Demodulators – Phase shift keying or quadrature amplitude demodulator
Reexamination Certificate
1999-09-28
2001-04-17
Grimm, Siegfried H. (Department: 2817)
Demodulators
Phase shift keying or quadrature amplitude demodulator
C375S324000, C375S329000
Reexamination Certificate
active
06218896
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention related generally to wireless communication systems. More specifically, the invention relates to digital demodulation of quadrature phase shift keying (QPSK) signals.
2. Description of the Related Art
The demodulation of QPSK signals can be accomplished digitally employing a tracking loop approach. A tracking loop generates the resample timing and is also used to remove the frequency and phase offsets, also referred to as the residual frequency and phase, from the incoming data symbols. Typically, such systems operate on one new sample pair at a time. Such systems employ feedback loops to track and correct for timing, frequency and phase offsets in the incoming data stream.
These feedback loops require careful setting of the tracking loop gains. In addition, they require an acquisition period over which the feedback loops have time to lock. Such known systems require the use of a relatively long preamble at the beginning of each new data transmission in order to provide the feedback loops time to lock. Such known systems also typically estimate timing, frequency and phase using localized loop metrics (typically over a few data samples), which can lead to increased error rates. Further, due to the local decisions used in tracking loops, implementing such systems in a digital signal processor chip limits the processing speeds with which such systems can operate.
Therefore, there is a need for method and apparatus for digitally demodulating QPSK signals, which overcomes these shortcomings.
SUMMARY OF THE INVENTION
The method and apparatus for digitally demodulating QPSK signals can comprise a first portion in which the digitally sampled data burst is resampled at a plurality of predetermined timing hypotheses. The maximum power of each of the hypotheses is determined. The hypothesis with the maximum power is used to interpolate a resampled timing estimation. The resampled timing estimation is then used to resample the data burst. Modulation of the resampled data burst is then removed by twice squaring the complex I/Q pairs (Z=I+j*Q). This Z
4
data represents frequency and phase that are four times the frequency and phase of the Z data. The data with the modulation removed is then subjected to a Chirp-Z transform to move the data into the frequency domain.
The spectral power over the data set of the Chirp-Z data transform is then determined. The highest spectral power is determined and quadratically interpolated. This interpolated value is 4 times the residual demodulation frequency.
The phase of the data is estimated by derotating the Z
4
data by a vector of data rotating at negative 4 times the residual frequency (four times the frequency rotating in the opposite direction). The vector used for derotating has a starting phase of 0 and a magnitude of 1. The resulting derotated complex data are summed over the data set. The arc tangent of the resulting sum is 4 times the desired starting phase. The frequency estimation and phase estimation are then used to derotate and dephase the resampled data, which results in resampled data corrected for timing, frequency and phase.
The Chirp-Z Transform can offer several advantages when used to estimate residual frequency in QPSK demodulation. Digital demodulation of QPSK signals generally employs frequency estimation to remove residual frequency from the incoming data symbol information. The Fast Fourier Transform (FFT) or a series of small overlapped FFTs is generally used to perform this estimation.
One embodiment of the invention uses a Chirp-Z Transform approach to frequency estimation, which provides three principal advantages over the FFT and Direct Fourier Transform (DFT) approaches.
1) An arbitrary frequency range can be specified over which to perform the estimation. The FFT requires the frequency range to equal the sampling rate (Fs), from −Fs/2 to Fs/2.
2) An arbitrary number of frequency estimation points may be specified (and therefore arbitrary frequency estimation resolution). The FFT requires that the number of frequency estimation points equal the number of input points (N). Therefore in the FFT the frequency estimation resolution is fixed at Fs/N.
3) Compared to DFT processing (which has the same flexibility as Chirp-Z Transform estimation) the Chirp-Z Transform estimator operates 5.6 times faster than the DFT for a 97-point estimate, 9.7 times faster than the DFT for a 193-point estimate, and 17.8 times faster than the DFT for a 385-point estimate.
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Becker Donald W.
Leigh William E. L.
Grimm Siegfried H.
Knobbe Martens Olsen & Bear LLP
Tachyon, Inc.
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