KR101552442B1 - OFDM cooperative communication method based on cell and System - Google Patents

Abstract

The present invention relates to a cell-based OFDM cooperative communications method and a system for the same. The method of the present invention comprises the steps of: allowing a mobile terminal having a plurality of antennas located in a first cell to receive a first signal from a first base station installed in the first cell during a first time slot; allowing the mobile terminal to receive a first signal demodulated from a first repeater during a second time slot, to receive a second signal from the first base station, and to receive a second signal demodulated from a second repeater located in a second cell; and allowing the mobile terminal to estimate the received first and second signals. According to the present invention, by introducing the cell-based OFDM cooperative communications method and the system for the same, the mobile terminal additionally receives a signal from an adjacent base station even if the distance between the mobile terminal and the base station is far away, thereby obtaining diversity gain and increasing data throughput. In addition, the present invention can perform reliable communications by decreasing a bit error rate.

Description

[0001] Cell-based OFDM cooperative communication method and system [0002]

The present invention relates to a cell-based OFDM collaborative communication method and system, and more particularly, to a method and system for collaborative communication in a cell to which a mobile terminal belongs and a base station of a cell adjacent to the cell, will be.

With the popularization of smart devices, demand for high-definition content and Internet services has increased sharply, and the demand for data traffic has increased rapidly. The number of wireless Internet users surpassed the number of wired Internet users, and Cisco expects global mobile traffic, which was 597 petabytes in 2011, to reach 10,804 petabytes in 2016, with global mobile traffic growing at an annual average rate of 78% Respectively. As a result, interest in traffic management on mobile communication networks has increased.

In addition, the service quality degradation problem in the base station coverage boundary area has increased, and the importance of providing uniform data service quality regardless of the location has expanded, and the best data service is provided in the coverage area of the base station by solving the mutual interference problem between the base stations Was studied. Research on Coordinated Multipoint (CoMP), which is a base station cooperative communication technology for LTE-Advanced, is actively being carried out.

Conventionally, when a mobile terminal is located in a coverage area of two base stations, a signal received from a base station to which the mobile station belongs is reduced, and a data rate is remarkably slowed due to interference of an outgoing signal from an adjacent base station.

On the other hand, when the CoMP technique is applied, the mutual interference between the base stations and the data disconnection phenomenon can be prevented by sharing and managing the communication state information between the base stations, even if the mobile terminal moves to the coverage area of the two base stations.

However, the CoMP (Cooperative Mobile Station) technology can cause data quality degradation due to cooperative communication when moving between a plurality of base stations, which may cause a delay in speed or a disconnection phenomenon.

1 is a diagram for explaining cooperative communication in a wireless communication system according to the prior art.

1, a repeater and a mobile station have two antennas in an OFDM-based wireless communication according to the related art, and the base station can transmit a signal to a mobile terminal moving away from the base station by extending a range of the cell using a repeater.

The base station transmits the signal to the repeaters and the mobile terminal, and the repeater modulates the demodulated signal through the DF (Decode and Forward) process and transmits the modulated signal to the mobile terminal. That is, the mobile terminal directly receives a signal from a base station of a cell to which the mobile terminal belongs, and further receives a signal using a repeater to obtain a diversity gain.

However, the prior art shown in FIG. 1 has a performance degradation when a repeater is added or the mobile terminal moves away from the base station. Particularly, as the mobile terminal is located at the edge of the cell, a significant performance degradation occurs.

The technology of the background of the present invention is disclosed in Korean Patent Laid-Open No. 10-2012-0059989 (published on Jun. 11, 2012).

The present invention relates to a cell-based OFDM collaborative communication method and system, and more particularly, to a method and system for collaborative communication in a cell in which a mobile terminal belongs and a base station of a cell adjacent to the cell in cooperation with a mobile terminal using multiple antennas .

According to another aspect of the present invention, there is provided a cell-based OFDM collaborative communication method including: a first mobile station having a plurality of antennas located in a first cell; Receiving a first signal demodulated from the first repeater during a second time slot and receiving a second signal from the first base station, receiving demodulated signals from a second repeater located in an adjacent second cell, Receiving the second signal, and estimating the received first signal and the second signal.

