Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1998-05-11
2001-10-23
Pham, Chi (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
Reexamination Certificate
active
06307889
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a system continuously transmitting a fast data signal having multicarrier in a digital subscriber line. More particularly, it relates to a method and system for reducing initial delay time occurring on an initialization process by using bit swapping in a multicarrier system, simultaneously capable of transmitting point-to-point and point-to-multipoint.
DESCRIPTION OF THE RELATED ART
A T1.413 of ANSI (American National Standards Institute) adopted a DMT (Discrete Multi-Tone) system as standardization for the ADSL (Asymmetric Digital Subscriber Lines) system.
FIG. 1
to
FIG. 5
illustrate a basic concept, characteristic, and basic architecture of the DMT system.
FIG. 1
illustrates a basic architecture of a transmitter/receiver of the DMT system. The transmitter
10
is composed of a serial-to-parallel converter
14
, a multicarrier modulator
16
and a pre-transmit processing part
18
. The receiver
12
is composed of a post-receive processing part
20
, a multicarrier demodulator
22
and a parallel-to-serial converter
24
. DSL or other types of communication channels can be connected between the transmitter and the receiver.
The serial input data transmitted at the rate of b
total
/T (bits/sec) form a multicarrier symbol according to accumulating b
total
bits each by the serial-to-parallel converter
14
, and then the symbol period is T. Bits in each multicarrier symbol are modulated by the modulator
16
, and the modulator
16
modulates b
total
bits of the total N subchannels according to the modulating b
i
bit of each ith subchannel.
The modulator
16
makes each multicarrier symbol having b
total
bits into N
s
time domain signals by using IFFT (Inverse Fast Fourier Transform). In this case, N
s
is equal to 2N.
Also, the receiver
12
can embody the multicarrier demodulator
22
demodulating the b
i
bit of an ith subchannel by using FFT (Fast Fourier Transformation). The modulation method embodied above is referred to as DMT modulation and
FIG. 2
illustrates a use of frequency for each subchannel of the DMT system.
As illustrated to
FIG. 2
, each subchannel of the DMT system uses a frequency band of 1/T (Hz), with the entire frequency band of N/T (Hz) being used by the DMT system.
FIG. 3
illustrates a normal architecture for the operation of the modulator/demodulator of the DMT system. {X
0
, X
1
, . . . , X
N−1
} signifies an original complex input data symbol, {X
k
} signifies a modulated data sequence (before adding a cyclic prefix), {h
k
} signifies a discrete time channel response, {n
k
} represents an additional noise sequence, {y
k
} signifies a received sequence (after removing the cyclic prefix), {{tilde over (X)}
0
, {tilde over (X)}
1
, . . . , {tilde over (X)}
N−1
} represents a decoded complex data symbol. P
i
and P
i
*
represent a modulating vector and a demodulating vector respectively, and preferably are orthogonal functions. That is, P
i
·P
i
*
=&dgr;
ij
where, · is the dot product of two vectors,
δ
ij
=
1
(
i
=
j
)
=
0
(
i
≠
j
)
.
Each vector can be expressed by using IDFT (Inverse Discrete Fourier Transform) and DFT (Discrete Fourier Transform):
X
m
=
1
2
N
∑
n
=
0
2
N
-
1
x
n
ⅇ
-
j2π
nm
2
N
,
x
m
=
1
2
N
∑
n
=
0
2
N
-
1
X
n
ⅇ
j2π
nm
2
N
The DMT system having N subchannels in the frequency domain uses the 2N-sized DFT, and samples in a time domain are real numbers due to conjugate symmetry in the frequency domain. IDFT and DFT use the IFFT and the FFT algorithm respectively. Cyclic prefix is a technology for preventing data from causing interblock interference, in which a start point of a block is affected by the end point of a block in the DMT system, and transmits data in front of the corresponding block according to copying the last v samples in the time domain after IDFT. The value v is determined by the impulse response length of a channel.
FIG. 4
is a block diagram illustrating the detailed operation for the transmitter
30
and the receiver
32
of the DMT system. The serial input data are converted into parallel data by accumulating a block of data, and encoded by the encoder
36
. The parallel output data from the encoder
36
are modulated by IFFT
38
, and converted again into serial data by the parallel-to-serial converter
40
. These modulated signals are converted into the analog signals by the digital-to-analog converter
42
after adding a cyclic prefix, and are transmitted to the communication channel
34
via the low-pass filter
44
and the DC isolating transformer
46
. The analog signals received in the receiver
32
are converted into the digital signals via the analog-to-digital converter
50
after passing through the DC isolating transformer and the low-pass filter
48
. These signals are demodulated by the FFT
56
after passing through a TEQ (Time domain Equalizer)
52
having a FIR (Finite Impulse Response) filter to minimize the real memory length of this channel, and then removing the cyclic prefix and converting into parallel signals by the serial-to-parallel converter
54
.
However, because the component of amplitude for the each frequency is different from the delay characteristics in the frequency band used, there occurs a difference between the transmitted signal and the received signal. That is, there is a difference between the parallel input of the decoder and the parallel output of the encoder. To solve this problem, the DMT system uses a simple type of a FEQ (Frequency domain Equalizer)
58
, that uses N one-tap complex equalizers to adjust attenuation and delay of each subchannel. The signals passing through the FEQ are converted into serial signals via the decoder
58
. Generally, the output data stream of the serial decoder
58
are identical with the input data stream of the serial encoder
36
in an ideal system.
In the DSL (Digital Subscriber Lines) using a telephone line having a large amount of channel attenuation and frequent ISI (Inter Symbol Interference) in the communication channel, an equalizer has to be used to minimize the ISI. Also, it is needed to optimize transmission bandwidth by using line-by-line for the best performance because of very different transmission environments of the subscribers.
In the conventional single carrier QAM (Quadrature Amplitude Modulation) system, symbol rate and carrier frequency determine transmission bandwidth. But, the single carrier system cannot sensitively regulate the variation of the symbol rate. That is, the symbol rate for the fixed data rate cannot help being regulated by the rate of b
symbol
/(b
symbol
+1). Herein, b
symbol
means the number of bits transmitted per data symbol. For example, in a case of the single carrier QAM having the symbol rate of 10 MHz, the transmission bandwidth of 40 Mbps (4 bit/symbol), 50 Mbps (5 bit/symbol) is determined by the number of bit transmitted per symbol. That is, the transmission bandwidth can only be adjusted in units of 10 MHz. In the DMT system having the symbol rate of 10 MHz and having 256 subchannels, each subchannel is assigned an adequate number of bits according to SNR (signal-to-noise ratio) of each channel. The DMT system can regulate a more sensitive transmission band than the single carrier system because what increases/decreases one bit for one subchannel among them, sensitively affects (10 MHz/256 bps) the whole transmission bandwidth. This is called “bit loading”, and this is performed in the initialization of the DMT system. That is, since the subchannel having good SNR characteristics assigns a large number of bits and the subchannel of bad SNR assigns few bits, the DMT system can gain the best performance.
Also, the multicarrier system can reduce error probability that can happen during transmission by moving one or two bits from a subchannel to the other subchannel
Pham Chi
Samsung Electronics Co,. Ltd.
The Law Offices of Eugene M. Lee, PLLC
Tran Khai
LandOfFree
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