Abstract:
There is disclosed an ACP monitoring circuit for use in a wireless network comprising a plurality of base stations for communicating with a plurality of mobile devices. Each of the plurality of base stations comprises an RF transmitter that receives an input baseband signal and an RF carrier signal and produces a modulated RF output signal that is then amplified. The ACP monitoring circuit monitors adjacent channel power (ACP) noise in the RF output signal. The ACP monitoring circuit comprises: 1) an RF mixer having a first input receiving the RF output signal and a second input receiving the RF carrier signal and produces a scaled output signal on an output of the RF mixer; and 2) a first power detection circuit coupled to the RF mixer output that determines a power level of the ACP noise outside an allocated channel bandwidth of the RF transmitter.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to wireless communications systems and, more specifically, to a system for monitoring adjacent channel power in a base station transmitter in a wireless communication network. 
     BACKGROUND OF THE INVENTION 
     Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered by cellular service providers, by the regional Bell companies, and by the national long distance operators. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population. 
     To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof. 
     In order to further 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 are eager to implement any innovations 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. 
     Every wireless network base station has an RF power amplifier for transmitting voice and/or data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, pagers, and the like) and a receiver for receiving voice and/or data signals from the mobile units. The FCC requires an RF power amplifier (PA) to be operated in such a manner that the adjacent channel power (ACP) noise (distortion) remains under certain limits (i.e., a mask) defined in a standard (i.e., ACP profile). ACP is the ratio of adjacent channel power in a specified bandwidth to the power of the desired transmitter output. 
     To ensure that the ACP profiles of network base stations remain within tolerance, wireless service providers frequently measure the RF output power and signal quality of each base station transmitter. However, the test equipment used typically includes a spectrum analyzer that costs far more that the base station transmitter itself. Due to this great cost, the test equipment rarely is implemented as part of the base station itself. Instead, maintenance crews transport the test equipment from cell site to cell site to perform ACP profile tests. Unfortunately, this does not provide real-time monitoring of ACP noise and distortion products in a wireless network. RF transmitters that are out-of-tolerance are not detected until a maintenance crew finally tests the equipment. 
     There is therefore a need in the art for test equipment that may be implemented as part of the base station. In particular, there is a need for test equipment that provides continuous monitoring of adjacent channel power (ACP) noise in wireless network base stations. More particularly, there is a need for ACP monitoring equipment that is reliable, adds the minimum amount of cost to a base station, and provides a remote monitoring capability for ACP noise. 
     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 an ACP monitoring circuit for use in a wireless network comprising a plurality of base stations capable of communicating with a plurality of mobile devices, wherein each of the plurality of base stations comprises an RF transmitter capable of receiving an input baseband signal and an RF carrier signal and producing therefrom an amplified modulated RF output signal. In an advantageous embodiment of the present invention, the ACP monitoring circuit, capable of monitoring adjacent channel power (ACP) noise in the RF output signal, comprises: 1) an RF mixer having a first input capable of receiving the RF output signal and a second input capable of receiving the RF carrier signal and producing therefrom a scaled output signal on an output of the RF mixer; and 2) a first power detection circuit coupled to the RF mixer and capable of determining a power level of the ACP noise outside an allocated channel bandwidth of the RF transmitter. 
     In one embodiment of the present invention, the ACP monitoring circuit further comprises a filter coupled to the RF mixer for isolating the ACP noise, wherein the first power detection circuit measures the power level of the ACP noise at an output of the filter. 
     In another embodiment of the present invention, the ACP monitoring circuit further comprises a second power detection circuit coupled to the RF mixer and capable of determining a power level of the RF output signal in the allocated channel bandwidth of the RF transmitter. 
     In still another embodiment of the present invention, the ACP monitoring circuit further comprises a filter coupled to the RF mixer for isolating the RF output signal, wherein the second power detection circuit measures the power level of the RF output signal at an output of the filter. 
