Abstract:
A method and apparatus blindly detects a received signal&#39;s modulation type characterizing an impairment component of the received signal for each postulated modulation type by determining spatial correlations between In-phase and Quadrature components of the received signal. The blind detection circuit then detects the modulation type based on the characterized impairment component. A metric generator generates a postulation metric for each postulated modulation type based on the characterized impairment component. After evaluating the postulation metrics, an evaluation circuit identifies the postulated modulation type having the best postulation metric as the modulation type of the received signal. According to an exemplary embodiment, the blind detection circuit determines a whitened noise estimate for each postulated modulation type and generates the postulation metrics based on the whitened noise estimate to reduce interference effects in the postulation metrics.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to processing received communication signals and particularly relates to interference cancellation during detection of a received signal&#39;s modulation type.  
         [0002]     In response to an ever-increasing demand for wireless services, wireless providers continue exploring new ways to increase the capacity of current wireless networks. One way to increase network capacity, commonly referred to as frequency reuse, involves reusing the same frequencies across the wireless network. When a wireless network employs frequency reuse, multiple cells within the wireless network reuse predefined radio frequencies to transmit/receive wireless signals within the boundaries of each cell. Ideally, every cell in the wireless network reuses the same frequencies to maximize network capacity. However, this 1/1 frequency reuse plan results in significantly increased interference effects, particularly co-channel and adjacent channel interference effects. These effects cause poor speech quality, lower data throughput, call dropout, etc. As a result, current wireless networks may instead use a 1/4 frequency reuse plan to reduce the interference while still obtaining some network capacity improvement.  
         [0003]     Interference arising from frequency reuse becomes particularly problematic in communication networks where different communication signal formats are used for different kinds of services, or to achieve different data rates, etc. For example, GSM/EDGE (Global System for Mobile communications/Enhanced Data rates for Global Evolution) networks use both Gaussian Minimum Shift Keying (GMSK) modulation and 8-ary Phase Shift Keying (8PSK) modulation. A given receiver in such networks may be expected to detect the modulation type of an incoming received signal such that it properly adjusts its received signal processing.  
         [0004]     Such detection is referred to as “blind” detection because the receiver is expected to determine the received signal&#39;s modulation type without being provided any explicit indication. A wrong guess on the receiver&#39;s part is costly in that nonsensical results flow from processing the received signal using the wrong modulation type assumption. Significantly, received signal interference, including co-channel and adjacent channel interference arising from aggressive frequency reuse, can seriously impair conventional blind detection processing. Such impairment can be particularly problematic in single-antenna receivers.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention comprises a method and apparatus for interference cancellation as part of received signal processing, and particularly comprises a method and apparatus for blind detection of a received signal&#39;s modulation type based on correlations between In-phase and Quadrature components of the received signal. According to the present invention, a blind detection circuit in a wireless receiver characterizes an impairment component of the received signal for each of two or more postulated modulation types by determining the correlations between In-phase and Quadrature components of the received signal. Using the correlations, the blind detection circuit identifies the modulation type of the received signal.  
         [0006]     In one or more exemplary embodiments, the blind detection circuit identifies the modulation type based on the characterized impairment component. The blind detection circuit may include a metric generator configured to generate a postulation metric for each postulated modulation type based on the characterized impairment component. After evaluating the postulation metrics, an evaluation circuit identifies the postulated modulation type corresponding to the best postulation metric as the modulation type of the received signal.  
         [0007]     In another exemplary embodiment, the blind detection circuit characterizes the impairment component by characterizing the interference effects for each of the two or more postulated modulation types based on the spatial correlations between In-phase and Quadrature components of the received signal. The blind detection circuit uses the characterized interference to reduce interference in the postulation metric determined for each postulated modulation type.  
         [0008]     According to another embodiment of the present invention, the blind detection circuit characterizes the impairment component by applying a characteristic phase de-rotation for each postulated modulation type to the received signal to generate a de-rotated signal for each postulated modulation type, and determines interference correlations for each de-rotated signal by spatially and temporally correlating In-phase and Quadrature components of each de-rotated signal. Using the interference correlations, the blind detection circuit reduces interference in the impairment components of the postulated modulation types so that a calculated metric corresponding to the correct postulated modulation type outperforms a calculated metric corresponding to an incorrect postulated modulation type. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  illustrates an exemplary mobile terminal according to the present invention.  
