Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-03-24
2001-08-07
Banks-Harold, Marsha D. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S012100, C455S067150
Reexamination Certificate
active
06272340
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a communications system and, more particularly, to a load shedding method to enhance uplink margin with combined FDMA/TDMA uplinks in a satellite based cellular communications system.
2. Discussion of the Related Art
In satellite based cellular communications systems, a central terrestrial control processor or network operations center (NOC) generally controls one or more communications satellites operating within the communications system. Each communications satellite within the communications system services multiple users located in multiple geographic areas, known as ground cells. The communications satellites receive and transmit data signals to and from the multiple users or terrestrial terminals positioned at the different locations within the separate ground cells on a point-to-point manner. In a frequency division multiple access (FDMA)/time division multiple access (TDMA) communications system, signals on the same frequency, polarization and time slot are generally reused by different users within the spatially separated ground cells because of bandwidth constraints.
An antenna on each communications satellite generates a multitude of spot beams to illuminate a surface on the earth where the ground cells are located in order to accommodate the re-use of the frequencies throughout the communications system. Antenna patterns for each spot beam covering each ground cell typically roll off very fast towards the edge of the beam, and thus with even small antenna or satellite pointing errors, the user on the ground may see significant signal attenuation due to this mispointing. Moreover, since modern satellite communications systems now operate at much higher carrier frequencies (for example, Ka-band), these signals are vulnerable to large attenuations due to rain, scintillation, and other atmospheric effects. Because of this, the terrestrial user terminals will have degraded performance unless attenuation mitigation is employed.
Conventional ways of mitigating attenuation caused by atmospheric effects, roll-off, etc., include transmitting at a lower data rate or at a higher power level to enhance or increase the transmitted energy per bit. Alternatively, additional error control coding may be used where the coding rate is increased without increasing the occupied bandwidth. This, however, reduces the amount of information bits being transmitted since more error control bits are generated with the higher error control rate. Increasing the coding also causes the demodulator on-board the satellite to be more complex. Increased power also means that the transmitter amplifier is oversized in nonfade conditions, thereby increasing overall system cost without any tangible benefits.
What is needed then is a load shedding method and apparatus to enhance uplink margin which does not suffer from the above-mentioned disadvantages. This will, in turn, provide a satellite based cellular communications system which utilizes multiple FDMA carrier channels simultaneously to operate in a multi-carrier fashion; enable “shedding” of carrier channels as fading or attenuation levels increase, thereby increasing the transmitted energy per bit to overcome the increased attenuation; provide a means for monitoring and controlling the appropriate number of carrier channels to be used by the terrestrial terminal; eliminate the requirement of multiple rate demodulators on-board the satellite to reduce satellite complexity; and provide terrestrial terminals that can support different numbers of carrier channels to provide varying fade capability such that the terrestrial terminals are scaleable in size and cost. It is, therefore, an object of the present invention to provide a satellite based cellular communications system which utilizes a load shedding method to enhance uplink margin with combined FDMA/TDMA uplinks.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, a load shedding method to enhance uplink margin between a terrestrial terminal and a satellite in a satellite based cellular communications system is provided. The load shedding method provides a means to efficiently and cost effectively provide enhanced uplink margin by utilizing multiple uplink carrier channels simultaneously. The power level of the carrier channels or number of carrier channels may then be adjusted depending on the particular attenuation levels.
In one preferred embodiment, a load shedding method to enhance uplink margin between a terrestrial terminal and a satellite in a satellite based communications system includes generating a first signal having a first carrier channel and a second signal having a second carrier channel. The first signal having the first carrier channel and the second signal having the second carrier channel are transmitted from the terrestrial terminal to the satellite. A transmit power level of the first signal and the second signal transmitted from the terrestrial transmitter is determined and a received power level of the first signal and the second signal received at the satellite is determined. Transmission of the second signal having the second carrier channel is terminated when the transmit power level of the first signal and the second signal transmitted from the terrestrial terminal exceeds a first threshold.
In another preferred embodiment, a load shedding method to enhance uplink margin between a terrestrial terminal and a satellite in a satellite based communication system includes generating a plurality of uplink carrier channels. The plurality of uplink carrier channels are transmitted from the terrestrial terminal to the satellite. A transmit power level and a receive power level of the plurality of uplink carrier channels are determined. An attenuation level on the plurality of uplink carrier channels based upon the transmit power level and the receive power level is also determined. The transmit power level of the plurality of uplink carrier channels is adjusted based upon the attenuation level.
In yet another preferred embodiment, a terrestrial terminal for enhancing uplink margin between the terrestrial terminal and a satellite in a satellite based communication system includes a multi-carrier network operable to generate a plurality of uplink carrier channels. An RF transmit and receive system transmits the plurality of uplink carrier channels to the satellite. A terminal control unit determines an attenuation level on the plurality of uplink carrier channels transmitted to the satellite and instructs the multi-carrier network to increase or decrease the number of uplink carrier channels to be transmitted.
Use of the present invention provides a load shedding method to enhance uplink margin in a combined FDMA/TDMA uplink used in a satellite based cellular communications system. As a result, the aforementioned disadvantages associated with the current communications systems have been substantially reduced or eliminated.
REFERENCES:
patent: 5956619 (1999-09-01), Gallagher et al.
Wilcoxson Donald C.
Wright David A.
Banks-Harold Marsha D.
Keller Robert W.
TRW Inc.
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