Patent Publication Number: US-6906681-B2

Title: Multicarrier distributed active antenna

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to antenna systems used in the provision of wireless communication services and, more particularly, to an active antenna array adapted to be mounted on a tower or other support structure for providing wireless communication services. 
     BACKGROUND OF THE INVENTION 
     Wireless communication systems are widely used to provide voice and data communication between multiple mobile stations or units, or between mobile units and stationary customer equipment. In a typical wireless communication system, such as a cellular system, one or more mobile stations or units communicate with a network of base stations linked at a telephone switching office. In the provision of cellular services within a cellular network, individual geographic areas or “cells” are serviced by one or more of the base stations. A typical base station includes a base station control unit and an antenna tower (not shown). The control unit comprises the base station electronics and is usually positioned within a ruggedized enclosure at, or near, the base of the tower. The control unit is coupled to the switching office through land lines or, alternatively, the signals might be transmitted or backhauled through backhaul antennas. A typical cellular network may comprise hundreds of base stations, thousands of mobile stations or units and one or more switching offices. 
     The switching office is the central coordinating element of the overall cellular network. It typically includes a cellular processor, a cellular switch and also provides the interface to the public switched telephone network (PSTN). Through the cellular network, a duplex radio communication link may be established between users of the cellular network. 
     In one typical arrangement of a base station, one or more passive antennas are supported at the tower top or on the tower and are oriented about the tower to define the desired beam sectors for the cell. A base station will typically have three or more RF antennas and possibly one or more microwave backhaul antennas associated with each wireless service provider using the base station. The passive RF antennas are coupled to the base station control unit through multiple RF coaxial cables that extend up the tower and provide transmission lines for the RF signals communicated between the passive RF antennas and the control unit during transmit (“down-link”) and receive (“up-link”) cycles. 
     The typical base station requires amplification of the RF signals being transmitted by the RF antenna. For this purpose, it has been conventional to use a large linear power amplifier within the control unit at the base of the tower or other support structure. The linear power amplifier must be cascaded into high power circuits to achieve the desired linearity at the higher output power. Typically, for such high power systems or amplifiers, additional high power combiners must be used at the antennas which add cost and complexity to the passive antenna design. The power losses experienced in the RF coaxial cables and through the power splitting at the tower top may necessitate increases in the power amplification to achieve the desired power output at the passive antennas, thereby reducing overall operating efficiency of the base station. It is not uncommon that almost half of the RF power delivered to the passive antennas is lost through the cable and power splitting losses. 
     More recently, active antennas, such as distributed active antennas, have been incorporated into base station designs to overcome the power loss problems encountered with passive antenna designs. Typical distributed active antennas include one or more sub-arrays or columns of antenna elements with each antenna element having a power amplifier provided at or near the antenna element or associated with each sub-array or column of antenna elements. The array of elements may be utilized to form a beam with a specific beam shape or multiple beams. One example of a distributed active antenna is fully disclosed in U.S. Ser. No. 09/846,790, filed May 1, 2001 and entitled Transmit/Receive Distributed Antenna Systems, which is commonly assigned with the present application and the disclosure of which is hereby incorporated herein by reference in its entirety. 
     The power amplifiers are provided in the distributed active antenna to eliminate the high amplifying power required in cellular base stations having passive antennas on the tower. By moving the transmit path amplification to the distributed active antennas on the tower, the significant cable losses and splitting losses associated with the passive antenna systems are overcome. Incorporating power amplifiers at the input to each antenna element or sub-array mitigates any losses incurred getting up the tower and therefore improves antenna system efficiency over passive antenna systems. 
     One problem encountered with distributed active antennas is that if one or more power amplifiers fail on the tower, the antenna elements associated with those failed power amplifiers become non-functional. This results in a loss of radiated power for the distributed active antenna and also a change in the shape of the beam or beams formed by the antenna array. Until the failed power amplifiers are repaired or replaced, the beam forming characteristics of the distributed active antenna are altered or, depending on the extent of the failure, the antenna becomes non-functional. 
     Therefore, there is a need for a distributed active antenna that is less susceptible to failure of the power amplifiers associated with the antenna elements in the transmit path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a schematic block diagram of a distributed active antenna in accordance with one aspect of the present invention. 
         FIG. 2  is a schematic block diagram of a distributed active antenna in accordance with another aspect of the present invention. 
         FIG. 3  is a schematic block diagram of a predistortion circuit in accordance with the principles of the present invention for use in the distributed active antenna of FIG.  3 . 
