U.S. patent number RE34,036 [Application Number 07/399,583] was granted by the patent office on 1992-08-18 for data transmission using a transparent tone-in band system.
This patent grant is currently assigned to National Research Development Corporation. Invention is credited to Andrew Bateman, Joseph P. McGeehan.
United States Patent |
RE34,036 |
McGeehan , et al. |
August 18, 1992 |
Data transmission using a transparent tone-in band system
Abstract
A communication system which uses a transmitter and a receiver.
The transmitter divides a band of interest in the frequency
spectrum into upper and lower portions, and frequency translates
one of these portions in order to provide a frequency notch between
the portions. The receiver of the system includes a receiver
processor which receives the upper and lower portions, and which
restores the original frequency spectrum. The receiver processor at
least partially determines the final position of the restored
portion.
Inventors: |
McGeehan; Joseph P. (Wiltshire,
GB2), Bateman; Andrew (Avon, GB2) |
Assignee: |
National Research Development
Corporation (London, GB2)
|
Family
ID: |
27016679 |
Appl.
No.: |
07/399,583 |
Filed: |
August 28, 1989 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
617733 |
Jun 6, 1984 |
04691375 |
Sep 1, 1987 |
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Current U.S.
Class: |
455/71; 375/270;
375/301; 375/321; 375/344; 455/257; 455/47 |
Current CPC
Class: |
H04B
1/68 (20130101) |
Current International
Class: |
H04B
1/68 (20060101); H04B 001/76 () |
Field of
Search: |
;455/46,47,4,68,70,71,103,104,105,118,201,202,257,258,265
;375/37,47,71,97 ;380/33,34,38,39,40 ;370/110.1,110.4,111 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
IEEE on Communications Equipment Systems, Apr. 20-23, 1982 pp.
121-126. .
IEEE Conference Publication No. 222 pp. 153 to 157, Jun. 7-9, 1983.
.
The Institute of Electrical and Electronics, Inc., May 1983, pp.
369-373, J. P. MaGeehan et al "Speech Communication over a 942 MHz
Tone-Above-Band Single Sideband Radio Channel (6.25 KHz)
Incorporating Feed Forwward Signal Regeneration". .
IEEE International Conference on Communications; pp. 245-249; A. J.
Bateman et al; Jun. 19-22 1983. .
IEEE Transactions on Communications, vol. COM-32, No. 1, Jan. 1984
pp. 81-87..
|
Primary Examiner: Safourek; Benedict V.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
We claim:
1. A receiver for a communication system which employs a
transmitter comprising frequency selection and translation means
for dividing a band of interest in the frequency spectrum into
upper and lower portions with at least one signal present at a
significant level in both portions and carrying out frequency
translation, the output signal of the frequency selection and
translation means having one said portion which is translated in
frequency to provide a frequency notch between the lower and upper
portions, the receiver comprising:
a receiver processor for restoring the original frequency
spectrum,
means for deriving first and second signals representative of the
phase of the said one signal in those parts of the restored
spectrum derived from the lower and upper portions, respectively,
and
comparison means for comparing the first and second signals to
derive a control signal which when applied to the receiver
processor tends to reduce any phase difference between the first
and second signals, the output of the comparison means being
coupled to the said receiver processor.
2. A receiver according to claim 1 wherein the processor comprises
a local oscillator and a mixer coupled to the local oscillator to
receive a carrier signal therefrom, and the control signal is
applied to control the frequency of the local oscillator.
3. A receiver according to claim 1 for a communication system in
which the transmitter includes a mixer and an oscillator for moving
the position of the upper portion upwards in the spectrum,
wherein the receiver comprises a local oscillator and a further
mixer connected to receive the output signal of the said local
oscillator and the upper portion before restoration, the output
signal of the said further mixer being the upper portion restored
to its original position in the frequency spectrum.
4. A receiver according to claim 1 in which the transmitter
includes a first oscillator and a first mixer for reducing the
frequencies of the upper portion of the band, and a second
oscillator and a second mixer for increasing the frequencies of the
output signals of the first mixer to provide the final position of
the upper portion at the output of the transmitter
wherein the receiver comprises third and fourth mixers and a third
oscillator, the third mixer and the said local oscillator or the
third oscillator reducing the frequencies of the signals received
from the transmitter, and the fourth mixer and the remaining
receiver oscillator increasing the frequencies of the output
signals from the fourth mixer to restore the upper portion of the
hand to its original position in the spectrum.
5. A receiver for a communication system which employs a
transmitter comprising frequency selection and translation means
for dividing a band of interest in the frequency spectrum into
upper and lower portions with at least one signal present at a
significant level in both portions and for frequency translation,
the output signal of the frequency selection and translation means
having one said portion which is translated in frequency to provide
a frequency notch between the lower and upper portions after
translation, the receiver comprising:
a receiver processor for restoring the original frequency
spectrum,
means for deriving first and second signals representative of the
phase of the said one signal in those parts of the restored
spectrum derived from the lower and upper portions,
respectively,
comparison means for comparing the first and second signals to
derive a control signal which when applied to the receiver
processor tends to reduce any phase difference between the first
and second signals, the output of the control means being coupled
to the said receiver processor,
wherein the receiver includes first and second delay means for
delaying the lower and upper portions, respectively,
the comparison means is connected to receive signals on the input
side of the first and second delay means as the said first and
second signals,
the first and second delay means being constructed to impart delays
which allow sufficient time for the said reduction of phase
difference relating to a temporal position in the receiver input
signals to be made by the time the translated portion containing
that temporal position is restored to its original position.
