Apparatus and method for 4-beamforming using radio units having two paths in wireless communication system

A base station is capable of performing 4-beamforming using Radio Units (RU), each having two paths in a wireless communication system. The base station includes a Digital Unit (DU), a first RU, and a second RU. The DU performs a mutual conversion function between an information bit line and a digital signal, calculates beamforming coefficients for 4-beamforming, and performs the 4-beamforming using the beamforming coefficients. The first RU has two Radio Frequency (RF) paths, extracts a clock signal from a digital signal from the DU, and provides the clock signal to the second RU. The second RU has two RF paths and operates according to the clock signal provided from the first RU.

The present application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Jan. 19, 2009 and assigned Serial No. 10-2009-0004094, the entire disclosure of which is hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a wireless communication system. More particularly, the present invention relates to an apparatus and a method for performing 4-beamforming using radio units (RU) each having two paths in a wireless communication system.

BACKGROUND OF THE INVENTION

In a wireless communication system, a system performance and capacity deteriorate due to phenomenon such as an interference occurring between cells or inside cells, multi-path fading, the Doppler effect, and the like. One of techniques for increasing a system capacity by addressing the above-listed performance deteriorating factors is a beamforming technique. The beamforming technique is one of smart antenna schemes, and increases a signal gain at a reception end by properly combining phases and sizes of signals transmitted, respectively, via a plurality of antennas. Generally, the beamforming technique is realized by using four or more transmission antennas. That is, to apply the beamforming technique, the transmission end needs to have four or more transmission antennas.

Presently, base stations having two transmission antennas have been developed. However, since four or more transmission antennas are required in order to apply the beamforming technique, a Radio Unit (RU) having two Radio Frequency (RF) paths of the base stations needs to be replaced by an RU having four or more RF paths. In this case, the RU having the two RF paths is not utilized and redundancy in a hardware resource occurs. Therefore, to prevent redundancy in the hardware resource, an alternative for realizing a 4-beamforming technique using an RU having two RF paths is required.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is a primary aspect of the present invention is provide an apparatus and a method for performing 4-beamforming using RUs each having two RF paths in a wireless communication system.

Another aspect of the present invention is to provide an apparatus and a method for supplying a clock signal to one RU having newly added two RF paths in a wireless communication system.

Still another aspect of the present invention is to provide an apparatus and a method for performing calibration of a base station including two RUs each having two RF paths in a wireless communication system.

Yet another aspect of the present invention is to provide an apparatus and a method for performing verification for calibration of a base station including two RUs each having two RF paths in a wireless communication system.

In accordance with an aspect of the present invention, an apparatus of a base station in a wireless communication system is provided. The apparatus includes a Digital Unit (DU) for performing a mutual conversion function between an information bit line and a digital signal, calculating beamforming coefficients for 4-beamforming, and performing the 4-beamforming using the beamforming coefficients, a first Radio Unit (RU) having two Radio Frequency (RF) paths, for extracting a clock signal from a digital signal from the DU, and providing the clock signal to a second RU, and the second RU having two RF paths, that operates according to the clock signal provided from the first RU.

In accordance with another aspect of the present invention, a method for operating a base station including two separated Radio Units (RU) each having two Radio Frequency (RF) paths in a wireless communication system is provided. The method includes calculating beamforming coefficients for 4-beamforming, performing 4-beamforming using the beamforming coefficients, and transmitting 4-beamformed RF signals via a first RU having two RF paths, and a second RU operating using a clock signal extracted by the first RU and having two RF paths.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present invention provide a technique for performing 4-beamforming using RUs having two RF paths.

FIG. 1illustrates a base station in a wireless communication system according to an exemplary embodiment of the present invention.

Referring toFIG. 1, the base station includes a Digital Unit (DU)110, a first RU130, and a second RU150.

The DU110is responsible for a digital signal process, and includes a modem112and a unit interface114.

