Abstract:
A system and method for a radio controlled clock receiver adapted to extract timing and time information from a phase modulated signal. The official time signal is broadcast from a central location using the modified modulation scheme of the present invention, which adds phase modulation that allows for greatly improved performance. The information modulated onto the phase contains a known synchronization sequence having good autocorrelation properties, error-correcting coding for the time information and notifications of daylight-saving-time (DST) transitions that are provided months in advance. The modulation scheme is based on a form of phase modulation, such as binary-phase-shift-keying (BPSK) or phase reversal keying (PRK). A superframe comprising multiple frames with repeated information allows for the accumulation of received energy over multiple frames to provide for a corresponding gain in the receiver.

Description:
REFERENCE TO PRIORITY APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 13/345,084, filed Jan. 6, 2012, now U.S. Pat. No. 8,233,525, entitled “Timing and Time Information Extraction from a Phase Modulated Signal in a Radio Controlled Clock Receiver,” incorporated herein by reference in its entirety. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    This invention was made with Government support under National Institute of Standards and Technology SBIR Grant No. NB401000-11-04154. The Government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates to the field of wireless communications, and more particularly relates to a radio controlled clock receiver adapted to extract timing and time information from a phase modulated signal. 
       BACKGROUND OF THE INVENTION 
       [0004]    Radio-controlled-clock (RCC) devices that rely on time signal broadcasts have become widely used in recent years. A radio-controlled-clock (RCC) is a timekeeping device that provides the user with accurate timing information that is derived from a received signal, which is broadcast from a central location, to allow multiple users to be aligned or synchronized in time. Colloquially, these are often referred to as “atomic clocks” due to the nature of the source used to derive the timing at the broadcasting side. In the United States, the National Institute of Standards and Technology (NIST) provides such broadcast in the form of a low-frequency (60 kHz) digitally-modulated signal that is transmitted at high power from radio station WWVB in Fort Collins, Colo. The information encoded in this broadcast includes the official time of the United States. This also includes information regarding the timing of the implementation of daylight saving time (DST), which has changed in the United States over the years due to various considerations. 
         [0005]    Reception of the time signal, however, is being challenged by a growing number of sources of electromagnetic interference. In particular, the on-frequency interference from the MSF radio station in the United Kingdom has been identified as a particularly challenging jammer for receivers on the East Coast. 
         [0006]    There is thus a need for a new protocol for time signal broadcasts, such as that provided by WWVB, that attempts to cost-effectively address the reception challenges. Such a new protocol should preserve existing amplitude modulation properties of the transmitted signal, in order to maintain backwards compatibility and not impact existing devices. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is a system and method for a radio controlled clock receiver adapted to extract timing and time information from a phase modulated signal. The system and method of the present invention provide a modified modulation scheme for transmission of the official time signal that is broadcast from a central location, and a receiver adapted to extract the timing and time information from this broadcast. The modified modulation scheme adds phase modulation that allows for greatly improved performance. The information modulated onto the phase contains a known synchronization sequence, error-correcting coding for the time information and notifications of daylight-saving-time (DST) transitions that are provided months in advance. 
         [0008]    The structure and method of operation of the receiver allows the timekeeping functionality of a device to be accurate, reliable and power efficient. The communication protocol of the present invention is adapted to allow prior-art devices to operate in accordance with the legacy communication protocol such that they are unaffected by the changes introduced to the protocol by the present invention, whereas devices adapted to operate in accordance with the present invention benefit from various performance advantages. These advantages include (1) greater robustness of the communication link; (2) allowing reliable operation at a much lower signal-to-noise-and-interference-ratio (SNIR); (3) greater reliability in providing the correct time; and (4) reduced energy consumption which leads to extended battery life in battery-operated devices. 
         [0009]    In one embodiment of the present invention, the modulation applied to the carrier is limited to its phase, thereby allowing existing devices that operate in accordance with the legacy communication protocol, whereby the information may be extracted through envelope detection, to continue to operate with the modified protocol without being affected. Although this backward compatibility property of the communication protocol of the present invention may represent a practical need when upgrading an existing system, the scope of the invention is not limited to the use of this modulation scheme and to operation in conjunction with an existing communication protocol. 
         [0010]    The enhanced robustness offered by the present invention, resulting in reliable reception at lower SNIR values with respect to those required for proper operation of prior art devices, is a result of the use of (1) a known synchronization sequence having good autocorrelation properties; (2) coding that allows for error detection and correction within the fields of information bits that are part of each data frame; and (3) the use of a superior modulation scheme, such as binary-phase-shift-keying (BPSK) (also known as phase-reversal keying or PRK) in one embodiment of the present invention. The PRK modulation, representing an antipodal system, provides the largest distance in the signal space with respect to signal power, whereas the historical modulation schemes that are used for time broadcasting worldwide are based on pulse width modulation (PWM) that relies on amplitude demodulation, requiring a higher SNIR to achieve the same decision error probability or bit-error-rate (BER). 
