Note: Descriptions are shown in the official language in which they were submitted.
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SATELLITE COMMUNICATION SYSTEM EMPLOYING A COMBINATION OF
TIME DIVISION MULTIPLEXING AND NON-ORTHOGONAL
PSEUDORANDOM NOISE CODES AND TIME SLOTS
I. FIELD OF THE INVENTION
The present invention relates to cellular telephone systems. More
specifically, the present invention relates to new and improved systems and
methods for communicating information in mobile cellular telephone systems or
satellite mobile telephone systems employing spread spectrum communication
signals.
II. BACKGROUND OF THE INVENTION
Historically, the telephone, which comes from the Greek word 'tele',
meaning from afar, and 'phone', meaning voice or voice sound, is said to have
been invented on March 10, 1876 in Boston, Massachusetts by Alexander
Graham Bell. The principle of the telephone was conceived as early as 1874
combining electricity and voice which led to Bell's actual invention of the
telephone in 1876.
U. S. Patent No. 174,465 issued March 3, 1876 for improvements in
telegraphy is now considered to be the most valuable patent ever issued.
Telstar, the world's first international communications satellite, years later
was placed into orbit on July 10, 1962 in a collaboration between NASA and the
Bell System. Today satellites in geosynchronous orbit are used mostly for long
distance service.
The basic concept of cellular phones which began in 1947 with crude
mobile car phones resulted in the realization that using small cells or range
of
service area with frequency re-use could increase the traffic capacity of
mobile
phones substantially. However, at this point in time the technology was
nonexistent. The cellular telephone is in fact a type of two-way radio which
in
1947 AT&T proposed at the FCC allocated large number of radio spectrum
frequencies so that widespread mobile phone service could become feasible and
provide AT&T an incentive to research the new technology. The FCC's decision
to limit the cellular phone frequencies in 1947 resulted in the possibility of
only 23
cellular phone conversations which could occur simultaneously in the same
service area. In 1968 this was increased. Thereafter, a cellular phone system
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was proposed by Bell Laboratories. In 1977 AT&T Bell Labs constructed and
operated a prototype cellular phone system. In 1981 Motorola and America
Radio Phone started a second U. S. cellular radio phone system test in the
Washington/Baltimore area. Suddenly consumer demand quickly outstripped the
cellular phone system's 1982 standards so that by 1987 cellular phone
subscribers exceeded one million and the airwaves were crowded. To stimulate
the growth of new cellular phone technology, the FCC declared in 1987 that
cellular phone licenses may employ alternative cellular phone technologies in
the
800 megahertz band.
Digital wireless and cellular find their roots back in the 1940s when
commercial mobile telephony began. On June 17, 1946 in St. Louis, Missouri,
AT&T and Southwestern Bell introduced the first American commercial mobile
radio telephone service and mobile telephony a channel is a pair of
frequencies,
one frequency to transmit on and one to receive.
A cell phone is a portable telephone which receives or sends messages
through a cell site or transmitting tower. Radio waves are used to transfer
signals
to and from the cell phone, each cell site having a range of 3-5 miles and
overlapping other cell sites. All of the cell sites are connected to one or
more
cellular switching exchanges which can detect the strength of the signal
received
from the telephone. As the telephone user moves or roams from one cell area to
another, the exchange automatically switches the call to the cell site with
the
strongest signal. The term 'cell phone' is uncommon outside the United States
and Japan. However, almost all mobile phones use cellular technology including
GSM, CDMA and the old analog mobile phone systems. Hence, the term 'cell
phone' has been regarded by many to designate any mobile telephone system.
An exception to mobile phones which employ cellular technology are satellite
phones; for example, the Iridium phone system which is very much like a cell
phone system except the cell sites are in orbit. Marine radio telephone
satellites
administered by Inmarsat have a completely different system. The lnmarsat
satellite system simply retransmits whatever signals it receives with a mobile
station's actually logging into a ground station.
With the advent of the Globalstar satellite telephone system, a great
advance in the art was recognized by virtue of a basic telephonic satellite
technology which provided a constellation of 48 satellites in low earth orbit
which
were much simpler to build and less expensive than those of Iridium employing
a
radically different technology which employs code division multiple access, or
CDMA, technology, converting speech signals into a digital format and then
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transmitting it from the Globalstar satellite phone up to satellite systems
and
down to the ground station. Every call on the Globalstar system possesses its
own unique code which distinguishes it from the other calls sharing the
airwaves
at the same time, and employing CDMA provides signals which are free of
interference, cross-talk or static. CDMA was introduced in 1995 and soon
became the fastest growing wireless technology and one that was chosen by
Globalstar for use in its satellite communications network, which service
Globalstar launched in 2000.
The key features of the Globalstar satellite phone employing CDMA
provide unique forward and reverse links, direct sequence spread spectrum,
seamless soft handoff, universal frequency re-use, propagation through
multiple
overlapping beams on multiple satellites for diversity, and variable rate
transmission.
The Globalstar satellite phone service is delivered through 48 low earth
orbiting satellites providing both voice and data services. The so-called
Globalstar LEO constellation consists of satellites arranged in a Walker
constellation, and each satellite is approximately 700 miles from the earth
which
allows for the highest quality voice clarity of any satellite phone in the
industry. At
the heart of the Globalstar system as initially proposed is Qualcomm's
adaptation of code division multiple access technology which provides
Globalstar's digital satellite service, resulting in a technology which
provides
signal security, superior quality, fewer dropped calls and greater
reliability. Calls
can be made from any gateway via any satellite of the system to any user
terminal, as long as the satellite is co-visible from both gateway and user
terminal. This co-visibility is what defines a gateway service area; at least
24
gateways around the globe are used to provide worldwide coverage. Each
satellite serves at least 2,000 simultaneous users.
