Pulse or digital communications – Testing – Data rate
Reexamination Certificate
2000-03-10
2002-12-24
Chin, Stephen (Department: 2634)
Pulse or digital communications
Testing
Data rate
C375S240200, C375S222000
Reexamination Certificate
active
06498808
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to communication systems and methods using multicarrier modulation. More particularly, the invention relates to communication multicarrier systems and methods using rate adaptive multicarrier modulation.
BACKGROUND OF THE INVENTION
Multicarrier modulation (or Discrete Multitone Modulation (DMT)) is a transmission method that is being widely used for communication over difficult media. Multicarrier modulation divides the transmission frequency band into multiple subchannels (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 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 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. Applications include Asymmetric Digital Subscriber Line (ADSL) systems, Wireless LAN systems, Power Line communications systems, and other applications. ITU standards G.992.1 and G.992.2 and the ANSI T1.413 standard specify standard implementations for ADSL transceivers that use multicarrier modulation.
The block diagram 100 for a standard compliant ADSL DMT transmitter known in the art is shown in FIG.
1
.
FIG. 1
shows three layers: the Modulation layer
110
, the Framer/FEC layer
120
, and the ATM TC layer
140
, which are described below.
The Modulation layer
110
provides functionality associated with DMT modulation. DMT modulation is implemented using an Inverse Discrete Fourier Transform (IDFT)
112
. The IDFT
112
modulates bits from the Quadrature Amplitude Modulation (QAM)
114
encoder into the multicarrier subchannels. ADSL multicarrier transceivers modulate 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 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 21.5 dB is the required SNR to transmit 4 QAM bits with a 1×10
−7
BER. Other subchannels can have a different SNR and therefore may have a different number of bits allocated to them at the same BER. The 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 in 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. The BAT is the main parameter used in the Modulation layer
110
of FIG.
1
. The BAT is used by the QAM
114
and IDFT
112
blocks for encoding and modulation. Table 1 shows an example of a BAT for a DMT system with 16 subchannels.
TABLE 1
Example of BAT for multicarrier system with 16 subchannels
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
80
Per DMT
symbol
In ADSL systems the DMT symbol rate is approximately 4 kHz. This means that a new DMT symbol modulating a new set of bits, using the modulation BAT, is transmitted every 250 microseconds. If the 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 kilobits 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 (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, 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 AM broadcast radio) 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
includes an Asynchronous Transfer Mode Transmission Convergence (ATM TC) block
142
that transforms bits and bytes in cells into frames.
The next layer in an ADSL system is the Frame/FEC layer
120
, which provides functionality associated with preparing a stream of bits for modulation, as shown in FIG.
1
. This layer contains the Interleaving (INT) block
122
, the Forward Error Correction (FEC) block
124
, the scrambler (SCR) block
126
, the Cyclic Redundancy Check (CRC) block
128
and the ADSL Framer block
130
. Interleaving and FEC coding provide impulse noise immunity and a coding gain. The FEC
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
128
is used to provide error detection at the receiver. The ADSL Framer
130
frames the received bits from the ATM framer
142
. The ADSL framer
130
also inserts and extracts overhead bits from module
132
for modem to modem overhead communication channels (known as EOC and AOC channels in the ADSL standards).
The key parameters in the Framer/FEC layer
120
are the size of the R-S codeword, the size (depth) of the interleaver (measured in number of R-S codewords) and the size of the ADSL frame. As examples, a typical size for an R-S codeword may be 216 bytes, a typical size for interleaver depth may be 64 codewords, and the 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
(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
, where S is a positive integer greater than 0. (1)
This constraint can also be expressed as: One R-S codeword contains an integer number of DMT symbols. The R-S codeword contains data bytes and parity (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 (greater than 200 bytes in a DMT ADSL system) along with a 16 checkbytes (8% of 200 bytes) will provide close to the maximum coding gain of 4 dB. If the codeword size is smaller and/or the percentage of checkbyte overhead is high (e.g. ≦30%) the coding gain may be very small or even negative. In general, it is best to have the ADSL system operating with the largest possible R-S codeword (the maximum possible is 255 b
Aware Inc.
Chin Stephen
Kim Kevin
Nixon & Peabody LLP
Vick Jason H.
LandOfFree
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