6. A communication system comprising:
a transmitter including a transmitter processor for dividing a band
of interest in the frequency spectrum into upper and lower
portions, the transmitter processor including:
(a) translation means for translating one of the portions in
frequency to provide a frequency notch between the lower and upper
portion after translation,
(b) a transmitter local oscillator connected to the translation
means to at least partly determine the position of the translated
portion in the spectrum,
(c) transmitter multiplying means, connected to the transmitter
local oscillator, for multiplying a frequency determined by the
transmitter local oscillator, and
(d) transmitter operating means, connected to said transmitter
multiplying means, for operating on signals passing by way of the
transmitter in accordance with the output of said transmitter
multiplying means; and
a receiver including a receiver process for receiving the upper and
lower portions from said transmitter, the receiver processor
comprising:
(a) restoring means for restoring the original frequency
spectrum,
(b) a receiver local oscillator connected to said restoring means
to at least partially determine the final position of a restored
portion of the frequency spectrum,
(c) receiver multiplying means connected to said receiver local
oscillator for multiplying a frequency determined thereby, and
(d) receiver operating means, connected to said receiver
multiplying means, for operating on signals passing by way of the
receiver in accordance with the output of said receiver multiplying
means.
7. A communication system according to claim 6 wherein the
transmitter and receiver each employ a further local oscillator for
use in translating the said one portion and the transmitter and the
receiver multiplying means multiply the difference frequency
between the frequencies of the two transmitter local oscillators
and the two receiver local oscillators, respectively.
8. A communication system according to claim 6 comprising
transmitter and receiver modems, the transmitter modem being
connected at the input to the means for dividing the band of
interest and to receive a clock signal from the transmitter
multiplier means, and the receiver modem being connected at the
output of the receiver processor and to receive a clock signal from
the receiver multiplier means.
9. A communication system according to claim 6 wherein the
transmitter includes means for inserting the output of the
transmitter multiplying means into the notch, and the receiver
includes phase-error correction means for removing phase errors in
the receiver output requiring the application of a receiver
reference signal having a frequency substantially equal to a
reference signal included in the transmitted signal, and means for
applying the output signal of the receiver multiplying means as the
receiver reference signal.
10. A receiver for a communication system which employs a
transmitter comprising frequency selection and translation means
for dividing a band of interest in the frequency spectrum into
upper and lower portions with at least one signal present at a
significant level in both portions and for carrying out frequency
translation, the output signal of the frequency selection and
translation means having one said portion which is translated in
frequency to provide a frequency notch between the lower and upper
portions, the receiver comprising:
a receiver processor for restoring the original frequency
spectrum,
means for deriving a correcting signal dependent upon any
difference in frequency and phase in the said one signal in those
parts of the restored spectrum derived from the lower and upper
portions respectively, and
error-reduction means coupled to the said receiver processor for
reducing any said frequency and phase difference in the output of
the said receiver processor in accordance with the correcting
signal.
11. A receiver according to claim 10 wherein the correcting signal
is applied to reduce frequency and phase difference at a position
in the receiver which, as far as signal transmission is concerned,
follows the position from which the correcting signal is
derived.
12. A receiver according to claim 11 including
delay means for delaying the lower and upper portions of the band
before the restoration of the translated portion to its original
position, the delay means being arranged to impart delay sufficient
for the error-reduction means to reduce any frequency and phase
difference relating to a temporal position in the receiver input
signals by the time the translated portion containing the temporal
position is restored to its original position.
13. A receiver according to claim 12 wherein the means for deriving
a correcting signal includes
means for deriving first and second signals representative of the
frequency and phase of the lower and upper portions, respectively,
of the band before restoration of one portion to its original place
in the frequency spectrum,
comparison means for so comparing the first and second signals that
a control signal is derived which when applied to the error
reduction means tends to reduce any frequency and phase difference
between the first and second signals,
the means for deriving the first and second signals receiving the
lower and upper portions as input signals from first and second
points in processing carried out by the receiver, respectively, and
the error reduction means receiving the lower and/or upper portions
as input signals from the first and/or second points.
14. A receiver according to claim 10 wherein the error reduction
means forms part of the receiver processor.
15. A receiver according to claim 12 wherein the delay means
includes first and second delay portions for delaying the lower and
upper portions of the spectrum, respectively, and
the comparison means is connected to receive signals on the input
side of the first and second delay portions as the said first and
second signals.
16. A receiver according to claim 14 wherein the processor
comprises a local oscillator and a mixer coupled to the local
oscillator to receive a carrier signal therefrom, and the
correcting signal is applied to control the frequency of the local
oscillator.
17. A receiver according to claim 14 for a communication system in
which the tansmitter includes a mixer and an oscillator for moving
the position of the upper portion upwards in the spectrum,
wherein the receiver comprises a local oscillator and a further
mixer connected to receive the output signal of the said local
oscillator and the upper portion before restoration, the output
signal of the said further mixer being the upper portion restored
to its original position in the frequency spectrum.
18. A receiver according to claim 17 wherein
the means for deriving a correcting signal comprises a mixer
connected to receive at respective inputs the lower and upper
portions or signals derived therefrom, to derive the correcting
signal and to apply the correcting signal to control the frequency
of the local oscillator, and
quadrature phase change means connected to change the phase of one
of the input signals to the mixer, or its output signal or the
output signal of the local oscillator.
19. A receiver according to claim 14 for a communication system in
which the transmitter includes a first oscillator and a first mixer
for reducing the frequencies of the upper portion of the band, and
a second oscillator and a second mixer for increasing the
frequencies of the output signals of the first mixer to provide the
final position of the upper portion at the output of the
transmitter
wherein the receiver comprises third and fourth mixers, the third
mixer and the said local oscillator reducing the frequencies of the
signals received from the transmitter, and the fourth mixer
increasing the frequencies of the output signals from the third
mixer to restore the upper portion of the band to its original
position in the spectrum.
20. A receiver according to claim 19 wherein the means for deriving
a correcting signal comprises
a fifth mixer arranged to receive the lower and upper portions or
respective signals derived therefrom and to supply a signal to the
fourth mixer to mix with the output of the third mixer.
21. A receiver according to claim 20 including phase locked loop
means and/or an averaging filter connected between the fifth and
fourth mixers.
