Abstract:
A wireless communications system includes a first multiplexer distribution network fed by a radio frequency input; a plurality of multi-stage power amplifiers fed by the first multiplexer distribution network, wherein each one of the multi-stage power amplifiers includes: a pre-distortion linearizer fed from the first distribution network; a first combiner receiving input from the pre-distortion linearizer; a second combiner; a plurality of power amplifier cells fed by the first combiner and feeding the second combiner; and a second multiplexer distribution network, wherein the second multiplexer distribution network is fed by the second combiner and feeds a radio frequency output.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/245,106, filed Sep. 23, 2009, which is incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present disclosure generally relates to millimeter-wave and microwave communications and, more particularly, to power combining for solid state power amplifiers implemented with monolithic microwave integrated circuit (MMIC) technology. 
         [0003]    Commercial and military satellite communication systems may look to achieve higher signal capacities by employing complex modulation schemes in channels that are closely spaced in frequency. Nonlinearities in the wireless communication links, particularly power amplifiers, can cause spectral re-growth, wherein extraneous power from one channel interferes with signals from adjacent channels. Adjacent channel interference adversely affects communication data rates and reliability. 
         [0004]    Thus, there is a need in the art for technology to minimize adjacent channel interference while increasing overall transmission power in the design and implementation of millimeter wave broadband power amplifiers. 
       SUMMARY 
       [0005]    According to one embodiment, a wireless communications system includes. a first multiplexer distribution network fed by a radio frequency input; a plurality of multi-stage broadband power amplifiers fed by the first multiplexer distribution network, wherein each one of the multi-stage broadband power amplifiers includes: a pre-distortion linearizer fed from the first distribution network; a first combiner receiving input from the pre-distortion linearizer; a second combiner; a plurality of power amplifier cells fed by the first combiner and feeding the second combiner; and a second multiplexer distribution network, wherein the second multiplexer distribution network is fed by the second combiner and feeds a radio frequency output. 
         [0006]    According to another embodiment, a solid state power amplifier includes: a splitting network having an input and plurality of outputs, wherein each splitting network output is selective of a channel of a full bandwidth; a plurality of pre-distortion linearizers each fed through one of a plurality of drivers from one output of the splitting network; a divider fed from one of the pre-distortion linearizers; a power amplifier fed from the divider and comprising a carrier amplifier and a peaking amplifier connected in a Doherty configuration, wherein the carrier amplifier is configured to operate in class B/AB and the peaking amplifier is configured to operate in class C; a combiner fed from the power amplifier; and a combining network having an input fed from the combiner, the combining network having a plurality of inputs and configured to combine signal power from the plurality of channels input into a signal output comprising the full bandwidth. 
         [0007]    According to another embodiment, a method of amplifying a wideband radio frequency signal includes: splitting the radio frequency signal to a plurality of narrow band pre-distortion linearizers; pre-distorting the radio frequency signal to create a gain expansion inverse to a compression point in a power amplifier; splitting the pre-distorted signal among a plurality of microwave power amplifiers; amplifying the split signal using a carrier amplifier and peaking amplifier transistor pair; combining the amplified signal using one of a plurality of combiners; and combining signals output from the plurality of combiners into an output signal. 
         [0008]    The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a system block diagram showing a combination of types of power combining for implementing a microwave power amplifier in accordance with one embodiment of the present invention; 
           [0010]      FIG. 2A  is an isometric view of a waveguide divider core element in accordance with an embodiment; and  FIG. 2B  is a frequency graph illustrating S-parameters for the element shown in  FIG. 2A ; 
           [0011]      FIG. 3A  is modified waveguide divider core element in accordance with another embodiment; and  FIG. 3B  is a frequency graph illustrating S-parameters for the element shown in  FIG. 3A ; 
           [0012]      FIG. 4A  is a diagram illustrating a four-finger Lange coupler in accordance with an embodiment.  FIGS. 4B and 4C  are frequency graphs showing, respectively, phase and gain for the Lange coupler shown in  FIG. 4A . 
