KR20130047189A - Method canceling interference using adaptive digital filter in on-frequency rf repeaters - Google Patents

Abstract

The present invention relates to a method for canceling interference in a radio frequency repeater in a marine telematics environment using a LMS (Least Mean Square) filter, and a method for canceling interference in a radio frequency repeater according to the present invention is a feedback. Estimating the channel state of the feedback signal based on the output signal of the least mean square (LMS) filter, and removing the feedback signal by controlling the step size of the LMS filter based on the estimated channel state of the feedback signal. The estimating may include determining fast fading when the power change of the output signal changes over a predetermined change level as time passes; And determining slow fading when the power change of the output signal changes below a predetermined change level over time.

Description

Method for Canceling Interference Using Adaptive Digital Filter in On-Frequency RF Repeaters in Marine Telematics Environment

The present invention relates to a method for canceling interference in a radio frequency repeater, and more particularly, to a method for canceling interference in radio frequency repeaters in a marine telematics environment.

Generally, in the wireless communication system, the same frequency wireless repeater is installed to eliminate the shadow area, but in the marine telematics environment, the wireless repeater is a high gain amplification when the part of the signal transmitted from the repeater is fed back to the wireless repeater. You will have a rash.

To solve this problem, ICS (Interference Canceling System), which distinguishes the signal from the base station and the signal transmitted from the base station using an adaptive digital filter and removes only the feedback signal, is used.

Various algorithms are known for the adaptive digital filter, but the linear filter uses the LMS (Least Mean Square) algorithm which is easy to implement in hardware because of its high stability and simple structure.

However, when the LMS filter is used, it is difficult to infer a fading-affected signal caused by many obstacles when the transmission signal of the wireless repeater is reflected and returned to the wireless repeater in a marine telematics environment.

In more detail, due to the rapid development of the wireless mobile communication system, there is a demand for a data service that facilitates communication anytime and anywhere. However, wireless communication services are difficult to use in the region where the distance between the base station and the mobile is far away or where radio waves are not reached by buildings or other obstacles.

In a marine telematics environment, RF repeaters are devices used to reduce these shaded areas and to extend wireless coverage.

The basic function of the wireless repeater is to amplify the RF signal received from the receiving antenna by a gain required through a high power amplifier (HPA) and transmit it through the transmitting antenna.

In the maritime telematics environment, the wireless repeater only amplifies and retransmits the received signal, so the signal transmitted from the repeater occupies the same on-frequency band as the received signal.

In a marine telematics environment, a part of the signal transmitted from the wireless repeater is reflected by obstacles such as surrounding buildings or hills, and a feedback is returned to the receiving antenna of the repeater, which is a high amplification gain required for the wireless system. This causes the system to oscillate.

If the direction of the receiving antenna receiving the signal from the base station and the transmitting antenna that amplifies and radiates the signal can be completely isolated from each other so that there is no coupling between the two antennas, system oscillation by the feedback signal will not occur but Isolation is practically impossible.

If the isolation between the antennas is not sufficient, inevitably lower the amplification gain of the wireless repeater to avoid oscillation, in this case there is a problem that can not achieve the original purpose of the wireless repeater.

The present invention was created in view of the above problems, and estimates the channel state of the feedback signal and controls the step size of the LMS filter to improve the performance of the Interference Cancellation System (ICS) radio repeater. An object of the present invention is to provide a method for eliminating interference in a repeater.

That is, in case of estimating and removing the feedback signal and amplifying and transmitting only the signal to be relayed, even if the feedback signal is received, the signal is not amplified and transmitted so that the relay system can be maintained in a stable state. The purpose of the present invention is to provide a method of removing the feedback signal from the signal to be relayed and amplifying only the signal to be relayed to the service area to prevent oscillation occurring in the wireless repeater.

In order to achieve the above object, the interference cancellation method in the same frequency radio repeater in a marine telematics environment using a LMS (Least Mean Square) filter according to an aspect of the present invention is the feedback signal (Least feedback) LMS (Least) Estimating a channel state of the feedback signal based on an output signal of a mean square filter; And removing the feedback signal by controlling the step size of the LMS filter based on the estimated channel state of the feedback signal, wherein the estimating includes: a change in power of the output signal as time passes. When changing above a set level of change, determining fast fading; And determining slow fading when the power change of the output signal changes below a predetermined change level over time.

According to the present invention, when the variable step size LMS filter of the present invention is applied, it is possible to prevent the feedback channel from radiating under any circumstances.

In an ICS RF repeater, it is possible to make sure that the feedback channel is always stable regardless of the Doppler frequency.

