Abstract:
A receiver is provided for receiving a digitally modulated signal in a communication system. The receiver comprises a signal input unit adapted for determining at least one in-phase and at least one quadrature-phase value of the received signal. The receiver further comprises a signal generator connected to receive the in-phase and quadrature-phase values and to generate at least one modified in-phase value and at least one modified quadrature-phase value of a rotated phase constellation system. The receiver further comprises a signal processing unit that is adapted for processing the received signal dependent on the in-phase and quadrature-phase values and the modified in-phase and quadrature-phase values. The signal generator is a passive impedance network. Further, a corresponding integrated circuit chip and operation method are provided. Using a passive impedance network may simplify the hardware implementation by avoiding the need to provide an active amplifier.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to processing digitally modulated signals received in a communication system such as a WLAN (Wireless Local Area Network) system, and in particular to a receiver and an operation method that may be used for estimating the power of a received signal to compensate input power changes and to keep the receiver output constant. 
     2. Description of the Related Art 
     A wireless local area network is a flexible data communication system implemented as an extension to, or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility. 
     Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems. 
     The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to the 802.11b standard, that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible as far as it relates to the direct sequence spread spectrum technology, and both standards adopt various digital modulated techniques. 
     A digitally modulated signal in a wireless local area network has to be processed to compensate the influence of disturbances and to keep the output power constant. For compensating power changes in the input digitally modulated signal, usually an automatic gain control loop unit is provided in the receiver. A typical block diagram of such an automatic gain control loop unit is illustrated in FIG.  1 . The unit of  FIG. 1  comprises an amplifier  100  and a feedback loop having a power calculation unit  110  and a gain control unit  120 . The power calculation unit  110  calculates the current power of the output signal of said amplifier  100 , and the gain control unit  120  delivers a gain control signal to the amplifier  100 . 
     The amplitude or power of any digitally modulated signal may be represented by I (in-phase) and Q (quadrature-phase) values and the I and Q values can be displayed in a complex diagram. The I value represents the real part and the Q value represents the imaginary part of the signal. When the power calculation unit  110  calculates the output power it has to calculate a square root of the sum of the squared I value and the squared Q value for each received pair of I and Q values. The conventional techniques for calculating the output power comprise unnecessary and complicated calculation steps. In particular the calculation of the squared I and Q components and the calculation of the square root is disadvantageous. It has been found that circuits used for calculation of the power are needed to be of significant complexity and are therefore responsible for high development and manufacturing costs. 
     SUMMARY OF THE INVENTION 
     An improved receiver, integrated circuit chip and operation method are provided that may allow for performing a power estimation in a simple and less complex implementation. 
     In one embodiment, there is provided a receiver for receiving a digitally modulated signal in a communication system. The receiver comprises a signal input unit adapted for determining at least one in-phase and at least one quadrature-phase value of the received signal. The receiver further comprises a signal generator connected to receive the in-phase and quadrature-phase values and to generate at least one modified in-phase value and at least one modified quadrature-phase value of a rotated phase constellation system. The receiver further comprises a signal processing unit that is adapted for processing the received signal dependent on the in-phase and quadrature-phase values and the modified in-phase and quadrature-phase values. The signal generator is a passive impedance network. 
     In a further embodiment, an integrated circuit chip may be provided for processing a digitally modulated signal received in a communication system. The integrated circuit chip comprises a signal input circuit that is adapted for determining at least one in-phase and at least one quadrature-phase value of a received digitally modulated signal. The integrated circuit chip further comprises a signal generator circuit adapted for generating at least one modified in-phase value and at least one modified quadrature-phase value of a rotated phase constellation system, and a signal processing circuit adapted for processing the received signal dependent on the in-phase and quadrature-phase values and the modified in-phase and quadrature-phase values. The signal generator circuit is an integrated passive impedance network. 
