U.S. patent application number 15/279072 was filed with the patent office on 2018-03-29 for backward-compatible signal variations for data augmentation.
This patent application is currently assigned to The MITRE Corporation. The applicant listed for this patent is The MITRE Corporation. Invention is credited to Brady O'HANLON.
Application Number | 20180091339 15/279072 |
Document ID | / |
Family ID | 61629839 |
Filed Date | 2018-03-29 |
United States Patent
Application |
20180091339 |
Kind Code |
A1 |
O'HANLON; Brady |
March 29, 2018 |
BACKWARD-COMPATIBLE SIGNAL VARIATIONS FOR DATA AUGMENTATION
Abstract
A system and method for augmenting the data capacity of
pre-existing communications channels is provided. In one example, a
subcarrier waveform of the system can be dithered based on data
generated by an additional source and then transmitted. The
dithered subcarrier can be passed through a plurality of matched
filters so as to ascertain which dither pattern was used, thus
ultimately allowing for the demodulation of the additional data
source. The system and methods provided herein can be implemented
with minimal impact to legacy users of the system as implementation
of the dithering scheme can have minimal impact to the performance
of receivers that are not equipped to demodulate the dithered
waveforms.
Inventors: |
O'HANLON; Brady; (Tewksbury,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The MITRE Corporation |
McLean |
VA |
US |
|
|
Assignee: |
The MITRE Corporation
McLean
VA
|
Family ID: |
61629839 |
Appl. No.: |
15/279072 |
Filed: |
September 28, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S 19/02 20130101 |
International
Class: |
H04L 27/20 20060101
H04L027/20; G01S 19/30 20060101 G01S019/30 |
Claims
1. A method for augmenting a data rate of a global navigation
satellite system transmitter, the method comprising: generating a
subcarrier waveform; applying a first dithering scheme to a first
portion the generated subcarrier waveform based on a first data,
wherein the first portion includes a plurality of sub-portions, and
wherein the first dithering scheme includes shifting a first
sub-portion of the first portion of the generated subcarrier
waveform in time in a first direction; and if a second data is
different from the first data: applying a second dithering scheme
to a second portion of the generated subcarrier waveform, wherein
the second portion includes a plurality of sub-portions, and
wherein the second dithering scheme includes shifting a first
sub-portion of the second portion of the generated waveform in time
in a second direction.
2. The method of claim 1, wherein the first dithering scheme
includes shifting the first sub-portion of the first portion of the
generated subcarrier waveform in the first direction and shifting a
second sub-portion of the first portion of the generated subcarrier
waveform in the second direction.
3. The method of claim 2, wherein a number of the sub-portions of
the plurality of sub-portions of the first portion that are shifted
in the first direction is equal to a number of the sub-portions of
the plurality of sub-portions of the first portion that are shifted
in the second direction.
4. The method of claim 1, wherein the second dithering scheme
includes shifting the first sub-portion of the second portion of
the generated subcarrier waveform in the second direction and
shifting a second sub-portion of the second portion of the
generated subcarrier waveform in the first direction.
5. The method of claim 4, wherein a number of the sub-portions of
the plurality of sub-portions of the second portion that are
shifted in the second direction is equal to a number of the
sub-portions of the plurality of sub-portions of the second portion
that are shifted in the first direction.
6. The method of claim 1, wherein the transmitter is configured to
transmit binary offset subcarrier signals.
7. A method for receiving an augmented data rate global navigation
satellite system transmission, the method comprising: generating an
expected data sequence; generating a subcarrier waveform; applying
a first dithering scheme to a first portion the generated
subcarrier waveform based on a first data, wherein the first
portion includes a plurality of sub-portions, and wherein the first
dithering scheme includes shifting a first sub-portion of the first
portion of the generated subcarrier waveform in time in a first
direction; applying a second dithering scheme to the first portion
of the generated subcarrier waveform, wherein the second dithering
scheme includes shifting the first sub-portion of the first portion
of the generated waveform in time in a second direction; generating
a first matched filter based on the expected data sequence and the
first subcarrier waveform dithering pattern; generating a second
matched filter based on the expected data sequence and the second
subcarrier waveform dither pattern; receiving a transmitted signal;
applying the first matched filter to the received transmitted
signal to generate a first demodulated signal; applying the second
matched filter to the received transmitted signal to generate a
second demodulated signal; and comparing the first demodulated
signal and the second demodulated signal to determine whether the
received transmitted signal included the first dithering pattern or
the second dithering pattern.
8. The method of claim 7, wherein the first dithering scheme
includes shifting the first sub-portion of the first portion of the
generated subcarrier waveform in the first direction and shifting a
second sub-portion of the first portion of the generated subcarrier
waveform in the second direction.
9. The method of claim 8, wherein a number of the sub-portions of
the plurality of sub-portions of the first portion that are shifted
in the first direction is equal to a number of the sub-portions of
the plurality of sub-portions of the first portion that are shifted
in the second direction.
10. The method of claim 7, wherein the second dithering scheme
includes shifting the first sub-portion of the second portion of
the generated subcarrier waveform in the second direction and
shifting a second sub-portion of the second portion of the
generated subcarrier waveform in the first direction.
