U.S. patent application number 13/563246 was filed with the patent office on 2013-05-16 for timing and time information extraction from a phase modulated signal in a radio controlled clock receiver.
This patent application is currently assigned to XW LLC dba Xtendwave. The applicant listed for this patent is Oren E. Eliezer. Invention is credited to Oren E. Eliezer.
Application Number | 20130121398 13/563246 |
Document ID | / |
Family ID | 46800747 |
Filed Date | 2013-05-16 |
United States Patent
Application |
20130121398 |
Kind Code |
A1 |
Eliezer; Oren E. |
May 16, 2013 |
Timing and Time Information Extraction from a Phase Modulated
Signal in a Radio Controlled Clock Receiver
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.
Inventors: |
Eliezer; Oren E.; (Plano,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Eliezer; Oren E. |
Plano |
TX |
US |
|
|
Assignee: |
XW LLC dba Xtendwave
Dallas
TX
|
Family ID: |
46800747 |
Appl. No.: |
13/563246 |
Filed: |
July 31, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13345084 |
Jan 6, 2012 |
8270465 |
|
|
13563246 |
|
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|
61559966 |
Nov 15, 2011 |
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Current U.S.
Class: |
375/238 ;
375/329 |
Current CPC
Class: |
G04R 20/10 20130101;
G04G 7/02 20130101 |
Class at
Publication: |
375/238 ;
375/329 |
International
Class: |
H04L 27/22 20060101
H04L027/22 |
Goverment Interests
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.
Claims
1. 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, said timing information based on a
known synchronization sequence; and a circuit operative to extract
said timing and time information from the phase of said received
signal.
2. The device according to claim 1, wherein said phase modulation
comprises antipodal binary phase shift keying (BPSK)
modulation.
3. The device according to claim 1, wherein said phase modulation
comprises non-antipodal binary phase shift keying (BPSK)
modulation.
4. The device according to claim 1, wherein said phase modulation
comprises discontinuous binary phase shift keying (BPSK)
modulation.
5. The device according to claim 1, wherein said extraction circuit
is operative to correlate said phase modulated received signal
against a known synchronization sequence to recover said timing
information.
6. The device according to claim 1, wherein said timing information
is used to determine bit boundaries (symbol timing) and frame
timing (minute boundaries).
7. The device according to claim 1, wherein said circuit is
operative to limit phase demodulation to the last 0.5 second of
each bit of a legacy PWM/ASK modulation scheme.
8. The device according to claim 1, wherein said circuit is
operative to limit phase demodulation to those portions of each bit
that are at high amplitude in accordance with a legacy PWM/ASK
modulation scheme.
9. The device according to claim 1, wherein said circuit performs
phase demodulation on the low as well as high portions of each bit
of a legacy PWM/ASK modulation scheme.
10. A radio receiver method, said method comprising: receiving a
phase modulated (PM), pulse width modulated (PWM)/amplitude shift
keyed (ASK) broadcast signal encoded with timing and time
information, said timing information based on a known
synchronization sequence; and extracting said timing and time
information from the phase of said received signal.
11. The method according to claim 10, wherein said phase modulation
comprises antipodal binary phase shift keying (BPSK)
modulation.
12. The method according to claim 10, wherein said phase modulation
comprises non-antipodal binary phase shift keying (BPSK)
modulation.
13. The method according to claim 10, wherein said phase modulation
comprises discontinuous binary phase shift keying (BPSK)
modulation.
14. The method according to claim 10, wherein said phase modulated
received signal is correlated against a known synchronization
sequence to recover said timing information.
15. The method according to claim 10, wherein extracting said
timing and time information comprises limiting phase demodulation
to the last 0.5 second of each bit of a legacy PWM/ASK modulation
scheme.
16. The method according to claim 10, wherein extracting said
timing and time information comprises limiting phase demodulation
to those portions of each bit that are at high amplitude in
accordance with a legacy PWM/ASK modulation scheme.
17. The method according to claim 10, wherein extracting said
timing and time information comprises performing phase demodulation
on the low as well as high portions of each bit of a legacy PWM/ASK
modulation scheme.
18. A radio receiver method for use in a timekeeping device, said
method comprising: receiving a phase modulated (PM), pulse width
modulated (PWM)/amplitude shift keyed (ASK) broadcast signal
encoded with timing and time information, said timing information
based on a known synchronization sequence; extracting said timing
and time information from the phase of said received signal; and
correlating said timing information against a known synchronization
sequence so as to establish frame and symbol timing.
