Abstract:
An antenna assembly for a cellular telecommunications system has an antenna having an array ( 1 ) of radiating elements, for example patches, situated in front of a panel ( 2 ) having an electrically conductive face. An amplifier ( 6 ) for amplifying RFS signals received by the antenna is situated on the opposite side of the panel ( 2 ) from the radiating elements. The amplifier is situated at least partially within the signal shadow cast by the panel ( 2 ) so that the latter shields the radiating elements from the amplifier. The amplifier can thus be situated close to the antenna without adversely affecting the performance of the latter.

Description:
FIELD OF THE INVENTION 
   This invention relates to an antenna assembly for a cellular telecommunications system and to an RF signal amplifier for such an assembly. 
   BACKGROUND TO THE INVENTION 
   A base station of a radio cellular telecommunications system is connected to one or more amplifier assemblies via which the base station receives signals from and transmits signals (from the network) to mobile units within the range of the base station. Many such antenna assemblies will include (in the antenna radome) phase shifters for altering the antenna beam tilt. Due to handset power and battery life restrictions on the mobile units, the signals received from the mobile units can be of a very low intensity. In previous generations of cellular mobile telephone systems, mast head amplifiers were occasionally required in order to boost the signals received by the antennas. 
   However, during the deployment of the current 3G UMTS telecommunications system it has become apparent that an amplifier is required at each antenna. A mast head amplifier needs to include various high frequency filters which take the form of filter cavities. As a result, mast head amplifiers tend to be relatively large metal objects, which, if placed too close to the radiating elements of the antenna arrays, could cause pattern interference. 
   Consequently, a mast head amplifier is conventionally mounted in a position spaced from the radome which contains the radiating elements of the antenna. Thus the mast head amplifier needs to be provided with its own weather proofing, and therefore considerably adds to the size and cost of the mast head amplifier and antenna assembly. In addition, the increased size of the assembly can lead to possible problems in obtaining planning permission for erecting the apparatus. 
   Furthermore, the loses associated with the cabling used to connect the mast head amplifier to the antenna radome and the radome ports to be the phase shifter can degrade the received signal. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, there is provided an antenna assembly for a cellular telecommunications system, the assembly comprising an antenna having an array of radiating elements for transmitting and/or receiving RF signals, a panel having an electrically conductive face, said panel being situated adjacent to the radiating elements, and an amplifier for amplifying RF signals received by the antenna, wherein the amplifier is situated on the opposite side of the panel from the radiating elements so that said panel shields the radiating elements from the amplifier. 
   Preferably the panel provides a back plane for the amplifier. 
   Since the amplifier is shielded by the plate/back plane, it can be situated in close proximity to the antenna, thus enabling the assembly to be of relatively compact construction. 
   Preferably, the antenna is contained within a radome, which also contains the amplifier. 
   The arrangement avoids the need for jumper cables to connect the amplifier to sockets to the base of the radome, since instead the amplifier can be connected directly to the antenna or its phase shifter in the radome and the radome RF input/output ports can constitute the input/output ports for the amplifier. Thus the insertion losses usually associated with such cables are also avoided. 
   Preferably, the antenna is situated wholly within an area bounded by a perpendicular projection from the panel&#39;s perimeter. Consequently, the amplifier is situated in the signal shadow of the antenna assembly (cast by the back plane) so that the amplifier does not affect the antenna beam pattern. 
   This enables the amplifier to be situated very close to the antenna, thus further facilitating the compact construction mentioned above. 
   Preferably, the panel of the antenna is elongate, and the amplifier is contained in an elongate amplifier housing, the elongate axes of the panel and the amplifier housing being substantially parallel. 
   Preferably, both the panel and the amplifier housing are rectangular, when viewed in front elevation. 
   Preferably, the amplifier housing is of the same width as the panel. This enables the use of the available area provided by the shadow of the antenna to be maximised. 
   Preferably, the array of radiating elements comprises an array of patch assemblies. 
   It has been found at patch assemblies enable the antenna to be of relatively compact construction, and more particularly to be considerably less deep than antennas which use other types of radiating element. Consequently, situating the amplifier behind the panel does not require a significantly deeper radome than is used in conventional antenna assemblies. 
   The antenna assembly may to advantage also include a remotely operable beam tilting device, also situated in the housing, the device being operable to enable the beam tilt of the array of radiating elements to be remotely adjusted. 
   Preferably, this device is also located behind the panel, adjacent to the amplifier. 
   This enables the antenna phase shifter to be connected to the amplifier through one or more relatively short links/cables. In conventional MHA/Antenna configurations there can be up to 1 dB insertion loss between the output of the MHA and the inputs of the phase shifter. This extra insertion loss results in the degradation of the noise figure of the MHA/antenna system. 
   The invention enables these insertion losses to be reduced or eliminated. 
   In conventional Antenna/Mha configurations there can also be problems with the build up of return losses of each component. For a standard configuration the component return losses are typical built up as shown below. 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               MHA 
               −18 dB 
             
