KR100696947B1 - Reduction of Multiple Access Interference in ODFDMA Uplink System - Google Patents

Abstract

The present invention relates to a method for reducing multiple access interference between users in an ODFMA uplink system, the method comprising: translating an OFDM symbol vector in a frequency domain transmitted in a first symbol period; Fast Fourier inverse transforming (IFFT) the OFDM symbol vector transposed in the frequency domain to generate a valid symbol signal in the time domain; Adding an cyclic prefix (CP) to the valid symbol signal to generate an OFDM symbol signal in a time domain and transmitting the second symbol interval subsequent to the first symbol interval. According to the present invention, the frequency diversity effect can be obtained by transmitting the same data during two consecutive OFDM symbol intervals, thereby significantly reducing the interference component due to multiple access.

OFDMA, MAI, Uplink, UPLINK, Frequency Diversity, Cycle Time Transition

Description

TECHNICAL FOR REDUCING MULTIPLE ACCESS INTERFERENCES IN AN OFDMA UPLINK SYSTEM}

1 is a graph comparing MAI strength according to a time synchronization error model in a conventional OFDMA uplink system.

2 is a structural diagram of an OFDM symbol signal transmitted in an OFDMA uplink according to the first and second embodiments of the present invention.

3 is a graph comparing MAI strength in a conventional OFDMA uplink transmission method and a transmission method according to the first and second embodiments of the present invention.

4 is a graph comparing bit error rate (BER) in a conventional OFDMA uplink transmission method and a transmission method according to the first and second embodiments of the present invention.

The present invention relates to an orthogonal frequency division multiple access (hereinafter referred to as " OFDMA ") system, and more particularly, to a method for mitigating interference and improving performance in an uplink of an OFDMA system.

Recently, an orthogonal frequency division multiplexing (OFDM) -based transmission system has been selected and commercialized in various standardization standards due to advantages such as high speed transmission and fading coping capability. In addition, the development of OFDM-based 4G system is in progress in the future, and several multiple access schemes are being studied to accommodate an increasing number of system users. Multiple access schemes of the OFDM technique that have been studied so far include OFDM-TDMA, OFDM-FDMA, OFDM-CDMA, and FH-OFDM. OFDM-TDMA is a multiple access scheme similar to TDMA, in which all subcarriers spread over the entire bandwidth are allocated to different users at different times. OFDM-FDMA is similar to FDMA, in which each subchannel is allocated to different users. It is a method to transmit user data. OFDM-CDMA applies a code spreading scheme to each subchannel after assigning each user to a different subchannel.

On the other hand, OFDMA consists of different users sharing fast Fourier transform (FFT) on the uplink, each user transmitting one or more subchannels. That is, by dividing the subchannels, a form of frequency division multiple access (FDMA) is taken. However, in an OFDMA-based communication system, a multiple access interference (MAI) component is generated due to a time error between uses during uplink transmission.

A simple way to reduce the MAI component is to insert a guard band (GB) between the user subcarriers, which reduces the transmission efficiency. Another way to reduce the MAI component is to further use cyclic suffixes. However, the method using cyclic suffix is helpful for the removal of MAI component, but has the disadvantage that the user transmission rate is lowered due to the use of additional cyclic suffix.

Regarding the MAI component generated in the OFDMA system, it will be described in more detail as follows.

개의 부반송파를 가지며, 최대 동시 사용자수는 부반송파수 할당 방식에 따라

을 지원한다. 여기서,

는 각 사용자에게 할당되는 부반송파수이다. OFDMA 상향 링크의 경우

번째 사용자에게 부대역

을 할당하는데, 예컨대 다음과 같은 연속 대역 할당 방식을 고려할 수 있다.OFDMA uplink system

Subcarriers, and the maximum number of concurrent users depends on the subcarrier allocation method.

Support. here,

Is the subcarrier assigned to each user. For OFDMA Uplink

To the first user

For example, the following continuous band allocation scheme may be considered.

번째 심볼 구간에 전송되는

번째 사용자의

번째 부반송파 신호는 다음과 같다.Next, in the time domain

Transmitted in the first symbol interval

Of the first user

The subcarrier signal is as follows.

