U.S. patent application number 10/719696 was filed with the patent office on 2004-09-30 for process for synchronization of communication involving multiple user equipments with a head-end equipment by means of ofdm modulation in the upstream channel.
Invention is credited to Blasco Claret, Jorge Vicente, Gavillero Martin, Juan Miguel, Iranzo Molinero, Salvador, Riveiro Insua, Juan Carlos.
Application Number | 20040190638 10/719696 |
Document ID | / |
Family ID | 31996621 |
Filed Date | 2004-09-30 |
United States Patent
Application |
20040190638 |
Kind Code |
A1 |
Blasco Claret, Jorge Vicente ;
et al. |
September 30, 2004 |
Process for synchronization of communication involving multiple
user equipments with a head-end equipment by means of OFDM
modulation in the upstream channel
Abstract
Where synchronization in time and frequency in the downstream
channel has been previously carried out, and determined by the
sending of synchronization sequences (3) in a communication between
a head-end (1) and user equipment (2); the present invention is
characterized because it corrects sampling frequency in the user
equipments (2) by starting from the estimation carried out during
frequency synchronization in the downstream channel, to synchronize
the upstream channel in frequency, removing the need for the
head-end to make corrections in reception in the upstream channel.
The user equipment (2) estimates (9) the moment the OFDM symbols
(7) should be sent to the head-end (1) so that these are received
by it at previously fixed moments in time. Furthermore, this
invention includes interrogation of the user equipment (2), which
allows them to place requests for access to the upstream channel
with the head-end then making the distribution between the requests
received. It is applicable to sending of data in bi-directional
communication over the electric network.
Inventors: |
Blasco Claret, Jorge Vicente;
(Valencia, ES) ; Riveiro Insua, Juan Carlos;
(Valencia, ES) ; Gavillero Martin, Juan Miguel;
(Burjasot, ES) ; Iranzo Molinero, Salvador;
(Valencia, ES) |
Correspondence
Address: |
KLAUBER & JACKSON
4th Floor
411 Hackensack Avenue
Hackensack
NJ
07601
US
|
Family ID: |
31996621 |
Appl. No.: |
10/719696 |
Filed: |
November 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10719696 |
Nov 21, 2003 |
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09765781 |
Jan 19, 2001 |
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6711901 |
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60177218 |
Jan 21, 2000 |
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Current U.S.
Class: |
375/260 |
Current CPC
Class: |
Y10T 29/49346 20150115;
C23C 30/00 20130101; F02K 9/974 20130101; H04L 5/023 20130101; F05D
2300/224 20130101; F05D 2300/2263 20130101; F05D 2300/502 20130101;
C23C 4/00 20130101; F05D 2300/13 20130101 |
Class at
Publication: |
375/260 |
International
Class: |
H04K 001/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 23, 2001 |
ES |
P200101179 |
May 21, 2002 |
WO |
PCT/ES02/00239 |
Claims
1. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN UPSTREAM CHANNEL, applicable to two-way communication
over the electricity network, including means to add and extract a
cyclic prefix (10) from OFDM symbols (7) in order to avoid
intersymbol interferences (ISI), and in which a synchronization in
frequency and time is carried out in a downstream channel,
determined by the communication from the head-end equipment (1)
with the user equipments (2) by means of sending synchronization
sequences (3); said Process for Synchronization of Communication
comprising: frequency synchronization by means of correcting a
sampling frequency in the multiple user equipments (2) from an
estimation carried out in the frequency synchronization in the
downstream channel; Characterized in that said Process for
Synchronization of Communication further Comprises:
pre-compensation, in the user equipments (2), of a rotation that
various carriers suffer on being sent in the upstream channel from
an estimation of the rotation suffered by the carriers in the
downstream channel, avoiding corrections in reception in the
upstream channel by the head-end equipment (1); time
synchronization by means of an estimation, carried out by the user
equipments (2) and the head-end equipment (1), of a moment when the
OFDM symbols (7) are sent to the head-end, in order to make the
head-end receive the OFDM symbols in previously fixed moments of
time; interrogate the user equipments (2) by means of assignation
by the head-end of intervals selected from: time intervals,
frequency intervals, and combinations thereof in the upstream
channel, slots (19), divided in fragments (27) for the
interrogation of the user equipments (2) where the user equipments
(2) reply to the head-end (1) if and only if user equipments (2)
intend to access the upstream channel; from which the head-end
equipment (1) distributes the upstream channel between petitions
received and sends said distribution to the user equipments (2) in
order to make the user equipments (2) transmit without
collisions.
2. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 1,
characterized in that said Process for Synchronization of
Communication comprises the use of equal oscillators to generate
the transmission and reception sampling frequencies both in the
head-end equipment and the user equipments, in order to make
frequency error in the upstream channel proportional to frequency
error in the downstream channel and to carry out synchronization in
frequency in the upstream channel at the same time that
synchronization in frequency in the downstream channel.
3. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 2,
characterized in that the pre-compensation of the rotation carried
out in the user equipments is accomplished by means of a rotor
included in the transmitter, estimated from the estimation carried
out in each one of the carriers of the received signals in the
downstream channel in the user equipment, avoiding rotation
correction of the signal received from the various user equipments
in the head-end equipment.
4. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 1,
characterized in that time synchronization comprises generating and
sending at least one synchronization sequence from the user
equipment (2) to the head-end (1) at request of the head-end
equipment; the synchronization sequence comprising two identical
synchronization symbols, and said synchronization sequence being
detected in the head-end by maximizing the maximum likelihood
criteria, in order to carry out the time synchronization from a
calculation of maximum correlation the sequence samples sent by the
various users, said maximum being determined as the mid point in a
flat zone of a correlation peak whose size in number of samples, is
equal to the number of samples of the cyclic prefix (10) without
intersymbol interference.
5. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 4,
characterized in that time synchronization comprises an acquisition
stage and a tracking stage in which the moment of transmission of
the OFDM symbols (7) is modified in the different user equipments
(2), from an estimation carried out in order to make said symbols
be received by the head-end equipment in previously established
fixed moments in time.
6. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 5,
characterized in that the time synchronization acquisition stage is
determined by an open loop in which the user equipments (2)
estimate (12) the start (8) of transmission for each OFDM symbol
(7) beginning from the time synchronization obtained in the
downstream channel; by having estimated the moment of arrival (9)
of the OFDM symbols (7) received in the downstream channel, the
user equipment compensates the delay introduced by the filters in
its receiver and transmitter, as well as the average error which is
established due to the cyclic prefix (10) and transmits the OFDM
symbols (7) in the moment (9) estimated following the
compensation.
7. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 5 or 6,
characterized in that the time synchronization tracking stage is
determined by a closed loop in which the head-end equipment (14)
estimates, by means of the received signal (13) that was sent in
the time synchronization acquisition stage by a user equipment (2),
the number of samples that the sending of OFDM symbols (7) must be
advanced or delayed by said user equipment in order to receive them
exactly in the moment expected; therefore the head-end equipment
sends (15) said number of samples to the user equipment in the
downstream channel as a parameter of the SAM message (26) that the
head-end conventionally sends to the user equipment, and from this,
the user equipment (16) carries out said advance or delay in
sending the OFDM symbols, continually undertaking the open loop of
the time synchronization, so that the estimation carried out by the
closed loop compensates the delay introduced by the channel.
8. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 4,
characterized in that the head-end equipment indicates to the
various user equipments when to send synchronization sequences (3),
and grants slots by means of SAM messages sent in the downstream
channel, so as to carry out adequate synchronization with respect
to the user equipment, so that the detection of synchronization
sequences (3) for a user in the upstream channel is only carried
out that user was ordered to send the synchronization sequence.
9. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 1,
characterized in that the head-end equipment assigns interrogation
slots (19), divided in fragments (27) by means of sending SAM
messages (26) to the various user equipments (2), said user
equipment, in the case where they need to transmit, transmitting a
POLLING symbol (20) in the assigned fragments (27), in order to
make the head-end equipment determine which user equipment intends
to transmit information and optimises the distribution algorithms
of the upstream channel between the user equipments.
10. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 9,
characterized in that the interrogation slots (19) are divided into
small fragments that are equal to or greater than the size of the
POLLING symbol (20) in order to interrogate.various user equipments
at the same time, therefore, the head end, by means of SAM
messages, assigns a fragment to at least one user equipments that
intend to interrogate and in which the user that intends to
transmit replies to the interrogation by means of a POLLING symbol
(20) in the assigned fragment.
11. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 10,
characterized in that in each fragment the users send only one
POLLING symbol (20), leaving the rest of the fragment in silence
(21) when said fragment occupies more than one symbol, in order to
avoid overlapping of POLLING symbols (20) in reply to the
interrogation of the users, when replying in two distinct fragments
while the user equipments are not correctly synchronized in
time.
12. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 11,
characterized in that the detection of the POLLING symbol (20) is
carried out by means of the correlation of the received signal and
of a POLLING symbol previously stored in the head-end equipment, by
means of a matched filter in order to carry out detection.
