KR101208519B1 - apparatus and method for transmitting data using a plurality of carriers - Google Patents

Abstract

The present invention relates to a method for transmitting and receiving data in a communication system, and more particularly, to include OFDM symbols having different lengths in an orthogonal frequency division multiplexing (OFDM) based communication system. A method of forming an OFDM frame. The OFDM frame forming method according to the present invention is a method for transmitting data in an OFDM frame unit including a plurality of OFDM symbols corresponding to input data and a cyclic prefix for each of the OFDM. Forming an OFDM frame comprising one OFDM symbol and at least one second OFDM symbol having an FFT length less than the first OFDM symbol; And transmitting the formed OFDM frame to a receiving side, wherein a ratio of the FFT length of the cyclic prefix to the second OFDM symbol and the second OFDM symbol is determined by the first OFDM symbol and the first symbol. It is characterized by being larger than the ratio of the FFT length of the cyclic prefix to one OFDM symbol.

OFDM frame, cyclic prefix, FFT length, OFDM symbol

Description

Apparatus and method for transmitting data using a plurality of carriers}

1 is a diagram illustrating an example of an OFDM transmission apparatus used in the prior art and the present invention.

2 is a diagram illustrating a structure of an OFDM symbol.

3 is a diagram illustrating a structure of an OFDM frame.

4A illustrates an OFDM frame including OFDM symbols of the same size.

4B is a diagram illustrating an OFDM frame including OFDM symbols of different sizes.

5A illustrates an OFDM frame including two short OFDM symbols and m-1 long OFDM symbols.

5B is a diagram illustrating an OFDM frame including three short OFDM symbols and m-1 long OFDM symbols.

The present invention relates to a method for transmitting and receiving data in a communication system, and more particularly, to include OFDM symbols having different lengths in an orthogonal frequency division multiplexing (OFDM) based communication system. A method of forming an OFDM frame.

Recently, the demand for high-speed data transmission is increasing, and OFDM is a suitable method for such a high-speed transmission, and has been recently adopted as a transmission method of various high-speed communication systems. Hereinafter, OFDM will be described. The basic principle of OFDM is to divide a data stream with high rates into a large number of data streams with low rates and transmit them simultaneously using multiple carriers. Each of the plurality of carriers is called a subcarrier. Since orthogonality exists between the multiple carriers of the OFDM, the frequency components of the carriers can be detected at the receiving end even if they overlap each other. The high data rate data stream is converted into a plurality of low data rate data streams through a serial to parallel converter, and each subcarrier is included in the parallel data streams. After multiplying, the respective data strings are summed and sent to the receiving end. OFDMA is a multiple access method for allocating subcarriers according to a transmission rate required by multiple users in the OFDM.

The above-described OFDM communication method is used in various systems (IEEE 802.11a / g, HiperLAN, IEEE 802.16, DSL, DAB, DVB, etc.), and is effective when the communication channel exhibits selective fading. to be. The OFDM scheme uses a plurality of sub-carriers, so that the selective attenuation may appear to be flat-fading, and the technique for compensating for the attenuation in the entire system may be simplified.

1 is a diagram illustrating an example of an OFDM transmission apparatus used in the prior art and the present invention. First, digital modulation is performed on a data stream transmitted to a user using various constellation mapping methods such as QPSK and 16 QAM. The modulation symbols are converted into parallel symbols by passing through an S / P converter. The parallel symbol is mapped to an assigned subcarrier among arbitrary subcarriers. The signal mapped to the subcarrier is input to an inverse fast fourier transform (IFFT) module having a specific size and converted into a time domain signal. The signal in the time domain is transmitted to the receiving end after the cyclic prefix is added.

