Technique for mitigating rain fading in a satellite communications system using quadrature phase shift keying

Claims

What is claimed is:

1. A communications system for transmitting digital data from a first location to a second location, comprising:

means at said first location for providing to digital data in first and second input data streams at first and second data rates, respectively, wherein the second data rate is faster than the first data rate, and for synchronizing the first and second input data streams in a fixed timing relationship, with at least said first input data stream providing timing for the first and second input data streams;

means electrically connected to the means for providing digital data, for generating a first phase modulated output signal having a phase relative to a reference phase that is responsive to the data of the first and second input data streams, said phase relative to said reference phase having a first component responsive to the data of said first input data systems and having a second component responsive to the data of said second input data stream, said second component variable independent of said first component, said first component and said second component of said first phase modulated output signal providing two distinct, noninterfering data communications channels at the same time;

means responsive to the first phase modulated output signal for transmitting a radio frequency signal modulated by said first phase modulated output signal;

means at said second location for receiving a radio frequency signal corresponding to said transmitted radio frequency signal and for generating a second phase modulated output signal corresponding to the first phase modulated output signal; and

means for demodulating the second phase modulated output signal to provide first and second output data streams at said first and second data rates, respectively, wherein said first and second output data streams correspond, respectively, to said first and second input data streams, and wherein the means for demodulating provides the first output data stream independently of the second output data stream so that timing information encoded on said first output data stream can be decoded independent of the second output data stream, said first data rate selected to be sufficiently slower than said second data rate so that said means for demodulating continues to provide said first output data stream in the presence of signal attenuation between said first and second locations that prevents said means for demodulating from providing said second output data stream, said first output data stream providing timing for said second output data stream.

2. The communications system as defined in claim 1 wherein said first phase modulated output signal has a first modulation power level responsive to said first input data stream and a second modulation power level responsive to said second input data stream.

3. The communications system as defined in claim 2 wherein one of said first and second modulation power levels is greater than the other of said first and second modulation power levels.

4. A communications system for transmitting digital data from a first location to a second location, comprising:

means at said first location for providing the digital data in first and second input data streams at first and second data rates, respectively, wherein the second data rate is faster than the first data rate and said second data rate is an integer multiple of said first data rate, and for synchronizing the first and second input data streams in a fixed timing relationship, with at least said first input data stream providing timing for the first and second input data streams;

means electrically connected to the means for providing digital data, for generating a first phase modulated output signal having a phase relative to a reference phase that is responsive to the data of the first and second input data streams, said means for generating said first phase modulated output signal including means for spreading the data in said first input data stream to provide a spread data stream at said second data rate, said first phase modulated output signal responsive to said spread data stream so that said first phase modulated output signal has substantially the same modulation bandwidth for each of said first and second input data streams;

means responsive to the first phase modulated output signal for transmitting a radio frequency signal modulated by said first phase modulated output signal;

means at said second location for receiving a radio frequency signal corresponding to said transmitted radio frequency signal and for generating a second phase modulated output signal corresponding to the first phase modulated output signal; and

means for demodulating the second phase modulated output signal to provide first and second output data streams at said first and second data rates, respectively, wherein said first and second output data streams correspond, respectively, to said first and second input data streams, and wherein the means for demodulating provides the first output data stream independently of the second output data stream so that timing information encoded on said first output data stream can be decoded independent of the second output data stream.

5. The communications system as defined in claim 4 wherein said means for demodulating said second phase modulated output signal comprises:

means for separating said second phase modulated output signal into first and second unstabilized output data streams; and

means for converting said first and second unstabilized output data streams into first and second stabilized output data streams corresponding to said first and second output data streams.

6. The communications systems as defined in claim 5 wherein said first stabilized output data stream corresponds to said spread data stream at said second data rate, and wherein said demodulating means includes means for converting said first stabilized output data stream to said first output data stream at said first data rate.

7. A communication system for reliably transmitting digital data from a first location to a second location, comprising:

means for generating first and second source data streams having first and second data rates, said second data rate higher than said first data rate;

means for synchronizing the first and second source data streams in a fixed timing relationship with each other, said means for synchronizing further providing data synchronization information as part of said first source data stream that synchronized said first source data stream with said second source data stream;

means for quadrature phase modulating a radio frequency carrier with said first and second source data streams, said first source data stream shifting the phase of said radio frequency carrier to provide a first phase shift component, said second source data stream shifting the phase of said radio frequency carrier to provide a second phase shift component, said first and second phase shift components selected to be non-interfering to provide two simultaneous distinct communications channels on said radio frequency carrier;

means for transmitting said carrier from the first location to the second location;

means at said second location for receiving a signal corresponding to said transmitted signal; and

means for demodulating said received signal to generate first and second received data streams, said first received data stream corresponding to said first source data stream and including said data synchronization information, said second received data stream corresponding to said second source data stream,

wherein said lower data rate of said first source data stream is selected to be sufficiently lower than said second data rate to enable said first received data stream to be demodulated in the presence of signal attenuation that prevents demodulation of said second received data stream, said first received data stream continuing to provide said synchronization information for said second received data stream regardless of whether said second received data stream is demodulated.

