METHODE ET SYSTEME DE SYNCHRONISATION D'EMETTEURS-RECEPTEURS A DUPLEXAGE PAR REPARTITION DANS LE TEMPS

METHOD AND SYSTEM FOR SYNCHRONIZING TIME-DIVISION-DUPLEXED TRANSCEIVERS

Abrégé français

Description de techniques améliorées pour synchroniser les transmissions et les réceptions d'un système de transmission de données utilisant le duplexage par répartition dans le temps. Les techniques de synchronisation améliorées utilisent la nature variable dans le temps de l'énergie des données reçues pour obtenir la synchronisation. Dans une réalisation, un traitement de synchronisation amélioré (500) mesure l'énergie (502) dans n trames consécutives de données reçues. Une estimation d'erreur d'alignement est alors calculée (504) en fonction des valeurs d'énergie mesurées pour les n trames consécutives. Dans une autre réalisation, la technique de synchronisation améliorée utilise les signaux de sortie d'une unité de modulation multiporteuse (unité FFT) et offre ainsi la capacité d'éviter les tonalités de fréquence qui sont sensibles au brouillage RF. Les techniques de synchronisation améliorées peuvent également utiliser des niveaux de brouillage par diaphonie pour obtenir une synchronisation. Les récepteurs à distance dans le système de transmission de données sont en mesure de se synchroniser avec les émetteurs centraux, les récepteurs centraux dans le système de transmission de données sont en mesure de se synchroniser avec les émetteurs à distance et les émetteurs centraux sont en mesure de se synchroniser entre eux.

Abrégé anglais

Improved techniques for synchronizing transmissions and receptions of a data transmission system utilizing time division duplexing is disclosed. The improved synchronization techniques utilize the time-varying nature of the energy of the received data to obtain synchronization. In one embodiment, an improved synchronization processing (500) measures energy (502) in n consecutive frames of received data. An alignment error estimate is then computed (504) based on the measured energy values for the n consecutive frames. In another embodiment, the improved synchronization technique uses the output signals from a multicarrier modulation unit (FFT unit) and thus provides the ability to avoid frequency tones that are susceptible to RF interference. The improved synchronization techniques can also utilize crosstalk interference levels to obtain synchronization. Remote receivers in the data transmission system are able to synchronize to central transmitters, central receivers in the data transmission system are able to synchronize to remote transmitters, and central transmitters are able to synchronize with one another.

Revendications

Claims
1. A method for adjusting an alignment for a first transceiver to receive
frames of data
transmitted from a second transceiver over a transmission medium to the first
transceiver, the
first transceiver and the second transceiver being associated with a data
transmission system
providing two-way data communication using time division duplexing, said
method
comprising the steps of:
measuring an energy amount for each of a plurality of consecutive frames of
received
data;
detecting an edge in the plurality of consecutive frames of the received data
based on
the measured energy amounts, the edge detected being a burst edge, said
detecting including
at least the acts of:
computing successive energy differences in the plurality of the measured
energy
amounts;
identifying a largest one of the successive energy differences, the largest
one of the
successive energy differences corresponding to the burst edge; and
determining an alignment error estimate using the edge detected in the
plurality of
consecutive frames.
2. The method of claim 1, wherein the alignment error estimate is an estimated
alignment
error as a fraction of a frame.
3. The method of claim 1, wherein the data transmission system transmits data
using a
superframe structure having a plurality of frames, a first set of the frames
in the superframe
transmit data in a first direction, and a second set of the frames in the
superframe transmit
data in a second direction.
4. The method of claim 3, wherein the first transceiver uses a frame boundary
pointer to
identify a beginning of a frame in the superframe being received; and
30

wherein said method further comprises:
adjusting the frame boundary pointer in accordance with the alignment error
estimate.
5. The method of claim 4, wherein the alignment error estimate is an estimated
alignment
error as a fraction of a frame.
6. The method of claim 4, wherein said method further comprises:
comparing the alignment error estimate with a threshold amount;
repeating all of the above identified steps until said comparing step
indicates that the
alignment error estimate is less than the threshold amount.
7. A computer readable medium containing program instructions for adjusting an
alignment for a first transceiver to receive frames of data transmitted from a
second
transceiver over a transmission medium to the first transceiver, the first
transceiver and the
second transceiver being associated with a data transmission system providing
two-way data
communication using time division duplexing, said computer readable medium
comprising:
first computer readable code devices for measuring an energy amount for each
of a
plurality of consecutive frames of received data;
second computer readable code devices for detecting an edge in the plurality
of
consecutive frames of the received data based on the measured energy amounts,
the edge
detected being a burst edge;
third computer readable code devices for computing successive energy
differences in
the plurality of the measured energy amounts;
fourth computer readable code devices for identifying a largest one of the
successive
energy differences, the largest one of the successive energy differences
corresponding to the
burst edge; and
fifth computer readable code devices for determining an alignment error
estimate
using the edge detected in the plurality of consecutive frames.
31

8. A receiver for a data transmission system using time division duplexing to
alternate between transmission and reception of data, said receiver
comprising:
an analog-to-digital converter, said analog-to-digital converter receives
analog data
that has been transmitted over a channel to said receiver and converts the
received analog signals into received digital signals;
an input buffer for temporarily storing the received digital signals;
a multicarrier demodulation unit, said multicarrier demodulation unit
demodulates the
received digital signals from said input buffer to frequency domain data for a
plurality of different carrier frequencies;
frame synchronization means for synchronizing a frame boundary for said
multicarrier
demodulation unit based on the energy of the frequency domain data produced
by said multicarrier demodulation unit, wherein the frame synchronization
means is configured to:
measure an energy amount for each of a plurality of consecutive frames of
received data;
detect an edge in the plurality of consecutive frames of the received data
based
on the measured energy amounts, the edge detected being a burst edge,
by computing successive energy differences in the plurality of the
measured energy amounts, and identifying a largest one of the
successive energy differences, the largest one of the successive energy
difference corresponding to the burst edge; and
determine the alignment error estimate using the edge detected in the
plurality
of consecutive frames;
a bit allocation table, said allocation table stores bit allocation
information used in
transmitting data being received at said receiver;
32

a data symbol decoder, said data symbol decoder receives the frequency domain
data
and decodes bits associated with the frequency domain data from the carrier
frequencies based on the bit allocation information stored in said bit
allocation
table; and
an output buffer for storing the decoded bits as recovered data.
9. In a data transmission system having a plurality of transmitters at a
central site, the
transmitters transmit data in accordance with a superframe format including at
least one quiet
period, a method for synchronizing data transmissions by a given transmitter
to other of the
transmitters at the central site, said method comprising the steps of:
measuring the energy in the quiet period associated with the given transmitter
due to
data transmissions from the other of the transmitters at the central site;
comparing flee measured energy with a threshold amount; and
modifying the synchronization for the transmissions by the given transmitter
when
said comparing step indicates that the measured energy exceeds the threshold
amount.
10. A computer readable medium containing program instructions for
synchronizing data
transmissions in a data transmission system having a plurality of transmitters
at a central site
where an external clock signal is unavailable for synchronizing the
transmitters, the
transmitters transmit data in accordance with a superframe format including at
least one quiet
period, said computer readable medium comprising:
first computer readable code devices for measuring the energy in the quiet
period
associated with a given transmitter due to data transmissions from other of
the transmitters at
the central site;
second computer readable code devices for comparing the measured energy with a
threshold amount; and
third computer readable code devices for modifying the synchronization for the
transmissions by the given transmitter when said comparing indicates that the
measured
energy exceeds the threshold amount.
33

Description

CA 02254807 2004-12-O1
METHOD AND SYSTEM FOR SYNCHRONIZING
TIME-DIVISION-DUPLEXED TRANSCEIVERS
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data transmission systems, and more
particularly, to data
transmission systems utilizing time-division duplexing.
2. Description of the Related Art
Bi-directional digital data transmission systems are presently being developed
for
high-speed data communications. One standard for high-speed data
communications over
1 o twisted-pair phone lines that has developed is known as Asymmetric Digital
Subscriber Lines
(ADSL). Another standard for high-speed data communications over twisted-pair
phone lines
that is presently proposed is known as Very High Speed Digital Subscriber
Lines (VDSL).
The Alliance For Telecommunications Information Solutions (ATIS), which is a
group
accredited by the ANSI (American National Standard Institute) Standard Group,
has finalized a
15 discrete mufti tone based approach for the transmission of digital data
over twisted-pair lines.
The standard, known as ADSL, is intended primarily for transmitting video data
and fast Internet
access over ordinary telephone lines, although it may be used in a variety of
other applications as
well. The North American Standard is referred to as the ANSI T1.413 ADSL
Standard
(hereinafter ADSL standard). Transmission rates under the ADSL standard are
intended to
20 facilitate the transmission of information at rates of up to 8 million bits
per second (Mbits/s) over
twisted-pair phone lines. The standardized system defines the use of a
discrete mufti tone
(DMT) system that uses 256 "tones" or "sub-channels" that are 4.3125 kHz wide
in the forward

