210 0163-6804/14/$25.00 © 2014 IEEE IEEE Communications Magazine • May 2014

Yingsi Liang and Dinesh

Rajan are with Southern

Methodist University.

Oren Eliezer is with EverSet Technologies. John

Lowe is with National

Institute of Standards and

Technology.

WWVB AND THE

LEGACY AM/PWM-BASED

BROADCAST FORMAT

WWVB is a time-signal broadcast station located near Fort Collins, Colorado, operated by the

National Institute of Standards and Technology

(NIST). WWVB continuously broadcasts digitally represented time information that is derived

from an accurate atomic-clock signal source. The

time information in the broadcast includes the

date and time, as well as notifications related to

upcoming daylight saving time (DST) transitions

and leap seconds [1]. The broadcast may also

carry short messages, such as emergency alerts.

WWVB transmits in the Low-Frequency (LF)

band at 60 kHz with an effective radiated power

of 70 kW, allowing propagation to great distances, particularly during the night, when the

conditions of the ionosphere are favorable. Millions of radio-controlled clocks (RCCs) throughout North America, commonly known as “atomic

clocks,” periodically synchronize to the station

for accurate time-keeping. The legacy WWVB

broadcast format, developed in the 1960s, was

designed to enable low-cost envelope detectionbased receivers, similar to those that were widely

used for AM audio broadcasting reception. The

amount of information transmitted by WWVB is

less than 50 bits each minute at a bit rate of 1 bit

per second, occupying a bandwidth of a few

hertz [2]. WWVB transmits 24 hours a day and

every frame aligns with the start of a minute,

enabling the seconds to be extracted based on

the frame boundaries.

Figure 1 shows the simulated coverage area

of the WWVB legacy broadcast at night, during

which time the reduced absorption in the ionosphere allows for optimal propagation. The simulated coverage area assumes that a RCC will

receive successfully if the field intensity exceeds

100mV/m. This assumption is valid for a typical

commercial RCC having a ferrite-rod antenna

that is oriented correctly towards Fort Collins

[3]. It does not account for possible interference

experienced in the receiver or for additional

propagation (shielding) losses that may be experienced indoors.

However, these assumptions are often invalid.

First, the receiver’s antenna is likely to be oriented randomly, potentially at an orientation for

which it experiences a low gain in the direction

of the broadcast. Second, man-made noise and

radio-frequency interference are likely to be present and dominate the receiver performance.

Third, the signal may experience shielding losses,

representing a major challenge, particularly in

large buildings.

Consequently, many RCCs, based on the

legacy broadcast format, experience reception

failures despite being located within the coverage area shown in Fig. 1. This unreliability has

ABSTRACT

WWVB is a broadcast station, operated by

the National Institute of Standards and Technology, which has been broadcasting time information to radio-controlled clocks (RCC)

throughout the continental US since 1965. The

transmitted information includes the year

(excluding the century), day, hour, minute, and

notifications for leap seconds and for daylight

saving time (DST) transitions. By receiving this

information from WWVB, RCCs can acquire

and maintain accurate timing and automatically

adjust for DST transitions and leap seconds. The

legacy amplitude modulation (AM)-based broadcast format, introduced in the 1960s, while allowing for a simple implementation of a receiver

with the technology of that era, exhibits low efficiency. Consequently, RCCs based on it often

fail to receive reliably, particularly in locations

that are distant from the station, such as on the

East Coast. To improve the station’s coverage,

an enhanced broadcast format, based on the

addition of phase modulation (PM), was

deployed in 2012. This article presents an

overview of this new broadcast format and its

enhanced features, including its modulation

scheme, information encoding, channel coding,

and transmission modes. These features enable

more reliable receiver performance and greater

time-keeping accuracy at reduced power consumption. Additionally, various challenges and

considerations associated with the design of

receivers for the new broadcast format are presented.

ACCEPTED FROM OPEN CALL

Yingsi Liang, Oren Eliezer, Dinesh Rajan, and John Lowe

WWVB Time Signal Broadcast: 

An Enhanced Broadcast Format and

Multi-Mode Receiver
IEEE Communications Magazine • May 2014 211

impeded the use of RCC solutions in many applications, such as digital cameras and microwave

ovens [4].

The legacy WWVB broadcast uses pulsewidth modulation (PWM) in which the binary

symbols “0” and “1” are represented by different

full-power durations, as shown in Fig. 2a [3].

