RU2830857C1 - Radio receiver with digital nonlinearity correction - Google Patents

Abstract

FIELD: reception and processing of signals.

SUBSTANCE: invention relates to equipment for receiving and processing signals and can be used to create radio equipment. Radio receiving device comprising preamplifier (1), analogue-to-digital converter (2), correcting nonlinear element (3.1) and digital signal processing (DSP) unit (4), N blank filters (5.1...5.N), N-1 correcting nonlinear elements (3.1...3.N) and adder (6), the output of which is connected to the input of DSP unit (4). Inputs of N blank filters (5.1…5.N) are combined and connected to the output of ADC (2), and the outputs of N blank filters (5.1…5.N) are connected to the inputs of the corresponding N correcting nonlinear elements (3.1…3.N).

EFFECT: reduced nonlinear distortions of the received signal in the entire operating frequency range of the radio receiver.

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Description

The invention relates to signal reception and processing technology and can be used to create radio devices with a programmable radio architecture with digital signal processing for operation in complex electromagnetic environments. The invention makes it possible to increase the linearity of the transfer characteristic of an analog-to-digital path (ADP) by correcting nonlinear distortions in a wide frequency band of received signals.

Such receivers are described, for example, in the book "Software defined radio: enabling technologies", John Wiley & Sons, Chichester, UK, 2002 - p.p. 440 by W. Tuttlebee, Ed and "Digital front-End in wireless communications and broadcasting: circuits and signal processing / Cambridge University Press 2011.

The essence of the known devices is to amplify and discretize signals at radio frequency and then receive them using digital signal processing (DSP) algorithms.

The reason for the appearance of signal distortions is the nonlinearity of the transfer characteristic of the ADC of the radio receiver, caused by the non-ideality of its components. The nonlinear transfer characteristic of the ADC is formed by the mutual influence of the nonlinearity of two key components: the analog-to-digital converter (ADC) and the preamplifier (PA).

Modern RF ADCs are wideband and the nonlinearity of the conversion characteristic depends on the frequency of the digitized signal. For example, the popular RF ADC AD9680 from Analog Devices has different nonlinearity at different frequencies: intermodulation distortion measured by the two-tone method (IMD) is -82 dBFS for the frequencies of the measurement signal tones of 185 and 188 MHz and -78 dBFS for the frequencies of the measurement signal tones of 449 and 452 MHz [1].

The reason for the nonlinearity of the PU is the imperfection of manufacturing technologies and the purity of materials. The nonlinearity of the PU also depends on the frequency: for example, the high linearity Low Noise Amplifier (LNA) TQP3M9035 from Qorvo has an OIP3 of 34 dBm at a frequency of 50 MHz, and about 40 dBm at a frequency of 100 MHz [2].

Existing radio receivers with digital correction of nonlinearity are built according to the scheme with serial inclusion of the correcting nonlinear element in the digital domain. The correcting nonlinear element has a transfer characteristic, which is intended to compensate for nonlinear distortions of the signal that arose in the ADC. Synthesis of the transfer characteristic of the correcting nonlinear element is usually carried out by measuring the values of a signal of a previously known shape at the output of the ADC.

The closest analogue to the proposed one is the device described in the scientific article “Spectral method of analysis and synthesis of digital nonlinear elements”, Theory and technology of radio communication. - 2010. - No. 4. - P. 55-65, author Makoviy V.A.

Fig. 1 shows a functional diagram of the prototype device, where the following is indicated:

1 - pre-amplifier (PA);

2 - analog-to-digital converter (ADC);

3 - correcting nonlinear element (CNE);

4 - digital signal processing (DSP) block.

The prototype device contains a series-connected preamplifier 1, ADC 2, correcting nonlinear element 3 and DSP unit 4, the output of which is the information output of the device. The input of PU 1 is the input of the device.

Blocks 2, 3, 4 have standard clock inputs to which signals are supplied that ensure synchronization of the operation of the device as a whole (not shown in Fig. 1).

The prototype device works as follows.

The input radio frequency signal, which is an additive mixture of the useful signal and concentrated interference, arrives at the input of the device and passes through the preamplifier 1, where it is amplified and undergoes nonlinear distortions. From the output of the PU 1, the amplified signal arrives at the input of the ADC 2, which converts the signal into the digital domain and also introduces distortions caused by differential and integral nonlinearity. From the output of the ADC 2, the signal arrives at the input of the KNE 3, which is an artificial nonlinearity, under the influence of which the distortions of the digitized signal are corrected. Then the linearized signal arrives at the DSP block 4 for further selection and detection.

