Abstract:
In response to a first signal having modulation representing information and a modulation index, embodiments of the present invention generate a second signal that differs from the first signal in its modulation index, and demodulate the second signal. In one embodiment, an arrangement having a signal processor is adapted to receive a first signal having modulation representing information and having a first modulation index and to generate, in response to the first signal, a second signal having frequency modulation representing the information and having a second modulation index different from the first modulation index, and a demodulator coupled to the signal processor so as to receive and demodulate the second signal. In another embodiment, a method comprises the steps of providing a first signal having modulation representing information and having a first modulation index; generating, in response to the first signal, a second signal having modulation representing the information and having a second modulation index different from the first modulation index; and demodulating the second signal.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to processing of modulated signals, including frequency-modulated signals such as Bluetooth signals. More particularly, this invention relates to processing of modulated signals that includes processing to alter the modulation index of such signals. 
   Various signaling or communication systems require modulated signals with particular modulation characteristics. For instance, the Bluetooth specification requires the use of a Gaussian frequency shift keyed (“GFSK”) modulation format with a modulation index, m, that is between 0.28 and 0.35. The performance of a demodulator may depend on the characteristics of the signal it demodulates. For instance, demodulators exist that have good performance with frequency shift keyed (“FSK”) signals that are nearly orthogonal, i.e., that have a modulation index of about 1. The performance of such demodulators can be lower when demodulating signals with a different modulation index, such as the lower modulation index signals required by the Bluetooth standard. 
   SUMMARY OF THE INVENTION 
   In response to a first signal having modulation representing information and a modulation index, embodiments of the present invention generate a second signal that differs from the first signal in its modulation index, and demodulate the second signal. 
   In one embodiment, the present invention is an arrangement having a signal processor adapted to receive a first signal having modulation representing information and having a first modulation index and to generate, in response to the first signal, a second signal having frequency modulation representing the information and having a second modulation index different from the first modulation index, and a demodulator coupled to the signal processor so as to receive and demodulate the second signal. 
   In another embodiment, the present invention is a method comprising the steps of providing a first signal having modulation representing information and having a first modulation index; generating, in response to the first signal, a second signal having modulation representing the information and having a second modulation index different from the first modulation index; and demodulating the second signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which: 
       FIG. 1  is a block diagram illustrating a receiver in accordance with one embodiment of the present invention. 
       FIG. 2  is a block diagram illustrating another receiver in accordance with an alternative embodiment of the present invention. 
       FIG. 3  is a block diagram illustrating a preferred implementation of the receiver of  FIG. 2 . 
       FIG. 4  is a graph illustrating the spectrum of a frequency-modulated signal at a first carrier frequency. 
       FIG. 5  is a graph illustrating the spectrum of a frequency-modulated signal at a second carrier frequency. 
       FIG. 6  is a graph illustrating the spectrum resulting from amplitude limiting the signal of  FIG. 5 . 
       FIG. 7  is a graph illustrating the spectrum resulting from filtering the signal of  FIG. 6 . 
       FIG. 8  is a graph illustrating in greater detail a portion of the spectrum of  FIG. 7 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram illustrating a receiver circuit in accordance with one embodiment of the present invention. The receiver circuit receives, as an input signal, a first signal S 1  having modulation representing information and having a first modulation index m 1 . The receiver circuit includes a signal processor  100  that generates, in response to first signal S 1 , a second signal S 2  having modulation representing the information contained in S 1  and having a second modulation index m 2  that differs from m 1 . The second signal S 2  is supplied as an input to demodulator  102 , which demodulates S 2  to generate a demodulated output signal. Signal processor  100  may comprise a nonlinear circuit, for example. In accordance with this embodiment of the invention, a receiver, in response to a first signal having modulation representing information and a modulation index, generates a second signal that differs from the first signal in its modulation index, and demodulates the second signal. 
