Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1999-07-23
2001-06-05
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transceivers
Modems
C375S260000
Reexamination Certificate
active
06243414
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to telecommunications. More particularly, the present invention relates to the transmission of data utilizing discrete multitone technology (DMT). The present invention is advantageously utilized in digital subscriber line (DSL) technology, although it is not limited thereto.
2. State of the Art
Recently, digital subscriber line (DSL) technology has been touted as the answer for the ever-increasing demand for transfer of information, and the requirement for higher and higher information transfer rates. DSL modems provide a much higher data rate than the convention V.34 and V.90-type modems. The DSL modems utilize discrete multitone (DMT) technology to transfer information. In DMT technology, a plurality of predefined frequencies (tones) are simultaneously subjected to quadrature amplitude modulation (QAM) in order to transfer information across a channel. In recently promulgated standards such as G.Lite and G.dmt standards (Recommendations G.992.1 and G.992.2 ITU-Telecommunication Standardization Sector, Study Group 15, MA-007 and MA-008, Melbourne Australia Mar. 29-Apr. 2, 1999) both of which are hereby incorporated by reference herein in their entireties, one hundred twenty-eight and two hundred fifty-six tones are specified respectively, with an integer number of bits of up to fifteen being transferred per tone. The actual tones utilized depends upon the signal-to-noise ratio (SNR) distribution of the channel. In particular, during a handshake sequence, the channel is scanned, and the SNR distribution and/or other parameters are measured.
The actual bit rate provided by a DMT-based system actually depends on the signal-to-noise ratio (SNR) distribution at the input of a receiver. The higher SNR per tone, the more bits the tone can carry (transfer). In turn, the SNR distribution is a function of signal attenuation and the noise power spectral density (PSD). It is well known that the signal attenuation is often a non-monotonic function of frequency, with one or more deep notches located along the frequency spectrum. In addition, PSD is not a flat function of frequency. As a result, the SNR is generally a multiextreme function of frequency.
An example of the SNR distribution for a 16 kft subscriber line is shown in Table 1. In this example, the first five tones would not be used as they would interfere with the “plain old telephone service” (POTS). The sixth tone has a SNR=29.51 dB. If the desired bit error rate (BER) is set equal to 10
−7
, the sixth tone can carry six bits, as a SNR=27 dB is required for transmission of six bits, while a SNR=30 dB is required for transmission of seven bits. The seventeenth tone, on the other hand, having a SNR=10.46 dB cannot even transmit a single bit, because a SNR of at least 11 dB is required to transmit one bit when the BER=10−
7
.
Some SNR adjustment is possible in DMT-based systems. For example, the transmitted level of the sixth tone in Table 1 may be increased by 0.49 dB to allow the tone to bear seven bits, with the transmitted level of the seventeenth tone may be increased by 0.54 dB to allow the tone to carry one bit. Thus, according to Section 11.12.14 and Section 11.11.13 of the G992.2 standard, and Sections 10.8.13 and 10.9.14 of the G992.1 standard, during initialization, the transmitting modem is provided information by the receiving modem regarding the number of bits to be sent (B) and the gain (G) for each tone being transmitted. However, the permissible signal gain is usually restricted. According to the previously incorporated G.lite standard, the maximum gain for any one tone is set equal to 2.5 dB. As a result, with a BER=10
−7
, no tone having a SNR<8.5 dB can be used for data transmission. Using this criteria, it will be appreciated that in the case corresponding to Table 1, all tones with numbers 20 to 75 and 107 to 128 (as shown in bold type) cannot be used for data transmission. As a result, the actual bit rate is significantly reduced.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide methods, apparatus, and systems for transmitting data utilizing DMT technology.
It is another object of the invention to provide methods, systems, and apparatus for increasing the bit rate in DMT-based systems by utilizing low-SNR tones which would otherwise not be utilized in existing systems.
It is a further object of the invention to provide methods, systems, and apparatus for providing a high bit rate DMT signal by transmitting information on low-SNR tones in parallel with either other low-SNR tones or with higher SNR tones, and coherently combining such tones at the receiver.
It is an additional object of the invention to provide methods, systems, and apparatus for transmitting initialization signals within existing standards which identify the combination of low-SNR tones with other tones.
In accord with the objects of the invention, a method of providing a high bit rate DMT signal includes providing information on a plurality of DMT tones, with at least two of the plurality of DMT tones sending information in parallel (i.e., the tones are taken from a single DMT symbol). Of the two tones sending information in parallel, at least one of those tones has a SNR too low to be individually used for the number of bits it is transmitting. According to the method of the invention, the tones are coherently “combined” at the receiver in order to generate a SNR sufficient to carry, at the desired BER, the number of bits being transmitted in parallel. Thus, two or more tones which alone cannot carry a single bit of information may be used together via parallel transmission to provide a sufficient SNR at the receiver at the desired BER to transmit one or more bits. Similarly, two or more tones which alone cannot carry two bits of information (the minimum requirement of certain standards) may be used together via parallel transmission to provide a sufficient SNR at the receiver at the desired BER to transmit two or more bits. Further, the bit-carrying capacity of a first tone which can carry one or more bits may be increased by transmitting an increased number of bits in parallel with one or more additional tones which cannot carry any bits of information. Further yet, the energy margin of an active tone carrying one or more bits may be increased by transmitting the same number of bits in parallel with one or more additional tones which alone cannot carry a single bit of information.
According to another embodiment of the invention, rather than sending two or more DMT tones in parallel, one or more DMT tones are repeated over a plurality of DMT symbols, and “combined” coherently in order to generate a SNR sufficient to carry, at the desired BER, the number of bits being carried by the repeated tone. As with the embodiment which sent tones in parallel in a single symbol (also called the “frequency-diversity” technique), the repeated tone arrangement (also called the “time-diversity” technique) permits a single tone which alone cannot carry a single bit of information to provide a sufficient SNR at the receiver at the desired BER to transmit one or more bits. Similarly, a tone which alone cannot carry two bits of information may be repeated over two or more symbols to provide a sufficient SNR at the receiver at the desired BER to transmit two or more bits. It should be appreciated that the time-diversity technique has certain advantages and disadvantages relative to the frequency-diversity technique. In particular, a disadvantage is that it introduces an at least one-symbol delay into the signal processing. An advantage is that for several consecutive symbols, the SNR ratio for any particular tone will be close to each other, and consequently the aggregate SNR increases quickly.
According to one embodiment of the invention, the time-diversity technique can be combined with the frequency-diversity technique to increase the bit rate of the system. It will be appreciated
Drucker Vitaly
Goldstein Yuri
Hanna William
Okunev Yuri
Chin Stephen
Deppe Betsy L.
Gallagher Thomas A.
Gordon David P.
Jacobson David S.
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