KR20090105534A - Method of Uplink Transmission Reducing Interference - Google Patents

Abstract

PURPOSE: An uplink transmission method for reducing interference for improving data transfer rate is provided to reduce interference between cells and improve signal to noise ratio of the terminal. CONSTITUTION: An uplink transmission method for reducing interference for improving data transfer rate is as follows. Scheduling information about the allocation of an uplink frequency band is received. According to the scheduling information, the up link transfer is performed. The uplink frequency band is divided by the first and the second sub band. The first sub band is allocated to the cell boundary terminal. The second sub band is commonly allocated to the cell boundary terminal.

Description

Method of Uplink Transmission Reducing Interference

The present invention relates to wireless communication, and more particularly, to an uplink transmission method for reducing interference.

The wireless communication system has a cell structure for efficient system configuration. A cell is a subdivision of a large area into smaller areas in order to use frequency efficiently. Multiple access systems generally include multiple cells. In general, the base station is installed in the cell to relay the terminal.

The wireless communication system supports downlink and uplink for a plurality of terminals. Here, downlink means communication from the base station to the terminal, and uplink means communication from the terminal to the base station. A base station (eg Node-B) operates a cell, and a scheduler located at the base station determines which terminal for which data is to be transmitted in the cell. The terminal may be a mobile or fixed device that is walking or operated by a person in a vehicle.

When there are a plurality of terminals in a cell, the plurality of terminals may simultaneously perform uplink transmission or the base station may simultaneously perform downlink transmission to the plurality of terminals. The problem that may arise at this time is interference. Interference mostly includes thermal noise, power transmitted from other cells, dedicated channel power transmitted within the cell, power for shared channels transmitted within the cell, and the like. Interference leads to a reduction in the signal to interference ratio (SIR). Reasons for the reduction of the signal-to-noise ratio include the following.

In the wireless mobile communication system based on the cell structure, the difference in the channel environment according to the position of the terminal in the cell shows a large difference in the signal-to-noise ratio of the terminal. In general, the terminal in the center of the cell close to the base station has a small propagation loss due to the short propagation distance of the signal, while the terminal of a cell boundary located far from the base station suffers a large propagation loss due to the long propagation distance of the signal. This leads to a reduction in signal-to-noise ratio.

Another cause of worsening signal-to-noise ratio is interference between terminals using the same frequency resource. Data transmission by the terminal in the same cell can prevent intra-cell interference through multiplexing having orthogonality. However, data transmission by the terminal in different cells may not be orthogonal, and the terminal may experience inter-cell interference from other cells.

For example, orthogonal frequency division multiplexing (OFDM) is one of the techniques using multiple carriers. OFDM partitions the overall system bandwidth into a plurality of subcarriers with orthogonality and carries data on subcarriers for transmission. In a multi-cell environment, a system using multiple carriers such as OFDM may cause interference to users when adjacent cells use the same subcarrier.

The signal-to-interference ratio can be improved by using different frequencies between cells. This is called a frequency reuse technique. For example, if the number of adjacent cells is three, the entire frequency band is divided into three parts so that the frequency bands do not overlap each other, thereby preventing inter-cell interference. However, this lowers the overall spectral efficiency. Meanwhile, a partial frequency reuse technique may be used to improve the signal-to-interference ratio near the boundary of the cell. However, the terminal near the cell boundary may have a low signal-to-noise ratio due to propagation loss during uplink transmission.

Meanwhile, the signal-to-interference ratio can also be improved through power control. A terminal having a large propagation loss located at a cell boundary compensates for power loss due to radio waves by increasing transmission power. As a result, the signal strength of the user received at the base station can be maintained at a required level. On the other hand, the terminal located in the center of the cell with low propagation loss lowers the transmission power in the range of maintaining the signal strength to receive the necessary service. Signal-to-noise ratio improvement by adjusting the transmission power can improve the signal-to-noise ratio, except the power amplifier and battery consumption of the terminal, if there is no overlapping frequency band between the terminals belonging to the boundary of the adjacent cell.

 However, in a system with a frequency reuse factor of 1, when a terminal located at a cell boundary performs uplink transmission with high transmission power, it is a strong interference to an adjacent cell. The procedure of power regulation should be followed. Furthermore, in addition to reducing the efficiency of using frequency and limiting the gain of using selective frequency in the fading channel, the complexity of the scheduler is increased, thereby increasing the system latency.

Therefore, there is a need for an uplink transmission method that can effectively reduce interference.

An object of the present invention is to provide an uplink transmission method for reducing interference.