         [0010]      FIG. 2  illustrates an exemplary wireless receiver in the mobile terminal of  FIG. 1 .  
         [0011]      FIG. 3  illustrates an exemplary embodiment of the blind detection circuit in the wireless receiver shown in  FIG. 2 .  
         [0012]      FIG. 4  illustrates an exemplary embodiment of the interference reduction circuit/metric generator in the blind detection circuit shown in  FIG. 3 .  
         [0013]      FIG. 5  illustrates an exemplary blind modulation detection technique according to the present invention.  
         [0014]      FIG. 6  illustrates another exemplary blind modulation detection technique according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]      FIG. 1  illustrates an exemplary mobile terminal  100  according to the present invention. As used herein, the term “mobile terminal” may include cellular telephones, satellite telephones, personal communication services (PCS) devices, personal data assistants (PDAs), palm-top computers, laptop computers, pagers, and the like. Further, those skilled in the art should note the present invention is described in one or more exemplary embodiments relating to GSM/EDGE wireless communication networks, but such descriptions are not limiting. Therefore, it should be understood that the present invention has a broad range of applicability, including other wireless communication standards including, but not limited to, Universal Mobile Telecommunication System (UMTS), TIA/EIA-136, Code Division Multiple Access (CDMA), cdmaOne, cdma2000, and Wideband CDMA.  
         [0016]     As illustrated, mobile terminal  100  includes a transmitter  110 , a receiver  120 , an antenna switch  160 , a system controller  170 , and a user interface  172 . In an exemplary embodiment, system controller  170  stores and executes program instructions that control transmitter  110 , receiver  120 , and antenna switch  160 . Further, system controller  170  interfaces the communication electronics (transmitter  110  and receiver  120 ) with the user interface  172 . When antenna switch  160  connects transmitter  110  to antenna  122 , transmitter  110  transmits wireless communication signals according to a predefined communication standard. Similarly, when antenna switch  160  connects receiver  120  to antenna  122 , receiver  120  receives and processes wireless communication signals according to a predefined communication standard.  
         [0017]      FIG. 2  illustrates an exemplary embodiment of the wireless communication receiver  120  shown in  FIG. 1 . Receiver  120  includes front-end  124 , channel estimator  125 , demodulator  126 , additional processor  128 , and blind detection circuit  130 . Front-end  124  processes a desired signal and one or more interfering signals received at antenna  122  to provide a received baseband signal r(n) to blind detection circuit  130  using filters, amplifiers, analog-to-digital converters, etc., as known in the art. Blind detection circuit  130  implements an exemplary blind modulation detection algorithm according to the present invention that uses knowledge of the training sequence included in the received signal to identify the modulation type of the received signal r(n), as discussed further below.  
         [0018]     For example, receiver  120  may be configured such that blind detection circuit  130  identifies the modulation type of the received signal as either Gaussian Minimum Shift Keying (GMSK) modulation or 8-ary Phase Shift Keying (8PSK) modulation. Based on such detection, demodulator  126  then correctly demodulates the received signal. However, it will be appreciated by those skilled in the art that the present invention is not limited to these two modulation types, and it should be understood that the modulation postulates used for blind detection can include additional or different modulation formats.  
         [0019]     Channel estimator  125  generates channel estimates from the received signal based on the modulation type identified by blind detection circuit  130 . Alternatively, channel estimator refines channel estimates generated by blind detection circuit  130 . In any case, demodulator  126  demodulates the received signal r(n) based on the modulation type identified by blind detection circuit  130  and the provided channel estimates. Alternatively, demodulator  126  demodulates a de-rotated version of the received signal provided by the blind detection circuit  130  based on the modulation type identified by the blind detection circuit  130  and the provided channel estimates. That is, detection circuit  130  may apply characteristic signal de-rotations to the received signal as part of its detection operations, and the de-rotated signal corresponding to the detected modulation type may be provided to the demodulator  126  or the demodulator  126  can operate directly on the received signal. While  FIGS. 1 and 2  illustrate a receiver  120  with a separate channel estimator  125 , those skilled in the art will appreciate that because the blind detection circuit  130  may provide channel estimates to demodulator  126 , channel estimator  125  is optional and may be omitted.  
         [0020]     Regardless, the demodulated values generated by demodulator  126  are then processed further, as necessary, in additional processor  128 . For example, additional processor  128  may include a convolutional decoder or a turbo decoder (not shown) that performs error correction based on the demodulated values provided by demodulator  126  to determine information bit values.  
         [0021]     As mentioned above, blind detection circuit  130  implements a blind modulation detection algorithm to identify the modulation type of the received signal r(n). The received signal r(n) provided to blind detection circuit  130  may be modeled as the sum of an impairment v(n) with the convolution of the radio channel h(n) and the transmitted symbols s(n), as shown in Equation 1:  
                 r   ⁡     (   n   )       =         e   jϕn     ⁢       ∑     k   =   0     L     ⁢           ⁢       h   ⁡     (   k   )       ⁢     s   ⁡     (     n   -   k     )             +     v   ⁡     (   n   )           ,           (     Eq   .           ⁢   1     )             
 