는 사이클릭 프리픽스(cyclic prefix, 이하 "CP"라 함)의 길이를 나타낸다. here,

Denotes the length of a cyclic prefix (hereinafter referred to as "CP").

When the signal of Equation 2 transmitted from each user passes through an independent fading channel, it is received by the base station in the following form.

은 가산성 백색 가우스 잡음(AWGN),

는

번째 사용자의 시간 동기 오차를 의미한다. 또한, 채널 주파수 응답은

으로서,

는 i.i.d. 레일리 분포를 가지며,

는

에서 균일 분포를 가진다. here

Silver additive white Gaussian noise (AWGN),

Is

The time synchronization error of the first user. In addition, the channel frequency response is

As

Has an iid Rayleigh distribution,

Is

Has a uniform distribution at

번째 사용자 및

번째 사용자의 시간 동기 오차를 각각

,

라 할 때,

번째 사용자에 대한

번째 사용자의 상대적인 시간 동기 오차(

)는 다음과 같이 표현된다.As described above, in the OFDMA uplink system, signals transmitted from each user pass through independent fading channels to reach the base station, and thus each user has a different time synchronization error. To model this time synchronization error,

User and

Each time synchronization error

,

When we say

For the first user

Relative time synchronization error of the second user (

) Is expressed as

는

이하인 최초의 정수이고,

는 OFDM 심볼 주기를 의미한다. 이 때, 상대적인 시간 동기 오차(

)의 크기에 따라 다음과 같은 세가지 시간 동기 오차 모델을 고려할 수 있다. 첫번째 모델(Model 1)은

인 경우이고, 두번째 모델(Model 2)은

(

는 보호구간의 주기임)인 경우이며, 마지막으로 세번째 모델(Model 3)은

인 경우이다. 한편, 수학식 3과 같이 정의된

는 전술한 각 모델 구간 내에서 균일한 분포를 가진다고 가정한다.here,

Is

The first integer less than or equal to

Denotes an OFDM symbol period. In this case, the relative time synchronization error (

Depending on the size of), three time synchronization error models can be considered: The first model (Model 1)

If the second model (Model 2)

(

Is the period of protection interval). Finally, the third model (Model 3)

If On the other hand, it is defined as

Assumes a uniform distribution within each model section described above.

) 가 [0,25%] 구간에서 발생하게 되면, 이들 사용자간의 MAI 성분은 OFDM에서 사용하는 사이클릭 프리픽스(cyclic prefix)로 인해 사라지게 된다. 반면에, 시간 동기가 [-25%,0] 내에서 발생할 경우에는, 이들 사용자 간에 MAI가 발생하게 되므로, 이에 대한 MAI 완화 기법이 요구된다.1 is a result showing the power (Power) of the MAI component generated by the three time synchronization error model described above. As can be seen from FIG. 1, in the case of time synchronization error model 3 (Model 3), it can be seen that almost no MAI component exists. However, the most realistic model among the three models is the time synchronization error model 2, in which an uplink signal of each user must arrive within ± 25% of the guard interval. In the time synchronization error model 2, the relative time synchronization error of different users (

) Occurs in the interval [0,25%], the MAI component between these users disappears due to the cyclic prefix used in OFDM. On the other hand, when time synchronization occurs within [-25%, 0], since MAI occurs between these users, a MAI mitigation technique is required.

개의 null 신호를 삽입함으로써, 인접 신호간의 간섭 성분을 일정 부분 제거해주는 방식이다. 그런데,

가 증가할수록 인접 신호간의 간섭이 줄어들어 시스템 성능은 향상되나, 이로 인해 사용자 전송률이 줄어든다는 단점이 존재한다. 한편, 사이클릭 서픽스를 사용하는 방식은 시간동기가 [-25%,0]내에서 발생할 경우에 효과적으로 간섭성분을 제거해줄 수 있지만, 추가적인 사이클릭 서픽스의 사용으로 인하여 사용자 전송률이 저하된다는 단점이 있다. As described above, conventional methods for mitigating such MAI include a protection interval (GB) insertion method and a cyclic suffix (CS). The GB insertion scheme is used between OFDMA uplink user signals.