           [0013]      FIG. 5A  is a circuit diagram showing a nonlinear core of a pre-distorter in accordance with an embodiment; 
           [0014]      FIG. 5B  is a circuit diagram showing a pre-distortion linearizer in accordance with an embodiment; 
           [0015]      FIG. 6  is a graphical diagram depicting gain expansion at a pre-distortion linearizer, such as that illustrated by  FIG. 5 , in accordance with an embodiment; 
           [0016]      FIGS. 7A and 7B  are input power graphs illustrating, respectively, amplitude and phase components of a gain expansion of a linearizer, such as that illustrated by  FIG. 5 , in accordance with an embodiment; 
           [0017]      FIG. 8  is a circuit block diagram illustrating the connection of a pre-distortion linearizer, such as that illustrated by  FIG. 5 , with a three-stage power amplifier, such as those illustrated in  FIG. 1 , in accordance with an embodiment; 
           [0018]      FIG. 9  is a before and after pair of frequency graphs comparing two-tone third-order intermodulation distortion (IMD3) measurement for a microwave power amplifier, such as that illustrated in  FIG. 1 , before and after tuning a pre-distortion linearizer, such as that illustrated by  FIG. 5 , in accordance with an embodiment; 
           [0019]      FIG. 10  is a before and after pair of frequency graphs comparing spectrum input versus output for a 16-QAM channel before and after pre-distortion linearization, in accordance with an embodiment; 
           [0020]      FIG. 11  is a circuit diagram illustrating a power amplification stage of a microwave power amplifier, in accordance with an embodiment; and 
           [0021]      FIG. 12  is a backoff power level graph comparing efficiencies of two different classes of microwave power amplifiers, in accordance with one or more embodiments. 
       
    
    
       [0022]    Embodiments and their advantages are best understood by referring to the detailed description that follows. Like reference numerals are used to identify like elements illustrated in one or more of the figures. 
       DETAILED DESCRIPTION 
       [0023]    In accordance with one or more embodiments of the present invention, systems and methods disclosed herein provide for microwave power amplification using power combining in an array of solid state power amplifiers (SSPA) to provide an effective output power such as 30 watts (W) in a frequency range of about 71-76 Giga Hertz (GHz). Due to its narrowband of operation, switching power amplifiers with efficiency of better than 45% at 76 GHz may also be used. A more compact array size may be achieved, although the array size may generally be constrained by the frequency of operation and the size of the corporate adders due to the transistor size being small in comparison to the size of the passive devices. The entire SSPA array may be monolithic and may be implemented using monolithic microwave integrated circuit (MMIC) process, with the combiner being manufactured by micro-machining Such an array configuration—implementing at once both power and frequency combining—for a broadband microwave power amplifier reduces adjacent channel interference that adversely affects communications data rates, and thus can improve satellite communications as well as the performance of other communication systems that use portions of the microwave spectrum. 
         [0024]    In a more conventional approach of distributing the input signal power with, for example, 5 GHz bandwidth into an array without altering the pass band frequency bandwidth, the full bandwidth of the input signal may be fed to a corporate divider at each branch. All branches may be designed identically with each linearizer operating over the entire 5 GHz bandwidth at each amplifier (cell of the array). This approach provides several system benefits such as identical design for all the branches of divider network, combiner network, and power amplifier cells. This approach, however, may complicate the process of hardware design, especially in using narrowband, efficiency-enhancement power cell methods, e.g., for Doherty power amplifiers and the pre-distortion linearizer circuits. 
         [0025]    In the approach, according to one or more embodiments, of distributing the input power with channelized pass band frequency—i.e., frequency as well as power combining—the input signal (with, for example, 5 GHz bandwidth) may be fed to an input splitter  104  (e.g., first distribution network  104 , see  FIG. 1 ) configured as a multiplexer (also referred to as filtered corporate combining) to provide output of 64 branches, each, for example, 80 MHz wide (which may include some overlap between adjacent channels). The very narrow, channelized band (e.g., 80 MHz) of frequency may facilitate the design of power cells  102  that can be highly linearized, using, for example, Doherty power amplifier configuration and the pre-distortion circuit. 
