KR20070089107A - Stfbc-ofdm system and its communication method - Google Patents

Abstract

An STFBC-OFDM system and a communication method thereof are provided to preserve the orthogonality of codes by reducing the size for a repeatedly transmitted signal and to reduce interference in a decoding process. A STFBC-OFDM system includes a transmitter(100) and a receiver(200). The transmitter(100) includes a SP(Serial/Parallel) converter(101), an encoder(103), inverse fast Fourier transformers(105,105'), parallel/serial converters(107,107'), and CP(Cyclic Prefix) inserters(109,109'). The receiver(200) includes a CP remover(201), an S/P converter(203), a fast Fourier transformer(205), a decoder(207), and a P/S converter(209). The SP converter(101) converts serial data symbols into parallel data symbols and supplies the parallel data symbols to the encoder(103). The PS(Parallel/Serial) converters(107,107') convert a parallel data vector operated as a baseband inputted from IFFT(Inverse Fast Fourier Transformers)(105,105') to a serial data vector and supplies the converted vector to the CP inserters. The CP remover removes a CP signal from the received data vector. The S/P converter converts data vector with no CP signal to the parallel data vector. The FFT performs the FFT operation of the parallel data vector as an OFDM reception symbol. The decoder decodes the OFDM reception symbol by using subcarrier data. The P/S converter converts decoded parallel OFDM reception symbol signals to the serial signal.

Description

STFBC-OFDM system and its communication method {STFBC-OFDM SYSTEM AND ITS COMMUNICATION METHOD}

The present invention relates to a Space Time Frequency Block Code (STFBC) -Orthogonal Frequency Division Modulation (OFDM) system and a communication method thereof, and more particularly to Multi-In Multi-Out (MIMO) -OFDM. In generating an OFDM signal, by sequentially mapping the same signal in the time domain and the frequency domain, reducing the size of the repeated transmission signal, and preserving the orthogonality of the code, the decoding process The present invention relates to a system and a method for reducing the interference generated in the system and preventing the performance degradation.

MIMO means an antenna system capable of multiple inputs and outputs. This MIMO allows a plurality of antennas to operate simultaneously to enable high speed data exchange. In other words, MIMO is a technology mainly used for Wibro, which is a mobile Internet service. To use this technology, both a base station or an access point and a terminal must support MIMO to communicate with each other.

In addition, OFDM is a digital modulation scheme for multiplexing orthogonal carrier signals, which avoids the use of high-speed equalizers, reduces multi-path fading and pulsed noise, and can make full use of available bandwidth. That is, as one type of multi-carrier transmission, a single data stream is transmitted using multiple carriers of low data rate.

The aforementioned MIMO-OFDM is called a multi-antenna orthogonal frequency multiplexing technique, and has been in the spotlight as the communication technology is rapidly developed.

On the other hand, space-time code (Space-time code) has been a lot of research as a technique for the diversity gain in a multi-antenna system. Among them, the technique called Alamouti code is evaluated as the most efficient code with simple and powerful performance (Ref. 1).

Although many studies have been conducted to apply Alamouti codes to OFDM systems, time-selective fading and frequency-selective fading are limited due to the limitation that they can be correctly recovered only when the channel values of neighboring times or adjacent frequency bands are the same. In the frequency-selective fading environment, there is a disadvantage in that performance is reduced (Documents 2 and 3).

The reason is that a simple decoding method, which is a great advantage of the Alamouti code, is applied. Although the Alamouti code can easily recover the transmission signal by using the orthogonality of the code, in the fading environment, the Intersymbol Interference (ISI) is generated during the decoding process. Efforts have been made to solve this problem because it is much larger than the Inter Carrier Interference (ICI) (Document 4, Document 5, Document 6).

Efforts to solve this problem have been studied using a space-time block code (STBC) other than the Alamouti code. That is, in the case of STBC using four antennas, the above-mentioned problem becomes more serious, and the problem is solved by mapping consecutive codes in two time and two frequency bands in four time or frequency bands (see Document 5). , Document 6).

