DAC architecture for analog echo cancellation

Details DAC architecture for analog echo cancellation DAC architecture for analog echo cancellation

1998-04-30

2003-09-09

Eng, George (Department: 2743)

Telephonic communications

Echo cancellation or suppression

Residual echo cancellation

C379S406110, C379S406120

Reexamination Certificate

active

06618480

ABSTRACT:

BACKGROUND OF THE INVENTION
Digital communication over wired transmission media is achieved by launching electromagnetic signals onto the wires at one end and capturing them at the other end of transmission lines. Wired transmission media include metallic wires, pairs or sets of metallic wires used together, coaxial cables, and even optical fiber. A point-to-point communication link over a wired transmission line can support two-way communications using transceivers at each end of the wire that feature both transmit and receive capabilities. Full-duplex modems support simultaneous transmit and receive capabilities by applying electronic and/or modulation techniques to separate the received signal from the transmitted signal.
An illustrative digital subscriber line (xDSL) implementation of two modems
10
,
12
communicating over a twisted-pair copper wire transmission line
14
connecting a telephone company central office (CO) to a customer premises is shown in FIG.
1
. At the transmission line input to the near end modem, just past line coupling circuitry
15
, the total signal is formed by superposition of the signal transmitted from the near end
10
and the signal received from the far end
12
. To support simultaneous communication in both directions (full-duplex operation), some method of separating the received signal from the transmitted signal must be employed so that a transmitter
16
and receiver
18
in each modem
10
,
12
can operate simultaneously. A popular method incorporates balanced line coupling circuitry to ensure that 100% of the signal to be transmitted is transferred to the transmission line
14
and 0% is reflected back into the receive path, eliminating all interference into the received signal. In the case of twisted-pair copper wire telephone transmission lines, the line coupling circuitry
15
converts the two-wire bidirectional transmission line
14
to a four-wire path with separate unidirectional transmit and receive circuits
21
,
22
. This line coupling circuit (and converter) is often referred to as a “hybrid.” See, Whitman D. Reeve, “Subscriber Loop Signaling and Transmission Handbook: Digital” (IEEE Press, Piscataway, N.J. 1995), pages 54-56, incorporated herein by reference. The two-wire transmit and receive paths
21
,
22
shown can accommodate differential signals like those conveyed on the phone line or single-ended signals, depending on the hybrid design. For single-ended signals, one wire of the pair conveys the signal and the other provides a ground signal reference.
FIG. 2
shows an electronic hybrid
24
that converts a differential two-wire telephone line
14
to separate single-ended transmit and receive paths
21
,
22
. (Electronic hybrids featuring differential transmit and receive terminal outputs are very similar, so are not discussed further here.) A transformer
26
provides magnetic inductive coupling of the hybrid circuitry
24
to the telephone line
14
. The signal received from the far end passes through transformer
26
to terminal A (FIG.
2
), which is connected directly to the positive terminal of a receive amplifier
27
. The transmit signal input to transformer terminal A is determined by a voltage divider formed by a resistor R (
28
) and an impedance Zi, where Zi is the input impedance looking into transformer
26
. Zi is a function of the transformer impedance as well as the phone line impedance. To remove the transmit signal from the receive path
22
, a cancellation voltage is generated at terminal B by another voltage divider formed by a resistor R (
31
) and a balance impedance Zb (
32
). The cancellation voltage is input to the negative terminal of the receive amplifier
27
and only the difference between the negative and positive terminals of the amplifier
27
is passed through as the received signal (common-mode voltages are rejected). See, Paul Horowitz and Winfield Hill, “The Art of Electronics: second edition” (Cambridge University Press, 1989), incorporated herein by reference. If Zb=Zi, then the cancellation voltage exactly matches the transmit voltage and no transmit signal is passed through the receive amplifier
27
. This is a complete electronic cancellation.
If Zb is not exactly equal to Zi, an impedance mismatch occurs at line coupling circuitry,
15
and a portion of the transmitted signal, called the echo, will be included in the output of the receive buffer. Any echo that leaks into the receive path
22
will interfere with the attenuated signal from the far end. Characteristics of the echo signal are determined by the specific circuit elements used in the hybrid
24
and the input impedance Zi of transformer
26
. The echo can be described by the convolution of the transmitted signal x(t) and h(t), the impulse response describing the echo path through the hybrid
24
. The severity of the impedance mismatch determines the magnitude of the echo, which is often defined as echo return loss (ERL), a logarithmic measure of the ratio of power of the transmitted signal to the power of the echo. A very high ERL indicates that very little transmit signal is echoed back, while a low ERL means that the echo is large. The ERL in an xDSL transceiver may be as low as 6 dB in some cases and the signal from the far end may be attenuated by as much as 70 dB. Thus, the received signal may be as much as 64 dB below the echo!
Under controlled conditions, where the hybrid line coupling can be tuned to match the line, the echo can be made arbitrarily small. However, manual tuning is undesirable for cost effective deployment of wireline modems. A better approach is to design the matching circuitry to provide an acceptable level of echo suppression over a wide range of anticipated line conditions. The system should be designed to operate reliably in the presence of echoes produced under the range of anticipated line conditions. This can be achieved with the proper selection and design of modulation techniques and receive circuitry.
Various modulation techniques are available for signal separation. To achieve co-existing transmission and reception in the presence of significant echo due to impedance mismatch in the hybrid circuit
24
, a data duplexing method can be incorporated into the modulation technique to help separate the bidirectional data traffic. For example, frequency-division duplexing (FDD) or time-division duplexing (TDD) can be used to separate the outgoing and incoming signals from one another in frequency or time, respectively. However, both FDD and TDD systems sacrifice channel capacity to facilitate signal separation. The idea in both FDD and TDD systems is to consider the transmitted signal as an unwanted and unknown interference into the received signal. The aim of both systems is to remove the unwanted component.
FDD systems place the transmitted signal in a different portion of the frequency spectrum than is occupied by the received signal. The ANSI T1E1.413 specification (ADSL standard) supports FDD operation. See, “T1.413-1995: Telecommunications—Asymmetric Digital Subscriber Line (ADSL) Metallic Interface ” (1995), incorporated herein by reference. Both transmit and receive functions are operated simultaneously, but throughput capacity in either direction is sacrificed because the full bandwidth is not used. TDD techniques separate the outgoing and incoming signals by turning off the transmit portion in order to extract the incoming signal without interference. When transmit mode is entered, the transceiver no longer attempts to receive data. This method has been proposed for the evolving VDSL standard. See, ANSI Contribution T1E1.4/96-329R1, DMT Group VDSL PMD Draft Standard Proposal (February 1997), incorporated herein by reference. Several VDSL proposals refer to the TDD scheme as “ping pong” because the modems take turns sending the information back and forth. TDD also sacrifices throughput capacity because transmission and reception do not occur simultaneously—only a portion of the total time is used for each.
Because the transmitter and receiver are co-located

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