KR100653282B1 - Pilot sequence transmission for channel estimation in ofdm cellular systems - Google Patents

Abstract

A method for transmitting a pilot sequence for channel estimation in an OFDM(Orthogonal Frequency Division Multiplexing) cellular system is provided to improve channel estimation performance and reception performance of the cellular system, by minimizing interference during a channel estimation process. In a method for transmitting a pilot sequence for channel estimation in an OFDM cellular system, the total or a part of a pilot signal sub carrier for channel estimation is used in common through equal time and frequency resource, in a number of adjacent cells. As to a time section of a channel to be estimated and a protection section for delay of an interference signal, a pilot signal sequence of an adjacent random cell 1 is modulated and then transmitted in order to be multiplied by a pilot signal sequence of a random cell 0 adjacent to an interference signal sequence where time response of the interference signal sequence satisfies a condition that does not exceed a specific threshold.

Description

Pilot Sequence Transmission for Channel Estimation in OFDM Cellular Systems

1 is a view showing an example of a pilot training signal subcarrier insertion structure of an OFDM-based cellular system,

2 is a diagram illustrating an OFDM receiver structure including a channel estimator using a training signal sequence;

3 is a diagram illustrating an example of interference due to training signal string collision between two adjacent cells in an OFDM-based cellular system,

4 is a diagram illustrating an example of a time response relationship between a channel impulse response and a controlled interference signal sequence in an OFDM-based cellular system;

FIG. 5 is a diagram illustrating mutual definitions and relationships of training signal interference signal sequences for three cells in an OFDM-based cellular system.

6 is a diagram illustrating a modulation interference signal sequence search and generation of interference signal sequence sets in an OFDM-based cellular system.

7 is a diagram illustrating an example of interference signal sequence aggregation using repetition characteristics in an OFDM-based cellular system,

8 is a diagram illustrating an example of a modulation process of a pilot training signal using an interference signal sequence set generated by repetition characteristics in an OFDM-based cellular system.

9 illustrates an example of an OFDM-based cellular system using an interference signal sequence set and a cluster.

The present invention relates to a method for transmitting a training signal sequence for channel estimation in an orthogonal frequency division multiplexing (OFDM) based cellular system. More particularly, in an OFDM based cellular system, channel estimation from a base station of a cell adjacent to each other is performed. The present invention relates to a training signal sequence transmission method for transmitting a training signal sequence to minimize mutual interference in a channel estimation process when all or a portion of the training signal sequence to be transmitted uses the same resources in time and frequency.

The present invention can improve the channel estimation performance and the reception performance of the cellular system by estimating the channel using the training signal sequence transmitted from each cell, the training signal sequence satisfying such a condition, and the training signal sequence.

Typical OFDM systems include DAB (Digital Audio Broadcasting), a European-style digital broadcasting system, and Digital Video Broadcasting-Terrestrial (DVB-T), IEEE802.11a and Hyperlan / 2, which are wireless LAN standards. The DAB system does not need channel estimation because it uses differential modulation between adjacent symbols. However, the DAB system cannot use a bandwidth efficient modulation scheme such as 16QAM or 64QAM, and has a disadvantage in that detection performance is degraded due to differential modulation.

In DVB-T system that requires higher bandwidth efficiency for transmission of video signal, the channel frequency response of each subcarrier is estimated by inserting a pilot signal known from the transmitter / receiver to some subcarriers of each symbol. According to this result, a change in the magnitude and phase of the received signal is compensated at the decision stage input. On the other hand, in the WLAN system, due to the characteristics of a packet having a low mobility and a short length, the channel is estimated by transmitting one or two symbols (Preamble) promised between the transceivers before the data symbol.

In a conventional OFDM system, a training signal sequence subcarrier or a symbol used for channel estimation is generally modulated into an arbitrary signal sequence known between transceivers. However, in cellular systems, channel performance is rapidly degraded due to interference caused by training signal sequences transmitted from adjacent cells, and channel estimation performance is maintained because this degraded channel estimation performance affects the transmission capacity of the entire system. There is a need for a technique.

