Abstract:
The number of power amplifiers required to amplify a plurality of transmission signals is reduced by using non-linear transmission lines (NTL) circuits. In general, a “combining” NTL circuit is used to combine the plurality of transmission signals to form a soliton pulse. The soliton pulse is then amplified such that each of its component transmission signals are amplified. A “dividing” NTL circuit is then used to divide the amplified soliton pulse into its component amplified transmission signals. The amplified transmission signals can therefore be transmitted over a communications channel without requiring a separate power amplifier for each.

Description:
GOVERNMENT INTEREST 
   The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon. 
   FIELD OF THE INVENTION 
   The present invention relates to the field of communications systems and, more particularly, to systems operating under multiple transmission conditions. 
   BACKGROUND OF THE INVENTION 
   Anytime a communications system is designed to transmit signals at more than one frequency (i.e. a multiple-transmission system), the designer and/or operator of the system is typically concerned with signal distortion. In some multiple-transmissions systems, a significant amount of signal distortion can be found when only one power amplifier is used to amplify all the transmitted signals. This is due to the fact that power amplifiers typically have the ability to provide distortion-free amplification for only a limited range of frequencies. Thus, if the multiple-transmission system requires the transmission of a signal having a frequency outside the distortion-free range of the power amplifier, the signal may be significantly distorted during transmission. 
   A solution to this problem is to design the multiple-transmission system to provide a separate power amplifier for each transmission signal. Since each amplifier may provide distortion-free amplification at the frequency of its respective signal, this solution insures that signal distortion is minimized during transmission. 
   Power amplifiers, however, are typically large and expensive devices that require large amounts of direct current (DC) power, and thus relatively large and expensive DC batteries, to operate. As a result, even though using separate amplifiers has been found to reduce signal distortion during transmissions, it can substantially increase the overall size and cost of the multiple-transmission systems in which it is implemented. Thus, from the above, it can be appreciated that designers of such multiple transmission systems are faced with the decision of designing a low-distortion system having a high cost—due to a large number of power amplifiers, or a high-distortion system having a low cost—due to a small number of power amplifiers. 
   SUMMARY OF THE INVENTION 
   We have realized that a low-distortion multiple-transmission system can be achieved, without the high cost of providing separate power amplifiers, by combining the transmission signals to form a so-called soliton pulse, amplifying the soliton pulse with a single amplifier, and dividing the soliton pulse back into its transmission signal components. The term soliton pulse as used herein refers to a pulse formed by combining a plurality of soliton waves, each soliton wave being a series of pulses composed of a phase-delayed portion of each transmission signal. 
   Amplifying the soliton pulse in accordance with the principles of the present invention has the effect of amplifying each portion of each transmission signal combined to form the soliton pulse. In addition, dividing the amplified soliton pulse in accordance with the principles of the present invention yields an amplified version of each of the original transmission signals that were combined to form the soliton pulse. Moreover, since the amplified transmission signals are obtained by using only a single amplifier, the amount of distortion produced during such amplification is substantially minimized. As a result, the present invention advantageously provides a means to reduce the number of amplifiers needed to provide low-distortion multiple-signal transmissions, and thus provides a means to reduce the overall size and cost of a multiple-transmission system, without substantially increasing signal distortion. 
   In particular embodiments, the soliton pulse is formed by a so-called non-linear transmission line (NTL) circuit. The term NTL circuit as used herein refers to a circuit composed of a plurality of non-linear sections, where each non-linear section has a series inductance, L, and a variable shunt capacitance, C, and where the product of L and C in a given non-linear section determines the phase velocity of a signal propagating in that section. In such embodiments, the soliton wave is obtained by inputting the plurality of transmission signals to the NTL circuit, and tapping the soliton pulse off a given non-linear section of the NTL circuit. The soliton pulse, and thus its component transmission signals can then be amplified by a conventional power amplifier with a relatively small amount of signal distortion, if any. Once amplified, the soliton pulse can then be fed into a “dividing” NTL circuit that is operable to divide the amplified soliton pulse into its component transmission signals. In such embodiments, the component transmission signals can then be fed to a common antenna for transmission over a communications channel. 
   These and other features of the invention will become more apparent from the Detailed Description when taken with the drawing(s). The scope of the invention, however, is limited only by the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an illustrative embodiment of a method for amplifying and transmitting a plurality of transmission signals, having different frequencies, in accordance with the principles of the present invention. 
       FIG. 2  is an illustrative embodiment of a nonlinear transmission circuit for combining a pair of transmission signals into a soliton pulse in accordance with the principles of the present invention. 