CA 02254807 1998-11-30
(downstream) direction. In the context of a phone system, the downstream
direction is
defined as transmissions from the central office (typically owned by the
telephone company)
to a remote location that may be an end-user (i.e., a residence or business
user). In other
systems, the number of tones used may be widely varied.
The ADSL standard also defines the use of reverse transmissions at a data rate
in the
range of 16 to 800 Kbit/s. The reverse transmissions follow an upstream
direction, as for
example, from the remote location to the central office. Thus, the term ADSL
comes from
the fact that the data transmission rate is substantially higher in the
downstream direction
than in the upstream direction. This is particularly useful in systems that
are intended to
to transmit video programming or video conferencing information to a remote
location over
telephone lines.
Because both downstream and upstream signals travel on the same pair of wires
(that
is, they are duplexed) they must be separated from each other in some way. The
method of
duplexing used in the ADSL standard is Frequency Division Duplexing (FDD) or
echo
15 canceling. In frequency division duplexed systems, the upstream and
downstream signals
occupy different frequency bands and are separated at the transmitters and
receivers by
filters. In echo cancel systems, the upstream and downstream signals occupy
the same
frequency bands and are separated by signal processing.
ANSI is producing another standard for subscriber line based transmission
system,
20 which is referred to as the VDSL standard. The VDSL standard is intended to
facilitate
transmission rates of at least about 6 Mbit/s and up to about 52 Mbit/s or
greater in the
downstream direction. Simultaneously, the Digital, Audio and Video Council
(DAVIC) is
working on a similar system, which is referred to as Fiber To The Curb (FTTC).
The
transmission medium from the "curb" to the customer is standard unshielded
twisted-pair
25 (UTP) telephone lines.
A number of modulation schemes have been proposed for use in the VDSL and FTTC
standards (hereinafter VDSL/FTTC). For example, some of the possible VDSL/FTTC
modulation schemes include mufti-carrier transmission schemes such as Discrete
Mufti-Tone
modulation (DMT) or Discrete Wavelet Mufti-Tone modulation (DWMT), as well as
single
30 carrier transmission schemes such as Quadrature Amplitude Modulation (QAM),
Carnerless
Amplitude and Phase modulation (CAP), Quadrature Phase Shift Keying (QPSK), or
vestigial sideband modulation.
Additionally, multicarner modulation transmission schemes have been receiving
a
large amount of attention due to the high data transmission rates they offer.
FIG. lA is a
35 simplified block diagram of a conventional transmitter 100 for a
multicarrier modulation
system. The conventional transmitter 100 is, for example, suitable for DMT
modulation in
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CA 02254807 1998-11-30
ADSL or VDSL systems. The transmitter 100 receives data signals to be
transmitted at a
buffer 102. The data signals are then supplied from the buffer 102 to a
forward error
correction (FEC) unit 104. The FEC unit 104 compensates for errors that are
due to crosstalk
noise, impulse noise, channel distortion, etc. The signals output by the FEC
unit 104 are
supplied to a data symbol encoder 106. The data symbol encoder 106 operates to
encode the
signals for a plurality of frequency tones associated with the multicarner
modulation. In
assigning the data, or bits of the data, to each of the frequency tones, the
data symbol encoder
106 utilizes data stored in a transmit bit allocation table 108 and a transmit
energy allocation
table 110. The transmit bit allocation table 108 includes an integer value for
each of the
l0 carriers (frequency tones) of the multicarner modulation. The integer value
indicates the
number of bits that are to be allocated to the particular frequency tone. The
value stored in
the transmit energy allocation table 110 is used to effectively provide
fractional number of
bits of resolution via different allocation of energy levels to the frequency
tones of the
multicarner modulation. In any case, after the data symbol encoder 106 has
encoded the data
onto each of the frequency tones, an Inverse Fast Fourier Transform (IFFT)
unit 112
modulates the frequency domain data supplied by the data symbol encoder 106
and produces
time domain signals to be transmitted. The time domain signals are then
supplied to a
digital-to-analog converter (DAC) 114 where the digital signals are converted
to analog
signals. Thereafter, the analog signals are transmitted over a channel to one
or more remote
receivers.
FIG. 1B is a simplified block diagram of a conventional remote receiver 150
for a
multicarner modulation system. The conventional remote receiver 150 is, for
example,
suitable for DMT demodulation in ADSL or VDSL systems. The remote receiver 150
receives analog signals that have been transmitted over a channel by a
transmitter. The
received analog signals are supplied to an analog-to-digital converter (ADC)
152. The ADC
152 converts the received analog signals to digital signals. The digital
signals are then
supplied to a Fast Fourier Transform (FFT) unit 154 that demodulates the
digital signals
while converting the digital signals from a time domain to a frequency domain.
The
demodulated digital signals are then supplied to a frequency domain equalizer
(FEQ) unit
156. The FEQ unit 156 performs an equalization on the digital signals so the
attenuation and
phase are equalized over the various frequency tones. Then, a data symbol
decoder 158
receives the equalized digital signals. The data symbol decoder 158 operates
to decode the
equalized digital signals to recover the data, or bits of data, transmitted on
each of the Garners
(frequency tones). In decoding the equalized digital signals, the data symbol
decoder 158
needs access to the bit allocation information and the energy allocation
information that were
used to transmit the data. Hence, the data symbol decoder 158 is coupled to a
received bit
allocation table 162 and a received energy allocation table 160 which
respectively store the
bit allocation information and the energy allocation information that were
used to transmit
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CA 02254807 1998-11-30
the data. The data obtained from each of the frequency tones is then forwarded
to the
forward error correction (FEC) unit 164. The FEC unit 164 performs error
correction of the
data to produce corrected data. The corrected data is then stored in a buffer
166. Thereafter,
the data may be retrieved from the buffer 166 and further processed by the
receiver 150.
Alternatively, the received energy allocation table 160 could be supplied to
and utilized by
the FEQ unit 166.
The bit allocation tables and the energy allocation tables utilized in the
conventional
transmitter 100 can be implemented as a single table or as individual tables.
Likewise, the bit
allocation tables and the energy allocation tables utilized in the remote
receiver 150 can be
1o implemented as a single table or as individual tables. Also, the
transmitter 100 is normally
controlled by a controller, and the remote receiver 150 is normally controlled
by a controller.
Typically, the controllers are programmable controllers.
The transmitter 100 and the remote receiver 150 illustrated in FIGS. lA and
1B,
respectively, optionally include other components. For example, the
transmitter 100 could
15 add a cyclic prefix to symbols after the IFFT unit 112, and the remote
receiver 150 can then
remove the cyclic prefix before the FFT unit 154. Also, the remote receiver
150 can provide
a time domain equalizer (TEQ) unit between the ADC 152 and the FFT unit 154.
Most of the proposed VDSL/FTTC transmission schemes utilize frequency division
duplexing (FDD) of the upstream and downstream signals. On the other hand, one
particular
20 proposed VDSL/FTTC transmission scheme uses time division duplexing (TDD)
of the
upstream and downstream signals. More particularly, the time division
duplexing is
synchronized in this case'such that periodic synchronized upstream and
downstream
communication periods do not overlap with one another. That is, the upstream
and
downstream communication periods for all of the wires that share a binder are
synchronized.
25 With this arrangement, all the very high speed transmissions within the
same binder are
synchronized and time division duplexed such that downstream communications
are not
transmitted at times that overlap with the transmission of upstream
communications. This is
also referred to as a (i.e. "ping pong") based data transmission scheme. Quiet
periods, during
which no data is transmitted in either direction, separate the upstream and
downstream
30 communication periods. When the synchronized time division duplexed
approach is used
with DMT it is often referred to as synchronized DMT (SDMT).
A common feature of the above-mentioned transmission systems is that twisted-
pair
phone lines are used as at least a part of the transmission medium that
connects a central
office (e.g., telephone company) to users (e.g., residence or business). Even
though fiber
35 optics may be available from a central office to the curb near a user's
residence, twisted-pair
phone lines are used to bring in the signals from the curb into the user's
home or business.
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The twisted-pair phone lines are grouped in a binder. While the twisted-pair
phone
lines are within the binder, the binder provides reasonably good protection
against external
electromagnetic interference. However, within the binder, the twisted-pair
phone lines
induce electromagnetic interference on each other. This type of
electromagnetic interference
is generally known as crosstalk interference which includes near-end crosstalk
(NEXT)
interference and far-end crosstalk (FEXT) interference. As the frequency of
transmission
increases, the crosstalk interference (NEXT interference) becomes substantial.
As a result,
the data signals being transmitted over the twisted-pair phone lines at high
speeds can be
significantly degraded by the crosstalk interference caused by other twisted-
pair phone lines
1o in the binder. As the speed of the data transmission increases, the problem
worsens. The
advantage of the synchronized TDD (such as SDMT) based data transmission is
that
crosstalk interference (NEXT interference) from other lines in a binder is
essentially
eliminated, provided all the lines transmit for the same duration (i.e., same
superframe
format).
A data transmission system normally includes a central office and a plurality
of
remote units. Each remote unit communicates with the central office over a
data link (i.e.,
channel) that is established between the central office and the particular
remote unit. To
establish such a data link, initialization processing is performed to
initialize communications
between the central office and each of the remote units. For purposes of the
discussion to
follow, a central office includes a central modem (or central unit) and a
remote unit includes
a remote modem. These modems are transceivers that facilitate data
transmission between
the central office and the remote unit. The central office thus normally
includes a plurality of
central side transceivers, each of which has a central side transmitter and a
central side
receiver, and the remote unit normally includes a remote side transceiver
having a remote
side transmitter and a remote side receiver.
One conventional frame synchronization technique required the transmission of
a
predetermined sequence of data which was received by a receiver and then
correlated with a
predetermined stored sequence of data to determine the adjustment required in
order to yield
synchronization. U.S. Patent No. 5,627,863 describes a frame synchronization
approach
suitable for systems (e.g., ADSL) using frequency division duplexing (FDD) or
echo
cancelling to provide duplexed operation. This frame synchronization technique
requires a
special start-up training sequence to obtain the frame synchronization.
However, the
described frame synchronization approach is not suitable for systems (e.g.,
synchronized
TDD or SDMT) using time division duplexing because synchronization in time is
not
necessary for FDD or echo cancelling as it is with TDD in order to reduce
crosstalk.
When a data transmission system is operating in a time-division duplexed (TDD)
manner, the transmitters and receivers of the central office and remote units
must be
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CA 02254807 1998-11-30
synchronized in time so that transmission and reception do not overlap in
time. In a data
transmission system, downstream transmissions are from a central side
transmitter to one or
more remote side receivers and upstream transmissions are from one or more
remote side
transmitters to a central side receiver. The central side transmitter and
receiver can be
combined as a central side transceiver, and the remote side transmitter and
receiver can be
combined as a remote side transceiver.
Generally speaking, in a time division duplexed system, upstream signals are
alternated with downstream signals. Typically, the upstream transmissions and
the
downstream transmissions are separated by a guard interval or a quiet period.
The guard
interval is provided to enable the transmission system to reverse the
direction in which data is
being transmitted so that a transmission can be received before the
transmission in the
opposite direction occurs. Some transmission schemes divide upstream and
downstream
transmissions into smaller units referred to as frames. These frames may also
be grouped
into superframes that include a series of downstream frames and a series of
upstream frames,
as well as guard intervals between the two.
Time-division duplexing is a simple method to share a channel (medium) between
two or more transceivers. Each transceiver is assigned a time slot during
which it may
transmit, and there are quiet periods (guard intervals) during which no unit
must transmit.
On channels subject to crosstalk (NEXT interference) between multiple
connections, if time-
division duplexing is used, synchronization must be established and maintained
among all
units so affected. An example is the VDSL service that uses the existing
twisted pair
telephone loop plant to transport up to 13-52 Mb/s on loops up to 1.5 km.
Pairs destined for
subscribers are bundled together in a cable consisting of 25-100 pairs. The
proximity and the
high frequency use (0.2-11 MHz signal bandwidth) leads to significant
crosstalk between
adjacent pairs in a binder. To get the desired data rate on loops up to 1.5 km
long, DMT is a
suitable multicarrier modulation scheme. This scheme makes excellent use of
time-division
duplexing since a single FFT unit can be used during transmission and
reception and avoids
the need for two such FFT units, and other savings in the analog circuitry.
Conventional frame synchronization techniques are not only not well suited for
synchronized TDD but also are unreliable when RF interference is present. Due
to the
potential for significant RF interference due to amateur radio frequency
bands, the RF
interference might have a signal power equal to, or perhaps greater than, the
desired receive
signal power under some conditions. However, in a synchronized TDD system, it
is
important that synchronization be established and maintained so that crosstalk
is mitigated
and controlled and/or received data is accurately recovered.
Accordingly, there is a need for improved synchronization techniques for time-
division duplexed systems.
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SUMMARY OF THE INVENTION
Broadly speaking, the invention relates to improved techniques for
synchronizing
transmissions and receptions of a data transmission system utilizing time
division duplexing.
According to one aspect of the invention, the improved synchronization
techniques utilize the
time-varying nature of the energy of the received data to obtain
synchronization. In one
embodiment, the improved synchronization technique uses the output signals
from a
multicarner modulation unit (e.g., FFT unit) and thus provides the ability to
avoid frequency
tones that are susceptible to RF interference. According to another aspect of
the invention,
the improved synchronization techniques utilize crosstalk interference levels
to obtain
synchronization. With the improved synchronization techniques, remote
receivers in the data
transmission system are able to synchronize to central transmitters, central
receivers in the
data transmission system are able to synchronize to remote transmitters, and
central
transmitters are able to synchronize with one another.
The invention can be implemented in numerous ways, including as an apparatus,
system, method, or computer readable media. Several embodiments of the
invention are
discussed below.
As a method for adjusting an alignment for a first transceiver to receive
frames of data
transmitted from a second transceiver over a transmission medium to the first
transceiver,
where the first transceiver and the second transceiver are associated with a
data transmission
system providing two-way data communication using time division duplexing, an
embodiment of the invention includes the operations of: measuring an energy
amount for
each of a plurality of consecutive frames of received data; detecting an edge
in the plurality
of consecutive frames of the received data based on the measured energy
amounts; and
computing an alignment error estimate using the edge detected in the plurality
of consecutive
frames. Additionally, the synchronization may thereafter be adjusted in
accordance with the
alignment error estimate. Optionally, the data transmission system transmits
data using a
superframe structure having a plurality of frames, a first set of the frames
in the superframe
transmit data in a first direction, and a second set of the frames in the
superframe transmit
data in a second direction.
As a computer readable medium containing program instructions for adjusting an
alignment for a first transceiver to receive frames of data transmitted from a
second transceiver
over a transmission medium to the first transceiver, the first transceiver and
the second
transceiver being associated with a data transmission system providing two-way
data
communication using time division duplexing, an embodiment of the invention
includes: first
computer readable code devices for measuring an energy amount for each of a
plurality of
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CA 02254807 1998-11-30
consecutive frames of received data; second computer readable code devices for
detecting an
edge in the plurality of consecutive frames of the received data based on the
measured energy
amounts; and third computer readable code devices for computing an alignment
error estimate
using the edge detected in the plurality of consecutive frames.
As a receiver for a data transmission system using time division duplexing to
alternate
between transmission and reception of data, an embodiment of the invention
includes: an
analog-to-digital converter, the analog-to-digital converter receives analog
data that has been
transmitted over a channel to the receiver and converts the received analog
signals into
received digital signals; an input buffer for temporarily storing the received
digital signals; a
multicarner demodulation unit, the multicarner demodulation unit demodulates
the received
digital signals from the input buffer to frequency domain data for a plurality
of different
carrier frequencies; a frame synchronization unit, the frame synchronization
unit
synchronizes a receive frame boundary for the multicarner demodulation unit
based on the
time-varying nature of the energy of the frequency domain data produced by the
multicarner
demodulation unit; a bit allocation table, the allocation table stores bit
allocation information
used in transmitting data being received at the receiver; a data symbol
decoder, the data
symbol decoder receives the frequency domain data and decodes bits associated
with the
frequency domain data from the carrier frequencies based on the bit allocation
information
stored in the bit allocation table; and an output buffer for storing the
decoded bits as
recovered data. Preferably, the data transmission system is a synchronized DMT
system, and
wherein the multicarrier demodulation unit includes a FFT unit.
As a receiver for a data transmission system using time division duplexing to
alternate
between transmission and reception of data, another embodiment of the
invention includes:
an analog-to-digital converter, the analog-to-digital converter receives
analog data that has
been transmitted over a channel to the receiver and converts the received
analog signals into
received digital signals; an input buffer for temporarily storing the received
digital signals; a
multicarrier demodulation unit, the multicarrier demodulation unit demodulates
the received
digital signals from the input buffer to frequency domain data for a plurality
of different
earner frequencies; frame synchronization means for synchronizing a receive
frame boundary
3o for the multicarrier demodulation unit based on the time-varying nature of
the energy of the
frequency domain data produced by the multicarner demodulation unit; a bit
allocation table,
the allocation table stores bit allocation information used in transmitting
data being received
at the receiver; a data symbol decoder, the data symbol decoder receives the
frequency
domain data and decodes bits;associated with the frequency domain data from
the carrier
frequencies based on the bit allocation information stored in the bit
allocation table; and an
output buffer for storing the decoded bits as recovered data.
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For a data transmission system having a plurality of transmitters at a central
site
where an external clock signal is unavailable for synchronizing the
transmitters, the
transmitters transmit data in accordance with a superframe format including at
least one quiet
period, a method for synchronizing data transmissions by a given transmitter
to other of the
transmitters at the central site according to an embodiment of the invention
includes the acts
of: measuring the energy in the quiet period associated with the given
transmitter due to data
transmissions from the other of the transmitters at the central site;
comparing the measured
energy with a threshold amount; and modifying the synchronization for the
transmissions by
the given transmitter when the comparing indicates that the measured energy
exceeds the
threshold amount.
As a computer readable medium containing program instructions for
synchronizing
data transmissions in a data transmission system having a plurality of
transmitters at a central
site where an external clock signal is unavailable for synchronizing the
transmitters, the
transmitters transmit data in accordance with a superframe format including at
least one quiet
period, an embodiment of the invention includes: first computer readable code
devices for
measuring the energy in the quiet period associated with a given transmitter
due to data
transmissions from other of the transmitters at the central site; second
computer readable
code devices for comparing the measured energy with a threshold amount; and
third
computer readable code devices for modifying the synchronization for the
transmissions by
2o the given transmitter when the comparing indicates that the measured energy
exceeds the
threshold amount.
The advantages of the invention are numerous. One advantage of the invention
is that
synchronization can be achieved even in the presence of radio frequency (RF)
interference,
such as due to amateur radio users. Another advantage of the invention is that
it is well
suited for data transmission systems utilizing time division duplexing such as
synchronized
DMT or synchronized VDSL. Yet another advantage of the invention is that it is
relatively
insensitive to noise in the data transmission system.
Other aspects and advantages of the invention will become apparent from the
following detailed description, taken in conjunction with the accompanying
drawings,
illustrating by way of example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be readily understood by the following detailed
description
in conjunction with the accompanying drawings, wherein like reference numerals
designate
like structural elements, and in which:
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CA 02254807 1998-11-30
FIG. lA is a simplified block diagram of a conventional transmitter for a
multicarner
modulation system;
FIG. 1B is a simplified block diagram of a conventional remote receiver for a
conventional multicarner modulation system;
FIG. 2 is a block diagram of an exemplary telecommunications network suitable
for
implementing the invention;
FIG. 3 is a block diagram of a processing and distribution unit 300 according
to an
embodiment of the invention;
FIG. 4 is a diagram illustrating an exemplary superframe format in which a
certain
to level of service is provided;
FIG. SA is a flow diagram of synchronization processing according to a basic
embodiment of the invention;
FIG. SB is a flow diagram of synchronization processing according to an
embodiment
of the invention;
FIGs. 6A and 6B are flow diagrams of synchronization processing according to a
more detailed embodiment of the invention;
FIG. 7 is a flow diagram of edge detection processing according to an
embodiment of
the invention;
FIG. 8 is a flow diagram of alignment error estimation processing according to
an
embodiment of the invention;
FIGs. 9A and 9B represent diagrams of energy values and energy difference
values
for received data over a sequence of twenty frames;
FIGS. l0A and l OB represent energy values and energy difference values for
received
data over a series of twenty frames for the example illustrated in FIGs. 9A
and 9B after an
alignment adjustment has been made in accordance with the invention;
FIG. 11 is a block diagram of a receiver according to one embodiment of the
invention; and
FIG. 12 is a flow diagram of a synchronization processing for synchronizing
adjacent
transmitters to compensate for small synchronization differences.
DETAILED DESCRIPTION OF THE INVENTION
The invention relates to improved techniques for synchronizing transmissions
and
receptions by a data transmission system utilizing time division duplexing. In
one aspect of
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CA 02254807 1998-11-30
the invention, the improved synchronization techniques utilize the time-
varying nature of the
energy of the received data to obtain synchronization. In another aspect of
the invention, the
improved synchronization techniques utilize crosstalk interference levels to
obtain
synchronization. With the improved synchronization techniques, remote
receivers in the data
transmission system are able to synchronize to central transmitters, central
receivers in the
data transmission system are able to synchronize to remote transmitters, and
central
transmitters are able to synchronize with one another.
The synchronization required in a time-division duplex system requires that
transmissions be synchronized with a superframe structure. Conventional time-
domain
methods which tend to correlate samples, such as first and last samples in a
frame to detect a
cyclic prefix, are not reliable because of the likely presence of RF
interference in the receive
signals which can be of equal power to be desired signals. However, the
invention provides
accurate techniques to synchronize transmissions in a time-division duplex
system even when
RF interference renders the time domain signal unreliable. The frequency
domain approach
to synchronization provided by the invention is able to obtain significant
immunity from RF
interference. In one embodiment, the improved synchronization technique
preferably uses
the output signals from a multicarrier modulation unit (FFT unit) and thus
provides the
ability to avoid frequency tones that are susceptible to radio frequency (RF)
interference.
Embodiments of the invention are discussed below with reference to FIGS. lA-
12.
However, those skilled in the art will readily appreciate that the detailed
description given
herein with respect to these figures is for explanatory purposes as the
invention extends
beyond these limited embodiments.
FIG. 2 is a block diagram of an exemplary telecommunications network 200
suitable
for implementing the invention. The telecommunications network 200 includes a
central
office 202. The central office 202 services a plurality of distribution posts
to provide data
transmission to and from the central office 202 to various remote units. In
this exemplary
embodiment, each of the distribution posts is a processing and distribution
unit 204 (node).
The processing and distribution unit 204 is coupled to the central office 202
by a high speed,
multiplexed transmission line 206 that may take the form of a fiber optic
line. Typically,
when the transmission line 206 is a fiber optic line, the processing and
distribution unit 204 is
referred to as an optical network unit (ONLI). The central office 202 also
usually interacts
with and couples to other processing and distribution units (not shown)
through high speed,
multiplexed transmission lines 208 and 210, but only the operation of the
processing and
distribution unit 204 is discussed~below. In one embodiment, the processing
and distribution
unit 204 includes one or more modems (central modems).
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CA 02254807 1998-11-30
The processing and distribution unit 204 services a multiplicity of discrete
subscriber
lines 212-1 through 212-n. Each subscriber line 212 typically services a
single end user. The
end user has a remote unit suitable for communicating with the processing and
distribution
unit 204 at very high data rates. More particularly, a remote unit 214 of a
first end user 216
is coupled to the processing and distribution unit 204 by the subscriber line
212-1, and a
remote unit 218 of a second end user 220 is coupled to the processing and
distribution unit
204 by the subscriber line 212-n. The remote units 214 and 218 include a data
communications system capable of transmitting data to and receiving data from
the
processing and distribution unit 204. In one embodiment, the data
communication systems
are modems. The remote units 214 and 218 can be incorporated within a variety
of different
devices, including for example, a telephone, a television, a monitor, a
computer, a
conferencing unit, etc. Although FIG. 2 illustrates only a single remote unit
coupled to a
respective subscriber line, it should be recognized that a plurality of remote
units can be
coupled to a single subscriber line. Moreover, although FIG. 2 illustrates the
processing and
distribution unit 204 as being centralized processing, it should be recognized
that the
processing need not be centralized and could be performed independently for
each of the
subscriber lines 212.
The subscriber lines 212 serviced by the processing and distribution unit 204
are
bundled in a shielded binder 222 as the subscriber lines 212 leave the
processing and
distribution unit 204. The shielding provided by the shielded binder 222
generally serves as
a good insulator against the emission (egress) and reception (ingress) of
electromagnetic
interference. However, the last segment of these subscriber lines, commonly
referred to as a
"drop" branches off from the shielded binder 222 and is coupled directly or
indirectly to the
end user's remote units. The "drop" portion of the subscriber line between the
respective
remote unit and the shielded binder 222 is normally an unshielded, twisted-
pair wire. In most
applications the length of the drop is not more than about 30 meters.
Crosstalk interference, including near-end crosstalk (NEXT) and far-end
crosstalk
(FEXT), primarily occurs in the shielded binder 222 where the subscriber lines
212 are
tightly bundled. Hence, when data is transmitted on some of the subscriber
lines 212 while
other subscriber lines are receiving data as is common when multiple levels of
service are
being provided, the crosstalk inference induced becomes a substantial
impairment to proper
reception of data. Hence, to overcome this problem, data is transmitted using
a superframe
structure over which bits of data to be transmitted are allocated. The
telecommunications
network 200, for example, is particularly well suited for a synchronized TDD
transmission
system (e.g., synchronized VDSL or SDMT) offering different levels of service.
Hence, referring to the SDMT transmission system shown in FIG. 2, data
transmissions over all lines 212 in the shielded binder 222 associated with
the processing and
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CA 02254807 1998-11-30
distribution unit 204 need to be synchronized. As such, all active lines
emanating from the
processing and distribution unit 204 could be transmitting in the same
direction (i.e.,
downstream or upstream) so as to substantially eliminate NEXT interference.
FIG. 3 is a block diagram of a processing and distribution unit 300 according
to an
embodiment of the invention. For example, the data processing and distribution
unit 300 is a
detailed implementation of the processing and distribution unit 204
illustrated in FIG. 2.
The data processing and distribution unit 300 includes a processing unit 302
that
receives data and sends data over a data link 304. The data link 304 could,
for example, be
coupled to a fiber optic cable of a telephone network or a cable network. The
processing unit
302 needs to operate to synchronize the various processed transmissions and
receptions of the
processing unit 302. The data processing and distribution unit 300 further
includes a bus
arrangement 308 and a plurality of analog cards 310. The output of the
processing unit 302 is
coupled to the bus arrangement 308. The bus arrangement 308 together with the
processing
unit 302 thus direct output data from the processing unit 302 to the
appropriate analog cards
310 as well as direct input from the analog cards 310 to the processing unit
302. The analog
cards 310 provide analog circuitry utilized by the processing and distribution
unit 300 that is
typically more efficiently performed with analog components than using digital
processing
by the processing unit 302. For example, the analog circuitry can include
filters,
transformers, analog-to-digital converters or digital-to-analog converters.
Each of the analog
2o cards 310 are coupled to a different line. Typically, all the lines for a
given data transmission
system 300 are bundled into a binder including about fifty (50) lines (LINE-1
through LINE-
SO). Hence, in such an embodiment, there are fifty (50) analog cards 310
respectively
coupled to the fifty (50) lines. In one embodiment, the lines are twisted-pair
wires. The
processing unit 302 may be a general-purpose computing device such as a
digital signal
processor (DSP) or a dedicated special purpose device. The bus arrangement 308
may take
many arrangements and forms. The analog cards 310 need not be designed for
individual
lines, but could instead be a single card or circuitry that supports multiple
lines.
In a case where the processing is not centralized, the processing unit 302 in
FIG. 3
can be replaced by modems for each of the lines. The processing for each of
the lines can
3o then be performed independently for each of the lines. In this case, the
modem may be
placed on a single card along with the analog circuitry.
The NEXT interference problem occurs on the lines proximate to the output of
the
processing and distribution unit 300. With respect to the block diagram
illustrated in FIG. 3,
the NEXT interference is most prevalent near the outputs of the analog cards
310 because this
is where the lines are closest to one another and have their largest power
differential (between
transmitted and received signals). In other words, from the output of the
processing and
distribution unit 300 the lines travel towards the remote units. Usually, most
of the distance
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CA 02254807 1998-11-30
is within a shielded binder that would, for example, hold fifty (50) twisted-
pair wires, and the
remaining distance is over single unshielded twisted-pair wires. Because all
these lines (e.g.,
twisted-pair wires) are held in close proximity in the binder and individually
offer little
shielding against electromagnetic coupling from other of the lines in the
binder, crosstalk
interference (namely NEXT interference and FEXT interference) between the
lines within the
binder is problematic.
Depending on the level of service being provided, data transmission
implemented
with SDMT can be symmetric or asymmetric with respect to upstream and
downstream
transmissions. With symmetric transmission, DMT symbols tend to be transmitted
in
alternating directions for equal durations. In other words, the duration in
which DMT
symbols are transmitted downstream is the same as the duration in which DMT
symbols are
transmitted upstream. With asymmetric transmission, DMT symbols tend to be
transmitted
downstream for a longer duration than upstream.
In VDSL it has been proposed to have a superframe structure including a fixed
number (e.g., 20) frames, with each frame being associated with a DMT symbol.
With such
a frame format, the number of frames being used for downstream transmissions
and the
number of frames being used for upstream transmissions can vary. As a result,
there are
several different superframe formats that can occur. Typically, a superframe
consists of a
downstream burst of several frames and an upstream burst of several frames.
Quiet frames
are inserted between the upstream and the downstream bursts to allow the
channel to settle
before the direction of transmission is changed.
FIG. 4 is a diagram illustrating an exemplary superframe format 400 in which a
certain level of service is provided. The superframe format 400 is an
asymmetric frame that
includes a downstream portion 402, a quiet portion 404, an upstream portion
406, and a quiet
portion 408. The quiet portions (quiet periods) 404 and 408 are positioned
between the
downstream and upstream transmissions. With this asyrrimetric superframe
format 400, the
downstream portion 402 is substantially larger (e.g., longer burst) than the
upstream portion
406. Such a superframe format is useful for situations in which downstream
traffic is
significantly greater than the upstream traffic. As an example, with respect
to FIG. 2, the
3o superframe format 400 can include 16 symbols downstream; 1 quiet period; 2
symbols
upstream; and 1 quiet period.
With proper synchronization at a central unit (processing and distribution
unit 204 or
processing unit 302) and uniform superframe formats, synchronized
transmissions of equal
duration are provided for all lines within a binder. Accordingly, the NEXT
interference
problem is effectively eliminated. The synchronization of the central unit and
the remote
units is also important for accurate data recovery. These synchronizations are
needed in
TI-27737 - 14

