RU2644609C1 - Digital demodulator ofdm with sample frequency decimation - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: implementation of the signal decimation operation is carried out in the FFT unit (direct Fast Fourier Transform) of the increased dimension, NL, where L=2^m. The FFT unit is configured to generate a parallel data stream at the K outputs, and its other NL-K outputs are not involved.

EFFECT: creation of a digital OFDM demodulator with decimation of the sample frequency with a simpler design with fewer discrete logic elements, and with a lower power consumption, by reducing the order of the channel digital filters I and Q and eliminating the I and Q channels from the decimator design.

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Description

The invention relates to the field of radio engineering, to broadband communication systems using orthogonal frequency division with many subcarriers (Orthogonal Frequency-Division Multiplexing - OFDM), namely to digital OFDM demodulators with decimation of the sampling frequency, and can be used in multi-user fixed or mobile communication systems based on OFDM technology.

Closest to the claimed invention is a digital OFDM demodulator described in patent US 7289765 B2, which contains a series-connected analog-to-digital Converter, in-phase (I) and quadrature (Q) channels, made identically and consisting of series-connected multipliers, the first inputs of which are connected with an analog-to-digital converter, digital low-pass filters with a finite impulse response and decimators, a digital generator of reference oscillations of an intermediate frequency, the output of which is It is connected to the other input of the channel I multiplier and through the phase shifter 90 degrees to the other input of the Q channel multiplier, and the outputs of the decimators are connected to the converter from consecutive OFDM symbols to parallel ones with the guard interval being dropped, the Fast Fourier Transform (FFT) block dimension N = 2 ⁿ (where in practice, as a rule, n = 6 ... 13), a converter from parallel data characters to serial ones. This digital OFDM demodulator is selected as a prototype of the claimed invention.

Digital OFDM demodulator prototype works as follows. From the output of the analog-to-digital converter, a digital intermediate frequency signal fpr is fed to the first inputs of the channel I and Q multipliers, while the second inputs of them supply 90-degree-shifted signals of the digital intermediate frequency reference generator. From the output of the channel I and Q multipliers, respectively, the real and imaginary components of the complex signal are fed to the input of digital low-pass filters, which suppress the spectra of mirror-frequency signals of the digital signal. Next, the quadrature components of the digital signal are fed to the corresponding inputs of the I and Q channel decimators, in which the sampling frequency fd is reduced by L = 2 ^m times (where m is a positive integer). As a result, the outputs of these decimators receive signals that have the sum of the spectra of mirror frequencies with a period of fd / L and are fed to the converter of consecutive OFDM symbols in parallel with the discarding of the guard interval at the beginning of the OFDM symbol, which is a copy of some of its final part. The I and Q spectrum of the OFDM symbol thus formed at the video frequency is fed to the N-input block FFT of dimension N = 2 ⁿ . At the output of the FFT block, a number of samples K, with a data rate of B / K bit / s in each, are fed to the converter from parallel to serial characters, the output of which is formed an output data stream with a speed of B bits / s. The K / N ratio determines the guard interval and is selected on the order of 0.7-0.8, which is sufficient to suppress harmful edge effects at the ends of the spectrum. The remaining NK outputs of the FFT block are discarded.

The use of decimators I and Q increases the requirements for the amplitude-frequency characteristic of channel digital low-pass filters due to a corresponding increase in the order of the filters. In FIG. Figure 1 shows the spectra of FFT signals before (a) and after (b) decimation at L = 2 ² = 4.

The disadvantage of the digital OFDM demodulator prototype demodulator with decimation of the sampling frequency is its hardware complexity, due to an increase in the order of channel digital low-pass filters and the implementation of decimators of I and Q channels, increasing with increasing L.

The technical result of the claimed invention is the creation of a digital OFDM demodulator with decimation of the sampling frequency with a simpler design with fewer discrete logic elements (gates), as well as with lower power consumption, by lowering the order of channel digital filters I and Q and eliminating the design of decimators I and Q channels, with the implementation of the operation of decimation of the signal in the FFT block (direct fast Fourier transform) of increased dimension? equal to NL, where L = 2 ^m , while the FFT block is configured to form a parallel data stream at K outputs, and its other NL-K outputs are not involved. A diagram characterizing the distribution of the “information” and “discarded” outputs of the FFT block is shown in FIG. 2. N values of FFT signal of dimension NL taken at the edges, in accordance with the direct Fourier transform property, coincide with the corresponding values of FFT of signal of dimension N minus the values of the remaining N (L-1) frequency “mirror” samples, which are discarded and do not fall into spectrum of “informational” readings as in decimation at the input of the FFT block.

