APPARATUS AND METHOD FOR AMPLITUDE VARIATION REDUCTION OF

A SIGNAL

Field of the invention

The invention relates to amplitude variation reduction for a signal and in particular to amplitude variation reduction for a complex base band signal .

Background of the Invention

A characteristic of many modulation techniques used in radio communication systems is that they have very high amplitude fluctuations. In order to avoid distortions to be introduced at the transmitter, it is important that the power amplifier has a sufficiently large dynamic range with a substantially linear transfer function (i.e. substantially constant gain) . However, achieving such a dynamic range is complex, cumbersome and expensive. Furthermore, in order to ensure that the transfer function is sufficiently linear, the amplifier is backed- off from the maximum power output and thus the peak power requirements results in RF power amplifiers typically operating well below their capability and hence operating very inefficiently.

One established technique for reducing the high peak to average ratios of a signal to be amplified is Crest limiting which reduces the peak to average ratio at the expense of some EVM (Error Vector Magnitude) degradation. Typically, some form of (digital) filtering is also used

so that the clipped peaks are rounded off to reduce the generation of spectral sidebands. With single carrier systems this is easily achieved by performing the clipping ahead of the channel filter.

One specific form of limiting is scalar limiting where the I and Q components of a signal are individually limited to a maximum absolute value. This results in a square constellation area for the constellation vectors to occupy which leads to a fairly large dynamic peak amplitude variation of up to 3dB depending on the angle of the vector being limited.

Another specific form of limiting is full circular limiting wherein the actual amplitudes of signal vectors are calculated and limited to a maximum allowed amplitude without affecting the angle of the vector. Although this may provide improved performance, it is very computationally intensive resulting in a complex and expensive implementation. A typical implementation of this approach uses a large FPGA (Field Programmable Gate Array) with multipliers, registers and look-up tables.

Hence, an improved amplitude limitation would be advantageous and in particular a limitation allowing increased flexibility, reduced peak to average amplitude variation, reduced distortion, improved amplifier efficiency and/or improved performance would be advantageous .

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Summary of the Invention

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided an apparatus for amplitude variation reduction of a signal, the apparatus comprising:means for receiving a complex base band signal having a first real component and a first imaginary component ;means for generating a limited signal by limiting the complex base band signal by independently limiting the first real component and the first imaginary component; phase rotating means for phase rotating the limited signal by a first phase value to generate a phase rotated signal having a second real component and a second imaginary component; means for generating a limited phase rotated signal by limiting the phase rotated signal by independently limiting the second real and second imaginary components ; and means for generating an amplitude limited signal from the limited phase rotated signal .

The invention may provide improved amplitude limitation and thus may provide a signal with reduced amplitude variations and especially a reduced peak to average amplitude ratio . A low complexity amplitude limitation can be achieved allowing facilitated implementation and/or reduced implementation cost.

According to an optional feature of the invention, the first phase value is substantially 45 degrees.

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This may provide improved performance and/or facilitated implementation. In particular, it may provide amplitude limitation relatively similar to a circular limiting while requiring only simple operations/functionality to be implemented.

According to an optional feature of the invention, the phase rotating means is arranged to generate at least one of the second real value and the second imaginary value in response to a subtraction of the limited first imaginary component from the limited first real component .

This may provide improved performance and/or facilitated implementation. In particular, it may provide a particularly advantageous and/or low complexity means of performing the phase rotation. The generation of the at least one of the secondary value and the second imaginary value may include a sign inversion of the subtraction result.

According to an optional feature of the invention, the phase rotating means is arranged to generate at least one of the second real value and the second imaginary value in response to an addition of the limited first imaginary component and the limited first real component.

This may provide improved performance and/or facilitated implementation. In particular, it may provide a particularly advantageous and/or low complexity means of performing the phase rotation. The generation of the at least one of the secondary value and the second imaginary

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value may include a sign inversion of the addition result .

According to an optional feature of the invention, the first limiting means is arranged to limit the absolute value of the first real component and of the second imaginary threshold to a first predetermined threshold.

This may provide improved performance and/or facilitated implementation. In particular, it may provide amplitude limitation relatively similar to a circular limiting while requiring only simple operations/functionality to be implemented.

According to an optional feature of the invention, the second limiting means is arranged to limit the absolute value of the second real component and of the second imaginary component to a second predetermined threshold.

