Abstract:
A technique includes storing in a memory a set of samples that are distorted so that the samples indicate a distorted representation of a modulation signal. The technique includes in response to the set of samples, generating a second signal that includes a substantially less distorted representation of the modulation signal. The distortion of the samples is used to at least partially compensate for a characteristic that is otherwise imparted to the second signal by the act of generating the second signal.

Description:
BACKGROUND  
       [0001]     The invention generally relates to a modulator.  
         [0002]     Content digital data typically is communicated over a wireless network in the form of radio frequency (RF) carrier signals, which are modulated to indicate the data.  
         [0003]     Gaussian Minimum Shift Keying (GMSK) is one form of modulation. Referring to  FIG. 1 , a conventional GMSK modulator  10  includes a data stream input terminal  12  that receives an incoming stream of “1” and “0” bits; and in response to the incoming bit stream, the GMSK modulator  10  generates a complex modulation waveform that includes two signal components: an in-phase signal (called “I” in  FIG. 1 ) and a quadrature signal (called “Q” in  FIG. 1 ) that are provided at output terminals  27  and  30 , respectively, of the modulator  10 .  
         [0004]     An encoder  14  of the modulator  10  encoding the incoming bit stream into an impulse stream of “+1” and “−1” impulses, which appear at an output terminal  16  of the encoder  14 . The impulse stream that is furnished by the encoder  14  is routed through a Gaussian filter  18 , and an integrator  20  integrates the resulting filtered signal from the Gaussian filter  18  to produce a signal on an output terminal  22  of the integrator  20 . A block  26  takes the cosine of the signal from the terminal  22  to produce the I in-phase signal; and a block  29  takes the sine of the signal from the terminal to produce the Q quadrature signal.  
       SUMMARY  
       [0005]     In an embodiment of the invention, a technique includes storing in a memory a set of samples that are distorted so that the samples indicate a distorted representation of a modulation signal. The technique includes in response to the set of samples, generating a second signal that includes a substantially less distorted representation of the modulation signal. The distortion of the samples is used to at least partially compensate for a characteristic that is otherwise imparted to the second signal by the act of generating the second signal.  
         [0006]     In another embodiment of the invention, a modulator includes a memory to store a set of samples that are distorted so that the samples indicate a distorted representation of a modulation signal. The modulator includes a controller to, response to the set of samples, generate a second signal that includes a substantially less distorted representation of the modulation signal; and the modulator uses the distortion of the samples to at least partially compensate further processing of the second signal.  
         [0007]     Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]      FIG. 1  is a block diagram of a GMSK modulator of the prior art.  
         [0009]      FIG. 2  is a block diagram of a GMSK modulator according to an embodiment of the invention.  
         [0010]      FIG. 3  is a flow diagram illustrating operation of the GMSK modulator of  FIG. 2  according to an embodiment of the invention.  
         [0011]      FIG. 4  is a block diagram illustrating an exemplary transmit path of a wireless device according to an embodiment of the invention.  
         [0012]      FIG. 5  depicts potential spectral energy that may be present in the modulated signal in the absence of compensation.  
         [0013]      FIG. 6  is a flow diagram illustrating a technique to use the GMSK modulator of  FIG. 2  to compensate the frequency response of the transmit path according to an embodiment of the invention.  
         [0014]      FIG. 7  is an illustration of a sampling technique used in connection with the GMSK modulator of  FIG. 2  according to an embodiment of the invention.  
         [0015]      FIG. 8  is an output waveform segment that is generated by the GMSK modulator of  FIG. 2  according to an embodiment of the invention.  
         [0016]      FIG. 9  illustrates a potential transfer function of a digital-to-analog converter.  
         [0017]      FIG. 10  is a flow diagram depicting a technique to use the GMSK modulator to compensate the systematic non-linearity of the digital-to-analog converter according to an embodiment of the invention.  
         [0018]      FIG. 11  is a schematic diagram of a wireless system according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]     Referring to  FIG. 2 , a Gaussian Minimum Shift Keying (GMSK) modulator  50  in accordance with some embodiments of the invention receives an incoming data bit stream (at an input terminal  54 ) and maps the incoming bit stream to a complex GMSK modulation signal (herein called the “modulation signal”). More specifically, the modulator  50  has two terminals that digitally indicate the components of the modulation signal: a terminal  75  that provides a digital signal that represents the in-phase component of the modulation signal and a terminal  76  that provides a digital signal, which represents the quadrature component of the modulation signal.  
