KR20050003904A - Interference Minimized OFDM Based Wireless Communication System and Method Therefor - Google Patents

Abstract

PURPOSE: OFDM(Orthogonal Frequency Division Multiplexing) wireless communication system and method having minimized interference are provided to suppress high power interferences by using an adaptive subcarrier allocation method. CONSTITUTION: An OFDM wireless communication system having minimized interference includes a transmitting terminal and a receiving terminal. The transmitting terminal receives and series/parallel converts(202) a source signal, inserts(204) a pilot signal, performs an IFFT(206) for modulation, inserts(208) a protection interval(208), performs a parallel/series conversion(210), a digital/analog conversion, and transmits the result over a wireless channel. The receiving terminal receives analog/digital converts a signal from the transmitting terminal, removes the protection interval(214), performs an FFT(216) for demodulation, equalizes(218) the result of FFT, and parallel/series converts(220) the result of the equalizer to output. The receiving terminal includes a received signal analyzer(222) which performs the equalization process when an output of the FFT is less than a predetermined threshold value while transmitting interference signal generation subcarrier information to the transmitting terminal. The transmitting terminal suppresses a subcarrier with which the interference signal is generated.

Description

Interference Minimized OFDM Based Wireless Communication System and Method Therefor}

The present invention relates to a wireless communication system of orthogonal frequency division multiplexing, and more particularly, orthogonal frequency division to minimize interference for improving transmission characteristics by minimizing interference between homogeneous or heterogeneous systems wirelessly communicating within a specified shared frequency. The present invention relates to a multiplex wireless communication system and a method thereof.

Orthogonal Frequency Division Multiplexing (hereinafter, referred to as 'OFDM') has been actively studied as a method suitable for high-speed data transmission in wired and wireless channels. When using a single carrier method for high-speed data transmission with short symbol periods in a wireless communication channel with multipath fading, the inter-symbol interference becomes more severe, whereas the complexity of the receiving end is greatly increased. Since the symbol period in each subcarrier can be extended by the number of subcarriers while maintaining the same, the simple equalizer can cope with the severe frequency selective fading channel by multipath with a simple equalizer.

In the OFDM scheme, since a plurality of carriers having mutual orthogonality are used, the frequency utilization efficiency is increased, and the process of modulating and demodulating the plurality of carriers at the transceiver stage is performed by inverse discrete Fourier transform (IDFT) and discrete Fourier transform (DFT), respectively. The result is a high speed implementation using an inverse fast Fourier transform (IFFT) processor and a fast Fourier transform (FFT) processor.

Meanwhile, in order to provide high-speed wireless communication service, efforts are being made to develop and standardize technologies in the IEEE 802.11 in the US, ETSI (HIPERLAN / 2) in Europe, and MMAC in Japan. The existing IEEE 802.11 WLAN supports a 2 Mbps data rate in the 2.4 GHz ISM band using the direct spread communication (DSSS), frequency hopping spread (FHSS), and infrared (IR) methods. However, these standards are not able to meet the increasing demand for higher transmission speeds, and new physical layer standards of IEEE 802.11a and IEEE 802.11b have been finalized. In particular, IEEE 802.11a adopts an OFDM modulation scheme to overcome the limitations of the DSSS scheme in the frequency range of 5 GHz and to obtain a higher transmission rate.

1 is a view for explaining a conventional OFDM wireless communication system.

In the OFDM wireless communication system 100, a bit string input to a transmitting end is encoded by a predetermined modulation scheme and then input to a serial / parallel converter 102 to be converted into a parallel data form, and the pilot symbol inserter 104 After the pilot symbols are inserted, they are input to the IFFT processing unit 106 and converted into inverse fast Fourier transforms. Subsequently, a guard interval longer than a channel impulse response is inserted in the guard interval inserter 108 to maintain orthogonality of subcarriers and to remove intersymbol interference, and then converts it to serial data in the parallel / serial converter 110. Digital / analog conversion is transmitted to the receiver through a wireless channel.

The receiving end converts data received from the transmitting end through a wireless channel into analog / digital conversion and converts serial data into parallel data by the serial / parallel conversion unit 112. The guard interval remover 114 removes the guard interval from the signal converted into parallel data, and the FFT processor 116 performs a fast Fourier transform to demodulate the signal. The demodulated signal is input to the equalization processor 118, where the pilot symbol is used to compensate for non-ideal characteristics, ie, various noises, interference between adjacent channels, and multipath. After the noise, distortion, and the like are compensated, the parallel / serial converter 120 outputs the serial data.