     FIG. 2  is a schematic diagram illustrating another receiver circuit in accordance with an alternative embodiment of the present invention. Elements in the various drawing figures that are the same are indicated by the same reference numerals. In addition to signal processor  100  and demodulator  102  as shown in  FIG. 1 , the receiver of  FIG. 2  includes a frequency converter  200 . The receiver circuit receives, as an input signal, a third signal S 3  having modulation representing information, a modulation index ml, and a carrier frequency f=f 3 . As used herein, “carrier frequency” means the frequency that is modulated by an information-containing modulating signal. Thus, as used herein the carrier frequency of a modulated transmission would be its transmission frequency; if the modulated transmission were mixed to an intermediate frequency (“IF”) signal, its carrier frequency would be the intermediate frequency; and the carrier frequency of the baseband modulating signal would be zero. Frequency converter  200  processes S 3  to generate an output signal having the same modulation index m 1  but a different carrier frequency f l . Frequency converter  200  may comprise a mixer, for example. The output of frequency converter  200  is supplied to signal processor  100  as its input signal S 1 . The remainder of the circuitry of  FIG. 2  functions as described above with respect to  FIG. 1 . In accordance with this embodiment of the invention, a receiver, in response to a third signal having modulation representing information, a carrier frequency, and a modulation index, generates a first signal that differs from the third signal in its carrier frequency, generates a second signal that differs from the first signal in its modulation index, and demodulates the second signal. 
   The Bluetooth standard specifies RF signals having a GFSK-modulated carrier with a modulation index between 0.28 and 0.35. The mean value of the Bluetooth modulation index range is about 0.32. FSK demodulators exist that have excellent performance. For instance, Agere Systems Inc. developed a 1-bit oversampled complex correlation demodulator architecture that was embodied in its products designated CSP1008/1009 and DSP1660 and is described in U.S. Pat. No. 6,288,618 B1. The performance of that demodulator is optimum when processing orthogonal, or nearly orthogonal, FSK signals, i.e., signals having a modulation index m 1.0. The performance of that demodulator is adversely impacted when processing relatively low modulation index signals, such as Bluetooth signals having a modulation index m≈0.32. The performance of a receiver that receives an input signal having a modulation index that is not optimum for its demodulator can be improved by providing circuitry to generate, from the received signal, another signal having a different modulation index that can be demodulated by the demodulator with better performance. For example, the above-referenced 1-bit oversampled complex correlation demodulator can be used as demodulator  102  in the receivers of  FIGS. 1–3  with near optimum performance in processing Bluetooth signals supplied to the receiver, if circuitry is provided to generate an m≈1.0 signal for demodulation from the m≈0.32 received Bluetooth signal. 
     FIG. 3  is a block diagram illustrating a preferred implementation of the receiver of  FIG. 2 . Blocks  302 ,  304 ,  306 ,  308 ,  310 ,  312 , and  314  illustrate a preferred implementation of the frequency converter  200  of  FIG. 2 , and blocks  316 ,  318 , and  320  illustrate a preferred implementation of the signal processor  100  of  FIG. 2 . The receiver of  FIG. 3  receives a radio frequency (“RF”) input signal  301  at a carrier frequency that may be, for example, a GFSK-modulated signal conforming to the Bluetooth standard. The RF input  301  is split by splitter  302  into a pair of signals  303  so that the in-phase (“I”) and quadrature (“Q”) components of RF input  301  can be processed separately. Signals and circuit elements in  FIG. 3  bear a suffix I or Q to indicate that they are, or process, in-phase or quadrature signals, respectively. Each of the split input signals  303  is supplied to the input of a mixer  306 . A reference signal source  304 , for example a local oscillator, generates in-phase and quadrature reference signals  305   I  and  305   Q  that are supplied to mixers  306   I  and  306   Q , respectively. Mixers  306   I  and  306   Q  generate in-phase and quadrature signals  307   I  and  307   Q , respectively, at a carrier frequency determined by the frequency of reference signal source  304 . Preferably mixers  306  mix signals  303  to baseband, but alternatively signals  303  could be mixed to an intermediate frequency. Signals  307   I  and  307   Q  are filtered by filters  308   I  and  308   Q , respectively, to improve channel selectivity. Filters  308  are preferably low-pass filters if signals  303  are mixed to baseband, and bandpass filters if signals  303  are mixed to an intermediate frequency. The filtered signals  309   I  and  309   Q  generated by filters  308  are input to mixers  312   I  and  312   Q . Mixers  312  mix signals  309  to an intermediate frequency that is determined by reference signal source  310 , which supplies reference signals  311  to mixers  312 . The choice of intermediate frequency will generally be based on implementation-specific design considerations. The intermediate frequency in-phase and quadrature signals  313   I  and  313   Q  generated by mixers  312  are summed by summer  314  to generate an intermediate frequency signal  315  that is no longer in I and Q. If mixers  306  mix signals  303  to an intermediate frequency rather than to baseband, then mixers  312  and reference signal source  310  might be omitted. 
