, or routed to 
other terrestrial gateway receivers with other signals using the same code 
set. 
SUMMARY OF THE INVENTION 
The present invention provides a method and a system that eliminates 
multipath fading and interference effects caused by an uplink transmission 
that is coupled into adjacent uplink beams, combined with a copy of itself 
or with signals from other beams using the same code sets, and 
downlink-routed to the same terrestrial gateway receiver. The advantages 
of the present invention are provided by a method and system for 
processing communication links in a satellite communications system. The 
satellite communications system includes a plurality of transmitter groups 
(TGs), a satellite and a plurality of destination receivers. Transmitter 
groups are determined according to which satellite uplink communications 
beam is assigned. All members of a TG are assigned to the same beam. Each 
TG is assigned a different beam and corresponding uplink resources. Each 
transmitter transmits a communications signal and is associated with only 
one transmitter group. Each respective communications signal is a code 
division multiple access (CDMA) signal selected from a predetermined code 
set that is associated with the transmitters of the transmitter group. 
Each respective code set is different from other code sets associated with 
the transmitters of selected transmitter groups so that they form a reuse 
pattern that is repeated for each beam group. The satellite receives the 
communications signals and groups together all received communications 
signals having a same predetermined code set on to a beam. The code sets 
of selected uplink communications signals in a group of received 
communications signals are translated to other code sets (including the 
possibility of being translated back to its own code set) and combined 
with signals from other beams so that the group of communications signals 
contains communications signals each having different codes. In the 
translation process, each signal is filtered for eliminating undesired 
multipath and interference from other beams. Bulk translation and 
filtering may be performed to simplify processing on board the satellite. 
The satellite then transmits each group of communications signals to a 
different destination receiver.

DETAILED DESCRIPTION 
The present invention eliminates multipath fading and interbeam 
interference occurring in bent-pipe satellite communications systems when 
an uplink signal is coupled into two spatially adjacent uplink beams along 
with other interfering signals which, in turn, are then downlink routed to 
the same destination receiver. To eliminate the effects caused by adjacent 
beam coupling, the present invention provides a method and an apparatus 
that ensures that potential multipath and same code set interference are 
removed prior to being downlinked to destination receivers. That is, the 
communications signals received by the satellite are sufficiently despread 
(though not demodulated) and sufficiently filtered to ensure removal of 
multipath and interference effects. The communications signals are 
respread (possibly on different code sets than used for uplink 
transmission), combined with transmissions received on other beams (using 
other downlink code sets to avoid interference), and then downlinked to 
destination receives. In this way, the efficiency and performance of a 
satellite communications system are improved dramatically. 
FIGS. 7 and 8 show illustrative embodiments of beam tiling patterns for a 
CDMA-based system that can be used with the present invention. Referring 
to FIGS. 1 and 8, four different code sets W1-W4 are utilized for 
separating adjacent uplink beams 12 and 13 in a well-known manner. 
FIG. 9 illustrates an exemplary coding technique that can be used with the 
present invention for allowing implementation of bulk partial despreading 
and filtering of CDMA signals. In FIG. 9, four synchronous orthogonal 
Walsh codes W11, W12, W21 and W22 are used to "sub-spread" the four 
distinct code sets used in each beam group, since each beam uses a 
different subspreading code common to all signals in a beam. The 
sub-spreading code can also be termed a "Beam" code. Other well-known 
orthogonal codes, such as a quadratic residue code, or nonorthogonal 
codes, such as Gold codes, can also be used. In asynchronous systems, 
coding schemes that are based on cyclic shift codes can be used. These 
"sub-spreading" codes then are used as building blocks for longer 
spread-spectrum codes that are used by individual transmitting earth 
stations. The transmit codes are formed by using various coding techniques 
(such as already mentioned) with the subspreading codes. The subspreading 
codes allow bulk partial despreading of all signals in a beam using a 
signal operation. The despreading occurs such that the signals from other 
adjacent beams are eliminated or become out-of-band responses that are 
eliminated by bandpass filtering (either RF or IF). Then, the signals are 
respread to their original bandwidth using the same or, if desired, a 
different subspreading code. The signals are then combined with signals 
from other beams using different downlink code sets so not to interfere, 
and downlinked to a common terrestrial gateway receiver. In one embodiment 
of the present invention, each individual signal received in a beam can be 
fully despread, filtered and respread by a bank of signal processors 
operating on the signals in a beam at either RF or IF. The key is that 
interference and multipath from other beams are eliminated in the spread 
spectrum signals prior to combining them with signals from other beams for 
downlink transmission to a common terrestrial gateway receiver. 