         [0011]    The enhanced reliability in assuming or setting the right time in a device of the present invention may be partly achieved through the use of a time-computing procedure that considers not only the information extracted from the received frame, but also the time that has been assumed in the timekeeping device. For example, if the information extracted from a received frame suggests that the year is many years ahead of what the timekeeping device has been assuming for a long time, it is likely that the reception is in error and should be disregarded. 
         [0012]    On a finer scale, when the correlation operation that makes use of the known synchronization sequence in the received signal produces a noisy result (i.e. the correlation peak is closer to the low-correlation results), based on which the timing extraction may be inaccurate, the receiver may apply averaging filtering, wherein the timing extracted from the received signal is weighted against the locally assumed time in the device such that the timing adjustment considers them both instead of being determined based solely on the received signal, as is typically done in existing prior art devices. 
         [0013]    Furthermore, the system is scalable in that it allows for receivers experiencing different reception conditions to use the received signal differently. In particular, it is designed to allow for the accumulation of received energy over multiple one-minute frames (i.e. throughout a one-hour superframe or a portion thereof), to provide for a corresponding gain in the receiver (e.g., reception for a whole hour may provide a gain of 60, or 18 dB, with respect to a single minute). 
         [0014]    The features described supra serve to greatly increase the robustness and reliability of the time signal communication system, allowing it to operate at signal-to-noise ratios that are several orders of magnitude lower than those required in the existing scheme, while exhibiting even higher gains in scenarios of on-frequency jamming, to which the existing receivers are particularly vulnerable. 
         [0015]    There is thus provided in accordance with the invention, a radio receiver comprising a receiver circuit operative to receive a phase modulated (PM), pulse width modulation (PWM)/amplitude shift keyed (ASK) broadcast signal encoded with timing and time information, the timing information based on a known synchronization sequence and a circuit operative to extract the timing and time information from the phase of the received signal. 
         [0016]    There is also provided in accordance with the invention, a radio receiver method, the method comprising receiving a phase modulated (PM), pulse width modulated (PWM)/amplitude shift keyed (ASK) broadcast signal encoded with timing and time information, the timing information based on a known synchronization sequence and extracting the timing and time information from the phase of the received signal. 
         [0017]    There is further provided in accordance with the invention, a radio receiver method for use in a timekeeping device, the method comprising receiving a phase modulated (PM), pulse width modulated (PWM)/amplitude shift keyed (ASK) broadcast signal encoded with timing and time information, the timing information based on a known synchronization sequence, extracting the timing and time information from the phase of the received signal and correlating the timing information against a known synchronization sequence so as to establish frame and symbol timing. 
         [0018]    There is also provided in accordance with the invention, a radio receiver method, the method comprising receiving a phase modulated (PM) broadcast signal encoded with timing and time information, wherein the timing and time information, intended for synchronization and time reference purposes, is conveyed in the phase of the carrier portion of the broadcast signal and extracting the timing and time information from the phase of the received signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0020]      FIG. 1  is a high level block diagram illustrating an example timing and time information transmitter of a system operating in accordance with the present invention; 
           [0021]      FIG. 2  is a high level block diagram illustrating an example timing and time information receiver constructed in accordance with the present invention; 
           [0022]      FIG. 3  is a diagram illustrating a first example pulse-width modulated AM signal representing a ‘0’ bit; 
           [0023]      FIG. 4  is a diagram illustrating a second example pulse-width modulated AM signal representing a ‘0’ bit; 
           [0024]      FIG. 5  is a diagram illustrating a first example pulse-width modulated AM signal representing a ‘1’ bit; 
           [0025]      FIG. 6  is a diagram illustrating a second example pulse-width modulated AM signal representing a ‘1’ bit; 
           [0026]      FIG. 7  is a diagram illustrating a first example pulse-width modulated AM signal representing a marker ‘M’; 
           [0027]      FIG. 8  is a diagram illustrating a second example pulse width modulated AM signal representing a marker ‘M’; 
           [0028]      FIG. 9  is a diagram illustrating the structure of an example data frame incorporating timing and time information; 
           [0029]      FIG. 10  is a diagram illustrating an example embodiment of phase modulation, shown at baseband, added to a pulse-width amplitude modulated carrier; 
           [0030]      FIG. 11  is a diagram illustrating the signal space representation of the prior art AM/pulse-width ‘0’ and ‘1’ signals, as well as that of the an example embodiment of the present invention, where PRK is added onto the AM/pulse-width modulation; 
           [0031]      FIG. 12  is a diagram illustrating an example receiver incorporating both amplitude and phase modulation receiver paths; 
           [0032]      FIG. 13  is a diagram illustrating an example receiver adapted to receive a phase modulated signal; 
           [0033]      FIG. 14  is a diagram illustrating a first example waveform of phase modulation added to a pulse-width amplitude modulated carrier in an example communication protocol; 
           [0034]      FIG. 15  is a diagram illustrating a second example phase modulation added to a pulse-width amplitude modulated carrier in an example communication protocol; 
           [0035]      FIG. 16  is a diagram illustrating an example phase modulated carrier in an example communication protocol; and 
