Apparatus and method for transmitting channel sounding signal in wireless communication system

Provided is an apparatus and method for transmitting a channel sounding signal from a user terminal in a multiple antenna system. In the method, a channel for each of a plurality of antennas is estimated. The receiving signal power of each of the antennas is calculated using the channel information obtained through the channel estimation. The antenna with the highest RX signal power is selected. Therefore, it is possible to enhance the signal reception performance of the user terminal.

This application claims priority under 35 U.S.C. §119(a) to an application filed in the Korean Intellectual Property Office on Feb. 5, 2007 and allocated Serial No. 2007-11488, and an application filed in the Korean Intellectual Property Office on Aug. 7, 2007 and allocated Serial No. 2007-78907, the contents of which are incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to an apparatus and method for transmitting a channel sounding signal in a wireless communication system, and in particular, to an apparatus and method for transmitting a channel sounding signal in a multiple antenna system based on a Time Division Duplex (TDD) scheme.

BACKGROUND OF THE INVENTION

A variety of multimedia services in wireless environments are required due to the rapid growth of the mobile communication market. Large-capacity data must be transmitted at a high speed in order to provide the multimedia services. Thus, research is being conducted on a multiple antenna system (e.g., a Multiple Input, Multiple Output (MIMO) system) for efficient use of limited frequency resources.

MIMO schemes for the multiple antenna system can be classified into an open-loop MIMO scheme and a closed-loop MIMO scheme depending on whether a transmitting side detects channel information. In the open-loop MIMO scheme, a transmitting side transmits data without knowing channel information. Examples of the open-loop MIMO scheme include a Space-Time Coding (STC) scheme and a Vertical-Bell Labs lAyered Space-Time (V-BLAST) scheme. In the closed-loop MIMO scheme, a transmitting side acquires channel information and transmits data using the acquired channel information. Examples of the closed-loop MIMO scheme include a Singular Value Decomposition (SVD) scheme and a Space Division Multiple Access (SDMA) scheme.

When the multiple antenna system uses the closed-loop MIMO scheme, a method for acquiring channel information by a transmitting side varies depending on the duplexing schemes of the multiple antenna system. For example, when the multiple antenna system uses a Frequency Division Duplex (FDD) scheme, a receiving side measures a channel through a signal received from a transmitting side and feeds the measured channel value back to the transmitting side. Thus, the transmitting side can acquire channel information. On the other hand, when the multiple antenna system uses a Time division Duplex (TDD) scheme, each of receiving sides transmits a channel sounding signal for discrimination between the receiving sides to a transmitting side. Then the transmitting side measures an uplink channel through the channel sounding signal and uses the measured uplink channel value as information about a downlink channel. This is based on the channel reciprocity property that the uplink and downlink channels are equal to each other.

In the multiple antenna system, a base station and a user terminal each have a plurality of antennas. The user terminal can receive signals through a plurality of receiving (RX) antennas, thereby achieving a diversity gain. Also, the base station can transmit a plurality of streams to a user terminal through a plurality of transmitting (TX) antennas, thus increasing the data transmission rate of the user terminal. However, in general, the user terminal uses only one TX antenna in a TX mode because it has a limited battery life and thus must transmit a signal at a much lower TX power than the base station. That is, the user terminal uses all of the antennas in an RX mode but uses only one of the antennas in a TX mode. Thus, when the multiple antenna system uses the closed-loop MIMO scheme, a user terminal transmits a channel sounding signal to a base station through only one TX antenna. In this case, the base station cannot acquire information about the entire downlink channel for the user terminal. This may reduce the data reception rate of the user terminal.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an apparatus and method for transmitting a channel sounding signal in a multiple antenna system.

Another object of the present invention is to provide an apparatus and method for enabling a user terminal to select an antenna for transmission of a channel sounding signal in a multiple antenna system on the basis of the power of a signal received through each antenna.

Still another object of the present invention is to provide an apparatus and method for performing a scheduling operation in a multiple antenna system by using channel sounding signals received from user terminals.