         [0020]     In some embodiments of the invention, the modulation signal contains spectral energy that spans over a certain frequency band, such as a baseband frequency band; and thus, in some embodiments of the invention, the modulation signal may be a baseband signal. However, the invention is not limited to baseband frequencies and baseband frequency modulators. Thus, in other embodiments of the invention, the modulation signal may have a spectral energy content that extends over a radio frequency (RF) band. Thus, many variations and applications of the modulator  50  are possible and are within the scope of the appended claims.  
         [0021]     In accordance with some embodiments of the invention, the modulator  50  digitally synthesizes the modulation signal. In this regard, the modulator  50  takes advantage of the recognition that, in general, a GMSK modulation signal may be represented by a finite collection of output waveform segments. The order in which the segments appear in the modulation signal is a function of the present and recent history of incoming data bit stream. In this regard, the modulator  50  relies on the recognition that a particular time slice of the incoming bit stream produces given I and Q waveforms. Therefore, the modulator  50  processes the incoming data bit stream in time slices, with such time slice being used as an index to select stored I and Q digital waveforms.  
         [0022]     More specifically, in accordance with some embodiments of the invention, potential I and Q waveforms are stored in a look-up table  70  of the modulator  50 . In this manner, each pair of I and Q waveforms correspond to a particular set of waveform samples that is stored in the GMSK modulation data  74 . Thus, each given time slice of the incoming data bit stream signal indexes a set of I and Q samples stored in the look-up table  70 . It is noted that for purposes of limiting the storage area for the GMSK modulation data  74 , in some embodiments of the invention, every possible incoming data bit waveform does not uniquely correspond to a set of I and Q samples (i.e., a 1:1 mapping may not be used). Rather, the modulator  50 , in some embodiments of the invention, may group certain input waveforms together for purposes of determining which set of I and Q samples to use.  
         [0023]     The modulator  50  includes a finite state machine (FSM)  60  that analyzes time slices of the incoming data bit stream to match each time slice to a corresponding set of I and Q samples of the GMSK modulation data  74 . Based on this match, the FSM  60  controls (as described below) an address decoder  80  and an up/down counter  90  to retrieve the corresponding I and Q samples from the memory  70  so that the samples appear on the terminals  75  and  76 .  
         [0024]     Digital-to-analog converters (DACs)  108  and  110  of the modulator  50  convert the digital signals that are provided by the terminals  75  and  76 , respectively, into corresponding analog signals. These analog signals, in turn, are filtered by image rejection filters  114  and  116  to produce an analog in-phase signal (called “I” in  FIG. 2 ), which appears at an analog output terminal  120  of the modulator  50  and an analog quadrature signal (called “Q” in  FIG. 2 ), which appears at another analog output terminal  124  of the modulator  50 .  
         [0025]     In accordance with some embodiments of the invention, the GMSK modulation data  74  only stores one half of the I and Q samples for each time slice of the modulation signal because, for each time slice, the I, Q signal is symmetrical about a midpoint of the time slice. The modulator  50  therefore takes advantage of the symmetry to minimize the storage space for the I and Q samples. In doing so, however, the modulator  50  uses two passes to read a given set of I and Q samples from the look-up table  70 : a first pass to read the I and Q samples for a particular output waveform segment the table  70  in a first order; and a second pass to retrieve the samples from the look-up table  70  in the opposite, or reverse, order for another output waveform segment. Depending on the current incoming bit stream, the above-described passess may read the same set of I and Q samples twice or may read two different sets of samples (one set of I and Q samples in the forward direction and another set of I and Q samples in the reverse direction).  
         [0026]     As a more specific example, in some embodiments of the invention, the modulator  50  may read a particular set of I and Q samples from consecutive memory locations, beginning with reading the first entry of I and Q samples and ending with reading the last entry of I and Q samples. Subsequently, the modulator  50  reads the entries from a particular set of I and Q samples (the same or another set of samples depending on the incoming bit stream) in the reverse order (to generate the remaining symmetrical halves of the I and Q waveforms) by reading the entries from the last entry to the first entry, beginning with the last sample and ending with the first sample.  
         [0027]     For purposes of implementing the above-described technique of storing and retrieving the GMSK modulation data  74  from the look-up table  70 , the FSM  60  controls operations of the address decoder  80  and the up/down counter  90 . More specifically, in accordance with some embodiments of the invention, to retrieve a particular set of I and Q samples from the look-up table  70 , the FSM  60  initializes the counter  90 , such as an action in which the FSM  60  resets the digital output signal from the counter  90  to be zero. For purposes of initializing the address decoder  80 , the FSM  60  may load the starting address or an index pointer to the starting address of the selected set of I and Q samples into the address decoder  80 .  
