Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to that disclosed in U.S. patent application Ser. No. 09/519,709, filed concurrently herewith, entitled “SYSTEM AND METHOD FOR MEASURING THE IMPEDANCE MATCH OF AN ANTENNA.” application Ser. No. 09/519,709 is commonly assigned to the assignee of the present invention. The disclosure of this related patent application is hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to wireless communication systems and, more specifically, to a combined system for monitoring receiver gain and 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 more than one type of test in a base transceiver station. More particularly, there is a need for integrated test equipment that can be used to calibrate the gain of the receive path of the receiver and that can also 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 measurement and calibration circuit for use in an RF transceiver comprising an antenna and an RF receiver coupled to the antenna and having a receive path comprising circuitry capable of amplifying an incoming signal received from the antenna. In an advantageous embodiment of the present invention, the measurement and calibration circuit comprises: 1) a test signal generator capable of generating a test signal having a known amplitude and a known frequency; 2) a switch having an input coupled to the test signal generator for receiving the test signal, a first output coupled to an input of the receive path, and a second output coupled to the antenna; 3) a test controller capable of causing the switch to directly inject the test signal into the input of the receive path and capable of causing the switch to inject the test signal into the antenna, wherein the antenna at least partially reflects the test signal into the receive path; and 4) a signal monitor coupled to an output of the receive path capable of measuring the direct injected test signal and the reflected test signal. 
     In one embodiment of the present invention, the signal monitor is capable of adjusting a gain of the receive path. 
     In another embodiment of the present invention, the known frequency of the test signal is the center frequency of the RF receiver. 
     In still another embodiment of the present invention, the test signal generator comprises a transmitter local oscillator capable of generating a transmitter carrier signal used by an RF transmitter of the RF transceiver, a test local oscillator capable of generating a single frequency reference signal, and an RF mixer capable of mixing the transmitter carrier signal and the single frequency reference signal. 
     In yet another embodiment of the present invention, a frequency of the single frequency reference signal is equal to a frequency difference between a center frequency of the RF transmitter and a center frequency of the RF receiver. 
     In a further embodiment of the present invention, the mixing by the RF mixer of the transmitter carrier signal and the single frequency reference signal is a subtractive mixing that generates the test signal at a center frequency of the RF receiver. 
     In a still further embodiment of the present invention, the signal monitor compares a measured value of the direct injected test signal and a measured value of the reflected test signal to determine an impedance match of the antenna. 
     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; 
     FIG. 4 illustrates an exemplary automatic gain control circuit in accordance with one embodiment of the present invention; and 
     FIG. 5 illustrates an exemplary flow diagram in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 through 5, 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 T1 line, a T3 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 calibration controller  251  which contains look-up table (LUT)  252 . Calibration controller  251 , in conjunction with test circuitry located in RF transceiver  250 , performs antenna impedance matching and receiver gain calibration. RF transceiver  250  and calibration controller  251  are described below in greater detail in connection with FIGS. 3 and 5. 
     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  240  and RF transceiver unit  250 . 
     Antenna array  255  transmits forward channel signals received 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 receive signal path through antenna array  255  and exemplary RF transceiver  250 . The RF receive path through RF transceiver  250  comprises duplexer  305 , RF coupler (RFC)  310 , RF amplifier  320 , band pass filter (BPF)  325 , RF amplifier  330 , RF mixer  335 , surface acoustic wave (SAW) filter  340 , and automatic gain control circuit (AGC)  345 . 
     RF transceiver  250  also comprises receiver (RX) local oscillator (LO)  365  and transmitter (TX) local oscillator (LO)  375 . Exemplary signal injection circuits associated with the present invention comprise test local oscillator (LO)  370 , RF mixer  380 , and switch  385 . The receiver IF signal output by RF transceiver  250  is a modulated intermediate frequency (IF) signal centered at 239 MHz. Besides providing the IF output, AGC  345  also generates a received signal strength indication (RSSI) signal that is provided as an output to calibration controller  251 . 