         [0026]    Also, using this channelized approach, all branches of the input splitter  111  (and, likewise, output combiner  113 ) may be identical, and in case one power amplifier  102  malfunctions, SSPA  100  can continue to perform acceptably with minimal loss in power. At the same time, i.e., in event of a total channel power amplifier failure, channel switching can circumvent the problem at system level. If a vector modulation like 16-QAM (quadrature amplitude modulation) is the modulation of choice, phase and amplitude of the waveforms may be required to be intact even after being split and recombined between two adjacent branches. Thus, the multiplexers  104 ,  106  may be designed in a way to guarantee phase and level integrity of the signals; in other words, the multiplexers  104 ,  106  may be seamless at transitions. In order to guarantee phase and level integrity of the signals, a one to 64 multiplexer combiner, e.g., multiplexers  104 ,  106 , may divide the 5 GHz, for example, bandwidth into 64 channels each 80 MHz wide. The filtered corporate combining may provide 18 dB of combining gain at this stage of SSPA  100 . Assuming that the communication channels distribute arbitrarily over the, for example, 5 GHz bandwidth, then SSPA  100  may be required to maintain phase and gain integrity (e.g., for 16-QAM modulation) while transitioning from one channel to the other. 
         [0027]      FIG. 1  is a system block diagram for an SSPA power combining switching amplifier array  100  (also referred to as SSPA  100 ) in accordance with one or more embodiments. The SSPA  100  shown in  FIG. 1  may be a planar array of power amplifier elements  102  (also referred to as cells  102  or power amplifiers  102 ). The integrated array in a monolithic substrate requires distribution networks  104  to bring power, and distribute radio frequency (RF) signals (e.g., a microwave input signal) from a central point (e.g. an RF input) to every power amplifier element  102  in the array, and combine the signals into a single signal output using a waveguide combiner  106 . Consideration may be given to the element level electronics, the inter-element connections, external interfaces, packaging, and the operation of the entire tile (array) of power amplifiers  102 . 
         [0028]    In order to provide a nominal goal of 45 dBm (decibels referenced to one millliwatt) of output power, several power cells  102  may be arranged in tandem. To attain a tandem arrangement, a combination of balanced and corporate power combining for cells  102  may be employed. Referring to  FIG. 1 , for example, a single power cell  102  of maximum output power 20 dBm may be multiplied by 1024 (64×16) to attain the required power level of about 45 dBm after deducting the implementation loss of the combiner network, e.g., splitting (or divider) network  104  and combiner network  106  (also referred to as first and second distribution networks). At each of the 64, for example, branches shown in  FIG. 1 , one or more first combiners  111  may operate as a splitter or divider to distribute signal to each of, in this example, 16 power amplifiers  102  and the output of the 16 power amplifiers  102  may be combined by one or more combiners  113 . Combiners  111 ,  113  may be connected at their respective inputs and outputs feeding to (from) distribution networks  104 ,  106  by hybrid couplers  109  as seen in  FIG. 1 . In order to reduce the combining loss, a system of waveguide-based combiner in conjunction with the planar balanced and corporate power combining may be used. Waveguide-based combining may provide high quality factor filter as well as a low loss transmission environment. 
         [0029]      FIG. 2A  is an isometric view of a waveguide divider core element  110  in accordance with one embodiment, and  FIG. 2B  is a frequency graph illustrating S 11  parameters for the core element  110 . In order to reduce the combining loss, a waveguide network may be used to implement the first splitting  104  and combining  106  network.  FIG. 2A  shows a core element  110  of such an arrangement. The same core element  110  may be used as a divider or a diplexer.  FIG. 2B  depicts the S 11  characteristics of the diplexer over the entire 71-76 GHz frequency range. 
         [0030]    As shown in  FIG. 3A , for implementation of a diplexer for splitting and combining networks  104 ,  106 , a modified core element  112  may introduce cavities—such as cavities  114 ,  116 , and  118 —to provide filtering of the channels at different frequency responses for adjacent branches.  FIG. 3B  is a frequency graph illustrating S 11  parameters for the modified core element  112 . 