             
                 
               Antenna 
               −15 dB 
             
             
                 
               Jumper Cable 
               −25 dB 
             
             
                 
                 
             
           
        
       
     
   
   In the worst case there can be losses of −12.9 dB which can cause alarms in the base station to protect amplifiers from large reflections. This poor return loss caused extra insertion loss due to mismatch further degrading system performance. 
   The invention allows the system to be tuned for better than −15 dB performance, a performance that could only be achieved by specifying excessively high component performance in a conventional configuration. 
   Preferably, the beam tilting device comprises a phase shifter for controlling the relative phases of signals fed to or received from the radiating elements. 
   Preferably, the phase shifter is an electromechanical device having a motor for adjusting the phase shifter to alter said relative phases. 
   Preferably, the gain of the amplifier and said beam tilting device are controlled by a common control module which controls both components in response to control signals from a remote location. 
   The common control module can be such as to enable the control signals for both the amplifier and phase shifter to be modulated on the same RF carrier at any one time. Thus the control signals can be supplied from the base station to the antenna assembly via one of the RF feeds connecting those two components. 
   In known systems, the separate amplifier or an antenna has its own control module, which is supplied with control signals by a dedicated control line. The phase shifter of such an arrangement also has a control module, which receives control signals via the RF feed from the base station. Such signals are modulated onto a carrier frequency. By having a common control module for both the amplifier and the phase shifter, it is possible to modulate control signals for both devices onto a carrier supplied through the RF feed. 
   A common integrated control module enables more devices to be accommodated on the antenna line. The number of separate addresses for the control signal is limited to 32. By controlling more than one device from one control module (corresponding to a respective address) the number of devices that can be supported is increased. 
   The common control module further reduces the possibility of address conflict and masking since, in a system having a plurality of such antenna assemblies, the number of control modules communicating (for example using RS 485 protocol) along a serial signal bus is lower than would be the case if each phase shifter and mast head amplifier had its own respective control module. 
   The invention also lies in an RF amplifier for use as a mast head amplifier in a cellular radio-telecommunications systems (preferably a 3G UMTS network), the amplifier being so sized and shaped as to fit within an antenna radome. To that end the amplifier housing preferably is of a length less than 450 mm, a width less than 130 mm and thickness less than 220 mm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the accompanying drawings in which 
       FIG. 1  is an exploded isometric view of an antenna assembly in accordance with the invention; 
       FIG. 2  is an isometric view of the exterior of the assembly; 
       FIGS. 3 ,  4  and  5  are cut-a-way views, in plan, side elevation and from one end (the lower end) respectively of the antenna assembly; 
       FIG. 6  is a block diagram of the components of the assembly; and 
       FIG. 7  shows an integrated mast head amplifier and control module forming part of the assembly. 
   

