ANTENNA SYSTEM FOR ENHANCING THE COVERAGE AREA, RANGE AND RELIABILITY OF WIRELESS BASE STATIONS

Abstract: An active antenna system which has a plurality of antenna elements arranged in a column with each element or subarray of elements integrated with an amplifier and other beam forming components. A separate amplifier and filter are disposed immediately adjacent and connected to each of the antenna elements or a subarray of antenna elements and a separate combiner/divider is connected to each of the amplifiers. The antenna elements, amplifier and filter are disposed on a common support. A base station is connected by the feed cables and is remote from each amplifier. A first group of the antenna elements with low power amplifiers forms a transmitting antenna system and/or a second group of the active antenna elements with low noise amplifiers forms a receiving antenna system. A variable attenuator and a variable phase shift circuit can be integrated with each amplifier and can be used for beam shaping and electronic beam pointing. For diversity combining, spatially separated or polarization diverse active antennas are used. For polarization diverse active antennas, implementation involves a shared column or two colocated orthogonally polarized columns.

Description: ANTENNA SYSTEM FOR ENHANCING THE COVERAGE AREA, RANGE AND RELIABILITY OF WIRELESS BASE STATIONS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to an antenna system and, more specifically, to an antenna system primarily for use in conjunction with base station in mobile communications systems. BRIEF DESCRIPTION OF THE PRIOR ART Mobile communication systems generally include a base station for receiving and transmitting electromagnetic radiations with the mobile terminal disposed within the coverage area of the base station for transmitting electromagnetic radiations to and receiving such radiations from the base station and where several such base stations are generally linked together through base station controllers (BSCs) and master station controllers (MSCs) to provide a seamless communication link between a mobile terminal and its calling party. Mobile communications are typically embodied within two bands. Those systems between approximately 850 and 950 MHZ are referred to as cellular and those systems between approximately 1.8 and 2.0 GHz are referred as personal communication systems (PCSs) . The total mobile applications which together cover both bands are often referred to as personal communication networks (PCNs) . This invention relates to all PCN systems, i.e. systems which operate in either or both frequency bands listed above. The area, range and reliability of base stations are generally limited in their coverage area by the base station receive noise figure and the transmit effective isotropic radiated power (EIRP) . The presently used PCN base station architecture utilizes a vertical column array comprising a plurality of spaced apart radiating elements for transmission and a separate such plurality of radiating elements for reception. The antenna elements are generally disposed in a vertical straight line on a support, the distance between extreme antenna elements often being quite large, often a few meters. To improve performance, the receive antenna configuration generally comprises two widely separated columns to provide spatial diversity or a single orthogonally polarized column comprising two orthogonal polarization outputs to provide polarization diversity. The radiating elements are generally electrically conductive members disposed on a support and are generally spaced between three fourths and one wavelength apart. The antenna elements are generally connected to a combiner via short transmission lines. For the transmit configuration, the radiating element is fed by a ground based high power amplifier through a long cable, typically between 50 and 200 feet. The placement of the power amplifiers within the base station also requires increased power from the amplifiers to overcome the insertion loss from the feed cable as well as the combiner. For the receive configuration, the combiner output is fed to a filter/low noise amplifier (LNA) combination through either a short transmission line (as in PCS systems for mast mounted LNAs) or through a long cable (as in cellular systems for base station integrated LNAs. Dual redundant amplifiers are typically provided when mast mounted electronics are used to improve reliability at the expense of complexity. For the receive configuration, the effective noise increase contributed by the ohmic losses in the array combiner and feed cable (depending upon its length) are amplified by the low noise amplifier and thus contribute to the increase in the system noise figure. There has been a constant desire to improve the range and coverage area capability for individual base stations used in wireless communication systems of the type described above. One way to solve this problem has been to place a redundant pair of LNAs on top of the tower and connect the LNAs to a passive (i.e. antenna elements with a combiner) antenna column with a short transmission line and a switch. The combiner loss and the switch loss still limits the receive system noise figure. SUMMARY OF THE INVENTION The above noted problem is minimized and there is provided an improvement in the coverage area, range and reliability of the base station by improving the transmit efficiency and receive sensitivity of the base station without sacrificing reliability. These improvements enable extension of the communication coverage area for a given base station. Briefly, the above improvement is accomplished by substituting for the presently existing antenna designs for use in conjunction with mobile communication systems an active phased array antenna. An active phased array antenna approach in accordance with the present invention incorporates a low power amplifier (for transmit) and/or a low noise amplifier (for receive) as closely adjacent as possible to each element of an array, generally spaced by a few centimeters or less from the associated element. The antenna elements are generally disposed in a column. As an alternative, a filter and amplifier can be coupled to a subgroup of the elements of the array, though this arrangement would provide less versatility as will be evident from the discussion hereinbelow. The active antenna approach thus involves distributing a plurality of amplifiers and filters, when required, across the