The Globalstar system employs redundancy with every call that a
customer places so that a call is routed through as many as four satellites
which
then combine the signal into a single static-free call. In the event that one
of the
paths to one of the satellites is blocked, the other satellites keep the call
from
terminating, applying the technology of path diversity which minimizes dropped
calls and enhances the quality of the Globalstar satellite phone service. The
Globalstar system employs bent pipe technology which allows a call to be
first
beamed up to the satellite and then retransmitted to a relatively close
gateway.
The call is then sent through its call destination through land line or
cellular
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networks. The Globalstare gateway carries out all the processing and switching
of the calls which improves the reliability of the call delivery, unlike the
Iridium
system which requires satellite-to-satellite transmission.
In addition, the Globalstare system, which provides reliable call delivery
with voice characteristics the same or better than conventional telephony,
complements the current cellular telephone systems in existence by allowing
the
user to first use conventional cellular, which is far less expensive but
totally
dependent upon the proximity of cell sites for its reliability, and then
allows the
user to select the Globalstar0 satellite system where cell sites are far too
distant
to be reliable or in remote locations where these sites are non-existent. Code
division multiple access, which refers to a multiple access scheme where
stations
use spread spectrum modulations and orthogonal codes to avoid interfering with
one another, is typically employed in Globalstar0 systems. The CDMA
modulation technique is one of several techniques for facilitating
communications
in which a large number of system users are present. Other multiple access -
communications system techniques such as time division multiple access
(TDMA), frequency division multiple access (FDMA), and AM modulation
schemes such as amplitude expanded single sideband (ACSSB) are known in
the art. The spread spectrum modulation technique of CDMA is found to have
significant advantages over these modulation techniques for multiple access
communications systems. CDMA techniques in multiple access communications
systems are disclosed in U. S. Patent No. 4,901,307 entitled Spread Spectrum
Multiple Access Communication System Using Satellite or Terrestrial Repeaters.
In this patent, a multiple access technique is disclosed where a large
number of mobile telephone system users each having a transceiver
communicate through satellite repeaters or terrestrial base stations, also
referred
to as cell sites stations, cell sites, or for short cells, using code division
multiple
access (CDMA) spread spectrum communication signals. Frequency spectrum
employed in CDMA can be reused multiple times, thus permitting an increase in
system user capacity. The CDMA is found to result in a much higher spectral
efficiency than can be achieved using other multiple access techniques.
Satellite channels employing this system typically experience fading that
is characterized as Rician. Accordingly, this signal is found to consist of a
direct
component summed with a multiple reflected component having a Rayleigh
fading statistic. A power ratio between the direct and reflected component is
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typically found to be on the order of 6 to 10 dBs depending upon the
characteristics of the mobile unit antenna and the environment about the
mobile
unit. Contrasted to the satellite channel, the terrestrial channel experiences
signal fading that typically consists of the Rayleigh faded component without
a
5 direct component. This terrestrial channel is found to present a more
severe
fading environment than the satellite channel in which the Rician fading is
the
dominant fading characteristic.
The Rayleigh fading characteristics experienced in the terrestrial signal is
found to be caused by the signal being reflected from many different features
of
the physical environment, resulting in a signal which arrives at a mobile unit
receiver from many directions with different transmission delays. In the UHF
frequency bands which are usually employed for mobile radio communications,
including cellular mobile telephone systems, there is found to be significant
phase
differences in signals traveling on different paths which provides the
possibility of
destructive summation of the signals causing occasional deep fades. Physical
position of the mobile unit is a strong function of the terrestrial channel
fading so
that small changes in the position of the mobile unit change the physical
delays of
all the signal propagation paths which further result in a different phase for
each
path. The motion of the mobile unit through the environment can result in a
rapid
fading process; for example, employing 850 MHz cellular radio frequency band,
the fading can typically be as fast as one fade per second per mile per hour
of the
vehicle speed. This level of fading is found to be extremely disruptive to
signals
in a terrestrial channel, resulting in poor communication quality. Quality may
be
improved by providing additional power to overcome the fading, which in itself
affects both the user in excessive power consumption and the system by
increased interference. Certain CDMA modulation techniques disclosed in U. S.
Patent 4,901,307 offer some advantages over narrow band modulation
techniques using communication systems employing satellite or terrestrial
repeaters. The terrestrial channel is found to pose special problems to any
communication system, particularly with respect to multiple path. These
problems may be overcome by using CDMA techniques which overcome the
special problems of the terrestrial channel by mitigating the adverse effect
of
multipath, for example fading, while also exploiting the advantages of
multipath.
CDMA cellular telephone systems allow the same frequency band to be
employed for communication in all calls. CDMA waveform properties that provide
processing gain are also used to discriminate between signals that occupy the
same frequency band. Furthermore, the high speed pseudo-noise PN
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modulation allows many different propagation paths to be separated provided
the
difference in path delay exceed the PN chip duration; i.e., 1/bandwidth. It is
found that if a PN chip rate of approximately one MHz is employed in a CDM
system, the full spread spectrum processing gain equal to the ratio of the
spread
bandwidth to system data rate can be employed against paths that differ by
more
than one microsecond in path delay from desired path. It is found that a one
microsecond path delay differential corresponds to differential path distance
of
approximately 1,000 feet, the urban environment typically providing
differential
path delays in excess of one microsecond and up to 10-20 microseconds in some
areas. When narrow band modulation systems are employed, such as at analog
FM modulation, by conventional telephone systems, the existence of multiple
paths results in severe multipath fading. By employing wideband CDMA
modulation, the different paths may be discriminated against in the
demodulation
process which greatly reduces the severity of multipath fading. Although
multipath fading is not totally eliminated using CDMA discrimination
techniques,
there will occasionally exist paths with delayed differentials of less than
the PN
chip duration for the particular system. For signals which possess path delays
on
this order, it is found that signals cannot be discriminated against in the
demodulator, resulting in some degree of fading.
It becomes apparent that some form of diversity is desirable which would
permit a system to reduce fading. One such system is diversity which mitigates
the deleterious effects of fading. The three major types of diversity which
may be
employed are time diversity, frequency diversity and space diversity. Time
diversity is found to be best obtained by the use of repetition, time
interleaving
and error detection and coding which is a form of repetition.