According to an aspect of the present invention, there is provided a method for performing uplink transmission in a cell-based system divided into a cell interior and a cell boundary. The method includes receiving scheduling information regarding allocation of an uplink frequency band, and performing uplink transmission according to the scheduling information. The uplink frequency band is divided into a first subband and a second subband, the first subband is allocated exclusively to a cell boundary terminal, and the second subband is a band sharing technique for the cell boundary terminal and a cell internal terminal. Commonly assigned by

The signal-to-noise ratio of the terminal located near the cell boundary having a relatively low signal-to-noise ratio can be improved and the inter-cell interference can be reduced. The use of frequency reuse factor 1 also provides band efficiency, frequency select gain, and increased data rate.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Like numbers refer to like elements throughout.

The following techniques can be used in various communication systems. Communication systems are widely deployed to provide various communication services such as voice, packet data, and the like. This technique can be used for downlink or uplink. Downlink means communication from a base station (BS) to a mobile station (MS), and uplink means communication from a terminal to a base station. A base station generally refers to a fixed station for communicating with a terminal and may be referred to as other terminology such as a node-B, a base transceiver system (BTS), an access point, or the like. The terminal may be fixed or mobile and may be called in other terms such as user equipment (UE), user terminal (UT), subscriber station (SS), wireless device (wireless device), and the like.

The present invention can be applied to any multiple access scheme that requires frequency division for multiple access. Multiple access schemes include frequency division multiple access (FDMA), orthogonal frequency division multiple access (OFDMA), multi-carrier code division multiple access (MC-CDMA), and combinations thereof.

More specifically with regard to OFDM / OFDMA based systems, OFDM uses multiple orthogonal subcarriers. OFDM uses orthogonality between an inverse fast Fourier Transform (IFFT) and a Fast Fourier Transform (FFT). At the transmitter, data is transmitted by performing an IFFT. The receiver performs FFT on the received signal to recover the original data. The transmitter uses an IFFT to combine multiple subcarriers, and the receiver uses a corresponding FFT to separate multiple subcarriers.

1 is a block diagram illustrating partial frequency reuse. Hereinafter, the cell boundary is defined as a concept covering an area near the cell boundary where the influence of an interference signal from an adjacent cell serves as a main cause of the link performance degradation. That is, the cell boundary is defined by the system as an area in which interference of adjacent cells has a major influence on system performance. In addition, the cell center region is defined as a concept encompassing the rest of the cell except for the cell boundary, that is, the region near the cell center.

Referring to FIG. 1, partial frequency reuse (FFR) is a method of using various frequency reuse coefficients in one cell or sector using orthogonality of subcarriers in an OFDM system. When the total frequency band available to the base station in each cell is F, in order to prevent intercell interference, the base stations of each cell have different frequencies f1 (for cell A), f2 (for cell B), Use f3 (for cell C). That is, since the cell boundary is easily affected by interference from neighboring cells, the frequency reuse factor 3 is applied to the cell boundary so that the bands do not overlap with the neighboring cells.

On the other hand, since the distance between adjacent cells is secured to some extent within the cell and the strength of the interference signal is weak, the frequency reuse factor 1 is applied to the cell.

In this way, the UE located at the cell boundary affected by the interference from neighboring cells (hereinafter, cell boundary terminal) is allocated only a limited portion for the cell boundary terminal in the entire frequency band, and has little influence on inter-cell interference from neighboring cells. By allocating resources in the entire frequency band to a terminal located within a receiving cell (hereinafter, referred to as a cell inside terminal), interference between cells can be effectively prevented and system capacity can be increased. On the other hand, when the frequency band available in each cell boundary is limited to some, the scheduler may be limited in assigning the frequency band to the terminal.

In addition, the terminal located in the cell boundary has a relatively low resource allocation rate in the scheduler for allocating resources to increase the system yield because the channel state is poor due to path loss, inter-cell interference, etc., compared to the terminal located in the cell. In addition, even if the terminal is allocated resources, the poor signal-to-noise ratio further restricts the yield since the modulation and encryption techniques that can obtain a high information rate cannot be used.

2 is a block diagram illustrating a method of allocating an uplink frequency spectrum of a cell to a terminal according to an embodiment of the present invention.

Referring to FIG. 2, the entire uplink frequency bands of cells A and B are divided into two subbands (first and second subbands). Whether to allocate the first or second subband to the terminal is determined by uplink scheduling of the base station. More specifically, the base station allocates a partial band to the terminal differently according to the position of the terminal. Of course, detailed subcarrier allocation in each subband may use AMC (Adapative Modulation and Coding) in consideration of channel quality. The location of the terminal is either cell boundary or cell interior (see FIG. 1). Since the cell A and the cell B use a partial frequency reuse scheme, the first partial band, which is a partial band allocated to a cell boundary terminal, is different for each cell.

As an example of how the base station obtains the position of the terminal, the terminal calculates its position in the cell and informs the base station. The terminal may use a reference signal (for example, a pilot signal) or a synchronization channel transmitted from the base station, but calculate its position from the degree of propagation attenuation of these signals.

