Abstract:
A robust differential receiver is described that may be used in any frequency modulated system, including short-range radio frequency (RF) communication devices. The differential receiver provides a preamble detection approach that reduces false preamble detection, a fine carrier frequency (CFO) estimation approach that provides an extended estimation range, and robust in-band and out-of-band interference detection. The described differential receiver assures that preamble detections are not falsely triggered, and that CFO estimates are based on accurately modeled preamble waveforms that have not been distorted by phase ambiguities or in-band distortion. In this manner, the described robust differential receiver assures that CFO estimates used to compensate remaining portions of detected signals are accurate, thereby reducing the likelihood that remaining portions of the detected signal will be affected by phase ambiguity distortions, thereby enhancing the differential receiver&#39;s ability to lock onto an otherwise unavailable communication channel, and/or reducing transmission errors and/or packet loss.

Description:
INCORPORATION BY REFERENCE 
     This application claims the benefit of U.S. Provisional Application No. 61/080,496, “A ROBUST DIFFERENTIAL RECEIVER FOR FREQUENCY MODULATED SYSTEM,” filed by Quan Zhou, Songping Wu and Hui-Ling Lou on Jul. 14, 2008, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Increasing demands for wireless ad-hoc interconnectivity between electronic devices has led to the development of a large number of wireless short-range communication protocols, such as Bluetooth and ultra low power (ULP) Bluetooth. Such protocols may be used to wirelessly exchange data over short distances, e.g., 0 to 100 meters, between fixed and/or mobile devices and may be used, for example, to replace wire-based protocols between two devices, to provide wireless connectivity to network access points, and to establish a wireless personal area network (PAN) between electronic devices within a limited physical distance of one another. 
     Such short-range communication protocols may be used to exchange information between a wide range of devices. For example, such short-range communication protocols may also be used by electronic devices, such as cell phones, hand-held radios, head-phones, personal recording devices and video game consoles, to facilitate short range information exchanges. In addition, such short-range communication protocols may be used by computing devices, such as laptop computers, hand-held computers, etc., to exchange information with peripheral equipment and accessories, such as printers, keyboards, wireless pointing devices, scanners, cameras and GPS receivers, and/or to exchange information with other computing devices either directly or via network access point. 
     SUMMARY 
     A robust differential receiver is described that may be used in any frequency modulated system, including short-range radio frequency (RF) communication devices. The differential receiver provides a preamble detection approach that reduces false preamble detection, a carrier frequency offset (CFO) estimation approach that provides an extended estimation range, and robust in-band and out-of-band interference detection. 
     In existing differential receivers, phase ambiguity, i.e., incorrect estimates of a received signal&#39;s phase, may occur in the presence of large frequency offset. For example, phase detectors within existing differential receivers may have a phase range from −π to π. However, large frequency offsets in a received signal during the channel acquisition process may cause the phase accumulation to exceed ±π, resulting in a distorted waveform and an inaccurate estimate of the CFO used to process remaining portions of the signal. Further, in-band and out-of band interference during the channel acquisition process may falsely trigger preamble detection, which may also result in inaccurate estimates of the CFO used to process remaining portions of the signal. Incorrect CFO estimates may prevent a frequency modulated device from being able to lock onto and/or maintain a communication connection via an otherwise useful communication channel, and/or may result in transmission errors and/or packet loss. 
     The described robust differential receiver may correct such deficiencies by detecting and repairing phase distortions in the received signal during the channel preamble detection process, thereby reducing the likelihood of a falsely triggered preamble detection and thereby allowing an accurate estimate of the CFO of the received signal. Further, the described robust differential receiver may generate a CFO estimate for a detected signal based on an average of the peak values identified within the corrected preamble or, alternatively, based on an average of all values between the first and last peaks of the corrected preamble. 
     In addition, the described robust differential receiver may include interference detection techniques that may be used to identify the presence of in-band and out-of-band interference in a received signal during the preamble detection process. The use of such interference detection techniques further reduces the likelihood of a falsely triggered preamble detection, and prevents a CFO estimate from being generated based on a detected preamble that includes distorted signal values that would lead to errors in the generated CFO estimate and, therefore, adversely affect subsequent processing of remaining portions of the signal. 
     Using the above techniques, the described robust differential receiver is able to assure that preamble detection is not falsely triggered, and that CFO estimates are based on accurately modeled preamble waveforms that have not been distorted by phase ambiguities or in-band distortion. Using such techniques, the described robust differential receiver assures that the CFO estimate used to compensate remaining portions of a detected signal is accurate such that the detected signal is centered at desired receiving channel, thereby reducing the likelihood that remaining portions of the detected signal will be affected by phase ambiguity distortions, enhancing the differential receiver&#39;s ability to lock onto an otherwise unavailable communication channel, and/or reducing transmission errors and/or packet loss. 
     One example embodiment of the described robust differential receiver may include, a phase detector that may generate phase values based on a stream of baseband data, and a preamble detection module that may include, a phase monitoring unit that may monitor the generated phase values and may detect an ambiguity in the phase values, a phase ambiguity elimination module that may remove the detected ambiguity to produce corrected phase values, and a preamble detection unit that may detect a communication channel preamble sequence based on the corrected phase values. 
