Patent Publication Number: US-6701137-B1

Title: Antenna system architecture

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of pending application Ser. No. 09/299,850, filed Apr. 26, 1999, now U.S. Pat. No. 6,583,763 and application Ser. No. 09/422,418, filed Oct. 21, 1999, now U.S. Pat. No. 6,597,325. 
    
    
     BACKGROUND OF THE INVENTION 
     Steered beam antenna systems have been used in defense electronics for radar systems, or for direction finding (DF) applications. These technologies have been making their way into commercial communications, for interference reduction and/or capacity enhancement. The generally accepted term in the latter industry is smart antennas; however, the term has been used to describe many different techniques and technologies. The earlier technologies were based on RF (radio frequency) beam steering, which used selection of one of a number of highly directional antennas. In these technologies, tower top antennas were typically completely passive, with the beams formed via Butler matrices, or by selecting antennas individually. The independent beam signals were then delivered to the base station via separate coaxial RF lines, with signal selection and RF switching performed at the base station. 
     Digitally adaptive systems, which might use any type of antennas at the tower top, and digital signal processing techniques (DSP) at the base station, have been tested and are slowly making their way into the commercial markets. However, most of these technologies are still based on using passive antennas at the tower top, bringing the RF signals from the tower to the base station via coaxial (RF) cables. The frequency conversion, digital conversion, and beamformer processing is then performed at the base station. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, an antenna system architecture is based on installing the RF electronics at the tower top, with the antenna or within the antenna housing. Other aspects of the antenna system architecture of the invention include: 
     Tower top electronics; 
     Distributed amplifier system; 
     Frequency and digital conversion at the tower top; 
     Antenna/array inputs/outputs are time division multiplexed; 
     Final multiplexed digital signal is converted to fiber optics; 
     Single or multiple fiber optic delivery cables for backhaul, or convert to microwave for backhaul. 
     Additionally, this approach allows for a basic split of functionalities, as follows: 
     RF signal processing is performed at the tower top; 
     Beamforming (DSP) and channel coding is performed at another location, such as: 
     a) at the bottom of the tower (base station) or BTS (Base Transceiver System); 
     b) at the MSC (Mobile Switching Center); or 
     c) at the CO (Central Switching Office). 
     This approach allows all processing and software, as well as digital hardware, to be installed at a single location, rather than distributed among various cell sites; which should reduce initial installation costs, as well as maintenance and upgrade costs. 
     Briefly, in accordance with the foregoing, an antenna system, for tower-top installation, comprises an antenna array comprising an array of M×N antenna elements, a corporate feed for operatively interconnecting said antenna elements with a backhaul link for communicating with ground-based equipment, and radio frequency circuits for processing radio frequency signals between said antenna array and said backhaul link, said radio frequency circuits including substantially all of the circuits required for the processing of radio frequencing signals between said array and said backhaul link. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a simplified schematic diagram, partially in block form, of a transmit only configuration for a generalized beamformer/smart antenna system; having tower top mounted electronics; 
     FIG. 2 is a functional block diagram of the components in FIG. 1, and corresponding base station mounted components; 
     FIG. 3 is a simplified schematic diagram, partially in block form, of a receive only configuration, for a smart antenna/beamforming subsystem; 
     FIG. 4 shows the same basic configuration as FIG. 3, but with a low noise amplifier (LNA) circuit/component at each antenna element; 
     FIG. 5 is a simplified schematic diagram, partially in block form, of a first configuration for a transmit/receive smart antenna/beamforming subsystem, 
     FIG. 6 shows a similar configuration to FIG. 5, except that the receive mode signals (uplink) are amplified, via an LNA, before summing in the corporate feed network; 
     FIG. 7 shows a basic system architecture, 
     FIG. 8 shows a system architecture for a system using a microwave backhaul link; 
     FIG. 9 is a simplified schematic diagram, partially in block form, of the tower top components for a “third generation” (3G) transmit mode antenna system;  15 FIG. 10 is a simplified schematic diagram, partially in block form, of the tower top components for a “third generation” (3G) receive mode configuration with a single LNA at the output of the corporate feed for each branch; 