22. A communication system having a receiver according to claim 10
and a transmitter, wherein the transmitter includes
frequency selection and filtering means dividing a band of interest
in the frequency spectrum into upper and lower portions, and for
frequency translation, the output signal of the frequency selection
and translation means having one said portion which is translated
in frequency to provide a frequency notch between the lower and
upper portions,
the frequency selection and translation means including a
transmitter local oscillator having an output so connected that the
output signal thereof at least partly determines the position of
the translated portion in the spectrum,
multiplying means for multiplying the frequency of the transmitter
local oscillator or a frequency derived therefrom, and
means for carrying out an operation in the transmitter in
accordance with the output of the said multiplying means.
23. A method for use in a receiver of a communication system which
employs a transmitter comprising frequency selection and filtering
means dividing a band of interest in the frequency spectrum into
upper and lower portions with at least one signal present at a
significant level in both portions and for frequency translation,
the output signal of the frequency selection and translation means
having one said portion which is translated in frequency to provide
a frequency notch between the lower and upper portions,
the method comprising the steps of:
restoring the original frequency spectrum,
deriving a correcting signal dependent on any difference in
frequency and phase in said one signal in those parts of the
restored spectrum derived from the lower and upper portions
respectively, and
reducing any said frequency and phase difference using the
correcting signal.
24. A method according to claim 23 wherein the reduction of any
phase and frequency difference is carried out at a position in the
receiver processing which, as far as signal transmission is
concerned, follows the position in which the correcting signal is
derived.
25. A receiver for a communication system which employs a
transmitter comprising means for causing a band of frequencies to
carry information, frequency selection and translation means for
dividing the said band into upper and lower portions with at least
one signal present at a significant level in both portions and
carrying out frequency translation, the output signal of the
frequency selection and translation means having one said portion
which is translated in frequency to provide a frequency notch
between the lower and upper portions, the receiver comprising:
a receiver processor for recovering the said information,
means for deriving first and second signals representative of the
phase of the said one signal in the lower and upper portions,
respectively, allowing for said translation in frequency, and
comparison means for comparing the first and second signals to
derive a control signal which when applied to the receiver
processor tends to reduce any phase difference between the first
and second signals, the output of the comparison means being
coupled to the said receiver processor.
26. A receiver for a communication system which employs a
transmitter comprising means for causing a band of frequencies to
carry information, frequency selection and translation means for
dividing the said band into upper and lower portions with at lest
one signal present at a significant level in both portions and for
carrying out frequency translation, the output signal of the
frequency selection and translation means having one said portion
which is translated in frequency to provide a frequency notch
between the lower and upper portions, the receiver comprising:
a receiver processor for recovering the said information,
means for deriving a correcting signal dependent upon difference in
frequency and phase in the said one signal as present in the lower
and upper portions allowing for said translation in frequency,
and
error-reduction means coupled to the said receiver processor for
reducing any said frequency and phase difference in the output of
the said receiver processor in accordance with the correcting
signal.
27. A method for use in a receiver of a communication system which
employs a transmitter comprising means for causing information to
be carried by a band of frequencies, frequency selection and
translation means for dividing a band of interest in the frequency
spectrum into upper and lower portions with at least one signal
present at a significant level in both portions and for frequency
translation, the output signal of the frequency selection and
translation means having one said portion which is translated in
frequency to provide a frequency notch between the lower and upper
portions, the method comprising the steps of:
deriving a correcting signal dependent on any difference in
frequency and phase between the said one signal as present in the
lower and upper portions of the band allowing for said translation
in frequency,
reducing any said frequency and phase difference using the
correcting signal, and
recovering the said information. .Iadd.
28. A receiver for a communication system which employs a
transmitter comprising frequency selection and translation means
for carrying out a process of frequency selection and frequency
translation on a band of interest in the frequency spectrum to
divide said band into upper and lower portions with the output
signal of the frequency selection and translation means having a
frequency notch between the lower and upper portions, the receiver
comprising:
a receiver processor for removing said notch to combine the upper
and lower portions and provide a restored frequency spectrum,
means for deriving first and second signals, from the output
signal, having a frequency and phase difference which is
representative of a frequency and phase difference between edges of
the notch, and
comparison means for comparing the first and second signals to
derive a control signal which when applied to the receiver
processor tends to reduce any frequency and phase difference
between the upper and lower edges of those parts of the restored
spectrum which are derived from the lower and upper portions,
respectively. .Iaddend. .Iadd.
