Adaptive predistortion circuit with memory
First Claim
1. An adaptive predistortion circuit with memory for a digital transmission system which transmits input data to be represented by points of a signal constellation C and which occur at the rate of a symbol clock having a period T, and wherein a modulator and a power amplifier coupled to the predistortion circuit distort the signal by creating smeared spots (clouds) in the constellation, the circuit comprising:
- a predistortion circuit for predistorting in the reverse sense in-phase and quadrature input data symbols an =(a'"'"'n, a"n) prior to their entering the power amplifier in order to transmit expected levels, the predistortion circuit simultaneously taking into account a number L of received input symbols an+(L-1)/2. . . an . . . an-(L-1)/2 which are temporarily stored in a memory comprising input shift registers,a demodulator for deriving in-phase data symbols (y'"'"'n) and quadrature data symbols (y"n),an adaptation circuit for continuously adapting the predistortion circuit to a stream of input data symbols and comprising a comparator circuit that compares the in-phase (a'"'"'n) and quadrature (a"n) input data symbols with the demodulated in-phase (y'"'"'n) and the demodulated quadrature (y"n) data symbols, and an interface circuit for continuously updating the predistortion circuit in response to an output signal of the comparator circuit, characterized in that the adaptation circuit further comprises, coupled between the comparator circuit and the interface circuit, a set of counters/accumulators which determine the center of gravity of each smeared spot (cloud) of the received constellation by calculating a set of means errors that is used to adapt the predistortion circuit, anda first filter Fe connected before the modulator and operating on the in-phase and quadrature paths and a second filter Fr connected after the demodulator and also operating on the in-phase and quadrature paths and in a manner such that the product of the filtering performed by the first and second filters Fe and Fr corresponds to a Nyquist filtering.
1 Assignment
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Accused Products
Abstract
An adaptive predistortion circuit with memory for a digital transmission system includes a set (10) of input registers storing various consecutive data symbols a predistortion circuit (11) for predistorting the data of the consecutive data symbols before they pass through a modulator (14) and then through an amplifier (15) and an adaptation circuit (19) which, in response to a demodulation (16, 171, 172, 181, 182) of the stream of transmitted data symbols continuously adapts the predistortion circuit (11) to the stream of input data symbols. The adaptation circuit (19) includes a set (21) of counters/accumulators which determine the center of gravity of the smeared spots (clouds) created by the distortion by calculating a set of errors that is used for adapting the predistortion circuit (11). Preferably, the predistortion circuit is a random access memory. In order to reduce the size of this random access memory the symmetry of the constellation is used and therefore at data input the constellation is reduced to a quarter of the constellation. Reverse operations are thus necessary at the output and in the adaptation loop.
84 Citations
19 Claims
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1. An adaptive predistortion circuit with memory for a digital transmission system which transmits input data to be represented by points of a signal constellation C and which occur at the rate of a symbol clock having a period T, and wherein a modulator and a power amplifier coupled to the predistortion circuit distort the signal by creating smeared spots (clouds) in the constellation, the circuit comprising:
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a predistortion circuit for predistorting in the reverse sense in-phase and quadrature input data symbols an =(a'"'"'n, a"n) prior to their entering the power amplifier in order to transmit expected levels, the predistortion circuit simultaneously taking into account a number L of received input symbols an+(L-1)/2. . . an . . . an-(L-1)/2 which are temporarily stored in a memory comprising input shift registers, a demodulator for deriving in-phase data symbols (y'"'"'n) and quadrature data symbols (y"n), an adaptation circuit for continuously adapting the predistortion circuit to a stream of input data symbols and comprising a comparator circuit that compares the in-phase (a'"'"'n) and quadrature (a"n) input data symbols with the demodulated in-phase (y'"'"'n) and the demodulated quadrature (y"n) data symbols, and an interface circuit for continuously updating the predistortion circuit in response to an output signal of the comparator circuit, characterized in that the adaptation circuit further comprises, coupled between the comparator circuit and the interface circuit, a set of counters/accumulators which determine the center of gravity of each smeared spot (cloud) of the received constellation by calculating a set of means errors that is used to adapt the predistortion circuit, and a first filter Fe connected before the modulator and operating on the in-phase and quadrature paths and a second filter Fr connected after the demodulator and also operating on the in-phase and quadrature paths and in a manner such that the product of the filtering performed by the first and second filters Fe and Fr corresponds to a Nyquist filtering. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
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15. A digital transmission system which transmits input data to be represented by points of a signal constellation C at the rate of a symbol clock having a period T, said system comprising:
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a power amplifier that distorts the signal to create smeared spots (clouds) in the signal constellation, a memory apparatus for storing a number L of data input symbols a n+(L-1)/2 . . . an . . . an-(L-1)/2'"'"' a predistortion circuit coupled between said memory apparatus and said power amplifier and controlled by said stored data input symbols for predistorting, in a sense opposite to the distortion in the power amplifier, in-phase (a'"'"'n) and quadrature phase (a"n) input data symbols, a modulator, a first in-phase signal path and a second quadrature phase signal path coupled in parallel circuit between the predistortion circuit and the modulator, means coupling an output of the modulator to an input of the power amplifier whereby said predistortion circuit, said parallel circuit, said modulator and said power amplifier are coupled together in cascade, a demodulator coupled to an output of the power amplifier to produce at an output of the demodulator a stream of in-phase and quadrature phase demodulated data symbols (y'"'"'n, y"n ) in response to a stream of data symbols at the power amplifier output, an adaptation circuit having input means coupled to the memory apparatus and to the demodulator output and responsive to the input data symbols and the demodulated data symbols (y'"'"'n, Y"n) for continuously adapting the predistortion circuit to the input data symbols as a function of said demodulated data symbols, said adaptation circuit comprising a comparator circuit that compares in-phase and quadrature phase input data symbols (a'"'"'n, a"n) with said in-phase and quadrature phase demodulated data symbols (y'"'"'n, y"n), an interface circuit for continuously updating the predistortion circuit and, between the comparator circuit and the interface circuit, means for calculating a set of mean errors so as to determine a center of gravity of each smeared spot (cloud) of the signal constellation, first filter means (Fe) connected in said first and second signal paths, and second filter means (Fr) connected between the demodulator output and the comparator circuit and responsive to the in-phase and quadrature phase demodulated data symbols. - View Dependent Claims (16, 17, 18, 19)
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Specification