KR100764789B1 - Signal transmission apparatus and method for ofdm system - Google Patents

Abstract

An apparatus and method for Orthogonal Frequency Division Multiplexing (OFDM) signal transmission according to the present invention, in order to improve transmission quality, a subcarrier having a good channel condition transmits a system bit and a subcarrier having a relatively poor channel condition. Transmit redundant bits. That is, according to the present invention, by transmitting a signal (symbol) on each subcarrier in consideration of the channel condition and the importance of data in the OFDM system, the reliability of the signal received at the receiver can be improved and the reception efficiency can be improved.

Orthogonal Frequency Division Multiplexing System, OFDM

Description

SIGNAL TRANSMISSION APPARATUS AND METHOD FOR OFDM SYSTEM}

1 is a block diagram of a conventional Orthogonal Frequency Division Multiplex (OFDM) system.

2 is a configuration diagram of an Orthogonal Frequency Division Multiplex (OFDM) system according to the present invention.

3 is a diagram illustrating a 1/3 rate turbo code encoder as an example of the channel coding unit shown in FIG. 2;

4 is a flowchart illustrating a signal transmission method in an OFDM system according to the present invention.

******* Description of the symbols for the main parts of the drawing ********

3: transmitting end 4: receiving end

100: channel coding unit 110: bit rearrangement unit

120: modulator 130: demultiplexer

140: subcarrier allocation unit 150: inverse fast Fourier transform unit (IFFT)

160: fast Fourier transform unit (FFT) 170: multiplexer

180: channel prediction section 190: demodulation section

200: bit array restoration unit 210: channel decoding unit

The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) system, and more particularly, to an apparatus and method for transmitting a signal in an OFDM system.

Recently, orthogonal frequency division multiplexing (OFDM), which transmits data using multiple carriers, has been used as a method for high-speed data transmission in wired and wireless channels.

The OFDM scheme is a multicarrier modulation scheme that converts a serial data sequence into a parallel data sequence and modulates each of them into a plurality of subcarriers having mutual orthogonality, and has been developed since the 1970s. Was difficult to implement, so there were some limitations to the actual system application. However, the negative effects of the system on multipath and delay spread by using the Discrete Fourier Transform (DFT) in the OFDM scheme, and also using the guard interval and the cyclic prefix protection interval This has been reduced. In addition, the OFDM method has not been widely used due to hardware complexity, but recently, due to the development of various digital signal processing technologies including a fast fourier transform (FFT) and an inverse fast Fourier transform (IFFT). To be used.

For this reason, the OFDM method has already been adopted as a modulation method for digital audio broadcasting (DAB) and digital television, and has been adopted as a standard (802.11a, HiperLAN II) in the wireless LAN field.

1 is a block diagram of a conventional OFDM system.

Conventional OFDM systems are largely divided into a transmitter 1 serving as a base station and a receiver 2 serving as a terminal.

The transmitter 1 includes a modulator 10 for modulating an input serial data sequence, a demultiplexer 20 for demultiplexing a modulated serial data sequence into a parallel data sequence, and a demultiplexed parallel data sequence for each subcarrier. Each subcarrier is composed of an inverse fast Fourier transform unit (IFFT) 30 which performs inverse Fourier transform at high speed so as to maintain orthogonality.

The receiver 2 includes a fast Fourier transform unit (FFT) 40 for Fourier transforming each transmitted subcarrier at high speed, and multiplexing for multiplexing parallel data sequences transmitted to each of the fast Fourier transformed subcarriers into a serial data sequence. Section 50 and a demodulation section 60 for demodulating the multiplexed serial data.

The signal transmission method in the OFDM system configured as described above is as follows.

Referring to FIG. 1, the serial data string of an input modulated by the modulator 10 is converted into N parallel data strings by the demultiplexer 20, and the inverse fast Fourier transform unit (IFFT) 30 transmits a data rate. In order to increase, the transformed plurality of data streams are respectively transmitted on a sub-carrier. In this case, the IFFT unit 30 should select a subcarrier so that each subcarrier maintains orthogonality, and the maintenance of orthogonality of each subcarrier means that signal (symbol) can be superimposed on the spectrum of a wireless channel environment.

The receiver 2 may restore the original data by separating each subcarrier transmitted by the transmitter 1 using a simple signal processing technique.

That is, as shown in FIG. 1, the fast Fourier transform unit (FFT) 40 of the receiver 1 separates data from each subcarrier transmitted from the transmitter 1 and multiplexes the parallel data stream 50. The multiplexer 50 converts the parallel data stream into a serial data stream and outputs the same to the demodulator 60. Therefore, the demodulator 60 demodulates the serial data string converted by the multiplexer 50 to restore the original data.