 where impairment v(n) includes white noise and interference components due to the interfering signals received at antenna  122 , where L+1 represents the number of channel paths modeled by receiver  120 , where r(n), h(n), and v(n) are complex values, and where φ denotes a characteristic rotation angle for the particular modulation in use. For example, φ=π/2 for GMSK (GSM) and φ=3π/8 for 8PSK (EDGE). When signal s(n) is transmitted in a densely populated area, impairment v(n) is dominated by co-channel interference, which may be as large as the desired signal. Therefore, the signal-to-noise ratio (SNR) associated with the received signal r(n) is very low, resulting in a high probability that blind detection circuit  130  will incorrectly identify the modulation type of the received signal r(n). By reducing the energy associated with the impairment v(n), the performance of blind detection circuit  130 , and therefore, of receiver  120 , improves. 
 
         [0022]     The impairment v(n) can be modeled as an Auto-Regression (AR) process of order K (see Equation 2).  
               v   ⁡     (   n   )       =         ∑     k   =   1     K     ⁢           ⁢       a   ⁡     (   k   )       ⁢     v   ⁡     (     n   -   k     )           +     e   ⁡     (   n   )                 (     Eq   .           ⁢   2     )             
 
 As shown in Equation 2, the AR process uses the K previous impairment values plus a complex valued white noise e(n) to predict the current impairment v(n), and therefore, only models the correlations of v(n) in the time domain. Equation 2 may be rewritten as:  
                   v   ⁡     (   n   )       -       ∑     k   =   1     K     ⁢           ⁢       a   ⁡     (   k   )       ⁢     v   ⁡     (     n   -   k     )             =     e   ⁡     (   n   )         ,           (     Eq   .           ⁢   3     )             
 
 which illustrates that the white noise e(n) may be generated from impairment v(n) by filtering the impairment v(n) with a standard Finite Impulse Response (FIR) filter. As a result, the FIR filter is a whitening filter. 
 
         [0023]     Because the AR process only models the correlations of v(n) in the time domain, the whitening filter of Equation 3 is a temporal whitening filter that only cancels the interference components with strong temporal correlations from the impairment v(n). As a result, the whitened noise e(n) may still include interference components from interference signals that are weakly correlated in the time domain, such as co-channel interference signals.  
         [0024]     The present invention addresses interference that is strongly correlated in the “spatial” domain, as defined by the In-phase (I) and Quadrature (Q) components, as well as interference that is correlated in the time domain. By exploiting such correlations within the impairment v(n), the impairment energy can be significantly reduced. Specifically, the present invention generates two signals, namely the In-phase (I) and a Quadrature (Q) component signal, from the single complex received signal r(n). Therefore, Equation 1 may be rewritten as:  
                 r   ⁡     (   n   )       =       (             r   I     ⁡     (   n   )                   r   Q     ⁡     (   n   )             )     =         Φ   n     ⁢       ∑     k   =   0     L     ⁢           ⁢       (             h   I     ⁡     (   k   )                   h   Q     ⁡     (   k   )             )     ⁢     s   ⁡     (     n   -   k     )             +     (             v   I     ⁡     (   n   )                   v   Q     ⁡     (   n   )             )           ,     
     ⁢         where   ⁢           ⁢   Φ     ≡       (           cos   ⁢           ⁢   ϕ             -   sin     ⁢           ⁢   ϕ               sin   ⁢           ⁢   ϕ           cos   ⁢           ⁢   ϕ           )     ⁢           ⁢   and   ⁢           ⁢     r   ⁡     (   n   )           =         r   I     ⁡     (   n   )       +         r   Q     ⁡     (   n   )       ⁢         -   1       .                   (     Eq   .           ⁢   4     )             
 
 Equation 4 illustrates that the transmitted signal s(n) may be estimated using either the I component of the received signal, r 1 (n), or the Q component of the received signal, r Q (n). As a result, Equation 4 indicates there is a strong correlation between the I and Q components of the received signal r(n). Because the I and Q components of the received signal can mathematically be treated as two received signals from two different antennas, the correlation between the I and Q components of the received signal is referred to as a spatial correlation, which thereby permits a single-antenna receiver embodiment of the present invention to nonetheless employ spatial correlation processing for interference reduction. 
 
         [0025]     Using a Vector Auto-Regression (VAR) process of order 2, the I and Q components of the impairment v(n) may be modeled as shown in Equation 5. 
 
 v   I ( n )= a   0,0 (1) v   I ( n− 1)+ a   0,1 (1) v   Q ( n− 1)+ a   0,0 (2) v   I ( n− 2)+ a   0,1 (2) v   Q ( n− 2)+ e   I ( n ) 
 
 v   1 ( n )= a   1,0 (1) v   I ( n− 1)+ a   1,1 (1) v   Q ( n− 1)+ a   1,0 (2) v   I ( n− 2)+ a   1,1 (2) v   Q ( n− 2)+ e   Q ( n )   (Eq. 5) 
 
 Equation 5 illustrates that the I component of the modeled impairment v I (n) depends on both its own previous values (temporal correlation) and the previous values of the Q component (spatial correlation). Similarly, the Q component of the modeled impairment v Q (n) depends on both its own previous values (temporal correlation) and the previous values of the I component (spatial correlation). As such, Equation 5 models the correlation of the I and Q components of the impairment v(n) in both time and space. 
 