       FIG. 3  is an illustrative embodiment of a nonlinear transmission circuit for dividing a soliton pulse in accordance with the principles of the present invention. 
       FIG. 4  is an illustrative embodiment of an apparatus for amplifying and transmitting a plurality of transmission signals, having different frequencies, in accordance with the principles of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1  there is shown a method  10  for amplifying and transmitting a plurality of transmission signals, having different frequencies, in accordance with the principles of the present invention. As shown, method  10  begins with a combining step  11  wherein the plurality of transmission signals are combined to form a soliton pulse. The soliton pulse is then passed through a power amplifier at an amplifying step  12  to form an amplified soliton pulse. The amplified soliton pulse is then divided at a dividing step  13  into its amplified transmission signal components at their original undistorted frequencies. The amplified transmission signal components are then transmitted, at a transmission step  14 , through a common antenna over a communications channel. 
   The soliton pulse formed at combining step  11  consists of a plurality of soliton waves, each soliton wave being a series of pulses, where each pulse is composed of a phase-delayed portion of each transmission signal. Amplifying the soliton pulse, at amplifying step  12 , therefore has the effect of amplifying each portion of each transmission signal that was combined in combining step  11  to form the soliton pulse. Since only a single amplifier is needed to amplify the soliton pulse, the amount of signal distortion resulting from the amplification at amplifying step  12  is substantially minimized. Moreover, since the amplified soliton pulse is composed of amplified versions of the original transmission signals, dividing the soliton pulse at dividing step  13  yields amplified versions of the original transmission signals. Thus, in accordance with the principles of the present invention, method  10  provides a means for amplifying a plurality of transmission signals with a single amplifier, while reducing the risk of signal distortion. As a result, method  10  advantageously provides a means to reduce the overall size and cost of a multiple-transmission system. 
   We do not intend to limit the present invention to any particular method of combining the plurality of transmission signals, as described above. Thus, in accordance with the principles of the present invention, the plurality of transmission signals may be combined by any means that results in a soliton pulse described above. For example, in particular embodiments, the soliton pulse can be formed by feeding the plurality of transmission signals to a non-linear transmission line (NTL) circuit. The operation of NTL circuits is fully disclosed in U.S. Pat. No. 5,495,253, entitled “Soliton Rejection Filter”, issued Feb. 27, 1996 to Albert et al. incorporated herein by reference. In general, transmission signals input to an NTL circuit are passed through a series of non-linear sections, each section having a series inductance, L and a variable shunt capacitance, C, where the product of L and C in a given non-linear section controls the phase velocity of the signal propagating in that section. 
   Referring now to  FIG. 2  there is shown an illustrative embodiment of an NTL circuit  20  for combining a first transmission signal  28  having a frequency F 1 , and a second transmission signal  29  having a frequency F 2 , to form a soliton pulse  30  in accordance with the principles of the present invention. As shown, NTL circuit  20  is composed of non-linear sections  21 - 25 , each non-linear section having series inductance L and a variable shunt capacitance C. Non-linear section  21  is coupled to an input port  26  of NTL circuit  20  through an isolator  40 . Nonlinear section  25  is coupled to an input port  27  of NTL circuit  20  through resistor R and isolator  41 . A signal tap  35  is coupled between non-linear section  23  and output port  48 . 
   In operation, when first transmission signal  28  is input to input port  26  and second transmission signal  29  is input to input port  27 , a soliton pulse  30  is formed at non-linear section  23 . Soliton pulse  30  can be obtained at output port  48  by conventional means through signal tap  35 . The conversion/combination of, for example, transmission signals  28  and  29  to form, for example, soliton pulse  30  is fully described by Kolosick et al. in “Properties of Solitary Waves as Observed on a Nonlinear Dispersive Transmission Line”, published in Proceedings of the IEEE, Vol. 62, No. 5, pg. 578-581, May 1974. 
   As described therein, the nonlinear sections of an NTL circuit are both nonlinear and dispersive to a transmission signal, or RF sinusoidal wave. That is, the phase velocity of a sinusoidal wave (e.g. transmission signals  28  and  29 ) propagating in a given nonlinear section of the NTL circuit is proportional to the inverse of the square root of the series inductance, L, times the variable shunt capacitance, C, in that nonlinear section. Since the value of C in each nonlinear section of the NTL circuit changes as the amplitude of the transmission signal traveling therein changes, the phase velocity of the transmission signal will become spread out, or dispersive, over time. That is, as the transmission signal travels in the NTL circuit, the components of the transmission signal will propagate at different speeds over time. As a result, such transmission signals will become more and more dispersed as they travel through each nonlinear section of the NTL circuit. 
