Pulse or digital communications – Transceivers
Reexamination Certificate
2000-06-14
2004-05-11
Tse, Young T. (Department: 2734)
Pulse or digital communications
Transceivers
C375S231000, C375S346000
Reexamination Certificate
active
06735244
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a data transmission system and a receiver unit therefor. More particularly, the present invention relates to a data transmission system which transports data between a transmitter unit and a receiver unit over a telephone subscriber loop, as well as to the receiver unit used in this system.
2. Description of the Related Art
In recent years, the use of multimedia communication services such as the Internet has become increasingly ubiquitous in our daily activities, both at work and at home. To meet such user requirements, there is an urgent need to provide highly reliable, cost effective digital access network systems. However, constructing a completely new telecommunication infrastructure for multimedia services is extremely costly and time consuming. This has motivated various proposals of new high-speed data communication techniques that use existing telecommunication networks. Digital Subscriber Lines (xDSL), for example, are known as one of the enabling technologies for high-speed data communications over plain old phone lines. This xDSL technology is a collection of signal transmission techniques using subscriber lines, and in another aspect, it is an advanced modulation/demodulation technique. Such XDSL techniques are broadly divided into two groups: ones that provide symmetric upstream and downstream transmission rates, and ones that provide asymmetric rates. Here, the term “upstream” refers to the direction from a subscriber's premises to its nearest central office, and the “downstream” refers to the opposite direction. The ITU-T recommendations for Asymmetric DSL (ADSL) include the following two versions: G.992.1 (G.dmt) for a downstream transmission rate of about 6 Mbps, and G.992.2 (G.lite) for about 1.5 Mbps. Both versions use Discrete Multitone (DMT) techniques for signal modulation.
(1) DMT Modulation System
Conventional DMT modulation techniques will be described below with reference to
FIG. 14
, focusing on the downstream signal modulation and demodulation in a G.lite-based ADSL system. The upper half of
FIG. 14
shows a transmitter unit which comprises: a serial-to-parallel buffer
10
, an encoder
20
, an IFFT unit
30
, a parallel-to-serial buffer
40
, a D/A converter
50
, and a transmission bit map
60
. The serial-to-parallel buffer
10
stores transmission data for a single symbol period of 250 microseconds (i.e., the reciprocal of a symbol rate 4 kHz) and converts it into a parallel data format. The encoder
20
applies a prescribed modulation processing to the parallel data supplied from the buffer
10
. The inverse fast Fourier transform (IFFT) unit
30
processes the output data of the encoder
20
using IFFT algorithms. The parallel-to-serial buffer
40
converts the transformed data back into a serial data format, as well as adding a cyclic prefix (described later) to each symbol. The digital-to-analog (D/A) converter
50
converts the serial data to an analog signal at a sampling rate of 1.104 MHz and outputs it to a metallic subscriber line
70
. The transmission bit map
60
is an allocation table describing how many data bits should be assigned to each DMT carrier. This table is called the “bit map” in the ADSL terminology.
The lower half of
FIG. 14
shows a receiver unit, which comprises: an A/D converter
80
, a TEQ unit
90
, a buffer
100
, an FFT unit
110
, an FEQ unit
120
, a decoder
130
, a parallel-to-serial buffer
140
, a reception bit map
150
, and a TEQ training block
160
. The analog-to-digital (A/D) converter
80
receives a DMT-modulated analog signal transmitted over the metallic subscriber line
70
and converts it into digital form at a sampling rate of 1.104 MHz. The time domain equalizer (TEQ) unit
90
then processes this digital data signal in a prescribed manner, so that intersymbol interference (ISI) to a cyclic prefix, which has been added at the parallel-to-serial buffer
40
, will settle within the period of that cyclic prefix. The serial-to-parallel buffer
100
converts the output data of the TEQ unit
90
into parallel data, after removing a cyclic prefix from each symbol. The fast Fourier transform (FFT) unit
110
converts the output data of the serial-to-parallel buffer
100
into parallel data signals in the frequency domain by using FFT algorithms. The frequency domain equalizer (FEQ) unit
120
equalizes those frequency-domain data signals according to the transmission characteristics (or frequency response) of the metallic subscriber line
70
. The decoder
130
demodulates the output data of the FEQ unit
120
in a prescribed manner. The parallel-to-serial buffer
140
receives parallel data from the decoder
130
and converts it into serial data. As the counterpart of the transmission bit map
60
, the reception bit map
150
stores information about the number of data bits assigned to each carrier at the sending end. Based on this information, a decoder
130
and parallel-to-serial buffer
140
decodes the received data. The TEQ training block
160
adjusts the characteristics of the TEQ unit
90
, with reference to the output signals of the FFT unit
110
.
The above-described conventional system operates as follows. The transmitter unit accepts data to be transmitted at the serial-to-parallel buffer
10
, which actually stores data bits for a single symbol period of 250 microseconds (i.e., the reciprocal of the symbol rate 4 kHz). Those stored data bits are divided into groups according to the bit allocation previously defined in the transmission bit map
60
. They are then supplied to the encoder
20
, which maps each given bit sequence to specific code points in the signal constellation of quadrature amplitude modulation. Those constellation points are passed to the IFFT unit
30
. The IFFT unit
30
performs inverse fast Fourier transform to accomplish the quadrature amplitude modulation of each constellation point. The data signal modulated in this way is then output to the parallel-to-serial buffer
40
. Note here that the DMT modulation is realized by a combination of the encoder
20
and IFFT unit
30
.
The parallel-to-serial buffer
40
now chooses the 240th to 255th samples of the IFFT output data and adds a copy of those sixteen samples to the beginning of a DMT symbol. This is called the “cyclic prefix,” the details of which will appear in a later section. The symbol data with a cyclic prefix is now sent from the parallel-to-serial buffer
40
to the D/A converter
50
. The D/A converter
50
converts it into an analog signal at the sampling rate of 1.104 MHz and transmits it toward the remote subscriber over the metallic subscriber line
70
.
At the subscriber's end, the A/D converter
80
converts the received signal to a digital data signal at the rate of 1.104 MHz and supplies it to the TEQ unit
90
. This digital data signal has been impaired with intersymbol interference. The TEQ unit
90
processes it in such a way that the effect of intersymbol interference will be confined within a limited period of the 16-sample cyclic prefix. The processed signal is then stored in the serial-to-parallel buffer
100
, the length of which is one DMT symbol period. The serial-to-parallel buffer
100
converts this signal into parallel form, removing a cyclic prefix from each symbol. The FFT unit
110
demodulates the resultant parallel data signals with fast Fourier transform algorithms, thereby reproducing the original constellation points. Those reproduced constellation points are then fed to the FEQ unit
120
to compensate for the amplitude and phase distortion that has occurred during the travel over the metallic subscriber line
70
. This equalization is performed for individual carriers having different frequencies. The decoder
130
then decodes the equalized signals according to the reception bit map
150
, which is identical to the transmission bit map
60
. (More details will be provided in a later section, about the signal processing path from the TEQ unit
90
to the decoder
130
.) F
Hasegawa Kazutomo
Koizumi Nobukazu
Miyoshi Seiji
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Tse Young T.
LandOfFree
Data transmission system and receiver unit thereof does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Data transmission system and receiver unit thereof, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Data transmission system and receiver unit thereof will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3209388