     Duplexer  305  serves as an RF filter and couples antenna array  255  to RF transmitter and the RF receiver portions of transceiver  250 . For example, duplexer  305  isolates the received signals in the 1850-1910 MHz frequency range from the transmitted RF output signal in the 1930-1990 frequency range. RFC  310  couples the incoming reverse channel signal from antenna  255  to the input of amplifier  320  and allows test signals to be injected into antenna array  255  or the receive path using the RFC  310  inputs labeled “A” and “B.” 
     Under normal conditions, RFC  310  transfers the received reverse channel signal from duplexer  305  to the receive signal path through amplifier  320 . During testing and calibration procedures, switch  385 , under control of calibration controller  251 , may inject a test signal at input A of RFC  310  toward antenna array  255  or may inject a test signal at input B of RFC  310  toward the receive path through amplifier  320 . 
     Amplifier  320  amplifies the output of RFC  310  to an intermediate level. BPF  325  filters the output of amplifier  320  to remove noise outside of the desired receiver frequency range of 1850-1910 MHz. (The difference between forward and reverse channel frequencies is constant for the frequency band and applicable country. For example, in conventional CDMA systems, the reverse (receive) channel frequency is 80 MHz below the forward (transmit) channel frequency.) BPF  325  may be centered at 1880 MHz. 
     RF amplifier  330  further amplifies the output of BPF  325  to the level needed for use by RF mixer  335 . RF mixer  335  down-converts the output of RF amplifier  330  by mixing it with the reference signal in the 1611-1671 MHz frequency range from RX LO  365 , to produce an intermediate frequency signal. RF mixer  335  effectively shifts the spread spectrum signal centered around the receiver RF frequency down to an intermediate frequency (IF) signal centered around 239 MHz. At this point, the signal output by RF mixer  335  may have spurious signals outside of the desired frequency range which have been amplified and/or introduced by the numerous amplification steps. Surface acoustic wave (SAW) filter  340  is an extremely narrow filter that blocks all but the desired frequencies from reaching AGC  345 . 
     AGC  345  automatically adjusts the output of SAW filter  340  in order to maintain a constant level of IF output power, as described in greater detail below. RX LO  365  provides a single carrier frequency output in the 1611-1671 MHz range which is 239 MHz down from the TX LO  375  transmit carrier frequency, as previously discussed. Test LO  370  provides a stable reference signal, 80 MHz for example, that may be used for injecting a known frequency component into the forward or reverse signals for internal testing purposes. These signal selections have advantages since 80 MHz represents the frequency separation between the forward and reverse signals, and the TX LO and RX LO signals are already available at precisely known power levels in transceiver  250 . Typically, test LO  370  comprises a multiplier or a phase-locked loop (PLL) referenced to a very stable 10 MHz reference clock and the necessary amplification circuits to generate a stable 80 MHz local oscillator signal with a known precise frequency and amplitude for input to RF mixer  380 . 
     Similarly, TX LO  375  generates a stable single frequency output signal in the 1930-1990 MHz range, providing an RF carrier frequency for exemplary CDMA transmissions. As described previously, the transmitter carrier frequency is 80 MHz above the receiver carrier frequency present in the reverse channel. In an exemplary embodiment, TX LO  375 , used by the transmitter portion of RF transceiver  250 , includes automatic gain control capabilities that provide a very accurate output within +/−0.5 dB of the desired power level. RF mixer  380  uses the 80 MHz reference signal from test LO  370  to modulate the transmitter carrier signal from TX LO  375  to generate a stable test injection signal with a single carrier frequency in the 1850-1910 MHz range in which the receiver operates. 
     Switch  385 , under control of calibration controller  251 , injects the test signal from RF mixer  380  into either input A or input B of RFC  310 . When BTS  220  is operating under normal conditions, switch  385  disables the transfer of the test output from RF mixer  380  to test outputs A or B. Under software control, for instance when the operating temperature exceeds a predetermined level or BTS  220  is reset, switch  385  receives a test indication from calibration controller  251  which causes input B to be enabled for calibration of RF transceiver  250 . 
