KR20110012266A - Apparatus for adaptive impulse postfix ofdm transmission depending on channel conditions - Google Patents

Abstract

PURPOSE: An adaptive impulse postfix OFDM(Orthogonal Frequency Division Multiplexing) transmission device is provided to improve the performance of an IP(Impulse Postfix)-OFDM by controlling the length of an impulse postfix according to a channel state. CONSTITUTION: A converting part(401) generates an OFDM symbol by executing the IFFT(Inverse Fast Fourier Transform) of input symbols. A zero padding part(402) executes a zero padding process for the OFDM symbols. An IP addition part(403) adds the impulse postfix which is defined by the OFDM symbol. The IP addition part determines the length of the impulse postfix based on the channel information which is feedback-transmitted from the reception side.

Description

Adaptive impulse postfix OPM transmitter according to channel environment {APPARATUS FOR ADAPTIVE IMPULSE POSTFIX OFDM TRANSMISSION DEPENDING ON CHANNEL CONDITIONS}

The present invention relates to an apparatus for adaptive impulse postfix (IP) orthogonal frequency division multiplexing (OFDM) according to a channel environment, and more particularly, to generating an impulse postfix (OFDM) symbol. An adaptive impulse postfix OFDM transmitter according to a channel environment capable of improving system performance by adaptively adjusting the length of an impulse postfix according to channel information (CIR length, SNR, etc.) fed back from a receiver. will be.

The present invention is derived from a study performed as part of the Ministry of Knowledge Economy's knowledge economy technology innovation project [Task Management Number: 2007-S-029-03, Task name: Development of home / company WiBro system technology].

Impulse Postfix OFDM (IP-OFDM) systems, unlike classical OFDM systems that use pilots for channel estimation, impulse samples behind a symbol in zero padded OFDM (ZP-OFDM). By adding an "Impulse Postfix" consisting of a plurality of zero samples and a zero sample, the structure is simple but provides a powerful channel estimation performance.

IP-OFDM has two important design parameters when compared to conventional OFDM. One is how much power is allocated to the impulse sample (the size of the impulse sample). One concerns the "length" determination of Impulse Postfix (IP).

While there is considerable research on how to determine the power of an impulse sample, the method of determining the impulse postfix (IP) length is a very important parameter when considering the impact on performance. Although it is a parameter, very little research has been conducted. Therefore, research on this is urgently needed.

In particular, in the prior art, the length of the impulse postfix (IP) is determined to be long and is used as 'fixed' in consideration of a bad channel condition, which is not efficient when the channel condition is good, and also impulse postfix (IP). If the length of R is too long, noise is added to the impulse postfix (IP) portion, which reduces the accuracy of the channel state estimation.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art as described above, and to provide an adaptive impulse postfix OMD transmission apparatus capable of optimally adjusting the length of an impulse postfix according to channel conditions.

The objects of the present invention are not limited to the above-mentioned objects, and other objects and advantages of the present invention which are not mentioned can be understood by the following description, and will be more clearly understood by the embodiments of the present invention. Also, it will be readily appreciated that the objects and advantages of the present invention may be realized by the means and combinations thereof indicated in the claims.

In order to achieve the above object, the present invention adaptively adjusts the length of an impulse postfix according to channel information (CIR length, SNR, etc.) fed back from a receiver in generating an impulse postfix OFDM symbol. It is characterized by adjusting.

More specifically, the present invention provides an adaptive impulse postfix (IP) OFDM apparatus according to a channel environment, comprising: conversion means for generating an OFDM symbol by performing an inverse fast Fourier transform (IFFT) on an input symbol; Zero padding means for performing zero padding on the OFDM symbol; And IP adding means for adding "an impulse postfix (IP) whose length is determined based on channel information" to said zero-padded OFDM symbol.

As described above, the present invention can improve the performance of the IP-OFDM by appropriately adjusting the length of the impulse postfix (IP) according to the channel situation. This has the effect of providing a foundation for the service.

The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings, in which: There will be. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

1 is a block diagram of a general impulse postfix (IP) OFDM transmission apparatus.

