U.S. patent application number 12/113623 was filed with the patent office on 2009-11-05 for channel allocation for burst transmission to a diversity of satellites.
This patent application is currently assigned to Mr.Daniel A. Katz. Invention is credited to Daniel A. Katz.
Application Number | 20090274113 12/113623 |
Document ID | / |
Family ID | 41170376 |
Filed Date | 2009-11-05 |
United States Patent
Application |
20090274113 |
Kind Code |
A1 |
Katz; Daniel A. |
November 5, 2009 |
Channel Allocation for Burst Transmission to a Diversity of
Satellites
Abstract
A method for allocating transmission channels to devices
configured to communicate short data packets to a diversity of non
geostationary satellites is disclosed hereby. The method suggests a
dynamic cellular partitioning of the earth surface, based on the
smallest intersections of overlapping satellite service areas
("footprints"), defined as mega-cells, and reusing channels in
different mega-cells. In addition, a transmission cycle is defined
and divided to time slots, synchronized at each device by GPS
timing signals, and mega-cells served by more satellites are
allocated with fewer time slots, in order to increase the chance of
transmitters placed in mega-cells served by fewer satellites to be
detected. Further, each mega-cell is divided to cells, and
different channels and time slots are allocated to each cell, from
those allocated to the corresponding mega-cell. Consequently,
collision of transmissions from different mega-cells is reduced,
and collision of transmissions from different cells in a mega-cell
is avoided.
Inventors: |
Katz; Daniel A.; (Kiryat
Ono, IL) |
Correspondence
Address: |
DANIEL KATZ
87 TZAHAL ST.
KIRYAT-ONO
55451
IL
|
Assignee: |
Katz; Mr.Daniel A.
Kiryat Ono
IL
|
Family ID: |
41170376 |
Appl. No.: |
12/113623 |
Filed: |
May 1, 2008 |
Current U.S.
Class: |
370/330 ;
342/357.31 |
Current CPC
Class: |
H04B 7/18513
20130101 |
Class at
Publication: |
370/330 ;
342/357.12 |
International
Class: |
H04Q 7/00 20060101
H04Q007/00; G01S 1/00 20060101 G01S001/00 |
Claims
1. A method for allocating transmission channels to a plurality of
devices configured to communicate with a diversity of satellites,
comprising, at each device, the steps of: a) determining the
current time and self location via a Global Navigation Satellite
System (GNSS); b) determining the current service areas
("footprints") of said diversity of satellites on earth surface; c)
defining as a mega-cell each entire area covered by the same
footprints; d) determining a transmission cycle divided to time
slots, synchronized with said GNSS signals; e) allocating at least
one channel or one time slot to each mega-cell, wherein channels
and time slots are reused in different mega-cells, and at least for
two mega-cells, fewer time slots are allocated to the mega-cell
served by more satellites; f) dividing each mega-cell into cells,
at least one cell in a mega-cell; g) allocating different channels
and/or different time slots to different cells in a mega-cell, from
those allocated to the corresponding mega-cell; h) selecting a
specific channel and a specific time slot from those allocated to
the corresponding cell, according to at least one of: random or
pseudo-random selection; unique identification data (ID) stored at
each device; classification data stored and/or acquired at each
device; the nature of data to be transmitted; signal/control input;
current geographical location; coordination with other devices.
2. A method according to claim 1, wherein said channels are at
least: time slots for Time Division Multiple Access (TDMA); or
frequencies for Frequency Division Multiple Access (FDMA); or codes
for Code Division Multiple Access (CDMA); or a combination
thereof.
3. A system and method according to claim 1, wherein said GNSS is
one of: US Global Positioning System (GPS); Russian GLONASS (or
GLONAS); European Galileo; Chinese COMPASS; Indian IRNSS, or a
combination thereof.
4. A method according to claim 1, wherein a satellite footprint is
determined according to at least one of: the current time: the
satellite almanac: the satellite ephemeris: the satellite position;
wherein said satellite data is stored or received at said
device.
5. A method according to claim 1, wherein cells statistically
denser with devices are smaller, or cells statistically with more
devices are allocated with more channels or with more time slots,
or a combination thereof.
6. A method according to claim 1, wherein said devices transmit a
service request, and said satellites are configured to detect this
request and allocate a different channel to the requesting
device.
7. A method according to claim 1, wherein said devices periodically
transmit data packets, and for each transmission at least a channel
or a time slot is selected by each device.
8. A method according to claim 1, wherein each of said devices
comprises unique identification data (ID), and is configured to
transmit said ID along with data representing its location.
9. A method according to claim 1, wherein a device is configured to
skip at least one transmission cycle if data to be transmitted is
substantially the same as previously transmitted, or if a previous
transmission was acknowledged, or a combination thereof.
10. A method according to claim 8, wherein each of said devices is
attached to an object to be located, from the following non
limiting list: person, animal, pet, vehicle, weapon, ammunition,
valuable asset.
11. A computer program product comprising computer instructions
embodied in a computer readable storage medium for a method for
allocating transmission channels to a plurality of devices
configured to communicate with a diversity of satellites,
comprising: a) determining the current time and self location via a
Global Navigation Satellite System (GNSS); b) determining the
current service areas ("footprints") of said diversity of
satellites on earth surface; c) defining as a mega-cell each entire
area covered by the same footprints; d) determining a transmission
cycle divided to time slots, synchronized with said GNSS signals;
e) allocating at least one channel or one time slots to each
mega-cell, wherein channels and time slots are reused in different
mega-cells, and at least for two mega-cells, fewer time slots are
allocated to the mega-cell served by more satellites; f) dividing
each mega-cell into cells, at least one cell in a mega-cell; g)
allocating different channels and/or different time slots to
different cells in a mega-cell, from those allocated to the
corresponding mega-cell; h) selecting a specific channel and a
specific time slot from those allocated to the corresponding cell,
according to at least one of: random or pseudo-random selection;
unique identification data (ID) stored at each device;
classification data stored and/or acquired at each device; the
nature of data to be transmitted; signal/control input; current
geographical location; coordination with other devices.
12. A computer program product according to claim 11, wherein said
channels are at least: time slots for Time Division Multiple Access
(TDMA); or frequencies for Frequency Division Multiple Access
(FDMA); or codes for Code Division Multiple Access (CDMA); or a
combination thereof.
13. A computer program product according to claim 11, wherein cells
statistically denser with devices are smaller, or cells
statistically with more devices are allocated with more channels or
with more time slots, or a combination thereof.
14. A computer program product according to claim 11, wherein each
of said devices comprises unique identification data (ID), and is
configured to transmit said ID along with data representing its
location.
15. A computer program product according to claim 11, wherein a
device is configured to skip at least one transmission cycle if
data to be transmitted is substantially the same as previously
transmitted, or if a previous transmission was acknowledged, or a
combination thereof.
16. A communication system and device for allocating transmission
channels to a plurality of such devices configured to communicate
with a diversity of satellites, each of said devices comprising at
least an RF transmitter coupled to a Global Navigation Satellite
System (GNSS) receiver, wherein each device is configured to: a)
determine the current time and self location via said GNSS; b)
determine the current service areas ("footprints") of said
diversity of satellites on earth surface; c) define as a mega-cell
each entire area covered by the same footprints; d) determine a
transmission cycle divided to time slots, synchronized with said
GNSS signals; e) allocate at least one channel or one time slot to
each mega-cell, wherein channels and time slots are reused in
different mega-cells, and at least for two mega-cells, fewer time
slots are allocated to the mega-cell served by more satellites; f)
divide each mega-cell into cells, at least one cell in a mega-cell;
g) allocate different channels and/or different time slots to
different cells in a mega-cell, from those allocated to the
corresponding mega-cell; h) select a specific channel and a
specific time slot from those allocated to the corresponding cell,
according to at least one of: random or pseudo-random selection;
unique identification data (ID) stored at each device;
classification data stored and/or acquired at each device; the
nature of data to be transmitted; signal/control input; current
geographical location; coordination with other devices.
17. A communication system and device according to claim 16,
wherein said channels are at least: time slots for Time Division
Multiple Access (TDMA); or frequencies for Frequency Division
Multiple Access (FDMA); or codes for Code Division Multiple Access
(CDMA); or a combination thereof.
18. A communication system and device according to claim 16,
wherein cells statistically denser with devices are smaller, or
cells statistically with more devices are allocated with more
channels or with more time slots, or a combination thereof.
19. A communication system and device according to claim 16,
wherein each of said devices comprises unique identification data
(ID), and is configured to transmit said ID along with data
representing its location.
20. A communication system and device according to claim 16,
wherein a device is configured to skip at least one transmission
cycle if data to be transmitted is substantially the same as
previously transmitted, or if a previous transmission was
acknowledged, or a combination thereof.
Description
BACKGROUND OF INVENTION
[0001] When two communication devices simultaneously transmit on
the same channel, a communication conflict might occur, potentially
degrading the receiving probability of one or both of these
transmissions. Such communication conflicts are obviously
undesired, yet cannot be disregarded in the crowded communication
networks which are usually short in bandwidth.