This may provide improved performance and/or facilitated implementation. In particular, it may provide amplitude limitation relatively similar to a circular limiting while requiring only simple operations/functionality to be implemented.

According to an optional feature of the invention, the first threshold is different from the second threshold.

This may provide improved performance and/or facilitated implementation. In particular, it may provide a similar limitation of the real and imaginary components while allowing a phase rotation operation that scales the signal .

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According to an optional feature of the invention, the second threshold corresponds to the first threshold scaled by a factor of substantially the square root of two .

This may provide improved performance and/or facilitated implementation. In particular, it may provide a similar limitation of the real and imaginary components while allowing a phase rotation operation by simple addition/subtraction operations.

According to an optional feature of the invention, the means for generating the amplitude limited signal is arranged to perform a phase rotation of the limited phase rotated signal by an inverse phase value to the first phase value.

This may provide improved performance and/or facilitated implementation. In particular, it may provide amplitude limitation relatively similar to a circular limiting while requiring only simple operations/functionality to be implemented. In particular, it may allow amplitude limitation without significantly affecting the phase of the signal .

According to an optional feature of the invention, the means for generating the amplitude limited signal is arranged to generate at least one of real value and an imaginary value of the amplitude limited signal in response to a subtraction of the limited second imaginary component from the limited second real component.

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This may provide improved performance and/or facilitated implementation. In particular, it may provide a particularly advantageous and/or low complexity means of performing the phase rotation. The generation of the at least one of the secondary value and the second imaginary value may include a sign inversion of the subtraction result .

According to an optional feature of the invention, the means for generating the amplitude limited signal is arranged to generate at least one of real value and an imaginary value of the amplitude limited signal in response to an addition of the limited second imaginary component and the limited second real component.

This may provide improved performance and/or facilitated implementation. In particular, it may provide a particularly advantageous and/or low complexity means of performing the phase rotation. The generation of the at least one of the secondary value and the second imaginary value may include a sign inversion of the addition result .

According to an optional feature of the invention, the means for generating the amplitude limited signal is arranged to scale the amplitude limited signal by a factor of substantially 0.5.

This may provide improved performance and/or facilitated implementation. In particular, it may provide a similar limitation of the real and imaginary components while allowing a phase rotation operation by simple addition/subtraction operations. The real and imaginary

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components of the signal following the inverse phase rotation may each be divided by a factor of substantially two.

According to another aspect of the invention, there is provided a transmitter comprising: means for generating a complex base band signal having a first real component and a first imaginary component; means for generating a limited signal by limiting the complex base band signal by independently limiting the first real component and the first imaginary component; phase rotating means for phase rotating the limited signal by a first phase value to generate a phase rotated signal having a first real component and a first imaginary component; means for generating a limited phase rotated signal by limiting the phase rotated signal by independently limiting the second real and second imaginary components; means for generating an amplitude limited signal from the limited phase rotated signal; and a power amplifier for amplifying the amplitude limited signal.

According to another aspect of the invention, there is provided a method of amplitude variation reduction of a signal, the method comprising the steps of: receiving a complex base band signal having a first real component and a first imaginary component; generating a limited signal by limiting the complex base band signal by independently limiting the first real component- and the first imaginary component; phase rotating the limited signal by a first phase value to generate a phase rotated signal having a first real component and a first imaginary component; and generating a limited phase rotated signal by limiting the phase rotated signal by

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independently limiting the second real and second imaginary components .

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment ( s ) described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 illustrates an example of a radio transmitter comprising an apparatus for amplitude variation reduction of a signal in accordance with some embodiments of the invention;

FIGs. 2 to 6 illustrate exemplary signal trajectories for a signal being limited by an apparatus for amplitude variation reduction of a signal in accordance with some embodiments of the invention; and

FIGs. 7 to 10 illustrate exemplary analogue implementations of functionality of an apparatus for amplitude variation reduction of a signal in accordance with some embodiments of the invention.

Detailed Description of Some Embodiments of the Invention

FIG. 1 illustrates an example of a radio transmitter 100 comprising an apparatus for amplitude variation reduction

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of a signal in accordance with some embodiments of the invention.

The transmitter 100 comprises a base band modulator 101 which generates a complex base band signal. For example, the base band modulator 101 can generate a complex time domain signal representing the complex data symbols to be transmitted as well as the pulse shaping of these. Specifically, the base band modulator 101 generates the real and imaginary signal components typically referred to as the I and Q components .

