Abstract:
A wireless communication device comprises a power amplifier configured to amplify a power level of a transmit signal to a required transmit power level and a transmission line coupled with the power amplifier. The transmission line is configured to convey the amplified transmit signal. The wireless communication device also comprises a power control circuit that includes a bi-directional coupler detector coupled with the transmission line. The bi-directional coupler detector is configured to sense a forward power level and a reflected power level in the transmission line. The power control circuit may be configured to adjust the required transmit power level based at least in part on the forward and reflected power levels sensed by the bi-directional coupler detector. The power control circuit may also adjust an impedance of an impedance matching circuit based at least in part on the reflected power level sensed by the bi-directional coupler detector.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to wireless communication and more particularly to systems and methods for controlling the output power in a wireless communication device.  
           [0003]    2. Background  
           [0004]    There are several factors that impact the transmit power level in the transmitter of a wireless communication device. Two factors that limit the transmit power level, for example, are: 1) Specific Absorption Rate (SAR) requirements; and 2) Adjacent Channel Power Ratio (ACPR) requirements. SAR is a metric used to evaluate compliance of portable devices with the maximum permissible exposure limits as defined in the FCC guidelines on human exposure to Radio Frequency (RF) emissions. Effectively, the FCC guidelines place a limit on the maximum transmit power of a communication device in order to prevent exposure by users of such devices to excessive levels of RF energy.  
           [0005]    ACPR is generally defined as the ratio of the average power in the adjacent frequency channel to the average power in the transmitted frequency channel. In other words, a wireless communication device is configured to transmit over a specific frequency channel at any given time. But due to inherent linearity and other limitations of the components that comprise a communication device transmitter, it is very difficult to prevent the energy transmitted by the device from spreading over into adjacent channels. If too much energy resides in the adjacent channels, then it can interfere with devices operating on those channels. Therefore, many wireless communication standards define limits for ACPR, and even when the applicable standard does not define a limit, ACPR is still a practical limitation.  
           [0006]    In order to maintain acceptable SAR and ACPR limits, conventional communication device transmitters typically comprise a power detector, to detect the transmit power level, and an isolator to isolate the transmitter from reflected energy generated at the interface between the transmitter and the device&#39;s antenna. For example, in a Frequency Modulation (FM) transmitter, SAR is the limiting issue. Therefore, a power detector can be used to ensure that the output power of the transmitter does not exceed the FCC specified limits. In a transmitter that is implementing a complex modulation scheme, such as Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA), on the other hand, there are much more stringent linearity requirements. Thus, ACPR is the limiting issue, although SAR still applies. If the transmitter attempts to produce too much or excessive power is reflected back from the antenna into the transmitter, the linearity and, therefore, the ACPR can be substantially degraded. Accordingly, conventional devices often insert an isolator to block the reflected power and have some means to limit the maximum RF output power if there is a danger of exceeding the transmitter rating before reaching the SAR threshold  
           [0007]    While the conventional detector/isolator approach has certain advantages, it also has certain limitations that can substantially impact the performance of a wireless communication device. For example, the impedance of the transmission line that conveys the transmitted power to the antenna is designed to match the impedance of the antenna in order to reduce the amount of reflected energy and increase transmission efficiency. But when the communication device is placed next to the human head, for example, the impedance of the antenna changes due to the proximity with the head. As a result, more power is reflected back toward the transmitter. When this reflected energy reaches the isolator it is dissipated as heat. Therefore, the resulting radiated transmit power is much lower than it otherwise could be, even taking into account the SAR limitation.  
           [0008]    Additionally, the isolator introduces extra loss into the transmission path that is typically on the order of 0.5 dB. Therefore, the transmitter must supply an extra 0.5 dB of power in order to compensate for the extra loss. Increasing the power, however, also increases the ACPR, i.e., increases the amount of energy in the adjacent channels. Because ACPR is predominantly a 3rd order product, the resulting increase in ACPR is approximately 3 times the increase in transmit power, or 1.5 dB, which can lead to noncompliance with the ACPR requirements. Thus, as can be seen, the conventional detector/isolator approach can have a substantial negative impact on the performance of a wireless communication device.  
