Pulse or digital communications – Testing
Reexamination Certificate
2000-04-14
2004-06-01
Chin, Stephen (Department: 2634)
Pulse or digital communications
Testing
C375S227000
Reexamination Certificate
active
06744813
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is related to co-pending commonly assigned U.S. patent application entitled “System and Method For Adapting an Equalizer in the Presence of Non-Stationary Noise” filed on even date herewith, which is incorporated herein by reference.
TECHNICAL FIELD
The present invention is generally related to determination of data transmission capability parameters of communications systems employed in a network, and more particularly to a system and method for estimating signaling error probability in a communications link by determining the power levels of noise.
BACKGROUND OF THE INVENTION
With the increasing bandwidth demands from the advent of the Internet, service providers have looked for ways to increase data transmission performance over the copper wire local loop transmission lines that connect the telephone central offices (COs) to the customer premises (CPs). The customer premises equipment (CPE) is connected to the CO switches over the above mentioned transmission lines known as “local loops,” “subscriber loops,” “loops,” or the “last mile” of the telephone network. Historically, the public switched telephone network (PSTN) evolved with subscriber loops connected to a telephone network with circuit-switched capabilities that were designed to carry analog voice communications. Digital service provision to the customer premises is a more recent development. With it, the telephone network has evolved from a system capable of only carrying analog voice communications into a system which can simultaneously carry voice and digital data.
Because of the prohibitive costs of replacing or supplementing existing subscriber loops, technologies have been implemented that utilize existing subscriber loops to provide easy and low cost migration to digital technologies. Subscriber loops capable of carrying digital signals are known as digital subscriber lines (DSLs). Logical channels within a subscriber line which carry digital signals are known as DSL channels, while logical channels within a subscriber line which carry plain old telephone service (POTS) analog signals are known as POTS channels. Some DSL technologies, such as but not limited to integrated services digital network (ISDN), high-bit-rate digital subscriber line (HDSL), HDSL
2
and symmetric digital subscriber line (SDSL), may utilize portions of the POTS channel and therefore do not coexist with a POTS signal. Other digital technologies provide customers with additional flexibility and enhanced services by utilizing frequency-division multiplexing and/or time-division multiplexing techniques to fully exploit a subscriber loop with multiple logical channels. These newer multiple channel DSL technologies provide digital service to the customer premises without significantly interfering with the existing POTS equipment and wiring. The newer DSL technologies accomplish this functionality by frequency-division multiplexing (FDM) their digital signal above (at higher frequencies than) the 0 KHz to 4 KHz frequency range, within which standard analog POTS signals are carried. Multiplexing techniques and terminology are common to those skilled in the art, and are not described herein.
Several variations of new multiple channel DSL technology exist, such as but not limited to Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), Very High Speed DSL (VDSL), Multiple Virtual Lines (MVL™) and Tripleplay™, with this group generally referred to as xDSL. Communications systems carrying xDSL may multiplex xDSL signals and a POTS signal onto a single physical local loop.
Historically, the POTS subscriber loop was designed with the functions needed to communicate both analog, voice-conversation signals and subscriber loop signaling. The CO switch uses subscriber loop signaling to notify the customer premises about events in the telephone network, while customer premises equipment (CPE) use subscriber loop signaling to inform the CO to perform actions for the customer. Some examples of subscriber loop signaling include: the CO switch signaling to the CPE that an incoming call has arrived by ringing the phone, the CPE (e.g., a telephone) signaling to the CO switch that the CPE is initiating a call by an on-hook to off-hook transition of the telephone handset, and the CPE signaling to the CO switch that a call should be connected to a location by sending the phone number of the location.
Although the transmission of both digital signals and analog POTS signals over a subscriber loop offers many potential advantages for customers, several practical problems must be solved when implementing DSL solutions. One significant problem resulting from the POTS subscriber loop signaling functions is the generation of high-frequency interference or noise into DSL channels. This high-frequency noise interferes with the decoding of a received signal. One category of noise is predictable to a reasonable degree. This predictable noise is often referred to as stationary (or cyclo-stationary) noise. Noise that is stationary or slowly varying can be anticipated, and to a degree corrected for, in the transmission of a digital signal.
Another category of noise is commonly referred to in the art as non-stationary noise. Non-stationary refers to the statistically unpredictable nature of the noise over the time period of interest. That is, it is more difficult to anticipate when non-stationary noise will occur, anticipate the strength of the non-stationary noise, or anticipate the duration of the non-stationary noise. For instance, a telephony system on-hook/off-hook signal or a pulse-dialing signal are square waveforns which have high-frequency components and harmonics. Theoretically, these telephony system signals require infinite frequency bandwidth and are therefore difficult to anticipate and compensate.
Another source of noise is crosstalk. Crosstalk is undesirable interference or noise that is induced into a channel by signals travelling in adjacent subscriber loops sharing the same underground cable or overhead wire.
FIG. 1
is a schematic view of a prior art communication system showing a CO
22
connected to a CP
24
via a single subscriber loop
26
. Typically, many individual subscriber loops
28
are bundled together at convenient locations into one cable
30
. The cable
30
extends back to the CO
22
. The close proximity of the many subscriber loops
28
to subscriber loop
26
results in magnetic and/or capacitive coupling between subscriber loop
26
and some of the other subscriber loops
28
adjacent thereto. Undesirable interference may be induced into subscriber loop
26
as various communication signals are transmitted across the subscriber loops
28
. For example, one type of crosstalk occurs when a modem rapidly and repeatedly transitions between an ON state (transmitting data) and OFF state (not transmitting). During the ON state, the modem induces noise onto adjacent subscriber loop
26
which has characteristics similar to stationary noise. However, when the modem in the OFF state, no noise is induced into subscriber loop
26
. This noise, induced into subscriber loop
26
by a modem which is rapidly transitioning between the ON and OFF states, has characteristics of both stationary and non-stationary noise, and is referred to in the art as short-term stationary (STS) noise. Since an individual cable may contain up to several thousand subscriber loops, STS noise can be a commonly encountered noise source.
FIG. 2
shows a typical data constellation as would be used in carrierless amplitude/phase modulation (CAP), quadrature amplitude modulation (QAM), Discrete MultiTone (DMT), or similar DSL modulation techniques. In this illustrative example, a
16
point constellation
40
is shown as a series of points
410
through
425
aligned with X axis
42
and Y axis
44
on a two dimensional grid. Each point represents a symbol which may be sent from the DSL transmitter to the DSL receiver. The symbols may be corrupted during transmission by channel distortio
Betts William
Ko Ken
Chin Stephen
Odom Curtis
Paradyne Corporation
Thomas Kayden Horstemeyer & Risley LLP
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