Abstract:
A satellite communications system ( 10 ) that employs an array of separate and easily deployable antennas ( 12 ) for transmission and reception purposes to accommodate high data rate transmissions. The antennas ( 12 ) can be deployed randomly at a communications site and are physically separated. Each antenna ( 12 ) transmits and receives the same information. A coded signal is used to identify the transmission from each antenna ( 12 ) for calibration purposes to align the bits transmitted by each antenna ( 12 ) and provide carrier frequency phase matching. The coded signals are used to compare the phase and timing relationship between each antenna signal and a reference antenna signal when the separate antennas receive all of the coded signals. Correction computations are performed and specialized phase and data alignment systems ( 24, 32 ) are employed to delay and adjust the phases of the various transmitted signals relative to the reference antenna ( 12 ) to provide the desired alignment. Additionally, phase and timing systems ( 194 ) are used to determine and correct the phase and data variations between the data received by the antennas ( 12 ) so that they can be combined and processed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a communications array transceiver and, more particularly, to a transceiver for a satellite communications system that employs an array of small, readily transportable antennas that transmit signals that are in phase and aligned in time with each other. 
     2. Discussion of the Related Art 
     The military requires robust, reliable and increasingly wideband communications systems to provide for the rapid collection and dissemination of intelligence data and tactical command and control information. There is a great tactical value in providing timely data to, and reports from, mobile units in the field that may be in a hostile environment. Satisfaction of this need requires that communications links be established quickly between the field unit and a remote, sometimes transcontinental, site. It has been recognized that communications by satellite provides the required access in this type of environment. 
     Modern strategic and tactical communications of this type typically require wide bandwidth communications, for example 40 megabits per second. A certain amount of energy is required for each bit that is to be transmitted. The more bits transmitted per second, the more energy is required per unit time, and thus the more power for the transmission is required. Similarly, a certain amount of energy per bit is required to receive a communication, and wider bandwidth communications require more signal power to be received. The system&#39;s transmission power requirements can be reduced and its receiving power collection capacity can be increased by increasing the antenna gain, which is achieved by increasing the size of the antenna. Therefore, large reception and transmission apertures are usually necessary to supply the gain to handle wide bandwidth signals. For example, to transmit 40 megabits per second in the Ku frequency band, it is desirable to have an antenna that is about 10 meters in diameter. 
     State of the art satellite communications systems are almost exclusively constructed of a single antenna that has a large aperture and a corresponding large high power amplifier to achieve high sensitivity and high equivalent isotropic radiated power (EIRP) for wide bandwidth communications. Typically, the combination of the large size of the aperture and the amplifier provide a communications system that is unwieldy for rapid deployment in unfriendly terrains. It is possible to transmit the higher data rate signals at lower power by combining identical transmissions from a plurality of smaller, more readily deployable antennas. However, in order to provide such a system, the transmitted bits from each separate antenna must be aligned in time with each other, and the radio frequency carrier transmitted by each antenna must be in phase with each other. 
     It is known to use phased array antennas to improve sensitivity and EIRP by phasing transmitted and/or received signals. The phased array antennas are typically constructed of a fixed, permanent, rigid physical configuration with closely spaced antenna elements that do not require or implement delay compensations. A variation of this type of antenna is a phased array design that implements “true time delay” for each element as a means of adjusting the phase of each element. Known designs of this type, however, require and implement delays that have a known relationship from element to element and do not require and do not implement delays that are arbitrary as a result of an arbitrary physical disposition of the elements. 
     One known commercial satellite communications system that employs more than one antenna is the TACSTAR MK-II, available from Datron/Transco Inc. This system performs phase combining with two independent antenna elements. In this design, the antenna operates only in the receive mode with two closely spaced antenna elements for narrowband signals that do not require delay compensation. 
