Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
2000-10-31
2003-12-02
Corrielus, Jean B. (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C379S093050
Reexamination Certificate
active
06658051
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to telecommunications, and more particularly, to isolation techniques in a digital subscriber line (DSL) modem.
2. Description of the Related Art
A conventional telephone transmission line is typically comprised of a pair of copper conductors that connect a telephone set to the nearest central office, digital loop carrier equipment, remote switching unit or any other equipment serving as the extension of the services provided by the central office. This pair of copper conductors, which is also referred to as a twisted pair, has its leads named as tip and ring. The tip and ring nomenclature is derived from the electrical contacts of an old-style telephone plug.
In general, a digital subscriber line is comprised of two DSL modems coupled to one another by such a twisted pair. The transmit (Tx) and receive (Rx) signals of DSL communications are therefore carried on the twisted pair. Some DSL systems also allow for Plain Old Telephone Service (POTS) communications, such as voice and facsimile data, to be transmitted on the same twisted pair. In such a DSL system, various signal modulation, separation and isolation techniques can be employed to ensure that the high frequency DSL data is received by the intended DSL modem, and that the low frequency POTS data is received by the intended POTS line card.
For example, discrete multitone (DMT) is a modulation technique commonly employed in various digital subscriber line (XDSL) communication systems (e.g., asynchronous DSL (ADSL) systems). A DMT line code comprises multiple bins or subchannels implemented through a Discrete Fourier Transform (DFT). Each bin is independently modulated to some carrier frequency. The number of bins available to carry information is generally equal to or slightly less than half of the DFT size. Each bin of a DMT line code employs a two-dimensional signal (e.g., a phase and amplitude modulated signal) equivalent to that of a passband single carrier system. Quadrature amplitude modulation (QAM) is a modulation method that is used to encode a variable number of bits into such a two-dimensional signal, where the bits are mapped into the in-phase and quadrature components of a complex symbol that is typically converted to an analog form, and then transmitted in the corresponding bin.
In addition to such modulation techniques, various separation and isolation techniques are available to ensure that the high frequency DSL data is received by the intended DSL modem, and that the low frequency POTS data is received by the intended POTS line card. For example, a common separation technique is to use a splitter, which separates low frequency POTS data from high frequency DSL data. Other separation methods are available, depending on factors such as the particular system involved and whether the system is deployed at the central office or the customer premises.
Isolation techniques, on the other hand, are required to isolate the signal equipment, such as the DSL modem circuitry and the POTS line card circuitry, from the common-mode line voltage of the telephone transmission line. For instance, a discrete subscriber line interface circuit (SLIC) is typically used to isolate POTS line card circuitry from the common-mode line voltage of the transmission line. On the DSL side, however., a coupling transformer is typically used to isolate the DSL modem circuitry from common-mode line voltage of the transmission line. This transformer serves other purposes as well. For example, it is used to match the transmitter impedance to the DSL line impedance. Additionally, the transformer may provide echo cancellation and or line impedance matching as part of the hybrid balance network. Such a DSL coupling transformner provides an easy design. solution and can be used with a number of line drivers.
However, a DSL coupling transformer is also associated with. significant disadvantages. For example the per unit cost associated with such DSL coupling transformers, as well as the physical space (e.g., on a printed circuit board) that is consumed by the DSL coupling transformer are considerable. Furthermore, DSL coupling transformers exhibit various undesirable performance characteristics, such as non-linear distortion (e.g., typically around −70 dB) and limited bandwidth. Moreover, the is adverse effects of such undesirable performance characteristics increase as the bandwidth requirement of a communication system increases. For example, a symmetric high-bit-rate DSL (SHDSL) system requires approximately thirty times (in octaves) more bandwidth than an ADSL system. A DSL coupling transformer in an SHDSL system, therefore, is a significant performance limiting component.
In addition, there are numerous transformer parameters that adversely affect both the remote and local modems in a DSL communication system. One such parameter is self inductance, which is the ability of a transformner coil to oppose changes in signal current. With respect to the remote DSL modem, this parameter causes reflection of the Tx signal, as well as attenuation of the Rx signal at the lower bins of the DMT line code. On the local side of the communication system, the DSL modem experiences harmonic distortion of the Tx signal, and cross modulation of both the Rx and Tx signals. Moreover, attenuation of the Rx signal and degradation of echo cancellation occur at the lower bins of the line code.
Another transformer parameter that can adversely affect a DSL communication system is referred to as leakage inductance. This parameter is particularly undesirable when it combines with stray capacitance to form a resonant circuit that causes an oscillation that typically manifests as high-frequency noise. As a result, the remote DSL modem experiences reflection of the Tx signal, as well as attenuation of the Rx signal at the higher bins of the line code. On the local side, the DSL modem is subjected to echo cancellation degradation, and attenuation of the Rx signal at higher bins of the line code.
Also, the non-linearity of a transformer output subjects both the remote and local DSL modems to harmonic distortion at the lower bins, and cross modulation at all bins included in the line code. Another parasitic element of a transformer is its DC resistance, which contributes to power efficiency degradation at both the remote and local DSL modems. Additionally, the interwinding capacitance of the transformer causes increased noise floor due to common-mode noise. This is because interwinding capacitance degrades the isolation of the transformer, and noise can therefore pass between the primary and secondary windings. As such, the common-mode noise (noise common to both the tip and ring terminals) will increase.
Another problem associated with transformers in DSL applications is that the transformer is required to work over a large frequency span (e.g., a frequency ratio of 30 for ADSL) under fairly stringent conditions defined by various industry regulatory standards. In addition, the cumbersome magnetic component of a typical DSL coupling transformer is undesirable in that it increases costs, occupies significant physical space and has a non-trivial weight.
In light of all the problems associated with transformers, it is desirable to develop technologies that offer alternatives having better performance characteristics given the defined industry standards, and desirable physical characteristics (e.g., small size and low weight). One such technology is a modem design that uses optoisolators to isolate data, while using the current of the telephone transmission line to supply the isolated section with power. In the case of a DSL modem, however, the power required for the analog transceiver is much greater than could be supplied by the current of the telephone transmission line. As such, this approach is not practical in a DSL modem system.
There is a need, therefore, for an isolation technique that does not adversely affect DSL modem performnance. The technique should provide a solution that can be o
Centillium Communications Inc.
Corrielus Jean B.
Fenwick & West LLP
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