Narrowband noise canceller

Details Narrowband noise canceller Narrowband noise canceller

1999-12-30

2004-01-27

Le, Amanda T. (Department: 2634)

Pulse or digital communications

Cable systems and components

Transformer coupling

C375S349000, C455S303000, C455S311000, C333S012000

Reexamination Certificate

active

06683913

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to high-speed data communications, and specifically to methods and systems for rejection of radio frequency (RF) noise in Digital Subscriber Line (DSL) modems.
BACKGROUND OF THE INVENTION
Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire. This is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. DSL thus opens the most critical bottleneck in local-loop access to high-speed networks, such as Asynchronous Transfer Mode (ATM) and Internet Protocol (IP) networks, without requiring major investments in new infrastructure.
A range of DSL standards have been defined, known generically as “xDSL,” wherein the various standards have different data rates and other associated features but share common principles of operation. These standards include High-Speed DSL (HDSL), at relative low frequencies (<500 kHz); Asymmetric DSL (ADSL), with an intermediate frequency range (30-1100 kHz); and Very High Speed DSL (VDSL), in a high frequency range (0.3-20 MHz). VDSL modems support the highest possible data rates on existing twisted-pair wire, with downstream bit rates of up to 50 Mb/sec. It is expected that in the near future, VDSL systems will come to dominate subscriber equipment on the telephone network.
One problem of VDSL implementation is that the frequency band of VDSL (up to 20 MHz) overlaps with several RF bands used for amateur and commercial radio transmissions. Twisted-pair wire is particularly prone to picking up radio frequency interference (RFI) from external sources. To avoid interference from existing RF systems, VDSL modems must be designed with reduced sensitivity to RFI. This problem becomes more severe the higher the transmission frequency and can cause significant degradation of VDSL signals.
Bodies concerned with defining VDSL standards have published documents that relate to issues of RFI and suggest possible solutions. For example, document T1.E1.4/95-142, promulgated by the T1E1.4 Technical Subcommittee Working Group of the American National Standards Institute (ANSI) and incorporated herein by reference, presents balance measurements of typical VDSL drop wires. The document describes typical power levels of RFI that are encountered. It concludes that longitudinal conversion loss (LCL—representing RFI crosstalk from the common to the differential mode) of 30-35 dB should be taken into account in designing receivers to deal with interference from radio transmissions. The RFI amplitude in the differential signal may thus be as high as 0 dBm (300 mV). Interference between VDSL signals and radio transmissions is also described in other technical papers, such as T1E1.4 Technical Subcommittee Working Group document T1.E1.4/96-025, which is likewise incorporated herein by reference.
The most problematic kind of interference stems from single side band (SSB) modulated transmissions, as are used in amateur radio. Amateur band SSB transmissions are modulated by human speech, which can be modeled as a series of syllables interrupted by short quiet periods. Therefore, the interfering signals are generally stationary only over very short terms. Any noise cancellation that is used must typically adapt to changes in the RFI level within about a millisecond.
T1E1.4 Technical Subcommittee Working Group document T1.E1.4/96-084, which is incorporated herein by reference, describes stationarity properties of RFI sources. The document suggests a method of mitigating short-term stationary RFI in VDSL signals by implementing an adaptive noise canceling module. The suggested method uses a least mean square (LMS) algorithm to rapidly update the noise canceler during periods of silence of the VDSL signals. The document notes that adaptive updating of the module will typically suffer from a problem known as high eigenvalue spread, which slows convergence and exacerbates dynamic range needs of the signal processing. A number of possible solutions are suggested, which may alleviate but do not eliminate the problem.
Methods of noise cancellation for use in DSL systems are also described in PCT Patent Application PCT/US97/06381, published as WO 97/40587, whose disclosure is incorporated herein by reference. This application shows a receiver system for high-speed data communications, such as ADSL or VDSL, having a RF noise canceller. The noise canceller adaptively estimates the radio frequency noise coming into the receiver through twisted pair input lines. The estimate is used to generate a noise cancellation signal, which is subtracted from the signals coming into the receiver. The noise estimate is based on a common-mode reference noise signal, which is sampled at a transformer that couples the input lines to the receiver.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide improved methods and apparatus for mitigating RFI in high-speed data receivers, and particular in VDSL modems and systems.
It is a further object of some aspects of the present invention to provide a RFI canceller that rapidly tracks and adapts to changes in the characteristics of RFI signals, particularly signals that are short-term stationary, such as SSB signals.
It is yet a further object of some aspects of the present invention to provide a RFI canceller module for use in a data receiver, wherein the module operates substantially autonomously of other data receiver functions, and shuts off when RFI is absent.
In preferred embodiments of the present invention, a high-speed data receiver receives signals over an input line. The receiver comprises an adaptive, narrow-band RFI canceller module, which samples noise on the input line, typically common mode noise arising due to ambient RFI. The module comprises an adaptive bandpass filter (BPF), which rapidly locks onto a narrow frequency range of the strongest RF component in the sampled noise and filters out data and wideband noise that are outside this range. The module processes the resultant narrowband interference signal in order to derive a noise cancellation output, which is subtracted from the signals received over the input line.
Preferably, the module uses a normalized least-mean-square (LMS) method to determine filter coefficients by which the narrowband interference signal is multiplied in order to generate the noise cancellation output. This method converges rapidly to an optimal coefficient value and is therefore particularly useful when the RFI is only short-term stationary. The RFI canceller module is thus optimized to reject the type of common mode interference that arises due to radio transmissions picked up by the line, substantially without introducing any signal degradation outside the interference band.
In some preferred embodiments of the present invention, the adaptive BPF comprises a notch filter element and a bandpass filter element, coupled in parallel to receive the sampled input noise. The notch filter element is used to determine the frequency of the strongest RF component in the sampled noise, by adjusting the notch filter parameters to minimize the notch filter output power. The bandpass filter element uses this determined frequency, and optionally other filter parameters, as well, to lock onto the frequency range of the strongest RF component.
In some preferred embodiments of the present invention, the module comprises a shutoff mechanism, which monitors the amplitude of the RFI and shuts off the noise cancellation output when the amplitude drops below a given minimum level. Preferably, the mechanism monitors aspects of the narrowband signal passed by the BPF, most preferably by checking the values of the above-mentioned filter coefficients. When the absolute values of the coefficients rise above a certain threshold, it is an indication that the RFI has d

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