     What is needed is a satellite communications system that provides transmission and reception of wideband signals, and includes antennas and corresponding equipment that is easily deployable, rugged, reliable and secure. It is therefore an object of the present invention to provide such a communications system. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a satellite communications system is disclosed that employs an array of separate and easily deployable antennas for transmission and reception purposes to accommodate high data rate transmissions. The antennas can be deployed randomly at a communications site, and are physically separated. Each antenna transmits and receives the same data. A coded signal is used to identify the transmission from each antenna for calibration purposes to align the bits transmitted by each antenna in time and provide phase matching for the carrier wave of each antenna signal. The coded signals are used to compare the phase and timing relationship between each antenna signal and a reference antenna signal when the reference antenna receives all the coded signals for all of the antennas. Correction computations are performed and specialized phase and data alignment systems are employed to delay the various transmitted signals relative to the reference antenna to provide the desired alignment. Additionally, phase and timing systems are used to determine and correct the phase and data timing variations between the data received by the antennas so that they can be combined and processed. 
    
    
     Further objects, features and advantages of the present invention will become apparent from a consideration of the following description and the appended claims when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram of a transceiver array for a satellite communications system, according to an embodiment of the present invention; 
     FIG. 2 is a functional block diagram of a communications system incorporating a transceiver array of the invention used for laboratory verification; 
     FIG. 3 is a functional block diagram showing a transmission control and error estimation system for a channel of the communications system shown in FIG. 1; 
     FIG. 4 is a functional block diagram showing a receiver combining method of the communications system shown in FIG. 1; and 
     FIG. 5 is a block diagram of a system architecture for the communications system shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to a satellite communications system including an array of antennas is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
     FIG. 1 is a schematic block diagram of an antenna array transceiver  10 , according to an embodiment of the present invention. The transceiver  10  includes an array of antennas  12  that transmit to and receive signals from a satellite  14 . The satellite  14  then rebroadcasts the signal to another satellite and/or to an Earth based receiver that the transceiver  10  is in communication with. Each antenna  12  includes a transmitter  18  and a receiver  20 . Each combination of antenna  12 , transmitter  18  and receiver  20  is a separate channel of the transceiver  10 . The antennas  12  are positioned on the Earth at random locations at a communications site. Each antenna  12  transmits and receives the same data so that the combination of all the transmissions and receptions provides enough power for the necessary or required bandwidth for a particular application. The number of antennas  12  for a particular application would be determined by the bandwidth required in combination with the actual size of each antenna  12 . 
     Because the location of the antennas  12  on the Earth relative to the satellite  14  is arbitrary, a phase and bit alignment correction needs to be made to insure that the carrier signal associated with the transmitted signals from the antennas  12  arrive at the satellite  14  in phase with each other, and the bits being transmitted by each channel arrive at the satellite  14  at the same time. According to the invention, the phase relationship and the bit alignment relationship between the various signals transmitted by the antennas  12  are aligned by employing a unique calibration signal for each antenna  12  that is transmitted in combination with the desired data. Each calibration signal includes its own code so that the separate signals from each of the antennas  12  can be distinguished from each other. The calibration signal can be a binary pseudo-random sequence waveform that is transmitted at very low power and a low temporal duty cycle. In one embodiment, the calibration signals transmitted by the separate antennas  12  are coded by a spread spectrum code. The combined calibration signal and data signal are sent to the several transmitters  18  for each channel on line  22 . The calibration signal is modulated onto the same radio frequency carrier as the data signal so that the phase of the calibration signal and the phase of the data signal are locked together. 
     The combination of the data signal and the calibration signal are transmitted by the antennas  12  and received by the satellite  14 . The satellite  14  rebroadcasts the combined signal, at a different carrier frequency, to be received by each of the antennas  12 . Because the calibration signal is transmitted at a much lower power than the data signal, it does not interfere with the data signal. 
