Order calculation method in a quadrature amplitude modulation receiver

BACKGROUND OF THE INVENTION

Field of the Invention

0001The present invention relates to an order calculation method, and more particularly relates to an order calculation method in a quadrature amplitude modulation receiver.

Description of the Related Art

0002In recent years, a need (e.g. carrier recovery) to calculate an order of quadrature amplitude modulation (QAM) in a receiver of a communication system was arisen. A demodulator in a receiver can calculate the QAM order by sweeping (trying) every possibility in a predetermined range. The sweeping method may, however, take a long time.

0003The demodulator can also obtain the QAM order by gathering statistics of different amplitudes in a constellation diagram. For example, if there are 4 different amplitudes in the constellation diagram, the QAM order can be determined to be 16. In other words, the QAM technology utilized in the communication system is 16-QAM. However, the statistical method needs a huge amount of memory space to temporarily save the statistical values and the accuracy, is low, particularly in high order. <patcit id="pcit0001" dnum="US5185763A"><text>US 5 185 763</text></patcit> is directed toward a method and apparatus in a communications device for mapping a selected group of data bits into a predetermined number of two-dimensional symbols selected from a minimized alphabet of M two-dimensional symbols forming a symbol constellation, with one of the two-dimensional symbols being transmitter during each symbol (i.e., baud) interval. A plurality of symbol mappers in an adaptive communications device use a superframe of bits per baud. Each superframe includes at least two groups of frames, a low bit frame and a high bit frame. The superframe has A) one of these two types of frames uses whole bits per baud and the other uses fractional bits per baud or B) both of these two types of frames will use fractional bits per baud.

BRIEF SUMMARY OF THE INVENTION

0004A detailed description is given in the following embodiments with reference to the accompanying drawings.

0005An order calculation method in a quadrature amplitude modulation receiver comprises: calculating a maximum amplitude and a minimum amplitude value from a plurality of amplitudes in an input signal; calculating a ratio of the maximum amplitude to the minimum amplitude value; and determining a QAM order according to the ratio.

BRIEF DESCRIPTION OF THE DRAWING

0006The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

0007<figref idref="f0001">Fig.1</figref> shows a block diagram of a receiver according to an embodiment of the invention.

0008<figref idref="f0002">Fig.2</figref> shows a decision table utilized in the QAM order calculator.

0009<figref idref="f0003 f0004 f0005">Fig.3A∼3E</figref> show QAM constellation diagrams of different QAM orders in the decision table.

0010<figref idref="f0006">Fig.4</figref> shows a block diagram of the QAM order calculator in <figref idref="f0001">Fig. 1</figref>.

0011<figref idref="f0007">Fig.5</figref> shows another block diagram of the QAM order calculator in <figref idref="f0001">Fig. 1</figref>.

0012<figref idref="f0008">Fig.6</figref> is a flowchart of the order calculation method in a QAM receiver according to an embodiment of the invention.

DETAILED DESCRIPTION OF INVENTION

0013The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.

0014Please refer to <figref idref="f0001">Fig. 1. Fig.1</figref> shows a block diagram of a receiver according to an embodiment of the invention. The receiver 100 comprises an analog-to-digital converter (ADC) 110, a down converter 120, a filter 130, a timing recovery module 140, an equalization and normalization module 150, a carrier recovery module 160, a slicer 170, and a QAM order calculator 180. The ADC 110 converts an analog input signal R(t) to a digital input signal R[n]. The digital input signal R[n] is then sequentially down converted, filtered, equalized, and normalized to generate a normalized input signal N[n]. The carrier recovery module 160 performs carrier recovery of the normalized input signal N[n] according to a QAM order OD_QAM that is outputted from the QAM order calculator 180. The QAM order calculator 180 will be further described later. Finally, the slicer 170 performs decision according to a QAM constellation diagram to output a decided signal.