In general, OFDM is mathematically implemented by FFT / IFFT (Fast Fourier Transform / Inverse FFT). In the transmitting end, the signal multiplexed in the frequency domain is converted to the time domain through IFFT and then transmitted. In addition, the receiving end converts the signal in the time domain into the frequency domain through the FFT, and then recovers the original signal through the demultiplexing process. The signal converted on the time axis through the IFFT adds Cyclic Prefix, which is called an OFDM symbol. In addition, each sampled signal in the time domain constituting an OFDM symbol is called an OFDM sample. Meanwhile, the cyclic prefix refers to removing an end portion of an OFDM symbol and attaching it to a front portion of an OFDM symbol. The cyclic prefix removes interference between multipaths in a multipath channel environment. The OFDM symbol may be transmitted by grouping several OFDM symbols together in actual transmission, which will be referred to as an OFDM frame. 2 is a diagram illustrating a structure of an OFDM symbol. 1 and 2 illustrate the flow of signals transmitted through the transmitter of FIG. 1. As shown, data symbols from x ₀ to x _{N -1} are converted into time in the time domain through an IFFT operation, and the length of the signal in the time domain is determined according to the length of the IFFT. Each OFDM symbol is copied with a certain length of cyclic prefix. 3 is a diagram illustrating a structure of an OFDM frame. As shown, one OFDM frame is formed by at least one OFDM symbol. The OFDM frame shown in FIG. 2 or 3 may be generated by various types of transmitters. In the transmitter of FIG. 1, an FFT length of a specific OFDM symbol may be adjusted or an OFDM frame including a specific OFDM symbol may be formed using a method of controlling a bit stream input to the transmitter or controlling symbol-subcarrier mapping. The method of forming such an OFDM frame is obvious to those skilled in the art.

In general, in the transmission and reception of OFDM, since the IFFT / FFT has the same length and is mathematically inversed and inversely related, FFT will be collectively referred to as FFT for convenience unless there is room for confusion.

The length of the OFDM symbol is proportional to the OFDM sample time and the FFT length which is the number of subcarriers used when performing the FFT operation. That is, the length of the OFDM symbol is determined by the two variables (sample time, FFT length). The sample time refers to a time interval between each sample. If the OFDM sample time is fixed here, the length of the OFDM symbol is purely dependent on the FFT length. Since the FFT length means the number of data symbols that can be transmitted in one OFDM symbol, the length of the OFDM symbol is determined according to the size of the actual data symbol to be transmitted.

When several OFDM symbols constitute one OFDM frame, it is the simplest in the design that all OFDM symbols have the same FFT length. However, the FFT length of the OFDM symbol may be different. That is, the FFT size of the OFDM symbol may vary depending on the purpose for which each OFDM symbol is used. If the OFDM symbol for large data transmission has a long FFT length, while the OFDM symbol for low capacity data transmission such as a control signal, a short FFT length is sufficient. Therefore, in case of transmitting large capacity and low capacity data in one OFDM frame at the same time, some OFDM symbols may have a long FFT length, and other OFDM symbols may have a short FFT length.

Hereinafter, an OFDM frame according to the prior art will be described. For example, there are m OFDM symbols in an OFDM frame and the FFT length of all OFDM symbols may be N. In this case, the length of the cyclic prefix of each OFDM symbol is N _CP , the OFDM frame should be transmitted for the time of T _frame , the OFDM symbol should be transmitted for the time of T _symbol , and the sampling time of each OFDM sample is T _sample . . In this case, each of the variables maintains the relationship of Equations 1 and 2, and is represented as shown in FIG. 4A.

In the actual implementation of the transmission and reception apparatus, the FFT length has an advantage when having a power of 2, the FFT length is expressed as a multiplier of 2 below. In this case, if the OFDM symbol having an FFT length of N has a short FFT length, the possible FFT lengths are N / 2, N / 4, N / 8, N / 16,... Becomes 2.

If k OFDM symbols have a short FFT length in an OFDM frame, and j OFDM symbols have a long FFT length, the following equations (3) and (4) are given. In the following equation, the FFT length of an OFDM symbol having a _short FFT length is referred to as N _short , and the length of the cyclic prefix is referred to as N _{cp , short} . Similarly, the FFT length of an OFDM symbol having a _long FFT length is referred to as N _long , and the length of the cyclic prefix is referred to as N _{CP , long} . The OFDM frame configuration diagram at this time is as shown in FIG. 4B.