Full-power transmission refers to the portion

using maximum transmission power, and the

amplitude in full-power duration is normalized

to 1 in Fig. 2a. The suppressed duration refers to

the portion having a transmission power suppressed to –17 dB with respect to the full power.

The corresponding constellation diagram is

shown in Fig. 2b, where F1 is the normalized

waveform of “1” in AM, and F0 is the orthogonal basis function that is derived using the GramSchmidt orthogonalization process to span the

two-dimensional signal space. While the normalization ensures the maximum transmission power

is 1, the Euclidean distance between the legacy

“0” and “1” signals is only 0.52, implying that

this modulation scheme is inefficient. The data

representation in the legacy format is also inefficient, as it is based on binary coded decimal

(BCD) representations for each decimal digit.

Due to these inefficiencies in the legacy broadcast format, and to overcome the reception challenges mentioned above, NIST decided to

develop a new broadcast format that would

allow for reliable and cost-effective reception

throughout the continental United States.

Several other countries operate radio timesignal stations similar to WWVB (e.g., DCF-77

in Germany, MSF in England and JJY in Japan)

[5–7]. These transmitters operate in the range 40

to 80 kHz, with the same bit rate and frame

length as WWVB. Although these stations use

modulation schemes similar to that of the legacy

WWVB format to represent the time information, the signal broadcast from DCF-77 also

includes portions that are phase-modulated by a

higher-rate pseudo-random sequence. This higher-rate PM component allows time-keeping

equipment to achieve higher accuracy while

being robust to interference [5].

THE NEW PM-BASED WWVB

BROADCAST FORMAT AND ITS

FEATURES

NIST officially commenced transmission of the

new phase-modulation (PM)-based broadcast

format on October 29, 2012, while maintaining

the AM/PWM based modulation of the legacy

format. The phase modulation added to the

broadcast was designed such that it would not

affect the operation and performance of existing

commercial RCCs whose operation is based on

envelope detection, thus maintaining backward

compatibility. Reception equipment based on

carrier locking, either for synchronous AM

detection of the time information or for extraction of the carrier as a frequency reference

source, may be adapted to operate on the phasemodulated signal by employing techniques such

as squaring or a Costas loop.

The PM scheme uses antipodal binary phaseshift keying (BPSK), in which a binary “0” is

represented by maintaining the carrier phase,

and a binary “1” is represented by inverting the

carrier (i.e., 180° phase shift). Figure 3a shows

the four possible baseband waveforms for the

various combinations of AM and PM-based

information symbols. As seen in Fig. 3b, the corresponding minimum Euclidean distance in PM

(d¢) is about three times greater than that of the

legacy format (d), resulting in a performance

gain of 20 × log10 3  10 dB, when assuming that

both schemes are received in an optimal coherent receiver. It should be noted, however, that

typical legacy receivers are based on envelope

detection, such that the difference is greater

than 10 dB. Further, the inherently higher immunity of a BPSK receiver to interference, and particularly to an on-frequency non-modulated

continuous waveform, allows the PM-based

receiver to withstand higher levels of interference than those that may be tolerated in a legacy

envelope-detector receiver [3].

In addition to its improved modulation

scheme, the new broadcast format also offers

more efficient data representation. The minute,

hour, day and year fields are all combined into

one 26-bit field that represents the number of

minutes that have elapsed since the beginning of

the year 2000. A 5-bit parity word, derived from

a Hamming (31,26) linear-block code for error

correction and detection, is added, totaling 31

bits for the time representation. This number of

bits is, coincidently, identical to that used for the

time and date representation in the legacy format as well, but without the level of protection

offered by the new scheme. This Hamming code

is capable of correcting one erroneous bit and

detecting up to two bit errors, and thus increases

the robustness of the reception. The decoding

operations performed in the receiver, involving a

syndrome calculation, as well as the conversion

of the minute counter into the actual date and

Figure 1. Simulated coverage area for the legacy WWVB broadcast at 0800

UTC (Coordinated Universal Time) in October, where the shaded area is

the day-night boundary. The simulated coverage assumes the use of a

properly oriented antenna and the absence of interference and shielding

losses. These three assumptions are often invalid in indoor applications.
212 IEEE Communications Magazine • May 2014

time, are relatively simple operations in a modern implementation. The new format also eliminates the astronomical time error information

(UT1), which consumes seven bits in the legacy

format and is not of much use in RCCs, as well

as the leap-year indication bit, which is redundant when the year is known.

The new broadcast format supports the following transmission modes, having different

code rates, to accommodate various ranges of

receiver signal-to-noise ratio (SNR):

• Normal mode, which has the same frame

duration of one minute and bit rate of 1

bit/s as in the legacy format, includes information similar to that of a legacy AM

frame, as shown in Fig. 4.

