1. Field of the Invention
The present invention relates generally to satellite communications and, more particularly, to uplink power control and time synchronization in a two-way satellite communications system incorporating multiple simultaneous uplink ground terminals.
2. Description of Related Art
Communication satellites have the capability of receiving uplink signals from earth-based terminals, amplifying and translating the uplink signals into downlink signals, and retransmitting the downlink signals to receiving terminals. Such communications systems permit two types of communications. In a typical broadcast system, such as for distributing television programming, an uplink signal from a single broadcast transmitter is retransmitted by the satellite for simultaneous reception by multiple receiving terminals. In two-way systems, such as for normal telecommunications, uplink signals from many uplinking terminals are transmitted with no central coordination to a single satellite. Each uplink signal is addressed to and retransmitted for reception by one or more receiving terminals. Both types of systems permit concurrent receipt and processing of multiple uplink signals.
The satellites in both broadcast and two-way systems are capable of receiving uplink signals from multiple terminals using time-division multiple access (TDMA), frequency-division multiple access (FDMA) or a combination of the two access methods. In TDMA, each terminal uses the entire uplink bandwidth for a portion of the time. A synchronization arrangement which controls the time of transmission of each terminal is required. In an ideal situation, each terminal transmits uplink signals that arrive at the allotted time without overlap or gaps. Unfortunately, in the real world terminals sometimes transmit TDMA uplink signals that overstep their allotted time slot, thereby jamming the signals of other terminals. In these cases, the relative transmission timing for the offending terminals must be controlled to ensure the uplink signals arrive in the proper time slot.
In FDMA, the uplink bandwidth is subdivided and portions are assigned to different terminals. Power levels of the uplink signals must be controlled to reduce interference between the subdivisions. An FDMA uplink signal will leak, due to hardware imperfections, into the adjoining frequency bandwidth subdivision. If the uplink signal has too much power, the FDMA leakage will jam the adjoining subdivision. In this case, the relative signal powers of the uplink signals in adjoining subdivisions must be adjusted to minimize interference due to signal leakage.
In a broadcast satellite communications system, transmission stations transmit to a satellite uplink signals which are broadcast and simultaneously received by multiple receiver stations. The transmission station transmits an uplink signal at a predetermined uplink frequency to an orbiting satellite. The satellite receives the uplink signal and upconverts or downconverts the uplink signal to a downlink signal at a predetermined downlink frequency. The downlink signal is then retransmitted in a broadcast beam for simultaneous reception by multiple receiver stations. Typically, the transmission station is within the area encompassed by the broadcast beam and, therefore, is capable of receiving the downlink signal.
In broadcast systems as described, uplink power control and time synchronization can be performed at the transmission station. Power control is typically performed by the transmission station measuring the power of a beacon or a communications carrier signal in the broadcast beam from the satellite. The beacon or signal is transmitted with a predetermined power in either the uplink frequency band or the downlink frequency band. The transmission station measures the power of the received beacon or signal, and determines whether the uplink signal power should be adjusted to allow for variations in signal fade and interference at the uplink signal frequency. In these systems, the uplink signal powers can be increased or decreased as is necessary to overcome signal fade because the uplink signals from one transmission station do not interfere with the uplink signals of the other transmission stations of the system.
Time synchronization in broadcast satellite communications systems and systems having central coordination of uplink signal transmissions is usually performed by the transmission station measuring its own transmission as it appears in the downlink signal. The timing relationship between the uplink signal and the downlink signal is constant as the uplink signal is received, converted to the downlink signal, amplified and retransmitted by the satellite. Therefore, the time of receipt of the downlink signal at the transmission station can be used to adjust the transmission time of the uplink signal to ensure that the uplink signal arrives at the satellite at the allotted time.
Previous alternatives for uplink power control and timing synchronization are not applicable in the uncoordinated two-way systems as described herein. Specifically, the two-way system described herein interconnects multiple, geographically disparate spot beam coverage areas with a regenerative satellite payload and no centralized control station. The payload demodulates uplink signals into their constituent packetized bit streams and routes the packets to the downlink spotbeam(s) specified within the packet header. Thus, the originating terminal may or may not receive the downlink manifestation of the uplink signal. Moreover, both the timing and the signal-to-noise ratio of the uplink signal are removed by the demodulation and routing operations. Demodulation reduces the signal to the binary information stream, and routing introduces random queuing delays. Thus, neither the timing nor the received signal strength is discernible from the downlinked data.
Another problem exists in systems where low power uplink terminals are used to reduce size, cost and power consumption. In these systems, the uplink terminals cannot indiscriminately increase their uplink signal power to compensate for other uplink signals that leak into their subdivisions. Even in systems with uplink terminals capable of transmitting signals with higher uplink signal powers, the terminals cannot indescriminantly increase their uplink signal power lest system runaway occur. Consequently, a power control strategy is necessary in systems without central coordinated uplinks and using either low power or high power uplink terminals wherein the terminals are self-policing and each control their own uplink signal power.
For these reasons, a need exists for a method for satellite-based uplink power control and time synchronization in satellite communications systems.
The present invention is directed to a method for uplink power control and time synchronization that is achieved using satellite based measurements of uplink signal power and signal arrival time.
According to one aspect of the present invention, the uplink signal power at an originating terminal is adjusted based on uplink signal power measured at a satellite in a satellite communications system. The originating terminal transmits an uplink signal with a first uplink signal power. The satellite receives the uplink signal, measures the received uplink signal power, and transmits information of the received uplink signal power back to the originating terminal. The originating terminal receives the information of the received uplink signal power and transmits subsequent uplink signals at a second transmitted uplink signal power determined based on the information transmitted by the satellite.
Adjustment of the uplink signal power may be performed continuously as the uplink signals are received at the satellite. Alternatively, uplink signal power adjustments may be triggered by the occurrence of an event, such as the transmission of a measurement request from the originating terminal to the satellite. Additionally, the processing capability for determining an adjusted uplink signal power can reside either at the satellite or at each of the originating uplink terminals.
According to another aspect of the present invention, the transmission timing for uplink signals from an originating terminal is adjusted based on the uplink signal arrival time at a satellite in a satellite communications system. The originating terminal transmits an uplink signal at a predetermined time according to a first uplink signal transmission timing pattern. The satellite receives the uplink signal, determines the uplink signal arrival time, and transmits information of the uplink signal arrival time back to the originating terminal. The originating terminal receives the information of the uplink signal arrival time and transmits subsequent uplink signals according to a second uplink signal transmission timing pattern determined based on the information transmitted by the satellite.
Adjustment of the uplink signal transmission timing pattern may be performed continuously as the uplink signals are received at the satellite. Alternatively, transmission timing pattern adjustments may be triggered by the occurrence of an event, such as the transmission of a request from the originating terminal to the satellite. Additionally, the processing capability for determining an adjusted uplink signal transmission timing pattern can reside either at the satellite or at each of the originating uplink terminals.
According to a still further aspect of the present invention, adjustment of both the uplink signal power and the uplink signal transmission timing pattern at an originating terminal occur at the same time based on the received uplink signal power and arrival time of a given uplink signal. Additionally, the present invention as described herein is directed to use in TDMA-packet switching satellite systems, but its application is not limited to use solely in the satellite systems discussed herein which are intended to be illustrative only and not to be limiting on the invention.
The present invention may best be understood with reference to the following description when considered with the accompanying drawings.