The second base station located in the second cell receives the signal to be transmitted from the first base station in advance and transmits the second signal to the second repeater during the first time slot.

In addition, the mobile terminal receives the first signal and the second signal from the first repeater, the first base station, and the second repeater in STBC (space-time block code) do.

Here, y _ij denotes an i-th received signal received from the j-th antenna, n _ij denotes a noise value added to each received signal, h _0j denotes a channel between the base station and the j-th antenna of the mobile terminal, h _1j Denotes a channel between the first repeater and the jth antenna of the mobile terminal, and _h2j denotes a channel between the second repeater and the jth antenna of the mobile terminal.

The first and second repeaters demodulate the first signal or the second signal received from the first base station or the second base station through a Decode and Forward (DF) scheme.

In addition, estimating the first signal and the second signal estimates the first signal and the second signal using a Minimum Mean Square Error (MMSE) method.

A cell-based OFDM collaborative communication system according to another embodiment of the present invention includes a mobile terminal having a plurality of antennas located in the same cell, a first base station and a first repeater, Receiving a first signal from the first base station, receiving a first signal demodulated from the first repeater during a second time slot, receiving a second signal from the first base station, and receiving a second signal from a second repeater located in an adjacent cell Receives the demodulated second signal, and estimates the received first and second signals.

Therefore, according to the present invention, by using the cell-based OFDM cooperative communication method and system, even when the distance from the base station is increased, the mobile terminal can additionally receive signals from adjacent base stations to obtain diversity gain, . In addition, the bit error rate can be lowered to perform reliable communication.

1 is a diagram for explaining cooperative communication in a wireless communication system according to the prior art.
2 is a block diagram illustrating a cell-based OFDM collaborative communication system according to an embodiment of the present invention.
3 is a signal flow diagram illustrating data and signal flow in a cell-based OFDM collaborative communication system in accordance with an embodiment of the present invention.
FIG. 4 is a graph illustrating a performance change according to an attenuation environment in a conventional cooperative communication method.
5 is a graph illustrating BER performance according to an attenuation environment in a cell-based OFDM cooperative communication system according to an embodiment of the present invention.
6 is a graph illustrating data throughput according to an attenuation environment in a cell-based OFDM cooperative communication system according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention. 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.

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

First, a cell-based OFDM cooperative communication method according to an embodiment of the present invention will be described in detail with reference to FIG. 2 and FIG.

2 is a block diagram illustrating a cell-based OFDM collaborative communication system according to an embodiment of the present invention.

2, according to a cell-based OFDM collaborative communication system, a first cell 200 includes a first base station 210, a first repeater 230, a second cell 205 includes a second base station 210, (215) and a second repeater (235). It is assumed that the mobile terminal 250 is located in the first cell 200 as shown in FIG.

The first and second repeaters 230 and 235 and the mobile terminal 250 have a plurality of antennas and transmit and receive data using the STBC scheme. According to the cell-based OFDM cooperative communication system according to the embodiment of the present invention, the first repeater 230, the second repeater 235, and the mobile terminal 250 each have two antennas.

Here, the first repeater 230 and the second repeater 235 use one antenna as a transmitting antenna and the other antenna as a receiving antenna, out of the two antennas. On the other hand, the mobile terminal 250 uses both antennas as a reception antenna.

The first base station 210 and the second base station 215 transmit signals to the first and second repeaters 230 and 235 respectively and the first and second repeaters 230 and 235 transmit signals to the base station 210, and 215, and transmits the demodulated signal to the mobile terminal 250. The first base station 210 may also transmit a signal to the mobile terminal 250 without going through a repeater.

The base stations 210 and 215 connect the mobile terminal 250 and an exchange (not shown). The base stations 210 and 215 transmit signals by connecting the mobile terminal 250 with a channel. The first and second base stations 210 and 215 may transmit signals to the mobile terminal 250 through the first and second repeaters 230 and 235. In the case of the wireless communication system including the repeaters 230 and 235, And may transmit the signal to the direct mobile terminal 250.