[0002] The penalty for such transmission collisions is a lower
quality of service, more power consumption and more undesired RF
radiation. Therefore, multiple access communication networks employ
methods to properly allocate communication channels to devices, in
order to avoid such conflicts. Still, due to the high ratio of
devices per channel, and to the often unsynchronized transmissions,
among different devices and users that share these channels, such
conflicts are still an important issue to consider in communication
systems.
[0003] One communication sector particularly vulnerable to channel
allocation conflicts is related to networks comprising a multitude
of one-way transmitters that share a relatively small amount of
channels. In such networks, transmitters cannot coordinate with
each other the allocation of channels, so there is a chance that
two such devices will simultaneously transmit on the same channel
and interfere with each other. One such particular network that
employs millions of one-way communication devices sharing thirty
three or less channels is Cospas-Sarsat. Though the scope of the
present invention is far beyond that specific system, Cospas-Sarsat
is a good example to clarify the need and solution and a particular
embodiment disclosed by the present invention, so is specifically
enlightened here.
[0004] Cospas-Sarsat is a satellite communications system to assist
Search and Rescue (SAR) of people in distress, all over the world
and at anytime. The system was launched in 1982 by the USA, Canada,
France and the Soviet Union (now Russia) and since then, it has
been used for thousands of SAR events and has been instrumental in
the rescue of over 20,000 lives worldwide. The goal of the system
is to detect and locate signals from distress radio beacons and
forward these data to ground stations, in order to support all
organizations in the world with responsibility for SAR operations,
whether at sea, in the air or on land. The system uses
spacecraft--Low Earth Orbit (LEO); Geostationary (GEO) satellites;
and in the future also Medium Earth Orbit (MEO); as well as ground
facilities. Cospas-Sarsat radio beacons transmit in the 406 MHz
band (and 121.5 MHz until 2009). The position of the beacon is
determined either by the Doppler shift of the received beacon
signal or by position data provided by an embedded Global
Navigation Satellite System (GNSS) decoder (receiver), integrated
with the radio beacon. A detailed description of the Cospas-Sarsat
System is provided in the document "Introduction to the
Cospas-Sarsat System, C/S G.003", which can be accessed through the
following link--http://cospas-sarsat.org)/Documents/gDocs.htm.
[0005] Several types of Cospas-Sarsat beacons are approved for use,
differing mainly in their mechanical structure and activation
method, customized for different applications: a) Emergency
Position Indicating Radio Beacon (EPIRB) for marine use; b)
Emergency Locator Transmitter (ELT) for aviation use; and c)
Personal Locator Beacon (PLB) for personal and/or terrestrial
use.
[0006] Cospas-Sarsat beacons are deployed in large quantities, all
over the world, sharing a few transmission channels. Further, these
distress beacons are activated upon local triggering and their
transmissions are not synchronized in time, neither in frequency,
with each other (except of a very rough control through a
preliminary factory set frequency allocation). A narrow bandwidth
is allocated to all these beacons which are usually off, typically
for several years, and transmit only in rare occasions, for a short
time, normally in periodic bursts for several days. However, as
tens and hundreds of thousands of such beacons are deployed,
sharing one narrowband channel, simultaneous transmissions might
statistically occur, interfering with each other and decreasing the
probability of a distress message to be detected. Then, in order to
ensure a certain quality of service, i.e. a minimal probability for
a distress message to be detected within a specific period of time,
the number of transmitters per channel should be limited.
Obviously, such a system could be more efficient in exploitation of
the allocated spectrum, serve more beacons and/or improve the
quality of service if the transmission collision rate could be
reduced.
[0007] The total bandwidth allocated for Cospas-Sarsat is 100 KHz
(406.0-406.1 MHz), divided to 3 KHz bandwidth channels. According
to the present art, each beacon is factory set to one of these
channels, which cannot be reconfigured in the field. Then, each
channel (theoretically 33 channels, however practically much less
mainly due to Doppler shift limitations and system overhead) is
shared by tens or hundreds of thousands of beacons. When activated
(automatically or manually), a Cospas-Sarsat beacon transmits short
bursts, each one approximately 0.5 seconds long, every 50 seconds,
for several days, until its battery drains. In order to avoid
repetitive collisions between two active beacons, a beacon is
required to set its transmission cycle to 50+/-2.5 seconds, and the
period should be randomized around a mean value of 50 seconds, so
that time intervals between transmissions are randomly distributed
on the interval 47.5 to 52.5 seconds.
[0008] A significant augmentation of the Cospas-Sarsat satellite
segment is planed to be implemented in the near future.
Accordingly, compatible Cospas-Sarsat payloads will be installed
onboard positioning satellites belonging to the Galileo GNSS
constellation and possibly also onboard GPS satellites. Galileo,
the upcoming European GNSS, is planed to comprise 27 satellites,
while the US GPS comprises 24 operational satellites. Each of these
systems provides at least four satellites simultaneously in Line of
Sight (LOS) with any point on earth, as required for trilateral
positioning (fourth satellite usually solves clock ambiguity), i.e.
any point on earth will be always in at least four different
satellite footprints (service areas) of the Galileo GNSS and
another four GPS satellite footprints. Considering also the LEO and
GEO satellites that currently cover the earth for this SAR system,
it is expected that any beacon on earth will be at any moment in
ten or more footprints, which move quickly relatively to the earth
surface. Hence, many different intersections of footprints will be
introduced on earth surface, enabling beacons on different
intersections to communicate with different satellites. For the
clarity of this discussion, it is assumed that beacons are featured
with omnidirectional antennas, and satellites are installed with
wide beam antennas, yet this is definitely not mandatory.
[0009] The applicant has proposed a method to improve channel
allocation for communication networks, such as Cospas-Sarsat, in
"Increasing Channel Capacity of TDMA Transmitters in Satellite
based Networks", application Ser. No. 12/046,509, filed on 12 Mar.
2008. This reference shares with the present invention the aspect
of allocating time slots to Time Division Multiple Access (TDMA)
transmitters based on their position, yet does not discuss
allocation of frequencies and does not address overlapping of
satellite footprints, specifically dynamically moving, and a
cellular partitioning based on that.
[0010] Another aspect of communications vulnerable to channel
allocation conflicts is the initial approach of a device to an
access point asking for service. Often, a communications network
properly allocates operational channels to devices, and well
synchronizes these devices in order to avoid collisions. However,
before the network allocates these operational channels, devices
that initiate a service request do not use said operational
channels, which are controlled by the network (or by any
administering unit related to that network), but share a pool of
service requesting channels. At this preliminary phase, these
devices might not be synchronized with each other, for different
reasons, such as: random timing of access, no peer to peer
connection; communication peaks; etc. Improving the channel
allocation method for devices requesting service, could enable a
faster reaction of the network to such requests, and/or managing
more devices requesting service, over the same
channels/bandwidth.
[0011] U.S. Pat. No. 6,115,371 to Berstis (IBM) discloses a
satellite uplink separation using time multiplexed global
positioning system cell location beacon system. This method, for
allocating bandwidth to devices seeking to initiate contact with a
communication service, suggests using time slots according to self
location determined by GPS.
[0012] U.S. Pat. No. 7,304,963 to Amouris discloses a method and
system for dynamically allocating a set of broadcast TDMA channels
to a network of transceiver nodes. This method is based on timeslot
partitioning and geographic location.
[0013] U.S. Pat. No. 7,082,111 to Amouris discloses a method and
system for dynamically allocating time slots of a common TDMA
broadcast channel to a network of transceiver nodes. This invention
allocates time slots to TDMA devices according to their
geographical position.
[0014] Yet, none of these three U.S. patents addresses
intersections of footprints, i.e. areas served by several
satellites, and neither Berstis nor Amouris suggests discriminating
between devices placed in areas served by a different number of
satellites, for channel allocation purposes.
[0015] U.S. Pat. No. 5,268,694 to Jan et al. (MOTOROLA) discloses a
method of reusing spectrum on an approximately spherical surface,
based on two satellite footprints partially overlapping, each
footprint divided to cells. According to Jan, cells located in the
intersection of footprints are defined non active, and channels are
assigned only to active cells, spacing co-channel cells a
predetermined distance apart. Yet, Jan does not address cells
contained in other than one or two footprints, and neither suggests
allocating active transmission channels to devices in overlapping
footprints, specifically not according to the number of overlapping
footprints.
[0016] WO/2001/095522 to Yung, Hagen and Chang, (HUGHES ELECTRONICS
CORPORATION) discloses a RESOURCE ALLOCATION METHOD IN A SATELLITE
DIVERSITY SYSTEM. This invention teaches allocating system
resources to user terminals communicating with a multiple of
satellites, wherein a ground hub compensates for differential
propagation delays to any one of these remote users. Yet, this
invention does not consider the various satellite footprint
intersections as a basis for resource allocation.
[0017] European Patent EP0935351 to Bains, Navjit Singh (ICO
Services) discloses a Radio resource management in a mobile
satellite telephone system. This method teaches allocating radio
resources to a plurality of mobile user terminals in a satellite
mobile telephone system, in which a position of each of the user
terminals within the footprint of a given satellite is capable of
being classified, specifically denying resources from terminals
placed at the edge of the satellite footprint, which obtain a
significant path delay for a signal to be communicated to the given
satellite. Still, this method does not address intersections of
footprints for the purpose of resource allocation.
