Abstract:
The present invention provides a system and method for automatically compensating for transmission line losses at a wireless base station antenna site. Transmission line losses are measured and the result of the measurement is used to adjust a variable gain amplifier which receives reception signals from the transmission line.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of wireless communications. More specifically, it relates to a system for automatically sensing changes/variations in receive path gain and compensating for those changes/variations when detected. 
     2. Description of the Related Art 
     In order for a wireless communications service provider to claim certain geographic areas (e.g., cells) of coverage, that service provider must be able to guarantee certain minimum criteria of the received signal within that geographic area. That is, the service provider must be able to guarantee a minimum sustained level of signal strength within an entire cell of its service area. 
     One of the limitations placed upon the service provider is the fact that its radio receivers at the cell site equipment are capable of detecting only those signals that are at least 10 dB above the noise level (a largely fixed value) for a time division multiple access (TDMA) system and those signals at or below the noise level for a code division multiple access (CDMA) system. 
     Typically, as depicted in FIG. 1, the receiving equipment at a cell site consists of a receiving antenna  100  mounted atop a cell site tower  140  which feeds, via receiver cable  130 , into the cell site equipment  135 . The cable length varies of course with the height of the cell site tower, however, typically this height can be between 20-200 feet. The cell site equipment  135  typically includes a receiver filter  105 , a fixed gain low noise amplifier (LNA)  110 , and the remainder of the receiver path  120 . 
     The receiver filter  105 , as is known in the art, is used for eliminating both signal noise and any of the signal that is not within a predetermined bandwidth. The output of the receiver filter  105  is then coupled to a fixed gain low noise amplifier (LNA)  110  for amplifying the received signal. The output of the LNA  110  is then coupled to the remainder of the receive path  120  of the cell site equipment  135  (e.g., radio frequency (RF) splitters, etc) and eventually to individual radio receiver ports, as is known in the art. 
     A problem with using the FIG. 1 receive path to feed into the cell site equipment  135 , is that the noise figure experienced between the receiver antenna  100  and the filter  105 /LNA  110  combination (i.e., over the 20-200 foot receiver cable  130 ) is high. In addition, there are variable losses within the cable  130  itself. These cable losses can vary with ambient conditions including temperature, etc. The higher the losses experienced within the cable  130 , the lower the signal strength at the cell site equipment  135 . For example, approximately 2 dB can be lost in the cable  130  between the receiving antenna  100  and the filter  105 /LNA  110  combination. These losses contribute to reducing a service provider&#39;s coverage area inasmuch as the coverage area is defined by, among other things, how well the strength of a received signal is maintained from the receiver antenna to the receiver radios and how well that provider can keep the noise figure associated with the received signal to a minimum. 
     Turning to FIG. 2, an alternative system for receiving wireless signals at a cell site is depicted for reducing the noise figure of the received signal. Similarly to FIG. 1, the FIG. 2 receiver contains the same receiving components (i.e., antenna  100 , receiver filter  105 , and fixed gain LNA  110 ), however, these components have been placed on top of the receiver tower  140  along with the receiver antenna  100  at the cell site. Placing the receiving equipment (i.e., filter  105 /fixed gain LNA  110 ) on top of the tower  140 , sometimes referred to as the masthead, results in a reduction of the noise figure of the received signal but does not change the receive path gain. These improvements allow the service provider to expand the geographic area that defines the cell site, thereby expanding wireless coverage while effectively using the same receiving equipment. 
     For example, instead of being able to detect only those signals that are 10 dB above the noise level, with the addition of the fixed gain LNA  110  at the masthead, the antenna may now receive all signals of e.g., only 7 dB above the noise level and still be able to detect those signals at the remainder of the receive path  120 . This improvement effectively increases the radius of coverage of the cell. 