     In another embodiment, there is provided a method of operating a receiver in a communication system. The method comprises determining at least one in-phase and at least one quadrature-phase value of a received digitally modulated signal, generating at least one modified in-phase value and at least one modified quadrature-phase value of a rotated phase constellation system, and processing the received signal dependent on the in-phase and quadrature-phase values and the modified in-phase and quadrature-phase values. The generation of the modified values is performed by means of a passive impedance network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are incorporated into and form a part of the specification for the purpose of explaining the principles of the invention. The drawings are not to be construed as limiting the invention to only the illustrated and described examples of how the invention can be made and used. Further features and advantages will become apparent from the following, and more particular description of the invention as illustrated in the accompanying drawings, wherein: 
         FIG. 1  is a typical block diagram of a conventional automatic gain control loop for controlling the gain of the amplifier; 
         FIG. 2  is a block diagram of a power estimation unit according to an embodiment and indicates the signal flow between the different units; 
         FIG. 3  is a flow chart illustrating the process of performing the power estimation according to an embodiment; 
         FIG. 4  illustrates a phase constellation system wherein the I and Q values represent a phase constellation point of the received signal, and the IX and QX values represent the phase constellation point rotated by 45°; 
         FIG. 5  is a block diagram of a power estimation unit according to another embodiment, implementing a resistor network; 
         FIG. 6  is a flow chart illustrating the process of performing the power estimation using the resistor network of  FIG. 5 ; 
         FIG. 7  illustrates another phase constellation system; 
         FIG. 8  illustrates the realization of the power estimation unit with electronic devices in electronic circuits according to an embodiment; 
         FIG. 9  illustrates the resistor network of  FIG. 5  with its input and output terminals; 
         FIG. 10  shows the constellation of the rotated phase constellation system, relating to the original phase constellation system; 
         FIG. 11  illustrates the function of the deviation of the maximum value from the true value depending on the signal phase angle; 
         FIG. 12  illustrates a BPSK modulated I signal represented in the time domain for a subset of phase shifts between 0° and 36°; 
         FIG. 13  illustrates a BPSK modulated Q signal represented in the time domain for a subset of phase shifts between 0° and 36°; 
         FIG. 14  illustrates the IX signal represented in the time domain for a subset of phase shifts between 0° and 36°; 
         FIG. 15  illustrates the QX signal represented in the time domain for a subset of phase shifts between 0° and 36°; 
         FIG. 16  illustrates an overlay of the absolute values of the Q signal, I signal, QX signal and IX signal, represented in the time domain for a subset of phase shifts between 0° and 36°; 
         FIG. 17  illustrates the envelope of the maximum of all absolute values of the Q signal, I signal, QX signal and IX signal represented in the phase domain; 
         FIG. 18  illustrates a BPSK modulated I signal represented in the time domain for phase shifts between 0° and 360°; 
         FIG. 19  illustrates a BPSK modulated Q signal represented in the time domain for phase shifts between 0° and 360°; 
         FIG. 20  illustrates the IX signal represented in the time domain for phase shifts between 0° and 360°; 
         FIG. 21  illustrates the QX signal represented in the time domain for phase shifts between 0° and 360°; 
         FIG. 22  illustrates an overlay of the absolute values of the Q signal, I signal, QX signal and IX signal, represented in the time domain for phase shifts between 0° and 360°; 
         FIG. 23  illustrates the envelope of the maximum of all absolute values of the Q signal, I signal, QX signal and IX signal represented in the phase domain for phase; 
         FIG. 24  illustrates a QPSK modulated I signal represented in the time domain for a subset of phase shifts between 0° and 36°; 
         FIG. 25  illustrates a QPSK modulated Q signal represented in the time domain for a subset of phase shifts between 0° and 36°; 
         FIG. 26  illustrates a QPSK modulated IX signal represented in the time domain for a subset of phase shifts between 0° and 36°; and 
         FIG. 27  illustrates a QPSK modulated QX signal represented in the time domain for a subset of phase shifts between 0° and 36°. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The illustrative embodiments of the present invention will be described with reference to the figure drawings, wherein like elements and structures are indicated with like reference numbers. 
