GB2346773A

GB2346773A - Pre-clipping module for a non-linear high power amplifier

Info

Publication number: GB2346773A
Application number: GB9900554A
Authority: GB
Inventors: Peter Close; John Stuart Curtis
Original assignee: NDS Ltd; Tandberg Television AS
Current assignee: Synamedia Ltd; Ericsson Television AS
Priority date: 1999-01-12
Filing date: 1999-01-12
Publication date: 2000-08-16

Abstract

A method of transmitting an analogue signal, said signal comprising an amplitude modulated component and a modulation envelope, through a channel having a non-linear characteristic that introduces a distortion frequency spectrum to the signal, said non-linear channel comprising a clipping region and said distortion frequency spectrum comprising an out-of-band component and an in-band component, the method comprising the step of controlling the excursion of the modulation envelope into the clipping region of the channel by adding a distortion frequency spectrum which reduces the out-of-band component and increases the in-band component to the signal prior to its transmission through the channel. Apparatus for transmitting an analogue signal, said signal comprising an amplitude modulated component and a modulation envelope, through a channel having a non-linear characteristic that introduces a distortion frequency spectrum to the signal, said non-linear channel comprising a clipping region and said distortion frequency spectrum comprising an out-of-band component and an in-band component, the apparatus comprising means for controlling the excursion of the modulation envelope into the clipping region of the channel by adding a distortion frequency spectrum, said means reducing the out-of-band component and increasing the in-band component to the signal prior to its transmission through the channel.

Description

METHOD AND APPARATUS FOR TRANSMITTING A SIGNAL The present invention relates to the field of transmitting data, and more specifically to a method and apparatus for reducing the effects of non-liner distortion through use of distortion pre-correction.

In order to achieve high efficiency in transmitters, high-power amplifiers are frequently driven into their non-linear region. This may result in the undesired effect of introducing distortion into the resultant amplified signal. Precorrection of a signal prior to amplification can reduce the magnitude of the effects of the non-liner characteristic of the amplifier by providing a perceived extension of the linear region of the amplifier. However, this does not eradicate the broadband distortion introduced by an amplifier due to its finite clippinglevel.

The out-of-band distortion introduced by an amplifier can be reduced subsequently by processing with an appropriate filter. As the power of the signal to be amplified increases, the costs of purchase and running of amplifier and also the complexity of the necessary filter arrangement also increase. Although running the amplifier at a lower clipping level can significantly reduce amplifier associated costs, this necessitates more expensive and bulky filtering.

It is an aim of system designers, and especially those working with highpower amplifiers, to reduce distortion introduced by the process of amplification without increasing the complexity of the output filter. Alternative aims include the increase of amplifier efficiency without an associated increase of out-of-band distortion or output filter complexity.

According to one aspect of the invention there is provided a method of transmitting an analogue signal, said signal comprising an amplitude modulated component and a modulation envelope, through a channel having a non-liner characteristic that introduces a distortion frequency spectrum to the signal, said non-linear channel comprising a clipping region and said distortion frequency spectrum comprising an out-of-band component and an in-band component, wherein the method comprises the step of controlling the excursion of the modulation envelope into the clipping region of the channel by adding a distortion frequency spectrum which reduces the out-of-band component and increases the in-band component to the signal prior to its transmission through the channel.

According to a second aspect of this invention there is provided apparatus for transmitting an analogue signal, said signal comprising an amplitude modulated component and a modulation envelope, through a channel having a non-liner characteristic that introduces a distortion frequency spectrum to the signal, said non-linear channel comprising a clipping region and said distortion frequency spectrum comprising an out-of-band component and an in-band component, wherein the apparatus comprises means for controlling the excursion of the modulation envelope into the clipping region of the channel by adding a distortion frequency spectrum, said means reducing the out-of-band component and increasing the in-band component to the signal prior to its transmission through the channel.

Reference will now be made, by way of example only, to the accompanying drawings, in which: Figure 1 is a schematic diagram of typical amplifier characteristics with and without known pre-correction.

Figure 2 is a number of schematic diagrams showing the typical effects of amplification and post amplification band-pass filtering on a typical ideal frequency spectrum.

Figure 3 is a number of schematic diagrams showing a modulation envelope of an amplitude modulated carrier, and the effects upon such an envelope of known pre-correction techniques and also of clipping induced by an amplifier.