     Another example embodiment of a method of implementing a robust differential receiver may include, generating phase values based on a stream of baseband data, detecting an ambiguity in the generated phase values, correcting the detected ambiguity thereby producing corrected phase values and detecting a communication channel preamble sequence based on the corrected phase values. 
     Yet another example embodiment of an RF device with a robust differential receiver may include, a phase detector that may generate phase values based on a stream of baseband data, and a preamble detection module that may include, a phase monitoring unit that may monitor the generated phase values and may detect an ambiguity in the phase values, a phase ambiguity elimination module that may remove the detected ambiguity to produce corrected phase values, and a preamble detection unit that may detect a communication channel preamble sequence based on the corrected phase values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments of a robust differential receiver for a frequency modulated system will be described with reference to the following drawings, wherein like numerals designate like elements, and wherein: 
         FIG. 1  is a block diagram of an example of an RF receiver/transmitter that includes an example embodiment of the robust differential receiver; 
         FIG. 2  is a block diagram of an example embodiment of the robust differential receiver shown in  FIG. 1 ; 
         FIG. 3  is a block diagram of an example embodiment of a preamble detection and CFO acquisition module shown in  FIG. 2 ; 
         FIG. 4  and  FIG. 5  show a flow-chart of an example process for preamble detection and CFO acquisition in an example embodiment of the robust differential receiver described above with respect to  FIG. 2 ; 
         FIG. 6  shows a flow-chart of an example process for performing in-band and out-of-band interference detection in an example embodiment of the robust differential receiver described above with respect to  FIG. 2 ; 
         FIG. 7  shows a plot of an example signal that suffers from multiple example phase ambiguities; 
         FIG. 8  shows a plot of an example signal that suffers from an example phase ambiguity; 
         FIG. 9  shows a plot of  FIG. 8 , after the example phase ambiguity has been corrected by an example embodiment of the preamble detection and CFO acquisition module shown in  FIG. 2  and  FIG. 3 ; 
         FIG. 10  shows plots associated with an example signal affected by an example of out-of-band distortion that may be detected and corrected by an example embodiment of the preamble detection and CFO acquisition module shown in  FIG. 2  and  FIG. 3 ; and 
         FIG. 11  shows plots associated with an example signal affected by an example of in-band distortion that may be detected and corrected for by an example embodiment of the preamble detection and CFO acquisition module shown in  FIG. 2  and  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a system level block diagram of an example of a radio frequency (RF) transceiver  100  with an RF receiver  114  that includes the described robust differential receiver for a frequency modulated system. As shown in  FIG. 1 , RF transceiver  100  may include an RF antenna  102 , an RF interface  104 , a processor  106  and device components  108 . RF interface  104  may include a transmitter/receiver switch  110 , a low noise amplifier  120 , a transmitter  112  and a receiver  114 . Receiver  114  may include an RF filter  122 , a down-conversion module  124 , a local oscillator  126 , an amplifier  130 , an analog-to-digital converter  132 , a received signal strength indicator (RSSI) module  134 , and a differential receiver module  136 . In the example embodiment shown in  FIG. 1 , RF filter  122 , down-conversion module  124 , local oscillator  126 , amplifier  130 , and analog-to-digital converter  132 , may be referred to collectively as an example embodiment of a receiver front end. 
     Although not shown in  FIG. 1 , device components  108  may include features such as a data interface unit and a physical interface unit that allows the RF transceiver to connect to and provide RF data communication services for a device that does not inherently support such a capability. For example, in an example embodiment in which the RF transceiver is a Bluetooth RF communication device, device components  108  may include a Universal Serial Bus (USB) compatible data interface unit and a USB compatible physical interface that allows the RF transceiver to connect to and provide short range private area network (PAN) communication for any device with a USB compatible port, such as a laptop, desktop or handheld computer. However, in other example embodiments the RF transceiver may be integrated within a device such as a laptop computer, GPS receiver, cellular telephone, calculator, keyboard, mouse, printer, scanner, home appliance, hand-held radio, etc., or virtually any device that would benefit from the ability to support short range communications, such as Bluetooth class  1 , class  2  and/or class  3  based communication or ULP based communication. In such embodiments, device components  108  may also include features such as a device memory, a rechargeable battery, and a user interface which may include a display, a keyboard, a speaker and/or microphone and/or any other components associated with the device in which the RF transceiver is integrated. 
     It is noted that although the example RF device shown in  FIG. 1  is a transceiver, other embodiments may not require the ability to transmit an RF signal. In such an embodiment, the transmit/receive switch  110  and transmitter  112  shown in  FIG. 1 , and described above, are optional, but the RF device would retain RF receiver  114  that includes the described robust differential receiver module  136  for a frequency modulated system. 
     Further, it is noted that processor  106  may execute numerous signal analysis processes that may be used to generate and manage control parameters used by processor  106  to control operation of transmitter  112  and receiver  114 . 
     In operation as a receiver, processor  106  receives from differential receiver module  136  a demodulated data stream containing, for example, digitized data received by RF transmission from a device within a PAN network. Processor  106  passes the digitized data stream to device components  108  which directs the digital data to an appropriate data destination. 