Considering the fact that the number of “discarded” outputs of the FFT block is at least half of its dimension, the implementation of the FFT block of dimension NL is slightly more complicated with respect to the gain resulting in lowering the order of channel digital filters and excluding channel decimators. Usually, in practice, L is chosen equal to 4 or 8. Moreover, the greatest efficiency is achieved in the case of L = 2.

The claimed technical result is achieved by creating a digital OFDM demodulator with decimation of the sampling frequency, containing an analog-to-digital converter, the output of which is connected to the inputs of the in-phase I and quadrature Q channels, made identically and consisting of two series-connected multipliers of I and Q channels, the first inputs of which connected to an analog-to-digital converter, and two digital low-pass filters with a finite impulse response of I and Q channels, while the output of a digital generator op oscillations of the intermediate frequency are connected to the second input of the channel I multiplier and through the phase shifter 90 degrees to the second input of the Q channel multiplier, and the outputs of the digital low-pass filters of the I and Q channels are connected to the converter input from consecutive OFDM symbols to parallel ones with the guard interval being discarded, the output which is connected to the input of the FFT unit (direct fast Fourier transform), the output of which is connected to the input of the converter from parallel data characters to serial, moreover, analog-to-digital The new converter is configured to receive an analog signal at an intermediate frequency at the output frequency from the output of an external high-frequency unit, as well as to generate a valid digital signal and feed it to the inputs of channel I and Q multipliers, capable of transferring an OFDM signal spectrum from an intermediate frequency to the video frequency using a generator and a phase shifter, which are made with the possibility of forming cos and sin components of the reference fucking at an intermediate frequency in digital form, while the I and Q channel multipliers are capable of parallel supplying the real and imaginary components of the complex OFDM signal to the corresponding digital low-pass filters with a finite impulse response of I and Q channels configured to pre-filter or OFDM correction signal before applying to the converter 8 consecutive OFDM symbols in parallel with the discard of the guard interval at the beginning of the OFDM symbol, which is a copy the final part of the OFDM symbol.

In a preferred embodiment of the digital OFDM demodulator, the FFT block dimension is NL, where L = 2m, N = 2n, n usually takes values from 6 to 13, am is a positive integer, and the FFT block is configured to form a parallel data stream on K outputs, and its other NL-K outputs are not involved.

For a better understanding of the claimed invention the following is a detailed description with the corresponding graphic materials.

FIG. 1. Spectra of FFT signals before (a) and after (b) decimation at L = 2 ² = 4, performed according to the prototype.

FIG. 2. The distribution diagram of the "information" and "discarded" outputs of the FFT block, made according to the invention.

FIG. 3. Block diagram of a digital OFDM demodulator with decimation of the sampling frequency, made according to the invention.

Items:

1 - analog-to-digital Converter;

2 - channel I multiplier;

3 - Q channel multiplier;

4 - digital generator of reference oscillations of the intermediate frequency;

5 - phase shifter 90 degrees;

6 - digital low-pass filter of channel I;

7 - digital low-pass filter of the Q channel;

8 - converter from consecutive OFDM symbols to parallel;

9 - block FFT;

10 - Converter from parallel data characters to serial.

Let us consider in more detail the operation of the claimed digital OFDM demodulator with decimation of the sampling frequency (Fig. 3), which contains an analog-to-digital converter 1, multipliers of 2-3 I and Q channels, a digital generator 4 of reference oscillations of the intermediate frequency fpr, phase shifter 5 by 90 degrees, digital low-pass filters 6 and 7 with a finite impulse response of I and Q channels, a converter 8 from consecutive symbols to parallel ones with the removal of the guard interval of the OFDM symbol, block 9 FFT of dimension 2 ^{n + m} and converter 10 from parallel characters to consecutive.

