Patent Publication Number: US-2009233644-A1

Title: Multiple carrier radio systems and methods employing polar active antenna elements

Description:
FIELD OF THE INVENTION 
     The present invention relates to wireless communications. More specifically, the present invention relates to the generation and transmission of multiple modulated radio frequency carrier signals in wireless communications networks. 
     BACKGROUND OF THE INVENTION 
     A cellular communications network includes geographically dispersed base transceiver stations (BTSs) that provide an interface between cellular handsets and the network. To limit the number of BTSs that must be built and maintained in the network, the BTSs are equipped with radio systems that handle multiple calls at the same time. Traditionally, this has been accomplished by employing a multiple carrier BTS radio system  100  like that shown in  FIG. 1 . The multiple carrier BTS radio system  100  includes a plurality of transceivers (TX 1 , TX 2 , . . . , TX )  102 - 1 ,  102 - 2  . . .  102 - m  (m is an integer that is ≧2), an associated plurality of linear power amplifiers (PAs)  104 - 1 ,  104 - 2 , . . . ,  104 - m , a combiner  106  and an antenna  108 . Modulation signals X 1 , X 2 , . . . , X m  are modulated onto the multiple radio frequency (RF) carrier signals generated by the plurality of transceivers  102 - 1 ,  102 - 2  . . .  102 - m , and amplified by the associated plurality of linear PAs  104 - 1 ,  104 - 2 , . . . ,  104 - m , thereby generating multiple modulated RF carrier signals. The multiple modulated RF carrier signals are combined by the combiner  106 , to generate a single multi-carrier modulated RF carrier signal, which is coupled to the antenna  108  and finally radiated over the air to a remote receiver (e.g., a receiver of a cellular handset). 
     While the multiple carrier BTS radio system  100  does succeed in transmitting multiple modulated RF carrier signals at the same time, from a power consumption perspective it does so very inefficiently. The high inefficiency is attributable to the linear PAs  104 - 1 ,  104 - 2 , . . . ,  104 - m  and the combiner  106 , typically a cavity type combiner, which dissipate large amounts of power. Together, the linear PAs  104 - 1 ,  104 - 2 , . . . ,  104 - m  and high-power combiner  106  limit the radio system  100  to an efficiency of only about 10%. 
       FIG. 2  is a diagram of an alternative BTS radio system  200 , which avoids the use of a high-power combiner by combining the multiple carrier signals prior to being amplified. Similar to the BTS radio system  100  in  FIG. 1 , the multi-carrier BTS radio system  200  in  FIG. 2  comprises a plurality of transceivers  202 - 1 ,  202 - 2  . . .  02 - m  that generates a plurality of modulated RF carrier signals. However, different from the multiple carrier radio system  100  in  FIG. 1 , the modulated RF carrier signals are combined before being amplified. Specifically, the combiner  204  combines the plurality of modulated RF carrier signals, thereby creating a single multi-carrier signal, which is amplified by a single multi-carrier power amplifier (MCPA)  206 . 
     Because the modulated RF carrier signals in the multi-carrier radio system  200  in  FIG. 2  are combined prior to being amplified, a high-power combiner is not required. Instead, a low power combiner  204  may be used. Unfortunately, the MCPA  206  is less efficient than the collective efficiency of the plurality of PAs  104 - 1 ,  104 - 2 , . . . ,  104 - m  in the multiple carrier BTS radio system  100  in  FIG. 1 . Consequently, very little efficiency gain is achieved over the multiple carrier BTS radio system  100  in  FIG. 1 , even though a low power combiner  204  can be used. 
