Low earth orbit communication satellite gateway-to-gateway relay system

This invention teaches the use of overlapping footprints in a LEO satellite communications system to increase the overall connectivity of the system, thus providing a wide service availability. In particular, this invention teaches the use of at least one terrestrial LEOS relay station (70) that is positioned within an overlap of at least two satellite coverage areas for relaying a communication from a gateway (18A) associated with a first coverage area to a gateway (18B) associated with a second coverage area. A plurality of LEOS relay stations can be so provided to enable a communication, such as a voice communication, to be routed through a plurality of coverage areas and gateways, thereby bypassing a substantial portion of an underlying terrestrial communication system.

FIELD OF THE INVENTION 
This invention relates generally to communications systems and, in 
particular, to a low earth orbit (LEO) satellite-based communications 
system. 
BACKGROUND OF THE INVENTION 
Satellite-based communications systems are well are represented in the 
prior art. By example, reference is made to U.S. Pat. No. 5,303,286, 
issued on Apr. 12, 1994 to Robert A. Wiedeman, and which is entitled 
"Wireless Telephone/Satellite Roaming System". Reference is also made to 
the numerous U.S. Patents, foreign patents, and other publications that 
are of record in U.S. Pat. No. 5,303,286. 
Of particular interest herein is a class of satellite-based communications 
systems that employs multiple satellites in a low earth orbit, referred to 
as a `LEO` system or LEOS. LEOS are characterized by moving patterns of 
signal `footprints` on the ground, where each footprint corresponds to the 
coverage area of one or more beams that are transmitted and received by a 
given satellite as it orbits the earth. The satellites communicate with 
terrestrial stations which may be referred to as `gateways`. 
It is often the case that two or more satellites of a constellation of LEO 
satellites will have overlapping footprints or coverage areas. The 
presence of overlapping coverage areas enables a ground-based receiver to 
simultaneously receive a communication signal from and transmit a 
communication signal through a plurality of satellites whose coverage 
areas overlap. For a receiver that receives multiple copies of the same 
signal through a plurality of satellites the effects of multi-path fading 
and signal blockage can be greatly reduced. Reference in this regard can 
be had to U.S. Pat. No. 5,233,626, issued Aug. 3, 1993 to Stephen A. Ames 
and entitled "Repeater Diversity Spread Spectrum Communication System", 
the disclosure of which is incorporated by reference herein in its' 
entirety. 
Communication systems that make use of repeater diversity generally use 
spread spectrum (SS) techniques, and possibly also code division multiple 
access (CDMA) as the modulation scheme in order to maximize the 
communications capability. In such systems there is a desire to cause the 
satellite footprints and any interior beams generated to have the maximum 
overlap possible to maximize the use of diversity techniques to combat 
fading and blockage. 
SUMMARY OF THE INVENTION 
This invention is directed to a unique use of overlapping footprints in a 
LEO satellite communications system to increase the overall connectivity 
of the system, thus providing a wide service availability. In particular, 
this invention teaches the use of at least one terrestrial LEOS relay 
station that is positioned within an overlap of at least two satellite 
coverage areas for relaying a communication from a gateway associated with 
a first coverage area to a gateway associated with a second coverage area. 
A plurality of LEOS relay stations can be so provided to enable a 
communication, such as a voice communication, to be routed through a 
plurality of coverage areas and gateways, thereby bypassing a substantial 
portion of an underlying terrestrial communication system. 
More particularly, this invention teaches a ground-based repeater station 
for use with a plurality of low earth orbit (LEO) communication satellites 
individual ones of which have an associated ground coverage area. The 
repeater station includes a first transceiver for receiving a downlink 
transmission from a first one of the LEO communication satellites 
associated with a first coverage area and for transmitting the received 
transmission on an uplink to a second one of the LEO communication 
satellites associated with a second coverage area that overlaps the first 
coverage area. The repeater station further includes a second transceiver 
for receiving a downlink transmission from the second one of the LEO 
communication satellites associated with the second coverage area and for 
transmitting the received transmission on an uplink to the first one of 
the LEO communication satellites associated with the first coverage area. 
The repeater station further includes a demodulator for demodulating a 
call request transmission that is received from the first LEO 
communication satellite; a controller for extracting call destination 
information from the demodulated call request transmission and for 
selecting a LEO communication satellite to receive the uplink 
transmission. 
In a preferred embodiment of this invention the demodulator includes 
circuitry for despreading and tracking a spread spectrum signal that is 
received from the first LEO communication satellite.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a presently preferred embodiment of a satellite 
communication system 10 that is suitable for use with the presently 
preferred embodiment of this invention. Before describing this invention 
in detail, a description will first be made of the communication system 10 
so that a more complete understanding may be had of the present invention. 
The communications system 10 may be conceptually subdivided into a 
plurality of segments 1, 2, 3 and 4. Segment 1 is referred to herein as a 
space segment, segment 2 as a user segment, segment 3 as a ground 
(terrestrial) segment, and segment 4 as a telephone system infrastructure 
segment. 