         [0026]     When rewritten as shown in Equation 6, the modeled impairment {umlaut over (V)}(n) looks very similar to the modeled impairment of Equation 3, except that the complex-valued impairment v(n) and white noise e(n) are replaced by the corresponding I/Q vectors, {umlaut over (V)}(n) and Ë(n), and the complex-valued filter coefficients, a(k), are replaced by 2×2 matrices of filter coefficients A(k), (k=1, 2).  
                     V   ¨     ⁡     (   n   )       -       ∑     k   =   1     2     ⁢           ⁢       A   ⁡     (   k   )       ⁢       V   ¨     ⁡     (     n   -   k     )             =       E   ¨     ⁡     (   n   )         ⁢     
     ⁢         where   ⁢           ⁢       V   ¨     ⁡     (   n   )         =     (             v   I     ⁡     (   n   )                   v   Q     ⁡     (   n   )             )       ,       A   ⁡     (   k   )       =     (             a     0   ,   0       ⁡     (   k   )               a     0   ,   1       ⁡     (   k   )                   a     1   ,   0       ⁡     (   k   )               a     1   ,   1       ⁡     (   k   )             )       ,       and   ⁢           ⁢       E   ¨     ⁡     (   n   )         =       (             e   I     ⁡     (   n   )                   e   Q     ⁡     (   n   )             )     .                 (     Eq   .           ⁢   6     )             
 
 As a result, Equation 6 also represents an FIR filtering of the vector-valued impairment {umlaut over (V)}(n) by a matrix FIR filter with the following filter taps:  
                 W   ⁡     (   0   )       =     (         1       0           0       1         )       ,       W   ⁡     (   1   )       =     (             a     0   ,   0       ⁡     (   1   )               a     0   ,   1       ⁡     (   1   )                   a     1   ,   0       ⁡     (   1   )               a     1   ,   1       ⁡     (   1   )             )       ,       and   ⁢           ⁢     W   ⁡     (   k   )         =     (             a     0   ,   0       ⁡     (   2   )               a     0   ,   1       ⁡     (   2   )                   a     1   ,   0       ⁡     (   2   )               a     1   ,   1       ⁡     (   2   )             )               (     Eq   .           ⁢   7     )             
 
         [0027]     Because the FIR filter in Equation 6 filters the impairment in both space and time, and because the filtered result is a whitened noise vector Ë(n), which typically has a lower power than {umlaut over (V)}(n), the FIR filter of Equation 6 is a spatial-temporal whitening filter. Therefore, in addition to canceling the adjacent channel interference, the FIR filter of Equation 6 also cancels co-channel interference. In other words, the spatial and temporal properties of the vector-valued impairment model of Equation 5 may be exploited to reduce both co-channel and adjacent channel interference present in a received signal received by a single antenna receiver  120 .  
         [0028]     The above formulation illustrates how separating the complex-valued impairment v(n) into its I and Q components reduces spatially and temporally correlated interference from an impairment component of the received signal by using the I and Q components to generate a whitened noise estimate for each postulated modulation type and comparing the resulting white noise estimates to determine which estimate has less energy, and therefore, less interference. The blind detection circuit  130  can use this result to detect the modulation type of the received signal. In other words, blind detection circuit  130  gains a much improved characterization of the impairment component v(n) in a received signal by exploiting the spatial and temporal correlations between the I and Q components of the received signal, thereby improving the accuracy of blind detection circuit  130 . Further, in one or more embodiments, the characterization of interference thus gained can be used to benefit demodulation in demodulator  126 .  
         [0029]      FIG. 3  illustrates an exemplary embodiment of the blind detection circuit  130  according to the present invention. Blind detection circuit comprises a signal rotator  132 , a channel estimator  134 , an evaluation circuit  136 , and an impairment characterization unit  140 . While shown as separate components, those skilled in the art will appreciate that two or more of these components may be combined into the same functional circuit. Further, those skilled in the art will appreciate that one or more of these circuits may be embodied in hardware and/or software (including firmware, software, micro-code, etc.), including an application specific integrated circuit (ASIC), field programmable gate array (FPGA), etc  
         [0030]     Signal rotator  132  applies a predetermined phase de-rotation corresponding to a postulated modulation type to the received signal r(n). In an exemplary embodiment, signal rotator  132  de-rotates the received signal r(n) by π/2 to generate a GMSK postulated received signal r G (n), and de-rotates the received signal r(n) by 3π/8 to generate an 8PSK postulated received signal r 8 (n). Using the de-rotated signals, channel estimator  134  estimates the radio channel(s) h(n) associated with each of the GMSK postulated received signal r G (n) and the 8PSK postulated received signal r 8 (n).  
         [0031]     Based on the channel estimates and the postulated (de-rotated) received signals, impairment characterization unit  140  characterizes the impairment component v G (n) of the GMSK postulated received signal r G (n) by determining spatial-temporal correlations between the I and Q components of the GMSK postulated received signal r G (n). Similarly, impairment characterization unit  140  characterizes the impairment component v 8 (n) of the 8PSK postulated received signal r 8 (n) by determining spatial-temporal correlations between the I and Q components of the 8PSK postulated received signal r 8 (n). Evaluation circuit  136  evaluates the characterized impairment components to identify the modulation type of the received signal r(n), as discussed further below.  
         [0032]     To characterize the impairment components for each postulated received signal, impairment characterization unit  140  includes an l/Q splitter  142  and an interference reduction circuit/metric generator  144 . Based on the channel estimates and the postulated received signals, I/Q splitter  142  separates at least a portion of each postulated received signal into their I and Q components. Interference reduction circuit/metric generator  144  characterizes the impairment components of the postulated received signals through the determination of the spatial and temporal correlations in the I and Q components.  
         [0033]     Once the interference has been suppressed from the impairment components of the postulated received signals, interference reduction circuit/metric generator  144  generates a postulation metric for each postulated modulation type that quantifies the energy present in each impairment characterization. Evaluation circuit  136  compares the resulting postulation metrics to determine the modulation type of the received signal. In general, because lower impairment component energies signify better interference cancellation, evaluation circuit  136  identifies the postulated modulation type corresponding to the smallest postulation metric as the modulation type of the received signal.  
         [0034]     An exemplary interference reduction circuit/metric generator  144  according to the present invention, shown in  FIG. 4 , comprises a whitening circuit  146 , a whitening filter generator  148 , and a metric generator  150 . According to an exemplary embodiment, whitening filter generator  148  generates a whitening filter based on the spatial and temporal correlations between the I and Q components. Whitening circuit  146  applies the whitening filter to the received signal to reduce interference from the channel estimate and the impairment components using the VAR impairment model described in Equations 5-7. Because the spatial and temporal correlations of the interference signals are stronger for the postulated received signal corresponding to the correct modulation type, the whitened noise estimate corresponding to the correct modulation type should have less energy than the “whitened” noise estimate corresponding to the incorrect modulation type(s). Therefore, the whitened noise estimates may be used to generate postulation metrics used by evaluation circuit  136  to identify the modulation type of the received signal.  
         [0035]     In summary, blind detection circuit  130  characterizes an impairment component of the received signal for each postulated modulation type by using the spatial and temporal correlations between the I and Q components of each postulated received signal to reduce interference from the impairment component. By reducing interference from each characterized impairment component based on the spatial and temporal correlations, blind detection circuit  130  generates a postulation metric corresponding to each postulated modulation type, and identifies the modulation type corresponding to the best postulation metric as the modulation type of the received signal.  
         [0036]      FIGS. 5 and 6  illustrate exemplary modulation detection methods  200 ,  200   a  for implementing the blind detection process of the present invention. As mentioned above, blind detection circuit  130  uses its knowledge of the training sequences in the transmitted signal to identify the modulation type of the received signal. As such, prior to executing either blind detection method, training sequence information is stored in system controller  170 . In exemplary embodiments, the following data are stored: 
        eight 26-symbol binary training sequences {s i (n) n−0   25 :i=0,1, . . . 7};     a window of possible starting sync positions m in the training sequence W={61, 62, 63, 64, 65, 66, 67, 68, 69}; and     eight L×(26-L) training sequence matrices {T i =(S i   HS   i ) −1 S i   H : i=0,1, . . . 7}, one for each training sequence, where S i  is computed according to:  
               S   i     =     [             s   i     ⁡     (   L   )               s   i     ⁡     (     L   -   1     )           …           s   i     ⁡     (   0   )                   s   i     ⁡     (     L   +   1     )               s   i     ⁡     (   L   )           …           s   i     ⁡     (   1   )               :       :       :       :               s   i     ⁡     (   25   )               s   i     ⁡     (   24   )           …         s   ⁡     (     25   -   L     )             ]             (     Eq   .           ⁢   8     )             
         