19. The method according to claim 18, further comprising applying
averaging filtering whereby the timing information extracted from
the received signal is weighted against the local currently assumed
time in said timekeeping device.
20. The method according to claim 19, wherein said averaging and
weighting are adaptive and performed using coefficients adapted to
consider the level of confidence in the received signal, based on
the reception conditions, against the level of confidence in the
current time, based on the time that has elapsed since the last
timing correction and the monitored statistics of historical timing
adjustments.
21. The method according to claim 18, further comprising
determining a current time as a nonlinear threshold-comparison
function of the timing and time information extracted from a
received frame, said threshold-comparison function adapted to
determine whether the distance between said extracted information
and the local currently assumed time in said timekeeping device is
greater than a predefined threshold.
22. The method according to claim 18, wherein the time information
extracted from a received frame is compared against local assumed
time allowing the device to validate said local assumed time or
replace it with said extracted time information based on whether or
not the reception conditions suggest high confidence in the
corresponding received information.
23. The method according to claim 22, wherein the time information
extracted from a received frame is validated against successive
receptions obtained in one or more subsequent frames before being
used in place of the local assumed time in said timekeeping
device.
24. The method according to claim 18, further comprising
accumulating received energy over multiple one-minute frames
thereby providing a corresponding gain in reception.
25. The method according to claim 18, further comprising receiving
superframes consisting of sets of 60 frames used to extract hourly
timing information.
26. The method according to claim 18, wherein synchronization
sequence fields in the frames of a superframe are phase modulated
in accordance with a known pattern.
27. A radio receiver method, said method comprising: receiving a
phase modulated (PM) broadcast signal encoded with timing and time
information, wherein said timing and time information, intended for
synchronization and time reference purposes, is conveyed in the
phase of the carrier portion of said broadcast signal; and
extracting said timing and time information from the phase of said
received signal.
28. The method according to claim 27, wherein said phase modulation
comprises antipodal binary phase shift keying (BPSK)
modulation.
29. The method according to claim 27, wherein said phase modulation
comprises non-antipodal binary phase shift keying (BPSK)
modulation.
30. The method according to claim 27, wherein said phase modulation
comprises discontinuous binary phase shift keying (BPSK)
modulation.
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.
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.sub.0(t)), `1` (denoted x.sub.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.sub.0(t)), `1`
(denoted x.sub.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.sub.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.sub.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.sub.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.sub.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.sub.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.sub.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.sup.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.degree. difference in the carrier'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.sub.0(t)=x.sub.0(t) (waveform 80) and s.sub.1(t)=-x.sub.1(t)
(waveform 84), respectively, as shown in FIG. 10. Waveform 82
represents the original `1` symbol s.sub.1(t)=x.sub.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'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.sub.0
(referenced 88) and -x.sub.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.sub.0 and x.sub.1 (referenced 90),
thereby allowing for more robust reception in the presence of
additive noise. Note that the existing symbols x.sub.0 and x.sub.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.sub.0 and x.sub.1 is shown to be 0.47, whereas the
Euclidean distance for the two phase modulated waveforms x.sub.o
and -x.sub.1 increases to 1.55. Therefore, the modulation gain
(denoted m.sub.g) representing the power ratio by which the
detection capability in the presence of additive noise is improved,
is given by
m g = 20 log 10 ( 1.55 0.47 ) = 10.36 dB ( 1 ) ##EQU00001##
[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.sub.1(t) 84 in FIG. 10. In an example embodiment, with
independent data being communicated through the carrier'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'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.degree., e.g., .+-.45.degree., .+-.25.degree., .+-.13.degree.,
etc. Use of a lower value such as .+-.13.degree. 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.degree. 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.sub.b/N.sub.o, is given by
BER = Q ( 2 E b N o ) ( 2 ) ##EQU00002##
where E.sub.b is the energy per bit and N.sub.o is the noise
density.
[0073] The E.sub.b/N.sub.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.sub.b/N.sub.o=SNR @
BW=R.sub.b, where R.sub.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
BER = Q ( d 2 2 N o ) ( 3 ) ##EQU00003##
where Q(x) is the tail probability of the normal distribution,
i.e.
Q ( x ) = 1 2 .pi. .intg. x .infin. exp ( - u 2 2 ) u ( 4 )
##EQU00004##
[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'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.
* * * * *