According to one aspect of the present invention, a method for transmitting a channel sounding signal from a user terminal in a multiple antenna system includes: estimating a channel for each of a plurality of antennas; calculating the RX signal power of each of the antennas by using the channel information obtained through the channel estimation; and selecting the antenna with the highest RX signal power.

According to another aspect of the present invention, an apparatus for transmitting a channel sounding signal from a user terminal in a multiple antenna system includes: a channel estimator for estimating a channel for each of a plurality of antennas; an RX signal power calculator for calculating the RX signal power of each of the antennas by using the channel information obtained through the channel estimation; and an antenna selector for selecting the antenna with the highest RX signal power.

According to still another aspect of the present invention, a method for transmitting data from a base station in a multiple antenna system includes: receiving a channel sounding signal transmitted from user terminals through an antenna with the highest RX signal power among a plurality of user terminal antennas; estimating a channel between a channel sounding signal transmitting antenna of each user terminal and base station antennas by using the received channel sounding signal from each user terminal; and calculating a beamforming weight for each user terminal by using the channel information of each antenna obtained through the channel estimation.

According to even another aspect of the present invention, an apparatus for transmitting data from a base station in a multiple antenna system includes: a TX and RX switch for receiving a channel sounding signal transmitted from user terminals through an antenna with the highest RX signal power among a plurality of user terminal antennas; a channel estimator for estimating a channel between a channel sounding signal transmitting antenna of each user terminal and base station antennas by using the received channel sounding signal from each user terminal; and a beamforming controller for calculating a beamforming weight for each user terminal by using the channel information of each antenna obtained through the channel estimation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is intended to provide a scheme for transmitting a channel sounding signal in a multiple antenna system.

In the following description, a multiple antenna system includes a base station with Mtantennas and a user terminal with Mrantennas. The user terminal receives signals through the Mrantennas and transmits signals through the N (smaller than Mr) antennas. The following description is made on the assumption that the user terminal has two antennas and transmits signals through one TX antenna. However, it is to be clearly understood that the present invention is also applicable to any case where the user terminal has a plurality of antennas and transmits signals through TX antennas fewer than the plurality of antennas.

Also, the following description is made on the assumption that the multiple antenna system uses a beamforming technique. However, it is to be clearly understood that the present invention is also applicable to any other multiple antenna system that uses a preceding technique. Furthermore, the present invention is not limited to a closed-loop MIMO scheme. Also, the following description is made in the context of a TDD multiple antenna system, to which the present invention is not limited. Thus, it is to be clearly understood that the present invention is also applicable to any other communication system such as an Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) system.

FIG. 1is a block diagram of a multiple antenna system using a beamforming technique according to the present invention.

Referring toFIG. 1, the multiple antenna system includes a base station with a plurality of antennas and a user terminal with a plurality of antennas. The user terminal includes a TX/RX switch101, a channel estimator103, an RX signal power calculator105, and an antenna selector107. The base station includes a TX/RX switch111, a channel estimator113, a beamforming controller115, a scheduler117, an encoder119, a modulator121, and a beamformer123.

First, a detailed description will be given of the configuration of the user terminal. Based on the TDD scheme, the TX/RX switch101receives signals from the base station through the antennas during a downlink period and transmits signals to the base station through some of the antennas during an uplink period. Also, using a sounding channel allocated by the base station, the TX/RX switch101transmits a channel sounding signal to the base station through the antenna selected by the antenna selector107during the uplink period.

The channel estimator103estimates a channel for each antenna by using a downlink signal received from the TX/RX switch101. For example, the channel estimator103estimates a channel for each antenna by using a pilot signal included in the downlink signal.

The RX signal power calculator105calculates the RX signal power of each antenna by using the estimated channel information received from the channel estimator103.

The antenna selector107selects the antenna with the highest RX signal power by comparing the calculated RX signal power levels of the respective antennas received from the RX signal power calculator105. For example, the antenna selector107selects the antenna with the best Signal-to-Noise Ratio (SNR) in order to transmit an uplink signal through the antenna with the best downlink channel condition.