13. PROCESS FOR SYNCHRONIZATION OF COMMUNICATION INVOLVING MULTIPLE
USER EQUIPMENTS WITH A HEAD-END EQUIPMENT BY MEANS OF OFDM
MODULATION IN THE UPSTREAM CHANNEL, according to claim 12,
characterized in that the POLLING symbol (20) has X equal parts to
use a finite response matched filter with N/X products (23) and
delays (22), where N is the number of samples in the POLLING symbol
(20) sent by the user in the fragment assigned by the interrogation
slot (19), when requiring to transmit in the upstream channel.
Description
RELATED APPLICATIONS
[0001] The present application is a Continuation of co-pending PCT
Application No. PCT/ES02/00239, filed May 21, 2002, which in turn,
claims priority from Spanish Application Serial No. 200101179,
filed May 23, 2001. Applicants claim the benefits of 35 U.S.C.
.sctn.120 as to the PCT application and priority under 35 U.S.C.
.sctn.119 as to said Spanish application, and the entire
disclosures of both applications are incorporated herein by
reference in their entireties.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention pertains to the telecommunications
sector, in particular it is applicable to two-way communication
between a head-end and various users so as to carry out
synchronization in the upstream channel, that is the link between
the users and the head-end, where the electricity network is used
as a means of communication to offer multiple services to the
users.
OBJECT OF THE INVENTION
[0003] The objective of the present invention is to provide a new,
highly secure, time and frequency synchronization process for the
upstream channel in the electricity network which avoids problems,
typically due to high noise levels and selectivity in frequency
inherently present during the transmission of data over the
electricity network, thereby avoiding the production of false
indications of synchronization in the upstream channel.
[0004] By means of the process in this invention, synchronization
in frequency is undertaken whereby the various user transmitters
synchronize at the frequency used by the head-end receiver,
starting from the signal received in the downstream channel, which
is defined as a communication running from the head-end equipment
to the user equipment, so that the head-end equipment does not have
to make any correction in frequency.
[0005] Furthermore, the process described herein comprises
synchronization in time, with the objectives of estimating the
moment in which the user equipment must send the OFDM symbols so
that the head-end equipment will receive them at previously
established fixed moments and the user equipment modifies the
moment of transmission of the OFDM symbols so as to comply with
this condition, and therefore the head-end equipment will not have
to undertake correction in time.
[0006] Since synchronization in the upstream channel as described
in the present invention is based on the synchronization carried
out in the downstream channel, it must be mentioned that
synchronization in the upstream as foreseen herein is based
preferably on the synchronization process for the downstream as
described in Spanish Patent application No. 200102254 (still
unpublished).
BACKGROUND OF THE INVENTION
[0007] Various methods of synchronizing communication in the
upstream channel for transmission from multiple user equipments
using OFDM modulation are known in the prior art, but none of these
reaches the levels of high security needed to avoid false
indications of synchronization in point to multipoint systems where
the means of transmission is the electricity network.
[0008] It is well known that the electricity network is a difficult
transmission medium due to the fact that the connection and
disconnection of different apparatus on the network produces
voltage peaks and impedance variation on the line, that cause
channel response to vary over time.
[0009] Among the synchronization methods known, it is important to
mention the method described in U.S. Pat. No. 5,732,113, concerning
a time synchronization process using one synchronization symbol
with two equal halves, where, to carry out the synchronization a
lesser number of samples is used than the number proposed by the
present invention, and where impulse noise, which is very common in
the electricity network, affects the synchronization to a much
greater degree due to the fact that by definition these are
occasional noises affecting a small number of samples, and
therefore affect to a much greater degree the process in the U.S.
Pat. No. 5,732,113, as already mentioned, more than the solution
proposed in the present invention, therefore, the method of using
only one synchronization symbol is not desirable in a transmission
system using the electricity network.
[0010] One must also indicate that the use of one synchronization
symbol means greater variance in the calculation or estimation of
the synchronization due to the use of a smaller number of samples
to carry out the synchronization.
[0011] P. Moose published the concept of using two equal symbols
in: "A technique for orthogonal frequency division multiplexing
frequency offset correction. IEEE Trans. on Comm., vol.42,
pp2908-2914, October 1994", but these symbols have never been
considered or used to carry out time synchronization, as occurs in
the present invention, instead, Moose used them to estimate the
error in analog translation frequency.