     Referring now to the drawings, in particular to  FIG. 2 , a block diagram is depicted of the power estimation unit according to an embodiment. An antenna receives a digitally modulated signal and the received signal is split into its I and Q components by an I value determination unit  200  and a Q value determination unit  210 , respectively. The I and Q values represent the real part and the imaginary part, respectively, of the power of the received digitally modulated signal. 
     A calculation unit  220  determines an IX value by calculating the difference of the I value and the Q value and dividing the result by a factor of two: 
       IX   =       I   -   Q     2         
 
     The calculation unit  220  further determines a QX value by calculating the sum of the I value and the Q value, and dividing the result by a factor of two: 
       QX   =       I   +   Q     2         
 
     The IX and QX values are then input to a weighting unit  230 . The weighting unit  230  multiplies the IX value and the QX value with a factor of the square root of two, i.e. √{square root over (2)}. The weighted IX and QX values represent a point of a phase constellation system that is rotated by 45°. 
     The weighted IX and QX values and the originally received I and Q values are input to an absolute value determination unit  240  for calculating the absolute values of each of the I, Q, IX and QX values. The absolute value determination unit  240  is connected to a peak detector  250  which identifies the maximum of all absolute values that are input to the peak detector  250 . The now available peak value is input to a power calculation unit  260  for calculating a power estimate of the received signal. The power estimate may be used to control the gain of a subsequent amplifier to compensate for input power changes and to keep the output power constant. 
     With reference to  FIG. 3 , the illustrated flow chart describes a method of the power estimation according to an embodiment. In the first step  310 , I and Q values are measured simultaneously. The next step  320  comprises the calculation of the IX and QX values, i.e. the creation of a new complex signal within a 45° rotated constellation system. 
     The power estimation process comprises in the subsequent step the weighting  330  of the IX and QX values by multiplying the values with a factor of the square root of two (√{square root over (2)}). 
     The I and Q values and the weighted IX and QX values are now available for being rectified in step  340 . The rectified I, Q, IX and QX values are then processed in a maximum determination step  350  to determine a maximum value of the rectified values, and provides the maximum value for the power calculation step  360 . This step  360  calculates a power estimate which may be used to control the gain of a subsequent amplifier. 
     At the end of the power estimation process, the entire process may step back to repeat the power estimation with new measured I and Q values. 
       FIG. 4  illustrates a phase constellation system wherein the I and Q values define the position of the received signal point. As apparent from  FIG. 2 , the weighted IX and QX values are determined by the calculation unit  220  and the weighting unit  230 . The weighted IX and QX values define the position of the received signal point, rotated by 45°. 
     As mentioned above, a power calculation may be performed in an automatic gain control loop. For this purpose, the receiver may use a passive impedance network. 
     Referring now to  FIG. 5 , a block diagram is depicted of the power estimation unit according to another embodiment. As above, an antenna receives a digitally modulated signal and the received signal is split in its I and Q components by an I value determination unit  200  and a Q value determination unit  210 , respectively. The I and Q values in the complex diagram will be explained in more detail later. 
     The I and Q values are input signals to a signal generator  500 . The signal generator  500  generates an IN and IP value, and a QN and QP value. The IN value is phase shifted by 180° relative to the phase of the IP value. The QN value is phase shifted by 180° relative to the phase of the QP value. The generated IN and IP values represent the negative or positive originally received I value and the generated QN and QP values represent the negative or positive originally received Q value, respectively. The signal generator is connected to a resistor network  510  and the generated IN, IP, QN and QP values are input to the resistor network. 
     The resistor network  510  comprises a plurality of resistors which are connected to scale down the input signals by a predetermined damping factor and to output the down scaled signals QN 71 , QP 71 , IN 71  and IP 71 . The resistors are further connected for providing the output signals IXN, IXP, QXN and QXP. The resistor network will be explained in more detail later. 