array aperture. The advantages of the active antenna system for transmit and receive configurations are briefly discussed below. For receive configurations, by maintaining the low noise amplifiers close to the radiating elements, system noise is reduced by as much as 4.5 dB over the conventional approach in which the LNA is integrated with the base station and by as much as 1.5 dB where an LNA is integrated with the passive antenna column at the tower top or the mast of the antenna system. Since a significant percentage of the effective noise degradation in the antenna system is a result of the loss in the feed cables and power combiners, it can be seen that, in the case of the receiving section of the active antenna system, the noise picked up by the feed cables is never amplified whereas this noise is amplified in the prior art system. Accordingly, a much smaller effective noise element arrives at the base station relative to the prior art system described above. Similarly, for transmit con igurations, the low power amplifiers, when integrated with the radiating elements, increase the EIRP by as much as 4.5 dB (for the same amplifier power output) over the conventional approach where the power amplifiers are integrated with the base station. The distributed nature of the amplifiers also improves reliability since the system can be designed to be fully compliant even after failure of one or more of the receive or transmit amplifiers. Even when sufficient failures occur to reduce overall system performance, the performance degradation is graceful rather than catastrophic. In the transmit configuration, using active antenna systems, noise in the feed cable amplified by the amplifiers is of no concern, since the signal power is several tens of dB higher than the noise power. The main advantage of active antenna systems for the transmit case is that, since the power amplifiers are placed between the antenna elements and the feed cable which is connected to the base station driver amplifier, there is no reduction in the amplified signal power due to the cable and combiner loss as is the case in the existing PCN base stations. Another advantage of distributed power amplifiers is that the amplifiers operate in a reduced thermal density environment, yielding enhanced reliability. The reduced thermal density results from the fact that the heat is distributed across the full area occupied by the antenna elements in the array rather than in a concentrated small area as in the case of unitary high power amplifiers used in the existing PCN base stations. Since the impact of lossy elements following low noise amplifiers or preceding low power amplifiers on the system noise figure or EIRP, respectively, is small, the active antenna system is capable of a great deal of versatility in that variable attenuators and/or phase shifters can be placed in the signal path of each antenna element. As is well known in the art, these variable attenuators and phase shifters can operate to provide electronic beam shaping and pointing capability. The active antenna systems of the present invention can be used for receive systems using either spatial or polarization diversity and in fact are valid independent of the diversity approach employed. Therefore, all of the advantages claimed for active antennas apply to all diversity receive systems. Furthermore, in a polarization diverse receive system, both orthogonal polarizations are implemented with each polarization port fed to its own filter/LNA network and combiner. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a diagram of a prior art PCN base station architecture for an antenna system for both transmission and reception of electromagnetic radiations; FIGURE 2 is a diagram of an active antenna system architecture in accordance with a first embodiment of the present invention; and FIGURE 3 is a diagram of an active antenna system architecture in accordance with a second embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGURE 1, there is shown a system utilizing the prior art PCN base station antenna architecture. FIGURE 1 is typical of the existing state of the art for PCS antenna systems. However, the discussion that follows applies to existing cellular systems as well. The system includes a transmit antenna system 1 and a receive antenna system 7. The transmit antenna system 1 includes a support 3 having thereon a plurality of radiating antenna elements 5 disposed in a straight line, the distance from the topmost element to the bottommost element being a few meters. The transmit antenna system 1 has a high power amplifier (HPA) 13 and a filter 15 disposed at the base station 17 with the amplifier/filter being connected to each of the radiating antenna elements 5 via a combiner 54 and a feed cable 31 with the length of the feed cable 31 varying anywhere between 15 and 70 meters, in general. As can be seen, the long cable 31 leads to a loss of 2 to 3 dB of power transmitted by the ground based power amplifier. The receive antenna system 7 includes a support 9 thereon a plurality of radiating antenna elements 11 disposed in a straight line with dimensions as in the transmit antenna system. The receive antenna system 7 has a pair of filters 19, ' 21, filter 19 coupled to a pair of amplifiers 23 and 25 and filter 21 coupled to a pair of amplifiers 27 and 29. Each radiating antenna element 11 is coupled to each of the filters 19 and 21 via power combiners 55 and 56 via feed cable 33 to provide vertical and horizontal outputs. Although the embodiment is shown for a receiving system with polarization diversity, the same discussion applies to the receiving system utilizing spatial diversity. For the purpose of illustration, , the filter/LNA combination is shown mounted on the mast, as is the common practice for PCS base stations. As can be seen, the ohmic losses in the combiners and short transmission lines between combiners and the filter/LNA combination contribute to the degradation in the system noise figure. Referring to FIGURE 2, there is shown an active antenna architecture in accordance with the present invention. The antenna is similar to that of FIGURE 1 except that all of the radiating antenna elements 5 and 11 are on the same support 35 and, rather than having a single filter 15 and amplifier 13 for the transmit section and the filter 19, 21 and associated amplifiers 23, 25, 27, 29 for the receive section as shown in FIGURE 1, each radiating antenna element has its own filter and