Patent Document

FIELD OF THE INVENTION 
     This invention relates generally to a digital communications receiver, and more specifically, to a method and apparatus for reducing the required dynamic range of the digital communications receiver. 
     BACKGROUND 
     Wireless communication systems have grown tremendously in popularity and are a widely used link in today&#39;s modem communications systems. In general, wireless communications systems comprise a base unit and one or more mobile units serviced by the base unit. Each of the mobile and/or base units comprise a receiver and a transmitter. The information that is exchanged via these base and mobile units may include, for example, voice or data information. One example of a wireless communications system is a cordless telephone system which can be found in many homes and businesses. Another is a cellular phone system. 
     One problem that currently exists in wireless communications systems is the variation in received signal power resulting from variations or changes in distance between receivers and transmitters in the system. For example, in a wireless communications system comprising a cordless telephone and an associated base unit, the power of the signal received by the cordless phone (mobile unit) is significantly greater when the phone is proximate the base unit than it is when the phone is remote from the base unit. In many cases, this variation in received signal power may be of the same order of magnitude as the variation in signal power due to fading, i.e., 60 dB. 
     This variation in received signal power translates into a wider dynamic range requirement of receivers employed in the system. For example, in a wireless communications system in which the variation in power due to fading is about 60 dB, and that due to variations in distance between the transmitter and receiver is about 40 dB, the required dynamic range of the receiver is about 100 dB. The increase in the required dynamic range due to distance variations is about 40 dB. 
     This increase in the required dynamic range of the receiver translates into a receiver which is more complex and expensive, and consumes more power and space than a receiver not subject to this requirement. The problem is particularly acute for digital receivers in which an analog-to-digital (A/D) converter is employed to convert the received signal into a digital format. The increase in the dynamic range requirement directly translates into increased cost, space, and power consumption of the A/D converter. However, for many markets, such as the consumer market, the increase in cost, space, and power consumption which results makes it infeasible to use a digital receiver in the wireless communications system. Although these increases can be offset somewhat by reducing the resolution of the A/D converter, e.g., from 16 to 8 bits, in many cases this results in unacceptable deterioration of signal quality. 
     Accordingly, an object of the subject invention is a method and apparatus for reducing the required dynamic range of a digital communications receiver configured for use in a wireless communications system without significant deterioration in signal quality. 
     Another object is a method and apparatus which overcomes the disadvantages of the prior art. 
     Further objects include utilization or achievement of the foregoing objects alone or in combination. 
     Additional objects and advantages are set forth in the description which follows or will be apparent to those of ordinary skill in the art who practice the invention. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing objects, and in accordance with the purpose of the invention as broadly described herein, there is provided a digital communications receiver comprising: an antenna for receiving a signal; a demodulator coupled to the antenna for demodulating the signal to obtain an information signal, the information signal having a dynamic range; a signal level adjustment circuit coupled to the demodulator for adjusting the signal level of the information signal; a control circuit coupled to the signal level adjustment circuit for controlling the same responsive to a parameter of the received signal; a digitizer for digitizing the reduced signal to obtain a digital signal, the digital signal having an amplitude; and a scaling circuit scaling the digital signal. A related method and computer readable media are also provided. 
     It is contemplated that the present invention may find application in a number devices including but not limited to cordless telephones, cellular telephones, whether CDMA, GSM, or TDMA, two-way radio systems, package or personal tracking devices, personal communications devices, wireless remote controls, baby monitors, and other wireless communications devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a block diagram of an example base unit of the subject invention. 
     FIG. 1B is a block diagram of an example mobile unit of the subject invention. 
     FIG. 2 is a block diagram of the front end of a digital communications receiver in accordance with an embodiment of the subject invention. 
     FIG. 3 is a generalized graph of received power intensity versus transmission distance in a wireless communication system. 
     FIG. 4 is a comparative plot of attenuator response time and automatic gain control response time in an exemplary embodiment of the subject invention. 
     FIG. 5 is an operational flow diagram illustrating operation of one embodiment of the present invention. 
     FIG. 6 is an operational flow diagram illustrating operation of the attenuation level decision process of one embodiment of the present invention. 
     FIG. 7 is a block diagram of the front end of a digital communications receiver in accordance with an embodiment of the invention. 
