Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-07-13
2001-05-15
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S428000, C455S429000
Reexamination Certificate
active
06233451
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to cellular and satellite communications. More particularly, the invention relates to a method and a system for ensuring that a user terminal, when operating in a GEM system nearly always selects the best spot beam to access the system which maximizes the power-limited capacity of the system to handle phone calls for ongoing voice communications between a mobile transmitter and a satellite in a time division multiple access (TDMA) system.
The spot beam selection and reselection procedure is the first of four (4) processes performed in the idle mode. These four (4) processes are spot beam selection and reselection, Public Land Mobile Network (PLMN) selection, global positioning system (GPS) position determination, and location updating, in accordance with the invention.
A mobile satellite communication system such as the Geosynchronous Earth Orbit Mobile (GEM) network discussed herein, typically includes one or more satellites, at least one fixed ground terminal such as a gateway system (GS) and several mobile access terminals (ATs). The access terminals typically communicate with the public switched telephone network (PSTN) or other mobile terminals via an air communication interface between the satellite and the gateway. Using the mobile access terminals, the satellite system provides a variety of telephony services. Satellite telephony systems as described herein share call processing information with terrestrial systems such as the GSM cellular system to allow compatibility between the satellite, cellular, and the public switch telephone network services. The terrestrial standards such as GSM may not apply directly to the mobile satellite communication system, more particularly the satellite air interface poses physical constraints not accounted for in the GSM architecture.
A number of communication systems utilizing satellites and small mobile terminals provide voice and other information communication. In all such systems, the satellite power associated with the communication links may be an expensive resource, e.g., constrained by satellite solar power. In addition, the mobile access terminals, such as hand-held terminals (HHTs), are often moved from one spot beam to another spot beam as users travel. Thus, to establish and maintain an optimal communication channel, an appropriate spot beam selection procedure must be employed for a user terminal in a GEM satellite system.
If too many mobiles select the wrong spot beam or a spot beam with lower RSS, then the system capacity will suffer. This is because the satellite capacity to serve telephone calls is constrained by available transmit power which is limited by the solar panels, batteries, and output power limitation of the transmit amplifiers.
If 50% of all user terminals select the spot beam with the weaker signal and that signal is 2 dB less than the strongest signal, then the capacity of the satellite to handle phone calls based on power limitations is reduced by 23%.
Thus, there exists a need for a method and a system for fast and efficient spot beam selection in a GEO Mobile Satellite System to provide acceptable performance with a minimum amount of power being used by the satellite and access terminal systems.
A satellite system is different from a cellular system in that all signals originate from the same point source, i.e., the satellite for all spot beams. In addition, all spot beam signals take the same propagation path from the satellite to the handset and therefore the measurement that the handset makes on the signals are highly correlated. Furthermore, the signals can be synchronized to a master timing reference source or oscillator in the satellite. These differences with the cellular system provide the opportunity to organize and control channels in the different spot beams to be time orthogonal.
SUMMARY OF THE INVENTION
Accordingly, the present invention addresses selecting the best spot beam based on relative received power or using the received signal strength indicator (RSSI) or any other signal strength measurements in a Geosynchronous Earth Orbit Mobile Satellite System, i.e., a Geo Mobile (GEM) system, which is critical to the performance of the entire satellite system. The invention thus provides a method and a system of organizing the control channel signals being broadcast by the satellite in its spot beams to enable rapid determination of the best spot beam by the user access terminal.
Briefly summarized, the present invention relates a system for employing an access terminal for initiating spot beam selection in a satellite communication system, in which the access terminal includes a receiver for measuring the received signal strength (RSS) of a multiplicity of radio frequency communication spot beam links. The access terminal is further provided with a controller for comparing the received signal strengths from each of the multiplicity of spot beams to initiate information communication via a communication channel of the satellite communication system. In a described embodiment, the controller of the access terminal may compare six or more spot beam links at the same time to determine whether to initiate information communication with one of the several spot beams received. Additionally,.a memory coupled to the controller of the access terminal is used for storing spot beam identification information including the spot beam links assigned to the access terminal via the satellite communication system. The selection procedure employed by the system and method described facilitate a rapid selection of an appropriate spot beam identified from the multiplicity of radio frequency spot beam links received at a mobile access terminal.
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Demers Stephanie
Gao Zhengping
Joshi Chandra
Noerpel Anthony
Parr Michael
Chow Charles
Hughes Electronics Corporation
Hunter Daniel
Sales Michael W.
Whelan John T.
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