Patent Publication Number: US-7912436-B2

Title: LNA gain adjustment in an RF receiver to compensate for intermodulation interference

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of and claims priority to U.S. patent application having an application Ser. No. 11/523,332; filed Sep. 19, 2006; which application is a continuation of and claims priority to U.S. patent application having an application Ser. No. 09/966,060; filed Sep. 28, 2001, now U.S. Pat. No. 7,120,410; and in which both applications are incorporated herein by reference in this application. 
    
    
     SPECIFICATION 
     1. Field of the Invention 
     This invention relates generally to wireless communications; and more particularly to the operation of a Radio Frequency (RF) receiver within a component of a wireless communication system. 
     2. Background of the Invention 
     The structure and operation of wireless communication systems is generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e. operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc. By complying with to these operating standards, equipment interoperability is achieved. 
     In a cellular system, a governmental body licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies within the frequency spectrum that support the subscribers&#39; subscriber units within the service area. These carrier frequencies are typically equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. 
     In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., GSM cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies and reverse link communications (from subscriber units to the base station) on a second set carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most, if not all cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized but so that system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results. 
     Both base stations and subscriber units include Radio Frequency (RF) transmitters and RF receivers. These devices service the wireless links between the base stations and subscriber units. Each RF receiver typically includes a low noise amplifier (LNA) that receives an RF signal from a coupled antenna, a mixer that receives the output of the LNA, a band-pass filter coupled to the output of the mixer, and a variable gain amplifier coupled to the output of the mixer. These RF receiver components produce an Intermediate Frequency (IF) signal that carries modulated data. 
     In order to improve the signal-to-noise ratio of an RF signal presented to the mixer by the LNA, the gain of the LNA is adjusted. In adjusting the gain of the LNA, great care must be taken. While maximizing the gain of the LNA serves to increase the Signal to Noise Ratio (SNR) of the RF signal, if the LNA gain is too large, the mixer will be driven into non-linear operation and the IF signal produced by the mixer will be distorted. Such is the case because a non-linear operating region of the mixer resides at an upper boundary of its operating range of the mixer. The input power level at which non-linearity is a problem for the mixer is often referred to as a 1 dB compression level. Thus, it is desirable to have the LNA provide as great an amplification of the received RF signal as possible prior to presenting the amplified RF signals to the mixer without driving the mixer into non-linear operation. During most operating conditions, the gain of the LNA may be set by viewing the input power present at the LNA and by setting the LNA gain to produce an output that causes the mixer to operate in a linear region. 
     However, when intermodulation interference exists, this technique for setting the LNA gain does not work. Intermodulation interference occurs when the mixer receives RF carriers (in addition to the desired signal) that cause the mixer to produce intermodulation components at the IF, the same frequency as the desired signal. This problem is well known and is a non-linear phenomenon associated with the operation of the mixer. The intermodulation component at the frequency of the desired signal is a third-order intermodulation component, IM3. In order to minimize intermodulation interference, the gain of the LNA should be reduced. However, reducing the gain of the LNA also reduces the SNR of the signal produced by the mixer. Thus, competing operational goals exist by the competing goals of increasing SNR by increasing the gain of the LNA and by reducing the effects of intermodulation interference by reducing the gain of the LNA. 