CA 02254807 1998-11-30
synchronized VDSL and SDMT systems. According to the invention, improved
synchronization techniques are described below with respect to FIGS. 5-12.
FIG. SA is a flow diagram of synchronization processing 500 according to a
basic
embodiment of the invention. Initially, the synchronization processing 500
measures 502
energy in n consecutive frames of received data. An alignment error estimate
is then
computed 504 based on the measured energy values for the n consecutive frames.
Following
block 504, the synchronization processing 500 is complete and ends.
FIG. SB is a flow diagram of synchronization processing 550 according to an
embodiment of the invention. Initially, the synchronization processing 550
measures 552
energy in n consecutive frames of received data. Next, an edge is detected 554
in the
received data based on the measured energy values for the n consecutive
frames. An
alignment error estimate is then computed 556 from the position of the edge
that has been
detected. Thereafter, the synchronization processing 550 is able to adjust 558
its
synchronization reference in accordance with the alignment error estimate.
Following block
558, the synchronization processing 550 is complete and ends.
By determining and adjusting synchronization of receivers of the remote units
to
transmissions from a central unit in accordance with the synchronization
processing 500 or
550, the remote units are able to establish synchronization with the central
unit. Once
synchronized the central unit and the remote units are able to share a channel
(transmission
line) in a time-division duplexed manner. Also, the synchronization processing
500 or 550 is
suitable for determining and adjusting synchronization of receivers at the
central unit with
transmissions from the remote units.
FIGS. 6A and 6B are flow diagrams of synchronization processing 600 according
to a
more detailed embodiment of the invention. Once the synchronization processing
600 is
initiated, FFT outputs are obtained 602 for n consecutive frames of received
data. Typically,
a receiver side of a transceiver will receive data from a transmission line
and forward the
received data to an analog-to-digital converter and then to a FFT unit, such
as illustrated in
FIG. 1B. Hence, the FFT outputs may be obtained from the output of the FFT
unit. The FFT
outputs are frequency domain signals.
Next, the FFT outputs that are susceptible to RF interference are dropped 604.
The
remaining FFT outputs are then used for subsequent processing. Typically, a
frame includes
a plurality of different frequency tones. Each of the frequency tones is
capable of having data
encoded thereon for transmission. However, certain of the frequency tones are
more
susceptible to RF interference than others. In the case where the RF
interference is caused by
amateur radio users, it is usually known which frequency tones of the frame
are likely
subjected to the RF interference due to amateur radio users. In the case of a
remote unit of a
TI-27737 - 15