[0012] Furthermore, as has already been mentioned, synchronization
in frequency in the upstream channel is undertaken by starting from
frequency synchronization in the downstream channel, and as
indicated in Spanish patent application ES-2188370-A1, frequency
synchronization by means of the arc tangent of the correlation to
correct the error in analog translation frequency is known in the
prior art, for example in U.S. Pat. No. 5,732,113 or in the article
by Moose; however in the case of the current invention,
synchronization in frequency is carried out by means of estimating
the sampling frequency error in the analog/digital converters in
each one of the user receivers, which is substantially different
from what is known in the prior art.
DESCRIPTION OF THE INVENTION
[0013] To carry out frequency and time synchronization in the
upstream channel for multiple users in a point to multipoint
transmission system with OFDM modulation using the electricity
network as the transmission means, the current invention comprises
frequency synchronization by means of correcting sampling frequency
in the various user equipment starting from the estimation carried
out in frequency synchronization in the downstream. Furthermore, it
comprises carrying out pre-compensation in the user equipment to
take account of the rotation suffered by the various carriers on
being sent by the upstream channel that is calculated based on the
estimation of the rotation suffered by the carriers in the
downstream channel. Both these characteristics allow frequency
synchronization in the upstream to be carried out without the
head-end equipment carrying out corrections in reception in the
upstream channel, due to the fact that synchronization is
undertaken in the user equipment.
[0014] Furthermore, the process in this invention comprises time
synchronization whereby the user equipment estimates the moment to
send the OFDM symbols to the head-end equipment, so that is will
receive them in previously established fixed moment of time, and so
that time synchronization is carried out from the signal received
in the downstream channel, without the head-end having to undertake
corrections in reception in the upstream channel.
[0015] The invention is further characterized because the user
equipment is questioned by the head-end equipment by means of
assignation slots (fragments of time and/or frequency), said user
equipments replying to the head-end equipment when they want to
send petitions to access the upstream channel, the head-end
distributing band width in the upstream channel between the users
that have requested to transmit and sending this distribution to
the user equipments so that they transmit without collisions.
[0016] Time synchronization is carried out by means of the user
equipment sending the OFDM symbols in the moments estimated from
the signal received in the downstream channel, so that the head-end
receives them at a given moment in time, or at least in a time
window with a small number of samples, thereby avoiding the
production of interferences due to not having correctly windowed
the user signals.
[0017] In consequence, the head-end equipment never advances or
delays a reception window; rather it maintains it in predetermined
fixed moments, to which the user equipment adjusts.
[0018] Synchronization in frequency in the upstream channel is
carried out from frequency synchronization in the downstream
channel, because the same oscillators are used to generate the
transmission and reception sample frequencies in both the head-end
equipment and the user equipments.
[0019] Pre-compensation for the rotation carried out in the user
equipment is undertaken by means of a rotor beginning with the
rotation estimation in each one of the carriers in the received
signals over the downstream channel in the user equipment and for
which each user equipment has a rotor device in the transmitter as
well as that used in the receiver, both of these having a similar
function. Therefore, the rotor in the transmitters in the user
equipment will function in the same manner as described in Spanish
Patent application No. 200102254 (still unpublished).
[0020] Pre-compensation for the rotation allows the signal to
arrive at the head-end equipment as if there had been no error in
frequency, due to the fact that the rotation carried out on
transmission by means of this invention pre-compensates for the
rotation produced by the sending of data over the electricity
distribution network.
[0021] Furthermore, the invention is characterized because time
synchronization comprises the generation and sending of one or more
synchronization sequences from the user equipment to the head-end
when the head-end equipment requires it; the synchronization
sequence being made up of two identical synchronization symbols.
The synchronization sequence is detected in the head-end by
maximizing the maximum likelihood criteria (known in the prior
art), so that time synchronization is carried out from the
calculation of the maximum correlation of the sequence samples sent
by the various users, where this maximum is determined as the mid
point in the flat zone of the correlation peak the size of which in
number of samples is equal to the number of samples of the cyclic
prefix (a prefix that is conventionally introduced to avoid
interference between symbols) without inter-symbol interference
(ISI), and all of this being the same as that which occurs in the
downstream, except with the one difference, that the head-end
equipment knows approximately the moment in which the
synchronization sequence will arrive from a specific user
equipment, due to the fact that a prior request has been made to do
this by means of assigning a slot following a slot allocation
message (SAM) or assigning resources in the upstream to the users
which is conventionally sent by the head-end equipment to the user
equipment, therefore the synchronization sequence is sent in the
slot assigned to the user.
[0022] The process of time synchronization in the upstream channel
comprises an acquisition stage and a tracking stage where the
moment of transmission of the OFDM symbols in the various user
equipments is estimated, so that said symbols will be received by
the head-end equipment in the previously established fixed moments
in time, as has been previously mentioned.