The modem112generates baseband digital transmission signals and recovers an information bit line from baseband digital reception signals by processing transmission/reception data according to a communication standard. For example, in the case where an Orthogonal Frequency Division Multiple Access (OFDM) communication standard is used, the modem112performs conversion between a frequency domain signal and a time domain signal by performing Inverse Fast Fourier Transform (IFFT) and FFT. More particularly, the modem112performs 4-beamforming on transmission signals. That is, the modem112performs 4-beamforming by multiplying signals to be transmitted to respective RF paths by beamforming coefficients calculated for beamforming. Here, the beamforming coefficients are calculated with consideration of a channel status for each antenna with a reception end, a mutual interference between antennas, and the like. In addition, the modem112controls a calibration process for four transmission/reception paths. That is, the modem112determines a transmission correction coefficient and a reception correction coefficient according to a channel estimate value calculated using an uplink reference signal and a downlink reference signal, and applies the transmission correction coefficient and the reception correction coefficient to transmission signals and reception signals.

The unit interface114provides an interface for data exchange with the RUs130and150. The unit interface114operates according to a Common Public Radio Interface (CPRI) standard or an Open Base Station Architecture Initiative (OBSAI) standard, and transmits/receives data using a predetermined frame structure.

The first RU130has two RF paths and includes a unit interface132, a first RF transmission path134, a first RF reception path136, a first duplexer138, a second RF transmission path144, a second RF reception path146, and a second duplexer148. The unit interface132provides an interface for data exchange with the DU110. The unit interface132operates according to the CPRI standard or the OBSAI standard, and transmits/receives data using a predetermined frame structure. More particularly, the unit interface132extracts a clock signal from a digital signal received from the DU110, and provides the extracted clock signal to the second RU150. Therefore, clock synchronization between the first RU130and the second RU150is performed, so that a phase deviation and jitter between a clock of the first RU130and a clock of the second RU150are removed.

The first RF transmission path134generates an RF transmission signal by performing up-conversion, amplification, and so forth, on a baseband signal provided from the unit interface132. The first RF reception path136recovers a baseband signal by performing amplification, down-conversion, and so forth, on an RF reception signal provided from the first duplexer138. The first duplexer138outputs an RF transmission signal to an antenna, and provides an RF reception signal input via the antenna to the first RF reception path136according to a Time Division Duplexing (TDD) scheme or a Frequency Division Duplexing (FDD) scheme.

The second RF transmission path144generates an RF transmission signal by performing up-conversion, amplification, and so forth, on a baseband signal provided from the unit interface132. The second RF reception path146recovers a baseband signal by performing amplification, down-conversion, and so forth, on an RF reception signal provided from the second duplexer148. The second duplexer148outputs an RF transmission signal to an antenna, and provides an RF reception signal input via the antenna to the second RF reception path146according to the TDD scheme or the FDD scheme.

The second RU150has two RF paths, and includes a unit interface152, a third RF transmission path154, a third RF reception path156, a third duplexer158, a fourth RF transmission path164, a fourth RF reception path166, a fourth duplexer168, a divider172, a first switch174, a second switch176, a third switch178, and a CALibration (CAL) processor182.

The unit interface152provides an interface for data exchange with the DU110. The unit interface152operates according to the CPRI standard or the OBSAI standard, and transmits/receives data using a predetermined frame structure. More particularly, the unit interface152receives a clock from the unit interface132of the first RU130. Therefore, clock synchronization between the first RU130and the second RU150is performed, so that a phase deviation and jitter between a clock of the first RU130and a clock of the second RU150are removed.

The third RF transmission path154generates an RF transmission signal by performing up-conversion, amplification, and so forth, on a baseband signal provided from the unit interface152. The third RF reception path156recovers a baseband signal by performing amplification, down-conversion, and so forth, on an RF reception signal provided from the third duplexer158. The third duplexer158outputs an RF transmission signal to an antenna, and provides an RF reception signal input via the antenna to the third RF reception path156according to the TDD scheme or an FDD scheme.

The fourth RF transmission path164generates an RF transmission signal by performing up-conversion, amplification, and so forth, on a baseband signal provided from the unit interface152. The fourth RF reception path166recovers a baseband signal by performing amplification, down-conversion, and so forth, on an RF reception signal provided from the fourth duplexer168. The fourth duplexer168outputs an RF transmission signal to an antenna, and provides an RF reception signal input via the antenna to the fourth RF reception path166according to the TDD scheme or the FDD scheme.