In particular, slow fading can improve the performance of the LMS.

1 is a block diagram illustrating an ICS wireless repeater for removing a feedback signal according to an embodiment of the present invention.
2 is a view for explaining the power of the feedback signal when the Doppler frequencies are 10Hz and 100Hz.
3 is a view for explaining the SNR of the ICS RF repeater when the Doppler frequencies are 10 Hz and 100 Hz.
4 is a block diagram illustrating a variable step size according to an embodiment of the present invention.
5 is a diagram for explaining an SNR according to a change in a feedback channel state.

Advantages and features of the present invention, and methods of achieving the same will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. And is intended to enable a person skilled in the art to readily understand the scope of the invention, and the invention is defined by the claims. It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. In the present specification, the singular form includes plural forms unless otherwise specified in the specification. It is noted that " comprises, " or "comprising," as used herein, means the presence or absence of one or more other components, steps, operations, and / Do not exclude the addition.

In a marine telematics environment, the wireless repeater continuously amplifies and diverges the received signal when it is received as a received signal. ICS uses the received signal and the signal output from the filter to prevent this phenomenon. It is a system to remove the signal.

First, the feedback signal is predicted through the correlation between the signal transmitted from the base station and the ICS wireless repeater output signal, and the feedback signal information generated by the output signal of the ICS wireless repeater is analyzed through the information of the ICS RF repeater output signal. It is used to regenerate the feedback signal, and it is the principle that only the signal of the mother station can be extracted by using the last received signal (BTS signal + feedback signal).

Hereinafter, a method of eliminating interference in a same frequency wireless repeater in a marine telematics environment according to an embodiment of the present invention will be described with reference to FIGS. 1 to 5. 1 is a block diagram illustrating an ICS wireless repeater for removing a feedback signal according to an embodiment of the present invention, Figure 2 is a view showing the power of the feedback signal when the Doppler frequency is 10Hz and 100Hz, 3 is a diagram showing the SNR of the ICS RF repeater when the Doppler frequencies are 10 Hz and 100 Hz, FIG. 4 is a block diagram illustrating a variable step size according to an embodiment of the present invention, and FIG. 5 is a diagram illustrating a feedback channel state. It is a figure for demonstrating SNR according to a change.

As shown in FIG. 1, signal r (t) is a signal received from a base station, signal f (t) is a signal fed back from the transmitting antenna of the ICS to the ICS receiving antenna, and the wireless repeater is fed back to operate without oscillation. The signal f (t) must be effectively removed.

The input signal y (t) of the RF repeater is an r (t) + f (t) signal, which is the sum of the signal sent from the base station and the incoming signal, and the adaptive digital filter calculates an estimate of the f (t) signal to be removed. This estimate is subtracted from the received signal to output only signal r (t) as e (t).

The x (t) signal extracted from the receiving antenna is used as a signal for driving the adaptive digital filter after an appropriate delay time that is not correlated with the currently received r (t) signal. The received signal from which the feedback signal is removed is amplified by the HPA to a proper gain and then transmitted to the service area.

There are many known algorithms for adaptive digital filters. Among them, LMS (Least Mean Square) algorithm is the most widely used as linear filter. Can be divided into stages.

First, Equation 1 expressing the coefficients of the filter tap, second Equation 2 representing the feedback error, and finally, Equation 3 representing the result of the filter.

here

Denotes the respective input, output, and error at n, the current time of the FIR filter, and d _n denotes the desired signal and denotes the step size of μ filter coefficient convergence.

The signal received from the ICS wireless repeater is a signal from the base station and the feedback signal reflected from the signal of the ICS wireless repeater reflected in a building or other obstacle.

The signal from the ICS radio repeater enters the ICS radio repeater through Rayleigh fading in the same way as the base station signal, and the feedback channel causes a lot of fading.

Through experiments, the Doppler frequency, which is an indicator of Rayleigh fading, is measured and the power of the signal received by the ICS wireless repeater is measured as follows.

That is, the Doppler frequency is measured by setting the 10 Hz and 100 Hz, respectively, and the power of the signal is as shown in FIG. 2, and when the signal shown in FIG. When calculated by measuring the SNR, and expressed in decibels (dB) using the output signal of the ICS wireless repeater and the signal from the base station, and expressed in a table, it is as shown in FIG.

The step sizes are tested at 0.01, 0.03, 0.05, 0.07, 0.09, 0.1, 0.3, and 0.5, respectively. As can be seen from the experiment, the SNR of the LMS filter varies greatly depending on the step size. It can be seen from FIG. 3 (a) that the step size should be small because the Doppler frequency of the feedback channel is 10 Hz, but the system may not hunt even if the step size is too small.