[0018] The present art methods described above have not yet
provided satisfactory solutions to the problem of allocating
communication channels to devices configured to transmit bursts of
data to a diversity of satellites, specifically non geostationary
satellites, sharing relatively few channels, particularly when
having a certain amount of data transmission redundancy.
[0019] It is an object of the present invention to provide a system
and method for allocating communication channels to devices
configured to communicate with a diversity of satellites,
particularly non geostationary satellites, reducing transmission
collisions and exploiting the allocated bandwidth.
[0020] It is also an object of the present invention to provide a
system and method for allocating communication channels to devices
configured to communicate with non geostationary satellites,
particularly devices that have no means to communicate with each
other or cannot coordinate channel allocation among them.
[0021] It is another object of the present invention to provide a
system and method for allocating momentary communication channels
to devices configured to communicate with non geostationary
satellites, in particular devices which are distress radio
beacons.
[0022] It is yet another object of the present invention to provide
a method for improving present or/and future systems for Search and
Rescue (SAR), such as Cospas-Sarsat and Galileo.
[0023] It is as well an object of the present invention to provide
a system and method for allocating momentary communication channels
to devices configured to communicate with non geostationary
satellites, based on the various intersections of overlapping
footprints of satellites on the earth surface.
[0024] It is still an object of the present invention to provide a
system and method for allocating communication channels to devices
configured to communicate with non geostationary satellites, based
on time synchronization and positioning information provided by a
GNSS such as GPS or Galileo or GLONASS.
[0025] It is also an object of the present invention to provide a
system and method for allocating communication channels to devices
configured to communicate with non geostationary satellites, based
on additional data such as statistics of geographical distribution
of devices, redundancy of data, location variation and
acknowledgement of transmissions.
[0026] It is still another object of the present invention to
provide a system and method for allocating communication channels
to devices configured to communicate with non geostationary
satellites, minimizing cost and size and power consumption of said
devices.
[0027] It is nonetheless an object of the present invention to
provide an apparatus and method for allocating communication
channels to devices configured to communicate with non
geostationary satellites, wherein said channels are either time
slots for Time Division Multiple Access (TDMA); or frequencies for
Frequency Division Multiple Access (FDMA); or digital codes for
Code Division Multiple Access (CDMA); or a combination thereof.
[0028] Other objects and advantages of the invention will become
apparent as the description proceeds.
SUMMARY OF INVENTION
[0029] The invention is directed to a method for allocating
transmission channels to a plurality of devices configured to
communicate with a diversity of satellites, comprising, at each
device, the steps of:
a) determining the current time and self location via a Global
Navigation Satellite System (GNSS); b) determining the current
service areas ("footprints") of said diversity of satellites on
earth surface; c) defining as a mega-cell each intersection of
footprints which is not divided to further intersections; d)
determining a transmission cycle divided to time slots,
synchronized with said GNSS signals; e) allocating channels and
time slots to each mega-cell, wherein channels and time slots are
reused in different mega-cells, and at least for two mega-cells,
fewer time slots are allocated to the mega-cell served by more
satellites; f) dividing each mega-cell into cells, at least one
cell in a mega-cell; g) allocating different channels and/or
different time slots to different cells in a mega-cell, from those
allocated to the corresponding mega-cell; h) selecting a specific
channel and a specific time slot from those allocated to the
corresponding cell, according to at least one of: random or
pseudo-random selection; unique identification data (ID) stored at
each device; classification data stored and/or acquired at each
device; the nature of data to be transmitted; signal/control input;
current geographical location; coordination with other devices.
[0030] Defining a mega-cell as an intersection of footprints which
is not divided to further intersections means that all points in a
mega-cell, i.e. all devices placed in a mega-cell, are in Line of
Sight (LOS) with, and served by, exactly the same satellites. The
disclosed invention is not limited to one antenna/beam/footprint
per satellite, yet for clarification purposes, one footprint per
satellite is assumed. Then, as a skilled person may appreciate, any
two adjacent mega-cells are served by at least one different
satellite; otherwise these mega-cells would have been merged to
one, following the mega-cell definition. Moreover, according to
this cellular partitioning, any mega-cell is served by a specific
set of satellites, and all its adjacent mega-cells are served by
this set of satellites plus or minus one satellite. In other words,
any two adjacent mega-cells are served by a different number of
satellites, where devices in the mega-cell served by more
satellites are in LOS with all the satellites serving the adjacent
mega-cell, and with one specific satellite more. Hence, if devices
in adjacent mega-cells simultaneously transmit on the same channel,
there is a significant chance that the transmission from the
mega-cell served by fewer satellites will be blocked, while the
transmission from the mega-cell served by more satellites will be
successfully detected by one satellite.
[0031] Actually, as a skilled person would probably appreciate, for
any two mega-cells, devices placed in a mega-cell covered by more
footprints basically obtain a better probability to be detected
than devices placed in a mega-cell served by fewer satellites.
Ultimately, transmissions from mega-cells served by the largest
number of satellites, compared to all other mega-cells, at a
specific moment, obtain the best detection probability, at least
according to this aspect.
[0032] As already mentioned, adjacent mega-cells are served by one
different satellite. Then, as mega-cells are more distant from each
other, they are usually less correlated regarding to satellites, so
devices placed in distant mega-cells less interfere with each
other. Ultimately, mega-cells which are more than the largest
footprint diameter away from each other, do not share any same
satellites; moreover, as a skilled person may realize, according to
the disclosed cellular partitioning, each of any two non adjacent
mega-cells is probably served by at least one satellite which does
not serve the other mega-cell, so two devices placed in non
adjacent mega-cells will probably be detectable even if
transmitting simultaneously on the same channel; in particular,
mega-cells served by the largest number of satellites, at a
specific moment, are obviously non adjacent mega-cells, and
probably do not share exactly the same satellites. Hence, a
transmitter placed in a mega-cell served by the largest number of
satellites is practically not interfered by any transmitter placed
in any other mega-cell.
[0033] Then, in order to improve the transmission probability of
the underprivileged devices, placed in mega-cells covered by fewer
satellites compared to adjacent mega-cells, the present invention
suggests allocating fewer time slots to mega-cells served by more
satellites, and particularly to mega-cells momentarily served by
the largest number of satellites. This ensures that devices placed
in mega-cells served by more satellites will less interfere with
devices placed in nearby mega-cells served by fewer satellites.
Since devices placed in mega-cells served by more satellites have a
better chance to be detected, then reducing the number of allocated
time slots for these mega-cells provides a sort of equalization,
sharing the network resources more evenly among devices. This
strategy might have a trade off, since the detection probability
for devices placed in the same mega-cell allocated with fewer time
slots might decrease, yet the probability for this case is
relatively smaller compared to other cases which are improved by
the present invention.
[0034] The current method can be further refined, by dividing each
mega-cell into cells and allocating different channels and/or
different time slots to different cells in a mega-cell, from those
allocated to the corresponding mega-cell. A geographically
uniformed cellular partitioning is clearly effective if devices are
geographically distributed more or less uniformly in mega-cells.
Then, there is a good chance that different devices will be placed
in different cells, so will use different channels and/or different
time slots and will not interfere with each other. Otherwise, this
secondary partitioning to cells may take into consideration further
parameters, such as the statistics of the geographical density of
devices (i.e. number of devices per area) and/or number of devices
per cell. Then, cells that are statistically denser with devices,
e.g. in urban areas, can be defined to be smaller, in order to keep
a substantially similar number of channels and/or time slots per
same number of devices. Similarly, cells which statistically
contain more devices, e.g. near ports or traffic hubs, can be
allocated with more channels and/or time slots, allocating a
substantially similar number of channels and/or time slots per
devices.
[0035] Then, in the next step, a specific channel and a specific
time slot are selected from those allocated to the corresponding
cell, according to at least one of: random or pseudo-random
selection; unique identification data (ID) stored at each device;
classification data stored and/or acquired at each device; the
nature of data to be transmitted; signal/control input; current
geographical location; coordination with other devices. The skilled
person may note that in cases where said devices cannot communicate
with each other or are not aware of each other and cannot
coordinate the channel allocation among them, the present invention
provides some "open loop" techniques to reduce the chance for a
transmission collision, such as a random selection. Also, some of
these techniques may provide a statistical advantage to specific
devices, or specific types of devices, or specific types of
messages, or specific types of inputs to devices, as might be
required by different applications. Also, a third geographical
partitioning may be performed, dividing cells to finer sectors and
allocating a specific channel and specific time slot to each such
sector. Then, "close loop" techniques may provide even less channel
conflicts and transmission collisions when nearby devices,
typically placed in the same cell, are able to coordinate channel
allocation among them.
[0036] In the scope of the present invention, communication
channels may be of several kinds, including: time slots for Time
Division Multiple Access (TDMA); frequencies for Frequency Division
Multiple Access (FDMA); codes for Code Division Multiple Access
(CDMA); or a combination thereof. For example, according to one
embodiment, devices use a fixed frequency, and can only manipulate
the time of transmission, i.e. allocate and select time slots.
According to another embodiment, devices can set, during operation,
the transmission frequency as well as the transmission time, so can
select a combination of frequency and time slot, from a pool of
frequencies and time slots.