A general impulse postfix OFDM transmission apparatus includes an inverse fast Fourier transform unit (IFFT) 11, a zero padding unit 12, and an impulse postfix addition unit 13, as shown in FIG. 1. An Inverse Fast Fourier Transform (IFFT) 11 performs an Inverse Fast Fourier Transform (IFFT) on an input symbol, and the zero padding unit 12 outputs the output symbols of the Inverse Fast Fourier Transform (IFFT) 11 (FIG. There is zero padding for " 21 " in 2a (see " 22 " in FIG. 2a). Then, the impulse postfix adding section 13 adds a fixed length impulse postfix IP to the second half of the "ZP-OFDM symbol" which is the output of the zero padding section 402 ("23" in Fig. 2A). .

길이만큼 제로 패딩(Zero Padding)하고 임펄스 포스트픽스(IP) 길이를

라고 할 때, IP-OFDM 심볼 벡터(symbol vector)는 다음의 [수학식 1]과 같이 나타낼 수 있다.With Guard Interval of OFDM

Zero Padding by Length and Impulse Postfix (IP) Length

In this case, the IP-OFDM symbol vector can be represented by Equation 1 below.

OFDM 데이터 벡터(data vector)이고,

는

제로 벡터(zero vector)이며,

는 채널추정을 위한

임펄스 포스트픽스 벡터(IP vector)이다. Where x is

OFDM data vector

Is

Is a zero vector,

For channel estimation

Impulse postfix vector (IP vector).

는 하나의 임펄스 응답(impulse sample)(s)과

개의 제로 샘플(zero sample)로 구성되어 있는데, 다음과 같이 나타낼 수 있다.

Is an impulse response (s) and

It consists of two zero samples, which can be represented as follows.

은

제로 벡터(zero vector)를 나타낸다.here,

silver

Represents a zero vector.

2A and 2B are conceptual explanatory diagrams of a general impulse postfix (IP) OFDM.

FIG. 2A shows an IP-OFDM transmission signal, and FIG. 2B shows an IP-OFDM signal (IP-OFDM received signal) after a channel effect in the absence of Additive White Gaussian Nois (AWGN). In the drawings, "21" and "24" denote subcarrier portions, "22" and "25" denote zero padding portions, and "23" and "26" denote impulse postfix portions.

=

, 8탭(Tap), 지수함수적으로 감소하는 레일리 페이딩 채널인 경우를 나타낸다.2A and 2B show the number of subcarriers N = 64,

=

, 8 taps, a Rayleigh fading channel that decreases exponentially.

3A and 3B are comparative explanatory diagrams of a channel impulse response (CIR) according to whether additional white Gaussian noise (AWGN) is added. FIG. 3A is a case in which AWGN is not added. Indicates.

을 시간 영역(time domain)에서 표현하면 다음의 [수학식 3] 다음과 같이 나타낼 수 있다.IP-OFDM reception signal in general communication system

When expressed in the time domain can be expressed as follows.

은 AWGN,

은

의 n 번째 요소(element)이다.

범위에서는

은

과 동일하다. Channel impulse response (CIR),

Silver AWGN,

silver

The nth element of.

In scope

silver

Is the same as

이라고 정의하면 [수학식 3]은 다음의 [수학식 4]와 같게 된다. if,

Equation 3 is the same as Equation 4 below.

Equation 4 may be simply expressed as in Equation 5 below.

은 추정된 CIR을 나타낸다.here,

Represents the estimated CIR.

보다 더 길다면 전체 채널 정보가 임펄스 포스트픽스(IP)에 캡쳐(capture) 되지 않을 것이다. 반면에, 단순하게 생각할 때

를 CIR보다 길게 정의하면 전체 채널정보를 캡쳐(capture)하여 채널 추정이 용이할 것으로 예상되지만, [수학식 5]를 보면 알 수 있듯이 AWGN(

)이 더해지기 때문에 CIR이 영(zero)인 지역에서는 AWGN이 채널정보로 인식되어 채널추정 성능이 떨어진다. 이러한 현상은 도 3a 및 도 3b를 통하여 알 수 있다. Looking at Equation 5, the concept of the IP-OFDM channel estimation technique can be understood. If CIR

If longer, the entire channel information will not be captured in the impulse postfix (IP). On the other hand, when you think simple

If is defined as longer than CIR, it is expected to capture the entire channel information to facilitate channel estimation, but as shown in Equation 5, AWGN (

), AWGN is recognized as channel information in the region where the CIR is zero, resulting in poor channel estimation performance. This phenomenon can be seen through FIGS. 3A and 3B.