     While this problem is well known, its solution is not. Some prior techniques have simply avoided high LNA gain when wideband received signal strength (across some portion of the operating range of the RF receiver) was approximately equal to the narrowband received signal strength (at the IF). Further, when the wideband received signal strength was significantly greater than the narrowband received signal strength, the gain of the LNA was set to a low level. These operations addressed the issue of the existence of interferers. However, it did not consider whether intermodulation interference existed. 
     Thus, there is a need in the art to improve the operational characteristics of the LNA in order to maximize signal-to-noise ratio and to minimize the effects of intermodulation interference. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to apparatus and methods of operation that are further described in the following Brief Description of the Drawings, the Detailed Description of the Embodiments of the Invention, and the Claims. Other features and advantages of the present invention will become apparent from the following detailed description of the embodiments of the invention made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings wherein: 
         FIG. 1  is a system diagram illustrating a cellular system within which the present invention is deployed; 
         FIG. 2  is a block diagram illustrating a Radio Frequency (RF) unit that operates according to the present invention; 
         FIG. 3A  is a system diagram illustrating a portion of the system of  FIG. 1  in which intermodulation interference is produced; 
         FIGS. 3B and 3C  are diagrams illustrating how the mixer of an RF unit of a subscriber unit produces an intermodulation interference signal, IM3, when strong adjacent channel interferers are present; 
         FIG. 4A  is a diagram illustrating the structure of TDMA Time Slots employed in a system that operates according to the present invention; 
         FIG. 4B  is a diagram illustrating the structure of a Time Slot according to the present invention; 
         FIG. 5  is a logic diagram illustrating generally operation according to the present invention; 
         FIGS. 6 and 7  are logic diagrams illustrating in more detail operation according to the present invention; 
         FIG. 8  is a block diagram illustrating the structure of a subscriber unit constructed according to the present invention; 
         FIG. 9  is a block diagram illustrating the structure of a base station according to the present invention; and 
         FIG. 10  is a graph showing the power of received signals present at the input of a mixer during operation according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a system diagram illustrating a cellular system within which the present invention is deployed. The cellular system includes a plurality of base stations  102 ,  104 ,  106 ,  108 ,  110 , and  112  that service wireless communications within respective cells/sectors. The cellular system services wireless communications for a plurality of wireless subscriber units. These wireless subscriber units include wireless handsets  114 ,  120 ,  118 , and  126 , mobile computers  124  and  128 , and desktop computers  116  and  122 . When wirelessly communicating, each of these subscriber units communicates with one (or more during handoff) of the base stations  102  through  112 . Each of the subscriber units of  FIG. 1 , both subscriber units and base stations includes radio frequency circuitry. 
     The services provided by the cellular system include both voice service and data services. Such services are provided according to a cellular networking standard such as the GSM standards, the IS-136 standards, the IS-95 standards, the 1xRTT standards, the 1XEV-DO standards, the 1XEV-DV standards, other operating standards in which communications are carried on a number of carriers across a frequency spectrum. 
       FIG. 2  is a block diagram illustrating a Radio Frequency (RF) unit that operates according to the present invention and that is present in the subscriber units  114 - 128  and/or the base stations  102 - 112  of  FIG. 1 . As shown, the RF unit  200  includes an antenna  202  that couples to a transmit/receive block  204 . The transmit/receive block  204  couples to transmit circuitry  206 . The transmit circuit  206  receives an Intermediate Frequency (IF) transmit signal, up-converts the IF transmit signal to an RF transmit signal, and couples the RF transmit signal to the transmit/receive block  204  that couples the RF transmit signal to the antenna  202 . 