     All output signals of the resistor network  510  are input to an absolute value determining unit  520 . The absolute value determining unit  520  determines the absolute values for each input value. The absolute value determining unit  520  is connected to a peak detector  530  which identifies the maximum of the absolute values delivered from the absolute value determining unit  520 . The identified maximum value is now input to the power calculation unit  260 . The power calculation unit  260  calculates a power estimate that may be used to control a gain of a subsequent amplifier. 
     The power estimation process performed by the device of  FIG. 5  will now be explained with reference to FIG.  6 . In the flow chart, the I and Q values are measured simultaneously in step  300 . In step  600 , the IN, IP, QN and QP values are generated using the measured I and Q values. As mentioned above, the IN value is phase shifted by 180° relative to the phase of the IP value and the QN value is phase shifted by 180 ° relative to the phase of the QP value. 
     As shown in  FIG. 6 , the power estimation process then splits into step  610  of scaling the values down and step  620  of calculating the rotated values. Both steps of the power estimation process may be performed simultaneously, using the IN, IP, QN and QP values previously generated. 
     Step  610  scales down the input values by a factor of the inverse (reciprocal) of the square root of two, and delivers the down scaled values IN 71 , IP 71 , QN 71  and QP 71  to the rectifying unit  520 . Step  620  calculates the IXN, IXP, QXN and QXP values which are also delivered to the rectifying unit  520 . 
     The rectifying step  630  determines the absolute value for each previously determined value, and in step  640 , the maximum of all rectified values is identified. The identified maximum value is now used in the power calculation step  360  to calculate a power estimate which may then be used to control the gain of a subsequent amplifier. 
     Again, the power estimation process may return to step back to repeat the entire process using a resistor network with new measured I and Q values. 
     As mentioned before, the I and Q values can be displayed in a phase constellation system.  FIG. 7  shows a phase constellation system wherein the I and Q values define a phase constellation point of a received signal. Downscaling the phase constellation point of the received signal by a factor of an inverse of the square root of two (1/√{square root over (2)}) results in a downscaled signal point located on the depicted dashed circle line. This dashed circle line represents a level of a phase constellation system that is shrunken by the factor of an inverse of the square root of two. 
     The downscaled signal point represents the related pair of the output signals IN 71 , IP 71 , QN 71  and QP 71  of the resistor network  510  in FIG.  5 . In addition, when operating the resistor network  510 , a signal point in the shrunken phase constellation system is generated that is rotated by 45°. The position of the rotated signal point in the phase constellation system is defined by the IX and QX values. The rotated signal point represents the related pair of the resistor network output signals IXN, IXP, QXN and QXP. 
     Turning now to  FIG. 8 , the electronic devices are shown that may be used to perform the power estimation process. The depicted resistors  840  are connected to form the modified value calculation unit  220  and the resistors of the present embodiment have all the same resistor values. 
     Four peak detector devices  800  to  830  are implemented in the circuit of  FIG. 8  for identifying the maximum of the respective signal. Each peak detector is connected to receive a clear signal to be reset. Further, four electronic switches  850  to  880  are provided to switch the output signals to the output terminals, thereby acting as diodes. 
     The function of the resistor network will now be explained in more detail with reference to FIG.  9 . The signal generator  500  is connected to the resistor network  510  and delivers the IN, IP, QN and QP values to respective input terminals of the resistor network  510 . Between the IN and IP input terminals, and between the QN and QP input terminals, there is provided a resistor divider, to generate the downscaled values. Further, the resistor network comprises a plurality of resistors which each have the same resistor value, to generate the rotated signal point values. 
     The output terminals IN 71  and IP 71  provide the I input signals, downscaled by a first resistor divider connected between the IN and IP input terminals. The output terminals QN 71  and QP 71  provide the Q input signals, downscaled by a second resistor divider that is connected between the QN and QP input terminals. The downscale factor for the IN 71 , IP 71 , QN 71  and QP 71  is the inverse of the square root of two. Thus, downscaling effects a shift of the received signal point in the phase constellation system down to the dashed circle line of FIG.  7 . 
