Multiplex communications – Communication techniques for information carried in plural... – Adaptive
Reexamination Certificate
1999-10-08
2004-01-06
Olms, Douglas (Department: 2661)
Multiplex communications
Communication techniques for information carried in plural...
Adaptive
C370S441000, C370S493000, C370S474000, C370S412000, C370S329000, C375S220000, C375S296000, C375S377000
Reexamination Certificate
active
06674768
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a data transmission device and a data transmission method, and in particular, to a data transmission device and a data transmission method which are applied to XDSL (X Digital Subscriber Line (X: A, S, V, etc.)) which realizes high speed data transmission of the order of Mbit/s by use of metal cables such as telephone lines.
1. Description of the Prior Art
These days, techniques concerning XDSL (X Digital Subscriber Line: (X: A (Asymmetric), S (Symmetric), V (Very high speed), etc.)) for realizing high speed data transmission of the order of Mbit/s via metal cables such as telephone lines are attracting attention. Most of all, ADSL (Asymmetric Digital Subscriber Line), whose bit rate asymmetry between upstream data transmission and downstream data transmission is suitable for the access to the Internet, is receiving much attention.
In an ADSL transceiver unit, digital signals are converted to analog signals according to a modulation/demodulation method called DMT (Discrete Multi-Tone), and the analog signals are transmitted by the ADSL transceiver unit. In the DMT method, modulation according to QAM (Quadrature Amplitude Modulation) is executed to 256 carriers, and the modulated carriers are multiplexed together by means of IDFT (Inverse Discrete Fourier Transform), and the multiplexed signal is transmitted. On the receiving side, each modulated carrier is extracted from the multiplexed signal by means of DFT (Discrete Fourier Transform), and each QAM-modulated carrier is demodulated, and thereby high speed data transmission is implemented.
However, in the case of the ADSL transceiver unit, if a cable used for ADSL (an ADSL cable) is included in a cable bundle that also includes an ISDN (Integrated Services Digital Network) cable, noise is caused in the ADSL cable due to a variety of effects of the ISDN cable and thereby the transmission bit rate of the ADSL cable is necessitated to be deteriorated. Most of all, crosstalk noise from the ISDN cable has a large effect on the ADSL cable.
In the following, the crosstalk noise which occurs to the ADSL transceiver unit in the case where a TCM(Time Compression Multiplex)-ISDN line is placed adjacent to an ADSL line will be explained referring to FIG.
1
.
FIG. 1
shows crosstalk noise which occurs to an ADSL terminal ATU-R (ADSL Transceiver Unit at the Remote end) due to data transmission on the TCM-ISDN line when downstream data transmission is being executed on the ADSL line.
In the TCM-ISDN, upstream data transmission (1.25 msec) and downstream data transmission (1.25 msec) are alternately executed. If upstream data transmission on the TCM-ISDN line is executed when downstream data transmission is being executed on the ADSL line, high power signals on the TCM-ISDN line before attenuation exert effects on attenuated signals on the ADSL line, and thereby NEXT (Near End X(cross)-Talk) occurs to the terminal ATU-R. If downstream data transmission on the TCM-ISDN line is executed when downstream data transmission is being executed on the ADSL line, signals on the TCM-ISDN line exert effects on attenuated signals on the ADSL line, and thereby FEXT (Far End X(cross)-Talk) occurs to the terminal ATU-R. Incidentally, the same effects also occur on the side of the central office ATU-C (ADSL Transceiver Unit at the Central office).
FIG. 2
is a schematic diagram showing noise levels of the NEXT and the FEXT. As shown in
FIG. 2
, the noise level of NEXT is higher than that of FEXT, since high power signals on the TCM-ISDN line before attenuation exert effects on attenuated signals on the ADSL line in the case of NEXT. There has been proposed a data transmission method which pays attention to the difference of the noise level, in which the data rate; (data transmission bit rate) in FEXT periods (i.e. periods in which the lower noise due to the FEXT occurs) is set larger than the data rate in NEXT periods (i.e. periods in which the higher noise due to the NEXT occurs) as shown in the bottom of FIG.