• Message mode, which also has the same

frame length and bit rate as the legacy AMbased format, contains no time information,

but can convey other types of information,

such as emergency alerts and control commands.

• Fast mode, which has a much faster bit rate

and shorter frame duration than the Normal mode, requires a shorter reception

time and lower processing energy, while

including all the information of Normal

mode.

• Medium mode, which targets receivers that

experience very low SNR by using frames

of 6 minute duration every half hour, conveys only the time and DST state.

• Long mode, which is designed to accommodate reception under even lower SNR conditions than the Medium mode, uses 17

minute frames that are transmitted once a

day. The time of day information can be

derived from the frame’s timing, and two

separate dedicated Long frames, broadcast

Figure 2. Waveforms and Euclidean distance for AM-based legacy broadcast format: a) baseband

waveforms of the WWVB legacy broadcast format; b) signal space representation of the legacy

broadcast format.

0

0.80

1.00

“1”

“0”

φ1

φ0

(a)

(b)

Marker

0.8

d = 0.52

t

0.2 0.4 0.6

0.14

1

0.8

t

0.2 0.4 0.6

0.14

1

0.8 1.0 1.0 1.0

t

0.2 0.4 0.6

0.14

1

“0” “1”

Figure 3. Waveform and Euclidean distance comparison of legacy (AM)

and PM-based signals: a) baseband waveform of the WWVB transmitted

signals; and b) Signal space representation of the legacy and PM-based

modulations.

AM "1"

PM "0"

AM "1"

PM "0"

AM "1"

PM "1"

AM "0"

PM "1"

-0.80 0.80

AM "1"

PM "1"

AM "0"

PM "0"

AM "0"

PM "0"

1.00

1.00

AM "0"

PM "1"

0

(a)

(b)

d’ = 1.60

d = 0.52

0.2 0.4 0.6 0.8 1.0 t

-1

-0.14

0.14

1

0.2 0.4 0.6 0.8 1.0 t

-1

-0.14

0.14

1

φ1

φ0
IEEE Communications Magazine • May 2014 213

at designated times of day, represent the

DST state and date, respectively. 

The following subsections provide additional

details for these transmission modes.

THE NORMAL MODE

The new frame structure, shown in Fig. 4, is

designed to improve the robustness of information recovery and reduce the overall energy consumption associated with reception. The markers

in the legacy frame, denoted by M in Fig. 4, are

used only for frame synchronization in legacy

receivers. Due to their lower energy content,

these were not assigned information bits in the

new PM-based frame. The leap-second notification and DST current state are merged into a 5-

bit codeword in the new format, denoted by

D+L. The schedule for the next DST transition

is represented by a 6-bit codeword in the DST

Next field of the new format.

Different Receiver Operations Enabled —

There are three receiver operations for RCCs

whose reception is based on the Normal frame.

First, when a radio-controlled clock is new or

reset, it will perform acquisition to synchronize

with WWVB and obtain the current time, date,

and additional information in the frame. Subsequently, after successful acquisition, the device

periodically performs tracking to compensate for

the time drift in its internal oscillator. A common commercial RCC may have to compensate

for up to a few seconds of time drift, if its periodic tracking operations are performed once a

day. Note that the propagation delay in the continental US is less than 15 ms [8], and hence the

time drift is dominated by the frequency error of

the RCC’s crystal rather than the geographic

location of the receiver. The tracking operation

is based on the reception of the synchronization

word (or sync word), from which the correct timing is derived, and does not include reception of

information. A third operation, notification

reception, is used to obtain information regarding

the schedule for the next DST transition and the

possible implementation of a leap second. The

different receiver operating modes are compared

in Table 1, wherein the time and date information includes minute, hour, date, and DST state,

FS timing represents frame and symbol timing,

and tm represents the maximal time drift to be

compensated.

Most legacy RCCs perform acquisition daily,

requiring that at least one full 60-second frame

be received successfully each time. By contrast,

the tracking operation offers energy consumption reductions due to its shorter operation

duration, as well as greater chances of being successful [9]. The notification-reception operation

involves the extraction of fewer information bits

from the received signal, when compared to the

acquisition operation, and is done when the timing (i.e., minute boundaries) is known.