29. A receiver for a communication system which employs a
transmitter comprising frequency selection and translation means
for carrying out a process of frequency selection and frequency
translation of a band of interest in the frequency spectrum to
divide said band into upper and lower portions with the output
signal of the frequency selection and translation means having a
frequency notch between the lower and upper portions, the receiver
comprising:
a receiver processor for removing said notch to combine the upper
and lower portions and provide a restored frequency spectrum,
means for deriving a correction signal representative of any
difference in frequency and phase between edges of the notch from
the output signal, and
error-reduction means coupled to the said receiver processor for
reducing in accordance with the correction signal, any frequency
and phase differences between the upper and lower edges of those
parts of the restored spectrum which are derived from the lower and
upper portions, respectively. .Iaddend. .Iadd.30. A receiver for a
communication system which employs a transmitter comprising
frequency selection and translation means for carrying out a
process of frequency selection and frequency translation on a band
of interest in the frequency spectrum to divide said band into
upper and lower portions with the output signal of the frequency
selection and translation means having a frequency notch between
the lower and upper portions, the receiver comprising:
a receiver processor for removing said notch to combine the upper
and lower portions and provide a restored frequency spectrum,
means for deriving a correcting signal from the output signal
representative of any difference in frequency and phase between the
upper and lower edges of those parts of the restored spectrum
derived from the lower and upper portions respectively, and
error-reduction means coupled to the said receiver processor for
reducing any said frequency and phase difference in the output of
the said receiver processor in accordance with the correcting
signal. .Iaddend. .Iadd.31. A method for use in a receiver of a
communication system which employs a transmitter comprising
frequency selection and filtering means carrying out a process of
frequency selection and frequency translation on a band of interest
in the frequency spectrum to divide said band into upper and lower
portions with the output signal of the frequency selection and
translation means having a frequency notch between the lower and
upper portions,
the method comprising the steps of:
removing said notch to combine the upper and lower portions and
provide a restored frequency spectrum,
deriving a correcting signal from the output signal representative
of any difference in frequency and phase between edges of the
frequency notch, and
reducing, in accordance with the correcting signal, any frequency
and phase difference between the upper and lower edges of those
parts of the restored spectrum which are derived from the lower and
upper portions,
respectively. .Iaddend. .Iadd.32. A method for use in a receiver of
a communication system which employs a transmitter comprising
frequency selection and filtering means for carrying out a process
of frequency selection and frequency translation on a band of
interest in the frequency spectrum to divide said band into upper
and lower portions with the output signal of the frequency
selection and translation means having a frequency notch between
the lower and upper portions,
the method comprising the steps of:
removing said notch to combine the upper and lower portions and
provide a restored frequency spectrum,
deriving a correcting signal from the output signal representative
of any difference in frequency and phase between the upper and
lower edges of those parts of the restored spectrum derived from
the lower and upper portions respectively, and
reducing any said frequency and phase difference using the
correcting signal. .Iaddend. .Iadd.33. A receiver for a
communication system which employs a transmitter comprising
frequency selection and translation means for carrying out a
process of frequency selection and frequency translation on a band
of interest in the frequency spectrum to divide said band into
upper and lower portions with the output signal of the frequency
selection and translation means having a frequency notch between
the lower and upper portions, the receiver comprising:
means for deriving a signal representative of at least one of
frequency difference between the edges of the frequency notch and
phase difference between edges of the frequency notch, and
means for removing said notch to combine the upper and lower
portions and restore said band of interest, using said signal to
reduce any frequency and phase difference between the upper and
lower edges of those parts of the restored band of interest which
are derived from the lower and upper portions, respectively.
.Iaddend.
Description
The present invention relates to the provision of facilities for
improved data transmission using transparent tone-in-band (TTIB)
systems.
TTIB systems are described by J. P. McGeehan, A. J. Bateman and D.
F. Burrows in "The Use of `Transparent` Tone-In-Band (TTIB) and
Feedforward Signal Regeneration (FFSR) In Single Sideband Mobile
Communication Systems", IEE Conference on Communications Equipment
and Systems 82, pages 121 to 126, 1982. In a TTIB system the
spectrum of a baseband signal, for example from 300 Hz to 3 kHz, is
split into two approximately equal segments, for instance from 300
Hz to 1.7 kHz and 1.7 kHz to 3 kHz. The upper frequency band is
translated upward in frequency by an amount equal to the width of
an intervening "notch" and added to the lower frequency band. If
for example the required "notch" width or band separation is 1.2
kHz the circuit output will comprise a signal extending from 300
kHz to 1.7 kHz and from 2.9 to 4.2 kHz. A low level reference tone
may then be added at the centre of the resulting notch which in
this example would be 2.3 kHz and the composite signal is then
tranmitted using conventional techniques, such as single sideband
(SSB), with the pilot tone in the notch acting as the reference for
subsequent pilot-based processing. In the receiver, the final
stages of audio processing remove the pilot in the usual way (for
use in, for example automatic gain control and automatic frequency
control purposes) and perform a complementary downwards frequency
translation of the upper half of the spectrum thereby regenerating
the original 300 Hz to 3 kHz baseband signal. Thus TTIB gives a
complete transparent channel from the baseband input of the
transmitter to the baseband receiver output avoiding the
disadvantage of removing a section of the band in order to insert
the pilot tone but obtaining the advantages of high degree of
adjacent channel protection, good correlation between fades on the
pilot tone and fades on the audio signal, and a large symmetrical
pull-in range for the frequency control to operate.
In order to use a TTIB system in data communications with a bit
error rate performance equal to or better than that obtained with
pilot carrier and above-band tone systems particularly in mobile
radio where amplitude and phase fading occurs, the TTIB transmitter
and receiver must be locked in frequency and phase to eliminate the
mid-band interference region of the unlocked system. An apparent
requirement is a frequency and phase reference to be generated from
the received signal for use in a phase locked tracking circuit.
Unfortunately it is not possible to use the pilot tone transmitted
with TTIB for this purpose since the tone suffers from the effects
of multipath propagation and possible frequency error due to
misalignment of the transmitter and receiver RF/IF local
oscillators.
In this specification a TTIB communication system comprises
a transmitter including means for dividing a band of interest in
the frequency spectrum into upper and lower portion, and means for
translating one of the portions in frequency to provide a frequency
notch between the lower and upper portions after translation,
and
a receiver including a receiver processor for restoring the
translated portion to its original place in the frequency
spectrum.
If a baseband is considered extending from DC upwards in frequency
it will always be necessary to translate the upper portion upwards
in the frequency spectrum to provide the frequency notch.
The transmitter and receiver are usually parts of an SSB system and
advantages are mainly expected to be gained in mobile radio
employing such systems. Other areas of application include
land-line communication links.
According to a first aspect of the present invention there is
provided a receiver for a TTIB communication system as hereinbefore
specified including
means for deriving first and second signals representative of the
phase of the lower and upper portions, respectively, of the band
before restoration of one portion to its original place in the
frequency spectrum,
comparison means for so comparing the first and second signals that
a control signal is derived which when applied to the receiver
processor tends to reduce any phase difference between the first
and second signals.
The receiver processor typically comprises a local oscillator and a
mixer, and the control signal is applied to control the frequency
of the local oscillator.
The main advantage of the first aspect of the invention is that
data communication systems employing this aspect are expected to
have a bit error rate comparable with or better than that obtained
with pilot carrier and above-band tone systems, especially in
mobile radio applications.
Two main possible ways of translating a portion of the frequency
band of interest are now given as alternatives.
