KR20060002173A - Method for mai decease in ofdma communication systems - Google Patents

Abstract

The present invention relates to OFMA communication, comprising the steps of: inserting a cyclic prefix in front of each OFDM uplink symbol in a multiple access interference (MAI) reduction method in the uplink of an OFDMA communication system; It provides a method of reducing MAI comprising inserting a cyclic suffix at the back. In addition, a method of reducing MAI by setting a guard band between each uplink symbol is provided. According to the present invention, it is possible to reduce the performance degradation of the OFDMA communication system due to the interference caused by the synchronization time error in the simultaneous access of multiple users.

OFDMA, MAI, sync, guard band, cyclic prefix, cyclic suffix

Description

Interference Reduction Method for Multiple Users in Orthogonal Frequency Division Multiple Access Communication System {METHOD FOR MAI DECEASE IN OFDMA COMMUNICATION SYSTEMS}

1A is a conceptual diagram illustrating a conventional OFDMA uplink subscriber station (SS) model.

1B is a conceptual diagram illustrating a conventional OFDMA uplink base station (BS) model.

2A is a diagram illustrating a time synchronization error model of an uplink to which the method of the present invention is applied.

2B is a graph showing MAI intensity according to each model of FIG. 2A.

3 is a diagram of a conventional OFDM frame structure using cyclic prefix.

4A illustrates a scheme of inserting guard bands in the uplink of an OFDMA system according to the present invention.

Figure 4b is a graph showing the MAI mitigation effect for each time synchronization error model after the guard band is inserted in accordance with the present invention.

5A illustrates an OFDM frame structure using both cyclic prefix and cyclic suffix according to the present invention.

5b is a BER performance of each modulation scheme of the OFDMA system uplink using the OFDM frame structure according to the present invention.

The present invention relates to OFDM communications, and more particularly, to a method and apparatus for mitigating interference between multiple users during uplink from a terminal to a base station.

OFDM is a wideband modulation scheme that divides a frequency bandwidth allocated for a communication session into a plurality of narrowband frequency subbands, each subband including a radio frequency (RF) subcarrier, each subcarrier being included in each of the other subchannels. Is orthogonal to the received RF subcarriers. The orthogonality of the subcarriers allows their individual spectra to overlap without interference with other carriers. By dividing the frequency bandwidth into a plurality of orthogonal subbands, the OFDM scheme enables high data rates and very efficient bandwidth usage.

On the other hand, in the uplink of an Orthogonal Frequency Division Multiple Access (OFDMA) system, the system performance of the base station is degraded due to a plurality of user-specific time synchronization errors. However, the research on the system performance degradation that occurs during the uplink from each user to the base station is insufficient.

In order to solve the above problems, an object of the present invention is to reduce a phenomenon in which system performance of a base station is degraded due to time synchronization error per user in an uplink of an OFDMA system.

In order to achieve the above technical problem, the present invention first provides a guard band insertion technique for inserting a guard band to reduce the performance degradation due to time synchronization error of the uplink, and then a cyclic prefix (CP) In addition, an OFDM frame structure having a cyclic suffix (CS) is proposed.

The uplink subscriber station (SS) of the OFDMA system considered in the present invention has N subcarriers as shown in FIG. 1A, and the maximum number of concurrent users is K = N / M _d according to the subcarrier allocation scheme. It is designed to support it. M _d is the subcarrier assigned to each user. Subband to d-user for uplink In allocating S _d , a continuous band allocation scheme, such as Equation 1 below, is considered.

The n th subcarrier signal of the d th user transmitted in the time domain is expressed by Equation 2.

Where N _G represents the cyclic prefix length. The transmitted signal of each user passes through an independent fading channel and is received by the base station in the following form as shown in FIG.

으로써

는 i.i.d. 레일리 분포를 가지며

는 [- pi , pi )에서 균일 분포를 가진다. Where w _n is AWGN (Additive White Gaussian Noise) and τ _d is the time synchronization error of the d-th user. The channel frequency response is also H _{k, d} =

As

Has iid Rayleigh distribution

Has a uniform distribution at [-pi, pi).