4 is a diagram illustrating an embodiment of an adaptive impulse postfix OFDM transmission apparatus using first feedback information (CIR length) according to the present invention. Hereinafter, the adaptive postfix (IP) addition method performed in the adaptive impulse postfix OFDM transmission apparatus will also be described.

The present invention improves the performance of the overall system by increasing the channel estimation performance by adjusting the impulse postfix (IP) length appropriately according to the channel situation, wherein the channel status information fed back from the receiving side (RX) to the transmitting side (TX) (Channel information) is used. Here, the feedback information (channel information) includes "channel impulse response (CIR) length" (first feedback information), "signal-to-noise ratio (SNR)" (second feedback information), "channel impulse response (CIR) length and signal-to-noise ratio". (SNR) "(third feedback information). The second and third feedback information will be described with reference to FIGS. 6 and 8. A large amount of feedback information provides better performance, but may be a burden on the processing of the feedback information.

The adaptive impulse postfix OFDM transmitter 40 according to the present invention has an inverse fast Fourier transform unit (IFFT) 401, a zero padding unit 402, and an adaptive impulse postfix addition as shown in FIG. A portion 403 is included. An Inverse Fast Fourier Transform (IFFT) 401 performs an Inverse Fast Fourier Transform (IFFT) on an input symbol, and a zero padding unit 402 outputs an output symbol of an Inverse Fast Fourier Transform (IFFT) 401 (FIG. There is zero padding for " 21 " in 2a (" 22 " in FIG. 2a). Then, the adaptive impulse postfix adding unit 403 adds an impulse postfix (IP) to the second half of the "ZP-OFDM symbol" which is the output of the zero padding unit 402 ("23" in FIG. 2A), where The length L ₂ of the added impulse postfix IP is determined using channel information fed back from the receiving side.

Next, a case in which the CIR length is transmitted as feedback information from the receiving side RX to the transmitting side TX will be described in detail with reference to FIG. 4.

The adaptive IP-OFDM transmitting apparatus 40 allows the entire CIR to be captured in the impulse postfix IP by sufficiently lengthening the impulse postfix IP at initial setting.

Due to the nature of IP-OFDM, CIR length estimation is very easy. The receiver channel estimator 41 estimates the CIR length information based on the channel information captured by the impulse postfix (IP), and then transmits the CIR length information through the feedback channel 42. Send it to).

The adaptive impulse postfix adding unit 403 of the adaptive IP-OFDM transmitting device 40 adjusts the IP length based on the CIR length information received through the feedback channel 41 and receives the IP-OFDM signal accordingly. To the side. In other words, the adaptive impulse postfix adder 403 adjusts the IP length to match the CIR length.

How often the channel estimator 41 on the receiving side should send feedback information to the transmitting side depends on the channel environment, the performance desired by the system designer, and the like.

5 is an explanatory diagram of a performance comparison between the adaptive IP-OFDM transmission apparatus of FIG. 4 and the prior art according to the present invention.

CIR is assumed to be a random variable uniformly distributed with [1,65]. Then, the number of subcarriers (N) = 256, the zero padding length (L ₁ ) = 64, and in the case of Normal, the IP length (L ₂ ) = 65 was fixed.

In the present invention (the embodiment of FIG. 4), since the CIR length information is accurately received through the feedback channel and the IP length is adjusted, as shown in FIG. 5, the system performance is significantly improved (2.5 dB or more).

6 is a diagram illustrating an embodiment of an adaptive impulse postfix OFDM transmitter using second feedback information (SNR) according to the present invention.

In the embodiment as shown in FIG. 4, "CIR length" is transmitted as feedback information. However, assuming that the adaptive IP-OFDM transmitting apparatus on the transmitting side already knows the CIR length, even if the SNR (second feedback information) is sent as feedback information, the performance can be improved.

In the case of the embodiment shown in FIG. 6, the amount of feedback information may vary depending on how the SNR range is divided, but generally, a smaller amount of feedback information is sent than a CIR length.

