Filters for communication systems
First Claim
1. A root-raised-cosine filter chain circuit having an output at a first data rate, the filter chain circuit comprising:
- a digital filter with programmable coefficients which operates at a second data rate that is lower than the first data rate; and
an interpolation filter connected in series with the digital filter and after the digital filter in the filter chain circuit, the interpolation filter configured to increase the data rate from the second data rate to the first data rate, wherein the coefficients of the interpolation filter are selected to introduce controlled distortions, andwherein the programmable coefficients of the digital filter are selected to compensate for the controlled distortions introduced by the interpolation filter.
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Accused Products
Abstract
The invention relates to a cascaded scheme in which an RRC filter, a modified RRC filter or other digital filter is implemented at a relatively low data rate, such as twice the symbol or chip rate, or 2×. Interpolation filters are used to increase the data rate to a higher data rate, such as 8×. Decimation filters are used to reduce the data rate from a higher rate, such as 8×, to a lower rate, such as 2×. The coefficients of the digital filter may be adjusted to compensate for characteristics of other components across the entire filter chain. Most of the implementation complexity of the filter chain is consolidated into the relatively low rate (such as 2×) digital filter while interpolation or decimation filters can be implemented at very low cost. The compensation capability provided by the digital filter makes design of simple decimation or interpolation filters much easier. The compensation capability provided by the digital filter, along with the relatively high over-sampling rate for the output of the interpolation filter (or the input of the decimation filter), also makes the design of the preceding (or following) analog filters much easier.
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Citations
46 Claims
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1. A root-raised-cosine filter chain circuit having an output at a first data rate, the filter chain circuit comprising:
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a digital filter with programmable coefficients which operates at a second data rate that is lower than the first data rate; and an interpolation filter connected in series with the digital filter and after the digital filter in the filter chain circuit, the interpolation filter configured to increase the data rate from the second data rate to the first data rate, wherein the coefficients of the interpolation filter are selected to introduce controlled distortions, and wherein the programmable coefficients of the digital filter are selected to compensate for the controlled distortions introduced by the interpolation filter. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
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15. A method for filtering a signal in a root-raised-cosine filter chain circuit and providing an output at a first data rate, comprising:
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filtering the signal in a digital filter with programmable coefficients at a second data rate that is lower than the first data rate; and filtering the output of the digital filter in an interpolation filter that increases the data rate from the second data rate to the first data rate, wherein the coefficients of the interpolation filter are selected to introduce controlled distortions and wherein the programmable coefficients of the digital filter are selected to compensate for the controlled distortions introduced by the interpolation filter. - View Dependent Claims (16, 17, 18, 19, 20)
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21. A root-raised-cosine filter chain circuit having an input at a first data rate, comprising:
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a decimation filter which receives the input at the first data rate and decreases the data rate from the first data rate to a second data rate that is lower than the first data rate, wherein the coefficients of the decimation filter are selected to introduce controlled distortions; and a digital filter with programmable coefficients connected in series with the decimation filter, wherein the digital filter operates at the second data rate, wherein the programmable coefficients of the digital filter are configured to compensate for the controlled distortions introduced by the decimation filter. - View Dependent Claims (22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34)
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35. A method for filtering an input signal in a root-raised-cosine filter chain circuit at a first data rate, comprising:
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filtering the input signal in a decimation filter that decreases the data rate from the first data rate to a second data rate that is lower than the first data rate; and filtering the output of the decimation filter in a digital filter with programmable coefficients at the second data rate, wherein the coefficients of the decimation filter are selected to introduce controlled distortions and wherein the programmable coefficients of the digital filter are selected to compensate for the controlled distortions introduced by the decimation filter. - View Dependent Claims (36, 37, 38, 39, 40)
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41. A root-raised-cosine filter chain circuit having an output at a first data rate, the filter chain circuit comprising:
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a digital filter with programmable coefficients which operates at a second data rate that is lower than the first data rate; a timing adjustment module connected in series with the digital filter to decrease the second data rate received from the digital filter to a third data rate; and an interpolation filter connected in series with the timing adjustment module and after the digital filter in the filter chain circuit, the interpolation filter configured to increase the data rate from the third data rate to the first data rate, wherein the coefficients of the interpolation filter are selected to introduce controlled distortions, and wherein the programmable coefficients of the digital filter are selected to compensate for the controlled distortions introduced by the interpolation filter. - View Dependent Claims (42, 43, 44, 45, 46)
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Specification