Apparatus and method for uplink channel sounding in a wireless communication system

An uplink channel sounding apparatus and method in a wireless communication system are provided, in which in an MS, a baseband MODEM generates as many channel sounding signals as the MS has antennas and outputs the channel sounding signals to an antenna switch. The antenna switch switches the channel sounding signals to the antennas in a one-to-one basis transmitting the channel sounding signals to a BS through all of the antennas of the MS.

PRIORITY

This application claims priority under 35 U.S.C. §119 to an application filed in the Korean Intellectual Property Office on Jun. 5, 2006, entitled “Apparatus and Method for Uplink Channel Sounding in a Wireless Communication System” and assigned Serial No. 2006-50159, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a wireless communication system, and in particular, to an apparatus and method for uplink channel sounding.

2. Description of the Related Art

Multiple-Input Multiple-Output (MIMO) is under intense study as one of the promising technologies for high-speed data transmission.

MIMO techniques are categorized into open-loop MIMO and closed-loop MIMO. In open-loop MIMO techniques, including Space-Time Coding (STC) and Vertical-Bell Labs Layered Space-Time (V-BLAST), a transmitter transmits data without knowledge of channel information. In closed-loop MIMO techniques, such as Singular Value Decomposition (SVD) and Spatial Division Multiple Access (SDMA), the transmitter acquires channel information and transmits data based on the channel information.

How the transmitter acquires the channel information depends on the duplexing mode. In Frequency Division Duplexing (FDD), a receiver measures the channel and feeds back the measured channel information to the transmitter. In Time Division Duplexing (TDD), the receiver transmits channel sounding signals to the transmitter and the transmitter measures the channels using the channel sounding signals and applies the uplink channel information to downlink channels. This is possible by exploiting the channel reciprocity of TDD between downlink and uplink transmissions.

A scenario where a Base Station (BS) uses a plurality of antennas and a Mobile Station (MS) uses a single antenna is a typical one considered for real implementation of a MIMO system because of the limit of the physical distance between antennas and the complexity and cost of the MS. Nonetheless, this system is called a MIMO system rather than a Multiple-Input Single-Output (MISO) system in that the BS can communicate with a plurality of users, i.e. a plurality of antennas when SDMA, for example, is used. In this scenario, the MS transmits and receives signals in the same manner as done in a conventional single-antenna system and the BS performs transmission and reception in MIMO.

FIG. 1illustrates an uplink channel sounding scheme for a single-antenna MS.

Referring toFIG. 1, each MS101transmits a channel sounding signal distinguishable from those of other MSs to a BS103. BS103measures the uplink channel of MS101using the channel sounding signal and uses the measured uplink channel information as downlink channel information in downlink data transmission.

Another scenario for a real MIMO system is that the MS uses a single Transmit (Tx) antenna and two Receive (Rx) antennas. The use of the two Rx antennas offers the benefits that diversity gain increases cell coverage and that as the BS can transmit two streams to the MS, the downlink data rate for the MS increases.

In general, the MS transmits at a far lower power level than the transmit power of the BS due to the lifetime of its battery. Thus, the MS can be designed to have two Rx antennas and one Tx antenna. For example, in the illustrated case ofFIG. 2, the MS can use one physical antenna207(second antenna) as an Rx antenna and another one205(first antenna) for the dual purpose of transmission and reception. In this case, a Tx-Rx switch203is provided between the Tx-Rx antenna205and a baseband Modulator-Demodulator (MODEM)201, for switching between transmission and reception.

FIG. 3illustrates an uplink channel sounding scheme for the above-described multi-antenna MS.

Referring toFIG. 3, an MS301transmits channel sounding signals to a BS303through the Tx-Rx antenna205. BS303measures uplink channels using the channel sounding signals. However, the measured uplink channel information does not suffice for use as downlink channel information because it is confined to uplink channels between Tx-Rx antenna205and BS303with no regard to uplink channels between Rx antenna207and BS303.

In the above scenario with one Tx antenna and two Rx antennas in an MS, given Ntantennas in a BS, 2Ntdownlink channels exist for Ntuplink channels. Therefore, the BS does not get full knowledge of the downlink channels using channel sounding signals received from the MS.

SUMMARY OF THE INVENTION

An aspect of the present invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an aspect of the present invention is to provide an apparatus and method for uplink channel sounding in a wireless communication system.

Another aspect of the present invention is to provide an apparatus and method in an MS with different numbers of Tx antennas and Rx antennas, for transmitting to a BS a channel sounding signal through all antennas that the MS has in a wireless communication system.

