KR20080049697A - Bio radar system based on orthogonal frequency division multiplexing - Google Patents

Abstract

A bio radar system using an OFDM(Orthogonal Frequency Division Multiplexing) signal is provided to overcome deterioration of performance by overcoming noise and interference. A bio radar system using an OFDM carrier includes an OFDM transmitter and receiver structure. The bio radar system further includes an adaptive filter for separating a transmission signal from a received signal, and a low pass filter and a band pass filter for separating a heartbeat and respiration signal from a detected complex heartbeat and respiration signal. The bio radar system utilizes the OFDM signal in order to overcome noise and interference. The bio radar system extracts a reflected signal of a pure target from a mixed reception signal.

Description

Bio Radar system based on Orthogonal Frequency Division Multiplexing}

Figure 1: Bio-Radar Application Diagram

FIG. 2 shows a structural diagram of using an OFDM transmitter and a receiver as a bio radar and differs from a general OFDM transmitter and receiver in a heartbeat and respiration mixed with an adaptive filter for extracting only a signal reflected from a living body from a received signal at a receiver. A low pass filter and a band pass filter are added to extract the.

3: QAM constellation diagram distorted by the Doppler phenomenon. The QAM constellation plot according to the Doppler frequency shift index e is shown. It can be seen that the center of the constellation also rotates on the unit circle as the value of e increases and at the same time the dispersion (randomness) increases.

Figure 4 shows the mixed signal extracted from the computer simulation using the reference signal of the heart rate and respiration mixed by the method presented in the present invention.

Figure 5 shows the frequency spectrum of the extracted heart rate and respiration and shows the bandpass filter frequency response characteristics for extracting the heart rate signal from it.

6 shows the frequency spectrum of the extracted heart rate and respiration and shows the frequency response of the low pass filter for extracting respiration signals.

Figure 7 shows the heart rate signal and the reference signal extracted after passing the bandpass filter from the mixed signal of the heart rate and breath.

8 shows the heart rate signal and the reference signal extracted after passing the band pass filter from the mixed signal of the extracted heart rate and respiration.

[Patent 1] Patent Registration 0801645: Doppler shift frequency measurement and correction method using reception chart in OFDM system

[2] B.S.Lee, "Doppler effect compensation scheme based on constellation estimation for OFDM System" IEE, Electronic Letter 2008. Jan Vol 44, no 1.PP38-40

As a radar using the Doppler effect, a weather radar for checking the distribution of clouds and a speed radar for detecting the speed of a vehicle are widely used. Recently, the research and development of the bio radar for detecting the heartbeat and respiration of the living body in a non-contact form is actively progressing. However, in the case of a bio radar, since the target is a human body, the power of a radio frequency (RF) transmission signal that can be radiated is strictly limited, and the signal reflected from the human body is also very weak, and thus, it is very vulnerable to ambient noise and interference. Therefore, there is a need for an effective and stable method for detecting Doppler frequency shifts by detecting and processing received signals in such poor signal environments and extracting heartbeat and respiration signals from them.

The conventional Doppler radar operates by using a single carrier wave (CW) as a transmission signal and extracting the Doppler transition component from the received signal reflected from the target to extract the velocity of the target. The reason for using a single carrier is to extract the Doppler transition frequency because it is simpler in system configuration and has advantages in frequency band efficiency than in the case of multi-frequency. If the target is not a living organism, the transmission signal can be increased to secure the required reception signal level. If the target object is a metal, the reflected signal level is relatively large, so it is easy to detect the Doppler frequency by a single carrier. On the other hand, as a disadvantage, it is susceptible to fading in a multipath environment, and there may be a limitation on the distance between the target and the radar due to the presence of a null point related to the wavelength of a single carrier.

In the case of bio radar, the power of the transmission signal that can be transmitted to the human body is limited, and the radio wave reflection coefficient of the human body is low, so the level of the received signal reflected from the target is extremely weak, which is inevitable to ambient noise and interference. In order to overcome this problem, the OFDM signal, which is an orthogonal multicarrier, can be used to measure the Doppler frequency shift very stably since the error in one carrier does not affect the interpretation through the entire carrier. Can be detected.

However, when implementing an OFDM-based Doppler radar, it may be technically impossible to mount the Doppler frequency measurement module on each of the 1024 frequencies assuming IFFT / FFT length is 1024. none. However, when using the method of [Document 1], it is possible to effectively detect the Doppler frequency shift in the whole carrier from the constellation diagram of the symbol detected through the conventional FFT receiver, and the number of symbols utilized statistically for the Doppler frequency calculation is FFT Assuming a length of 1024 and a symbol transmission rate of 100 symbols / second, the number of available data for one second is 102,400 symbols, so that an error that can occur on several carriers does not affect the detection of Doppler frequency throughout. . Therefore, using the proposed method, it is possible to construct a bio radar that is robust to noise and interference but is simple and very economical.

The configuration of the present invention basically has an OFDM transmitter and receiver structure as shown in FIG. 2, and an adaptive filter for separating a transmitted signal from a received signal, and a separated heartbeat and respiratory signal from the detected complex heartbeat and respiratory signals. A low pass filter and a band pass filter are added for this purpose.

