Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2000-06-19
2002-08-06
Chin, Vivian (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S069000, C455S012100, C455S427000, C370S321000, C370S326000
Reexamination Certificate
active
06430418
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to satellite communication systems. In particular, the present invention relates to controlling the uplink power in a satellite communication system.
Satellites have long been used to provide communication services to large regions of the globe. Historically, communication satellites have used frequencies in the range—of 3 to 12 GHz (C or Ku band) to produce an antenna beam which covers a large portion of a continent. Modern satellites may operate at frequencies of 20 to 30 GHz (Ka band) to produce a beam which may cover an area (or “cell”) with a diameter of 300 to 400 miles. Many such cells may be needed to provide communications to a region which previously may have been serviced by a single antenna beam. A modern cellular communication satellite may employ many antennas to generate a large number of beams used for transmitting downlink signals to, and receiving uplink signals from, various User Earth Terminals (UET) distributed over the surface of the earth.
In order for communication to occur on the uplink, signals generated by the UET must be of sufficient power to be received by the satellite. Thus, the antenna gain of the satellite's uplink antenna coupled with the transmission power of the UETs must be sufficient to allow communication to occur. Typically, communication satellite systems are designed with a predetermined, fixed satellite uplink antenna gain. Thus, the transmission power of the UET is typically controlled to enable and ensure communication.
In practice, several factors exists which may negatively impact the uplink communication channel. That is, certain undesired influences may cause the actual antenna gain to vary from the predetermined, designed antenna gain or may cause attenuation of, or interference with, a signal transmitted by a UET. For example, antenna gain may be affected by gain roll-off which may cause the antenna gain to vary spatially over the cell or, alternatively, antenna gain may vary over the cell as a result of pointing errors in the antenna. Atmospheric attenuation, also known as “rain loss,” or interference among several UETs, also known as Co-Channel Interference (CCI), may also affect the quality of a signal transmitted from a UET. Each of these conditions, gain roll-off, antenna pointing errors, atmospheric attenuation, and CCI is further discussed below.
1) Gain Roll-Off
The pattern of cells on the surface of the earth is known as the cellular pattern of the satellite communication system. The cellular pattern in a modern satellite communication system may be defined on the surface of the earth such that the maximum gain of a satellite antenna beam is directed toward the center of its assigned cell. The boresight of a satellite antenna beam may be defined as the maximum gain point in the satellite antenna beam, and is typically directed to the center of a cell. The edge of a cell may be defined by determining the angular deviation from the antenna boresight at which the gain of the antenna beam drops to a predetermined value below the maximum gain value, typically at least 3 dB below the maximum gain value. The decrease in antenna beam gain with increasing angular deviation from boresight is known as gain roll-off. In terms of uplink power, a communications signal which is transmitted to the satellite from a UET located at the edge of a cell may be received by the satellite antenna with a gain which is at least 3 dB lower than the gain of a signal which is transmitted from a UET located at the antenna boresight, or center of the cell. Thus, the transmission power level of a terminal located at the edge of a cell must be at least 3 dB higher than that of a terminal located at the center of a cell in order to achieve the same level of performance. In other words, if the edge of a cell is defined as the angle from boresight at which the satellite antenna gain has decayed to 3 dB below the maximum antenna gain at the boresight, a UET at the edge of the cell may need to use a transmission power level 3 dB higher than a UET at the center of the cell in order to compensate for the reduced antenna gain at the edge of the cell. By transmitting at the 3 dB higher transmission power level, the signal from the UET at the edge of the cell may be received at the satellite with a power that is approximately equal to the power of a signal from the UET at the center of the cell. In order to simplify and reduce the cost of uplink components installed on the satellite, it is desirable to maintain a similar received power level for each UET in the cell. Thus, it is desirable to modify the transmission power of each UET in the cell to compensate for any reduction in the antenna gain at each UET resulting from the UET's position within the cell.
2) Antenna Pointing Errors
In practice, the antenna beams of a cellular communication satellite are generally not directed precisely toward the centers of their assigned cells. Slight mis-orientations of the antenna boresights and deviations from a perfectly circular, zero-inclination satellite orbit give rise to pointing errors. These pointing errors may cause the location of the maximum gain of an antenna beam to deviate from the cell center. Some pointing errors may also cause the maximum gain of an antenna beam pattern to change measurably over the course of a day. In other words, the antenna beam gain distribution across the cell may change with time.
The antenna beam gain at the edge of a cell typically rolls off rapidly as the distance from the center of the cell increases, that is, as the angular deviation from boresight increases. Thus, a pointing error corresponding to only 10% of a cell diameter may cause the antenna beam gain at the edge of a cell to vary by 2 dB or more. Because it is desirable to maintain a similar received power for each UET in the cell, it is desirable to adjust the transmission power of each UET in the cell to compensate for antenna beam pointing errors.
3) Atmospheric Attenuation
Achieving satisfactory communication performance for a signal transmitted from a UET to a satellite generally depends upon receiving a requisite level of signal power at the satellite. That is, each user terminal must transmit a signal with sufficient power to be received. The relationship between the power of the signal transmitted by the terminal and the power of the signal received by the satellite receiver depends in part upon the amount of attenuation of the signal as it passes through the earth's atmosphere. At Ka-band frequencies, the amount of atmospheric attenuation varies considerably as meteorological parameters and weather patterns change. In particular, the occurrence of rain has a pronounced effect on the attenuation of a Ka-band communication signal. The attenuation of the communication signal is known as rain loss or rain fade, although other meteorological phenomena may also provide attenuation. Such atmospheric conditions and/or weather patterns may change rapidly and may vary among different UETs in a cell depending upon the UET's position within the cell. Because it is desirable to maintain a similar received power for each UET in the cell, it is desirable to adjust the transmission power of each UET in the cell to compensate for the attenuation experienced by the UET's signal due to rain loss.
4) Co-Channel Interference
Immediately adjacent cells in a cellular satellite communication system typically use different frequencies for transmitting signals. However, non-adjacent cells may use the same frequency. Such frequency re-use among cells within a cellular pattern serves to reduce the overall frequency bandwidth necessary for the satellite communication system. However, imperfections in satellite antenna beams such as, for example, sidelobe generation, may cause signals transmitted from a UET located in a first cell to be received by a satellite antenna beam which is assigned to receive signals from UETs located in a second cell which uses the same frequency as the first cell. Signal
Caso Gregory S.
Munoz Michael S.
Nivens Dennis A.
Stephens Scott A.
Wright David A.
Chin Vivian
McAndrews Held & Malloy Ltd.
Moore James K
TRW Inc.
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