CDMA by its inherent nature possessing a wide band signal which offers a
form of frequency diversity by spreading the signal energy over a wide
bandwidth,
resulting in a small part of the CDMA signal bandwidth experiencing selective
fading effects.
Space or path diversity is obtained by providing multiple signal paths
through simultaneously links from a mobile user through two or more cell
sites.
Path diversity may be obtained by exploiting the multipath environment through
spread spectrum processing by allowing a signal arriving with different
propagation delays to be received and processed separately. In U. S. Patent
No.
5, 101,501 entitled Soft Handoff in a CDMA Cellular Telephone System, and
U. S. Patent No. 5,109,390 entitled Diversity Receiver in a CDMA Cellular
Telephone System, examples of path diversity are illustrated. Further control
of
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deleterious effects in a CDMA system may be realized by controlling
transmitter
power. Such a system for cell site mobile unit power control is disclosed in
U. S.
Patent No. 5,056,109 entitled Method and Apparatus for Controlling
Transmission
Power in a CDMA Cellular Mobile Telephone System. Techniques as disclosed
in U. S. Patent No. 4,901,307 contemplate the use of coherent modulation and
demodulation for both directions of the link in mobile satellite
communications. A
pilot carrier signal as a coherent phase reference for the satellite to mobile
link
and the cell to mobile link is disclosed. It is found, however, that the
severity of
multipath fading experienced in the terrestrial cellular environment with the
resulting phase disruption of the channel precludes usage of coherent
demodulation techniques for the mobile to cell link.
Relatively long PN sequences with each user channel being assigned a
different PN sequence are also disclosed in U. S. Patent No. 4,901,307. The
different user signals may be discriminated upon reception employing the cross
correlation between different PN sequences and the auto correlation of a PN
sequence for all time shifts other than zero where both have a zero average
value. Although the cross correlations average zero for a short time interval,
such as an information bit time, the cross correlation follows a binomial
distribution since PN signals are not orthogonal. As such, signals interfere
with
each other much the same as if they were wide bandwidth Gaussian noise
resulting in other user signals or mutual interference noise ultimately
limiting the
achievable capacity.
Multipath can provide path diversity to a wide band PN CDMA system
which uses greater than 1 MHz bandwidth if two or more paths are available
with
greater than one microsecond differential path delay. Two or more PN receivers
can be employed to separately receive these signals. These signals typically
will
exhibit independence in multipath fading, i.e., they usually do not fade
together,
the outputs of the two receivers can be diversity combined. It is found that a
loss
in performance in this situation only occurs when both receivers experience
fades
at the same time, hence two or more PN receivers in combination with a
diversity
combiner may be employed utilizing a waveform that permits path diversity
combining operations to be performed.
In U. S. Patent No. 4,901,307 filed October 17, 1986, issued February 13,
1990, a communication system which accommodates a large number of users
throughout a variety of user environments from high density urban to very low
density rural is provided which results in a multiple access communication
system
having high simultaneous user capacity.
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In U. S. Patent No. 5,101,501 filed November 7, 1989, issued March 31,
1992, there is disclosed a CDMA cellular telephone system wherein the same
frequency band is used for all cells employing CDMA waveform properties that
provide processing gains which are also used to discriminate between signals
that occupy the same frequency band.
In U. S. Patent No. 5,103,459 filed June 25, 1990, issued April 7, 1992,
there is disclosed spread spectrum communication techniques, particularly
CDMA techniques, in the mobile cellular telephone environment which provide
features to vastly enhance system reliability and capacity over other
communication system techniques overcoming fading and interference while
providing greater frequency reuse and enabling a substantial increase in the
number of system users.
In U. S. Patent No. 5,109,390 filed November 7, 1989, issued April 28,
1992, there is disclosed a CDMA cellular telephone system where the same
frequency band is used for communication in all cells to provide a cellular
telephone system in which a receiver design facilitates reception and
processing
of the strongest signals transmitted from one or more cell sites, the signals
being
multipath signals from a single cell site or signals transmitted by multiple
cell
sites.
In U. S. Patent No. 5,233,626 filed May 11, 1992, issued August 3, 1993,
there is disclosed a repeater diversity spread spectrum communication system
providing substantially fade free communications between a transmitter (1) and
a
receiver (7). A transmitted signal is relayed through a plurality of linear
communications repeaters (3-6) that produce copies of the transmitted signal,
the
copies each arriving through an independently fading signal path. The receiver
processes the received signal copies to equalize them to one another in delay,
frequency, and phase, and then combines the multiple received and equalized
signal copies to produce a composite signal having a greatly reduced fading
depth.
In U. S. Patent No. 5,267,261 filed March 5, 1992, issued November 30,
1993, there is provided a system for directing handoff in mobile station
communication between base stations which employ code division multiple
access techniques.
In U. S. Patent No. 5,267,262 filed October 8, 1991, issued November 30,
1993, there is disclosed a CDMA cellular mobile telephone wherein the
transmitter power of the mobile units are controlled so as to produce at the
cell
site a nominal received signal power from each and every mobile unit
transmitter
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operating within the cell. Thus, the transmitter power is controlled in the
terrestrial channel and the cell diversity environment so as to overcome
deleterious fading without causing unnecessary system interference.
In U. S. Patent No. 5,303,286 filed March 29, 1991, issued April 12, 1994,
there is disclosed a radio communication system capable of servicing a roaming
user or the like outside the range of terrestrial relay stations including a
packet
switched network and database of roaming users, a satellite communications
system having at least one, but usually a plurality of orbiting satellites
over a
terrestrial satellite service area, a satellite control center and a plurality
of
terrestrial communication links wherein call setup is controlled by processors
and
databases onboard the orbiting satellites and wherein only after the satellite
link
for the communication channels is completed, does control and switching rely
on
ground base system such that the orbiting satellites are integrated into a
ground
based telephone network and tariff structure.