The divider172is designed for calibration, and transfers signals coupled at the four antennas to the fourth RF reception path166via the first switch174and the third switch178, or transfers a reference signal provided from the fourth RF transmission path164via the second switch176and the first switch174to the four antennas.

The CAL processor182is designed for calibration, calculates a downlink channel estimate value using a downlink reference signal, and provides the downlink channel estimate value to the modem112via the unit interface152. In addition, the CAL processor182generates an uplink reference signal for calculating an uplink channel estimate value, and provides the uplink reference signal to the divider172via the fourth RF transmission path164, the second switch176, and the first switch174.

A calibration process is described below in detail based on the construction illustrated inFIG. 1. The calibration is a process required for applying a beamforming technique. The purpose of the calibration is to correct phase differences and bandwidth differences between four RF paths. In exemplary embodiments of the present invention, the calibration is separately performed for an uplink and a downlink.

Downlink reference signals for downlink calibration move along paths illustrated inFIG. 2. Referring toFIG. 2, the modem112generates downlink reference signals, and sequentially provides the downlink reference signals to the four RF transmission paths134,144,154, and164included in the two RUs130and150via the unit interface114. The downlink reference signals provided to the RF transmission paths134,144,154, and164pass through the RF transmission paths134,144,154, and164and reach the antennas, and are coupled at the antennas and provided to the divider172.

The divider172provides the downlink reference signals that have been sequentially received after having passed through the RF transmission paths134,144,154, and164, to the fourth RF reception path166via the first switch174and the third switch178. The CAL processor182receives the downlink reference signals via the fourth RF reception path166and the unit interface152. In addition, the CAL processor182calculates downlink channel estimate values using the downlink reference signals, and provides the downlink channel estimate values to the modem112via the unit interface152. At this point, the downlink channel estimate values include four values corresponding to the RF transmission paths134,144,154, and164, respectively.

The modem112calculates signal distortion rates at respective RF transmission paths134,144,154, and164using the downlink channel estimate values, and determines inverse distortion rates for the signal distortion rates, that is, transmission correction coefficients. After that, when performing communication, the modem112applies the transmission correction coefficients to downlink signals.

Uplink reference signals for uplink calibration move along paths illustrated inFIG. 3. Referring toFIG. 3, the CAL processor182generates an uplink reference signal, and provides the uplink reference signal to the divider172via the fourth RF transmission path164, the second switch176, and the first switch174. The divider172divides the uplink reference signal to the same four uplink reference signals, which are input to the four antennas. Therefore, the uplink reference signals pass through the RF reception paths136,146,156, and166, and reach the modem112.

The modem112calculates uplink channel estimate values using uplink reference signals that have been received after having passing through the RF reception paths136,146,156, and166. In addition, the modem112calculates signal distortion rates at respective RF reception paths136,146,156, and166using the uplink channel estimate values, and determines inverse distortion rates for the signal distortion rates, that is, reception correction coefficients. After that, when performing communication, the modem112applies the reception correction coefficients to uplink signals.

As described with reference toFIGS. 2 and 3, reference signals are used for calibration. For example, the reference signal is configured as illustrated inFIG. 4.FIG. 4illustrates one frame section. That is, one frame is used for the calibration procedure, and data communication is suspended while the calibration procedure is performed. As illustrated inFIG. 4, downlink reference signals occupy four symbols411to414in a downlink subframe. That is, since a downlink reference signal is used on one signal basis per transmission path, a total of four downlink reference signals are sequentially transmitted. In the case of an uplink reference signal, even when only one reference signal is used, the reference signal may be provided to all reception paths. However, to increase reliability of uplink calibration, uplink reference signals may be repeatedly transmitted via a plurality of symbols421to424inside an uplink subframe as illustrated inFIG. 4. In addition, the downlink reference signals may be repeatedly transmitted within a range of the number of available symbols. The frequency of repetition of transmission of the uplink reference signal and transmission of the downlink reference signal may change depending on the intention of an operator of an exemplary embodiment of the present invention.

