Description

INTERFERENCE CANCELLATION APPARATUS AND METHOD IN WIBRO REPEATER

Technical Field

[1] The following description relates to an interference cancellation system (ICS), and more particularly, to feedback channel interference cancellation for processing interference signals caused by the echo and feedback of output signals in a Wibro repeater. Background Art

[2] Interference cancellation systems (ICSs) are used to cancel interference signals caused by the echo and feedback of signals output from repeaters so that only original signals without interference signals can be processed.

[3] Since the input and output terminals of a general wireless repeater operate at the same frequency band, the output signals of the repeater may be fed back to the input terminal, or multi-path signals, etc. formed by reflection of the output signals to obstacles or moving objects, etc. may be fed back to the input terminal, when isolation between the antennas at the input and output terminals is not sufficient.

[4] As such, in the case where an output signal is fed back to the input terminal through a feedback channel, signal-to-noise ratio (SNR) may deteriorate or the system may become unstable or oscillate. Accordingly, a repeater has to perform preprocessing on received signals in order to relay only original input signals without interference signals.

Disclosure of Invention Technical Problem

[5] Accordingly, disclosed is an interference cancellation apparatus and method for cancelling interference from an input signal, by estimating the characteristics of a feedback channel based on the input and output signals to estimate an echoic signal and subtracting the echoic signal from the input signal, in a Wibro repeater for relaying a received signal based on orthogonal frequency division multiplexing (OFDM).

[6] Also, disclosed is a Wibro repeater for relaying an input signal after estimating the characteristics of a feedback channel based on the input and output signals to estimate an echoic signal and cancelling the echoic signal from the input signal. Technical Solution

[7] According to an aspect of the present invention, there is provided a Wibro repeater for relaying a received signal based on Orthogonal Frequency Division Multiplexing (OFDM), including: an interference cancellation unit cancelling an interference signal

calculated through feedback channel estimation from the received signal, in a manner to extract an original signal from the received signal by estimating characteristics of the feedback channel using a pilot signal included in the received signal; and an amplifier amplifying the extracted original signal and transmitting the result of the amplification.

[8] In a preamble period of the received signal, the interference cancellation unit estimates the characteristics of the feedback channel by subtracting an original signal calculated based on pre-known information from the received signal, and in a data period of the received signal, the interference cancellation unit estimates the characteristics of the feedback channel by estimating an original signal using the pilot signal, and subtracting the estimated original signal from the received signal.

[9] The interference cancellation unit includes: a preamble detector detecting preamble components of the received signal to obtain a synchronizing signal of the received signal and a pattern of the preamble components of the received signal; a map decoder decoding a uplink or downlink map message to obtain information about a location and modulation value of a pilot assigned to each OFDM symbol of the received signal; a reference signal generator generating a reference signal using the synchronizing signal and the information about the location and modulation value of the pilot; a pilot signal processor processing the pilot signal included in the received signal using the reference signal and the synchronizing signal; and a channel estimator estimating the characteristics of the feedback channel using the synchronizing signal, the reference signal and the processed pilot signal.

[10] If the received signal uses a partial frequency band, the Wibro repeater further includes a pseudo noise generator for adding pseudo noise to the received signal.

[11] According to another aspect of the present invention, there is provided an interference cancellation apparatus for cancelling interference components of a feedback channel, in a Wibro repeater for relaying a received signal based on Orthogonal Frequency Division Multiplexing (OFDM), the interference cancelling apparatus includes: a preamble detector detecting preamble components of the received signal to obtain a synchronizing signal of the received signal and a pattern of the preamble components of the received signal; a map decoder decoding a uplink or downlink map massage to obtain information about a location and modulation value of a pilot assigned to each OFDM symbol of the received signal; a reference signal generator generating a reference signal using the synchronizing signal and the information about the location and modulation value of the pilot; a pilot signal processor processing a pilot signal included in the received signal using the reference signal and the synchronizing signal; and a channel estimator estimating characteristics of the feedback channel using the synchronizing signal, the reference signal and the processed pilot

signal.

[12] The pilot signal processor estimates the characteristics of the feedback channel, using a pilot signal remaining after performing orthogonal projection on the received signal in a direction of pilot components, and the channel estimator estimates the characteristics of the feedback channel using a Least Mean Square (LMS) algorithm, after subtracting an original signal from the received signal using the synchronizing signal, the reference signal and the processed pilot signal.

[13] According to another aspect of the present invention, there is provided an interference cancellation method for cancelling interference components of a feedback channel, in a Wibro repeater for relaying a received signal based on orthogonal frequency division multiplexing (OFDM), the interference cancellation method including: detecting preamble components of the received signal to obtain a synchronizing signal of the received signal and a pattern of the preamble components of the received signal; decoding a uplink or downlink map massage to obtain information about a location and modulation value of a pilot assigned to each OFDM symbol of the received signal; generating a reference signal using the synchronizing signal and the information about the location and modulation value of the pilot; processing a pilot signal included in the received signal using the reference signal and the synchronizing signal; and estimating characteristics of the feedback channel using the synchronizing signal, the reference signal and the processed pilot signal.