[0023] The time synchronization acquisition stage is determined by
an open loop in which the user equipment estimate the start of
transmission for each OFDM symbol beginning from the time
synchronization obtained in the downstream channel; so that having
estimated the moment of arrival of the OFDM symbols received in the
downstream channel, the user equipment compensates for delays
introduced by the filters in its receiver and transmitter, as well
as average error which is established due to the cyclic prefix and
transmits the OFDM symbols in the moment estimated in such a way
that the moment when the symbols begin in the electricity line will
be approximately equal in the upstream and the downstream.
[0024] The time synchronization tracking stage is determined by a
closed loop feed back in which the head-end equipment estimates, by
means of the received signal sent by the user equipment, the number
of samples that must be advanced or delayed in the sending of OFDM
symbols by said user equipment so as to receive them at the
head-end exactly in the moment expected; therefore, the head-end
equipment sends information in the downstream channel to the user
equipment on the calculation carried out as a parameter of a SAM
message and starting from this, the user equipment carries out said
advance or delay in sending the OFDM symbols.
[0025] The open loop in time synchronization is undertaken
continually so that the estimation carried out by the closed loop
compensates for the delay introduced by the channel, thereby
achieving adequate synchronization.
[0026] When the head-end equipment wishes to know which users are
interested in transmitting via the upstream channel, it assigns
interrogation slots to the user equipments, in which the user
equipments concerned send a predetermined symbol (a POLLING symbol
that indicates a reply to an interrogation) when they wish to
transmit in the upstream channel.
[0027] The interrogation slots are divided into small fragments of
one or more symbol to interrogate various user equipments at the
same time, and head-end assigns a fragment to one or more user
equipments that it wishes to interrogate by means of SAM, so that
the user that wishes to transmit replies to the interrogation using
this fragment and not a complete slot.
[0028] In each fragment the users send only one POLLING symbol,
leaving the rest of the fragment in silence when this occupies more
than one symbol, so that there is no overlapping of POLLING symbols
in reply to the interrogation of the users that may occur, when
replying in two distinct fragments when the user equipment is not
correctly synchronized in time.
[0029] The detection of the POLLING symbol that is the response to
an interrogation request is carried out by means of the correlation
of the received signal and that of the POLLING symbol previously
stored in the head-end equipment. To carry out this function one
preferably uses a matched filter.
[0030] The POLLING symbol has X equal parts so that during
detection one may use a finite response matched filter with N/X
products and delays, where N is the number of samples in the
POLLING symbol sent by the user in the fragment assigned to the
interrogation slot when it needs to transmit in the upstream
channel.
[0031] Therefore, by means of the POLLING symbol the head-end
equipment knows which user equipment wished to transmit, and
therefore if necessary, asks for the synchronization sequence by
means of a SAM.
[0032] The following drawings are provided to facilitate a better
understanding of the present invention and while forming an
integral part of the detailed description and the claims, they
offer an illustrative but not limited representation of the
principles of this invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1. Schematically shows the time synchronization
acquisition stage constituted by an open loop for the transmission
by the user equipment in the upstream channel.
[0034] FIG. 2. Details the sending of OFDM symbols in the upstream
channel following the time synchronization acquisition stage.
[0035] FIG. 3. Shows a time diagram of the time synchronization
tracking stage carried out by means of a closed loop.
[0036] FIG. 4. Shows an example of the assignation of interrogation
slots to determine which users wish to transmit.
[0037] FIG. 5. Shows a possible example of the filter used to
detect the POLLING symbol.
DESCRIPTION OF AN EMBODIMENT OF THE INVENTION
[0038] The following is a description of the invention based on the
drawings as described previously.
[0039] As stated previously, the invention is applicable to two-way
communication over the electricity network between a head-end 1 and
various users 2 and has as its objective the provision of a process
that makes synchronization possible in the upstream channel in a
point to multipoint system with orthogonal frequency division
multiplexing (OFDM) modulation, such as the process described in
Spanish Patent application ES-2184587-A1.
[0040] As known in the state of the art, the head-end equipment
comprises an analog/digital converter that is connected to certain
analog components (a separator and filters) that allow the signal
to be introduced into the electricity network, and through which
the signals are sent to the different user equipments 2, where
receivers take the signal from the electricity network by means of
a separator and analog filter, and give this signal to an
analog/digital converter, so that together, all of this allows
communication to take place in the downstream channel.