As a first alternative, the transmitter may include a mixer and an
oscillator for moving the position of the upper portion upwards in
the spectrum. The receiver may then include a further mixer
connected to receiver the output signal of the said local
oscillator and the upper portion before restoration, the output
signal of the mixer being the upper portion restored to its
original position in the frequency spectrum.
As a second alternative the tranmitter may include a first
oscillator and a first mixer for reducing the frequencies of the
upper portion of the band, and a second oscillator and a second
mixer for increasing the frequencies of the output signals of the
first mixer to provide the final position of the upper portion at
the output of TTIB transmitter processing. The receiver may then
include third and fourth mixers and a third oscillator, the third
mixer and the said local oscillator or the third oscillator
reducing the frequencies of the signals received from the
transmitter, and the fourth mixer and the remaining receiver
oscillator increasing the frequencies of the output signals from
the fourth mixer to restore the upper portion of the band to its
original position in the spectrum.
The second alternative is more flexible than the first because the
width of the notch can be chosen as required whereas using the
first alternative the notch must be of sufficient width to avoid
spectral overlap in the desired upper-band segment.
In both the first and second alternatives a number of filters are
required to remove unwanted signals at different positions in the
circuit as will be apparent from the description which follows.
According to a second aspect of the invention there is provided a
receiver for a TTIB communication system as hereinbefore specified
including
means for deriving first and second signals representative of the
phase of the lower and upper portions, respectively, of the band
before restoration of one portion to its original place in the
frequency spectrum,
comparison means for so comparing the first and second signals that
a control signal is derived which when applied to the receiver
processor tends to reduce any phase difference between the first
and second signals,
wherein the receiver includes first and second delay means for
delaying the lower and upper portions, respectively,
the comparison means is connected to receive signals on the input
side of the first and second delay means as the said first and
second signals,
the first and second delay means being constructed to impart delays
which allow sufficient time for the said reduction of phase
difference relating to a temporal position in the receiver input
signals to be made by the time the translated portion containing
that temporal position is restored to its original position.
The main advantage of the second aspect of the invention is that
the whole of a portion of data can be transmitted without errors
since feedforward from the inputs of the first and second delay
means allows phase correction to be carried out before restoration
of the translated poriton to its original position is
completed.
The second aspect of the invention can be employed with the first
and second alternatives mentioned above.
According to a third aspect of the present invention there is
provided a TTIB communication system as hereinbefore specified
wherein
the means for translating one portion of the input frequency
spectrum at the transmitter includes a transmitter local oscillator
having an output so connected that the output signal thereof at
least partly determines the position of the translated portion in
the spectrum,
means for multiplying the frequency of the transmitter local
oscillator or a frequency derived therefrom, and
means for carrying out an operation in the transmitter in
accordance with the output of the multiplying means, and
wherein the receiver includes a receiver local oscillator having an
output so connected that the output signal thereof at least
partially determines the final position of the restored
portion,
means for multiplying the frequency of the receiver local
oscillator or a frequency derived therefrom, and
means for carrying out an operation in the receiver in accordance
with the output of the receiver multiplying means.
Both the first and second alternatives mentioned above may be used
in the third aspect of the invention, and when the second is used
the multiplying means of both the transmitter and the receiver may
multiply the difference frequency between the frequencies of the
two transmitter and the two receiver oscillators, respectively.
The transmitter and receiver of the third aspect of the invention
may include complementary circuits, such as modems, requiring
reference signals in order to operate, the output of the
transmitter multiplying means and the receiver multiplying means
may then be used as the required reference signals, an advantage
being that a simple technique for achieving bit synchronisation
with TTIB systems is provided. Consequently the technique has wide
applications including line satellite and mobile
communications.
In SSB, and particularly mobile radio SSB, the pilot tone suffers
from phase variations but this problem can be overcome by using
feedforward signal regeneration (FFSR) as described for TTIB in the
1982 paper by McGeehan, Bateman and Burrows (see FIG. 5). In this
solution the frequency of an oscillator in the receiver is made
equal to that of the pilot tone. However a problem remains where a
received signal is to be coherently demodulated after SSB
transmission and demodulation. The third aspect of the invention
may be used to allow coherent demodulation if the transmitter
includes means for inserting the output of the transmitter
multiplying means into the notch, and the receiver includes
phase-error correction means for removing phase errors in the
receiver output requiring the application of a receiver reference
signal having a frequency substantially equal to a reference signal
included in the transmitted signal, and means for applying the
output signal of the receiver multiplying means as the receiver
reference signal.
The phase-error correction means preferably employs a feedforward
arrangement.
The invention also includes methods of tranmitting and/or receiving
data transmitted using a TTIB system corresponding to any of the
above aspects of the invention.
Certain embodiments of the invention will now be described by way
of example, with reference to the accompanying drawings, in
which:
FIG. 1 is a block diagram of a TTIB system employing a receiver
according to a first aspect of the invention,
FIG. 2 is a block diagram of a TTIB system employing a receiver
according to a second aspect of the invention,
FIG. 3 is a block diagram of a first TTIB system according to the
third aspect of the present invention providing bit
synchronisation, and
FIG. 4 is a block diagram of a second TTIB system according to the
third aspect of the invention employing FFSR phase correction.
A simple form of TTIB is first described and then a method of phase
locking this form is explained.
In FIG. 1 a baseband signal is applied to a low-pass filter 10 to
remove any high frequency components outside the band which may be
present. At the output of the filter 10 the signal is passed to a
further low-pass filter 11 which selects a low frequency portion of
the band for application to a combining circuit 12. The output of
the filter 10 is also applied to a mixer 13 receiving an additional
signal from an oscillator 14 at a frequency equal to the required
notch width. The upper sideband of the output of the modulator 13
is selected using a bandpass filter 15 and applied to the combining
circuit 12 together with a tone, typically an SSB pilot tone, which
is supplied to a terminal 16. The composite signal so produced is
passed through another lowpass filter 17 to ensure that no signals
outside the required band remain and it is then transmitted, using,
for example, an SSB transmitter, over mobile radio to a receiver
where the pilot tone is first extracted using a bandpass filter
(not shown) for use in pilot-based correction of the SSB signal.