     The transmit/receive switch  204  also couples the antenna  202  to an RF SAW circuit  208  so that the RF SAW circuit  208  receives an RF receive signal. The output of the RF SAW circuit  208  is received by low noise amplifier (LNA)  210 . Associated with the LNA  210  is a LNA gain (LNA_G). The LNA  210  amplifies the signal received at its input from the RF SAW  208  by the LNA_G receives to produce an output which it applies to mixer  212 . Mixer  212  mixes a Local Oscillator (LO) input with the output of the LNA  210  to produce an IF receive signal. The output of the mixer  212  is received by band-pass filter (BPF)  214 , which filters the output of the mixer  212  within a frequency band of interest. Residing within this frequency band of interest is an IF signal that carries modulated data. The output of the BPF  214  is amplified via a Variable Gain Amplifier (VGA)  216 . The VGA  216  produces the IF received signal as its output. 
     According to the present invention, the RF unit  200  includes two received signal strength indicators (RSSIs). A first RSSI, RSSI_A  218 , measures the signal strength of a wideband signal produced by the mixer  212 . The wideband signal whose strength the RSSI_A  218  measures the combined strength of a plurality of carriers that the RF unit  200  operates upon and that are down converted by the mixer. A second RSSI, RSSI_B  220 , receives as its input the output of the BPF  214 . Thus, the RSSI_B  220  measures the received signal strength within the frequency band output by the BPF  214 , a narrowband signal. The narrowband frequency corresponds to the frequency of the IF receive signal that contains modulated data intended for a wireless device containing the RF unit  200  (a signal of interest). In a first alternate embodiment, the RSSI_B  220  receives as its input the output of the VGA  216 . In a second alternate embodiment, an RSSI_B  221  is present in a coupled baseband/IF processor that is separate from the RF unit  200 . Thus, as is illustrated in these alternate embodiments, the RSSI_B need not be directly coupled to the output of the BPF  214  but must be able to measure narrowband signal strength in the received signal path. 
     LNA gain adjustment block  222  receives the measured received signal strengths from the RSSI_A  218  and the RSSI_B  220 . Based upon these two inputs, the LNA gain adjustment block  222  produces the LNA_G for the LNA  210 . The LNA gain adjustment block  222  satisfies various competing criterion. Generally speaking, the LNA gain adjustment block  222  selects an LNA_G to maximize the Signal to Noise Ratio (SNR) of the IF signal produced by the RF unit  200  while operating the mixer  212  in its linear range. Further, the LNA gain adjustment block  222  detects intermodulation interference and, when detected, adjusts the LNA_G so that the mixer  212  operates well within its linear range. 
     The LNA gain adjustment block  222  may be dedicated hardware, may be a combination of hardware and software, or may be implemented in software. Further, the RSSI_B  220 , when contained in the RF unit  200 , will be implemented mostly/fully implemented in hardware. However, the RSSI_B  221  contained in the baseband/IF processor may be implemented partially/fully in software. 
       FIG. 3A  is a system diagram illustrating a portion of the system of  FIG. 1  in which intermodulation interference is produced. The example of  FIG. 3A  shows one possible operating condition of the system of  FIG. 1  in which a large third order modulation product (IM3) is present. Of course, many various other operating conditions may also produce large IM3 products. As shown in  FIG. 3A , subscriber unit  114  receives forward link transmissions from base station  106  at a carrier frequency of 880.2 MHz. However, these forward link transmissions from the base station  106  to the subscriber unit  114  pass through an obstacle  300  that weakens the received signal. Base station  102  transmits forward link signals to other subscriber units at the carrier frequencies of 881.0 MHz and 881.8 MHz. 
       FIGS. 3B and 3C  are diagrams illustrating how the mixer  212  of an RF unit  200  of the subscriber unit  114  produces an intermodulation interference signal, IM3, when the strong adjacent channel interferers are present.  FIG. 3B  illustrates the mixer input power present within the RF unit of subscriber unit  114 . As is shown, interfering carrier (INT_A) is present at 881.0 MHz and interfering carrier (INT_B) is present at 880.2 MHz. The signal separation between INT_A and INT_B is 800 KHz. Further, the separation between INT_A and the desired carrier that carries the signal of interest at 880.2 MHz is 800 KHz and the LO input is at a higher frequency than each of the carrier frequencies. The frequency separation between the LO and the desired carrier is equal to the IF. 