The FFT output value corresponding to the l th subcarrier of the q th user at the receiver is expressed as follows.

,

이다.here

,

to be.

로 정의하면, 상대적인 시간동기 오차는 다음과 같이 표현된다.As shown in FIG. 1A, each user has a different time synchronization error because each user reaches a base station through a fading channel independent of each other. In order to model this time synchronization error, the time synchronization error of the d user relative to the l user is calculated.

Relative time, the relative time synchronization error is expressed as

에 대한 다음과 같은 세 가지 시간동기 오차 모델을 고려하며, 도 2a 및 도 2b에 도시된 3가지 모델도 아래의 각 모델에 해당한다. Where | x | = first integer ≤ x and T _S denotes an OFDM symbol period. In the present invention

Considering the following three time synchronization error models for the three models shown in Figures 2a and 2b also corresponds to each model below.

(모델 1)

(Model 1)

(모델 2)

(Model 2)

(모델 3)

(Model 3)

는 고려하는 각 모델 구간내에서 균일한 분포를 가진다고 가정한다. Where T _G represents the period of the guard period. The temporal concepts for the three time synchronization error models considered above are shown in FIGS. 3 and 4. Defined

Assumes a uniform distribution within each model interval under consideration.

  2A and 2B show the power of multiple access interference (MAI) generated by three time synchronization error models. In FIG. 2A, a reference user signal is an uplink OFDM signal from a user who first accesses a base station, which is a reference. The thick dotted line indicates the start and end of a set of Guard Intervals and User Data in the reference user signal. The uplink signal from another user that deviates from the dotted line has a time synchronization error. As a result, the performance of the overall OFDMA system is degraded.

The most realistic of the three models is the time synchronization error model 2, where each user's uplink signal arrives within ± 25% of the protection interval. In practice, random access such as time synchronization error model 1 is not allowed in the OFDMA uplink. On the other hand, in the case of time synchronization error model 3, as can be seen in Figures 2a and 2b it can be seen that almost no MAI component. In addition, when the MAI component occurs in the error range [0,25%] interval MAI components between different users disappear. Therefore, the present invention focuses on providing a method for MAI mitigation when time synchronization occurs within [-25%, 0].

That is, the conventional OFDM symbol minimizes the influence of interference between symbols using a cyclic prefix (CP), but when applying the OFDM frame structure using the CP to the OFDMA system, as shown in FIG. As in the synchronization error model 2, even if the time synchronization is within ± 25% of the protection interval, the MAI component exists between users. This MAI component occurs when time synchronization is achieved within [-25%, 0], which causes a considerable amount of interference from the MAI strength point of view. If the MAI component occurs in the [0,25%] section, the MAI component between other users disappears. Therefore, it is not necessary to consider the MAI component. will be.

EMBODIMENT OF THE INVENTION Hereinafter, the structure of this invention is described in detail with reference to an accompanying drawing.

1. Guard band insertion technique

In the uplink of the OFDMA system, as described above, system performance is degraded due to time synchronization error for each user. As the simplest method for solving this problem, a technique of inserting guard bands (GB) between user signals may be used.

As shown in FIG. 4A, the GB insertion method inserts Ma null signals between uplink user signals of an OFDMA system to partially remove ISI components between adjacent signals. In the frequency spectrum shown in FIG. 4A, a solid line is a data region, and a dotted line represents a guard band filled with a null signal. In the figure, G _P-1 ^u denotes a guard band inserted after the previous symbol, and G _P ^l denotes a guard band inserted before the current symbol. As Ma increases, system performance is improved by reducing interference between adjacent signals. However, since this reduces the user data rate, it is desirable to find a suitable Ma in each case in consideration of the trade-off relationship between the interference reduction and the data rate reduction.

FIG. 4B is a graph comparing MAI intensities when three modeled time synchronization error models and GBs are inserted. As shown in the figure, the time synchronization error model 2 shows that the MAI intensity decreases with increasing GB.