     For example, in operation as a receiver, low noise amplifier  120  receives an RF signal from antenna  102  via transmission/receiver switch  110 . Low noise amplifier  120  amplifies the received signal by a predetermined gain and passes the amplified signal to RF filter  122 . 
     RF filter  122  may be configured to pass a range of frequencies. The frequency range passed by RF filter  122  may include multiple communication channels, as described in greater detail below and, therefore, may pass to down-conversion module  124  a filtered RF signal that includes frequency components for multiple communication channels. 
     Down-conversion module  124  down-converts the received filtered RF signal using a local oscillator signal having a frequency that retains communication channel frequency components, and passes the down-converted signal to amplifier  130 . 
     Amplifier  130  amplifies the down-converted signal and passes the amplified, down-converted signal to analog-to-digital converter  132 . 
     Analog-to-digital converter  132  is configured to sample the down-converted signal at a predetermined sampling rate and generate a stream of baseband digital data based on the sampled values, which stream is provided to RSSI module  134  and differential receiver module  136 . 
     Differential receiver module  136  receives the stream of baseband digital data produced by analog-to-digital converter  132 , demodulates a portion of the digital data stream associated with a currently selected communication channel, and provides the demodulated digital data stream to processor  106  for further processing and/or for delivery to one or more device components  108 , as described above. 
     RSSI module  134  generates a received signal strength estimate, for example, a received signal strength indicator (RSSI) that is provided to processor  106  for use in monitoring and controlling operation of RF interface  104  and provided to differential receiver module  136  for use in detecting in-band interference, as described below. 
       FIG. 2  is a block diagram of an example embodiment of the robust differential receiver module  136  described above with respect to  FIG. 1 . For example, differential receiver module  136  receives the baseband digital data stream generated by analog-to-digital converter  132  and delivers a demodulated digital data stream to processor  106 . As described in greater detail below, differential receiver module  136  processes the baseband digital data to detect a communication channel within the baseband digital data and to extract data from the detected channel. For example, in one example embodiment, the communication channel is configured to detect, lock onto and decode a Gaussian Frequency Shift Keying (GFSK) encoded channel. 
     As shown in  FIG. 2 , differential receiver module  136  includes a carrier frequency offset (CFO) compensation module  202 , a channel filter  204 , a differential detector  206 , a phase detector  208 , a preamble detection and CFO acquisition module  210 , an adder  212 , a digitally controlled multiplexor  214 , a symbol timing module  216  and a data detection module  218 . 
     In operation, CFO compensation module  202  receives baseband digital data from analog-to-digital converter  132  and applies a CFO compensation signal produced by preamble detection and CFO acquisition module  210  to compensate the received signal, in the frequency domain, for a carrier frequency offset (CFO) determined by preamble detection and CFO acquisition module  210 , as described in greater detail below. CFO compensation module  202  passes the CFO compensated baseband digital data stream to channel filter  204 . In one embodiment, channel filter  204  is a low pass filter. 
     Channel filter  204  filters the received CFO compensated baseband digital data stream and filters out frequency components that are outside of a desired frequency range and passes the filtered baseband digital data stream to differential detector  206 . For example, in one example embodiment, channel filter  204  is an adjacent channel rejection (ACR) channel filter which is configurable to filter high frequency components outside of a frequency band associated with a selected channel range. 
     Differential detector  206  determines a change in frequency, Δf, over a predetermined period, e.g., a one-symbol period, f n −f n-1 , and passes the determined Δf data to phase detector  208 . 
     In one embodiment, phase detector  208  determines a change in phase, Δψ n , e.g., over a one-symbol period, ψ n −ψ n-1 , based on the change in frequency, Δf, data provided by differential detector  206  based on the relationship shown in equation 1.
 
Δψ=2 πΔf*T   EQ. 1
         Where Δψ is a change in phase;   Δf is a change in frequency over a symbol period; and   T is a predetermined time period, e.g. a one-symbol period.       

     As shown in  FIG. 2 , the output of phase detector  208  is provided to each of data detection module  218  (via connection  201 ), preamble detection and CFO acquisition module  210  (via connection  203 ), and symbol timing module  216  (via digital multiplexor  214  and one of connections  205  and  209 ). For example, digital multiplexor  214  may forward to symbol timing module  216  output of phase detector  208  received on connection  205 , or corrected phased detector output received from adder  212  via connection  209 , based on a control signal provided by preamble detection and CFO acquisition module  210 . Processing performed by preamble detection and CFO acquisition module  210 , symbol timing module  216  and data detection module  218 , as well as the operation of multiplexor  214  and adder  212  are described in greater detail below. 
     Preamble detection and CFO acquisition module  210  processes the output of phase detector  208  to perform preamble detection and CFO acquisition. In performing preamble detection, preamble detection and CFO acquisition module  210  correlates the phase detector output with a predetermined preamble pattern in the phase domain. During preamble detection, signal timing may be determined based on the peak output of the correlator. Once a preamble is detected, preamble detection and CFO acquisition module  210  performs CFO acquisition. 