       FIG. 3C  illustrates the output power at the mixer of the RF unit in the receive path for subscriber unit  114 . As is shown, interfering signals INT_A and INT_B produce intermodulation components of the third order, IM3, that coexist with the IF of the desired signal. When the desired signal is relatively weak, the IM3 component is relatively large as compared to the power of the desired signal. The band-pass filter can do nothing to remove the IM3 component which is additive to the desired signal at the IF. Thus, the RF unit simply passes both the desired signal and the IM3 component at the IF to demodulating section of the subscriber unit  114 . 
     With the relatively large IM3 component at the IF, the demodulating section of the subscriber unit  114  will have difficulty extracting modulated data. Thus, high bit error rates and lost packets may occur when the large IM3 component exists. Thus according to the present invention, during a guard period when the desired signal is not present, the gain of the LNA  210  is adjusted by the LNA gain adjustment block  222  to minimize the impact of the IM3 component by ensuring that the mixer  212  operates well within its linear region. These operations are described in detail with reference to  FIGS. 4A through 7 . 
       FIG. 4A  illustrates the structure of TDMA time slot employed in a system that operates according to the present invention. As is shown in  FIG. 4A , transmissions on any carrier within the wireless communication system occur in time slots. Such is the case in almost all systems but particularly in TDMA systems. These time slots are transmitted somewhat continually by the base station to a subscriber unit during an ongoing communication. 
       FIG. 4B  is a diagram illustrating the structure of a time slot according to the present invention. As is shown, the time slot  400  includes a guard period  402  and an active transmission period  404 , followed by the guard period of a subsequent time slot. During the guard period  402 , the carrier corresponding to the desired signal is not present. During this guard period, the LNA gain adjustment block  222  operates to adjust the LNA_G according to the present invention. 
       FIG. 5  is a logic diagram illustrating generally operation according to the present invention. As shown in  FIG. 5 , operation commences after the guard period has commenced (step  502 ). When the guard period commences, a guard period timer may be set to expire when the guard period ends. Then, the wideband received signal strength (RSSI_A) is measured (step  504 ). Then, it is determined whether mixer non-linearity (1 dB compression) is caused based upon the strength of the wideband signal present at the mixer (step  506 ). If the determination at step  506  is affirmative, the LNA_G is reduced at step  508  until it is determined that the mixer is no longer driven into 1 dB compression due to the wideband signal. 
     If the mixer is not driven into 1 dB compression due to the wideband signal, as determined at step  506 , it is next determined whether the wideband signal strength, RSSI_A, exceeds a particular threshold (THRS_B) to indicate that the wideband signal alone will cause the mixer to operate in its upper range. If this threshold is exceeded at step  510 , operation proceeds to step  512  where the narrowband receive signal strength is measured via RSSI_B (step  512 ). With both the wideband received signal strength and narrowband received signal strength, measured operation is performed to determine whether intermodulation interference exists (step  514 ). 
     In determining whether IM3 exists, IM3 is first characterized by Equation (1) as: 
                       IM   ⁢           ⁢   3   ⁢     (     dB   ⁢           ⁢   m     )       =       3   *     Pin   ⁡     (   dB   )         -     2   *   IIP   ⁢           ⁢   3   ⁢     (     dB   ⁢           ⁢   m     )           ⁢     
     ⁢     where   ⁢     :       ⁢     
     ⁢       Pin   =     Input   ⁢           ⁢   Power   ⁢           ⁢   of   ⁢           ⁢   the   ⁢           ⁢   Mixer       ;     ⁢     
     ⁢   and   ⁢     
     ⁢       IIP   ⁢           ⁢   3     =       3   rd     ⁢           ⁢   Order   ⁢           ⁢   Intercept   ⁢           ⁢   Point   ⁢           ⁢   of   ⁢           ⁢   the   ⁢           ⁢     Mixer   .                 Equation   ⁢           ⁢     (   1   )                 
Thus, when an IM3 component exists, its existence will be revealed by:
         (1) measuring RSSI_B;   (2) altering the LNA_G;   (3) measuring again RSSI_B; and   (4) determining that the change in RSSI_B (dB) was greater than the alteration in LNA_G (dB). Such is the case because the IM3 component is not a linear function of LNA_G.       