11. The method of claim 10, wherein a number of the sub-portions of
the plurality of sub-portions of the second portion that are
shifted in the second direction is equal to a number of the
sub-portions of the plurality of sub-portions of the second portion
that are shifted in the first direction.
12. The method of claim 7, wherein the global navigation satellite
system transmits and receives binary offset subcarrier signals.
13. A non-transitory computer readable storage medium having stored
thereon a set of instructions for augmenting a data rate of a
global navigation satellite system transmitter that when executed
by a computing device, cause the computing device to: generate a
subcarrier waveform; apply a first dithering scheme to a first
portion the generated subcarrier waveform based on a first data,
wherein the first portion includes a plurality of sub-portions, and
wherein the first dithering scheme includes shifting a first
sub-portion of the first portion of the generated subcarrier
waveform in time in a first direction; and if a second data is
different from the first data: apply a second dithering scheme to a
second portion of the generated subcarrier waveform, wherein the
second portion includes a plurality of sub-portions, and wherein
the second dithering scheme includes shifting a first sub-portion
of the second portion of the generated waveform in time in a second
direction.
14. The non-transitory computer readable storage medium of claim
13, wherein the first dithering scheme includes shifting the first
sub-portion of the first portion of the generated subcarrier
waveform in the first direction and shifting a second sub-portion
of the first portion of the generated subcarrier waveform in the
second direction.
15. The non-transitory computer readable storage medium of claim
14, wherein a number of the sub-portions of the plurality of
sub-portions of the first portion that are shifted in the first
direction is equal to a number of the sub-portions of the plurality
of sub-portions of the first portion that are shifted in the second
direction.
16. The non-transitory computer readable storage medium of claim
13, wherein the second dithering scheme includes shifting the first
sub-portion of the second portion of the generated subcarrier
waveform in the second direction and shifting a second sub-portion
of the second portion of the generated subcarrier waveform in the
first direction.
17. The non-transitory computer readable storage medium of claim
16, wherein a number of the sub-portions of the plurality of
sub-portions of the second portion that are shifted in the second
direction is equal to a number of the sub-portions of the plurality
of sub-portions of the second portion that are shifted in the first
direction.
18. The non-transitory computer readable storage medium of claim
13, wherein the transmitter is configured to transmit binary offset
subcarrier signals.
19. A non-transitory computer readable storage medium having stored
thereon a set of instructions for receiving an augmented data rate
global navigation satellite system transmission that when executed
by a computing device, cause the computing device to: generate an
expected data sequence; generate a subcarrier waveform; apply a
first dithering scheme to a first portion the generated subcarrier
waveform based on a first data, wherein the first portion includes
a plurality of sub-portions, and wherein the first dithering scheme
includes shifting a first sub-portion of the first portion of the
generated subcarrier waveform in time in a first direction; apply a
second dithering scheme to the first portion of the generated
subcarrier waveform, wherein the second dithering scheme includes
shifting the first sub-portion of the first portion of the
generated waveform in time in a second direction; generate a first
matched filter based on the expected data sequence and the first
subcarrier waveform dithering pattern; generate a second matched
filter based on the expected data sequence and the second
subcarrier waveform dither pattern; receive a transmitted signal;
apply the first matched filter to the received transmitted signal
to generate a first demodulated signal; apply the second matched
filter to the received transmitted signal to generate a second
demodulated signal; and compare the first demodulated signal and
the second demodulated signal to determine whether the received
transmitted signal included the first dithering pattern or the
second dithering pattern.
20. The non-transitory computer readable storage medium of claim
19, wherein the first dithering scheme includes shifting the first
sub-portion of the first portion of the generated subcarrier
waveform in the first direction and shifting a second sub-portion
of the first portion of the generated subcarrier waveform in the
second direction.
21. The non-transitory computer readable storage medium of claim
20, wherein a number of the sub-portions of the plurality of
sub-portions of the first portion that are shifted in the first
direction is equal to a number of the sub-portions of the plurality
of sub-portions of the first portion that are shifted in the second
direction.
22. The non-transitory computer readable storage medium of claim
19, wherein the second dithering scheme includes shifting the first
sub-portion of the second portion of the generated subcarrier
waveform in the second direction and shifting a second sub-portion
of the second portion of the generated subcarrier waveform in the
first direction.
23. The non-transitory computer readable storage medium of claim
22, wherein a number of the sub-portions of the plurality of
sub-portions of the second portion that are shifted in the second
direction is equal to a number of the sub-portions of the plurality
of sub-portions of the second portion that are shifted in the first
direction.
24. The non-transitory computer readable storage medium of claim
19, wherein the global navigation satellite system transmits and
receives binary offset subcarrier signals.
25. A computing system comprising: one or more processors, the one
or more processors configured to: generate a subcarrier waveform;
apply a first dithering scheme to a first portion the generated
subcarrier waveform based on a first data, wherein the first
portion includes a plurality of sub-portions, and wherein the first
dithering scheme includes shifting a first sub-portion of the first
portion of the generated subcarrier waveform in time in a first
direction; and if a second data is different from the first data:
apply a second dithering scheme to a second portion of the
generated subcarrier waveform, wherein the second portion includes
a plurality of sub-portions, and wherein the second dithering
scheme includes shifting a first sub-portion of the second portion
of the generated waveform in time in a second direction.