   Depending on the value of C and L in each nonlinear section of the NTL circuit, the transmission signals input to the NTL circuit will begin to look like a series of pulses with sharp discontinuities that resemble a shock wave, or what is called a soliton wave. Moreover, transmission signals input to the input ports of the NTL circuit will collide at a predetermined location (e.g. nonlinear section  23 , shown in  FIG. 2 ), and form a single soliton pulse (e.g. soliton pulse  30 , shown in  FIG. 2 ). As a result, the soliton pulse is obtained by signal-tapping (e.g. through signal tap  35 , shown in  FIG. 2 ) that predetermined location of the NTL circuit. In addition, by isolating the input ports of the NTL circuit (e.g. by isolators  40  and  41 , shown in  FIG. 2 ), the soliton waves that travel past that predetermined location are blocked from interfering with transmission signals input to the input ports of the NTL circuit. 
   As described above, once the soliton pulse is obtained it is amplified and then input to an NTL circuit operable to divide the soliton pulse into amplified versions of the original transmission signals input to the NTL circuit that formed the soliton pulse. It is not our intention to limit the present invention to any particular method of dividing an amplified soliton pulse into amplified versions of the original transmission signals. Thus, in accordance with the principles of the present invention, the soliton pulse may be divided by any means that results in obtaining such amplified versions of the original transmission signals. For example, in particular embodiments, the amplified versions of the original transmission signals can be obtained by inputting the soliton pulse to a “dividing” non-linear transmission line (NTL) circuit. 
   Referring now to  FIG. 3 , there is shown an illustrative embodiment of an NTL circuit  50  for dividing a soliton pulse  51  into its component amplified transmission signals  52  and  53 , where amplified transmission signal  52  has a frequency F 1  and amplified transmission signal  53  has a frequency F 2 , in accordance with the principles of the present invention. As shown, NTL circuit  50  is composed of non-linear sections  54 - 58 , each non-linear section having series inductance, L, and a variable shunt capacitance, C. Non-linear section  54  is coupled to an output port  59  of NTL circuit  50 , nonlinear section  58  is coupled to output port  60  of NTL circuit  50 , and non-linear section  56  is coupled to input port  61  of NTL circuit  50 . 
   In operation, when soliton pulse  51  is input to input port  61 , amplified transmission signals  52  and  53  are formed at output ports  60  and  59 , respectively. The physical conversion/dividing of soliton pulse  51  into amplified transmission signals  52  and  53  is fully described by Kolosick et al. in “Properties of Solitary Waves as Observed on a Nonlinear Dispersive Transmission Line”, published in Proceedings of the IEEE, Vol. 62, No. 5, pg. 578-581, May 1974. 
   As described therein, an NTL circuit configured in such a manner basically behaves opposite to the behavior of an NTL configured to form a soliton pulse, as shown in  FIG. 2  and described above. For example, as the soliton pulse travels into NTL circuit  50  through input port  61 , it splits such that a first portion of soliton pulse  51  travels through nonlinear sections  54  and  55  to output port  59 , and a second portion travels through nonlinear section  57  and  58  to output port  60 . The portion that reaches output port  59  is amplified transmission signal  53 , and the portion that reaches output port  60  is amplified transmission signal  52 . 
   Referring now to  FIG. 4 , there is shown an illustrative embodiment of an apparatus  70  for amplifying and transmitting a plurality of transmission signals, having different frequencies, in accordance with the principles of the present invention. As shown, apparatus  70  is composed of a power amplifier  73  connected to an output port  81  of a “combining” NTL circuit  71  and an input port  82  of a dividing NTL circuit  72 . Combining NTL circuit  71  has input ports  74  and  75 , and dividing NTL circuit  72  has output ports  76  and  77 . 
   Since the operation of combining NTL circuit  71  and dividing NTL circuit  72  have been fully described above in connection with  FIGS. 2 and 3 , respectively, it should be appreciated that feeding transmission signals  85  and  86  to input ports  74  and  75  results in the output of amplified versions  87  and  88  of transmission signals  85  and  86  from output ports  76  and  77 , where the amplified versions  87  and  88  of transmission signals  85  and  86  can be fed to common antenna for transmission over a communication channel. 
   The terms and expressions used herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or any portions thereof. Also, it is recognized that various modifications are possible within the scope of the invention claimed.