     If intermodulation interference does exist, the LNA_G is set to optimize the signal-to-interference ratio of the mixer (step  516 ) by causing the mixer to operate well within its linear range. In one particular embodiment of step  516 , the LNA_G is reduced until the IM3 component, as indicated by RSSI_B is lower than a predefined threshold. From both step  516  and when RSSI_A did not exceed THRS_B, operation proceeds to step  518  wherein the LNA_G is set so that the narrowband signal exceeds a minimum. 
       FIGS. 6 and 7  are logic diagrams illustrating in more detail operation according to the present invention as was described in  FIG. 5 . In the description of  FIG. 6 , THRS_A, is chosen as −23 dBm, THRS_B is selected as −43 dBm, THRS_C is selected as −102 dBm and a Delta, D, is selected at 6 dB. From step  504  of  FIG. 5 , operation proceeds to step  602  of  FIG. 6  in the more detailed description of  FIG. 6 . In such case, the wideband received signal strength, RSSI_A, is compared to THRS_A (−23 dBm). If RSSI_A exceeds THRS_A, operation proceeds to step  604  where LNA_G is reduced by one step. In the particular example of  FIG. 6 , the LNA_G may be set at any of 20, 16, 12, 8, 4, and 0 dB. Thus, the gain step is 4 dB, the max gain is 20 dB and the minimum gain is 0 dB. From step  604 , operation proceeds to step  606  where it is determined whether the current LNA_G is greater than LNA_G_MIN (0 dB). If so, operation proceeds again to step  602 . If not, operation ends and the minimum LNA_G of 0 dB is used until the operation of  FIG. 6  is again employed to adjust LNA_G. 
     When RSSI_A does not exceed THRS_A, operation proceeds to step  608  where it is determined whether RSSI_A exceeds THRS_B (−43 dBm). If not, operation proceeds to step  702  of  FIG. 7 . If so, the narrowband received signal strength RSSI_B is measured (step  610 ). Next, it is determined whether RSSI_B exceeds THRS_C (−102 dBm) (step  612 ). If not, operation proceeds to step  702  of  FIG. 7 . If so, it is determined whether the LNA_G is greater than the LNA_G_MIN (step  614 ). If not, operation ends. 
     If so, the LNA_G is reduced by one step and the narrowband received signal strength is again read (step  616 ). Next, it is determined whether an intermodulation interference component IM3 is present (step  618 ). In making this determination, it is assumed that intermodulation component IM3 will be revealed when RSSI_B decreases non-linearly with a decrease in LNA_G. Thus, the dB decrease in RSSI_B caused by the dB reduction in LNA_G (measurement at step  610  versus measurement at step  616 ) is compared to the dB reduction in LNA_G. If the dB decrease in RSSI_B is not linearly related to the dB decrease in LNA_G, IM3 is present. In one particular embodiment, the inequality of Equation (2) is employed to detect IM3: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
                               
