METHOD AND APPARATUS FOR SPECTRUM-PRESERVING AMPLITUDE COMPRESSION OF A MODULATED SIGNAL

Abstract

A method and associated apparatus for preserving frequency spectrum in a linear modulation scheme, by adding a compensation signal to a modulated signal, the compensation signal having the same spectrum as the modulated signal. In the present invention, peaks above a pre-determined maximum threshold and/or minima below a pre-determined minimum threshold are searched. When a peak is found, the amount by which the signal peak exceeds the maximum threshold, as well as the signal phase at the peak is calculated. A compensation signal is generated with the same peak amplitude as the signal exceeds the maximum threshold, and with opposite phase. Then the shape of the compensation signal is chosen to be the same as the transmitter pulse of the modulated signal. Finally, the compensation signal is added to the modulated signal.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the following section, the invention will be described with reference to exemplary embodiments illustrated in the figures, in which:

FIG. 1 is a block diagram of an apparatus for transmitting digital data over a wireless channel;

FIG. 2 is a graph illustrating an example of peak compression;

FIG. 3 is a graph illustrating an example of minimum compression;

FIG. 4 is a graph of a portion of a signal where both peak and minimum compression have been applied;

FIG. 5 is a graph illustrating the signal power distribution of 8 PSK, 16 QAM and 16 QAM with minimum/maximum compression;

FIG. 6 is a graph illustrating the spectrum of 8 PSK, 16 QAM and 16 QAM with minimum/maximum compression;

FIG. 7 is a graph illustrating the BLER performance impact of smooth minimum compression;

FIG. 8 is a graph that illustrates the BLER performance impact of smooth maximum compression;

FIG. 9 is a graph illustrating how the BLER performance impact of simultaneous minimum and maximum compression;

FIG. 10 is a flow chart of an embodiment of the method of the present invention; and

FIGS. 11A-11C are block diagrams of each of the three embodiments of an apparatus of the present invention.

DETAILED DESCRIPTION

As used herein, the following abbreviations or terms shall have the following meanings:

16 QAM 16-ary QAM

32 QAM 32-ary QAM

64 QAM 64-ary QAM

8 PSK 8-ary PSK

BLER Block Error Ratio

DAC Digital-to-Analog Converter

dB decibel

dBc decibel over carrier

EDGE Enhanced Data rates for GSM Evolution

GMSK Gaussian Minimum Shift Keying

GSM Global System for Mobile telephony

MAR Minimum-to-Average Ratio

MCS Modulation and Coding Scheme

PA Power Amplifier

PAR Peak-to-Average Ratio

PSK Phase Shift Keying

QAM Quadrature Amplitude Modulation

The present invention is a method an apparatus adapted to reduce the amplitude variations of a modulated signal without changing its spectrum. The method comprises adding a compensation signal to the modulated signal, the compensation signal having the same spectrum as the modulated signal. The apparatus comprises a means for adding a compensation signal to the modulated signal, the compensation signal having the same spectrum as the modulated signal.

The present invention further is adapted to reduce the peaks of the modulated signal as well as to increase minima, i.e., so as to avoid zero crossings. There are different methods for generating the appropriate compensation signal.

A first method and associated apparatus iteratively searches for peaks above a pre-determined maximum threshold and/or minima below a pre-determined minimum threshold. When a peak is found, the amount by which the signal peak exceeds the maximum threshold, as well as the signal phase at the peak, is calculated. A compensation signal is generated with the same peak amplitude as the signal exceeds the maximum threshold, and with opposite phase. The shape of the compensation signal is chosen to be the same as the transmitter pulse (the impulse response of the pulse shaping filter) of the modulated signal. The compensation signal is then added to the modulated signal. FIG. 2 is a graph 200 illustrating an example of peak compression.

Similarly, when a minimum is found, the amount by which the signal minimum is below the minimum threshold, as well as the signal phase at the minimum, is calculated. A compensation signal is generated with the same peak amplitude as the signal is below the minimum threshold, and with the same phase. The shape of the compensation signal is chosen to be the same as the transmitter pulse of the modulated signal. The compensation signal is then added to the modulated signal. FIG. 3 is a graph 300 illustrating an example of minimum compression.