26. The computing system of claim 25, wherein the first dithering
scheme includes shifting the first sub-portion of the first portion
of the generated subcarrier waveform in the first direction and
shifting a second sub-portion of the first portion of the generated
subcarrier waveform in the second direction.
27. The computing system of claim 26, wherein a number of the
sub-portions of the plurality of sub-portions of the first portion
that are shifted in the first direction is equal to a number of the
sub-portions of the plurality of sub-portions of the first portion
that are shifted in the second direction.
28. The computing system of claim 25, wherein the second dithering
scheme includes shifting the first sub-portion of the second
portion of the generated subcarrier waveform in the second
direction and shifting a second sub-portion of the second portion
of the generated subcarrier waveform in the first direction.
29. The computing system of claim 28, wherein a number of the
sub-portions of the plurality of sub-portions of the second portion
that are shifted in the second direction is equal to a number of
the sub-portions of the plurality of sub-portions of the second
portion that are shifted in the first direction.
30. The computing system of claim 25, wherein the transmitter is
configured to transmit binary offset subcarrier signals.
31. A computing system comprising: one or more processors, the one
or more processors configured to: generate an expected data
sequence; generate a subcarrier waveform; apply a first dithering
scheme to a first portion the generated subcarrier waveform based
on a first data, wherein the first portion includes a plurality of
sub-portions, and wherein the first dithering scheme includes
shifting a first sub-portion of the first portion of the generated
subcarrier waveform in time in a first direction; apply a second
dithering scheme to the first portion of the generated subcarrier
waveform, wherein the second dithering scheme includes shifting the
first sub-portion of the first portion of the generated waveform in
time in a second direction; generate a first matched filter based
on the expected data sequence and the first subcarrier waveform
dithering pattern; generate a second matched filter based on the
expected data sequence and the second subcarrier waveform dither
pattern; receive a transmitted signal; apply the first matched
filter to the received transmitted signal to generate a first
demodulated signal; apply the second matched filter to the received
transmitted signal to generate a second demodulated signal; and
compare the first demodulated signal and the second demodulated
signal to determine whether the received transmitted signal
included the first dithering pattern or the second dithering
pattern.
32. The computing system of claim 31, wherein the first dithering
scheme includes shifting the first sub-portion of the first portion
of the generated subcarrier waveform in the first direction and
shifting a second sub-portion of the first portion of the generated
subcarrier waveform in the second direction.
33. The computing system of claim 32, wherein a number of the
sub-portions of the plurality of sub-portions of the first portion
that are shifted in the first direction is equal to a number of the
sub-portions of the plurality of sub-portions of the first portion
that are shifted in the second direction.
34. The computing system of claim 31, wherein the second dithering
scheme includes shifting the first sub-portion of the second
portion of the generated subcarrier waveform in the second
direction and shifting a second sub-portion of the second portion
of the generated subcarrier waveform in the first direction.
35. The computing system of claim 34, wherein a number of the
sub-portions of the plurality of sub-portions of the second portion
that are shifted in the second direction is equal to a number of
the sub-portions of the plurality of sub-portions of the second
portion that are shifted in the first direction.
36. The computing system of claim 31, wherein the global navigation
satellite system transmits and receives binary offset subcarrier
signals.
Description
FIELD OF THE INVENTION
[0001] This disclosure relates to augmenting the data capacity of
pre-existing communication channels in a way so as to minimally
affect legacy users of the communications channel. More
specifically, this disclosure relates to system and methods for
increasing the data capacity of a communications channel that
employs binary offset carrier (BOC) modulation by dithering a
subcarrier signal during a pre-defined time period in order to
convey additional data over the channel.
BACKGROUND OF THE INVENTION
[0002] As modern electronics become smaller and are able to produce
data at quicker rates, often times the transmission of data becomes
constrained by the bandwidth of the communications channel rather
than the transmission rates of a device. Therefore, while advances
in transmission speeds have the potential to speed up end-to-end
communications between devices, often that potential is not
realized due to the bandwidth constraints of the communications
channel between the devices over which the data is transmitted.
[0003] In an attempt to maximize the amount of data that can be
sent over a given communications channel at a given time, various
methods of modulating and demodulating data have been used to
increase the throughput of a channel. For instance various
modulation techniques such as binary phase shift keying (BPSK),
amplitude shift keying (ASK), or code shift keying (CSK) have been
utilized to make efficient use of the bandwidth constraints
presented by a given channel.
[0004] However, once a communications system is established, it can
be a challenge to increase the throughput of a given channel
without affecting legacy users of an existing communication
channel. For instance, if a new modulation technique is employed by
a transmitter of a given communication systems, legacy users of the
system may not have the required demodulation hardware needed to
work with the new modulation technique and thus these legacy users
would not be able to participate in the new system without having
to replace their existing hardware.
SUMMARY OF THE INVENTION
[0005] Accordingly, a system and method for augmenting the
throughput of a communications system while minimizing the impact
to legacy users of the communication system is provided. The system
and method can include generating shift patterns in the data to be
modulated so as to encode additional data onto the modulated data
stream. At the receiver, the modulated data can pass through a
plurality of matched filters to not only determine the contents of
the original data but also determine the content of the additional
data. The system can be configured so that legacy users of the
system can still receive the original data without having to
replace or modify their existing receiver architectures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 illustrates an exemplary binary offset carrier
modulator according to examples of the disclosure.