After such demodulation a signal comprising the lower band, the
notch, the reference tone and the upper band is applied to a
low-pass filter 20 where the lower band is extracted and applied to
a combining circuit 21. A bandpass filter 22 separates the upper
band and applies it to a mixer 23 receiving signals from a local
oscillator 24 which restores the upper band to its original place
in the spectrum so that when combined with the lower portion in the
first combining circuit 21 the original baseband is restored. A
lowpass filter 25 ensures that no spurious frequencies outside the
baseband are passed to output.
The TTIB system described so far with its unlocked local oscillator
24 in the receiver is satisfactory for informal voice communication
but not for conventional data transmission systems such as
frequency shift keying (FSK) and differential phase shift keying
(DPSK).
In order to overcome this problem it is necessary to lock together
the frequency and phase of the transmitter and receiver translation
oscillators; this has the effect of ensuring phase integrity in the
restored band at the output of a combining circuit 21. See the
paper "Phase-Locked Transparent Tone-In-Band (TTIB): A new Spectrum
Configuration Particularly Suited to the Transmission of Data Over
SSB Mobile Radio Networks" by J. P. McGeehan and A. J. Bateman,
IEEE Transactions on Communications, COM-32, No. 1, Jan. 1984, pp
81-87.
Control for the local oscillator 24 is derived by taking the
signals from the lower and upper portions of the band after
restoration and applying these signals by way of bandpass filters
18 and 19, and limiters 26 and 27 to a phase sensitive detector 28,
the phase of the signal of the upper portion of the band being
changed by 90.degree. in a circuit 29 before application to the
detector 28. The output of the detector 28 passes by way of a
low-pass filter 30 before being applied as a control signal for the
oscillator 24. The circuits 18, 19, 23, 24 and 26 to 30 form a
phase locked loop and the filter 30 determines the order and type
of the loop.
The control for the oscillator 24 depends on the outputs of the
filters 20 and 22 containing signals in the transition region at
the edges of the notch which should nominally be at the same
frequency and phase after restoration of the upper portion.
Detection of frequency and phase differences in these signals by
the detector 28 provides the control signal for the oscillator 24.
These signals may be up to 40 to 50 dB below the passbands of the
filters. The filters 18 and 19 have passbands of typically 200 Hz
and since the upper portion has been restored in frequency at the
output of the mixer 23 both of the filters 18 and 19 are centred on
the transition region at the lower edge of the notch. The filters
18 and 19 may sometimes by omitted. If necessary the filter 30 is
given a frequency characteristic which depends on the roll-off of
the filters 11 and 15.
In the following more detailed description of the operation of FIG.
1, it is assumed that the appropriate time delays in all paths are
matched, and that the input signal to the filter 10 is represented
by
where .alpha. and .beta. represent the amplitude and phase terms,
respectively, of a signal frequency component centred at
.omega..sub.s, where .omega..sub.s is within the transition region
between the upper and lower bands. This signal is then in turn
mixed with the frequency translating carrier, .omega..sub.1,
generated by the oscillator 14 and expressed as:
where .psi. is an arbitrary phase term. The resulting summed and
filtered output of the transmitter processing at the output of the
filter 17 is then
where A.sub.1 A.sub.2 represent the attenuation coefficients of the
filters 11 and 15, respectively , t.fwdarw.t-D.sub.1 -D.sub.2 means
replace t by t-D.sub.1 -D.sub.2, D.sub.1 is the delay of the filter
10, and D.sub.2 is the delay of the filters 11 and 15.
On reception in a mobile environment for example, the signal is
assumed corrupted by both random amplitude and phase modulation,
r(t) and .phi.(t), and, after demodulation, has a residual
frequency offset error of .DELTA..omega., due to oscillator drift.
The input to the receiver processing at the filters 20 and 22 can
then be written as
If the free-running centre frequency of the voltage controlled
oscillator (VCO) 24 is expressed as
where .delta. is an arbitrary phase term, then the two input
signals to the summing circuit 21 are given by
and ##EQU1## where it is assumed that the passbands of the filters
20 and 22 are sufficiently wide to pass the input signal without
distortion except for a constant attenuation A.sub.3, A.sub.4 and
time delay D.sub.3.
For satisfactory combining of these two signals defined, the
frequency and phase of the oscillator 24 must be adjusted such
that
After filtering to remove the "2f" components from the mixer
processing and to improve the signal-to-noise ratio of the signals
being fed into the limiters, the two signals are mixed and filtered
to obtain the difference term y.sub.1 (t), which is used to control
the oscillator 24. The bandpass filters and limiters are included
in the circuit to extract the frequency components in the sub-band
overlap region and to remove the effects of envelope fading and
speech modulation so that the input to the double balanced mixer
and the loop gain, K, are held constant. In some circumstances it
can be advantageous to exclude the limiters to reduce the noise
sensitivity of the system. The resulting difference term is given
by ##EQU2## which is independent of the multipath induced fading,
containing only the required information concerning the frequency
and phase error between the transmitter and receiver translating
oscillators, and phase shifts introduced by the transmission link.
This signal is used to modify the frequency and phase of the
oscillator 24 such that the frequency error is eliminated and the
phase error minimised.
Inherent in the application of feedback phase lock techniques for
correct sub-band recombination is a small but finite time delay in
which correction of the local oscillator 24 is achieved. Standard
acquisition aids can be applied here to advantage. The inherent
delay may cause temporary errors while correction is applied but
this problem can be overcome in a way which will now be described
with reference to FIG. 2 but by way of illustration another form of
TTIB is employed in the transmitter and receiver. In FIG. 2 as in
FIGS. 3 and 4 component circuits which have the same function as in
an earlier Figure or Figures have the same disignations and are not
further described.
