GB2376611A

GB2376611A - Method of adjusting received constellation points

Info

Publication number: GB2376611A
Application number: GB0114527A
Authority: GB
Inventors: David G Edwards; Brian Herbert Beech
Original assignee: Tandberg Television AS
Current assignee: Ericsson Television AS
Priority date: 2001-06-14
Filing date: 2001-06-14
Publication date: 2002-12-18
Also published as: GB0114527D0

Abstract

A digital data transmission channel link uses modulation techniques (eg. PSK, QAM etc.) to produce a constellation which represents digital data. A constellation point is measured at a receiver (137) and compared to an optimum constellation point as determined at the receiver. A dynamic precorrector (120) is used to adjust the magnitude and phase of the transmitted signal in order to align the subsequent measured point with the optimum point. The method may make use of a graticule (fig. 11) in which optimum constellation points are marked on concentric circles and a grid of orthogonally disposed intersecting lines.

Description

METHOD OF ADJUSTING RECEIVED CONSTELLATION POINTS AND A TUNING AID THEREFOR This invention relates to a method of adjusting constellation points of a digital data transmission channel link and to a tuning aid used in said method.

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. Typical techniques are those of phase shift keying (PSK) and quadrature amplitude modulation (QAM). Common techniques are quadrature phase shift keying (QPSK) 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.

For 16 QAM operation, the highest drive level which can be used practically for a peak power limited satellite channel is that which forces the satellite transponder to saturation on the corners of the 16 QAM constellation, and such a constellation diagram is shown in Figure 1 having axes I and Q. It is desirable to allow the satellite

transponder to operate at, or very close to, saturation and such operation is known as"corners at saturation" operation.

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 non-linear 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 WO-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-ofband 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 at least 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 dynamic pre-corrector comprising 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.

In the use of such a dynamic pre-corrector, where on a symbol-by-symbol basis the position of transmitted constellation points are re-positioned with knowledge of the non-linearity in the transmission channel, the requirement is that at the receiving station the received constellation is the same as that which would be transmitted with no pre-correction, i. e. it is the same as though passed through a linear channel. Such a process works correctly only if the response of the non-linearity can be accurately predicted.

Dynamic pre-correction is primarily of benefit for travelling wave tube (TWT) satellite power amplifiers and/or high-power amplifiers operated with a low value of output back-off. However, such dynamic pre-correction may also be used with solid-state power amplifiers used in the satellite.

An exemplary known satellite system having power control for a satellite transponder not having AGC is shown in Figure 2. In the system, plural encoders 1 provide input to a multiplexer 2 through an asynchronous serial interface (not shown) and the multiplexer 2 provides input to a modulator 3, also through an asynchronous serial interface (not shown). The modulator 3 provides an intermediate frequency output to a super high frequency up converter 4 which provides output to a high power amplifier 5, output of which is transmitted by, for example, a parabolic dish 6 to a satellite 7. The satellite retransmits the received signal to a receiving satellite dish 8 and the received signal is demodulated by a demodulator 9 and applied to one or more decoders 10.

In the satellite system there are non-linearities in the high power amplifier 5 and in an amplifier in the satellite 7. The dynamic pre-corrector is required to correct for both of these non-linearities. The nonlinearity has both a magnitude non-linearity and a phase non-linearity. Typical non-linearities for a satellite are shown in Figures 3 and 4, where Figure 3 shows a graph of output power against input power and Figure 4 shows a graph of output phase against input power. Figure 5 shows a 16QAM constellation in which magnitude and phase of the constellation points is indicated.

From the foregoing it will be observed that output magnitude is a function of input magnitude and that output phase is also a phase of input magnitude.

From Figure 2 it will be noted that the signal firstly passes through the non-linearity of the high power amplifier 5. This implies that the behaviour of the signal at the output of the satellite 7 is a function of the input signal modified by the high power amplifier and then modified by the satellite.

The signal path may be visualised as shown in the block schematic diagram of Figure 6 where the inputted signal is subject to magnitude and phase distortion in the high power amplifier, and output magnitude of the signal from the high power amplifier is subjected to magnitude and phase distortion in the satellite, whereas phase distortion in the high power amplifier is subject to phase distortion in the satellite. Thus, the total output magnitude distortion is dependent upon the magnitude non-linearities, but the total phase distortion is dependent upon the phase non-linearities plus the magnitude non-linearities.

It will, therefore, be understood that the method of adjusting the dynamic pre-corrector is extremely complex and difficult when performed on a trial and error basis by inspecting the received constellation and making changes to the pre-corrector.

The present invention seeks to provide a method of adjusting the received constellation points of a digital data transmission channel link and to a tuning aid that may be used in such a method.