     FIG. 11 is a simplified schematic diagram, partially in block form, of the tower top components for a “third generation” (3G) the receive mode configuration with an LNA on each antenna element, prior to the corporate feed network; 
     FIG. 12 is a simplified schematic diagram, partially in block form, of the tower top components for a “third generation” (3G) a transmit/receive mode configuration with a single LNA on each receive branch; and 
     FIG. 13 is a simplified schematic diagram, partially in block form, of the tower top components for a “third generation” (3G) a transmit/receive mode configuration with an LNA on each element, prior to the corporate feed network. 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
     Referring now to the drawings, FIG. 1 shows a transmitter system configuration  20  for a beamformer/smart antenna system, using tower-top mounted electronics for all of the RF circuits. The illustrated embodiment takes digital IF (intermediate frequency) signals (from an optical carrier or fiber optic cable  22 ), converts, at a fiber converter (FC)  24  from optical to a high speed digital signal and at a high speed time multiplexer (T-MUX)  26  de-multiplexes the high speed digital signal into M lower speed digital signals. The transmitter  20  next converts to analog via digital to analog converters (DAC)  28  and upconverts, at upconverters (UC)  30 , the analog IF signals to RF. The transmitter  20  then amplifies the signals via a distributed antenna approach, resulting in a beamformed collection of signals. This distributed antenna approach, in the embodiment illustrated in FIG. 1, comprises an M by N array of antenna elements  40 , such as patch/microstrip antenna elements, and a power amplifier (PA)  42  closely coupled to each of the antenna elements  40 , for example, at the feedpoint of each antenna element  40 . Thus, each of the upconverters  30  feeds one of M composite antennas, each comprising a total of N antenna elements. 
     In operation, after conversion from fiber (optical IF) to digital, at a selected data rate X, the high speed digital signal is de-multiplexed into M streams of digital signals, at data rates of X/M. These signals contain the digital beamforming weights and adjustments for phase and amplitude (determined and fixed at a central processing site-BTS, MSC, or CO). It will be noted that digital IF signals may be fed to/from the T-MUX by a twisted pair or coaxial cable rather than using a fiber optic cable and converter as shown in FIG.  1  and the below-described drawings. Also, a DC power cable/system for delivering DC power from the ground to the tower top has been omitted in the drawings for simplicity, but will be understood to be included in such systems. 
     The diagram of FIG. 1 shows M columns of N antenna elements forming an antenna array  45 , each connected via a series corporate feed network. Parallel corporate feed arrangements could also be used here and throughout the rest of the described embodiments hereinbelow. The corporate feed network could be microstrip, stripline, or RF coaxial cables. 
     Each antenna element  40  is fed with a power amplifier (PA) module  42 , in similar fashion to the active/distributed antenna architecture described in the above-referenced copending applications. 
     A common local oscillator (LO)  32  is used for all of the upconverters  30 , thus assuring coherent phase for each of the M paths. This LO  32  can be a fixed frequency crystal, or a synthesizer. 
     The fiber optic input(s)  22  to the fiber to digital converter (FC)  24  can be separate lines (e.g., multi-mode fiber), or a single line (e.g., single mode fiber). 
     FIG. 2 shows the tower-top components of FIG. 1 in functional block form (shown on the left hand side of FIG.  2 ), and (on the right side of FIG. 2) a ground-based central processing site (BTS, MSC or CO). In FIG. 2, voice and or data channels  50  are fed into a DSP block  52  which performs all channel processing (vocoder, code spreading/code division multiple access (CDMA), time multiplexing/time division multiple access (TDMA), equalization, etc.) and beamforming and/or spatial processing. This block  52  may be referred to as the “Common DSP Block”. It is a collection of DSP processors, programmed for each specific task (channel and spatial processing). The output from this block  52 , in either digital baseband (I&amp;Q—in phase and quadrature) or digital IF, is converted to an optical carrier via a digital fiber optic (OF) converter  54 . In one embodiment of the invention, this block  52  and the converter  54  can be located at the tower base (cell site) BTS, MSC, or CO (Central Office). 
     The fiber signals are then carried to the tower via a single cable or combination of multimode or singlemode fiber cables, indicated by reference numeral  22 . 