This procedure is iterated until there are no peaks above the maximum threshold and/or no minima below the minimum threshold (possibly limited by a maximum number of iterations for complexity reasons). FIG. 4 is a graph 400 of a portion of a signal where both peak and minimum compression have been applied.

A second method and associated apparatus calculates the compensation signal by first calculating the (complex) difference between the modulated signal and the signal that would be the result if the amplitude of the modulated signal was hard limited to be below the maximum threshold and/or above the minimum threshold (without changing the phase). Second, the compensation signal is fed through a filter whose impulse response is the same as the transmitter pulse. Finally, the filtered compensation signal is added to the modulated signal.

A third method and associated apparatus uses a least squares method to obtain the compensating signal. The least squares method will cancel out the peaks and minima of the signal that lie outside the linear region of the PA in a least squares sense.

In the third method and associated apparatus, the signal amplitudes above and below the pre-defined maximum and minimum limits are detected. Given these values, the signal with equal amplitude as the exceeding parts but with opposite phase is defined. Using this signal and the pre-defined pulse shaping filter, the least squares method is applied to obtain the compensating signal. Since the same pulse shape is used for all symbols, the least squares approach gives a low complexity solution where a pre-calculated matrix can be used. A short example of output of the method and apparatus is described below. Mathematical notations used herein are set forth below:

TABLE 1
Notation Explanation
ā        Vector a
A        Matrix A
AT       Matrix transpose of A
A−1      Matrix inverse of A
ALS/āLS  Least-Squares solution of matrix A/vector a

From the constellation mapping of digital bits, as described hereinabove, a symbol vector, s, is constructed. The symbol vector is filtered with a pulse shaping filter, P, before the signal is sent to the digital-to-analog converter 102 as seen in FIG. 1. The filtered signal, s_tx, could have a wider signal dynamic, i.e. too large and/or too small signal amplitudes, than what is required for the power amplifier (PA) to have optimum performance. Thus, a compensating signal, c_tx, needs to be defined that cancels out the signal amplitudes outside of the linear region of the PA.

s _tx=P s (signal to be transmitted)

c _tx (compensating signal defined from s_tx and the amplitude constraints of the PA).

By using the compensating signal and the known pulse shaping filter, a least squares solution can be defined:

c _tx ^LS=P^LSĉ_tx; where P^LS=P(P^TP)⁻¹P^T.

It is seen that the least squares method only consists of a matrix/vector multiplication, where the matrix, P^LS, can be pre-calculated.

There are several advantages of the method and apparatus of the present invention. Advantages include reducing PAR and MAR to arbitrarily chosen levels. Further, it does not change the frequency spectrum of the signal or require extra carriers. It has a small impact on performance (for relevant levels of PAR and MAR), and it can be applied on single-carrier signals as well as multi-carrier signals.

The signal power distribution of 8 PSK, 16 QAM and 16 QAM with minimum/maximum compression is shown in the graph 500 of FIG. 5. In this example, the minimum and maximum compression limits are set to −15 and 4 dBc, respectively.

FIG. 6 shows a graph 600 illustrating the spectrum of 8 PSK, 16 QAM and 16 QAM with minimum/maximum compression. It can be seen that the spectrum is not affected by the smooth compression.

FIG. 7 is a graph 700 illustrating the BLER performance impact of smooth minimum compression. In FIG. 7, it can be seen that the minimum compression has a very limited impact on the performance, even when the lower limit is −14 to −13 dBc. (The lowest level of EDGE (8 PSK modulation with 3π/8 rotation is −13.4 dBc.).

TABLE 2
Lower limit	Unlimited	−17 dBc	−14 dBc	−13 dBc
Loss @ 10% BLER	—	0.02 dB	0.08 dB	0.14 dB
Maximum compression

FIG. 8 is a graph illustrating the BLER performance impact of smooth maximum compression. The losses are summarized in Table 3 below.

Limiting the peaks to 4-4.5 dBc has only a minor performance impact. When the upper limit is set to 3.25 dBc (i.e., the same peak level as EDGE), there is a 1.5 dB loss. Note though that this reduces the PAR by 2 dB (from 5.3 dB to 3.25 dB) and therefore allows the output power to be increased by 2 dB. Thus, there is a net gain of 0.5 dB in coverage limited situations.