[0007] FIG. 2 illustrates an exemplary binary offset carrier
receiver according to examples of the disclosure.
[0008] FIG. 3 illustrates exemplary signals generated by a
transmitter according to examples of the disclosure.
[0009] FIG. 4 illustrates an exemplary binary offset carrier
transmitter with carrier waveform dithering according to examples
of the disclosure
[0010] FIG. 5 illustrates an exemplary carrier waveform dithering
scheme according to examples of the disclosure.
[0011] FIG. 6 illustrates an exemplary zero average shift dithering
scheme according to examples of the disclosure.
[0012] FIG. 7 illustrates an exemplary binary offset carrier
receiver configured to receive data that is encoded by carrier
waveform dithering according to examples of the disclosure.
[0013] FIG. 8 illustrates an example of a computing device in
accordance with one embodiment of the disclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0014] Described herein are systems and methods for augmenting the
data capacity of a pre-existing communications channel that
minimizes the impacts to legacy users of the communications
channel. The systems and methods described herein can be used to
allow certain users of a communications channel to receive a larger
amount of data while at the same time allowing legacy users of the
communications channel to continue receiving a normal amount of
data without requiring them to modify their modulation and
demodulation systems.
[0015] The system and methods can employ an alternate data source
that is used to apply a dither to a carrier waveform. The original
data source can be modulated using the dithered carrier waveform.
On the receiver side the received transmission can be demodulated
by passing the received signal through a plurality of matched
filters, wherein each matched filter represents a different dither
pattern along with the original data source. The resulting outputs
of the matched filter can be compared to determine which dither
pattern was most likely received. In this way, legacy users can
still receive the original data source while new users can receive
data from both the original data source and the alternate data
source.
[0016] The discussion below utilizes the example of global
positioning system (GPS) systems and specifically to communication
channels that utilize binary offset carrier (BOC) modulation to
explain the various systems and methods discussed above. However,
the disclosure should not be seen as limiting and could be
applicable to other types of communication systems.
[0017] Current GPS communication channels utilize low data rate
signals to allow for the robust signal tracking in challenging
environments in which the received signal power can be low. The use
of a low data rate however can constrain the ability to add various
signal integrity checks to a GPS signal such as self-contained
authentication capability. For instance, the fact that the GPS
signal data rate is low can make inclusion of a digital signature
of the data difficult to achieve, because the additional data can
cause the transmitted GPS signal to exceed the data capacity of the
legacy GPS system. If the GPS system were to increase the data rate
of the signal so as to incorporate a digital signature, legacy
users of the GPS system may be required to modify their existing
communications hardware so as to ensure that they can continue to
receive GPS signals. Given that GPS systems have been employed by
numerous types of devices, requiring such legacy users to modify
their equipment could be costly and impractical.
[0018] Legacy GPS systems can employ binary offset carrier
modulation to convey data from a GPS satellite to an end user
device. FIG. 1 illustrates an exemplary binary offset carrier
modulator according to examples of the disclosure. The system 100
can employ a conventional GPS transmitter that utilizes BOC
modulation. In the example of FIG. 1, the system 100 can include a
code division multiple access (CDMA) symbol generator 104 that can
generate the CDMA spreading sequence used by GPS devices to
determine the position of a GPS enabled device via trilateration.
The CDMA symbol generator 104 can mix a pseudo-random noise (PRN)
sequence with a subcarrier waveform (discussed below with respect
to FIG. 3) to produce its output. The output of the CDMA generator
104 can be mixed at mixer 106 with a carrier signal generated by
carrier signal generator 102 that can shift the signal generated
CDMA generator 104 into a radio frequency (RF) band. In some
examples, the signal outputted at mixer 106 can be further
modulated by modulator 108 and then transmitted by antenna 110. In
one example, modulator 108 can be a binary phase shift keying
(BPSK) modulator. In other examples, the output of mixer 106 can be
directly transmitted by antenna 110.
[0019] FIG. 2 illustrates an exemplary binary offset carrier
receiver according to examples of the disclosure. The receiver 200
illustrated in FIG. 2 can be configured to receive GPS signals
transmitted from transmitters like the one discussed above with
respect to FIG. 1. The receiver 200 can include a CDMA sequence
generator 204. The CDMA sequence generator can be programmed to be
synchronized with a CDMA sequence generator 104 of the transmitter
100 discussed with respect to FIG. 1. Being synchronized can mean
that both the CDMA sequence generator 104 of the transmitter and
the CDMA sequence generator 204 of the receiver can generate the
same CDMA chip sequence mixed with a subcarrier waveform of
identical frequencies.
[0020] The output of the CDMA sequence generator 204 can be mixed
at mixer 214 with a carrier generator 212 that operates in
substantially the same manner as carrier generator 102 of the
transmitter discussed with respect to FIG. 1. In one example, the
carrier generator 214 can be tuned to have a frequency equal to
that of the frequency set at carrier generator 102 so as to shift
the signal from the RF back to a passband or baseband.