One method for maintaining channel estimation performance in an OFDM-based cellular system is to design a training signal sequence pattern using a high frequency reuse rate (EP1178641, "Frequency Reuse Scheme for OFDM systems," European Patent, SONY, 2002). Reference). This method uses a higher frequency reuse rate for the training signal subcarriers than the frequency reuse factor (FRF) used for the data traffic channel to reduce the interference in channel estimation by the training signal sequence transmitted from adjacent cells. It is a technique. This method has the advantage of not requiring perfect time synchronization between base stations, but it is a general assumption that the time synchronization between base stations must be aligned for managing inter-cell interference of data traffic channels. In addition, when the frequency selective characteristic of the channel is strong, there is a disadvantage that it is difficult to allocate a sufficient number of training signal subcarriers as much as accurate channel estimation due to the high frequency reuse rate applied to the training signal sequence.

Another method for maintaining channel estimation performance in an OFDM-based cellular system is to modulate the interfering characteristics of training signal sequences transmitted from adjacent cells to average the interference in channel estimation. It is a signal string modulation method. However, this method reduces interference only by the ratio of the number of training signal subcarriers and the length of the channel, and thus has a disadvantage in that channel estimation performance deteriorates rapidly as the number of training signal subcarriers does not increase or approaches the cell boundary. It is difficult to apply in a cellular system that guarantees.

The present invention has been proposed to solve the problems of the prior art, in the OFDM-based cellular system, all or part of the training signal sequence transmitted for the channel estimation from the base station of the cell adjacent to each other is equal to each other in time and frequency When using resources, in order to minimize the mutual interference in the channel estimation process, the mutual condition of the training signal sequence transmitted from each cell, the training signal sequence satisfying these conditions, and the channel by estimating the channel using the training signal sequence It is an object of the present invention to provide a training signal transmission method for channel estimation in an OFDM-based cellular system that can improve the estimation performance and the reception performance of a cellular system.

Other objects and advantages of the present invention 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.

The present invention for achieving the above object, Orthogonal Frequency Division Multiplexing (OFDM) based cellular in which all or part of a pilot training signal subcarrier for channel estimation is commonly used through the same resources of time and frequency in a plurality of adjacent cells. In the method of transmitting a training signal sequence for channel estimation in a system, an interference signal satisfying a condition that an interference signal sequence time response does not exceed a specific threshold for a guard interval for a time interval of a channel to be estimated and a delay of an interference signal. And modulating and transmitting the training signal sequence of any cell 1 adjacent to each other so as to multiply the pilot training signal sequence of any cell 0 adjacent to the column.

As such, the present invention proposes a correlation between the training signal sequence and the design method for increasing the effective frequency reuse rate in channel estimation in order to minimize mutual interference caused by the training signal sequence between adjacent cells in an OFDM-based cellular system. To this end, the present invention modulates the pilot training signal to reduce intercell interference in an OFDM-based cellular system. In the process of transmitting a pilot training signal through a set of subcarriers corresponding to the same time and frequency resources in adjacent cells and estimating a channel response using the same, a channel to which interference due to the pilot training signal transmitted from adjacent cells is to be estimated By designing the pilot training signal to exist at a different time delay than the impulse response of, it is possible to increase the frequency reuse efficiency for the channel estimation of the cellular system and to increase the transmission capacity of the entire system.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, whereby those skilled in the art may easily implement the technical idea of the present invention. There will be. In addition, in describing the present invention, when it is determined that the detailed description of the known technology related to the present invention may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a diagram illustrating an example of a pilot training signal subcarrier insertion structure of an OFDM-based cellular system, and shows a symbol structure of an OFDM-based cellular system according to an embodiment of the present invention.

One OFDM symbol 100 is composed of several subcarriers, and most subcarriers are data subcarriers that transmit bit information strings that are modulated by BPSK, QPSK, or QAM according to a transmission rate or error probability required by a system. 110). In addition, some of the remaining subcarriers are used as a signal or pilot subcarrier 120 used for channel estimation or synchronization by transmitting a signal sequence known in advance at the transceiver end.

In a cellular OFDM system, a common training signal subcarrier 121 using the same resource as a pilot at the same time and frequency position as an adjacent cell and a subcarrier 122 transmitting a training signal at a time and frequency position designated to transmit data in an adjacent cell. Can be mixed and used. In addition, a common subcarrier used at the same time and frequency location and a subcarrier using separate resources may be different for each adjacent cell.

In an OFDM cellular system using a preamble or a midamble, since all cells use one OFDM symbol at a specific time, it can be considered that all subcarriers constituting the preamble or the midamble are composed of a common training signal subcarrier.

FIG. 2 is a diagram illustrating an OFDM (A) receiver structure including a channel estimator using a training signal sequence, and illustrates a receiver structure generally used in an OFDM (A) system.