In the presently preferred embodiment of this invention there are a total 
of 48 satellites in, by example, a 1414 km Low Earth Orbit (LEO). The 
satellites 12 are distributed in eight orbital planes with six 
equally-spaced satellites per plane (Walker constellation). The orbital 
planes are inclined at 52 degrees with respect to the equator and each 
satellite completes an orbit once every 114 minutes. This approach 
provides approximately full-earth coverage with, preferably, at least two 
satellites in view at any given time from a particular user location 
between about 70 degree south latitude and about 70 degree north latitude. 
As such, a user is enabled to communicate to or from nearly any point on 
the earth's surface within a gateway (GW) 18 coverage area to or from 
other points on the earth's surface (by way of the PSTN), via one or more 
gateways 18 and one or more of the satellites 12, possibly also using a 
portion of the telephone infrastructure segment 4. 
It is noted at this point that the foregoing and ensuing description of the 
system 10 represents but one suitable embodiment of a communication system 
within which the teaching of this invention may find use. That is, the 
specific details of the communication system are not to be read or 
construed in a limiting sense upon the practice of this invention. 
Continuing now with a description of the system 10, a soft transfer 
(handoff) process between satellites 12, and also between individual ones 
of 16 spot beams transmitted by each satellite (FIG. 3B), provides 
unbroken communications via a spread spectrum (SS), code division multiple 
access (CDMA) technique. The presently preferred SS-CDMA technique is 
similar to the TIA/EIA Interim Standard, "Mobile Station-Base Station 
Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular 
System" TIA/EIA/IS-95, July 1993, although other spread spectrum and CDMA 
techniques and protocols can be employed. 
The low earth orbits permit low-powered fixed or mobile user terminals 13 
to communicate via the satellites 12, each of which functions, in a 
presently preferred embodiment of this invention, solely as a "bent pipe" 
repeater to receive a communications traffic signal (such as speech and/or 
data) from a user terminal 13 or from a gateway 18, convert the received 
communications traffic signal to another frequency band, and to then 
re-transmit the converted signal. That is, no on-board signal processing 
of a received communications traffic signal occurs, and the satellite 12 
does not become aware of any intelligence that a received or transmitted 
communications traffic signal may be conveying. 
Furthermore, there need be no direct communication link or links between 
the satellites 12. That is, each of the satellites 12 receives a signal 
only from a transmitter located in the user segment 2 or from a 
transmitter located in the ground segment 3, and transmits a signal only 
to a receiver located in the user segment 2 or to a receiver located in 
the ground segment 3. 
The user segment 2 may include a plurality of types of user terminals 13 
that are adapted for communication with the satellites 12. The user 
terminals 13 include, by example, a plurality of different types of fixed 
and mobile user terminals including, but not limited to, handheld mobile 
radio-telephones 14, vehicle mounted mobile radio-telephones 15, 
paging/messaging-type devices 16, and fixed radio-telephones 14a. The user 
terminals 13 are preferably provided with omnidirectional antennas 13a for 
bidirectional communication via one or more of the satellites 12. 
It is noted that the fixed radio-telephones 14a may employ a directional 
antenna. This is advantageous in that it enables a reduction in 
interference with a consequent increase in the number of users that can be 
simultaneously serviced with one or more of the satellites 12. 
It is further noted that the user terminals 13 may be dual use devices that 
include circuitry for also communicating in a conventional manner with a 
terrestrial cellular system. 
Referring also to FIG. 3A, the user terminals 13 may be capable of 
operating in a full duplex mode and communicate via, by example, L-band RF 
links (uplink or return link 17b) and S-band RF links (downlink or forward 
link 17a) through return and forward satellite transponders 12a and 12b, 
respectively. The return L band RF links 17b may operate within a 
frequency range of 1.61 GHz to 1.625 GHz, a bandwidth of 16.5 MHz, and are 
modulated with packetized digital voice signals and/or data signals in 
accordance with the preferred spread spectrum technique. The forward S 
band RF links 17a may operate within a frequency range of 2.485 GHz to 2.5 
GHz, a bandwidth of 16.5 MHz. The forward RF links 17a are also modulated 
at a gateway 18 with packetized digital voice signals and/or data signals 
in accordance with the spread spectrum technique. 
The 16.5 MHz bandwidth of the forward link is partitioned into 13 channels 
with up to, by example, 128 users being assigned per channel. The return 
link may have various bandwidths, and a given user terminal 13 may or may 
not be assigned a different channel than the channel assigned on the 
forward link. However, when operating in the diversity reception mode on 
the return link (receiving from two or more satellites 12), the user is 
assigned the same forward and return link RF channel for each of the 
satellites. 
The ground segment 3 includes at least one but generally a plurality of the 
gateways 18 that communicate with the satellites 12 via, by example, a 
full duplex C band RF link 19 (forward link 19a (to the satellite), return 
link 19b (from the satellite)) that operates within a range of frequencies 
generally above 3 GHz and preferably in the C-band. The C-band RF links 
bi-directionally convey the communication feeder links, and also convey 
satellite commands to the satellites and telemetry information from the 
satellites. The forward feeder link 19a may operate in the band of 5 GHz 
to 5.25 GHz, while the return feeder link 19b may operate in the band of 
6.875 GHz to 7.075 GHz. 
The satellite feeder link antennas 12g and 12h are preferably wide coverage 
antennas that subtend a maximum earth coverage area as seen from the LEO 
satellite 12. In the presently preferred embodiment of the communication 
system 10 the angle subtended from a given LEO satellite 12 (assuming 
10.degree. elevation angles from the earth's surface) is approximately 
110.degree.. This yields a coverage zone that is approximately 3600 miles 
in diameter. 