     FIG. 8 is an operational flow diagram illustrating operation of one embodiment of the subject invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1. Example Environment Of The Subject Invention 
     An exemplary environment in which the subject invention can be beneficially employed is a cordless telephone system, the major components of which are illustrated in FIGS. 1A and 1B. As can be seen, the system comprises a base unit  100  and one or more mobile handsets  102  serviced by the base unit. Base unit  100  is generally stationary, and mobile handsets  102  are capable of being moved in relation to the base unit. Although systems are possible in which multiple handsets and multiple base units are employed, in the ensuing discussion, it will be assumed that the system comprises a single handset and base unit. 
     Communication systems of this nature commonly operate at 900 MHz frequency. Power is provided to the mobile unit via a battery pack or other similar power source. The distance between the mobile unit and base unit may vary between a few feet, such as when the caller is standing right next to the base unit, to 1000 feet or more. 
     Base Unit 
     The base unit  100  includes a primary jack  110  which is coupled to transmitter circuit  112  and receiver circuit  122 . In addition, the primary jack is coupled to a wire-based system such as a public switched telephone network (PSTN), which is not shown. Transmitter circuit  112  is coupled to a modulator  114 , which in turn is coupled to an antenna  116 . The antenna  116  is also coupled to a demodulator  120  and the demodulator is coupled to receiver circuit  122 . Both the receiver circuit  122  and transmitter circuit  112  are optionally coupled to combined microphone and speaker  124  which support a speaker phone feature. Receiver circuit  122  is also coupled to a secondary jack, identified with numeral  126 , as may be required to connect the base unit  100  to an answering machine (not shown). Together, the antenna  116 , demodulator  120 , and receiver circuit  122  comprise what is commonly known as a receiver, and antenna  116 , modulator  114 , and transmitter circuit  112  comprise what is commonly known as a transmitter. 
     In a transmission from the base unit to the mobile unit, transmitter circuit  112  receives an information signal, either from the primary jack, the secondary jack, or the microphone component of element  124 . Then, the transmitter circuit prepares the signal for transmission over a wireless medium, and passes the same to modulator  114 . The modulator  114  modulates the signal onto an RF carrier frequency and passes the modulated signal to the antenna  116 . The antenna  116  transmits the modulated signal to mobile unit  102 . 
     In a transmission from the mobile unit to the base unit, the antenna  116  receives a modulated signal, and passes the same to demodulator  120 . The demodulator  120  demodulates the signal to provide an information signal and provides the same to receiver circuit  122 . Receiver circuit  122  then processes the signal and provides the resulting signal to either the primary jack, the secondary jack, or the speaker component of element  124 . 
     Mobile Unit 
     The mobile unit  102  includes receiver circuit  150 , transmitter circuit  152 , antenna  154 , modulator  156 , demodulator  158 , attenuation circuit  160 , optional data port  172 , speaker  170 , microphone  174 , and optional data port  176 . The attenuation circuit  160  is part of the subject invention, and is described in greater detail below in relation to FIGS. 2-6. It is fully contemplated that the attenuation circuit  160  shown in the mobile unit  102  may reside in either or both of the base unit  100  and mobile unit  102 . 
     The antenna  154  is coupled to demodulator  158  which is turn is coupled to attenuation circuit  160 . Attenuation circuit  160  is also coupled to receiver circuit  150  which in turn is coupled to data port  172  and speaker  170 . Microphone  174  and data port  176  are coupled to transmitter circuit  152 , which in turn is coupled to modulator  156 . Modulator  156  is then coupled to antenna  154 . Together, antenna  154 , demodulator  158 , attenuation circuitry  160 , and receiver circuit  150  comprise a receiver, and antenna  154 , modulator  156 , and transmitter circuit  152  comprise a transmitter. 
     In a transmission from the base unit to the mobile unit, a signal is received by antenna  154  and provided to demodulator  158 . Demodulator  158  demodulates the signal to obtain an information signal, and provides the same to attenuation circuit  160 . In accordance with the subject invention, attenuation circuit  160  attenuates the signal in a manner to be described below in relation to FIGS. 2-6 and provides an attenuated signal to receiver circuit  150 . Receiver circuit  150  processes the signal, and provides the resulting signal to the speaker  170  for voice reproduction or optionally to data port  172 . 