[0041] Similarly, the receiver of the head-end equipment comprises
an analog/digital converter while the transmitter in the user
equipment also contains an digital/analog converter to allow
communication in the upstream channel.
[0042] In one embodiment of the invention, an error in sampling
frequency exists between the head-end receiver and the user
transmitter, due to the differences between the oscillators in
their converters and because of which the sampling frequency used
in these equipments will not be exactly the same.
[0043] The error in sampling frequency causes the constellation in
each of the carriers in the modulation to rotate symbol by symbol.
Furthermore, errors in sampling frequency also cause attenuation
and noise in the system; therefore it is necessary to correct this
error by means of synchronization in the upstream channel so that
sampling frequency in the head-end receiver and sampling frequency
in the transmitters of the user equipments will be the same.
[0044] A further problem also exists in that the head-end receiver
may receive different information from different users through any
of the carriers associated with the upstream channel; therefore it
is necessary to undertake the transformation for the entire
spectrum in the received signal.
[0045] Furthermore, the head-end receiver must know the moment in
which the user equipments will send the OFDM symbols, and for which
synchronization in time must be undertaken so as to avoid the
production of interferences due to not having correctly detected
the signals sent by the user equipments.
[0046] Sampling frequency synchronization is carried out in the
transmitters of the user equipments from synchronization in
frequency in the downstream channel.
[0047] Therefore, frequency synchronization in the transmitters of
the users is carried out from the reference transmitted by the
downstream channel for synchronizing the user receivers, as has
been described in the Spanish patent application ES-2188370-A1.
[0048] The frequency used by the head-end equipment to transmit in
the downstream channel is the same as the sampling frequency it
uses to receive in the upstream channel (or a multiple or
sub-multiple of it), and therefore, the frequency used in the
downstream channel has the same error with respect to the nominal
frequency which is that used to receive in the upstream channel.
Therefore, when the users receive synchronization information in
the downstream channel and they synchronize in frequency, at the
same time they synchronize in frequency in the upstream channel and
as a result there is no need to add any further additional steps
for synchronization in frequency of multiple user equipments in
transmission to the head-end in the upstream channel.
[0049] As described in Spanish patent application ES-2188370-A1, if
in the downstream channel, error correction in frequency is carried
out by means of a VCXO (voltage controlled crystal oscillator),
then on carrying out frequency correction in reception for the
users, at the same time frequency correction in transmission for
the users is also carried out in the upstream channel, so that
frequency correction in both channels is carried out at the same
time. If on the other hand, error correction in frequency in the
downstream channel is carried out by means of a resampler, as
described in the Spanish patent application ES-2188370-A1, then on
the user equipment it will be necessary to use a similar re-sampler
in their transmission of same so as to pre-compensate this error in
transmission and to ensure that the signal received by the head-end
equipment will not have frequency error.
[0050] Furthermore, to correct the problem of the rotation of
carriers in the upstream channel, it would be necessary to use a
rotor in the head-end equipment, for each one of the users in the
system, which would suppose a huge memory and process capacity over
the signal received by the upstream channel, since multiple users
transmit at the same time over said upstream channel, and therefore
this is a significant difficulty.
[0051] To compensate for this difficulty, this invention has
foreseen that the process of the invention undertakes a
pre-compensation of the rotation in transmission by each one of the
users and each user equipment includes a rotor in the transmitter
as well as that used in its receiver as has been described in the
Spanish patent application ES-2188370-A1.
[0052] The rotor of the transmitter in the user equipment estimates
how much the carriers must be rotated in the upstream starting from
the amount rotated by the constellation in each one of the carriers
for the received signal from the downstream channel.
[0053] Therefore, the signal arrives at the head-end equipment as
if it had no rotation in the carriers, because the rotation
produced in transmission over the electricity network, has been
pre-compensated.
[0054] The operation of the rotor in the transmitters in the user
equipment uses the same concept as that used to operate the rotor
in reception used in synchronization in the downstream channel, and
which has been described in the Spanish patent application
ES-2188370-A1, already mentioned.
[0055] Time synchronization in the upstream channel comprises a
phase in which the user equipments estimate the moment to send the
OFDM symbols to the head-end equipment, so that they receives them
at previously established fixed moments of time, that is to say,
the head-end never advances or delays its reception window; rather,
it is the users that carry out the adjustment in their transmission
window, advancing or delaying the moment when transmission of the
OFDM symbols begins so that these adequately arrive in the
reception window of the head-end equipment.
[0056] To carry out this function, time synchronization in the
upstream channel is carried out by means of an acquisition stage
and a tracking stage.