Since the total length of the OFDM frame must be constant, the results of Equations 2 and 4 must be identical. Therefore, we obtain the following relation.

Hereinafter, an OFDM frame configuration according to the prior art will be described, but a case in which a realistic assumption is made will be described. The number j of OFDM symbols having a long FFT length is m−1, and the FFT length N _long at this time is equal to the original FFT length N. Also assume that the number k = 2 of OFDM symbols having a short FFT and the short FFT length N _short is N / 2 which is half of the original FFT length. The relation is then as shown in Equation 6 below. The OFDM frame at this time is as shown in Figure 5a.

In summary, a short length OFDM symbol having a short FFT length divided by one conventional OFDM symbol is added, and thus the number of OFDM symbols increased by one more than the number of existing OFDM symbols. That is, instead of a total of m OFDM symbols in one OFDM frame, m + 1 OFDM symbols are included.

가 되어서 상기 짧은 길이의 OFDM 심볼의 순환 전치는 원래 순환 전치 길이의 절반이 된다. 수신 측의 원활한 수신을 위해서는 충분한 순환 전치 길이가 보장되어야하고, 충분한 길이의 순환 전치가 다중경로 채널환경에서 다중 경로 간의 채널 간섭현상을 막아줄 수 있는데, 종래 기술에 따르면 순환 전치의 길이가 절반으로 감소하여, 상기 짧은 길이의 OFDM 심볼은 다중경로 채널환경에서 성능이 나빠지게 된다. Here, if the cyclic prefix length N _{CP , long} for the long OFDM symbol is equal to the original cyclic prefix length N _CP ,

The cyclic prefix of the short OFDM symbol is half of the original cyclic prefix length. In order to receive the receiver smoothly, sufficient cyclic prefix length must be ensured, and sufficient cyclic prefix can prevent channel interference between multiple paths in a multipath channel environment. According to the related art, the cyclic prefix length is halved. As a result, the short OFDM symbol is degraded in a multipath channel environment.

이 되므로, 각각의 순환 전치의 길이는 원래의 순환 전치 길이 N_CP보다 짧아지게 된다. 따라서 이 경우에는 상기 짧은 길이의 OFDM 심볼뿐만 아니라, 상기 긴 길이의 OFDM 심볼 역시 다중경로 채널에서 성능이 열화된다. On the other hand, if N _{CP , long} and N _{CP , short} are set equal

Therefore, the length of each cyclic prefix is shorter than the original cyclic prefix length N _CP . In this case, therefore, not only the short OFDM symbol, but also the long OFDM symbol deteriorate performance in a multipath channel.

As described above, as the length of the cyclic prefix decreases, performance deteriorates in a multipath channel environment even when there are three OFDM symbols having a short length as shown in FIG. 4B.

The present invention has been proposed to solve the above-mentioned problems of the prior art, and an object of the present invention is to form an OFDM frame including OFDM symbols having different lengths, and to maintain or increase the length of the cyclic prefix to facilitate smooth communication. To make it possible.

Summary of the Invention

In order to achieve the above object of the present invention, the present invention, in a method for transmitting data in units of OFDM frames including a plurality of OFDM symbols corresponding to the input data and a cyclic prefix for each of the OFDM Forming an OFDM frame comprising at least one first OFDM symbol and at least one second OFDM symbol having a smaller FFT length than the first OFDM symbol; And transmitting the formed OFDM frame to a receiving side, wherein a ratio of the FFT length of the cyclic prefix to the second OFDM symbol and the second OFDM symbol is equal to the first OFDM symbol. It is characterized by being larger than the ratio of the FFT length of the cyclic prefix to the first OFDM symbol.

In addition, the transmitting apparatus according to the present invention is a transmitting apparatus for transmitting data in an OFDM frame unit including a plurality of OFDM symbols corresponding to input data and a cyclic prefix for each of the OFDM, at least one An OFDM frame forming module for forming an OFDM frame including a first OFDM symbol and at least one second OFDM symbol having a smaller FFT length than the first OFDM symbol; And a transmitting module for transmitting the formed OFDM frame to a receiving side, wherein an FFT length ratio of a cyclic prefix to the second OFDM symbol and the second OFDM symbol is determined by the first OFDM symbol and the first OFDM symbol. It is characterized by being larger than the ratio of the FFT length of the cyclic prefix to one OFDM symbol.