The claimed digital OFDM demodulator operates as follows. From the output of the high-frequency unit, the signal in analog form at an intermediate frequency fpr is fed to the input of the analog-to-digital converter 1. Next, a valid digital signal is fed to channel multipliers 2 and 3 of I and Q, where the OFDM signal is transferred from the intermediate frequency fpr to the video frequency using generator 4 and phase shifter 5, which form cos and sin components shifted by 90 degrees relative to each other, reference oscillation components at an intermediate frequency fpr in digital form. Then, the real and imaginary components of the complex OFDM signal are fed in parallel to the corresponding digital low-pass filters 6 and 7 with a finite impulse response of I and Q channels. In these filters, the OFDM signal can be pre-filtered or corrected before applying consecutive OFDM symbols to the converter 8 in parallel with discarding the guard interval at the beginning of the OFDM symbol, which is a copy of the final part of the OFDM symbol. With K outputs of block 9, FFT with a dimension of 2 ^{n + m} K data streams with a speed of V / K bit / s are fed to a converter 10 of parallel characters into sequential ones, the output of which forms an output data stream with a speed of B bits / s, and the other 2 ^{n m} - K outputs of block 9 FFT are discarded.

The essence of the claimed invention is to create a digital OFDM demodulator with decimation of the sampling frequency, containing a series-connected analog-to-digital converter, in-phase (I) and quadrature (Q) channels made identically and consisting of series-connected multipliers, the first inputs of which are connected to analog-to-digital converter, and digital low-pass filters with finite impulse response, a digital intermediate frequency reference oscillator, the output of which is connected it is connected to another input of the channel 1 multiplier and through the phase shifter 90 degrees to the other input of the Q channel multiplier, and the outputs of the digital low-pass filters are connected to the converter from consecutive OFDM symbols to parallel ones with the guard interval being discarded, the direct fast Fourier transform unit, the dimension of which is increased 2 ^m times, converter from parallel data characters to serial.

The invention can be carried out on the corresponding elemental base for standard technologies.

Although the embodiment described above has been set forth to illustrate the claimed invention, it is clear to those skilled in the art that various modifications, additions and substitutions are possible without departing from the scope and meaning of the claimed invention disclosed in the attached claims.

Claims (2)

1. Digital OFDM demodulator with decimation of the sampling frequency, containing an analog-to-digital converter, the output of which is connected to the inputs of the in-phase I and quadrature Q channels, made identically and consisting of two series-connected multipliers of I and Q channels, the first inputs of which are connected to analog-digital a converter, and two digital low-pass filters with a finite impulse response of I and Q channels, while the output of a digital generator of reference oscillations of the intermediate frequency is connected to the second input to the channel I multiplier and through the phase shifter 90 degrees to the second input of the Q channel multiplier, and the outputs of the digital low-pass filters of the I and Q channels are connected to the converter input from consecutive OFDM symbols to parallel ones with the guard interval being discarded, the output of which is connected to the input of the FFT block ( direct fast Fourier transform), the output of which is connected to the input of the Converter from parallel data characters to serial, moreover, the analog-to-digital Converter is configured to receive on de signal in analog form at an intermediate frequency from the output of an external high-frequency unit, as well as with the possibility of generating a valid digital signal and supplying it to the inputs of channel I and Q multipliers, configured to transfer the OFDM signal spectrum from an intermediate frequency to a video frequency using a generator and a phase shifter which are made with the possibility of forming cos and sin components shifted 90 degrees relative to each other of the reference oscillation at an intermediate frequency in digital form, while I and Q channel multipliers are capable of parallel supplying the real and imaginary components of a complex OFDM signal to the corresponding digital low-pass filters with a finite impulse response of I and Q channels configured to pre-filter or correct the OFDM signal before applying consecutive OFDM symbols to the converter 8 parallel with the discard of the guard interval at the beginning of the OFDM symbol, which is a copy of the final part of the OFDM symbol. 2. The digital OFDM demodulator according to claim 1, characterized in that the dimension of the FFT block is NL, where L = 2 ^m , N = 2 ⁿ , n usually takes values from 6 to 13, am is a positive integer, while the FFT block configured to form a parallel data stream at the K outputs, and its other NL-K outputs are not involved.

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