amplifier positioned as closely adjacent to the antenna element as possible. This is shown in FIGURE 2 wherein, for the transmit portion, each antenna element 5 is connected to its own filter 37 and amplifier 39 with the feed cable 41 extending from the base station 43 to each of the amplifiers 39 through power divider 54. In the receive portion, each radiating antenna element 11 has two orthogonally polarized outputs as in FIGURE 1 with each output have its own filter 45 and amplifier pair 47 and 49. The outputs of the amplifiers 47 and 49 are combined in the power combiners 55 and 56 and are fed to the base station via feed cables 51 and 53 respectively. The receive configuration with polarization diversity is shown here for the purpose of illustration. However, the claim of active antenna application holds as well for the receive configuration with spatial or any other diversity as well. For the receive configuration, it should be understood that the positions of the amplifiers and associated filters can be reversed where amplification can take place prior to filtering or filtering can take place prior to amplification. When the filter is placed ahead of the amplifier, the impact of the filter loss on the system noise figure is reduced, but the amplifier stage must be designed to have high dynamic range, so that any expected interference can be passed by the amplifier to the filter without generating any significant intermodulation products. When the amplifier is placed ahead of the filter to provide out-of-band rejection of signals prior to in-band signal amplification (as shown in FIGURE 2) , the filter must have low power loss with implementation preferably performed in waveguides. Referring now to FIGURE 3, there is shown a variation of the architecture of FIGURE 2 wherein a variable phase shifter 51 and a variable attenuator 53 are placed in series with each combination of amplifier and filter of FIGURE 2. By varying the phase and amplitude, there is provided the ability to electronically tilt the beam and shape the beam in elevation, depending upon the traffic patterns and the topography of the cell being served. The elevation beam shaping and switching can be controlled dynamically by the service provider through a remote controller. A second embodiment of the invention comprises only the receive portion of the above described structure and a third embodiment of the invention comprises only the transmit portion of the above described structure. Though the invention has been described with respect to specific preferred embodiments thereof, many variations and modifications will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.

Claims: CLAIMS 1. An antenna system which comprises: (a) an active antenna which comprises: (i) a plurality of antenna elements; (ii) a separate one of at least one of a transmit or receive amplifier disposed immediately adjacent each of said antenna elements; and (iii) a separate combiner/divider connected to each of said amplifiers; and (b) a feed cable connecting said active antenna to a base station. 2. The system of claim 1 further including a separate filter connected to each said amplifier. 3. The system of claim 1 further including a support, said antenna elements and each said amplifier disposed on said support. 4. The system of claim 2 further including a support, said antenna elements, each said amplifier and each said filter disposed on said support . 5. The system of claim 1 wherein a first group of said plurality of antenna elements comprises a transmitting antenna system and a second group of said plurality of antenna elements comprises a receiving antenna system. 6. The system of claim 4 wherein a first group of said plurality of antenna elements comprises a transmitting antenna system and a second group of said plurality of antenna elements comprises a receiving antenna system. . The system of claim 5 wherein said transmitting antenna system and said receiving antenna system display a different type of diversity. 8. The system of claim 6 wherein said transmitting antenna system and said receiving antenna system display a different type of diversity. 9. The system of claim 5 wherein said transmitting antenna system and said receiving antenna system display spatial diversity. 10. The system of claim 6 wherein said transmitting antenna system and said receiving antenna system display spatial diversity. 11. The system of claim 5 wherein said transmitting antenna system and said receiving antenna system display polarization diversity. 12. The system of claim 6 wherein said transmitting antenna system and said receiving antenna system display polarization diversity. 13. The system of claim 3 further including at least one of a variable attenuator and a variable phase shift circuit connected to each said amplifier and disposed on said support. 14. The system of claim 4 further including at least one of a variable attenuator and a variable phase shift circuit connected to each said amplifier and disposed on said support. 15. The system of claim 6 further including at least one of a variable attenuator and a variable phase shift circuit connected to each said amplifier and disposed on said support. 16. The system of claim 3 further including a variable attenuator and a variable phase shift circuit connected to each said amplifier and disposed on said support. 17. The system of claim 4 further including a variable attenuator and a variable phase shift circuit connected to each said amplifier and disposed on said support. 18. The system of claim 6 further including a variable attenuator and a variable phase shift circuit connected to each said amplifier and disposed on said support. 19. The system of claim 8 further including a variable attenuator and a variable phase shift circuit connected to each said amplifier and disposed on said support. 20. The system of claim 10 further including a variable attenuator and a variable phase shift circuit connected to each said amplifier and disposed on said support. 21. The system of claim 12 further including a variable attenuator and a variable phase shift circuit connected to each said amplifier and disposed on said support.

Inventors: GILLILAND PAUL C, SANZGIRI SHASHIKANK M, VESPA ANTHONY J, VOLPI JOHN P, GILLILAND, PAUL, C., SANZGIRI, SHASHIKANK, M., VESPA, ANTHONY, J., VOLPI, JOHN, P.
Applicants: RAYTHEON TI SYST INC, RAYTHEON TI SYSTEMS, INC.
Application year: 1997
Main classification: H01Q23
IPC codes: H01Q3, H01Q21, H01Q23, H04B7