The invention Example

The present invention is to form an OFDM frame including a plurality of OFDM symbols, characterized in that the FFT length for the OFDM symbol is different. In particular, an OFDM symbol having a small FFT length may be generated by dividing an OFDM symbol having a large FFT length by a specific number (D). It is characterized in that it is smaller than the equivalent of a certain number (D).

In addition, the present invention transmits the first OFDM symbol and the second OFDM symbol of a different size, the length of the first OFDM symbol is characterized in that the larger than the second OFDM symbol, and the The ratio of the cyclic prefix to the FFT length is smaller than the ratio of the cyclic prefix to the FFT length of the second OFDM symbol.

The present invention does not adjust the size of the OFDM frame or adjust the sampling time of OFDM to give more cyclic prefix to a particular OFDM symbol. The present invention efficiently adjusts the size of an OFDM symbol having a small FFT length and applies a sufficiently long cyclic prefix to the OFDM symbols of various sizes. That is, according to the present invention, it is possible to prevent performance degradation in a multipath channel environment while including a larger number of OFDM symbols in one OFDM frame.

The construction, operation and effects of the present invention will be further embodied by one embodiment of the present invention described below. For convenience of explanation of one embodiment of the present invention described below, the present invention will be described by comparing the present embodiment. That is, the configuration, operation and effects of the present invention will be described by comparing the present embodiment with the prior art described with reference to FIGS. 5A or 5B and the like.

The present embodiment forms an OFDM frame including at least two OFDM symbols. The sizes of the OFDM symbols are not equal to each other. The OFDM symbols may be grouped into OFDM symbols having a FFT length of a constant first size and OFDM symbols having a FFT length of a second size. If the size (FFT length) of the first OFDM symbol is larger than the size of the second OFDM symbol, the second OFDM may be a partition of the first OFDM symbol by a specific number (D).

This embodiment is compared with the prior art shown in FIG. 5A as follows.

이 더욱 바람직하다. 만약, D=5인 경우 상기 제2 OFDM 심볼의 크기는 상기 제1 OFDM 심볼의 크기를 5등분한 것보다 작아야 하는바, 상기 제2 OFDM 심볼의 크기는 상기 제1 OFDM 심볼의 1/6, 1/7, 1/8, 1/9 등이 될 수 있으나,

이 더욱 바람직하다. 상술한 바와 같이 특정한 OFDM 심볼의 크기는 FFT 길이는 2의 멱승(power)을 갖는 경우 장점이 있기 때문이다.5A shows that the number k of OFDM symbols having a short FFT is 2, the number j of OFDM symbols having a long FFT length is m-1, and the FFT length N _long is equal to the original FFT length N. If it is. In the present embodiment, the number j of the first OFDM symbol is m-1, and the second OFDM symbol is obtained by dividing the first OFDM symbol by a specific number (D = 2), and the size of the second OFDM symbol is It is preferable that the size of the OFDM symbol of the first size is smaller than the equivalent of the specific number (D = 2). That is, when D = 2, since the second OFDM symbol is obtained by dividing the first OFDM symbol by a specific number (D = 2), the number of the second OFDM symbols is two, and the size of the second OFDM symbol is two. Is preferably smaller than dividing the size of the first OFDM symbol by two. However, the size of the second OFDM symbol is more preferably equal to the power of the power of the second OFDM symbol size of (2). For example, when D = 2, the size of the second OFDM symbol should be smaller than dividing the size of the first OFDM symbol by two. The size of the second OFDM symbol is 1 / the size of the first OFDM symbol. 3, 1/4, 1/5, 2/5, etc.

This is more preferable. If D = 5, the size of the second OFDM symbol should be smaller than the size of the first OFDM symbol divided by five. The size of the second OFDM symbol is 1/6 of the first OFDM symbol, 1/7, 1/8, 1/9, etc.,

This is more preferable. As described above, the size of a specific OFDM symbol is advantageous when the FFT length has a power of two.

