Adjustment of the sampling frequency in a multicarrier receiver

US 6,310,926 B1
Filed: 03/20/2000
Issued: 10/30/2001
Est. Priority Date: 09/25/1998
Status: Expired due to Term

First Claim

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1. A receiver of a multicarrier QAM system for receiving and demodulating an analog multicarrier QAM signal TM into a digital output signal (R_x) consisting of successive bits, comprising:

a) analog/digital conversion means (7, 8) for converting said analog multicarrier QAM signal TM into a digital time-domain multicarrier QAM signal (DS) using a predetermined sample frequency (f_sampR) from a receiver clock generator (NCO, 15);

b) time-domain/frequency-domain conversion means (10) for converting said digital time-domain multicarrier QAM signal (DS) into a digital frequency-domain multicarrier QAM signal consisting of a plurality of complex QAM data values (Y₁, Y₂. . . Y_k;

Y₀₀, Y₀₁, Y₁₀, Y₁₁);

c) decoding means (11, 12) for respectively selecting a complex default QAM data value (C_k, C₀₀, C₀₁, C₁₀, C₁₁) representing a known digital data symbol (00, 01, 10,

11) on the basis of said complex default QAM data values and for decoding said complex default QAM data values into said digital output signal; and

d) timing recovery means (14, 16, 19-21) for adjusting a phase of said receiver clock generator (NCO, 15) such that the sample timing in said analog/digital conversion means (7, 8) coincides with that (f_sampT) used in a transmitter (1-6) which generates said analog multicarrier signal;

characterized in that e) said timing recovery means (14, 16, 19-21) comprises;

e1) phase rotation direction determining means (221-224) for determining a plurality of phase rotation direction values (−

1, +1, 0;

d₁, d₂, d₃, d_k) of a respective complex QAM data value (Y_k) with respect to a corresponding complex default QAM data value; and

e2) average phase rotation direction determining means (219) for determining an average phase rotation direction value (d_k′) by averaging all respective rotation direction values; and

wherein f) said phase of said sample frequency (f_sampR) is adjusted on the basis of said average phase rotation direction value (d_k).

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Abstract

In a multicarrier QAM system, a receiver comprises a data directed phase error estimator which determines a phase rotation direction of each received complex QAM data value (Y_k) with respect to the complex default QAM data value C(k) used by a quantizer/decoder for decoding. The data directed phase error estimator averages the phase rotation directions for all carriers and adjusts the phase of a sample frequency (f_sampR) output by a numerically-controlled oscillator on the basis of this overall average phase rotation direction value (D). Thus, an overall estimate of the missing phase synchronization to the sample frequency used in the transmitter can be obtained such that the data symbols can be more accurately decoded.

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37 Claims

1. A receiver of a multicarrier QAM system for receiving and demodulating an analog multicarrier QAM signal TM into a digital output signal (R_x) consisting of successive bits, comprising:
- a) analog/digital conversion means (7, 8) for converting said analog multicarrier QAM signal TM into a digital time-domain multicarrier QAM signal (DS) using a predetermined sample frequency (f_sampR) from a receiver clock generator (NCO, 15);
  
  b) time-domain/frequency-domain conversion means (10) for converting said digital time-domain multicarrier QAM signal (DS) into a digital frequency-domain multicarrier QAM signal consisting of a plurality of complex QAM data values (Y₁, Y₂. . . Y_k;
  
  Y₀₀, Y₀₁, Y₁₀, Y₁₁);
  
  c) decoding means (11, 12) for respectively selecting a complex default QAM data value (C_k, C₀₀, C₀₁, C₁₀, C₁₁) representing a known digital data symbol (00, 01, 10,
  
  11) on the basis of said complex default QAM data values and for decoding said complex default QAM data values into said digital output signal; and
  
  d) timing recovery means (14, 16, 19-21) for adjusting a phase of said receiver clock generator (NCO, 15) such that the sample timing in said analog/digital conversion means (7, 8) coincides with that (f_sampT) used in a transmitter (1-6) which generates said analog multicarrier signal;
  
  characterized in that e) said timing recovery means (14, 16, 19-21) comprises;
  
  e1) phase rotation direction determining means (221-224) for determining a plurality of phase rotation direction values (−
  
