Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-05-04
2003-01-28
Hunter, Daniel (Department: 2749)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S427000
Reexamination Certificate
active
06512920
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a satellite communications system and its handover processing method, and in particular, to a satellite communications system using a group of orbital satellites according to a satellite fixed cell scheme, as well as a handover processing method therefor.
2. Description of the Prior Art
In the current satellite communications system using a group of orbital satellites, a plurality of satellites are arranged around the earth so as to revolve around it along certain orbits. One of known systems that are adapted for such a satellite communications system uses a group of low-orbital satellites that are principally non-stationary orbital satellites such as iridium satellites or global stars which operate on low earth orbits (LEO).
For example, iridium satellites revolve along six orbits that are substantially orthogonal with the equator. Eleven satellites are located on a single orbit so as to construct a cellular telephone network in which a total of 66 satellites cover every area of the earth. A variety of control centers (hereafter referred to as “ground stations”) on the earth monitor the satellite network, transmit instructions for attitude control, and inform each satellite of necessary information. An iridium system satellite has a switching function and an intersatellite communication function. If a user makes a telephone call from an iridium terminal to a public telephone network, a radio wave originated from the iridium terminal is first received by an overhead satellite and then passes through adjacent satellites to reach a gateway connected to a destined telephone network. The call further passes through the telephone network connected to the gateway before connecting to the destination.
In the case where a user makes a call from an iridium terminal to a subscriber telephone connected to a public telephone network, a call setup request is first sent to the nearest gateway. This is to authenticate this iridium terminal. Upon identifying the iridium terminal, the gateway then identifies a gateway corresponding to the destination, and delivers the setup request to this gateway via a satellite circuit. The gateway that has received the connection request identifies the destination and makes a call to a corresponding subscriber telephone network to make a connection. When the destination receives the call, it connects to a communication path joining the satellites together and then starts communication. If the iridium terminal is connected to a telephone connected to a public telephone network, the call setup processing involves two gateways.
In the case of communication between iridium terminals, the satellites also provide a switching function and a communication function. Switching processing, however, is not carried out only by the satellites but connections are always made through a circuit connection procedure via gateways.
The orbits of iridium satellites are as described above. Each of the satellites arranged on these orbits covers the overall global surface using beams impinging on the surface of the earth. A plurality of such communication beams emitted from a satellite cooperate with one another to form a coverage area of the global surface. Since the coverage area is formed by non-stationary orbital satellites, it moves relative to the global surface over time. When the iridium terminal is located in the area of a certain overhead satellite, it can connect to another terminal in the same area or to a public telephone network via a gateway in the same area. Alternatively, when the iridium terminal is to be connected to another iridium terminal or a public telephone network that belongs to a different area, a call is set up via an intersatellite communication link established with an adjacent satellite.
The coverage area formed by a plurality of beams from a satellite in corporation is formed of subsections each formed by a corresponding one of the plural beams. By dividing the coverage area into the subsections, the number of communication channels available in the system can be increased to augment the entire capacity (that is, the number of mobile stations that can be supported by the system).
Likewise, each subsection formed by a plurality of cover beams emitted by satellites corresponds to one dynamic geographical area (cell) that moves over the ground surface as the satellite obits the earth. A coverage beam emitted to each cell is assigned a particular frequency as a communication carrier, and each carrier frequency supports a plurality of channels, which each channel assigned to a specific terminal located within the cell. A terminal can communicate with a satellite at an assigned frequency on an assigned communication channel as long as this terminal is located within the associated cell.
When, however, the terminal leaves an old beam/cell and enters a new beam/cell, this terminal must be assigned a new communication channel. In addition, this terminal must adjust its communication frequency to the frequency associated with the new cell. Otherwise, the current communication channel (an ongoing call) will be dropped (that is, disconnected). Once the terminal has entered the new beam/cell, it uses the new channel for all communications with the satellite.
In addition, of course, such switching of communication channels is required not only within the same satellite but also between satellites. In a satellite communications system principally using a group of low-orbital satellites as described above, a terminal may shift from a satellite by which it is being serviced to a service area of a next satellite, and in this case, it must also switch to a new communication channel.
The process of switching channels carried out in this aspect is called “handover.” Handovers are necessary when a mobile station moves from one beam or cell to another beam or cell, and in order to carry out handover, the mobile station must switch to a new predetermined transmission or reception frequency.
In the handover process carried out in a conventional mobile communication system, when a terminal determines the necessity of handover, for example, depending on the intensity of electric field received from a satellite, it sends a handover request signal to a gateway belonging to a service area for the terminal or directly to the satellite. Then, the gateway or satellite that has received this signal transmits and receives information to and from the satellite or gateway, respectively, in order to switch to a new channel to the terminal.
To achieve a smooth handover process, timings when handover becomes necessary are preferably known beforehand. For example, as an example of a conventional technique relating to the handover process, a gateway makes analysis based on a report generated by a terminal as appropriate, and if a power level contained in the report has a predetermined threshold or lower, the gateway determines that the terminal is leaving the current coverage area and recognizes that the handover process will be required in the near future. In an alternative known technique, the gateway continuously monitors the position of the terminal and the position of an associated coverage beam to determine a timing when handover becomes necessary. The gateway, which has used the above method to recognize the timing when handover is to be carried out, transmits signals with appropriate timings starting with the handover request signal to establish a new channel between a corresponding satellite and the terminal.
Handover in the conventional intersatellite communications system will be described with reference to the sequence diagram shown in
FIGS. 1 and 2
.
With reference to
FIG. 1
, handover between a first satellite and a second satellite following the first satellite on the same orbit will be described. A ground station controls all the satellites such that a controller of each satellite is caused to register via its receiver a physical location immediately below the navigating satellite (the
Hunter Daniel
NEC Corporation
Sughrue & Mion, PLLC
Tran Tuan
LandOfFree
Satellite communications system and handover processing method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Satellite communications system and handover processing method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Satellite communications system and handover processing method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3036858