CA 02254807 1998-11-30
synchronized multicarrier VDSL system, where a frame has 256 frequency tones,
frequency
tones 6 through 40 are generally free from RF interference due to amateur
radio users, less
attenuated because lower frequency tones have less attenuation, and therefore
sufficient to
obtain a reliable synchronization result. Hence, in one embodiment, frequency
tones 6
through 40 from each of n consecutive frames are used for subsequent
processing.
Next, energy values for the n consecutive frames of the remaining FFT outputs
are
determined 606. As an example, if the frequency tones 6 through 40 are being
utilized, then
the corresponding outputs from the FFT unit are obtained and then converted to
energy
values and summed together so as to produce a single energy value for the
frame.
Preferably, the energy values are power values for the frames. As an example,
the single
energy value for a frame can be obtained by summing the squared moduli of all
outputs of
the FFT unit that are in use. Alternatively, the energy values could be
obtained by summing
the energies of time domain samples, after having filtered out those time
domain samples that
are subjected to substantial amounts of RF interference.
Once the energy values for the n consecutive frames have been determined 606,
the
synchronization processing 600 detects 608 a burst edge within the received
data based on
the determined energy values. By detecting the burst edge, a receiver is able
to identify when
the received data burst from a transmitter begins. The burst edge thus
identifies the
beginning (or end) of a received transmission from the transmitter and
additionally identifies
2o a synchronization for the frame. A trailing edge within the received data
and/or
characteristics of the superframe (superframe information) can also be
detected.
Next, an alignment error estimate for a frame boundary setting is determined
610 with
respect to the detected burst edge. Here, using the burst edge that has been
detected 608, the
alignment error estimate can be determined 610 for a frame boundary setting.
In particular,
from the determined energy values in the burst edge, the remote unit
synchronization
processing 600 is able to determine the alignment error for a frame (i.e.,
error in frame
synchronization). Typically, the alignment error is estimated as a fraction of
a frame.
Thereafter, the frame boundary can be adjusted 612 in accordance with the
alignment error
estimate.
Once adjusted 612, the frame synchronization should be established. However,
preferably, the synchronization processing 600 continues to confirm that the
synchronization
has been achieved. Specifically, following block 612, a decision block 614
determines
whether the absolute value of the alignment error estimate is less than a
predetermined
threshold. If the alignment error estimate is not less than a predetermined
threshold, then the
synchronization processing 600 returns to repeat block 602 and subsequent
blocks so as to
iteratively reduce the magnitude of the error. On the other hand, when the
decision block 614
determines that the alignment error estimate is less than the predetermined
threshold, then the
TI-27737 - 16