[0037] The GNSS in the scope of this invention is preferably the
presently operative US GPS, however may be of other types, such as
the upcoming European Galileo; the Russian GLONASS (or GLONAS); the
Chinese COMPASS; the Indian IRNSS, or a combination thereof. As a
person skill in the art may appreciate, different GNSS receivers
could be coupled to different communication devices, according to
the present invention, as long as such a receiver provides accurate
timing signals and accurate timing and position data.
[0038] Non geostationary satellites provide footprints which are
dynamically moving on the earth surface. In order to determine
these footprints, a device may use the momentary spatial
coordinates of every satellite and of the earth center, as well as
the earth radius (assuming a spherical earth). The satellites
coordinates may be derived, based on the current time reading
acquired from the coupled GNSS receiver, according to records
stating the satellites position and/or almanac and/or ephemeris,
which may be partially or entirely stored in the memory of each
device or/and received at said device. For example, positioning
satellites as the GPS periodically transmit these data, and more
often transmit their spatial position coordinates, embedded in the
navigation messages. For global navigation and location purposes,
the earth center is usually the origin point of a spatial
coordinate system, even though it is said that the earth is
orbiting around the sun. Obviously, in case that the device can
detect signals transmitted by a satellite, this is a clear
indication for being in this satellite footprint.
[0039] Having the satellite spatial coordinates, in addition to the
direction and width of the satellite antenna beam, as well as the
spatial coordinates of the earth center and the earth radius (or a
more accurate mathematical model of the geoid), may be used to
calculate this satellite communication footprint on the earth
surface, as well known in the art.
[0040] Typically, the dynamically moving footprints of
non-geostationary satellites provide, according to the present
method, a dynamically cellular partitioning and consequently a
dynamic channel allocation. Hence, this channel allocation is
effectively valid for a short period, so typically suitable for
relatively short periods. In one embodiment of the disclosed
method, this channel allocation is applied by devices which
transmit a service request, before been allocated with operational
channels that are administered by the network or system. In this
case, the requesting devices may share a pool of channels for this
preliminary phase of communications, from which the disclosed
method can function. Then, upon establishing an initial contact
with a satellite, the system may take control and allocate a
different channel to the requesting device.
[0041] In a second embodiment of the disclosed method, this channel
allocation is applied by devices that periodically transmit data
packets, and for each transmission a channel and/or time slot is
selected by each device, according to the present method. A typical
relevant application is a Search and Rescue (SAR) satellite system,
such as Cospas-Sarsat, employing distress radio beacons. This
system is basically allocated (according to international
regulations) with 33 frequency channels (though not all
operational), and compatible beacons placed in active mode are
configured to transmits short bursts of about 0.5 seconds each,
every transmission cycle of 50 seconds. The disclosed method may
then be applied on a subset of these 33 channels and 100 possible
time slots (theoretically a pool of 3300 orthogonal selections), in
order to improve the system capacity and/or quality of service.
[0042] Typically, a device according to the present invention may
comprise unique identification data (ID), and be configured to
transmit said ID along with data representing its location. This is
particularly relevant to location systems and applications. In such
cases, said device may be attached to an object to be located, such
as: a person, animal, pet, vehicle, weapon, ammunition, valuable
asset and so on.
[0043] The current method can be further refined and further
improve the network capacity and/or quality of service, by reducing
the collision rate among devices, in specific scenariii. One
scenario is related to location applications, where the most
important data transmitted by a device is its position. Then, if
the position of a device does not change significantly between
successive reports, this device may skip one or more transmission
cycles, avoiding transmission of redundant data and reducing the
chance to interfere with another device. This logic may be broaden
to similar scenario, so a device skips a transmission cycle if
other type of data to be transmitted is substantially the same as
previously transmitted, for example a sensor reading that did not
change lately.
[0044] Another scenario is related to systems which provide
transmission acknowledgement. In such a case, if a device is
configured to periodically transmit data packets, as a distress
radio beacon, and if one transmission is been acknowledged, then
this device may skip one or more transmission cycles in order to
reduce a potential transmission collision with another device,
knowing that its own message was already been delivered.
[0045] The invention is further directed to a computer program
product in a computer readable medium for a method for allocating
transmission channels to a plurality of devices configured to
communicate with a diversity of satellites, comprising, for each
device, the steps of:
a) determining the current time and self location via a Global
Navigation Satellite System (GNSS); b) determining the current
service areas ("footprints") of said diversity of satellites on
earth surface; c) defining as a mega-cell each intersection of
footprints which is not divided to further intersections; d)
determining a transmission cycle divided to time slots,
synchronized with said GNSS signals; e) allocating channels and
time slots to each mega-cell, wherein channels and time slots are
reused in different mega-cells, and at least for two mega-cells,
fewer time slots are allocated to the mega-cell served by more
satellites; f) dividing each mega-cell into cells, at least one
cell in a mega-cell; g) allocating different channels and/or
different time slots to different cells in a mega-cell, from those
allocated to the corresponding mega-cell; h) selecting a specific
channel and a specific time slot from those allocated to the
corresponding cell, according to at least one of: random or
pseudo-random selection; unique identification data (ID) stored at
each device; classification data stored and/or acquired at each
device; the nature of data to be transmitted; signal/control input;
current geographical location; coordination with other devices.
[0046] This computer program product may refer to channels which
are at least: time slots for Time Division Multiple Access (TDMA);
or frequencies for Frequency Division Multiple Access (FDMA); or
codes for Code Division Multiple Access (CDMA); or a combination
thereof.
[0047] According to this computer program product, cells
statistically denser with devices can be defined smaller, and/or
cells statistically with more devices can be allocated with more
channels and/or time slots.
[0048] Further, this computer program product may refer to a device
comprising unique identification data (ID), and configured to
transmit said ID along with data representing its location.
[0049] Then, according to this computer program, a device may be
configured to skip at least one transmission cycle if data to be
transmitted is substantially the same as previously transmitted,
or/and if a previous transmission was acknowledged.
[0050] The invention is also directed to a communication system and
device for allocating transmission channels to a plurality of such
devices configured to communicate with a diversity of satellites,
each of said devices comprising at least an RF transmitter coupled
to a Global Navigation Satellite System (GNSS) receiver, wherein
each device is configured to:
a) determine the current time and self location via said GNSS; b)
determine the current service areas ("footprints") of said
diversity of satellites on earth surface; c) define as a mega-cell
each intersection of footprints which is not divided to further
intersections; d) determine a transmission cycle divided to time
slots, synchronized with said GNSS signals; e) allocate channels
and time slots to each mega-cell, wherein channels and time slots
are reused in different mega-cells, and at least for two
mega-cells, fewer time slots are allocated to the mega-cell served
by more satellites; f) divide each mega-cell into cells, at least
one cell in a mega-cell; g) allocate different channels and/or
different time slots to different cells in a mega-cell, from those
allocated to the corresponding mega-cell; h) select a specific
channel and a specific time slot from those allocated to the
corresponding cell, according to at least one of: random or
pseudo-random selection; unique identification data (ID) stored at
each device; classification data stored and/or acquired at each
device; the nature of data to be transmitted; signal/control input;
current geographical location; coordination with other devices.
[0051] For this communication system and this device, said channels
are at least: time slots for Time Division Multiple Access (TDMA);
or frequencies for Frequency Division Multiple Access (FDMA); or
codes for Code Division Multiple Access (CDMA); or a combination
thereof.
[0052] Further, for this communication system and this device,
cells statistically denser with devices may be defined to be
smaller, and/or cells statistically with more devices may be
allocated with more channels and/or time slots.
[0053] Also, for this communication system and this device, each of
said devices may comprise unique identification data (ID), and
configured to transmit said ID along with data representing their
location.
[0054] Then, each of said devices may be attached to an object to
be located, such as: a person (particularly the young, the old and
the disabled), animal, pet, vehicle, weapon, ammunition, valuable
asset.
[0055] In addition, said device may be configured to skip at least
one transmission cycle if data to be transmitted is substantially
the same as previously transmitted, or/and if a previous
transmission was acknowledged.
[0056] Other objects and advantages of the invention will become
apparent as the description proceeds.
BRIEF DESCRIPTION OF DRAWINGS
[0057] The above and other characteristics and advantages of the
invention will be better understood through the following
illustrative and non-limitative detailed description of preferred
embodiments thereof, with reference to the appended drawings,
wherein:
[0058] FIG. 1 illustrates an Overview of a System for Burst
Transmission to a Diversity of Satellites. The figure shows six
satellites orbiting around the globe, detecting a transmission of a
device (not shown) placed in Africa, while five of these satellite
also detect a transmission of another device (not shown) placed in
the Atlantic Ocean.
[0059] FIG. 2 shows a Block Diagram of a Device for Burst
Transmission to a Diversity of (SAR) Satellites, according to a
preferred embodiment of the present invention. It shows a
microcontroller (including RAM and EPROM memory, not specifically
depicted), coupled to a GNSS (e.g. GPS) receiver. The
microcontroller is also coupled to an RF transmitter and an RF
synthesizer.
[0060] FIG. 3 (3a and 3b) illustrates the footprint of a satellite
on earth surface, and is divided to FIG. 3a and FIG. 3b.