The transfer characteristic of the KNE 3 can be described by a look up table (LUT). The correspondence between the values of the input and output code can be found using a harmonic measuring signal [3].

The disadvantage of the prototype device is the lack of consideration of the dependence of the nonlinearity of the transfer characteristic of the analog-digital path on the frequency. The prototype device allows for effective correction of distortions only in the vicinity of the frequency of the measuring signal, based on the results of which the transfer characteristic of the KNE was synthesized. At frequencies significantly different from the frequency of the measuring signal, the prototype device may not work effectively and may even lead to deterioration of the parameters compared to the path without correction. This limits the use of the prototype device for the linearization of modern radio receivers with a wide range of received frequencies.

The objective of the proposed technical solution is to increase the efficiency of the digital corrector of nonlinear distortions in radio receivers with a wide range of received frequencies.

In order to solve the stated problem, N blank filters are introduced into a radio receiver with digital nonlinearity correction, comprising a series-connected preamplifier (PA) and an analog-to-digital converter (ADC) unit, a first correcting nonlinear element (CNE), and also a digital signal processing (DSP) unit, the output of which is the output of the radio receiver, wherein the input of the preamplifier is the input of the radio receiver, according to the invention, as well as N-1 correcting nonlinear elements; the outputs of the N correcting nonlinear elements are connected to the corresponding inputs of an adder, the output of which is connected to the input of the DSP unit, wherein the inputs of the N blank filters are combined and connected to the output of the ADC, the outputs of the N blank filters are connected to the inputs of the corresponding N correcting nonlinear elements.

This allows for the correction of distortions in the received signal, taking into account the frequency dependence of the nonlinearity of the analog-digital path.

The functional diagram of the claimed device is shown in Fig. 2, where it is indicated:

1 - pre-amplifier (PA);

2 - analog-to-digital converter (ADC);

3.1-3.N - from the first to the Nth correcting nonlinear elements (KNE);

4 - digital signal processing (DSP) block.

5.1-5.N - from the first to the Nth blank filters;

6 - adder.

The claimed device comprises a series-connected preamplifier 1, an analog-to-digital converter 2, the output of which is connected to the inputs of N blank filters 5.1…5.N, the outputs of which are connected to the inputs of N corresponding correcting nonlinear elements (CNE) 3.1…3.N, the outputs of which are connected to the corresponding N inputs of an adder 6, the output of which is connected to the input of a digital signal processing unit (DSP) 4, the output of which is the information output of the device. The input of PU 1 is the input of the device.

Blocks 2, 3.1…3.N, 4, 5.1…5.N, 6 have standard clock inputs to which signals are supplied that ensure synchronization of the operation of the device as a whole (not shown in Fig. 2).

The claimed device operates as follows.

The operating principle of the proposed device is explained in Fig. 3, where it is indicated:

1 - pre-amplifier (PA);

2 - analog-to-digital converter (ADC);

3.1-3.N - from the first to the Nth correcting nonlinear elements (KNE);

4 - digital signal processing (DSP) block;

5.1-5.N - from the first to the Nth blank filters;

6 - adder.

The input radio frequency signal, which is an additive mixture of the useful signal and concentrated interference, arrives at the input of the device and passes through PU 1, where it is amplified and undergoes nonlinear distortions. From the output of PU 1, the amplified signal arrives at the input of ADC 2, which converts the signal into the digital domain and also introduces distortions caused by differential and integral nonlinearity. From the output of ADC 2, the digital signal arrives at the inputs of blank filters 5.1…5.N, which divide the frequency range of the input signal into N sub-ranges. Digital signals from the output of each blank filter 5.1…5.N arrive at the corresponding inputs of KNE 3.1…3.N, where they undergo post-distortion individually for signals in each sub-range. From the outputs of KNE 3.1…3.N, the digital signals arrive at adder 6, where they are synchronously summed count by count. The result of the summation is fed to the input of the digital signal processing unit 4 for further selection and detection.

Blank filters 5.1…5.N are bandpass filters, the sum of the amplitude-frequency response (AFR) of which in the radio frequency range of the receiver is equal to one.

The KNE 3.1…3.N blocks are a memory device that stores readings of the transfer characteristic synthesized individually for each of the N sub-ranges based on the results of the passage of the measuring signal at the central frequency of each blank filter.