When the SNR estimator 61 on the receiving side estimates the SNR from the output signal (receive signal) of the RF receiver and transmits the SNR to the adaptive IP-OFDM transmitting apparatus 60 through the feedback channel 62, the adaptive IP-OFDM The transmitting device 60 (more precisely, the "adaptive impulse postfix addition 603") adjusts the length of the impulse postfix IP according to the range of its SNR. It can be seen from the simulation results of FIGS. 7 and 8 that the optimal IP length may vary depending on the range of the SNR. However, the length of the impulse postfix (IP) is determined to be inversely related to the SNR size.

7 and 8 are explanatory diagrams of the relationship between the SNR and the impulse postfix (IP) length, and the SNR and the IP length L _{2 in} the case of applying the adaptive IP-OFDM transmission scheme of FIG. Represents a relationship.

Fig. 7 shows the BER simulation results of IP-OFDM and QPSK when CIR = 31 is fixed (the transmitting side already knows the CIR length), and Fig. 8 shows the 16QAM BER simulation results under the same conditions.

In the case of FIG. 7 (QPSK), when the SNR corresponds to the reference value (“SNR Switching Threshold”), for example, 20 dB or less, the IP length is smaller than the CIR length (CIR = 31), for example. For example, select "16" (L ₂ = 16), and if it is 20 dB or more, the IP length is the same / similar to the CIR length (CIR = 31) within a certain error range, for example "32" (L ₂ = 32). If you select, you can expect a 1dB performance improvement. Longer IP lengths can add more noise within the range that allows the Post Prefix (IP) to maintain its original functionality, so if the channel conditions are poor, such as when the SNR is less than the reference value, the IP length Is smaller than the CIR length. In some embodiments, a plurality of reference values may be used, and the length of the impulse postfix (IP) is determined to be inversely related to the SNR. In the case of Fig. 8 (16QAM), the reference value (SNR switching threshold) is changed to 17 dB. Therefore, the SNR switch threshold may vary according to the modulation method selection.

9 is a configuration diagram of an adaptive impulse postfix OFDM transmitter using third feedback information (CIR length and SNR) according to the present invention.

The channel / SNR estimator 91 on the receiving side estimates the "CIR length" and "SNR" from the output signal (receive signal) of the RF receiver and adaptive IP-OFDM transmission device 90 on the feedback channel 92. To send).

Then, the adaptive IP-OFDM transmitting apparatus 90 (more precisely, the "adaptive impulse postfix adding unit 903") obtains an optimal IP length using the CIR length and the SNR, and then ZP-OFDM symbol. Insert "IP as much as the optimal IP length" at the end of to generate the IP-OFDM transmission signal and send it to the receiver. In this case, the adaptive impulse postfix adding unit 903 uses a table (TABLE) that presents an optimal IP length according to the CIR length and the SNR (a CIR length mapping table configured according to the CIR length and the SNR).

In general, when the SNR is low (when the SNR is smaller than the predetermined reference value), shortening the IP length rather than the actual CIR length may reduce the influence of noise, and when the SNR is high (the SNR is above the predetermined reference value) ) Improves performance by making IP length equal / similar to CIR length within a certain error range. Here, the reference value is " SNR switching threshold ", for example, 20 dB.

A mapping table (TABLE) indicating an optimal IP length according to the CIR length and the SNR may be determined through simulation. For example, the mapping table may be represented as shown in Table 1 below. Here, the mapping table TABLE is configured such that the length of the impulse postfix IP is inversely related to the SNR in the case of a specific CIR length.

[Table 1] shows the optimal IP length according to CIR length and SNR when the QPSK signal and CIR = 32. If the modulation scheme is changed, the optimal IP length specified in [Table 1] is also changed. As shown in [Table 1], if CIR = 32 and SNR = 10dB, the IP length can be set to “16”, and if the SNR = 25dB in the same CIR, the IP length can be set to “20”.

 In Table 1, simulation results are displayed only when CIR = 32, and very few CIR, SNR, and IP length levels are used, but in the actual system, more CIR, SNR, and IP length levels are created. If you use it, you can improve performance further.