In U. S. Patent No. 5,309,474 filed March 27, 1992, issued May 3, 1994,
there is disclosed spread spectrum communication techniques, particularly
CDMA, in a mobile cellular telephone environment which provides features to
vastly enhance system reliability and capacity over other communication system
techniques.
In U. S. Patent No. 5,416,797 filed January 24, 1992, issued May 16,
1995, there is disclosed a system for constructing PN sequences that provide
orthogonality between the users so that mutual interference will be reduced
allowing higher capacity and better link performance, employing spread
spectrum
communication techniques, particularly CDMA, in a mobile cellular telephone
environment.
In U. S. Patent No. 5,715,297 filed September 15, 1995, issued
February 3, 1998, there is disclosed a radio communication system capable of
servicing a roaming user or the like outside the range of terrestrial relay
stations
which includes a packet switched network and database of roaming users, a
satellite communications system having at least one, but usually a plurality
of
orbiting satellites over a terrestrial satellite service area, a satellite
control center
and a plurality of terrestrial communication links, wherein call setup is
controlled
by processors and databases onboard the orbiting satellites and wherein only
after the satellite link for the communication channels is completed, does
control
and switching rely on ground based equipment such that the orbiting satellites
are integrated to a ground based telephone network and tariff structure.
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In U.S. Patent No. 5,903,837 filed September 22, 1997, issued May 11,
1999, there is disclosed a radio communication system capable of servicing a
roaming user or the like outside the range of terrestrial relay stations which
includes a packet switched network and database of roaming users, a satellite
5 communications system having at least one, but usually a plurality of
orbiting
satellites over a terrestrial satellite service area, a satellite control
center and a
plurality of terrestrial communication links wherein call setup is controlled
by
processors and databases onboard the orbiting satellites and wherein only
after
the satellite link for the communication channel is completed, does control
and
10 switching rely on ground based equipment such that the orbiting
satellites are
integrated into a ground based telephone network and tariff structure.
In U. S. Patent No. 6,072,768 filed September 4, 1996, issued June 6,
2000, there is disclosed a communication system having a satellite
communication component comprising at least one satellite and at least one
terrestrial gateway and also a wireless terrestrial communication component
comprising at least one repeater and at least one mobile switching center, the
gateway and switching center coupled together by a first mobile applications
part
network, the gateway and the mobile switching center further coupled to a
terrestrial communication network, further including at least one dual mode or
higher tri-mode user terminal comprising a first transceiver for
bidirectionally
communicating with the gateway through the satellite, a second transceiver for
bidirectionally communicating with the mobile switching center through the
repeater and a controller responsive to one of a user selected or a gateway
selected protocol for selectively enabling either the first or the second
transceiver
for conveying a user communication to a terrestrial communication network.
In U. S. Patent No. 6,529,485 filed October 20, 1998, issued March 4,
2003, there is disclosed a method for generating a Doppler-free local clock in
a
communications network having a master reference terminal (400) and a terminal
(200) exchanging references and management bursts, includes steps for
determining a transmit timing correction value responsive to the management
burst received by the master reference terminal (400), determining a receive
timing correction value responsive to the reference burst received by the
terminal
(200), and adjusting the frequency of a clock responsive to both the transmit
timing correction value and the receive timing correction value to thereby
generate the Doppler-free local clock.
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In U. S. Patent No. 6,640,236 filed August 31, 1999, issued October 28,
2003, there disclosed an apparatus for generating a PN sequence with an
arbitrary number of bits where the number of bits is provided in parallel with
each
clock pulse, allowing the sequences to be generated at high speed when needed
and allowing parallel processing in the acquisition and demodulation
processes.
In U. S. Patent No. 6,693,951 filed July 23, 1999, issued February 17,
2004, there is disclosed implementation of spread spectrum communication
techniques, particularly CDMA, in a mobile cellular telephone environment
which
provides features that vastly enhance system reliability and capacity over
other
communication system techniques, overcoming, for example, fading and
interference while promoting greater frequency reuse, enabling a substantial
increase in the number of system users.
In U. S. Patent No. 6,714,780 filed June 12, 2001, issued March 30, 2004,
there is disclosed a multibeam communication system having a user terminal, a
communications station for transmitting information to and receiving
information
from the user terminal and a plurality of beam sources where each beam source
projects a plurality of beams and where a communication link between the user
terminal and the communications station is established on one or more beams,
providing a system and method for reducing call dropping rates while
maintaining
a desired level of beam source diversity.
In U. S. Patent No. 6,813,259 filed July 15, 1998, issued November 2,
2004, there is disclosed a method and apparatus for providing a low 2-point
Cell
delay variation (CDV) for cell or packet transmissions via a TDMA or TDM
network, where the cells or packets are assembled in bursts or slots for
transmission. In order to permit a TDMA or TDM network that carries cells or
packets between source and destination pairs to guarantee that a desired 2-
point
CDV will be met, for example, a 3 ms CDV required for Class 1 traffic, each
cell is
associated with a transmitted TDMA or TDM frame. Using a time counter and a
frame counter in a transmitter interface, the cell or packet has appended to
it a
time count and a frame count that is sent across the network and made
available
to the receiving TDMA/TDM terminal. The receiving terminal uses this timing
information to perform traffic shaping of the cell or packet stream, thereby
reducing the impact of the 2-point CDV as well as the effect of cell clumping
prior
to distribution on a terrestrial network.
In U. S. Patent No. 6,839,007 filed September 9, 2002, issued January 4,
2005, there is disclosed embodiments which address the need for reliable
transmission of higher priority data within a frame wherein an inner code is
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applied to one or more partial segments of a transmitted data frame, in
addition to
an outer code applied to the entire frame, the inner code segment being
retained
when the inner decoding decodes without error providing the benefit of
reducing
the number of retransmissions of higher priority data, as well as reducing
delay
for time sensitive segments of the frame.