         FIG. 4  is a schematic block diagram of an intermodulation generation circuit for use in the predistortion circuit of FIG.  3 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION 
     Referring now to the Figures, and to  FIG. 1  in particular, a distributed active antenna  10  in accordance with one aspect of the present invention is shown. The distributed active antenna  10  comprises a sub-array  14  of N transmit antenna elements  12  that are arranged in either a vertical or horizontal column, although other configurations of the transmit antenna elements  12  are possible as well without departing from the spirit and scope of the present invention. It will be understood that components of the receive antenna elements associated with the distributed active antenna are not shown for purposes of clarity and only the transmit components of the distributed active array are described herein. Those of ordinary skill in the art will readily appreciate the components of the receive antenna elements suitable for use in the distributed active antenna  10  of the present invention. 
     In this embodiment, each transmit antenna element  12  of the sub-array  14  is coupled to a respective power amplifier module  16  comprising a parallel combination of power amplifiers  18 . The number of transmit antenna elements  12  in the sub-array  14  can be scaled to achieve suitable size and antenna directivity. 
     Each parallel combination of power amplifiers  18  has inputs and combined outputs for driving the respective transmit antenna element  12  associated with each parallel combination of power amplifiers  18 . The inputs to each parallel combination of power amplifiers  18  are coupled to an M-way power splitter  24  and the outputs of each parallel combination of power amplifiers  18  are coupled to an M-way power combiner  26 . The number of power amplifiers  18  can be scaled to achieve the desired radiated output power for each element  12 . 
     Each transmit antenna element  12  is operatively coupled to one of the respective M-way power combiners  26 . The M-way power splitters  24  are coupled to an N-way common power splitter  28 . In one embodiment of the present invention, each power amplifier  18  comprises a multicarrier linear power amplifier although other power amplifiers are suitable as well without departing from the spirit and scope of the present invention. 
     In use of the distributed active antenna  10  during a transmit cycle, an RF signal is applied from the control unit (not shown) of the base station (not shown) to the N-way power splitter  28 . The N-way power splitter  28  splits the RF signal N-ways and applies the split RF signals to the M-way power splitters  24 . The M-way power splitters  24  associated with each transmit antenna element  12  further split the RF signals M-ways across the inputs of the parallel power amplifiers  18  and apply the split RF signals to the parallel combination of power amplifiers  18  associated with each transmit antenna element  12 . 
     Each power module  16  amplifies the split RF signals with the parallel combination of power amplifiers  18  and the amplified split RF signals are then combined by the M-way power combiner  26  at the outputs of the parallel combination of power amplifiers  18 . Each transmit antenna element  12  forms a beam by transmitting the combined amplified RF signal. 
     The parallel combination of power amplifiers  18  associated with each transmit antenna element  12  provides several advantages. First, the power required to drive each transmit antenna element  12  is less than for a passive antenna design because amplification of the RF signal is performed on the tower at or near each transmit antenna element  12 . The reliability of the distributed active antenna  10  is improved because a failure of one or more power amplifiers  18  only decrements the output power by a small amount so the operating performance of the distributed active array  10  is not significantly degraded. In an N antenna element array with M power amplifiers  18  per antenna element, the loss of power in response to a power amplifier failure is approximately given by: 
       Δ   =     10   ·     log   ⁡     (     1   -     κ     N   ·   M         )             
 
where “k” is the number of amplifier failures. In addition, because the required output power of each power amplifier  18  is low, the power amplifier can be chosen to be small, inexpensive and simple to implement.
 
       FIG. 2  illustrates a distributed active antenna  30  in accordance with another aspect of the present invention and is similar in configuration to the distributed active antenna  10  of  FIG. 1 , where like numerals represent like parts. In this embodiment, linearization of the signals at the transmit antenna elements  12  is provided by predistortion circuits  32  that are each operatively coupled to the M-way power splitter  24  associated with each transmit antenna element  12 . Power amplifiers, such as multi-carrier power amplifiers, generate undesired intermodulation (IM) products in the signal which degrade the signal quality. As will be described in detail below, the predistortion circuits  32  are operable to reduce or eliminate the generation of intermodulation distortion at the outputs of the transmit antenna elements  12  so that a linearized output is achieved. 