       FIG. 3  is a drawing illustrating how an MCPA-based radio system  304 , like the MCPA-based radio system  200  in  FIG. 2 , is configured within a BTS  300 . The amplified multi-carrier signal at the output of the MCPA-based radio system  304  is conveyed to passive antenna elements  314  mounted on the BTS tower  310 , via an RF antenna feed (typically a coaxial cable)  312 . To limit intermodulation distortion products and thereby comply with air interface standards, the MCPA-based radio system  304  must operate with high linearity. Unfortunately, linear power amplifiers are very inefficient, and results in the MCPA-based radio system  304  being very large and heavy. In fact the MCPA-based radio system  304  is usually so large and heavy that it cannot, as a practical matter, be mounted in the BTS tower  310 . Instead, it is housed within a cabinet  302  positioned on the ground near the base of the BTS tower  310 . An air conditioning unit  308  is also typically included within the cabinet  302 , to cool the MCPA-based radio system  304 . The cabinet  302  protects the MCPA-based radio system  304  and the air conditioning unit  308  from the elements and from being vandalized. 
     While the multi-carrier BTS  300  in  FIG. 3  does provide the ability to generate and transmit multiple modulated RF carrier signals, it has a number of significant shortcomings. First, the MCPA-based radio system  304  presents a single point of failure, since it only employs a single PA. Consequently, should the PA of the MCPA-based radio system  304  fail, the entire BTS  300  will fail. Second, the requirement that the MCPA-based radio system  304  operate with high linearity results in large amounts of wasted power. Not only is this wasted power costly, it is also harmful to the environment. Third, because the MCPA-based radio system  304  is so inefficient and must be mounted on the ground, power is lost in the antenna feed  312  as the amplified multi-carrier signal at the output of the MCPA-based radio system  304  is conveyed to the passive antenna elements  314  mounted in the BTS tower  310 . This power loss can be substantial (8-15 dB, depending on the length of the antenna feed  312 ). Finally, due again to the inefficiency and size of the MCPA-based radio system  304 , an expensive and power consuming air conditioning unit  308  is needed. When all of these inefficiencies are considered, the MCPA-based BTS  300  typically only operates with an overall efficiency between 1 to 5%. 
     Given the foregoing limitations and problems of prior art BTS radio systems, it would be desirable to have methods and systems for amplifying and transmitting multiple carrier signals that are efficient, inexpensive, do not require large air conditioning system, and are not subject to a single point of failure. 
     BRIEF SUMMARY OF THE INVENTION 
     Multiple carrier radio systems and methods that employ polar active antenna elements (PAAE&#39;s) are disclosed. An exemplary multiple carrier radio system includes a baseband subsystem and a plurality of polar modulators. The polar modulators are mounted at antenna locations (e.g., in a base transceiver station (BTS)) above the ground), and are configured to receive modulation signals generated by the baseband subsystem. The plurality of polar modulators generates a plurality of modulated radio frequency (RF) carrier signals from the modulation signals. The modulated RF carrier signals are radiated by a corresponding plurality of antenna elements coupled to the plurality of polar modulators. 
     The multiple carrier radio systems and methods of the present invention have a number of significant advantages over prior art multi-carrier radio transmitter systems. First, the PAAE&#39;s employ nonlinear switch-mode PAs, rather than linear PAs. Consequently, the efficiency of the multiple carrier radio methods and systems of the present invention are substantially higher than prior art linear-PA-based approaches. Second, because the PAAE&#39;s used in the methods and systems of the present invention dissipate substantially less power compared to prior art linear-PA-based approaches, large air conditioning units and high power consuming AC/DC converters are not needed. Third, because RF power generation is performed up in the tower close to the antenna elements, and not on the ground as in prior art approaches, RF feed losses resulting from transferring RF power from the base of the BTS tower to the antenna elements are avoided. Fourth, because the systems and methods of the present invention use a plurality of PAs to generate multiple modulated RF carrier signals, the single point of failure problem caused by using only a single multi-carrier PA (MCPA) is avoided. Finally, but not necessarily lastly, high power consumption combiners are not needed to combine multiple RF carrier signals in the methods and systems of the present invention. Instead, in circumstances in which it is desired to combine RF carrier signals, the RF carrier signals are combined in space (i.e., spatially) after being radiated by the antennas of the PAAE&#39;s. 