	TABLE 3
	Upper limit
	Unlimited	5 dBc	4.5 dBc	4.25 Bc	4 dBc	3.25 dBc
Loss @	—	0.01 dB	0.1 dB	10.2 dB	0.35 dB	1.5 dB
10%
BLER
Net	—	0.3 dB	0.7 dB	0.8 dB	0.95 dB	0.5 dB
coverage
gain
Minimum and maximum compression

FIG. 9 is a graph 900 illustrating the BLER performance impact of simultaneous minimum and maximum compression. The losses are summarized in Table 4 below.

Comparing Table 2 with Table 3, it can be seen that the losses from minimum and maximum compression are roughly additive.

	TABLE 4
	Lower limit
	Unlimited	−14 dBc	−13 dBc	−13 dBc	−13 dBc
	Upper limit
	Unlimited	4.5 dBc	4.25 Bc	4 dBc	3.25 dBc
Loss @	—	0.15 dB	0.32 dB	0.5 dB	1.6 dB
10% BLER
Net coverage	—	0.65 dB	0.7 dB	0.8 dB	0.4 dB
gain

Referring now to FIG. 10, a flow chart of an embodiment of the method of the present invention is presented. As seen therein, in step 1001, peaks above a pre-determined maximum threshold and/or minima below a pre-determined minimum threshold are iteratively searched. In step 1002A, when a peak is found, the amount by which the signal peak exceeds the maximum threshold, as well as the signal phase at the peak, is calculated. In step 1003A, a compensation signal with the same peak amplitude as the signal exceeds the maximum threshold, and with opposite phase is generated. In step 1004A, the shape of the compensation signal is chosen to be the same as the transmitter pulse of the modulated signal. In step 1005A, the compensation signal is added to the modulated signal.

In step 1002B, when a minimum is found, the amount by which the signal minimum is below the minimum threshold is calculated, as well as the signal phase at the minimum. In step 1003B, a compensation signal is generated with the same peak amplitude as the signal is below the minimum threshold and with the same phase. In step 1004B, the shape of the compensation signal is chosen to be the same as the transmitter pulse of the modulated signal; and in step 1005B, the compensation signal is added to the modulated signal.

Referring now to FIG. 11A, a block diagram 1100A of the first embodiment of an apparatus of the present invention is presented. As seen therein, the digital information is mapped onto a predefined set of (complex) digital signals using a digital modulator 1101A. The digital modulator also consists of upsampling and passing the signal through a transmit pulse shape filter. The signals are then sent through the iterative compensation signal generator, 1106A, where compensating signals are iteratively added to the modulated signals. The compensating signals are using the same transmit pulse shape filter as the modulated signal. The signals are then converted to analog signals using a digital-to-analog converter (DAC) 1102A, converted to a suitable carrier frequency using an up-converter 1103A, amplified to get a sufficient transmit power using a power amplifier (PA) 1104A and converted to a radio signal by the antenna 1105A.

Referring now to FIG. 11B, a block diagram 1100B of the second embodiment of an apparatus of the present invention is presented. As seen therein, the digital information is mapped onto a predefined set of (complex) digital signals using a digital modulator 1101B. The digital modulator also consists of upsampling and passing the signal through a transmit pulse shape filter. The signals are then sent through the compensation signal generator, 1106B, which adds compensating signals to the modulated signals using the same transmit pulse shape. These are then converted to analog signals using a digital-to-analog converter (DAC) 1102B, converted to a suitable carrier frequency using an up-converter 1103B, amplified to get a sufficient transmit power using a power amplifier (PA) 1104B and converted to a radio signal by the antenna 1105B.

Referring now to FIG. 11C, a block diagram 1100C of the third embodiment of an apparatus of the present invention is presented. As seen therein, the digital information is mapped onto a predefined set of (complex) digital signals using a digital modulator 1101C. The digital modulator also consists of upsampling and passing the signal through a transmit pulse shape filter. The signals are then sent through the compensation signal generator, 1106C, which adds compensating signals, using the same transmit pulse shape filter, to the modulated signals. The compensating signals are generated in a least squares sense. The compensated signals are converted to analog signals using a digital-to-analog converter (DAC) 1102C, converted to a suitable carrier frequency using an up-converter 1103C, amplified to get a sufficient transmit power using a power amplifier (PA) 1104C and converted to a radio signal by the antenna 1105C.

Although preferred embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing Detailed Description, it is understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the scope of the invention. The specification contemplates all modifications that fall within the scope of the invention defined by the following claims.