FIG. 10 shows a schematic block diagram of a portion of a satellite 1000 
that is part of a satellite communications system that uses four uplink 
beam code sets (WV, WX, WY and WZ). All received uplink beam signals from 
the same code set are respectively grouped together in a well-known 
manner. According to the invention, each code set WV, WX, WY and WZ can be 
a single code when subspreading is used, or be a separate code set. Each 
group of uplink beam carrier signals respectively pass through a first 
bandpass filter 1001a-1001d and then are respectively amplified by a 
low-noise amplifier LNA 1002a-1002d. After low-noise amplification, each 
respective uplink beam signal is down-converted at mixers 1003a-1003d into 
four separate IF signals using a first local oscillator LO1. The four IF 
signals then respectively pass through a second bandpass filter 
1004a-1004d before being respectively despread by mixers 1005a-1005d. The 
despread IF signals are respectively bandpass filtered at bandpass filters 
(BPFs) 1006a-1006d and respread using a different code set (W1, W2, W3 and 
W4) at mixers 1007a-1007d before being input to an IF combiner 1008 to 
form a single IF signal. IF combiner 1008 is a standard 4:1 IF combiner. 
The IF signal output from IF combiner 1008 is up-converted at mixer 1009 
to a selected RF frequency using a second local oscillator LO2. The RF 
signal then passes through a fourth bandpass filter 1010 before being 
amplified by a power amplifier PA 1011 for downlink transmission to a 
terrestrial gateway. 
Alternatively, the down-conversion and filtering process performed by 
mixers 1003a-1003d and BPFs 1004a-1004b, indicated by 1012, can be omitted 
so that the bulk despreading and respreading occurs at RF frequencies. 
Accordingly, the center frequency of bandpass filters 1006a-1006d is the 
center frequency of each respective uplink transmission signal, IF 
combiner 1008 is replaced by an standard 4:1 RF combiner and local 
oscillator LO2 converts the RF signal output from the RF combiner to an 
appropriate downlink frequency in a well-known manner. 
When each code set WV, WX, WY and WZ shown in FIG. 10, is a set of codes, 
each subspreading code is despread accordingly. FIG. 11 shows a schematic 
block diagram of a sub-despreading/sub-respreading circuit that would be 
substituted for despreading/respreading block 1013. While the circuit of 
FIG. 11 replaces only despreading/respreading functional block indicated 
by 1013, a circuit similar to that of FIG. 11 will be used for each code 
set WV-WZ. 
In FIG. 11, the IF signal output from BPF2 1004d is sub-despread by a 
plurality of mixers 1101a-1101n, where n is the number of codes forming 
the sub-spreading code. Each sub-despread signal is respectively bandpass 
filtered by bandpass filters BPF4 1102a-1102n and respective sub-respread 
by mixers 1103a-1103n. The output of mixers 1103a-1103n are combined by a 
standard n:1 IF combiner to form a single IF that is input to IF combiner 
1008 (FIG. 10). 
When the down-conversion and filtering process indicated by 1012 in FIG. 10 
is not used, the sub-despreading and sub-respreading functions shown in 
FIG. 11 occurs at RF frequencies and, consequently, IF combiner is 
replaced by a standard n:1 RF combiner. 
To facilitate radio processing, four groups of uplink beam signals are 
shown formed in FIG. 10 because four distinct beam code sets are used, but 
any number of groups can be formed as long as the number of groups formed 
equals the number of distinct beam code sets used in the satellite 
communications system. An advantage of this approach in the present 
invention is that all received uplink beams using the same code set must, 
by definition, be at least second tier with respect to other uplink beams 
using the same code set. Thus, by using this approach, no substantial 
multipath and/or interbeam interference situations exist at a given 
destination receiver. 
While the present invention has been described using an exemplary 
orthogonal coding scheme, the present invention can be equally applied to 
any satellite communications system where two adjacent beams received at 
the satellite are sent to the same destination receiver. Further, it will 
be appreciated and understood that modifications may be made without 
departing from the true spirit and scope of the invention.