According to a first aspect of this invention there is provided a method of adjusting received constellation

points of a digital data transmission channel link including the steps of providing a dynamic pre-corrector arranged to correct magnitude and phase distortion in said channel, providing constellation point measuring means at a receiving station, providing optimum constellation point determining means at said receiving station, comparing a constellation point measured by said measuring means with said optimum constellation point provided by said determining means, and adjusting the magnitude of the transmitted signal with said dynamic pre-corrector to align the measured constellation point with the optimum constellation point.

Preferably, there is provided the further step of adjusting the phase of the transmitted signal by comparing the constellation point phase measured by said measuring means with the optimum constellation point and adjusting the phase of the transmitted signal using said dynamic precorrector.

Advantageously, the transmitted signal is 16 QAM.

Preferably, said measuring means is an oscilloscope.

In a preferred embodiment, the optimum constellation point determining means is a graticule having marked thereon optimum constellation points for a predetermined quadrature amplitude modulation type, said optimum constellation points being equi-circumferentially arranged on one or more concentric circles and a grid of orthogonally disposed lines is arranged to intersect at each said optimum constellation point.

In a further embodiment, the optimum constellation points are stored in memory means and processor means is provided to compare said optimum constellation points with

the measured constellation points derived by said measuring means, and to provide an output signal to adjust said dynamic pre-corrector.

According to a further aspect of this invention there is provided a tuning aid for a digital signal channel dynamic pre-corrector including graticule means having marked thereon optimum constellation points for a predetermined quadrature amplitude modulation type, said optimum constellation points being equi-circumferentially arranged on one or more concentric circles, and a grid of orthogonally disposed lines intersecting one another at each said optimum constellation point.

Preferably, for 16 QAM there is provided three concentric circles and said grid comprises four lines in one direction and four lines in an orthogonal direction.

Advantageously, said graticule is made of plastics material.

Conveniently, said one or more circles and said grid are one of engraved and painted on said graticule.

Preferably, diametric lines are provided on the graticule for use in correcting phase distortion.

According to another aspect of this invention there is provided a method of using the tuning aid comprising the steps of providing a dynamic pre-corrector arranged to correct magnitude and phase distortion in a digital data transmission channel, providing constellation point measuring means at a receiving station, providing said tuning aid on an oscilloscope at said receiving station, measuring the amplitude and phase of a received signal at the receiving station, comparing the magnitude of the measured constellation point with the optimum constellation

point marked on said tuning aid, varying characteristics of said dynamic pre-corrector to adjust the received constellation point to appear on a requisite circle of said tuning aid, which circle is representative of correct magnitude of all phase conditions, and adjusting the phase of the received signal by adjusting said dynamic precorrector to shift the constellation point to align with a diametric line passing through the optimum constellation point.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which : Figure 1 shows a constellation diagram with"corners at saturation"for 16 QAM, Figure 2 shows a known satellite transmission system, Figure 3 shows a typical satellite magnitude nonlinearity characteristic, Figure 4 shows a typical satellite phase non-linearity characteristic, Figure 5 shows magnitude and phase shown on a 16 QAM constellation, Figure 6 shows the signal path through transmission channel link elements producing non-linearities, Figure 7 shows, in block schematic form, a satellite transmission apparatus used in the present invention, Figure 8 shows a dynamic pre-corrector used in this invention, Figure 9 shows a known 64 QAM graticule, Figure 10 shows a tuning aid in the form of a graticule for a 16 QAM embodiment in accordance with this invention, and

Figures 11 and 12 respectively show uncorrected and corrected constellations for 16 QAM from an actual satellite channel link.

In the drawings like references denote like parts.

A satellite communication link used in this invention will now be described with reference to Figure 7. The transmitter side has a modulator 119 including a precorrector 120, the modulator having an input 118 for receiving a stream of data bits and the modulator produces complex, I and Q, modulated outputs. The pre-corrector will be described in detail hereinafter with reference to Figure 8. Output from the modulator is applied to an up sampler 121 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 122, 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. Output from the filter 122 is applied to an I, Q modulator 123 which provides an output to an up-converter 124, output of which is applied to a high power amplifier 125 and then transmitted by, for example, the parabolic dish 6 to a satellite 7.

The satellite 7 has a receiving antenna 28 applying a signal to an input multiplexer (IMUX) filter 70, thence to a power amplifier 80 and an output multiplexer (OMUX) filter 90. Output from the OMUX filter is applied to a transmitting antenna 29 and a signal is received by, for example, the parabolic dish 8 at a receiver side.

An output R. F. signal from the dish 8 is applied to a down converter 131. Output from the down converter 131 is

applied to an I, Q demodulator 132 which, in turn, provides output to a root Nyquist band pass filter 133. The output of the filter 133 is applied to down-sampler 134 and the I, Q down sampled outputs are demodulated by demodulator 135 to provide digital data transmitted by the symbols within the modulation scheme and provide an IQ output at output terminal 136.