In general, as in the OFDM method, a signal transmission method using a plurality of orthogonal subcarriers has a longer interval between data (symbols) than a method of sequentially transmitting data using one carrier, resulting in a delay time and Less affected by impulse noise. When the OFDM method is used, InterSymbol Interference (ISI) of consecutive data (symbols) can be reduced, and in terms of hardware, an equalizer structure can be easily designed. In addition, the OFDM scheme is strong in frequency selective fading and multipath fading, and thus has an advantage of increasing efficiency of the frequency spectrum compared to the general frequency division scheme.

However, since the data (symbols) carried on each subcarrier in the OFDM system undergo independent fading, the channel state of each subcarrier transmitted by the transmitter is changed in real time. In such a channel environment, the conventional OFDM system has a problem in that reception efficiency is reduced at the receiving end by simply transmitting data on a subcarrier without considering the channel condition of each subcarrier and the importance of data.

Accordingly, an object of the present invention is to provide an apparatus and method for transmitting a signal in an OFDM system that can increase the reception efficiency of a receiver.

In order to achieve the above object, there is an Orthogonal Frequency Division Multiplexing (OFDM) system which demultiplexes input data and then transmits a plurality of subcarriers having orthogonality to each other. Channel coding with a second bit; Identifying channel status of each subcarrier; And allocating the first or second bit to each subcarrier differently according to the identified channel condition.

Preferably, the first bit is the same system bit as the input data, and the excess bit is a redundant bit generated to prevent signal distortion, such as intersymbol interference, on a wireless channel.

Preferably, a first bit is allocated to a subcarrier having a good channel condition, and a second bit is allocated to a subcarrier having a relatively good channel condition.

In order to achieve the above object, there is an orthogonal frequency division multiplexing (OFDM) system for demultiplexing input data and then transmitting the data to a plurality of subcarriers having mutual orthogonality. A channel coding unit for channel coding the first and second bits; A bit rearrangement unit for rearranging the channel-coded first and second bits for each bit; A modulator for modulating the rearranged bits; A demultiplexer for demultiplexing the modulated signal; A subcarrier assignment unit for allocating the demultiplexed signal to each subcarrier differently according to a channel condition of each subcarrier; And an inverse fast Fourier transform unit which transmits by inverse Fourier transform the subcarrier to which the demultiplexed signal is assigned.

Preferably, the channel status of each subcarrier is provided from the receiving end.

Preferably, the first bit is the same system bit as the input data, and the second bit is a redundant bit generated to prevent signal distortion, such as intersymbol interference on the wireless channel.

Preferably, the subcarrier assignment unit allocates a first bit to a subcarrier having a good channel condition and a second bit to a subcarrier having a relatively poor channel condition.

The present invention is implemented in an Orthogonal Frequency Division Multiplexing (OFDM) system using multiple orthogonal subcarriers. However, the present invention can also be applied to a wireless communication system operating according to other standards. Hereinafter, preferred embodiments of the present invention will be described in detail.

In general, coded bits encoded through channel coding are divided into system bits and redundancy bits. Since the system bits contain information related to the input data, they are more important than the redundant bits that contain information for preventing transmission distortion on the channel, such as inter-symbol interference in terms of signal recovery of the receiver. can do.

Accordingly, the basic idea of the present invention is to improve the signal transmission quality by transmitting system bits in subcarriers having good channel conditions and by transmitting extra bits in subcarriers having relatively poor channel conditions. That is, the present invention proposes a method of transmitting a desired signal (symbol) through each subcarrier in consideration of the channel condition and the importance of data in an OFDM system.

2 is a block diagram of an OFDM system according to the present invention. In particular, FIG. 2 is an example implemented assuming an FDD system in which a forward channel state is unknown at a transmitting end.

As shown in Fig. 2, the OFDM system applied to the present invention is largely divided into a transmitter 3 and a receiver 4. Here, the transmitting end 3 and the receiving end 4 mean a signal transmission device and a signal reception device, respectively.

The transmitting end 3 further includes a channel coding unit 100, a bit reordering unit 110, and a subcarrier allocating unit 140 in addition to the conventional transmitting end 1 shown in FIG. In addition to the conventional receiver 2 of FIG. 1, the channel prediction unit 180, the bit array restoration unit 200, and the channel decoding unit 210 are further included. At this time, the modulation unit 10, demultiplexer 20, IFFT unit 30, FFT unit 40, multiplexer 50, demodulator 60 of the conventional OFDM system shown in FIG. Only the modulator 100, the demultiplexer 130, the IFFT unit 150, the FFT unit 160, the multiplexer 170, the demodulator 190, and the drawing numbers of the present invention shown in FIG. Its configuration and function are the same.

As shown in FIG. 3, the channel coding unit 100 codes input data into one system bit (Xk) and two redundancy bits (Zk, Z'k). Implemented with a / 3 rate turbocode encoder. The 1/3 rate turbo code encoder is an encoder currently included in the 3GPP standard, and includes a first and second constituent encoders 104 and 106 and an internal interleaver 102. .

