Pulse or digital communications – Cable systems and components
Reexamination Certificate
2000-02-09
2001-09-04
Pham, Chi (Department: 2731)
Pulse or digital communications
Cable systems and components
C379S413020
Reexamination Certificate
active
06285718
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apparatus and methods for wideband transmission of digital signals over telephone wires.
BACKGROUND OF THE INVENTION
A CAP (carrierless amplitude and phase) modulator is described in U.S. Pat. No. 4,924,492 to Gitlin et al.
Use of adaptive notch filters having constrained poles and zeros to eliminate narrow-band or sine wave components with unknown frequencies from observed time series is described in Stoica, P. and Nehorai, A., “Performance analysis of an adaptive notch filter with constrained poles and zeros”, IEEE Transactions on Acoustics, Speech and Signal Processing, 36(6), June 1988, and in Ng, T. S., “Some aspects of an adaptive digital notch filter with constrained poles and zeros”, IEEE Transactions on Acoustics, Speech and Signal Processing, ASSP-35(2), February 1987.
The disclosures of all publications mentioned in the specification and of the publications cited therein are hereby incorporated by reference.
SUMMARY OF THE INVENTION
The present invention seeks to provide improved modem apparatus.
The present invention relates to the transmission, at high bit rate, of digital signals over copper telephone wires. The pervading demand for high speed digital traffic requires the use of a high capacity physical medium as the backbone of a network. This is usually implemented via broadband optical fiber or coaxial cable. These cables interconnect the geographically distant central offices of the service providers (e.g. telephone companies) and also interconnect the central communications areas and curbside optical network units.
However, it is still extremely expensive to deploy fiber optic or coaxial cable to the individual premises of the customers. Therefore, the transmission over the local loop, typically the last 1-2 miles from an adjacent central office or from a curbside optical network unit to the customers' premises utilizes the existing infrastructure of twisted copper wires.
The local telephone line connecting a central office to the customers' premises is conventionally used for analog transmission of voice signals. DSL (digital subscriber line) technologies are used by telephone companies to provide high speed digital transmission over the local loop at reasonable cost. The use of copper local loops in conjunction with the backbone optical network enables broadband services to be provided to customers. There are various DSL technologies which vary in their transmission symmetries and transmission rates. The rate of transmission limits the service reach.
VDSL (very high bit rate digital subscriber line) is a type of DSL technology now being developed to carry high speed data for applications such as ATM (asynchronous transfer mode) and digital compressed video. VDSL technology increases the downstream data rate to about 13 to 52 Mb/s and the upstream data rate to about 1.6 to 6.5 Mb/s. These high rates require the use of frequency bands which are not used by other DSL technologies. A difficulty to be overcome is that the transmitted spectrum of a VDSL signal overlaps AM broadcasts and amateur radio transmissions causing RF (radio frequency) egress and RF ingress. RF egress is the radiation from the VDSL transmission into amateur radio receivers. Field measurements have shown that the transmitted PSD (power spectral density) at amateur radio bands should be maintained at a low level to minimize interference, however the low level of PSD cannot be allowed to significantly compromise performance.
RF ingress is the induced power of the amateur radio transmission onto the VDSL receiver. The received interference may be stronger than the VDSL signal and state of the art digital receiving schemes may not be adequate to handle such a high level of noise. The frequency of amateur radio transmissions tends to drift, necessitating careful monitoring by the VDSL receiver.
There is thus provided, in accordance with a preferred embodiment of the present invention, apparatus for transmission of high speed data over communication channels, the apparatus including a modulator operative to modulate an outgoing stream of digital data, thereby to generate an outgoing signal, and a demodulator operative to demodulate an incoming signal, thereby to generate an incoming stream of digital data, wherein the modulator includes a band suppressor for suppressing portions of the outgoing signal which have specified frequencies.
Further in accordance with a preferred embodiment of the present invention, the specified frequencies include fixed predetermined frequencies and/or programmable frequencies.
Still further in accordance with a preferred embodiment of the present invention, the specified frequencies include a plurality of frequency bands.
Additionally in accordance with a preferred embodiment of the present invention, the band suppressor includes a plurality of band suppressor elements operative to suppress portions in the outgoing signal whose frequencies fall within the plurality of frequency bands respectively.
Still further in accordance with a preferred embodiment of the present invention, the modulator includes a single carrier modulator such as a quadrature amplitude modulator (QAM).
Also provided, in accordance with another preferred embodiment of the present invention, is apparatus for transmission of high speed data over communication channels, the apparatus including a modulator operative to modulate an outgoing stream of digital data, thereby to generate an outgoing signal, and a demodulator operative to demodulate an incoming signal, thereby to generate an incoming stream of digital data, wherein the demodulator includes a noise canceller including an adaptive IIR (infinite impulse response) digital filter operative to cancel narrow band interfering signals.
Further in accordance with a preferred embodiment of the present invention, the band suppressor includes at least one infinite impulse response (IIR) digital filter.
Still further in accordance with a preferred embodiment of the present invention, the demodulator also includes a noise canceller including a first adaptive digital filter receiving a first input signal and a second adaptive digital filter receiving a second input signal wherein the second input signal is received as an error signal by the first adaptive digital filter, wherein adaptation of the first adaptive digital filter is performed in accordance with the error signal and wherein an output of the noise canceller includes a combination of the outputs of the first and second adaptive digital filters.
Further in accordance with a preferred embodiment of the present invention, the demodulator includes a decision feedback equalizer and wherein the band suppressor is characterized in that any distortion of the incoming signal caused by the band suppressor is compensated substantially totally by the decision feedback equalizer.
Also provided, in accordance with another preferred embodiment of the present invention, is apparatus for transmission of high speed data over communication channels, the apparatus including a communication channel, a modulator operative to modulate an outgoing stream of digital data, thereby to supply an outgoing signal to the communication channel, and a demodulator operative to receive an incoming signal from the communication channel and to demodulate the incoming signal, thereby to generate an incoming stream of digital data, wherein the demodulator includes a band suppressor for suppressing portions of the incoming signal which have specified frequencies.
Further in accordance with a preferred embodiment of the present invention, the communication channel includes a two-wire telephone local loop.
Still further in accordance with a preferred embodiment of the present invention, the band suppressor includes an infinite impulse response digital filter.
Additionally in accordance with a preferred embodiment of the present invention, the infinite impulse response digital filter has an infinite impulse response which decays rapidly to a level close to zero.
Still fu
Darby & Darby
Orckit Communication Ltd.
Pham Chi
Tran Khai
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