The frequency axis received signal obtained after the Discrete Fourier Transform requires a channel estimator 200 to compensate for magnitude and phase distortion by the channel.

In order to estimate the channel, the training signal subcarrier extractor 210 first extracts a reception value corresponding to the training signal subcarrier position from the frequency axis received signal. The training modulated signal transmitted from the received value extracted by the training signal subcarrier extractor 210 may be divided to obtain a channel frequency response at a corresponding frequency position. In general, since the training signal is modulated into a PSK signal having a constant size for convenience of implementation, the division operation may be replaced with a complex complex operation of the complex and the received signal of the training signal. To this end, a training signal sequence complex multiplier 220 is provided.

Since the obtained channel frequency response at the training signal subcarrier location is severely distorted by noise and interference, the distortion due to noise or interference is reduced by using the estimation filter 230 using correlations in the time and frequency axis of the channel. Let's do it. The length of the channel impulse response is designed to eliminate inter-symbol interference (ISI), which is generally shorter than the length of the guard interval, so that the channel frequency response at different subcarrier positions is correlated and therefore appropriate. The implementation of the estimation filter reduces the effect of noise on channel estimation. Estimation filters using short lengths of channels are described in M. Morelli and U. Mengali, “A Comparison of Pilot-Aided Channel Estimation Methods for OFDM Systems,” IEEE Trans. Signal Processing , vol. 49, No. 12, pp. 3065-3073, Dec. In 2001, an estimation filter using the statistical characteristics of the channel through the power delay function (Multipath Intensity Profile) and the noise average power is described by O. Edfors, M. Sandell, JJ van de Beek, SK Wilson, and PO Borjesson, "OFDM Channel Estimation. by Singular Value Decomposition, " IEEE Trans. Commun. , vol. 46, pp. 931 939, July 1998. However, these estimation filters show good performance in a noisy environment having no correlation for each training symbol subcarrier, and when applied as it is in a cellular system with a large influence of interference, the gain of channel estimation performance is not large due to inter-cell interference.

The channel frequency interpolation filter 240 is used to determine the channel response of the data subcarrier position using the channel frequency response of the training signal subcarrier which has higher accuracy through the estimation filter 230. As the interpolation method, a linear interpolator or a second order polynomial interpolator is used. In particular, an interpolation method using a discrete Fourier transform may be combined with the estimation filter 230 and implemented as a single device. .

3 illustrates an example of interference due to training signal sequence collision between two adjacent cells in an OFDM-based cellular system, through the same resource in the time and frequency axes in two adjacent cells (Cell 0 and Cell 1). Shows the definition of the interference signal sequence between training symbols generated by transmitting the training signal sequence for channel estimation.

When trying to estimate the channel response between the cell and the terminal by using the training symbol received from a specific cell (Cell 0) currently serving as a terminal, for this purpose, the terminal first trains a signal of any cell 0 signal. The signals corresponding to the positions of the subcarriers in the column and the channel frequency response of the subcarriers are extracted. In this process, the channel frequency response of many subcarriers can be used by storing the frequency response of the channel in the buffer for several symbol intervals to improve the performance through the channel estimation filter. It is generally assumed that during the symbol interval 310 the channel has hardly changed in time.

Other cells adjacent to each other in the time and frequency resources corresponding to the training signal sequence subcarrier position 320 transmitted from the cell 0 may transmit the training signal sequence, transmit data traffic, or transmit no signal. .

During the buffer time in which the training signal sequence is stored, the pilot training signal of the cell 1 is transmitted from the pilot training signal sequence transmitted by the cell 0 and the cell 1 to the same time and frequency resources. Observe the interference on the estimation process.

번째 부반송파에서의 주파수 수신 신호는 아래의 [수학식 1]과 같이 표현될 수 있다.Assuming that each cell has the same time synchronization with each other and that the time delay error due to transmission is small enough, intersymbol interference (ISI) is removed by the guard interval,

The frequency reception signal in the first subcarrier may be expressed as Equation 1 below.

은 셀 a 기지국과 단말 사이에 연결된 채널에 대한

번째 부반송파의 주파수 응답을,

은 셀 a 기지국이

번째 부반송파에 전송하는 신호를,

은

번째 부반송파에서의 열잡음을 각각 나타낸다.In [Equation 1]

Is the channel connected between the cell a base station and the terminal

Frequency response of the first subcarrier,

Cell a base station

The signal to be sent to the first subcarrier,

silver

The thermal noise of the first subcarrier is respectively shown.