[0021] At mixer 206, the signal generated by the CDMA sequence
generator 204 mixed with the carrier waveform generated by carrier
generator 212 can further be mixed with the signal received by
antenna 202. Since the CDMA sequence generator 204 generates an
identical code as the CDMA sequence generator 104 of the
transmitter 100, the mixing occurring at mixer 206 can act as a
matched filter for the signal received at antenna 202 (with the
CDMA code generated by the generator 204 being shifted to account
for propagation delay). The mixed signal at the output of mixer 206
can be accumulated at accumulator 208 and can be processed by
processor 210 for further processing.
[0022] The system described with respect to FIGS. 1 and 2 can
illustrate the constraints of transmitting additional data through
the communications channel. Any additional data sent across the
channel could require additional processing at the receiver. With
respect to legacy receivers, implementing the additional processing
required to receive an additional data source may not feasible as
many legacy systems are not accessible to be modified (i.e., the
legacy receiver is on a mobile device in use). A change for
instance to the data rate of the transmitter 100 via the CDMA
sequence generator 104 could require that the CDMA generator 204 of
the receiver 200 be altered identically so that the matched filter
of the receiver maintains signal fidelity. In other words, a change
to the GPS transmitter could require a change in hardware or
software for legacy receiver systems or even require that legacy
user update their systems or risk being unable to enable GPS
functionality on their devices. In another example, a change to the
modulator 108 rate, could entail a software change in processor
210, accumulator 208, or both.
[0023] Ideally if an increased data rate is desired, for example to
enable digital signing of the GPS signal data, the transmitted
signal from the GPS transmitter would need to be altered so that
legacy users can still employ the system even if they do not
upgrade their software or hardware. Thus, the GPS transmitter
signal should be altered so as to minimize the increase in error
rate that would be associated with a GPS receiver receiving a
signal that is not matched to the signal transmitted. In this way,
legacy users of the system can still employ GPS capability on their
devices, while users who wish to utilize the higher data rate
signal can employ a GPS receiver that is capable of receiving the
"new" signal.
[0024] In order to illustrate methods by which a transmitter can be
altered to increase the data rate outputted, the signals generated
within the transmitter can be examined to determine ways in which
the data rate can be augmented. FIG. 3 illustrates exemplary
signals generated by a transmitter according to examples of the
disclosure. In the example of FIG. 3, signal 304 can represent the
output of the CDMA sequence generator 104 of FIG. 1, signal 302 can
represent the subcarrier signal produced internally by the CDMA
sequence generator 104, that is mixed with the PRN code as
discussed with respect to FIG. 1, and signal 306 can represent the
output of mixer 106 of FIG. 1 when the signals generated by CDMA
sequence generator 104 and carrier signal generator 102 are mixed.
The X-axis of each graph can represent the chip number. For
instance values between 0 and 1 on the x-axis of each graph can
represent the state of the signal when the first chip is generated
by the CDMA sequence generator 104. The Y-axis of each graph can
represent the voltage of the signal. For instance for signal 304,
the voltage can vary between -1 and 1V.
[0025] In order to transmit additional data, rather than modifying
the data bit rate of the communications system discussed with
respect to FIGS. 2 and 3, modifying the signal being generated by
the CDMA generator 104, and more specifically the subcarrier
waveform employed by the CDMA generator 104 (e.g., signal 302 of
FIG. 3) can be altered to increase the data rate of a
communications system with minimal impact to legacy users. In other
words, while conventional communications systems modify the data
bit rate (i.e., signal 304) to the detriment of legacy users, using
the subcarrier waveform to encode additional data into the
transmission stream of a transmitter can yield increased data
capacity in the communications systems while at the same time
minimizing the impact on legacy users of the system. By minimizing
the impact to legacy users, the additional encoding of data on the
carrier waveform may not require a legacy user to upgrade their
hardware.
[0026] FIG. 4 illustrates an exemplary binary offset carrier
transmitter with carrier waveform dithering according to examples
of the disclosure. The exemplary system of FIG. 4 can employ
dithering of the subcarrier waveform in order to encode an
additional data source into the transmitted data stream thereby
increasing the data rate of the binary offset carrier transmitter
while minimizing the impact to legacy users. The example of FIG. 4
can include both data symbol generator 402 and CDMA sequence
generator 404. CDMA sequence generator 404 can operate in
substantially the same way as the CDMA generator 104 and 204
discussed with respect to FIGS. 1 and 2. Data symbol generator 402
can be used to generate symbols associated with additional data
that a user of the system depicted in FIG. 4 desires to have
transmitted. For instance, data symbol generator 402 can generate
symbols associated with a scheme to generate digital signatures of
the GPS data (assuming that the transmitter in FIG. 4 is employed
by a GPS communications system).
[0027] The symbols generated by data symbol generator 402 can be
passed to CDMA sequence generator 404. The output of data symbol
generator 402 can be used by CDMA sequence generator 404 to
"dither" the subcarrier waveform based on the symbols generated by
data symbol generator 402. "Dithering" can refer to shifting in
time of the periodic carrier waveform over one or more periods. The
direction of the shift in time can be based on the value of the
symbol generated by the data symbol generator 402. The dithered
signal can be further mixed with a carrier waveform 406 at mixer
408 to shift the signal from the baseband/passband to the RF
frequency band. Additional modulation such as BPSK can be applied
by modulator 410, and the signal can then be transmitted by antenna
412 in substantially the same manner as described with respect to
FIG. 1.
