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 greater than 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 xe2x80x9cstorage methodxe2x80x9d and the xe2x80x9cVolterra series methodxe2x80x9d 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=LM 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=LM 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.
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 Nxe2x88x921 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 Nxe2x88x921 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 system including a transmitter receiving data to be transmitted on a transmit channel, the transmitter passing the data to a transmission line and limiting a transmission pulse duration of a message signal to N*T, where N is an integer number and T is a symbol period of the message signal, a nonlinear echo compensator, including:
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 the message signal having L levels, L being an integer number;
a selection circuit connected to the plurality of groups of coefficient memories and to be connected to the 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 and Nxe2x88x921 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 Nxe2x88x921 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.
In other words, the object of the invention is achieved by a nonlinear echo compensator for an L-level message signal, which has a plurality of groups of coefficient memories, wherein each group is assigned to at least one tupel of N successive symbols of the message signal, a selection circuit which is connected to a transmit channel in order to receive an outgoing message signal, and which is able to use a value currently being received and Nxe2x88x921 preceding symbols of the message signal to select the group associated with the tupel formed by these symbols, and a superposition circuit, which is able to superpose the coefficients of the group, successively and according to a symbol clock, on a message signal arriving on a receive channel or reception channel.
Digital transversal filters are preferably used as the filter elements.
In this case, each group of coefficients may form a filter element in the form of a digital transversal filter.
The selection circuit is expediently able to use the value currently being received and the Nxe2x88x921 preceding symbols of the message signal to excite the filter element associated with the tupel formed by these symbols, and the superposition circuit is set up in order to superpose the response signals from the filter elements on the arriving message signal. This measure ensures that, in each symbol period, the arriving message signal has superimposed thereon the first coefficient of a currently excited filter element, if required the second coefficient of a filter element excited in the previous symbol period, and generally the n-th coefficient of a symbol period that occurred nxe2x88x921 symbol periods before, thus producing an accurate model of the nonlinearities caused successively in the transmitter.
Alternatively, each group of coefficients may form a column of a memory matrix which is organized in rows and columns, wherein the selection circuit is able to select the first coefficient of the group associated with the tupel in one symbol period and respectively to select subsequent coefficients in the group in subsequent symbol periods, and the superposition circuit superposes the sum of the selected coefficients on the arriving message signal, in order in this way to produce an accurate model of the nonlinearities which are caused successively in the transmitter.
In accordance with another feature of the invention, the selection circuit includes a number of memory cells for storing tupels. The number of the memory cells corresponds to a number of the rows in the memory matrix, and the selection circuit replaces, during each of the symbol periods, an oldest one of the tupels stored in the memory cells with a current one of the tupels.
In accordance with a further feature of the invention, each of the groups of coefficient memories is assigned to at least one tupel of two successive symbols of the message signal.
In accordance with yet another feature of the invention, each of the groups of coefficient memories is assigned to at least one tupel of N successive symbols of a message signal having two levels.
The number of coefficients in each group is expediently chosen to correspond to the duration of the echo signal component caused by the associated tupels. In this case, the number of coefficients may be matched to the length of the respective echo signal component based on the duration of the longest echo signal component, jointly for all the groups, or individually.
In accordance with another feature of the invention, the groups of coefficient memories are assigned only to tupels of successive symbols causing an echo signal component exceeding a given minimum value.
In accordance with yet another feature of the invention, the groups of coefficient memories store adjustable coefficients.
The nonlinear echo compensator according to the invention can advantageously be used combined with a linear echo compensator. Since the linear component in the echo signal predominates, the linear compensator can carry out a coarse compensation so that the amplitudes of the responses to be produced by the individual groups are reduced, and their number of coefficients can be kept lower.
In accordance with another feature of the invention, a linear echo compensator is to be connected to the transmit channel. The linear echo compensator is a digital transversal filter and provides an output signal. A further superposition circuit is connected to the linear echo compensator for superposing the output signal from the linear echo compensator onto the message signal arriving on the receive channel.
In accordance with yet another feature of the invention, the superposition circuit and the further superposition circuit are configured as an addition element.
In accordance with a further feature of the invention, a linear echo compensator is to be connected to the transmit channel. The linear echo compensator is a digital transversal filter. The linear echo compensator and the nonlinear echo compensator provide respective output signals. A fixed recursive filter is connected in series with the linear echo compensator.
In accordance with yet a further feature of the invention, a linear echo compensator is to be connected to the transmit channel. The linear echo compensator is a digital transversal filter, and the linear echo compensator and the nonlinear echo compensator provide respective output signals. An addition circuit has an output connected to an input of the fixed recursive filter for adding the respective output signals from the nonlinear echo compensator and from the linear echo compensator.
In accordance with another feature of the invention, the fixed recursive filter has a transfer function f=1/(1xe2x88x92(1xe2x88x922n)zxe2x88x921) where n is an integer number and z is a complex value.
The echo compensator can advantageously be used in a data transmission system having a transmitter which receives data to be transmitted on the transmission channel and passes such data to a transmission line, wherein the transmission pulse duration of the message signal is limited in the transmitter to N*T, where T is the symbol period of the message signal. This transmission pulse duration limit implies that the signal emitted from the transmitter is defined at any time by a maximum of N symbols, with these N symbols also defining the nonlinearities in the output from the transmitter.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a nonlinear echo compensator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.