By inserting two null signals, a part of interference components between adjacent signals is removed. By the way,

As, increases the performance of the system by reducing the interference between adjacent signals, but there is a disadvantage that the user transmission rate is reduced. On the other hand, the method of using cyclic suffix can effectively remove the interference component when time synchronization occurs within [-25%, 0], but the user transmission rate is lowered due to the use of additional cyclic suffix. There is this.

In order to solve the above problems, an object of the present invention is to provide a method for effectively reducing the time synchronization error occurring in an OFDMA-based uplink system and at the same time improving the performance, and a structure of the OFDMA transmission symbol accordingly. have.

In order to achieve the above object, according to a first aspect of the present invention, there is provided a method for reducing multiple access interference (MAI) in the uplink of an orthogonal frequency division multiple access (OFDMA) system, in a first symbol period Transposing the OFDM symbol vector in the transmitted frequency domain; Fast Fourier inverse transforming (IFFT) the OFDM symbol vector transposed in the frequency domain to generate a valid symbol signal in the time domain; Adding an cyclic prefix (CP) to the valid symbol signal to generate an OFDM symbol signal in a time domain and transmitting the second symbol interval subsequent to the first symbol interval.

In this case, the transposing step may divide a subcarrier band allocated to the OFDM symbol vector transmitted in the first symbol period into two regions and transpose the positions of the two regions.

According to a second aspect of the present invention, there is provided a method for reducing an MAI component in an uplink of an OFDMA system, comprising: transposing an OFDM symbol vector in a frequency domain transmitted in a first symbol period; Fast Fourier inverse transforming (IFFT) the OFDM symbol vector transposed in the frequency domain to generate a valid symbol signal in the time domain; Cyclically shifting the valid symbol signal on a time axis to generate a second valid symbol signal; Adding an cyclic prefix (CP) to the second valid symbol signal to generate an OFDM symbol signal in a time domain and transmitting the second symbol interval subsequent to the first symbol interval.

In this case, preferably, the effective symbol signal may be cyclically shifted in the reverse direction on the time axis by the cyclic prefix.

According to still another aspect of the present invention, there may be provided a mobile terminal or mobile station apparatus, an OFDMA uplink system equipped with a device for performing each of the steps of the first and second aspects of the present invention described above.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings.

A) OFDMA (FD-OFDMA) with frequency diversity

According to a first embodiment of the present invention, there is provided a method and frame structure for mitigating MAI components by applying frequency diversity to an OFDMA uplink system, which is referred to as "FD-OFDMA".

번째 심볼 구간에서

번째 사용자의 신호(

)는 주파수축에서 다음과 같이 정의된다.first

In the first symbol interval

Signal from user

) Is defined in the frequency axis as

는 예컨대, 전술한 수학식 1에 따라 2l번째 심볼 구간에서

번째 사용자에게 할당된

개의 부반송파를 통해 전송되는 OFDM 심볼 벡터이며,

는

를 두 개의 영역으로 분할한 것으로서, 각각

차원의 OFDM 심볼 벡터를 나타낸다. 그리고,

번째 심볼 구간에서

번째 사용자의 신호는 고속 푸리에 역변환(IFFT)을 거쳐, 시간축에서 다음과 같이 표현할 수 있다.here,

For example, in the second symbol period according to Equation 1 described above

Assigned to the first user

OFDM symbol vector transmitted on subcarriers

Is

Is divided into two regions, each of

Represents a two-dimensional OFDM symbol vector. And,

In the first symbol interval

The user's signal can be expressed as follows on the time axis through fast Fourier inverse transform (IFFT).

이다. 수학식 6에서

과

는 예컨대, 전술한 수학식 1에 따라

번째 사용자에게 할당된 부반송파 대역을 두 개의 영역으로 분할한 것으로서, 아래와 같이 정의된다.here,

to be. In equation (6)

and

For example, in accordance with Equation 1 described above

The subcarrier band allocated to the first user is divided into two regions and is defined as follows.

