Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-08-13
2003-06-24
Pham, Chi (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S285000, C375S222000
Reexamination Certificate
active
06584160
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to digital transmission systems, and more particularly relates to a system and method for reducing the effects of clipping in a DMT transceiver.
2. Discussion of the Related Art
In order to make high data rate interactive services such as video conferencing available to more residential and small business customers, high-speed data communication paths are required. Although fiber optic cable is the preferred transmission media for such high data rate services, it is not readily available in existing communications networks, and the expense of installing fiber optic cable is prohibitive. Current telephone wiring connections, which consist of copper twisted-pair media, are not designed to support the data rates, or bandwidth, required for interactive services. Asymmetric Digital Subscriber Lines (ADSL) technology has been developed to increase the effective bandwidth of existing twisted-pair connections, allowing interactive services to be provided without requiring the installation of new fiber optic cable.
Discrete Multi-Tone (DMT) is a multicarrier technique that divides the available bandwidth of twisted-pair connections into many subchannels. The DMT technique has been adopted by the ANSI T1E1.4 (ADSL) committee for use in ADSL systems. In ADSL, DMT is used to generate 250 separate 4.3125 kHz subchannels from 26 kHz to 1.1 MHz for downstream transmission to the enduser, and 26 subchannels from 26 kHz to 138 kHz for upstream transmission by the enduser. The transmission capability of the individual subchannels are evaluated for each connection, and data is allocated to the subchannels according to their transmission capabilities (the number of bits each subchannel can support). Subchannels that are not capable of supporting data transmission are not used, whereas the bit-carrying capacity of subchannels that can support transmission is maximized. Thus, by using DMT in an ADSL system, the transmission capability of each twisted-pair connection is maximized over the fixed bandwidth.
Once the transmission capability of a connection has been established, the data transfer process begins by encoding the data. Data in an ADSL system is grouped in frames, where a frame represents a time-slice of the data to be transmitted. Bits from the frames are assigned to the subchannels based on the number of bits that each subchannel can support, and the subchannels are encoded by creating a frequency-domain vector set. Frequency-domain vectors in the vector set use phase and magnitude components to encode the values of the bits. An Inverse Fast Fourier Transform (IFFT) performs a frequency-to-time conversion of the frequency-domain vectors, resulting in digital time-domain information. A digital-to-analog converter (DAC) then converts the digital information to an analog signal which a transmitter transmits onto the copper twisted-pair media. The ANSI T1E1.4 standard defines the average power requirement of the signal for transmission on the twisted pair media, and in order to satisfy the power requirement, an amplifier is required.
When the analog signal from the DAC overshoots a magnitude threshold, which is dependent on the power supply used in the system, clipping of the signal can occur. Peaks in the analog signal occur when the vectors in the frequency-domain vector set are combined through the IFFT. Each frequency-domain vector contributes to the magnitude of the time-domain signal, and if the frequency-domain vectors are such that their contributions are concentrated in one area of the time-domain signal, peaks can result. Clipping occurs when the Integrated Circuit (IC) on which the transmitter is fabricated cannot support the dynamic range requirements of the peaking signal and can result in the loss of information. Section 7.11.1 of the T1E1.4 standard addresses this problem and limits the information loss by specifying that the probability of the signal clipping be less than one in 10 million.
The probability of a peak exceeding the magnitude threshold (maximum signal power on the IC) is based on the Peak-to-Average Ratio (PAR) of the signal, which is a ratio of the maximum power of the signal to the average power of the signal. If the average power is small compared to the magnitude threshold, a large peak can occur without exceeding the point where clipping occurs. Therefore, one method of reducing the number of peaks exceeding the magnitude threshold for a fixed PAR is to reduce the average power of the signal. Although this reduces the occurrence of clipping, lower signal strength increases susceptibility to noise, which can cause other transmission problems. Another method of reducing the probability of clipping utilizes a larger power supply, which raises the magnitude threshold where clipping occurs. A larger power supply, however, increases cost and consumes excessive power and adds additional regulatory requirements.
Therefore, a need exists for a method and/or apparatus to reduce the occurrence of signal peaks in a DMT transmitter such that the power supply of the system can be reduced, the signal strength can be raised, and/or the probability of the signal clipping can be reduced.
Several approaches to address this problem have been made by systems known in the prior art. For example, U.S. Pat. No. 5,835,536 discloses one such system. As illustrated in
FIG. 1
, U.S. Pat. No. 5,835,536 discloses a system having a DMT transmitter including a symbol generator
104
, a magnitude comparator
112
, and a magnitude adjuster
114
. The DMT transmitter receives framed data
102
at the symbol generator
104
and generates a time-domain DMT symbol
110
based on the framed data
102
. In an ADSL system, the symbol generator
104
includes an ADSL constellation encoder
106
and an IFFT block
108
. The ADSL constellation encoder
106
encodes the framed data
102
by mapping the values of the data bits to frequency-domain vectors on subchannels within the bandwidth used for ADSL transmission. The number of bits that can be encoded on each subchannel may be determined by sending a training signal. The IFFT block
108
transforms the frequency-domain vectors to the time-domain, resulting in a time-domain DMT symbol
110
.
The magnitude comparator
112
compares the magnitude of the time-domain DMT symbol
110
to a magnitude threshold to determine if clipping will occur. The magnitude adjuster
114
includes a magnitude adjusting symbol
116
, a multiplexer or mux
118
, and an adder
120
. When the magnitude comparator
112
determines that the magnitude of the time-domain DMT symbol
110
is such that clipping will occur, it directs the mux
118
to pass the magnitude adjusting symbol
116
to the adder
120
which adds it to the time-domain DMT symbol
110
such that magnitude of the time-domain DMT symbol
110
is reduced, effectively reducing the PAR of the system.
Such a system, however, always makes the same magnitude of adjustment, regardless of how much the magnitude exceeds the clipping threshold.
As illustrated in
FIG. 2
, U.S. Pat. No. 5,835,536 also discloses an alternative DMT transmitter which includes a symbol generator
204
, a magnitude comparator
210
, and a Symbol modifier
208
. The symbol generator
204
generates a time-domain DMT symbol
206
based on the framed data
202
. The magnitude comparator
210
compares the magnitude of the time-domain DMT symbol
206
to a magnitude threshold to determine if clipping will occur. When the magnitude of the time-domain DMT symbol
206
compares unfavorably to the magnitude threshold, the symbol modifier
208
modifies the time-domain DMT symbol
206
to produce a modified time-domain DMT symbol
212
of reduced magnitude. The symbol modifier
208
may modify the symbol by altering the mapping function used for encoding the data, altering certain vectors in the frequency-domain representation of the DMT symbol, etc. The symbol modifier
208
may also produce a modification signal
207
, wherein the modification signal
2
Amrany Daniel
Cai Lujing
Liu Wei-min
GlobespanVirata Inc.
Pham Chi
Thomas Kayden Horstemeyer & Risley
Tran Khanh
LandOfFree
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