KR20180059111A - Method and appratus for removing inter carrier interference by frequency offset in noma system - Google Patents

Abstract

Provided are a method and apparatus for removing intercarrier interference due to a frequency offset in a non-orthogonal multiple access system (NOMA). According to an embodiment of the present invention, the apparatus for removing intercarrier interference due to frequency offset in a NOMA system comprises: a frequency offset compensating unit compensating overall for a plurality of frequency offsets generated in each of the sub-blocks, with respect to a plurality of sub-blocks included in a received signal, wherein the frequency offsets are present for each user sharing resources of a corresponding sub-block; a signal processing unit demodulating and decoding the received signal, for which the frequency offsets are compensated, for each sub-block, and outputting a demodulated signal; and an interference removing unit removing the sum of interference signals, which are signals of other users, from a signal of a corresponding user for each user sharing resources of a corresponding sub-block in each sub-block, decoding the signal, and outputting a final demodulated signal, wherein the frequency offset compensating unit applies a min-max algorithm to an absolute value obtained by subtracting a specific candidate value from the user-specific frequency offset to extract a value compensating overall for the plurality of frequency offsets.

Description

METHOD AND APPARATUS FOR REMOVING INTER CARRIER INTERFERENCE BY FREQUENCY OFFSET IN NOMA SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a technique for eliminating inter-carrier interference due to frequency offset in a Non-Orthogonal Multiple Access (NOMA) system.

In general, a NOMA system is a system in which multiple users simultaneously access the same time and frequency resources.

Since this NOMA technology can serve many users compared to orthogonal multiple access technology, it is considered as one of the most promising multi-access technologies in the Massive Machine Type Communication (MMTC) have.

In fact, NOMA technologies that maximize the number of users can define up to 300% of users compared to existing Orthogonal Frequency Division Multiple Access (OFDMA) by defining non-orthogonal resources using both time / frequency / It can be serviced without major performance degradation.

However, when the uplink frequency offset occurs in the conventional NOMA system, inter-carrier interference (ICI) occurs, which causes severe reception performance degradation.

Generally, frequency offset correction is performed in a time domain in a multicarrier system in which ICI occurs. However, in the case of a NOMA system, a frequency offset can not be corrected in a time domain because multiple users access the same time / frequency resource .

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems of the conventional art, and it is an object of the present invention to provide a method for eliminating inter-carrier interference due to the occurrence of a frequency offset between a transmitter and a receiver of various users in a NOMA system.

In order to achieve the above object, an apparatus for eliminating inter-carrier interference due to frequency offset in a non-orthogonal multiple access (NOMA) system according to an embodiment of the present invention includes: A plurality of frequency offsets generated in the corresponding sub-block for each sub-block for each user sharing resources of the corresponding sub-block with respect to a sub-block of the sub-block, A signal processor for demodulating and decoding the received signal with the frequency offset compensated for each of the subblocks to output a demodulated signal, and a demodulator for demodulating the received signal, The sum of interference signals, which are signals of users, is removed and decoded to generate interference Wherein the frequency offset compensating unit applies a min-max algorithm to an absolute value obtained by subtracting a specific candidate value from the frequency offset for each user, thereby extracting a value that totally compensates for the plurality of frequency offsets. do.

According to an aspect of the present invention, there is provided a method of canceling inter-carrier interference due to a frequency offset in a non-orthogonal multiple access (NOMA) a) a plurality of frequency offsets generated in a corresponding sub-block for each of a plurality of sub-blocks included in a received signal, and exists for each user sharing a resource of the corresponding sub-block (B) demodulating and decoding the received signal with the frequency offset compensated for each of the subblocks, and outputting a demodulation signal; and (c) The sum of the interference signals which are signals of the other users is removed from the signal of the user for each shared user and decoded to output the final demodulation signal Wherein the step (a) comprises applying a min-max algorithm to an absolute value obtained by subtracting a specific candidate value from a frequency offset for each user, and extracting a value for totally compensating the plurality of frequency offsets .

