Abstract:
There is disclosed a system for determining the impedance match of an antenna, for use in an RF transceiver comprising an RF transmit path comprising circuitry capable of amplifying an RF signal and transmitting the amplified RF signal to the antenna coupled to the RF transceiver. The system comprises: 1) a test signal generator coupled to the RF transmit path for generating a first test signal and injecting the first test signal into the RF transmit path, wherein the RF transmit path amplifies the first test signal; 2) an RF mixer coupled to an output of the RF transmit path for down-converting the amplified first test signal to produce a second test signal at a frequency within a receive frequency band of the RF transceiver; 3) an RF coupler coupled to an output of the RF mixer for transmitting the second test signal into the antenna and for receiving a reflection of the second test signal; and 4) a first signal monitor coupled to the RF coupler for measuring the reflection of the second test signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to that disclosed in U.S. patent application Ser. No. 09/475,604, filed Dec. 30, 1999, entitled “COMBINED SYSTEM FOR CALIBRATING RECEIVER GAIN AND MEASURING ANTENNA IMPEDANCE MATCH AND METHOD OF OPERATION.” U.S. patent application Ser. No. 09/475,604 is commonly assigned to the assignee of the present invention. The disclosure of the related patent application is hereby incorporated by reference in the present application as if fully set forth herein. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to wireless communications systems and, more specifically, to a system for measuring receiver antenna impedance match in a base station in a wireless network. 
     BACKGROUND OF THE INVENTION 
     In order to increase the number of subscribers that can be serviced in a single wireless network, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the greater number of base transceiver stations increases infrastructure costs. To offset this increased cost, wireless service providers continually implement any improvements that may reduce equipment costs, maintenance and repair costs, and operating costs, or that may increase service quality and reliability, and the number of subscribers that the cellular system can service. 
     Wireless service providers use a variety of test equipment to monitor the performance of the RF receiver and the RF transmitter of a base transceiver station (BTS). The test equipment may monitor a variety of signal parameters in the RF transmitter, including adjacent channel power ratio (ACPR), spectral purity (including in-band and out-of-band spurious components), occupied bandwidth, RHO, frequency error, and code domain power. The test equipment may also perform a variety of test functions in the RF receiver, including receive antenna impedance matching and receiver calibration. Preferably, the signal parameters are remotely monitored from a central location, so that a wireless service provider can avoid the expense of sending maintenance crews into the field to test each BTS individually. Additionally, a remote monitoring system can detect the failure of an RF transmitter or an RF receiver nearly instantaneously. 
     Unfortunately, adding some types of test equipment, such as spectrum analyzers, to a BTS significantly increases the cost of the BTS. In some cases, the cost of the test equipment may be greater than the cost of the BTS itself. As a result, wireless service providers frequently do not install test equipment in base transceiver stations, or install only a limited amount of test equipment to test only some of the functions of the BTS. The remaining functions must be monitored by maintenance crews using portable test equipment. 
     There is therefore a need in the art for inexpensive test equipment that may be implemented as part of the base station. In particular, there is a need for integrated test equipment that can perform reuse some of the existing circuitry in a base transceiver station. More particularly, there is a need for integrated test equipment that can be used to measure the impedance match of the receiver antenna. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a system for determining the impedance match of an antenna for use in an RF transceiver comprising an RF transmit path comprising circuitry capable of amplifying an RF signal and transmitting the amplified RF signal to the antenna coupled to the RF transceiver. In an advantageous embodiment of the present invention, the system comprises: 1) a test signal generator coupled to the RF transmit path and capable of generating a first test signal and injecting the first test signal into the RF transmit path, wherein the RF transmit path amplifies the first test signal; 2) an RF mixer coupled to an output of the RF transmit path and capable of down-converting the amplified first test signal to produce a second test signal at a frequency within a receive frequency band of the RF transceiver; 3) an RF coupler coupled to an output of the RF mixer and capable of transmitting the second test signal into the antenna and capable of receiving a reflection of the second test signal; and 4) a first signal monitor coupled to the RF coupler capable of measuring the reflection of the second test signal. 
     According to one embodiment of the present invention, the test signal generator generates the first test signal at a center frequency of a transmit frequency band of the RF transceiver. 
     According to another embodiment of the present invention, the second test signal is at a center frequency of the receive frequency band. 
     According to still another embodiment of the present invention, the system further comprises a second signal monitor coupled to the RF mixer and capable of determining at least one parameter of the second test signal. 
     According to yet another embodiment of the present invention, the system further comprises a signal splitter coupled to the RF mixer output capable of generating the second test signal on a first split output and a second split output signal on a second split output, wherein the second split output signal is substantially equal to the second test signal. 