The L-band and the S-band antennas are multiple beam antennas that provide 
coverage within an associated terrestrial service region. The L-band and 
S-band antennas 12d and 12c, respectively, are preferably congruent with 
one another, as depicted in FIG. 3B. That is, the transmit and receive 
beams from the spacecraft cover the same area on the earth's surface, 
although this feature is not critical to the operation of the system 10. 
As an example, several thousand full duplex communications may occur 
through a given one of the satellites 12. In accordance with a feature of 
the system 10, two or more satellites 12 may each convey the same 
communication between a given user terminal 13 and one of the gateways 18. 
This mode of operation, as described in detail below, thus provides for 
diversity combining at the respective receivers, leading to an increased 
resistance to fading and facilitating the implementation of a soft handoff 
procedure. 
It is pointed out that all of the frequencies, bandwidths and the like that 
are described herein are representative of but one particular system. 
Other frequencies and bands of frequencies may be used with no change in 
the principles being discussed. As but one example, the feeder links 
between the gateways and the satellites may use frequencies in a band 
other than the C-band (approximately 3 GHz to approximately 7 GHz), for 
example the Ku band (approximately 10 GHz to approximately 15 GHz) or the 
Ka band (above approximately 15 GHz). 
The gateways 18 function to couple the communications payload or 
transponders 12a and 12b (FIG. 3A) of the satellites 12 to the telephone 
infrastructure segment 4. The transponders 12a and 12b include an L-band 
receive antenna 12c, S-band transmit antenna 12d, C-band power amplifier 
12e, C-band low noise amplifier 12f, C-band antennas 12g and 12h, L band 
to C band frequency conversion section 12i, and C band to S band frequency 
conversion section 12j. The satellite 12 also includes a master frequency 
generator 12k and command and telemetry equipment 121. 
Reference in this regard may also be had to U.S. Pat. No. 5,422,647, by E. 
Hirshfield and C. A. Tsao, entitled "Mobile Communications Satellite 
Payload" (U.S. Ser. No. 08/060,207). 
The telephone infrastructure segment 4 is comprised of existing telephone 
systems and includes Public Land Mobile Network (PLMN) gateways 20, local 
telephone exchanges such as regional public telephone networks (RPTN) 22 
or other local telephone service providers, domestic long distance 
networks 24, international networks 26, private networks 28 and other 
RPTNs 30. The communication system 10 operates to provide bidirectional 
voice and/or data communication between the user segment 2 and Public 
Switched Telephone Network (PSTN) telephones 32 and non-PSTN telephones 32 
of the telephone infrastructure segment 4, or other user terminals of 
various types, which may be private networks. 
Also shown in FIG. 1 (and also in FIG. 4), as a portion of the ground 
segment 3, is a Satellite Operations Control Center (SOCC) 36, and a 
Ground Operations Control Center (GOCC) 38. A communication path, which 
includes a Ground Data Network (GDN) 39 (see FIG. 2), is provided for 
interconnecting the gateways 18 and TCUs 18a, SOCC 36 and GOCC 38 of the 
ground segment 3. This portion of the communications system 10 provides 
overall system control functions. 
FIG. 2 shows one of the gateways 18 in greater detail. Each gateway 18 
includes up to four dual polarization RF C-band sub-systems each 
comprising a dish antenna 40, antenna driver 42 and pedestal 42a, low 
noise receivers 44, and high power amplifiers 46. All of these components 
may be located within a radome structure to provide environmental 
protection. 
The gateway 18 further includes down converters 48 and up converters 50 for 
processing the received and transmitted RF carrier signals, respectively. 
The down converters 48 and the up converters 50 are connected to a CDMA 
sub-system 52 which, in turn, is coupled to the Public Switched Telephone 
Network (PSTN) though a PSTN interface 54. As an option, the PSTN could be 
bypassed by using satellite-to-satellite links. 
The CDMA sub-system 52 includes a signal summer/switch unit 52a, a Gateway 
Transceiver Subsystem (GTS) 52b, a GTS Controller 52c, a CDMA Interconnect 
Subsystem (CIS) 52d, and a Selector Bank Subsystem (SBS) 52e. The CDMA 
sub-system 52 is controlled by a Base Station Manager (BSM) 52f and 
functions in a manner similar to a CDMA-compatible (for example, an IS-95 
compatible) base station. The CDMA sub-system 52 also includes the 
required frequency synthesizer 52g and a Global Positioning System (GPS) 
receiver 52h. 