         [0028]     In some embodiments of the invention, the counter  90  initially counts in an upward direction to cause the address decoder  80  to generate a sequence of increasing addresses to retrieve the selected set of I and Q samples from the look-up table  70 . After the selected set of samples are retrieved (for one half of each of the corresponding I and Q waveforms), the FSM  60  re-initializes the up/down counter  90  to cause the counter  90  to begin counting in a downward direction. In response to the counter&#39;s counting in the downward direction, the address decoder  80  decrements the addresses that are provided to the look-up table  70 . As a result, the same set of samples is read from the look-up table  70  in the reverse order.  
         [0029]     In summary, the modulator  50  may operate pursuant to a technique  150  that is generally depicted in  FIG. 3 . Pursuant to the technique  150 , the FSM  60  identifies (block  152 ) the next segments of the I and Q signals based on the present and recent past history of the incoming data bit stream. Next, pursuant to the technique  150 , the FSM  60  initializes the address decoder  80  with the address of the selected set of samples and initializes the counter  90 , as depicted in block  154 . The initialization of the counter  90  includes initializing the counter  90  to count in a particular direction, such as a direction in which the output signal from the counter  90  increases in value with each count. FSM  60  then begins reading the I and Q entries from the look-up table  70 , as depicted in block  155 . The read I and Q samples are provided to the output terminals  75  and  76 . The reading of the I and Q samples continues until the FSM  60  determines (diamond  156 ) that each of the I and Q waveforms are complete. Next, the FSM  60  allows the continued retrieval of the samples from the look-up table  70 .  
         [0030]     If generation of one half of the output waveform segment is complete, then the FSM  60  returns to block  152  where the FSM  60  targets a set of I and Q samples (pursuant to block  152 ); and the FSM  60  intializes the counter  90  to count in the opposite direction and initializes the address decoder  80  with an address for the targeted set of I and Q samples.  
         [0031]     Thus, in some embodiments of the invention, the direction in which the samples are read from the look-up table  70  alternates each times another pass occurs through the blocks  152 ,  154 ,  155  and  156 .  
         [0032]     Referring to  FIG. 4 , in accordance with some embodiments of the invention, the modulator  50  may be part of a transmit path  200  of a wireless system. As an example, in accordance with some embodiments of the invention, the GMSK modulator  50  may generate a baseband modulation signal. The baseband modulation signal that is provided by the GMSK modulator  50  may ultimately be modulated by a quadrature modulator  205 . The quadrature modulator  205 , in turn, may translate the baseband modulation signal to RF frequencies for purposes of forming a modulated RF carrier signal to be communicated to a wireless network by an antenna  210 .  
         [0033]     In accordance with some embodiments of the invention, the modulation signal that is produced by the GMSK modulator  50  may have a spectral energy that ideally is contained with a given frequency band. However, because the look-up table  70  stores a finite, or limited set of samples, the modulation signal may contain inherent distortion, which introduces spectral energy beyond the targeted band. This may present problems in that this spectral energy may ultimately interfere with an alternate adjacent frequency band generated by another wireless system. More particularly, referring also to  FIG. 5 , if not for the features of the modulator  50  that are described herein, a spectral energy  300  of the modulation signal that is produced by the modulator  50  may include spectral energy  310  that is generally confined within a band (whose upper limit appears at a frequency called “f 1 ”) and an additional unwanted spectral component  304  that appears at a higher out-of-band frequency (called “f 2 ” in  FIG. 5 ). Due to the spectral component  304 , unwanted noise may appear in an alternate frequency band.  
         [0034]     For purposes of preventing the out-of-band spectral component  304  from appearing in the modulation signal that is produced by the modulator  50 , the GMSK modulation data  74  (see  FIG. 2 ) is purposefully pre-distorted to cancel, if not significantly diminish, the spectral component  304 .  
         [0035]     Referring to  FIG. 6 , to summarize, a technique  350  may be used in connection with the modulator  50  in accordance with some embodiments of the invention. The technique  350  includes obtaining (block  352 ) samples of a modulated signal waveform. The samples are distorted (block  354 ) to compensate for an undesired spectral component that may otherwise be present in the modulation signal. These distorted samples are stored in the lookup table  70 , as depicted in block  356 . The distorted samples are then used (block  360 ) by the modulator  50  to produce a reconstructed modulated signal waveform, a waveform that whose spectral frequency components are within the desired band.  