When correction coefficients are determined as described above, the modem112corrects transmission signals and reception signals according to the correction coefficients. At this point, when it is determined that the correction coefficients are not suitable values, accurate beamforming cannot be performed. Therefore, a verification procedure for the correction coefficients is performed according to an exemplary embodiment of the present invention.

A verification signal is used for the verification procedure. For example, the verification signal is configured as illustrated inFIG. 5. Referring toFIG. 5, five symbols511to515are used for downlink verification, and N symbols521to520+N are used for uplink verification. Downlink verification signals are [1 0 0 0], [1 1 1 1], [a −a b −b], [a b −a −b], and uplink verification signals are all [1 1 1 1].

The verification signals are multiplied by correction coefficients determined via the above-described calibration procedure, and then transmitted via the same path as the transmission path of reference signals. That is, downlink verification signals are transmitted by the modem112, pass through the paths illustrated inFIG. 2, and then are input to the modem112. Uplink verification signals are transmitted by the CAL processor182, pass through the paths illustrated inFIG. 3, and then are input to the modem112.

In the case of downlink verification procedure, verification signals are multiplied by correction coefficients, pass through the transmission paths134,144,154, and164, and then are received in the modem112. In addition, the modem112reviews accuracy of the correction coefficients using the received signals. A specific verification procedure for each verification signal is described below.

Downlink verification signals [1 0 0 0] are described. The modem112normalizes downlink verification signals by multiplying the downlink verification signals by transmission correction coefficients, transmits a signal corresponding to 1 via the first RF transmission path134, and transmits 0 via the second RF transmission path144, the third RF transmission path154, and the fourth RF transmission path164. Therefore, the modem112measures power of a signal received via the divider172.

Downlink verification signals [1 1 1 1] are described. The modem112normalizes downlink verification signals by multiplying the downlink verification signals by transmission correction coefficients, and simultaneously transmits signals corresponding to 1 via the transmission paths134,144,154, and164. Signals that pass through the four transmission paths134,144,154, and164constructively interfere with one another while passing through the divider172. Therefore, if the correction coefficients are accurate, a power value of a signal received via the divider172becomes four times greater than a power value measured when [1 0 0 0] is transmitted. That is, the modem112measures power of a signal received via the divider172, and determines whether the measured power value is four times greater than the power value measured when [1 0 0 0] is transmitted. However, since the four times greater value is a theoretical value of an ideal case, the modem112determines whether a difference between the four time greater value than the power value measured when [1 0 0 0] is transmitted, and the power of a signal received via the divider172is less than a relevant threshold.

Downlink verification signals [a −a b −b], [a b −a −b], [a b −b −a] are described. The modem112normalizes downlink verification signals by multiplying the downlink verification signals by transmission correction coefficients, and transmits the verification signals multiplied by the correction coefficients to the transmission paths134,144,154, and164. Signals that pass through the four transmission paths134,144,154, and164interfere with one another while passing through the divider172. At this point, a and −a destructively interfere with each other, and b and −b destructively interfere with each other. Therefore, if the correction coefficients are accurate, a power value of a signal received via the divider172becomes 0. That is, the modem112measures power of a signal received via the divider172, and determines whether the measured power value is 0. However, since 0 is a theoretical value of an ideal case, the modem112determines whether the measured power value is less than a threshold.

In the case of an uplink verification procedure, uplink verification signals [1 1 1 1] pass through the RF reception paths136,146,156, and166, and then are received in the modem112. In addition, the modem112divides the received signals by reception correction coefficients, and then reviews accuracy of the correction coefficients using results of division. A specific verification procedure for each verification signal is described below.

The modem112that has received first verification signals controls signals received via the RF reception paths136,146,156, and166to the same size and the same phase by dividing the received signals by reception correction coefficients. At this point, if the reception correction coefficients are accurate, sum of power values of the signals received via the RF reception paths136,146,156, and166is four times greater than a power value of a signal corresponding to one RF reception path. Therefore, the modem112sums power values of signals arithmetically, and determines whether a summed result is four times greater than a power value of a signal corresponding to one RF reception path. However, since the four times greater value is a theoretical value of an ideal case, the modem112determines whether a difference between the summed result and the four times greater value than the power value of the signal corresponding to the reception path is less than a relevant threshold.