     FIG. 3 shows a receive-only system configuration, for a smart antenna/beamforming subsystem  120 . RF signals are received via an M×N array of antenna elements  140 , here shown as a collection of patch/microstrip elements. Each column in the array is summed via a series corporate feed, which could alternatively be a parallel corporate feed. In this particular configuration, the summed signals are amplified, via a low noise amplifier (LNA)  144 , after the corporate feed. After each signal is amplified, it is downconverted at a downconverter (DC)  160  to IF, and digitized by an analog to digital converter (ADC)  128 . The digitized signals are then time division multiplexed by a T-MUX  126 , into a single high speed digital signal, which is fed to a fiber converter (FC)  124 , which translates/modulates the high speed digital signal onto an optical carrier  122 . This carrier  122  may be a single, or multiple, fiber optic cables, for delivering signals to the BTS, MSC, or CO. Similar to the transmit mode (see FIG.  1 ), a common LO  132  is used to coherently translate all column/array signals from RF to IF. The systems of FIGS. 1 and 3 may be combined to form a transmit/receive system, which could in turn be combined with the ground-based components of FIG. 2 to define an antenna system architecture in accordance with one embodiment of the invention. 
     FIG. 4 shows the same basic architecture (a receive-only subsystem  120   a ) as FIG. 3, but with an LNA circuit/amplifier module  142  at each antenna element  140 . Thus the signals are amplified prior to being summed via the corporate feeds. This configuration may be more expensive, in terms of the costs of the additional LNA components, but will achieve increased sensitivity (lower system noise figure) since the signals are amplified prior to any losses in the corporate feed circuits. 
     FIG. 5 shows one embodiment of a transmit/receive smart antenna/beamforming subsystem  220 . This system utilizes a single LNA  244  for each branch (i.e., column of the M×N array), similar to the receive-only configuration of FIG.  3 . At each antenna element  240 , a frequency diplexer (D)  262  is used to separate the transmit and receive power, on separate frequency bands. The receive power is summed, via a series corporate feed (could be parallel), and fed to an LNA  244  at the bottom of each branch (column, i.e., of the M×N array). The amplified RF signals are then downconverted to IF at downconverters (DC)  260  and digitized at A/D converters  264 , and fed to the high speed T-MUX (time domain multiplexer)  226 . Similarly, transmit mode signals (from the BTS, MSC, or CO) are converted, de-multiplexed, digitized, and upconverted from IF to RF at FC  224 , T-MUX  226 , DACs  228  and UCs  230 . The converted signals are then distributed to the antenna elements, on each branch, via the corporate feed (series or parallel) and amplified (at each antenna element  240 ) by PAs  242 . The amplified signals pass through the frequency diplexer (D)  262  to the antennas  240  to be radiated into space. The same LO source  232  can be used for both the upconversion and downconversion operations, for all of the branches. 
     The fiber optic cables  222  thus carry digital IF on an optical carrier in both directions. This can be accomplished on a single OF (fiber optic) cable via wavelength division multiplexing, or on multiple OF cables, one (or more) for each path. 
     FIG. 6 shows a similar architecture to FIG. 5 for a transmit/receive system  220   a , except that the receive mode signals (uplink) are amplified by LNAs  244  at the antenna elements  240 , before summing in the corporate feed network. This is similar to the receive-only configuration of FIG.  4 . 
     FIG. 7 shows a basic architecture for the tower-top beamformer subsystem, for all of the embodiments of FIGS. 1-6. A panel antenna system  300 , with a fiber converter (FC)  324 , is shown with fiber optic transmission line(s) or cable(s)  322 . The subsystem  300  may include all of the components of any of the subsystems of FIGS. 1-6, up to the FC (fiber converter)  324 . The advantage of this arrangement is that all of the RF functionality is performed at a single location, i.e. at the tower top. This minimizes the lengths of RF transmission lines throughout the system. For example, there is no need to transmit RF back to the base station (BTS), MSC or CO  310 . This results in minimizing ohmic and power losses, as well as increasing the overall system performance (noise figure, etc.). This is also the part of the system that is most likely to remain static (i.e. not requiring performance-oriented changes as often). 
     The section of the beamforming system that will likely change, due to improved DSP availability and algorithms, software updates, etc. can be centralized in a single location  310  (e.g., BS/BTS, MSC, or CO). This section may include beamformer, digital signal processing (DSP) and channel processing components as indicated by reference numeral  352  in FIG.  7 . 
     At the other end of the fiber cable  322  is a fiber converter (FC)  354  to convert to digital IF, and a digital multiplexer  312 , which may be part of the base station  310 . The above-described arrangement allows all the high cost “digital processing” segment of the beamformer to be placed in a central location, to facilitate algorithm and software upgrades, as well as hardware (DSP) changes. 
     FIG. 8 shows an architectural approach for microwave backhaul link to replace the fiber connection  22  ( 122 ,  222 ,  322 ). All of the prior embodiments described the high-speed backhaul link being performed using fiber optic cable. However, currently many cell sites use microwave (2-40 GHz range) links for the trunking/backhaul, and this may be substituted for the fiber link shown in the above-described embodiments without departing from the invention. 
