DC interference removal in wireless communications

• US 7,266,359 B2
• Filed: 10/12/2004
• Issued: 09/04/2007
• Est. Priority Date: 03/18/2003
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Chen, Weizhong, Molina, Robert
• Primary Examiner(s): Trinh, Sonny
• Application Number:
  US10/963,387
• Publication Number:
  US 20050079850A1
• Time in Patent Office: 1,057 Days
• Field of Search: 455/63.1, 455/67.13, 455/114.2, 455/222, 455/278.1, 455/296, 455/234.1, 455/226.1, 455/226.3, 455/130, 375/316, 375/319, 375/340, 375/345, 375/346, 375/148
• US Class Current: 455/296
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22
• CPC Class Codes: H04L 2027/0095 in a preamble or similar structure
• CPC Class Codes: H04L 25/062 Setting decision thresholds using feedforward techniques only
• CPC Class Codes: H04L 25/064 Subtraction of the threshold from the signal, which is then compared to a supplementary fixed threshold

Abstract

A method for removing direct current (DC) interference from a signal received by a communication receiver is provided that removes both a DC offset signal induced by the communication receiver and transmitter. The method includes removing the estimated DC offset from the received signal, correcting a frequency shift in the received signal, estimating a second DC offset signal induced by a source of the received signal, such as a transmitter and removing the estimated second DC offset from the received signal. The receiver DC offset signal is estimated and removed prior to performing a timing carrier offset correction using Barker code manipulation to remove receiver-induced DC offset interference and to sum all Barker chips after effectively multiplying Barker codes to correlate to a Barker sequence unaffected by the receiver DC offset signal.

Claims

1. A method for removing direct current (DC) interference from one or more signals received by a communication receiver, the method comprising:

estimating a DC offset signal induced by the communication receiver;

removing the estimated DC offset from the received signal;

correcting a frequency shift in the received signal;

estimating a second DC offset signal induced by a source of the received signal;

removing the estimated second DC offset from the received signal; and

tracking a residual DC offset by the source of the received signal by (a) receiving DC offset estimates produced over a symbol, (b) accumulating a plurality of DC offset estimates produced over N symbols, (c) averaging the accumulated DC offset estimates to produce an averaged DC offset estimate, and (d) updating the received DC offset estimates using the averaged DC offset estimate.

2. The method of claim 1 wherein the DC offset estimate is supplied to first summer, the first summer operating with a register to accumulate the DC offset estimates produced over N symbols.

3. The method of claim 1 wherein the averaging is performed by dividing the accumulated DC offset estimates by the number of symbols accumulated to determine the average DC offset estimate.

4. The method of claim 1 wherein the updating includes:

receiving the averaged DC offset estimate;

accumulating the averaged DC offset estimate in a register; and

providing the accumulated averaged DC offset estimate to a subractor to complete a DC tracking loop.

5. The method of claim 1 wherein the estimating the DC offset signal includes:

determining at least one sum of the received signal; and

applying one or more functions to estimate the DC offset, the functions selected according to predetermined criteria and the sums of the received signals.

6. The method of claim 1 wherein the source of the received signal is a transmitter.

7. The method of claim 1 wherein the receiver DC offset signal is estimated and removed prior to performing a timing and carrier offset correction.

8. The method of claim 1 wherein the receiver DC offset signal is estimated and removed using Barker code manipulation to remove receiver-induced DC offset interference.

9. The method of claim 1 wherein the DC offset signal is estimated by rearranging received Barker signals to separate the estimated DC signal from a de-spread Barker output.

10. A method for removing direct current (DC) interference from a received signal, the method comprising:

operating on an equal number of positive and negative Barker chips to determine a sum of the DC offset estimate;

dividing the sum by the number of Barker chips to form an estimate of the DC offset signal; and

removing the DC estimate from a remaining chip term to produce a at least a first part of a Barker de-spread output with the DC offset signal removed.

11. The method of claim 10 wherein a second part of the Barker de-spread output is produced by:

operating on an equal number of positive and negative Barker chips;

using the first part and the second part of the Barker de-spread output to produce a total Barker de-spread output.

12. A communication receiver comprising:

a detection component configured to remove receiver induced direct current (DC) offset interference from a received signal;

a timing and carrier offset correction component coupled to the detection component;

a symbol recovery component coupled to the timing and carrier offset correction component, the symbol recover component configured to estimate a transmitted symbol by examining a frequency corrected signal with estimated transmitter-induced DC offset interference removed; and

a transmitter DC estimation component coupled to the symbol recovery component, the transmitter DC estimation component configured to receive the estimated transmitted symbol and subtract the symbol to produce an estimate of the transmitter-induced DC offset interference.

13. The communication receiver of claim 12 further comprising a DC offset tracking component including:

a first register configured to operate with a first summer to receive DC offset estimates produced over a symbol and accumulate a plurality of DC offset estimates produced over N symbols;

a multiplier coupled to the register via a switch, the multiplier configured to average the accumulated DC offset estimates to produce an averaged DC offset estimate;

a second register configured to operate with a second summer to accumulate a plurality of averaged DC offset estimates received by the multiplier to determine an update; and

a subtractor coupled to the second register, the subtractor configured to receive the accumulated averaged DC offset to complete a DC tracking loop.