Sync Word for Synchronization — The new

broadcast format uses a 13-bit sync word with

good auto-correlation properties to enable

robust FS synchronization at the receiver. The

last PM bit in each frame, coinciding with the

marker at second 59 of the legacy frame, is fixed

at zero and may be considered an additional bit

in the sync word, extending the sync word to 14

bits. Since the duration of this word is shorter

than a quarter of the frame, and its approximate

timing is known when the RCC is tracking

(rather than acquiring), the duration and energy

consumption of the tracking operation are correspondingly lower. Although the sync word is also

used in the acquisition operation, to determine

the start time of the frame, the sync word design

was based primarily on performance metrics

associated with the tracking operation. The reasons for this are:

• Tracking is the most frequent operation in

the RCCs

• The false synchronization probability in

acquisition can be reduced by leveraging

the channel-coding schemes employed in

the data in order to verify the validity of

the various fields in the frame.

The bits in the sync word may have three different levels of energy, depending on whether they

are accompanying a “0”, “1” or marker bit in the

legacy format. Therefore, if a traditional synchronization sequence were chosen, such as the

Barker code, it may not have exhibited its known

auto-correlation properties, thus resulting in

inferior performance. Hence, the sync word was

designed to maximize the reliability of the tracking operation by treating the sync word as a

waveform rather than a sequence of bits.

Emphasis on Daylight Saving Time — DST

is observed throughout most of the US, and

automatically adjusting the time when DST transitions occur is considered one of the most significant benefits of RCCs. Hence, in the new

broadcast format, a new 6-bit field was introduced, DST Next, consuming 10 percent of the

entire 60-second frame, intended primarily to

convey the schedule for the next DST transition.

Additionally, a 5-bit field conveys both the curFigure 4. Frame structure of AM (Amplitude Modulation) and PM (Phase Modulation) in Normal

mode, where M denotes marker, R denotes reserved, D+L denotes DST state and leap second, LY

denotes leap year, and LS denotes leap second.

Bit index 1

AM

PM Sync word Time and Date

M Minute

2 3 4 5

R Minute

10

M

11

R

12

R

15

R

6

Hour

13

Hour

14 16 17 18 19 20

M

30

M

R

21

R

22

R

25

Day R

23

Day

7 8 9 24 26 27 28 29

Bit index 31

AM

PM Time and Date Time and Date

Day

32 33 34 35

R R UTI

40

M

41 42 45

R

36

UTI

43

Year

44 46 47 48 49 50

M

60

M

R D+L R D+L DST Next 0

51 52 55

Year R LY LS

53

DST

37 38 39 54 56 57 58 59

Most legacy RCCs

perform acquisition

daily, requiring that

at least one full 60-

second frame be

received successfully

each time. By contrast, the tracking

operation offers

energy consumption

reductions due to its

shorter operation

duration, as well as

greater chances of

being successful.
214 IEEE Communications Magazine • May 2014

rent DST state and a leap second notification for

the end of the current month. There are four

possible DST states: DST in effect for more than

a day (i.e., within summer time); DST not in

effect for more than a day (i.e., within winter

time); DST starting today; or DST ending today

[1]. There are three possible leap-second notifications: no leap second this month, or a positive

or negative leap second to occur at the end of this

month [1]. Leap seconds are usually not scheduled for at least ten months in a year, and DST

is in effect for about 2/3 of the year. Therefore,

to increase the overall probability of successful

decoding, given the unequal probabilities for the

different codewords, the codeword representing

DST in effect for more than a day and no leap

second was chosen to have the largest minimum

distance (dmin) in the codebook. Over the last

decade, the implementation of leap seconds has

been discussed internationally and it has been

suggested that leap seconds be abolished altogether [10]. The 5-bit error correction code was

therefore designed such that if leap seconds are

eventually abolished, it will become a systematic

block code, with the most commonly used codewords for DST (summertime and wintertime)

having the maximum dmin.

The 6-bit DST Next field in Fig. 4 informs the

RCCs of the next DST transition well ahead of

time, eliminating the criticality of decoding the

DST-state field daily. There are 24 possible

upcoming DST transition schedules for both the

beginning and the end of the DST period. When

transitioning into DST (in March or April), an

advance notification of about seven months is

provided for the ending of the DST period (to

occur in October or November), whereas at the

end of a DST period, an advance notification of

about five months is provided for the beginning

of next year’s DST period. The 24 combinations

represented by the 6-bit codewords cover eight

different Sundays and three different times for

implementing the transition: 1 a.m., 2 a.m., or 3

a.m. [1]. Additional codewords serve to represent possibilities such as a DST transition occurring outside of those 24 possibilities [1]. As with

the 5-bit codeword for the DST state and leap

second notification, the most probable DST

transition schedules were chosen to have the

greatest dmin.