   Signal  315  is input to limiter  316 , the output of which, signal  317 , is an amplitude-limited analogue of signal  315 . Limiter  316  may, for example, comprise a comparator, a diode limiter, or other circuit type depending on implementation-specific design considerations. Amplitude limiting is a type of nonlinear signal processing, and generates an output signal having components that are harmonically related to the input signal. Hard limiting by limiter  316  produces a substantially square wave signal  317  having a spectrum that is rich in odd harmonics. Each harmonic component of signal  317  is a frequency-multiplied version of the intermediate frequency signal  315 . Each harmonic component of signal  317  has a frequency deviation that is larger than that of signal  315  by a factor equal to its harmonic number. However, the symbol rate of each harmonic component of signal  317  is the same as that of signal  315 . Thus, each harmonic component of signal  317  has a modulation index that is larger than that of signal  315  by a factor equal to its harmonic number. Amplitude limiting provides modulation index multiplication in each harmonic component of the output. A harmonic component may be selected for demodulation in accordance with the modulation index desired in the signal to be demodulated. 
   If demodulator  102  is sensitive primarily to a particular component in the spectrum of signal  317 , or if demodulator  102  is otherwise capable of demodulating signal  317  to provide an output signal acceptably representing the modulating information notwithstanding the harmonically-related components in signal  317 , then signal  317  could be supplied directly to the input of demodulator  102  and be demodulated by it. However, to reduce adverse effects that might be caused by directly demodulating signal  317 , such as effects due to the presence of a fundamental component having greater power than that of a harmonic component to be demodulated, the receiver of  FIG. 3  processes signal  317  before demodulation. Filter  318  filters signal  317  to enhance the relative strength of a desired harmonic component in the filter output signal  319 . Filter  318  may, for example, be a bandpass filter having a passband preferentially passing a particular harmonic component present in signal  317 . Filter  318  may also, for example, be a high pass filter preferentially attenuating frequencies below a particular harmonic present in signal  317 . Limiter  320 , which may for example be a self-biasing comparator, processes filtered signal  319  to provide a controlled-amplitude input signal  321  to demodulator  102 . 
   If RF input  301  is a Bluetooth-modulated RF signal, either as transmitted or as mixed to an intermediate frequency, it will have a modulation index that is nominally about 0.32. If the third harmonic of intermediate frequency signal  315  is selected by filter  318 , then the input signal  321  to demodulator  102  will have a modulation index that is nominally about (0.32*3) or about 0.96. This is close to the optimum modulation index of the above-referenced 1-bit oversampled complex correlation demodulator, and so the circuitry of  FIG. 3  can process Bluetooth modulated signals to enable improved demodulation performance by such a demodulator. 
   The mixing of the filtered baseband signals  309  back up to an intermediate frequency allows AC coupling between circuit elements (i.e., AC signals are coupled between DC-isolated circuit elements) and hard limiting in the subsequent stages, which is an advantage because it minimizes the DC offset problems inherent in DC coupled direct conversion receiver architectures. AC coupling may be employed between any circuit elements processing signals after they have been mixed to I and Q components, for example, between mixers  312  and summer  314 , and/or between summer  314  and limiter  316 . 