Inserting the guard band in the OFDM communication scheme is basically accompanied to maintain the orthogonality of the carrier, but this is the case in the downlink, the guard band insertion according to the present invention inserts the guard band in the OFDM symbol of the uplink In this respect, there is a remarkable difference from the conventional technology.

2. OFDM frame structure

The present invention proposes a new OFDM frame structure using CP and CS simultaneously as another method for generating MAI component. 5A illustrates a new OFDM frame structure using CP and CS. The conventional method and the CP and the sum of the T _CP and T _CS in new OFDM frame structure to ensure the same rate with respect to a new way of using both the CS using only the CP equal to the T _CP from the existing OFDM frame structure To be designed.

As shown in FIG. 5A, interference components may be removed by the CS and the CP in the early-arriving symbol and the late-arriving symbol of another user, respectively. Therefore, when the time synchronization error generated between users satisfies the condition of Equation 9, the MAI component generated by the time synchronization error may be removed for both the early arrival symbol and the delay arrival symbol.

This situation corresponds to the time synchronization error model 2, and does not apply to the time synchronization error model 1. As described above, random access such as time synchronization error model 1 is not allowed in the OFDMA uplink and cannot be considered. In case of error model 3, the occurrence of MAI does not occur. There is no need to apply the MAI mitigation technique either.

FIG. 5B shows the uplink BER performance for the conventional OFDMA and the proposed OFDMA frame structure for each modulation scheme. The dotted line (Ordinary OFDMA) represents the result of inserting the guard band in the conventional OFDM frame structure, the solid line (Modified OFDMA) shows the BER result of the OFDM uplink symbol modified by inserting both CP and CS in accordance with the present invention It is a graph. In FIG. 5B, the number of users is 16, and time synchronization error model 2 is used. Also shown is the BER performance of the existing OFDMA system uplink with GB insertion.

As shown in FIG. 5B, as the GB is increased, the MAI component is reduced, so that the BER performance is slightly improved compared to the OFDMA system uplink without applying the conventional GB scheme. However, the performance increase is not as great as the GB increase. However, it can be seen that the BER performance of the OFDMA system uplink using the OFDM frame structure according to the present invention is almost similar to the uplink in the case of a single user. In addition, it can be seen that the interference component is removed by the CS and CP of the early arrival symbol and the delay arrival symbol of another user, respectively.

As described above, the sum of T _CP and T _CS in the new OFDM frame structure is equal to the existing OFDM frame structure in order to guarantee the same data rate for the existing method using only CP and the new method using both CP and CS. It is designed to be equal to T _CP at.

As described above, according to the present invention, it is possible to effectively reduce the MAI effect caused by the time synchronization error of the OFDM uplink and to be easily applied to the existing OFDMA system due to the simple configuration, thereby improving the high performance of the OFDMA system at low cost. You can expect.                     

In addition, the present invention is applicable to all next-generation communication systems (IEEE 802.16a / e, IEEE 802.20, etc.) employing OFDMA.

Claims (5)

In the method of reducing multiple access interference (MAI) in the uplink of the OFDMA communication system, Inserting a cyclic prefix in front of each OFDM uplink symbol, Inserting a cyclic suffix after each OFDM uplink symbol MAI reduction method comprising a. The method of claim 1, wherein the cyclic prefix and the cyclic suffix are

(here,

} Is the time synchronization error of the d user relative to the l user, and Tcp and Tcs are the time periods of the cyclic prefix and the suffix, respectively) MAI reduction method that follows the relationship. In the MAI reduction method in the uplink of the OFDMA communication system, Inserting a guard band of a predetermined length between each OFDM uplink symbol. In the MAI reduction method in the uplink of the OFDMA communication system, Inserting a cyclic prefix in front of each OFDM uplink symbol, Inserting a cyclic suffix after each OFDM uplink symbol, Inserting a guard band of a predetermined length between each OFDM uplink symbol MAI reduction method comprising a. The method of claim 4, wherein the cyclic prefix and the cyclic suffix are

(here,

Is the time synchronization error of the d user relative to the l user, and Tcp and Tcs are the time periods of the cyclic prefix and the cyclic suffix, respectively) MAI reduction method that follows the relationship.

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