ADDITIONAL TRANSMISSION MODES

Due to the diverse locations and environments

where receivers may operate, different transmission modes were designed to accommodate the

wide range of receiver SNRs and impulse noise

scenarios.

The Fast mode, which has not yet been

deployed, will allow high SNR users to have

shorter reception duration and thus minimizes

energy consumption. A Fast frame is transmitted

at a bit rate of 100 bit/sec during the suppressed

duration of a marker bit, and can therefore be

transmitted simultaneously with a frame from

any other mode. The Fast mode may also be

considered an effective solution against impulse

noise, assuming that the reception of a Fast

frame, having a duration of less than one second, occurs at an instance in which sufficiently

high SNR is experienced.

While the Fast mode is intended for RCCs

having higher SNRs, the Medium mode is targeted at RCCs that are affected by noise, interference and/or shielding losses to the extent that

they cannot operate in Normal mode. In the

Medium mode, a 106 bit sync word is used for

frame synchronization, surrounded by two mirrored 127-bit PN sequences that are chosen

from a set of 124 PN sequences of length 127

bits, representing the current minute, hour and

DST state. The Medium frame is transmitted

twice every hour at minute 10 and 40 [1].

The Long mode, which has not yet been

deployed, is intended for receivers that do not

have sufficient SNR to operate in Medium

mode. In Long mode, a 1023-bit PN sequence

truncated to 1020 bits, is broadcast at a designated time of day. After a certain time gap, a second truncated 1023-bit PN sequence is

broadcast, chosen from a set of two truncated

1023-bit PN sequences, to indicate the current

DST state. There are also Long frames dedicated to represent the date, which are broadcast at

designated times of day. The Long frames are

transmitted after midnight in the Continental

US for two reasons:

• The propagation conditions are better during the night

• The probability of a RCC performing initial

acquisition at such time would be low, thus

minimizing the population of RCCs that

would be deprived of the opportunity to

acquire based on the Normal frames, which

are overridden by the Long sequences.

Table 2 provides an overview of the various

transmission modes, where CNR refers to Carrier-to-Noise Ratio in a bandwidth of 1 Hz. The

effective SNR is about 2 dB lower on average

than the CNR due to the coexisting AM/PWM

in the transmitted BPSK symbols. The minimum

Table 1. Comparison of the different receiver operations in Normal mode.

RX operation Frequency of

operation

A-priori

knowledge Purpose Receptionduration

Information to be

recovered

Acquisition When batteries

are replaced None Acquire time information 2 min FS timing, time and date,DST state

Tracking Daily Approximate time Compensate for time drift 14 s + tm FS timing

Notification

Reception Twice a year Approximate time Obtain DST schedule andLS notification 1 min + tm FS timing, DST scheduleand LS notification
IEEE Communications Magazine • May 2014 215

receiver CNR is defined for reliable recovery of

100 percent of the frame’s content, such that the

probability of an error in a recovered frame is

about 10–4. Existing RCC receivers based on the

legacy format require a minimum receiver CNR

of about 28 dB, for which their probability for

erroneous recovery is unspecified and is likely

higher than 10–4. Hence, receivers based on the

new broadcast format have a performance

advantage in the range of 18 to 41 dB. The performance advantage in Normal mode has been

shown in simulations [11], reception trials and

lab testing [9].

RECEIVER DESIGN CHALLENGES

There are several unique design considerations

for WWVB receivers when compared to conventional communication systems. First, consecutive

data frames are highly correlated and, therefore,

repeated reception may be employed to ensure

reliability. Second, the receiver SNR can vary

significantly depending on the reception time,

location and interference level, requiring that

the receivers be designed to accommodate a

wide range of SNR. Third, the broadcast’s bit

rate is very low, resulting in relatively large frequency offsets that may be experienced during

coherent demodulation. Finally, energy consumption must be minimized for battery-operated devices.

MULTIPLE RECEPTIONS FOR

INCREASED ROBUSTNESS

In traditional data communication systems,

repeated transmission/reception can result in an

undesired decrease in throughput. In contrast, in

the WWVB broadcast, apart from the infrequent

changes in the DST and LS fields, consecutive

frames are strongly correlated, such that successfully decoding one frame is equivalent to recovering data from other frames. Therefore,

multiple reception attempts can be made to

increase reception robustness without loss of

information. The only penalty associated with

repeated reception is increased energy consumption and a possible delay in obtaining the result.

