Apparatus and method for resource allocation in mobile communication system

An apparatus and method for providing a resource allocation of a Base Station (BS) in a mobile communication system includes, when a Mobile Station (MS) is located in a cell boundary region, locating a transmission frame region for the MS in a duty cycle reduction region.

CLAIM OF PRIORITY

This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Mar. 26, 2009 and assigned Serial No. 10-2009-0025690, the entire disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for improving transmission efficiency by reducing an influence of other-cell interference in such a manner that a frame region for duty cycle reduction in a conventional transmission frame region is partially allocated to a user by incorporating Zero Forcing (ZF) beamforming.

2. Description of the Related Art

A resource reuse scheme for dividing a system frequency band in a frequency domain or a time domain has conventionally been deployed to reduce inter-cell interference. In this scheme, resource allocation is achieved such that a resource used in each cell (or sector) is different from that used in its neighbor cell.

FIG. 1Aillustrates a case where a conventional frequency reuse scheme is used when a reuse factor N is 1. As shown, a Base Station (BS) is located in a center of each cell where data transmission is achieved using the same frequency band.

FIG. 1Billustrates a case where the conventional frequency reuse scheme is used when the reuse factor N is 3. As shown, a BS is located in a center of each cell. By dividing the system frequency band into 3 bands, all BSs can be prevented from using the same frequency resource used in neighbor cells, thereby reducing interference.

The frequency (or time) reuse scheme allows the neighbor cells to use different resources. Thus, there is an advantage in that inter-cell interference can be reduced.

However, an increase in the reuse factor N results in a decrease in a cell throughput which leads to a decrease in a performance gain. This is because a system frequency bandwidth is limited, and an increase in the frequency reuse factor N results in a decrease in an amount of frequency resources that can be used in each cell. Further, since a required transmission amount varies from one user to another, resources may be insufficient or wasted. Therefore, when the frequency reuse factor N is 1, the system operates in a most ideal manner in terms of overall system efficiency, and a highest cell transmission throughput can be determined. However, since all cells use the same frequency-band resource in this case, there is a problem in that reception performance significantly deteriorates when a user is located in a cell boundary.

SUMMARY OF THE INVENTION

An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an apparatus and method for providing an improved resource allocation in a mobile communication system.

Another aspect of the present invention is to provide an apparatus and method for reducing inter-cell interference so as to be adaptive to a channel environment in such a manner that a conventional region having a frequency reuse factor N=1 in a conventional transmission frame region of a mobile communication system consisting of a single-antenna Base Station (BS) and a multi-antenna Mobile Station (MS) is allocated to an MS having a high Signal to Interference signal Ratio (SIR), and a region that can be used by employing a beamforming scheme and a frequency reuse scheme in an incorporative manner is partially allocated to an MS having a low SIR, such as an MS located in a cell boundary.

In accordance with an aspect of the present invention, a resource allocation method of a BS in a mobile communication system includes: when an MS is located in a cell boundary region, providing a transmission frame region for the MS in a duty cycle reduction region; when the MS is located in a cell center region providing the transmission frame region for the MS in a conventional region, completing a transmission frame, and transmitting the completed transmission frame.

In accordance with another aspect of the present invention, a BS apparatus for allocating a resource in a mobile communication system includes a controller for providing a transmission frame region for an MS in a duty cycle reduction region when the MS is located in a cell boundary region, and for providing the transmission frame region for the MS in a conventional region when the MS is located in a cell center region, and a modem for completing a transmission frame and for transmitting the completed transmission frame.

In accordance with another aspect of the present invention, a method of receiving a transmission frame of an MS in a mobile communication system includes, when a region of the MS belongs to a duty cycle reduction region in the received transmission frame, receiving the transmission frame by using Zero Forcing (ZF) beamforming.

In accordance with another aspect of the present invention, an MS apparatus for receiving a transmission frame in a mobile communication system includes a controller for receiving the transmission frame by using ZF beamforming when a region of the MS belongs to a duty cycle reduction region in the received transmission frame, and a modem for demodulating and decoding the received transmission frame.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present invention will be described below with reference to the accompanying drawings. For the purposes of clarity and simplicity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail.