CA 02254807 1998-11-30
superframe information is output 616. As an example, the superframe
information can
indicate the beginning of the received transmission and the end of the
received transmission
and/or the number of frames in the burst. Following block 616, the
synchronization
processing 600 is complete and ends.
Normally, when the frame synchronization is adjusted 612 by a significant
amount,
the alignment error estimate is greater then the predetermined threshold.
Hence, the
synchronization processing 600 will repeat and should produce a small
alignment error
amount which is less than the predetermined threshold. Then, the
synchronization processing
600 is able to proceed to block 616. Alternatively, the decision block 614 can
be eliminated
l0 when the alignment error estimate is produced accurately with a high degree
of confidence.
FIG. 7 is a flow diagram of edge detection processing 700 according to an
embodiment of the invention. The edge detection processing 700 describes
additional details
on the block 608 in FIG. 6A where the burst edge is detected. The edge
detection processing
700 initially computes 702 successive energy differences for the n determined
energy values.
These successive energy differences may be indexed from 1 to i. Next, the
largest energy
difference and its index (j) are determined 704. The energy differences at
indices (j-1) and
(j+1) are then stored 706 for later retrieval. Following block 706, the edge
detection
processing 700 is complete and the processing returns to block 610 of the
synchronization
processing 600.
FIG. 8 is a flow diagram of alignment error estimation processing 800
according to an
embodiment of the invention. The alignment error estimation processing 800
describes
additional details on the block 610 in FIG. 6A where an alignment error
estimate is
determined. The alignment error estimation processing 800 initially determines
802 a
difference amount from the energy values at indices (j+1) and (j-1). The
energy values at
indices (j+1) and (j-1) are the energy values immediately proceeding and
immediately
following the largest energy difference at index (j). The energy values can,
for example, be
power values. Next, the difference amount is normalized 804 to produce the
alignment error
estimate. In this embodiment, the alignment error estimate represents a
fractional part of a
frame. Accordingly, the synchronization of the receiver to the data
transmission unit would
be off by this fractional part of the frame. Following block 804, the
alignment error
estimation processing 800 is complete and the processing returns to block 612
of the
synchronization processing 600.
FIGs. 9A and 9B represent diagrams of energy values (e) and energy difference
values (De) for received data over a sequence of twenty frames. In FIG. 9A, a
diagram 900
plots the energy values (e) for the twenty frames shows a burst of data in the
vicinity of
frames 6 through 15. As an example, the energy values (e) are produced by
block 606 in
FIG. 6A. In FIG. 9B, a diagram 902 plots successive energy difference values
(De) for the
TI-27737 - 17