           [0009]    [0009]FIG. 1 illustrates an exemplary wireless communication transceiver  100 . Such a transceiver can be included in a wireless communication device, thus enabling the device to communicate over a wireless communication channel  124  in a wireless communication system. Transceiver  100  actually comprises a receive path  106  and a transmit path  110 . Preferably, both paths are interfaced with antenna  102  via a duplexer  108 . Duplexer  108  essentially acts as a filter that is configured to shunt incoming RF signals received by antenna  102  to receive path  106 . Duplexer  108  is further configured to send outgoing RF signals from transmit path  110  to antenna  102 , while providing isolation between paths  106  and  110  so that the incoming and outgoing signals do not interfere with each other.  
           [0010]    The received RF signals are then demodulated and processed so as to extract a baseband information signal in the receive portion of transceiver  100  (not shown). Preferably, the baseband information signal is then decoded and processed in a baseband processor (not shown), such as a Mobile Station Modem (MSM). The MSM, or equivalent, is also preferably responsible for generating and encoding baseband information signals that are to be transmitted over communication channel  124 . The baseband information signals generated by the MSM (not shown) are then modulated with a RF carrier in the transmit portion of transceiver  100 , which generates a RF transmit signal to be transmitted via antenna  102 .  
           [0011]    The transmit portion of transceiver  100  is also preferably configured to set the power level of the RF transmit signal. In general, Power Amplifier (PA)  120  in conjunction with Variable Gain Amplifier (VGA)  122  generate the required power level as demanded by the MSM. PAs are typically key components in any high frequency RF transmitter design. This is because RF transmitters typically require high output power to compensate for path losses in communication channel  124  and to ensure an adequate signal strength at the base station associated with channel  124 . Since the base station can be miles away, it can be difficult to achieve adequate receive power at the base station. At the same time, if the signal power at the base station is too high, then it may interfere with reception by the base station of transmit signals from other devices within the communication system. Transmitting at higher power levels also reduces battery operating time. Therefore, while it is important to ensure an adequate transmit power level, it is also important to ensure that the transmit power level is not too high. Thus, power control in a wireless communication device is an important aspect of wireless communication.  
           [0012]    In conventional wireless communication systems, power control is often performed in the wireless communication device. For example, the base station can be configured to measure the power level of a received transmit signal and determine if it is too high or too low. The base station can then be configured to transmit commands to the wireless communication device instructing the device to turn its power up or down. CDMA communication systems, for example, use such a power control loop. In a CDMA system, the goal of the base stations is to receive signals from each of the devices with which it is communicating at the same receive power level. In fact, such power equalization at the base station for each of the devices in communication with the base station is a critical aspect of CDMA operation. Thus, power control is a critical component of device operation in a CDMA system, although it is similarly important in many types of wireless communication systems.  
           [0013]    For illustrative purposes, a simplified power control loop operation for a CDMA system is described in the following paragraphs in conjunction with the flow chart of FIG. 2. The process of FIG. 2 is intended to illustrate the need for power control and the role it plays in wireless communication. It should not, however, be seen as limiting the systems and methods described herein to any particular type of power control, or any particular power control approach. Nor should it be seen as limiting the systems and methods described herein to any particular type of wireless communication system.  
           [0014]    Again, in a CDMA system, such as an IS-95 compliant system, the transmit power is controlled in the communication device so that devices communicating with the same base station appear to have the same signal strength at the base station. In each device, the transmit power is variable to compensate for changes in the signal strength as received by the base station. The signal strength at the base station can vary due to changing distances between a communication device and the base station and such factors effecting communication channel  124  as multipath fading, varying terrain topology, and varying environmental conditions.  
           [0015]    Referring to FIG. 2, the power control loop in a CDMA system, begins by entering an open loop power control sequence  234  in step  202 . Once in open loop sequence  234 , the device will estimate an initial transmit power in step  204 . For example, the initial estimate can be made using a predetermined loop power equation such as the following equation:  
             Rx  power+ Tx  power=−73  dBm.   (1)  
           [0016]    In equation (1), Rx power is the signal strength of a RF signal received from the bases station over communication channel  124  by antenna  102 . Once this received power level is determined, e.g., via a Received Signal Strength Indication (RSSI) measurement, then it can be used by loop equation (1) to determine the initial transmit power, or Tx power, in step  204 . The device will then transmit a signal at this initial power level to the base station in step  206  and wait for an acknowledgement from the base station in step  210 . If the device does not receive an acknowledgement in step  212 , then it will increase the transmit power in step  214 , transmit again in step  216 , and again wait for acknowledgement (step  210 ). Typically, a device may need to increase its power 1 or 2 times before receiving the acknowledgement.  