           [0036]      FIG. 17  is a diagram illustrating the structure of an example super-frame incorporating timing and time information. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    A high level block diagram illustrating an example timing and time information transmitter a system operating in accordance with the present invention is shown in  FIG. 1 . The equipment at the transmitter end, generally referenced  10 , comprises a high accuracy clock source (frequency source)  12  from which a clock signal (timing information) is derived, a time-code-generator  14  having user-interface  16 , a source of time data  13 , a transmitter  18  generating a TX signal  19  and coupled to transmitting antenna  11 . 
         [0038]    The time code generator  14  keeps track of time based on the high-accuracy frequency source input to it from source  12 , constructs the frames of data representing the time information received from time data source  13  and other information that is to be transmitted, modulates the data frames onto the RF carrier in accordance to a protocol and allows time initialization and other controls to be set in it through its user interface  16 . The transmitter  18  amplifies the modulated signal to generate an output TX signal  19  at the desired levels, e.g., 50 kW, and drives the antenna  11  that is used for the wide-coverage omnidirectional broadcasting of the signal. 
         [0039]    A high level block diagram illustrating an example timekeeping device constructed in accordance with the present invention is shown in  FIG. 2 . Typically, the timekeeping device is incorporated into low cost consumer market products, but may be implemented in any device that requires a precision time reference. The timekeeping device, generally referenced  20 , comprises receiving antenna  21 , receiver module  24  operative to receive RX signal  22 , processor and controller  26 , timekeeping function  30 , internal or external clock source  31 , display  32  and user interface  34 . 
         [0040]    The receiver module  24  extracts timing and time information from the received signal  22 , in accordance with the modulation scheme and protocol in use, and provides the processing and control function  26  with the extracted timing and time information. Controller function/processor  26  appropriately enables/disables the operation of the receiver module through control line  28  such that it is limited to the intervals of interest to minimize energy consumption in those applications where it may be critical to do so (e.g., wrist watches). The timekeeping function  30  keeps track of the time based on pulses provided by clock source  31  having limited accuracy. Note that the clock source  31  may comprise any suitable clock source or clock signal such as a crystal oscillator and may be provided internal to the timekeeping device  20  or supplied from a source external to the timekeeping device. 
         [0041]    The timekeeping may be adjusted by the processor/controller in accordance with an estimated drift at a specific instant, which is either measured or calculated or a combination of the two. The display function  32  may be used to display the time as well as various indications to the user, including reception quality, estimated bound for error in displayed time, battery status, etc. The user interface function  34 , based on pushbuttons, slide-switches, a touch-screen, keypad, computer interface, a combination therefrom, or any other form of human interface, may be used to set the initial time, define the maximal allowed timing error, the time-zone according to which time is to be calculated, the use of daylight saving time, etc. 
         [0042]    In one embodiment of the invention, the timekeeping device is operative to extract timing and time information conveyed in a broadcast signal. Timing information denotes information related to synchronization and tracking and is used, e.g., for bit and frame synchronization. Time information denotes information related to the current time being communicated, such as the date and the time of day (hours, minutes, etc.), as well as scheduled events, such as an upcoming DST transition, leap second, etc. 
         [0043]    Typical available time-broadcast signals employ some form of amplitude modulation combined with some form of pulse width modulation (PWM) to send binary data bits. As an example consider the WWVB signal broadcast from Fort Collins, Colo. in the United States of America. The WWVB signal comprises a 60 second frame consisting of 60 one second bits. Each bit, of one second duration, is sent as a pulse width modulated signal where carrier signal is transmitted at a low amplitude or a high amplitude for different portions of the bit. The frame also consists of several marker bits spread out evenly through the frame, which serve only to indicate timing and do not convey time information. Representations of the different possible signal waveforms transmitted by WWVB are presented below. 
         [0044]    The existing WWVB system transmits a pulse-width modulated amplitude-shift keyed waveform on a 60 kHz carrier. The one-second duration ‘0’ and ‘1’ symbols are represented by a power reduction of −17 dB at the start of the second for 0.2 s and 0.5 s, respectively.  FIGS. 3 ,  5 ,  7  show the baseband waveforms for the ‘0’ (denoted x 0 (t)), ‘1’ (denoted x 1 (t)) and Marker (‘M’) symbols for the existing WWVB system where the low portion of the symbols are reduced in power −17 dB, corresponding to an amplitude reduction to about 0.14 of the high amplitude.  FIGS. 4 ,  6 ,  8  show the baseband waveforms for the ‘0’ (denoted x 0 (t)), ‘1’ (denoted x 1 (t)) and Marker (‘M’) symbols for an example broadcast system where the low portion of the symbols are zero amplitude. 
         [0045]    A diagram illustrating a first example pulse width modulated AM signal representing a ‘0’ bit is shown in  FIG. 3 . The signal x 0 (t)  40  (upper diagram) represents the envelope or baseband waveform of a ‘0’ bit and consists of 0.2 seconds of low amplitude carrier (e.g., 0.14 amplitude) and 0.8 seconds of high amplitude carrier. The lower diagram shows the corresponding carrier waveform  42  of 0.2 seconds low amplitude followed by 0.8 seconds of high amplitude. 
         [0046]    A diagram illustrating a second example pulse width modulated AM signal representing a ‘0’ bit is shown in  FIG. 4 . The signal x 0 (t)  44  (upper diagram) represents the envelope or baseband waveform of a ‘0’ bit and consists of 0.2 seconds of zero amplitude carrier and 0.8 seconds of high amplitude carrier. The lower diagram shows the corresponding carrier waveform  46  of 0.2 seconds zero amplitude followed by 0.8 seconds of high amplitude. 
         [0047]    A diagram illustrating a first example pulse width modulated AM signal representing a ‘1’ bit is shown in  FIG. 5 . The signal x 1 (t)  48  (upper diagram) represents the envelope or baseband waveform of a ‘1’ bit and consists of 0.5 seconds of low amplitude carrier (e.g., 0.14 amplitude) and 0.5 seconds of high amplitude carrier. The lower diagram shows the corresponding carrier waveform  50  of 0.5 seconds low amplitude followed by 0.5 seconds of high amplitude. 