However, in the design of such an OFDM wireless communication system, a plurality of wireless systems are shared within a predetermined frequency between homogeneous and heterogeneous systems, and a problem due to frequency allocation for heterogeneous systems has emerged. There is a demand for a technique for investigating the amount of interference between heterogeneous systems and reducing / removing interference within a frequency band. In the present invention, an interference signal is defined as a signal that affects each subcarrier of an OFDM signal with an irregular size.

In order to solve this problem, researches and analyzes problems in frequency sharing are proposed, and several techniques are proposed to effectively reduce interference. In the OFDM-based system, the channel coding method or the IS is used to reduce the influence of the interference signal. To reduce the interference by the interference mitigation technique using OFDM (Interference Suppressing OFDM).

2A and 2B are diagrams for describing an interference minimization method using a channel encoding method in an OFDM wireless communication system, and show a case of channel coding at a data rate of 1/2 at a 10 MHz interference signal and a case in which no coding is performed.

In this measurement example, the signal bandwidth is 20 MHz, the number of subcarriers is 64, the interval of subcarriers is 312 KHz, QPSK modulation, the bandwidth of the interference signal is 10 MHz, and the channel environment is AWGN (Additive White Gaussian Noise). ), And the jamming-to-signal ratio (hereinafter referred to as 'JSR') was changed to -10, 0, and 10 dB.

As shown in FIGS. 2A and 2B, when the JSR is -10 dB, the influence of the interference decreases as the OFDM signal power increases. However, when the JSR is 0 dB or more, the interference signal generates continuous errors in the subcarriers, so there is no performance improvement due to channel coding, and it is a factor that has a constant BER performance.

FIG. 3 is a diagram for describing a method for minimizing interference by a channel encoding method in an IS-OFDM wireless communication system, and measurement conditions are the same as those of FIG. 2.

The IS-OFDM regenerates data by spreading the number of subcarriers using Walsh codes and then parallelizing them again by a specified number (N). As N data is spread on each subcarrier, the influence of interference signal on one data is also reduced. This is a gain that is obtained by distinguishing the characteristics of the Walsh code, that is, the magnetic signal and reconstructing the signal by N times when demodulated at the receiver, so that it can have higher power than the interference signal. However, this method has a disadvantage in that BER performance cannot be improved when the power of the interference signal is large (when JSR is 10 dB in FIG. 3).

As described above, in the OFDM wireless communication system, when the power of the interference signal is strong, interference between heterogeneous systems cannot be improved by channel coding or IS-OFDM.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and the receiving end of the OFDM wireless communication system estimates an error-prone subcarrier according to an interference signal and informs the transmitting end of the subcarrier so that the interference does not use for signal transmission As a result, there is a technical problem to effectively remove the strong interference to improve the transmission characteristics in the wireless communication system.

1 is a view for explaining a wireless communication system of the general OFDM scheme;

2A and 2B are diagrams for describing an interference minimization method by a channel encoding method in an OFDM communication wireless communication system;

3 is a view for explaining an interference minimization method by a channel encoding method in an IS-OFDM wireless communication system;

4 is a configuration diagram of a wireless communication system of the OFDM method with minimized interference according to the present invention;

5 is a flowchart illustrating a method for minimizing interference in an OFDM communication wireless communication system according to the present invention;

6a and 6b are views for explaining a signal processing process by the interference minimization method according to the present invention;

7 is a view for explaining the BER performance after applying the interference minimization method according to the present invention.

<Description of the symbols for the main parts of the drawings>

100, 200: OFDM wireless communication system 102, 202: serial / parallel conversion unit

104, 204: pilot symbol insertion unit 106, 206: ITTF processing unit

108, 208: guard section insertion section 110, 210: bottle / serial conversion section

112, 212: serial / parallel conversion section 114, 214: protection section removal section

116, 216: FFT processor 118, 218: equalizer

120, 220: bottle / serial converter 222: receiving signal analyzer

The present invention provides a source signal and a radio channel section in an OFDM receiver that receives a signal transmitted from an OFDM transmitter in order to remove a large amount of power generated by sharing a frequency between homogeneous or heterogeneous systems in an OFDM wireless communication system. Analyze the interference component (especially narrowband interference signal) included in the AMS. If the received signal sample size is larger than the specified threshold, the interference is considered to be notified and the transmitter is not transmitted to the corresponding subcarrier. . From the subsequent data transmission, the transmitter does not transmit data on the corresponding subcarrier, thereby minimizing the influence of interference.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

4 is a block diagram of a wireless communication system of the OFDM method with the interference minimized according to the present invention.