     In a transmission from the mobile unit to the base unit, an information signal is first obtained either via the microphone  174  or optional data port  176 . The information signal is provided to transmitter circuit  152  which processes the signal in preparation for transmission. Transmitter circuit  152  then provides the resulting signal to modulator  156 , which modulates the signal onto an RF carrier frequency. The modulator  156  then provides the modulated signal to antenna  154 , which transmits it to the base unit. 
     2. Embodiments Of The Subject Invention 
     FIG. 7 illustrates a first embodiment of a digital communications receiver in accordance with the subject invention. As shown, an antenna  210  is coupled to a demodulator  212 . The output of the demodulator  212  is coupled to a signal level adjustment circuit  214 , which has an output coupled to an analog to digital (A/D) converter  220 . The output of the A/D is coupled to a scaling circuit  244 . The output of the scaling circuit  244  is coupled to receiver circuit  240 . A control module  232  is coupled to signal level adjustment circuit  214 , and a signal parameter estimator  230  is coupled to the control module  232 . Control module  232  controls the level of signal level adjustment performed by signal level adjustment circuit  214  responsive to the output of signal parameter estimator  230 . Signal parameter estimator  230  estimates a parameter of the signal representative of received signal power. Consequently, signal level adjustment circuit  214  is configured to reduce the power of the incoming signal from demodulator  212  responsive to the estimate of the parameter of the incoming signal. This circuitry can be beneficially employed in either of the base unit  100  or the mobile unit  102  in the example environment discussed above in relation to FIG.  1 . 
     A/D converter  220  digitizes the signal provided by signal level adjustment circuit  214 . The scaling circuit  244  is configured to adjust or scale the digital samples of the digitized signal provided by A/D converter  220 . The scaling circuit  244  adjusts the average power of the digital signal to be at or close to a pre-set level. Accordingly the scaling circuit  244  changes the amplitude of the incoming signal from A/D converter  220  to compensate for average power variation due to fading and other short term influences on the amplitude of the signal. Receiver circuitry  240  is standard circuitry found at the back end of digital communications receivers. 
     In operation, the antenna  210  receives a signal and passes the same to demodulator  212 . Demodulator  212  demodulates the incoming signal to remove the carrier frequency therefrom and obtain an information signal. Signal level adjustment circuit  214  receives the information signal and adjusts the power level thereof at a level controlled by control module  232  responsive to an input from signal parameter estimator  230 , which estimates a parameter of the signal that is used to select a signal level adjustment. Control module  232  receives this parameter estimate from signal parameter estimator  230 , and responsive thereto, it controls the level of signal level adjustment undertaken by signal level adjustment circuit  214 . The reduced signal produced by signal level adjustment circuit  214  is then passed to A/D converter  220 . A/D converter  220  receives the adjusted signal and produces therefrom a digital signal. Scaling circuit  244  receives the digital signal and adjusts the average power of the digital signal responsive to a pre-set value. In one embodiment the pre-set value is dependant on the type of circuitry in receiver components  240 . Accordingly, scaling circuit  244  adjusts the average power of the signal to be at or close to a level desired by receiver componentry  240 . The scaled signal is then provided to standard backend receiver circuitry  240 . Other standard components (not shown) may provide the signal to a speaker or other device. 
     A method of operation of this embodiment is illustrated in FIG.  8 . As shown, in step  510 , an incoming signal from a corresponding transmitter is received and demodulated. Next, in step  520 , a parameter of the signal is estimated. Responsive to this estimate, in step  530 , a signal adjustment level is determined. In step  540 , the power of the signal is adjusted by the level determined in step  530 . In addition, the signal is digitized. During this step, the power level of the digital signal is scaled to compensate for fading which has occurred in the signal. Additional processing may also be performed in this step, including, for example, removal of certain transmitting codes and decompression. Concurrently with the execution of steps  520 ,  530 , and  540 , the parameter of the incoming signal is periodically if not continuously estimated, and the level of signal adjustment performed responsive to this estimate. Such is indicated by the loop back to step  520  from step  540  in FIG.  8 . The purpose is to ensure that the proper adjustment is applied to the incoming signal at substantially all times, and to prevent saturation of downstream receiver components. Finally, in step  550 , the signal is provided to its desired destination, e.g., a speaker or a data port. These steps are performed for the substantial duration of time that a communications link is established between the transmitter and receiver. 