  1, +1, 0;
  
  d₁, d₂, d₃, d_k) of a respective complex QAM data value (Y_k) with respect to a corresponding complex default QAM data value; and
  
  e2) average phase rotation direction determining means (219) for determining an average phase rotation direction value (d_k′) by averaging all respective rotation direction values; and
  
  wherein f) said phase of said sample frequency (f_sampR) is adjusted on the basis of said average phase rotation direction value (d_k).
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
- - 2. A receiver according to claim 1, characterized in that
3. A receiver according to claim 1, characterized in thatsaid analog multicarrier QAM signal (TM) contains a pilot tone (PT) corresponding to a known digital data symbol (e.g. 01), wherein said timing recovery means (14, 16, 19-21) further comprises a pilot tone phase determining means (19) for determining a pilot tone phase rotation direction value of a complex QAM data value (y_k(PD)) corresponding to said received pilot tone with respect to a corresponding complex default QAM data value corresponding to said known digital data symbol of said pilot tone, wherein said phase of said sample frequency (f_sampR) is adjusted on the basis of a combination (22) of said average phase rotation direction value (D2) and said pilot tone phase direction (D1).
4. A receiver according to claim 3, characterized in thatsaid combination (22) is a weighted sum of said average phase rotation direction value (D2) and said pilot tone phase direction (D1).
5. A receiver according to claim 1, characterized in that:
- said receiver clock generator (15, NCO) is a numerically controlled oscillator (NCO) incorporated in a phase locked loop (PLL) and said phase rotation directions (d_k) are coded as 2 bit values.
6. A receiver according to claim 1, characterized in thatsaid phase rotation direction determining means (221-224) comprises:
- e11) a subtractor (211) for calculating a complex error value (e_k) by subtracting said complex QAM data value (y_k) from said corresponding complex default QAM data value (C_k);
  
  e12) a first complex sign extractor (212) for forming a first complex sign value (Y_k′) whose real and imaginary part is −
  
  1 when the real part and the imaginary part of said complex QAM data value (Y_k) is respectively negative, whose real and imaginary part is 0 when the real part and the imaginary part of said complex QAM data value (y_k) is respectively negative, and whose real and imaginary part is +1 when the real part and the imaginary part of said complex QAM data value (y_k) is respectively positive;
  
  e13) a second complex sign extractor (213) for forming a second complex sign value (e_k′) whose real and imaginary part is −
  
  1 when the real part and the imaginary part of said complex error value (e_k) is respectively negative, whose real and imaginary part is 0 when the real part and the imaginary part of said complex error value (e_k) is respectively negative, and whose real and imaginary part is +1 when the real part and the imaginary part of said complex error value (e_k) is respectively positive;
  
  e14) a first multiplier (216) for forming a third sign value (e_k″) by multiplying said first complex sign value (y_k′) with said second complex sign value (e_k′);
  
  e15) a second multiplier (214, 217) for forming a quadrant determining value (0, ±
  
  1) by multiplying the real part (±
  
  1) of said first complex sign value (y_k′) with the imaginary part (±
  
  1) of said first complex sign value (y_k′), said quadrant determining value indicating the quadrant in the complex plane where said complex QAM data value is located;
  
  e16) a third multiplier (215, 218) for forming said phase rotation direction value by multiplying the real part (215) of said third complex sign value (e_k″) with said quadrant determining value.
7. A receiver according to claim 1, characterized in thatsaid average phase rotation direction determining means (219) comprises an adder (219) which outputs as said average phase rotation direction value (d_k) a sum of all phase rotation direction values divided by the number of complex QAM data values.
8. A receiver according to claim 2, characterized in thatsaid average data symbol phase rotation direction determining means (20) is a low pass filter of a phase-locked loop (20) used for adjusting a phase of said receiver clock generator (NCO, 15).
9. A receiver according to claim 1, characterized in thatsaid phase rotation direction value has a value +1, when said phase rotation of said complex QAM data value is in a counterclockwise direction from said decoded complex default QAM data value, said phase rotation direction value has a value 0, when no phase rotation of said complex QAM data value with respect to said decoded complex default QAM data value exists and said phase rotation direction value has a value −
- 1, when said phase rotation of said complex QAM data value is in a clockwise direction with respect to said decoded complex default QAM data value.
10. A receiver according to claim 3, characterized in thatsaid complex default QAM data value corresponding to said sent pilot tone has only an imaginary part (+j, −
- j), wherein said pilot tone phase rotation direction determining means (19) determines said pilot tone phase rotation direction value by determining the real part of said pilot tone complex QAM data value.
11. A receiver according to claim 1, characterized in thatsaid timing recovery means further comprises a time domain sample frequency detector (TDPD, S1, S2) for recovering in the time domain said sample frequency (f_sampR) from an analog multicarrier QAM signal including a pilot tone, wherein said analog/digital conversion means (8) samples said received multicarrier QAM signal once per period of said pilot tone and the PLL of said receiver clock generator (15, NCO) is controlled based on the samples obtained.
12. A receiver according to claim 1, characterized in thatsaid time-domain/frequency domain conversion means (10) comprises a Fourier transform means (FFT) for performing a Fourier Transform on said time-domain multicarrier QAM signal (DS).
13. A receiver according to claim 1, characterized bysaid timing recovery means further comprises a time domain symbol rate detector (TDPD, S1, S2) for recovering in the time domain a symbol rate and a symbol start position of individual symbols from said multicarrier QAM signal.
14. A receiver according to claim 1, characterized in thata weighting means is provided for weighting each of said phase rotation direction values with a weighting factor.
15. A receiver according to claim 14, characterized in thatsaid weighting means calculates a SNR ratio for each carrier and said weighting factor depends on said calculated SNR ratio.