         [0031]    The power amplifier (e.g., cells  102 ) with a pre-distortion network implementation may be highly linear when operated close to its Psat (e.g., power saturation point) which means higher power-added efficiency. At the same time, required high efficiency implies that a class A amplifier is not a suitable choice. A Class AB amplifier biased closer to class B, however, and used in conjunction with a class C amplifier (e.g., in a Doherty arrangement) may provide a combination of higher efficiency and acceptable linearity. Implementing a class AB power amplifier cell  102  may be achieved by channelizing the entire, for example, 5 GHz bandwidth into smaller bands and then combining them to reproduce the original 5 GHz band of interest. 
         [0032]      FIG. 4A  illustrates the layout of a four-finger Lange coupler  121 . For example, Lange coupler  121  may be a 4-finger, 4-port Lange hybrid centered at 73 GHz. For Lange coupler  121 , if the top left port  1212  is the input (ml in  FIGS. 4B and 4C ), the bottom left port  1211  is the “coupled” port (m 2  in  FIGS. 4B and 4C ), the bottom right port  1214  is the “through” port (m 4  in  FIGS. 4B and 4C ), and the top right port  1213  is the “isolated” port (m 3  in  FIGS. 4B and 4C ). When the isolated port  1213  is terminated in 50 ohms, the Lange coupler  121  provides two equal coupled and straight outputs  1211 ,  1214  (3 dB loss) with 90 degrees phase shift. 
         [0033]      FIGS. 4B and 4C  are frequency graphs showing phase shift and insertion loss between two coupled and straight paths from port  1212  (ml) being the input of the Lange coupler  121 . 
         [0034]      FIG. 5A  is a schematic diagram showing a nonlinear core of a pre-distorter  120  (included, for example, in pre-distortion linearizer  108 ). Pre-distorter  120  is a through nonlinearity generator. By using one 90-degree coupler, the reflective nonlinearity happening at the back to back diodes  122  is converted into a through or transmission nonlinearity. The effect may be a gain compression (S 21 ) as a function of the input power. The reason to have an antiparallel diode pair  122  as the nonlinear element is to cancel out the nonlinearities of even order, mostly to target the third and fifth order nonlinearity. The amplifier  123  provides enough gain to drive the diodes  122  into the nonlinear region which eventually demonstrates as the gain compression in S 21 . The Lange coupler  125  may be a Lange coupler such as Lange coupler  121  described above with reference to  FIGS. 4A ,  4 B, and  4 C. 
         [0035]    By using a nonlinearity of signal gain before the power amplifier  102  in a method of analog pre-distortion (e.g., feeding the signal through pre-distortion linearizers  108  including pre-distorters  120 ), the gain compression effect of the power amplifier  102  may be compensated. Different semiconductor devices generally present different nonlinear behaviors. One approach is to choose the same process for the pre-distorter  120  as the power amplifier  102 . For example, simulations and evaluations may be based on a GaAs power pHEMT (pseudomorphic high electron mobility transistor) process.  FIG. 5A  shows a nonlinear core of a pre-distorter  120  comprised of two pairs of parallel reversed diodes  122  chosen from a pHEMT process. The configuration generates an odd order nonlinearity which may be suitable for third and fifth order linearization efforts. 
         [0036]      FIG. 5B  shows a pre-distortion linearizer  108  that may create a gain expansion inverse to the compression point in the power amplifier  102 . Predistorter  108  includes two paths  124  and  126 . A nonlinear path  126  includes the nonlinearity generator (pre-distorter)  120 , attenuator  136  and phase shifter  134 . A linear path  124 , which is a transmission line  127 , for example, a micro-strip transmission line. Micro-strip transmission line  127  provides the same phase shift as the nonlinear path  126  of the predistorter  120  while in the linear region with the phase shifter  134  and attenuator  136  in mid range. When this condition is met, there is maximum control over the gain and phase expansion characteristics (see  FIG. 6 ) over the range of the phase shifter  134  and attenuator 136 . 