     FIG. 2 illustrates a second embodiment of a digital communications receiver in accordance with the subject invention. As shown, an antenna  210  is coupled to a demodulator  212 . The output of the demodulator  212  is coupled to attenuation circuit  290 , which has an output coupled to an analog to digital A/D converter  220 . The output of the AID converter  220  is coupled to a digital automatic gain control (AGC) circuit  294  as is commonly found in wireless communication receivers. The output of AGC circuit  294  is coupled to receiver circuit  240 . An attenuation control module  232  is coupled to attenuation circuit  290 , and a power estimator  292  is coupled to the attenuation control module  232 . Attenuation control module  232  controls the level of attenuation provided by attenuation circuit  290  responsive to the output of power estimator  292 . Power estimator  292  estimates the strength of the demodulated signal provided by demodulator  212 . Consequently, attenuation circuitry  290  is configured to attenuate the amplitude of the incoming signal from demodulator  212  responsive to the signal strength of the incoming signal. This circuitry can be beneficially employed in either of the base unit  100  or the mobile unit  102  in the example environment discussed above in relation to FIG.  1 . 
     AID converter  220  digitizes the signal provided by attenuation circuit  290  and forwards the signal to the AGC circuit  294 . AGC circuit  294  is configured to scale the average power level of the digitized signal provided by A/D converter  220  responsive to receiver components  294 . Accordingly, AGC circuit  294  adjusts or scales the amplitude of the incoming digitized signal to compensate for fading that inherently occurs in wireless communication systems. Receiver circuitry  240  is standard circuitry found at the back end of digital communications receivers. 
     In operation, the antenna  210  receives a signal and passes the same to demodulator  212 . Demodulator  212  demodulates the incoming signal to remove the carrier frequency therefrom and obtain an information signal. Attenuation circuit  290  receives the information signal and attenuates the amplitude thereof at an attenuation level controlled by the attenuation control module  232  responsive to an input from receiver power estimator  292 , which estimates the power of the demodulated signal from demodulator  212 . The attenuation control module  232  receives this estimate of power from power estimator  292 , and responsive thereto, it controls the level of attenuation undertaken by attenuation circuit  290  such that the level of attenuation is determined by the strength of the incoming signal. 
     The attenuated signal produced by attenuation circuit  290  is then passed to A/D converter  220 . A/D converter  220  receives the attenuated signal and produces therefrom a digital signal. AGC  294  receives the digital signal and adjusts the average power of this signal to be at or close to a preset level. Accordingly, the AGC  294  changes the amplitude of the incoming signal from A/D  220  to compensate for signal variation due to fading and other short term influences on amplitude. The scaled signal from the AGC  294  is then provided to standard backend receiver circuitry  240 . Other standard components (not shown) may provide the signal to a speaker or other device. 
     A method of operation of this embodiment is illustrated in FIG.  5 . As shown, in step  510 , an incoming signal from a corresponding transmitter is received and demodulated. Next, in step  520 , the power level of the signal is estimated. Responsive to this estimate, in step  530 , an attenuation level is determined. In step  540 , the signal is attenuated at the level of attenuation determined in step  530 . In addition, the signal is digitized. The AGC  294  adjusts the amplitude of the digitized signal to bring the average power of the signal to a pre-set level which thereby compensates for signal fading. Additional processing may also be performed in this step, including, for example, removal of certain transmitting codes and decompression. Concurrently with the execution of steps  520 ,  530 , and  540 , the power of the incoming signal is periodically if not continuously estimated, and the level of attenuation determined responsive to this estimate. Such is indicated by the loop back to step  520  from step  540  in FIG.  5 . The purpose is to ensure that the proper attenuation level is applied to the incoming signal at substantially all times, and to prevent saturation of downstream receiver components. Finally, in step  550 , the signal is provided to its desired destination, e.g., a speaker or a data port. These steps are performed for the substantial duration of time that a communications link is established between the transmitter and receiver. 