Various satellite telephone systems have been proposed, including those
as depicted in the FCC filing for "Authority to Launch and Operate a Satellite
System to Provide Mobile Satellite Services in the 2 GHz Bands" dated
November 3, 2000, relating to the Globalstare system, which is hereby
incorporated by reference; the FCC filing in the matter of Mobile Satellite
Ventures Subsidiary, LLC for "Minor Amendment of Application to Launch and
Operate a Replacement L Band Mobile Service Satellite at 101 West" dated
November 18, 2003; and the FCC filing by Thoraya which depicts a one GEO
satellite system to provide a satellite telephone service.
Thus, it can be seen from the inception of the telephone through its
various phases of improvement, cellular to satellite cellular telephony, a
vast
number of advances have been made which provide a modern, efficient and
affordable telephone system which today, in many cases, supplants the existing
telephone system and may in the future do so on an increasing basis.
There is, however, a continuing demonstrated need to provide improved
satellite constellation systems, preferably LEO systems, which provide
multiple
beams to a plurality of users and employ at least one gateway connected to a
PSTN communicating with a user over the constellation where each of the users
within a given frequency band is distinguished from another employing
orthogonal codes.
Although previous patents such as U. S. Patent No. 4,901,307 describe or
reference a multi-beam satellite system, these beams are considered to cover
fixed regions on the ground, which requires a GEO satellite. In this case, the
same sort of hand-off of a user terminal from beam to beam can be used as is
used in a terrestrial cellular system. However, the '307 patent does not
address
the case where the beams and satellites are rapidly moving as they are in a
ME0
or LEO system, since it was written in an era that preceded the satellite
technology that enabled large numbers of relatively smaller satellites (such
as
Globalstars ) to be economically and reliably launched and controlled.
Therefore, the hand-off issues described in the '307 patent are much simpler
than
those encountered in the Globalstare system or similar LEO or ME0 systems, or
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even those encountered in GEO systems which have dynamically varying beam
shapes, which is another technological advance that is now feasible in
satellite
systems. That patent also does not address packet data services, since those
were not widely used in the time frame of the patent. Other patents that
address
packet data services also do not address the LEO, ME0 or dynamic beam GEO
systems. The present invention describes a multi-beam LEO, ME0 or GEO
satellite system that can be used to provide packet data services (in addition
to
voice) for mobile users, that can be either initiating or receiving packet
data calls
over the system, while communicating with either a fixed or mobile user
anywhere in the world.
OBJECTS OF THE INVENTION
It is therefore an object of an aspect of this invention to provide an
improved satellite communication system comprising at least one satellite
emplOying multiple beams to a plurality of users where each of the users
within a
given frequency band is distinguished from another employing a combination of
time division multiplexing (TDM) and non-orthogonal pseudorandom noise
(NOPN) codes and time slots.
A further object of an aspect of this invention is to provide an improved
LEO satellite system which provides multiple beams to a plurality of users
employing at least one gateway connected to a PSTN wherein each of said UTs
within a given frequency band is distinguished from another of said UTs
employing a combination of TDM and NOPN codes.
Still another object of an aspect of this invention is to provide an improved
satellite communication system which provides multiple beams to a plurality of
users employing at least one gateway connected to an Internet wherein each of
said users within a given frequency band is distinguished from another of said
UTs employing a combination of TDM and NOPN codes wherein each of said
users within a given frequency band is distinguished from another of said
users
employing a combination of NOPN codes and time slots,
Again another object of an aspect of this invention is to provide an
improved ME0 satellite system providing multiple beams to a plurality of users
employing at least one gateway connected to a PSTN or the Internet wherein
each of said users within a given frequency band is distinguished from another
of
said users employing a combination of NOPN codes and time slots.
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Yet again another object of an aspect of this invention is to provide an
improved satellite communication system which provides multiple beams to a
plurality of users employing at least one gateway connected to either a PSTN
or
the Internet wherein the users within a given frequency band are distinguished
one from the other employing a combination of NOPN codes and time slots.
IV. SUMMARY OF THE INVENTION
These and other objects of the instant invention are accomplished,
generally speaking, by providing an improved satellite communication system
employing at least one satellite using multiple beams to a plurality of user
terminals wherein a gateway is employed to connect to either a PSTN or an
Internet, communicating with a user terminal over the constellation so that
the
users within a given frequency range are distinguished one from the other
employing a combination of NOPN codes and time slots.
Accordingly, in one-aspect there is provided a satellite communication
system, comprising at least one satellite which provides multiple beams; and
at
least one gateway in communication with at least one of said at least one
satellite, said gateway also communicating with a public switched telephone
network or a data network, said satellite communication system communicating
with user terminals on a forward link and a reverse link through said at least
one
satellite, where each user terminal communicates on at least one of said
multiple
beams, wherein on at least one portion of the forward link or return link,
said
satellite communication system separates each of said user terminal's
communications by time division multiplexing employing time slots, where a
single time slot is occupied by only a single user terminal communication,
said
satellite communication system distinguishing time slot communications on
adjacent beams of each satellite of said at least one satellite by scrambling
codes
comprising non-orthogonal pseudorandom noise sequences, where each of said
beams is associated with a unique non-orthogonal pseudorandom noise
sequence.
CA 02589369 2014-03-21
14a
Thus, for example, in a preferred embodiment an improved LEO satellite
constellation system is provided comprising approximately 40 to 48 satellites
as
presently employed in the GlobalstarO system, employing multiple beams which
may reach a plurality of user terminals. This is more fully described in U. S.
Patent No. 6,272,325. A gateway is employed connected to either a PSTN or the
Internet and communicating with a user terminal over the constellation so that
each user within a given frequency band is distinguished from another of said
users employing a combination of TDM and NOPN codes.
The system described herein employs NOPN codes to serve fixed
terminals. The system includes TOM on the forward link from a gateway through
the satellite to the UT. The forward link transmission is divided into data
frames
with multiple slots per frame. Each slot is assigned to a separate UT so that
users are distinguished from each other by means of the time slots in each
frame.