         [0037]    As is shown in  FIG. 5B , the input signal may pass through a phase compensating transmission line  124  (lower branch  124 ) without any compression and then at the upper branch  126  the signal gets compressed by the pre-distorter  120 . The compressed signal may go through two 90 degrees hybrids (e.g., Lange couplers  128 , which may be Lange couplers such as Lange coupler  121  described above with reference to  FIGS. 4A ,  4 B, and  4 C), and with adjusting the attenuator  136  and the phase shifter  134  the compressed version of the signal (e.g. upper branch  126 ) may be subtracted from the linear version of the signal (e.g., lower branch  124 ) and generate a gain expansion  130  (e.g., at pre-distortion linearizer output  130 ) as shown in  FIG. 6 . 
         [0038]    The schematic in  FIG. 5B  uses the diode and actual designed hybrids to create the gain expansion  130  as seen in  FIGS. 7A and 7B , having amplitude component  131  shown in  FIG. 7A  and phase component  132  shown in  FIG. 7B . To be as effective as possible, pre-distortion should address both gain offset distortion—also referred to as amplitude-to-amplitude modulation (AM/AM)—and phase distortion—also referred to as amplitude-to-phase modulation (AM/PM). The phase shifter  134  and the attenuator  136  may be adjusted at the same time. Beside the limited improvement (mostly due to its open loop operation) that RF pre-distortion provides, another shortcoming of RF pre-distortion in general is the narrow band of operation over which it is effective. Both AM/AM and AM/PM may be compensated by first dividing the pass band bandwidth (e.g., 5 GHz) into smaller sub-bands and then using Doherty based amplifiers for power cells  102 .  FIGS. 7A and 7B  indicate phase ( FIG. 7B ) and gain ( FIG. 7A ) expansion of the schematic shown in  FIG. 5  for a pre-distortion linearizer  108 . 
         [0039]      FIG. 8  shows a three stage power amplifier  140  including a driver  107  feeding a pre-distortion linearizer  108  preceding a power amplifier  102 . After tuning the phase shifter  134  and the attenuator of the pre-distortion linearizer  108 , an improvement, as shown in  FIG. 9 , of at least approximately 13 dB in two-tone third-order intermodulation distortion (IMD3) measurement may be observed in a two-tone setup. In a similar simulation, a single 16-QAM waveform is introduced to the input of the three stage amplifier  140 , and as shown in  FIG. 10 , before and after tuning, an adjacent channel power ratio (ACPR) improvement of 10 dB may be observed. 
         [0040]      FIG. 11  is a circuit diagram for a microwave power amplifier  102  employing a Doherty transistor pair. The upper (carrier) transistor  141  is operated in class B/AB while the lower (peaking) transistor  142  is biased at class C. The prototypical Doherty power cell and most of the low frequency implementations of Doherty power cells bias the carrier transistor at class B, but at higher microwave bands, however, the available gain is reduced and using a class B bias might not offer enough gain, so a compromise between gain and bias (the tradeoff being determined by how deep in class AB the carrier transistor  141  is biased) may be made. 
         [0041]    For example, Doherty power amplifiers are extensively used at RF and lower microwave frequencies. The generally highest frequencies at which they are operated are at about 60 GHz (for CMOS 65 nanometers(nm)) and about 45 GHz (for GaAs pHEMT 0.15 um). When using a class B or Class AB power cell, in order to achieve a minimum efficiency of 30% for a 6 dB back off for 16-QAM modulation (the back off would be less if there is a coding) the maximum power efficiency of the power cell should be very high. The extreme example would be using a %100 efficient power cell at its maximum RF power; after 6 dB power back off, the efficiency would yield to merely 25%. One solution to circumvent these conflicting requirements is to use an efficiency enhancement method for the power cell. Thus,  FIG. 11  shows a Doherty transistor pair  141 ,  142  that comprises a class B/AB carrier amplifier (transistor  141 ) which manages enhancing the average power, and a class C peaking amplifier (transistor  142 ) which manages the peak power amplifications. Using this configuration the impact of power back off on reducing the efficiency diminishes. 
         [0042]      FIG. 12  is a backoff power level graph comparing efficiencies of two different classes of microwave power amplifiers.  FIG. 12  shows that even after 6 dB backoff, a class B Doherty pair maintains the same efficiency as an ordinary class B amplifier at maximum power. 
         [0043]    Embodiments described herein illustrate but do not limit the disclosure. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present disclosure. Accordingly, the scope of the disclosure is best defined only by the following claims.