Multiplex communications – Duplex – Time division
Reexamination Certificate
1998-09-14
2002-11-12
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Duplex
Time division
C370S470000, C370S479000, C370S506000, C370S516000, C375S260000, C375S362000, C375S371000, C714S701000, C714S752000, C714S774000, C714S776000
Reexamination Certificate
active
06480475
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data transmission systems, and more particularly, to data transmission systems utilizing time-division duplexing.
2. Description of the Related Art
Bi-directional digital data transmission systems are presently being developed for high-speed data communications. 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). In general, these high-speed data communications techniques are referred to as xDSL systems.
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 twisted-pair phone lines. The standard, known as ADSL, 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), and is hereby incorporated by reference. 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.
The ADSL standard also defines the use of reverse transmissions at a data rate in the range of 16 to 800 Kbit/s. The reverse transmissions follow 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 cancel 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 about 4 Mbit/s and up to about 52 Mbit/s or greater in the downstream direction. 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 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). For example, some of the possible VDSL/FTTC modulation schemes include 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.
Additionally, multicarrier modulation transmission schemes have been receiving a large amount of attention due to the high data transmission rates they offer.
FIG. 1A
is a simplified block diagram of a conventional transmitter
100
for a multicarrier modulation system. The conventional transmitter
100
is, for example, suitable for DMT modulation in ADSL or VDSL systems. The transmitter
100
receives data signals to be transmitted at a buffer
102
. The data signals are then supplied from the buffer
102
to a forward error correction (FEC) unit
104
. The FEC unit
104
compensates for errors that are due to crosstalk noise, impulse noise, channel distortion, etc. The signals output by the FEC unit
104
are supplied to a data symbol encoder
106
. The data symbol encoder
106
operates to encode the signals for a plurality of frequency tones associated with the multicarrier modulation. In assigning the data, or bits of the data, to each of the frequency tones, the data symbol encoder
106
utilizes data stored in a transmit bit allocation table
108
and a transmit energy allocation table
110
. The transmit bit allocation table
108
includes an integer value for each of the carriers (frequency tones) of the multicarrier modulation. The integer value indicates the number of bits that are to be allocated to the particular frequency tone. The value stored in the transmit energy allocation table
110
is used to effectively provide fractional number of bits of resolution via different allocation of energy levels to the frequency tones of the multicarrier modulation. In any case, after the data symbol encoder
106
has encoded the data onto each of the frequency tones, an Inverse Fast Fourier Transform (IFFT) unit
112
modulates the frequency domain data supplied by the data symbol encoder
106
and produces time domain signals to be transmitted. The time domain signals are then supplied to a digital-to-analog converter (DAC)
114
where the digital signals are converted to analog signals. Thereafter, the analog signals are transmitted over a channel to one or more remote receivers.
FIG. 1B
is a simplified block diagram of a conventional remote receiver
150
for a multicarrier modulation system. The conventional remote receiver
150
is, for example, suitable for DMT demodulation in ADSL or VDSL systems. The remote receiver
150
receives analog signals that have been transmitted over a channel by a transmitter. The received analog signals are supplied to an analog-to-digital converter (ADC)
152
. The ADC
152
converts the received analog signals to digital signals. The digital signals are then supplied to a Fast Fourier Transform (FFT) unit
154
that demodulates the digital signals while converting the digital signals from a time domain to a frequency domain. The demodulated digital signals are then supplied to a frequency domain equalizer (FEQ) unit
156
. The FEQ unit
156
performs an equalization on the digital signals so the attenuation and phase are equalized over the various frequency tones. Then, a data symbol decoder
158
receives the equalized digital signals. The data symbol decoder
158
operates to decode the equalized digital signals to recover the data, or bits of data, transmitted on each of the carriers (frequency tones). In decoding the equalized digital signals, the data symbol decoder
158
needs access to the bit allocation information and the energy allocation information that were used to transmit the data. Hence, the data symbol decoder
158
is coupled to a received bit allocation table
162
and a received energy allocation table
160
which respectively store the bit allocation information and the energy allocation information that were used to transmit the data. The data obtained from each of the frequency tones is then forwarded to the forward error corr
Chow Jacky S.
Modlin Cory S.
Tang Eugene Yuk-Yin
Tong Po
Brady III W. James
Hernandez Pedro P.
Hsu Alpus H.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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