According to an embodiment of the present invention, the absolute amount of the frequency offset can be reduced.

In addition, it is possible to mitigate inter-carrier interference (ICI) due to frequency offset.

It is also possible to improve the performance of the system by reducing the absolute amount of the frequency offset and by mitigating the ICI due to the frequency offset.

It should be understood that the effects of the present invention are not limited to the above effects and include all effects that can be deduced from the detailed description of the present invention or the configuration of the invention described in the claims.

1 is a diagram illustrating a configuration of a NOMA system according to an embodiment of the present invention.
Figure 2 illustrates the effect of frequency offset in the NOMA system of the present invention.
3A is a block diagram illustrating a configuration of an apparatus for eliminating inter-carrier interference due to a frequency offset in a NOMA system according to an embodiment of the present invention.
FIGS. 3B through 3E are detailed views of the respective components shown in FIG. 3A.
4 is a flowchart illustrating a process of eliminating inter-carrier interference due to a frequency offset in a NOMA system according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

In order to clearly illustrate the present invention, parts not related to the description are omitted, and similar parts are denoted by like reference characters throughout the specification.

Throughout the specification, when a part is referred to as being "connected" to another part, it includes not only "directly connected" but also "indirectly connected" .

Also, when an element is referred to as "comprising ", it means that it can include other elements, not excluding other elements unless specifically stated otherwise.

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

1 is a diagram illustrating a configuration of a NOMA system according to an embodiment of the present invention.

The NOMA system according to an embodiment of the present invention may include a plurality of user equipment (UE) 100 and a base station 200.

The system shown in FIG. 1 is a system in which a plurality of UEs 100 simultaneously access the same time and frequency resources, in which a plurality of UEs 100 access the base station 200 and transmit data, to be.

For reference, in the embodiment of the present invention, a basic unit of time and frequency resources that can be allocated to the UE 100 is referred to as a resource block (RB), and a plurality of resource blocks are divided into one sub- block.

The subblock is a basic unit that can be decoded in the NOMA system, and a plurality of subblocks can be included in one symbol.

In FIG. 1, the UE 100 may transmit a signal x to be transmitted using an allocated power p.

That is, in the embodiment of FIG. 1, all of the K users (UEs) can transmit their own signal x using the same power band p allocated to themselves at the same time using the same frequency band.

For reference, the total number of UEs 100 connected to the base station 200 can be expressed as a product of the number of subblocks M and the number of UEs per subblock, that is, the number of UEs sharing resources of one subblock have.

On the other hand, the base station 200 can simultaneously receive signals y from a plurality of UEs 100.

At this time, different frequency offsets are generated for each UE 100, and the frequency offset causes inter-carrier interference (ICI) .

Here, the 'frequency offset' is a phenomenon that the signal received from each UE 100 by the base station 200 is different from the original signal of each UE 100.

It is very difficult to correct ICI in the time domain because a plurality of UEs 100 share the resource blocks included in the same time and frequency resource subblocks.

The base station 200 reduces the absolute amount of frequency offset of each UE 100 sharing the resource block of the corresponding subblock in each subblock in the time domain and removes the interference signal of another UE for each UE 100 Correction of ICI can result in mitigating performance degradation due to frequency offset.

Figure 2 illustrates the effect of frequency offset in the NOMA system of the present invention.

Assuming that the frequency offset does not occur, the peak and zero-crossing must match in accordance with the sync function, but the frequency offset is actually generated and the frequency is shifted as shown in FIG. 2 .

There is a problem that the peak and zero crossing do not coincide due to the frequency offset, and the channel estimation error due to the channel conversion effect and the carrier interference with respect to the zero crossing occur with respect to the peak.

Accordingly, in the present invention, an absolute amount of an average frequency offset is reduced by compensating a frequency offset in a time domain for each sub-block, an ICI replica signal is generated from the received signal by demodulating the received signal, The problem can be solved.