The PSTN interface 54 includes a PSTN Service Switch Point (SSP) 54a, a 
Call Control Processor (CCP) 54b, a Visitor Location Register (VLR) 54c, 
and a protocol interface 54d to a Home Location Register (HLR). The HLR 
may be located in the cellular gateway 20 (FIG. 1) or, optionally, in the 
PSTN interface 54. 
The gateway 18 is connected to telecommunication networks through a 
standard interface made through the SSP 54a. The gateway 18 provides an 
interface, and connects to the PSTN via Primary Rate Interface (PRI). The 
gateway 18 is further capable of providing a direct connection to a Mobile 
Switching Center (MSC). 
The gateway 18 provides SS-7 ISDN fixed signalling to the CCP 54b. On the 
gateway-side of this interface, the CCP 54b interfaces with the CIS 52d 
and hence to the CDMA sub-system 52. The CCP 54b provides protocol 
translation functions for the system Air Interface (AI), which may be 
similar to the IS-95 Interim Standard for CDMA communications. 
Blocks 54c and 54d generally provide an interface between the gateway 18 
and an external cellular telephone network that is compatible, for 
example, with the IS-41 (North American Standard, AMPS) or the GSM 
(European Standard, MAP) cellular systems and, in particular, to the 
specified methods for handling roamers, that is, users who place calls 
outside of their home system. The gateway 18 supports user terminal 
authentication for system 10/AMPS phones and for system 10/GSM phones. In 
service areas where there is no existing telecommunications 
infrastructure, an HLR can be added to the gateway 18 and interfaced with 
the SS-7 signalling interface. 
A user making a call out of the user's normal service area (a roamer) is 
accommodated by the system 10 if authorized. In that a roamer may be found 
in any environment, a user may employ the same terminal equipment to make 
a call from anywhere in the world, and the necessary protocol conversions 
are made transparently by the gateway 18. The protocol interface 54d is 
bypassed when not required to convert, by example, GSM to AMPS. 
It is within the scope of the teaching of this invention to provide a 
dedicated, universal interface to the cellular gateways 20, in addition to 
or in place of the conventional "A" interface specified for GSM mobile 
switching centers and vendor-proprietary interfaces to IS-41 mobile 
switching centers. It is further within the scope of this invention to 
provide an interface directly to the PSTN, as indicated in FIG. 1 as the 
signal path designated PSTN-INT. 
Overall gateway control is provided by the gateway controller 56 which 
includes an interface 56a to the above-mentioned Ground Data Network (GDN) 
39 and an interface 56b to a Service Provider Control Center (SPCC) 60. 
The gateway controller 56 is generally interconnected to the gateway 18 
through the BSM 52f and through RF controllers 43 associated with each of 
the antennas 40. The gateway controller 56 is further coupled to a 
database 62, such as a database of users, satellite ephemeris data, etc., 
and to an I/O unit 64 that enables service personnel to gain access to the 
gateway controller 56. The GDN 39 is also bidirectionally interfaced to a 
Telemetry and Command (T&C) unit 66 (FIGS. 1 and 4). 
Referring to FIG. 4, the function of the GOCC 38 is to plan and control 
satellite utilization by the gateways 18, and to coordinate this 
utilization with the SOCC 36. In general, the GOCC 38 analyses trends, 
generates traffic plans, allocates satellite 12 and system resources (such 
as, but not limited to, power and channel allocations), monitors the 
performance of the overall system 10, and issues utilization instructions, 
via the GDN 39, to the gateways 18 in real time or in advance. 
The SOCC 36 operates to maintain and monitor orbits, to relay satellite 
usage information to the gateway for input to the GOCC 38 via the GDN 39, 
to monitor the overall functioning of each satellite 12, including the 
state of the satellite batteries, to set the gain for the RF signal paths 
within the satellite 12, to ensure optimum satellite orientation with 
respect to the surface of the earth, in addition to other functions. 
As described above, each gateway 18 functions to connect a given user to 
the PSTN for both signalling, voice and/or data communications and also to 
generate data, via database 62 (FIG. 2), for billing purposes. Selected 
gateways 18 include a Telemetry and Command Unit (TCU) 18a for receiving 
telemetry data that is transmitted by the satellites 12 over the return 
link 19b and for transmitting commands up to the satellites 12 via the 
forward link 19a. The GDN 39 operates to interconnect the gateways 18, 
GOCC 38 and the SOCC 36. 
In general, each satellite 12 of the LEO constellation operates to relay 
information from the gateways 18 to the users (C band forward link 19a to 
S band forward link 17a), and to relay information from the users to the 
gateways 18 (L band return link 17b to C band return link 19b). This 
information includes SS-CDMA synchronization and paging channels, in 
addition to power control signals. Various CDMA pilot channels may also be 
used to monitor interference on the forward link. Satellite ephemeris 
update data is also communicated to each of the user terminals 13, from 
the gateway 18, via the satellites 12. The satellites 12 also function to 
relay signalling information from the user terminals 13 to the gateway 18, 
including access requests, power change requests, and registration 
requests. The satellites 12 also relay communication signals between the 
users and the gateways 18, and may apply security to mitigate unauthorized 
use. 