     However, a problem associated with the FIG. 2 receiving system is that the remainder of the receive path  120  is still at the bottom of the tower  140  within the cell site equipment  135 , while the receiving components (i.e., antenna  100 , filter  105 /LNA  110 ) are mounted atop the tower  140  with approximately 20-200 feet of receiver cable  130  separating them. As a result, receive path gain, while somewhat improved by placing the filter  105 /LNA  110  combination atop the tower  140 , varies as a function of the instability of e.g., the RF losses and impedance of the receiver cable  130 . This instability in the receive path gain effectively reduces the amount of coverage area in a cell site since the required minimum levels of signal strength are not maintained. That is, while the service provider should realize the full 3 dB advantage by placing the LNA  110  on top of the tower, only approximately 1 dB improvement can be realized due to the losses in the cable. Thus, a system and method for automatically sensing cable losses and compensating for those losses as measured at the cell site equipment  135  is desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and method for automatically sensing cable losses and adjusting for those losses as measured at the cell site equipment from a receiver antenna and filter/LNA combination that is located remotely from the cell site equipment. A circuit is provided for sensing a voltage at an exit port to the cell site equipment. If the sensed voltage is different from a predetermined reference voltage, thereby signifying the presence of cable losses, a control signal is delivered from an error amplifier to a variable gain LNA for varying its gain and thereby adjusting (up or down) the strength of the received signal so as to maintain a constant receive path gain. The circuit also provides a source of power for the masthead containing a portion of the receiver circuitry atop the tower. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention will become more readily apparent from the following detailed description which is provided in connection with the accompanying drawings in which: 
     FIG. 1 illustrates a typical cell site receiver system-, 
     FIG. 2 illustrates another typical cell site receiver system; 
     FIG. 3 illustrates a receiver system according to a first embodiment of the present invention; 
     FIG. 4 illustrates a receiver system according to a second embodiment of the present invention; and 
     FIG. 5 illustrates a flowchart depicting operation of a control circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 3 illustrates a receiver system according to the present invention in which, similar to the receiver system of FIG. 2, the receiver antenna  100  and the receiver filter  105 /fixed gain LNA  110  combination are located atop a receiver tower  140 , wherein the output of the antenna is coupled to the input of the filter  105 /LNA  110  combination. The output of the filter  105 /LNA  110  combination is coupled to a receiver cable  130  for carrying the received signal to the cell site equipment  135 . 
     The cell site equipment  135  of the FIG. 3 receiver system contains a circuit  300  for maintaining a constant receive path gain. The received signal is fed into the cell site equipment  135  and more specifically into receiver port  325 . Also coupled to port  325  is a constant current source  320  for providing a source of power, via the receiver cable  130 , to the receiver filter  105 /fixed gain LNA  110  combination as will be described more fully below. The other side of constant current source  320  is coupled to ground. As is known in the art, a receiver cable, such as e.g., a coaxial receiver cable, contains a center conductor surrounded by a return sheath with foam insulation therebetween to help provide fixed, controlled RF parameters. It is via the center conductor that the constant current source  320  supplies power to the masthead (i.e., to the LNA  110  within the masthead). That is, the constant current source  320  causes a certain known source voltage at the receiver port  325 . As current is delivered via the center conductor of cable  130  to power up the LNA  110  via a fixed, known resistance (e.g., via a resistor, a zener diode biasing the LNA, etc.), it is returned back down to receiver port  325  via the outer sheath of cable  130 . The voltage of the center conductor is then read at port  325 . Since the source current is known at source  320  and the impedance and associated voltage drop at the LNA  110  is also known, the only variable (i.e., in the voltage loop) is the voltage drop in the cable  130  measured at port  325  (i.e., Vmeas). It has been determined that this measured voltage Vmeas is directly proportional to the amount of RF losses in the cable. 
     Therefore, when the two voltages Vmeas and Vref are known, the losses of the cable are also known. It is these known cable losses for which the invention seeks to compensate. The magnitude of the cable loss is common to all signals travelling through the cable  130 . That is, e.g., a 2 dB RF loss in the cable will reduce a 10 dB signal to 8 dB, and a 6 dB signal to 4 dB, and so on. 
     The receive port  325  then feeds Vmeas to one input of an error amplifier  310  (e.g., a differential amplifier). The other input of error amplifier is Vref. If there is a difference between the two voltages, the error amplifier  310  is configured to send a control signal  335  to a variable gain LNA  310  for varying the gain of the LNA  310  up or down, dependent upon how the two voltages compare with each other so that a constant receive path gain is achieved. In addition, before the received signal reaches the variable gain LNA  310 , it may be sent through a “mop up” receiver filter  305  for both sharpening the received signal and eliminating any unwanted frequencies. After the strength of the received signal is adjusted by the variable gain LNA  310 , the signal is forwarded to the remainder of the receive path  120  (e.g., RF splitters, etc.) with a constant receive path gain. For example, if the antenna  100  receives a signal at 5 dB above the noise level (normally undetectable), the fixed gain LNA  110  raises it to a detectable signal level of 10 dB above the noise level. Thereafter, although there is a known loss of (e.g., 2 dB) in the cable  130 , the invention restores that signal to the original 10 dB above the noise level (i.e., replacing the signal strength lost in the cable  130 ), thereby effectively turning an undetectable signal (of 8 dB above the noise level) into a detectable signal of 10 dB above the noise level. This advantage allows the service provider to increase the area of wireless coverage with very little added equipment and cost to the cell site equipment  135 . 
