Receiver architecture for pilot based OFDM systems
View Patent ↗The invention relates to a novel methodology and apparatus for clock-offset compensation and common-phase offset correction in Frequency Division Multiplexing based wireless local area network (WLAN) environment, such as an Orthogonal Frequency Division Multiplexing (OFDM) environment. A curve fit, such as a threshold-based, least mean squares (LMS) fit of phase of the pilot sub-carriers in each OFDM symbol is used to estimate and counteract the rotation of the data sub-carriers due to residual frequency offset, low frequency phase noise, and clock offset. The invention is particularly well suited to wireless channels with multipath where pilots typically undergo frequency-selective fading. The thresholding LMS is implemented in a hardware-efficient manner, offering cost advantages over a weighted-LMS alternative. Additionally, the invention uses a unique phase-feedback architecture to eliminate the effects of phase wrapping, and avoid the need to refine channel estimates during packet reception.
1. A method for reducing phase error in a pilot-based, frequency-division-multiplexing (FDM) receiver configured to receive FDM symbols from a remote source, each symbol including a data sub-carrier and a plurality of pilot sub-carriers, the method comprising:
simultaneously rotating the plurality of pilot sub-carriers and the data sub-carrier by an accumulated phase offset associated with a carrier frequency offset between the FDM receiver and the remote source;
calculating a residual phase offset for each of the plurality of rotated pilot sub-carriers;
determining a mean residual phase offset for the calculated residual phase offsets of the plurality of rotated pilot sub-carriers; and
updating the accumulated phase offset using the mean residual phase offset.
2. The method of claim 1 , wherein rotating the plurality of pilot sub-carriers comprises multiplying each pilot sub-carrier by a phasor having an argument including an inverse of the accumulated phase offset.
3. The method of claim 1 , wherein calculating the residual phase offset comprises calculating a respective arctangent for each of the rotated plurality of pilot sub-carriers.
4. The method of claim 1 , wherein determining the mean residual phase offset comprises fitting a curve to the calculated residual phase offset versus pilot sub-carrier frequency for the plurality of pilot sub-carriers.
5. The method of claim 4 , wherein the fitted curve defines a zero-frequency crossing indicative of the mean residual phase offset.
6. The method of claim 4 , wherein fitting the curve comprises determining a least mean square (LMS) solution.
7. The method of claim 1 , wherein rotating the data sub-carrier comprises multiplying the data sub-carrier by a phasor having an argument an inverse of the updated accumulated phase offset.
8. The method of claim 1 , further comprising, prior to rotating the plurality of pilot sub-carriers by the accumulated phase offset, rotating the pilot sub-carriers to a predetermined region.
9. The method of claim 8 , wherein the predetermined region corresponds to the “+1” decision region of a binary-phase-shift-keying (BPSK) constellation.
10. The method of claim 1 , further comprising:
detecting sample timing errors; and
adjusting samples of the received FDM symbols responsive to detecting sample timing errors.
11. The method of claim 1 , wherein the FDM symbols are orthogonal frequency division multiplexing (OFDM) symbols.
12. The method of claim 11 , wherein the OFDM symbols are defined by a protocol selected from the group consisting of: IEEE 802.11a, IEEE 802.11g, and HYPERLAN/2.
13. An apparatus for correcting phase error in a pilot-based, frequency-division-multiplexing (FDM) receiver configured to receive FDM symbols from a remote source, each symbol including a data sub-carrier and a plurality of pilot sub-carriers, comprising:
an accumulator storing an accumulated phase offset associated with a carrier frequency offset between the FDM receiver and the remote source;
a first multiplier continuously coupled to the accumulator, the multiplier receiving a plurality of pilot sub-carriers and rotating the plurality of pilot sub-carriers by the accumulated phase offset;
a phase error processor coupled to the first multiplier and the accumulator, to calculate a residual phase offset for each of the plurality of rotated pilot sub-carriers, to determine a mean residual phase offset for the calculated residual phase offsets of the plurality of rotated pilot sub-carriers, and to provide the mean residual offset to the accumulator for updating the accumulated phase; and
a second multiplier continuously coupled to the accumulator, the second multiplier receiving the data sub-carrier and rotating the data sub-carrier by the accumulated phase offset.
14. The apparatus of claim 13 , wherein multiplier receives from the accumulator, a phasor having an argument including an inverse of the accumulated phase offset.
15. The apparatus of claim 13 , wherein phase error processor includes an arctangent function for calculating the residual phase offset for each of the rotated plurality of pilot sub-carriers.
16. The apparatus of claim 13 , wherein phase error processor comprises a curve fitting function to the calculated residual phase offset versus pilot sub-carrier frequency for each of the plurality of pilot sub-carriers, the fitted curve defining a slope indicative of the phase gradient versus frequency and a zero-frequency crossing indicative of the mean residual phase offset.
17. The apparatus of claim 16 , wherein a curve fitting function comprises a least mean square (LMS) function.
18. The apparatus of claim 13 , wherein the FDM symbols are orthogonal frequency division multiplexing (OFDM) symbols.
19. The apparatus of claim 18 , wherein the OFDM symbols are defined by a protocol selected from the group consisting of: IEEE 802.11a, IEEE 802.11g, and HYPERLAN/2.
20. A system for reducing phase error in a pilot-based, frequency-division-multiplexing (FDM) receiver configured to receive FDM symbols from a remote source, each symbol including a data sub-carrier and a plurality of pilot sub-carriers, the method comprising:
means for simultaneously rotating the plurality of pilot sub-carriers and the data sub-carrier by a current accumulated phase offset associated with a carrier frequency offset between the FDM receiver and the remote source;
means for calculating a residual phase offset for each of the plurality of rotated pilot sub-carriers;
means for determining a mean residual phase offset for the calculated residual phase offsets of the plurality of rotated pilot sub-carriers; and
means for the current accumulated phase offset using the mean residual phase offset.