As another example of how the base station obtains the position of the terminal, the base station directly calculates the position of the terminal. The location of the terminal in the cell can be directly calculated by the base station using an uplink reference signal (for example, a sounding reference signal) or a signal to interference ratio (SIR) transmitted from the terminal. have.

When the position of the terminal is determined by the above method, the base station allocates an uplink frequency band to the terminal as follows. The first subband is a frequency band allocated to a cell boundary terminal. The second subband is additionally allocated to the cell boundary terminal as well as the cell internal terminal. That is, a cell internal terminal may be allocated only the second subband, but a cell boundary terminal may be allocated only the first subband or both the first and second subbands. If it is determined by the scheduling which partial band is allocated to the terminal, the base station transmits scheduling information to the terminal.

The uplink frequency band is divided into two subbands, but this is only an example and may be divided into three or more subbands. For example, when the uplink frequency band is divided into three subbands, the cell boundary terminal may simultaneously perform uplink transmission through at least two of the subbands or simultaneously uplink through the three subbands. You can also perform the transfer.

Now, a method for performing uplink transmission by a terminal receiving scheduling information is disclosed.

3 is a block diagram illustrating a method of performing uplink transmission of a terminal according to an embodiment of the present invention.

Referring to FIG. 3, the terminal receiving the scheduling information performs uplink transmission as follows. UE # 1 is a cell boundary terminal, UE # 2 is an intra-cell terminal, and UE # 1 and UE # 2 both belong to the same cell. UE # 1 simultaneously performs uplink transmission on the first subband and the second subband. Of course, not all of the first subband and all of the second subband are allocated to UE # 1, and since each subband is frequency division multiplexed with another cell boundary terminal, some subcarriers are allocated to UE # 1. UE # 2 performs uplink transmission on the second subband. That is, the second subband is frequency division multiplexed between terminals in a cell.

For clarity of description, data transmitted by the UE # 1 through the first subband is data A, and data transmitted through the second subband is called data A ', and UE # 2 is referred to as the second part. Let data B be transmitted through the band. Here, the data A and the data A 'may be the same data or different data. When the data A and A 'are different data, the data rate may be improved. When the data A and A 'are the same data, transmission diversity and reliability of data transmission can be improved.

Since the first subband is frequency division multiplexed (for example, FDMA or OFDMA) among cell boundary terminals, interference does not occur, but the second subband is allocated to UE # 1 as well as UE # 2. In transmission, the data A 'and the data B may interfere with each other. In order to remove such interference, the data A 'of UE # 1 is transmitted using a band sharing technique in the second subband allocated to UE # 2.

As an example of the band sharing technique, a Collaborate Spatial Multiplexing (CSM) receiver may be used. CSM is a technique for removing interference inside a cell to increase the user capacity of the system. As another example of the band sharing technique, Layered Superposed-OFDMA (LS-OFDMA) may be used. LS-OFDMA is a non-orthogonal multiplexing technique in which subcarriers of an uplink frequency band are shared by a plurality of terminals in a cell.

When using a partial frequency reuse scheme, UE # 1 performs uplink transmission in the first subband and simultaneously shares band like CSM or LS-OFDMA in a second subband allocated to an intra-cell UE for the same data. Uplink transmission is performed again using the scheme. As such, the cell boundary terminal may obtain frequency diversity gain as well as diversity gain by the OFDM scheme by performing redundant uplink transmission on different frequency bands for the same data.

4 is a block diagram illustrating a method of performing uplink transmission of a terminal according to another embodiment of the present invention.

Referring to FIG. 4, the terminal performs an IFFT on a first subband with respect to data D ₁ and performs an IFFT on a _second subband with respect to data D ₂ interleaving the data D ₁ . That is, the data transmitted through the second subband is data in which a signal sequence of data transmitted through the first subband is changed by an interleaver. This is to solve the interference between terminals in the cell and to facilitate signal detection of the terminal in the cell.

Data transformed into a signal in the time domain by an IFFT is transmitted to a base station, and the base station receives the data D ₁ in the first subband and the data D ₂ in the second subband through an FFT process. Get The data D ₂ thus obtained is deinterleaved again, and the original signal may be restored by combining the data D ₁ obtained by the deinterleaving with the data D ₁ obtained in the _first subband. Whether the cell boundary terminal performs interleaving separately while performing redundant uplink transmission as described above may be known as signaling of the base station.

In each cell using the OFDMA system of the prior art, the terminals of adjacent cells using the first subband, which is the same frequency band as the cell boundary terminals, are spatially separated by a partial frequency reuse technique. Therefore, the cell boundary terminal may use sufficient power to overcome the path attenuation. This is because interference between cells is less likely to occur. However, at the same time, since the band which can be allocated to the cell boundary terminal is limited to the first subband, the frequency selective gain is reduced. However, according to the present invention, since the cell boundary terminal can also use the second subband as necessary, the frequency selective gain can be increased by increasing the frequency band for uplink transmission.