     In performing CFO acquisition, (in one embodiment) preamble detection and CFO acquisition module  210  uses the peak values of the signal preamble determined during the preamble detection process. For example, a carrier frequency offset may be generated as an average of the determined peak values, or may be generated as an average over all the samples within the preamble to provide a more precise estimation. Once the carrier frequency offset is determined, preamble detection and CFO acquisition module  210  generates a CFO compensation signal that is provided to CFO compensation module  202  and used to compensate the received baseband digital data stream for the determined carrier frequency offset in the frequency domain. Further, once the carrier frequency offset is determined, preamble detection and CFO acquisition module  210  generates an immediate CFO compensation signal that may be provided to adder  212  and that may be used to compensate the output of phase detector  208  prior to delivery to symbol timing module  216 , as described in greater detail below. 
     Symbol timing module  216  is responsible for processing the output of phase detector  208  to generate symbol timing data used by data detection module  216  to decode the received payload. For example, a communication data channel data stream may contain a predetermined preamble that is the same for all communication channel data streams, followed by a fixed length access code, followed by a variable length data payload. The access code, e.g., a 32-bit access code, may contain symbol timing information and data payload length data that is needed by symbol timing module  216  and data detection module to correlate and demodulate the received data stream. Once the access code is extracted, symbol timing module  216  uses the symbol timing information contained within the access code to generate symbol timing data for the data payload based on CFO compensated phase detector output, and provides payload symbol timing data to data detection module  218  data until the full data payload is demodulated. 
     As described above, preamble detection and CFO acquisition module  210  may perform phase ambiguity correction for channel signals that exhibit phase ambiguity during the preamble detection process, and may generate a CFO compensation signal that is provided to CFO compensation module  202  to compensate the output of phase detector  208  based on the generated CFO. By correcting for phase ambiguity during the preamble detection process, the differential receiver is able to detect channels that otherwise may have been ignored and may generate a more accurate CFO value. By compensating the remaining portions of the channel signal for a CFO based on the corrected preamble, the differential receiver assures that ambiguities that would likely occur with respect to the access code and payload are avoided. The use of such preamble correction and CFO compensation techniques results in a differential receiver that is more robust and more reliable than other differential receivers with respect to the ability to lock onto and to maintain a stable channel connection. 
     It is noted that CFO compensation may be performed regardless of whether or not phase ambiguities are detected within the preamble. Compensating for a detected CFO, maximizes the reliability, stability and robustness of the differential receiver by minimizing the likelihood that phase ambiguities in the access code and payload portions of the signal are encountered. 
     As described above, the access code of a channel data stream immediately follows the preamble of the channel data stream. By compensating for a determined CFO in the phase domain via CFO compensation module  202 , portions of the channel data stream which have already passed from the CFO compensation module  202  before the CFO compensation signal from preamble detection and CFO acquisition module  210  is applied are not CFO compensated. For example, uncompensated portions of the channel data stream which have passed from the CFO compensation module  202  before arrival of the CFO compensation signal, to be further processed by channel filter  204 , differential detector  206  and phase detector  208 , may continue to emerge from phase detector  208  for a processing delay period, or loop delay, that is equal to the combined processing delay introduced by channel filter  204 , differential detector  206  and phase detector  208 . 
     Therefore, in addition to the CFO compensation signal sent to CFO compensation module  202  to compensate for the determined CFO in the frequency domain, preamble detection and CFO acquisition module  210  may also generate an immediate CFO compensation signal that may be added via adder  212  to the output of phase detector  208 . For example, in one example embodiment, preamble detection and CFO acquisition module  210 , at the start of a preamble detection process, applies a first control signal, e.g., 00, to digital multiplexor  214  that blocks any input signals from passing through digital multiplexor  214  to symbol timing module  216 . However, once a preamble is detected and an immediate CFO compensation signal has been applied to adder  212 , CFO acquisition module  210  applies a second control signal, e.g., 01, to digital multiplexor  214  that allows the compensated phase detector output signal to pass from adder  212  to symbol timing module  216 , but does not allow the uncompensated phase detector output signal to pass from phase detector  208  to symbol timing module  216 . After a predetermined period of time equal to the loop delay of channel filter  204 , differential detector  206  and phase detector  208 , preamble detection and CFO acquisition module  210  applies a third control signal, e.g.,  10 , to digital multiplexor  214  that allows a compensated phase detector output signal to pass directly from phase detector  208  to symbol timing module  216 , and shuts off the data stream received from adder  212 . In this manner, once the output of phase detector  208  is based on portions of the channel signal stream which have been CFO compensated by CFO compensation module  202 , the compensated phase detector output is passed directly to symbol timing module  216 , and use of adder  212  may be discontinued. 
     Data detection module  218 , upon receiving symbol timing data from symbol timing module  216  begins processing CFO compensated output received directly from phase detector  208 . For example, assuming that the output of phase detector  208  represents GFSK encoded data, data detection module  218  may apply a GFSK demodulator to generate a demodulated digital data stream based on the phase data received from phase detector  208  and the timing data received from symbol timing module  216 . 