     Referring now to  FIG. 3 , each predistortion circuit  32  receives an RF carrier signal from the N-way power splitter  28  at an input  34  of the predistortion circuit  32 . Along the top path  36 , the carrier signal is delayed by a delay circuit  38  between the input  34  and an output  40 . Part of the RF carrier signal energy is coupled off at the input  34  for transmission through a bottom intermodulation (IM) generation path  42 . An adjustable attenuator  44  is provided at the input of an intermodulation (IM) generation circuit  46  to adjust the level of the coupled RF carrier signal prior to being applied to the intermodulation (IM) generation circuit  46 . 
     The intermodulation (IM) generation circuit  46  is illustrated in FIG.  4  and includes a 90° hybrid coupler  48  that splits the RF carrier signal into two signals that are applied to an RF carrier signal path  50  and to an intermodulation (IM) generation path  52 . In the RF carrier signal path  50 , the RF carrier signal is attenuated by fixed attenuator  54  of a sufficient value, such as a 10 dB attenuator, to ensure that no intermodulation products are generated in amplifier  58 . The signal is further phase adjusted by variable phase adjuster  56 . The attenuated and phase adjusted RF carrier signal is amplified by amplifier  58 , but do to the attenuation of the signal, the amplifier  58  does not generate any intermodulation (IM) products at its output so that the output of the amplifier  58  is the RF carrier signal without intermodulation (IM) products. The RF carrier signal in the RF carrier signal path  50  is attenuated by fixed attenuator  60  and applied to a second 90° hybrid coupler  62 . 
     Further referring to  FIG. 4 , in the intermodulation (IM) generation path  52 , the RF carrier signal is slightly attenuated by a fixed attenuator  64 , such as a 0-1 dB attenuator, and then applied to an amplifier  66 . The amplifier  66  has a similar or essentially the same transfer function as the transfer function of the power amplifiers  18  coupled to the transmit antenna elements  12  and so will generate the similar or essentially the same third, fifth and seventh order intermodulation (IM) products as the power amplifiers  18  used in the final stage of the transmit paths. This insures that characteristics between the IM products of the predistortion circuit are correlated to the amplifier module IM products and characteristics. The amplifier  66  amplifies the RF carrier signal and generates intermodulation (IM) products at its output. The amplified RF carrier signal and intermodulation (IM) product are then applied to a variable gain circuit  68  and a fixed attenuator  70 . The phase adjustment of the RF carrier signal by the variable phase adjuster  56  in the RF carrier signal path  50 , and the gain of the RF carrier signal and intermodulation (IM) products by the variable gain circuit  68  in the intermodulation (IM) generation path  52 , are both adjusted so that the RF carrier signal is removed at the summation of the signals at the second hybrid coupler  62  and only the intermodulation (IM) products remain in the intermodulation (IM) generation path  52 . 
     Referring now back to  FIG. 3 , the intermodulation (IM) products generated by the intermodulation (IM) generation circuit  46  of  FIG. 4  are amplified by amplifier  72  and then applied to a variable gain circuit  74  and variable phase adjuster  76  prior to summation at the output  40 . The RF carrier signal in the top path  36  and the intermodulation (IM) products in the intermodulation (IM) generation path  42  are 180° out of phase with each other so that the summation at the output  40  comprises the RF carrier signal and the intermodulation (IM) products 180° out of phase with the RF carrier signal. 
     The combined RF carrier and intermodulation (IM) products signal is applied to the parallel combination of power amplifiers  18  coupled to each transmit antenna element  12  at the final stages of the transmit paths so that the RF carrier signal is amplified and the intermodulation (IM) products at the output of the power amplifiers  18  are cancelled. 
     Further referring to  FIG. 3 , a carrier cancellation detector  78  is provided at the output of the intermodulation (IM) generation circuit  46  to monitor for the presence of the RF carrier signal at the output. If the RF carrier signal is detected, the carrier cancellation detector  78  adjusts the variable phase adjuster  56  and the variable gain circuit  68  of the intermodulation (IM) generation circuit  46  until the RF carrier signal is canceled at the output of the intermodulation (IM) generation circuit  46 . An intermodulation (IM) cancellation detector  80  is provided at the output of each parallel combination of power amplifiers  18 . If intermodulation (IM) products are detected, the intermodulation (IM) cancellation detector  80  adjusts the variable gain circuit  74  and variable phase adjuster  76  in the bottom intermodulation (IM) generation path  42  until the intermodulation (IM) products are canceled at the outputs of each parallel combination of power amplifiers  18 . In this way, the predistortion circuits  32  suppress generation of intermodulation (IM) products by the power amplifiers  18  so that the outputs of the transmit antenna elements  12  are linearized. 
     While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.