The complex base band signal generated by the base band modulator 101 is amplified in a power amplifier 103 which is coupled to an antenna 105. In the example, the power amplifier 103 furthermore comprises upconversion circuitry for upconverting the base band signal to the desired transmit carrier frequency.

In the transmitter of FIG. 1, the modulated complex base band signal is not fed directly from the base band modulator to the power amplifier 103 but is fed to an apparatus which reduces the amplitude variations of the signal by limiting the signal. This amplitude limitation results in a reduced peak to average amplitude ratio for the signal and accordingly in an increased efficiency and facilitated design and implementation of the power amplifier 103.

The base band modulator 101 is specifically fed to a first limiter 107 which generates a limited signal by limiting the complex base band signal by independently limiting the first real component and the first imaginary

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component. The limitation can specifically be by clipping the real and imaginary signal components to a predetermined maximum value.

The first limiter 107 is coupled to a first phase rotator 109 which is arranged to phase rotate the limited signal by a first phase value to generate a phase rotated signal having a first real component and a first imaginary component. As will be shown later, a phase rotation of 45 degrees can be achieved by a simple addition and subtraction of the real and imaginary signal components .

The first phase rotator 109 is coupled to a second limiter 111 which generates a limited phase rotated signal by limiting the phase rotated signal by independently limiting the real and imaginary components. Again the limitation can specifically be a clipping of the real and imaginary signal components to a predetermined maximum value .

The second limiter 111 thus provides scalar clipping along a different angle than the first limiter 107 thus resulting in a clipping or limitation which is closer to a fully circular clipping or limitation. Specifically, for a two stage clipping with a phase angle of 45 degrees an octagonal clipping characteristic is achieved. Hence, the limited signal generated by the second limiter 111 has a reduced amplitude variation compared to a conventional scalar clipping but is much simpler to generate as it does not require a determination of the actual amplitude as is required for circular limitation.

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It will be appreciated that although the described embodiment uses only two limiters with a 45 degree phase rotation between them, other numbers of limiters and phase rotations can be applied. For example, a three stage scalar limiting can be combined with two phase rotations of 30 degrees.

The second limiter 111 is coupled to an optional second phase rotator 113. The second phase rotator performs an inverse phase rotation to the first phase rotator 109 such that the resulting signal is phase aligned with the real and imaginary values of the original complex base band signal . The second phase rotator 113 is coupled to the power amplifier 103 which is fed the phase rotated signal.

In some embodiments, the second limiter 111 can be coupled to another functional unit which provides a suitable interface to the power amplifier 103. For example, a scaling of the resulting signal can be performed or a simple forwarding or feeding of the signal output of the second limiter 111 to the power amplifier can be implemented (corresponding e.g. to the second limiter being directly coupled to the power amplifier 103) .

It will be appreciated that in many embodiments, only the relative phase rather than the absolute phase of the transmitted signal is important (as the absolute phase is estimated in the receiver) and that the second phase rotator may therefore be left out in many embodiments and/or be replaced with a simple scaling functionality. The second phase rotator 113 may be particularly useful

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in embodiments where the transmitted signal is associated with other signals transmitted from the transmitter 100 and for which the relative phase between the signals is important .

As will be described in more detail in the following, the amplitude reduction can be achieved without any requirements for implementation of multiplication, division or trigonometric functions. An octagonal clipping shape which provides a relatively good approximation to a circular clipping can achieved. This may provide a peak limitation range of 0.7dB range (as a function of angle) in comparison to a range of 3 dB for a scalar clipping. Furthermore, the system is easily implemented by digital or analogue circuitry.

In the following a specific example will be described with reference to the signals illustrated in FIGs. 2 to 6 and the exemplary circuitry in FIGs. 7 to 10.

FIG. 2 illustrates a complex base band signal as may be received by the first limiter 107 from the base band modulator 101.

The first limiter limits the real and imaginary components individually by limiting the value of the real and imaginary component to a maximum value. Specifically, the limited values can be determined as:

IF X <= Max Xi = X ELSE Xi = Amax

IF Xi >= -Amax X ₂ = Xl ELSE X ₂ = -Amax

IF Y <= Amax Yi = Y ELSE Y _x = Amax

IF Yi >= -Amax Y ₂ = Yl ELSE Y ₂ = -Amax

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where X and Y are the input real and imaginary signal values, X ₂ and Y ₂ are the limited output values of the first limiter 103 and Amax is the applied maximum clipping level (which in the present example is symmetric around the origin and identical for the real and imaginary axes) .