     Each of the receivers  20  receives all of the coded calibration signals transmitted by all of the antennas  12 . Each of the calibration signals from each of the  10  receivers  20  is sent to a calibration phase/delay error measurement system  24  on lines  26  within a processor  28 . One of the channels is designated a reference channel, and is the channel with the longest round trip time to and from the satellite  14 . The measurement system  24  uses the calibration signals received by the reference channel to separate and identify the signals by their codes. In other words, the calibration signals from the receiver  20  of the reference antenna are used by the measurement system  24  to determine the phase relationship between the carrier frequency of the reference channel and the carrier frequency of all of the other channels. Additionally, the measurement system  24  measures the time delay between the calibration signal for the reference channel and the calibration signal from the other channels. 
     The measurement of the phase and delay between the signal from the reference channel and the signal from each of the other channels identified by the measurement system  24  is then applied to a phase/delay correction computation system  32  that determines how much the transmissions from the various antennas  12  must be delayed in time and changed in phase relative to the transmission from the reference antenna so that the carrier waves from each antenna  12  arrive at the satellite  14  in phase, and all of the data is aligned in time. This information from the computation system  32  is applied to the transmitters  18  on line  34 . Because the data signal is phase locked to the calibration signal, the corrected calibration signal causes the data signal from each antenna  12  to also be in phase and aligned in time. 
     Phase and data alignment must also be provided for the signals received by the antennas  12  from the remote communications site. To provide this alignment, each of the received signals from the receivers  20  are also sent to a receiver combining system  38 . The combining system  38  processes the various signals so that the carriers are aligned in phase, and data aligned in time, and sums the aligned signals together. Various receiver combining schemes are known in the art that provide this type of function. In one particular scheme, the various signals received by the antennas  12  are cross-correlated relative to each other. The cross correlation between the received signals gives the phase difference between the signals and their relative delay. 
     FIG. 2 is a schematic block diagram of a communications system  50  showing a laboratory depiction of the phase alignment technique to align the transmitted signals of the invention described above. The system  50  includes a transmitter  52  and a receiver  54 . The transmitter  52  includes three separate channels  56 , where each channel transmits a separate coded calibration signal. Because each channel  56  is the same, only one channel will be described with the understanding that the other two channels operate in the same manner. The channel that is described is the reference channel. 
     Each channel  56  includes an antenna  60  for transmitting the combined calibration and data signal. Each channel  56  also includes a carrier synthesizer  62  that generates a carrier signal, 70 MHz in this example. The carrier signal is sent to a divider  64  that divides the signal into first and second paths. The first path is connected to a linear recursive sequence random number generator  66 . The generator  66  provides a predetermined sequence of zero and one bits that defines the calibration code for that channel. The calibration code modulates the carrier frequency from the synthesizer  62 . The generator  66  also receives a signal from a chip reference synthesizer  68 . The chip reference synthesizer  68  is a clock input to the generator  66  that determines the rate at which the zero and one bits are generated in the generator  66 . The coded modulated carrier wave from the generator  66  is applied to a summer  70  through an amplifier  74 . 
     The second split carrier signal from the divider  64  is applied to the summer  70  through an attenuator  72  as an unmodulated signal. The unmodulated signal represents the data signal even though it is not modulated with actual data in this laboratory example. It is not necessary to transmit data in this example because it is the calibration signal that is the focus. The attenuator  72  and the amplifier  74  combine to set the relative power between the data signal and the coded signal so that they have different powers and do not interfere with each other. The summer  70  combines the data signal and the calibration signal so that they are locked in phase. The summed signal from the summer  70  is applied to a multiplier  76  along with a high frequency signal from a local oscillator  78 . The local oscillator signal upconverts the signal to be transmitted by the antenna  60  and generates, for example, a 12 GHz+/−70 MHz signal. Each channel  56  generates a separately coded signal that is transmitted at the same carrier frequency, where the data signal is phase locked to the calibration signal. 