[0028] FIG. 5 illustrates an exemplary carrier waveform dithering
scheme according to examples of the disclosure. The dithering
scheme of FIG. 5 illustrates three separate waveforms. The first
waveform 502 illustrates an undithered subcarrier waveform. The
second waveform 504 illustrates a subcarrier waveform that is
dithered in response to receiving a "one" symbol from the carrier
waveform generator 506. The third waveform 506 illustrates a
subcarrier waveform that is dithered in response to receiving a "0"
symbol from the data symbol generator 402.
[0029] In the example of FIG. 5, and as discussed above, subcarrier
waveform 502 can represent an undithered subcarrier waveform. For
the purpose of illustration, and without unduly narrowing the scope
of the disclosure, the undithered waveform 502 can have a period
equal to approximately one half of a chip period. As illustrated,
the subcarrier waveform can have a first rising edge at
approximately 0.25 of the chip symbol period and a first falling
edge at approximately 0.5 of the chip symbol period. This pattern
can repeat itself in a periodic manner during the duration of a
CDMA sequence transmission.
[0030] Subcarrier waveform 504 can represent a dither pattern
applied to a subcarrier waveform when a "one" is generated by data
symbol generator 402. In order to understand the dither pattern,
some parameters are defined and referenced in the disclosure so as
to explain how the dither pattern is applied. S.sub.L can be
defined as shift length 514. Shift length 514 can be defined as the
period of time in which a particular shift pattern is applied to
the subcarrier waveform. In the example of FIG. 5, the shift
pattern S.sub.L 514 can be approximately four times the subcarrier
(i.e., undithered subcarrier waveform) period. S.sub.P 516 can be
defined as the total shift period. The total shift period can
represent one period of a dithered waveform.
[0031] In the example waveform of 504, when a one is generated by
the data symbol generator 402, the subcarrier waveform can be
shifted (i.e., dithered) by shifting the subcarrier waveform in
time to the "right" which can mean delaying the first rising edge
by a definite amount of time. In the figure as illustrated, the
first rising edge 518 can be delayed by approximately 0.2 of a
subcarrier period. After applying the shift, the subcarrier
waveform can progress normally, rising and falling in a periodic
manner with the same timing as an undithered waveform (other than
the initial shift of the waveform as discussed above). This shifted
waveform can be allowed to progress until the end of the shift
length 514.
[0032] At end of the shift length period 514, the dithered
subcarrier waveform, rather than experiencing a falling edge in the
undithered example of 502, can still be in mid-waveform. In the
example of 504, the waveform at the end of the shift length can be
abruptly terminated at 520 (i.e., have a falling edge occur at the
end of the shift length). From the end of the shift length 514 and
for the rest of the shift period 516, the subcarrier waveform can
be returned to its undithered timing sequence similar to that
illustrated in waveform 502.
[0033] In the event of a zero being generated by data symbol
generator 402, the dither pattern illustrated by waveform 506 can
be applied. In the example of waveform 506, rather than being
shifted to the right, the waveform can be shifted to the "left" or
in other words the first rising edge of the subcarrier waveform can
be encountered earlier in comparison to the undithered example of
waveform 502. In the example of undithered waveform 502, the first
rising edge can be encountered at 0.25 of a chip period. In the
example of waveform 506, the first rising edge 522 can be
encountered earlier (approximately 0.05 of the chip period) thereby
shifting the waveform to the left in time. Therefore, whereas in an
unshifted example the first falling edge would have occurred at
time 508, as the waveform is shifted to the left, it can occur at
time 510.
[0034] In substantially the same manner as discussed above with
respect to waveform 504, after the initial shift, the rest of the
waveform can be allowed to progress with the same frequency and
period as the undithered waveform until the end of the shift length
514. At the end of the shift length 514, the last waveform can be
prolonged so that the falling edge can coincide with the end of the
shift length. In the same way as subcarrier waveform 504, the rest
of the subcarrier waveform can proceed unshifted between the end of
the shift length 514 and the rest of the shift period 516.
[0035] By applying waveform 504 for a one generated by data symbol
generator 402, and applying waveform 506 for a zero generated by
data symbol generator 402, the data generated can be encoded into
the transmission without altering the chip rate and without
substantially altering the bandwidth of the data transmitted over
the communications channel. In other words, the data capacity of
the channel can be increased with minimal changes to the bandwidth
of the signal transmitted.
[0036] To further minimize the bandwidth and spectral changes
associated with dithering, the average shift of the entire
transmission can be kept at substantially zero using the scheme
illustrated in FIG. 6. FIG. 6 illustrates an exemplary zero average
shift dithering scheme according to examples of the disclosure. In
the example of FIG. 6, the dithering scheme can be expressed by
waveforms 602 and 604. Waveform 602 illustrates an exemplary
dithering scheme associated with a "one" symbol being produced by
the data symbol generator 402 and can represent a zoomed out in
time version of waveform 504 of FIG. 5.