For convenience of explanation, the cell to which the mobile terminal 250 belongs is referred to as a first cell 200, the base station located in the first cell 200 is referred to as a first base station 210, It is assumed that the cell closest to one cell 200 is the closest.

The first and second repeaters 230 and 235 are provided to improve the propagation shaded area in the cells of the first and second base stations 210 and 215 and include first and second repeaters 230 and 235 The cell sizes of the first and second base stations 210 and 215 can be increased. The first and second repeaters 230 and 235 receive signals from the first and second base stations 210 and 215 and retransmit the signals to the mobile terminal 250.

The mobile terminal 250 is located at the end of the user side on the wireless link, and performs a terminal function of providing a service to the user. The mobile terminal 250 receives signals from the first base station 210 of the first cell 200, the first repeater 230 and the second repeater 235 of the second cell 205 through a plurality of antennas .

Hereinafter, a cell-based OFDM cooperative communication method according to an embodiment of the present invention will be described in more detail with reference to FIG.

3 is a signal flow diagram illustrating data and signal flow in a cell-based OFDM collaborative communication system in accordance with an embodiment of the present invention.

In the cell-based OFDM collaborative communication system according to the embodiment of the present invention, the base stations 210 and 215 transmit a first signal (x ₁ ), a second signal x ₂ ), i.e., two signals are transmitted.

First, during a first time slot, the first base station 210 transmits a first signal x ₁ to the first repeater 230 and the mobile terminal 250 (S310). The first base station 210 receives the g ₀₁ And transmits the first signal (x ₁ ) to the first repeater 230 through the channel. Also, the first base station 210 transmits the first signal x ₁ to the first and second antennas of the mobile terminal 250 through the channels h ₀₁ and h ₀₂ , respectively.

Equation 1 is a formula for calculating the vector of the signal received by the first and second repeaters 230 and 235 or the mobile terminal 20. The signal received by the first and second repeaters 230 and 235 and the signal received by the mobile terminal 250 can be calculated using Equation (1).

In Equation 1, Y is a vector value of a signal received by the first and second repeaters 230 and 235 or the mobile terminal 250, H is a channel matrix used for signal reception, S is a matrix of transmitted signals And N denotes a noise vector added when transmitting and receiving a signal.

The signal received by the first repeater 230 is obtained by adding a noise vector to the product of the channel matrix used by the first repeater 230 and the signal matrix transmitted by the first base station 210 as shown in Equation 1 Can be calculated. The signal received by the mobile terminal 250 through the first antenna and the second antenna may also be calculated using Equation (1).

Next, the second base station 215 transmits the second signal (x ₂ ) previously received from the first base station 210 to the second repeater 235 during the first time slot (S320). At this time, the second base station 215 transmits g ₀₂ And transmits the second signal (x ₂ ) through the channel.

)를 이동 단말기(250)로 전송한다(S330). 여기서 제1 중계기(230)는 DF Relaying 기법을 사용하여 수신한 신호를 복조하고, 복조 결과값인 복조한 제1 신호(

)를 이동 단말기(250)로 전송한다. 이때, 제1 중계기(230)는 채널 h₁₁, h₁₂을 통하여 이동 단말기(250)의 첫 번째 및 두 번째 안테나로 각각 복조된 제1 신호 (

)를 전송한다. During the second time slot, the first repeater 230 demodulates the first signal received in step S310, and outputs the demodulated first signal

To the mobile terminal 250 (S330). Here, the first repeater 230 demodulates the received signal using the DF Relaying technique, and outputs the demodulated first signal (

To the mobile terminal (250). At this time, the first repeater 230 demodulates the first signal demodulated by the first and second antennas of the mobile terminal 250 through the channels h ₁₁ and h ₁₂

).