8. A communications system for reliably transmitting digital data from a first location to a second location, comprising:

means for generating first and second source data streams having first and second data rates, said second data rate higher than said first data rate;

means for synchronizing the first and second source data streams in a fixed timing relationship with each other;

means for quadrature phase modulating a radio frequency carrier with said first and second source data streams;

means for transmitting said carrier from the first location to the second location;

means at said second location for receiving a signal corresponding to said transmitted signal; and

means for demodulating said received signal to generate first and second received data streams, said first received data stream corresponding to said first source data stream and said second received data stream corresponding to said second source data stream, said means for demodulating said received signal comprising:

a translation circuit for converting said received signal into first and second digital data signals each having a magnitude that varies in response to said first and second source data streams, said translation circuit comprising:

a controllable oscillator for generating first and second mixing signals at substantially the same frequency as said received signal;

first and second frequency mixers responsive to said first and second mixing signals, respectively, and responsive to said received signal for generating first and second analog signals; and

first and second analog-to-digital convertors for receiving said first and second analog signals and for generating said first and second digital data signals at a sampling rate that is at least twice the second data rate; and

a set of tracking circuits for receiving said first and second digital data signals from said translation circuit and for converting said first and second digital data signals into first and second digital reproductions of said first and second source data streams, respectively, said first digital reproduction comprising data at said first data rate and said second digital reproduction comprising data at said second data rate, said set of tracking circuits further providing a frequency control signal to said translation circuit to control the frequency of said controllable oscillator and a sampling signal to control the sampling rate of said analog-to-digital convertors,

wherein said lower data rate of said first data stream enable s said first data stream to be demodulated in the presence of signal attenuation that prevents demodulation of said second data stream.

9. The communications system as defined in claim 8, wherein said set of tracking circuits comprises:

a coordinate conversion circuit for receiving said first and second digital data signals and for converting said first and second digital data signals into first and second intermediate data streams at said second data rate, said second intermediate data stream corresponding to said second digital reproduction of said second source data stream, said coordinate conversion block further producing an angular output signal having a magnitude proportional to an angular phase difference between said received signal and one of said first and second mixing signals;

a despread circuit for receiving said first and second intermediate data streams at said second data rate and for converting said first and second intermediate data streams to first and second despread data streams at said first data rate, said first despread data stream corresponding to said first digital reproduction of said first source data stream;

a frequency tracking circuit for receiving said angular output signal from said coordinate conversion circuit and for generating an angular correction signal to said coordinate conversion circuit to correct for angular phase differences between said received signal and said one of said first and second mixing signals, said circuit further generating said frequency control signal to control the frequency of said controllable oscillator in said translation circuit; and

a clock oscillator circuit for generating said sampling signal to control the time at which said first and second analog-to-digital convertors sample said first and second analog signals, said time controlled so that said first and second analog signals are samples at times corresponding to maximum and minimum magnitudes of said first and second analog signals.

10. A method of transmitting digital data from a first location to a second location, comprising the steps of:

generating first and second source data streams, said first source data stream having a first data rate and said second source data stream having a second data rate higher than said first data rate

synchronizing said second source data stream with said first source data stream so that said second source data stream has a fixed timing relationship with said first source data stream;

providing timing information in said first source data stream for said first source data stream and said second source data stream;

quadrature phase modulating a radio frequency carrier with said first and second source data streams such that said radio frequency carrier is shifted in phase by a first component responsive to said first source data stream and shifted in phase by a second component responsive to said second source data stream, said second component independent of said first component so that two independent data communications channels are provided on said radio frequency carrier;

transmitting said carrier form a first location to a second location;

receiving said transmitted signal at second location as a received signal;

demodulating said received signal to generate first and second received data streams, said first received data stream corresponding to said first source data stream and said second received data stream corresponding to said second source data stream, said first received data stream including timing information for said first received data stream and said second received data stream;

selecting said lower data rate of said first source data stream to be sufficiently lower than said higher data rate of said second source data stream so that said first received data stream can be demodulated in said demodulating step in the presence of signal attenuation that prevents demodulation of said second received data stream.