CA 02254807 1998-11-30
determined energy values. The successive energy difference values (0e)
identify regions
associated with edges or transition points in the received data. A first edge
represents an
initial edge or the start of a burst of data and is somewhere within a region
904, and a second
edge 906 represents a trailing edge, or an end of a burst of data, and is
somewhere within a
region 906. As an example, the energy difference values (0e) are determined by
block 702 in
FIG. 7.
As seen in FIGS. 9A and 9B, the receiver is not properly synchronized with the
incoming transmitted data from a remotely located transmitter. In particular,
the beginning
of the burst of data received from the transmitter begins somewhere within
frame 6. To be
properly synchronized, the burst of data from the transmitter would begin
exactly at the
beginning of frame 6 in this example. By using the energy difference values
(De), the
technique achieves substantial immunity to noise levels on the received data.
The diagram
902 shows that the initial edge of the burst of data is within the region 904,
i.e., somewhere
within frame 6, and that the trailing edge of the burst of data is within the
region 906, i.e.,
somewhere within frame 14.
FIGS. l0A and l OB represent energy values (e) and energy difference values
(0e) for
received data over a series of twenty frames for the example illustrated in
FIGs. 9A and 9B
after an alignment adjustment has been made in accordance with the invention,
that is, with
proper synchronization. In FIG. l0A a diagram 1000 indicates a burst of data
between
2o frames 6 and 14 with an initial edge 1002 at the beginning of frame 6 and a
trailing edge
1004 at the end of frame 14. In FIG. l OB, a diagram 1006 illustrates the
successive energy
difference values (0e) over the twenty frames, including an initial maximum
point 1008 and
a trailing maximum point 1010. The initial edge (frame 6) of the burst of data
indicates the
starting frame of the received burst of data, while the negative edge (frame
15) indicates the
frame following the end of the burst of data. From this information, the
received burst length
can be inferred (9 frames), and the superframe format can be identified (9-1-9-
1).
During synchronization the successive differences in the energy values
observed in
each frame of the superframe will show a positive and negative peak. The
positive peak
indicates the leading edge of a burst, while the negative edge indicates the
end of a burst.
According to one embodiment of the invention, the edge detection processing
adjusts the
frame alignment so that the maximum difference is increased, the right-hand
neighboring
energy difference is forced to zero. When synchronization has been obtained,
the result is as
shown in FIG. l OB. Note that tie edge detection processing is relatively
insensitive to the
absolute amplitudes being obseived. The successive differences approach
requires only that
the energy in the "quiet" frames (which are not truly quiet due to noise) be
smaller than the
energy in the active frames and that the energy be approximately constant for
each type of
frame.
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CA 02254807 1998-11-30
When the data transmission system operates to delete a cyclic prefix at the
receiver, a
dead-zone may be created in the frame/superframe alignment that is the width
of the cyclic
prefix because the removal of the cyclic prefix drops samples useful for frame
synchronization but thus unavailable from the FFT unit. One technique to
resolve this dead-
zone where the frame has 512 samples and the cyclic prefix has 40 samples is
to use energy
estimates from samples 41 through 552 as well as using samples 1 to 512, and
then take the
mean of these energy estimates to get a combined energy estimate which is then
used in the
burst detection processing.
The synchronization processing discussed above is generally applicable to
remote
side and central side synchronization. For synchronization processing at a
remote unit, a
receiver at the remote unit acquires and maintains synchronization with data
transmissions
(bursts) with a transmitter of the central unit. As for synchronization
processing at a central
unit, a receiver at the central unit acquires and maintains synchronization
with data
transmissions (bursts) with a transmitter of a remote unit. In one embodiment,
the
synchronization is managed by setting or adjusting receive frame alignment for
the recovery
of data transmissions (bursts) at a receiver.
Due to the round-trip delay of a line (or channel), the time at which an
upstream
transmission from a remote unit reaches a central unit will vary and will
appear to be late by
the by the length of the round-trip delay if no correction is made.
Accordingly, the central
unit needs to adjust its receive frame alignment so that the correct receive
samples are used in
the receiver at the central unit. The processing carned out at the central
unit to adjust its
receive frame alignment is similar to the synchronization processing discussed
above for the
remote unit. Generally, the energy in upstream frames being received is
measured over a
number of frames corresponding to the length of the upstream transmission
burst from the
remote unit. These energy values are used to identify the start of the
upstream transmission
burst and then determines an alignment correction to align the receive frame
boundary
pointer with the frames of data received from the remote unit.
FIG. 11 is a block diagram of a receiver 1100 according to one embodiment of
the
invention. The receiver 1100 is part of a time domain duplexed transmission
system. The
construction of the receiver 1100 illustrated in FIG. 11 may be used in either
or both of the
central office transceiver and the remote unit transceiver.
The receiver 1100 receives analog signals 1102 that have been transmitted over
a
channel from a transmitter (e.g.,;a central office transmitter). The received
analog signals are
then supplied to an analog-to-digital (ADC) 1104 which converts the received
analog signals
to digital signals. The digital signals are then supplied to an input buffer
1106 that
temporarily stores the digital signals. The FFT unit 1108 retrieves a frame of
data from the
TI-27737 - 19

CA 02254807 1998-11-30
input buffer 1106 in accordance with a receive frame boundary pointer 1110,
and then
produces frequency domain signals.
In accordance with the invention, the FFT unit 1108 outputs the frequency
domain
signals 1112 to a frame synchronization unit 1114. The frame synchronization
unit 1114
operates to perform the synchronization processing discussed above with
reference to FIGS.
5-10B. The frame synchronization unit 1114 outputs an alignment error estimate
1116 to a
controller 1118. The controller 1118 then adjusts the receive frame boundary
pointer 1110
for accessing the received data from the input buffer 1106. Hence, the frame
synchronization
unit 1114 provides for frame synchronization in the time domain duplexed
transmission
l0 system in a manner that is substantially immune from RF interference (e.g.,
such as from
amateur radio users). The controller 1118 also controls the overall operation
of the receiver
1100. The controller 1118, for example, controls the receiver 1100 to perform
the
initialization processing and to monitor steady-state data transmission. For
example, the
controller 1118 can be implemented by a digital signal processor, a
microprocessor or
microcontroller, or specialized circuitry. In the case where the receiver 1100
forms part of a
transceiver, the controller 1118 can be used by both transmit and receive
sides of the
transceiver, shared among a plurality of transceivers, or individually
provided for each
transmitter and receiver. Likewise, the frame synchronization unit 1114 can be
implemented
by a digital signal processor, a microprocessor or microcontroller, or
specialized circuitry.
Returning to the receive data path the frequency domain signals 1112 output by
the
FFT unit 1108 are then equalized by the FEQ unit 1120. The equalized signals
are then
supplied to a data symbol decoder 1122. The data symbol decoder 1122 operates
to decode
the equalized signals to recover data that has been transmitted on each of the
frequency tones
of the symbol being received. The decoding by the data symbol decoder 1122 is
performed
based on bit allocation information stored in a receive bit and energy
allocation table 1124.
The decoded data is then supplied to the FEC unit 1126 and then stored in an
output buffer
1128. Thereafter, recovered data 1130 (stored decoded data) may be retrieved
from the
output buffer 1128 as needed.
The receiver 1100 illustrated in FIG. 11 optionally includes other components.
For
example, when a corresponding transmitter adds a cyclic prefix to symbols
after an IFFT
unit, the receiver 1100 can remove the cyclic prefix before the FFT unit 1108.
Also, the
receiver 1100 can provide a time domain equalizer (TEQ) unit between the ADC
1104 and
the FFT unit 1106. Additional details on TEQ units are contained in U.S.
Patent No.
5,285,474 and U.S. Application Serial No. 60/046,244 (Att.Dkt.No.: AMATP021+),
filed
May 12, 1997 and entitled POLY-PATH TIME DOMAIN EQUALIZATION, which are
hereby incorporated by reference.
TI-27737 - 20