Also, the first base station 210 transmits the second signal x ₂ to the mobile terminal 250 during the second time slot (S340). The first base station 210 transmits the second signal (x ₂ ) to the first and second antennas of the mobile terminal 250 via the channel h ₀₁ and the channel h ₀₂ , respectively.

)를 이동 단말기(250)로 전송한다(S350). 이때, 제2 중계기(235)는 채널 h₂₁, h₂₂을 통하여 각각 이동 단말기(250)의 첫 번째 및 두 번째 안테나로 복조한 제2 신호(

)를 전송한다. During the second time slot, the second repeater 235 demodulates and demodulates the second signal x ₂ received from the second base station 215 in step S320,

To the mobile terminal 250 (S350). At this time, the second repeater 235 demodulates the first and second antennas of the mobile terminal 250 through the channels h ₂₁ and h ₂₂ , respectively,

).

), 복조한 제2 신호(

)를 STBC 방식으로 수신한다. 이동 단말기(250)가 수신한 수신 심볼을 행렬로 나타내면 수학식 2로 표현할 수 있다. As described above, the mobile terminal 250 transmits the first signal x ₁ , the second signal x ₂ , the demodulated first signal x ₁ ,

), A demodulated second signal (

) In the STBC manner. If the received symbols received by the mobile terminal 250 are represented by a matrix, Equation (2) can be expressed.

X denotes an OFDM symbol received by the mobile terminal 250. The first row denotes a signal received from the first base station 210, the first repeater 230 and the second repeater 235 in the first time slot . The second row represents signals received from the first base station 210, the first repeater 230, and the second repeater 235 in the second time slot.

As shown in Equation (2), during a first time slot, the mobile terminal 250 receives the first signal x ₁ only from the first base station 210. Here, during the first time slot, the second repeater 235 receives the second signal x ₂ from the second base station 215.

)를 수신하며, 제2 중계기(235)로부터 제2 중계기(235)가 DF과정을 통하여 복조한 제2 신호(

)를 수신한다. During the second time slot, the mobile terminal 250 receives the second signal x ₂ from the first base station 210 and the first repeater 230 receives the second signal x ₂ from the first repeater 230 through the DF process The demodulated first signal (

) From the second repeater 235 to the second signal demodulated through the DF process by the second repeater 235

).

The signal received by the mobile terminal 250 having two antennas for each time slot during each time slot can be expressed by Equation (3).

Here, y _ij represents a signal received by the mobile terminal 250 at the j-th antenna during an i-th time slot, n _ij represents a noise value added to each received signal, h _0j represents a noise value added to the received signals by the base stations 210 and 215, It means that the channel between the j-th antenna at 250), and, h _1j indicates a channel between the j-th antenna of the first repeater 230 and the mobile terminal 250, and, h _2j is the second repeater 235 and the mobile Denotes a channel between the j < th >

That is, y ₁₁ denotes a signal received by the first antenna during the first time slot of the mobile terminal 250, and the mobile terminal 250 receives the signal from the first base station 210 through the channel h ₀₁ during the first time slot. And receives the first signal (x ₁ ) with the first antenna. y ₁₂ denotes a signal received by the mobile terminal 250 through the second antenna during the first time slot and the mobile terminal 250 transmits the first signal from the first base station 210 to the second antenna through the channel h _02, And receives the signal x ₁ . Also, the y ₁₁ signal and the y ₁₂ signal include noise of n ₁₁ and n ₁₂ , respectively.

)를 수신하며, h₂₁채널을 통하여 제2 중계기(235)로부터 첫 번째 안테나로 복조한 제2 신호(

)를 수신한다. y₂₁신호에는 잡음인 n₂₁도 포함되어 있다. And y ₂₁ denotes a signal received by the first antenna of the mobile terminal 250 during the second time slot. The mobile terminal 250 receives the second signal x ₂ from the first base station 210 through the h ₀₁ channel during the second time slot to the first antenna and transmits the second signal x ₂ from the first relay 230 through the h ₁₁ channel The first signal demodulated with the first antenna (

And a second signal demodulated from the second repeater 235 to the first antenna through the h ₂₁ channel

). The y ₂₁ signal also includes noise, n ₂₁ .