번째 부반송파 번호를

라고 표기하고, 두 셀이 공통의 자원을 사용하지 않고 파일롯 훈련신호 간의 충돌이 발생하지 않는 훈련 부반송파의 집합 중에서

번째 부반송파 번호를

라고 하면, 셀(Cell) 0에서 서비스하고 있는 단말이 관찰하는 공통 파일롯 훈련 부반송파 위치에서의 채널 주파수 응답의 추정값을 다음의 [수학식 2]와 같이 표현할 수 있다.Among subcarrier sets of pilot training signal sequence in resources shared by two cells (Cell 0 and Cell 1)

Subcarrier number

In the set of training subcarriers in which two cells do not use common resources and no collision between pilot training signals occurs.

Subcarrier number

In this case, the estimated value of the channel frequency response at the common pilot training subcarrier location observed by the UE serving the cell 0 can be expressed as Equation 2 below.

는 셀 1의

번째 부반송파 위치에 송신하는 파일롯 훈련신호의 변조신호열을 나타내고,

는 셀 0에서 송신하는 변조신호열을 의미한다. 이 중 셀 0의 채널 추정 과정에서 셀 1의 훈련신호가 미치는 간섭을 아래의 [수학식 3]과 같이 간섭신호열(330)의 형태로 표현할 수 있다.In Equation 2 above

Of cell 1

A modulation signal sequence of a pilot training signal transmitted to a first subcarrier position,

Denotes a modulated signal string transmitted from cell 0. The interference of the training signal of the cell 1 in the channel estimation process of the cell 0 can be expressed in the form of the interference signal sequence 330 as shown in Equation 3 below.

From [Equation 2] and [Equation 3], the effect of the pilot training signal of the cell 1 on the channel estimation between the cell 0 and the terminal is expressed by the product of the interference signal sequence and the channel response between the terminal and the cell 1 Able to know.

4 is a diagram illustrating an example of a time response relationship between a channel impulse response and a controlled interference signal sequence in an OFDM-based cellular system. FIG. 4 illustrates a relation between a channel response of cell 0 to be estimated and an interference signal sequence of a training signal of an adjacent cell 1. FIG. Represents a relationship.

은 이산 푸리에 역변환(IFFT)을 통해서 채널 임펄스 응답(410)으로 변환될 수 있다. 채널 임펄스 응답으로의 변환은, 채널 추정 성능을 높이기 위해서 사용되는 추정 필터의 구현 과정에서 직접적인 방법이나 채널 상관 행렬을 이용한 간접적인 방법으로 대부분 적용된다.Frequency response of channel estimated from product of frequency axis received signal and pilot training signal

May be transformed into the channel impulse response 410 through a discrete Fourier inverse transform (IFFT). The conversion to the channel impulse response is mostly applied in the direct method or indirect method using the channel correlation matrix in the implementation of the estimation filter used to improve the channel estimation performance.

The estimated value of the channel impulse response may be approximately defined as shown in Equation 4 below.

At this time, the number of pilot subcarriers is sufficient, and the closer to equal intervals, the expression of [Equation 4] is approximately identical. In the estimation process for the channel impulse response, it can be seen that the time response of the interference signal sequence defined between the cell 0 and the pilot training signal sequence of the cell 1 affects the channel delay connected between the cell 1 and the terminal. Here, the time response of the interference signal sequence by the training signal sequence transmitted from the neighbor cell 1 is defined as in Equation 5 below.

에 대해서 고르게 확산되어서 간섭량을 감소시키게 되지만, 파일롯 부반송파의 개수가 충분히 크지 않거나 셀의 경계 지역에서 두 셀로부터 수신되는 신호의 크기가 비슷하면 간섭량은 매우 커진다.In general, when the pilot training signal sequences transmitted from two adjacent cells are modulated by different PN sequences, etc. so that they do not have correlation with each other, the time-base interference signal sequences are all time intervals.

Although it spreads evenly to reduce the amount of interference, the amount of interference is very large if the number of pilot subcarriers is not large enough or if the signals received from the two cells are similar in the cell boundary area.

In order to prevent this phenomenon, the time response 430 of the interference signal sequence and the time responses 440 delayed by the multipath channel between the cell 1 and the terminal are minimized within the interval 420 of the channel impulse response to be estimated. do. In the present invention, the interference signal sequence defined by the complex multiplication of the pilot training signal sequence of the adjacent cell 1 and the pilot training signal sequence of the cell 0 and its time response are designed to reduce the channel impulse response.