In the OFDM wireless communication system 200, a bit string input to a transmitting end is encoded by a predetermined modulation scheme and then input to a serial / parallel converter 202 to be converted into a parallel data form, and the pilot symbol inserter 204 After the pilot symbols are inserted, they are input to the IFFT processor 206 and are inverse fast Fourier transformed. Subsequently, a guard interval longer than a channel impulse response is inserted in the guard interval inserter 208 to maintain orthogonality of the subcarriers and remove intersymbol interference, and the parallel / serial converter 210 converts the serial data into serial data. / Analog conversion and transmission to the receiver through wireless channel.

In this way, the signal output from the transmitting end is shown in [Equation 1].

Here, b _{l, k} denotes a symbol in the k-th subcarrier and the l-th frame, T _s = T + T _g denotes an entire time interval, T denotes a symbol period, and T _g denotes a guard interval. In addition, the sample in the time interval is represented by T _N = T / N, f (t) is a square wave having a value of 1 from the interval 0 to T _s .

The signal s (t) output from the transmitter is affected by an interference signal in a wireless environment. In the present invention, it is assumed that there is no intersymbol interference and intercarrier interference component, and only a narrowband interference signal is applied. Narrow-band interference signal is a signal size that uniformly interferes the entire signal interval, and the frequency interference band is defined as a narrow band smaller than that of the OFDM signal, and the interference signal source is generated as a signal sampled as a guard interval. The channel environment is frequency-flat fading. Accordingly, the signal r (t) received through the wireless channel is defined as follows.

Here, I (t) is a narrowband interference signal, the bandwidth of the interference signal is defined as a subcarrier and has a band smaller than the band of the OFDM signal, and n (t) is AWGN (Additive White Gaussian Noise). Gaussian noise). And the signal sampled at t _n = nT _N is shown in [Equation 3].

The receiving end converts the serial data r (n) received from the transmitting end through a wireless channel into analog / digital conversion and then receives it from the serial / parallel converter 212 and converts the serial data r (n) into parallel data. The guard interval remover 214 removes the guard interval from the signal converted into parallel data, and the FFT processor 216 demodulates the signal by performing a fast Fourier transform. A signal obtained by removing N guard signals from the l-th frame (or OFDM sample) from which the guard interval is removed is represented by a vector and is represented by Equation 4 below.

Here, r _{n, l} is r ((n + N _g + lN _s ) T), which is a signal modulated by the FFT processing unit 216. Assuming that the environment of the channel does not change in the interval of the OFDM frame, the output value of the FFT processor 216 in the l-th frame and the k-th subcarrier is expressed by Equation 5 below.

From here, Where H _{l, k} is the channel transfer function at the frequency of the subcarrier f _k = k / T and during the l-th frame, and η denotes noise generated in the l-th frame and the k-th subcarrier. In addition, I _{l, k} and η _{l, k} are also output values of the FFT processing unit 216. The channel transfer function H _{l, k} is expressed by considering the transmission filter and the channel response as follows.

The signals output from the FFT processing unit 216 are input to the equalization processing unit 218, where the pilot symbols are used to compensate for non-ideal characteristics, ie, various noises, interference between adjacent channels, and multipath. do. After the noise, distortion, and the like are compensated, the parallel / serial converter 220 outputs the serial data.

The power of the interference signal with respect to the OFDM signal is represented by JSR and is represented by Equation 7 below.

Where P _I is the average power of the interfering signal and P _S is the average power transmitted. This affects the constant interference power ratio for OFDM. The total power in the received signal is then P _Z = P _U + P _I + P _η , P _U is the average power of the useful OFDM signal received with the interference signal added, P _η is the average noise power, and for all subcarriers Has The signal-to-noise ratio (SINR) for analyzing the performance of the OFDM signal by the interference signal is as follows.

From here, Is the normalized value of the useful signal in each channel value. L is a value smaller than the number N of subcarriers.

The sample value appearing in each subcarrier of the OFDM signal received in the AWGN environment changes differently from the size of the sample transmitted by the AWGN. Therefore, the size of the received signal sample that varies according to the transmission power of the OFDM signal is analyzed and the threshold is determined based on the data having the largest error by the noise power. The determined threshold is defined as K, and is expressed by Equation 5 without considering the narrowband interference signal.