In other words, if the step size is 0.01, the size is too small, so the SNR becomes smaller and larger, and if the Doppler frequency of the feedback channel is 10 Hz, the step size is 0.05, and the SNR is about -20 dB, indicating that the system has the best performance. Can be.

It can be seen from (b) of FIG. 3 that the Doppler frequency is so large that the change in the signal being fed back is very large. Is 100Hz, the larger the step size, the more stable the system.

However, even in this case, it can be seen that the step size is not good. For example, if the step size is 0.3, the performance is best, and if it is larger or smaller than that, the performance is poor.

That is, each step size can be tested to determine what is the best step size.

However, the situation of the feedback channel is not always known in the situation of the actual ICS RF repeater. If the LMS filter step size of the ICS RF repeater is set to a small value, it can be assumed that the feedback channel can perform well with slow fading, but the LMS filter will diverge if the channel is affected by fast fading. can do.

On the contrary, if the step size of the LMS filter is set large so that it does not divergence even in a fast fading situation, a problem occurs that the LMS filter may perform better even in a channel having a slow fading.

The variable step size technique of the present invention estimates an incoming channel fed back in an ICS repeater environment and changes the step size accordingly. The output signal is measured by measuring the ratio of the error signal and the output signal power of a conventional variable step size LMS filter. Is larger than the error signal, the tracking size is increased by increasing the step size, and if the output signal is smaller than the error signal, the tracking capacity is increased by the smaller step size.

However, this method is established under the condition that the change in the feedback signal does not change much like the ICS radio repeater environment. In the ICS radio repeater environment, the feedback signal has passed the fast fading environment and is slow. Since it may pass through the environment of fading, that is, the environment of the channel is greatly changed, there is a limit in applying it to the actual situation.

On the other hand, as shown in Fig. 4, the channel estimation method of the present invention uses the output signal of the LMS is the information of the feedback signal, it is determined that the change in the signal power is fast fading if the change is quite severe over time If the signal power changes slowly with time, it is determined to be slow fading.

The estimation of the channel is slow fading or rapid change over time by measuring the average of variances over a certain period of the feedback signal. In the opposite case it is determined to be slow fading.

By using the present invention by varying the Doppler frequency of the feedback channel while measuring the SNR of the ICS wireless repeater, it can be represented as shown in FIG.

The dotted circle is the point where you change the Doppler frequency of the feedback channel. In the circle of the first dotted line, change the Doppler frequency to 10 Hz, and in the second circle, change the Doppler frequency of the 100 Hz to 10 Hz feedback channel. A variable step size LMS filter using only a fixed step size and lambda _fast and a variable step size LMS filter of the present invention are compared.

When the Doppler frequency is 10 Hz, the variable step size LMS filter of the present invention initially converges with the tracking as slow fading, and the performance of the SNR continues to improve until the Doppler frequency changes by 100 Hz.

When the Doppler frequency is set to 100 Hz, the variable step size LMS filter of the present invention is more stable than the other step sizes.

The Doppler frequency of the feedback channel is changed from 100Hz to 10Hz alternately. In practice, when the Doppler frequency is about 100 Hz, a phenomenon occurs when a signal is reflected by a vehicle or other moving medium around it, and the characteristics of the channel change in a short time.

Based on this result, the system using the variable step size LMS filter of the present invention does not diverge under any circumstances of the feedback channel. For this reason, ICS RF repeaters can improve LMS performance in the case of slow fading while keeping the feedback channel stable at all times regardless of Doppler frequency.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined not only by the scope of the following claims, but also by the equivalents of the claims.

Claims (2)

In the interference cancellation method in the same frequency radio repeater in a marine telematics environment using a linear mean square filter (LMS),
Estimating a channel state of the feedback signal based on an output signal of the least mean square (LMS) filter, which is a feedback signal; And
Controlling the step size of the LMS filter based on the estimated channel state of the feedback signal to remove the feedback signal;
Wherein the estimating step comprises:
Determining fading when the power change of the output signal changes over a predetermined change level as time passes; And
Determining a slow fading when the power change of the output signal changes below a predetermined change level over time.
Method of eliminating interference in a co-frequency repeater. The method of claim 1,
The channel state estimation measures a variance average of the output signal for a preset period, determines fast fading when the measured variance average is greater than or equal to a predetermined value, and slow fading when the measured variance average is less than a predetermined value. Deciding
Method of eliminating interference in a co-frequency repeater.

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