16. A multicarrier QAM communication system for transmitting information using a multicarrier QAM between a transmitter and a receiver, comprising at said transmitter:
- a) coding means (1, 3) for coding an input bit stream (T_x) into a digital frequency-domain multicarrier QAM signal consisting of a number (N_cT) of complex QAM data values using a multicarrier QAM technique;
  
  b) frequency-domain/time-domain conversion means (4) for converting said digital frequency-domain multicarrier QAM signal into a digital time-domain multicarrier QAM signal consisting of number of separate QAM samples; and
  
  c) digital/analog conversion means (5, 6) for converting said samples of said digital time-domain multicarrier QAM signal into an analog multicarrier QAM signal (TM) using a predetermined transmitter sample frequency (f_sampT) from a transmitter clock generator; and
  
  at said receiver;
  
  d) analog/digital conversion means (7, 8) for converting said analog multicarrier QAM signal (TM) into a digital time-domain multicarrier QAM signal (DS) using a predetermined sample frequency (f_sampR) from a receiver clock generator (NCO, 15);
  
  e) time-domain/frequency-domain conversion means (10) for converting said digital time-domain multicarrier QAM signal (DS) into a digital frequency-domain multicarrier QAM signal consisting of a plurality of complex QAM data values ((Y₁, Y₂. . . Y_k;
  
  Y₀₀, Y₀₁, Y₁₀, Y₁₁);
  
  f) decoding means (11, 12) for respectively selecting a complex default QAM data value (C_k, C₀₀, C₀₁, C₁₀, C₁₁) representing a known digital data symbol (00, 01, 10,
  
  11) on the basis of said complex default QAM data values and for decoding said complex default QAM data values into a digital output signal; and
  
  g) timing recovery means (14, 16, 19-21) for adjusting a phase of said receiver clock generator (NCO, 15) such that the sample timing in said analog/digital conversion means (7, 8) coincides with that (f_sampT) used by said digital/analog conversion means (5) in said transmitter (1-6);
  
  characterized in that h) said timing recovery means (14, 16, 19-21) comprises;
  
  h1) phase rotation direction determining means (221-224) for determining a plurality of phase rotation direction values (−
  
  1, +1, 0;
  
  d₁, d₂, d₃, d_k) of a respective complex QAM data value (Y_k) with respect to a corresponding complex default QAM data value; and
  
  h2) average phase rotation direction determining means (219) for determining an average phase rotation direction value (d_k) by averaging all respective rotation directions; and
  
  wherein i) said phase of said sample frequency (f_sampR) is adjusted on the basis of said average phase rotation direction value (d_k).
- View Dependent Claims (17, 18, 19, 20, 21, 22, 23, 24, 25, 26)
- - 17. A system according to claim 16, characterized in that
18. A system according to claim 16, characterized in thatsaid transmitter sends a pilot tone corresponding to an input bit stream of successive occurrences of a known digital data symbol (e.g. 01), wherein said timing recovery means (14, 16, 19-21) further comprises a pilot tone phase determining means (19) for determining a pilot tone phase rotation direction value of a complex QAM data value (Y_k(PD)) corresponding to said received pilot tone with respect to a corresponding complex default QAM data value corresponding to said known digital data symbol of said pilot tone, wherein said phase of said sample frequency (f_sampR) is adjusted on the basis of a combination (22) of said average phase rotation direction value (D2) and said pilot tone phase direction value (D1).
19. A system according to claim 18, characterized in thatsaid combination (22) is a weighted sum of said average phase rotation direction value (D2) and said pilot tone phase direction value (D1).
20. A system according to claim 16, characterized in thatsaid phase rotation direction determining means (221-224) comprises:
- e11) a subtractor (211) for calculating a complex error value (e_k) by subtracting said complex QAM data value (Y_k) from said corresponding complex default QAM data value (C_k);
  
  e12) a first complex sign extractor (212) for forming a first complex sign value (Y_k′) whose real and imaginary part is −
  