           [0017]    The open loop process is a coarse estimate of the required transmit power. Thus a tolerance of +/−9 db is, for example, allowed on the open loop estimate of the required power. Once the device receives an acknowledgement (step  212 ), however, it enters, in step  218 , a closed loop power sequence  236  in which the transmit power level estimate is refined. The goal of closed loop power control sequence  236  is to ensure that the power received at the base station is the minimum level of power required for each device with which the base station is communicating.  
           [0018]    Once in closed loop sequence  236 , the base station measures the received power-to-interference-ratio (Eb/Io) and compares the measured value to a minimum and a maximum threshold in step  222 . If the base station determines that the measured Eb/Io is above the maximum threshold in step  224 , then, in step  226 , it sends a command to the device to reduce its power. If, on the other hand, the base station determines in step  228  that the Eb/Io is below the minimum threshold, then it sends a command to the device to raise its transmit power level in step  230 . Of course, the measured Eb/Io may be between the minimum and maximum thresholds in which case there would be no need for the device to modify its power. In such a situation, the device can be instructed to maintain the same transmit power level in step  232 . The measurement (step  220 ) and comparison ( 222 ) is preferably repeated periodically, e.g., every 1.25 ms, or 800 times per second. Thus, it can be seen that power control plays an important role in proper operation of a communication device within a wireless communication system.  
           [0019]    Referring again to FIG. 1, there are several ways that a device can control the output power in transceiver  100 . Because the transmit power may have to be varied over a large range typically in excess of 70 dB, one way to control the output power is by varying the gain of VGA  122 . Further to ensure improved transmitter efficiency at lower power levels, PA bias  126  may also be adjusted as required. VGA  122  can be configured to amplify the transmit signal before it is sent to PA  120 . How much VGA  122  amplifies the transmit signal can be controlled via a TX POWER CONTROL signal  128 , which can be generated by an MSM (not shown) or some other baseband control circuit (not shown).  
           [0020]    Proper control of the transmit power level, as explained above, can be critical for efficient operation of a wireless communication device in a wireless communication system. There are other limits, however, on the transmit power level in transceiver  100 . For example, as explained above, SAR limitations may restrict the transmit power level regardless of what the power control loop operation may dictate. To ensure that the SAR limitations are not exceeded, conventional wireless communication devices typically employ some sort of power detector  114 . In transceiver  100 , power detector  114  comprises a diode  118 . The output  130  of detector  114  is then sent to a MSM (not shown) or some other baseband control circuit (not shown).  
           [0021]    The analog voltage generated by the detector  114  represents the generated transmit power level and can be converted to a digital number, by means of an analog-to-digital converter for example, such that the MSM (not shown) can adjust the gain of VGA  122  accordingly to meet the desired transmit power level. Notably, however, such a power detection scheme does not take into account reflected power that is dissipated in isolator  112 . Isolator  112  is included because the reflected power can have an adverse effect, e.g., increased ACPR especially at high transmit power levels, if it is allowed to interact with the transmit signal being generated by PA  120 .  
           [0022]    Reflected power occurs because of mismatches in the impedance between the transmission line  132  conveying the transmit signal and antenna  102 . The amount of reflected power is determined by the reflection coefficient, which is a measure of the mismatch in impedance between antenna  102  and transmission line  132 . To lower the reflection coefficient, and thereby reduce the amount of reflected power, conventional devices typically include matching circuit  104 . The purpose of matching circuit  104  is to match the impedance of transmission line  132  with that of antenna  102 . In practice, however, it is very difficult to achieve a perfect match. Therefore, some of the transmit power is reflected back toward PA  120 . The reflected power generates a standing wave on transmission line  132  from the interaction between the forward and reflected signals. Voltage Standing Wave Ratio (VSWR) is a metric used to determine how much of the transmitted power is making it out at antenna  102 . VSWR can be defined by the following equation:  
             VSWR =( Vfwd+Vref )/( Vfwd−Vref ).  (2)  
           [0023]    In equation (2), Vfwd is a measure of the voltage level of the transmit signal on transmission line  132  and Vref is a measure of the voltage level of the reflected signal. If impedance matching circuit  104  provides a perfect match, then the ratio is 1:1 and maximum power will be delivered to antenna  102 . Any deviation from this condition, i.e., a VSWR greater than 1:1, and less than maximum power is delivered to antenna  102 .  