     The RSSI calibration is made by injecting a continuous wave (CW) tone into the receiver front-end through input B of RFC  310 . The receive path of RF transceiver  250  treats the injected CW tone from RFC  310  as a normal component of the received forward channel signal which is amplified and then adjusted and filtered by AGC  345  to produce the RSSI signal. Calibration controller  251  compares the resultant RSSI signal with the known injected test signal to generate attenuation correction factors for storage in look-up table  252  and for transfer to the operator. Thus, compensation factor data stored in and transferred from look-up table  252  represents the attenuation characteristic curve for RF transceiver  250 . In one embodiment of the invention, the attenuation factors stored in look-up table  252  may represent different attenuation factors exhibited by RF transceiver  250  in different temperature ranges. 
     During normal operating conditions, calibration controller  251  may periodically measure the RSSI level and adjust the measured RSSI value using the RSSI correction factors for the current temperature stored in look-up table  252 . Calibration controller  251  may then periodically transfer the corrected RSSI result to the wireless service provider (i.e. , remote operator) that operates wireless network  100 . 
     BTS  220  determines the antenna match from the voltage standing wave ration (VSWR) of the antenna by comparing the RSSI level of the receive path injected test signal to the RSSI level of the test signal injected into and reflected by antenna array  255 . After the previously described calibration measurement procedure has been performed, calibration controller  251  initiates the VSWR measurement by using switch  385  to inject a test signal of a known precise amplitude into the receive path through input B of RFC  310 . Calibration controller  251  measures the level of the RSSI signal corresponding to the injected test signal and determines any necessary correction factor to compensate for losses in the receive path. The RSSI correction factors are stored in LUT  252 . 
     Next, calibration controller  251  uses switch  385  to inject the test signal from RF mixer  380  into antenna array  255  through input A of RFC  310 . Some portion of the injected test signal is then reflected by antenna array  255 , depending on the impedance match of antenna array  255 . Ideally, no signal is reflected if there is a perfect impedance match. RF transceiver  251  filters and amplifies the reflected test signal as part of the normal receiver input signal, providing an RSSI level for measurement by calibration controller  251 . Calibration controller  251  measures the component of the RSSI signal corresponding to the reflected test signal and adjusts the measured RSSI value of the reflected test signal according to the RSSI correction factors stored in LUT  252 . The corrected result may be stored for comparison purposes and/or transmitted to the remote operator. 
     As described, exemplary transceiver  250  and calibration controller  251  provide the capability for calibrating the receiver gain and for measuring the impedance match of antenna array  255  and the power gain of the receiver. Advantageously, the present invention provides these capabilities while BTS  220  continues to process forward channel information. 
     RF transceiver  250  performs indicated tests while the system is operational by injecting test signals A or B at the receiver carrier frequency. Thus, RF transceiver  250  down-converts the test signal to zero frequency (i.e., DC signal) The test signal does not affect the demodulation process in RF transceiver  250  which occurs in a separate demodulator circuit that converts the 239 MHz IF output to separate I and Q baseband outputs, well known to those familiar with the present art. Depending on the relative power difference between the injected input A or input B test signal and the reverse channel signal, the reduction in signal level at the demodulator may represent a degradation of the receive signal, but not to a level which prevents demodulation of the desired reverse channel information. Besides allowing normal operation to continue, exemplary BTS  220  minimizes introduction of additional production costs by re-using existing receiver amplifier and detector circuits for measurement of the impedance match of the receive path for antenna  255 . 
     The RSSI reading is customarily accomplished with some type of temperature compensation circuitry which adjusts variations in gain, attenuation, and detector slopes over temperature and frequency. The prior art methods provide varying degrees of accuracy with fluctuations due to device and component tolerances and changes over temperature and across frequency ranges. Prior art circuit analysis is typically characterized during preproduction, with the resultant characterization information being stored in memory for individual radios or mobile stations. This characterization process requires analysis of many units and increased time for developing an adequate profile for compensation purposes. Component substitutions and lot-to-lot device variations through-out the production interval introduce additional variations which are not considered during the radio characterization process. 
     The present invention provides a stable injection test signal with the corresponding RSSI reading being measured at any instantaneous temperature and operating channel, eliminating the necessity for characterization and compensation circuitry and continually adjusting for changes in component performance. 