Output from the demodulator is applied to an oscilloscope 137. Alternatively, the output from the demodulator may be connected to a processor such as a personal computer 138 which, in any event, is connected over a transmission path to the modulator 119.

The pre-corrector 120 shown in Figure 8, has an 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 pre-corrector 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 places 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

- n 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-lineai 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 all input of the forward mode. 42 which is a pre-calculated forward model representative of the satellite transmission/ reception channel from the input of 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 (1) that are acted upon by the forward mode) 42. The subtracter 44 output, which is an error signal given by Vi-f (Vi), is appJied to an amplifier 45 and thence to one input of an adder 46, the other input of adder 46 being derived from jnput line 41 which are delayed by a delay (not shown) representative of the dc] ay 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 4V 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 precorrected 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 predistortion, each approximating to the required predistortion 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.

Initially, to set up the system, typically, firstly magnitude is adjusted and then phase is adjusted. Assuming 16 QAM modulation with the satellite operating at"corners at saturation", i. e. the four outer corners of the constellation shown in Figure 1 are located with the satellite transponder operating at its saturation point.

The constellation diagram of Figure 1 is displayed on the oscilloscope 137 and the correction parameters of precorrector 120 are adjusted to obtain the corners at saturation constellation diagram that is desired.

It is known to use graticules over the CRT of the oscilloscope 137 to show a visual indication of perfect performance or an allowable tolerance of the subject being measured. A known graticule is shown in Figure 9 where each of the points in the 64 QAM constellation are deemed to be acceptable if they fall within the boundaries of their dedicated squares. The problem with graticules of the type shown in Figure 9 is that they simply show an area of acceptability. In the tuning process of setting up the dynamic pre-corrector 120, the received constellation starts off in a very distorted uncorrected state and needs to be tuned to as near a perfect state as possible. The graticule shown in Figure 9 neither indicates a manner of correcting the uncorrected state nor clearly indicates the perfectly corrected state.

The tuning aid of the present invention is for use in adjustment of the dynamic pre-corrector. It will be appreciated from the foregoing that dynamic pre-correction is a technology where, on a symbol-by-symbol basis, the position of transmitted constellation points are repositioned with knowledge of the non-linearity that occurs

in the transmission path. The object is that at the receiver side the received constellation is as perfect in shape as if it passed through a linear channel.

The parameters that describe the non-linear model are adjusted by inspecting the received constellation against a reference and by making changes to the model, i. e. by making changes in the dynamic pre-corrector, so the received constellation points are adjusted to align with the reference.

Referring to Figure 10, an embodiment of the invention showing a 16 QAM tuning aid has a graticule which, firstly, shows the position of each constellation points 201-216 in the ideal situation, i. e. to act as the reference. The graticule marks the magnitude of the constellation points by way of three concentric circles 241-243. With three circles each of differing diameter it is possible to describe the magnitude of all of the sixteen constellation points for all angles of phase. It is this information which is of great importance when tuning the dynamic precorrector to model the magnitude non-linearity of the satellite channel. Straight lines 221-228 are drawn through each of the constellation points to form a grid of four lines in one direction (in the embodiment, a horizontal direction) and four lines in an orthogonal direction (in the embodiment, a vertical direction). The grid is useful because the combination of the constellation points and the circles on their own creates an optical illusion that the constellation has become"pin cushion"in shape.

The graticule preferably also includes diametric lines 231-236 passing to the centre of the constellation

through the optimum constellation points so as to mark values of correct phase for all values of magnitude. By such a marking expedient, it is possible to tune the precorrector for both magnitude and phase.

In operation, the received constellation points are adjusted in terms of magnitude to appear on a relevant circle, and once the magnitude correction has been made by the dynamic pre-corrector, so it is then possible to alter the phase of the transmitted signal using the dynamic precorrector so that all the constellation points will align with their optimum (target) positions to correct phase.

Figure 11 shows a 16 QAM constellation from an actual satellite link running at saturation in which the constellation is uncorrected. When examining the constellation by comparing it with the information on the graticule of the tuning aid, an understanding of how the constellation has been modified by the non-linearities in the transmission path can readily be understood.

