WO2002073920A1

WO2002073920A1 - Distortion pre-corrector for communication apparatus

Info

Publication number: WO2002073920A1
Application number: PCT/GB2002/001153
Authority: WO
Inventors: Brian Herbert Beech; David G. Edwards; Ravin Perinpanayagam
Original assignee: Tandberg Television Asa
Priority date: 2001-03-13
Filing date: 2002-03-13
Publication date: 2002-09-19
Also published as: GB2389494A; GB0318255D0; GB2389494B; GB0106160D0

Abstract

A pre-corrector for producing non-linear distortion in a signal channel containing a root Nyquist bandpass filter (52, 57) in transmission and reception portions of the channel has plural cascaded groups of stages (78a, 70b). Each group has a series connection of a static pre-distorting section (48a, 48b) configured to represent an inverse function of magnitude and phase distortion estimated to occur in the channel, and a dynamic pre-distortion section (40a, 40b) including a forward model (42) configured to represent magnitude and phase distortion in the channel. Each successive stage group is connected to receive output from a preceding stage group so that distortion in the channel is successively reduced towards zero with each successive stage group.

Description

DISTORTION PRE-CORRECTOR FOR COMMUNICATION APPARATUS

This invention relates to a distortion pre-corrector for a communication apparatus and in particular to a pre- corrector for producing non-linear channel distortion in a communication channel .

In a digital data transmission channel link, particularly a satellite transmission channel link, it is known for modulation techniques to use symbols arranged as points in a particular constellation pattern to represent digital data. The constellation shows all possible combinations of complex (I and Q) samples of the data being transmitted and the constellation pattern is an overlay of all possible positions of each data sample at a particular point. Thus, for each symbol, we may have four sampling points. Typical techniques are those of phase shift keying (PSK) and quadrature amplitude modulation (QAM) . Common techniques are quadrature phase shift keying (QPS) which is used for digital satellite transmission for consumer TV applications, and 8 PSK which is used, for example, for satellite news gathering applications. It is a desire to utilise higher order modulation methods such as 16 PSK and 16 QAM to permit transmission at a higher bit rate so as to facilitate a greater number of channels to be carried within a predefined bandwidth of a particular transmission link.

As is well known, transmission of a modulated signal through a transmission channel such as a terrestrial link, cable or satellite results in distortion of the signal. The distortion is due, at least in part, to nonlinear effects upon a signal as it passes through the transmission link. The distortion, in terms of magnitude and/or phase, results in a change in location of the constellation points for any given modulation scheme and an increase in the order of modulation results in a decrease in the distance between constellation points, thereby leading to distortion having a greater effect. Such distortion has the disadvantage of producing errors in demodulation.

It is known to compensate for such non-linear distortion effects within transmission links by use of a pre-correction compensator. Signal pre-distortion performed at radio frequencies (RF) , intermediate frequencies (IF) or base band frequencies is often carried out by application of an inverse function of the distortion to be expected of the signal in the transmission path. Such pre-distortion is disclosed in O-A-95132561 and US-A-4992754. Such forms of pre- correction tend to generate out-of-band components which are passed through to amplifiers in the transmission channel. Where the amplifier has an input filter, as is common for amplifiers used in satellite transmission links, then these out-of-band components are usually filtered out prior to amplification. Thus, the input signal to the amplifier is not the entire signal. This means that pre-correction is not effective for correction of amplifiers contained within satellite transponders where the bandwidth of the incoming signal is high in relation to the bandwidth of the transponder. Further, for higher order modulation schemes, such a form of pre- correction requires very high clocking rates in order to generate the wide-band pre-distortion components. The foregoing problems are partially mitigated by the apparatus disclosed in WO-A-0025495, which discloses an arrangement for pre-distorting a signal so as to offset later distortion of the signal during transmission across a satellite transmission link which contains root Nyquist bandpass filters in respective up and down links. The apparatus includes a plurality of identical pre- distorting stages each of which generates an approximation of the required pre-distortion. Each successive stage receives an approximation from the preceding stage so that errors in successive approximations converge towards zero with increase in the number of stages. However, the total non-linearity of the channel may be substantial with as much as 60° phase shift over the signal level range.