Advantageous Effects

[14] Therefore, a Wibro repeater according to the present invention can easily cancel interference signals through a simple structure. That is, when an interference cancellation method according to the present invention is applied over partial frequency bands and full frequency bands, the simulation result shows that RCE values are smaller than in the case of cancelling interference only using the conventional Least Mean Square (LMS) algorithm. Brief Description of the Drawings

[15] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention.

[16] FIG. 1 is a view for explaining a phenomenon where signal interference occurs by a feedback channel in a Wibro repeater.

[17] FIG. 2 is a block diagram of a repeater including an interference cancellation unit according to an embodiment.

[18] FIG. 3 is a detailed block diagram of the interference cancellation unit illustrated in

FIG. 2.

[19] FIG. 4 shows a frequency spectrum of a received signal when a projection-based

Least Mean Square (LMS) algorithm is applied on the received signal using pilot signals.

[20] FIG. 5 is a block diagram of a repeater including a pseudo noise generator and an interference cancellation unit, according to another embodiment.

[21] FIG. 6 shows a frequency spectrum of a received signal when the projection-based

LMS algorithm is applied on the received signal using pseudo noise inserted into the received signal.

[22] FIG. 7 is a flowchart of an interference cancellation method according to an embodiment. Mode for the Invention

[23] The invention is described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like reference numbers in the drawings denote like elements.

[24] FIG. 1 is a view for explaining a phenomenon where signal interference occurs by a feedback channel in a Wibro repeater.

[25] Referring to FIG. 1, a repeater 100 receives and amplifies a downlink signal from a base station 110 and then transfers the downlink signal to a plurality of mobile terminals 120a, 120b and 120c. Also, the repeater 100 receives and amplifies a plurality of uplink signals from the mobile terminals 120a, 120b and 120c and then transfers the uplink signals to the base station 110.

[26] During this process, the downlink signal processed and amplified by the repeater 100 has to be transferred only to the respective mobile terminals 120a, 120b and 120c, but the output signal (that is, the downlink signal) of the repeater 100 may be fed back to the input terminal of the base station 110, due to echoes caused by insufficient isolation between antennas at the input and output terminals of the repeater 100 or due to the reflection of the output signal to obstacles, etc.

[27] FIG. 2 is a block diagram of a repeater 100 including an interference cancellation unit 210 according to an embodiment.

[28] Referring to FIG. 2, the repeater 100 includes an interference cancellation unit 210, a delay unit 220, and an amplifier 230. The interference cancellation unit 210 estimates

the characteristics of a feedback channel 150 using pilot components included in a received signal, calculates an interference signal through the feedback channel estimation, and then cancel the interference signal from the received signal, thereby restoring an original input signal.

[29] That is, the interference cancellation unit 210 estimates the characteristics of the feedback channel 150 by subtracting a pre-known original signal from the received signal in a preamble period of the received signal, and in a data period of the received signal, estimates the characteristics of the feedback channel 150 by estimating an original signal using the pilot components of the received signal and subtracting the estimated original signal from the received signal. The interference cancellation unit 210 may be a finite impulse response (FIR) filter, and the detailed construction and operation of the interference cancellation unit 210 will be described later with reference to FIG. 3.

[30] The delay unit 220 delays a signal that is to be input to the interference cancellation unit 210, and matches the timing of the delayed signal with the timing of a signal input to the repeater 100. The amplifier 230 amplifies the restored original signal from which interference has been cancelled by the interference cancellation unit 210, and transmits the amplified original signal.

[31] FIG. 3 is a detailed block diagram of the interference cancellation unit 210 illustrated in FIG. 2.

[32] Referring to FIG. 3, the interference cancellation unit 210 includes a preamble detector 310, a reference signal generator 320, a pilot signal processor 330, a channel estimator 340, and a map decoder 350.

[33] The preamble detector 310 detects preamble components of a received signal to obtain a synchronizing signal of the received signal and a pattern of the preamble components. The map decoder 350 decodes an uplink or downlink map message to obtain information about the location and modulation value of a pilot included in each orthogonal frequency division multiplexing (OFDM) symbol of the received signal. The reference signal generator 320 generates a reference signal, using the synchronizing signal and the locations and modulation values of pilots included in the OFDM symbols of the received signal.

[34] The process of generating the reference signal will be described in detail below. The reference signal is generated in different ways in the preamble and data periods. In the preamble period, a reference signal is generated by detecting the pattern of preamble components. In the data period, a reference signal is an OFDM signal consisting of only pilot frequency components except for data frequency components. That is, in the data period, a reference signal is generated for each data symbol, by performing Binary Phase Shift Key (BPSK) modulation using the modulation values of pilots obtained

from the uplink or downlink map message to obtain BPSK symbols, mapping the BPSK symbols respectively to the pilot components obtained from the map message, performing Inverse Fast Fourier Transform (IFFT) on the result of the mapping, and then inserting Cyclic Prefixes (CPs) respectively into the results of the IFFT.