       FIG. 3  is a block diagram of an example embodiment of preamble detection and CFO acquisition module  210 , as described above with respect to  FIG. 2 . As shown in  FIG. 3 , the example embodiment of preamble detection and CFO acquisition module  210  includes a preamble detection and CFO acquisition module controller  302 , a phase monitoring unit  304 , a phase discontinuity assessment unit  306 , a phase ambiguity elimination unit  308 , a low-pass filter monitoring unit  310 , an RSSI monitoring unit  312 , a preamble detection unit  314 , and a CFO acquisition unit  316 . 
     As described below, operation of phase monitoring unit  304 , phase discontinuity assessment unit  306  and phase ambiguity elimination unit  308  may be coordinated by preamble detection and CFO acquisition module controller  302  to identify and correct phase ambiguities in a received data stream so that preamble detection unit  314  may work to correlate an ambiguity free data stream with a predetermined preamble sequence. 
     As also described below, operation of low-pass monitoring unit  310  and RSSI monitoring unit  312  may be coordinated by preamble detection and CFO acquisition module controller  302  to identify in-band and out-of-band interference in a received data stream while preamble detection unit  314  works to correlate the received corrected/ambiguity-free data stream with a predetermined preamble sequence. Detecting such interference during the preamble detection process may prevent preamble detection unit  314  from detecting a preamble based on a weak or distorted signal, and thereby may prevent CFO acquisition unit  316  from generating erroneous CFO estimates which would otherwise likely result in transmission errors and possible packet loss. 
     In operation, preamble detection and CFO acquisition module controller  302  maintains a workflow state machine, and/or control parameters that allow each of the respective units described below to perform its assigned task. For example, preamble detection and CFO acquisition module controller  302  monitors the output of low-pass filter monitoring unit  310  and RSSI monitoring unit  312  and reinitiates the preamble detection process upon determining that the preamble detection has been triggered by out-of-band interference, or upon determining that a CFO calculated based on a detected preamble may be incorrect due to the presence of in-band interference. Further, preamble detection and CFO acquisition module controller  302  may monitor the output of phase monitoring unit  304  and may initiate operation of phase discontinuity assessment unit  306  and phase ambiguity elimination unit  308 , as described below, upon detection of a phase ambiguity during the preamble detection process. In addition, preamble detection and CFO acquisition module controller  302  may generate the control signals provided to digital multiplexor  214 , shown in  FIG. 2 , based on a state machine, control parameters and timers maintained by preamble detection and CFO acquisition module controller  302  based on feedback received from preamble detection unit  314  and CFO acquisition unit  316 . 
     Phase monitoring unit  304  may be initiated by preamble detection and CFO acquisition module controller  302  to monitor the output of phase detector  208  during the preamble detection process performed by preamble detection unit  314 . Phase monitoring unit  304  assesses the variation in the phase in over a predetermined period, e.g., a one-symbol period, and if the variation exceeds a predetermined threshold, e.g., π, phase monitoring unit  304  informs preamble detection and CFO acquisition module controller  302  that a phase ambiguity has been detected. 
     Phase discontinuity assessment unit  306  may be initiated by preamble detection and CFO acquisition module controller  302  in response to a notification from phase monitoring unit  304  that a phase ambiguity has been detected. For example, the phase ambiguity may be eliminated by comparing two adjacent phase values. If a frequency offset is large enough to make a phase difference of two adjacent phase values larger than 2π, the value may be wrapped back by adding or subtracting π. For example, assuming that α 1  and α 2  are consecutive phase values generated by phase detector  208 , shown in  FIGS. 2 , and 0&lt;α 1 &lt;π, and 0&lt;α 2 &lt;2π, then α 2  may be wrapped to α′ 2  by subtracting 2π from α 2 , i.e., α′2=α 2 −2π. Therefore, |α′ 2 −α 1 |=|α 2 −α 1 −2π|&gt;=|2π−|α 2 −α 1 ∥&gt;π. Hence, a discontinuous phase value that is the result of a phase ambiguity may be wrapped back by adding or subtracting π. Using such an approach, phase discontinuity assessment unit  306  assesses consecutive phase values α 1  and α 2  and determines whether +π or −π should be added to α 2  to eliminate the ambiguity. 
     Phase ambiguity elimination unit  308  may be initiated by preamble detection and CFO acquisition module controller  302  to repair ambiguities in the phase data generated by phase detector  208  prior to passing the affected phase values to preamble detection unit  314 . As described above with respect to phase discontinuity assessment unit  306 , phase ambiguity elimination unit  308  may eliminate the ambiguity by adding either +π or −π to α 2 , as determined by phase discontinuity assessment unit  306 . 
     Low-pass filter monitoring unit  310  may be initiated by preamble detection and CFO acquisition module controller  302  to monitor the output of channel filter  204 , shown in  FIG. 2 , to detect sudden changes in the output of channel filter  204 . For example, in one example embodiment, low-pass filter monitoring unit  310  is configured with two reporting thresholds. A first threshold may be triggered upon detection of a significant change, e.g., a sudden positive or negative change of greater than 150% over a period of 10-20 μsec. A second threshold may be triggered upon detection of a large change, e.g., a sudden positive or negative change of greater than 100% over a period of 10-20 μsec. Each time the output of channel filter  204  exceeds a reporting threshold, the event may be reported to preamble detection and CFO acquisition module controller  302 , as described in greater detail below. 