Hereinafter, a method and apparatus for eliminating inter-carrier interference due to frequency offset in a NOMA system will be described in detail with reference to FIGS.

FIG. 3A is a block diagram showing a configuration of an apparatus for eliminating inter-carrier interference due to a frequency offset in a NOMA system according to an embodiment of the present invention, and FIGS. 3B to 3E are detailed views of the respective components.

A device 200 for eliminating inter-carrier interference due to frequency offset in a NOMA system according to an embodiment of the present invention includes a frequency offset compensator 210, a signal processor 220, An interference removing unit 230, a control unit 240, and a storage unit 250.

The carrier-interference cancellation apparatus 200 shown in FIG. 3A can be included in the base station 200 shown in FIG. 1, so that it uses the same identification number.

The frequency offset compensator 210 may compensate a plurality of frequency offsets generated in the subblock for each subblock for a plurality of subblocks included in one symbol.

Here, frequency offset compensation is performed for each sub-block because a plurality of sub-blocks are orthogonal but a plurality of resource blocks included in one sub-block are non-orthogonal.

The frequency offset compensator 210 compensates the J frequency offsets generated in the mth subblock among the M subblocks by 竜_m to reduce the absolute amount of the overall frequency offset of the corresponding subblock.

Here, the J frequency offsets represent the respective frequency offsets for the J UEs sharing the resource block of the corresponding sub-block.

The frequency offset compensator 210 can reduce the absolute amount of the overall frequency offset of the corresponding sub-block using the min-max algorithm.

For example, if the frequency offset generated at the j-th UE among the J UEs in the m-th sub-block is ε _{m, j} , the compensation value ε _m of the overall frequency offset for the corresponding sub- Can be expressed by the following equation (1). &Quot; (1) "

Here, the frequency offset values? _{M, 1} ,? _{M, 2} , ... epsilon _{m and J} are values that can be estimated in advance by transmitting a preamble having orthogonality.

In Equation (1), the signal interference due to the frequency offset becomes worse as the absolute value of the frequency offset becomes larger, and when the absolute value of the largest frequency offset exists, the performance of the system deteriorates, Epsilon < / RTI > that minimizes the absolute value is obtained as a compensation value ϕ m of the overall frequency offset for the subblock. For reference, the compensation value unit of the frequency offset is Hz.

As an example, when the estimated frequency offset values of UE 1, UE 2, and UE 3 are 2, 2, and 3, respectively, and? Is 2, according to Equation 1, {| 2-2 | 2 |, | 3-2 |}, the performance of the system is degraded due to '1' of the UE 3 having the maximum value of max {0, 0, 1}.

In another embodiment, when the estimated frequency offset values of UE 1, UE 2, and UE 3 are 2, 2, and 3, respectively, and? Is 2.5, 2-2.5 |, | 3-2.5 |}, it becomes max {0.5, 0.5, 0.5} and becomes the optimal state without the largest absolute value.

As a result, when? Is 2.5, the compensation value? M of the overall frequency offset for the sub-block m is 2.5.

Conventional OFDMA does not need to consider a frequency offset in which a plurality of UEs overlap because at most one UE can connect to one resource block.

However, in the NOMA system, since a plurality of UEs overlap in one resource block, it is necessary to compensate for the frequency offset.

3B is a detailed diagram of the frequency offset compensator 210. The absolute value of a frequency offset is calculated based on a min-max algorithm for each of a plurality of UEs sharing a resource block in one sub-block, Lt; _{RTI ID} = 0.0 _{> m} , < / RTI > of the overall frequency offset for that resource block.

Although only the m-th sub-block is representatively shown in FIG. 3B, in practice, the process of FIG. 3B may be performed for every sub-block.

