RU209338U1 - DEVICE FOR DEMODULATION OF OFDM SIGNAL WITH SAMPLING FREQUENCY DECIMATING - Google Patents

Abstract

The utility model relates to the field of radio engineering. The technical result of the utility model is to increase the input sampling rate to values that allow you to work directly from the antenna input without tightening the requirements for the low-pass filter due to preliminary decimation. A device for demodulating an OFDM signal with sampling frequency decimation comprises a decimation module between I and Q multipliers and channel low-pass filters. 2 ill.

Description

The utility model relates to the field of radio engineering.

Closest to the claimed device is a digital OFDM demodulator with sampling rate decimation, patent RU 2644609 C1, 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 serially connected I and Q channel multipliers , the first inputs of which are 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 the digital intermediate frequency reference oscillation generator is connected to the second input of the I channel multiplier and through a 90 degree phase shifter to the second to the 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 input of the converter from serial OFDM symbols to parallel with the rejection of the guard interval, the output of which is connected to the input of the FFT block (direct fast Fourier transform), the output of which is connected en with the input of a converter from parallel data symbols to serial ones, moreover, the analog-to-digital converter is configured to receive an input signal in analog form at an intermediate frequency from the output of an external high-frequency unit, as well as to generate a real digital signal and feed it to the inputs of multipliers I and Q channels configured to transfer the OFDM signal spectrum from an intermediate frequency to a video frequency using an oscillator and a phase shifter, which are configured to generate cos and sin components of the reference waveform shifted by 90 degrees relative to each other at an intermediate frequency in digital form, while the multipliers I and Q channels are made with the possibility of parallel supply of 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, made with the possibility of pre-filtering or correcting the OFDM signal before applying to the Converter 8 consecutive OFDM symbols 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.

The main disadvantage of the device is the low input sampling frequency, which prevents the ADC from being connected directly to the antenna input, forcing it to work only with the IF.

The purpose of the utility model is to increase the input sampling rate to values that allow you to work directly from the antenna input without tightening the requirements for the low-pass filter due to preliminary decimation.

This goal is achieved by applying a pre-decimation module between the I and Q multipliers and the low-pass filters of the channels, as shown in FIG. 1. The decimation module consists of a chain of links, as shown in FIG. 2. After each link, decimation is performed 2 times. The number of links N allows you to reduce the sampling rate at the inputs of the low-pass filters by 2 ^N times. Each link has its own order, which determines the parameters of this link. The input of the first-order link receives the sequence x(n), and at the output we get the sequence y(n). In this case, the sequences are related by the difference equation

The frequency response of such a link is given by the equation

Accordingly, the n-th link has a frequency response

If the useful signal is enclosed in the frequency range from 0 to a , then after thinning by 2 times at the output of the link, the spectrum of the useful signal is superimposed on the spectrum from F _d /2- a to F _d /2, where F _d is the sampling frequency. However, this spectrum is strongly weakened. For example, if a = 0.01, then the attenuation of the spectrum for the first order link will be 66 dB. If the spectrum is not weakened enough, then the order of link n should be increased.

The use of the proposed device makes it possible to demodulate an OFDM signal directly from the antenna output.

For a better understanding of the claimed invention, the following is a detailed description with the corresponding drawings.

Fig. 1. Block diagram of the device.

Fig. 2. Pre-decimation module.

Elements:

1 - analog-to-digital converter;

2 - multiplier of the I channel;

3 - Q channel multiplier;

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

5 - phase shifter by 90 degrees;

6 - digital low-pass filter of channel I;

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

8 - Converter from serial OFDM symbols to parallel;

9 - FFT block;

10 - converter from parallel data symbols to serial ones;

11 - preliminary decimation module;

12 - pre-decimation module.

Bibliographic list

1. Patent RU 2644609 C1: Digital OFDM demodulator with sampling rate decimation / Cascade Limited Liability Company (RU).

Claims (1)

A device for demodulating an OFDM signal with sampling frequency decimation, 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 I and Q channel multipliers, the first inputs of which are connected to the analog- digital converter, and two digital low-pass filters with a finite impulse response of I and Q channels, while the output of the digital intermediate frequency reference oscillator is connected to the second input of the I channel multiplier and through a 90 degree phase shifter to the second input of the Q channel multiplier, and the digital outputs low-pass filters of I and Q channels are connected to the input of the converter from serial OFDM symbols, characterized in that between the multipliers I and Q and the low-pass filters of the channels, a pre-decimation module is included.

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