         [0040]     According to a first method  200 , illustrated in  FIG. 5 , signal rotator  132  de-rotates the received signal  
         {     r   ⁡     (   n   )       }       n   =   0     155       
 
 by π/2 (multiplier  202 ) to generate the GMSK postulated received signal:  
                 r   G     ⁡     (   n   )       =       r   ⁡     (   n   )       ⁢       e       -   j     ⁢     π   2     ⁢   n       .               (     Eq   .           ⁢   9     )             
 
 For each possible sync position in the synchronization window m ε W, channel estimator  134  computes the channel estimates using the best sync position m* to determine an impairment estimate (block  204 ) according to the following process. First, channel estimator  134  computes candidate channel estimates {right arrow over (h)} m =(h m (0),h m (1), . . . ,h m (L)) T  for each sync position m according to: 
 
{right arrow over (h)} m =T i {right arrow over (y)} m ,   (Eq. 10) 
 
 where {right arrow over (y)} m =(r G (m+L),r(m+L+1), . . . ,r G (m+25)) T . Based on the candidate channel estimates {right arrow over (h)} m , channel estimator  134  computes a corresponding impairment disturbance estimate: 
 
α i,m   2   =∥{right arrow over (y)}   m   −S   i   {right arrow over (h)}   m ∥ 2 .   (Eq. 11) 
 
 The best sync position m is selected according to:  
               m   *     =         arg   ⁢           ⁢   min     mεW     ⁢       α     i   ,   m     2     .               (     Eq   .           ⁢   12     )             
 
 Channel estimator  134  then generates an initial GMSK channel estimate for the GMSK postulated received signal using the best sync position m* according to: 
 
 {right arrow over (h)}   G =( h   m* (0), h   m* (1), . . . , h   m* ( L )) T ,   (Eq. 13) 
 
 and calculates GMSK impairment estimate samples  
         {     v   ⁡     (   n   )       }       n   =   0       25   -   L         
 
 according to:  
               v   ⁡     (   n   )       =         r   G     ⁡     (       m   *     +   L   +   n     )       -       ∑     k   =   0     L     ⁢           ⁢           h   ⇀     G     ⁡     (   k   )       ⁢         s   i     ⁡     (     n   -   k   +   L     )       .                   (     Eq   .           ⁢   14     )             
 