Since the user terminal is time-synchronized with respect to cell 0 to estimate a channel, even though the two cells are synchronized with each other by perfect reference timing, the pilot training signal sequence of cell 1 is determined by the difference in distance between the terminal and the two base stations. It has a constant delay and is received by the terminal in the form of time spread 440 by the multipath channel between cell 1 and the terminal. Since the time response of the interference signal sequence may interfere with the channel impulse response to be estimated by the cyclic characteristics of the Discrete Fourier Transform when the time delay passes, the time response 430 of the interference signal sequence is timed to prevent the cyclic phenomenon. The amount of interference should also be designed for the interval 450 at the end of the response. However, when accurate time synchronization between each base station is obtained, as the time delay between the base station and the terminal of the neighboring cell increases, since the user terminal is closer to cell 0 than the cell 1, the received interference signal power is gradually attenuated. Does not affect estimation Therefore, the guard interval 450 according to the delay of the interference signal need not be designed to include both the channel length and the time delay between the cell 1 and the terminal.

The condition regarding the time response of the interference signal sequence including all the above relations can be simply expressed by Equation 6 below.

는 추정하고자 하는 이산 채널 임펄스 응답의 길이를 나타내고,

는 두 셀에서의 시간 지연차이와 인접 셀에서의 채널의 시간 확산의 합을 나타내는데, 이는 전력 감쇄를 무시할 수 없는 범위에 대해서 설정한 값이다. 즉, 추정하고자 하는 채널의 시간 구간과 간섭신호의 지연에 대한 보호구간에 대해서, 간섭신호열 시간응답이 특정한 임계치

를 넘어서지 않도록 설계되어야 한다.In Equation 6 above

Denotes the length of the discrete channel impulse response to be estimated,

Represents the sum of the time delay difference in the two cells and the time spread of the channel in the adjacent cell, which is set for a range in which power attenuation cannot be ignored. That is, for the time interval of the channel to be estimated and the protection interval for the delay of the interference signal, the interference signal sequence time response has a specific threshold value.

It should be designed not to exceed

The expression using the time response of the interference signal sequence described in Equation 6 is useful when the positions of pilot subcarriers commonly used in adjacent cells 0 and 1 are large enough and are arranged at substantially constant intervals on the frequency axis. In particular, when a base station of each cell wants to estimate a channel using a symbol (preamble) composed of almost all training signal subcarriers at the same time, it can be easily applied.If common training subcarriers are arranged at regular intervals, the subcarriers Since it is represented in the form of a repeated time response as much as the interval, the relationship as shown in Equation 6 can be obtained.

Also, even though the common pilot training signal subcarriers are not perfectly equidistant, it is common to distribute them as evenly as possible in order to observe many frequency components.

FIG. 5 is a diagram illustrating mutual definitions and relationships of training signal interference signal sequences for three cells in an OFDM-based cellular system. Example of a system for specifically illustrating a pilot training signal sequence modulation method by adjusting interference signal sequences in a cellular system. Is showing.

More general settings are needed to apply the adjusted interference signal sequence to cellular systems. First, for both cells (Cell 0 and Cell 1), not only does the pilot training signal of cell 1 interfere with the channel estimation of adjacent cell 0, but the pilot training signal sequence of cell 0 also interferes with the channel estimation of cell 1 , It should be adjusted in the form of mutual interference 510 between the two cells. The interrelationship of the two cells can be expressed in the form of [Equation 7] below according to the definition of the interference signal sequence, and the time axis response of the interference signal sequence according to the characteristic of Discrete Fourier Transform is shown in [Equation 8] Satisfy the relationship.

From this relationship, Equation 6 may be re-expressed as Equation 9 below as a condition for controlling mutual interference by pilot training signal sequences of two cells.

이 임의의 변조신호열을 가진다고 할 때, 상기 [수학식 9]를 만족하는 간섭신호열

과 훈련신호열

간의 곱셈이 되도록 셀 1의 파일롯 훈련신호열을 아래의 [수학식 10]과 같이 변조하여 전송하면, 셀 0과 셀 1의 채널추정에서는 파일롯 훈련신호에 의한 상호간섭이 최소화된다.That is, the pilot training signal sequence of cell 0

Assume that the random signal sequence has an interference signal sequence satisfying Equation 9 above.

And training signal sequence

If the pilot training signal sequence of cell 1 is modulated and transmitted as shown in Equation 10 below to multiply, the interference between the pilot training signals is minimized in the channel estimation of cells 0 and 1.