   DETAILED DESCRIPTION 
   With reference to  FIGS. 1-5 , the antenna assembly comprises a radiating patch array  1  mounted on a base plate  2  through perpendicular spacers such as spacer  4  to define a gap that extends along the length of the assembly and that accommodates a mast head amplifier  6 , a phase shifter assembly  8  and an actuating motor  10  for operating the phase shifter assembly  8 . 
   All the aforesaid components are contained in an elongate radome  12  which is open at its bottom end  14  to allow for the insertion of the components and is sealed at its top by means of a cap  16 . 
   The radiating patch assembly  1  is of the kind currently sold by the applicants, and is substantially as described in the UK patent specification no. GB 2364175B. The assembly thus comprises a linear array of radiating patch sub-assemblies, for example sub-assembly  18 , mounted via dielectric spacers on a panel  20 . The upper surface of the panel  20  is coated in copper, and the lower surface of the panel has a feed/reception network of transmission lines for use in connecting each patch sub-assembly to a respective feed port on the phase shifter assembly  8 . Each patch sub-assembly comprises an upper and a lower circular panel which are held in spaced relationship with each other by a central spacer element for example the element  22 , in the form of a dielectric column extending perpendicularly between the two circular panels (and protruding through a central hole in the upper circular panel). 
   It will be appreciated that the radiating patch array  1  is operable to send and receive signals in two polarities. Accordingly, the amplifier  6  has two input ports  24  and  26  each for a respective polarity of transmitted and received signals, and which protrude from a bottom end plate  5  constituted by a perpendicular projection of an end of the base plate  2 . 
   The phase shifter assembly  8  comprises a pair of microstrip antenna phase shifters, one for each respective polarity of signals sent/received by the radiating patch assembly  1 . Each phase shifter has an input feed  28  and  30  for connection to a respective RF input/output port  32  and  34  ( FIG. 6 ) of the amplifier  6 . Each microstrip phase shifter also has ten output feeds, each for connection to a respective patch sub-assembly (via the feed/reception network of transmission lines on the panel  20 ). One example of such an output feed shown at  36 . 
   The relative phases of signals at these feeds are controlled by means of a common dielectric slider  38  which is slideably mounted between the two phase shifters and is connected to the motor  10  by means of a worm drive  40 , although the motor&#39;s is shown as spaced from the drive  40  in the drawings for the sake of clarity. Consequently, the motor  10  controls the linear position of the slider  38 , and hence the relative phases of the signals (transmitted or received) at the output feed of the phase shifter assembly  8 . 
   The linear position of the slider  38  and the angular position of the output shaft of the motor  10  are monitored by means of an opto-electronic feedback system. The feedback system uses a series of LEDs and photo-transistors, collectively denoted by reference numeral  42  in  FIG. 6 , in a housing (not shown) which are connected to the phase shifter assembly by means of fibre optic cables (also not shown). 
   The form and function of the phase shifter assembly  8 , the motor  10  and the feedback system are as described in the applicant&#39;s existing PCT Patent Application No. PCT/EP2004/006054, the contents of which are incorporated herein by reference. 
   The components of the amplifier  6  are contained within a housing  44  which is generally rectangular in plan (and also in side and end elevation). 
   In order to fit within the antenna assembly, the amplifier  44  is more narrow than a conventional mast head amplifier. However, in order to accommodate the necessary components of the amplifier the housing  6  is longer than that of a conventional mast head amplifier. More specifically, the amplifier housing  44  is 130 mm wide, 46 mm thick and 442 mm long. 
   As can be seen from  FIG. 3 , the width of the housing  44  is slightly less than that of the panel  20 , so that the amplifier  6  is situated wholly behind the panel  20 . As a result, the amplifier  6  can be situated close to the rear of the panel  20  without extending into any path of signals transmitted or to be received by the antenna assembly. The amplifier  6  is thus within the ‘shadow’ or footprint of the panel  20 . 
