Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1998-04-24
2002-03-05
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transceivers
Modems
C375S229000, C375S349000, C375S350000, C708S312000, C708S323000
Reexamination Certificate
active
06353629
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data communications, and more particularly, to channel shortening techniques for data communications.
2. Description of the Related Art
Bi-directional digital data transmission systems are presently being developed for high-speed data communication. One standard for high-speed data communications over twisted-pair phone lines that has developed is known as Asymmetric Digital Subscriber Lines (ADSL). Another standard for high-speed data communications over twisted-pair phone lines that is presently proposed is known as Very High Speed Digital Subscriber Lines (VDSL).
The Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, has finalized a discrete multi tone based approach for the transmission of digital data over ADSL. The standard is intended primarily for transmitting video data and fast Internet access over ordinary telephone lines, although it may be used in a variety of other applications as well. The North American Standard is referred to as the ANSI T1.413 ADSL Standard (hereinafter ADSL standard). Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of up to 8 million bits per second (Mbits/s) over twisted-pair phone lines. The standardized system defines the use of a discrete multi tone (DMT) system that uses 256 “tones” or “sub-channels” that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is defined as transmissions from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user). In other systems, the number of tones used may be widely varied. However when modulation is performed efficiently using an inverse fast Fourier transform (IFFT), typical values for the number of available sub-channels (tones) are integer powers of two, as for example, 128, 256, 512, 1024 or 2048 sub-channels.
The ADSL standard also defines the use of a reverse signal at a data rate in the range of 16 to 800 Kbit/s. The reverse signal corresponds to transmission in an upstream direction, as for example, from the remote location to the central office. Thus, the term ADSL comes from the fact that the data transmission rate is substantially higher in the downstream direction than in the upstream direction. This is particularly useful in systems that are intended to transmit video programming or video conferencing information to a remote location over telephone lines.
Because both downstream and upstream signals travel on the same pair of wires (that is, they are duplexed) they must be separated from each other in some way. The method of duplexing used in the ADSL standard is Frequency Division Duplexing (FDD) or echo canceling. In frequency division duplexed systems, the upstream and downstream signals occupy different frequency bands and are separated at the transmitters and receivers by filters. In echo cancelled systems, the upstream and downstream signals occupy the same frequency bands and are separated by signal processing.
ANSI is producing another standard for subscriber line based transmission system, which is referred to as the VDSL standard. The VDSL standard is intended to facilitate transmission rates of at least 12.98 Mbit/s and up to 51.92 Mbit/s or greater in the downstream direction. To achieve these rates, the transmission distance over twisted-pair phone lines must generally be shorter than the lengths permitted using ADSL. Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a similar system, which is referred to as Fiber To The Curb (FTTC). The transmission medium from the “curb” to the customer premise is standard unshielded twisted-pair (UTP) telephone lines.
A number of modulation schemes have been proposed for use in the VDSL and FTTC standards (hereinafter VDSL/FTTC). Most of the proposed VDSL/FTTC modulation schemes utilize frequency division duplexing of the upstream and downstream signals. Another promising proposed VDSL/FTTC modulation scheme uses periodic synchronized upstream and downstream communication periods that do not overlap with one another. That is, the upstream and downstream communication periods for all of the wires that share a binder are synchronized. With this arrangement, all the very high speed transmissions within the same binder are synchronized and time division duplexed such that downstream communications are not transmitted at times that overlap with the transmission of upstream communications. This is also referred to as a (i.e. “ping pong”) based data transmission scheme. Quiet periods, during which no data is transmitted in either direction, separate the upstream and downstream communication periods. For example, with a 20-symbol superframe, two of the DMT symbols in the superframe are silent (i.e., quite period) for the purpose of facilitating the reversal of transmission direction on the phone line. In such a case, reversals in transmission direction will occur at a rate of about 4000 per second. For example, quiet periods of about 10-25 &mgr;s have been proposed. The synchronized approach can be used a wide variety of modulation schemes, including multi-carrier transmission schemes such as Discrete Multi-Tone modulation (DMT) or Discrete Wavelet Multi-Tone modulation (DWMT), as well as single carrier transmission schemes such as Quadrature Amplitude Modulation (QAM), Carrierless Amplitude and Phase modulation (CAP), Quadrature Phase Shift Keying (QPSK), or vestigial sideband modulation. When the synchronized time division duplexed approach is used with DMT it is referred to as synchronized DMT (SDMT).
Multicarrier modulation has been receiving a large amount of attention due to the high data transmission rates it offers.
FIG. 1A
is a basic block diagram of a conventional multicarrier transmitter
10
. The multicarrier transmitter
10
receives serial input data at a rate Mf
s
bit/s. The serial input data is grouped by a serial-to-parallel converter
12
into blocks of M bits at a symbol rate of f
s
. The M bits are used by modulators
14
to modulate N
c
carriers (m
n
bits for carrier n) which are spaced &Dgr;f
c
apart across a usable frequency band. The modulated signals are then summed by an adder
16
and transmitted. In a receiver, the received signal is demodulated by each of the N
c
carriers, and m
n
bits are recovered from each carrier. A more detailed discussion of the principals of multicarrier transmission and reception is provided in J. A. C. Bingham, “Multicarrier Modulation For Data Transmission: An Idea Whose Time Has Come,” IEEE Communications Mag., pp. 5-14, May 1990.
FIG. 1B
is a block diagram of a conventional multicarrier modulation system
100
. The multicarrier modulation system
100
is generally known in the art and discussed in, for example, U.S. Pat. No. 5,285,474, which is hereby incorporated by reference. The multicarrier modulation system
100
has a transmitter side and a receiver side. The transmitter side includes an encoder
102
that receives digital signals to be transmitted. The encoder
102
encodes the digital signals and then passes the encoded signals to a IFFT unit
104
that modulates the encoded signals on multiple carriers. The modulated signals are then converted to analog signals by a digital-to-analog converter
106
. The resulting analog signals are then transmitted to a receiver over a channel
108
.
The receiver side of the multicarrier modulation system
100
operates to receive the transmitted analog signals from the transmitter side through the channel
108
. The received analog signals are converted into digital signals by an analog-to-digital converter
110
. The digital signals are then supplied to a time-domain equalizer (TEQ)
112
that compensates for the attenuation and delay on each of the subchannels. The resulting signals are then supplied to
Brady III W. James
Chin Stephen
Ha Dac V.
Moore J. Dennis
Telecky , Jr. Frederick J.
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