     RSSI monitoring unit  312  may be initiated by preamble detection and CFO acquisition module controller  302  to monitor the output of RSSI module  134 , shown in  FIG. 1 , to detect sudden changes in the output of RSSI module  134 . For example, in one example embodiment, RSSI monitoring unit  312  is configured with a reporting threshold that may be triggered upon detection of a significant change, e.g., a sudden positive or negative change of greater than 200% over a period of 10-20 μsec. Each time the output of RSSI module  134  exceeds a reporting threshold, the event may be reported to preamble detection and CFO acquisition module controller  302 , as described in greater detail below. 
     In-band interference may include interference with a center frequency that is within 1 MHz from the desired signal. Out-of-band interference may include interference with a center frequency that at least 1 MHz from the desired signal. Each type of interference may falsely trigger a preamble detection, and/or may result in an incorrect CFO estimation for the desired channel. For this reason, preamble detection and CFO acquisition module  210 , monitors for both in-band and out-of-band interference during the preamble detection process, as described in greater detail below. 
     For example, a significant positive or negative change in the output of channel filter  204  during a preamble detection period may indicate that the preamble detection process was triggered by out-of-band interference. Therefore, if preamble detection and CFO acquisition module controller  302  receives a report from low-pass filter monitoring unit  310 , during a preamble detection period, indicating a significant positive or negative change in the output of channel filter  204 , preamble detection and CFO acquisition module controller  302  may instruct preamble detection unit  314  to drop any CFO values calculated during the current preamble detection period and to reinitiate the preamble detection process. 
     Further, a large positive or negative change in the output of channel filter  204  that coincides with a large change in the output of RSSI module  134 , during a preamble detection period, may indicate the presence of in-band interference that may adversely affect the accuracy of a CFO value generated from preamble data values that include points collected during the period of in-band interference. Therefore, if preamble detection and CFO acquisition module controller  302  receives, during a preamble detection period, a report from low-pass filter monitoring unit  310  indicating a large positive or negative change in the output of channel filter  204  that coincides with a report from RSSI monitoring unit  312  indicating a significant change in the output of RSSI module  134 , preamble detection and CFO acquisition module controller  302  may instruct preamble detection unit  314  to drop any CFO values calculated during the current preamble detection period and to reinitiate the preamble detection process. 
     Preamble detection unit  314  may be initiated by preamble detection and CFO acquisition module controller  302  to detect a preamble based on phase data generated by phase detector  208 . The phase data received by preamble detection unit  314  may be uncorrected/ambiguity-free phase data unchanged from that generated by phase detector  208  or may be phase data generated by phase detector  208  in which one or more phase ambiguities have been corrected. Preamble detection unit  314  may correlate the received phase values with a predetermined preamble pattern, as described above, and may report to a preamble match to preamble detection and CFO acquisition module controller  302 . 
     CFO acquisition unit  316  may be initiated by preamble detection and CFO acquisition module controller  302  upon receipt of a notification from preamble detection unit  314  that a preamble has been successfully detected. CFO acquisition unit  316  may be initiated to generate CFO correction values based on the peak data values or all data values associated with the detected preamble, as described above. The generated CFO value may be stored and may be used by preamble detection and CFO acquisition module controller  302  to generate the CFO compensation signal sent from preamble detection and CFO acquisition module  210  to CFO compensation module  202 , and to generate the immediate CFO compensation signal sent from preamble detection and CFO acquisition module  210  to adder  212 , as described above with respect to  FIG. 2 . 
       FIG. 4  and  FIG. 5  show a flow-chart of an example process for performing preamble detection and CFO acquisition in an example embodiment of the robust differential receiver described above with respect to  FIG. 2 . It is assumed that the process described below is performed in parallel with an in-band and out-of-band interference detection process, as described above, and as described below with respect to  FIG. 6 . The two respective processes may interact based on values stored and maintained by preamble detection and CFO acquisition module controller  302 . For example, the CFO value generated by CFO acquisition unit  316  and maintained by preamble detection and CFO acquisition module controller  302  may be accessible to both example processes. In the example process described above with respect to  FIG. 4  and  FIG. 5 , a null CFO value may indicate that a previously determined CFO value is no longer valid and that a preamble detection process is either in progress, or about to be initiated. A non-null CFO value may indicate that a preamble detection process has been successfully completed, i.e., free of in-band and out-of-band distortion, and a valid CFO value has been generated and stored. Further, an interference flag maintained by preamble detection and CFO acquisition module controller  302  may also be accessible to both processes. For example, an interference flag in a “set” state may indicate that one or more of in-band and out-of-bad interference has been detected during a preamble detection process, and an interference flag in a “reset” state may indicate that no such interference has yet been detected during a current preamble detection process. As shown in  FIG. 4 , operation of the process begins at step  402  and proceeds to step  404 . 
     In step  404 , preamble detection and CFO acquisition module controller  302  begins monitoring the value of the CFO value generated by CFO acquisition unit  316 , and operation of the process continues to step  406 . 