[0057] Time synchronization acquisition stage is carried out by
open loop, and consists of extracting the information of the time
synchronization obtained in the downstream channel, so as to
carrying out synchronization in time in the upstream channel. This
does not limit the system, because propagation times by the
electricity network for the frequencies used are very small
compared with the duration of the cyclic prefix that is
conventionally added to the OFDM symbols to avoid intersymbol
interference.
[0058] On receiving the synchronization signal sent by the head-end
in the downstream channel, each transmitter in the user equipment
estimates the start of the OFDM symbols. The error in this
estimation is negative due to system design, that is, it is
estimated that the start of the symbol is previous to its real
position so that it always falls within the cyclic prefix (which is
conventionally in front of the OFDM symbol); thereby avoiding
intersymbol interference due to bad estimations. In the absence of
a dispersive channel the average error in the estimation is half
the length of the cyclic prefix. In dispersive channels, such as is
the case of the electricity network, the error is somewhat less,
because the initial part of the cyclic prefix is affected by
intersymbol interference.
[0059] Individual user equipments insert the OFDM symbols to be
transmitted in the upstream channel starting from the moment that
has been estimated to be the start of the symbols in the downstream
channel. For this the receivers in the user equipment estimate the
moment of arrival of the symbols for the downstream channel, as for
example is undertaken in the Spanish patent application
ES-2188370-A1, already mentioned, and compensate for the delay due
to the reception filters in the downstream channel, the
transmission filters in the upstream channel and the average error
committed, which as has been mentioned, is half the length of the
cyclic prefix. By this means, from the point of view of the user
equipment, the symbols in the downstream channel and those in the
upstream commence in the same moment in the channel. It is evident
that any other temporary relationship between the symbols in the
upstream and downstream channels could have been chosen without
limiting the process described in this invention. The temporary
difference between the symbols sent in the downstream channel and
those received by the upstream, from the point of the view of the
head end, corresponds to round trip delay introduced by the
downstream and upstream channels.
[0060] FIG. 1 represents the acquisition stage carried out by means
of the open loop, known as blind acquisition, where the start 4 of
the synchronization 3 sequence symbols sent in the downstream
channel serve as a temporary reference for the user equipment 2,
which has been indicated by 5.
[0061] Reference 6 in FIG. 1 represents the rest of the symbols
sent in the downstream channel (data, equalization, etc).
[0062] FIG. 2 schematically represents the moment in which the OFDM
symbols 7 are sent in the upstream channel, the moment 8 calculated
as the arrival of an OFDM symbol 7 in the downstream channel
according to the synchronization of this channel, and starting from
this moment 8, estimating when to transmit 9 in the upstream
channel, where the error committed will always be negative and is
represented by the difference between 8 and 9. In this Figure the
numerical reference 10 represents the cyclic prefix.
[0063] Once blind acquisition has been carried out, the tracking
stage begins by means of a closed loop because blind acquisition
from synchronization in the downstream channel provides a temporary
reference for when to transmit in the upstream channel, and since
it can happen that this blind acquisition is not sufficiently
precise, the head-end equipment may not be able to discriminate the
signal received, because as previously described it always windows
the signal received at fixed moments.
[0064] With blind acquisition correctly undertaken there remains
one unknown value, from the point of view of the head-end
equipment, due to the round trip delay in the channel that is
estimated by means of the time synchronization in closed loop.
[0065] To this end, time synchronization is carried out in closed
loop, so the number of samples by which the user must advance or
delay its transmission to ensure that windowing in the head-end
equipment will be optimum, is calculated through the signal
received by the head-end from that user equipment.
[0066] This feedback information is sent in the downstream channel
to the user equipment implicated as a parameter of the SAM
message.
[0067] This process has been represented in a time scheme, starting
from synchronization in the downstream channel so that the head-end
sends synchronization sequences, 28, blind acquisition 12 is
carried out in the manner explained, and at this point the user
equipment sends data and the synchronization sequence, if this
sequence has been requested by the head end, after having received
in a SAM message 11 sent by the head end, and permission to use
certain frequencies and times, that is, one or more slots in the
upstream channel, as represented by the number 13. Starting from
the synchronization sequence sent from the user equipment, the
head-end calculates the number of samples that it must advance or
delay so as to window optimally, as has been represented by the
number 14. The head-end detects the moment of arrival of the OFDM
symbol from the user, and calculates the difference between this
moment and the exact moment in which it should have arrived. This
difference in the number of samples is the reference that is sent
to the user equipment involved so that it can correct the moment in
which the symbols are sent.