11. A system for transmitting digital data from a first ground location to a second ground location via an orbiting relay satellite, comprising:

a source of digital data, said digital data comprising a first input data stream having a first data rate, and a second input data stream having a second data rate higher than said first data rate, said first input data stream and said second input data stream synchronized so that said first input data stream and said second input data stream have a fixed timing relationship, said first input data stream providing timing information for said first input data stream and said second input data stream;

a quadrature phase modulator electrically connected to receive said first and second input data stream, said modulator generating a first phase modulated output signal having a phase relative to a reference phase that is responsive to the data of said first and second input data streams;

a transmitter electrically coupled to said modulated output signal for generating a first radio frequency signal modulated by said modulated output signal, and an antenna electrically connected to said transmitter for radiating said first radio frequency signal toward said satellite wherein a second radio frequency signal responsive to said first radio frequency signal is transmitted toward said second ground location;

an antenna and receiver combination at said second ground location for receiving said second radio frequency signal transmitted by said satellite, said receiver generating a second phase modulated output signal corresponding to said first phase modulated output signal; and

a demodulator electrically connected to receive said second phase modulated output signal from said receiver, said demodulator generating a first output data stream at said first data rate, said first output data stream corresponding to said first input data stream, and a second output data stream at said second data rate, said second output data stream corresponding to said second input data stream, wherein said first output data stream is demodulated independently of said second output data steam so that timing information provided by said first input data stream can be reproduced independent of whether said second output data stream is demodulated, said first data rate selected to be sufficiently lower than said second data rate so that said first output data stream can be demodulated in the presence of signal attenuation that prevents the demodulation of said second output data stream.

12. In a satellite communications system, a method of mitigating the loss of signals caused by atmospheric attenuation, comprising the steps of:

generating first and second source data streams having first and second data rates, said second data rate higher than said first data rate, said first source data stream including data synchronization information, said second source data stream having a fixed timing relationship to said first source data stream so that said second source data stream is synchronized with said data synchronization information of said first source data stream;

quadrature phase modulating a radio frequency carrier with said first and second source data streams to provide two independent communications channels on said radio frequency carrier for said first and second source data streams;

transmitting said carrier from a first location to a second location via an orbiting relay satellite;

receiving said transmitted signal at said second location as a received signal;

demodulating said received signal to generate first and second received data streams, said first received data stream corresponding to said first source data stream and said second received data stream corresponding to said second source data stream, said first received data stream including said data synchronization information for said first and second received data streams; and

selecting said lower data rate of said first source data stream to be sufficiently lower than said higher data rate of said second source data stream such that said first received data stream can be demodulated in said demodulating step in the presence of signal attenuation that prevents demodulation of said second received data stream, said first received data stream providing synchronization for both said first received data stream and said second received data stream regardless of whether said second data stream is demodulated.

13. A method for transmitting digital data through the earth's atmosphere at a frequency susceptible to attenuation by precipitation, comprising the steps of:

providing first and second source digital data streams at first and second digital data rates, said second digital data rate higher than said first digital data rate;

synchronizing said first source digital data stream with said second source digital data stream so that said second source digital data stream has a fixed timing relationship to said first source digital data stream;

providing timing information in said first source digital data stream for said first source digital data stream and said second source digital data stream;

phase modulating a radio frequency carrier with said first source digital data stream, to provide a first phase modulated signal;

phase modulating said radio frequency carrier with said second source digital data stream, to provide a second phase modulated signal in quadrature with said first phase modulated signal;

combining said first phase modulated signal and said second phase modulated signal to provide a combined quadrature phase shift modulated signal;

transmitting said combined signal;

receiving a combined signal responsive to said transmitted signal;

demodulating said received signal to generate a first output signal at said first digital data rate, said first output signal corresponding to said first source digital data stream and including said synchronization information, and to generate a second output signal at said second digital data rate, said second output signal corresponding to said second source data stream and synchronized with said synchronization information;

selecting said first source digital data rate to be sufficiently lower than said second source digital data rate such that said first output signal can be demodulated in said demodulating step to provide said synchronization information in the presence of precipitation attenuation that prevents the demodulation of said second output signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the transmission of data between two locations using a radio frequency carrier, and, more particularly, relates to the transmission of data from an orbiting satellite to the ground and a technique for mitgating the effect of heavy precipitation on transmissions between the orbiting satellite and an earth station when heavy precipitation causes attenuation of the signal transmitted from the satellite.