         [0048]    A diagram illustrating a second example pulse width modulated AM signal representing a ‘1’ bit is shown in  FIG. 6 . The signal x 1 (t)  52  (upper diagram) represents the envelope or baseband waveform of a ‘1’ bit and consists of 0.5 seconds of zero amplitude carrier and 0.5 seconds of high amplitude carrier. The lower diagram shows the corresponding carrier waveform  54  of 0.5 seconds zero amplitude followed by 0.5 seconds of high amplitude. 
         [0049]    A diagram illustrating a first example pulse width modulated AM signal representing a marker ‘M’ bit is shown in  FIG. 7 . The signal x 1 (t)  56  (upper diagram) represents the envelope or baseband waveform of a ‘M’ bit and consists of 0.8 seconds of low amplitude carrier (e.g., 0.14 amplitude) and 0.2 seconds of high amplitude carrier. The lower diagram shows the corresponding carrier waveform  58  of 0.8 seconds low amplitude followed by 0.2 seconds of high amplitude. 
         [0050]    A diagram illustrating a second example pulse width modulated AM signal representing a marker ‘M’ bit is shown in  FIG. 8 . The signal x 1 (t)  60  (upper diagram) represents the envelope or baseband waveform of a ‘M’ bit and consists of 0.8 seconds of zero amplitude carrier and 0.2 seconds of high amplitude carrier. The lower diagram shows the corresponding carrier waveform  62  of 0.8 seconds zero amplitude followed by 0.2 seconds of high amplitude. 
         [0051]    A diagram illustrating the structure of an example data frame incorporating timing and time information in an example communication protocol is shown in  FIG. 9 . The frame N, generally referenced  70 , comprises timing data  74 , time data  76  and a field of zero or more additional information bits  78 . The N th  transmitted frame is preceded by frame N−1  72  and followed by frame N+1  79 , both of which span 60 seconds and represent the minute before and the minute after frame N, respectively. 
         [0052]    In one embodiment, the transmitted frame  70  comprises a synchronization sequence  74  spanning m seconds, a field of information  73  spanning k seconds that precedes the synchronization sequence and a field  78  spanning the remaining time  60 −(m+k) seconds following the synchronization sequence, such that the four fields together span the total of 60 seconds. The values of m and k are preferably fixed and their sum is less than 60, such that the location of the synchronization sequence is predictable in a frame, allowing the receiver to search for it at the expected timing, while ignoring the information bits if there is no need to receive them. 
         [0053]    The timing data field  74  comprises a known synchronization sequence (e.g., barker code, modified barker code, pseudo random sequence, or any other known word or bit/symbol sequence) at a known timing within the one minute frame of 60 bits that is transmitted every 60 seconds. Note that in alternative embodiments the synchronization sequence may be placed within the frame such that it overlaps or straddles the frame N−1 before it or frame N+1 after it. 
         [0054]    In one embodiment of the invention, phase modulation is added to an amplitude modulated carrier. A diagram illustrating phase modulation added to an amplitude modulated carrier in an example communication protocol is shown in  FIG. 10 . This diagram describes the amplitude/pulse width modulation (PWM) used in the historical WWVB broadcast as well as the phase modulation introduced in accordance with an embodiment of the present invention. The diagram shows the baseband representation of the ‘0’ and ‘1’ symbols in both the historical WWVB modulation and in one that is modified in accordance with an example embodiment of the present invention. It is noted that the enhancement in the communication protocol offered by the present invention, in the form of independently defined phase modulation and the use of a known synchronization sequence, is not limited to the broadcast of WWVB and may be applied to other timing/time information broadcast systems such as those in other countries around the world where similar AM/pulse-width schemes are used or where no AM/pulse-width modulation needs to be supported, allowing for continuous BPSK to be used. 
         [0055]    In one embodiment, the additional phase modulation added to the signal is binary phase shift keying (BPSK) having an 180° difference in the carrier&#39;s phase between the ‘0’ and ‘1’ symbols, also known as antipodal phase modulation or Phase Reversal Keying (PRK). Hence, the modulated waveforms representing these symbols may be expressed as the products of the sinusoidal 60 kHz carrier (in the case of WWVB) and the baseband waveforms s 0 (t)=x 0 (t) (waveform  80 ) and s 1 (t)=−x 1 (t) (waveform  84 ), respectively, as shown in  FIG. 10 . Waveform  82  represents the original ‘1’ symbol s 1 (t)=x 1 (t) that is replaced by its inverse waveform  84  in one example embodiment of the present invention. As is shown in  FIG. 10 , the enhanced modulation scheme can be accomplished through simple sign inversion for the waveform representing the ‘1’ symbol. It is noted that since the existing envelope detector based receivers designed to receive and decode the current WWVB AM/PWM based broadcast signal do not consider the carrier&#39;s phase, they are not impacted by the modification of phase inversion of the ‘1’ symbol. 
         [0056]    A diagram illustrating the signal space representation of AM only and PM over AM ‘0’ and ‘1’ symbols is shown in  FIG. 11 . As shown in the diagram, the new pair of waveforms, x 0  (referenced  88 ) and −x 1  (referenced  86 ), having the same amount of energy (corresponding to their distances from origin), exhibit a much greater distance between the ‘0’ and ‘1’ symbols (as compared to waveform pair x 0  and x 1  (referenced  90 ), thereby allowing for more robust reception in the presence of additive noise. Note that the existing symbols x 0  and x 1  are strongly correlated, i.e. they have a very short distance between them in the signal space with respect to their energies. 
         [0057]    The Euclidean distance between the two amplitude modulated waveforms x 0  and x 1  is shown to be 0.47, whereas the Euclidean distance for the two phase modulated waveforms x o  and −x 1  increases to 1.55. Therefore, the modulation gain (denoted m g ) representing the power ratio by which the detection capability in the presence of additive noise is improved, is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           m 
                           g 
                         