     Prior art VSWR measurements on the transmit antenna are usually accomplished with a circulator or detector located at the output of the transmit path where high power levels are routed, switched, and detected. Since reflected high power levels have a tendency to radiate from any discontinuity in the reflected path and to load down the power amplifier, the gain characteristics of the amplifier may continually change, producing an inaccurate power reading. The present invention eliminates the use of a circulator, padding, and switches in favor of using a low power level signal for obtaining an accurate measurement of VSWR. 
     In addition, prior art implementations typically provide a precision oscillator frequency which is injected at one receiver band frequency for calibration when traffic is not present. The present invention provides the exemplary 80 MHz test carrier signal which allows calibration of the receiver at the operating frequency in the presence of traffic so that system operation is not interrupted. 
     FIG. 4 illustrates exemplary AGC  345  in accordance with one embodiment of the present invention. Exemplary AGC  345  comprises amplifier  405 , voltage variable attenuator  410 , power splitter  415 , power detector  420 , integrator  425 , and filter  430 . Amplifier  405  amplifies the incoming IF signal from SAW  340  to an intermediate power level. Voltage variable attenuator  410  adjusts the input voltage from amplifier  405  in proportion to the input from integrator  425  to provide an adjusted power level for input to power splitter  415 . Power splitter  415  splits the signal on its input into two modulated IF signals with known, related power levels (for instance two signals of equal power), one for IF output and a second signal for input to power detector  420 . 
     Power detector  420  adjusts the IF signal on its input to provide a rectified DC signal for input to integrator  425 . Integrator  425  integrates the difference between the signal from power detector  420  and the known AGC reference (ref) signal provided by BTS controller  225 . The output of integrator  425  is the control voltage for voltage variable attenuator  410  and filter  430 . Finally, filter  430  filters the output of integrator  425  to generate a “raw” received signal strength indication (RSSI) signal for input to calibration controller  251  for measurement and test purposes. 
     FIG. 5 depicts flow diagram  500 , which illustrates the operation of base transceiver station (BTS)  220  according to one embodiment of the present invention. Under control of calibration controller  251 , the output of TX LO  375  is set to a known and precise amplitude (process step  505 ). RF mixer  380  mixes the outputs from test LO  370  and TX LO  375  to create an RF test output signal with known amplitude and frequency. The test signal frequency is equal to the receiver carrier frequency (i.e., TX LO output—80 MHZ) (process step  510 ). 
     Next, calibration controller  251  causes switch  385  to inject the test signal from RF mixer  380  directly into the receiver through input B of RF coupler  310 . The directly injected test signal passes through the receive path, and the resulting RSSI signal from AGC  345  is measured by calibration controller  251 . Calibration controller  251  compares the measured RSSI signal with the known output level of RF mixer  380  to determine the gain of the receive path for the current ambient temperature (process step  515 ). 
     Since the difference between the injected test signal and the measured RSSI signal is know, calibration controller  251  may determine one or more correction factors for the RSSI signal level from AGC  345  at a variety of temperature levels for storage in look-up table  252 . This process is repeated for different ambient temperatures (process step  520 ). 
     When the gain of the receive path is calibrated, calibration controller  251  causes switch  385  to switch the test signal on the B input of RFC  310  to the A input of RFC  310 . Thus, the test signal is injected into antenna array  255 , which reflects some portion of the test signal back into the receive path of RF transceiver  250 . RF transceiver  250  amplifies and filters the reflected test signal through the receive path. Calibration controller  251  then measures the reflected test signal components in the RSSI signal (process step  525 ). Next, calibration controller  251  compares the measured RSSI levels for the direct injected test signal and the reflected test signal to determine the antenna voltage standing wave ratio (VSWR) match for antenna array  255  and stores this result in look-up table  252  for eventual transfer to the operator (process step  530 ). Thereafter, calibration controller  251  periodically measures and corrects the RSSI for normal traffic receive signals and reports the corrected RSSI values to the system operator (process step  535 ). 
     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.

Technology Category: 5