[Documentation information of the prior art]

Literature 1. S. Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J. Select. Areas Commun., Vol. 16, pp. 1451-1458, Oct. 1998.

Document 2. K.F.Lee and D.B Williams, “A space-frequency transmitter diversity technique for OFDM systems,” in Proc. Globecom, vol. 3, pp. 1473-1477, Nov. 2000.

Literature 3. K.F.Lee and D.B Williams, “A space-time coded transmitter diversity technique for frequency selective fading channels,” in Proc. IEEE Sensor Array and Multichannel Signal Processing Workshop. 149-152, Mar. 2000.

Literature 4. J. W. Wee, J. W. Seo, K. T. Lee, Y. S. Lee and W. G. Jeon, “Successive interference cancellation for STBC-OFDM systems in a fast fading channel,” in Proc. IEEE Vehicular Tech. Conf., Vol. 2, pp. 841-844, May 2005.

Document 5. K. Suto and T. Ohusuki, “Space-time-frequency block codes over frequency selective fading channels,” IEICE Trans. Commun. vol. E87-B, No. 7, pp. 1939-1945, Jul. 2004.

Literature 6. G. Bauch, “Space-time block codes versus space-frequency block codes,” in Proc. IEEE Vehicular Tech. Conf., Pp. 567-571, May 2003.

In the prior art operating as described above, the STBC-OFDM system using the Alamouti code degrades as the value of Doppler spread increases in time-selective fading. That is, this phenomenon can be seen that the bit error rate (BER) performance of the STBC-OFDM is degraded in the time-selective fading environment shown in FIG. 1, and SFBC-OFDM in the frequency-selective fading environment shown in FIG. As can be seen that the performance of the BER decreases, the SFBC-OFDM system decreases as the value of the multipath delay spread increases in frequency selective fading.

The STBC-OFDM system and SFBC-OFDM system can be summarized as follows. First, the encoding method of STBC-OFDM uses the Alamouti code shown in FIG. Data to be subjected to the IFFT process by applying the STBC-OFDM encoding scheme is represented by Equation 1

In order to further explain the decoding process, only the k-th subcarrier signal of consecutively received OFDM blocks is represented by Equation 2

를 곱하는 디코딩과정을 적용하면 수학식 3Can be expressed as On both sides

Applying the decoding process to multiply Equation 3

It is expressed as

와 α_ST를 이용하여 송신한 신호를 검출하면 신호를 복원하는 과정이 종료된다.Here, if the channel values of two successive OFDM blocks are equal, β _ST will be '0', otherwise β _ST will act as ISI. And

When the signal transmitted by using and α _ST is detected, the process of restoring the signal ends.

Subsequently, the encoding method of SFBC-OFDM uses the Alamouti code shown in FIG. 4. The data to be subjected to the IFFT process by applying the SFBC-OFDM encoding scheme is represented by Equation 4

It is expressed as

Unlike the STBC-OFDM, the SFBC-OFDM uses only two adjacent subcarrier signals as it enters a decoding process using adjacent subcarrier signals in one OFDM block.

를 곱하는 디코딩 과정을 적용하면 수학식 6Expressed on both sides

Applying the decoding process to multiply the equation (6)

It is expressed as

와 α_ST를 이용하여 송신한 신호를 검출하면 신호를 복원하는 과정이 종료된다.Here, if two adjacent subcarriers have the same channel value, β _ST will be '0', otherwise β _ST will act as ISI,

When the signal transmitted by using and α _ST is detected, the process of restoring the signal ends.

That is, when the STBC-OFDM and SFBC-OFDM systems undergo time-selective fading and frequency-selective fading, respectively, the most deteriorating performance is expressed by β _ST and α _SF . Comparing the code with its size, β _STF of the proposed code is the sum of β _ST and β _SF half.

It is expressed as

Therefore, in the time-selective fading environment, only about half of the ISI experienced by STBC-OFDM is affected. In the frequency-selective fading environment, only half of the ISI is experienced by SFBC-OFDM. In other words, the present investigation sought a solution to be less affected by ISI than at least one system unless both time and frequency selective fading were severe.