     Turning now to FIG. 4, a second embodiment of the invention is depicted. The FIG. 4 embodiment is identical to the FIG. 3 embodiment except that the error amplifier has been replaced with a controller  330 . Controller  330  may be a central processing unit (CPU), a microprocessor, etc. In this embodiment, controller  330  receives the measured voltage Vmeas at an input and then compares that voltage Vmeas with a reference voltage Vref. If the two voltages are equal, or within a predetermined range, the controller  330  is configured to do nothing. If Vmeas is not equal with Vref, then controller  330  is configured to increase (or decrease) the gain of variable gain LNA  310 . Thereafter, the received signal is forwarded to the remainder of the receive path  120  with constant receive path gain. 
     Turning now to FIG. 5, a flowchart is depicted as describing a process flow within circuit  300  and controller  330  of the FIG. 4 system. At step S 500 , the error amplifier  315  (FIG. 3) or controller  330  (FIG. 4) receives the voltage measured Vmeas at port  325  as returned by the return sheath. At step S 505 , a decision is made whether Vmeas=Vref. If the two voltages are equal, or within a predetermined range, the error amplifier  315  (FIG. 3) or controller  330  (FIG. 4) is configured to do nothing. That is, at step S 510 , no control signal is sent to variable gain LNA  310 , but rather the received signal passes through to the remainder of the receive path  120  without having its strength adjusted. 
     If at step S 505 , Vmeas is determined not to be equal with Vref, a decision is made as to whether Vmeas is less than Vref at step S 515 . If Vmeas&lt;Vref, then error amplifier  315  or controller  330  sends a control signal  335  to variable gain LNA  310 , at step S 520 , so that a positive bias voltage is applied to the variable gain LNA  310  to increase its gain, thereby increasing the strength of the received signal such that Vmeas=Vref. Thereafter, at step S 525 , the received signal is forwarded to the remainder of the receive path  120  with constant receive path gain. 
     If at step S 515 , it is determined that Vmeas is not less than Vref, the error amplifier  315  or controller  330  concludes that Vmeas is greater than Vref at step S 535  and sends a control signal  335  to variable gain LNA  310  so that a negative bias voltage is applied to decrease its gain, thereby decreasing the strength of the received signal such that Vmeas=Vref. Thereafter, at step S 540 , the received signal is forwarded to the remainder of the receive path  120  with constant receive path gain. 
     The present invention provides a system for sensing signal strength and automatically adjusting receive path gain after a received signal has reached the cell site equipment from a receiver antenna and filter/LNA combination that is located remotely from the cell site equipment (e.g., atop a receiver tower). The system senses a voltage at a receiver port of the cell site equipment. If the sensed voltage is different from a predetermined reference voltage, a control signal is delivered from an error amplifier to a variable gain LNA for varying the gain of the received signal so as to maintain a constant receive path gain. The circuit also provides a source of power for the masthead containing a portion of the receiver circuitry atop a tower. In an alternative embodiment, the error amplifier is replaced with a controller. 
     Since the invention reduces variations in receive path gain, it also improves the matching between the two diversity receive paths traditionally provided in cellular base stations. Therefore, the invention allows for the use of tighter tolerances on internal diagnostic tests that involve the receive path. 
     While preferred embodiments of the invention have been described and illustrated, it should be apparent that many modifications can be made to the invention without departing from its spirit or scope. For example, although systems have been described as containing certain specific components coupled together in a specific manner for sensing a measured voltage and adjusting strength of the received signal, it should be readily apparent that any combination of components may be substituted for the disclosed schematic diagram so long as the object of the circuit is to provide constant receive path gain by sensing and adjusting strength of a received signal at a point within the receiver system that is remote from the actual receiver antenna position and also remote from at least one receiver filter/receiver amplifier stage. Furthermore, although the invention has not been described in connection with any one specific multiple user topology, it should be readily apparent that the invention may be used within a time division multiple access (TDMA) system, and also within a code division multiple access (CDMA) system. Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.