     The transmitted signals from the antennas  60  for each channel  56  are received by a receiver antenna  82  in the receiver  54 . The antenna  82  represents any one of the antennas  12  and is preferably the reference channel. The signals received by the antenna  82  are multiplied with a local oscillator signal from a local oscillator  84  in a multiplier  86  to provide a difference signal that will be used as an intermediate frequency for downconversion purposes. In this example, the frequency of the local oscillator  84  is 11,860 GHz to provide the intermediate frequency of about 70 MHz, as used in the transmitter  52 . A low pass filter/bandpass filter  88  filters out the sum signal and the harmonics from the multiplier  86 , and passes the intermediate frequency signal through to be amplified by an amplifier  90 . The amplified intermediate signal is sent to a power meter  92  to provide a measurement of the received power. 
     The amplified intermediate frequency signal is also sent to three separate channels  94  in the receiver  54  to separate the codes for each of the channels  56 . Each channel  94  operates in the same manner, and therefore only one channel will be described with the understanding that the other two channels operate in the same manner. 
     The signal from the bandpass filter  88  includes all three of the coded calibration signals from the channels  56 . This signal is applied to a multiplier  96  in each channel  94 . Each code that was generated in the transmitter  52  is also reconstructed in the receiver  54 . To accomplish this, a code generator  100  is used to generate the codes, and is similar to the generator  66 . The generator  100  receives a despread intermediate frequency signal, for example 70 MHz, from a despreader synthesizer  102 , that is modulated by the particular zero and one bit code in the code generator  100 . A divider  98  is used to divide the signal from the synthesizer  102  so that each channel  94  receives the same carrier frequency. A chip despreader synthesizer  104  provides the clock input to the code generator  100  to provide the rate at which the ones and zeros are generated. The coded signal is thus generated in the same manner as in the transmitter  52 . The coded signal at the intermediate frequency from the code generator  100  is then applied to the multiplier  96  to be multiplied with the intermediate frequency signal received by the antenna  82 . By multiplying the received calibration signal with the locally generated coded signal, the like codes cancel out. Because the signal from the antenna  82  includes all three codes, only the particular code generated by the code generator  100  is cancelled. The remaining two codes are still present from the output of the multiplier  96 . This signal is filtered by a lowpass filter (LPF)  106  that only passes the low frequency carrier of the signal. Thus, only the carrier for the first calibration signal is passed by the LPF  106 . 
     Therefore, for each channel  94 , a separate one of the codes is output to an oscilloscope  108 . The oscilloscope  108  displays the carriers of the various codes, and provides the phase difference between them. The phase difference between the first coded signal and the second coded signal is supplied to a computer  112 , which provides a command to the carrier synthesizer  62  in the second channel in the transmitter  52 , and the phase difference between the first coded signal and the third coded signal is applied to the carrier synthesizer  62  in the third channel of the transmitter  52  to provide the phase relationship correction. A spectrum analyzer  110  is also provided to display the power of the received and combined data signal. 
     FIG. 3 is a functional block diagram  120  showing how the signals to be transmitted are aligned in phase and are timed relative to each other in the manner described above. The block diagram  120  includes a transmission control system  122  for an n channel that represents any channel that is not the reference channel. The calibration signal, generated as discussed above, in this channel is applied to a delay device  124  for bit alignment purposes, as will be discussed below. Because the calibration signal is digital, it is converted to an analog signal by a digital-to-analog (D/A) converter  126  for transmission. Likewise, the digital data signal to be transmitted is sent through a delay device  128 , and then to a digital-to-analog converter  130  to be converted to an analog signal for transmission. Amplifiers  132  and  134  amplify the calibration signal and the data signal, respectively. The amplified calibration and data signals are phase locked together in a summer  136  for transmission. The combined calibration signal and data signal is applied to a base-band (BB) to IF conversion system  138  that modulates the base-band data and the calibration signal onto an IF carrier wave. The intermediate frequency carrier signal is then upconverted to a high frequency (12 GHz) by an upconverter  140  suitable for transmission. 