Accordingly, the technical problem of the present invention is to solve the above-described solution, in the MIMO-OFDM, the signal is orthogonality by continuously mapping the signal in the time domain and the frequency domain and reducing the size of the repeated transmission signal, By preserving the present invention, the present invention provides an STFBC-OFDM system and a communication method thereof, which can reduce interference generated during decoding to prevent performance degradation.

In the STFBC-OFDM system according to an aspect of the present invention, a transmitting terminal means for transmitting a data vector encoded with STFBC-OFDM encoding and mapped to each subcarrier, and a data vector received from the transmitting terminal means. Receiving terminal means for decoding the original signal using only subcarrier data.

A communication method of an STFBC-OFDM system according to another aspect of the present invention includes the steps of converting data symbols in parallel, and encoding the STFBC-OFDM encoding to satisfy the orthogonal design conditions for the multiple transmit antennas in parallel. Providing a data vector of a coding array of STFBC-OFDM generated data through encoding and mapping to each subcarrier, IFFT performing the encoded data vector on a baseband, and converting the IFFT calculated data vector serially And adding a CP signal to the serially converted data vector, removing the CP signal from the received data vector after receiving the transmitted data vector, and removing the CP signal from the received data vector. Converting the P into parallel, performing FFT operation on the parallel converted data vector into OFDM received symbols, and receiving the FFT computed OFDM And decoding the symbol signal using only subcarrier data, and converting and decoding the decoded parallel OFDM received symbol signal in series.

As shown in FIG. 7, the present invention can confirm that a loss using a modulation modulation method having a high modulation order to compensate for the rate is almost canceled by a signal-to-noise ratio gain. Similarly, we can see that the performance of STFBC-OFDM is better because we get the signal-to-noise ratio gain and the ISI is less affected by the decoding process.

In addition, in the MIMO-OFDM, the present invention continuously maps the time domain and the frequency domain, thereby reducing the size of the repeatedly transmitted signal, preserving the orthogonality of the code, and reducing the interference caused by the decoding process. There is an effect that can be prevented.

Hereinafter, with reference to the accompanying drawings will be described in detail the operating principle of the present invention. In the following description of the present invention, when it is determined that a detailed description of a known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted. Terms to be described later are terms defined in consideration of functions in the present invention, and may be changed according to intentions or customs of users or operators. Therefore, the definition should be made based on the contents throughout the specification.

Looking at the specific technical gist of the present invention, in MIMO-OFDM, by continuously mapping to the time domain and the frequency domain, the size of the repeated transmission signal is reduced, the orthogonality of the code is preserved, and the interference generated in the decoding process is reduced. It is possible to easily achieve what is to be achieved in the present invention through a technology that reduces and prevents performance degradation.

5 is a block diagram of an STFBC-OFDM system according to a preferred embodiment of the present invention, and includes a transmitting terminal 100 and a receiving terminal 200.

The transmitting terminal 100 includes a serial / parallel (S / P) converter 101, an encoder 103, an inverse fast fourier transformer (IFFT) 105, 105 ', and a parallel / serial ( Parallel / Serial, P / S) conversion units 107 and 107 ', and cyclic prefix (CP) insertion units 109 and 109'.

The S / P converter 101 converts serial data symbols in parallel and provides them to the encoder 103.

The encoder 103 reduces the size of the repeated subcarrier signal by using orthogonal design conditions (eg, orthogonal design conditions) with respect to the plurality of transmit antennas S1 and S1 ′ in parallel with the data symbols input from the SP converter 101. And the signal received by the receiving antenna S2 through the wireless channels H1, H2, G1, and G2 through the encoding of STFBC-OFDM.

(Here, the number of subcarriers actually used and the number of subcarriers used for the guard band are expressed as N _a and N _v , respectively.)

개 만큼 데이터를 생성하여 각 부반송파에 매핑시킨 도 6에 도시된 STFBC-OFDM의 부호화 어레이의 데이터 벡터는 수학식 9Can be defined as In this process through the encoding of the encoding of STFBC-OFDM

The data vector of the coding array of STFBC-OFDM shown in FIG. 6, in which data is generated and mapped to each subcarrier, is represented by Equation 9

The expressed data vector is provided to the IFFT 105, 105 '.