번째 전송되는

번째 사용자의 신호가 아래와 같이 설계되어야 한다. At this time, in order to obtain the maximum frequency diversity gain in the subcarrier band allocated to each user,

Being sent th

The user's signal should be designed as follows.

번째 심볼 구간에서 전송되는 신호는

번째 심볼 구간에서 전송된 주파수 영역의 심볼 벡터를 전치한 후, 통상의 방법에 따라 고속 푸리에 역변환(IFFT) 및 CP 삽입 과정을 거쳐 전송되며, 수학식 8의 전치 과정에 의하여 주파수 다이버시티 이득을 얻을 수 있다.As can be seen from Equation 8, according to the first embodiment of the present invention,

The signal transmitted in the first symbol interval is

After transposing the symbol vector of the frequency domain transmitted in the first symbol interval, it is transmitted through a fast Fourier inverse transform (IFFT) and a CP insertion process according to a conventional method, and the frequency diversity gain is obtained by the transposition process of Equation 8. Can be.

번째 심볼 구간에서

번째 사용자의 신호는 시간축에서 다음과 같이 표현된다.Finally, when using the transmission scheme defined in Equation 8,

In the first symbol interval

The signal of the first user is expressed as follows on the time axis.

  B) OFDMA (FD-OFDMA with CTS) applying frequency diversity and cyclic time shift

According to a second embodiment of the present invention, there is provided an interference mitigation method and frame structure applying Cyclic Time Shift (CTS) together with the above-described frequency diversity of FD-OFDMA. It is called.

FIG. 2 illustrates a structure of an OFDMA transmission symbol (FD-OFDMA with CTS) to which frequency diversity and a cyclic time shift are applied according to a second embodiment of the present invention. For reference, the symbol of the aforementioned FD-OFDMA transmission scheme is described. The structure is shown together.

번째 심볼 구간에서

번째 사용자의 시간축 신호를

만큼 순환 시간 천이를 시켜서 전송한다. As shown in FIG. 2, in the FD-OFDMA with CTS transmission scheme according to the second embodiment of the present invention, in order to remove an interference component of another user signal that arrives in advance of a reference user signal without lowering a transmission rate,

In the first symbol interval

Signal of the first user

Transmit as many times as the cycle time.

동안에 전송되는 신호는 아래와 같은 벡터로 정의할 수 있다.In more detail, first, two symbol intervals in the time axis

The transmitted signal can be defined by the following vector.

이고,

는 도 2에 도시된 바와 같이 시간축 상에서 각각

개의 샘플로 이루어진 유효 신호이며,

는 길이가

인 사이클릭 프리픽스(CP)를 나타낸다. here,

ego,

Are each on the time axis as shown in FIG.

Is a valid signal of samples

Has a length

In cyclic prefix (CP).

구간에서 전송되는 신호는

심볼 구간에서 전송된 유효 심볼 신호를 수학식 6에 따라 주파수 영역에서 전치시킨 후, 고속 푸리에 역변환(IFFT) 및 CP 삽입을 거쳐 전송되며, 이는 전술한 수학식 10 및 도 2의 (가)에 도시된 바와 같다. According to the FD-OFDMA transmission scheme of the first embodiment described above,

The signal transmitted in the interval

The effective symbol signal transmitted in the symbol period is transposed in the frequency domain according to Equation 6, and then transmitted through fast Fourier inverse transform (IFFT) and CP insertion, which is shown in Equation 10 and FIG. As it is.

구간에서 전송되는 신호는

심볼 구간에서 전송된 유효 심볼 신호를 수학식 6에 따라 주파수 영역에서 전치시킨다. 그리고, 이를 고속 푸리에 역변환(IFFT)하고, 시간축에서 유효 신호를

만큼 순환 천이함으로써 새로운 유효 심볼 신호를 생성하며, 이어서 사이클릭 프리픽스(CP)를 삽입하여 전송한다. 그리고, 이러한 방법에 따르면, 수학식 12로부터 다음과 같은

구간의 전송 신호를 얻을 수 있다. However, according to the second embodiment of the present invention,

The signal transmitted in the interval

The effective symbol signal transmitted in the symbol period is transposed in the frequency domain according to Equation 6. Then, the fast Fourier inverse transform (IFFT) is performed, and the effective signal is

By cyclic shifting, a new valid symbol signal is generated, and then a cyclic prefix (CP) is inserted and transmitted. And, according to this method,

The transmission signal of the section can be obtained.