     The RF transmission from the transmission control system  122  is sent to the satellite  14 . All of the antennas  12  receive all of the calibration signals from all of the channels. In the reference channel, the antenna  12  sends the received signals to an amplifier  144  in an error measurement system  146  in the receiver  20 . A downconverter  148  converts the high frequency carrier signal to a suitable IF for processing. A despreader  150  is provided to decode the reference channel signal and a despreader  152  is provided to decode the n channel signal. The despreaders  150  and  152  each provide a frame sync output that is indicative of the timing of the data and calibration code of the received signal for the reference channel and the n channel. The frame sync signals are received by a time difference system  154  that acts to identify the relative alignment between the frame sync signals. The output of the time difference system  154  is a signal indicative of the alignment between the data and calibration code in the n channel and the data and calibration code in the reference channel. The alignment between the signal for each channel and the reference channel is performed in this manner. 
     The despreaders  150  and  152  decode the signals by removing the digital code for that channel and leaving the IF carrier for a particular signal. In other words, the despreader  150  receives all of the coded signals for all the channels, but only outputs the carrier signal for the particular code associated with the reference channel because the code in the despreader  150  only selects the code for that channel. The despreader  152  does the same for the n channel. The despreaders  150  and  152  separate the carrier signals for the particular code into in-phase and quadrature-phase signals. The in-phase signals from the despreaders  150  and  152  are sent to a multiplier  156 , and the quadrature-phase signals from the despreaders  150  and  152  are sent to a multiplier  158 . The multiplied in-phase and quadrature-phase signals from the reference channel and the n channel are then applied to a summer  160  that subtracts the signals to generate a difference signal that gives the sine of the phase difference between the carrier signals. The difference signal is sent to an accumulator  162  that accumulates the sine difference to provide a phase error output of the difference in phase of the carrier signals for the reference channel and the n channel. 
     The in-phase and quadrature-phase signals from the despreaders  150  and  152  are also applied to multipliers  164  and  166 . The multiplied signals from the multipliers  164  and  166  are then applied to a summer  168  that adds the signals to provide the cosine of the phase difference between the signals. An accumulator  170  accumulates the added cosines and provides a lock indicator output indicative of when the phase error between the reference channel and the n channel is reduced to zero, indicating the signals are in-phase. 
     Both the delay error signal from the difference system  154  and the phase error signal from the accumulator  162  are applied to a correction computation system  172  that determines the amount of delay needed to align the n channel with the reference channel, and the phase adjustment needed to cause the n channel carrier signal to be in phase with the reference channel carrier signal. A delay correction signal from the correction computation system  172  is then sent to the delay devices  124  and  128  to delay the calibration and data signals of the n channel and align them with the calibration signal and data signals in the reference channel. A phase correction signal is sent to the conversion system  138  to provide a phase correction to the n channel carrier signal. Therefore, the RF signal transmitted by the antenna  12  in the n channel is aligned in time and in phase, as it is seen by the satellite  14 , with the RF signal transmitted by the reference channel. This delay and phase adjustment process is done for all the channels relative to the reference channel so that all of the channels are aligned in time and in phase with the reference channel, and thus with each other. 
     FIG. 4 is a functional block diagram  180  showing how signals received from a remote communications site are aligned in phase and in time, and combined, for all the channels. Each one of the channels is represented in FIG. 4, including the reference channel  1 . The receiver functions of the reference channel  1  will be discussed below, with the understanding that the other channels receive and process the signals in the same manner. Each antenna  12  receives the same signals from the satellite  14 . The signals received by the antenna  12  in the reference channel are downconverted by a downconverter  184  to an intermediate frequency, and then from an intermediate frequency to base-band by a converter  186 . The base-band signal is then converted to a digital signal by an analog-to-digital converter  188 . The digital signal is then sent to a first-in first-out (FIFO) delay register  190 . The downconverted, digital signal from the FIFO register  190  is then sent to a digital receiver  192  that provides digital filtering around an optimum band and further downconversion by an applied frequency f. The digital representation of the signal allows for frequency phase control. 