IFFT (105, 105 ') is IFFT operation in the baseband for each data vector input from the encoder 103,

The expressed data vector is provided to the P / S converter 107, 107 '.

The P / S converters 107 and 107 'convert the parallel data vectors calculated in the baseband input from the IFFT 105 and 105' into serial and provide them to the CP inserters 109 and 109 '.

The CP inserting units 109 and 109 'add CP signals to the serial data vectors input from the P / S converters 107 and 107' to prevent the destruction of orthogonality that may be caused by the delay of the subcarriers. Transmit to the receiving terminal 200 through each antenna (S1, S1 'S2).

The receiving terminal 200 uses a CP remover 201, an S / P converter 203, a Fast Fourier Transformer (FFT) 205, a decoder 207, and a P / S converter 209. Include.

)를 S/P 변환부(203)에 제공한다.The CP removing unit 201 removes only the CP signal from the data vector into which the CP signal received from the transmitting terminal 100 is inserted, and then removes the CP signal from the data vector (

) Is provided to the S / P conversion unit 203.

The S / P converter 203 converts the serial data vectors input from the CP remover 201 in parallel and provides them to the FFT 205.

In this case, when the FFT operation of the parallel data vector transformed by the S / P converter 203 into the OFDM received symbol is performed, Equation 11

And the represented OFDM received symbol signal is provided to the decoder 207.

If the decoder 207 decodes the input OFDM received symbol signal using only three subcarrier data,

This signal is provided to the P / S converter 209.

과 α_STF를 이용하여 송신한 신호를 검출하면 신호를 복원하는 과정이 종료된다. Then, the P / S converter 209 demodulates the original OFDM signal by serially converting the parallel OFDM received symbol signal input from the decoder 207. Here, if the channel values between two consecutive OFDM channels and two adjacent subcarriers are the same, the value of β _STF becomes '0'.

And detecting the signal transmitted using α _STF , the process of restoring the signal ends.

Accordingly, referring to FIG. 7, FIG. 7 compares the BER performance of the present invention with the conventional STBC-OFDM and SFBC-OFDM in a weak fading environment and a severe environment, and uses a signal-to-noise ratio gain and a different modulation and demodulation scheme. The loss caused by the comparison is compared. In other words, the dotted line shows the experimental results in an environment with almost no fading effect with Δf = 0.005 and τ _rms = 0.5 Hz, and the solid line shows Δf = 0.05 and τ _rms = 4 심한 for both time and frequency selective fading. Experimental results in the environment show that, as shown in the results, the loss of using a modulation modulation method with a high modulation order to compensate for the rate is almost offset by the signal-to-noise ratio gain. .

In addition, referring to FIG. 8, FIG. 8 is a diagram comparing the BER performance of the STBC-OFDM and the present invention in an environment in which time-selective fading is severe. The experimental environment is τ _rms = 0.5 ms, and Δf is 0.005 and 0.1. Experiments with the environment show that the STFBC-OFDM proposed in the present invention is better because the signal-to-noise ratio gain and the decoding process are less affected by the ISI, even though a better modulation and demodulation method is used. .

Finally, referring to FIG. 9, FIG. 9 is a diagram comparing BER performance of a system using SFBC-OFDM and a code proposed by the present invention in an environment where frequency selective fading is severe. Δf is 0.005. Experiments were conducted in two environments of τ _rms = 5 ms and 3 ms, which shows that the performance of the STFBC-OFDM proposed by the present invention is better, similar to the results of STBC-OFDM.

Therefore, in the MIMO-OFDM, the present invention continuously reduces the size of a repeatedly transmitted signal by preserving the orthogonality of the code and reduces the interference generated in the decoding process by sequentially mapping the time domain and the frequency domain. Can be prevented.

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications are possible without departing from the scope of the present invention. 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 those equivalent to the scope of the claims.