   With reference to  FIG. 6 , the amplifier  6  includes amplification circuitry  46  (shown in more detail in  FIG. 7 ) and also incorporates a common control module  48  connected to the feedback system  42  and motor  10  via ports  50  and  52  respectively. The control module  48  is also connected to a current injector level  1  converter (Ciloc)  54 . This converter is connected to the RF port  26  and includes a DC module  56  that taps off DC power from the RF line for powering the amplifier circuitry  46  and the motor  10 . The Ciloc  54  also includes a communications modem  58  that decodes serial control signals modulated onto a carrier signal on the RF line connected to the port  26  (by an equivalent device at the base station end of the RF feeder cable). The modem  58  converts this type of communication to a digital serial communication to the control module  48 . The control communication conveys information about the desired down tilt of the antenna assembly and gain used by the amplifier circuitry  46  to amplify the RF signals received by the radiating patch array  1 . 
   As can be seen from  FIG. 7 , the amplifier circuitry  46  defines two sets of components, referenced  60  and  62 , one for each polarity of signal. The sets of components  60  and  62  are identical to each other, and only the set  60  will therefore only be described. Three low noise amplifiers  64 ,  66  and  68 , are connected in series between two Rx band pass filters  70  and  72 . The filters pass signals in the band 1920-1980 MHz, whilst rejecting signals outside that band. A bypass connector  74  is also connectable between the filters  70  and  72  to bypass the amplifiers  64 ,  66  and  68 . Such connection is achieved by means of electronic switches (not shown), and the bypass connection is closed in the event of a failure in the power supply to the amplifiers. In that case, received signal (albeit reduced magnitude due to insertion losses of less than 3 dB) will still be conveyed from the terminal  32  to the terminal  24  even if the amplifiers are unable to boost or pass any signal as the result of the power failure. 
   The circuitry also includes a transmission path, generally indicated as  76  in which there is provided a transmission filter  78  which is a band pass filter passing signals in band 2110-2170 MHz. 
   The gain levels in the amplification the signals received at the terminal  32  can be changed by switching on a variable number of the amplifiers  64 ,  66  and  68 . Each amplifier has a gain of 12 dB, so that, in 12 dB mode only one of the amplifiers is used, in 32 dB mode or 36 dB mode all three of the amplifiers are used. Any amplifier which is not switched on will be bypassed by bypass circuitry (not shown), and the activation of the amplifiers is controlled by means of the module  48 . 
   In conventional arrangements, the beam tilt data for controlling the phase shifter is modulated on a carrier of specific frequency 2.1 KHz on the signal supplied to the control module for the phase shifter. This is then de-modulated for conversion back to RS485 format, in which there will be a command set including command such get-tilt, set-tilt, calibrate etc. In the conventional arrangements, the mast head amplifier would be controlled (via a dedicated control line) by means of a similar command set except gain would replace tilt so as to give get-gain and set-gain. Thus, in the known systems, the mast head amplifier and the phase shifter would appear to the control software as two different devices which have different addresses and identities. In the presently described arrangement, however, the antenna assembly appears as a single device with a number of extra parameters to control, by means of the control signal modulated onto the carrier fed to the terminal  26 . 
   Thus the control signal will have an address code identifying the control module (and hence the phase shifter and amplifier) and will convey data on the required tilt of the antenna beam (and hence whether any movement of the slider  38  is required) and the necessary gain for the amplifier (i.e. the number of the low noise amplifiers that need to be operated). These signals are defined by the AISG protocols or similar. 
   A further advantage of the invention is that the amplifier  6  is positioned very close to the input feeds  28  and  30  of the phase shifter assembly  8  so that only relatively short lengths of cable are required to connect the amplifier to the phase shifter. 
   Furthermore, various site specific information such as sector, bearing and site location which are relevant to the operation of the phase shifter and the amplifier only have to be programmed into one control module, thus facilitating the set-up of the assembly.