           [0024]    If it were not for isolator  112 , the reflected power would travel back toward PA  120 , reflect again, and travel back toward antenna  102 . Therefore, at least some portion of the reflected power would eventually get out at antenna  102 . Thus, a transceiver can be designed for a VSWR of approximately 2:1 and still have sufficient performance. But in transceiver  102 , the reflected power is actually dissipated in isolator  112  as heat. Thus, any deviation from a VSWR of 1:1 results in wasted transmit power and reduced talk time. But detector  114  does not take into account the effect of isolator  112  and, as a result, transceiver  100  can actually be operating well below SAR limits when the device is limiting the PA output due to the measurements from detector  114 .  
           [0025]    For example, it is not uncommon for the VSWR to degrade from 2:1 to approximately 3:1 when a wireless communication device is placed next to a human head during operation. A VSWR of 3:1, however, means that 25% of the transmit power is reflected back into the device, where it is dissipated as heat in isolator  112 . Because this power is wasted, the power level is much lower than expected. This not only results in inefficient operation of transceiver  100 , but can actually cause the device to lose its connection with the base station. Even if detector  114  is not causing the transmit power level to be limited, PA  120  must operate at excessive power levels in order to compensate for the transmit power wasted in isolator  112 , which not only reduces battery capacity but can also raise ACPR.  
           [0026]    As mentioned above, isolator  112  also typically adds approximately 0.5 dB of loss to the transmission path, which requires PA  120  to increase the transmit power level to compensate. Not only does this result in inefficient operation of PA  120  and reduces battery life among other things, but it also causes the ACPR to increase. Because ACPR is a 3rd order product, a 0.5 dB increase in transmit power will result in approximately a 1.5 dB increase in ACPR, which may cause excessive interference in the adjacent channel.  
           [0027]    In view of the above discussion, it can be seen that the use of detector  114  and isolator  112  can have a substantial negative impact on the performance of transceiver  100 .  
         SUMMARY OF THE INVENTION  
         [0028]    Thus, it is an objective of the invention to eliminate the need for an isolator in the transmitter of a wireless communication device. It is a further objective to provide dynamic control of the transmit power level in the transmitter in order to maintain maximum output power, while still meeting such limitations as SAR and ACPR. In one aspect of the invention, the systems and methods for controlling output power in a communication device use a power control circuit to sense both the forward and reflected power levels in the transmission path between the transmitter and the antenna. The power control circuit is then configured to control the transmit power level of the transmitter based on the power levels that it senses.  
           [0029]    For example, in one particular implementation the transmitter power level can be optimally controlled by adjusting a Power Amplifier (PA) drive signal in accordance with inputs from a MSM, such as a demanded transmit power input, and by forward/reverse powers detected by the power control circuit. For example in an extreme case where forward and reflected powers are identical (100% power reflection), this invention would reduce drive power to the PA to a very low level and avoid the condition of dissipating excess power as heat. Further the power control system would adjust the PA bias to the most appropriate point for a given output level. For higher power levels where high linearity is required the bias would be increased. For lower power levels the bias point would be decreased, but not to a point where ACPR is impacted.  
           [0030]    In another aspect of the invention, the power control circuit may also adjust an impedance of an impedance matching circuit based at least in part on the reflected power level sensed by the bi-directional coupler detector.  