)를 수신하며, h₂₂채널을 통하여 제2 중계기(235)로부터 두 번째 안테나로 복조한 제2 신호(

)를 수신한다. 또한 y₂₂에는 잡음 신호인 n₂₂도 포함되어 있다. Finally, y ₂₂ denotes a signal received by the second antenna of the mobile terminal 250 during the second time slot, and the mobile terminal 250 transmits a signal from the first base station 210 to the second antenna through the channel h _02, 2 signal x ₂ and demodulates the first signal from the first repeater 230 to the second antenna through the h ₁₂ channel

And a second signal demodulated from the second repeater 235 to the second antenna through the h ₂₂ channel

). Y ₂₂ also includes the noise signal n ₂₂ .

는 추정 값을 나타내는

로 표현할 수 있으며,

를 이용하여 수학식 3을 행렬로 표현하면 수학식 4와 같다. x _i and

Represents an estimated value

As shown in FIG.

The equation (3) can be expressed by the following equation (4).

The mobile terminal 250 can know the signals received from the first base station 210, the first repeater 230, and the second repeater 235 at each antenna during each time slot using Equation (4).

Finally, the mobile terminal 250 estimates the received first signal x ₁ and the received second signal x ₂ (S360).

According to an embodiment of the present invention, a method of estimating a minimum mean square error (MMSE) is used to demodulate a received signal. Here, MMSE is a method designed to compensate for the disadvantage of average value filtering, and it is a method to obtain an optimal estimate of an unknown variable by minimizing the mean squared error that is easy to operate mathematically.

Meanwhile, according to the embodiment of the present invention, for convenience of description, two signals are transmitted during two time slots. However, it is possible to design a plurality of signals to be transmitted and estimated during a plurality of time slots in real time.

Hereinafter, simulation results using a cell-based OFDM cooperative communication system according to an embodiment of the present invention will be described in detail with reference to FIG. 4 through FIG.

For simulating a cell-based OFDM cooperative communication system according to an embodiment of the present invention, a Fast Fourier Transform (FFT) size is set to 256 and a Guard-Interval length of an OFDM symbol is set to 64, 16QAM (Quadrature Amplitude Modulation) method was used. The signal is coded with a convolutional code with a code rate of 1/2 and a constraint length of 3 and is subjected to a 7-path Rayleigh fading channel with quasi-static characteristics, Are assumed to be uniformly allocated.

FIG. 4 is a graph illustrating a performance change according to an attenuation environment in a conventional cooperative communication method.

A large degree of attenuation means that the mobile terminal is far from the base station, and the attenuation level is -12 dB, which means that the distance from the base station is longer than when the attenuation is -3 dB.

As shown in FIG. 4, the larger the degree of attenuation is, the higher the bit error rate (BER) is, and the performance is degraded. When the SNR value is 30, the bit error rate (BER) is stable to 10 ^-3 or less in the attenuation environments of -3 dB and -6 dB, but the bit error rate (BER) is high at -9 dB and -12 dB, . That is, it can be seen from FIG. 4 that the bit error rate (BER) increases as the mobile terminal moves away from the base station.

FIG. 5 is a graph illustrating bit error rate (BER) performance according to an attenuation environment in a cell-based OFDM collaborative communication system according to an embodiment of the present invention. 6 is a graph illustrating data throughput according to an attenuation environment in a cell-based OFDM cooperative communication system according to an embodiment of the present invention.

Referring to FIG. 5 and FIG. 6, when a cell-based OFDM cooperative communication system according to an embodiment of the present invention is applied, performance in an attenuation environment of -9 dB and -12 dB can be confirmed by comparison.