                         = 
                           
                          
                         
                           20 
                            
                           
                               
                           
                            
                           
                             
                               log 
                               10 
                             
                              
                             
                               ( 
                               
                                 1.55 
                                 0.47 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           10.36 
                            
                           
                               
                           
                            
                           dB 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0058]    Thus, by simply adding such phase modulation, an order of magnitude of improvement may be achieved when assuming additive white Gaussian noise (AWGN). This analysis implicitly assumes that the receivers for both schemes would be optimal, i.e. based on correlation or matched filtering. In practice, the BPSK receiver may be implemented digitally in a near-optimal fashion, whereas the receivers for the existing AM/pulse-width scheme, not designed as a classical digital-communications system, are based on envelope detection, as previously noted. This adds an additional gap of 2 to 4 dB between the two when only AWGN is considered. In the presence of on-frequency interference, however, the gain offered by realizing a near-optimal BPSK receiver may be arbitrarily higher. Furthermore, additional gains can be offered, such as (1) through encoding of the information, (2) use of a known synchronization sequence, and (3) extended-duration reception in the receiver over multiple frames (i.e. superframes). 
         [0059]    In an embodiment of the present invention, the information represented by the phase modulation in each bit is independent from that represented by the existing (legacy) AM/pulse-width modulation, such that an inverted phase would not necessarily be tied to the shorter waveform  82 , represented by inverted waveform −x 1 (t)  84  in  FIG. 10 . In an example embodiment, with independent data being communicated through the carrier&#39;s phase, a phase inverted bit, which may represent a “1”, for example, may be combined with either a “0” or a “1” in the AM/PWM signal, resulting in the example waveform shown in  FIG. 14 . 
         [0060]    The receiver extracting the information from the phase may limit the phase demodulation operation to the last 0.5 sec of each bit, where both the “0” and “1” symbols of the AM/PWM scheme shown in this example are at high amplitude. Alternatively, in order to gain from the additional energy in the longer “0” pulses (0.8 sec in this example), the receiver may extend the demodulation of phase during those symbols to 0.8 sec when the content is of the AM/PWM modulation is known to be “0”. In the existing WWVB protocol, for example, there are several such bits fixed at “0”. Additionally, when a device operating in accordance with the present invention has already acquired the time and is tracking it, its reception of the phase modulated information may consider the predicted durations of the time-information bits as they are defined by the particular AM/PWM protocol, thereby further optimizing reception. 
         [0061]    Furthermore, a receiver operating in accordance with the present invention may also consider some or all of the energy that a transmitted bit may have in the low amplitude portion of it, if it is greater than zero. This is to be done by weighting that portion of the signal in accordance with the theory of matched filtering, i.e. if the lower amplitude portion is at a normalized level of 0.14, the correlation operation in the receiver must provide it with such weighting with respect to the weighting of 1 that is applied during the high level duration in the receiver symbol. 
         [0062]    In one embodiment, the receiver determines the current time in accordance with a nonlinear function that disregards the timing and time information extracted from the received frame (along with its weighting) if its distance from the local currently assumed time in the timekeeping device is greater than a predefined or dynamic threshold. This it to avoid incorrect timing adjustments that could be caused by erroneous reception of the timing or time information, the likelihood of which increases as the SINR conditions are more severe. 
         [0063]    In one embodiment, a dynamically adaptive threshold considers the duration over which the time-keeping device has been maintaining the time and the statistics of the time corrections applied throughout that duration. For example, a time keeping device that has been tracking the time for an entire year, while performing weekly timing adjustments averaging 0.8 sec, with the greatest correction being below 1.5 seconds in magnitude, may act to disregard a reception instance suggesting a timing correction of 4 seconds, whereas it would have been considered and weighted at an earlier point in time during that year. 
         [0064]    When the time-keeping device takes into account the timing information extracted by correlating the appropriate portion of the received signal against the known synchronization sequence, an example embodiment of the present invention may perform such an operation utilizing linear combining wherein the coefficient applied towards the timing extracted from the received signal and the coefficient applied for the locally assumed time depend on the levels of confidence in these two timings variables. If, for example, the reception conditions are determined to be excessively noisy, for which the probability of inaccurate timing extraction is higher, whereas the locally assumed time is based on a relatively recent adjustment and a good record of successive timing adjustments suggests that not much drift could have been experienced up until the instance of the reception at question, then relatively low weighting may be applied towards the received timing versus the locally assumed one. If, in contrast, the received timing is accompanied by an indication of high SINR, suggesting a high probability that it is accurate, then it may receive higher weighting compared to that of the locally assumed timing. 