CA 02254807 1998-11-30
Moreover, the invention provides techniques to synchronize transmissions at a
central
side (i.e., central unit). With synchronized transmissions at the central
side, the NEXT
interference is substantially eliminated, provided all lines of a binder offer
the same level of
service (i.e., superframe format). However, if the transmissions from the
central side over
lines in a binder are not properly synchronized, the NEXT interference is a
substantial
impediment to efficient and accurate operation of the data transmission
system. Hence, the
invention also pertains to techniques to adjust a transmit frame boundary at a
central side
transmitter of data transmission system. The general principle is to use NEXT
interference
from other central side transmissions. If the NEXT interference is not strong
enough to be
l0 detected for synchronization purposes, then it will be assumed to be
insignificant during
reception, and therefore synchronization is not necessary.
Conventionally, the various transmitters at the central side can synchronize
with one
another by all using a common master clock supplied to the central side.
However,
sometimes such a master clock is not available for one reason or another.
Also, even though
available, the various transmitters could be positioned at slightly greater
positions from the
master clock source so as to cause small synchronization differences between
the various
transmissions. Hence, the synchronization techniques according to the
invention can also be
used to synchronize various transmissions at the central side.
FIG. 12 is a flow diagram of a synchronization processing 1200 for
synchronizing
adjacent transmitters to compensate for small synchronization differences. If
these small
synchronization differences were to go uncorrected, over time the degree of
the lack of
synchronization worsens. The synchronization processing 1200 initially
measures 1202
energy received from other transmitters at the central side. Here, during
quiet periods (or
guard periods), energy being received from other transmitters at the central
side is measured
by the receivers associated with the transmitters (i.e., transceivers). The
transmissions from
the various transmitters all follow the same superframe format. Preferably,
the second quiet
period (i.e., after the upstream transmission) is used to measure the energy
because there
tends to be less echo present. Next, a decision block 1204 determines whether
the measured
energy is greater than a predetermined threshold amount. If the measured
energy during the
quiet period is determined to be greater than the predetermined threshold
amount, then the
presence of NEXT interference is detected. Since NEXT interference is
detected, it is known
that the transmitters at the central side are not synchronized. Hence, the
timing alignment at
the transmitter is modified 1202 in order to synchronize its alignment with
respect to the
other transmitters at the central side. For example, the timing alignment
could be modified
by altering an oscillator frequency or changing (increase or decrease) the
length of the
superframe. On the other hand, when the measured energy is determined to be
less than the
predetermined threshold amount, then the transmitters at the central side are
deemed to be
TI-27737 - 21

CA 02254807 1998-11-30
sufficiently aligned and thus block 1206 is bypassed. Following block 1206 or
following
block 1204 when the predetermined threshold is not exceeded, the
synchronization
processing 1200 is complete and ends.
The synchronization processing 1200 is performed by all the transceivers at
the
central side. By repeating the synchronization processing 1200, gradually the
alignment will
reach a more less steady state, particularly if adjustments to alignment are
made in only one
direction.
Recall, as illustrated in FIG. 4, the superframe format has two quiet periods
404 and
408. The synchronization processing 1200 uses one of the two quiet periods 404
and 408.
When a receiver at the central side hears the NEXT interference during the
quiet period 408,
it means that this transceiver is late and should transmit earlier.
Alternatively, if the receiver
at the central side uses the quiet period 404 and hears the NEXT interference
during the quiet
period 404, it means that this transceiver is early and should transmit later.
However, before
the transceiver adjust its timing alignment at the central side, it may inform
the corresponding
i5 remote unit of the change so that it also modifies its timing alignment.
This notification to
the remote can, for example, be performed over an overhead channel.
The synchronization technique needs to distinguish downstream NEXT
interference
from upstream FEXT interference. This can be achieved a number of different
ways. One
way to distinguish upstream transmissions from downstream transmissions, in
the case of
VDSL using DMT frames with 256 tones, is to use tone 128 which is Nyquist/2
only with
downstream transmissions. As noted above, the quiet period is used to measure
the
interference from adjacent downstream transmissions. If the downstream
distinguishing
feature is detected (greater than some threshold) it means that the clock in
this unit is running
faster than the interfering transmitter's clock.
The adjustment to the synchronization can be to modify the clock frequency of
the
particular transceiver's clock, such as with a voltage controlled oscillator.
Alternatively, an
extra cycle can be inserted into the superframe structure. In VDSL, if the
central side
transceiver's clocks are within 100 ppm of each other, then insertion of 1
sample per
superframe (11,040 samples) will be sufficient to monitor synchronization. If
the central side
transceivers can only insert, the central side transceivers will reach a
consensus at the lowest
clock frequency of the group (that has significant NEXT).
For example, the energy of tone 128 can be measured with a special single-tone
DFT:
a -
2 r z55 k,Z ~ 255 k+1 ~2
1281 y~xzk(-1) J +~~xzk+~(-1) J
k 0 k 0
TI-27737 - 22

CA 02254807 1998-11-30
If the measured energy is larger than a predetermined threshold, then insert a
sample (extra
cycle) in the subsequent downstream transmission.
The advantages of the invention are numerous. One advantage of the invention
is that
synchronization can be achieved even in the presence of radio frequency (RF)
interference,
such as due to amateur radio signals. Another advantage of the invention is
that it is well
suited for data transmission systems utilizing time division duplexing such as
synchronized
DMT or synchronized VDSL. Yet another advantage of the invention is that it is
relatively
insensitive to background or receiver noise.
l0 Thus, the present invention includes a method for adjusting an alignment
for a first
transceiver to receive frames of data transmitted from a second transceiver
over a
transmission medium to the first transceiver, the first transceiver and the
second transceiver
being associated with a data transmission system providing two-way data
communication
using time division duplexing the method comprising the acts of (a) measuring
an energy
amount for each of a plurality of consecutive frames of received data; and (b)
computing an
alignment error estimate based on the measured energy amounts.
Also included is the method as recited hereinabove, wherein the alignment
error
estimate is an estimated alignment error as a fraction of a frame.
Also included is the method as recited hereinabove, wherein the data
transmission
system transmits data using a superframe structure having a plurality of
frames, a first set of
the frames in the superframe transmit data in a first direction, and a second
set of the frames
in the superframe transmit data in a second direction.
Also included is the method as recited hereinabove, wherein the first
transceiver uses
a frame boundary pointer to identify a beginning of a frame in the superframe
being received,
and wherein the method further comprises: (c) adjusting the frame boundary
pointer in
accordance with the alignment error estimate.
Also included is the method as recited hereinabove, wherein the alignment
error
estimate is an estimated alignment error as a fraction of a frame.
Also included is the method as recited hereinabove, wherein the method further
comprises: (d) comparing the alignment error estimate with a threshold amount;
(e)
repeating steps (a) - (d) until the comparing (d) indicates that the alignment
error estimate is
less than the threshold amount.
Also included is the method as recited hereinabove, wherein the method further
comprises: (f) outputting superframe identification information.
Also included is the method as recited hereinabove, wherein the computing (b)
comprises: detecting an edge in the plurality of consecutive frames of the
received data based
TI-27737 - 23

CA 02254807 1998-11-30
on the measured energy amounts; and determining the alignment error estimate
using the
edge detected in the plurality of consecutive frames.
Also included is the method as recited hereinabove, wherein the edge detected
is a
burst edge.
Also included is the method as recited hereinabove, wherein the detecting
comprises:
computing successive energy differences in the plurality of the measured
energy amounts;
and identifying a largest one of the successive energy differences, the
largest one of the
successive energy difference corresponding to the burst edge.
Also included is the method as recited hereinabove, wherein the computing (b)
comprises: identifying a prior energy difference and a subsequent energy
difference, the
prior energy difference being the one of the successive differences
immediately preceding the
largest one of the successive energy differences, and the subsequent energy
difference being
the one of the successive differences immediately following the largest one of
the successive
energy differences; and determining the alignment error estimate based on the
prior energy
difference and the subsequent energy difference.
Also included is the method as recited hereinabove, wherein the determining of
the
alignment error estimate computes a difference amount between the subsequent
energy
difference and the prior energy difference.
Also included is the method as recited hereinabove, wherein the determining of
the
alignment error estimate computes a difference amount between the subsequent
energy
difference and the prior energy difference, and then normalizes the difference
amount to
produce the alignment error estimate.
Also included is a method as recited hereinabove, wherein the first
transceiver uses a
frame boundary pointer to identify a beginning of a frame in the superframe
being received,
and wherein the method further comprises: (c) adjusting the frame boundary
pointer in
accordance with the alignment error estimate.
Also included is a method as recited hereinabove, wherein the alignment error
estimate is an estimated alignment error as a fraction of a frame.
Also included is a method as recited hereinabove, wherein the method further
comprises: (d) comparing the alignment error estimate with a threshold amount;
(e)
repeating (a) - (d) until the comparing (d) indicates that the alignment error
estimate is less
than the threshold amount.
Also included is a method as recited hereinabove, wherein the method further
comprises: (f) outputting superframe identification information.
Also included is a method as recited hereinabove, wherein the first
transceiver is a
remote unit and the second transceiver is a central unit.
TI-27737 - 24

