Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-08-10
2001-05-01
Eisenzopf, Reinhard J. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S450000, C455S452200, C455S427000, C455S012100
Reexamination Certificate
active
06226521
ABSTRACT:
The present invention relates to a method and apparatus for controlling the allocation of channels to communications stations and for selecting channels for communication at communications stations.
A known example of channel management is employed in the Inmarsat (TM) Aero Services, which are designed for aeronautical satellite communications. Details of the Inmarsat (TM) Aero Services can be found, for example, in “Satellite Communications” by Calcutt & Tetley, First edition 1995.
The architecture of a part of the Inmarsat (TM) network is shown in
FIG. 1. A
geostationary satellite S
1
provides a satellite communications transponder for satellite communications by any one of a number of aircraft earth stations AES
1
, , , AES
n
located within the coverage region of the satellite S
1
. The aircraft earth stations AES comprise communications equipment located on aircraft, and can be used for satellite communications via the satellite S
1
when the aircraft earth stations AES are within the beam coverage area of the satellite S
1
. In this example, there are four satellites S
1
to S
4
which provide communications coverage over four different areas, usually referred to as “ocean regions”.
The aircraft earth stations AES communicate via the satellite S
1
to one of a plurality of ground earth stations (GES) GES
11
to GES
1
n which direct radio frequency signals to and receive radio frequency signals from the satellite S
1
. Each GES is connected to a network operated by a service provider, such as a public service telephone network (PSTN), a terrestrial cellular network or a local area network, so that fixed terminals connected to the network can communicate with any one of the aircraft earth stations AES.
The ground earth stations GES are operated by different service providers and are typically located in the country in which the service provider operates.
When the user of an AES initiates a call, the AES sends a signal via the satellite S
1
to a selected GES according to the service provider selected by the user. The selected GES sends a signal back to the AES indicating the carrier to be used for the call. The Aero circuit-mode service uses single channel per carrier (SCPC) full duplex transmission channels, so that the carrier required for a single communication comprises a pair of frequencies. Communications then proceed between the AES and the selected GES.
Likewise, if a terrestrial caller initiates a call to a selected AES, a call request signal is sent to a selected GES through the terrestrial network. The GES sends a calling signal through the satellite S
1
indicating the AES to which a call is to be set up. If the AES is within the coverage area of the satellite S
1
and is logged on to the network, it sends an acknowledgement signal via the satellite S
1
to the GES which in turn signals to the AES which carrier is to be used for the call.
From the above description, it will be appreciated that data must be available at each GES indicating which carriers may be used by that GES for communication with the AES. Each GES is therefore allocated a subset of all the channels which are available for communication via the satellite S
1
. The allocation of carriers to each GES is controlled by a network operations centre NOC which periodically updates the carrier allocation to the ground stations GES through an inter-station communications link ISL.
This system of carrier allocation, though simple and reliable, is inefficient because each GES must be allocated enough carriers to cope with peak demand at that GES. Therefore, at any one time a large proportion of the carriers are unused.
In case of failure of the geostationary satellite S
1
, a backup geostationary satellite S
1
′ is provided and is available for communication with each GES in the event of failure of the geostationary satellite S
1
.
Further, GESs GES
2
x
, GES
3
x
, GES
4
x
, are provided for communication with further satellites S
2
, S
3
, S
4
covering further ocean regions.
A backup network operation centre NOC′ is also provided, in case of failure by the primary network operation centre NOC.
An alternative network architecture, which is used in other Inmarsat (TM) services such as Inmarsat-M (TM) and Inmarsat-B (TM), is shown in FIG.
2
.
The same geostationary satellites are used for these services as for the Aero Services. In this example, a plurality of mobile terminals MES
1
to MES
n
communicate via the satellite S
1
with a plurality of land earth stations LES
11
to LES
1
n. However, this example differs from the previous example in the method employed to allocate carriers. Each LES is arranged to communicate with a network coordination station NCS
1
, through an inter-station communications link ISL, which in this example is a satellite communication link through the satellite S
1
. The network control station NCS
1
is also able to communicate directly with the MESs through the satellite S
1
.
When a call is initiated by an MES user, the MES sends a call request signal both to the NCS and to a selected LES. The NCS selects a suitable carrier for the communication and signals the selected carrier both to the MES and to the selected LES. The call then proceeds between the MES and the selected LES.
If a call is initiated by a terrestrial user connected to an LES, the LES transmits a calling signal through the selected satellite S and awaits a response from the called MES. If a response is received from the MES, the LES sends a channel request signal to the NCS, which selects a carrier for the communication and signals the carrier selected to both the MES and the LES.
One NCS and one backup NCS provided for each satellite S store a set of all the carriers available for communication via that satellite. This set of carriers is updated from time to time by data received from an NOC over an inter-network link INL.
In this system, carriers are assigned to the LESs by the NCS only as they are needed and unused channels are available to any of the LESs which request them. However, a considerable delay may be incurred between a call request and the completion of call set-up, because of the delay involved in sending a request to the NCS and in communicating the allocated carrier from the NCS to the LES. Moreover, if there is a high demand for calls at several of the LESs, the inter-station link ISL may become congested, leading to further delays in the allocation of carriers and possible failure of call set-up if an MES times out and terminates the call because no carrier has been allocated to it within a predetermined time.
Moreover, this architecture is vulnerable to failure by the NCS, in which case no carriers can be allocated to the LESs and the entire network for the associated ocean region may fail. A back-up network control station NCS′ is provided for each primary NCS so as to avoid such catastrophic failure. However, the back-up NCS′ must operate in synchronism with its associated primary NCS so that, if a failure occurs, the back-up NCS′ is immediately available for carrier allocation and is consistent with the state of the primary NCS immediately before the moment of failure. Providing such a back-up NCS is costly, particularly if the back-up NCS is remote from the primary NCS, which is desirable in order to provide an effective back-up against failure due to localised disruptions. If the back-up NCS also fails, the entire network for the associated satellite also fails.
Hence, this architecture, though efficient, is inherently less reliable than the distributed network architecture of the Inmarsat-Aero (TM) system.
The document WO94/18804 discloses a real-time channel borrowing method for a cellular communication system, in which a channel is “borrowed” from an adjacent cell in response to a channel request from a user, if no channels are available in the user's cell. Once the borrowed channel is released, it is returned to the adjacent cell.
The article “Comparisons of Channel-Assignment strategies in Cellular Mobile Telephone Systems” by Ming Zhang
Liu Ming
Sengupta Jay
Wasse Michael Petre
Banner & Witcoff , Ltd.
Eisenzopf Reinhard J.
International Mobile Satellite Organization
Ramos-Feliciano Eliseo
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