Nonlinear echo compensator

Details Nonlinear echo compensator Nonlinear echo compensator

2000-06-22

2003-05-13

Chin, Stephen (Department: 2631)

Pulse or digital communications

Equalizers

Automatic

C375S350000, C370S290000

Reexamination Certificate

active

06563870

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a nonlinear echo compensator configuration and in particular to an echo compensator configuration to be used in a data transmission system.
Echo compensators are used for a duplex data transmission between two terminals via a line. The echo compensators suppress echoes at the input of a receiver of a terminal. The echoes are caused by the transmitted signal being fed onto the line by the same terminal. This echo suppression is required when the same frequency band is used for transmission in both directions. Transmission systems with echo compensators are disclosed, for example, in U.S. Pat. No. 5,132,963, U.S. Pat. No. 4,464,545 and U.S. Reissue Pat. No. 31 253.
The requirements for such an echo compensator increase as the length of the line increases, and thus as the line attenuation increases since, in this case, the level of the signal received by a terminal decreases, while the level of the echo produced predominantly by this terminal itself remains approximately unchanged. In very long lines, the level of the echo signal is many times greater (30-40 dB) than the received signal level, so that the compensation accuracy is subject to stringent requirements. Depending on the number of levels of the transmitted signal and the overall system noise suppression requirements, it is necessary to achieve a signal-to-noise ratio between the received signal and the residual echo signal of more than 30 dB after compensation.
FIG. 6
is a block diagram of a typical duplex transmission system with two data transmission devices each of which includes a transmitter
2
, a receiver
4
and a line interface
6
, which is also referred to as a hybrid or four-to-two wire circuit. The function of the line interface
6
is to input data from the transmitter
2
into a transmission line
8
, and to pass data arriving via the transmission line on to the receiver
4
. Normally, part of the echo is compensated by using an analog matching circuit which is accommodated in the line interface
6
. Trick et al., ntz-Archive Volume 10, pages 59-68 (1988) describe a number of variants of such a line interface. Due to the wide variety of lines that may be connected and due to the tolerances of the components used, such matching circuits allow to compensate only part of the echo signal that occurs.
The majority of the echo that occurs is therefore compensated for using a digital system (digital echo compensator), as shown in FIG.
7
.
FIG. 7
shows the schematic configuration of a data transmission device having a line coder
16
, which converts arriving data to the signal format used on the transmission line
8
and outputs the data on a transmission channel
12
via which the data are supplied to both the transmitter
2
and the echo compensator
10
. The transmitter
2
includes a pulse former
18
for smoothing the message signal in the time domain and for spectrally limiting the message signal to be transmitted. The pulse former is followed by an amplifier or line driver
20
. This amplifier is a major source of nonlinear distortion, whose extent depends on the implementation complexity and the required power loss.
The parameters for the compensator
10
must be set such that the output signal from the echo compensator provides as accurate a match as possible to the residual echo signal on a reception channel
14
to which the output of the echo compensator is connected. Linear echo components, which result from a convolution of the data stream being carried on the transmission channel with an impulse response h(t) of the transmitter, have to be taken into account. Furthermore, nonlinear components are present in the output signal from the transmitter. The nonlinear components result from the sequence of symbols in the data stream and are caused by the fact that the transient behavior or dynamic performance of the transmitter may be different for two different transmitted signal symbols, depending on the combination of these symbols. This results in interference pulses at the receiver input, which decay or fade out over a number of symbol periods T and cannot be suppressed using a linear echo compensator.
In the case of conventional compensator structures to compensate for nonlinear echoes, no distinction is drawn between the source of the nonlinearities (transmitter or receiver). If the echo impulse response decays over M symbol periods, where M>N, that is to say has a duration of M*T, it follows from this that all M symbols transmitted in this time period must be taken into account in the echo compensation. The “storage method” and the “Volterra series method” are normally used for this purpose.
In the storage method, all the echo values that occur in the receiver are stored as a function of the values of the previously transmitted symbols. Although this allows nonlinearities from both the transmitter and the receiver to be corrected, the number of memory or storage locations required to do so rises exponentially with the length of the impulse response, and is S=L
M
for an L-level transmitted signal.
In the Volterra series method, the echo signal is first of all developed to form a Volterra series in which the contributions of all the combinations of transmitted symbols are taken into account up to a length M of the combination to form the echo signal. Once again, memory space is required, in the general case, for S=L
M
different combinations. Although, depending on the extent of the nonlinearity, the series may be terminated prematurely, thus reducing the number of coefficients to be considered, the complexity is still considerable for increasing echo impulse response lengths and multi-level transmission.
U.S. Pat. No. 5,146,494 discloses a nonlinear echo compensator which has a number of coefficient memories, to each of which one symbol from the message signal is assigned, and which are addressed using this symbol. Furthermore, a superposition device is provided, through the use of which the coefficients read from the memories are superposed on a received signal.
U.S. Pat. No. 5,148,427 discloses an echo compensator which is formed from a linear echo compensator and a nonlinear echo compensator. The linear echo compensator has a digital transversal filter.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a compensator structure which overcomes the above-mentioned disadvantages of the heretofore-known compensator structures of this general type and whose memory complexity is considerably reduced. The compensator structure according to the invention is particularly suitable for compensating nonlinearities that arise in the transmitter.
With the foregoing and other objects in view there is provided, in accordance with the invention, an echo compensator configuration, including a nonlinear echo compensator having:
a plurality of groups of coefficient memories, each of the groups of coefficient memories storing respective coefficients and being assigned to at least one tupel of N successive symbols of a message signal having L levels, N and L being integer numbers;
a selection circuit connected to the plurality of groups of coefficient memories and to be connected to a transmit channel for receiving an outgoing message signal including symbols, the selection circuit using a currently received one of the symbols of the outgoing message signal and N−1 preceding ones of the symbols of the outgoing message signal for selecting one of the groups of coefficient memories assigned to a tupel formed by the currently received one of the symbols and the N−1 preceding ones of the symbols; and
a superposition circuit to be connected to a receive channel and connected to the selection circuit for superposing, successively and according to a symbol clock, the respective coefficients of the selected one of the groups of coefficient memories on a message signal arriving on the receive channel.
With the objects of the invention in view there is also provided, in a data transmission

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