Based on the location of the user, the gateway can assign a specific beam of a
separate satellite. In order to minimize interference between two users who
are
assigned the same time slot in adjacent beams, each transmission is further
modulated by a scrambling code that is a PN, or pseudorandom noise, sequence
uniquely assigned to each beam. Cross-correlation between any two of these PN
sequences is minimal, so as to reduce interference between beams. If a user's
location is covered by two different satellites, the gateway transmits to that
UT on
CA 02589369 2007-05-14
both satellites, and diversity combining is used in the UT to combine these
two
signals and improve bit error rate (BER) performance.
The power allocated to each UT in each time slot is predetermined by the
gateway and is used to vary the data rate to the UT as its propagation
5 environment changes. This technique is also referred to in the art as
HSDPA, or
high speed digital packet access in the terrestrial WCDMA standard, or
wideband
CDMA. An alternative is to use power control similar to what is employed in
the
current generation of Globalstar where the UT data rate is kept constant and
the power transmitted to the UT is varied according to propagation
environment.
10 The center frequency of the signal transmitted to each UT is adjusted
to
pre-compensate for Doppler between the gateway and satellite, thus minimizing
the search time and window that the UT needs to lock on to the signal. This
technique is currently used in the Globalstar system. Similarly, the timing
of
signals in each time slot transmitted to each UT is adjusted by the gateway
based
15 on a calculated position of each UT; this calculation may be done either
by
incorporating GPS into each UT, which informs the gateway of its coordinates,
or
by other known methods of position location, such as the techniques currently
employed in the Globalstar system which is predicated on triangulation using
multiple different delays from different satellites.
A separate narrowband control signal is transmitted from the gateway to
each UT having a fixed frequency for all UTs and is employed to inform the UTs
as to the center frequency to be used in transmitting forward link signals in
that
gateway service area.
In the reverse link from UT to satellite to gateway, each user is assigned a
different phase shift of a long PN code. These phase shifts ensure that the
cross-
correlation between different user signals at the gateway is minimal. This
technique is referred to as NOPN in this invention since these PN codes are
not
orthogonal, although they have low cross-correlation. Transmissions through
multiple satellites are combined at the gateway as in the current Globalstar
system. Each transmission from a UT consists of a short preamble which is used
to reduce burst acquisition complexity at the gateway. Each preamble
identifies
all users transmitting at a unique data rate. Reverse link power control may
be
performed as in the current Globalstar system, where data rate is fixed and
power is varied as needed to meet the link budget, or by varying the data rate
and keeping UT power fixed, as was mentioned for the forward link recited
above.
Typically, this presents a tradeoff which needs to be made between allowing a
CA 02589369 2007-05-14
16
greater number of UT data rates to improve granularity of power utilization
versus
hardware complexity at the gateway.
In this system, the gateway receiver compensates for the gateway to
satellite Doppler based on accurately known satellite positions and for the
less
precisely known UT locations.
Typically, for the forward link each user is assigned a fixed time slot of a
frame such as 5 ms slot in a 40 ms frame and different users are time division
multiplexed, or TDM, onto a single frequency carrier. In the return direction,
the
data from the different UTs is distinguished using different time slots which
may
typically be 10 MS long; a group of users assigned a particular time slot and
a
particular phase shift of a very long pseudorandom code, or PN code. Different
phase shifts of such a code are used to increase the number of users supported
since the number of time slots of a single code would limit the number of
users
that can be supported. This describes the conventional technique referred to
as
NOPN, or non-orthogonal PN code usage.
Any suitable satellite may be employed in the system of the instant
invention. Typical satellites include bent pipe repeaters, satellites equipped
with
low end processing power to those that include high processing systems.
Any suitable gateway may be employed in the system of the instant
invention. Typical gateways include the Globalstare gateway which is more
fully
described in U. S. Patent 6,804,514, Fig. 2B.
The gateway consists of the following major subsystems:
a) Transceivers and associated RE antennas, which transmit RF
signals to the satellite constellation and receive RE signals from
the satellite constellation. A typical gateway for a satellite system
has two or more antennas each of which is able to track one of the
two or more satellites visible to the gateway.
b) A TDM/NOPN code subsystem that modulates/demodulates and
spreads/despreads the CDMA signals that are being
transmitted/received by the transceivers.
c) A Gateway Controller (GC) that is used to control the operation of
all the gateway subsystems.
d) A Baseband Processing Subsystem (BPS) that processes and
transmits the baseband signals between the CDMA subsystem
and an IS-41 switch and/or a GSM switch, both of which connect
to the PSTN and enable the mobile satellite users' calls to be
routed to and from terrestrial callers on the PSTN.
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e) A Call Control Processor (CCP) that generally handles radio
setup and channel assignments, along with other call related
functions. The CCP may include the gateway Visitor Location
Register (VLR) that enables roaming between gateways.
f) Current gateways comprise a Global Mobile System Interface
(GSMI) or a router which connects to the Internet. The router
routes data packets to/from the Internet or other packet data
network. The GSMI detects the presence of a GSM call and
routes it to the GSM switch and enables GSM roaming. Optimal
systems would not have a GSM!.
The signal is received at the MSS Gateway and, after downconverting,
demodulating in transceivers and CDMA system, and otherwise being processed,
is delivered to a BPS. The signal after processing by the BPS is provided as
an
output. This output signal may be sent to a Mobile Switching Center (MSC),
such
as an IS-41 switch or a GSM switch (that contains the GSM VLR), or to a
Router,
or it may be provided directly to the HS/LS Interface in the High Speed
System.
Depending on the means chosen, the signal is either routed via an internal or
external network to the Operations Center (also referred to herein as the User
Control Center). The signal is then processed by the Operations Center and,
depending on the nature of the call setup desired, is routed to the external
network for interaction with the Media provider, or is used otherwise in the
Operations Center. The Operations Center may be collocated with the Gateway,
or it may be at a remote location and connected though the external network.