)를 출력할 수 있다. 이에 대한 내용이 도 3c에 도시되어 있다.On the other hand, the signal processing unit 220 changes the received signal y in parallel, removes the CP, passes through the multi-carrier demodulator, passes through the NOMA decoder for each sub-block,

Can be output. This is illustrated in Figure 3c.

은 m번째 서브 블록의 복조 신호 벡터를 의미하며 [1×J]의 사이즈를 가질 수 있다.For reference, the primary demodulation signal

Denotes a demodulation signal vector of the m-th sub-block and may have a size of [1 x J].

Meanwhile, the interference eliminator 230 can extract the sub-blocks of the corresponding index according to the stream index using the primary demodulation signal of each sub-block.

For reference, the extraction of the sub-block may be performed by the interference eliminator 230 or may be performed by a separate sub-block extracting unit (not shown) according to the embodiment.

에서 해당 서브 블록의 자원을 공유하는 다른 UE의 신호(즉, 간섭 신호)들의 합을 제거할 수 있다.The interference eliminator 230 receives the signal of the j < th > UE of the m &

(I.e., interference signals) of other UEs sharing the resource of the corresponding sub-block.

The interference signal may be represented by I _{u, (m, j)} . This is the interference signal that the u-th UE gives to the j-th UE of the m-th sub-block.

The interference eliminator 230 may repeat the ICI removal process for each sub-block, and then may pass the NOMA decoder to output a second demodulation signal, which is a final demodulation signal for each sub-block.

This is shown in FIG. 3D. In FIG. 3D, the interference signal for each UE is shown to include its own signal, but the actual value is zero.

At this time, the interference eliminator 230 may increase the reception reliability by removing ICI in parallel. For this, the output likelihood ratio (LLR) of each sub-block is calculated and the sequential ICI removal is performed according to the calculation result .

For reference, in order to determine whether LLR is 0 or 1 based on a received bit,

(0 probability / 1 probability) of the probability of becoming 0 and 1 (probability of probability / 1 probability), and determines whether the received bit is 0 or 1 based on whether LLR is larger or smaller than 0 as a logarithmic value.

The interference eliminator 230 can determine that the reliability is higher as the LLR is larger. That is, a sub-block having a high output LLR is determined as a high-reliability sub-block, and an ICI removal for a sub-block having a high reliability is first performed.

If ICI is removed sequentially according to the result of calculation of the output LLR of each sub-block, if the interference for the previous sub-block is removed, ICI is removed in the state where the interference is removed in the subsequent sub-block, .

In the conventional OFDMA, the interference is canceled in the order of the channel gain of a specific UE. However, since the order of the channel gain and the actual reception performance are different in the NOMA system, the ICI can be removed in the order of the LLR instead of the channel gain .

FIG. 3E is an illustration of the combined operation of FIG. 3B and FIG. 3E. In FIG. 3B, the operations described above are performed for each subblock and the interference is removed in parallel. A separate subblock The sub-block extracting unit is present separately from the interference eliminating unit 230.

The controller 240 may control the components of the base station 200 such as the frequency offset compensator 210, the signal processor 220 and the interference eliminator 230 to perform the above-described operations. And the storage unit 250 can also be controlled.

Meanwhile, the storage unit 250 may store an algorithm for causing the controller 240 to perform the above-described operation and various data generated in the process.

4 is a flowchart illustrating a process of eliminating inter-carrier interference due to a frequency offset in a NOMA system according to an embodiment of the present invention.

4 can be performed by the carrier interference cancellation apparatus 200 shown in FIG. 3 and the carrier interference cancellation apparatus 200 can be included in the base station 200, ).

The base station 200 compensates a plurality of frequency offsets generated in the subblock for each subblock for a plurality of subblocks included in the received signal (S401).

At this time, the base station 200 may calculate the compensation value of the overall frequency offset for the corresponding sub-block using the min-max algorithm of Equation (1).

After step S401, the base station 200 changes the reception signal y compensated for the frequency offset in parallel, removes the CP, passes through the multi-carrier demodulator, passes through the NOMA decoder for each sub-block, and outputs a primary demodulation signal (S402).

