Pulse or digital communications – Spread spectrum
Reexamination Certificate
1999-11-02
2001-05-29
Vo, Don N. (Department: 2631)
Pulse or digital communications
Spread spectrum
C370S318000, C455S013400, C455S059000
Reexamination Certificate
active
06240124
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to repeater-based communication systems and, in particular, to satellite-based communications systems having bidirectional communication signal links between one or more satellites and at least one ground station.
BACKGROUND OF THE INVENTION
Satellite-based communications systems are well represented in the prior art. By example, reference is made to U.S. Pat. No. 5,303,286, which issued on Apr. 12, 1994 to one of the inventors of this patent application, and which is entitled “Wireless Telephone/Satellite Roaming System”. Reference is also made to the numerous U.S. patents, foreign patents, and other publications that are of record in U.S. Pat. No. 5,303,286.
Low earth orbit satellite systems have been proposed for worldwide for mobile, cellular-like communications. These systems provide an ability to use low cost, hand-held communication devices, or user terminals, for communicating via satellite to parties in remote, rural, suburban and other environments.
As one example, user links to and from one or more satellites may operate on a relatively low frequency, such as a UHF signal. The user links are connected by the one or more satellites to ground station-originated feeder links that operate at a higher frequency, e.g., 3 GHz to 40 GHz or more. The feeder links are connected to a terrestrial gateway which allows the user to gain access to the public switched telephone network (PSTN), a private network, or some other terrestrial communications facility.
In general, if the feeder link frequency is below 7 GHz there is small potential for signal impairment. However, for frequencies above 7 GHz the effect of rain on the links to and from a satellite becomes increasingly significant. Research by NASA and others have quantified this rain effect, and have found the impairment effect to be more severe in what are termed ‘rain cells’ that are distributed around the site of a satellite uplink transmitter operating above 7 GHz.
A further consideration in a wireless communication system is the control of transmission power. By example, individual user links may be power controlled by a central site, such as a base station, after link impairment information between the user terminal and the base station is exchanged. This technique is generally referred to as user terminal power control. A function of this power control is to mitigate fading caused by trees, buildings and other RF-impairing factors within the user link. These impairments have the characteristic of reducing the signal power level to a lower level. To compensate for the reduction in signal level, the user terminal can be commanded to increase its transmitted power. Correspondingly, the user terminal may be able to request that the central station transmit at a higher power level.
However, and in a satellite-based communication system that uses satellites as repeaters, an increase in transmitted power from the user terminal or from a ground station, such as a gateway, can result in increased power being required for the satellite repeater. In that satellite power is a primary resource to be provided to and partitioned between many users, any increase in the power consumption of the satellite is undesirable. Furthermore, and for battery-powered user terminals, an increase in transmission power can have a detrimental impact on the number and duration of calls that can be made before the battery is required to be recharged.
This problem is compounded if the feeder link itself becomes impaired, in that the effect will be a reduction in signal power in all associated user links. To compensate for the reduction in signal power, all user terminals may request the ground station to increase its output power, thereby significantly increasing the satellite power consumption.
It is therefore desirable to provide a power control function for a satellite-based communications system that overcomes these and other problems.
SUMMARY OF THE INVENTION
This invention is directed to a satellite communication system, and a method executed by same, for providing adaptive closed loop power control. In accordance with a method of this invention for operating a satellite communication system having at least one satellite and at least one ground station, the method comprises the steps of: measuring a quality of at least one reference signal received by a user terminal, transmitting the measured quality of the reference signal received by the user terminal to the ground station, comparing the measured quality, and adjusting a transmit power of the ground station on the basis of the comparison. The reference signal being transmitted by the ground station through the satellite to the user terminal. The measured quality is compared with a predetermined reference. The transmit power of the ground station is adjusted in response to a difference between the predetermined reference and the measured quality so that a flux density of a downlink beam is substantially constant at the user terminal independent of the location of the user terminal in the beam.
Further in accordance with this invention there is provided a satellite communication system that includes at least one satellite and at least one ground station. In accordance with this invention the satellite communication system further includes means for transmitting an uplink reference signal, the satellite comprising a receiver and a transmitter, at least one user terminal and a processor coupled to the ground station. The satellite receiver receives the uplink reference signal. The satellite transmitter transmits the reference signal as a repeated downlink reference signal. The user terminal has a receiver for receiving the reference signal repeated by the satellite. The user terminal further has means for measuring a quality of the reference signal received by the user terminal, and also has means for transmitting the measured quality. The processor is coupled to the ground station for adjusting a transmit power of the ground station based on the measured quality transmitted by the user terminal. The processor is programmed to at least one of adjust the transmit power such that a flux density of a downlink beam from the satellite is substantially constant at the user terminal independent of the location of the user terminal in the beam or adjust the transmit power to compensate for predicted variation in satellite gain and maintain the flux density of the beam above a predetermined threshold at the location of the user terminal.
In another embodiment of the invention, the plurality of user terminals include a plurality of types of user terminals, and the method for operating the satellite communication system in accordance with the teachings found herein includes steps of measuring a quality of reference signals received by the user terminals, transmitting the measured quality of the reference signals received by the user terminals to the ground station, comparing the measured quality with a predetermined reference, and adjusting the transmit power of the ground station based on the comparison. The reference signals are transmitted by the ground station through the satellite to the user terminals. The transmit power of the ground station is preferably adjusted to at least one of provide a predetermined percentage of all user terminals in a given one of the downlink beams with downlink signal quality above a common predetermined threshold, or to provide a predetermined percentage of each type of user terminal in the beam with downlink signal quality above a corresponding predetermined threshold that is set independently for each type of user terminal.
REFERENCES:
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patent: 4261054 (1981-04-01), Scharla-Nielsen
patent: 4309764 (1982-01-01), Acampora
patent: 4731866 (1988-03-01), Muratani et al.
patent: 4752925 (1988-06-01), Thompson et al.
patent: 4752967 (1988-06-01), Bustamante et al.
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patent: 4941199 (1990-07-01), Saam
patent: 5010317 (1991-04-01), Schwende
Sites Michael J.
Wiedeman Robert A.
Globalstar L.P.
Ohlandt Greeley Ruggiero & Perle L.L.P.
Vo Don N.
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