  1 when the real part and the imaginary part of said complex QAM data value (Y_k) is respectively negative, whose real and imaginary part is 0 when the real part and the imaginary part of said complex QAM data value (Y_k) is respectively negative, and whose real and imaginary part is +1 when the real part and the imaginary part of said complex QAM data value (Y_k) is respectively positive;
  
  e13) a second complex sign extractor (213) for forming a second complex sign value (e_k′) whose real and imaginary part is −
  
  1 when the real part and the imaginary part of said complex error value (e_k) is respectively negative, whose real and imaginary part is 0 when the real part and the imaginary part of said complex error value (e_k) is respectively negative, and whose real and imaginary part is +1 when the real part and the imaginary part of said complex error value (e_k) is respectively positive;
  
  e14) a first multiplier (216) for forming a third sign value (e_k″) by multiplying said first complex sign value (y_k′) with said second complex sign value (e_k′);
  
  e15) a second multiplier (214, 217) for forming a quadrant determining value (0, ±
  
  1) by multiplying the real part (±
  
  1) of said first complex sign value (Y_k′) with the imaginary part (±
  
  1) of said first complex sign value (Y_k′), said quadrant determining value indicating the quadrant in the complex plane where said complex QAM data value is located; and
  
  e16) a third multiplier (215, 218) for forming said phase rotation direction value by multiplying the real part (215) of said of said third complex sign value (e_k″) with said quadrant determining value.
21. A system according to claim 16, characterized in thatsaid average phase rotation direction determining means (219) comprises an adder (219) which outputs as said average phase rotation direction value (d_k) a sum of all phase rotation direction values divided by the number of complex QAM data values.
22. A system according to claim 17, characterized in thatsaid average data symbol phase rotation direction determining means (20) is a low pass filter of a phase-locked loop (20) used for adjusting a phase of said receiver clock generator (NCO, 15).
23. A system according to claim 16, characterized in thatsaid phase rotation direction value has a value +1, when said phase rotation of said complex QAM data value is in a counterclockwise direction from said decoded complex default QAM data value, said phase rotation direction value has a value 0, when no phase rotation of said complex QAM data value with respect to said decoded complex default QAM data value exists and said phase rotation direction value has a value −
- 1, when said phase rotation of said complex QAM data value is in a clockwise direction with respect to said decoded complex default QAM data value.
24. A system according to claim 16, characterized in thatsaid time-domain/frequency domain conversion means (10) comprises a Fourier transform means (FFT) for performing a Fourier Transform on said time-domain multicarrier QAM signal (DS) wherein said frequency-domain/time domain conversion means (10) comprises a Inverse Fourier transform means (FFT) for performing an Inverse Fourier Transform on said frequency-domain multicarrier QAM signal (DS).
25. A system according to claim 16, characterized in thatin said receiver a weighting means is provided for weighting each of said phase rotation direction values with a weighting factor.
26. A system according to claim 25, characterized in thatsaid weighting means calculates a SNR ratio for each carrier and said weighting factor depends on said calculated SNR ratio.

27. A method in a multicarrier QAM system for receiving and demodulating an analog multicarrier QAM signal (TM) into a digital output signal (R_x) consisting of successive digital data symbols, comprising the following steps:
- a) converting said analog multicarrier QAM signal (TM) into a digital time-domain multicarrier QAM signal (DS) using a predetermined sample frequency (f_sampR) from a receiver clock generator (NCO, 15);
  
  b) converting said digital time-domain multicarrier QAM signal (DS) into a digital frequency-domain multicarrier QAM signal consisting of a plurality of complex QAM data values (Y₁, Y₂. . . Y_k;
  
  Y₀₀, Y₀₁, Y₁₀, Y₁₁);
  
  c) selecting respectively a complex default QAM data value (C_k, C₀₀, C₀₁, C₁₀, C₁₁) representing a known digital data symbol (00, 01, 10,
  
  11) on the basis of said complex default QAM data values said complex default QAM data values and decoding said complex default QAM data values into said digital output signal; and
  
  d) adjusting a phase of said receiver clock generator (NCO, 15) such that the sample timing used in said analog/digital conversion step coincides with that (f_sampT) used in a transmitter (1-6) which generates said multicarrier QAM signal;
  
  characterized by the following steps;
  
  e1) determining respectively a plurality of phase rotation direction values (−
  
  1, +1, 0;
  
  di, d₂, d₃, d_k) of a respective complex QAM data value (Y_k) with respect to a corresponding complex default QAM data value; and
  
  e2) determining an average phase rotation direction value (d_k) by averaging all respective phase rotation direction values; and
  
  f) adjusting said phase of said sample frequency (f_sampR) on the basis of said average phase rotation direction value (d_k).
- View Dependent Claims (28, 29, 30, 31, 32, 33, 34, 35, 36, 37)
- - 28. A method according to claim 27, characterized in that
29. A method according to claim 27, characterized in that:
- said analog multicarrier QAM signal (TM) contains a pilot tone (PT) corresponding to a known digital data symbol (e.g.
  