In operation, the satellites 12 transmit spacecraft telemetry data that 
includes measurements of satellite operational status. The telemetry 
stream from the satellites, the commands from the SOCC 36, and the 
communications feeder links 19 all share the C band antennas 12g and 12h. 
For those gateways 18 that include a TCU 18a the received satellite 
telemetry data may be forwarded immediately to the SOCC 36, or the 
telemetry data may be stored and subsequently forwarded to the SOCC 36 at 
a later time, typically upon SOCC request. The telemetry data, whether 
transmitted immediately or stored and subsequently forwarded, is sent over 
the GDN 39 as packet messages, each packet message containing a single 
minor telemetry frame. Should more than one SOCC 36 be providing satellite 
support, the telemetry data is routed to all of the SOCCs. 
The SOCC 36 has several interface functions with the GOCC 38. One interface 
function is orbit position information, wherein the SOCC 36 provides 
orbital information to the GOCC 38 such that each gateway 18 can 
accurately track up to four satellites that may be in view of the gateway. 
This data includes data tables that are sufficient to allow the gateways 
18 to develop their own satellite contact lists, using known algorithms. 
The SOCC 36 is not required to known the gateway tracking schedules. The 
TCU 18a searches the downlink telemetry band and uniquely identifies the 
satellite being tracked by each antenna prior to the propagation of 
commands. 
Another interface function is satellite status information that is reported 
from the SOCC 36 to the GOCC 38. The satellite status information includes 
both satellite/transponder availability, battery status and orbital 
information and incorporates, in general, any satellite-related 
limitations that would preclude the use of all or a portion of a satellite 
12 for communications purposes. 
An important aspect of the system 10 is the use of SS-CDMA in conjunction 
with diversity combining at the gateway receivers and at the user terminal 
receivers. Diversity combining is employed to mitigate the effects of 
fading as signals arrive at the user terminals 13 or the gateway 18 from 
multiple satellites over multiple and different path lengths. Rake 
receivers in the user terminals 13 and the gateways 18 are employed to 
receive and combine the signals from multiple sources. As an example, a 
user terminal 13 or the gateway 18 provides diversity combining for the 
forward link signals or the return link signals that are simultaneously 
received from and transmitted through the multiple beams of the satellites 
12. 
In this regard the disclosure of U.S. Pat. No. 5,233,626, issued Aug. 3, 
1993 to Stephen A. Ames and entitled "Repeater Diversity Spread Spectrum 
Communication System", is incorporated by reference herein in its 
entirety. 
The performance in the continuous diversity reception mode is superior to 
that of receiving one signal through one satellite repeater, and 
furthermore there is no break in communications should one link be lost 
due to shadowing or blockage from trees or other obstructions that have an 
adverse impact on the received signal. 
The multiple, directional, antennas 40 of a given one of the gateways 18 
are capable of transmitting the forward link signal (gateway to user 
terminal) through different beams of one or more satellites 12 to support 
diversity combining in the user terminals 13. The omnidirectional antennas 
13a of the user terminals 13 transmit through all satellite beams that can 
be "seen" from the user terminal 13. 
Each gateway 18 supports a transmitter power control function to address 
slow fades, and also supports block interleaving to address medium to fast 
fades. Power control is implemented on both the forward and reverse links. 
The response time of the power control function is adjusted to accommodate 
for a worst case 30 msec satellite round trip delay. 
The block interleavers (53d, 53e, 53f, FIG. 5) operate over a block length 
that is related to vocoder 53g packet frames. An optimum interleaver 
length trades off a longer length, and hence improved error correction, at 
the expense of increasing the overall end-to-end delay. A preferred 
maximum end-to-end delay is 150 msec or less. This delay includes all 
delays including those due to the received signal alignment performed by 
the diversity combiners, vocoder 53g processing delays, block interleaver 
53d-53f delays, and the delays of the Viterbi decoders (not shown) that 
form a portion of the CDMA sub-system 52. 
FIG. 5 is a block diagram of the forward link modulation portion of the 
CDMA sub-system 52 of FIG. 2. An output of a summer block 53a feeds a 
frequency agile up-converter 53b which in turn feeds the summer and switch 
block 52a. The telemetry and control (T&C) information is also input to 
the block 52a. 
An unmodulated direct sequence SS pilot channel generates an all zeros 
Walsh Code at a desired bit rate. This data stream is combined with a 
short PN code that is used to separate signals from different gateways 18 
and different satellites 12. If used, the pilot channel is modulo 2 added 
to the short code and is then QPSK or BPSK spread across the CDMA FD RF 
channel bandwidth. The following different pseudonoise (PN) code offsets 
are provided: (a) a PN code offset to allow a user terminal 13 to uniquely 
identify a gateway 18; (b) a PN code offset to allow the user terminal 13 
to uniquely identify a satellite 12; and (c) a PN code offset to allow the 
user terminal 13 to uniquely identify a given one of the 16 beams that is 
transmitted from the satellite 12. Pilot PN codes from different ones of 
the satellites 12 are assigned different time/phase offsets from the same 
pilot seed PN code. 