     The subject invention results in a decrease in the required dynamic range of a digital communications receiver incorporating the invention. That in turn leads to a receiver which is less expensive, consumes less power, and requires less space than a receiver not incorporating the invention. For example, consider a system in which the required dynamic range of a receiver is 100 dB because of power variations due to fading and distance variations between the receiver and transmitter. Assuming an audio telephone application utilizing spread spectrum coding in which 16 bits of resolution is required to handle the 100 dB dynamic range, the A/D converter in such a system must be capable of providing 5 Msamples/sec. @ 16 bits/sample, or 80 Mbits/sec. That is to be contrasted with a system incorporating the subject invention in which the required dynamic range is reduced to 60 dB. Again assuming an audio telephone application in which only 10 bits of resolution is required to handle the 60 dB dynamic range, the A/D converter in such a system need only be capable of handling 50 Mbits/sec. The reduction in required bandwidth and bit resolution allows use of an A/D converter which is less expensive, consumes less power, and consumes less space than an A/D converter capable of greater bandwidth and resolution. 
     EXAMPLE 
     In an exemplary embodiment, the estimate of signal strength determined by receiver power estimator  292  is a received signal strength indicator (RSSI), an average measurement of signal strength determined by averaging the signal received from demodulator  212  over a predetermined time period. In this exemplary embodiment, the predetermined time period is advantageously in the range of about 0.5 seconds to about 3 seconds. Based on the RSSI, the attenuation control module  232  estimates the location of the receiver in relation to the transmitter. One of three possible categories are determined: short range, mid-range, and long-range. The long-range category is determined if the RSSI is approximately two-thirds or more of the total variation in power of the incoming signal attributable to the combined efforts of fading and variations or changes in distance between the transmitter and receiver. The mid-range category is chosen if the RSSI is between about two-thirds and about one-third of the total power variation. The short range category is selected if the RSSI is less than about one-third of the total power variation. 
     Based on the category which is selected, the attenuation level which should be applied is determined. For the short range category, the attenuation level to be applied is about 40 dB; for the mid-range category, the attenuation level selected is about 20 dB; and for the long range category, the attenuation level selected is about 0 dB. 
     It is further contemplated that the thresholds power levels between short range, mid-range, and long range be determined to prevent excessive attenuation level changes. To achieve this objective, advantageously, information such as the history of past attenuation levels and/or distance categories is stored in a memory, and the next distance category and thus attenuation level determined responsive to this information as well as the RSSI. Thus, in the exemplary embodiment, if the RSSI indicates a transition to a distance category with a higher attenuation level, the actual threshold level at which the transition to the second distance category becomes effective is greater than the case in which the RSSI indicates a transition from the second distance category to the first distance category. In accordance with the foregoing, in the exemplary embodiment, the threshold used to transition from a short range to mid-range distance category is greater than the threshold used to transition from the mid-range to short range distance category. In one example, the threshold level used to transition from the short range to mid-range distance categories is 0.4 of the total power variation, while the threshold level used to transition from the mid-range to short range distance levels is 0.3 of the total power variation. This staggering of threshold levels is referred to as hysteresis. 
     In the exemplary embodiment, the attenuation control module  232  instructs the attenuation circuit  290  to attenuate the incoming signal at an attenuation level selected from about 40 dB, about 20 dB, or about 0 dB based on the selected distance category. The attenuation level corresponding to the short range category is about 40 dB; that corresponding to the mid-range category is about 20 dB; and that corresponding to the long range category is about 0 dB. 
     The foregoing principles are illustrated in FIG. 3, which represents a plot of received signal power versus distance of the receiver from the transmitter in a typical cordless telephone system. The vertical axis  302  corresponds to received signal power, and the horizontal axis  304  corresponds to distance between the receiver and transmitter. In this example, 100 dB is the maximum received power. As shown, the horizontal axis is labeled with the three predetermined distance categories, short range, mid-range, and long range, indicated with identifying numerals  330 ,  332 , and  334  respectively. At the top of the figure, the horizontal axis is also labeled with the attenuation levels, about 40 dB, about 20 dB, and about 0 dB, indicated with identifying numerals  340 ,  342 , and  344  respectively, corresponding to each of the predetermined distance categories. Line  310  represents a plot of maximum received power strength as a function of distance between the receiver and transmitter, while line  312  represents a plot of minimum received power strength as a function of distance between the receiver and transmitter. As can be seen, the two lines are vertically displaced relative to one another by about 60 dB, the variation in received power strength due to fading. 