0015Please refer to <figref idref="f0002">Fig.2</figref> and <figref idref="f0003 f0004 f0005">Fig.3</figref> at the same time. <figref idref="f0002">Fig.2</figref> shows a decision table utilized in the QAM order calculator 180. <figref idref="f0003 f0004 f0005">Figs.3A∼3E</figref> show QAM constellation diagrams of different QAM orders in the decision table (<figref idref="f0002">Fig.2</figref>). For example, if the ratio of the outer meter (maximum amplitude) R1 to the inner meter (minimum amplitude) R2 in the QAM constellation diagram is 3, the QAM order calculator 180 looks up in the decision table to output the QAM order OD_QAM equal to 16. Additionally, a window length W of the input signal varies with different QAM order. For example, if the window length W is equal to 1024 in 256QAM, the window length W of the 128QAM should be 512 (1024/2), the window length W of the 64QAM should be 256 (1024/4), etc... The window length W should be at least 4 times larger than the QAM order to maintain a higher accuracy. A detailed description of finding the outer and inner meters is provided in the following.

0016Please refer to <figref idref="f0006">Fig.4. Fig.4</figref> shows a block diagram of the QAM order calculator 180 in <figref idref="f0001">Fig.1</figref>. The QAM order calculator 180 comprises an amplitude calculator 410, an inner-and-outer meter calculator 420, and an order calculator 430. The amplitude calculator 410 calculates and generates a plurality of amplitudes from each symbol in the normalized signal N[n]. The formula of calculating amplitude is shown in the following:

0017<maths id="math0001"><math display="block"><mi>A</mi><mo>=</mo><msqrt><msup><mi>I</mi><mn>2</mn></msup><mo>+</mo><msup><mi>Q</mi><mn>2</mn></msup></msqrt></math><img file="EP1868340B1_D0001.tif" /></maths>

0018wherein A is the amplitude and I and Q are values from I-Channel and Q-Channel. An approximate calculation is providing in the following to reduce cost:

0019<maths id="math0002"><math display="block"><mi>A</mi><mo>=</mo><mrow><mo>{</mo></mrow><mtable columnalign="left"><mtr><mtd><mfenced open="|" close="|"><mi>I</mi></mfenced><mo>+</mo><mfrac><mfenced open="|" close="|"><mi>Q</mi></mfenced><mn>16</mn></mfrac><mo>+</mo><mfrac><mfenced open="|" close="|"><mi>Q</mi></mfenced><mn>64</mn></mfrac><mspace width="2em" /><mi mathvariant="italic">when</mi><mspace width="2em" /><mfenced open="|" close="|"><mi>Q</mi></mfenced><mo><</mo><mfrac><mfenced open="|" close="|"><mi>I</mi></mfenced><mn>4</mn></mfrac></mtd></mtr><mtr><mtd><mfenced open="|" close="|"><mi>I</mi></mfenced><mo>+</mo><mfrac><mfenced open="|" close="|"><mi>Q</mi></mfenced><mn>4</mn></mfrac><mo>+</mo><mfrac><mfenced open="|" close="|"><mi>Q</mi></mfenced><mn>16</mn></mfrac><mspace width="2em" /><mi mathvariant="italic">when</mi><mspace width="2em" /><mfenced open="|" close="|"><mi>Q</mi></mfenced><mo><</mo><mfrac><mfenced open="|" close="|"><mi>I</mi></mfenced><mn>2</mn></mfrac><mspace width="2em" /><mi mathvariant="italic">and</mi><mspace width="2em" /><mfenced open="|" close="|"><mi>Q</mi></mfenced><mo>></mo><mfrac><mfenced open="|" close="|"><mi>I</mi></mfenced><mn>4</mn></mfrac></mtd></mtr><mtr><mtd><mfenced open="|" close="|"><mi>I</mi></mfenced><mo>+</mo><mfrac><mfenced open="|" close="|"><mi>Q</mi></mfenced><mn>4</mn></mfrac><mo>+</mo><mfrac><mfenced open="|" close="|"><mi>Q</mi></mfenced><mn>32</mn></mfrac><mspace width="2em" /><mi mathvariant="italic">when</mi><mspace width="2em" /><mfenced open="|" close="|"><mi>Q</mi></mfenced><mo><</mo><mfrac><mrow><mn>3</mn><mfenced open="|" close="|"><mi>I</mi></mfenced></mrow><mn>4</mn></mfrac><mspace width="2em" /><mi mathvariant="italic">and</mi><mspace width="2em" /><mfenced open="|" close="|"><mi>Q</mi></mfenced><mo>></mo><mfrac><mfenced open="|" close="|"><mi>I</mi></mfenced><mn>2</mn></mfrac></mtd></mtr><mtr><mtd><mfenced open="|" close="|"><mi>I</mi></mfenced><mo>+</mo><mfrac><mfenced open="|" close="|"><mi>Q</mi></mfenced><mn>4</mn></mfrac><mo>+</mo><mfrac><mfenced open="|" close="|"><mi>Q</mi></mfenced><mn>8</mn></mfrac><mo>+</mo><mfrac><mfenced open="|" close="|"><mi>Q</mi></mfenced><mn>64</mn></mfrac><mspace width="2em" /><mi mathvariant="italic">when</mi><mspace width="2em" /><mfenced open="|" close="|"><mi>Q</mi></mfenced><mo><</mo><mfenced open="|" close="|"><mi>I</mi></mfenced><mspace width="2em" /><mi mathvariant="italic">and</mi><mspace width="2em" /><mfenced open="|" close="|"><mi>Q</mi></mfenced><mo>></mo><mfrac><mrow><mn>3</mn><mfenced open="|" close="|"><mi>I</mi></mfenced></mrow><mn>4</mn></mfrac></mtd></mtr></mtable></math><img file="EP1868340B1_D0002.tif" /></maths>