개의 샘플 포인트로 이루어진 유효 구간 범위에서

를

만큼 시간축에서 순환 천이하면,

와 같은 유효 심볼 신호를 얻을 수 있으며, 통상의 CP 생성 방법에 따라

가 유효 신호의 선두에 CP로서 추가되는 것이다.In other words,

In the range of valid intervals of

To

By cyclic shift on the time axis,

According to the conventional CP generation method, an effective symbol signal such as

Is added as CP at the head of the valid signal.

번째 심볼 구간에서

번째 사용자의 신호는 시간축에서 다음과 같다.As a result, after the above process

In the first symbol interval

The signal of the first user is as follows on the time axis.

Signals transmitted in two sections from the above-described equations (6) and (12) have the following relationship on the frequency axis.

에 대해서,

이 작은 값을 가지게 되면 아래와 같이

은

로 근사화될 수 있다.In addition, the guard interval index in Equation (9)

about,

If you have this small value,

silver

Can be approximated by

번째 심볼 구간에서 전송 신호

의 사이클릭 프리픽스(CP)중에서 앞부분의 몇 샘플들은 2l~번째 심볼 구간에서 전송된 신호

의 사이클릭 서픽스(CS)로 해석될 수 있다. 이를 이용하면, 기존의 방식에서 추가로 사이클릭 서픽스(CS)를 사용하는 것과는 달리, 별도의 사이클릭 서픽스를 사용하지 않고도 동일한 효과를 얻을 수 있다. 반면에, 수학식 13과 수학식 14로부터 알 수 있는 바와 같이, 두 심볼 구간동안 전송되는 신호는 동일함을 알 수 있고, 이로 인한 전송률 감소는 기존 방식보다 고차의 변조 방식을 사용함으로써 해결할 수 있다.This assumption applies to the large number of users in an OFDMA uplink system. therefore,

Transmission signal in the first symbol interval

The first few samples of the cyclic prefix (CP) are the signals transmitted in the 2l-th symbol interval.

It can be interpreted as the cyclic suffix of. With this, the same effect can be obtained without using a separate cyclic suffix, unlike using an additional cyclic suffix CS in the conventional scheme. On the other hand, as can be seen from the equations (13) and (14), it can be seen that the signals transmitted during the two symbol intervals are the same, and thus the reduction of the transmission rate can be solved by using a higher-order modulation method than the conventional method. .

C) Experimental results

을 사용하였다.3 and 4 show the results of comparing the performance of the OFDMA system according to the first and second embodiments of the present invention in terms of MAI power and bit error rate (BER) performance.

Was used.

FIG. 3 compares MAI power of OFDMA transmission schemes (FD-OFDMA and FD-OFDMA with CTS) according to the first and second embodiments of the present invention in the above-described time synchronization error model. In the case of the time synchronization error model 2 (Model 2), it can be seen that the MAI component is reduced and the transmission rate is not reduced compared to the conventional GB insertion method.

4 shows BER performance of an OFDMA system according to the first and second embodiments of the present invention. In FIG. 4, the conventional scheme (Ordinary OFDMA) uses the QPSK modulation scheme, and the proposed scheme according to the present invention uses the 16QAM modulation scheme. Referring to FIG. 4, it can be seen that the proposed method of the present invention has better BER performance than the conventional method.

Although the preferred embodiment according to the present invention has been described above, this is merely exemplary and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. For example, in the above-described embodiment, only a method for reducing MAI and an OFDM symbol structure have been described, but the present invention can be implemented in the form of a mobile terminal equipped with means for performing each step of the method. Therefore, the protection scope of the present invention should be defined by the following claims.