The bit rearranger 110 rearranges the coded bits coded by the channel coder 100 according to the importance of the data. That is, the bit rearranger 110 includes information directly related to input data (bits), and thus separates and rearranges important system bits and redundant bits that are relatively less important than the system bits. do.

The subcarrier allocator 140 feeds back the output of the bit reordering unit 110, that is, the demultiplexed parallel data (bit) strings, received through the modulator 120 and the demultiplexer 130, from the receiver 4. It assigns to the subcarriers according to the channel status of each subcarrier. At this time, the subcarrier assignment unit 140 allocates system bits to subcarriers having a good channel condition, and assigns surplus bits to subcarriers having a relatively poor channel condition.

The channel predictor 180 of the receiver 4 detects channel conditions such as signal power or signal transmission distortion of each subcarrier from the received signal and feeds back to the subcarrier allocator 140 of the transmitter 3. However, when the present invention is applied in the TDD system, since the transmitter 3 already knows the channel status of each subcarrier, it is not necessary to implement the channel predictor 180 in the receiver 4.

The bit array restoration unit 200 arranges the signals output from the demodulation unit 190 in the same form as the system bits and the redundant bits encoded by the channel coding unit 100 of the transmitting end 3, and the channel decoding unit ( 210 decodes the system bits and the redundant bits output from the bit array restoration unit 200 and outputs input data (bits).

The operation of the OFDM system according to the present invention configured as described above will be described with reference to FIGS.

First, in order to prevent signal distortion, such as inter-symbol interference (ISI), on the wireless channel, the channel coding unit 100 inputs the input data into a system bit (first bit) and a surplus of second bit. Channel coding is performed using redundancy bits (S10).

For example, assuming that the input data (bits) are composed of 2 bits (X₁, X₂), the two bits of input data (X₁, X₂) are respectively represented by the 1/3 rate turbo code encoder shown in FIG. X₁, Z₁, Z'₁ and X₂, Z₂, Z'₂.

Among the coded bits, X 'and X2 are system bits, and Z', Z '', Z2 and Z'2 are redundant bits. The system bit is the same as the input data, and is a highly informationable bit used for restoring data (bit) at the receiving end 4, and the surplus bit is a signal distortion such as inter-symbol interference (ISI) phenomenon on a wireless channel. It is a bit generated to prevent the data, and it is less important data because it is relatively less informative than the system bit.

The bit rearranger 110 rearranges the channel coded system bits (X₁X₂) and the redundant bits (Z₁Z'₁Z₂Z'₂) for each bit by the channel coding unit 100, and arranges the rearranged data (bits). Becomes X₁X₂Z₁Z₂Z'₁Z'₂. That is, the bit reordering unit 110 rearranges the coded bits for each of the excess bits output from the system bits and the first and second component encoders 104 and 106.

The coded bits (X₁X₂Z₁Z₂Z'₁Z'₂) rearranged by the bit rearranger 110 are modulated by the modulator 120 to be shaped (symbolized) in a form suitable for transmission on a wireless channel. In the modulation scheme, for example, if the QPSK scheme of shaping two bits into one symbol is used, three symbols (X₁X₂, Z₁Z₂ and Z formed by two bits in the modulator 120 as a result of the modulation operation) are obtained. 'Z'₂) is produced. Since the generated symbols (X₁X₂, Z₁Z₂ and Z'₁Z'₂) are serial signals (symbols), the demultiplexer 130 demultiplexes them into parallel signals (symbols) in order to increase the signal transmission rate.

The channel predicting unit 180 grasps channel information such as signal power or signal transmission distortion of each subcarrier to determine which subcarrier channel is better for transmitting data (S20). The subcarrier assignment unit 140 allocates the de-neutralized parallel signal (symbol) to each subcarrier based on the identified channel condition of each subcarrier. The channel condition of each subcarrier is fed back from the channel predicting unit 180 of the receiving end 4 in the case of the FDD OFDM system, and the subcarrier allocating unit 140 is known from the receiving end 4 in the case of the TDD method. There is no need to get feedback.

For convenience of explanation, the present invention assumes an FDD OFDM system, and assumes a case of transmitting the de-neutralized parallel signal (symbol) using three subcarriers A, B, and C. In this case, it is assumed that the channel condition of the subcarrier C is better than the channel condition of the subcarriers A and B among the three subcarriers A, B, and C.

That is, the subcarrier assignment unit 140 assigns a symbol (X₁X₂) in which a system bit is shaped to a subcarrier (C) having a relatively good channel condition, and the subcarriers (A) and (B) having a relatively poor channel condition. Are assigned symbols (Z₁Z₂) and symbols (Z'₁Z'₂), each of which has a surplus bit.