     If, in step  406 , preamble detection and CFO acquisition module controller  302  determines that the CFO value has been reset to null, operation of the process continues to step  408 , otherwise, operation of the process continues to step  404 . 
     In step  408 , preamble detection and CFO acquisition module controller  302  updates the control signal provided to digital multiplexor  214 , shown in  FIG. 2 , to stop the flow of phase values to symbol timing module  216 , and operation of the process continues to step  410 . 
     In step  410 , preamble detection and CFO acquisition module controller  302  receives phase data from phase detector  208  and initiates phase monitoring unit  304 , low-pass filter monitoring unit  310  and RSSI monitoring unit  312 , and operation of the process continues to step  412 . 
     If, in step  412 , preamble detection and CFO acquisition module controller  302  receives a report from phase monitoring unit  304  that a phase ambiguity has been detected, operation of the process continues to step  414 , otherwise, operation of the process continues to step  418 . 
     In step  414 , phase discontinuity assessment unit  306  determines the magnitude of the detected ambiguity, and operation of the method continues to step  416 . 
     In step  416 , phase ambiguity elimination unit  308  corrects the detected ambiguity based on the magnitude of the detected ambiguity determined by phase discontinuity assessment unit  306 , and operation of the process continues to step  418 . 
     If, in step  418 , preamble detection unit  314  reports a preamble detection, operation of the process continues to step  420 , otherwise, operation of the process continues to step  410 . 
     In step  420 , CFO acquisition unit  316  locates peak values of the detected preamble, and operation of the process continues to step  422 . 
     In step  422 , CFO acquisition unit  316  calculates an average based on either the detected peak values, or all phase values, associated with the detected preamble, and operation of the process continues to step  424 . 
     If, in step  424 , preamble detection and CFO acquisition module controller  302  determines that the interference flag is in a “set” state, i.e., indicating that at least one of an out-of-band and in-band interference was detected during the preamble detection process, operation of the process continues to step  426 , otherwise, operation of the process continues to step  428 . 
     In step  426 , preamble detection and CFO acquisition module controller  302  resets the interference flag, and operation of the process continues to step  410 . 
     In step  428 , CFO acquisition unit  316  saves the calculated average as the new CFO value, and operation of the process continues to step  430 . 
     In step  430 , preamble detection and CFO acquisition module controller  302  generates CFO compensation signals provided to CFO compensation module  202  and adder  212 , and operation of the process continues to step  432 . 
     In step  432 , preamble detection and CFO acquisition module controller  302  updates the control signal provided to digital multiplexor  214  to allow corrected phase detector output to pass from adder  212  to symbol timing module  216 , as described above, and may set a loop delay timer, as described above, and operation of the process continues to step  434 . 
     If, in step  434 , a loop delay timer timeout is detected, operation of the process continues to step  436 , otherwise, operation of the process continues to step  434 . 
     In step  436 , preamble detection and CFO acquisition module controller  302  updates the control signal provided to digital multiplexor  214  to allow phase detector output to pass from phase detector  208  to symbol timing module  216 , as described above, and operation of the process continues to step  438 . 
     If, in step  438 , a power down of the receiver device is detected, operation of the process continues to step  440  and the process terminates, otherwise, operation of the process continues to step  404 . 
       FIG. 6  shows a flow-chart of an example process for performing in-band and out-of-band interference detection in an example embodiment of the robust differential receiver described above with respect to  FIG. 2 . It is assumed that the process described below is performed in parallel with the preamble detection and CFO acquisition process described above with respect to  FIG. 4  and  FIG. 5 . The two respective processes may interact based on values stored and maintained by preamble detection and CFO acquisition module controller  302 . For example, the CFO value generated by CFO acquisition unit  316  and maintained by preamble detection and CFO acquisition module controller  302  may be accessible to both example processes. In the example process described below with respect to  FIG. 6 , a null CFO value indicates that a previously determined CFO value is no longer valid and that a preamble detection process is either in progress, or about to be initiated. A non-null CFO value indicates that a preamble detection process has been successfully completed, i.e., free of in-band and out-of-band distortion, and a valid CFO value has been generated and stored. Further, an interference flag maintained by preamble detection and CFO acquisition module controller  302  may also be accessible to both processes. For example, an interference flag in a “set” state may indicate that one or more of in-band and out-of-bad interference has been detected during a preamble detection process, and an interference flag in a “reset” state may indicate that no such interference has yet been detected during a current preamble detection process. As shown in  FIG. 6 , operation of the process begins at step  602  and proceeds to step  604 . 
     In step  604 , preamble detection and CFO acquisition module controller  302  begins monitoring the interference flag, described above, and operation of the process continues to step  606 . 
     It in step  606 , preamble detection and CFO acquisition module controller  302  determines that the interference flag is in a reset state, operation of the process continues to step  608 , otherwise, operation of the process continues to step  604 . 
     In step  608 , low-pass filter monitoring unit  310  begins receiving/monitoring output generated by channel filter  204  (e.g., a low-pass filter), and operation of the process continues to step  610 . 
     In step  610 , RSSI monitoring unit  312  begins receiving/monitoring RSSI values generated by RSSI module  134 , and operation of the process continues to step  612 . 
