Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1998-01-28
2001-10-23
Pham, Chi (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
Reexamination Certificate
active
06307893
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communication systems, and more specifically, to a system and method which allows for the shaping of a transmit signal spectrum in a modem in order to control the amount of power transmitted in a particular frequency bandwidth.
BACKGROUND OF THE INVENTION
Modems, or remote terminal units, convey information from one location to another over a data communications connection. Digital Subscriber Line (DSL) technology now enables modems to communicate large amounts of data. Modems normally communicate by modulating a baseband signal carrying digital data, precoding the transmit signal to match channel impulse response characteristics, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation, differentially encoding the information, and modulating the information. The signal space constellation can include both analog and digital information or merely digital information.
Communication signals, such as those used in digital subscriber line (DSL) technology, that are transmitted over an analog copper wire pair must meet specific power and frequency requirements as required by government regulations. These requirements dictate that a limited amount of energy per unit of bandwidth be transmitted for a given portion of the frequency spectrum. Typically, higher power is permitted to be transmitted at lower frequencies, while as transmit frequency increases, the power transmitted must be decreased. These government regulatory requirements are designed to limit interference and are imposed on all services that share the analog communications system. Previously, there has been no way to provide the required spectral shaping without unduly limiting the available transmit power.
Furthermore, conventional preceding techniques as described in U.S. Pat. No. 5,396,515, U.S. Pat. No. 5,559,835 and U.S. Pat. No. 5,159,610, and specified in the International Telecommunications Union-Telecommunications (ITU-T) V0.34 standard require a receiver to reply to a transmitter with information regarding the channel response of the communications channel during a training period so as to enable the transmitter to set its precoder. This requires a lengthy training period in which a modem transmitter awaits information from a receiver in order to set the transmitter's precoder in order to maximize transmission efficiency.
SUMMARY OF THE INVENTION
It is desirable to have a precoder in a modem transmitter that also contains a shaping filter that can shape the transmit signal spectrum to meet regulatory requirements without requiring a training period. The present invention provides a system and method for shaping a transmit signal spectrum. The system includes a filter having a set of filter coefficients defining a particular transmit spectrum. The defined transmit spectrum is one that meets regulatory requirements regarding power transmitted within particular frequency limits. In the preferred embodiment, a finite impulse response (FIR) filter is illustrated, however other filter configurations are possible. The filter is designed to receive a signal space mapped signal from a mapper as is known in the art of modem communications. This signal space mapped signal can be a multidimensional mapped signal, for example a quadrature amplitude modulated (QAM) signal, or it can be a one-dimensional signal, such as that generated by pulse amplitude modulation (PAM). This signal space mapped signal has subtracted from it a substantially flat spectrum signal from a modulo operator. The output of the filter is added to the signal space mapped substantially flat spectrum signal and provides as an output a precoded signal having the desired transmit signal spectrum.
The filter signal is fed back into a feedback loop designed to receive the same signal space mapped substantially flat spectrum signal that was input to the filter and, due to feedback, generate the inverse transfer function, resulting in an output that has a frequency response exactly opposite that of the output of the filter. A modulo operator in the feedback loop follows the filter. The modulo operator is designed to operate on the signal from the filter in order to restrict the magnitude of the output of the filter resulting in a substantially flat spectrum signal. The modulo operator operates on the signal from the filter whereby if the voltage of the signal is greater than a preset value, twice that value is subtracted from the signal. A subtractor designed to subtract the output signal of the modulo operator from the signal space mapped signal provides the input to the filter. The input to the filter is essentially a substantially flat spectrum signal and results in a precoded transmit signal output from the filter that is supplied to an adder, which is configured to add the output of the filter to the substantially flat spectrum signal and output a transmit signal that meets the desired transmit spectrum characteristics.
The invention has numerous advantages, a few of which are indicated hereafter, as examples.
An advantage of the present invention is that it allows the shaping of a transmit signal using a precoder without the requirement of a receiver sending back to a transmitter a signal containing channel response information. For example, the present invention can be implemented as a standard preset precoder that uses existing coefficients or in combination with a standard precoder using remotely computed coefficients.
Another advantage of the present invention is that it optimizes performance in a modem that implements spectral shaping.
Another advantage of the present invention is that modem training is not required, thus allowing the transmitter to develop the required transmit signal spectrum including precoding without the need for modem training. This allows the modem to be used in broadcast applications where multiple receivers monitor a common transmitter.
Another advantage is that the modem can be used in multipoint applications where multiple transmitters are used to transmit to a common receiver.
Other features and advantages of the present invention will become apparent to one of skill in the art upon review of the following drawings and the detailed description of the preferred embodiment.
REFERENCES:
patent: 4791483 (1988-12-01), Miller
patent: 5249200 (1993-09-01), Chen et al.
patent: 5388124 (1995-02-01), Laroia et al.
patent: 5396519 (1995-03-01), Betts et al.
patent: 5446758 (1995-08-01), Eyuboglu
patent: 5488633 (1996-01-01), Laroia
patent: 5512957 (1996-04-01), Hulyalkar
patent: 5878077 (1999-03-01), Betts
Betts William L.
Bremer Gordon
Burd Kevin M.
Paradyne Corporation
Pham Chi
Thomas Kayden Horstemeyer & Risley
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