Further components of the MSS Gateway include a Call Control
Processor (CCP) that generally handles radio setup and channel assignments,
among other call-related functions. The CCP can include the Gateway VLR. A
GSMI detects the presence of a GSM call and routes the call to the GSM switch,
enabling the possibility of GSM roaming. These various components can be
included with or within a signaling system seven (SS-7 ) server unit. If
present,
the HLR could be part of the SS-7 server.
The Gateway Controller (GC) provides overall control of the Gateway, and
that provides an interface to and controls the operation of the set of High
Speed
Equipment.
It should be noted that if the media or data flowing towards the user is low
speed data, the signal after processing by the Operations Center is sent to
the
MSS system for delivery via the satellites to the UT in the normal manner of
the
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MSS system. The decision logic or point of which path (LS or HS) to use may be
located in the Operations Center, or may be located in the HS/LS Interface.
The UT can be used for the delivery of tracking and terminal control
signals, as well as for low speed (MSS) data delivery and transmission. The
MSS
system receives supervision and control signals from the Operations Control
Center or from any external facility. Alternatively, the UT and the Dual
Terminal
can be controlled from the HS Data System collocated with the MSS Gateway.
Commands and other signals are sent via the MSS low speed data system over
Control and Supervision Links. In an alternative embodiment, the commands and
other signals may be sent over the High Speed system. As was mentioned
above, a packet data modem could also be used, as could more than one UT.
Tracking signals are important when the Dual Terminal is fitted with
tracking antennas. A MSS Ground Operations Control Center (GOCC) provides
information over a Ground Data Network (GDN) as to which satellite(s) of the
constellation to use and for other transmission parameters, such as power at
which to transmit, frequencies to use, which RF antenna(s) are to be used,
etc.
Antenna pointing information is sent to the Operations Center, which is
preferably
also connected to the GDN. The tracking and other information is sent over the
Control and Supervision links to the UT and, after processing, to the baseband
unit of the Dual Terminal. The baseband unit converts the information to
control
signals used by a Track Information unit to point and track the antenna or
antennas of the Dual Terminal.
Also located in the High Speed Equipment System of the Dual Gateway
is a Control and Billing Management system. The Billing and Management
system is preferably connected to the GOCC via the GDN, but may instead be
connected to the Gateway Management System (GMS) of the MSS Gateway.
The Billing and Management system accounts for system usage and provides
Call Detail Records and other information such that the user can be charged
appropriately, and so that the air time used can be correctly charged to the
system provider.
System control is exercised so that priorities of transmission are
accounted for. For example, High Speed Data may be restricted during certain
periods of time in order to allow maximum MSS voice circuit usage during high
voice traffic periods. Conversely, more of the MSS bandwidth can be allocated
to
the High Speed Data Services during periods of lower MSS voice/data traffic
demand. In this case the high speed data can be spread over a wider bandwidth,
enabling higher data rates. It should be noted that in some embodiments it may
CA 02589369 2007-05-14
19
not be necessary to share the in-band spectrum between the LS/HS services, as
adjacent spectrum may be employed for providing the HS services (and/or for
providing the LS services). Gateway provider control can be used for these
purposes, or the control may be dictated by the GOCC under the direction of
the
system operator.
Any suitable satellite constellation may be employed to practice the
system of the instant invention. Typical satellite constellations include LEO,
ME0
and GEO. Preferred of these is the LEO satellite which provides the requisite
signal reception, reliability and clarity.
Any suitable user terminal may be employed in the system of the instant
invention. Typical user terminals include mobile phones, PDAs, laptops, fixed
phones, satellite data modem, car kits, airplane phones, and any devices or
sensors that can be interfaced to any of the above. Preferable of these is the
Globalstar satellite phone GSP 1600, Iridium satellite phone, and the like.
Any suitable gateway may be employed in the system of the instant
invention. Typical gateways include those described in assignee Globalstars
U. S. Patents 6,775,251, 6,735,440, 6,661,996, 6,253,080, 6,134,423,
6,067,442,
5,918,157, 5,884,142, 5,812,538, 5,758,261, 5,634,190 and 5,592,481. A
preferable one of these is the gateway as described in Fig. 2B of U. S. Patent
No.
6,804,514.
IV. BRIEF DESCRIPTION OF THE DRAWINGS
The above set forth and other features of the invention are made more
apparent in the ensuing Detailed Description of the Invention when read in
conjunction with the attached drawings, wherein:
FIG. 1 is a block diagram of a satellite communication system that is
constructed and operated in accordance with a presently preferred embodiment
of this invention;
FIG. 2 is a block diagram of one of the gateways of FIG. 1;
FIG. 3A is a block diagram of the communications payload of one of the
satellites of FIG. 1;
FIG. 3B illustrates a portion of a beam pattern that is associated with one
of the satellites of FIG. 1; and
FIG. 4 is a block diagram that depicts the ground equipment support of
satellite telemetry and control functions.
CA 02589369 2007-05-14
VI. DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates a presently preferred embodiment of a satellite
5 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 communication system 10 may be conceptually sub-divided into a
10 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 terrestrial system infrastructure segment; e.g., a
telephone infrastructure.
In the presently preferred embodiment of this invention there are a total of
15 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
20 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 terrestrial
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 combination of time division and phase shifts of long PN
codes.
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21
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
CA 02589369 2007-05-14
22
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 bidirectionally 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 100 elevation angles from the earth's surface) is approximately
1100.
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 12c and 12d, 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 are 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
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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
io 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 terrestrial
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. Patent No. 5,422,647,
by E. Hirshfield and C. A. Tsao, entitled "Mobile Communications Satellite
"Payload", which discloses one type of communications satellite payload that
is
suitable for use with the teaching of this invention.
The terrestrial infrastructure segment 4 is comprised of existing terrestrial
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, Internet 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 terrestrial 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.
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This portion of the communication 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 subsystems each comprising a
dish antenna 40, antenna drive 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 baseband
subsystem 52 which, in turn, is coupled to the Public Switched Telephone
Network (PSTN) through a PSTN interface 54. As an option, the PSTN could be
bypassed by using satellite-to-satellite links.