     In yet another embodiment of the present invention, the ACP monitoring circuit further comprises: 1) a first filter coupled to the RF mixer for isolating the ACP noise; and 2) a second filter coupled to the RF mixer for isolating the RF output signal, wherein the first power detection circuit measures the power level of the ACP noise at an output of the first filter and measures a power level of the RF output signal in the allocated channel bandwidth of the RF transmitter at an output of the second filter. 
     In a further embodiment of the present invention, the ACP monitoring circuit further comprises a switch having a first input coupled to the first filter output, a second input coupled to the second filter output, and an output coupled to the first power detection circuit. 
     In a still further embodiment of the present invention, the ACP monitoring circuit further comprises at least one bandpass filter coupled to the RF mixer for isolating the ACP noise, wherein the first power detection circuit measures the power level of the ACP noise in a first selected frequency band at an output of the bandpass filter. 
     In a yet further embodiment of the present invention, the ACP monitoring circuit further comprises a plurality of bandpass filters coupled to the RF mixer for isolating the ACP noise, wherein the first power detection circuit measures the power level of the ACP noise in a plurality of selected frequency bands at an output of the bandpass filter. 
     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 incorporating an exemplary adjacent channel power measurement circuit in accordance with one embodiment of the present invention; 
     FIG. 4 illustrates in greater detail an exemplary bandwidth power measurement circuit in accordance with another embodiment of the present invention; and 
     FIG. 5 is a flow diagram illustrating the operation of the exemplary RF transceiver 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 cell 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 shapes, depending on the cell configuration selected and natural and man-made obstructions. 
     In one embodiment of the present invention, each of 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 . 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 know, 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 , and antenna array  255 . 
     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, transceiver  250  may contain an antenna selection unit to select among different antennas in antenna array  255  during both transmit and receive operations. 
     In an advantageous embodiment of the present invention, the transmitters in RF transceiver unit  250  comprise variable-power RF amplifiers that are capable of varying the amplitude of the RF output signal transmitted by base station  101 . In so doing, the RF amplifiers in the transmitters may occasionally produces distortion signals (or distortion products) in the adjacent channels. To monitor this condition, the present invention provides inexpensive means for remotely monitoring the ACP profile of a base station transmitter. For the purpose of simplicity, the ACP distortion products generated by the RF amplifier(s) described below may be referred to as ACP “noise” signals or ACP “distortion” signals. 
     FIG. 3 illustrates in greater detail exemplary RF transceiver unit  250  in accordance with one embodiment of the present invention. The transmit path of exemplary RF transceiver unit  250  comprises modulator  310 , transmitter local oscillator (TX LO)  315 , power amplification stage  320 , and RF coupler (RFC)  330 . RF transceiver unit  250  also comprises ACP measurement circuit  350 . 
     Modulator  310  receives an in-phase (I) baseband signal and a quadrature (Q) baseband signal from channel element  240  and uses them to modulate an RF carrier signal provided by TX LO  310  to produce a modulated RF output signal. The RF carrier signal produced by TX LO  310  is a stable single frequency signal in the 1930-1990 MHz range used for exemplary CDMA transmissions. This same TX LO reference signal is used by ACP measurement circuit  350 . 
     Power amplification stage  320  amplifies the RF spread spectrum output from modulator  305  to produce the required power level for broadcast to the cell site through RFC  330  and antenna array  255 . Power amplification stage  320  comprises a first power amplifier  322 , second power amplifier  324 , which may include an automatic gain control adjustment circuit, and perhaps one or more additional power amplifiers, such as power amplifier  326 . Power amplifier  322  amplifies the output of modulator  310  to an intermediate power level. Power amplifier  324  and any succeeding power amplifiers, including power amplifier  326  continue to amplify the output of power amplifier  322  until the desired RF output power level is reached for output to RFC  330 . RFC  330  transmits one copy of the RF output signal, RF OUT to RF antenna array  255  and transmits another copy of the RF OUT signal to ACP measurement circuit  350 . 