         [0041]     I/Q splitter  142  separates the impairment estimate samples into their I and Q components (block  206 ): 
 
 v   I ( n )= Re{v ( n )}
 
 v   Q ( n )= Im{v ( n )}′  (Eq. 15) 
 
 and interference reduction circuit/metric generator  144  generates the GMSK postulated metric according to the following process. First, interference reduction circuit/metric generator  144  calculates covariance matrices C(k) for k={0, 1, 2} according to:  
               C   ⁡     (   k   )       =       ∑     n   =     L   +   k       25     ⁢           ⁢         [             v   I     ⁡     (   n   )                   v   Q     ⁡     (   n   )             ]     ⁡     [         v   I     ⁡     (     n   -   k     )       ⁢           ⁢       v   Q     ⁡     (     n   -   k     )         ]       .               (       Eq   .           ⁢   16     ⁢   a     )             
 
 As shown above in Equation 5, the I/Q impairment is modeled as a VAR process, resulting in:  
               |             v   I     ⁡     (   n   )                   v   Q     ⁡     (   n   )             |     +     A   1       |             v   I     ⁡     (     n   -   1     )                   v   Q     ⁡     (     n   -   2     )             |     +     A   2       |             v   I     ⁡     (     n   -   2     )                   v   Q     ⁡     (     n   -   2     )             |     =       e   ⁡     (   n   )       .             (       Eq   .           ⁢   16     ⁢   b     )             
 
 Next, interference reduction circuit/metric generator  144  estimates the GMSK VAR coefficient matrices iteratively using a Whittle-Wiggins-Robinson Algorithm (WWRA) according to the following steps: 
        Initialize the WWRA algorithm D 0 =c(0), S 0 =C(0)     Calculate the VAR(1) filter coefficients: 
 
 A   1 (1)=− C (1) C   −1 (0),   (Eq. 17a) 
 
 B   1 (1)= C   T (1) C   −1 (0),   (Eq. 17b) 
 
 D   1   =D   0   +A   1 (1) C   T (1), and   (Eq. 17c) 
 
 S   1   =S   0   +C   T (1) B   1   T (1), P   1   =C (2)+ C (1) B   1   T (1).   (Eq. 17d) 
    Calculate the VAR(2) coefficients: 
 
 A   2 (2)= P   1   S   1   −1 ,   (Eq. 18a) 
 
 A   2  (1)= A   1 (1)+ A   2 (2) B   1 (1), and   (Eq. 18b) 
 
 D   2   =D   1   +A   2 (2)P 1   T .   (Eq. 18c) 
 
 Those skilled in the art will appreciate that matrix D 2  represents a “whitened noise estimate”. By taking the determinant of D 2 , metric generator 150 generates a GMSK postulation metric λ GMSK , shown in Equation 19, which may be used by evaluation circuit  136  to identify the correct modulation type. 
 
λ GMSK   =det ( D   2 )   (Eq. 19) 
       
 
         [0045]     The above process associated with Equations 9-19 is repeated for the 8PSK postulated modulation type. Specifically, signal rotator  132  de-rotates the received signal  
         {     r   ⁡     (   n   )       }       n   =   0     155       
 
 by 3π/8 (multiplier  220 ) to generate the 8PSK postulated received signal (Equation 20).  
                 r   8     ⁡     (   n   )       =       r   ⁡     (   n   )       ⁢     e       -   j     ⁢       3   ⁢   π     8     ⁢   n                 (     Eq   .           ⁢   20     )             
 
 Using Equations 10-12, channel estimator  134  computes a channel estimate for the 8PSK postulated received signal (block  222 ) using the best sync position m*, resulting in: 
 
 {right arrow over (h)}   8 =( h   m* (0), h   m* (1),. . . , h   m* ( L )) T ,   (Eq. 21) 
 
 and calculates 8PSK impairment estimate samples  
         {     v   ⁡     (   n   )       }       n   =   0       25   -   L         
 
 (block  222 ) according to:  
               v   ⁡     (   n   )       =         r   8     ⁡     (       m   *     +   L   +   n     )       -       ∑     k   =   0     L     ⁢           ⁢           h   ⇀     8     ⁡     (   k   )       ⁢         s   i     ⁡     (     n   -   k   +   L     )       .                   (     Eq   .           ⁢   22     )             
 