In the same manner, the training signal sequence of the cell 2 may be selected such that the interference signal sequence 520 with the cell 0 satisfies Equation 9, thereby minimizing mutual interference of channel estimation. However, the result of Equation 9 must also be satisfied with respect to the interference 530 between the pilot training signal sequence of the cell 1 and the cell 2 selected based on the cell 0. To satisfy these characteristics, the time and frequency positions of the pilot training signal subcarriers commonly used by cells 0 and 1 and the positions of the pilot training signal subcarriers commonly used by cells 1 and 2 or cells 0 and 2 If they do not coincide with each other, it is relatively easy to adjust by using subcarrier signal sequences that do not overlap each other.

라고 했을 때, 아래의 [수학식 11]과 같이 표현할 수 있으며, 모든 신호열 원소가 켤레 복소 연산에 대해서 닫혀있어야 한다고 해석할 수 있으며, 집합의 크기만큼 서로 다른 셀이 인접해서 채널 추정에 대한 간섭을 최소화하도록 조절할 수 있다.On the contrary, when all adjacent cells transmit the pilot training signal through the same resource, the set of all modulation signal sequences satisfying Equation (9) is obtained.

In this case, it can be expressed as Equation 11 below, and it can be interpreted that all signal sequence elements should be closed for the conjugate complex operation. Can be adjusted to minimize

FIG. 6 is a diagram illustrating a modulation interference signal sequence search and an interference signal sequence set in an OFDM-based cellular system. FIG. 6 is a view illustrating whether an interference signal sequence can be used to modulate a pilot training signal sequence in adjacent cells. The process of construction is shown.

개로 표현한다. 여기서,

은 변조 심볼의 종류를 나타내는 값으로, 파일롯 훈련신호열이 BPSK이면 2이고 QPSK일 때 4의 값을 가진다. 만약, 64개의 공통 훈련신호 부반송파에 대해서 BPSK로 변조된 파일롯 훈련신호열에 대하여 검색해야하는 모든 신호열의 개수는

로 매우 많다. 이 중 하나의 신호열을 발생시키고(610), 이산 푸리에 역변환을 통하여(620) 얻은 간섭신호 시간응답이 상기 [수학식 9]를 만족하는 신호열인 지를 확인한다(630). 확인 결과, 만족하는 경우에는 [수학식 11]의 곱셈에 대한 닫힘 조건을 만족하는지 검사하고(640), 만족하는 경우 간섭신호열 집합을 구성한다(650).First, the number of all possible interference signal sequences

Express as a dog. here,

Is a value indicating the type of modulation symbol and has a value of 2 when the pilot training signal sequence is BPSK and a value of 4 when the QPSK. If the 64 common training signal subcarriers, the number of all signal sequences to be searched for the pilot training signal sequence modulated by BPSK is

There are so many. One signal sequence is generated (610), and it is checked whether the interference signal time response obtained through the discrete Fourier inverse transformation (620) is a signal sequence satisfying Equation 9 (630). As a result, if it is satisfied, it checks whether the closing condition for the multiplication of Equation 11 is satisfied (640), and if it is satisfied, configures the interference signal string set (650).

Since this process has many kinds of interference signal sequences, it requires a very complicated operation such as a corresponding number of discrete Fourier inverse transform operations. For simpler and easier searching, the number of interference signal strings considered in 610 should be reduced.

개의 부반송파를 주기로 반복된다면, 고려되는 간섭신호열도

를 주기로 반복시켜서 반복신호열을 생성하면(611) 신호열의 개수를 줄일 수 있다. 여기서,

는

개의 부반송파의 한 주기에 포함된 공통 훈련부반송파의 개수를 나타낸다. 주파수 축에서의 반복 특성이 시간 축에서 주기적인 0을 만들어내는 이산 푸리에 변환의 특성에 따라, 반복된 신호열은 상기 [수학식 9]에서 정의된 간섭신호의 시간 응답의 전력을 주기적으로 0이 되도록 만든다. 이러한 주기적인 신호열을 고려하면, "620" 과정에서 사용하는 이산 푸리에 역변환의 크기도, 반복되는 공통 훈련부반송파 위치의 주기

로 작아진다.The location of the common training carrier

If repeated in subcarriers, the interference signal sequence to be considered

By repeating at a cycle to generate a repeating signal sequence (611) it is possible to reduce the number of signal sequences. here,

Is

This indicates the number of common training subcarriers included in one period of the two subcarriers. Depending on the nature of the Discrete Fourier Transform, where the repetition characteristic on the frequency axis produces a periodic zero on the time axis, the repeated signal sequence is such that the power of the time response of the interference signal defined in Equation 9 is periodically zero. Make. Considering this periodic signal sequence, the magnitude of the discrete Fourier inverse transform used in the process 620 is also repeated in the common subcarrier position.