On the other hand, the method of the present invention as described above can be written in a computer program. And the code and code segments constituting the program can be easily inferred by a computer programmer in the art. In addition, the written program is stored in a computer-readable recording medium (information storage medium), and read and executed by a computer to implement the method of the present invention. The recording medium may include any type of computer readable recording medium.

The present invention described above is capable of various substitutions, modifications, and changes without departing from the technical spirit of the present invention for those skilled in the art to which the present invention pertains. It is not limited by the drawings.

1 is a block diagram of a general impulse postfix (IP) OFDM transmission apparatus,

2A and 2B are conceptual explanatory diagrams of a general impulse postfix (IP) OFDM;

3A and 3B are comparative explanatory diagrams of a channel impulse response (CIR) according to whether additional white Gaussian noise (AWGN) is added;

4 is a configuration diagram of an adaptive impulse postfix OFDM transmission apparatus using first feedback information (CIR length) according to the present invention;

5 is an explanatory diagram of a performance comparison between the adaptive IP-OFDM transmitting apparatus of FIG. 4 and the prior art according to the present invention;

6 is a configuration diagram of an adaptive impulse postfix OFDM transmission apparatus using second feedback information (SNR) according to the present invention;

7 and 8 are explanatory diagrams showing the relationship between the SNR and the impulse postfix (IP) length;

9 is a configuration diagram of an adaptive impulse postfix OFDM transmitter using third feedback information (CIR length and SNR) according to the present invention.

* Explanation of symbols on the main parts of the drawings

40, 60, 90: Adaptive IP-OFDM transmitting device 41: Channel estimating unit

401: Inverse fast Fourier transform unit 402: Zero padding unit 403, 603, 903: Adaptive impulse postfix addition unit

61: SNR estimator 91: channel / SNR estimator

Claims (10)

In the adaptive impulse postfix (IP) OFDM apparatus according to the channel environment, Transform means for inverse fast Fourier transform (IFFT) the input symbol to generate an OFDM symbol; Zero padding means for performing zero padding on the OFDM symbol; And IP adding means for adding "an impulse postfix (IP) whose length is determined based on channel information" to said zero-padded OFDM symbol. Adaptive IP-OFDM transmission device comprising a. The method of claim 1, The IP adding means, Adaptive IP-OFDM transmission apparatus for determining the length of the impulse postfix (IP) based on the channel information that is transmitted feedback from the receiving side. The method of claim 2, The channel information, And an at least one of a channel impulse response (CIR) length and a signal-to-noise ratio (SNR). The method of claim 3, wherein The IP adding means, And if the channel information is a channel impulse response (CIR) length, determine the length of the impulse postfix (IP) to match the channel impulse response (CIR) length. The method of claim 3, wherein The IP adding means, And if the channel information is a signal-to-noise ratio (SNR), determining the length of the impulse postfix (IP) to be inversely related to the signal-to-noise ratio (SNR). The method of claim 5, The IP adding means, If the channel information is a signal-to-noise ratio (SNR), if the signal-to-noise ratio (SNR) is less than a predetermined reference value, the length of the impulse postfix (IP) is determined to be smaller than a predetermined CIR length, and the signal-to-noise ratio (SNR) is determined. And transmitting the impulse postfix (IP) to the same / similarity within the predetermined error range. The method of claim 3, wherein The IP adding means, If the channel information is a channel impulse response (CIR) length and a signal-to-noise ratio (SNR), the length of the impulse postfix (IP) is determined using a mapping table configured according to the CIR length and the signal-to-noise ratio (SNR). Adaptive IP-OFDM Transmission Device. The method of claim 7, wherein The mapping table is, And the length of the impulse postfix (IP) is inversely related to the signal-to-noise ratio (SNR). The method of claim 8, The mapping table is, If the signal-to-noise ratio SNR is smaller than a predetermined reference value, the length of the impulse postfix IP is determined to be smaller than the CIR length. If the signal-to-noise ratio SNR is greater than or equal to the reference value, the impulse postfix IP is determined. Adaptive IP-OFDM transmission apparatus, characterized in that the length is configured to be determined the same / similar to the CIR length within a predetermined error range. The method according to claim 6 or 9, The reference value, Adaptive IP-OFDM transmission apparatus, characterized in that set based on the modulation scheme.

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