     According to a further embodiment of the present invention, the second signal monitor monitors the second split output signal to determine the at least one parameter of the second test signal. 
     According to a yet further embodiment of the present invention, the RF coupler is coupled to, and receives the second test signal from, the second split output of the signal splitter. 
     According to a still further embodiment of the present invention, the system further comprises a controller capable of comparing at least one parameter of the reflection of the second test signal with the at least one parameter of the second test signal. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
     FIG. 1 illustrates an exemplary wireless network according to one embodiment of the present invention; 
     FIG. 2 illustrates in greater detail an exemplary base station in accordance with one embodiment of the present invention; 
     FIG. 3 illustrates in greater detail an exemplary RF transceiver in accordance with one embodiment of the present invention; and 
     FIG. 4 illustrates an exemplary flow diagram in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 through 4, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged wireless network. 
     FIG. 1 illustrates an exemplary wireless network  100  according to one embodiment of the present invention. The wireless telephone network  100  comprises a plurality of cell sites  121 - 123 , each containing one of the base stations, BS  101 , BS  102 , or BS  103 . Base stations  101 - 103  are operable to communicate with a plurality of mobile stations (MS)  111 - 114 . Mobile stations  111 - 114  may be any suitable cellular devices, including conventional cellular telephones, PCS handset devices, portable computers, metering devices, and the like. 
     Dotted lines show the approximate boundaries of the cells sites  121 - 123  in which base stations  101 - 103  are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites may have other irregular shapes, depending on the cell configuration selected and natural and man-made obstructions. 
     In one embodiment of the present invention, BS  101 , BS  102 , and BS  103  may comprise a base station controller (BSC) and a base transceiver station (BTS). Base station controllers and base transceiver stations are well known to those skilled in the art. A base station controller is a device that manages wireless communications resources, including the base transceiver station, for specified cells within a wireless communications network. A base transceiver station comprises the RF transceivers, antennas, and other electrical equipment located in each cell site. This equipment may include air conditioning units, heating units, electrical supplies, telephone line interfaces, and RF transmitters and RF receivers, as well as call processing circuitry. For the purpose of simplicity and clarity in explaining the operation of the present invention, the base transceiver station in each of cells  121 ,  122 , and  123  and the base station controller associated with each base transceiver station are collectively represented by BS  101 , BS  102  and BS  103 , respectively. 
     BS  101 , BS  102  and BS  103  transfer voice and data signals between each other and the public telephone system (not shown) via communications line  131  and mobile switching center (MSC)  140 . Mobile switching center  140  is well known to those skilled in the art. Mobile switching center  140  is a switching device that provides services and coordination between the subscribers in a wireless network and external networks, such as the public telephone system. Communications line  131  may be any suitable connection means, including a T 1  line, a T 3  line, a fiber optic link, a network backbone connection, and the like. In some embodiments of the present invention, communications line  131  may be several different data links, where each data link couples one of BS  101 , BS  102 , or BS  103  to MSC  140 . 
     In the exemplary wireless network  100 , MS  111  is located in cell site  121  and is in communication with BS  101 , MS  113  is located in cell site  122  and is in communication with BS  102 , and MS  114  is located in cell site  123  and is in communication with BS  103 . The MS  112  is also located in cell site  121 , close to the edge of cell site  123 . The direction arrow proximate MS  112  indicates the movement of MS  112  towards cell site  123 . At some point, as MS  112  moves into cell site  123  and out of cell site  121 , a “handoff” will occur. 
     As is well known, the handoff procedure transfers control of a call from a first cell to a second cell. For example, if MS  112  is in communication with BS  101  and senses that the signal from BS  101  is becoming unacceptably weak, MS  112  may then switch to a BS that has a stronger signal, such as the signal transmitted by BS  103 . MS  112  and BS  103  establish a new communication link and a signal is sent to BS  101  and the public telephone network to transfer the on-going voice, data, or control signals through BS  103 . The call is thereby seamlessly transferred from BS  101  to BS  103 . An “idle” handoff is a handoff between cells of a mobile device that is communicating in the control or paging channel, rather than transmitting voice and/or data signals in the regular traffic channels. 
     FIG. 2 illustrates in greater detail exemplary base station  101  in accordance with one embodiment of the present invention. Base station  101  comprises base station controller (BSC)  210  and base transceiver station (BTS)  220 . Base station controllers and base transceiver stations were described previously in connection with FIG.  1 . BSC  210  manages the resources in cell site  121 , including BTS  220 . BTS  220  comprises BTS controller  225 , channel controller  235 , which contains representative channel element  240 , transceiver interface (IF)  245 , RF transceiver unit  250 , antenna array  255  and impedance measurement controller  251 . 