The bio radar signal s ( t ) transmitted from the OFDM transmitter is represented by Equation 1.

Equation 1

Where X (i) is a QAM symbol, N is the number of OFDM sub-carriers, T is the symbol width, f _s is the sub-carrier bandwidth, f _c denotes a carrier frequency transmission. s ( t ) is transmitted and received signals are represented by equations (2) and (3), respectively.

Equation 2

Here, if the specified distance to the target is d _o , and the displacement due to respiration and heartbeat is x ( t ), the distance that the radio waves travel is expressed as 2 d ( t ) = 2d _o +2 x ( t ), and c is a radio wave. It means the speed of propagation. Α is a leakage coefficient directly transmitted from a transmitting antenna to a receiving antenna, and β is a reflection coefficient representing a magnitude of a signal reflected from an object. In Equation 2, if only the term reflected from the target object is represented by R ( t ), Equation 3 is obtained.

Equation 3

Therefore, the phase delay value according to the time delay to the target including the displacement of the target is given by Equation 4,

Equation 4

From this, the Doppler frequency shift can be obtained as shown in Equation 5.

Equation 5

In the present invention, however, the Doppler transition frequency is obtained by using a pattern in which the symbols carried on each OFDM subcarrier are distorted by the Doppler frequency transition after recovering the symbol at the receiving end without calculating the Doppler transition frequency. For this purpose, if the signal through the low pass filter is displayed in a complex form after down conversion in the demodulation process of the receiver, it is expressed as in Equation 6.

Equation 6

As shown in Equation 7 in which Equation 6 passes through the FFT stage, it is possible to recover a transmitted symbol.

Equation 7

Where S ( lk ) is equal to Equation 8 and represents an Inter Carrier Interference (ICI) coefficient due to Doppler frequency shift.

Equation 8

Where e is the Doppler frequency shift divided by the subcarrier frequency interval, ie e = f _d / f _s . In Equation 7, the first term αX ( k ) is a term directly induced from the transmitting antenna to the receiving antenna and does not include distortion due to the Doppler frequency shift in the symbol. Therefore, after effectively removing the first term, it is necessary to measure the Doppler frequency by processing Y _d ( k ), which is a component of which the constellation is distorted by the Doppler frequency shift. Since the Doppler radar system of this α value in accordance with the isolated (isolation) the degree of the receive antenna for the transmit antenna it is determined easily obtained by measurement when the building α value may know in advance. Unlike the general communication system, since the symbol values carried in the respective OFDM subcarriers are also known in the bio radar, the pure Y _d ( k ) value can be approximated as shown in Equation (9).

Equation 9

Β value, which is a reflection coefficient between a target and a radar in a bio radar, can also be approximated by obtaining an expectation of an absolute value of Y _d ( k ) as shown in Equation 10.

Equation 10

Equation 11 can be obtained by normalizing Y _d ( k ) with the obtained β value.

Equation 11

Equation Y ( k ) means a symbol that is purely distorted by the Doppler transition, divides it by a known transmission symbol X ( k ), and obtains an expected value.

Equation 12

That is, when the received symbol is divided by the transmitted symbol and the expected value is obtained, the phase component of the obtained complex value becomes ( πe ). Thus , the Doppler shift frequency f _d can be obtained using Equation 13 with this value.

Equation 13

Since the Doppler shift frequency has a functional relationship with x ( t ), which is a function of the target's movement, it is possible to extract displacement signals according to the heartbeat and respiration of the target body.

Doppler shift frequency information represented by Equation 13 includes human variation information according to the heartbeat and respiration, and because each frequency information is different, it passes through the low pass filter and the bandpass filter as shown in [FIG. 5] and [FIG. 6]. 7 and 8 to extract the pure heartbeat signal and the respiratory signal.

Using OFDM signal the variation of the human body is very small, such as when Wo of the bio radar when trying to measure the Doppler frequency this Doppler frequency shift is small, resulting in a deviation (e) on the phase small through which the Doppler frequency appearing in the receive constellation according It may be difficult to measure. However, this can induce an optimal constellation displacement by adjusting the OFDM subcarrier spacing f _s as shown in FIG. In other words, if f _d is small, f _s may be reduced to maintain an appropriate value of e .

Equation 14

By using a multicarrier bio radar using OFDM, the conventional biocarrier using a single carrier is extremely weak in receiving signals, which is vulnerable to noise and interference, and can overcome performance degradation due to the presence of a NULL point according to a single wavelength. have. Furthermore, since OFDM transmitter and receiver are already widely used in communication machines and have been miniaturized and standardized, it is possible to easily implement economic and excellent bio radar when implemented as a bio radar system.

Claims (4)

A system that utilizes OFDM signals, which are multiple orthogonal carriers, in a bio (Doppler) radar to overcome noise and interference [FIG. 1] and the method specified by Equation 13 in Equation 1 A method for deriving a signal reflected from a pure target in the mixed received signal represented by Equation (9) How to calculate the reflection coefficient between the target and the bio radar as shown in Equations 10 and 11 By adjusting the OFDM subcarrier spacing as shown in Equation 14, a method of effectively measuring the Doppler frequency shift as a result of maintaining an appropriate e value for a small Doppler frequency shift

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