                                 RSSI_B 
                                 - 
                                 RSSI_B 
                               
                               ’ 
                             
                             &gt; 
                             
                               ( 
                               
                                 LNA_G 
                                 + 
                                 3 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                             
 
                           
                           ⁢ 
                           
                             where 
                             ⁢ 
                             
                               : 
                             
                           
                           ⁢ 
                           
                             
 
                           
                           ⁢ 
                           
                             
                               RSSI_B 
                               = 
                               
                                 measurement 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 at 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 step 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 610 
                               
                             
                             ; 
                           
                           ⁢ 
                           
                             
 
                           
                           ⁢ 
                           RSSI_B 
                         
                         ’ 
                       
                       = 
                       
                         measurement 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         at 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         step 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         616 
                       
                     
                     ; 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   and 
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     LNA_G 
                     = 
                     
                       LNA_G 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       at 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       step 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       610. 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
     When this equation is satisfied, a third order intermodulation product IM3 is present. If IM3 product is not present, as determined at step  618 , operation ends. If IM3 is present, as determined at step  618 , it is next determined whether the LNA_G is greater than the minimum LNA_G (step  620 ). If not, operation ends. If so, the LNA_G is reduced by one step (step  622 ) and operation proceeds again to step  610 . Therefore, when the mixer is operating in its upper region and IM3 is detected, LNA_G is reduced until RSSI_B does not exceed THRS_C (−102 dBm). Such adjustment of LNA_G will cause the mixer to operate well within its linear range. 
     Referring now to  FIG. 7 , at step  702 , RSSI_B is compared to the difference between THRS_C (−102 dBm) and Delta (6 dBm). If RSSI_B is less than (THRS_C+Delta), it is next determined whether LNA_G is less than the LNA_G_MAX (step  704 ). If so, the LNA_G is increased by one step, the RSSI_B is again measured (step  706 ) and operation returns to step  702 . If at step  702  or  704  a negative determination is made, operation ends. 
       FIG. 8  is a block diagram illustrating the structure of a subscriber unit  802  constructed according to the present invention. The subscriber unit  802  operates with a cellular system, such at that described with reference to  FIG. 1  and according to the operations described with reference to  FIGS. 2-7 . The subscriber unit  802  includes an RF unit  804 , a processor  806 , and a memory  808 . The RF unit  804  couples to an antenna  805  that may be located internal or external to the case of the subscriber unit  802 . The processor  806  may be an Application Specific Integrated Circuit (ASIC) or another type of processor that is capable of operating the subscriber unit  802  according to the present invention. The memory  808  includes both static and dynamic components, e.g., DRAM, SRAM, ROM, EEPROM, etc. In some embodiments, the memory  808  may be partially or fully contained upon an ASIC that also includes the processor  806 . A user interface  810  includes a display, a keyboard, a speaker, a microphone, and a data interface, and may include other user interface components. The RF unit  804 , the processor  806 , the memory  808 , and the user interface  810  couple via one or more communication buses/links. A battery  812  also couples to and powers the RF unit  804 , the processor  806 , the memory  808 , and the user interface  810 . 
     The RF unit  804  includes the components described with reference to  FIG. 2  and operates according to the present invention to adjust the LNA gain. The structure of the subscriber unit  802  illustrated is only an example of one subscriber unit structure. Many other varied subscriber unit structures could be operated according to the teachings of the present invention. 
       FIG. 9  is a block diagram illustrating the structure of a base station  902  constructed according to the present invention. The base station  902  includes a processor  904 , dynamic RAM  906 , static RAM  908 , EPROM  910 , and at least one data storage device  912 , such as a hard drive, optical drive, tape drive, etc. These components (which may be contained on a peripheral processing card or module) intercouple via a local bus  917  and couple to a peripheral bus  920  (which may be a back plane) via an interface  918 . Various peripheral cards couple to the peripheral bus  920 . These peripheral cards include a network infrastructure interface card  924 , which couples the base station  902  to a wireless network infrastructure  950 . 
     Digital processing cards  926 ,  928  and  930  couple to Radio Frequency (RF) units  932 ,  934 , and  936 , respectively. Each of these digital processing cards  926 ,  928 , and  930  performs digital processing for a respective sector, e.g., sector  1 , sector  2 , or sector  3 , serviced by the base station  902 . The RF units  932 ,  934 , and  936  couple to antennas  942 ,  944 , and  946 , respectively, and support wireless communication between the base station  902  and subscriber units. Further, the RF units  932 ,  934 , and  936  operate according to the present invention. The base station  902  may include other cards  940  as well. 
       FIG. 10  is a graph showing the power of received signals present at the input of a mixer during operation according to the present invention. Time increases to the right in the graph, commending with a guard period, during which no desired signal is present. However, thermal noise and an intermodulation interference component IM3, are both present. During a first portion of the guard period (period  1002 ), the LNA_G is set to a maximum gain and the LNA Gain Adjustment Circuit determines that IM3 is present. At transition  1004 , the LNA_G is reduced by one step and, resultantly, the IM3 component decreases non-linearly and the thermal noise increases linearly. Thus, during period  1006 , the IM3 component has decreased non-linearly while the thermal noise floor has increased linearly. 
     At transition  1008 , the LNA_G is reduced by another step. Resultantly, IM3 decreases again non-linearly while the thermal noise floor increases linearly. After the transition  1008 , during period  1010 , the magnitude of IM3, which is decreasing substantially, meets the level of the noise floor. At transition  1012 , the guard period expires and the desired signal is present. At this time, the signal of interest will be present with a carrier-to-interference ratio (C/I). This is denoted as C/I (with). If the gain of the LNA had not been adjusted, the C/I ratio (without) would have been less and a coupled modulator would have had more difficulty in extract data from the desired signal. 
     The invention disclosed herein is susceptible to various modifications and alternative forms. Specific embodiments therefore have been shown by way of example in the drawings and detailed description. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the claims.