Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1996-03-29
2001-12-11
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Receivers
Angle modulation
C375S343000
Reexamination Certificate
active
06330291
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to frequency tracking loops used in communication systems, and more particularly to a frequency tracking loop for use in a communication system employing orthogonal Walsh modulation.
II. Description of the Related Art
In every communication system employing modulated communication signals, some mechanism must be provided to demodulate received signals. Further, in order to ensure accurate demodulation, the demodulator must be able to compensate for variations or shifts in the carrier frequency of the received signals.
One conventional technique used to implement such carrier shift tracking in a communication system demodulator is to use a Phase Locked Loop (“PLL”). This type of demodulator works well when the transmitted signal is modulated using conventional modulation techniques, and can be reasonably optimized for either a wide frequency range response or high accuracy. Where more sophisticated modulation schemes are used, however, certain properties of these modulation schemes can make traditional techniques much less useful or responsive.
This is particularly true in digital spread spectrum type communication systems employing M-ary orthogonal Walsh code modulation. As part of such techniques, groups of data symbols to be transferred are mapped into Walsh functions or codes that are transmitted. The received signal is demodulated with respect to a set of such Walsh codes to establish a likelihood as to which codes were transmitted to establish what data symbols are being transferred. However, frequency errors in tracking such signals, especially in the presence of noise, greatly reduces the ability to distinguish which Walsh code was received, and it can quickly become problematic for conventional tracking techniques to maintain frequency tracking.
Therefore, what is needed is a frequency tracking loop designed to take advantage of certain properties associated with M-ary orthogonal Walsh modulation to provide improved tracking abilities. Such an apparatus and tracking method would be useful in implementing more effective communication signal demodulators.
SUMMARY OF THE INVENTION
The present invention comprises apparatus and method for tracking the carrier frequency used in a communication system employing M-ary orthogonal modulation. The invention may be implemented alone, or as part of a larger demodulation system. A preferred embodiment of the invention operates in an environment or system, such as a wireless spread spectrum communication system, where M-ary orthogonal Walsh modulation is employed. In this embodiment, the invention includes a frequency tracking loop comprising a rotator or rotation means, a correlator or correlation means, a discriminator or discrimination means, and a filter or filtering means.
The rotation means receives an input signal and a frequency offset estimate and produces a frequency shifted input signal, the frequency shift being proportional to the frequency offset estimate. The correlation means determines the correlation between a set of Walsh functions and the frequency shifted input signal, and produces a correlation vector. The correlator output may also be used as a data output as well. The discrimination means receives the correlation vector and produces a frequency error signal (“current error”). The filtering means accumulates the resulting frequency error signals to produce the frequency offset estimate used by the rotator means (“residual error”).
The correlation vector produced by the correlator or correlation means includes a plurality of correlation results, each correlation result being the result of a correlation between the frequency shifted input signal and a single Walsh function. Each correlation result has an index value, which may be represented in binary notation, with each index value corresponding to a particular Walsh function.
In one embodiment, the discrimination means determines the current error of the frequency tracking loop as follows. First, the Walsh index of the correlation result with the highest signal energy contained in the correlation vector is determined. This value is selected as representing the most likely transmitted Walsh function or code, and as containing the largest amount of the transmitted signal component. One of the bits of the binary representation of the Walsh index associated with this correlation result is then inverted to generate a Walsh index of another Walsh function. The correlation result from the correlation means having this second index is selected, and a cross product is formed between this second result and the first selected result. That is, the imaginary part of a product between the correlation result with the highest energy level and the complex conjugate of a second correlation result corresponding to a bit reversed index for the first result, is then determined. The resulting cross product value that is determined from this process is proportional to the current error of the frequency tracking loop.
Generally, the most significant bit (MSB) of the binary representation of the Walsh index associated with the highest energy output from the correlator is the bit that is reversed. The correlation means output for the Walsh index selected by reversing the first MSB is predicted to contain the second largest signal component when there is a frequency tracking error.
In other embodiments, the discriminator determines the current error of the frequency tracking loop by substituting correlation results corresponding to other inverted Walsh index bits into the cross product calculation. The correlation means output corresponding to reversing the second MSB is predicted to provide the third largest transmitted signal component, the correlation means output for reversing the third MSB is predicted to provide the fourth largest signal component, and so forth. Therefore, each particular index bit is selected for reversal based on a desired or predicted amount of signal energy, and its relative output offset, to use for determining the current error of the frequency tracking loop. Selecting higher order significant bits generally provides better steady state loop performance while lower order bits provide a higher pull-in range.
In still other embodiments, the discriminator determines the current error by averaging various combinations of correlation results, each determined through bit manipulation or reversal, as described above. Where even greater accuracy is required, the average of two or more results obtained from differing types of processing (cross products), as described above, can be produced.
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Agrawal Avneesh
Vijayan Rajiv
Bocure Tesfaldet
Ogrod Gregory D.
Qualcomm Inc.
Wadsworth Philip R.
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