The modem112that has received second verification signals controls signals received via the RF reception paths136,146,156, and166to the same size and the same phase by dividing the received signals by reception correction coefficients. Subsequently, the modem112determines whether a destructive interference occurs by multiplying the controlled signals by [a −a b −b] and then summing them. At this point, if the reception correction coefficients are accurate, a perfect destructive interference occurs, and a summed result becomes 0. However, since 0 is a theoretical value of an ideal case, the modem112determines whether the measured power value is less than a threshold.

The modem112that has received third verification signals controls signals received via the RF reception paths136,146,156, and166to the same size and the same phase by dividing the received signals by reception correction coefficients. Subsequently, the modem112determines whether a destructive interference occurs by multiplying the controlled signals by [a b −a −b] and then summing them. At this point, if the reception correction coefficients are accurate, a perfect destructive interference occurs, and a summed result becomes 0. However, since 0 is a theoretical value of an ideal case, the modem112determines whether the measured power value is less than a threshold.

The modem112that has received fourth verification signals controls signals received via the RF reception paths136,146,156, and166to the same size and the same phase by dividing the received signals by reception correction coefficients. Subsequently, the modem112determines whether a destructive interference occurs by multiplying the controlled signals by [a b −b a] and then summing them. At this point, if the reception correction coefficients are accurate, a perfect destructive interference occurs, and a summed result becomes 0. However, since 0 is a theoretical value of an ideal case, the modem112determines whether the measured power value is less than a threshold.

The above-described calibration procedure and verification procedure are performed during an operation of a base station. One frame is used for the calibration, and one frame is used for the verification procedure. That is, communication is suspended while the calibration and the verification procedure are performed.

The calibration procedure is performed at a period of predetermined several minutes or predetermined tens of minutes, and may be re-performed depending on the calibration result. Here, the calibration result denotes a combination of a verification procedure result and an amount of change in a channel estimate value before and after the calibration procedure, and is derived as one of ‘success’, ‘suddenly-change-success’, and ‘failure’. ‘Success’ denotes that an amount of change in the channel estimate value is less than a threshold and simultaneously, the verification procedure is successful, ‘suddenly-change-success’ denotes that an amount of change in the channel estimate value is greater than the threshold and simultaneously, the verification procedure is successful, and ‘failure’ denotes that the verification procedure fails regardless of an amount of change in the channel estimate value. Here, an amount of change in the channel estimate value is expressed in terms of an inner product of channel estimate values before and after the calibration procedure and a difference value of 1. An operation of a base station depending on the calibration result is illustrated in Table 1.

That is, when a performance period of a calibration procedure arrives, the modem112performs the calibration procedure, calculates an amount of change in the channel estimate value, and then performs the verification procedure. In addition, the modem112determines a calibration result depending on an amount of change in the channel estimate value and whether the verification procedure is successful, determines whether to re-perform the calibration procedure and whether to activate a beamforming function according to Table 1, and operates according to the determination. Activation and inactivation of the beamforming function are valid during one calibration performance period, and may change depending on a calibration result of the next period.

FIG. 6is a view illustrating a procedure for operating a base station in a wireless communication system according to an exemplary embodiment of the present invention.

Referring toFIG. 6, the base station performs communication in step601. At this point, the base station performs 4-beamforming using two RUs each having two RF paths. That is, the base station calculates beamforming coefficients for the 4-beamforming, and performs the 4-beamforming using the beamforming coefficients. At this point, one of the two RUs provides a clock signal to the other RU. However, when a beamforming function is inactivated according to a previous calibration result, the base station does not perform the beamforming.

While performing the beamforming, the base station determines whether a performance period of a calibration procedure arrives in step603. That is, the calibration procedure is performed at a period of several minutes or tens of minutes.