         [0065]    In one embodiment, a time-keeping device operating in accordance with the present invention applies non-linear logic in its reception of time information when a locally assumed time is available and has been validated over time. If the device attempts to extract from a received frame not only the timing information, for the purpose of timing adjustment, but also time information, despite such information already being available to it, then rather than computing a new time based on a linear combination of the received time and the locally assumed one, it is to select one of the two. If the locally assumed time has been validated over time and the received frame is received with errors or is accompanied by a low SINR indication, then the device may disregard the information extracted from the receiver. If, however, the device&#39;s confidence in its locally assumed time is low and the received signal is accompanied by an indication of reliable reception, then the received time may be selected, or one or more additional frames may be received to further increase the confidence in the received information. 
         [0066]    In an alternative embodiment, non-antipodal phase modulation can be used to modulate the PWM signal. For example, the magnitude of phase modulation applied may be set at any value less than 180°, e.g., ±45°, ±25°, ±13°, etc. Use of a lower value such as ±13° ensures that the modulated signal is contained within a narrow bandwidth and does not escape the narrow filtering in typical existing AM receivers, which is on the order of 10 Hz. Note that such narrowband PM is not comparable in performance to antipodal BPSK, where the two symbols are 180° apart exhibiting a correlation factor of −1. 
         [0067]    A diagram illustrating an example receiver incorporating both amplitude and phase modulation receiver paths is shown in  FIG. 12 . In this example embodiment, the receiver is operative to receive both a legacy PWM/AM modulated broadcast signal as well as a phase modulated signal which is transmitted over the legacy PWM/AM signal. The receiver, generally referenced  100 , comprises an AM receiver block  104  and a PM receiver block  102 , both of which are connected to antenna  106  at their input and to processor  124  at their output. 
         [0068]    Amplitude modulation receiver  104  comprises an envelope-detector-based receiver of the type that is typically used in consumer market RCC devices. The AM receiver  104  comprises band pass filter (e.g., crystal filter)  110 , envelope detector  112  and threshold block  114 . As shown in this block diagram, the AM signal is converted into an analog equivalent baseband signal by use of a conventional nonlinear envelope detector  112  (similar to the diode-based circuit in traditional AM receivers). A threshold operation  114  that follows serves to determine the middle level, around which the voltages below it would be converted to a logic low level and the voltages above it to a logic high level. The digital processing stage that follows this operation measures the pulse durations and accordingly recovers the symbols (‘1’, ‘0’, or ‘marker’). Note that, with such a receiver topology, an on-frequency interferer can cause the receiver to decode that symbol incorrectly. Typically, the effect of the interferer is greatest when the signal is at a “low”. If the interferer is exactly on-frequency, however, then it has a very significant effect when it is out of phase and added to the high state of the transmitted signal (e.g., the WWVB signal). 
         [0069]    In operation of a typical envelope detector based receiver, the modulated signal input to the receiver has two different amplitude levels with the information represented in the durations of each of these levels. The high/low decision is made by following the “low” and “high” levels with dedicated peak holders (with appropriate time-constants) and deriving the middle (average) of these two. A threshold operation (e.g., a simple comparator) is then used to create the logic level signals for the digital stage that follows where the pulse durations are measured and the ‘1’/‘0’/‘marker’ symbol decision is made. 
         [0070]    The phase modulation receiver  102  comprises a demodulator  118 , correlator  120  and decoder  122 . In one embodiment, the PM receiver  102  is operative to receive the signal broadcast from WWVB in Fort Collins, Colo. This broadcast signal adds phase modulation (PM) to the WWVB broadcast while maintaining the existing AM code, so as not to impact the existing time-of-day RCC devices. 
         [0071]    A diagram illustrating an example receiver adapted to receive a phase modulated signal is shown in  FIG. 13 . In one embodiment, the receiver, generally referenced  130 , comprises a coherent BPSK optimal receiver that may be implemented digitally. The PM receiver  130  comprises antenna  132  coupled to analog front end (AFE)  134 , low pass filter (LPF)  136 , analog to digital converter (ADC)  138 , mixer  140 , local synthesized carrier (e.g., local oscillator (LO))  146 , correlator  143  and threshold detector  144 . The filtering of the signal is based on the correlation operation which is followed by a decision that is made in the presence of AWGN. 
         [0072]    The bit-error-rate (BER) performance of the receiver, for a signal to noise ratio E b /N o , is given by 
         [0000]    
       