It should be noted that, in the prior art, in contrast to selecting N / 2, where a short length OFDM symbol is half of the original FFT length, in this embodiment, the size (FFT length) of the second OFDM symbol is 1 of the first OFDM symbol. Less than / 2 For example, the size of the second OFDM symbol may be 1/4 of the first OFDM symbol. If the size of the second OFDM symbol is 1/4 of the size of the first OFDM symbol, the size of the OFDM frame is determined by the following equation. In the following equation, the FFT length of the first OFDM symbol is N _, the length of the cyclic prefix of the first OFDM symbol is N _{CP, long} , and the number of first OFDM symbols is m−1. In the following equation, the FFT length of the first OFDM symbol is N / 4, the length of the cyclic prefix of the first OFDM symbol is N _{CP , short} , and the number of first OFDM symbols is 2 (= D).

In summary, the number of OFDM symbols may be increased by adding a second OFDM symbol having a short FFT length obtained by dividing one first OFDM symbol by one quarter. That is, one first OFDM symbol is replaced with D (= 2) second OFDM symbols, and the FFT length of the second OFDM symbol is N / 4 smaller than N / D. That is, one OFDM frame has m + 1 OFDM symbols in total m OFDM symbols. Finally, the relation of circular translocation is as follows.

If the length of the cyclic prefix N _{CP , new} used in the present embodiment is equal to N _{CP , long} and N _{CP , short} ,

This relationship holds true.

In general, since the length of the cyclic prefix is designed to be very small compared to the FFT length of the first OFDM symbol having the largest size, the length of the cyclic prefix is smaller than N / 2. Therefore _, N _{cp , new} according to the present embodiment has a larger value than N _cp according to the prior art. In addition, according to the present embodiment, not only the short OFDM symbol, that is, the second OFDM symbol, but also the long OFDM symbol, that is, the first OFDM symbol, are more robust to the multipath channel.

The data transmission method according to the present embodiment may be described by the ratio of the cyclic prefix for each OFDM symbol and each OFDM symbol. In the above-described prior art, the ratio of the length of the OFDM symbol of the specific size and the cyclic prefix to the OFDM symbol of the specific size is constant. That is, the above-described prior art is characterized in that the ratio of the large OFDM symbol and its cyclic prefix is equal to the ratio of the small OFDM symbol and its cyclic prefix. However, in the present embodiment described above, the ratio of the length of the cyclic prefix to the first OFDM symbol and the first OFDM symbol is equal to the ratio of the length of the cyclic prefix to the second OFDM symbol and the second OFDM symbol. Small compared to By this feature, the additional second OFDM symbol may be allocated a cyclic prefix enough to allow normal reception at the receiving side.

The OFDM symbol represents a set of data processed by one IFFT operation and is transmitted together for a specific time unit.

In the prior art of FIG. 5B, the number k of OFDM symbols having a short FFT length is 3, the number j of OFDM symbols having a long FFT length is m-1, and the FFT length N _long is Assume that the original FFT length is equal to N. In contrast to the prior art described above, in the present embodiment, the number j of the first OFDM symbols is m−1, and the second OFDM symbol is obtained by dividing the first OFDM symbol by a specific number (D = 3). Preferably, the size of the 2 OFDM symbols is smaller than the size of the OFDM symbols of the first size is divided by the specific number (D = 3). That is, when D = 3, since the second OFDM symbol is obtained by dividing the first OFDM symbol by a specific number (D = 3), the number of the second OFDM symbols is three, and the size of the second OFDM symbol is Is smaller than the size of the first OFDM symbol divided by three. For example, the size of the second OFDM symbol may be equal to the quadrant of the size of the first OFDM symbol. This case can be summarized as follows.

In summary, the number of OFDM symbols may be increased by adding a second OFDM symbol having a short FFT length obtained by dividing one first OFDM symbol by one quarter. That is, one first OFDM symbol is replaced with D (= 3) second OFDM symbols, but the length of the second OFDM symbol is N / 4 smaller than N / D. That is, one OFDM frame has m + 2 OFDM symbols in total of m OFDM symbols. Finally, the relational expression of the cyclic transposition according to the present embodiment is as follows.