Simulation of the convergence properties of the apparatus disclosed in WO-A-0025495 has shown that the rate of convergence is dependent on the degree of non- linearity being corrected. For severe non-linearities, for example where the phase shift variation is greater than 60°, the apparatus may not converge the error as required.

It is an object of the present invention to provide a pre-corrector which will provide errors in successive approximations of a pre-distorting signal which will converge toward zero, even when the non-linearity being corrected is severe, by which term is meant, for example, more than 45° of phase shift and when the magnitude error, i.e. output voltage divided by input voltage, is greater than 2. It is also an object of this invention to provide a pre-corrector which will allow error convergence more quickly than the prior art so as to reduce the number of stages and hardware employed. According to a first aspect of this invention there is provided a method of pre-distorting a signal which has been modulated to carry signals representative of digital data so as to reduce non-linear distortion in a signal channel containing root Nyquist bandpass filters in transmission and reception portions of said channel, including the steps of providing plural cascaded groups of stages, each group including a series connection of a static pre-distortion section configured to represent an inverse function of magnitude and phase distortion estimated to occur in said channel, and a dynamic pre- distortion section including a forward model configured to represent magnitude and phase distortion in said channel, and passing said signal through said cascaded groups of stages, whereby each successive stage group receives input from an output of a preceding stage group so that distortion in said channel is successively reduced towards zero with each successive stage group. Preferably, at least two groups of stages are provided.

Advantageously, said channel is one of a satellite, cable or terrestrial channel .

Preferably, said channel includes a satellite having a cascaded input multiplexer filter, a power amplifier and an output multiplexer filter, and said forward model further comprises a model of magnitude response of said input multiplexer filter and output multiplexer filter, whereby magnitude response of said channel may be corrected.

Advantageously, said stage groups are substantially identical to one another.

In a preferred embodiment, said forward model comprises a series connection of an up-sampler, a model representative of magnitude and phase of: a transmission side root Nyquist filter, a transmission side high power amplifier, a satellite input multiplexer filter, a satellite power amplifier, a satellite output multiplexer filter, a receiver side root Nyquist filter, and a down sampler. Conveniently, said satellite power amplifier is one of a travelling wave tube and solid state power amplifier.

Preferably, an initial approximator is connected in an input path to said corrector means, said initial approximator comprising a static pre-distortion approximation model representative of channel magnitude and phase non-linearity and group delay of said channel.

Conveniently, said initial approximator is substantially an inverse function of the forward model of said corrector means.

Advantageously, said signal is modulated in accordance with one of 16 QAM, 32 QAM, and 16 PSK.

According to a further aspect of this invention there is provided a pre-corrector for producing non- linear distortion in a signal channel containing a root Nyquist bandpass filter in transmission and reception portions of said channel, said pre-corrector including plural cascaded groups of stages, each group including a series connection of a static pre-distorting section configured to represent an inverse function of magnitude and phase distortion estimated to occur in said channel, and a dynamic pre-distorting section including a forward model configured to represent magnitude and phase distortion in said channel, each successive stage group connected to receive input from an output of a preceding stage group, whereby distortion in said channel is successively reduced towards zero with each successive stage group. Preferably, at least two groups of stages are provided.

Advantageously, said channel is one of a satellite, cable, or terrestrial channel. Preferably, said channel includes a satellite having a cascaded input multiplexer filter, a power amplifier and an output multiplexer filter and said forward model further comprises a model of magnitude response of said input multiplexer filter and output multiplexer filter, whereby magnitude response of said channel may be corrected.

Advantageously, said stage groups are substantially identical to one another.

In a preferred embodiment, said forward model comprises a series connection of an up-sampler; a representation of magnitude and phase distortion produced by a transmitting side root Nyquist filter, a transmitting side high power amplifier, a satellite input multiplexer filter, a satellite power amplifier, a satellite output multiplexer filter, a receiving side root Nyquist filter; and a down sampler.