If used, each pilot channel that is transmitted by the gateway 18 may be 
transmitted at a higher or lower power level than the other signals. A 
pilot channel enables a user terminal 13 to acquire the timing of the 
forward CDMA channel, provides a phase reference for coherent 
demodulation, and provides a mechanism to perform signal strength 
comparisons to determine when to initiate handoff. The use of the pilot 
channel is not, however, mandatory, and other techniques can be employed 
for this purpose. 
The Sync channel generates a data stream that includes the following 
information: (a) time of day; (b) transmitting gateway identification; (c) 
satellite ephemeris; and (d) assigned paging channel. The Sync data is 
applied to a convolution encoder 53h where the data is convolutionally 
encoded and subsequently block interleaved to combat fast fades. The 
resulting data stream is modulo two added to the synchronous Walsh code 
and QPSK or BPSK spread across the CDMA FD RF channel bandwidth. 
The Paging channel is applied to a convolutional encoder 53i where it is 
convolutionally encoded and is then block interleaved. The resulting data 
stream is combined with the output of a long code generator 53j. The long 
PN code is used to separate different user terminal 13 bands. The paging 
channel and the long code are modulo two added and provided to a symbol 
cover where the resulting signal is modulo two added to the Walsh Code. 
The result is then QPSK or BPSK spread across the CDMA FD RF channel 
bandwidth. 
In general, the paging channel conveys several message types which include: 
(a) a system parameter message; (b) an access parameter message; and (c) a 
CDMA channel list message. 
The system parameter message includes the configuration of the paging 
channel, registration parameters, and parameters to aid in acquisition. 
The access parameters message includes the configuration of the access 
channel and the access channel data rate. The CDMA channel list message 
conveys, if used, an associated pilot identification and Walsh code 
assignment. 
The vocoder 53k encodes the voice into a PCM forward traffic data stream. 
The forward traffic data stream is applied to a convolutional encoder 53l 
where it is convolutionally encoded and then block interleaved in block 
53f. The resulting data stream is combined with the output of a user long 
code block 53k. The user long code is employed to separate different 
subscriber channels. The resulting data stream is then power controlled in 
multiplexer (MUX) 53m, modulo two added to the Walsh code, and then QPSK 
or BPSK spread across the CDMA FD RF communication channel bandwidth. 
The gateway 18 operates to demodulate the CDMA return link(s). There are 
two different codes for the return link: (a) the zero offset code; and (b) 
the long code. These are used by the two different types of return link 
CDMA Channels, namely the access channel and the return traffic channel. 
For the access channel the gateway 18 receives and decodes a burst on the 
access channel that requests access. The access channel message is 
embodied in a long preamble followed by a relatively small amount of data. 
The preamble is the user terminal's long PN code. Each user terminal 13 
has a unique long PN code generated by a unique time offset into the 
common PN generator polynomial. 
After receiving the access request, the gateway 18 sends a message on the 
forward link paging channel (blocks 53e, 53i, 53j) acknowledging receipt 
of the access request and assigning a Walsh code to the user terminal 13 
to establish a traffic channel. The gateway 18 also assigns a frequency 
channel to the user terminal 13. Both the user terminal 13 and the gateway 
18 switch to the assigned channel element and begin duplex communications 
using the assigned Walsh (spreading) code(s). 
The return traffic channel is generated in the user terminal 13 by 
convolutionally encoding the digital data from the local data source or 
the user terminal vocoder. The data is then block interleaved at 
predetermined intervals and is applied to a 128-Ary modulator and a data 
burst randomizer to reduce clashing. The data is then added to the zero 
offset PN code and transmitted through one or more of the satellites 12 to 
the gateway 18. 
The gateway 18 processes the return link by using, by example, a Fast 
Hadamard Transform (FHT) to demodulate the 128-Ary Walsh Code and provide 
the demodulated information to the diversity combiner. 
The foregoing has been a description of a presently preferred embodiment of 
the communication system 10. A description is now made of a presently 
preferred embodiment of the gateway-to-gateway relay system that is 
illustrated in FIGS. 6 and 7. 
In accordance with the teaching of this invention multiple transceiver 
repeaters are located within overlapped coverage areas of two or more of 
the satellites 12. Any number of these repeaters may be employed, however 
for the purpose of the ensuing description two gateways 18 (designated A 
and B) and two satellites 12 and 12', having overlapping coverage areas 1 
and 2 (respectively), are described in the context of a single LEOS relay 
station 70. 