This process is determined by the received signal analyzer 222 that receives the output value of the FFT processor 216 of the receiver. The received signal analyzer 222 extracts samples of a signal that is equal to or larger than a specified threshold value among the output values of the FFT processor 216, and transmits the samples to the serial / parallel converter 202 at the front end of the IFFT processor 206 for interference. Notify which frequency band the signal is located. In this sense, the present invention may be referred to as an adaptive subcarrier assignment method.

That is, the OFDM signal received in the environment to which the narrowband interference signal defined in Equation 5 is added affects a specific subcarrier, and when the power of the interference signal is large, the sample value of the signal having a value as large as the band of the interference signal. Will be detected. That is, it is analyzed in the equation defined in Equation 5, and in the received signal, the sample value represented by the size larger than the envelope of the source signal affecting only a specific subcarrier of OFDM can be seen.

5 is a flowchart illustrating a method for minimizing interference in an OFDM wireless communication system according to the present invention.

The bit string input to the transmitting end of the OFDM wireless communication system is encoded by a predetermined modulation scheme and then input to the serial / parallel converter 202 (S101), converted into a parallel data form (S102), and a pilot symbol is inserted. After (S103), the inverse fast Fourier transform is performed (S104).

Subsequently, the guard interval inserting unit 208 inserts a guard interval to maintain the orthogonality of the subcarriers and remove the intersymbol interference (S105), and converts it into serial data in the parallel / serial converter 210 and converts it into digital / analog. The signal is converted and transmitted to the receiving end through the wireless channel (S106).

When transmitting a signal from a transmitter to a receiver, a narrowband interference signal generated in a radio channel section is included in the source signal. In the present invention, it is assumed that the intersymbol interference component and the intercarrier interference component are not considered in addition to the narrowband interference signal. .

The receiving end receiving the signal including the narrowband interference signal in the source signal converts the serial data received from the transmitting end through the wireless channel into analog data, converts the serial data into parallel data, and then removes the guard interval from the guard interval removing unit 214. (S107). The FFT processor 216 demodulates the signal by performing fast Fourier transform on the signal from which the guard interval is removed (S108). Accordingly, the signal output from the FFT processing unit 216 becomes as shown in [Equation 5].

After the signal is demodulated, the received signal analyzer analyzes the samples of all demodulated received signals to determine whether the size of the received signal samples is greater than or equal to a specified threshold (S109). That is, the result value of [Equation 5] is compared with the specified threshold value. At this time, the specified threshold is a value determined by the above-described Equation (9).

If the sample size of the received signal is less than the specified threshold, the received signal is input to the equalization processor 118, and the pilot symbol is used to detect non-ideal characteristics such as various noises, interference between adjacent channels, and multipath. Compensation (S110), perform parallel / serial data conversion (S111), and output in serial data form (S112).

If it is determined in step S109 that the sample size of the received signal is greater than or equal to the specified threshold, the received signal analyzer 222 determines that an error has occurred and the serial / parallel converter 202 in front of the IFFT processor 206 of the transmitter. Informs the location of the subcarrier where the interference is located (S113). Accordingly, the transmitter does not transmit data on the subcarrier where interference is located when transmitting the next data.

6A and 6B are diagrams for describing a signal processing process by an interference minimization method according to the present invention.

It represents the sample size of OFDM subcarriers when interference signals are added in the frequency domain, and when the above-mentioned threshold value (K value) is applied to the received signal as shown in [Equation 5], subcarriers larger than the threshold value can be estimated. Will be.

6A and 6B illustrate a process of estimating subcarriers in which an error occurs through a determined K value and informing a position of a subcarrier in which an error occurs when the next data is transmitted, thereby not transmitting data to a subcarrier in which interference occurs.

7 is a view for explaining the BER performance after applying the interference minimization method according to the present invention. The measurement conditions were the signal bandwidth of 20 MHz, the number of subcarriers 64, the subcarrier spacing 312 KHz, the QPSK modulation, and the bandwidth of the interference signal 10 MHz, and the channel environment AWGN (Additive White Gaussian Noise).

As shown, when the JSR changes to -10, 0, 10 dB, it can be seen that the BER performance is remarkably improved compared to that shown in FIG. 3 even when the power of the interference signal is large.

In the present invention described above, the degree of estimation according to the JSR is different. When the JSR is large, the position of the interference signal can all be estimated, but when the JSR is 2 dB or less, it is difficult to estimate the threshold applied. As the transmission power of OFDM increases, the received signal decreases the influence on the AWGN, so that the size of the received signal is closer to the size of the transmitted signal and the level at which the threshold value is also lowered. Therefore, the present invention has the greatest advantage for the case where the power of the interference signal is large. If the entire band of OFDM is B _T and the band of the interference signal is B _I , the bandwidth efficiency becomes (B _T -B _I ) / B _T , and the data rate is also followed. The transmission rate will also increase.