Preferably, a static pre-distorter approximation model representative of channel magnitude and phase non- linearity and group delay of said channel is evaluated and said signal passed through said initial approximator prior to passage to said forward model .

Preferably, said approximation model is an initial approximation arranged to be substantially an inverse function of the forward model. The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows, in block schematic form, a satellite transmission apparatus embodying this invention,

Figure 2 shows, in block schematic form, a known pre-corrector for reducing signal channel distortion, Figure 3 shows, in block schematic form, a pre- corrector in accordance with this invention,

Figure 4 shows, in block schematic form, a forward model of the elements for which pre-distortion is derived. In the Figures like reference numerals denote like parts .

A channel link which, by way of example is shown as a satellite channel link, will now be described with reference to Figure 1. The transmitter side has a modulator 19 having an input 18 for receiving a stream of data bits and the modulator produces complex, I and Q, modulated outputs which are input to a pre-corrector 20 for reducing signal channel distortion. The corrector will be described in detail hereinafter. Output from the corrector is applied to an up sampler 21 which multiplies the input bit rate by a factor of 2 or more so as to provide a required output facilitating operation of a root Nyquist filter 22, which is usually a bandpass filter. It is usual to use Nyquist filtering within a transmission link in order to constrain the bandwidth of the transmitted signal, but such root Nyquist filters impose a linear distortion upon the modulated signal. Output from the filter 22 is applied to an I , Q modulator 23 which provides an up-converter 24, output of which is amplified by a high power amplifier 25 and then transmitted by, for example, a parabolic dish 26 to a satellite 6. The satellite 6 has a receiving antenna 28 applying data to an input multiplexer (IMUX) filter 7, thence to a power amplifier 8 and an output multiplexer (OMUX) filter 9. Output from the OMUX filter is applied to a transmitting antenna 29 and data is received by, for example, a parabolic dish 30 at a receiver side.

An output R.F. signal from the dish 30 is applied to a down converter 31. Output from the down converter 31 is applied to an I , Q demodulator 32 which, in turn, provides output to a root Nyquist band pass filter 33.

The output of the filter 33 is applied to down-sampler 34 and the I, Q down sampled outputs are demodulated by demodulator 35 to provide digital data transmitted by the symbols within the modulation scheme and provided at output terminal 36.

The pre-corrector 20 is arranged in accordance with this invention to apply pre-distortion to the incoming signal to compensate for the magnitude and phase distortion subsequently applied to that signal during its passage through the transmission/reception channel and the pre-corrector is also preferably arranged to substantially correct for group delay distortions and distortions caused by truncating the frequency spectrum due to filtering. The pre-corrector 20 is shown in greater detail with reference to Figures 3 and 4.

However, before describing the pre-corrector of this invention, a prior art pre-corrector will be described with reference to Figure 2.

Referring to Figure 2, input signal Vi on line 41, although shown as a single signal input line, is a complex signal representative of magnitude and phase and, similarly, output from the apparatus shown in Figure 2 is also a complex, I, Q signal. It will be understood by those skilled in the art that the inputs and outputs may be Cartesian or in polar form.

The input signal Vi is applied to an initial approximation approximator 48 in input line 41 which is arranged to provide an output which is approximately the inverse of the distorting function of a forward model 42. For pre-distortion of an amplifier such as a TWT or solid state power amplifier, the initial approximator 48 may be a function which bases the constellation points in the correct place for pre-distortion but which does not dynamically change their position from symbol to symbol. Such an initial approximator is known in the art as a static pre-distorter. Such a static pre-distorter may comprise equal and opposite pre-distorters for distortion in the channel caused by non-linearity and group delay. The initial approximator disclosed in WO-A-0025495 produces an approximation of the non-linear distortion within the satellite. For combined non-linear and group delay correction, the approximator 48 may be a known non- linear corrector cascaded with a conventional group delay corrector.