CA 02254807 1998-11-30
Also included is a method as recited hereinabove, wherein the second
transceiver is a
remote unit and the first transceiver is a central unit.
Also included is a method as recited hereinabove, wherein the energy amount is
a
power amount.
Also included is a method as recited hereinabove, wherein the data
transmission
system transmits data using a supeiframe structure having a plurality of
frames, some of the
frames transmit data in a first direction, some of the frames transmit data in
a second
direction, and some of the frames contain a cyclic prefix for the superframe
structure, and
wherein the measuring (a) of the energy amounts comprises: measuring energy
l0 amounts of a first set of consecutive frames of received data for the
superframe structure;
measuring energy amounts of a second set of consecutive frames of received
data for the
superframe structure, the second set of the consecutive frames being offset
from and
overlapped with the first set of the consecutive frames; and combining
together the energy
amounts from respective consecutive frames from the first and second sets of
the consecutive
frames to produce the energy amounts for the computing (b).
Also included is the method as recited hereinabove, wherein the number of
frames in
the first and second sets of the consecutive frames is equal to the length of
the superframe
structure less the length of the cyclic prefix.
Also included is the method as recited hereinabove, wherein the combining
determines mean energy amounts for each of the frames of the superframe
structure including
the cyclic prefix.
Also included is a computer readable medium containing program instructions
for
adjusting an alignment for a first transceiver to receive frames of data
transmitted from a
second transceiver over a transmission medium to the first transceiver, the
first transceiver
and the second transceiver being associated with a data transmission system
providing two-
way data communication using time division duplexing, the computer readable
medium
comprising: first computer readable code devices for measuring an energy
amount for each of
a plurality of consecutive frames of received data; and second computer
readable code
devices for computing an alignment error estimate based on the measured energy
amounts.
Also included is the computer readable medium as recited hereinabove, wherein
the
second computer readable medium comprises: computer readable code devices for
detecting
an edge in the plurality of consecutive frames of the received data based on
the measured
energy amounts; and computer readable code devices for determining the
alignment error
estimate using the edge detected in the plurality of consecutive frames.
Also included is the computer readable medium as recited hereinabove, wherein
the
second computer readable medium further comprises: computer readable code
devices for
computing successive energy differences in the plurality of the measured
energy amounts;
TI-27737 - 25

CA 02254807 1998-11-30
and computer readable code devices for identifying a largest one of the
successive energy
differences, the largest one of the successive energy difference corresponding
to a burst edge.
Also included is the computer readable medium as recited hereinabove, wherein
the
second computer readable medium comprises: computer readable code devices for
identifying a prior energy difference and a subsequent energy difference, the
prior energy
difference being the one of the successive differences immediately preceding
the largest one
of the successive energy differences, and the subsequent energy difference
being the one of
the successive differences immediately following the largest one of the
successive energy
differences; computer readable code devices for determining the alignment
error estimate
based on the prior energy difference and the subsequent energy difference.
Also included is the computer readable medium as recited hereinabove, wherein
the
data transmission system transmits data using a superframe structure having a
plurality of
frames, some of the frames transmit data in a first direction, some of the
frames transmit data
in a second direction, and some of the frames contain a cyclic prefix for the
superframe
structure, and wherein the first computer readable code devices for measuring
of the energy
amounts comprises: computer readable code for measuring energy amounts of a
first set of
consecutive frames of received data for the superframe structure; computer
readable code for
measuring energy amounts of a second set of consecutive frames of received
data for the
superframe structure, the second set of the consecutive frames being offset
from and
overlapped with the first set of the consecutive frames; and computer readable
code for
combining together the energy amounts from respective consecutive frames from
the first and
second sets of the consecutive frames to produce the energy amounts for the
second computer
readable code devices.
Also included is the computer readable medium as recited hereinabove, wherein
the
combining determines mean energy amounts for each of the frames of the
superframe
structure including the cyclic prefix.
Also included is the computer readable medium as recited hereinabove, wherein
the
number of frames in the first and second sets of the consecutive frames is
equal to the length
of the superframe structure less the length of the cyclic prefix.
Also included is a receiver for a data transmission system using time division
duplexing to alternate between transmission and reception of data, the
receiver comprising:
an analog-to-digital converter, the analog-to-digital converter receives
analog data that has
been transmitted over a channel to the receiver and converts the received
analog signals into
received digital signals; an input buffer for temporarily storing the received
digital signals; a
multicarner demodulation unit; the multicarrier demodulation unit demodulates
the received
digital signals from the input buffer to frequency domain data for a plurality
of different
Garner frequencies; a frame synchronization unit, the frame synchronization
unit
synchronizes a receive frame boundary for the multicarrier demodulation unit
based on the
TI-27737 - 26

CA 02254807 1998-11-30
time-varying nature of the energy of the frequency domain data produced by the
multicarrier
demodulation unit; a bit allocation table, the allocation table stores bit
allocation information
used in transmitting data being received at the receiver; a data symbol
decoder, the data
symbol decoder receives the frequency domain data and decodes bits associated
with the
frequency domain data from the carrier frequencies based on the bit allocation
information
stored in the bit allocation table; and an output buffer for storing the
decoded bits as
recovered data.
Also included is the receiver as recited hereinabove, wherein the frame
synchronization unit determines an alignment adjustment amount, wherein the
receiver
further comprises a controller for controlling overall operation of the
receiver, the controller
receives the alignment adjustment amount from the frame synchronization unit
and
accordingly adjusts a receive frame boundary pointer for the input buffer.
Also included is the receiver as recited hereinabove, wherein at least one of
the frame
synchronization unit and the controller are implemented by a processor.
Also included is the receiver as recited hereinabove, wherein the frame
synchronization unit is a processor.
Also included is the receiver as recited hereinabove, wherein the received
data signals
undesirably include radio frequency interference, and wherein the frame
synchronization unit
ignores the portion of the frequency domain data that overlaps with frequency
ranges of the
radio frequency interference.
Also included is the receiver as recited hereinabove, wherein the data
transmission
system is a synchronized DMT system, and wherein the multicarrier demodulation
unit
includes a FFT unit.
Also included is a receiver for a data transmission system using time division
duplexing to alternate between transmission and reception of data, the
receiver comprising:
an analog-to-digital converter, the analog-to-digital converter receives
analog data that has
been transmitted over a channel to the receiver and converts the received
analog signals into
received digital signals; an input buffer for temporarily storing the received
digital signals; a
multicarrier demodulation unit, the multicarrier demodulation unit demodulates
the received
digital signals from the input buffer to frequency domain data for a plurality
of different
carrier frequencies; frame synchronization means for synchronizing a frame
boundary for the
multicarner demodulation unit based on the time-varying nature of the energy
of the
frequency domain data produced by the multicarrier demodulation unit; a bit
allocation table,
the allocation table stores bit allocation information used in transmitting
data being received
at the receiver; a data symbol decoder, the data symbol decoder receives the
frequency
domain data and decodes bits associated with the frequency domain data from
the carrier
frequencies based on the bit allocation information stored in the bit
allocation table; and an
output buffer for storing the decoded bits as recovered data.
TI-27737 - 27

CA 02254807 1998-11-30
Also included is, in a data transmission systems having a plurality of
transmitters at a
central site, the transmitters transmiting data in accordance with a
superframe format
including at least one quiet period, a method for synchronizing data
transmissions by a given
transmitter to other of the transmitters at the central site, the method
comprises the acts of:
(a) measuring the energy in the quiet period associated with the given
transmitter due to data
transmissions from the other of the transmitters at the central site; (b)
comparing the
measured energy with a threshold amount; and (c) modifying the synchronization
for the
transmissions by the given transmitter when the comparing (b) indicates that
the measured
energy exceeds the threshold amount.
l0 Also included is the method as recited hereinabove, wherein the data
transmission
system transmits data using time division duplexing, and wherein the
transmitters are part of
transceivers at the central site.
Also included is the method as recited hereinabove, wherein the data
transmission
system is a multicarrier data transmission system.
Also included is the method as recited hereinabove, wherein the modifying (c)
comprises adjusting timing alignment to reduce crosstalk interference.
Also included is the method as recited hereinabove, wherein the adjusting
increases or
decreases the length of the superframe format.
Also included is the method as recited hereinabove, wherein the adjusting
alters the
frequency of a local clock for the given transmitter.
Also included is the method as recited hereinabove, wherein the data
transmission
system is a multicarrier data transmission system, and wherein an external
clock signal is
unavailable for synchronizing the transmitters, and wherein the modifying (c)
comprises
adjusting timing alignment to reduce crosstalk interference.
Also included is the method as recited hereinabove, the measuring (a) of the
energy in
the quiet period associated with the given transmitter due to the data
transmissions from the
other of the transmitters at the central site operates to distinguish between
outgoing data
transmissions from the other of the transmitters and incoming data receptions,
so that the
measuring (a) measures the energy in the quiet period due to the outgoing data
transmissions
from the other of the transmitters and not due to the incoming data
receptions.
Also included is a computer readable medium containing program instructions
for
synchronizing data transmissions in a data transmission systems having a
plurality of
transmitters at a central site where an external clock signal is unavailable
for synchronizing
the transmitters, the transmitters transmit data in accordance with a
superframe format
including at least one quiet period, the computer readable medium comprising:
first computer
readable code devices for measuring the energy in the quiet period associated
with a given
transmitter due to data transmissions from other of the transmitters at the
central site; second
computer readable code devices for comparing the measured energy with a
threshold amount;
TI-27737 - 28

CA 02254807 2005-06-09
and third computer readable code devices for modifying the synchronization for
the
transmissions by the given transmitter when the comparing indicates that the
measured
energy exceeds the threshold amount.
Also included is the computer readable medium as recited hereinabove, wherein
the
data transmission system is a multicarrier data transmission system that
transmits data uses
time division duplexing, and the transmitters are part of transceivers at the
central site, and
wherein the third computer readable code devices operates to adjust timing
alignment to
reduce crosstalk interference.
The many features and advantages of the present invention are apparent from
the
i0 written description, and thus, it is intended by the appended claims to
cover all such features
and advantages of the invention. Further, since numerous modifications and
changes will
readily occur to those skilled in the art, it is not desired to limit the
invention to the exact
construction and operation as illustrated and described. Hence, all suitable
modifications and
equivalents may be resorted to as falling within the scope of the invention.
29

Dessin représentatif