[0068] Following this, the head-end sends the SAM message
represented by the reference 15 in which it indicates the advance
or delay that the user equipment must include in transmission of
the OFDM symbols 7 in the upstream channel. The reference 16
represents detection of this information sent by the head-end where
the user equipment carries out the correction previously mentioned
and sends the OFDM symbols 7 in the corrected moment, and
including, as an option, a synchronization sequence as represented
by the reference 17, so that the OFDM symbols arrive at the
head-end at the moment when windowing is carried out, as indicated
by reference 18.
[0069] It must be indicated that the acquisition stage is carried
out continuously each time that a synchronization sequence is
detected in the downstream channel, where the moment of
transmission is corrected with the open loop algorithm as described
previously. Due to the fact that certain error may exist between
the head-end and user equipments in the sampling frequency,
windowing varies slowly and it is necessary to correct it by means
of the acquisition stage described previously, before transmission
is advanced or delayed so much that the receiver does not window
correctly.
[0070] In the process that has been described, one of the problems
that must be solved by the head end consists of the identification
of the user equipments that wish to transmit at a given moment.
[0071] To do this, when a user wants to transmit in the upstream
channel, it listens to the downstream channel and waits until the
head-end 1 indicates by means of the information included in the
SAM message 26, that one or more slots in the upstream channel are
used for interrogation. In the example shown in FIG. 4,
interrogation slots are assigned to each one of the three user
equipments 2 shown and that may or may not wish to transmit. An
interrogation slot 19, divided into various fragments 27 (where
each fragment is assigned to one of the users to be interrogated),
is assigned and each user interested in transmitting replies in
this slot if and only if it is interested in accessing the upstream
channel and by sending a predetermined symbol, known as a POLLING
symbol 20, to indicate the reply to the interrogation in the
fragment 27 assigned by the interrogation slot 19. It is foreseen
that requests by the user equipment may collide when various
equipments from the same group in the same section of the slot 19
are being interrogated, and therefore a process to solve contention
may be put to work or, on the contrary, the users affected will be
left to send their access requests again later.
[0072] The interrogation slot 19 is divided in fragments 27 greater
or equal in size to the POLLING symbol 20 leaving the rest of the
fragment in silence so as to avoid the overlapping of POLLING
symbols 20 in response to the interrogation of various users
replying in the various fragments 27. Thus, the probability of
interference is reduced, because the user equipments are already
synchronized in time by blind acquisition, that is, in sub-optimum
form, since part of the POLLING symbols 20 could be overlapped and
this is avoided by means of introducing the silences 21.
[0073] To detect the POLLING symbols 20, the head-end equipment
uses a special device that includes a filter with the form of the
POLLING symbol as shown in FIG. 5. This filter includes a series of
delays 22 through which the response received in the interrogation
slot 19 passes.
[0074] These delays 22 are determined by the sample interval, that
is, the inverse of the sampling frequency used to sample the
signal.
[0075] The output of the delays 32 are multiplied by filter
coefficients C.sub.i, which are equal to the form of the POLLING
symbol 20 due to the fact that the head-end knows this symbol.
These coefficients are the complex conjugations of the samples that
make up the POLLING symbol so that if the head-end is receiving a
POLLING symbol 20, multiplying this by a multiplier 23 with the
conjugated values and adding these by means of an adder 24, the
maximum will be reached and therefore the arrival of the POLLING
symbol can be detected, and this is used in the head-end to
indicate the arrival of a response to an interrogation message.
[0076] Due to the fact that N is relatively high and therefore the
number of products necessary is also high, the invention foresees
reducing the number of operations so that a POLLING symbol 20 with
X equal parts is sent and therefore N multipliers 23 and delayers
22 are not necessary since all that is required is N/X. The maximum
is detected when the POLLING symbol arrives and X maximums appear.
In summary, the detection of the POLLING symbols is carried out by
means of correlating the signal received and the POLLING symbol
previously stored in the head-end equipment and which is detected
by means of a matched filter as described previously.
[0077] With module 25 a signal indicating detection of the POLLING
symbol is generated when the module detects a total of X peaks with
approximately the same power level, and spaced at N/X samples.
[0078] Once it has been detected that one or more users wish to
transmit, the head-end equipment communicates by means of SAM
messages 26, which user or users may have access to the upstream
channel and the slots (time and/or frequency intervals) that may be
used, these SAM messages being periodically sent in the downstream
channel, as used for example in the process described in the
Spanish patent application ES-2186531-A2 and, in case of necessity,
the sending of a synchronization sequences following blind
acquisition 12 may be requested, as already described
previously.
* * * * *