Output from approximator 48 is applied to an input of the forward model 42 which is a pre-calculated forward model representative of the satellite transmission/ reception channel from the input to the up sampler 21 to the output of the down sampler 34. It will be understood that the forward model is based upon the linear and nonlinear transfer function f of the channel. Output 43 of the forward model is applied to one input of a subtractor 44, the other input of which is supplied from input line 41. The input to the subtractor 44 from line 41 is delayed by a delay (not shown) to provide delayed symbols representative of digital data for time = t(l) so as to align the data with the symbols at time = t(l) that are acted upon by the forward model 42. The subtractor 44 output, which is an error signal given by Vi-f (Vi) , is applied to an amplifier 45 and thence to one input of an adder 46, the other input of adder 46 being derived from input line 41 which are delayed by a delay (not shown) representative of the delay through components 42, 44 and 45. The amplification A by amplifier 45 is chosen to achieve the highest convergence rate for a given forward model distorting function.

It will be realised by those skilled in the art that an output 47 of adder 46 provides an estimate of the required transmitted signal and concerns symbols representative of digital data for time = t(l), whereby a first stage of approximation of the input signal pre- corrected for channel distortion is provided which is given by A [Vi-f (Vi) ] +Vi . The initial approximator 48 thus forms a static pre-distorting section and the elements 42 - 46 form a first dynamic pre-distorting stage 40. Because the output 47 of the first, i.e. single stage is not mathematically the required corrected signal, i.e. A [Vi-f (Vi) ] +Vi ≠ Vi , so further dynamic pre- distorting stages 40 are provided which are identical to the first stage 40 so as to provide cascaded, successive stages of pre-distortion, each approximating to the required pre-distortion necessary for correction of the signal at the output 36. It has been found by computer simulation that errors in successive approximations converge toward zero with increase in the number of stages. In the example shown, there are second and further successive, cascaded, stages. It has been found that in the prior art six dynamic pre-distorting stages of successive approximation provides a reasonable balance between convergence towards zero and hardware implementation of the corrector. By using a number of successive stages of approximation, the error converges to zero and the final output becomes the required transmitted signal .

During passage of symbols representative of digital data for time = t(l) through the second stage, the first stage will be supplied with symbols representative of digital data for time = t(l+n), where n represents the pipeline delay.

The invention will now be described with reference to Figure 3 where like parts to the Figure 2 are given like references.

In the pre-corrector of Figure 3, a first stage 70a is formed by the initial approximator 48a forming a static pre-distorting section and elements 42 - 46 forming a dynamic pre-distorting section 40a. However, unlike the prior art of Figure 2, output from adder 46 is applied to another initial approximator 48b forming a static pre-distorting section of a second stage 70b.

Output from the initial approximator 48b is applied to a further dynamic pre-distorting stage 40b. Output from the second stage may be applied to an initial approximator (not shown) of a third stage 70c. In distinction from the prior art where there is a single static pre-distorting stage followed by plural dynamic pre-distorting stages, in the present invention there is provided plural stages each of which comprises a static pre-distorting stage and a dynamic pre-distorting stage.

For convenience, the static pre-distorting section of each stage may be identical to one another and the dynamic pre-distorting section of each stage may be identical to one another, although it is to be understood that such identicality is not essential to the present invention.

Thus, in the present invention the amount of non- linearity processed by each stage is similar and the non- linearity is reduced by a factor related to the number of stages compared with the total for non-linearity being corrected.

The pre-corrector of the present invention shown in Figure 3 results in more rapid convergence and more successful convergence than in the prior art.

The components of the forward model 42 will now be described with reference to Figure 4.