     Further aspects of the invention are described and claimed below, and a further understanding of the nature and advantages of the invention may be realized by reference to the remaining portions of the specification and the attached drawings in which like reference numbers are used to indicate like or similar items. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of a traditional approach to generating a multi-carrier communications signal in a base transceiver station (BTS); 
         FIG. 2  is a drawing of a BTS radio system that utilizes a multi-carrier power amplifier (MCPA); 
         FIG. 3  is a drawing illustrating how a MCPA-based radio system is configured within a BTS; 
         FIG. 4  is a drawing of a BTS equipped with polar active antenna elements (PAAE&#39;s), according to an embodiment of the present invention; 
         FIG. 5  is a drawing showing a PAAE, which may be used in the BTS in  FIG. 4  or in the BTSs of other embodiments of the present invention; 
         FIG. 6  is a drawing of a multiple carrier radio transmitter system that employs a plurality of PAAE&#39;s, according to an embodiment of the present invention; 
         FIG. 7  is a drawing illustrating how a plurality of PAAE&#39;s may be configured in a polar active antenna array, according to an embodiment of the present invention; and 
         FIG. 8  is a conceptual drawing illustrating how multiple polar active antenna arrays may be mounted in a BTS tower, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 4 , there is shown a base transceiver station (BTS)  400  equipped with polar active antenna elements (PAAE&#39;s)  404 , according to an embodiment of the present invention. The BTS  400  comprises a communications tower  406 , upon which one or more radio frequency (RF) transceivers  402  having PAAE&#39;s  404  are mounted, and a baseband subsystem  408 . A digital communications bus  410  comprising, for example, one or more fiber optic cables, is coupled between the RF transceivers  402  and the baseband subsystem  408  for conveying digital baseband data, status and control signals between the baseband subsystem  408  and the RF transceivers  402 . A power source  412  provides power to the baseband subsystem  408  and to the RF transceivers  402  via power lines  414  and  416 . 
     The baseband subsystem  408  of the BTS  400  is operable to send and receive digital messages to and from a base station controller (BSC) or mobile switching center. It may be placed in a cabinet or other protective housing at the base of the tower  406 , mounted in the tower  406 , or located at some other location. A digital message received by the baseband subsystem  408 , and which is destined for transmission by one or more of the PAAE&#39;s  404 , is first processed by a digital signal processor (DSP) within the baseband subsystem  408 , to generate a digital baseband modulation signal. The digital baseband modulation signal is comprised of modulation symbols having modulation states defined by an applicable wireless communications standard, e.g., the Global System for Mobile Communications (GSM) standard, the Wideband Code Division Multiple Access (W-CDMA) standard, or other wireless communications standard. 
     According to one aspect of the invention, generating the digital baseband modulation signal includes converting the modulation symbols of the digital baseband modulation signal from rectangular (i.e., Cartesian) coordinates (x, y) to polar coordinates (ρ, θ), where ρ=√{square root over (x 2 +y 2 )} and θ=arc tan(y/x). The resulting digital polar modulation signal is communicated over the communications bus  410  to the tower-mounted PAAE&#39;s  404 . In an alternative embodiment, the Cartesian signal coordinates are communicated to the PAAE&#39;s  404  and the Cartesian-to-polar conversion is performed at each PAAE  404 . 
       FIG. 5  is a drawing showing a PAAE  404  in more detail. The PAAE  404  comprises a polar modulator  500  and a dedicated antenna element  502 . (The PAAE  404  may also include processing circuitry for converting Cartesian signal coordinates to polar coordinates, as was discussed in the previous paragraph.) The polar modulator  500  has an envelope path that includes a first digital-to-analog converter (DAC)  504 , envelope modulator  506 , power controller  508  and power regulator  510 ; a phase path that includes a second DAC  512  and a voltage controlled oscillator (VCO)  514 ; and a power amplifier (PA)  516 . The first and second DACs  504  and  512  of the polar modulator  500  have digital inputs labeled “ρ” and “θ”, to indicate that they are to receive the digital envelope and phase components of digital baseband modulation signals, respectively, from the baseband subsystem  408 . The power controller  508  has a power control signal input that is labeled “P”, to indicate that it is configured to receive a power control signal. 