Next, a detailed description will be given of the configuration of the base station. Based on the TDD scheme, the TX/RX switch111transmits signals received from the beamformer123to user terminals located in a service area through the antennas during a downlink period and receives signals from the user terminals through the antennas during an uplink period. In particular, using a sounding channel allocated to the user terminals, the TX/RX switch111receives a channel sounding signal from the user terminals during the uplink period. Herein, each of the user terminals selects the best antenna for transmission of the channel sounding signal and transmits the channel sounding signal through the selected antenna.

The channel estimator113estimates a channel between each antenna of the base station and the channel sounding signal transmitting antenna of each user terminal by using the channel sounding signal of each user terminal received from the TX/RX switch111.

The beamforming controller115calculates a beamforming weight of each user terminal by using the channel information of each user terminal received from the channel estimator113.

Using the channel information of the user terminals received from the channel estimator113, the scheduler117selects the user terminals to be provided with a service in the current frame among the user terminals located in the service area. For example, when a service is provided to k user terminals among N user terminals located in the service area, the scheduler117constructs user terminal sets of all possible combinations including the k user terminals among the N user terminals and selects the user terminal set to be provided with a service among the user terminal sets on the basis of the channel information of the user terminals received from the channel estimator113, thereby selecting the user terminals to be provided with a service. Herein, the scheduler117selects the user terminal set using Equation 1:

p_=arg⁢⁢minp∈P⁢tr⁡((Hp⁢HpH)-1),[Eqn.⁢1]
wherepdenotes the selected user terminal set, P denotes the user terminal set of all possible combinations including the k user terminals among the N user terminals located in the service area, Hpdenotes a matrix including the channel information of the user terminals included in the user terminal set, and tr( ) denotes a trace function for obtaining the sum of all diagonal elements of a matrix in ( ).

The encoder119encodes a signal, which is to be transmitted to the user terminals selected by the scheduler117, at a predetermined modulation level (e.g., a predetermined Modulation and Coding Scheme (MCS) level).

The modulator121modulates the encoded signal, which is received from the encoder119, at a predetermined modulation level.

The beamformer123copies the modulated signal, which is received from the modulator121, for the corresponding antennas. Thereafter, the beamformer123multiplies the copied signals by the beamforming weights received from the beamforming controller115, and outputs the resulting signals to the TX/RX switch111.

FIG. 2is a block diagram of the TX/RX switch of the user terminal in the multiple antenna system according to the present invention.

Referring toFIG. 2, the TX/RX switch of the user terminal includes an antenna switch203, a first switch205, a second switch207, a first antenna209, and a second antenna211. The following description is made on the assumption that the first antenna209is used to both transmit and receive a data signal and the second antenna211is used to only receive a data signal.

A baseband modem201includes the channel estimator103, the RX signal power calculator105, and the antenna selector107that are illustrated inFIG. 1. That is, the baseband modem201selects the best antenna for transmission of a channel sounding signal and provides an antenna selection signal to the antenna switch203. Also, the baseband modem201outputs a data signal and a channel sounding signal, which are to be transmitted to the base station, to the antenna switch203during an uplink period, and receives a data signal from the base station through the first switch205and the second switch207during a downlink period.

Based on the TDD scheme, the antenna switch203outputs the data signal, which is received from the baseband modem201, to the first switch205during the uplink period. Also, in response to the antenna selection signal received from the baseband modem201, the antenna switch203outputs the channel sounding signal, which is received from the baseband modem201, to the switch205or207connected to the corresponding antenna. In another embodiment, in response to the antenna selection signal received from the baseband modem201, the antenna switch203may output the data signal, which is received from the baseband modem201, to the switch205or207connected to the corresponding antenna.

The first switch205outputs a signal received from the antenna switch203to the first antenna209, and outputs a signal received from the first antenna209to the baseband modem201.

Likewise, the second switch207outputs a signal received from the antenna switch203to the second antenna211, and outputs a signal received from the second antenna211to the baseband modem201.

FIG. 3is a flowchart illustrating a process for performing a beamforming operation of a base station on the basis of a channel sounding signal in the multiple antenna system according to an embodiment of the present invention.