Where N _{CP , long} and N _{CP , short} are equal to N _{cp , new} ,

Has a relationship.

In general, since the length of the cyclic prefix is designed to be very small compared to the FFT length of the first OFDM symbol having the largest size, the length of the cyclic prefix is smaller than N / 3. Therefore _, N _{cp , new} according to the present embodiment has a larger value than N _cp according to the prior art. Accordingly, according to the present embodiment, not only the short OFDM symbol, that is, the second OFDM symbol, but also the long OFDM symbol, that is, the first OFDM symbol, are more robust to the multipath channel.

Even in the above-described case, the ratio of the length of the cyclic prefix to the OFDM symbol varies with the OFDM symbol. That is, in the case of an OFDM symbol A having a relatively long FFT length, the ratio of the length of the cyclic prefix to the OFDM symbol A and the OFDM symbol A is relatively small. In the case of an OFDM symbol B having a relatively short FFT length, the ratio of the length of the cyclic prefix to the OFDM symbol B and the OFDM symbol B is relatively large. With this feature, the OFDM symbol according to the present embodiment shows more robust performance for the multipath channel.

Those skilled in the art through the above description can be seen that various changes and modifications can be made without departing from the spirit of the present invention. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

The effect obtained by the present invention is as follows.

The present invention applies a cyclic prefix of a sufficient length to OFDM symbols having different FFT lengths without adjusting the OFDM frame and OFDM sample time, which is advantageous in that performance degradation is prevented in a multipath channel environment.

Claims (10)

A method of transmitting data in an OFDM frame unit including a plurality of OFDM symbols corresponding to input data and a cyclic prefix for each of the OFDM, Forming an OFDM frame comprising at least one first OFDM symbol and at least one second OFDM symbol having a smaller FFT length than the first OFDM symbol; And Transmitting the formed OFDM frame to a receiving side, The second OFDM symbol is obtained by dividing the first OFDM symbol into 2 ^N (where N> 1), and the FFT length of the second OFDM symbol is 1/2 ^N of the FFT length of the first OFDM symbol. Less than The ratio of the FFT length of the cyclic prefix to the second OFDM symbol and the second OFDM symbol is compared to the ratio of the FFT length of the cyclic prefix to the first OFDM symbol and the first OFDM symbol. Big thing A method for transmitting data using a plurality of carriers characterized in that. The method of claim 1, The FFT length of the cyclic prefix for the first OFDM symbol is equal to the FFT length of the cyclic prefix for the second OFDM symbol. A method for transmitting data using a plurality of carriers characterized in that. The method of claim 1, Sample times for the plurality of OFDM symbols are the same A method for transmitting data using a plurality of carriers characterized in that. delete delete A transmitting apparatus for transmitting data in an OFDM frame unit including a plurality of OFDM symbols corresponding to input data and a cyclic prefix for each of the OFDM, An OFDM frame forming module for forming an OFDM frame including at least one first OFDM symbol and at least one second OFDM symbol having a smaller FFT length than the first OFDM symbol; And It comprises a transmitting module for transmitting the formed OFDM frame to the receiving side, The second OFDM symbol is obtained by dividing the first OFDM symbol into 2 ^N (where N> 1), and the FFT length of the second OFDM symbol is 1/2 ^N of the FFT length of the first OFDM symbol. Less than The FFT length ratio of the cyclic prefix to the second OFDM symbol and the second OFDM symbol is larger than the FFT length ratio of the cyclic prefix to the first OFDM symbol and the first OFDM symbol. A transmitting device for transmitting data using a plurality of carriers characterized in that. The method of claim 6, The FFT length of the cyclic prefix for the first OFDM symbol is equal to the FFT length of the cyclic prefix for the second OFDM symbol. A transmitting device for transmitting data using a plurality of carriers characterized in that. The method of claim 6, Sample times for the plurality of OFDM symbols are the same A transmitting device for transmitting data using a plurality of carriers characterized in that. delete delete

Images