         [0036]      FIG. 7  illustrates a technique that may be used to pre-distort the GMSK modulation data  74  in accordance with some embodiments of the invention. In particular,  FIG. 7  depicts an exemplary output waveform segment  400  (a segment of the I or Q signal) of the modulation signal and illustrates the associated samples that are stored in the GMSK modulation data  74 , as further described below. The waveform segment  400  may be viewed as being divided into two portions  401  and  402  that are symmetrical about a midpoint  403 . Thus, to generate the portion  401 , samples that correspond to times T 0  to time T 7  may be read from the lookup table  70  in sequence; and to generate the portion  402 , the samples that correspond to times T 7  to time T 0  are read from the look-up table  70  in sequence.  
         [0037]     Times T 0 -T 7  represents uniform sampling times, i.e., times at which corresponding samples (such as an exemplary sample  406  that corresponds to uniform sampling time T 2 ) may be provided at the output of the modulator  50  to reproduce a non-distorted version of the portion  401  or  402  of the output waveform segment  400 . Although the modulator  50  reproduces a corresponding output waveform segment pursuant to uniform sampling times that correspond to the uniform sampling times T 0 -T 7 , the GMSK modulation data  74  is purposefully time-shifted to distort the samples. More specifically, as depicted in  FIG. 7 , the first half  401  of the waveform  400  is, instead of being sampled at the sampled points that correspond to the uniform sampling times T 0 -T 7 , sampled at times T 0 *-T 7 *, which are time-shifted versions of times T 0 -T 7 . Therefore, although the samples are taken at times T 0 *-T 7 *, the modulator  50  uses the uniform sampling times T 0 -T 7  to reproduce a version of the output waveform segment  408  at its output.  
         [0038]     As a more specific example, exemplary sampling time T 2  corresponds to exemplary sample  406  if no distortion is introduced. However, instead of storing the sample  406  in the GMSK modulation data  74 , exemplary sample data  408 , taken at time T 2 *, is instead used and thus, stored as part of the GMSK modulation data  74 .  
         [0039]     Referring to  FIG. 8 , the above-described time shifting of the samples causes the modulator  50  to produce a waveform segment  450 . Contrasting the waveform segment  400  of  FIG. 7  with the waveform  450  segment, the waveform  450  is distorted in time in that the waveform  450  includes a discontinuous peak  451  at its midpoint. This distortion in the time domain, in turn, compensates the frequency domain of the modulation signal.  
         [0040]     Thus, as described above, the GMSK modulation data  74  (see  FIG. 2 ) may be time-shifted for purposes of distorting the modulation signal to eliminate if not significantly reduce out-of-band spectral energy.  
         [0041]     The GMSK modulation data  74  may also be pre-distorted for purposes of compensating for characteristics other than frequency characteristics that are introduced downstream of the modulator  50 . For example, referring to  FIG. 2  in conjunction with  FIG. 9 , the DAC  108 ,  110  may have a systematic non-linear transfer function  508 , which is a relationship between the analog output signal from the DAC  108 ,  110  and the digital code that is received at the input terminals of the DAC  108 ,  110 . Ideally, a DAC has a linear transfer function  500 . In general, the closer the transfer function of a DAC is to an ideal linear transfer function is a function of the complexity and die area of the DAC. However, by pre-distorting the GMSK modulation data  74  to compensate for the non-linearity of a DAC, a significantly less complex and smaller DAC may be used.  
         [0042]     More specifically, in accordance with some embodiments of the invention, the magnitudes of the sample values of the GMSK modulation data  74  are pre-distorted to account for the non-linearity of the DAC  108 ,  110 . For example, a particular digital input code called “Code A” in  FIG. 9  that is received by the DAC  108 ,  110  should ideally produce an certain analog output voltage (called “V A ” in  FIG. 9 ) from the DAC  108 ,  110 . However, due to the non-linearity of the DAC  108 ,  110 , the DAC  108 ,  110  instead produces an analog output voltage called “V B ” in  FIG. 9 .  
         [0043]     To compensate for the difference between the ideal linear and non-ideal non-linear response of the DAC  108 ,  110 , the samples that are stored in the look-up table  70  are pre-distorted in amplitude, in some embodiments of the invention. Thus, in some embodiments of the invention, the samples are both time-shifted for purposes of frequency compensation and are amplitude adjusted to compensate for the systematic non-linearity of each of the DACs  108  and  110 .  