In FIG. 2, the output from the low-pass filter 10 is applied to a
mixer 32 which also receives a signal from an oscillator 33 so that
the whole baseband is moved up in frequency as a result of mixing.
The lower portion of the lower sideband of the resultant signal now
corresponds to the upper portion of the baseband signal and this
upper portion from the mixer 32 is removed using a low-pass filter
34. The resulting signal corresponding to the upper portion of the
baseband, but reversed in frequency, is applied to a further mixer
35 receiving another signal from an oscillator 36 and the resultant
signal is applied to the combining circuit 12. This arrangement is
flexible in terms of notch width since the notch width equals the
frequency of the oscillator 36 minus that of the oscillator 33.
Such a transmitter is described in the 1984 paper by McGeehan and
Bateman where FIG. 2 illustrates the frequency translations
used.
Assuming SSB transmission, the receiver signal obtained, after SSB
demodulation using the pillot tone, is applied to a mixer 37 also
coupled to an oscillator 38. The upper portion of the original
received signal now forms part of the lower sideband of the output
from the mixer 37 and the whole of this output except the said
upper portion is removed by means of a low-pass filter 40 and then
applied by way of a delay circuit 41 to another mixer 42 coupled to
an oscillator 43. The lower portion of the baseband signal
available at the output of the low-pass filter 20 is passed through
a delay circuit 44 before it is reunited with the upper portion in
the combining circuit 21. The delays of the circuits 41 and 44 are
such that parts of the upper and lower portions which correspond in
time arrive at the combining circuit 21 at the same time.
In order to apply phase locking the output signals of the low-pass
filters 20 and 40 are applied via bandpass filters 18 and 19' and
limiters 26 and 27 to a mixer 55 the output of which is used to
control a phase locked loop (PLL) 56 comprising the oscillator 43,
a mixer 57 and a low-pass filter 58. The oscillator output is fed
via a 90.degree. phase-shift circuit 59 to the mixer 42. By taking
output signals from the filters 20 and 40 before the delay circuits
41 and 44 and using these signals to derive a signal to correct the
frequency of the oscillator 43, this correction can be provided for
the whole of any input signal to the receiver including the initial
portion, at the expense of delaying the output signal by the
interval imparted by the delays of the circuits 41 and 44, this
interval being chosen to be sufficient for such correction to be
carried out.
Since the output of the filter 40 has not been translated to its
original position in the frequency spectrum, the passband of the
filter 19' is centred on the transition region of the upper band
segment.
The circuits 43, 57 and 58 can be regarded as selecting a frequency
nominally equal to the frequency of the oscillator 36 and for this
reason these circuits may be replaced by a bandpass filter or a
combination of a bandpass filter and a phase locked loop.
In some circumstances it is thought possible that the input signals
for the filters 18 and 19' may be taken from the inputs of the
filters 20 and 40 instead of their outputs. As in FIG. 1, there are
arrangements in which the filters 18 and 19' can be omitted.
Making the same assumption as were made for FIG. 1, but omitting
the phase term .alpha., the arbitrary phase terms .beta., .psi. and
random amplitude modulation r(t), the input signal to the receiver
combining circuit is ##EQU3## where T.sub.1 and T.sub.2 are the
frequencies of the oscillators 33 and 36, respectively.
The output from the filter 20 is
and that from the filter 40 is
where R.sub.1 is frequency of the oscillator 38. Thus the sum term
output signal from the mixer 55 is
The PLL 56 comprising the components 43, 57 and 58 cause the
frequency R.sub.2 of the oscillator 43 to be given by
that is
Hence the output of the mixer 42 is
which is combined with the output of the filter 20 to give the
baseband signal assuming the delays imparted by the circuits 41 and
44 are equal.
The feed forward technique shown in FIG. 2 can equally be applied
to the circuit of FIG. 1 by introducing delays following the
filters 20 and 22 and taking the signals for the circuits 26 and 29
from the inputs of these delays. A further mixer is however
necessary in order to translate the frequency of the output from
the delay following the filter 22 to the correct part of the
spectrum.
Where a data stream is to be transmitted in mobile radio SSB the
need to obtain bit synchronisation in a receiver modem arises. The
required clock timing signal is required to be independent of both
randmom phase fluctuations and demodulation frequency error. Such a
signal can be obtained from the arrangement shown in FIG. 3 where a
data input is applied to a transmitter modem 62 and then to the
TTIB transmitter processor which is the same as that of FIG. 2 and
is designated 60 in both FIG. 2 and FIG. 3. The clock frequency for
the modem is derived from the signals of oscillators 33 and 36
which play the same part in TTIB processing as they do in the
arrangement of FIG. 2. The output of the oscillators 33 and 36 is
applied to a mixer 63 which derives an output signal having a
frequency equal to the difference frequency of its input signals
and the output of this mixer after multiplication in a multiplier
64 is applied as the clock signal for the modem 62.
After transmission by any convenient means and demodulation the
signal containing data is applied to a low-pass filter 20 as in
FIGS. 1 and 2. The receiver input signal is also applied to the
circuit branch comprising the mixers 37 and 42 and the low-pass
filter 40. This arrangement is the same as shown in FIG. 2 except
that since feedforward control is not used in this example the
delay 41 is omitted. The oscillator 43 is controlled by an
oscillator control circuit 61 which is the same as that indicated
by the designation 61 and the dotted line in FIG. 1. The output
signal of the low-pass filter 25 is applied to a receiver modem 65
which generates the data output. The clock frequency for the modem
65 is derived from the oscillators 38 and 43 by passing their
output signals to a mixer 66 which derives a signal having a
frequency which equals the difference between the frequencies of
the oscillators 38 and 43. The output of the mixer 66 is connected
by way of an adjustable phase shift circuit 67 and a multiplier 68
to the input clock terminal of the modem 65. The multipliers 64 and
68 multiply the output frequencies of the mixers 63 and 66,
respectively, by a convenient factor N to provide a suitable clock
frequency.