As shown in FIG. 5, in the attenuation environments of -9 dB and -12 dB, the bit error rate (BER) is remarkably lowered and the performance is higher than the conventional technique. When a cell-based OFDM cooperative communication method according to an embodiment of the present invention is applied, a gain of about 3 dB can be obtained in an attenuation environment of -9 dB since it has a bit error rate (BER) similar to that of the existing attenuation environment of -6 dB . Also, in the -12 dB attenuation environment, it has a bit error rate (BER) similar to the bit error rate (BER) at the existing attenuation environment of about -7 dB, so that it can have a gain of about 5 dB.

Also, as shown in FIG. 6, when the signal-to-noise ratio (SNR) is 10 or less, the data throughput in the attenuation environment is slightly higher when the conventional method is applied. However, if the SNR exceeds 10, It can be seen that the method according to the embodiment significantly increases the data throughput compared to the conventional method.

As described above, according to the embodiment of the present invention, even when the mobile terminal is distant from the base station, the mobile terminal can additionally receive a signal from an adjacent base station to obtain a diversity gain and increase data throughput. In addition, the bit error rate can be lowered to perform reliable communication.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, Of the right.

200: first cell 205: second cell
210: first base station 215: second base station
230: first repeater 235: second repeater
250: mobile terminal

Claims (10)

A cooperative communication method using a cell-based OFDM cooperative communication system,
A mobile terminal having a plurality of antennas located in a first cell includes receiving a first signal from a first base station installed in the first cell during a first time slot,
Receiving a first signal demodulated from a first repeater during a second time slot and receiving a second signal from the first base station, receiving a second demodulated signal from a second repeater located in an adjacent second cell, And
Estimating the first signal and the second signal received,
And a second base station located in the second cell,
Wherein the second base station receives the signal to be transmitted from the first base station in advance and transmits the second signal to the second repeater during the first time slot. delete The method according to claim 1,
The mobile terminal includes:
Based OFDM cooperative communication (hereinafter, referred to as " cell-based OFDM cooperative communication ") represented by the following equation: < EMI ID = Way:

Here, y _ij denotes an i-th received signal received from the j-th antenna, n _ij denotes a noise value added to each received signal, h _0j denotes a channel between the base station and the j-th antenna of the mobile terminal, h _1j Denotes a channel between the first repeater and the jth antenna of the mobile terminal, and _h2j denotes a channel between the second repeater and the jth antenna of the mobile terminal. The method of claim 3,
Wherein the first and second repeaters comprise:
And a demodulator for demodulating the first signal or the second signal received from the first base station or the second base station through a DF (Decode and Forward) scheme. The method according to claim 1,
Wherein estimating the first signal and the second signal comprises:
A cell-based OFDM collaborative communication method for estimating a first signal and a second signal using a Minimum Mean Square Error (MMSE) method. In a cell-based OFDM cooperative communication system including a mobile terminal having a plurality of antennas located in the same cell, a first base station and a first repeater,
The mobile terminal includes:
Receiving a first signal from the first base station during a first time slot,
Receiving a first signal demodulated from the first repeater during a second time slot and receiving a second signal from the first base station, receiving a second demodulated signal from a second repeater located in an adjacent cell,
Estimating the received first and second signals,
And a second base station located in the adjacent cell,
Wherein the second base station receives the signal to be transmitted from the first base station in advance and transmits the second signal to the second repeater during the first time slot. delete The method according to claim 6,
The mobile terminal includes:
Based OFDM cooperative communication (hereinafter, referred to as " cell-based OFDM cooperative communication ") represented by the following equation: < EMI ID = system:

Here, y _ij denotes an i-th received signal received from the j-th antenna, n _ij denotes a noise value added to each received signal, h _0j denotes a channel between the base station and the j-th antenna of the mobile terminal, h _1j Denotes a channel between the first repeater and the jth antenna of the mobile terminal, and _h2j denotes a channel between the second repeater and the jth antenna of the mobile terminal. 9. The method of claim 8,
Wherein the first and second repeaters comprise:
And a first signal or a second signal received from the first base station or the second base station through a DF (Decode and Forward) scheme. The method according to claim 6,
The mobile terminal includes:
A cell-based OFDM collaborative communication system for estimating a first signal and a second signal using a Minimum Mean Square Error (MMSE) method.

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