     In the envelope path of the polar modulator  500 , the first DAC  504  converts the digital envelope component of a digital baseband modulation signal into an analog baseband envelope modulation signal. The analog baseband envelope modulation signal is coupled to a first input of the envelope modulator  506 , which operates to modulate a DC power supply voltage, Vsupply, according to amplitude variations in the analog baseband envelope modulation signal. The resulting amplitude modulated power supply signal, Vs, is coupled to the power controller  508 , which may comprise, for example, a multiplying DAC. A power control signal generated by the baseband subsystem  408  and conveyed to the PAAE  404  via the digital communications bus  410 , is applied to the power control signal input, P, and is used to control the output power of the PA  516 . In other words, the power of the amplitude modulated power supply signal, Vs, from the envelope modulator  506  is scaled by a factor, k, by the power controller according to the digital power control signal, thereby providing a scaled amplitude modulated power supply signal, kVs. The scaled amplitude modulated power supply signal, kVs, is applied to the power regulator  510 , which operates to generate an amplitude modulated power setting signal, Venv, for the modulator&#39;s PA  516 . The PA  516  comprises a highly-efficient switch-mode PA that is driven repeatedly between a heavily compressed state (switch closed) and a cut-off state (switch open), in response to a constant-amplitude RF phase modulated signal generated in the phase path of the polar modulator  500  (discussed in more detail below). When driven into the heavily compressed state, the output power of the PA  516  is proportional to the square of the amplitude modulated power setting signal, Venv. 
     While the amplitude modulated power setting signal, Venv, is generated in the envelope path, a constant-amplitude RF phase modulated signal (i.e., an RF phase modulated signal having an amplitude that remains constant over time) is generated in the phase path. The constant-amplitude RF phase modulated signal is generated by first converting the phase component of the digital baseband modulation signal received from the baseband subsystem  408  into a constant-amplitude analog phase component signal, and then using the constant-amplitude analog phase component signal to phase modulate an RF carrier signal generated by the VCO  514 . The resulting RF phase modulated signal is coupled to the RF input of the PA  516 . 
     The amplitude modulated power setting signal, Venv, generated in the amplitude path of the polar modulator  500  and the RF phase modulated signal generated in the phase path of the polar modulator  500  are combined at the PA  516 , thereby generating a fully modulated RF carrier signal. The fully modulated RF carrier signal is coupled to the antenna element  502 , which radiates the fully modulated RF carrier signal to a remote receiver. 
     A plurality of PAAE&#39;s  404  may be configured to form a highly efficient multiple carrier radio system for a BTS of a cellular communications network.  FIG. 6  is a drawing of an exemplary multiple carrier radio system  600  that employs multiple PAAE&#39;s  404 , according to an embodiment of the present invention. The multiple carrier radio system  600  comprises a baseband subsystem  602  and a plurality of PAAE&#39;s  404 . Each PAAE  404  contains a polar modulator  604  having a switch-mode PA, and an antenna element  606 , similar to the polar modulator  500  shown in  FIG. 5 . The baseband subsystem  602  generates polar modulation signals S 1  (ρ 1 , θ 1 ), S 2 (ρ 2 , θ 2 ), . . . , Sn(ρ n , θ n ), where n is an integer that is ≧2. The polar modulation signals S 1 (ρ 1 , θ 1 ), S 2 (ρ 2 , θ 2 ), . . . , Sn(ρ n , θ n ) are applied to the plurality of PAAE&#39;s  404 , via a digital communication bus  610 . Separate and independently controllable power control signals, P 1 , P 2 , . . . , P n , which affect the individual output powers of the PAAE&#39;s  404 , are also provided by the baseband subsystem  602  to the PAAE&#39;s  404 , via the digital communications bus  610 . The polar modulators  604  modulate the polar modulation signals S 1 (ρ 1 , θ 1 ), S 2 (ρ 2 , θ 2 ), . . . , Sn(ρ n , θ n ) onto a plurality of RF carrier signals, which are then amplified by the individual switch-mode PAs of the polar modulators  604 , to generate multiple modulated RF carrier signals. The multiple modulated RF carrier signals are coupled to the antenna elements  606 , which radiate the multiple modulated RF carrier signals over the air to a remote receiver. 