1 illustrates BER performance of STBC-OFDM in a time selective fading environment according to the prior art;

2 is a diagram illustrating BER performance of SFBC-OFDM in a frequency selective fading environment according to the prior art;

3 is a coded array diagram of an STBC-OFDM using an Alamouti code according to the prior art;

4 is a coded array diagram of an SFBC-OFDM using an Alamouti code according to the prior art;

5 is a block diagram of an STFBC-OFDM system according to a preferred embodiment of the present invention;

6 is a diagram of an encoding array of STFBC-OFDM according to the present invention;

7 is a view comparing the BER performance of the present invention and the conventional STBC-OFDM, SFBC-OFDM in the weak fading environment and severe environment,

8 is a diagram comparing the BER performance of the STBC-OFDM and the present invention in an environment of severe time selective fading,

FIG. 9 is a diagram comparing BER performance of a system using SFBC-OFDM and a code proposed by the present invention in an environment with severe frequency selective fading. FIG.

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

100: transmitting terminal 101: S / P conversion unit

103: Encoder 105,105 ': IFFT

107,107 ': P / S converter 109,109': CP insertion part

200: receiving terminal 201: CP removing unit

203: S / P converter 205: FFT

207: decoder 209: P / S converter

S1, S1 ', S2: Antenna

Claims (11)

Transmitting terminal means for transmitting the data vector encoded by STFBC-OFDM encoding and mapped to each subcarrier; Receiving side terminal means for decoding the data vector received from the transmitting terminal means into an original signal using only subcarrier data STFBC-OFDM system comprising a. The method of claim 1, The transmitting terminal means, An S / P converter for converting the data symbols in parallel; Provides a data vector of the STFBC-OFDM encoded array in which the data symbols converted in parallel are generated through encoding of the STFBC-OFDM encoding to satisfy the orthogonal design conditions for a plurality of transmit antennas and mapped to each subcarrier. With the encoder, An IFFT for performing IFFT operation on the encoded data vector at baseband; A P / S converter for converting the IFFT-calculated data vector into a serial; CP insertion unit for adding a CP signal to the serially converted data vector STFBC-OFDM system comprising a. The method of claim 2, The orthogonal design condition is to reduce the size of the repeating subcarrier signal. The method of claim 2, The coding array of the STFBC-OFDM is, Equation

STFBC-OFDM system, characterized in that represented by. The method of claim 2, The arbitrary number is

STFBC-OFDM system characterized by the individual. The method of claim 2, The IFFT operation is Equation

STFBC-OFDM system, characterized in that represented by. The method of claim 1, The receiving terminal means, A CP removing unit for removing a CP signal from the data vector received from the transmitting terminal means; An S / P converter for converting the data vector from which the CP signal is removed in parallel; An FFT for performing FFT operation on the parallel converted data vector with an OFDM received symbol; A decoder for decoding the FFT-operated OFDM received symbol signal using only subcarrier data; P / S converter for converting the decoded parallel OFDM received symbol signal in series STFBC-OFDM system comprising a. The method of claim 7, wherein The FFT operation is Equation

STFBC-OFDM system, characterized in that represented by. The method of claim 7, wherein Decoding of the decoder, Equation

STFBC-OFDM system, characterized in that defined as. (a) converting the data symbols in parallel, (b) a data vector of a coding array of STFBC-OFDM in which data is generated through encoding of STFBC-OFDM encoding and mapped to each subcarrier so that the parallel-converted data symbols satisfy quadrature design conditions for a plurality of transmitting antennas; Providing a, (c) performing IFFT operation on the encoded data vector at baseband; (d) converting the IFFT computed data vector into a serial; (e) adding and transmitting a CP signal to the serially converted data vector Communication method of the STFBC-OFDM system comprising a. The method of claim 10, The method, (f) removing the CP signal from the received data vector after receiving the data vector transmitted in step (e); (g) converting the data vector from which the CP signal has been removed in parallel; (h) performing an FFT operation on the parallel converted data vector with an OFDM received symbol; (i) decoding the FFT-operated OFDM received symbol signal using only subcarrier data; (j) converting and demodulating the decoded parallel OFDM received symbol signal in series Communication method of the STFBC-OFDM system further comprising.

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