     This downconversion and digitizing process as just described is provided for all of the n channels. The digitized signal for each channel is then sent to a delay/phase error system  194 . The error system  194  separately computes the delay difference and the phase difference between the digital reference channel signal and the digital signal for each of the other channels. This delay and phase error determination can be done in any number of different ways known to those skilled in the art. One example is a cross-correlation technique. The delay t 1n  and the phase error kφ 1n  computed by the system  194  for each channel are applied to the delay register and the digital receiver, respectively, in each of the channels to align them with the reference channel. The frequency f plus the phase error kφ 1n  between the n channel and the reference channel is applied to a digital receiver  196  in the n channel so that the phase of the low frequency narrow band signal in the digital receiver  196  is matched to the frequency in the digital receiver  192 . Likewise, the time difference signal t 1n  is applied to a FIFO register  198  in the n channel to provide a delay to the received signal to align the n channel with the reference channel  1 . Therefore, the low frequency signal from the digital receiver  196  is aligned in time and phase with the signal from the digital receiver  192 . This process is performed for the other channels relative to the reference channel  1 . 
     All of the aligned signals from all of the channels  1 -n are sent to a combiner  200  that adds the signals to a single signal representative of the received signal. The combined signal is then sent to a digital demodulator where the digital low frequency carrier wave is removed and the digital data is identified. 
     The round trip time T RT  of the transmission of the calibration signal from the antennas  12  to the satellite  14  and then from the satellite  14  to the antennas  12  is typically on the order of one-quarter of a second. For land based deployment of the array transceiver, the phase and time differences between channels change sufficiently slowly that this round trip time does not affect the measurement and correction process as just described. However, if the communication site is on a ship or the like, where the relative orientation between the antennas  12  and the satellite  14  may change significantly during the transmission round trip time of the calibration signal, relative phase changes due to the movement of the antennas  12  relative to the satellite  14  and each other need to be compensated for during this time. Therefore, an output signal from the system  194  is provided that is representative of the continually measured phase difference between the reference channel and each n channel, and is sent to a phase accumulator  204 . Additionally, the round trip time T RT  is applied to the phase accumulator  204 . The phase accumulator  204  continually adds up the phase differences for each of the channels for the round trip time, and outputs the phase change as Δφ 1n  in to the correction computation system  172 . The correction computation system  172  computes the phase correction at the transmission frequency that compensates for the short term phase change Δφ 1n  that was measured at the receiving frequency. The short term phase change due to transceiver motion is thereby accounted for. 
     FIG. 5 shows an example of a system architecture  210  for a particular implementation of the system described above. The architecture  210  includes an antenna platform  212  that includes an antenna feed  214  connected to the antenna  12 . The received signals from the antenna  12  go through a transmission reject system  216 , a low noise amplifier (LNA)  218 , and are downconverted by a down-converter  220  to generate the intermediate frequency received signal. The signals to be transmitted are sent to an up-converter  222  to upconvert the signal to a higher frequency, and then to a high power amplifier (HPA)  224 , through a receiver reject system  226  and then to the antenna feed  214 . A frequency reference input signal is applied to the downconverter  220  and the upconverter  222  from a system clock  230  to lock the signals to a particular frequency. 
     The system clock  230 , in a control platform  232 , provides timing for the various operations. A modem  234  is provided for each channel, where the modem  234  includes everything in the error measurement system  146  after the downconverter  148 , and also includes the converter  186 , the analog-to-digital converter  188 , the FIFO register  190 , and the digital receiver  192 . A digital summer  236  represents the combiner  202 . A track processing system  238  includes the phase accumulator  204 , the delay-phase error system  194  and the correction computation system  172 . A digital demodulator  240  demodulates the digital data received from the summer  236 . 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications or variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.