As such, those skilled in the art will appreciate that the present invention can be implemented in other specific forms without changing the technical spirit or essential features thereof. Therefore, the above-described embodiments are to be understood as illustrative in all respects and not as restrictive. The scope of the present invention is shown by the following claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present invention. do.

As described above, according to the present invention, in order to remove a large amount of interference caused by sharing a frequency between homogeneous or heterogeneous systems in an OFDM wireless communication system, an OFDM receiver receives a signal transmitted from an OFDM transmitter. Analyzes the interference components (especially narrowband interference signals) included in the signal and radio channel intervals, and if the received signal sample size is larger than or equal to a specified threshold, the interference is considered to be generated and the signal is not transmitted on the corresponding subcarrier. According to the so-called adaptive subcarrier allocation method of notifying the transmitting end, if the power of the interference signal is large, the interference can be effectively removed to improve data transmission performance.

Claims (6)

An OFDM wireless communication system, Transmitter that receives the source signal, converts it in parallel / parallel, inserts pilot signal, performs inverse fast Fourier transform, modulates it, inserts a guard interval, performs parallel / serial conversion, digital / analog conversion, and transmits it to the wireless channel. ; And Receives the signal transmitted from the transmitter through the wireless channel, performs analog / digital conversion, performs serial / parallel conversion, removes the guard interval, performs fast Fourier transform, demodulates, and equalizes the result of the fast Fourier transform. And a receiving end for outputting the parallel / serial conversion; The signal received by the receiving end includes a narrowband interference signal inserted in the radio channel, and the receiving end compares the parallel demodulation result by the Fourier transform with a specified threshold value so that the demodulation result is equal to or less than the threshold value. A reception signal analyzer configured to perform the equalization process and to transmit interference signal generation subcarrier information on which a signal exceeding the threshold is transmitted to the transmitter, when a value exceeding the threshold is generated among the demodulation results; Further, the transmitting end by referring to the interference signal generation subcarrier information received from the received signal analysis unit, orthogonal frequency division multiplexing with minimal interference, characterized in that the sub-carrier to generate the interference signal during data transmission Wireless communication system. The method of claim 1, The threshold is a sum of a product of a data symbol and a channel transfer function of a subcarrier on which the data symbol is transmitted and an error generated in the subcarrier, and are stored in the received signal analyzer. Split wireless communication system. The method of claim 1, And the received signal analyzer transmits the interference signal generation subcarrier information in which the signal exceeding the threshold is transmitted to a serial / parallel converter of the transmitter. An interference minimization method in an OFDM wireless communication system including an OFDM transmitter and an OFDM receiver, Receiving, at the OFDM transmitter, a source signal in serial data form, converting the source signal into parallel data form, and inserting a pilot symbol; Inverse fast Fourier transforming and modulating the data sequence into which the pilot symbols are inserted; Inserting a guard interval after performing the modulation process, converting the data into serial data, and then digitally / analog converting the data to be transmitted over a wireless channel; At the OFDM receiving end received through the wireless channel, converting data into analog / digital data and converting the data into parallel data and removing the guard interval; Demodulating the signals by performing fast Fourier transform on the signals from which the guard intervals are removed; Comparing an absolute value of each demodulation result with a specified threshold value; When the at least one of the absolute values of the demodulation result value exceeds the threshold, transmitting the interference signal generating subcarrier information to which the demodulation result value is transmitted to the OFDM transmitter; And Equalizing the demodulated received signal when the absolute value of the demodulation result value is less than or equal to the threshold value, performing parallel / serial data conversion, and outputting the serial data in serial data form; Method of minimizing interference in orthogonal frequency division multiplexing wireless communication system comprising a. The method of claim 4, wherein The threshold value is a method of minimizing interference in the orthogonal frequency division wireless communication system, characterized in that the product of the sum of the channel transfer function of the data symbol and the subcarrier to which the data symbol is transmitted and the error occurred in the subcarrier. The method of claim 4, wherein The OFDM receiver transmits interference signal generation subcarrier information in which a signal exceeding the threshold is transmitted before performing the serial / parallel conversion in the OFDM transmitter, before performing the inverse fast Fourier transform in the OFDM transmitter. The interference minimization method in the orthogonal frequency division multiplexing wireless communication system characterized by referring to the interference signal generation subcarrier information.