14. The communication receiver of claim 12 wherein the detection component includes at least a Barker de-spread with DC estimation component and an estimated receiver DC averaging component.

15. The communication receiver of claim 12 wherein the detection component, the timing and carrier offset correction component, the symbol recovery component, and the transmitter DC estimation component are implemented digitally.

16. The communication receiver of claim 12 wherein data is encoded in the received signal as per the WLAN 802.11 wireless protocol.

17. A system for removing direct current (DC) interference from one or more signals received by a communication receiver, the system comprising:

means for estimating a DC offset signal induced by the communication receiver;

means for removing the estimated DC offset from the received signal;

means for correcting a frequency shift in the received signal;

means for estimating a second DC offset signal induced by a source of the received signal; and

means for removing the estimated second DC offset from the received signal;

wherein the means for removing direct current (DC) interference from a received signal includes (a) means for operating on an equal number of positive and negative Barker chips to determine a sum of the DC offset estimate, (b) means for dividing the sum by the number of Barker chips to form an estimate of the DC offset signal, and (c) means for removing the DC estimate from a remaining chip term to produce a at least a first part of a Barker de-spread output with the DC offset signal removed.

18. The system of claim 17 wherein the estimated the DC offset signal is determined by:

means for calculating at least one sum of the received signal; and

means for applying one or more functions to estimate the DC offset, the functions selected according to predetermined criteria and the sum of the received signals.

19. The system of claim 17 wherein the predetermined criteria includes cancellation of at least a portion of the received signal attributable to one or more transmitted chips.

20. The system of claim 17 wherein the receiver DC offset signal is estimated and removed prior to performing a timing carrier offset correction.

21. The system of claim 17 wherein a second part of the Barker de-spread output is produced via:

means for operating on an equal number of positive and negative Barker chips;

means for using the first part and the second part of the Barker de-spread output to produce a total Barker de-spread output.

22. The system of claim 17 wherein the DC offset signal is estimated by rearranging received Barker codes to separate the estimated DC signal from a de-spread Barker output.

23. A method for determining a direct current (DC) offset signal in a received symbol in a wireless communication system, the method comprising:

determining a sum of chips in the received symbol, the received symbol according to a complementary code keying (CCK) code; and

isolating the DC offset signal by method as determined by the sum of the chips by (a) identifying one of a plurality of groups to which the received symbol belongs according to the sum of the chips; and

(b) applying a function according to the identified group.

24. The method of claim 23 wherein the identifying one of the groups includes examining a remodulated symbol that represents an estimate of a sent signal.

25. The method of claim 23 wherein the function is independent of subtracting an estimated received symbol.

26. The method of claim 23 wherein the DC component is found by examining the chips of the remodulated symbol, the chips of the received symbol corresponding to the real (+1/−

1) chips of the remodulated symbol being referred to as r1; and

the chips of the received symbol corresponding to the imaginary (+j/−

j) chips of the remodulated symbol being referred to as r2, the real and imaginary parts of the received chips, defined according to the r1 and r2 designations are organized as;

r1=a1+jb1 and r2=a2+jb2.

27. The method of claim 26 wherein if the sum of the remodulated symbols yields (2+2j) or (−

2−

2j), a part of the DC is found using the equation;

⅛

(Σ

r1+Σ

r2)=⅛

[(Σ

a1+Σ

a2)+j(Σ

b1+Σ

b2)].

28. The method of claim 26 wherein if the sum of the remodulated symbols yields (2+2j) or (−

2−

2j), a part of the DC is found using the equation;

⅛

j(Σ

r1−

Σ

r2)=⅛

[(Σ

b1−

Σ

b2)+j(Σ

a2−

Σ

a1)].

29. The method of claim 26 wherein if the sum of the remodulated symbols yields (2+2j) or (−

2−

2j), the DC components are found using the equation

dc=⅛

[(Σ

a1+Σ

a2+Σ

b1−

Σ

b2)+j(Σ

b1+Σ

b2+Σ

a2−

Σ

a1)].

30. The method of claim 26 wherein if the sum of the remodulated symbols yields (2−

2j) or (−

2+2j), a part of the DC is found using the equation;

⅛

(Σ

r1+Σ

r2)+⅛

[(Σ

a1+Σ

a2)+j(Σ

b1+Σ

b2)].

31. The method of claim 26 wherein if the sum of the remodulated symbols yields (2−

2j) or (−

2+2j), a part of the DC is found using the equation;

⅛

j(Σ

r1−

Σ

r2)+⅛

[(Σ

b2−

Σ

b1)+j(Σ

a1−

Σ

a2)].

32. The method of claim 26 wherein if the sum of the remodulated symbols yields (2−

2j) or (−

2+2j), the DC components are found using the equation

dc=⅛

[(Σ

a1+Σ

a2+Σ

b2−

Σ

b1)+j(Σ

b1+Σ

b2+Σ

a1−

Σ

a2)].