As described above, according to the present invention, a frequency diversity effect can be obtained by transmitting the same data during two consecutive OFDM symbol intervals, thereby significantly reducing the interference component due to multiple access.

In addition, by transmitting the OFDM symbol signal in a cyclic time shift with the aforementioned frequency diversity, the same effect can be achieved without the addition of a cyclic suffix as well as the effect of mitigating interference components of other users. By not using the fix, there is no problem of lowering the rate. Further, according to the present invention, since the interference area from other user signals is reduced, it is easy to perform OFDM time synchronization.

Claims (10)

A method for reducing Multiple Access Interference (MAI) in the uplink of an Orthogonal Frequency Division Multiple Access (OFDMA) system, Transposing the OFDM symbol vector of the frequency domain transmitted in the first symbol period; Fast Fourier inverse transforming (IFFT) the OFDM symbol vector transposed in the frequency domain to generate a valid symbol signal in the time domain; Adding an cyclic prefix (CP) to the valid symbol signal to generate an OFDM symbol signal in a time domain, and transmitting the second symbol interval subsequent to the first symbol interval; OFDMA uplink transmission method comprising a. The method of claim 1, wherein the transposing step The subcarrier band allocated to the OFDM symbol vector transmitted in the first symbol period is divided into two regions, and the positions of the two regions are transposed. A method for reducing Multiple Access Interference (MAI) in the uplink of an Orthogonal Frequency Division Multiple Access (OFDMA) system, Transposing the OFDM symbol vector of the frequency domain transmitted in the first symbol period; Fast Fourier inverse transforming (IFFT) the OFDM symbol vector transposed in the frequency domain to generate a valid symbol signal in the time domain; Cyclically shifting the valid symbol signal on a time axis to generate a second valid symbol signal; Adding an cyclic prefix (CP) to the second valid symbol signal to generate an OFDM symbol signal in a time domain, and transmitting the second symbol interval subsequent to the first symbol interval; OFDMA uplink transmission method comprising a. The method of claim 3, wherein the transposing step The subcarrier band allocated to the OFDM symbol vector transmitted in the first symbol period is divided into two regions, and the positions of the two regions are transposed. The method of claim 3, wherein the circular transition step And cyclically shifting the valid symbol signal in the reverse direction on the time axis by the size of the cyclic prefix. A mobile terminal apparatus for reducing multiple access interference (MAI) in the uplink of an orthogonal frequency division multiple access (OFDMA) system, Means for transposing the OFDM symbol vector in the frequency domain transmitted in the first symbol interval; Means for fast Fourier inverse transforming (IFFT) the OFDM symbol vector transposed in the frequency domain to generate a valid symbol signal in the time domain; Means for generating an OFDM symbol signal in a time domain by adding a cyclic prefix (CP) to the valid symbol signal, and transmitting the second symbol interval subsequent to the first symbol interval Mobile terminal device comprising a. The method of claim 6 wherein the transposition means And a subcarrier band allocated to the OFDM symbol vector transmitted in the first symbol period into two regions, and translocates the positions of the two regions. A mobile terminal apparatus for reducing multiple access interference (MAI) in the uplink of an orthogonal frequency division multiple access (OFDMA) system, Means for transposing the OFDM symbol vector in the frequency domain transmitted in the first symbol interval; Means for fast Fourier inverse transforming (IFFT) the OFDM symbol vector transposed in the frequency domain to generate a valid symbol signal in the time domain; Cyclic shift means for cyclically shifting the valid symbol signal on a time axis to generate a second valid symbol signal; Means for generating an OFDM symbol signal in a time domain by adding a cyclic prefix (CP) to the second valid symbol signal and transmitting it in a second symbol period subsequent to the first symbol period Mobile terminal device comprising a. The method of claim 8 wherein the transposition means And a subcarrier band allocated to the OFDM symbol vector transmitted in the first symbol period into two regions, and translocates the positions of the two regions. The method of claim 8, wherein the circular transition means And cyclically shift the valid symbol signal in the reverse direction on the time axis by the cyclic prefix magnitude.

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