     BTS controller  225  comprises processing circuitry and memory capable of executing an operating program that controls the overall operation of BTS  220  and communicates with BSC  210 . Under normal conditions, BTS controller  225  directs the operation of channel controller  235 , which contains a number of channel elements, including channel element  240 , that perform bi-directional communications in the forward channel and the reverse channel. A “forward” channel refers to outbound signals from the base station to the mobile station and a “reverse” channel refers to inbound signals from the mobile station to the base station. In an advantageous embodiment of the present invention, the channel elements operate according to a code division multiple access (CDMA) protocol with the mobile stations in cell  121 . Transceiver IF  245  transfers the bi-directional channel signals between channel controller  235  and RF transceiver unit  250 . 
     Impedance measurement controller  251 , in conjunction with circuitry located in RF transceiver  250 , measures the receiver gain and the impedance match for the receive portion of antenna  255 . Portions of RF transceiver  250  and the operation of impedance measurement controller  251  are described below in greater detail in connection with FIGS. 3 and 4. 
     Antenna array  255  transmits forward channel signals from RF transceiver unit  250  to mobile stations in the coverage area of BS  101 . Antenna array  255  also sends to transceiver  250  reverse channel signals received from mobile stations in the coverage area of BS  101 . In a preferred embodiment of the present invention, antenna array  255  is multi-sector antenna, such as a three sector antenna in which each antenna sector is responsible for transmitting and receiving in a 120° arc of coverage area. Additionally, RF transceiver  250  may contain an antenna selection unit to select among different antennas in antenna array  255  during both transmit and receive operations. 
     FIG. 3 illustrates a portion of exemplary RF transceiver  250  according to one embodiment of the present invention. RF transceiver  250  comprises a transmit signal path comprising radio frequency coupler (RFC)  310 , RF amplifiers  315 ,  320 ,  325 , and  330 , circulator  335 , RFC  340 , and duplexer  345 , which sends the RF output signals to antenna array  255 . RF transceiver  250  also comprises antenna impedance measuring circuitry comprising test signal generator  305 , switch  346 , 80 MHz signal generator  350 , RF mixer  355 , signal splitter  360 , signal monitor  365 , RF circulator  370 , and signal monitor  385 . Finally, RF transceiver  250  comprises a receive signal path, a portion of which comprises RFC  375 , which receives incoming RF signals from antenna array  255 , duplexer  380 , and receive path demodulation circuitry  399 . 
     In one embodiment of the present invention, test signal generator  305  generates an output test signal which is at the center frequency (for example, 1960 MHz) of the transmit (or forward) channel signal frequency band (for example, 1930-1990 MHz). Thus, the output test signal from test signal generator  305  is 80 MHz above the center frequency (for example, 1880 MHz) of the receive (or reverse) channel frequency band (for example, 1850-1910 MHz). In the illustrated embodiment, RFC  310  receives signals from the small signal transmit path or from test signal generator  305  and generates a combined signal as an input to RF amplifier  315 . In another embodiment of the present invention, the test signal may also be generated by, for example, channel element  240 . If channel element  240  is used to generate the test signal, RFC  310  receives the test signal from the small signal transmit path, rather than from test signal generator  305 . In fact, if the test signal is generated exclusively by channel element  240 , both RFC  310  and test signal generator  305  may be omitted and the test signal may be received by RF amplifier  315  directly from the small signal transmit path. 
     Impedance measurement controller  251  may selectively enable and disable the output of test signal generator  305 , depending upon whether test mode is enabled and whether an isolated test signal is desired. The low level voltage output of RFC  310  is first amplified in series stages by RF amplifiers  315  and  320 . Parallel RF amplifiers  325  and  330  amplify the output of RF amplifier  320  in order to increase the output current that ultimately drives antenna array  255 . 
     Circulator  335  transmits the amplified RF signal from parallel amplifiers  325  and  330  to RFC  340 . Duplexer  345  transmits the RF signal from RFC  340  to antenna array  255 . Duplexer  345  serves as an RF filter and directional coupler and permits sharing of antenna array  255  by the RF transmitter and RF receiver portions of transceiver  250 . Duplexer  345  may isolate the transmit frequencies in the exemplary 1930-1990 MHz frequency band from the receive signals in the exemplary 1850-1910 frequency band. 