Becomes smaller.

일 때의 간섭신호 시간응답은 원래 추정해야 하는 채널과 동일한 시간 위치에서 간섭신호를 발생시키므로 더욱 낮은 간섭 전력을 유지시켜야 한다. 이때의 시간응답은 "620" 과정과 "630" 과정의 연산과정 대신에 아래의 [수학식 12]를 통해서 간단히 표현된다.In Equation 9, which is a condition of the interference signal sequence,

Since the interference signal time response generates the interference signal at the same time position as the channel to be originally estimated, lower interference power must be maintained. At this time, the time response is simply expressed through Equation 12 below instead of the operation of the process "620" and "630".

Equation 12 denotes a condition 612 for a sum of 0 to a general modulation interference signal sequence. For example, in the case of BPSK, a signal sequence having the same number as 1 and -1 may be selected.

In particular, when the length of the discrete channel impulse response to be estimated satisfies the following Equation 13, the conditions of Equation 9 do not need to be checked separately. This can be omitted and intercell interference is completely eliminated.

FIG. 7 illustrates an example of an interference signal sequence set using repetitive characteristics in an OFDM-based cellular system. When all cells use the same common training signal subcarriers, and all the training signal subcarriers are equally spaced, a length of 4 is given. An example of a repeated interference modulated signal sequence is shown.

It is assumed that the length of the discrete channel impulse response is short to satisfy Equation 13 above. Since the interference signal sequence is defined by the phase difference of the pilot training signal, when each training signal sequence is modulated by BPSK, the interference signal sequence is [1, -1], and in QPSK, one of [1, j , -1, -j ] is selected. Is represented. For convenience of notation, BPSK is represented by the exponent of (-1) and QPSK is represented by the exponent of ( j ) as [0, 1] and [0, 1, 2, 3].

Interference signal sequence 710, which is the period 4 of the training signals modulated with BPSK, is all four closed for complex multiplication. Although there are 10 repetitive interference signal sequences 720 having a period of four for the training signal modulated by QPSK, only four subsets 721, 722, and 723 are closed. Therefore, using the interference signal sequence having a period of 4 can effectively increase the frequency reuse factor (FRF) by 4 times in channel estimation even if all cells transmit the pilot training signal to the same resource. In the case of the modulated interference signal sequence, the frequency reuse rate can be further increased by modulating the training signal sequence with interference signal sequences belonging to different subsets for cells that are not located close to each other.

8 is a diagram illustrating an example of a modulation process of a pilot training signal using an interference signal sequence set generated by repetition characteristics in an OFDM-based cellular system, and a pilot for a cellular system in which all cells use the same pilot training subcarrier positions. An example of the modulation process of the training signal is shown. The set of interference signal sequences used has a repetition length of 4, and each pilot training signal is modulated by BPSK.

The pilot training signal 810 of the reference cell (Cell 0) generates a random BPSK signal to be a sufficient random number using a similar noise generator or the like. If the position of each training signal subcarrier is repeated with a length of 4, and evenly distributed on each subcarrier as much as possible, the repeated interference signal sequence having a length of 4 as an interference signal sequence with a pilot training signal of another adjacent cell (Cell 1) [0, 0, 1, 1] can be selected. This interference signal sequence is repeated 811 in accordance with each training signal subcarrier. The pilot training signal 820 transmitted from the cell 1 may be obtained by complex multiplication of the pilot training signal 810 of the cell 0 as a reference and the repetitive interference signal sequence 811. Through a similar process, the pilot training signal 820 of cell 1 and the pilot training signal 830 to be transmitted by cell 2 are obtained by complex multiplication of another interference signal sequence 821 [0, 1, 1, 0]. Since the interference signal sequence 831 between cells 0 and 2, which were not considered in the generated pilot interference signal relationship of each cell, satisfies the element condition of the set of interference signal sequences that sum to 0 as [0, 1, 0, 1], In addition, it can be seen that the training signal interference between the two cells is controlled. In addition, since the interference signal sequence 821 used to define the pilot modulation relationship between the cell 1 and the cell 2 can be used again as the repetitive interference signal sequence 841 with the cell 3 based on the cell 0, The pilot training signal sequence 850 may be selected. In the above embodiment, a pilot training signal in which channel estimation interference does not occur may be generated for four adjacent cells (Cell 0 to Cell 3).