Hereinafter, an apparatus and method for resource allocation in a mobile communication system will be described.

As a scheme for cancelling inter-cell interference, the ZF beamforming cancels the inter-cell interference by configuring orthogonal signals between a Base Station (BS) from which a Mobile Station (MS) intends to receive a service and a BS which transmits an inter-cell interference signal.

When a mobile communication system consisting of K cells uses an MS having MRantennas, it is assumed that, during a feedback is made from the MS to a BS to use the ZF beamforming, a channel state does not change and channel information can be used.

An MR×K channel matrix H formed between K BSs and the MS having MRantennas is expressed by Equation (1) below.
H=[h1h2. . . hK]  (1)

In Equation (1), hkis [h1,(K)h2,(K). . . hMR,(K)]T(1≦k≦K), and k denotes a channel vector between a kthBS and the MS.

A signal y received by the MS is expressed by Equation (2) below.
y=Hx+n(2)

In Equation (2), x=[x1x2. . . xK]Tis a K×1 symbol vector transmitted by each of the K BSs, n=[n1n2. . . nMR]Tdenotes an MR×1 white noise vector of which an average is 0 and a variance is σ2.

A beamforming matrix W for cancelling the inter-cell interference signal consists of vectors determined by normalizing respective columns of a matrix which is a pseudo-inverse matrix of the channel matrix H. That is, a K×MRbeamforming matrix W=[w1w2wK] consists of a row vector w1(1≦1≦K), and is expressed by Equation (3) below.
W=normalize(pseudo-inverse(H))  (3)

In this case, the MS using the ZF beamforming can demodulate a symbol transmitted by the kthBS as expressed by yZF,(k)in Equation (4) below.
yZF,(k)=w1·Hx+w1·n, (w1·hk=0 for 1≠k).  (4)

Briefly, the mobile communication system of the present invention partially allocates a region for duty cycle reduction, during which a ZF beamforming reception scheme is used, in a conventional transmission frame region to a user so as to be adaptive to a channel environment, thereby reducing other-cell influence Further, the present invention proposes a partial frequency reuse method for minimizing transmission efficiency deterioration while improving performance deterioration caused by other-cell interference in a multi-cell environment.

FIG. 2illustrates a structure of a transmission frame according to an exemplary embodiment of the present invention.

Referring toFIG. 2, a duty cycle reduction region202is additionally allocated in a conventional region201according to the teachings of the present invention in order to minimize the performance deterioration caused by inter-cell interference in a cell boundary. A function and feature of each region are as follows.

The conventional region201is a region in which all cells use the same resource with a reuse factor N=1 in a BS (or a transmitter) and managed in the same manner as in the conventional mobile communication system.

The duty cycle reduction region202is a region in which partial frequency reuse is applied between BSs so that the BSs use different frequencies to reduce an influence of interference acting on a user located in a cell boundary region. Here, a receiver uses a ZF beamforming reception scheme to improve transmission efficiency.

FIG. 3illustrates a resource allocation method according to an exemplary embodiment of the present invention.

If the BSs301,302, and303use the same frequency resource, the MS1304located in a cell boundary experiences performance deterioration due to interference from the BS3303in all transmission frame regions A, B, and C.

In case of using the duty cycle reduction scheme and the ZF beamforming scheme of the present invention, that is, if the BSs301,302, and303allocate transmission frames of different frequency regions to the MS1304, the MS2305, and the MS3306as illustrated inFIG. 3, the MS1304receives a signal only from the BS1301. When ZF beamforming is used to improve a Signal to Interference signal Ratio (SIR), the MSs304,305, and306can remove MR−1 inter-cell interference signals by using Equation (4) above according to the number MRof receive antennas.