[0061] FIG. 3a shows an isometric view of the earth, and a
satellite at the zenith of a point which is the center of this
satellite footprint on the earth surface.
[0062] FIG. 3b shows a sectional view of the earth, and a satellite
at a distance of 4 times the earth radius from earth center. The
radius of the footprint is an arc of an angle denoted as
.theta..
[0063] FIG. 4 illustrates overlapping footprints of 24 satellites
on the earth surface. This illustration of the earth surface is
only two dimensional, and definitely not according to an accurate
geographical projection. Yet, it depicts the way that overlapping
footprints create intersections, which are the basis for the
present invention cellular partitioning.
[0064] FIG. 5 shows a flow chart illustrating the process of
channel allocation according to the present invention, to be
periodically implemented by each device, step by step.
[0065] FIG. 6 illustrates Allocation of channels and time slots to
cells in A-mega-cells (left side) and B-mega-cells or C-mage-cells
(right side). Each side of the picture is a table comprised of 100
rows representing 100 time slots, and 10 columns representing 10
channels. Each combination of ten rows and one column represents a
unique allocation of [channel+10 time slots] to a specific
cell.
DETAILED DESCRIPTION
[0066] The invention will now be described with respect to various
embodiments. The following description provides specific details
for a thorough understanding of, and enabling description for,
these embodiments of the invention. However, one skilled in the art
will understand that the invention may be practiced without these
details. In other instances, well-known structures and functions
have not been shown or described in detail to avoid unnecessarily
obscuring the description of the embodiments of the invention.
[0067] The invention is directed to a method for allocating
transmission channels to a plurality of devices configured to
communicate with a diversity of satellites, comprising, at each
device, the steps of:
a) determining the current time and self location via a Global
Navigation Satellite System (GNSS); b) determining the current
service areas ("footprints") of said diversity of satellites on
earth surface; c) defining as a mega-cell each intersection of
footprints which is not divided to further intersections; d)
determining a transmission cycle divided to time slots,
synchronized with said GNSS signals; e) allocating channels and
time slots to each mega-cell, wherein channels and time slots are
reused in different mega-cells, and at least for two mega-cells,
fewer time slots are allocated to the mega-cell served by more
satellites; f) dividing each mega-cell into cells, at least one
cell in a mega-cell; g) allocating different channels and/or
different time slots to different cells in a mega-cell, from those
allocated to the corresponding mega-cell; h) selecting a specific
channel and a specific time slot from those allocated to the
corresponding cell, according to at least one of: random or
pseudo-random selection; unique identification data (ID) stored at
each device; classification data stored and/or acquired at each
device; the nature of data to be transmitted; signal/control input;
current geographical location; coordination with other devices.
[0068] Considering the term "channels", the preferred embodiment of
the present invention refers to frequency channels. Another
embodiment to be discussed following references to channels which
are time slots, as it deals with devices with a single factory set
frequency (e.g. present Cospas-Sarsat beacons) and suggests
dynamically allocation only of time slots. Furthermore, in the
scope of this invention, "channels" may be time slots for Time
Division Multiple Access (TDMA) or frequencies for Frequency
Division Multiple Access (FDMA), as well as codes for Code Division
Multiple Access (CDMA) or any combination thereof.
[0069] FIG. 1 shows an Overview of a System for Burst Transmission
to a Diversity of Satellites. The globe is illustrated in this
figure, six satellites orbiting around it, receiving a transmission
from a device (not shown) placed in Africa. Five of these satellite
are also shown receiving a transmission from another device (nor
shown), placed in the Atlantic Ocean. In a preferred embodiment of
the present invention, these and similar devices are distress radio
beacons and said diversity of satellites (represented in FIG. 1 by
six satellites) are 24 Medium Earth Orbiting (MEO) satellites,
typically covering any point on earth surface with several
overlapping footprints. For example, FIG. 1 indicates that the
device placed in the Atlantic is in five overlapping footprints
while the device placed in Africa is covered by six overlapping
footprints. Preferably, each of said diversity of satellites
carries onboard two types of payloads: a) positioning; mainly for
transmitting signals enabling location determination via a
trilateral calculation; and b) SAR; mainly for receiving distress
signals from radio beacons by the earth surface and relay these
signals or related data to earth stations. Practically, these
satellites can be part of the upcoming European Galileo system
which is essentially a positioning system but also augments the
Cospas-Sarsat SAR system. Another embodiment may be based on the
future US GPS constellation, which is presently a positioning
system yet planned to be upgraded to support SAR such as
Cospas-Sarsat. Both GPS and Galileo employ a trilateral positioning
method, so at least 4 satellites of each of these constellations
are required to be in LOS with any receiver whose position is to be
determined. Practically, each of these constellations is designed
to provide at least 6 satellites in LOS (i.e. six overlapping
footprints), almost anytime, with almost any point on the earth
surface, but usually even more than six satellites, to provide a
good (low) Geometric Dilution of Precision (GDOP) at each receiving
device.
[0070] FIG. 2 depicts a Block Diagram of a Device for Burst
Transmission to a Diversity of SAR Satellites, according to the
preferred embodiment of the present invention. As shown in FIG. 2,
each of said devices comprises an RF transmitter, on the 406 MHz
band, basically according to the specifications of
Cospas-Sarsat--htt://www.cospas-sarsat.org//DocumentsTSeries/T1NOV
1.sub.--07_CompleteDoc.pdf. Each of said RF transmitters is coupled
to an RF synthesizer, and to a microcontroller. The microcontroller
is also coupled to a GNSS decoder/receiver which is preferably a
Galileo or a GPS receiver. These basic blocks might be part of
present art Cospas-Sarsat beacons (such as EPIRBs, PLBs or ELTs),
and theoretically, the implementation of this preferred embodiment
may be based on present art beacons, customized according to the
disclosed method. In many cases, this customization can be
implemented in software, largely enabling using current
Cospas-Sarsat beacon's hardware. Many present art beacons comprise
a factory set local oscillator, not a programmable synthesizer, and
such a single frequency device can be employed according to a
different embodiment of the present method, where frequency
channels are fixed at each device and only time slots are been
dynamically allocated. Yet, the preferred embodiment requires each
device to comprise a programmable RF synthesizer, coupled to the
internal processor, as shown in FIG. 2, enabling also a dynamic
frequency channel allocation. The microcontroller is then
configured to select a transmission channel from the 33 channels
(or a sub set of these channels) in the range of 406.0-406.1 MHz
which are allocated by the regulator to this SAR system. Hence, a
person skilled in the art wishing to build a device according to
the preferred embodiment of the present invention, can choose
either strategy: a design based on the component level, i.e. using
off the shelf integrated circuits (ICs), or a design starting with
higher integrated blocks, i.e. using/modifying radio beacons and/or
related printed circuit boards (PCBs).
[0071] Cospas-Sarsat radio beacons and GPS receivers are items
mostly available on the market, by a considerable number of
manufacturers, and very well documented (specific products as well
as systems--Cospas-Sarsat and GPS), in a way that a person skilled
in the art, particularly manufacturers of such beacons, can utilize
in the scope of the present invention. A design starting from
crash, can utilize components which are also popular in the market,
such as the Texas Instruments (TI) MSP430 Ultra-Low Power
Microcontrollers or the TI family of low power radio ICs,
CC1101/CC1110/CC1111, which can be used as RF transmitters and/or
RF synthesizers. Data sheets and specifications can be found
at--http://www.ti.com/ and http://focus.ti.com/general/docs/.
Further circuits and components, such as those required to
implement the biphase modem compatible with Cospas-Sarsat, are also
well known in the art.
[0072] According to the preferred embodiment, each active device
(i.e. radio beacon in distress mode), transmits no more than one
short packet of data every transmission cycle. This cycle is
divided to time slots, to improve the channel capacity, up to twice
compared to unsynchronized transmissions, due to less transmission
collisions among transmitters. The slotted method is well practiced
in the art, as in the "Slotted Aloha" protocol, originally used for
satellite communications in the Pacific Ocean region.
[0073] FIG. 5 shows a step by step Channel Allocation Flow Chart,
to be conducted at each device, every transmission cycle, according
to the preferred embodiment of the present invention. These steps
comprise of:
a) determining the current time and self location, based on said
GNSS signals and data; this is typically done by the
microcontroller and the coupled GNSS receiver, as well practiced in
the art; the preferred embodiment suggests employing either GPS or
Galileo as the GNSS (also know as SPS--Satellite Positioning
System); yet, the present invention may also be related to other
types of GNSS, presently operating or futuristic, such as the
Russian GLONASS (or GLONAS), the Chinese COMPASS, the Indian IRNSS
or a combination thereof. Furthermore, a skilled person may
appreciate that any other positioning system, been satellite based
or not, may be employed in conjunction with the present invention,
as long as it provides accurate positioning and timing information.
b) determining the current service area ("footprint") of each of
said diversity of satellites, as following: [0074] b1) determining
each satellite spatial coordinates, by receiving via the coupled
GNSS receiver the satellites almanac and ephemeris and for
satellites in LOS also receiving the position coordinates embedded
in the frequently transmitted navigation messages; also, the
satellites almanac and ephemeris may be stored in the device's
memory, enabling, at a known time, to determine the spatial
position of the satellites. [0075] b2) determining the current
footprints, through a trigonometric calculation, based on the ECEF
(Earth Centered Earth Fixed) satellite coordinates, assuming that
the earth center is in the origin point (0, 0, 0) of this Cartesian
coordinate system, and using the earth radius (assuming a sphere)
or a more accurate mathematical model of the geoid. FIG. 3
illustrates a satellite orbiting around the globe, where FIG. 3a
shows an isometric view of the satellite footprint on earth
surface, and FIG. 3b shows a sectional view of the earth, and a
satellite at a distance of 4 times the earth radius from earth
center. The radius of the footprint is an arc of an angle denoted
as .theta. (FIG. 3b). A skilled person may probably note that cos
.theta.=1/4, so .theta..apprxeq.75.degree., and such a footprint
covers about one third of the earth surface; also, the coordinates
of the center of this footprint may be determined noting that this
point is on the line connecting the satellite with earth center,
distant one earth radius away from the earth center; clearly, the
footprint center and its radius on the surface of the earth
uniquely define the footprint perimeter. According to a second
embodiment, said devices comprise also a receiver configured to
receive signals transmitted by said diversity of satellites, so the
indication that a device is in the footprint of a satellite is
simply detecting a signal from this satellite at this device.