           [0031]    These embodiments as well as other features, advantages, and embodiments are described below in conjunction with the following drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    Preferred embodiments of the present inventions taught herein are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which:  
         [0033]    [0033]FIG. 1 is a logical block diagram illustrating an exemplary wireless communication transceiver;  
         [0034]    [0034]FIG. 2 is a flow chart illustrating an exemplary power control loop process for controlling the transmit power level in the wireless transceiver of FIG. 1;  
         [0035]    [0035]FIG. 3 is a logical block diagram illustrating an example embodiment of a wireless communication transceiver in accordance with the invention;  
         [0036]    [0036]FIG. 4 is flow chart illustrating an example embodiment of a process for controlling the transmit power level in the wireless transceiver of FIG. 3 in accordance with the invention; and  
         [0037]    [0037]FIG. 5 is a logical block diagram illustrating an exemplary power amplifier for use in the transceiver of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    [0038]FIG. 3 illustrates an example embodiment of a transceiver  300  designed in accordance with the systems and methods described herein. As can be seen, transceiver  300  can comprise an antenna  302 , matching circuit  304 , duplexer  308 , receive path  310 , transmit path  328 , PA  312 , and VGA  314 , which operate in substantially the same manner as described in relation to the similar components of transceiver  100 . Instead of isolator  112 , however, transceiver  300  includes power control circuit  318 , which preferably comprises bi-directional coupler detector  316  and a processor  320 . Processor  320  can be a DSP, microcontroller, microprocessor, or any other type of controller capable of implementing the systems and methods as described herein. Further Processor  320  can be integrated within the MSM.  
         [0039]    Bi-directional coupler detector  316  is configured to sense both the forward and reflected power levels on transmission line  328 . By comparing these two power levels, power control circuit  318  can accurately determine the actual amount of transmit power reaching antenna  302  and sent over communication channel  124 . Thus, the transmit power level can be more accurately controlled in order to meet a variety of limitations and/or design criteria, such as SAR, power consumption (battery life), and base station receive power level requirements. Moreover, the reflected power can be monitored to ensure that it remains below an absolute predetermined level in order to avoid excessive ACPR. If the reflected power exceeds the absolute predetermined level, power control circuit  318  can be configured to cause the transmit power level to be reduced in order to bring the reflected power level and, therefore, the ACPR level down to an acceptable level.  
         [0040]    Preferably, processor  320  is configured to control the transmit power level in the same ways described above. Thus, processor  320  is preferably interfaced with VGA  314  in such a manner as to be able to modify the gain of the VGA  314 , which is preferably initially set in accordance with instruction from the MSM (not shown). It will be appreciated that power control circuit  318  may not include a processor  320 . In that case, power control circuit  318  would be coupled to some other suitable controller for performing the functions described herein with reference to processor  320 . A MSM (not shown), for example, could be configured to perform these functions.  
         [0041]    By eliminating isolator  112 , significant savings can be recognized in the construction of transceiver  300  including savings in cost, component count, and board area. Moreover, power control circuit  318  helps to provide greater control over the transmit power level, which results in better performance, reduced power consumption, and increased battery life.  
         [0042]    [0042]FIG. 4 is a flow chart illustrating an example method whereby power control circuit  318  can control VGA  314  and PA  312  in order to control the transmit power level in a transceiver such as transceiver  300 . First, in step  402 , the power control circuit  318  can receive a control signal instructing that the gain of VGA  314  be set to a predetermined level. For example, power control circuit  318  can be instructed to set the gain such that the transmit power level is equal to an initial power level as determined using equation (1) or an equivalent equation. Alternatively, the initial gain can be set by the MSM (not shown) or other baseband control circuit (not shown).  
         [0043]    Next, in step  404 , bi-directional coupler detector  316  senses the forward and reverse power levels on transmission line  324 . Then in step  406 , power control circuit  318  accurately determines the amount of transmit signal power reaching antenna  302  and actually being radiated by comparing the forward and reverse power levels. If the power level reaching antenna  302  and being radiated exceeds the SAR limitations as determined in step  408 , then in step  410  the power control circuit  318  can decrease the transmit power level by varying the gain of VGA  314 , for example. Alternatively, power control circuit  318  can be interfaced with the MSM (not shown), or other baseband control circuit (not shown), configured to control the gain. Thus, after making the determination in step  408 , power control circuit  318  can cause the MSM (not shown) or baseband control circuit (not shown) to reduce the transmit power level.  