5 is a block diagram illustrating a power splitting method for each subband according to an embodiment of the present invention. This is applied to uplink transmission by the cell boundary terminal. Assume a system with a frequency reuse factor (FRF) of one.

Referring to FIG. 5, in general, a cell boundary terminal performs uplink transmission with higher power compensation to compensate for path attenuation of a signal according to a distance between base stations. The transmission power at this time is referred to as P _com . . However, uplink transmission of P _com may cause interference between adjacent cells occurring in the same frequency band due to high power. Accordingly, the cell boundary terminal divides the given transmission power P _com into the first subband and the second subband, respectively, to perform uplink transmission. As described above, the information about the transmission power divided in the two bands may be transmitted from the base station, and the division of the transmission power may be adjusted by the terminal or the base station according to the degree of intercell interference.

That is, the cell boundary terminal performs uplink transmission in the first _subband with a power smaller than P _com , and performs uplink transmission in the second subband with power other than power consumed in the first subband. do. That is, since the reduced power can be used in the second subband in order to minimize the interference to the adjacent cell in the first subband, not only the interference acting on the adjacent cell can be reduced, but also the diversity gain is secured. It is possible to utilize the power required to fully utilize the limited transmission power efficiently. Also, it is effective to apply CSM technique that can get the best performance when the signal strength of two users sharing the band is similar.

When the cell boundary terminal simultaneously performs uplink transmission in two bands, interference with the cell boundary terminal of the neighboring cell does not occur. This is because resource allocation between cells is performed by a partial frequency reuse technique. On the other hand, this may cause interference with the terminal inside the cell. However, due to the fact that the internal UE has fewer terminals with overlapping bands than the interference between the UEs, the UE can easily recognize the pilot patterns of the UEs, and thus, it is relatively easy to estimate the channel state. It is possible to obtain a higher signal detection capability of the terminal than to remove the inter-cell interference.

All of the above functions may be performed by a processor such as a microprocessor, a controller, a microcontroller, an application specific integrated circuit (ASIC), or the like according to software or program code coded to perform the function. The design, development and implementation of the code will be apparent to those skilled in the art based on the description of the present invention.

Although the present invention has been described above with reference to the embodiments, it will be apparent to those skilled in the art that the present invention may be modified and changed in various ways without departing from the spirit and scope of the present invention. I can understand. Therefore, the present invention is not limited to the above-described embodiment, and the present invention will include all embodiments within the scope of the following claims.

1 is a block diagram illustrating partial frequency reuse.

2 is a block diagram illustrating a method of allocating an uplink frequency spectrum of a cell to a terminal according to an embodiment of the present invention.

3 is a block diagram illustrating a method of performing uplink transmission of a terminal according to an embodiment of the present invention.

4 is a block diagram illustrating a method of performing uplink transmission of a terminal according to another embodiment of the present invention.

5 is a block diagram illustrating a power splitting method for each subband according to an embodiment of the present invention.

Claims (12)

In a cell-based system divided into a cell and a cell boundary, in a method for performing uplink transmission, Receiving scheduling information regarding allocation of an uplink frequency band; And Performing uplink transmission according to the scheduling information, The uplink frequency band is divided into a first subband and a second subband, the first subband is allocated exclusively to a cell boundary terminal, and the second subband is a band sharing technique for the cell boundary terminal and a cell internal terminal. A data transmission method, commonly assigned by. The method of claim 1, The cell boundary is an area in which interference of adjacent cells has a major influence on system performance, and is defined by the system. The method of claim 1, And the second partial band does not overlap with a frequency band allocated to a cell boundary of an adjacent cell by partial frequency reuse (FFR). The method of claim 1, The cell boundary terminal performs uplink transmission in the first subband and the second subband simultaneously. The method of claim 4, wherein The data transmitted by the cell boundary terminal in the first subband and the data transmitted in the second subband are different from each other. The method of claim 5, wherein The data transmitted in the second subband is data interleaved with data transmitted in the first subband. The method of claim 4, wherein The data transmitted by the cell boundary terminal in the first subband and the data transmitted in the second subband are the same as each other. The method of claim 4, wherein The cell boundary terminal divides the transmission power into the first subband and the second subband, respectively. The method of claim 8, The information about the transmission power is transmitted from a base station. The method of claim 8, The division of the transmission power is adjusted according to the degree of intercell interference. The method of claim 1, The band sharing technique is a technique using a Collaborate Spatial Multiplexing (CSM) receiver that removes interference in a cell to increase a user capacity of a system. The method of claim 1, The band sharing technique is a technique using Layered Superposed-OFDMA (LS-OFDMA), which is a non-orthogonal multiplexing technique in which a plurality of terminals in a cell share subcarriers in an uplink frequency band.

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