Furthermore, since according to the preferred embodiment, the same
satellites transmit positioning signals and receive the (distress)
devices' transmissions, determining the satellites footprints is
straightforwardly done by detecting the navigation signals
transmitted by these satellites, i.e. almanac and ephemeris
(typically indicating the position of all the constellation
satellites) and position, at the GNSS receiver coupled to every
device; c) defining as a mega-cell each intersection of footprints
which is not divided to further intersections and recording how
many footprints currently overlap each mega-cell. FIG. 4 is an
illustration of overlapping footprints of 24 satellites on the
earth surface. Obviously, it is only a two dimensional
illustration, not even a cartographic projection. Still, it
basically depicts the overlapping footprints and intersections
which define the mega-cells, according to the present invention.
Since preferably, said diversity of satellites are 24 MEO
satellites, orbiting substantially evenly around the earth, at an
altitude of about 20K Kms above earth surface (i.e. about 4 times
the earth radius from the earth center), each of these satellite
footprints covers approximately 1/3 of the entire earth surface, as
shown in FIG. 3, thus these 24 satellite footprints accumulatively
cover the earth surface about eight times, i.e. averagely provide
eight satellites in LOS with any point on the earth. As shown in
FIG. 4, such satellite footprints create many intersections. The
current method considers the smallest intersections, which are not
further divided by any footprint circumference, and defines each of
these intersections as a mega-cell. FIG. 4 shows some mega-cells
covered by 6 footprints, denoted as A-mega-cells (the darkest
intersections); some mega-cells covered by 5 footprints, denoted as
B-mega-cells; and others covered by 4 overlapping footprints,
denoted as C-mega-cells. As already indicated, a GNSS as Galileo or
GPS ensures at least 4 satellites always in LOS with any point on
earth, and usually provides many more; the example illustrated in
FIG. 4 indicates the maximum number of satellites covering any
mega-cell as six; d) determining a transmission cycle of 50
seconds, divided to 100 time slots of 0.5 seconds each,
synchronized with said GNSS signals; preferably, said GNSS signals
are provided by the coupled GNSS decoder, and typically are: a 1
Pulse Per Second (PPS) signal and Time of Day (TOD) message (i.e.
the current date and time). Actually, the propagation delay
variation of transmissions traveling different time periods to the
same satellite is also to be considered, in order to avoid
collision of transmissions starting at different time slots. This
might require decreasing the maximum packet transmission period by
about 20 milliseconds; e) allocating channels and time slots to
each mega-cell, wherein channels and time slots are reused in
different mega-cells but at least for two mega-cells, fewer time
slots are allocated to the mega-cell served by more satellites; in
the preferred embodiment, all channels are allocated to all
mega-cells, and all time slots are allocated to all mega-cells
served by less than the largest number of satellites, while fewer
time slots are allocated to the mega-cells served by the largest
number of satellites. FIG. 6 illustrates Allocation of channels and
time slots to mega-cells and cells according to this preferred
embodiment. The left side of FIG. 6 depicts the channels and time
slots allocated to A-mega-cells, and the right side depicts the
allocation to B-mega-cells/C-mage-cells. This figure shows also the
allocation of channels and time slots to cells in mega-cells, which
will be discussed later. The step related to the mega-cell
allocation, is conducted as following: [0076] e1) determining which
mega-cells are served by the maximum number of satellites;
following the example depicted in FIG. 4, the maximum number of
satellites serving any mega-cell is six, and these mega-cells are
denoted A-mega-cells; there are six A-mega-cells in the area marked
by the thick black circle (still in FIG. 4), and about a total of
40 such A-mega-cells on the entire earth surface, as a rough
extrapolation based on this FIG. 4 example; [0077] e2) Determining
a pool of channels available for transmission, preferably 10
channels in the range of 406.0-406.1 MHz, each channel 3 KHz wide;
following the non limiting example illustrated in FIG. 6, the
center frequencies of these channels are: I=406.005 MHz; II=406.015
MHz; III=406.025 MHz; . . . ; X=406.095 MHz; each of these channels
is represented by a column in each of the left and right tables in
FIG. 6; [0078] e3) allocating all the 10 pool channels to each and
every mega-cell; as a skilled person may observe in both tables in
FIG. 6, all 10 channels are marked (each channel is marked by a
different graphical pattern) available for A-mega-cell and
B-mega-cells and C-mega-cells; [0079] e4) allocating all 100
transmission time slots per cycle to all B-mega-cells and all
C-mega-cells; as a skilled person may observe, all 100 time slots
(each time slot is represented by a different row) are marked
available (by a gray color) for B-mega-cells and C-mega-cells, in
the right table in FIG. 6; [0080] e5) allocating only 90 time slots
per cycle to A-mega-cells; according to the non limiting example
shown in the left table in FIG. 6, each 10.sup.th time slot (10th,
20th, 30th, . . . , 100th) is denied from A-mega-cells. f) dividing
each mega-cell into cells; preferably, each mega-cell is divided to
100 equal in area cells; in other cases, in order to provide an
even number of devices per cell, cells may be defined in different
sizes, to compensate for a non uniformly geographical distribution
of devices. For example, cells in areas known to be statistically
denser with devices, like urban areas or areas populated with many
devices (not necessarily related to people, such as remote sensors)
may be determined to be smaller g) allocating different channels
and/or different time slots to different cells in a mega-cell, from
those allocated to the corresponding mega-cell; basically, each
cell in a mega-cell may be allocated with the same number of
channels/time slots, yet it is possible to compensate for non
uniform distribution of devices per cell with allocating more
channels and/or time slots to statistically more populated cells;
still, according to the preferred embodiment, channels/time slots
allocated to a mega-cell are divided evenly among the cells in this
mega-cell, as following: [0081] g1) each cell in a B-mega-cell or a
C-mega-cell is allocated with 1 channel and 10 time slots, wherein
each channel is shared by 10 cells in a mega-cell (a column in FIG.
6) and each of these cells is allocated with different time slots
(groups of ten consecutive rows along a column in FIG. 6); [0082]
g2) each cell in A-mega-cells is principally allocated with 1
channel and 9 time slots (depicted in the left table in FIG. 6 as a
section of ten consecutive rows along one column, where only the
first 9 rows in a section are colored gray); yet practically, each
cell is allocated with a basic set of 10 time slots (i.e. all ten
time slots depicted in said section of the left table, both the 9
gray colored and 1 white colored), however is denied one of these
time slots (the white colored); h) selecting a specific channel and
a specific time slot from those allocated to the corresponding
cell, preferably in a pseudo-random way. According to this
preferred embodiment, each cell is allocated with only one channel,
so the random selection is performed only on the allocated time
slots. Practically, each active device randomly selects a time slot
from the basic set of 10 time slots allocated to its cell, and for
A-mega-cells, if the selected time slot happens to be denied from
this cell (i.e. the 10.sup.th time slot), this device does not
transmit during this transmission cycle.
[0083] In other embodiments, this selection can be done based on a
unique identification data (ID) stored at each device (e.g. the ID
modulo the number of channels and/or tome slots); or a
classification data stored and/or acquired at each device (e.g.
channels served for "more important" devices); or according to the
nature of data to be transmitted (e.g. channels served for
emergency messages); or based on a signal/control input (e.g.
channels served for SOS messaging, activated by a user); or current
geographical location (e.g. separation of channels according to a
finer cellular partitioning);
[0084] According to the second embodiment mentioned above, the
selection of a specific channel and a specific time slot from those
allocated to the corresponding cell is done at each device with
coordination with other devices. As already indicated, in this
second embodiment, devices comprise also RF receivers (beyond GNSS
receivers), so nearby devices are able to communicate with each
other and negotiate channels, in order to refine the channel
allocation among them and further reduce potential transmission
collisions at the satellite receivers.
