Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
2002-01-16
2003-11-25
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transceivers
Modems
C375S260000
Reexamination Certificate
active
06654410
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to Digital Subscriber Line (DSL) systems. In particular, this invention relates to a method of initializing modems in a DSL system.
2. Description of Related Art
Multicarrier modulation, or Discrete Multitone Modulation (DMT), is a transmission method that is being widely used for communication over media, and especially over difficult media. Multicarrier modulation divides the transmission frequency band into multiple subchannels, i.e., carriers, with each carrier individually modulating a bit or a collection of bits. A transmitter modulates an input data stream containing information bits with one or more carriers and transmits the modulated information. A receiver demodulates all of the carriers in order to recover the transmitted information bits as an output data stream.
Multicarrier modulation has many advantages over single carrier modulation. These advantages include, for example, a higher immunity to impulse noise, a lower complexity equalization requirement in the presence of a multipath, a higher immunity to narrow band interference, a higher data rate and bandwidth flexibility. Multicarrier modulation is being used in many applications to obtain these advantages, as well as for other reasons. The applications include, for example, Asymmetric Digital Subscriber Line (ADSL) systems, Wireless LAN systems, power line communications systems, and other applications. ITU standards G.992.1, G.992.2 and the ANSI T1.413 standard, each of which are incorporated herein by reference in their entirety, specify standard implementations for ADSL transceivers that use multicarrier modulation.
FIG. 1
illustrates an exemplary standard compliant ADSL DMT transmitter
100
. In particular, the ADSL DMT transmitter
100
comprises three layers: the modulation layer
110
, the Framer/Forward Error Correction (FEC) layer
120
, and the ATM TC (Asynchronous Transfer Mode Transmission Convergence) layer
140
.
The modulation layer
110
provides the functionality associated with DMT modulation. In particular, DMT modulation is implemented using an Inverse Discrete Fourier Transform (IDFT)
112
. The IDFT
112
modulates bits from the Quadrature Amplitude Modulation (QAM) encoder
114
into the multicarrier subchannels. The ADSL multicarrier transceiver modulates a number of bits on each subchannel, the number of bits depending on the Signal to Noise Ratio (SNR) of that subchannel and the Bit Error Rate (BER) requirement of the communications link. For example, if the required BER is 1×10
−7
, i.e., one bit in ten million is received in error on average, and the SNR of a particular subchannel is 21.5 dB, then that subchannel can modulate 4 bits, since 21.6 dB is the required SNR to transmit 4 QAM bits with a 1×10
−7
BER. Other subchannels can have a different number of bits allocated to them at the same BER. The current ITU and ANSI ADSL standards allow up to 15 bits to be modulated on one carrier.
A table that specifies how many bits are allocated to each subchannel for modulation is one DMT symbol is called a Bit Allocation Table (BAT). A DMT symbol is the collection of analog samples generated at the output of the IDFT by modulating the carriers with bits according to the BAT is the main parameter used in the modulation layer
110
. The BAT is used by the QAM encoder
114
and the IDFT 112 for encoding and modulation. The following Table illustrates an example of a BAT for an ememplary DMT system having 16 subchannels.
TABLE 1
Subchannel
Bits per
Number
Subchannel
1
5
2
9
3
3
4
2
5
4
6
0
7
5
8
7
9
8
10
3
11
0
12
5
13
6
14
8
15
4
16
3
Total Bits Per
80
DMT Symbol
In ADSL systems, the typical DMT symbol rate is approximately 4 kHz. This means that a new set of bits, using the modulation BAT, is transmitted every 250 microseconds. If the exemplary BAT in Table 1, which specifies 80 bits modulated in one DMT symbol, were used at a 4 kHz DMT symbol rate, the bit rate of the system would be 4000*80=320 kbits per second (kbps).
The BAT determines the data rate of the system and is dependent on the transmission channel characteristics, i.e., the SNR of each subchannel in the multicarrier system. A channel with low noise, i.e., a high SNR on each subchannel, will have many bits modulated on each DMT carrier and will thus have a high bit rate. If the channel conditions are poor, e.g., high noise, the SNR will be low and the bits modulated on each carrier will be few, resulting in a low system bit rate. As can be seen in Table 1, some subchannels may actually modulate zero bits. An example is the case when a narrow band interferer, such as an AM broadcast, is present at a subchannel's frequency and causes the SNR in that subchannel to be too low to carry any information bits.
The ATM TC layer
140
comprises an Asynchronous Transfer Mode Transmission Convergence (ATM TC) section
142
that transforms bits and bytes in cells into frames.
The Framer/FEC layer
120
provides the functionality associated with preparing a stream of bits for modulation. The Framer/FEC layer
120
comprises an Interleaving (INT) portion
122
, a Forward Error Correction (FEC) portion
124
, a scrambler (SCR) portion
126
, a Cyclic Redundancy Check (CRC) portion
128
and an ADSL Framer portion
130
. The Interleaving and FEC coding provide an impulse immunity and a coding gain. The FEC portion
124
in the standard ADSL system is a Reed-Solomon (R-S) code. The scrambler
126
is used to randomize the data bits. The CRC portion
128
is used to provide error detection at the receiver. The ADSL Framer portion
130
frames the received bits from the ATM framer
142
. The ADSL framer
130
also inserts and extracts overhead bits from the module
132
for modem to modem overhead communication channels, which are known as EOC and AOC channels in the ADSL standards.
The key parameters of the Framer/FEC layer
120
are the size of the R-S codeword, the size, i.e., depth, of the interleaver, which is measured in the number of R-S codewords, and the size of the ADSL frame. As an example, a typical size for an R-S codeword may be 216 bytes, a typical size for interleaver depth may be 64 codewords, and a typical size of the ADSL frame may be 200 bytes. It is also possible to have an interleaving depth equal to one, which is equivalent to no interleaving. In order to recover the digital signal that was originally prepared for transmission using a transmitter as discussed above, it is necessary to deinterleave the codewords by using a deinterleaver that performs the inverse process to that of the interleaver, with the same depth parameter. In the current ADSL standards, there is a specific relationship between all of these parameters in a DMT system. Specifically, the BAT size, N
BAT
, i.e., the total number of bits in a DMT symbol, is fixed to be an integer divisor of the R-S codeword size, N
FEC
, as expressed in Equation 1:
N
FEC
=S*N
BAT
, (1)
where S is a positive integer greater than 0.
This constant can also be expressed as one R-S codeword containing an integer number of DMT symbols. The R-S codeword contains data bytes and parity, i.e, checkbytes. The checkbytes are overhead bytes that are added by the R-S encoder and are used by the R-S decoder to detect and correct bit errors. There are R checkbytes in a R-S codeword. Typically, the number of checkbytes is a small percentage of the overall codeword size, e.g., 8%. Most channel coding methods are characterized by their coding gain, which is defined as the system performance improvement, in dB, provided by the code when compared to an uncoded system. The coding gain of the R-S codeword depends on the number of checkbytes and the R-S codeword size. A large R-S codeword, e.g., greater than 200 bytes in a DMT ADSL system, along with 16 checkbytes, i.e., 8% of the 200 bytes, will provide close to the maximum coding gain of 4 dB. If the codeword size is smaller and/or the percentage of checkby
Aware Inc.
Chin Stephen
Lugo David B.
Nixon & Peabody LLP
Vick Jason H.
LandOfFree
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