     The difference between the minimum and maximum received signal power for a given distance category defines the variation in received signal power for that category. Thus, for the short range category, identified in the figure with numeral  330 , the variation in received signal power is about 40 dB to about 100 dB, and the attenuation level corresponding to this category is about 40 dB; for the mid-range category, identified in the figure with numeral  332 , the variation in received signal power is between about 20 dB to about 80 dB, and the attenuation level corresponding to this category is about 20 dB; and for the long range category, the variation in received signal power is between about 0 dB to about 60 dB, and the attenuation level corresponding to this category is about 0 dB. 
     In this exemplary embodiment, AGC  294  monitors the digital samples and adjusts their values to thereby compensate for fading in a wireless communication system. To achieve this objective, in the exemplary embodiment, the level of scaling performed by AGC  294  is undertake to cause the average power of the digital samples to approximate a pre-set level. In one variation the pre-set level depends on the receiver components  240 . 
     FIG. 4 illustrates the rate at which the attenuator changes the power level of the incoming signal. The upper graph illustrates the incoming attenuation level  414  versus time  416  in seconds. The lower graph illustrates attenuation level  410  versus time  412  in milliseconds. As shown, at time 200 seconds, reference number  450 , the attenuation level changes from 20 dB to 40 dB. On a time scale  416  of seconds, the increase in attenuation appears as a vertical line  452 . However, as shown in the lower graph, the rate of change in attenuation level is selected to increase generally slowly in time as compared to the response time or slew rate of the AGC. In one variation the attenuation level changes over a period of approximately 200 milliseconds, shown in period  422 . In this variation this is a generally slow rate attenuation introduction. In contrast, a rate of change in power level due to fading is generally 20 dB per 100 milliseconds. Thus, in one variation the attenuation occurs gradually over a period of time larger than the average fade margin. 
     By configuring the attenuator  290  to generally slowly introduce an attenuation step into the incoming signal the attenuation appears as slow fading to AGC  294 . This desirably allows AGC  294  to accurately track the short term changes in average power level in the digital signal arriving at the AGC. For example, in the exemplary embodiment, the response time or slew rate of AGC  294  is greater than that of attenuation circuit  290  to ensure that the scaling level of AGC  294  accurately tracks the signal. 
     FIG. 6 is a flow chart depicting a method of operation of the exemplary embodiment. In step  612 , a call is made or accepted. At the initiation of the call, as indicated by step  614 , it is assumed that the distance between the receiver and transmitter is in the long range category, and thus that the attenuation level is set to about 0 dB. In step  618 , the incoming signal representative of the call is received by the receiver and demodulated to isolate the information component of the signal from the carrier component. 
     Next, in step  620 , the power of the demodulated signal from step  618  is evaluated to determine the distance category which is indicated. If the short range category is indicated, in step  620 , the attenuation level is set to that corresponding to the short term distance category. If the mid-range category is indicated, in step  628 , the attenuation level is set to that corresponding to the mid-range category. If the long range category is indicated, in step  632 , the attenuation level is set to that corresponding to the long range category. 
     Then, in step  624 , additional processing on the signal is performed, including attenuating the signal at the attenuation level set in steps  622 ,  628 , or  632 , digitizing the signal, and then scaling the signal to compensate for fading. 
     Throughout the duration of the call, the power of the incoming signal is continuously or at least periodically monitored, and responsive thereto, the distance category and attenuation level readjusted to account for changes in the distance between the receiver and transmitter. Such is indicated by step  634  in FIG.  6 . 
     In the exemplary embodiment, steps  620 ,  622 ,  626 ,  628 ,  630 , and  632  of the foregoing procedure is implemented in computer software executable on one or more digital signal processors (DSP). It is contemplated that such software be provided on computer readable media such as CD-ROMs, floppy disks, or the like. Digital signal processors are known by those of ordinary skill in the art and accordingly need not be described in great detail herein. These one or more DSPs are configured to work in conjunction with attenuation circuit  290  and AGC  294  to process the incoming signal in accordance with the foregoing principles. 
     In the exemplary embodiment, attenuation circuit  290  is a variable resistance network in which the level of resistance determines the level of attenuation which is performed. In the exemplary embodiment, the level of resistance is determined and controlled by attenuation control module  232 . 
     While embodiments and applications have been shown and described, it should be apparent to those of ordinary skill in the art that the foregoing example is merely illustrative, and that many other embodiments are possible without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be restricted, except as by the appended claims in light of the doctrine of equivalents.

Technology Category: 5