     If, in step  612 , low-pass filter monitoring unit  310  reports to preamble detection and CFO acquisition module controller  302  a significant positive or negative change in the output of channel filter  204 , operation of the process continues to step  616 , otherwise, operation of the process continues to step  614 . 
     If, in step  614 , preamble detection and CFO acquisition module controller  302  determines that a large positive or negative change in the output of channel filter  204 , reported by low-pass filter monitoring unit  310 , has occurred concurrently with a significant increase in the magnitude of RSSI values reported by RSSI monitoring unit  312 , and the CFO value is null, operation of the process continues to step  616 , otherwise, operation of the process continues to step  608 . 
     In step  616 , preamble detection and CFO acquisition module controller  302  places the interference flag in a “set” state, and operation of the process continues to step  618 . 
     In step  618 , preamble detection and CFO acquisition module controller  302  sets the CFO value to “null,” and operation of the process continues to step  620 . 
     If, in step  620 , a power down of the receiver device is detected, operation of the process continues to step  622  and the process terminates, otherwise, operation of the process continues to step  604 . 
       FIG. 7  shows a plot of an example signal  702  that suffers from multiple example phase ambiguities. As shown in  FIG. 7 , phase ambiguity  703  occurs between point  701  and  705 , phase ambiguity  709  occurs between point  707  and  711 , phase ambiguity  715  occurs between point  713  and  717 , and phase ambiguity  721  occurs between point  719  and  723 . As described above, even if such a signal could achieve preamble detection, a CFO value based on such preamble peaks or preamble data values would not likely be representative of the true CFO for the signal, i.e., would likely be highly inaccurate, and attempts to extract access codes and payload data after having adjusted a signal with such an inaccurate CFO may likely result in transmission errors and possible packet loss. 
       FIG. 8  shows a plot of an example signal  802  that suffers from an example phase ambiguity. As shown in  FIG. 8 , phase ambiguity  805  occurs between point  803  and  807 . 
       FIG. 9  shows a plot  902  based on the same data presented in the plot of  FIG. 8 , in which the phase ambiguity, shown in  FIG. 8  at  805 , has been corrected and a determined CFO offset has been calculated and applied to the data values using an example embodiment of the preamble detection and CFO acquisition module described above with respect to  FIG. 2  and  FIG. 3 , using the example processes described above with respect to  FIG. 4 ,  FIG. 5  and  FIG. 6 . 
       FIG. 10  shows on a common axis, plot  1002  of example output generated by analog-to-digital converter  132 , described above with respect to  FIG. 1 , plot  1004  of example output generated by channel filter  204 , described above with respect to  FIG. 2 , and plot  1006 , which is a filtered, or smoothed, version of plot  1004 . Plots  1004  and  1006  are typical of the type of channel filter output that may be monitored by low-pass filter monitoring unit  310 , as described above. The significant rise from a small value in the magnitude of the channel filter output, shown at  107 , is representative of a significant rise that, upon being detected by low-pass filter monitoring unit  310  maybe reported as indicating of the arrival of a true signal in the presence of out-of-band interference, as described above with respect to  FIG. 6 . 
       FIG. 11  shows on a common axis, plot  1102  of example output values generated by analog-to-digital converter  132 , described above with respect to  FIG. 1 , plot  1104  of example output values generated by channel filter  204 , described above with respect to  FIG. 2 , plot  1106 , which is a filtered, or smoothed, version of plot  1104 , and plot  1108  which is a plot of the RSSI values generated by RSSI module  134 , described above with respect to  FIG. 1 . Plots  1104  and  1108  are typical of the type of channel filter output and RSSI module output that may be monitored by low-pass filter monitoring unit  310  and RSSI monitoring unit  312 , respectively, as described above. The large rise from a small value in the magnitude of the channel filter output, shown at  1111 , occurs concurrently with a significant change in the RSSI values, shown at  1109 . Such concurrent changes in the respective output values, upon being detected by low-pass filter monitoring unit  310  and RSSI monitoring unit  312 , respectively, and reported to preamble detection and CFO acquisition module controller  302  may be interpreted as indicating the presence of in-band interference, as described above with respect to  FIG. 6 . 
     It is to be understood that various functions of the described robust differential receiver for a frequency modulated system is compatible with and may be seamlessly integrated within integrated circuit hardware, such as system on a chip (SoC) devices. Further, it is to be understood that the described approach may be distributed in any manner among any quantity (e.g., one or more) of hardware and/or software modules or units that may be interconnected with circuitry and/or software interfaces. 
     For purposes of explanation, in the above description, numerous specific details are set forth in order to provide a thorough understanding of the robust differential receiver for a frequency modulated system. It will be apparent, however, to one skilled in the art that the robust differential receiver for a frequency modulated system may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the features of the robust differential receiver for a frequency modulated system and the RF transmitter/receiver devices in which the robust differential receiver for a frequency modulated system may be used. 
     While the robust differential receiver for a frequency modulated system has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, embodiments of the robust differential receiver for a frequency modulated system as set forth herein are intended to be illustrative, not limiting. There are changes that may be made without departing from the spirit and scope of the invention.