The forward model 42 is arranged to operate at a higher sampling rate than the input signal and so the input signal is up-sampled by the up-sampler 21 multiplying the input bit rate by two or more so as to provide required operation of the root Nyquist bandpass filter 22 having model 52 representative of a model of magnitude and phase, i.e. output magnitude versus frequency and phase versus frequency, of the filter 22. Filtered output is applied to a model of non-linearity expressed in magnitude and phase of the high power amplifier by model 53. Output from the model 53 is applied to an IMUX filter model 54 which models the magnitude and phase response of the IMUX filter 7. Output of the IMUX filter is applied to a model of non- linearity expressed in terms of magnitude and phase of the satellite power amplifier using model 55. Output of the model 55 is applied to an OMUX model 56 which is a model of the magnitude and phase response of the OMUX filter 9. The output of the OMUX model is applied to a further root Nyquist bandpass filter model 57 representative of filter 33 magnitude and phase. Output of the model 57 is down-sampled by down-sampler 34 and then output .

The models 54 and 56 represent magnitude and phase response and preferably also the group delay of the filters 7 and 9. The magnitude response correction will also correct for the spectrum truncation caused by the filters 7 and 9.

The pre-corrector of this invention may, in principle, correct for any channel impairment provided that it is predeterminable and correction is achieved by successive approximation converging the error to zero.

Claims

CLAIMS :

1. A method of pre-distorting a signal which has been modulated to carry signals representative of digital data so as to reduce non-linear distortion in a signal channel containing root Nyquist bandpass filters in transmission and reception portions of said channel, including the steps of providing plural cascaded groups of stages, each group including a series connection of a static pre- distortion section configured to represent an inverse function of magnitude and phase distortion estimated to occur in said channel, and a dynamic pre-distortion section including a forward model configured to represent magnitude and phase distortion in said channel, and passing said signal through said cascaded groups of stages, whereby each successive stage group receives input from an output of a preceding stage group so that distortion in said channel is successively reduced towards zero with each successive stage group.

2. A method as claimed in claim 1, wherein at least two groups of stages are provided.

3. A method as claimed in claim 1 or 2 , wherein said channel is one of a satellite, cable or terrestrial channel .

4. A method as claimed in any preceding claim, wherein said channel includes a satellite having a cascaded input multiplexer filter, a power amplifier and an output multiplexer filter, and said forward model further comprises a model of magnitude response of said input multiplexer filter and output multiplexer filter, whereby magnitude response of said channel may be corrected.

5. A method as claimed in any preceding claim, wherein said stage groups are substantially identical to one another.

6. A method as claimed in any preceding claim, wherein said forward model comprises a series connection of an up-sampler, a model representative of magnitude and phase of: a transmission side root Nyquist filter, a transmission side high power amplifier, a satellite input multiplexer filter, a satellite power amplifier, a satellite output multiplexer filter, a receiver side root Nyquist filter, and a down sampler.

7. A pre-corrector for producing non-linear distortion in a signal channel containing a root Nyquist bandpass filter in transmission and reception portions of said channel, said pre-corrector including plural cascaded groups of stages, each group including a series connection of a static pre-distorting section configured to represent an inverse function of magnitude and phase distortion estimated to occur in said channel, and a dynamic pre-distorting section including a forward model configured to represent magnitude and phase distortion in said channel, each successive stage group connected to receive input from an output of a preceding stage group, whereby distortion in said channel is successively reduced towards zero with each successive stage group.

8. A pre-corrector as claimed in claim 7, wherein at least two groups of stages are provided.

9. A pre-corrector as claimed in claim 7 or 8 , wherein said channel is one of a satellite, cable, or terrestrial channel .

10. A pre-corrector as claimed in any of claims 7 to 9, wherein said channel includes a satellite having a cascaded input multiplexer filter, a power amplifier and an output multiplexer filter and said forward model further comprises a model of magnitude response of said input multiplexer filter and output multiplexer filter, whereby magnitude response of said channel may be corrected.

11. A pre-corrector as claimed in any of claims 7 to 10, wherein said stage groups are substantially identical to one another.

12. A pre-corrector as claimed in any of claims 7 to 11, wherein said forward model comprising a series connection of an up-sampler; a representation of magnitude and phase distortion produced by a transmitting side root Nyquist filter, a transmitting side high power amplifier, a satellite input multiplexer filter, a satellite power amplifier, a satellite output multiplexer filter, a receiving side root Nyquist filter; and a down sampler.