[0037] Waveform 602, during the total shift period 608
S.sub.P(discussed above), can be dithered in accordance with the
example discussed above with respect to waveform 504. Thus, during
the first shift length 606, the waveform can be shifted to the
right at 612a and then undithered in the time period between the
end of the shift length 606 and the end of the total shift period
608. At the end of the first total shift period 608, the waveform
602 can be shifted to the left at 612b during a second shift
length, and then left undithered between the end of the second
shift length and the end of the second total shift period. This
alternating pattern between shifting to the right and left can be
alternated as shown in the figure over a total symbol time 610. In
the example of waveform 602 the total symbol time 610 can be 20 mS.
In other words, data symbol generator 402 can produce one symbol
every 20 mS. Thus when data symbol generator 402 produces a "one"
symbol, the subcarrier waveform can be dithered in accordance with
the scheme of waveform 602 in which the subcarrier waveform is
first shifted to the right, and then shifted to left, and so on for
the duration that the data symbol generator 402 produces a one
bit.
[0038] In the event of a "zero" bit being produced by the data
symbol generator 402, the subcarrier waveform can be dithered using
the scheme depicted by waveform 604. The example of waveform 604
can be identical to that of waveform 602, except that the pattern
begins with waveform 612b and alternates back and forth between
612b and 612a as shown whereas the example of waveform 602 begins
with 612a.
[0039] In this way, whether a one or zero is generated by the data
symbol generator 402, the average time shift of the subcarrier
waveform can remain at zero since the waveform is dithered to the
right and to the left equally for every symbol generated by the
data symbol generator. The zero mean time shift can ensure that
systems sensitive to the shift patterns induced by the encoding are
minimally affected by the dithering scheme.
[0040] FIG. 7 illustrates an exemplary binary offset carrier
receiver configured to receive data that is encoded by subcarrier
waveform dithering according to examples of the disclosure. Similar
to the receiver discussed with respect to FIG. 2, the receiver 700
can utilize matched filtering to decode transmissions sent by a
transmitter equipped with subcarrier waveform dithering such as the
one discussed with respect to FIG. 4.
[0041] The receiver 700 can be configured to receive signals
transmitted by the transmitter described with respect to FIG. 4.
The receiver 700 can include a CDMA sequence generator 702 that is
substantially identical to the CDMA sequence generator 204 of FIG.
2. The CDMA sequence generator 702 can be synchronized with CDMA
sequence generator 402 of FIG. 4. For each CDMA sequence generated
by CDMA sequence generator 702, shift pattern generator 704 can
generate a subcarrier waveform that assumes a dither pattern
associated with waveform 602 of FIG. 6 associated with a "one"
symbol generated by subcarrier waveform generator 406 as discussed
above, and can generate a subcarrier waveform that assumes a dither
pattern associated with waveform 604 of FIG. 6 associated with a
"zero" symbol generated by carrier waveform generator 406 as
discussed above.
[0042] The shift patterns generated by shift pattern generator 704
can be mixed with the output of CDMA sequence generator 702 at
sequence shifter 706 to generate two distinct matched filters 708a
and 708b. Matched filter 708a can be associated with the CDMA
sequence when a "one" symbol is encoded via dither, and matched
filter 708b can be associated with the CDMA sequence when a "zero"
symbol is encoded via dither as discussed above.
[0043] When a signal is received by antenna 714 it can be mixed
with each individual matched filter 708a and 708b at 710a and 710b
respectively. The outputs of mixers 710a and 710b can be
accumulated at 712a and 712b respectively and compared to determine
which dither pattern was most likely received. Thus for example, if
the data subcarrier waveform 406 was dithered using the dithering
scheme associated with a "one" symbol than output of 712a would be
greater than the output of 712b and thus the receiver 700 can
determine that a "one" symbol was transmitted. If the data
subcarrier waveform 406 was dithered using the dithering scheme
associated with a "zero symbol" than output of 712b would be
greater than the output of 712a and thus the receiver 700 can
determine that a "zero" symbol was transmitted.
[0044] Thus, by having a priori knowledge of the CDMA sequence, and
the potential dither patterns associated with the various symbols
produced by the subcarrier waveform generator at the transmitter,
the receiver 700 can decode data encoded into the subcarrier
waveform via dithering.
[0045] The dithering schemes described above can have minimal
impact to legacy users of the communications system. For instance
if the receiver described with respect to FIG. 2 (i.e., a legacy
receiver) were to be employed to receive a transmission generated
by the transmitter described with respect to FIG. 4, the signal
degradation caused by the dithering can be minimal.
[0046] To illustrate the impact, and ignoring noise and carrier
mixing for the moment, and given a received signal amplitude A,
sampling frequency F.sub.S, and the various previously defined
parameters (discussed above), the expected accumulation value over
the shift period S.sub.P can be given by:
= F S A 2 ( T S + S p - S L + .intg. T S S L C ( t ) C ( t - T S )
dt ) ##EQU00001##
[0047] Where C(t) is the subchip sequence (i.e., a square wave).