2. Description of the Related Art

In domestic satellite communication systems, the Ku band, corresponding to frequencies in the range of 12-14 gigahertz, is widely used to transmit digital data signals from an orbiting satellite to a plurality of ground stations. Although the satellite is orbiting at a height such that it is virtually unaffected by weather conditions on the earth, the signals which propagate between the satellite and the ground stations must travel through the earth's atmosphere and are thus subjected to the weather conditions that exist along the line of sight between the satellite and any particular ground station. A major drawback associated with the use of the Ku band for communications between the ground stations and an orbiting satellite is that the signals in the 12-14 GHz range are attenuated by rainfall. For example, the passage of an active cumulonimbus storm cell across the line of sight between a ground station and the satellite can cause sufficient attenuation such that the ground station is no longer able to detect and discriminate the modulated signal being transmitted by the satellite. This attenuation is called "rain fading"; however, it can result from snow, sleet, or other atmospheric conditions. Although the same effect will occur for signals transmitted from the ground station to the satellite, typically a ground station is not power limited, and thus the attenuation can be overcome by increasing the power in the signal transmitted by the ground station. On the other hand, an exemplary satellite typically has a limited amount of power available to operate its transmitter and associated circuitry. Thus, it is generally not feasible to overcome the attenuation caused by precipitation by simply increasing the transmitted power of the satellite. For example, if precipitation is causing a 10 dB attenuation in the signal received by a particular ground station, the power in the signal transmitted by the satellite would have to be increased by 10 dB in order to overcome the attenuation. This corresponds to a ten-fold increase in the power requirements of the satellite (i.e., from 5 watts to 50 watts). To increase the power by this amount may require more than a ten-fold increase in cost. Furthermore, since it is likely that only one or a few of the ground stations are affected by rain-induced attenuation at any given time, the increase in the transmitted power would result in the unaffected ground stations receiving ten times the power which they need to receive. Thus, simply increasing the transmitted power of the satellite is not a desired solution to the problem.

Certain techniques have developed for overcoming the problem of rain fading. For example, U.S. Pat. No. 4,309,764, to Acampora, describes a technique in which the digital data transmitted from the satellite to the ground stations using a Time Division Multiple Access (TDMA) technique. The transmitted signal includes spare TDMA time slots in each frame sequence that are shared among all the ground stations and which can be allocated to a ground station experiencing rain fading. For example, a ground station experiencing rain fading may be provided with the equivalent of three additional time slots so that the data is transmitted in an encoded form that can be more readily detected in the presence of a high level of attenuation. The extra time slots require the frame sequence to be longer and thus this method causes a decrease in the overall data rate of the transmitted data.

In U.S. Pat. No. 4,287,598, to Langseth, et al., a system is described which provides land communication paths between the ground stations. In the event the signal from the satellite to a particular ground station is attenuated to the point of terminating the communication between the ground station and the satellite, the communication is redirected to a second ground station that is located outside the area of the fade condition and is then transmitted to the affected ground station via a land communication path. This, of course, partially eliminates one of the advantages of having a satellite communication system in that a land communication system is required to maintain communications. U.S. Pat. No. 3,896,382, to Magenheim, discloses a similar technique for rerouting the transmission from the satellite through an alternative ground station (FIG. 1) separated from the primary ground station by a relatively large distance. In the alternative, the Magenheim patent shows the transmission from the ground station to a second satellite at a different angle that avoids the rain cell causing the attenuation. Both of these techniques require additional equipment and are thus costly to implement.

U.S. Pat. No. 3,676,778, to Mori, discloses a satellite communication system in which the receivers and transmitters operate at a plurality of frequency bands having differing attenuations in the presence of precipitation. Mori describes a technique that mitigates the rain fading problem by assigning frequency bands having low attenuation to precipitation to those ground stations incurring attenuation at the primary frequency band, while maintaining communication at the primary frequency for the unaffected ground stations This, of course, requires the satellite to have at least two transmitters for transmitting on the two (or more) frequency bands. This requires additional hardware in the satellite and also requires additional power to operate the second transmitter.

A need continues to exist for a technique for communicating between a satellite and a ground station that provides a means for mitigating the effect of attenuation of the signal caused by precipitation in the line of sight between the satellite and the ground station without the need for ground communication channels or alternate satellite to ground signal paths.