     FIGS. 4–8  are graphs illustrating spectra of signals that may be present in a receiver according to  FIG. 3 . The spectral plots of  FIGS. 4–8  were generated by computer simulation using TESLA simulation software. 
     FIG. 4  is a plot of a GFSK signal spectrum at an arbitrarily-selected frequency of 10.7 MHz, with a time-bandwidth product (“BT”) of 0.5 and modulation index of 0.32, which is representative of the spectrum of a Bluetooth signal. The symbol rate of the signal is 1 megabit per second. The discussion below of  FIGS. 5–8  assumes that such a signal is presented as RF input  301 . 
     FIG. 5  is a plot of the signal spectrum of signals  313  resulting from mixing filtered baseband signals  309  up to an arbitrarily-selected intermediate frequency of 3 MHz. Because of the low modulation index, 0.32, the spectrum of  FIG. 5  lacks distinct FSK tone peaks. Signal  315 , the summed intermediate frequency in-phase and quadrature signals  313 , would also have a spectrum as shown in  FIG. 5 . 
     FIG. 6  is a plot of the signal spectrum of signal  317  resulting from limiting the 3-MHz IF signal  315 . The simulation modeled limiter  316  as a simple comparator that “slices” the 3-MHz IF to a square wave. This produces odd harmonics, of which for example the third, fifth, and seventh can be seen centered at 9 MHz, 15 MHz, and 21 MHz, respectively. The modulation index of each of these harmonic components of signal  317  is higher than that of the fundamental by a factor equal to its harmonic number. 
   As noted above, instead of bandpass filtering to select a particular harmonic for demodulation, a signal such as that of  FIG. 6  might be directly demodulated or high pass filtered and then demodulated. In either case, the signal presented to the demodulator would have several components with different modulation indices, including, if no filtering were done to remove the fundamental, a component with the same modulation index as that of the input. It should be understood that in the terminology used herein, a signal such as that of  FIG. 6  is a second signal, having a modulation index different from that of the first signal in response to which it was generated, if it includes a component that has a modulation index different from that of the first signal, and that component is demodulated. 
     FIG. 7  is a plot of the signal spectrum of signal  319  resulting from filtering the output of limiter  316 . The simulation modeled filter  318  as a 3-pole Chebyshev filter with passband centered at 9 MHz and a passband ripple of 1.0 dB. The plot of  FIG. 7  is at the same scale as the plot of  FIG. 6 . 
     FIG. 8  is also a plot of the signal spectrum of signal  319  resulting from filtering the output of limiter  316 . The plot of  FIG. 8  is at a different scale than the plot of  FIG. 7  to provide a closer, zoomed-in view of the spectrum shown in  FIG. 7 .  FIG. 8  shows more clearly the distinct tone peaks at 9±0.5 MHz (tone peak  800  at 8.5 MHz and tone peak  802  at 9.5 MHz), indicating a nearly orthogonal FSK signal. This spectrum can be processed efficiently and with good performance by demodulators such as the previously described 1-bit oversampled complex correlation demodulator. 
   It will be understood that limiters other than hard limiters may be used to provide outputs other than square waves having different harmonic content, and that nonlinear circuits other than limiters can be used to provide outputs having a modulation index different from their inputs. It will also be understood that embodiments of the invention may be used to process signals having modulation indices other than those of Bluetooth signals to yield signals for demodulation having modulation indices other than about 1. For example, to process a signal having a modulation index of 0.1 to provide a signal for demodulation having a modulation index of 0.5, the fifth harmonic may be generated and selected. Subharmonics of a signal may be generated and selected in order to provide a signal for demodulation with a reduced modulation index. Signals may also be generated for demodulation that are not harmonically related to received signals; the modulation indices of the received signal and the demodulated signal may differ other than by an integer factor 
   Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. 
   It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.