Figure 4 is a block diagram of an overview of a system according to this invention.

Figure 5 is a block diagram of a first embodiment of the base-band preclipping module of the system of Figure 4.

Figure 6 is a number of schematic diagrams showing the effects of precorrection in accordance with the present invention by the base-band preclipping module of Figure 5 on a typical modulation envelope of an amplitude modulated carrier.

Figure 7 is block diagram of a second embodiment of the base-band preclipping module of the system of Figure 4.

Figure 1 is a schematic diagram showing a known typical amplitude characteristic 10 of an amplifier. It is known in the art of design of systems incorporating amplifiers that the efficiency of an amplifier can be increased by extending its linear region by the use of a pre-correction characteristic 12 in order to achieve an extended linear region 14. Although theoretically such an extension can result in an infinie linear region, in practice the amplifier will impose a clipping to the length of the linear region as seen between points A and B of characteristic 14. Analogue pre-correction of this type is well known in the art, whilst digital pre-correction is described in our patent application GB 9819688. 4.

Figure 2 (a) shows an ideal frequency spectrum 20. It will be understood by those skilled in the art that spectrum 20 is representative of spectra produced by orthogonal frequency division multiplex (OFDM) signals.

Referring to Figure 2 (b), on transmission of ideal frequency spectrum 20 through an amplifier, the resultant frequency spectrum 21 will be substantially the same. However, the amplifier will have introduced broadband distortion frequency spectrum 22 to the output of the amplifier. This broadband distortion spectrum comprises two components, namely in-band distortion 23 and out-of-band distortion 24. The out-of-band distortion component is undesirable as it can lead to adjacent-channel interference. This is particularly true for applications such as digital terrestrial television transmission.

Broadband distortion spectrum 22 can be reduced by application of known pre-correction techniques as described with reference to Figure 1, resulting in broadband distortion spectrum 25 as seen in Figure 2 (c). Where the amplifier is driven such as to impose clipping, the magnitude of the broadband distortion increases to that of distortion frequency spectrum 26.

Processing of the output of an amplifier by a filter leads to a distortion spectrum having a reduced out-of-band component 27, whilst the in-band component will remain substantially the same. There are no known techniques for elimination of in-band distortion spectra by post amplification filtering. Reduction of out-of-band spectra below a power of-36dB (from a reference of peak spectrum power of OdB) result in filter embodiments that are costly for typical uses such as high-power amplifiers contained within confined spaces often sited at remote locations.

Figure 3 (a) shows schematically a modulation envelope 30 of a carrier with an amplitude modulation component. As was described earlier with reference to Figure 1, it is well known that amplifiers induce clipping of input amplitudes greater than a threshold particular to any given amplifier. This threshold is represented in Figure 3 as Lc, ip It can be seen from Figure 3 (b) that any peak amplitude within the envelope 30 which is greater than Ldjjp is clipped such that amplifier output amplitudes are no greater than those within the clipping level. Input amplitudes which are less than the threshold Lc, ip are unaffected by amplitude clipping.

Although the carrier of Figure 3 (a) is shown as a carrier having an amplitude modulation component, it will be understood by those skilled in the art that the term amplitude modulated component includes those waveforms which are modulated in a number of ways, one of which is that of amplitude modulation.

One example of this combination modulation is that of amplitude and phase modulation of carriers such as 64QAM.

One embodiment of a system of the present invention is shown in Figure 4.

Although this embodiment shows a digital implementation of the present invention, it will be understood that the invention may be implemented using analogue techniques. Digital data is provided as input 40 to Coded OFDM modulator 41 providing I and Q outputs as complex modulation of multiple base band frequency orthogonal carriers. The I and Q outputs are each input to base-band pre-clipping module 42. Each of these outputs is multiplied by the quadrature outputs of an intermediate frequency oscillator 43 at multipliers 44 and 45. The output of each multiplier is added by summer 46 and then converted to an analogue signal representative of the digital output of summer 46 by digital-to-analogue converter 47. The output of converter 47 is multiplied with the output of ultra high frequency (UHF) oscillator 48 by multiplier 49 and then filtered by filter 53 to provide a UHF analogue signal to high-power amplifier 50. The output of amplifier 50 is filtered by UHF filter 51 and then transmitted by UHF antenna 52. The system of Figure 4 differs from prior art systems due to the inclusion of base-band pre-clipping module 42.