         
           
             
               
                 
                   BER 
                   = 
                   
                     Q 
                      
                     
                       ( 
                       
                         
                           
                             2 
                             · 
                             
                               E 
                               b 
                             
                           
                           
                             N 
                             o 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where E b  is the energy per bit and N o  is the noise density. 
         [0073]    The E b /N o  ratio is equivalent to the ratio between the power of the signal and the power of the noise in a bandwidth that is equal to the bit rate, i.e. E b /N o =SNR @ BW=R b , where R b  represents the bit rate. The threshold decision block  144  is where the decisions are made and the errors occur, in direct relation to the variance of noise, which is assumed to have Gaussian nature and equal variances around the ‘0’ and ‘1’ symbols. The BER may also be expressed as a function of the distance between the symbols in the signal space, as follows 
         [0000]    
       
         
           
             
               
                 
                   BER 
                   = 
                   
                     Q 
                      
                     
                       ( 
                       
                         
                           
                             d 
                             2 
                           
                           
                             2 
                             · 
                             
                               N 
                               o 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where Q(x) is the tail probability of the normal distribution, i.e. 
         [0000]    
       
         
           
             
               
                 
                   
                     Q 
                      
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     
                       1 
                       
                         
                           2 
                            
                           π 
                         
                       
                     
                      
                     
                       
                         ∫ 
                         x 
                         ∞ 
                       
                        
                       
                         
                           exp 
                            
                           
                             ( 
                             
                               - 
                               
                                 
                                   u 
                                   2 
                                 
                                 2 
                               
                             
                             ) 
                           
                         
                          
                         
                             
                         
                          
                         
                            