Except during acquisition, this delay is not

noticed by the user, since the devices are mostly

in tracking mode, in which their timekeeping

continues even without time-drift correction. 

Developing a reliable metric for reception

quality is essential to determine the need for a

second reception attempt. Estimated CNR/SNR

is an example of such metric. The syndromes in

acquisition operations, derived from the errorcorrecting codes employed in the data, can also

indicate the reception quality, as long as the

number of errors does not exceed the detection

capability of the channel code.

RECEIVING AT THE DIFFERENT TX MODES

The receiver can operate over a wide range of

SNR values and leverage the multiple different

modes provided by the enhanced broadcast.

Therefore, a reliable quality measure and decision strategy are required to determine which

transmission mode would guarantee robust

reception for a particular RCC under specific

SNR conditions, while also considering response

time and energy consumption. While lower

SNRs generally require the use of lower code

rates, as offered by the Medium or Long modes,

interference of impulsive nature may be effectively addressed by the Fast mode, where the

entire frame may be received in less than one

second, allowing for greater chances of it not

being impacted. Note that since reception conditions may vary, a device forced to acquire using

the Medium mode, for example (possibly during

daytime reception and in the presence of interference), may perform periodic tracking based

on the Normal or even Fast modes at night,

when the received signal is stronger and the

interference levels may be lower. 

COMPENSATING FOR

TIME-VARYING FREQUENCY OFFSETS IN THE

COHERENT RECEIVER

Although the 60 kHz carrier generated by the

station is accurate, the low-cost crystals used in

most RCCs result in a time-varying frequency

offset between the transmitter and receiver.

Major factors that affect the frequency accuracy

of a crystal include temperature, crystal aging,

and retrace [12]. The relative frequency tolerance of crystals in commonly used clocks and

watches is up to 20 ppm, which translates into

Table 2. Comparison of the different transmission modes.

TX modes Bit rate Information represented Frame

duration

Frequency of

occurrence

Minimum

RX CNR

Fast 100 b/s Time, date, DST state, DST

schedule, LS notification 630 ms Every 10 s 30 dB

Normal 1 b/s Time, date, DST state, DST

schedule, LS notification 1 min Every 60 s 10 dB

Message 1 b/s Message 1 min Infrequent 10 dB

Medium 1 b/s Minute, hour, and DST state 6 min Every half hour –3 dB

Long 1 b/s Time, DST state, and date (in

separate frames) 17 min Every 24 hours –13 dB

Existing RCC

receivers based on

the legacy format

require a minimum

receiver CNR of

about 28 dB, for

which their 

probability for 

erroneous recovery is

unspecified and is

likely higher than

10–4. Hence,

receivers based on

the new broadcast

format have a 

performance advantage in the range of

18 to 41 dB.
216 IEEE Communications Magazine • May 2014

1.2 Hz for a 60 kHz local oscillator for frequency

down-conversion. Depending on the frequency

error, the phase error accumulated throughout

the coherent demodulation of a symbol may be

intolerable due to the long symbol period for

most transmission modes. Therefore, the frequency offset must be compensated during synchronization and data recovery.

Further, since the reception time period is on

the order of minutes, certain environmental

changes (such as temperature) may occur during

that period of time, which can affect the crystal

frequency. In an experiment performed in an

air-conditioned lab, where the temperature fluctuations are much smaller than those in outdoor

environments, the frequency fluctuations were

on the order of 0.01 Hz within a minute. In

Long mode, where the timing and information

recovery is made based on a 17-minute sequence,

frequency fluctuations of such magnitude can be

detrimental. Therefore, the corresponding

demodulation scheme must take into account

the non-negligible time-varying frequency offset

within the sequence. One possible approach is

Post Detection Integration (PDI), in which

coherent integration is performed within each

segment of a sequence, and non-coherent integration is used to combine the partial results

from different segments [13].

REDUCING RECEIVER ENERGY CONSUMPTION

In some RCC applications, such as battery-operated clocks and watches, the energy consumption

of the RCC receivers might represent a great

part of the energy consumed by the entire device.

By properly utilizing different reception operations, energy consumption may be minimized: 

•The acquisition operation should be performed only once throughout a battery lifetime,

unless the maximal time drift tm becomes greater

than ±30 s, which may occur only if the device

has not performed successful tracking operations

in a long period.

•The DST transition notification field may

change as frequently as twice a year, and leap seconds are usually scheduled only in June and

December. Therefore, by combining the efforts of

receiving these two fields into one reception, the

frequency of the notification reception operation

may be reduced to only twice a year: in June, at

which time the schedule for the end of the DST

period may be received, and in December, at

which time the beginning of the next DST period

may be received. In both instances, a leap-second

notification for that month may be received.