Figure 5 is a block diagram of one embodiment of base-band pre-clipping module 42 of Figure 4. The I and Q outputs from Coded OFDM modulator 41 are provided as inputs 58 and 59 respectively. These inputs are each squared by multipliers 60 and 61 respectively, the outputs of which are then fed as inputs to summer 62. The resultant signal is provided as an input to square root function module 63, which in turn feeds its output to gain module 64. The output from gain module is multiplied with each of I and Q inputs 58 and 59 at multipliers 66 and 67 respectively. The output from each of multipliers 66 and 67 is filtered by filters 68 and 69 respectively before being provided as inputs to multipliers 44 and 45 of Figure 4.

Although not shown in Figures 4, it is preferable for the clipping pre-corrector of the present invention to be implemented in a system that also incorporates known techniques of pre-correction such as described with reference to Figure 1.

The gain applied by module 64 is dependent upon the clipping threshold of amplifier 50, and can be expressed as: Gn = min {1, (klip-6)/en} equation (1) where en is the estimated amplitude envelop of the modulated base-band carrier or carriers, and 8 governs the extent of clipping pre-correction applied by pre-corrector 42. en is estimated by computing the magnitude of the complex base-band signal in the time domain.

The effects pre-correction in accordance with the present invention by baseband pre-clipping module 42 of Figure 5 on a typical modulation envelope of an amplitude modulated carrier can be seen in Figure 6. Figure 6 (a) shows a schematic diagram of an amplitude envelope 70 with reference to thresholds Lcjip and (t-d, p-8) where Lc ; p is the clipping threshold of amplifier 50. In order to ensure amplifier 50 does not clip its input, and therefore generate increased broadband distortion envelope 70 is modified to such that its peak amplitude is less than Lclip. All magnitudes of amplitude within the envelope are modified in accordance with equation (1), leading to a modified amplitude envelope 71 represented in Figure 6 (b). Filtering by filters 68 and 69 result in the amplitude envelope of Figure 6 (c). The amplifier will introduce no further distortion unless the filtered modified envelope exceeds Lois.

By appropriate selection of 8 and filters 68 and 69, it is possible to reduce the out-of-band distortion component to a desired level which is less than the36dB commonly accepted as the practical limit for output filters associated with power amplifiers. However, it will be understood by those skilled in the art that such a reduction in the out-of-band distortion component is accompanied by an increase in the in-band distortion component. Such increases can be tolerated by applications such as digital terrestrial television transmission links, where it is important to reduce the interference between channels that can be caused by out-of-band distortion components. Typically, such increases will be tolerable until the increase of the in-band distortion component leads to a component that is comparable to the noise floor inherent in the transmission link used.

The value of 8 may be varied during operation of pre-clipping module 42 such as to provide adaptive control of the gain applied by module 64. This allows for an improvement in the performance of the baseband pre-clipping module 42, i. e. a greater reduction of the out-of-band distortion component for a given in-band degradation.

Figure 7 is a block diagram of an alternative embodiment of pre-clipping module 42. The I and Q outputs from Coded OFDM modulator 41 are provided as inputs 80 and 81 respectively. These inputs are each squared by multipliers 82 and 83 respectively, the outputs of which are then fed as inputs to summer 84. The resultant signal is provided as an input to square root function module 85, which in turn feeds its output to computational module 86.

The computational module also takes I and Q inputs from 80 and 81. Each of these inputs enables the computational unit to compute a complex scaling parameter. An impulsive signal with appropriate spectral characteristics is provided by module 87, scaled by the complex scaling parameter output of module 86 by multipliers 88 and 89, and added to the base-band signal provided by I and Q inputs 80 and 81 by summers 90 and 91 respectively.

With careful selection of an impulsive signal having adequately low level outof-band frequency distortion components, then the pre-clipping can be limited to the in-band part of the spectrum and no further low-pass filtering, such as that of Figure 5, is required. For example, an impulse h (t), defined below, shapes the frequency spectrum of the added distortion so that it lies within the in-band portion of the distortion frequency spectrum. h (t) = [sin (x. t/X)]/x. t and X = 1/ [2fc] where fc is defined as the single-sided bandwidth of the I and Q signals 80 and 81.