     Exemplary ACP measurement circuit  350  receives the RF OUT signal from RFC  330  and creates an intermediate frequency (IF) measurement signal for output to MSC  140 . Exemplary ACP measurement circuit  350  comprises RF mixer  355 , low pass filter (LPF)  360 , high pass filter (HPF)  365 , power detectors  370  and  375 , and input/output interface (I/O IF) circuit  380 . RF mixer  355  down-converts the RF OUT signal by mixing it with the 1930-1990 MHz carrier signal from TX LO  315  to recover a baseband representation of the original I and Q signals. The down-converted baseband signal also contains adjacent channel noise (distortion) signals produced by the amplification of the I and Q signals. 
     LPF  360  and HPF  365  receive the resultant baseband and noise signals from RF mixer  355  and filter them to produce desired signals for use in measuring ACP characteristics. LPF  360  filters out the unwanted ACP signals to present the desired I and Q components to power detector  370 . HPF  365  filters out the I and Q components to present the ACP signal components to power detector  375 . Power detectors  370  and  375 , which may be simple peak detectors, measure the relative amplitudes of the desired baseband signals and the undesired ACP noise signals. 
     Subsequently, I/O IF  380  receives the power level outputs from power detectors  370  and  375  and calculates the ratio of the power of the undesired distortion signal (power detector  375  output) to the desired signal components (power detector  370  output). The ratio determined by I/O IF  380  provides a continuous measurement of ACP. I/O IF  380  subsequently converts the ACP measurement to the form required for input to MSC  140 . For instance, one, embodiment for I/O IF  380  is an analog-to-digital converter which provides an digital output representation of the ACP measurement. 
     In an alternate embodiment of the present invention, a switch may couple the outputs of LPF  360  and HPF  365  to a single, common power detector, thereby simplifying the circuit by eliminating the need for one power detector. 
     FIG. 4 is an illustration of an exemplary ACP measurement circuit  400  in accordance with an alternate embodiment of the present invention. ACP measurement circuit  400 , which may be used to replace ACP measurement circuit  350 , comprises RF mixer  355 , bandpass filters  410 ,  420  and  430  (hereafter referred to respectively as “BW 1 ,” “BW 2 ” and “BW 3 ”), switch  450 , and power detector  460 . Again, RF mixer  355  down-converts the RF OUT signal by mixing it with the 1930-1990 MHz carrier signal from TX LO  315  to recover a baseband representation of the original I and Q signals. The down-converted baseband signal also contains adjacent channel noise (distortion) signals produced by the amplification of the I and Q signals. 
     Next, each of BW 1 , BW 2 , and BW 3  filter the output of RF mixer  355 . Each of BW 1 -BW 3  provides an output which represents either the desired baseband signals or a selected frequency range of the ACP noise. Switch  450  then selectively switches the outputs of BW 1 -BW 3  to power detector  460 , which measures the power level in each bandpass and provides an output to MSC  140  indicating the measured power level in each bandpass. Thus, a greater amount of information regarding the amount of ACP noise in different frequency bands can be analyzed. 
     FIG. 5 depicts flow diagram  500 , which illustrates the operation of the exemplary RF transceiver  250  in accordance with one embodiment of the present invention. RF mixer  355  down-converts the copied RF output signal from RFC  330  using the carrier frequency from TX LO  315  as its reference signal (process step  505 ). HPF  365  isolates the ACP noise signal from the down-converted signal and LPF  360  isolates the amplified baseband frequency from the down-converted signal (process step  510 ). Subsequently, power detector  375  rectifies and detects the peak power of the ACP noise signal to generate an output DC signal which is proportional to the ACP noise portion of RF OUT from RFC  330  (process step  515 ). Similarly, power detector  370  provides an output DC signal which is proportional to the desired baseband (in-band) output of RFC  330  (process step  520 ). I/O IF  380  receives the power measurement signals from power detectors  370  and  375  and converts them to an output signal which is compatible with interfaces to MSC  140  or any other central location which performs fault isolation processes (process step  525 ). 
     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.