                           u 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0074]    As previously noted, the analysis presented for the improvement obtained through the introduction of the phase modulation scheme assumed only the presence of AWGN in the receiver. In the presence of radio frequency interference (RFI), and particularly on-frequency interference, the performance improvement could be much more significant and stems from the structure of the BPSK receiver, where the demodulation is based on correlation. 
         [0075]    A diagram illustrating a first example phase modulation added to an amplitude modulated carrier in an example communication protocol is shown in  FIG. 14 . The waveform illustrates three consecutive example bits in the transmission as a time-domain waveform  150 . The three bits  152 ,  154  and  156  each span a duration of one second. Each of the one second bits is divided into a first portion  160  for which the carrier power is low and a second portion  162  for which the carrier power is high. In the WWVB protocol, the information in each bit depends on the durations of these two portions with an even 0.5/0.5 sec partition representing a “1” bit, and the uneven 0.2/0.8 sec partition representing a “0” bit. A 0.8/0.2 sec partition represents a ‘marker’ bit, which may be used for timing identification, but does not carry information. The bits represented under the legacy PWM/AM modulation are indicated at the top portion of the diagram. For example, the three PWM/AM bits shown are “1”, “0” and “1”. 
         [0076]    In accordance with an embodiment of the present invention, information is added to the existing modulation using BPSK modulation. A “1” is represented by a carrier having an inverted phase, with the phase inversion  158  occurring at the beginning of the bit, as shown for the third bit  156  at t=2 sec. It is noted that the phase inversion may also be performed at any other instance, e.g., during the low amplitude portion of the carrier if the receiver&#39;s phase demodulation operation is limited to the high-amplitude duration and disregards the low amplitude portion. While the information represented by the pulse widths is shown to be “1”, “0”, “1”, the information that is sent in parallel, in accordance with the example BPSK (or PRK) protocol of the present invention, is shown to be “0”, “0”, “1” (as shown along the bottom portion of the diagram). Note that there is not necessarily any relationship between the bit pattern transmitted using PWM/AM and that transmitted using PM as they can be completely independent. It is noted that the carrier frequency is not shown to scale in the figure to enhance clarity, but it is preferable for the phase transitions to occur at zero crossing instances of the carrier. 
         [0077]    A diagram illustrating a second example phase modulation added to an amplitude modulated carrier in an example communication protocol is shown in  FIG. 15 . In this second example, the carrier amplitude transmitted during the low portions of a bit is zero rather than reduced to a lower value (e.g., −17 dB or 0.14 amplitude level) as is the case in  FIG. 14 . As in  FIG. 14 , the waveform illustrates three consecutive example bits in the transmission as a time-domain waveform  170 . The three bits  172 ,  174  and  176  each span a duration of one second. Each of the one second bits is divided into a first portion  178  for which the carrier power is zero and a second portion  180  for which the carrier power is high. 
         [0078]    In accordance with the present invention, the modulation of information is added to the existing modulation using BPSK modulation. A “1” is represented by a carrier having an inverted phase, with the phase inversion  182  occurring at the beginning of the bit as shown for the third bit  176  at t=2.5 sec. While the information represented by the pulse widths is shown to be “1”, “0”, “1”, the information that is sent in parallel, in accordance with the BPSK (or PRK) protocol of the present invention, is shown to be “0”, “0”, “1” (as shown along the bottom portion of the diagram). 
         [0079]    Note that there is not necessarily any relationship between the bit pattern transmitted using PWM/AM and that transmitted using PM as they can be completely independent. It is noted that the carrier frequency is not shown to scale in the figure to enhance clarity, but it is preferable for the phase transitions to occur at zero crossing instances of the carrier. 
         [0080]    A diagram illustrating an example phase modulated carrier in an example communication protocol is shown in  FIG. 16 . In this third example, the phase modulation is not added to a PWM/AM signal but rather is sent as the entire bit duration. The waveform illustrates three consecutive example bits in the transmission as a time-domain waveform  190 . The three bits  192 ,  194  and  196  each span a duration of one second. During each of the bits the carrier power is high. The modulation of information is performed using BPSK (or PRK) modulation, in accordance with an embodiment of the present invention. A “1” is represented by a carrier having an inverted phase, with the phase inversion  198  occurring at the beginning of the bit, as shown for the third bit  196  at t=2 sec. The information sent in accordance with the BPSK protocol of the present invention is shown to be “0”, “0”, “1” (as shown along the bottom portion of the diagram). It is noted that the carrier frequency is not shown to scale in the figure to enhance clarity, but it is preferable for the phase transitions to occur at zero crossing instances of the carrier, as may be implemented easily when a bit spans an integer number of carrier cycles, as is the case for WWVB, where the carrier frequency is 60 kHz (i.e. 60,000 cycles per bit). 
         [0081]    A diagram illustrating the structure of an example super frame incorporating timing and time information is shown in  FIG. 17 . In an alternative embodiment, information is recovered not only from the bits of a frame, but may also be recovered by using multiple consecutive frames making up a superframe. In this embodiment, additional information may be conveyed using the superframe, or the same information from each frame may be repeated to allow for improved reception based on the accumulated energy of multiple frames. 
         [0082]    The use of superframes can potentially improve performance of the receiver by nearly two orders of magnitude, which may be critical in low SNR conditions. In one embodiment, the polarity of each of the one-minute frames in an hour is modulated (e.g., differentially or otherwise) by a corresponding bit in a 60-bit hour-synchronization sequence. The preserved consistency between the polarities of the synchronization sequence and the information in each of one-minute frames permits the receiver to resolve the 180-degree phase ambiguity of BPSK reception. 
         [0083]    By correlating against multiple consecutive synchronization sequences, the receiver can accurately adjust its timing and can then use recorded data from an entire hour to perform long-term integration for the hour field (i.e. soft addition). This provides an improvement in gain of 60 (i.e. 18 d B), which enables operation at SNIR values well below 0 dB (when evaluated in a 1 Hz bandwidth). While the minute and parity fields for the time information vary from one minute to the next in the course of an hour, all other fields, however, remain fixed. Thus, simple addition can be used to increase the total amount of energy involved in the information recovery. Since the pattern according to which the minute frame is changing is also known, it too can serve in the extended reception operation. The receiver may determine its timing with respect to the beginning of an hour based on the identification of a portion of the hour-synchronization sequence (at least six bits, collected over seven minutes) with or without recovering information from the minute fields in the received frames. 
         [0084]    With reference to  FIG. 17 , a frame  216  comprises a synchronization sequence field  218 , hour field  220 , minute field  222  and zero or more additional fields  224 . A superframe (e.g., superframe P  212 ) is defined as a set of multiple frames (e.g., 60 frames) wherein the phase of one or more fields in each frame may be modulated to convey information on a superframe basis. For example, additional timing information can be conveyed by modulating the phase of the synchronization sequence field to define a super-synchronization sequence. Each synchronization sequence (i.e. sync seq  0 , sync seq  1 , . . . , sync seq  59 ) is assigned a particular phase wherein the pattern is known to all receivers. The receivers use their knowledge of the super-synchronization sequence to aid in adjusting their time to a particular minute within the hour without having to recover the information from the minute field. Such a super-synchronization sequence provides additional information for receivers to aid in acquisition and tracking at low SINR conditions. 
         [0085]    The use of superframes provides system scalability in that it allows for receivers experiencing different reception conditions to use the received signal differently. In particular, superframes (or the use of a number of multiple frames) allow for the accumulation of received energy over multiple one-minute frames to provide for a corresponding gain in the receiver. For example, reception for an entire hour may provide a gain of 60 or 18 dB with respect to reception over a single minute (i.e. a single frame). 
         [0086]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0087]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. As numerous modifications and changes will readily occur to those skilled in the art, it is intended that the invention not be limited to the limited number of embodiments described herein. Accordingly, it will be appreciated that all suitable variations, modifications and equivalents may be resorted to, falling within the spirit and scope of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.