For practical implementation, the impulse must be truncated (due to h (t) of above having infinite length, and thus a filter with impulse response h (t) is not realisable) preferably using a window function such as given below : co (t) = 0.54-0.46. cos (. t/Tw) for-Tw < t T m (t) = 0 for tt ! Z Tw where 2Tw is defined as the window width.

Alternative impulse and window fungous having characteristics appropriate to the desired level of out-of-band distortion may be used.

The above embodiments are described without reference to any other methods of pre-correction that may be used. Preferably embodiments of the present invention are combined with pre-correction techniques, typically those such as described with reference to Figure 1.

It will be understood by those skilled in the art that although the present invention allows control of the modulation envelope of a particular carrier or carriers such that the amplifier is not driven into its clipping region, it may in some circumstances by advantageous to tolerate a limited amount of amplifier clipping.

Claims

CLAIMS 1. A method of transmitting an analogue signal, said signal comprising an amplitude modulated component and a modulation envelope, through a channel having a non-liner characteristic that introduces a distortion frequency spectrum to the signal, said non-liner channel comprising a clipping region and said distortion frequency spectrum comprising an out of-band component and an in-band component, the method comprising the step of controlling the excursion of the modulation envelope into the clipping region of the channel by adding a distortion frequency spectrum which reduces the out-of-band component and increases the in-band component to the signal prior to its transmission through the channel.
2. A method according to Claim 1 wherein the signal is produced from a set of digital data giving a first digital signal and the step of controlling the excursion of the modulation envelope into the clipping region of the channel is carried out digital.
3. The method of Claim 2 wherein the step of controlling the excursion of the modulation envelope into the clipping region of the channel further comprises applying a clipping function to the first digital signal and filtering the clipped first digital signal.
4. The method of Claim 3 wherein the clipping function is: Gn = min {1, (Lc, ip-8)/en} where Lctip is the threshold at which clipping occurs during transmission of the analogue signal through the channel, en is the estimated amplitude envelop of the analogue signal, and 8 governs the extent of clipping.
5. The method of Claim 2 wherein the step of controlling the excursion of the modulation envelope into the clipping region of the channel further comprises adding a second digital signal to the first digital signal, wherein the second digital signal is representative of a signal having impulsive time domain characteristics.
6. A method according to Claim 5 wherein the signal having impulsive time domain characteristics is given as: h (t) = [sin (jet. t/T)]/7r. t and T = 1/ [2fc] where fc is defined as the single-sided bandwidth of the complex components of the second digital signal.
7. A method according to Claim 4 wherein 8 is varied during operation of the method.
8. Apparatus for transmitting an analogue signal, said signal comprising an amplitude modulated component and a modulation envelope, through a channel having a non-liner characteristic that introduces a distortion frequency spectrum to the signal, said non-liner channel comprising a clipping region and said distortion frequency spectrum comprising an out of-band component and an in-band component, the apparatus comprising means for controlling the excursion of the modulation envelope into the clipping region of the channel by adding a distortion frequency spectrum, said means reducing the out-of-band component and increasing the in band component to the signal prior to its transmission through the channel.
9. Apparatus according to Claim 8 wherein the signal is a first digital signal produced from a first set of digital data and the means for controlling the excursion of the modulation envelope into the clipping region of the channel is digital means.
10. The method of Claim 9 wherein the means for controlling the excursion of the modulation envelope into the clipping region of the channel further comprises means for applying a clipping function to the first digital signal and means for filtering the clipped first digital signal.
11. The method of Claim 10 wherein the clipping function is: Gn = min {1, (LcXip4)/en} where Lcp is the threshold at which clipping occurs during transmission of the analogue signal through the channel, en is the estimated amplitude envelop of the analogue signal, and 8 governs the extent of clipping.
12. The method of Claim 9 wherein the means for controlling the excursion of the modulation envelope into the clipping region of the channel further comprises means for adding a second digital signal to the first digital signal, wherein the second digital signal is a signal having impulsive time domain characteristics.
13. A method according to Claim 13 wherein the signal having impulsive time domain characteristics is given as: h (t) = [sin (n. t/T)]/. t and T= 1/ [2fc] where fc is defined as the single-sided bandwidth of complex components of the first digital signal.
14. A method according to Claim 11 wherein 8 is varied during operation of the method.