As a brief explanation of the operation of FIG. 3 the output signal
of the oscillator 43 can be regarded as being locked in frequency
and phase by the phase sensitive detector 61 to a predetermined
relationship with the oscillator 38, this relationship being the
same as that between the output signals of the oscillators 33 and
36 in the transmitter. Hence the difference frequencies of the
transmitter oscillators and the receiver oscillators can be used to
derive the modem clock signals.
In FIG. 3 the clock signal supplied to the modem 62 is
where N is the multiplication factor of the multiplier circuits 64
and 68.
.psi..sub.T1, .psi..sub.T2 are the phases of the oscillators 33 and
36, and ideally this signal e.sub.i (t) should also be supplied as
the clock signal for the modem 65 after it has suffered the same
delays as the output signal of the modem 62.
The clock signal for the modem 65 is given by
where .psi..sub.R1, .psi..sub.R2 are the phases of the oscillators
38 and 43.
Since, in view of the operation of the PSD 61, T.sub.2 -T.sub.1
+R.sub.2 -R.sub.1 =0 (see equation 2) and, as can be shown for a
phase locked TTIB system, the phase tracking error .psi..sub.e at
the output of the combining circuit 25 is ##EQU4## where D.sub.0,
D.sub.1, D.sub.3, are the delays of filters 10, 11 and 34, and 20
and 40, and D.sub.2 is the variable time delay due to transmitter
and receiver RF/IF processing and propagation delay.
Using equations 2 and 4 the expression for e.sub.j (t) can
therefore be rewritten as follows ##EQU5##
Any error in the required clock signal for the modem 65 is found by
taking equation 1 modified by the delays D.sub.0, D.sub.1, D.sub.2,
D.sub.3, and D.sub.4 (where D.sub.4 is the delay due to the filter
25) from the above expression for e.sub.j (t), with the result that
the difference in frequency .delta..omega., and phase .delta..psi.
between the desired and received data clock is
Hence there is no frequency error and the phase error is
independent of the variable system delay D.sub.2 and can be
compensated by the fixed phase shift 67 when correctly adjusted in
the receiver clock path. The only residual error is the loop phase
tracking error .psi..sub.e and by using narrowband tracking loops,
this error can be made arbitrarily small.
The principle of operation of the circuit of FIG. 3 can be applied
to that of FIG. 1 so that a transmit modem connected at the input
of the filter 10 receives its clock signal from a multiplier
connected to the oscillator 14 and in the receiver a receive modem
connected at the output of the filter 25 receives its clock signal
from the oscillator 24 by way of an adjustable delay circuit and a
further multiplier.
Where clock signals for modems are provided in the way described,
the feedforward technique of FIG. 2 may also be used; that is for
example the technique can be applied to the arrangement of FIG. 3
by and addition of delays 41 and 44 and the mixer 55.
The TTIB FFSR system described in the 1982 paper by McGeehan,
Bateman and Burrows reduces that frequency uncertainty between
transmitter and receiver local oscillators which results in
frequency error in the demodulated baseband signal. Where coherent
demodulation is required for example in data transmission the
remaining frequency error is still, though small, important. To
provide coherent demodulation within, for example modems designed
to make use of TTIB FFSR, the arrangement shown in FIG. 4 can be
employed. The baseband input is applied to the TTIB transmitter
processing circuit 60 and a control tone which is inserted in the
notch is derived from the difference frequency of the output
signals of the oscillators 33 and 36 which are applied as in FIG. 3
to a mixer 63. After multiplication in a multiplier circuit 71 to a
convenient frequency the output of the mixer 63 is applied to the
terminal 16 (see FIG. 2) for insertion in the notch.
At the receiver, envelope and phase differences between the
baseband input signal at the transmitter and that derived by the
receiver are corrected by an FFSR circuit 73 connected at the input
to a TTIB receiver processing circuit 70 which is as shown by the
same designation in FIG. 3. The FFSR circuit 73 is as shown in FIG.
5 of the 1982 paper by McGeehan, Bateman and Burrow except that the
local oscillator having an output frequency of .omega..sub.2 shown
in FIG. 5 is replaced by a signal derived from the output signals
of the oscillators 38 and 43. These output signals are applied to
the mixer 66 which derives the difference frequency. The output of
the mixer 66 is connected to a multiplier circuit 74 which provides
the required signal to replace .omega..sub.2. Multipliers 71 and 74
both multiply by the same convenient factor N.
From the 1982 paper it will be apparent that the signal y.sub.e (t)
at the output of the FFSR circuit 73 is
where .delta..omega..sub.k, .delta..psi..sub.k =frequency and phase
difference between the FFSR local oscillator (that is the output
signal of the multiplier circuit 74) and the delay transmitted tone
(that is as originating through application to the terminal
16).
The local oscillator input is given by equation 5 and the
corresponding delayed transmitted reference signal y.sub.1 (t)
is
Thus from equations 5 and 6, the difference frequency and phase
between the transmitted reference and the local FFSR reference
superimposed upon the baseband output signal is given by
Hence demodulation frequency error is eliminated and the phase
error .delta..psi..sub.k is independent of the time delay D.sub.2
and can therefore be compensated by an adjustable phase shift
circuit 75 connected between the multiplier 74 and the FFSR circuit
73. Consequently coherent demodulation of the received signal is
achieved.
Again it will be appreciated that the FFSR correction technique can
be applied to a circuit of the type shown in FIG. 1 by multiplying
the output frequency of the oscillator 14 and applying it to the
terminal 16 as a signal in the notch. At the receiver the output
signal from the oscillator 24 is multiplied by the same amount and
applied to an FFSR correction circuit connected at the input to the
filters 20 and 22. In addition the feedforward technique of FIG. 2
can be applied to FIG. 4 or FIG. 1 when modified to employ
FFSR.
Having explained a number of specific embodiments of the invention
it will be clear that the invention can be put into practice in
many other ways.
* * * * *