     RFC  340  transmits a copy of the RF transmit signal to one input of RF mixer  355 . The other input of RF mixer  355  is connected to the output of 80 MHz signal generator  350 , which may be selectively enabled and disabled by impedance measurement controller  251 . Impedance measurement controller  251  also opens and closes switch  346  depending on whether or not the return loss is being measured by impedance measurement controller  251 . If test measurements are not being performed, impedance measurement controller  251  opens switch  346  to prevent signals from being injected into the test signal path. RF mixer  355  down-converts the RF output from RFC  340  by the 80 MHz reference signal. Thus, the 1960 MHz test signal is down-shifted 80 MHz to produce a 1880 MHz test signal at the output of RF mixer  355 . The 1880 MHz test signal falls directly in the center of the receiver operating frequency band (1850-1910 MHz) and has a power level that is high enough to be discernable above expected noise levels. 
     The output from RF mixer  355  is connected to the input of splitter  360 . Splitter  360  generates two identical copies of the test signal at the center of the receiver frequency range for signal monitoring purposes. Under the direction of impedance measurement controller  251 , signal monitor  365  measures one or more signal parameters, such as amplitude, frequency, and the like, of one of the identical “split” test signals that signal impedance measurement controller  251  receives from splitter  360 . Signal monitor  365  provides an accurate measurement (i.e., an initial reference level) of the 1880 MHz test signal that is to be injected into antenna  255 . 
     Circulator  370  receives the other identical “split” test signal from splitter  360  and transmits the 1880 MHz test signal into RFC  375 . RFC  375  injects the test signal from circulator  370  into duplexer  380 . Under normal operating conditions, duplexer  380  filters the signals received from antenna array  255  and transmits the filtered reverse channel frequency band (i.e., 1850-1910 MHz) through RFC  375  to receive path demodulation circuitry  399 . Forward channel signals transmitted from antenna array  255  are filtered out by duplexer  380 . 
     Since the 1880 MHz test signal is in the center of the receive frequency band, duplexer  380  transmits the test signal into antenna array  255 . Under ideal conditions, if the impedance of antenna array  255  is properly matched, there will be no signal reflection from antenna array  255  back to duplexer  380 . However, under normal operating conditions, the injected test signal is at least partially reflected from antenna array  255  back to duplexer  380 . Duplexer  380  and RFC  375  transmit a portion of the reflected test signal back to circulator  370 . Circulator  370  transmits the reflected test signal to signal monitor  385 . 
     Signal monitor  385 , operating under the direction of impedance measurement controller  251 , measures the reflected test signal that signal impedance measurement controller  251  receives from circulator  370 . Signal monitor  385  provides an accurate measurement of the reflected 1880 MHz test signal that may be compared to the original test signal measured by test monitor  365 . 
     In one embodiment of the present invention, the monitor characteristic curve (V/dB) is stored in impedance measurement controller  251 . Impedance measurement controller  251  determines the antenna match from the voltage standing wave ratio (VSWR) of the antenna by comparing the power level (in dB) of the injected test signal measured by signal monitor  365  to the power level (in dB) of the reflected test signal measured by signal monitor  385 . 
     As described, exemplary RF transceiver  250  and impedance measurement controller  251  are capable of measuring the impedance match of the antenna array  255 . In an advantageous embodiment of the present invention, RF transceiver  250  and impedance measurement controller  251  perform these measurements while BTS  220  continues to process forward and reverse channel information. 
     FIG. 4 illustrates flow diagram  400 , which illustrates the operation of the exemplary antenna impedance matching test circuitry in RF transceiver  250  of base transceiver station (BTS)  220 . Initially, impedance measurement controller  251  causes test signal generator  305  to generate a test signal at the center frequency, typically 1960 MHz, of the transmitter operating frequency range (process step  405 ) Next, the test signal is amplified in the transmitter path to a sufficient level for input to RFC  340 . RFC  340  provides a copy of the RF transmitter output to duplexer  345  for transfer to antenna array  255  and a separate copy for output to RF mixer  355  (process step  410 ). RF mixer  355  mixes the amplified test signal with an 80 MHz reference signal to generate a new test signal at the center frequency of the receiver operating range, typically 1880 MHz (process step  415 ). The resultant test signal is measured in signal monitor  365  to establish an input test signal strength reference level (process step  420 ). The test signal is also injected into antenna array  255  via RFC  375  and duplexer  380  (process step  425 ). The reflected signal from antenna array  255  is measured in signal monitor  385 . Under the control of impedance measurement controller  251 , the measured reflected signal is compared to user-defined threshold values to determine a pass/fail status for the impedance match of antenna array  255  (process step  430 ). 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.