Claims

1. A method for preserving frequency spectrum in a linear modulation scheme, comprising the step of adding a compensation signal to a modulated signal, the compensation signal having the same spectrum as the modulated signal.

2. The method of claim 1, wherein the amplitude of the modulated signal is compressed.

3. The method of claim 1, further comprising the steps of: iteratively searching for peaks above a pre-determined maximum threshold and/or minima below a pre-determined minimum threshold;when a peak is found, calculating the amount by which the signal peak exceeds the maximum threshold, as well as the signal phase at the peak;generating a compensation signal with the same peak amplitude as the signal exceeds the maximum threshold, and with opposite phase;choosing the shape of the compensation signal to be the same as the transmitter pulse of the modulated signal; andadding the compensation signal to the modulated signal.

4. The method of claim 3, further comprising the steps of: when a minimum is found, calculating the amount by which the signal minimum is below the minimum threshold, as well as the signal phase at the minimum;generating a compensation signal with the same peak amplitude as the signal is below the minimum threshold and with the same phase;choosing the shape of the compensation signal to be the same as the transmitter pulse of the modulated signal; andadding the compensation signal to the modulated signal.

5. The method of claim 4, wherein the procedure is iterated until there are no peaks above the maximum threshold and/or no minima below the minimum threshold.

6. The method of claim 5, wherein the procedure is limited by a maximum number of iterations.

7. The method of claim 1, further comprising the steps of: calculating a compensation signal by first calculating the (complex) difference between a modulated signal and a signal that would be the result if the amplitude of the modulated signal was hard limited to be below the maximum threshold and/or above the minimum threshold, without changing the phase;feeding the compensation signal through a filter whose impulse response is the same as the transmitter pulse; andadding the filtered compensation signal to the modulated signal.

8. The method of claim 1, further comprising using a least squares method to obtain the compensating signal.

9. The method of claim 8 wherein the least squares method cancels out the peaks and minima of the signal that lie outside the linear region of the power amplifier in a least squares sense.

10. The method of claim 1 for use with an 8 PSK modulation scheme.

11. The method of claim 1, for use with a 16 QAM, 32 QAM or 64 QAM modulation scheme.

12. A linear modulation apparatus adapted to preserve frequency spectrum, comprising a means for adding a compensation signal to a modulated signal, the compensation signal having the same spectrum as the modulated signal.

13. The apparatus of claim 12, wherein the amplitude of the modulated signal is compressed.

14. The apparatus of claim 12, further comprising: means for iteratively searching for peaks above a pre-determined maximum threshold and/or minima below a pre-determined minimum threshold;means for calculating the amount by which the signal peak exceeds the maximum threshold, as well as the signal phase at the peak when a peak is found;means for generating a compensation signal with the same peak amplitude as the signal exceeds the maximum threshold, and with opposite phase;means for choosing the shape of the compensation signal to be the same as the transmitter pulse of the modulated signal; andmeans for adding the compensation signal to the modulated signal.

15. The apparatus of claim 14, further comprising: means for calculating the amount by which the signal minimum is below the minimum threshold, as well as the signal phase at the minimum, when a minimum is found;means for generating a compensation signal with the same peak amplitude as the signal is below the minimum threshold and with the same phase;means for choosing the shape of the compensation signal to be the same as the transmitter pulse of the modulated signal; andmeans for adding the compensation signal to the modulated signal.

16. The apparatus of claim 15, wherein the procedure is iterated until there are no peaks above the maximum threshold and/or no minima below the minimum threshold.

17. The apparatus of claim 16, wherein the procedure is limited by a maximum number of iterations.

18. The apparatus of claim 12, further comprising: means for calculating a compensation signal by first calculating the (complex) difference between a modulated signal and a signal that would be the result if the amplitude of the modulated signal was hard limited to be below the maximum threshold and/or above the minimum threshold, without changing the phase;means for feeding the compensation signal through a filter whose impulse response is the same as the transmitter pulse, andmeans for adding the filtered compensation signal to the modulated signal.

19. The apparatus of claim 12, further comprising means for using a least squares method to obtain the compensating signal.

20. The apparatus of claim 19, wherein the least squares method cancels out the peaks and minima of the signal that lie outside the linear region of the power amplifier in a least squares sense.