FIG. 9 illustrates an example of an OFDM-based cellular system using an interference signal sequence set and a cluster, and illustrates a cell structure of an OFDM cellular system transmitting a pilot training signal sequence generated using an interference signal sequence.

Depending on the length of the discrete channel and the size of the set of interference signal sequences defined in relation to the number of pilot training signal subcarriers, the number of pilot training signal sequences that each cell can transmit without mutual interference is determined.

When the number of pilot training signal sequences that each cell selects is three, three clusters 911, 912, and 913 adjacent to each other may be bundled to form one cluster 910. At this time, if the number of pilot training signal sequences configured is not large, interference between cells 911, 921, and 931 using the same pilot training signal sequence occurs in another adjacent cluster. When the number of available pilot training signal sequences is four, the number of cells constituting one cluster 950 increases to four (951, 952, 953, 954), and the same pilot in adjacent clusters 950 and 960. The distance between the cells 951 and 961 using the training signal sequence also increases as the number of pilot training signal sequences increases. However, since the number of interference signal sequences for the pilot training signal sequence is limited, the cells using the same pilot training signal sequence in adjacent clusters interfere with each other in channel estimation.

Inter-cluster interference is achieved by using a different random number signal sequence having a sufficiently low correlation with the pilot training signal sequence of the reference cell of each cluster as a scrambling sequence, or arranging common pilot subcarriers differently for each cluster. Can be averaged to mitigate.

As described above, the method of the present invention may be implemented as a program and stored in a recording medium (CD-ROM, ROM, RAM, floppy disk, hard disk, magneto-optical disk, etc.) in a computer-readable form. Since this process can be easily implemented by those skilled in the art will not be described in more detail.

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.

The present invention as described above, in the OFDM-based cellular system, when all or part of the training signal sequence transmitted for the channel estimation from the base station of the adjacent cells to each other using the same resources in time and frequency, in the channel estimation process In order to minimize the mutual interference between the signals, the training signal sequence transmitted from each cell, the training signal sequence satisfying these conditions, and the channel estimation using the training signal sequence can be used to improve the channel estimation performance and the reception performance of the cellular system. It can be effective.

Claims (7)

In multiple adjacent cells, transmit a training signal sequence for channel estimation in an orthogonal frequency division multiplexing (OFDM) -based cellular system that uses some or all of the pilot training signal subcarriers for channel estimation in common over the same resources in time and frequency In the method, For the time interval of the channel to be estimated and the guard interval for the delay of the interference signal, the multiplication between the interference signal sequence satisfying the condition that the interference signal sequence time response does not exceed a certain threshold and the pilot training signal sequence of any cell 0 adjacent to Training signal sequence transmission method characterized in that for modulating and transmitting the training signal sequence of any cell 1 adjacent to each other. The method of claim 1, The interference signal sequence for modulation is selected to satisfy the condition of Equation 14 below. [Equation 14]

here,

Is the length of the discrete channel impulse response to be estimated,

Is the sum of the time delay difference in two cells and the time spread of the channel in adjacent cells,

Is the time interval,

Each represents an arbitrary threshold. The method of claim 2, The interference signal sequence is Training signal sequence transmission method characterized in that the interference signal time response obtained through the discrete Fourier transform is a signal satisfying [Equation 14], and at the same time includes a signal sequence satisfying the closing condition for multiplication. The method according to any one of claims 1 to 3, The training signal sequence, A training signal sequence transmission method, characterized in that the channel impulse response from the interfering cell is designed to exist at a different time than the channel impulse response of the cell to be channel estimation. The method of claim 4, wherein The interference signal sequence is And generating a repetitive signal sequence based on the number of common training subcarriers included in one cycle of any of the subcarriers, wherein the sum of the generated repetitive signal sequences includes a signal sequence satisfying "0". The method of claim 4, wherein Comprising a cluster corresponding to the size of the interference signal sequence set for the adjacent cells, and generating and transmitting a pilot training signal sequence using a different random number signal sequence in each cluster Training signal sequence transmission method further comprising. The method of claim 4, wherein Comprising a cluster corresponding to the size of the interference signal sequence set for the adjacent cells, and generating and transmitting a training signal sequence by arranging different pilot training signals in each cluster so that interference between pilot training signals does not occur Training signal sequence transmission method further comprising.

Images