         [0046]     The 8PSK path may then further de-rotate the initial 8PSK impairment by π/8 (multiplier  224 ). To appreciate the benefits of this additional de-rotation, consider the following. Assuming that the desired signal and the interfering signal are both GMSK modulated, applying the π/2 de-rotation to the received signal r(n) correctly removes the rotation in both the desired signal and in the impairment. However, when multiplier  220  applies the 3π/8 de-rotation to the received signal, the desired signal and the impairment both have a residual rotation of π/8. Thus, the postulation metric corresponding to the signal de-rotated by π/2 will be better than the postulation metric corresponding to the signal de-rotated by 3π/8. As a result, blind detection circuit  130  detects the correct modulation type (GMSK).  
         [0047]     However, when the desired signal is 8PSK and the interfering signal is GMSK, applying the π/2 de-rotation to the received signal leaves a residual rotation in the desired signal and correctly removes the rotation from the interference signal, while applying the 3π/8 de-rotation to the received signal removes the rotation from the desired signal and leaves a residual rotation in the interference signal. Because of the residual rotation in the interfering signal in the 8PSK path, there is an increased probability that the signal de-rotated by π/2 will generate a better postulation metric than the signal de-rotated by 3π/8, resulting in an incorrect modulation type detection. To mitigate this probability, the 8PSK path may further de-rotate the impairment estimate samples by π/8 (multiplier  224 ) to generate modified impairment estimate samples  
         {       v   ′     ⁡     (   n   )       }       n   =   0       25   -   L         
 
 according to:  
                 v   ′     ⁡     (   n   )       =       v   ⁡     (   n   )       ⁢     e       -   j     ⁢     π   8     ⁢     (     n   +     m   *     +   L     )                   (     Eq   .           ⁢   23     )             
 
 to make the GMSK impairment stationary for the subsequent interference cancellation. 
 
         [0048]     I/Q splitter  142  separates the modified impairment estimate samples into their I and Q components (block  226 ): 
 
 v   I ( n )= Re{v ′( n )}
 
 v   Q ( n )= Im{v ′( n )}′  (Eq. 24) 
 
 and interference reduction circuit/metric generator  144  uses the I and Q components of Equation 24 to generate the 8PSK VAR coefficient matrices according to Equations 16-18. As with the GMSK VAR coefficient matrices, D 2  is simply the whitened noise estimate for the 8PSK postulated received signal. By taking the determinant of D 2 , metric generator  150  generates an 8PSK postulation metric λ 8PSK , shown in Equation 25, which may be used by evaluation circuit  136  to identify the correct modulation type. 
 
λ 8PSK   =det (D 2 )   (Eq. 25) 
 
         [0049]     According to a second modulation detection method  200   a,  shown in  FIG. 6 , signal rotator  132  de-rotates the received signal  
         {     r   ⁡     (   n   )       }       n   =   0     155       
 
 by π/2 and by 3π/8 to generate the GMSK and 8PSK postulated received signals, respectively, as shown in Equations 9 and 20 (multipliers  202  and  220 ). Using Equations 10-12, channel estimator  134  generates the postulated GMSK and 8PSK channel estimates for each sync position to determine the best sync position m* for each postulated received signal (blocks  204  and  222 ). 
 
         [0050]     Using the best sync position m*, the postulated received signal r x (n), where x represents the modulation type, over the known training sequence is provided to I/Q splitter  136 , where each postulated received signal is separated into their respective I and Q components (blocks  205  and  225 ), as shown in Equations 26a and 26b. 
 
 r   8 ( m+n )=[ r   8,I ( m*+n ) r   8,Q ( m*+n )] T    (Eq. 26a) 
 
 r   G ( m+n )=[ r   G,I ( m*+n ) r   G,Q ( m*+n )] T    (Eq. 26b) 
 
 Based on the I and Q components of the postulated received signals, interference reduction circuit/metric generator  144  constructs a vector of postulated received symbols R x (n) for each postulated received signal, as shown in Equations 27a and 27b. 
 
 R   8 ( n )=[ r   8,I ( n− 1) r   8,Q ( n− 1) r   8,I ( n−   2 ) r   8,Q ( n− 2)] T    (Eq. 27a) 
 
 R   G ( n )=[ r   G,I ( n −1) r   G,Q ( n −1) r   G,I ( n −2) r   G,Q ( n −2)] T    (Eq. 27b) 
 
 Using the postulated received signal, r x (n)=[r x,I (n) r x,Q (n)] T , the vector of postulated received symbols R x (n), and vectors of the known transmitted training sequence symbols, 
 
 S   8 ( n )=[ S   8 ( n ) s   8  ( n −1) . . .  s   8 ( n−L −2)] T  and (Eq. 28a) 
 
 S   G ( n )=[ s   G ( n ) s   G ( n −1) . . .  s   G ( n−L −2)] T ,   (Eq. 28b) 
 
 interference reduction circuit/metric generator  144  computes covariance matrices according to the indirect Generalized Least Squares (iGLS) algorithm, as shown in Equations 29a-29c, to measure the strength of the correlations between the postulated received signal r x (n), the generated vector of postulated received symbols R x (n), and the generated vectors of the known transmitted training sequence symbols (blocks  207  and  227 ).  
               C   rr     =       1     26   -   L   -   2       ⁢       ∑     n   =     L   +   2       25     ⁢         r   x     ⁡     (       m   *     +   n     )       ⁢       r   x   T     ⁡     (       m   *     +   n     )                     (       Eq   .           ⁢   29     ⁢   a     )                 C   rz     =       1     26   -   L   -   2       ⁢       ∑     n   =     L   +   2       25     ⁢           r   x     ⁡     (       m   *     +   n     )       ⁡     [             R   x     ⁡     (       m   *     +   n     )                 S   ⁡     (       m   *     +   n     )             ]       T                 (       Eq   .           ⁢   29     ⁢   b     )                 C   zz     =       1     26   -   L   -   2       ⁢       ∑     n   =     L   +   2       25     ⁢         [         R   x     ⁡     (       m   *     +   n     )       ⁢           ⁢     S   ⁡     (       m   *     +   n     )         ]     ⁡     [             R   x     ⁡     (       m   *     +   n     )                 S   ⁡     (       m   *     +   n     )             ]       T                 (       Eq   .           ⁢   29     ⁢   c     )             
 