Referring first to FIG. 6 for a forward link embodiment, the gateway A 
transmits a signal (which includes routing information and which may 
include other systems operation information) to a satellite 12 in the 
portion of the constellation that is currently overhead. The signal is 
received by antenna 12h (which may be a single beam or a beam of a 
multiple beam antenna) and is routed to the receiver which in turn sends 
the amplified signal, translated in frequency, to the transmitter and, 
thence, to transmit antenna 12d. The transmit antenna 12d may also be a 
single beam or a beam of a multiple beam array, and which forms a first 
coverage area on the earth. The second satellite 12' has the receive 
antenna 12c (which may be a single beam or a beam of a multiple beam 
antenna) with a second coverage area which overlaps the first coverage 
area within an overlap region designated 72. Located within the overlap 
region 72 is the LEOS relay station 70. The LEOS relay station 70 receives 
the signal transmitted from the satellite 12 with antenna 70d (which may 
be a directional, tracking, or omni-directional antenna) and with a 
receiver 70a. The receiver 70a demodulates the received signal to extract 
at least signal routing information therefrom. The LEOS relay station 70 
subsequently employs a transmitter 70b and antenna 70e (which may be a 
directional, tracking, or omni-directional antenna) to transmit the 
signal, shifted in frequency, to satellite 12'. The signal routing 
information that is extracted by the receiver 70a is processed by a 
controller 70c. The transmitted signal is received by satellite antenna 
12c (which may be a single beam or a beam of a multiple beam antenna) and 
sent to the receiver which in turn sends the amplified signal, translated 
in frequency, to the transmitter and, via antenna 12g (which may also be a 
single beam or a beam of a multiple beam antenna), to the gateway B. 
Gateway B serves the second coverage area. 
Referring to FIG. 7 for the return link, the gateway B transmits a signal 
(which includes routing information and may include other systems 
operation information) to the satellite 12'. The signal is received by 
antenna 12h (which may be a single beam or a beam of a multiple beam 
antenna) and sent to the receiver which in turn sends the amplified 
signal, translated in frequency, to the transmitter and, thence, to 
transmit antenna 12d. The transmit antenna 12d may also be a single beam 
or a beam of a multiple beam array, and which forms the first coverage 
area on the earth. The satellite 12 includes the receive antenna 12c 
(which may be a single beam or a beam of a multiple beam antenna) having a 
second coverage area which overlaps the first coverage area within the 
overlap region 72. The LEOS relay station 70 receives the signal 
transmitted from the satellite 12' with an antenna 70d' (which may be a 
directional, tracking, or omni-directional antenna) and with a receiver 
70a' of a second transmitter/receiver (transceiver) pair. The receiver 
70a' demodulates the received signal to extract at least the signal 
routing information therefrom. The LEOS relay station 70 subsequently 
employs transmitter 70b' and antenna 70e' (which may be a directional, 
tracking, or omni-directional antenna) to transmit the signal, shifted in 
frequency, to satellite 12. The transmitted signal is received by 
satellite antenna 12c (which may be a single beam or a beam of a multiple 
beam antenna) and is sent to the receiver which in turn sends the 
amplified signal, translated in frequency, to the transmitter and, via 
antenna 12g (which may be a single beam or a beam of a multiple beam 
antenna), to the gateway A. 
In accordance with the foregoing description, and referring also to FIG. 3A 
which shows the various transmit and receive antennas of a satellite 12, 
it can be seen that the LEOS relay station 70 receives and transmits SS 
signals with the frequencies normally employed by the user terminals 13. 
Only one LEOS relay station 70 is required to be located within a given 
overlap region 72, although more can be so positioned to provide immunity 
from fading and signal blockages due to obstructions as the elevation 
angles of the satellites 12 and 12' vary. Preferably a significant portion 
of the earth's surface has the overlapped coverage regions 72, and 
therefore a given one of the LEOS relay stations 70 is typically located 
at some intermediate distance between gateway A and gateway B. The 
controller 70c within the LEOS relay station 70 may provide alternate 
routing if required. 
Operation of the system occurs as is depicted in the flow chart of FIG. 8. 
Referring also to FIG. 6, it is assumed that a user employs the gateway A 
to place a call, via gateway B, to another user (Block A). The call set-up 
data is packetized or otherwise formatted in the gateway A to include call 
routing (destination code) information and a request for service is 
transmitted, with the routing information, via the satellite 12 to all 
LEOS relay stations 70 in view of the satellite 12 or within a specific 
beam of the satellite 12 (Blocks B and C). Each LEOS relay station 70 down 
converts the received signal to baseband (or to a point sufficient to 
extract the destination and any instructional information), using a 
suitable SS-CDMA despreader and demodulator, and extracts the destination 
code information from the received signal. The controller 70c of at least 
one of the receiving LEOS relay stations 70 selects a satellite 12, or 
broadcasts to all satellites in view of the LEOS relay station 70, to 
route the signal to a further gateway 18. The relay satellite may be 
selected as a function of the destination information included in the 
packetized call routing information, or may be selected based on a 
database look-up table. Assuming for this discussion that the gateway B is 
the selected gateway, the signal is then transmitted to gateway B via 
satellite 12' (Block D). Gateway B then down converts the received signal 
to baseband and, depending upon the destination code, makes a 
determination (Block E) if the gateway B is the final destination gateway. 
If the result of the determination is no, then control passes to Block B 
where the call may be routed through a further satellite 12, gateway 18, 
and possibly a further LEOS relay station 70. If the result of the 
determination at Block E is yes, the gateway B may connect the call to the 
local telephone infrastructure segment 4, such as routing the call to the 
local PSTN (Block F). Return messages, if any, are processed in the same 
manner. It can be appreciated that near real time voice, data and 
messaging is made possible using this technique. 