  01), a pilot tone phase rotation direction value of a complex QAM data value (Y_k(PD)) corresponding to said received pilot tone with respect to a corresponding complex default QAM data value corresponding to said known digital data symbol of said pilot tone is determined and said phase of said sample frequency (f_sampR) is adjusted on the basis of a combining (22) of said average phase rotation direction value (D2) and said pilot tone phase direction value (D1).
30. A method according to claim 29, characterized in thatin said combining step (22) a weighted sum of said average phase rotation direction value (D2) and said pilot tone phase direction value (D1) is calculated.
31. A method according to claim 27, characterized by the following steps:
- e11) calculating a complex error value (e_k) by subtracting said complex QAM data value (Y_k) from said corresponding complex default QAM data value (C_k);
  
  e12) forming a first complex sign value (Y_k′) whose real and imaginary part is −
  
  1 when the real part and the imaginary part of said complex QAM data value (Y_k) is respectively negative, whose real and imaginary part is 0 when the real part and the imaginary part of said complex QAM data value (Y_k) is respectively negative, and whose real and imaginary part is +1 when the real part and the imaginary part of said complex QAM data value (Y_k) is respectively positive;
  
  e13) forming a second complex sign value (e_k′) whose real and imaginary part is −
  
  1 when the real part and the imaginary part of said complex error value (e_k) is respectively negative, whose real and imaginary part is 0 when the real part and the imaginary part of said complex error value (e_k) is respectively negative, and whose real and imaginary part is +1 when the real part and the imaginary part of said complex error value (e_k) is respectively positive;
  
  e14) forming a third sign value (e_k″) by multiplying said first complex sign value (Y_k′) with said second complex sign value (e_k′);
  
  e15) forming a quadrant determining value (0, ±
  
  1) by multiplying the real part (+1) of said first complex sign value (Y_k′) with the imaginary part (±
  
  1) of said first complex sign value (Y_k′), said quadrant determining value indicating the quadrant in the complex plane where said complex QAM data value is located; and
  
  e16) forming said phase rotation direction value by multiplying the real part (215) of said of said third complex sign value (e_k″) with said quadrant determining value.
32. A method according to claim 27, characterized in that:
- said average phase rotation direction value (d_k) is determined as a sum of all phase rotation direction values divided by the number of complex QAM data values.
33. A method according to claim 28, characterized in thatsaid average data symbol rotation direction value (D) is determined by a low pass filter of a phase-locked loop (20) used for adjusting a phase of said receiver clock generator (NCO, 15).
34. A method according to claim 27, characterized in thatsaid phase rotation direction value has a value +1, when said phase rotation of said complex QAM data value is in a counterclockwise direction from said decoded complex default QAM data value, said phase rotation direction value has a value 0, when no phase rotation of said complex QAM data value with respect to said decoded complex default QAM data value exists and said phase rotation direction value has a value −
- 1, when said phase rotation of said complex QAM data value is in a clockwise direction with respect to said decoded complex default QAM data value.
35. A method according to claim 27, characterized in thatsaid conversion step b) comprises applying a Fourier transform (FFT) to said time-domain multicarrier QAM signal (DS).
36. A method according to claim 27, characterized in thateach of said phase directions is weighted with a weighting factor.
37. A method according to claim 36, characterized in thata SNR ratio is calculated for each carrier and said weighting factor depends on said calculated SNR ratio.

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Current Assignee
Optis Wireless Technology, LLC (Brevet Capital)
Original Assignee
Telefonaktiebolaget LM Ericsson
Inventors
Tore, Mikael André
Primary Examiner(s)
Ghebretinsae, Temesghen

Application Number

US09/531,336
Time in Patent Office

589 Days
Field of Search

375/355, 375/261, 375/316, 375/324, 375/344
US Class Current

375/355
CPC Class Codes

H04L 27/2657   Carrier synchronisation

H04L 27/2662   Symbol synchronisation

H04L 27/2679   Decision-aided

Adjustment of the sampling frequency in a multicarrier receiver

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Adjustment of the sampling frequency in a multicarrier receiver

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