 Interference reduction circuit/metric generator  144  then computes a noise covariance matrix D according to Equation 30 for each postulated modulation type. 
 
 D=C   rr   −C   rz   C   zz   −1   C   rz   T  (Eq. 30) 
 
 It will be appreciated that the mathematical operations of Equations 29a-29c and 30 operate to simultaneously estimate the whitened channel estimate and the whitening filter coefficients. As with the matrix D 2  of the first modulation detection method  200 , matrix D corresponds to a whitened noise estimate. By taking the determinant of D, metric generator  150  generates a GMSK postulation metric λ GMSK  and an 8PSK postulation metric λ 8PSK , as shown in Equations 19 and 25. 
 
         [0051]     Regardless of the modulation detection method  200 ,  200 a used, evaluation circuit  136  evaluates the postulation metrics to identify the modulation type of the received signal (block  210 ). For example, if λ GMSK &lt;λ 8PSK , then evaluation circuit  136  identifies GMSK as the modulation type of the received signal (block  212 ). If λ GMSK ≧λ 8PSK , then evaluation circuit  136  identifies 8PSK as the modulation type of the received signal (block  214 ). In either case, blind detection circuit  130  provides the modulation type indicator to the demodulator  126 . In some embodiments, blind detection circuit may also supply the de-rotated signal (r x (n) where x=G when the modulation type is GMSK and x=8 when the modulation type is 8PSK) and/or whitened channel estimates, as shown in  FIG. 2 , to the demodulator to further facilitate the demodulation process.  
         [0052]     As shown above, the first modulation detection method  200  of  FIG. 5  characterizes an impairment component for the received signal corresponding to each of the GMSK and 8PSK postulated received signals by determining spatial correlations between the I and Q components of the impairment component in the received signal (see Equations 16-18). Similarly, the second modulation detection method  200 a of  FIG. 6  also characterizes an impairment component for the received signal corresponding to each of the GMSK and 8PSK postulated received signals by determining spatial correlations between the I and Q components of the received signal (see Equations 29-30). As such, both methods characterize an impairment component of the received signal by determining spatial correlations between I and Q components of the received signal for each postulated modulation type, where the I and Q components may be the I and Q components of the received signal or may be the I and Q components of an impairment portion of the received signal.  
         [0053]     The first modulation detection method  200  uses an initial channel estimate and the originally received signal to calculate the impairment, models the impairment as a VAR process using the I and Q components of the calculated impairment to whiten the initial channel estimate, and solves for the whitened noise estimate using the WWRA algorithm. Because the first modulation detection method  200  involves simple 2×2 matrix operations, the first modulation detection method  200  is computationally efficient.  
         [0054]     Contrastingly, the second modulation detection method  200 a uses the I and Q components of the received signal r(n) for each modulation type to generate the whitened channel estimate and the spatial-temporal whitening filter coefficients (VAR coefficients) simultaneously using the iGLS algorithm so that the estimation of both the channel and the VAR coefficients may benefit from the interference cancellation. Because the second modulation detection method  200 a involves large covariance matrices, e.g., 13×13 matrices when L=6, the second modulation detection method  200 a is computationally more complex. However, due to the simultaneous estimation and interference cancellation, the identified modulation type resulting from the second modulation detection method  200 a is generally more accurate.  
         [0055]     Simulations of the exemplary modulation detection methods  200 ,  200 a of the present invention evaluated the first and second methods for the following signal-interference scenarios: 
        Case 1: GMSK modulated desired signal and interference;     Case 2: 8PSK modulated desired signal, GMSK modulated interference;     Case 3: GMSK modulated desired signal, 8PSK modulated interference;     Case 4: 8PSK modulated desired signal and interference. 
 
 The simulations showed an improved receiver performance of approximately 1.2 dB using the first modulation detection method  200 , and more than 3 dB using the second modulation detection method  200 a for case 1 as compared to the performance of current receivers using conventional blind detection methods. In fact, the simulations show that using the second modulation detection method  200 a, the receiver performance for case 1 approaches the performance of a receiver with perfect blind modulation detection. As a result, both modulation detection methods  200 ,  200 a improve the performance of the receiver  120  for case 1, where blind detection limits the performance of conventional receivers. 
       
 
         [0060]     The above describes an exemplary blind modulation detection method for single antenna receivers in EDGE systems. However, those skilled in the art will appreciate that the present invention is not so limited, and may therefore be used with multi-antenna receivers and/or other modulation types.  
         [0061]     The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.