The LEOS relay station 70 is preferably constructed with two independent 
transceivers operating in pairs (70a, 70b and 70a', 70b'). The LEOS relay 
station 70 operates under the control of the serving gateways 18, and 
programs included in the controller 70c enable real time or preprogrammed 
routing decisions to be made locally at the LEOS relay station 70. 
Directional antennas 70d, 70e (FIG. 6) and 70d', 70e' (FIG. 7) may be used 
at the LEOS relay station 70, although non-directional or omni-directional 
antennas may also be employed. Each LEOS relay station 70 includes 
circuitry, such as that illustrated in FIGS. 2 and 5, for despreading, 
demodulating, tracking, and transmitting a spread spectrum signal. Each 
LEOS relay station 70 preferably is constructed to have multiple finger 
rake receivers, or other suitable receiver types, that are capable of 
simultaneously receiving and tracking multiple SS signals. As a result, a 
single LEOS relay station 70 can be used for simultaneously relaying 
multiple communications, such as telephone calls, from one satellite 
coverage area to another. 
Reference is now made to FIG. 9 for illustrating a first example of the use 
of this invention. In this example a plurality of satellites 12A-12D each 
have an associated coverage area (CA) 1-4, respectively. Each of the 
coverage areas 1-4 is served by a gateway 18A-18D, respectively. Overlap 
regions 72 are formed between the various coverage areas. Within each 
overlap region is at least one LEOS relay station (RS) 70. In this 
example, a system user employs phone 1 to place a call via a terrestrial 
communication system (e.g., PSTN 1) to phone 2 connected to PSTN 2. Phone 
1 and phone 2 may be separated by thousands of kilometers and may be fixed 
or mobile. In response to placing the call from phone 1 the gateway 18A 
forms a call request packet that includes destination information and 
other instructions and transmits the call request packet to LEO satellite 
12A. Satellite 12A repeats the call request on the downlink. The call 
request is received by the LEOS RS 70 in the overlap region 72 between CA1 
and CA2. This LEOS RS 70 despreads and demodulates the transmission to 
extract the destination information from the call request packet. Based on 
the destination information, or in accordance with preprogrammed 
instructions, the LEOS RS 70 selects the LEO satellite 12B to receive the 
transmission. If an omnidirectional antenna is used the LEOS RS 70 can 
broadcast to all satellites 12 that are in view of the LEOS RS 70. 
Assuming that the transmission is directed to satellite 12B, the satellite 
relays the transmission to the LEOS RS 70 in the overlap region 72 between 
CA2, CA3 and CA4. This LEOS RS 70 extracts the destination information 
from the call request packet and selects the satellite 12C to receive the 
transmission. The satellite 12C repeats the transmission on the downlink 
which is received by the gateway 18C which, based on the destination 
information, connects the call to the PSTN 2 for delivery to the phone 2. 
It can be appreciated that the call is routed from PSTN 1 to PSTN 2 without 
using the PSTN that is connected to either the gateway 18B or the gateway 
18D. Furthermore, the call is routed without requiring any 
satellite-to-satellite communication links. This greatly simplifies the 
construction and operation of the satellites 12A-12B. 
FIG. 10 is a further example of the utility of this invention. In FIG. 10 a 
user terminal 13 within CA1 initiates a call to a mobile user having a 
mobile station (MS) that is coupled via a base station (BS) and a mobile 
switching center (MSC) to a PSTN that is connected to gateway 18D within 
CA4. The call initiated by mobile user 13 is relayed via satellite 12A to 
the gateway 18A in the manner described previously with respect to FIG. 1. 
The gateway 18A forms a call request packet that includes destination and 
other information and transmits the call request packet to LEO satellite 
12A. Satellite 12A repeats the call request on the downlink. The call 
request is received by the LEOS RS 70 in the overlap region 72 between CA1 
and CA2. This LEOS RS 70 despreads and demodulates the transmission to 
extract the destination information from the call request packet. Based on 
the destination information, the LEOS RS 70 selects the LEO satellite 12B 
to receive the transmission. Satellite 12B relays the transmission to the 
LEOS RS 70 in the overlap region 72 between CA2, CA3 and CA4. This LEOS RS 
70 extracts the destination information from the call request packet and 
selects the satellite 12D to receive the transmission. The satellite 12D 
repeats the transmission on the downlink which is received by the gateway 
18D which, based on the destination information, connects the call to the 
PSTN for delivery, via the MSC and BS, to the MS. 
Although described in the context of a SS-CDMA communication system the 
teaching of this invention is not so limited. That is, the teaching of 
this invention may also be used, with suitable adaptation, with other 
types of communication systems, such as time division multiple access 
(TDMA), frequency division multiple access (FDMA), and hybrid systems, 
such as a TD-SS communication system. Furthermore, aspects of this 
invention can also be employed in non-LEO satellite systems, such as in 
mid-earth orbit satellite systems (e.g., inclined orbits in the range of 
approximately 5000 to 10,000 nautical miles). 
Thus, while the invention has been particularly shown and described with 
respect to preferred embodiments thereof, it will be understood by those 
skilled in the art that changes in form and details may be made therein 
without departing from the scope and spirit of the invention.