2
. Such a method is called “dual bit-map method”.
Since the noise level changes periodically in the ADSL transceiver unit as mentioned above, generally, the SNR (Signal to Noise Ratio) of each carrier through the ADSL line is measured both in the upstream data transmission and in the downstream data transmission of the TCM. ISDN line (that is, both in the NEXT periods and in the FEXT periods), and thereby bit distribution and gain distribution onto the carriers are determined with respect to FEXT periods and with respect to NEXT periods, based on the SNR.
FIG. 3
shows an example of such bit distribution. In
FIG. 3
, the horizontal axis denotes the frequency of each carrier to be used for the data transmission, and the frequency difference between adjacent carriers is 4.3125 kHz, and the number of carriers is 256. Each carrier is modulated when data is transmitted. The SNR of each carrier is evaluated in the NEXT periods and in the FEXT periods, as a result of transmission of a pseudo random signal which will be described below. Thereafter, actual data transmission is executed according to the bit distribution and the gain distribution which have been determined based on the measurement of the SNR.
In the following, a concrete example of the SNR evaluation and the bit/gain distribution calculation will be described referring to FIG.
3
through FIG.
5
.
FIG. 4
is a block diagram showing an example of an ADSL data communication system. Referring to
FIG. 4
, an ATU-C
21
is provided to the system as the central office, and an ATU-R
22
is provided to the system as a terminal. The ATU-C
21
includes a transmission section
23
A and a reception section
24
A, and the ATU-R
22
includes a transmission section
23
B and a reception section
24
B.
FIG. 5
is a block diagram showing the composition of the ATU-C
21
and the ATU-R
22
of the ADSL data communication system of FIG.
4
. The ATU-C
21
on the left-hand side of
FIG. 5
includes the transmission section
23
A, the reception section
24
A, a pseudo random signal generation section
25
A, an SNR measurement section
26
A, a bit/gain distribution calculation section
27
A and a bit/gain distribution table
28
A. The transmission section
23
A includes an IDFT (Inverse Discrete Fourier Transform) section
10
A, and the reception section
24
A includes a DFT (Discrete Fourier Transform) section
29
A. Similarly, the ATU-R
22
on the right-hand side of
FIG. 5
includes the transmission section
23
B, the reception section
24
B, a pseudo random signal generation section
25
B, an SNR measurement section
26
B, a bit/gain distribution calculation section
27
B and a bit/gain distribution table
28
B. The transmission section
23
B includes an IDFT section
10
B, and the reception section
24
B includes a DFT section
29
B.
In the ATU-C
21
, the pseudo random signal generation section
25
A generates a pseudo random signal which includes all the carriers (256 carriers according to the standard of ANSI (American National Standards Institution), for example) and sends the pseudo random signal to the IDFT: section
10
A of the transmission section
23
A so as to be transmitted to the ATU-R
22
. The SNR measurement section
26
A calculates the SNR of each carrier of a pseudo random signal supplied from the ATU-R
22
, with respect to the FEXT periods and the NEXT periods. The bit/gain distribution calculation section
27
A calculates and determines the bit distribution and the gain distribution of the carriers with respect to the. FEXT periods and the NEXT periods, based on the SNR of each carrier in the FEXT periods and the SNR of each carrier in the NEXT periods which have been obtained by the SNR measurement section
26
A. The bit/gain distribution table
28
A receives bit distribution and gain distribution of the carriers which have been calculated and determined by the bit/gain distribution calculation section
27
B of the ATU-R
22
, and stores the bit distribution and the gain distribution.
In the ATU-R
22
, the pseudo r
NEC Corporation
Nguyen Van
Olms Douglas
Young & Thompson
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