Referring toFIG. 3, the base station determines a data transmission scheme in step301. For example, the base station determines whether to use a closed-loop MIMO scheme or an open-loop MIMO scheme. Herein, the user terminal may determine the data transmission scheme and request the base station to use the determined data transmission scheme. Alternatively, the base station may determine the data transmission scheme on the basis of feedback information received from the user terminal.

In step303, the base station determines whether a closed-loop MIMO scheme is selected as the data transmission scheme. If a closed-loop MIMO scheme is not selected as the data transmission scheme, the base station transmits data in other data transmission scheme. On the other hand, if a closed-loop MIMO scheme is selected as the data transmission scheme, the base station allocates a sounding channel to user terminals located in a service area, in step305.

In step307, the base station determines whether a channel sounding signal is received from the user terminals over the sounding channel. Herein, the user terminals transmit the channel sounding signal using the antenna with the highest RX signal power among the antennas of the user terminals.

If the channel sounding signal is received from the user terminals, the base station estimates a channel between the channel sounding transmitting antenna of each user terminal and the antennas of the base station by using the received channel sounding signal, in step309. In step311, the base station calculates a beamforming weight for each user terminal using channel by using the channel information of the user terminal obtained through the channel estimation. Herein, the base station calculates the beamforming weight wifor each user terminal by using Equation 2:

wi=hihi[Eqn.⁢2]
where hidenotes the channel information of the ithantenna of the user terminal obtained through the channel estimation and * denotes a complex conjugate transpose.

Herein, the division of hi* by ||hi|| in Equation (2) is to normalize the beamforming weight wito a unit norm, thereby preventing a power increase due to beamforming.

In step313, the base station multiplies a TX data signal by the calculated beamforming weight and transmits the resulting signal to the user terminal through the antennas. Thereafter, the base station ends the process.

In order to use a beamforming technique as a preceding technique in the multiple antenna system, the above-described embodiment generates the beamforming weight using the channel information of each user terminal. However, when other preceding technique is used in the multiple antenna system, the base station calculates a preceding matrix using the channel information of each user terminal and precodes a TX signal using the preceding matrix.

Also, when there is a plurality of user terminals in the service area, the base station estimates a channel using the channel sounding signal received from the user terminals, and performs a scheduling operation using the channel information obtained through the channel information, thereby selecting the user terminals to be provided with a service. Herein, the base station performs the scheduling operation using Equation 1.

FIG. 4is a flowchart illustrating a process for transmitting a channel sounding signal from a user terminal in the multiple antenna system according to an embodiment of the present invention.

Referring toFIG. 4, the user terminal is allocated a sounding channel from a base station in step401. In step403, the user terminal estimates a channel using a downlink signal received from the base station through each antenna. For example, the user terminal estimates the channel using a pilot signal included in the downlink signal.

In step405, the user terminal calculates the RX signal power of each antenna using the channel information obtained through the channel estimation. In step407, the user terminal selects an antenna with the highest RX signal power (e.g., the highest SNR) among its antennas. Herein, the user terminal selects the antenna using Equation 3:

Herein, the user terminal obtains information about a 1×Ntchannel row vector hicorresponding to the ithantenna of the user terminal by using an Mr×Mtchannel matrix H of the downlink, and calculates the beamforming weight wiof the ithantenna by using the channel information hiof the ithantenna as Equation 2. Herein, the user terminal calculates the RX signal power ||Hwi||2of the ithantenna by using the channel matrix H and the beamforming weight wiof the ithantenna. Thus, the user terminal antenna with the highest RX signal power can be selected by comparing the RX signal power levels of all the user terminal antennas.

In step409, the user terminal transmits a channel sounding signal to the base station through the selected antenna. Herein, the channel sounding signal is transmitted to the base station over the sounding channel allocated from the base station. Thereafter, the user terminal ends the process.

As described above, in the TDD multiple antenna system, the user terminal compares the power levels of signals received through the respective antennas and transmits the channel sounding signal through the antenna with the highest RX signal power. Therefore, the signal reception performance of the user terminal can be enhanced through an antenna selection scheme with low complexity.