         [0044]    In step  408 , bi-directional coupler detector  316  determines if the reflected power level is excessive enough to raise the ACPR to an unacceptable level. If it is, then the process again goes to step  410  and the transmit power level is reduced.  
         [0045]    Power control circuit  318  can be implemented in hardware or software. Preferably, however, the power control circuit  318  comprises a processor, e.g., processor  320 , to executing software/firmware instructions in such a manner as to perform the steps described in the above paragraphs. This allows flexibility to reprogram power control circuit  318 , which can be useful in the face of varying SAR or ACPR requirements, for example.  
         [0046]    A push-pull PA design can be used for PA  312  to further reduce ACPR and enhance power conservation within transceiver  300 . FIG. 5 illustrates an example push-pull PA  500  that can be used in transceiver  300 . PA  500  comprises an in-phase path and an out-of-phase path, which originate at splitter  502 . Splitter  502  receives a transmit signal and sends an in-phase signal to amplifier  504 , which is supplied by BIAS  506 . Splitter  502  also sends an out-of-phase signal to amplifier  508 , supplied by BIAS  510 . The out-of-phase signal is 180° out of phase with the in-phase signal. After amplification in amplifiers  504  and  508 , respectively, the two signals are then balanced and combined in combiner  512 .  
         [0047]    The transmitter linearity performance can be controlled by varying the amplifier biases BIAS  506  and BIAS  510 , respectively. The total transmit power level is the combination of the power levels of the signals in both the in-phase and out-of-phase signal paths. Thus, one advantage of PA  500  is that one or the other of amplifiers  504  and  508  can be completely shut off when the total transmit power requirement is low. This can be accomplished, for example, by simply removing the appropriate bias signal. Thus the overall efficiency at low powers is greatly improved.  
         [0048]    In another embodiment, the power control circuit  318  can be configured to control the impedance of matching circuit  304  in order to reduce the amount of reflected power. This capability allows the power control circuit  318  further means of lowering the ACPR and ensuring more efficient operation of transceiver  300 . Thus, for example, the actual impedance presented by antenna  302  can change as the communication device is brought near the human head, for example. The change in impedance increases the amount of reflected energy and increases the VSWR. In transceiver  300 , the increased amount of reflected power will also increase ACPR, since there is no isolator to prevent the reflected power from mixing with the transmit signal. To prevent this from occurring, power control circuit  318  can be configured to modify the impedance of matching circuit  304  so as to provide a better match and reduce the amount of reflected power.  
         [0049]    In one implementation, the power control circuit  318  can be configured to control the impedance of matching circuit  304  by switching in and out more or less impedance. A matching circuit  304  typically comprises various inductive and capacitive components in various configurations. Thus, for example, the power control circuit  318  can be configured to switch in and out inductive and/or capacitive components to alter the impedance of matching circuit  304 .  
         [0050]    Impedance matching using fixed valued inductive components and capacitors is difficult to achieve and highly application specific. Therefore, only a limited amount of impedance control can practically be achieved in the manner just described. In another implementation, however, matching circuit  304  can comprise ferroelectric tunable inductive components or ferro-electric tunable capacitors or both, the impedance of which can be much more easily controlled to provide dynamic impedance matching for transceiver  300 . Systems and methods for controlling the impedance of a ferro-electric tunable matching circuit are described more fully in U.S. patent application Ser. No. ______ (Applicant Docket No. UTL-00004), entitled “Tunable Matching Circuit,” filed Aug. 10, 2001, which is fully incorporated herein by reference. Thus, by using the systems and methods described in U.S. patent application Ser. No. ______ (Applicant Docket No. UTL-00004), the advantages of the systems and methods described herein are even further enhanced and extended.  
         [0051]    It should be noted that the above examples are provide for illustration only and are not intended to limit the invention to any particular type of transceiver architecture or to any particular type of wireless communication device. Moreover, the systems and methods described herein do not necessarily need to be implemented in a transceiver, they can, for example, be implemented in a device that solely comprise a transmitter incorporating a power control circuit  318 . Thus, while embodiments and implementations of the invention have been shown and described, it should be apparent that many more embodiments and implementations are within the scope of the invention. Accordingly, the invention is not to be restricted, except in light of the claims and their equivalents.