Accordingly, the IFFT unit 150 transmits after inverse Fourier transforming the subcarriers (A, B, and C) to which the symbol is assigned in order to increase the signal transmission rate by maintaining orthogonality between the subcarriers.

On the other hand, when a signal transmitted from the transmitter 3 is received through a wireless channel, the FFT unit 160 of the receiver 4 detects the demultiplexed subcarriers A, B, and C by Fourier transforming the received signal. The multiplexer 170 multiplexes the parallel signals, i.e., symbols X₂X₂, Z₂Z₂ and Z'₁Z'₂, carried on the detected subcarriers A, B, and C into serial signals (symbols). At this time, the channel predictor 180 grasps the channel status of each subcarrier (A, B, C subcarrier) output from the FFT 160 and feeds it back to the subcarrier allocator 140 of the transmitter 3.

Subsequently, the demodulator 190 demodulates the serial signal (symbol) and outputs serial coded bits (X₁X₂Z₁Z₂Z'₁Z'₂), and the serial coded bits are transmitted from the channel coding unit 100 and the bit rearrangement unit 110. It has the same arrangement as the coded coded bits.

Accordingly, the bit array restoration unit 200 restores the demodulated coded bits (X₁X₂Z₁Z₂Z'₁Z'₂) to coded bits having an array such as X₁Z₁Z'₁X₂Z₂Z'₂, and the channel decoder 210 transmits such as inter-symbol interference. In order to remove the surplus bits generated to prevent distortion, the decoded coded bits X (Z₁Z'₁X₂Z₂Z'₂ are decoded to obtain output data XX, X₂ which is the same as the input data XX, X₂.

Although the present invention has described a data transmission method using an FDD-based OFDM system as an example, in the case of the TDD system, since the forward channel state is known at the transmitter, the configuration and method of the present invention are applied as it is.

In addition, although the present invention has been described with reference to the embodiments illustrated in the drawings, this is merely exemplary and will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. . Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

As described above, in the present invention, important data (system bits) having a lot of information on input data are transmitted on a subcarrier having a relatively good channel state, and thus the reception end can increase reception efficiency for important data.

Claims (15)

In an Orthogonal Frequency Division Multiplexing (OFDM) system, which demultiplexes input data and then transmits a plurality of subcarriers having mutual orthogonality, Channel coding the input data into first and second bits; Identifying channel status of each subcarrier; A signal transmission method comprising the step of differently assigning the first or second bits to each subcarrier according to the identified channel condition, and transmitting the same. The first bit is the same system bit as the input data; And the second bit is a redundant bit. The method of claim 1, wherein the channel coding step Channel coding the input data into first and second bits to form encoded bits; And separating and rearranging the encoded bits for each of the first bit and the second bit. delete delete The method of claim 1, wherein a first bit is allocated to a subcarrier having a good channel condition, and a second bit is allocated to a subcarrier having a relatively poor channel condition. The channel condition of claim 1, wherein In the case of the FDD system, a signal transmission method comprising receiving feedback from a receiving end. In an Orthogonal Frequency Division Multiplexing (OFDM) system, which demultiplexes input data and then transmits a plurality of subcarriers having mutual orthogonality, As a signal transmission method of identifying a channel condition of each subcarrier, and assigning and transmitting a first bit to a subcarrier having a good channel condition, and assigning and transmitting a second bit to a subcarrier having a relatively poor channel condition, The first bit is the same system bit as the input data; And the second bit is a surplus bit. delete delete The method of claim 7, wherein the channel situation is In the case of the FDD system, a signal transmission method comprising receiving feedback from a receiving end. There is an Orthogonal Frequency Division Multiplexing (OFDM) system that demultiplexes input data and then transmits a plurality of subcarriers with mutual orthogonality. A channel coding unit for channel coding the input data into first and second bits; A bit rearranger for rearranging the channel-coded first and second bits for each bit; A modulator for modulating the rearranged bits; A demultiplexer for demultiplexing the modulated signal; A subcarrier assignment unit for allocating the demultiplexed signal to each subcarrier differently according to a channel condition of each subcarrier; And an inverse fast Fourier transform unit for transmitting the inverse multiplexed subcarrier to which the demultiplexed signal is assigned by inverse Fourier transform at high speed. 12. The channel state of each subcarrier, wherein Signal transmission device characterized in that provided from the receiving end. The method of claim 11, wherein the first bit Signal transmission device, characterized in that the same system bits as the input data. The method of claim 11, wherein the second bit And a surplus bit generated to prevent signal distortion such as intersymbol interference on a wireless channel. 12. The method of claim 11, wherein the subcarrier allocation unit And a first bit is allocated to a subcarrier having a good channel condition, and a second bit is allocated to a subcarrier having a relatively good channel condition.

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