0020After obtaining amplitudes from the amplitude calculator 410, the inner-and-outer meter calculator 420 then generates the maximum amplitude R1 and the minimum amplitude R2 according to the plurality of symbol amplitudes. A detailed description of the inner-and-outer meter calculator 420 is provided in the following. The inner-and-outer meter calculator 420 further comprises a sorter 510, memory modules 520 and 540, and extractors 530 and 550. The sorter 510 sorts the plurality of amplitudes from a minimum value to a maximum. The memory modules 520 stores the largest M amplitudes L(1), L(2), ..., L(M) and the memory modules 540 stores the least N amplitudes S(1), S(2), ..., S(N). Finally, the extractor 530 chooses one amplitude L(M') in a middle range of the memory module 520 as the maximum amplitude R1 according to the parameter M'; the extractor 550 also chooses one amplitude S(N') in a middle range of the memory module 540 as the minimum amplitude R2 according to the parameter N'. After the maximum and minimum amplitudes R1 and R2 are determined, the order calculator 430 looks up the ratio of these amplitudes in the decision table and finds the corresponding QAM order OD_QAM.

0021Please refer to <figref idref="f0007">Fig. 5. Fig.5</figref> shows another block diagram of the QAM order calculator 180 in <figref idref="f0001">Fig.1</figref>. The QAM order calculator 180 comprises an amplitude calculator 610, an inner-and-outer meter calculator 620, an accumulator 630, an average module 640, and an order calculator 650. A detailed description of the elements 610∼620 is omitted for the sake of brevity since it has already been described. In order to increase the accuracy, the maximum and minimum amplitudes R1 and R2 are calculated repeatedly to accumulate in the accumulator 630 and then an average in the average module 640 is output as the averaged maximum and minimum amplitudes R1_avg and R2 _avg. After the averaged maximum and minimum amplitudes R_1avg and R2_1avg are determined, the order calculator 650 will look up the ratio of R_1avg to R2_1avg in the decision table and find the corresponding QAM order OD_QAM.

0022Please refer to <figref idref="f0008">Fig.6. Fig.6</figref> is a flowchart of the order calculation method in a QAM receiver according to an embodiment of the invention. A detailed description is provided in the following.

0023Step 702: Start. Setting initial values of parameters. The parameters comprise window length W(e.g. 64, 128, 256, 512, 1024), middle sample positions M' and N', and a QAM order OD_QAM (16, 32, 64, 128, 256).

0024Step 704: Adding one unit to a counter.

0025Step 706: Calculating an averaged maximum amplitude and an averaged minimum amplitude value.

0026Step 708: Calculating a ratio of the averaged maximum amplitude to the averaged minimum amplitude value.

0027Step 710: Comparing the ratio with a threshold value in a decision table.

0028Step 712: Updating parameters.

0029Step 714: Determining whether the value of the counter reaches a maximum supported QAM order number? If yes, choosing the current QAM order as the final QAM order in a receiver.

0030Step 716: Generating a normalized signal according to the parameters.

0031Step 718: End.

0032Compared with the related art, the QAM order calculation method of the invention requires less memory and raises the accuracy of calculation. Additionally, the higher value of the QAM order, the more accuracy.

0033While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.