The baseband subsystem 52 includes a signal summer/switch unit 52a, a
Gateway Transceiver Subsystem (GTS) 52b, a GTSC controller 52c, and a
baseband processor 52d. It also includes the required frequency synthesizer
52g
and a 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), or other
suitable means. 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 signaling to the CCP 54b. On
the gateway side of this interface, the CCP 54b interfaces with the baseband
processor 52d and hence to the baseband subsystem 52. The CCP 54b
provides protocol translation functions for the system Air Interface (Al).
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
CA 02589369 2007-05-14
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 signaling interface.
A user making a call out of the user's normal service area (a roamer) is
5 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.
10 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
15 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
20 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 Unit (TCU)
18A
25 (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
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26
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 signaling, voice and/or data communications and also to
generate date, 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). 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 signaling
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
CA 02589369 2007-05-14
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know 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.
Thus, for example, in a preferred embodiment an improved LEO satellite
constellation system is provided comprising approximately 40 to 48 satellites
as
presently employed in the Globalstar system, employing multiple beams which
may reach a plurality of user terminals. This is more fully described in U. S.
Patent No. 6,272,325 which is incorporated herein. A gateway is employed
connected to either a PSTN or the Internet and communicating with a user
terminal over the constellation so that each user within a given frequency
band is
distinguished from another of said users employing a combination of TDM and
NOPN codes.
The system described herein employs NOPN codes to serve fixed
terminals. The system includes TDM on the forward link from a gateway through
the satellite to the UT. The forward link transmission is divided into data
frames
with multiple slots per frame. Each slot is assigned to a separate UT so that
users are distinguished from each other by means of the time slots in each
frame.
Based on the location of the user, the gateway can assign a specific beam of a
separate satellite. In order to minimize interference between two users who
are
assigned the same time slot in adjacent beams, each transmission is further
modulated by a scrambling code that is a PN, or pseudorandom noise, sequence
uniquely assigned to each beam. Cross-correlation between any two of these PN
sequences is minimal, so as to reduce interference between beams. If a user's
location is covered by two different satellites, the gateway transmits to that
UT on
both satellites, and diversity combining is used in the UT to combine these
two
signals and improve bit error rate (BER) performance.
The power allocated to each UT in each time slot is predetermined by the
gateway and is used to vary the data rate to the UT as its propagation
environment changes. This technique is also referred to in the art as HSDPA,
or
high speed digital packet access in the terrestrial WCDMA standard, or
wideband
CDMA. An alternative is to use power control similar to what is employed in
the
CA 02589369 2007-05-14
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current generation of Globalstar where the UT data rate is kept constant and
the power transmitted to the UT is varied according to propagation
environment.
The center frequency of the signal transmitted to each UT is adjusted to
pre-compensate for Doppler between the gateway and satellite, thus minimizing
the search time and window that the UT needs to lock on to the signal. This
technique is currently used in the Globalstar system. Similarly, the timing
of
signals in each time slot transmitted to each UT is adjusted by the gateway
based
on a calculated position of each UT; this calculation may be done either by
incorporating GPS into each UT, which informs the gateway of its coordinates,
or
by other known methods of position location, such as the techniques currently
employed in the Globalstar system which is predicated on triangulation using
multiple different delays from different satellites.
A separate narrowband control signal is transmitted from the gateway to
each UT having a fixed frequency for all UTs and is employed to inform the UTs
as to the center frequency to be used in transmitting forward link signals in
that
gateway service area.
In the reverse link from UT to satellite to gateway, each user is assigned a
different phase shift of a long PN code. These phase shifts ensure that the
cross-
correlation between different user signals at the gateway is minimal. This
technique is referred to as NOPN in this invention since these PN codes are
not
orthogonal, although they have low cross-correlation. Transmissions through
multiple satellites are combined at the gateway as in the current Globalstar
system. Each transmission from a UT consists of a short preamble which is used
to reduce burst acquisition complexity at the gateway. Each preamble
identifies
all users transmitting at a unique data rate. Reverse link power control may
be
performed as in the current Globalstar system, where data rate is fixed and
power is varied as needed to meet the link budget, or by varying the data rate
and keeping UT power fixed, as was mentioned for the forward link recited
above.
Typically, this presents a tradeoff which needs to be made between allowing a
greater number of UT data rates to improve granularity of power utilization
versus
hardware complexity at the gateway.
In this system, the gateway receiver compensates for the gateway to
satellite Doppler based on accurately known satellite positions and for the
less
precisely known UT locations.
Typically, for the forward link each user is assigned a fixed time slot of a
frame such as 5 ms slot in a 40 ms frame and different users are time division
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multiplexed, or TDM, onto a single frequency carrier. In the return direction,
the
data from the different UTs is distinguished using different time slots which
may
typically be 10 MS long; a group of users assigned a particular time slot and
a
particular phase shift of a very long pseudorandom code, or PN code. Different
phase shifts of such a code are used to increase the number of users supported
since the number of time slots of a single code would limit the number of
users
that can be supported. This describes the conventional technique referred to
as
NOPN, or non-orthogonal PN code usage.
While the present invention has been particularly described with respect
to certain components in its preferred embodiment, it will be understood that
the
invention is not limited to these particular components described in the
preferred
embodiments, or the sequence in employing or methods of processing the
components. The scope of the claims should not be limited by the preferred
embodiments set forth herein, but should be given the broadest interpretation
consistent with the description as a whole.
In addition, other components may be employed in the system of the
instant invention as claimed as well as variations and alternatives to the
components disclosed and claimed with similar results with regard to the
operation and function of the system of the instant invention. In particular,
the
scope of the invention is intended to include, for example GEO satellites
equipped with dynamic beam forming which further enhances the performance of
the system, or equipped with a Digital Channelizer Router (DCR) or employing
virtual gateway techniques as set out in U. S. Patent No. 6,735,440,
especially in
Figs. 15B-C.
This may be combined with reconfigurable beam forming or dynamic
beam forming.