•The tracking operation can be performed

daily or once every few days, while considering

the tradeoff between the frequency of tracking

and the maximal time drift, which corresponds

to the maximum possible time drift experienced

in the RCC since the last tracking operation.

Since a particular crystal would be characterized by a relatively stable error over a fixed period of time, a cognitive receiver, after estimating

the error based on WWVB reception, could

adjust the time drift without receiving WWVB as

frequently, and the resulting tm can be narrowed.

In such case the amount of residual time drift

would mainly depend on the temperature stability in which the device is placed.

Tracking should be performed during nighttime, preferably at 0700 or 0800 UTC

(Coordinated Universal Time), for four

reasons:

1. The signal strength is stronger during nighttime in locations that are distant from the

station;

2. Human activity is at a much lower level at

night, leading to lower levels of interference;

3. All the AM bits are “0” (longer duration of

high power), resulting in higher overall signal energy;

4. The transmission of Long frames and Medium frames avoids whole hours.

SUMMARY

The new enhanced WWVB broadcast, based on

BPSK, offers much improved performance for

RCCs when compared to the legacy broadcast,

allowing receivers to recover time and timing

information reliably from signals that are orders

of magnitude weaker, or to operate in the presence of interference that may be orders of magnitude stronger than what can be tolerated by

the legacy receivers. The performance superiority of the new broadcast has been demonstrated

in multiple locations throughout North America

with EverSet receivers that were designed to

receive it [9]. The new broadcast format is backward compatible, such that legacy envelopedetection based receivers are not impacted by its

introduction. Various transmission modes are

supported, having different bit rates and code

rates, to accommodate a wide range of SNRs. In

order to reduce receiver energy consumption

and enhance reliability, the new broadcast format also enables different receiver operations,

such as tracking, allowing a receiver to perform

timing adjustments based on a robust and energy-efficient data-aided synchronization operation. Data representation was made more

efficient and reliable in the new format by

employing compact data encoding and error

control coding. The new broadcast format allows

new RCCs to deliver higher reliability and accuracy, and enables many new applications to be

based on the WWVB broadcast. Several consumer market products are underway based on

EverSet receivers, and it is anticipated that the

use of this enhanced broadcast will greatly

exceed that of its predecessor. 

ACKNOWLEDGMENT

The work presented here was partially supported

by the National Institute of Standards and Technology through government grants

SB134110SE0700 and SB134111CN00095 and by

the National Science Foundation under grant #

EEC-0946373 to the American Society for Engineering Education.

Prof. Waleed Khalil from The Ohio State

University is acknowledged for initiating the

research in 2010 and for taking part in its early

stages.

Carl Kugel from the U.S. Navy SPAWAR

Systems Center is acknowledged for the simulation results for the broadcast’s coverage and for

the discussions related to propagation.

The new broadcast

format is backward

compatible, such

that legacy envelopedetection based

receivers are not

impacted by its introduction. Various

transmission modes

are supported, having different bit rates

and code rates, to

accommodate a

wide range of SNRs.
IEEE Communications Magazine • May 2014 217

REFERENCES

[1] J. Lowe, “Enhanced WWVB Broadcast Format,” 2013;

http://www.nist.gov/pml/div688/grp40/wwvb.cfm.

[2] P. Hansen and D. Gish, “WWVB Baseline Measurements:

Summary, Findings, and Recommendations,” U.S. Navy,

Tech. Rep. Feb., 1995.

[3] J. Lowe et al., “New Improved System for WWVB

Broadcast,” Proc. Annual PTTI Meeting, 2011.

[4] J. Burrus, “New NIST Time Code to Boost Reception for

Radio-Controlled Clocks,” Tech Beat, 2013, available:

http://www.nist.gov/pml/div688/wwvb-030513.cfm.

[5] Physikalisch-Technische Bundesanstalt (PTB) Braunschweig, “Time and Standard Frequency Station DCF77

(Germany),” 1984; http://www.eecis.udel.edu/~mills/ntp/

dcf77.html.

[6] Japan Standard Time Group, “The Method of Emitting

Standard Time and Frequency Signal Emission,” available: http://jjy.nict.go.jp/jjy/trans/index-e.html.

[7] NPL Time & Frequency Services, “MSF 60 kHz Time and

Date Code,” 2007, http://www.npl.co.uk/upload/pdf/

MSF_Time_Date_Code.pdf. 