An examination of Figure 11 shows that the four outer corner points are displaced with respect to the optimum points shown on the graticule and it is clear that the received outer constellation points that have been received are not in the correct position. By comparing the received constellation corner points with the outermost circle it is possible to secure an indication of the extent of the magnitude error. It should be noted that it is not readily possible to gauge how much magnitude error is present by comparing the received constellation corner points to the optimum constellation point because the received constellation points also have a phase error as well as the magnitude error, thereby making direct comparison

difficult. Similarly, it is possible to examine the constellation corner points and by comparing their position with the radial phase lines of the graticule, it is possible to obtain an indication of the phase error. Once the magnitude and phase errors are known then correction can be made using the dynamic pre-corrector to overcome the error. The same inspection procedure can be repeated for any other chosen constellation point. As stated previously, the magnitude error is firstly corrected by moving the received constellation point that is in error onto the appropriate magnitude circle and then to correct for the phase error by shifting the constellation point to the optimum point where the radial line intersects the circle.

Figure 12 shows the received constellation where the dynamic pre-corrector has been correctly adjusted and where the received constellation points are now located directly over the optimum points.

It will be appreciated that the graticule of the present invention may be adapted for other quadrature amplitude modulation types.

It will also be appreciated that the optimum constellation points could be held as reference points in a memory store and a computer algorithm run by the PC 138 could inspect the received constellation registering the position of the received symbols and comparing the received points to the stored magnitude references representative of the circle, and then to compare the received phase with the stored phase references representative of the radial lines. Knowing the error in magnitude and phase, the algorithm can then steer the correction parameters of the dynamic pre-

corrector in such a manner which minimises the magnitude and phase error to achieve a substantially perfectly corrected constellation.

Claims

CLAIMS: 1. A method of adjusting received constellation points of a digital data transmission channel link including the steps of providing a dynamic pre-corrector arranged to correct magnitude and phase distortion in said channel, providing constellation point measuring means at a receiving station, providing optimum constellation point determining means at said receiving station, comparing a constellation point measured by said measuring means with said optimum constellation point provided by said determining means, and adjusting the magnitude of the transmitted signal with said dynamic pre-corrector to align the measured constellation point with the optimum constellation point.
2. A method as claimed in claim 1, wherein there is provided the further step of adjusting the phase of the transmitted signal by comparing the constellation point phase measured by said measuring means with the optimum constellation point and adjusting the phase of the transmitted signal using said dynamic pre-corrector.
3. A method as claimed in claim 1 or 2, wherein the transmitted signal is 16 QAM.
4. A method as claimed in any preceding claim, wherein said measuring means is an oscilloscope.
5. A method as claimed in any preceding claim, wherein the optimum constellation point determining means is a

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graticule having marked thereon optimum constellation points for a predetermined quadrature amplitude modulation type, said optimum constellation points being equicircumferentially arranged on one or more concentric circles and a grid of orthogonally disposed lines is arranged to intersect at each said optimum constellation point.
6. A method as claimed in any preceding claim, wherein, the optimum constellation points are stored in memory means and processor means is provided to compare said optimum constellation points with the measured constellation points derived by said measuring means, and to provide an output signal to adjust said dynamic pre-corrector.
7. A tuning aid for a digital signal channel dynamic precorrector including graticule means having marked thereon optimum constellation points for a predetermined quadrature amplitude modulation type, said optimum constellation points being equi-circumferentially arranged on one or more concentric circles, and a grid of orthogonally disposed lines intersecting one another at each said optimum constellation point.
8. A tuning aid as claimed in claim 7, wherein for 16 QAM there is provided three concentric circles and said grid comprises four lines in one direction and four lines in an orthogonal direction.
9. A tuning aid as claimed in claim 7 or 8, wherein said graticule is made of plastics material.

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,, e 14 4 M11
10. A tuning aid as claimed in claim 3, wherein said one A n cl a or more circles and said grid are one of engraved and painted on said graticule.
11. A tuning aid as claimed in any of claims 7 to 10, wherein diametric lines are provided on the graticule for use in correcting phase distortion.
12. A method of using the tuning aid claimed in claim 8, comprising the steps of providing a dynamic pre-corrector arranged to correct magnitude and phase distortion in a digital data transmission channel, providing constellation point measuring means at a receiving station, providing said tuning aid on an oscilloscope at said receiving station, measuring the amplitude and phase of a received signal at the receiving station, comparing the magnitude of the measured constellation point with the optimum constellation point marked on said tuning aid, varying characteristics of said dynamic pre-corrector to adjust the received constellation point to appear on a requisite circle of said tuning aid, which circle is representative of correct magnitude of all phase conditions, and adjusting the phase of the received signal by adjusting said dynamic pre-corrector to shift the constellation point to align with a diametric line passing through the optimum constellation point.
13. A method substantially as herein described with reference to and as shown in Figures 10 to 12 of the accompanying drawings.

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14. A tuning aid substantially as herein described with reference to and as shown in Figures 10 to 12 of the accompanying drawings.
15. A method using a tuning aid substantially as herein described with reference to and as shown in Figures 10 to 12 of the accompanying drawings.