         [0044]     Therefore, for the example that is depicted in  FIG. 9 , although Code A is the correct code for a linear DAC, Code A is pre-distorted to be a large digital value called “Code B.” As depicted in  FIG. 9 , in view of the non-linear transfer function  508 , Code B produces the V A  analog output voltage from the DAC  108 ,  110 . Therefore, by pre-distorting the GMSK modulation data  74  in the appropriate manner, the pre-distorted data effectively produces a linear transfer function for the DAC  108 ,  110 .  
         [0045]     To summarize,  FIG. 10  depicts a technique  550  that may be used in accordance with some embodiments of the invention. Pursuant to the technique  550 , an analog signal waveform is sampled (block  554 ) to generate sampled data. This sampled data is distorted (block  560 ) to compensate for the re-occurring, or systematic, non-linearity of a digital-to-analog converter. The technique  550  may be used in connection with the technique  350  (see  FIG. 6 ) to produce the GMSK modulation data  74  for the look-up table  70  which compensates the spectral frequency of the modulation signal as well as compensates for the systematic non-linearity of the DACs  108  and  110 .  
         [0046]     Referring to  FIG. 11 , the GMSK modulator  50  may be used in a wireless system  600  in accordance with some embodiments of the invention. The wireless system  600  may include a transceiver  610  that is coupled to a microphone  708  for purposes of receiving an input speech signal and a speaker  710  for purposes of producing an audio sound from the system  600 . Depending on the particular embodiment of the invention, the transceiver  610  may also be coupled to a keypad  700  to receive input user data and a display  702  for purposes of displaying applications, content data, etc., on the wireless device  600 . Furthermore, the transceiver  610  may be coupled to an antenna  720  for purposes of communicating modulated RF carrier with a wireless network.  
         [0047]     Depending on the particular embodiment of the invention, the wireless system  600  may be, as examples, a handheld device such as a personal digital assistant (PDA) or a cellular telephone. In other embodiments of the invention, the wireless system  600  may be a notebook or a less portable device, such as a desktop computer (as an example).  
         [0048]     The transceiver  610  may be fabricated on a single die that is part of a semiconductor package in accordance with some embodiments of the invention. However, in other embodiments of the invention, the transceiver  610  may be fabricated on multiple dies on a single semiconductor package, may be formed from more than one semiconductor package, etc. Thus, many variations are possible and are within the scope of the appended claims.  
         [0049]     The GMSK modulator  50  may receive its incoming bit stream from a digital signal processor (DSP)  612  of the modulator  50 . As depicted in  FIG. 11 , the modulator  50  provides the modulation signal to a radio  624 .  
         [0050]     For transmissions, the radio  624  receives the modulation signal from the modulator  50  and translates the baseband frequencies to RF frequencies for purposes of transmitting a modulated RF carrier signal over a wireless network via the antenna  720 . For purposes of receiving content from the wireless network, the radio  624  may receive a modulated RF carrier signal from the antenna  720  and translate the RF frequencies of the signal to baseband frequencies to produce an analog modulated baseband signal that is provided to analog-to-digital converter (ADCs)  630 . The ADCs  630  convert the analog modulated baseband signal from the radio  624  into a digital signal that is processed by the DSP  612 . The DSP  612  may implement a de-modulator for purposes of recovering content from the received signal.  
         [0051]     Among the other features of the transceiver  610 , in accordance with some embodiments of the invention, the transceiver  610  may include a microcontroller unit (MCU)  650  that may be coupled to the DSP  612  to generally control and coordinate operations of the transceiver  610 . Depending on the particular embodiment of the invention, the MCU  650  may be coupled to a keypad scanner  652  that receives signals from the keypad  700  and a display driver  656  that generates signals to drive the display  702 . As also depicted in  FIG. 11 , the transceiver  610  may include a speech ADC path  640  for purposes of processing a speech signal received from the microphone  708  and a speech DAC path  644  for purposes of converting a digital speech signal into an analog audio signal that is provided to the speaker  710 .  
         [0052]     It is noted that  FIG. 11  depicts one out of many possible wireless systems in accordance with the numerous possible embodiments of the invention. It is noted that in other embodiments of the invention, other wireless systems may incorporate the GMSK modulator, architectures for the GMSK modulator other than the one that is depicted in  FIG. 2  may be used.  
         [0053]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.