[8] M. A. Lombardi, “NIST Time and Frequency Services,”

Special Publication 432, 2002, http://www.nist.gov/

pml/div688/grp40/wwvb.cfm.

[9] Y. Liang et al., “A New Broadcast Format and Receiver

Architecture for Radio Controlled Clocks,” Proc. IEEE

MWSCAS, Columbus, OH, 2013. 

[10] M. Kuhn, “Efforts to Abolish Leap Seconds,” available:

http://www.cl.cam.ac.uk/~mgk25/time/leap/.

[11] Y. Liang et al., “Receiver Design of Radio-Controlled

Clocks Based on the New WWVB Broadcast Format,”

IEEE Trans. Wireless Commun., Submitted. 

[12] H. Zhou et al., “Frequency Accuracy and Stability

Dependencies of Crystal Oscillators,” Carleton University, Systems and Computer Engineering, Technical

Report SCE-08-12, Nov. 2008. 

[13] G. Corazza and R. Pedone, “Generalized and Average

Likelihood Ratio Testing for Post Detection Integration,”

IEEE Trans. Commun., vol. 55, no. 11, 2007, pp.

2159–71.

BIOGRAPHIES

YINGSI LIANG (y.liang@ieee.org) received her B.E. degree in

communication engineering from South China Normal University, Guangzhou, in 2008, and her M.S. and Ph.D.

degrees in electrical engineering from Southern Methodist

University, Dallas, Texas, in 2010 and 2014, respectively.

Her research has resulted in major contributions to the

design of the enhanced WWVB broadcast, which the

National Institute of Standards and Technology (NIST)

deployed in 2012, as well as to the development of the

EverSet® receiver technology for this broadcast. Her

research interests include wireline and wireless communication systems, synchronization, channel coding, interference

mitigation, and digital signal processing.

OREN ELIEZER (OrenE@ieee.org) received his Bachelor’s

and Master’s degrees in electrical engineering from TelAviv University, Israel, in 1988 and 1996, respectively,

and his Ph.D. degree in microelectronics from the University of Texas at Dallas in 2008. He served in The Israel

Defense Forces from 1988 to 1994, where he specialized

in wireless communications, and after his military service

cofounded Butterfly VLSI Ltd and served as the company’s chief engineer. Following Butterfly’s acquisition by

Texas Instruments (TI) in 1999, he was relocated to Dallas in 2002, where he took part in the development of

TI’s Digital RF Processor (DRPTM) technology and was

elected a Senior Member of Technical Staff. Since 2009

he has been serving as chief technology officer for

Xtendwave (now EverSet® Technologies), where he has

led the development of the enhanced WWVB broadcast

and the EverSet® receiver technology for it. He serves on

advisory boards for academia and on the steering committees of several IEEE conferences, and was awarded

the IEEE Circuits and Systems award for outstanding voluntary work in his region in 2010. He has authored and

coauthored over 50 papers and over 30 issued patents,

and has given over 40 invited talks and seminars related

to communication systems, digital transceiver architectures, interference mitigation, built-in testing, and builtin calibration and compensation in wireless transceiver

integrated circuits.

DINESH RAJAN (rajand@lyle.smu.edu) received his B.Tech

degree from the Indian Institute of Technology (IIT),

Madras in electrical engineering. He was also awarded M.S

and Ph.D degrees from Rice University, Houston, Texas,

both in the areas of electrical and computer engineering.

He is currently Department Chair of and a professor in the

Electrical Engineering Department at Southern Methodist

University. His current research interests include communications theory, wireless networks, information theory, and

computational imaging. He received a National Science

Foundation CAREER award in 2006 for his work on applying information theory to the design of mobile wireless

networks.

JOHN LOWE (lowe@boulder.nist.gov) received a B.S. degree

in electrical engineering from the University of Missouri,

Kansas City in 1982 and an M.S. in electrical engineering

from the University of Colorado in 1984. Since 1985 he has

been with the Time and Frequency Division of the National

Institute of Standards and Technology (NIST) in Boulder,

Colorado. His first position was in the Atomic Standards

Group, where he took part in building the National Frequency Reference (NIST–7), for which he was awarded the

Department of Commerce’s Gold Medal. In 1995 he was

assigned to the Time and Frequency Services Group, which

he now leads. He is also the Broadcast Manager for NIST’s

radio stations WWV, WWVH, and WWVB in Colorado and

Hawaii, and oversees their operation, maintenance, and

enhancements.