When the performance period of the calibration procedure arrives, the base station performs a first calibration procedure and a verification procedure in step605. That is, the base station suspends communication, and then transmits downlink reference signals via the paths illustrated inFIG. 2, and determines transmission correction coefficients using the downlink reference signals that have passed through the paths. In addition, the base station transmits uplink reference signals via the paths illustrated inFIG. 3, and determines reception correction coefficients using the uplink reference signals that have passed through the paths. Subsequently, the base station calculates an amount of change in the channel estimate value before and after the calibration procedure, and performs the verification procedure. At this point, the base station performs the verification procedure using verification signals illustrated inFIG. 5.

After performing the first calibration procedure and the verification procedure, the base station determines a first calibration result in step607. Here, the first calibration result denotes a combination of a result of the verification procedure and an amount of change in the channel estimate value before and after the calibration procedure, and is derived as one of ‘success’, ‘suddenly-change-success’, and ‘failure’. At this point, when the first calibration result is ‘success’, the base station goes to step621.

When the calibration result is ‘suddenly-change-success’, the base station performs a second calibration procedure and a verification procedure in step609. That is, the base station repeats the same process as step605. In other words, the base station determines correction coefficients by performing the calibration procedure using reference signals, calculates an amount of change in the channel estimate value, and performs the verification procedure using verification signals.

After performing the second calibration procedure and the verification procedure, the base station determines a second calibration result in step611. Here, the second calibration result denotes a combination of a result of the verification procedure and an amount of change in the channel estimate value before and after the calibration procedure, and is derived as one of ‘success’, ‘suddenly-change-success’, and ‘failure’. When the second calibration result is ‘success’, the base station goes to step621, and when the second calibration result is ‘suddenly-change-success’ or ‘failure’, the base station goes to step623.

When the first calibration result is ‘failure’ in step607, the base station performs the second calibration procedure and the verification procedure in step613. That is, the base station repeats the same process as step605. In other words, the base station determines correction coefficients by performing the calibration procedure using reference signals, calculates an amount of change in the channel estimate value, and performs the verification procedure using verification signals.

After performing the second calibration procedure and the verification procedure, the base station determines a second calibration result in step615. Here, the second calibration result denotes a combination of a result of the verification procedure and an amount of change in the channel estimate value before and after the calibration procedure, and is derived as one of ‘success’, ‘suddenly-change-success’, and ‘failure’. When the second calibration result is ‘failure’, the base station goes to step623.

When the second calibration result is ‘success’ or ‘suddenly-change-success’, the base station performs a third calibration procedure and a verification procedure in step617. That is, the base station repeats the same process as step605. In other words, the base station determines correction coefficients by performing the calibration procedure using reference signals, calculates an amount of change in the channel estimate value, and performs the verification procedure using verification signals.

After performing the third calibration procedure and the verification procedure, the base station determines a third calibration result in step619. Here, the third calibration result denotes a combination of a result of the verification procedure and an amount of change in the channel estimate value before and after the calibration procedure, and is derived as one of ‘success’, ‘suddenly-change-success’, and ‘failure’.

When the third calibration result is ‘success’, the base station activates a beamforming function in step621. Accordingly, the base station utilizes the beamforming function until a performance period of the next calibration arrives, and performs communication. In contrast, when the third calibration result is ‘suddenly-change-success’ or ‘failure’, the base station inactivates the beamforming function in step623. Accordingly, the base station performs communication without the beamforming function until a performance period of the next calibration arrives.

According to the above-described exemplary embodiment of the present invention, when a calibration performance period arrives, the base station performs a calibration procedure, and then performs a verification procedure. However, according to other exemplary embodiments of the present invention, the base station may perform the verification process before the calibration process is performed. That is, when a calibration performance period arrives, the base station performs the verification process, and then performs the calibration process only when the verification process fails. For example, in the operation procedure illustrated inFIG. 6, when the calibration performance period arrives, the base station performs a verification process. When the verification process fails, the base station performs steps605to623. When the verification process is successful, the base station returns to step601.

In a wireless communication system, one RU having two RF paths is added to a base station having an RU having two RF paths, so that a 4-beamforming system is easily realized. Furthermore, reliability of beamforming may be improved through a calibration procedure and a verification procedure of a base station including RUs each having two RF paths.