Unlike the prototype, the proposed device allows for effective correction of nonlinear distortions regardless of the frequency and bandwidth of the received signal.

The implementation of blocks 1 and 2 of the claimed device is similar to the blocks of the prototype device and can be performed in accordance with the monograph by Paul Horowitz and Winfield Hill "The Art of Circuit Design" in 2 volumes. Moscow Mir 1986. Blocks 3.1...3.N, 4, 5.1...5.N and 6 can be implemented in software on a hardware platform, which is a digital signal processor in conjunction with an FPGA, for example, TMS320C6748 (Texas Instruments) and XC7K325T-2FFG900I (Xilinx), or on a system on a chip, for example, the Zynq family (Xilinx).

The frequency response of blank filters 5.1…5.N can be synthesized using the window weighting method [4].

The transfer characteristic of the KNE 3.1…3.N can be synthesized using a harmonic measuring signal at the central frequency of each of the N blank filters [3] and implemented by a look up table (LUT) of the input and output values of the digital signal.

Let us provide evidence of the effectiveness of the claimed device.

Radio frequency ADCs and preamplifiers are key nonlinear elements of the analog-to-digital path. The characteristics of the ADC and preamplifier indicate nonlinearity parameters depending on the signal frequency. In fact, such an analog-to-digital path can be represented as N narrowband analog-to-digital paths with different transfer characteristics in terms of nonlinearity parameters.

If the analog-digital paths have different nonlinearity, then the use of a correcting nonlinear element with the same transfer characteristic, synthesized for one analog-digital path, as in the prototype device, for the remaining N-1 analog-digital paths will be ineffective.

Fig. 4 shows the transfer characteristics g ₁ (s)…g _N (s) N of the analog-to-digital paths (ADP) and the results of their interaction with the fixed transfer characteristic f(s) of the correcting nonlinear element (CNE).

Fig. 5 shows the transfer characteristics g ₁ (s)…g _N (s) N of the analog-to-digital paths (ADP) and the results of their interaction with various transfer characteristics f ₁ (s)…f _N (s) N of the correcting nonlinear elements (CNE).

The use of various transfer correcting nonlinear elements in the proposed device, synthesized separately for each of the N analog-to-digital paths, eliminates this problem.

Thus, in accordance with the set task, a radio receiving device was implemented in which the correction of nonlinear distortions of the analog-digital path is carried out more effectively in the entire range of received frequencies of the receiver, and the efficiency of the proposed device is proportional to the number of blank filters and KNE.

The achieved technical result is a reduction in nonlinear distortions of the received signal in the entire range of operating frequencies of the radio receiver.

Literature:

AD9680. 14-Bit, 1.25 GSPS/1 GSPS/820 MSPS/500 MSPS JESD204B, Dual Analog-to-Digital Converter / Analog Devices, inc. - 2019 (electronic resource https://www.analog.com/en/products/ad9680.html).

TQP3M9035. High Linearity LNA Gain Block / Qorvo, inc. - 2020 (electronic resource https://www.qorvo.com/products/p/TQP3M9035).

Makovy, V.A. Spectral method of analysis and synthesis of digital nonlinear elements / V.A. Makovy // Theory and technology of radio communication. - 2010. - No. 4. - P. 55-65.

Theory and Application of Digital Signal Processing / L. Rabiner, B. Gould. Translated from English by A.L. Zaitsev, E.G. Nazarenko, N.N. Tetekina. - Moscow: Mir Publishing House, 1978, pp. 103-106.

Claims (1)

A radio receiver with digital nonlinearity correction, comprising a series-connected preamplifier (PA) and an analog-to-digital converter (ADC) unit, a first correcting nonlinear element (CNE), and also a digital signal processing unit (DSP), the output of which is the output of the radio receiver, wherein the input of the preamplifier is the input of the radio receiver, characterized by in that N blank filters are introduced, which are bandpass filters, the sum of the amplitude-frequency characteristic (AFC) of which in the radio frequency range of the receiver is equal to one, as well as N-1 correcting nonlinear elements, wherein N correcting nonlinear elements are a memory device in which the readings of the transfer characteristic are stored, synthesized individually for each of the N sub-ranges based on the results of passing the measuring signal at the central frequency of each blank filter; the outputs of the N correcting nonlinear elements are connected to the corresponding inputs of the adder, the output of which is connected to the input of the DSP block, wherein the inputs of the N blank filters are combined and connected to the output of the ADC, the outputs of the N blank filters are connected to the inputs of the corresponding N correcting nonlinear elements.