For example, if a frequency reuse factor is 3 (see a Fractional Frequency Reuse (FFR) pattern1ofFIG. 4B, an FFR pattern2ofFIG. 4Cis a case where the frequency reuse factor is 2), unlike in the conventional case (i.e., the frequency reuse factor is 1, seeFIG. 4A), a BS for transmitting an interference signal to an MS differs according to a transmission frame region allocated to the MS.

Referring back toFIG. 3, if the MS1304is located in a position capable of receiving all signals of the BSs301,302, and303.

If the MS1304is allocated to the transmission frame region A, the MS1304is influenced by an interference signal transmitted from the BS2302. Further, if the MS1304is allocated to the transmission frame region C, the MS1304is influenced by an interference signal transmitted from the BS3303.

In a case where the MS has two antennas and uses the aforementioned ZF beamforming scheme, the MS can receive a signal without inter-cell interference in the transmission frame regions A and C.

Accordingly, an influence of inter-cell interference can be minimized when each BS cooperatively uses the duty cycle reduction scheme while an MS uses the ZF beamforming reception scheme in an incorporative manner. As the number of receive antennas of the MS increases, the inter-cell interference can be further cancelled, and thus an additional performance gain can be determined.

FIG. 5is a flowchart illustrating an operation of a BS according to an exemplary embodiment of the present invention.

Referring toFIG. 5, before resource allocation of the BS, a frequency reuse factor is determined (step510). The frequency reuse factor may be predetermined.

Thereafter, the BS allocates an MS located in a cell center to a conventional region of the aforementioned transmission frame (step520), and allocates an MS located in a cell boundary to a duty cycle reduction region of the transmission frame (step530). The BS can determine whether the MS is currently located in a cell center or a cell boundary on the basis of channel state information transmitted by the MS.

Thereafter, the BS completes a transmission frame to be transmitted (step540), and then transmits the transmission frame (step550).

FIG. 6is a flowchart illustrating an operation of an MS according to an exemplary embodiment of the present invention.

Referring toFIG. 6, the MS receives a frame transmitted by a BS (hereinafter, simply referred to as a transmission frame) (step610).

Thereafter, the MS performs channel estimation on the transmitted transmission frame (step620), and determines a weight by performing a weight operation for the ZF beamforming of the present invention (step630). A weight matrix used in this operation corresponds to the aforementioned beamforming matrix.

Thereafter, the MS performs the ZF beamforming on the received transmission frame by using the weight (step640), and performs a subsequent reception process.

If the transmission frame region allocated to the MS is a conventional region, the MS performs a conventional reception process, and if the allocated transmission frame region is a duty cycle reduction region, the MS performs the aforementioned ZF beamforming process.

FIG. 7is a block diagram illustrating a structure of a BS according to an exemplary embodiment of the present invention. For illustrative purposes, it is assumed herein that the wireless communication system uses a time division duplex scheme and an Orthogonal Frequency Division Multiplexing (OFDM) scheme. However, it should be noted that other transmission schemes can be implemented in accordance with the teachings of the present invention.

Referring toFIG. 7, the BS includes a Radio Frequency (RF) switch700, an RF processor702, an Analog/Digital Converter (ADC)704, an OFDM demodulator706, a decoder708, a message processor710, a scheduler712, a resource allocator714, a message generator716, a coder718, an OFDM modulator720, a Digital/Analog Converter (DAC)722, and an RF processor724.

The RF switch700connects an antenna to the RF processor702of a receiver during a reception period according to a time-division duplex signal, and connects the antenna to the RF processor724of a transmitter during a transmission period. During the reception period, the RF processor702converts an RF signal provided from the RF switch700into a baseband analog signal.

The ADC704converts the analog signal provided from the RF processor702into digital sample data. The OFDM demodulator706converts the time-domain sample data provided from the ADC704into frequency-domain data by performing Fourier transform on the digital signal. In this case, the OFDM demodulator706performs the Fourier transform using a Fast Fourier Transform (FFT) operator.

The decoder708selects data of subcarriers to be actually received from the frequency-domain data provided from the OFDM demodulator706. Thereafter, the decoder708demodulates and decodes the selected data according to a predetermined Modulation and Coding Scheme (MCS) level.