SUMMARY OF THE INVENTION

The present invention is an apparatus and method for transmitting communications signals between two locations. Two communications channels are provided on a single carrier frequency. One of the channels is particularly suitable for providing continuing communication in the presence of signal conditions that substantially prevent communication on the other channel, such as attenuation caused by precipitation in the signal path between the two locations. For example, the present invention is advantageously useable in an exemplary communications system having a satellite for receiving signals transmitted by one ground station and for retransmitting the received signals to one or more ground stations. The invention does not require an additional transmitter in the satellite, nor does it require the transmission power of the satellite to be increased to overcome the attenuation. Furthermore, the transmission continues to occur between the satellite and the affected ground station, rather than rerouting the transmission to an alternative ground station and then using land communication to complete the transmission link.

The present invention includes a satellite transmitter operating at a single frequency. The data transmitted from the satellite to a plurality of ground stations is imposed upon the transmission carrier frequency by quadrature phase shift keying. First and second data transmission channels are provided at the same frequency by shifting the phase of the carrier signal in accordance with the instantaneous state of the two data transmission channels. Thus, the phase of the signal received by the ground station has phase information which can be demodulated to reproduce the two separate digital data channels. In the preferred embodiments of the invention, both digital data channels are operated at the same channel symbol ("chip") rate. However, one of the digital data channels is selected to operate at a lower bit or data rate by encoding the data transmitted on that channel, using multiple chips per bit, so that the data can be more readily detected in the presence of signal fading.

In one application of the present invention, the higher data rate channel is the primary communications path from the satellite to a plurality of ground stations. The lower data rate channel can be advantageously used to transmit essential network control functions such as frame synchronization, cryptographic keys and initialization vectors (when implemented), and demand assignment directives. The frame synchronization that is transmitted on the lower data rate channel is used by the ground stations to maintain phase synchronization of both data channels with the signal being transmitted by the orbiting satellite. Thus, the use of the lower data rate channel for transmitting the frame synchronization information substantially increases the probability that each of the ground stations will maintain synchronization with the transmitted signal during periods of heavy rain fading which may preclude communications on the higher data rate channel.

In one particular embodiment of the present invention, the satellite communications system is used to transmit broadcast information to a plurality of ground stations, wherein each of the ground stations is intended to receive all of the information that is transmitted. For example, the satellite communications systems may be used by a news service to transmit news stories, stock market quotes, emergency information, and other information from a central location to a plurality of receiving stations which retransmit the information in a given area. In such an embodiment, the higher data rate channel is used to transmit routine information, such as news stories, for which the time of receipt is not critical. On the other hand, data having a degree of urgency, is transmitted on the lower data rate channel so that the probability that the data will be received and acted upon is substantially increased. For example, the lower data rate channel can be advantageously used to transmit emergency broadcasts, such as reports of hazardous weather conditions, current stock market quotations, or the like.

In alternative embodiments of the present invention, the satellite communications system is used for the transmission of addressed traffic. In other words, a portion of each data transmission frame is directed towards a particular ground station. The data communications to each ground station is preferentially transmitted on the higher data rate channel. The use of the higher data rate channel enables the system to transmit more information in a given unit of time than if the lower data rate channel were used exclusively. When a ground station is affected by precipitation that occurs in the line of sight between the orbiting satellite and the ground station, the data transmissions to that particular ground station are rerouted from the higher data rate channel to the lower data rate channel to increase the margin of the signal level over the noise level. The ground station then has a substantially higher probability of correctly receiving the data transmission directed to it. When the level of precipitation in the line of sight decreases to a level such that the attenuation caused by the precipitation no longer deleteriously affects the signal on the higher data rate channel, the communications to the affected ground station are redirected to the higher data rate channel so that the resources of the lower data rate channel are available for other ground stations which may become affected by precipitation. This is particularly advantageous when the precipitation is part of a storm cell which is moving across the country and is sequentially affecting a plurality of ground stations.

The present invention is particularly advantageous for mitigating signal attenuation between a satellite and a ground station (i.e., attenuation of the downlink). The present invention can also be advantageously used to mitigate attenuation on the unlink from the ground station to the satellite, particularly when the transmitting ground station is a mobile unit or other system having low transmission power output or another impairment.

In preferred embodiments of the present invention, the signal level in the higher data rate channel and the lower data rate channel are substantially equal. In alternative embodiments of the present invention, the signal level in one of the data channels can be increased to further increase the signal-to-noise ratio of the data transmitted on that channel. The signal levels can advantageously be adjusted so that the signal-to-noise ratio of the higher data rate channel in the absence of any precipitation is approximately the same as the signal-to-noise ratio of the lower data rate channel in the presence of heavy precipitation. Thus, the system resources (i.e., the available transmission power in the satellite) can be allocated so that neither channel has more or less than the power necessary to maintain communication.

BRIEF DESC