       FIG. 7  is a drawing illustrating how a plurality of PAAE&#39;s  404  may be configured in a polar active antenna array  700 , to amplify and transmit a plurality of modulate RF carrier signals, according to an embodiment of the present invention. A plurality of these polar active antenna arrays  700  may be mounted in a BTS tower to form a polar active phased array antenna system, as conceptually illustrated in  FIG. 8 . Each polar active antenna array  700  comprises a plurality of PAAE&#39;s  404  arranged in row-column array. Each PAAE  404  includes digital inputs configured to receive one of the digital polar modulation signals S 1 (ρ 1 , θ 1 ), S 2 (ρ 2 , ρ 2 ), . . . , Sn(ρ n , θ n ) from a baseband subsystem (not shown in the drawing). In this exemplary embodiment, eight PAAE&#39;s  404  are included in the array  700 . A different number may be used, as will be understood by those of ordinary skill in the art. Each active antenna element  404  has its own digital envelope and phase component inputs and its own independent power control input. Specifically, the digital envelope and phase component signal inputs and the power control inputs are labeled (ρ A , θ A , P A ), (ρ B , θ B , P B ), (ρ C , θ C , P C ) and (ρ D , θ D , P D ) for the PAAE&#39;s  404  in the first column of the array  700 , and are labeled (ρ E , θ E , P E ), (ρ F , θ F , P F ), (ρ G , ρ G , P G ) and (ρ H , θ H , P H ) for the PAAE&#39;s  404  in the second column of the array  700 . A switching matrix  702  disposed between the PAAE&#39;s  404  and the baseband subsystem operates to route each of the digital polar modulation signals, S 1 (ρ 1 , θ 1 ), S 2 (ρ 2 , θ 2 ), . . . , Sn(ρ n , θ n ) to any one or more of the PAAE&#39;s  404 , e.g., according to a predetermined routing algorithm used in a particular application. 
     According to one embodiment of the invention the power and/or phase relationships of the modulated RF signals radiated by the individual antenna elements of the plurality of PAAE&#39;s  404  are controlled to generate a spatially combined modulated RF carrier signal having a desired radiation pattern. Combining the individual modulated RF carrier signals from each PAAE  404  in space obviates any need for a conductive combiner. The output power of each PAAE  404 , and therefore the radiation pattern of the spatially combined modulated RF signal, can be optimized by varying the power control signals applied to the power control inputs P A , . . . , P H . Further optimization of the radiation pattern of the spatially combined RF carrier signal, including its directionality, can be controlled by controlling the phase relationships among the digital polar modulation signals, S 1 (ρ 1 , θ 1 ), S 2 (ρ 2 , θ 2 ), . . . , Sn(ρ n , θ n ), applied to the PAAE&#39;s  404 . Phase-shift and amplitude calibration elements  706  and  708  coupled between the switching matrix  702  and each of the PAAE&#39;s  404  provide for this optimization capability. The phase shifts and amplitude calibrations may be performed dynamically during operation (e.g., in response to phase-shift and amplitude calibration signals provided by the baseband subsystem) or by manual adjustments made to the phase-shift and/or amplitude calibration elements  706  and  708  prior to system operation. The ability to independently calibrate the phase shifts and amplitude of the modulation signals applied to the PAAE&#39;s  404 , together with the ability to independently control the power output of each PAAE  404  affords the ability to optimize the radiation pattern of the spatially combined modulated RF signal. 
     The present invention has been described with reference to specific exemplary embodiments. These exemplary embodiments are merely illustrative, and not meant to restrict the present invention. For example, while multiple PAAE&#39;s described in the embodiments above are suitable for amplifying and transmitting a plurality of carrier signals according to a single wireless communications standard, multiple PAAE&#39;s may also (or alternatively) be configured to amplify and transmit a plurality of carrier signals according to two or more different wireless communications standards. Further, while the various exemplary embodiments have been described in the context of cellular communications applications, those of ordinary skill in the art will appreciate and understand that the inventions disclosed and claimed herein are not necessarily limited to cellular communications applications. Hence, various changes, substitutions and alterations can be made without departing from the spirit and scope of the inventions as defined by the appended claims.