The message processor710decomposes a control message provided from the decoder708and provides the decomposition result to the scheduler712.

The scheduler712provides the resource allocator714with the result provided from the message processor710. Further, the scheduler712receives scheduling information provided from the resource allocator714and provides the scheduling information to the message generator716.

The resource allocator714generates the scheduling information on the basis of the control message provided from the message processor710. That is, on the basis of a result determined by analyzing the control message transmitted from an MS, the resource allocator714recognizes a location of the MS in a cell to determine whether the MS is located in a cell boundary or a cell center.

After recognizing the location of the MS, the resource allocator714determines a specific position of the MS in a transmission frame according to the location and provides information thereon to the message generator716. The resource allocator714may determine whether the MS is currently located in the cell center or the cell boundary on the basis of channel state information transmitted by the MS.

During the transmission period, the message generator716allocates the MS to a region of the transmission frame according to the scheduling information (i.e., information on a position in the transmission frame) determined by the scheduler712.

The coder718codes and modulates the message provided from the message generator716according to the MCS level.

The OFDM modulator720converts frequency-domain data provided from the coder718into time-domain sample data (i.e., an OFDM symbol) by performing inverse Fourier transform.

In this case, the OFDM modulator720performs the inverse Fourier transform by using an Inverse Fast Fourier Transform (IFFT) operator.

The DAC722converts the sample data provided from the OFDM modulator720into an analog signal. The RF processor724converts the analog signal provided from the DAC722into an RF signal of a specific channel.

In alternate embodiment, a controller (not shown) may perform functions of the constitutional elements714,712,710, and716. Accordingly, when the present invention is implemented as a product in practice, the functions of the constitutional elements714,712,710, and716may be entirely or partially processed by the controller. Further, a modem chip may be used to perform functions of the constitutional elements708,706,704,718,720, and722.

FIG. 8is a block diagram illustrating a structure of an MS according to an exemplary embodiment of the present invention.

Referring toFIG. 8, the MS may be an multi-antenna MS, and it includes a plurality of antennas, a plurality of RF processors810and815, a channel estimator820, a weight generator825, a ZF beamforming processor830, a demodulator835, and a decoder840.

The RF processors810and815convert RF signals provided from the antennas into baseband signals. It is assumed that the RF processors810and815also perform an ADC conversion process.

The channel estimator820performs channel estimation on the signals processed with baseband processing by the RF processors810and815and provides a result of the channel estimation to the weight generator825.

The weight generator825determines a weight for performing ZF beamforming on the basis of a channel estimation result value provided by the channel estimator820, and provides the weight to the ZF beamforming processor830. A weight matrix used in this operation corresponds to the aforementioned beamforming matrix.

The ZF beamforming processor830performs the aforementioned ZF beamforming process on the signal processed with the baseband processing on the basis of the weight provided by the weight generator825, and provides a resultant signal to the demodulator835.

The demodulator835demodulates the signal provided from the ZF beamforming processor830, and provides the signal to the decoder840to perform a decoding process.

If a transmission frame region allocated to the MS is a conventional region, the MS performs a conventional reception process, and if the allocated transmission frame region is a duty cycle reduction region, the MS performs the aforementioned ZF beamforming process.

In an alternate embodiment, a controller (not shown) may perform functions of the constitutional elements820,825, and830. Accordingly, when the present invention is implemented as a product in practice, the functions of the constitutional elements820,825, and830may be entirely or partially processed by the controller. Further, a modem chip may be used to perform functions of the constitutional elements835and840.

According to exemplary embodiments of the present invention, a conventional region having a frequency reuse factor N=1 is allocated to an MS having a high SIR, and a region that can be used by employing a beamforming scheme and a frequency reuse scheme in an incorporative manner is partially allocated to an MS having a low SIR, such as an MS located in a cell boundary. As apparent from foregoing, this type of adaptive approach to a channel environment is advantageous in that other-cell interference can be reduced.