FIG. 3 illustrates the effect of the clipping on the signal of FIG. 2 (with Amax=l) . FIG. 7 illustrates an example of an analogue implementation of the first limiter 107.

The signal is then fed to the first phase rotator 109 where it is phase rotated by 45 degrees (a corresponding effect can be achieved by a phase rotation of -45 degrees) .

The phase rotation can be given by:

X ₃ + iY ₃ = (X ₂ + iY ₂ )-e ^i-45ϊ = (X ₂ -Y ₂ )- cos(45 ^s ) +i(X ₂ +Y ₂ ) -sin (45 ^s )

As sin(45 ² ) =cos (45 ² ) = V2 , a phase rotation (with a scaling of V2 ) can simply be performed by the first phase rotator determining:

X ₃ =X ₂ -Y ₂

Y ₃ =X ₂ +Y ₂

The phase rotated version of the signal of FIG. 3 is illustrated in FIG. 4. FIG. 8 illustrates an example of

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an analogue implementation of the first phase rotator 109.

The phase rotated signals are fed to the second limiter 111 which performs a scalar limiting on the phase rotated real and imaginary components. However, as the phase rotation has scaled the signal, the applied maximum clipping level is adjusted accordingly. Thus, the maximum level of the second limiter 111, Bmax, is set equal to Amax times the scaling of the first phase rotator 109.

Thus, setting Bmax = (Amax*2 ⁰ ' ⁵ ) , the clipped output signals X ₃ and Y ₄ of the second limiter 111 can be determined by

IF X ₃ <= Bmax X ₄ = X ₃ ELSE X ₄ = Bmax

IF X ₄ >= - Bmax X ₅ = X ₄ ELSE X ₅ = - Bmax

IF Y ₃ <= Bmax Y ₄ = Y ₃ ELSE Y ₄ = Bmax

IF Y ₄ >= - Bmax Y ₅ = Y ₄ ELSE Y ₅ = - Bmax

It is worth noting that the clipping threshold Bmax can be predetermined and fixed at the manufacturing and design stage. The effect of the clipping on the signal of FIG. 4 is illustrated in FIG. 5. FIG. 9 illustrates an example of an analogue implementation of the second limiter 111.

The output of the second limiter 111 is thus a signal which has been clipped by a substantially orthogonal shape which provides a good approximation of a circular clipping. Furthermore, this clipping can be achieved by simple functionality and can be implemented with simple circuitry or low computational resource.

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In many embodiments, the output of the second limiter 111 can be fed directly to a power amplifier for transmission. Specifically, in many embodiments, the phase rotation and scaling of the signal introduced by the first phase rotator 109 can simply be taken into account when setting the power amplifier gain and by the phase estimation in the receiver.

However, in the specific example the double clipped signal is fed to a second phase rotator 113 which performs a second phase rotation to compensate the phase rotation of the first phase rotator 109.

Specifically, the signal of the second limiter 111 is phase rotated by -45 degrees. The phase rotation can be given by:

X ₃ + iY ₃ = (X ₂ + iY ₂ )-e ^i- - ^45s = (X ₂ -Y ₂ )- cos(-45 ² ) +i(X ₂ +Y ₂ ) °sin(-45 ² )

As sin(-45 ² ) =-V2 and cos(-45 ² )= V2 , a phase rotation (with a scaling of V2 ) can simply be performed by the first phase rotator determining:

XOUT = (X ₅ + Y ₅ ) YOUT = (Y5 - X5)

This introduces a second scaling by a factor of ^2 resulting in a total scaling by the two phase rotations of a factor of 2. To compensate for this scaling, the second phase rotator can simply divide the output result by two, thus

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XOΌT = (Xs + Y ₅ ) /2 Your = (Ys - X ₅ ) / 2

A division by a factor of 2 is generally very simple to implement and can for example in a digital implementation be achieved by a single bitwise right shift of a binary number

The effect of the phase rotation and scaling on the signal of FIG. 5 is illustrated in FIG. 6. FIG. 10 illustrates an example of an analogue implementation of the second limiter 111.

The described approach thus simplifies the process for applying crest factor reduction to complex base band signals .

It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization.

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The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these . The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units . As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous . Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally

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applicable to other claim categories as appropriate. Rather, the steps may be performed in any suitable order.

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