The autocorrelation function for a square wave with a circular
shift r can be expressed as:
R ( .tau. ) = C ( t ) C ( t - .tau. ) = 1 - 4 t s .tau. , 0
.ltoreq. .tau. < 0.5 t s ##EQU00002##
[0048] Thus the accumulation value (over S.sub.P subcarrier
periods) can be given by
I accum = F S A 2 ( T S + S p - S L + [ S L - T S ] [ 1 - 4 t s T S
] ) = F S A 2 ( S p - 4 T S t s ( S L - T S ) ) ##EQU00003##
[0049] The correlation loss L.sub.C for a signal can be computed as
the ratio of the expected accumulation value using T.sub.S=0 to the
expected accumulation value using the actual T.sub.S.
L C = 10 log 10 ( F S A 2 [ S P - 4 T S t s ( S L - T S ) ] ) - 10
log 10 ( F S A 2 [ S p ] ) = 10 log 10 ( 1 - 4 T S t s S p ( S L -
T S ) ) ) ##EQU00004##
[0050] Thus, for a BOC(1,1) signal (t.sub.s=1/1.023e6) with
S.sub.L=5t.sub.s, S.sub.P=100t.sub.s, and T.sub.S=0.1t.sub.s,
correlation loss can be computed as 0.0897 dB. In other words, the
computed correlation loss for a legacy receiver receiving a signal
that has been dithered using the scheme described with respect to
FIGS. 4-7 can be less than 0.1 dB.
[0051] As the above calculations illustrate, while the dithering
scheme can allow for additional data to be transmitted with minimal
spectral impacts on the channel, legacy receivers may only see
minimal performance degradation thus allowing for legacy users to
still participate in communications systems that employ
transmitters modified to encode additional data.
[0052] FIG. 8 illustrates an example of a computing device in
accordance with one embodiment. Device 800 can be a host computer
connected to a network. Device 800 can be a client computer or a
server. As shown in FIG. 8, device 800 can be any suitable type of
microprocessor-based device, such as a personal computer, work
station, server, or handheld computing device (portable electronic
device) such as a phone or tablet. The device can include, for
example, one or more of processor 810, input device 820, output
device 830, storage 840, and communication device 860. Input device
820 and output device 830 can generally correspond to those
described above, and can either be connectable or integrated with
the computer.
[0053] Input device 820 can be any suitable device that provides
input, such as a touchscreen, keyboard or keypad, mouse, or
voice-recognition device. Output device 830 can be any suitable
device that provides output, such as a touchscreen, haptics device,
or speaker.
[0054] Storage 840 can be any suitable device that provides
storage, such as an electrical, magnetic, or optical memory
including a RAM, cache, hard drive, or removable storage disk.
Communication device 860 can include any suitable device capable of
transmitting and receiving signals over a network, such as a
network interface chip or device. The components of the computer
can be connected in any suitable manner, such as via a physical
bus, or wirelessly.
[0055] Software 850, which can be stored in storage 840 and
executed by processor 810, can include, for example, the
programming that embodies the functionality of the present
disclosure (e.g., as embodied in the devices described above).
[0056] Software 850 can also be stored and/or transported within
any non-transitory, computer-readable storage medium for use by or
in connection with an instruction execution system, apparatus, or
device, such as those described above, that can fetch instructions
associated with the software from the instruction execution system,
apparatus, or device and execute the instructions. In the context
of this disclosure, a computer-readable storage medium can be any
medium, such as storage 840, that can contain or store programming
for use by or in connection with an instruction-execution system,
apparatus, or device.
[0057] Software 850 can also be propagated within any transport
medium for use by or in connection with an instruction-execution
system, apparatus, or device, such as those described above, that
can fetch instructions associated with the software from the
instruction-execution system, apparatus, or device and execute the
instructions. In the context of this disclosure, a transport medium
can be any medium that can communicate, propagate, or transport
programming for use by or in connection with an
instruction-execution system, apparatus, or device. The transport
readable medium can include, but is not limited to, an electronic,
magnetic, optical, electromagnetic, or infrared wired or wireless
propagation medium.
[0058] Device 800 may be connected to a network, which can be any
suitable type of interconnected communication system. The network
can implement any suitable communications protocol and can be
secured by any suitable security protocol. The network can comprise
network links of any suitable arrangement that can implement the
transmission and reception of network signals, such as wireless
network connections, T1 or T3 lines, cable networks, DSL, or
telephone lines.
[0059] Device 800 can implement any operating system suitable for
operating on the network. Software 850 can be written in any
suitable programming language, such as C, C++, Java, or Python. In
various embodiments, application software embodying the
functionality of the present disclosure can be deployed in
different configurations, such as in a client/server arrangement or
through a Web browser as a Web-based application or Web service,
for example.
[0060] The foregoing description, for purpose of explanation, has
made reference to specific embodiments. However, the illustrative
discussions above are not intended to be exhaustive or to limit the
disclosure to the precise forms disclosed. Many modifications and
variations are possible in view of the above teachings. The
embodiments were chosen and described in order to best explain the
principles of the techniques and their practical applications.
Others skilled in the art are thereby enabled to best utilize the
techniques and various embodiments, with various modifications,
that are suited to the particular use contemplated.
[0061] Although the disclosure and examples have been fully
described with reference to the accompanying figures, it is to be
noted that various changes and modifications will become apparent
to those skilled in the art. Such changes and modifications are to
be understood as being included within the scope of the disclosure
and examples as defined by the claims.
* * * * *