[0085] It is interesting to analyze the contribution of the
disclosed method to the message receive probability per
transmission cycle, i.e. the chance to receive a message from a
device, at least by one of the receiving satellites, in a
transmission cycle period. Observing FIG. 4, focusing on the area
of the footprint marked by a thick black circle, a skilled person
may detect 38 mega-cells: a) 6.times.A-type mega-cells; b)
20.times.B-type mega-cells (basically four B-mega-cells are
adjacent to each A-mega-cell, but the present analysis counts only
mega-cells placed inside the thick black circle); and c)
12.times.C-type mega-cells (six placed on the upper part of the
marked footprint and six in the lower part); some six smaller
intersections placed by the horizontal diameter of the marked
footprint are neglected, for the purpose of this discussion. A
skilled person may observe that for any two non adjacent of these
38 mega-cells, each mega-cell is covered at least by one footprint
that does not cover the other. In other words, two devices in this
area, placed in any non adjacent mega-cells, may be detected by at
least one satellite each, even if both devices are simultaneously
transmitting on the same channel. For satellites that cover the
earth surface substantially uniformly, as depicted in FIG. 4, such
communication isolation is probably provided all over the earth
surface. Hence, upon defining mega-cells as intersections of
footprints which are not divided to further intersections, as
indicated in step (c) of FIG. 5, communication conflicts may be
mainly due only to active devices placed in the same mega-cell or
in adjacent (sharing a bordering line) mega-cells. Furthermore, as
already mentioned, adjacent mega-cells differ by exactly one
satellite footprint, and devices placed in a mega-cell formed by
more footprints are not blocked by devices in an adjacent mega-cell
covered by one footprint less; therefore, post said step (c), an
active device might be interfered mainly only by another device
placed in the same mega-cell or a device in an adjacent mega-cell
which is covered by more footprints.
[0086] Then, the partitioning of mega-cells to cells and allocation
of different channels/time slot to different cells in a mega-cell,
as indicated in FIG. 5 steps (f)-(g), further limit potential
communication conflicts only to devices placed in cells that are
allocated with the same channels and time slots, either in the same
mega-cell or in adjacent mega-cells. Following, cells which are
allocated with the same channels/time slots (even related to
A-mega-cells, where cells are denied one time slot of ten) are
denoted as "matching cells".
[0087] Consequently, for two active devices, transmission collision
may occur mainly only in the following scenarii: [0088] i) a device
in an A-mega-cell interfered by another device in the same cell;
[0089] ii) a device in a B-mega-cell interfered by either a device
in the same cell or a device in a "matching cell" of an adjacent
A-mega-cell; [0090] iii) a device in a C-mega-cell interfered by
either a device in the same cell or a device in a "matching cell"
of an adjacent B-mega-cell (assuming no A-mega-cell adjacent to a
C-mega-cell).
[0091] Following is an analysis of the collision rate and message
receive probability per transmission cycle, in the above indicated
scenarii. This rough analysis is presented in order to clarify the
disclosed method and provide a person skilled in the art with some
practical design insights and considerations related to this
method. The following analysis assumes a uniform distribution of
devices on the earth surface, starting with two active devices. It
also assumes, extrapolated from FIG. 4, that A-mega-cells and
B-mega-cells and C-mega-cells have roughly the same area each, and
that the earth surface is divided to about 40.times.A-mega-cells;
160.times.B-mega-cells and about 120.times.C-mega-cells; wherein
each B-mega-cell has one adjacent A-mega-cell and each C-mega-cell
has 2-4 (averagely 3) adjacent B-mega-cells and no adjacent
A-mega-cells; Thus, roughly: the probability for a device to be in
an A-mega-cell is Pa=40/320=0.125; the probability for a device to
be in a B-mega-cell is Pb=160/320=0.5; the probability for a device
to be in a C-mega-cell is Pc=120/320=0.375; and the probability for
a device to be in a specific mega-cell (either type) is
Pm=1/320=0.003.
[0092] Then, since according to the preferred embodiment, a
mega-cell is divided to 100 equal in size cells, the probability
for a device to be placed in a specific cell in an A-mega-cell is
Pa/100, and correspondingly Pb/100 and Pc/100 and Pm/100.
[0093] This analysis also assumes a random selection of a specific
channel and a specific time slot in a cell, as indicated in step
(h) in FIG. 5.
[0094] Denoting the probability for receiving a message per
transmission cycle, from a device in an A-mega-cell as RPCa, and
correspondingly RPCb and RPCc for B-mega-cells and C-mega-cells and
RPC for the total weighted probability; the probability of a device
to transmit in this cycle as TP; the probability that two devices
in the same cell or in matching cells select exactly the same
channel and same time slot as CTP; then formulae 1-4 are:
RPCa=TPa*(1-CTP*Pm/100*TPa) (1)
RPCb=TPb*[1-CTP*Pm/100*(TPb+TPa)] (2)
RPCc=TPc*[1-CTP*Pm/100*(TPc+3*TPb)] (3)
RPC=Pa*RPCa+Pb*RPCb+Pc*RPCc (4)
[0095] At this point, if differently than according to the
disclosed method, step (e5) is cancelled and step (e4) is applied
to all mega-cells, i.e. uniformly allocating all 10 channels and
all 100 time slots to each and every mega-cell (hereafter: "default
approach"), then: each device transmits once in a cycle, so
TPa=TPb=TPc=1, and since preferably one channel and ten time slots
are allocated to each cell, then CTP=0.1 (probability to select the
same time slot of ten allocated to a cell); so:
"default approach", 1+1 devices: RPCa=1-CTP*Pm/100=0.999997;
RPCb=1-2*CTP*Pm/100; RPCc=1-4*CTP*Pm/100;
[0096] Considering back the preferred embodiment (including step
e5), TPa=0.9; TPb=TPc=1; so:
preferred embodiment, 1+1 devices:
RPCa=0.9*(1-0.9*CTP*Pm/100)=0.8999975 preferred embodiment, 1+1
devices: RPCb=1-1.9*CTP*Pm/100 preferred embodiment, 1+1 devices:
RPCc=1-4*CTP*Pm/100
[0097] For two active devices, the above analysis (applying the
above indicated specific parameters) shows that with the preferred
embodiment, RPCa is decreased, RPCb is increased and RPCc is the
same, compared to the "default approach".
[0098] However, networks as Cospas-Sarsat employ millions of
devices, from which more than two can simultaneously be active. If,
for example, a first device according to the present invention is
potentially interfered by n other devices, instead of 1, then
transmission collision may occur in the following scenarii: [0099]
i) a device in an A-mega-cell interfered by at least 1 of n other
devices in the same cell; [0100] ii) a device in a B-mega-cell
interfered by at least 1 of n other devices placed either in the
same cell or in a "matching cell" of an adjacent A-mega-cell;
[0101] iii) a device in a C-mega-cell interfered by at least 1 of n
other devices placed either in the same cell or in a "matching
cell" of an adjacent B-mega-cell.
[0102] Then, in formula (1), (1-CTP*Pm/100*TP) which is the
probability that no (second) device be in a specific cell and
select the same channel and time slot as a first device and
transmit during that cycle, is to be replaced by
(1-CTP*Pm/100*TP).sup.n which is the probability that none of n
devices be in a specific cell and select the same channel and time
slot as a first device and transmit during that cycle; placing
CTP=0.1 and Pm=0.003 and for the "default approach" TPa=TPb=TPc=1;
then
"default approach", 1+n devices:
RPCa=(1-CTP*Pm/100).sup.n=(1-0.000003).sup.n
RPCb=(1-2*CTP*Pm/100).sup.n; RPCc=(1-4*CTP*Pm/100).sup.n
[0103] And considering the preferred embodiment, placing TPa=0.9;
TPb=TPc=1; then:
preferred embodiment, 1+n devices:
RPCa=0.9*(1-0.9*CTP*Pm/100).sup.n preferred embodiment, 1+n
devices: RPCb=(1-1.9*CTP*Pm/100).sup.n preferred embodiment, 1+n
devices: RPCc=(1-4*CTP*Pm/100).sup.n
[0104] Clearly, for 1+n active devices, the above analysis shows
that comparing the preferred embodiment to the "default approach",
RPCb is increased and RPCc is the same, for any (positive) value of
n. Yet, a skilled person may probably appreciate that RPCa might be
increased or decreased by the preferred embodiment, compared to the
"default approach", depending on the value of n. By approximation,
RPCa for the "default approach" is
(1-0.000003).sup.n.apprxeq.1-n*0.000003; and RPCa for the preferred
embodiment is
0.9*(1-0.9*0.000003).sup.n.apprxeq.0.9*(1-0.9*n*0.000003); then, in
order for RPCa to increase by the preferred embodiment, i.e. have
0.9*(1-0.9*n*0.000003)>1-n*0.000003, it is required to have
approximately n>175,000.
[0105] Denoting m as the average number of devices per cell, and q
as the number of cells per mega-cell then m=n*Pm/q; also, denoting
ct as the total number of orthogonal channels and time slots
available for the whole system, and assuming that all ct are
allocated to every mega-cell (though some time slots are denied for
some mega-cells), then CTP=q/ct; so formulae (5) and (6) are a more
general presentation:
RPCa=TPa*(1-CTP*Pm/100*TPa).sup.n=TPa*(1-q/ct*Pm/q*TPa).sup.n=TPa*(1-Pm/-
ct*TPa).sup.n (58)
RPCa.apprxeq.TPa*(1-n*Pm/ct*TPa) (6)
[0106] RPCa is improved by step (e4) of the disclosed method
(compared to the "default approach" where TPa=1) if (approximately)
TPa*(1-n*Pm/ct*TPa)>1-n*Pm/ct; for a given n, as well as Pm and
ct, this is true if 1>TPa>1/(n*Pm/ct)-1; a skilled person may
appreciate that this is possible if n>ct/(2*Pm); checking again
on the preferred embodiment values (ct=1000; Pm=0.003), this means
approximately n>167,000.