[35] The pilot signal processor 330 processes a pilot signal included in the received signal, using the reference signal and the synchronizing signal. Here, the pilot signal processor 330 performs orthographic projection on the received signal in the direction of the pilot components, thus extracting only the pilot signal from the received signal.

[36] Then, the channel estimator 340 estimates the characteristics of a feedback channel, using the synchronizing signal, the reference signal and the pilot signal. In more detail, the channel estimator 340 subtracts an original signal from the received signal and then estimates the characteristics of the feedback channel using the Least Mean Square (LMS) algorithm. In other words, the channel estimator 340 estimates the characteristics of the feedback channel by estimating a coefficient of a FIR filter for modeling a feedback channel using the LMS algorithm.

[37] FIG. 4 shows a frequency spectrum of a received signal when a projection-based

Least Mean Square (LMS) algorithm is applied on the received signal using pilot signals.

[38] Referring to FIG. 4, when white noise is added to an original signal consisting of data components and pilot components, by performing orthographic projection on the original signal in the direction of the pilot components so as to leave only the pilot components and noise of pilot frequency bands, and analyzing the resultant signal at the pilot frequency bands, the characteristics of a feedback channel can be estimated.

[39] FIG. 5 is a block diagram of a repeater 100 including a pseudo noise generator 250 and an interference cancellation unit 210, according to another embodiment.

[40] The repeater 100 illustrated in FIG. 5 further includes the pseudo noise generator

250, differently from the repeater 100 illustrated in FIG. 2. The pseudo noise generator 250 generates pseudo noise for a received signal and adds the pseudo noise to the received signal, when the received signal uses partial frequency bands.

[41] In other words, since the interference cancellation apparatus according to the current embodiment estimates a feedback channel using only a pilot signal, except for the preamble period, the interference cancellation capability at partial frequency bands is lower than that at full frequency bands. Accordingly, in the current embodiment, pseudo noise is added to empty areas where no data or signal exists so that the interference cancellation capability of signals of partial frequency bands having a relatively small number of pilot subcarriers can be improved.

[42] A process of generating and adding pseudo noise will be described in detail below.

First, the locations (that is, frequencies) of pilot subcarriers and data subcarriers are

detected from the symbols of a received signal. Then, Quadrature Phase Shift Keying (QPSK) symbols having an energy level required for amplification are generated, and the QPSK symbols are mapped to the remaining subcarriers except for the data, pilot subcarriers and guide band subcarriers. Then, Inverse Fast Fourier Transform (IFFT) is performed on the results of the mapping, and Cyclic Prefixes (CPs) are inserted into the results of the IFFT, so that OFDM symbols are generated. The generated pseudo noise (that is, the OFDM symbols) is added to the output signal of the repeater 100, generated at the same time as when the pseudo noise is generated.

[43] FIG. 6 shows a frequency spectrum of a received signal when the projection-based

LMS algorithm is applied on the received signal using pseudo noise inserted into the received signal.

[44] Referring to FIG. 6, when white noise is added to an original signal consisting of data components and pilot components, by performing orthographic projection on the original signal in the direction of a guide frequency band and frequency bands including pseudo noise, the pilot components and data components are removed, and pseudo noise and noise of the guide frequency band remain. Then, by analyzing the noise signal, the characteristics of a feedback channel for the remaining frequency components except for the pilot components can be estimated accurately.

[45] FIG. 7 is a flowchart of an interference cancellation method according to an embodiment.

[46] First, preamble components of a received signal are detected to obtain a synchronizing signal of the received signal and a pattern of the preamble components (operation S710). Then, a downlink or uplink map massage is decoded, so that information about the locations and modulation values of pilots included in the received signal is obtained (operation S720). Next, a reference signal is generated using the synchronizing signal (operation S730). Then, a pilot signal included in the received signal is processed using the reference signal and the synchronizing signal (operation S740). In operation S740, orthographic projection is performed on the received signal in the direction of the pilot components so as to leave only the pilot components.

[47] Then, the characteristics of a feedback channel are estimated using the synchronizing signal, reference signal, and processed pilot components (operation S750). That is, the characteristics of the feedback is estimated by subtracting a pre-known original signal from the received signal in the preamble period of the received signal, and in the data period of the received signal, by estimating an original signal using the pilot signal, and subtracting the estimated original signal from the received signal. The respective operations of the interference cancellation method according to the current embodiment correspond to the operations of the respective elements of the interference cancellation apparatus as described above with reference to FIGS. 2 and 3.

[48] The interference cancellation method can be written as a program. Program codes and code segments constructing the program can be easily inferred by computer programmers skilled in this art. The programs can be stored in a computer-readable recording medium. The computer-readable medium includes a magnetic recording medium, an optical recording medium, and a carrier- wave medium.

[49] It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. Industrial Applicability

[50] As described above, an interference cancellation apparatus and method according to the present invention can be applied to wireless communications systems and repeaters.