     Still discussing  FIG. 9 , the resistors are connected for delivering both the IN 71 , IP 71 , QN 71  and QP 71  values and the IXN, IXP, QXN and QXP values. As apparent from the above formulas, IXN and IXP represent a difference of the related resistor network input signals and QXN and QXP represent the sum of the resistor network input signals, respectively. Determining the sum and the difference by means of the resistor network  510  effects both an amplitude reduction by a factor of the inverse of square root of two, and a rotation by 45° in the phase constellation system. Thus, the resulting signal point is positioned on the dashed circle of  FIG. 7 , in addition to the signal point that is generated by downscaling the input signals by means of the resistor dividers. 
     To summarize, the resistor network  510  provides the IN 71 , IP 71 , QN 71 , QP 71  output signals as well as the IXN, IXP, QXN and QXP output signals. In the power estimation process, the output signals of the resistor network  510  are then rectified and the maximum of the rectified signals is input to the power calculation unit  260  for calculating a power estimate. 
       FIG. 10  shows schematically the constellation of the input and the output terminals connected via resistors. Referring now to  FIG. 11 , the deviation of the maximum value depending on the signal phase angle is illustrated. The function of the deviation has a periodic form and shows a local maximum at the signal phase angle of 22.5°. The deviation function has its maximum recurring in steps of 45°. 
     Examples of signal waveforms will now be discussed for explaining in detail the operation of the power estimation unit according to one of the embodiments. For this purpose, reference is made to  FIGS. 12  to  27 . 
       FIGS. 12  to  15  illustrate the BPSK modulated I, Q, IX and QX signals in the time domain for a subset of phase shift angles between 0° and 36°. The phase shiftings influence the amplitude of the signal, and the direction of the variation of the amplitude of the signal is indicated by an arrow. 
     The diagram in  FIG. 16  illustrates an overlay of the rectified I, Q, IX and QX signals for a subset of phase shift angles between 0° and 36°. 
       FIG. 17  shows the envelope of the peak value depending on the phase that has a maximum peak value at 0° phase, and a maximum deviation at 22.5°. The curve of  FIG. 17  can be thought as corresponding to the most left portion of the curve FIG.  11 . 
       FIGS. 18  to  21  illustrates the I, Q, IX and QX signals in the time domain for a phase shift angle between 0° and 360°. The phase shiftings influence the amplitude of the signal and each plotted function represents a respective phase shift angle. 
       FIG. 22  illustrates an overlay of the rectified I, Q, IX and QX signals depicted for a phase shift angle between 0° and 360°. 
       FIG. 23  illustrates the envelope of the maximum of the rectified I, Q, IX and QX signals in dependency of the phase between 0° and 360°. The plotted maximum value shows a periodic form wherein the maximum is at 0° and is recurring in steps of 45°. 
       FIGS. 24  to  27  correspond to  FIGS. 12  to  15  but illustrate QPSK modulated I, Q, IX and QX signals in the time domain for a subset of phase shift angles between 0° and 36°. 
     As apparent from the foregoing description, all of the embodiments as described may advantageously provide a high-precision, high-accuracy and high-density technique that may be used in particular in an automatic gain control loop, thus improving overall efficiency. 
     The arrangements may have the advantage to allow for a process such as the power estimation process wherein solving of complicated formulas is no longer necessary. 
     Further, the arrangements may have the advantage due to the fact that a resistor network is used for voltage scaling of signals. This allows for evaluating the power of a digitally modulated signal without using an active amplifier having a gain of the square root of two. Avoiding active elements in the circuits reduces power consumption. 
     Moreover, the manufacturing is simplified and therefore, the above described embodiments effect lower production costs. 
     While the invention has been described with respect to the physical embodiments constructed in accordance therewith, it will be apparent to those skilled in the art that various modifications, variations and improvements of the present invention may be made in the light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention. In addition, those areas in which it is believed that those of ordinary skill in the art are familiar, have not been described herein in order not to unnecessarily obscure the invention described herein. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrative embodiments, but only by the scope of the appended claims.