After step S402, the base station 200 removes the sum of the interference signals, which are signals of other UEs sharing the resources of the corresponding subblock, from the primary demodulation signal for each subblock, passes through the NOMA decoder to generate a second demodulation signal (S403).

Herein, the base station 200 may increase the reliability of reception by removing ICI in parallel. For this purpose, the base station 200 may calculate the output likelihood ratio (LLR) of each sub-block and perform sequential ICI removal according to the calculation result.

It will be understood by those skilled in the art that the foregoing description of the present invention is for illustrative purposes only and that those of ordinary skill in the art can readily understand that various changes and modifications may be made without departing from the spirit or essential characteristics of the present invention. will be.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

For example, each component described as a single entity may be distributed and implemented, and components described as being distributed may also be implemented in a combined form.

The scope of the present invention is defined by the appended claims, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included within the scope of the present invention.

100: User terminal (UE)
200: base station (carrier interference cancellation device)
210: Frequency offset compensator
220: Signal processor
230: Interference elimination
240:
250:

Claims (8)

An apparatus for eliminating inter-carrier interference due to frequency offset in a non-orthogonal multiple access (NOMA) system,
A plurality of frequency offsets generated in the corresponding sub-block for each of a plurality of sub-blocks included in the received signal are present for each user sharing resources of the corresponding sub-block. A frequency offset compensator for compensating the frequency offset;
A signal processor for demodulating and decoding the received signal with the frequency offset compensated for each sub-block, and outputting a demodulated signal; And
In each sub-block, an interference eliminator for removing a sum of interference signals which are signals of other users in a signal of a corresponding user for each user sharing resources of the corresponding sub-block, decoding the decoded signal,
, ≪ / RTI &
The frequency offset compensator
Wherein the min-max algorithm is applied to an absolute value obtained by subtracting a specific candidate value from a frequency offset for each user, thereby extracting a value that totally compensates for the plurality of frequency offsets.
The method according to claim 1,
The interference elimination unit
(LLR) of each sub-block to determine a channel reliability order for each sub-block, and sequentially removes the sum of the interference signals in the determined order. Interference canceller.
3. The method of claim 2,
The interference elimination unit
And repeats the operation of removing the sum of the interference signals by a predetermined number of times.
The method according to claim 1,
A sub-block extracting unit for extracting each sub-block according to a stream index using the demodulation signal in a received signal copied in the same number of streams as the number of sub-blocks,
Further comprising: an antenna for receiving the carrier wave;
A method for eliminating inter-carrier interference due to frequency offset in a non-orthogonal multiple access (NOMA) system, the method comprising:
(a) a plurality of frequency offsets generated in a corresponding sub-block for each of a plurality of sub-blocks included in a received signal, and (b) a frequency offset generated for each user sharing the corresponding sub- Comprehensively compensating for the error;
(b) demodulating and decoding the received signal with the frequency offset compensated for each sub-block, and outputting a demodulated signal; And
(c) removing the sum of the interference signals, which are signals of other users, from the signal of the user for each user sharing the resource of the corresponding sub-block, decoding the decoded signal, and outputting the final demodulated signal
, ≪ / RTI &
The step (a)
Wherein the min-max algorithm is applied to an absolute value obtained by subtracting a specific candidate value from a frequency offset for each user, thereby extracting a value that totally compensates for the plurality of frequency offsets.
6. The method of claim 5,
The step (c)
Calculating an output log likelihood ratio (LLR) of each sub-block to determine a channel reliability order for each sub-block; And
Sequentially removing the sum of the interference signals in the determined order
And removing the carrier wave.
The method according to claim 6,
The step (c)
And repeating the operation of removing the sum of the interference signals by a predetermined number of times.
A computer program stored in a recording medium comprising a series of instructions for performing the method according to any one of claims 5 to 7.

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