[0107] It may also be noted that RPCb is improved by
(1-TPa)*n*Pm/ct, so if for example, TPa=0.9; n=200K; Pm=0.003;
ct=1000; then RPCb is improved compared to the "default approach"
by 6% and RPCa is improved by about 2%. A skilled person may
probably appreciate that this improvement increases linearly as n
increases.
[0108] Further, the present method is particularly effective for
communications with a diversity of non-geostationary satellites,
which provide many overlapping footprints on the earth surface.
Yet, by nature, these footprints move dynamically on the earth
surface, as these satellites dynamically orbit around the earth
sphere. Thus, the suggested channel allocation is valid for a
limited period of time.
[0109] In this context, in the second embodiment of the disclosed
method, already discussed above, channel allocation is applied to
devices which transmit a service request, before been allocated
operational channels that are administered by the network or
system. In this case, the requesting devices may share a pool of
channels for this preliminary phase of communications, from which
the disclosed method can function. Upon establishing an initial
contact with a satellite, the system may take control and allocate
a different channel to the requesting device. Clearly, this
embodiment assumes devices comprising an RF receiver configured to
detect the satellite transmissions, and a programmable RF
synthesizer, which can be set to different frequencies for
transmission and reception.
[0110] Still in the context of the momentary nature of the present
method, in the preferred embodiment of the disclosed method, the
channel allocation is applied by devices that periodically transmit
data packets, for each transmission a new channel is selected,
based on the updated satellite footprints, according to the
disclosed method. The disclosed method may then be applied on a
pool of channels and time slots derived from those allocated for
these transmissions.
[0111] Still referring to the preferred embodiment, a device
according to the present invention may comprise unique
identification data (ID), and be configured to transmit said ID
along with data representing its position coordinates. This is
particularly relevant to location systems and applications. In such
cases, said device may be attached to an object to be located, such
as: a person (particularly children and adult people), animal, pet,
vehicle, weapon, ammunition, valuable asset and so on.
[0112] Regarding location systems, the most important data
transmitted by a device is its position. Then, if its position does
not change significantly between successive reports, this device is
preferably configured to pass over one or more transmission cycles,
avoiding transmission of redundant data and improving the chance of
other devices to be detected. As a non limiting example, a
transmission cycle is preferably skipped if a device did not move
more than 100 meters compared to its previously transmitted
position. In another embodiment, a device may skip a transmission
cycle if its current data did not change significantly from the
previously transmitted data, not necessarily related to position,
but for example a sensor reading of temperature, humidity,
barometric pressure, water level, pollution level, etc.
[0113] In another case, a device transmission can be acknowledged
by the satellite. For example, such a feature is planed to be
implemented by the Galileo GNSS, so preferably, a device is
configured to pass over one or more transmission cycles upon
detecting one or more such acknowledgements, as indicated in step
(i) in FIG. 5. As a non limiting example, a transmission cycle is
skipped by a device whose transmission was acknowledged in the
previous cycle.
[0114] The invention is further directed to a computer program
product in a computer readable medium for a method for allocating
transmission channels to a plurality of devices configured to
communicate with a diversity of satellites, comprising, for each
device, the steps of:
a) determining the current time and self location via a Global
Navigation Satellite System (GNSS); b) determining the current
service areas ("footprints") of said diversity of satellites on
earth surface; c) defining as a mega-cell each intersection of
footprints which is not divided to further intersections; d)
determining a transmission cycle divided to time slots,
synchronized with said GNSS signals; e) allocating channels and
time slots to each mega-cell, wherein channels and time slots are
reused in different mega-cells, and at least for two mega-cells,
fewer time slots are allocated to the mega-cell served by more
satellites; f) dividing each mega-cell into cells, at least one
cell in a mega-cell; g) allocating different channels and/or
different time slots to different cells in a mega-cell, from those
allocated to the corresponding mega-cell; h) selecting a specific
channel and a specific time slot from those allocated to the
corresponding cell, according to at least one of: random or
pseudo-random selection; unique identification data (ID) stored at
each device; classification data stored and/or acquired at each
device; the nature of data to be transmitted; signal/control input;
current geographical location; coordination with other devices.
[0115] The invention is also directed to a communication system and
device for allocating transmission channels to a plurality of such
devices configured to communicate with a diversity of satellites,
each of said devices comprising at least an RF transmitter coupled
to a Global Navigation Satellite System (GNSS) receiver, wherein
each device is configured to:
a) determine the current time and self location via said GNSS; b)
determine the current service areas ("footprints") of said
diversity of satellites on earth surface; c) define as a mega-cell
each intersection of footprints which is not divided to further
intersections; d) determine a transmission cycle divided to time
slots, synchronized with said GNSS signals; e) allocate channels
and time slots to each mega-cell, wherein channels and time slots
are reused in different mega-cells, and at least for two
mega-cells, fewer time slots are allocated to the mega-cell served
by more satellites; f) divide each mega-cell into cells, at least
one cell in a mega-cell; g) allocate different channels and/or
different time slots to different cells in a mega-cell, from those
allocated to the corresponding mega-cell; h) select a specific
channel and a specific time slot from those allocated to the
corresponding cell, according to at least one of: random or
pseudo-random selection; unique identification data (ID) stored at
each device; classification data stored and/or acquired at each
device; the nature of data to be transmitted; signal/control input;
current geographical location; coordination with other devices.
[0116] Said computer program product, as well as said communication
system and device, may refer to channels which are at least: time
slots for Time Division Multiple Access (TDMA); or frequencies for
Frequency Division Multiple Access (FDMA); or codes for Code
Division Multiple Access (CDMA); or a combination thereof.
[0117] Also, according to said computer program product, and/or
according to said communication system and device, cells
statistically denser with devices can be defined smaller, and/or
cells statistically with more devices can be allocated with more
channels and/or time slots.
[0118] Further, said computer program product and/or said
communication system and device may refer to a device comprising an
ID, and configured to transmit said ID along with data representing
its location.
[0119] Then, according to said computer program product, and/or
said communication system and device, a device may be configured to
skip some transmission cycles if previously transmitted data (e.g.
location data) did not change significantly or/and if a previous
transmission was acknowledged.
[0120] Furthermore, each of said devices related to said computer
program product and/or said communication system and device, may be
attached to an object to be located, such as: a person
(particularly young, old and disabled), animal, pet, vehicle,
weapon, ammunition, valuable asset.
[0121] According to one aspect of the current invention, a
satellite based Search and Rescue (SAR) system is deployed, wherein
devices are distress radio beacons, programmed to transmit not more
than a data packet per transmission cycle, sharing a pool of
frequencies and time slots. The present invention is used by these
devices to select a specific frequency and time slot every
transmission cycle. When active, these radio beacons are configured
to transmit an ID and a self position, and skip a transmission
cycle if this position did not change at least by 100 meters from
the previously transmitted position.
[0122] According to another aspect of the invention, a satellite
based telephone network is deployed, wherein telephones request
service from the satellite network. Since it is expected that more
transmissions will come from urban areas, mega-cells in these areas
are divided to smaller cells, in a way that averagely a
substantially similar number of service requests will come from
each cell in a mega-cell. Further, when cells populated by many
devices are momentarily in B-mega-cells, then nearby A-mega-cells
are allocated with less time slots in order to less interfere with
devices placed in these dense cells.
[0123] According to an additional aspect of the invention, remote
sensors configured to be read by satellites are deployed worldwide.
The sensors are a multitude of low cost and low power (battery
operated) devices. In order to reduce cost, devices do not comprise
a receiver (beyond a GPS receiver), neither an RF synthesizer, thus
each device is factory set to a single frequency. The present
invention is then utilized to minimize transmission collisions
among devices, yet ensuring a sufficient probability to receive
data from these remote sensors by relatively few transmissions, in
order to save battery power. Assuming also a significant diversity
of satellites, many overlapping footprints are formed as well as
many footprint intersections, so the present invention relatively
employs many mega-cells and many cells. Then, this system allocates
fewer time slots to mega-cells served by more satellites (not only
the maximum number of satellites). For example, if three types of
mega-cells are momentarily formed, such as A-mega-cells and
B-mega-cells and C-mega-cells (using the terminology of FIG. 4),
C-mega-cells are allocated with all the available time slots per
cycle; B-mega-cells are allocated with 95% of the time slots and
A-mega-cells are allocated with 90% of the time slots per
cycle.
[0124] The above examples and description have of course been
provided only for the purpose of illustration, and are not intended
to limit the invention in any way. As will be appreciated by the
skilled person, the invention can be carried out in a great variety
of ways, employing more than one technique from those described
above, all without exceeding the scope of the invention.
* * * * *
References