Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
1998-12-11
2001-01-23
Cangialosi, Salvatore (Department: 2732)
Multiplex communications
Communication over free space
Repeater
C370S393000
Reexamination Certificate
active
06178163
ABSTRACT:
TECHNICAL FIELD
The present disclosure relates generally to mobile satellites and terrestrial wireless communication systems, and more particularly, to a routing method and system for enabling a Low Earth Orbit satellite communication system to efficiently transfer data packets among satellites for a continuous wireless service.
BACKGROUND OF THE INVENTION
In the modern satellite telecommunication industry, the Low Earth Orbit (LEO) satellite system is considered as one of the alternatives to the Geostationery satellite system for providing global wireless services. The significantly small round trip time delay between a ground station on earth and a satellite, i.e. about 14 mili-seconds, and low power consumption for transferring radio frequency signals, such as packets, have proved to be distinctive advantages. In order to achieve a global coverage, a LEO satellite system may have over a hundred satellites rotating around the earth.
FIG. 1
depicts such a global LEO satellite system
10
.
Turning now to
FIG. 2
, each satellite
12
of a LEO satellite system will have a footprint
14
. The footprint is a spherical coverage area on the surface of the earth that a satellite can cover. However, in order to eliminate redundant coverage in an overlapping area
16
of neighboring footprints, each covering satellite will only have a smaller responsible region
18
within its respective footprint. Accordingly, the surface of the earth is divided into a number of fixed responsible regions, the size of which being determined by the total number and the physical positions of satellites in a particular LEO satellite system.
A major problem of LEO satellite systems is that the satellites move around the earth at a speed much faster than the speed of the earth's rotation. The faster speed of the satellites is required in order for the satellites to be kept in a particular orbit, and therefore, at a particular position on the earth. As a result, wireless telecommunication services are handled by different covering satellites at different times. For example, with a LEO satellite system in a 1500 km orbit, the moving speed of satellite units is on the order of 6.35 km/sec (i.e., about 4 miles/sec). Consequently, a particular geographical location on the earth will have a change in its covering satellite about once every eight (8) minutes.
In order to have continuous wireless services in a particular geographic location on the earth, an incoming covering satellite must take over prescribed information about its new responsible region from an outgoing covering satellite. This is required in order not to interrupt any ongoing wireless service. For example, a mobile phone user may be talking on the phone while a corresponding covering satellite moves out from the responsible region or coverage area. Another satellite moves in to cover the same area, however, if the new satellite does not obtain information about the ongoing phone call, the user will be undesirably disconnected. To avoid this problem, information is exchanged between outgoing and incoming satellites through wireless communication connections between them known as inter-satellite links
20
, for example, as shown in FIG.
2
.
Due to technical limitations, at any specific time, a satellite may only have connections with at least one satellite north of it, another one south of it, two satellites west of it, and two more satellites east of it. For illustration purposes herein, each LEO satellite of the LEO satellite system is assumed to have only four such inter-satellite links to its neighboring satellites, as shown in a two dimensional view of FIG.
3
. Take a particular wireless service such as a long-distance phone call as an example, it is likely to be one that intends to reach a location outside of the responsible region for a particular covering satellite. For the long-distance call, packets are sent through multiple inter-satellite links to reach a covering satellite over the final destination, to be received by a receiver of the long-distance call. Traditionally, identifications of those satellites and the route for the packets are stored in each of the covering satellites, both for the caller and the receiver, in the form of a routing table. When there is a transition between two covering satellites for the caller, this routing table must be transferred from the outgoing covering satellite to the incoming one. The size of a routing table for a responsible region can be very large, for example, on the order of about 3 kilo bytes. Since there is always a bandwidth limitation on inter-satellite links, frequently passing of routing tables between satellites consumes valuable bandwidth of the inter-satellite links (i.e., congests messaging traffic), and as a result, prevents other services, such as billable phone calls, from going through. A traffic congestion control mechanism can be installed to mitigate this effect, however, the cost to maintain the LEO satellite system increases, and at the same time, the overall capacity of the system is reduced.
In order to find a feasible alternative to the frequent information exchange between satellites, a virtual network and virtual routing method is proposed in a co-pending application UK Patent No. 9,707,832, incorporated herein by reference. Consistent with the concept of a virtual network, as
FIG. 4
illustrates, the surface of the earth
22
is divided into a fixed number of virtual regions, each region being called a node
24
. Each region or node has the same size as a responsible region as determined by a LEO satellite system. Similar to the inter-satellite links, each node is assumed to be linked to four immediately adjacent nodes, for example, as indicated by reference numeral
26
. Because of the constant rotation of the satellites with respect to the earth, a satellite's responsible region is likely to cover a portion or part of multiple virtual nodes. If more than one quarter of the total area of a node is covered by a given satellite, then the given satellite is defined as a primary satellite to that node. For example, as shown in
FIG. 5
, the virtual network
22
is defined by a grid system with continuous lines, while the actual responsible regions
18
are represented by a grid system with dotted lines. Therefore, by definition, since the unit sizes of these two systems are the same, only one primary satellite exists at any time for a particular node. For example, Satellite “q” is a primary satellite for Node “Q”.
In co-pending patent UK Patent No. 9,707,832, a method is disclosed for finding a path between any two nodes in a virtual network, and then mapping this path to an actual satellite network to eliminate the need for passing routing tables between satellites. However, even utilizing the framework of a virtual network, wherein a shortest path for routing data packets is determined between any two satellites, undesired traffic congestion can still be triggered unexpectedly between the satellites if a normal shortest path algorithm is used. The undesired traffic congestion occurs because numerous wireless services will be taking the same shortest route between these satellites which leads to saturation of the bandwidth. Another disadvantage of using a normal shortest path algorithm is that since only the capacities of satellites on the route are used, other satellites in the entire satellite system cannot be taken advantage of or used, even if they are idle with no current service operations.
SUMMARY OF THE INVENTION
What is needed in a LEO satellite system with the implementation of a virtual network is a method to arrange an optimal route for data packets to travel, and to diversify the traffic between satellites, thus to enhance the efficient use of the entire satellite system. In order to achieve this goal, a satellite is randomly selected first as an intermediary satellite between a source satellite and a destination satellite, i.e., between a satellite which covers the region where the phone call is initiated and a satellite which covers the re
Subramanian Sairam
Yuan Wei
Cangialosi Salvatore
Haynes and Boone LLP
Northern Telecom Limited
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