     In FIG. 8, on the top left, is a block  300  denoted as “RF circuits”. This encompasses the antenna elements, LNAs, PA&#39;s, corporate feed networks, RF upconverters and downconverters, as well as A/Ds and DACs shown in the above-described embodiments. The digital signal is then fed into a composite high speed digital T-MUX  326  (as shown in the previous embodiments). However, rather than feed the time division digitally multiplexed signals into a fiber converter, the signals are directly translated, at the tower top, by a microwave (MW) converter (transceiver)  313 , and amplified through a PA (power amplifier)  317 , fed through a microwave frequency diplexer (D)  321 , to a radiating backhaul antenna  323 . This backhaul antenna  323  is similar to a terrestrial microwave antenna, or LMDS (local multipoint distribution service) antenna system. Similarly received uplink microwave signals, from the antenna  323 , are fed back through the frequency diplexer (D)  321 , amplified via a microwave LNA  319 , and downconverted to digital IF (high speed), back to the high speed T-MUX  326 . 
     Optionally, the high speed digital multiplexed signals from the beamformer/smart antenna subsystem  320  could be fed to an intermediate modulator (MOD)  315  (shown in phantom line), that modulates the IF signals to a format more efficient for microwave transmission, and then fed to the microwave converter  313 . 
     FIGS. 9-13 are respectively similar to FIGS.  1  and  3 - 6 , however, FIGS. 9-13 show third generation PCS and UMTS (universal mobile telecommunications service) (3G) systems. Two standards, designated as CDMA- 2000  and W-CDMA, are currently being developed for use as the worldwide roaming or mobile (cellularized) systems for voice and data transport. While architecturally very similar to the diagrams in FIGS.  1  and  3 - 6 , FIGS. 9-13 differ in that they use a QPSK (quadrature phase shift keying) modulator and RF upconverter block, designated in FIGS. 9-13 as a 3G (third generation CDMA) modulator block  410  ( 510 ,  610 ). This block assumes digital baseband I &amp; Q on the input (or output). Therefore, digital baseband (I&amp;Q) signaling is embedded in the fiber optic signal, which is assumed to be time division multiplexed. 
     FIG. 9 shows a 3G transmit mode smart antenna/beamformer subsystem  420 . The digital multiplexed (baseband I &amp; Q) signals, carried on a high speed stream, are converted from fiber to digital at FC  424  and de-multiplexed at T-MUX  426  into M lower speed streams. The 3G modulator block  410 , on each branch, converts the signals from digital to analog, performs a QPSK modulation, spreads the carriers (via the appropriate CDMA spreading codes) and upconverts to RF. The rest of FIG. 9 is similar to FIG.  1 . Also, all 3GM blocks  410  use the same local oscillator  432  to coherently upconvert to all branches. 
     FIG. 10 shows a receive mode configuration  520 , with a single LNA  544  at the output of the corporate feed for each branch. A 3G modulator block  510  has been separated into two blocks, a “demodulator” (downconverter, CDMA code despreader, and QPSK demodulator)  560  and an A/D  564 . The digital baseband (I &amp; Q) outputs are then time division multiplexed at T-MUX  526 , and fed to the digital to fiber converter (FC)  524 , which sends the multiplexed digital baseband signals on a fiber carrier  522 . 
     FIG. 11 shows a second receive mode configuration  520 , with an LNA  544  at each antenna element  540 , prior to the corporate feed network, on each branch, and is otherwise the same as FIG.  10 . 
     FIGS. 12 and 13 shows two configurations  620 ,  620   a  for a transmit/receive 3G beamformer/smart antenna system, with a 3G modulator block  610 ,  612  on each path (2-Way) on each branch. FIG. 12 shows a configuration with a single LNA  644  on each receive branch. FIG. 13 shows a configuration with an LNA  644  at each antenna element prior to the corporate feed network. In FIGS. 12 and 13, components similar to those used in the above-described embodiments are designated by similar reference numerals with the prefix  6 . Also in FIGS. 12 and 13, the 3G modulator block  610  includes the components of both the 3G modulator blocks  410  and  510  of FIGS. 9 and 10, as described above. 
     While the systems of FIGS. 9-13 illustrate a fiber carrier  422 ,  522 , etc., each could alternatively use a microwave backhaul link of the type shown in FIG.  8 . 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.