In accordance with an aspect of the present invention, there is provided a sounding apparatus for an MS in a wireless communication system, in which a baseband MODEM generates as many channel sounding signals as antennas of the MS and outputs the channel sounding signals to an antenna switch, and the antenna switch switches the channel sounding signals to the antennas in a one-to-one correspondence so as to transmit the channel sounding signals to a BS through all of the antennas of the MS.

In accordance with another aspect of the present invention, there is provided a channel sounding apparatus of a BS in a wireless communication system, in which a channel estimator estimates channels between the antennas of the MS and antennas of the BS upon receipt of channel sounding signals from antennas of an MS, a calculator calculates a preceding matrix according to a predetermined precoding scheme, when the channel estimation is completed on all antennas of the MS and the antennas of the BS, and a precoder multiplies transmission data by the preceding matrix and transmits the multiplied data to the antennas of the MS.

In accordance with a further aspect of the present invention, there is provided a channel sounding method of a BS in a wireless communication system, in which the BS estimates channels between antennas of the MS and antennas of the BS, upon receipt of channel sounding signals from the antennas of an MS, calculates a preceding matrix according to a predetermined precoding scheme, when the channel estimation is completed on all antennas of the MS and the antennas of the BS, multiplies transmission data by the precoding matrix, and transmits the multiplied data to the antennas of the MS.

In accordance with still another aspect of the present invention, there is provided a channel sounding method of an MS in a wireless communication system, in which the MS generates as many channel sounding signals as physical antennas of the MS, and transmits the channel sounding signals to a BS through all of the physical antennas of the MS.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses an uplink channel sounding apparatus and method in a wireless communication system. The following description of the present invention is made assuming an MS with Nrphysical antennas, which transmits signals through the Nrantennas and receives signals through N antennas fewer than the Nrantennas. While the present invention is described in the context that Nr=2 and N=1, it is obvious that the present invention is applicable to any case with N<Nr.

Referring toFIG. 4, the MS includes a baseband MODEM401, an antenna switch403, a first Tx-Rx switch405, a second Tx-Rx switch407, a first antenna409, and a second antenna411.

Baseband MODEM401generates a data channel signal and channel sounding signals for transmission to a BS and receives a data channel signal from the BS through the first and second Tx-Rx switches405and407.

Upon receipt of the data channel signal from baseband MODEM401, the antenna switch403outputs the data channel signal to first Tx-Rx switch405. Upon receipt of the channel sounding signals from baseband MODEM401, antenna switch403outputs a first channel sounding signal to first Tx-Rx switch405and a second channel sounding signal to second Tx-Rx switch407.

For transmission, first Tx-Rx switch405switches the data channel signal or the first channel sounding signal received from antenna switch403to first antenna409. For reception, first Tx-Rx switch405switches a data channel signal received from first antenna409to baseband MODEM401.

For transmission, second Tx-Rx switch407switches second channel sounding signal received from antenna switch403to second antenna411. For reception, second Tx-Rx switch407switches a data channel signal received through second antenna411to baseband MODEM401.

First antenna409transmits the data channel signal or the first channel sounding signal received from first Tx-Rx switch405to the BS and outputs a data channel signal received from the BS to first Tx-Rx switch405.

Second antenna411transmits the second channel sounding signal received from second Tx-Rx switch407to the BS and outputs a data channel signal received from the BS to second Tx-Rx switch407.

That is, a data channel signal is transmitted through first antenna409only and channel sounding signals are transmitted alternately through the first and second antennas409and411. As described above, the first and second channel sounding signals are transmitted to the BS through first and second antennas409and411, respectively, or vice versa. The switching varies depending on frame structures.

FIGS. 5A,5B and5C illustrate exemplary frame structures for a Time Division Duplexing-Orthogonal Frequency Division Multiplexing (TDD-OFDM) system. Thus, each frame is composed of a plurality of OFDM symbols.

Referring toFIG. 5A, if a plurality of symbols (or channels) in an uplink frame are allocated for channel sounding of an MS, the MS transmits a channel sounding signal through a first antenna in a first symbol501and through a second antenna in a second symbol503. In the same manner, the MS transmits the channel sounding signal in the following symbols.

Referring toFIG. 5B, if a single symbol (or channel) per uplink frame is allocated for channel sounding of an MS, the MS transmits a channel sounding signal through a first antenna in a symbol505of a frame and through a second antenna in a symbol507of the next frame. In the same manner, the MS transmits the channel sounding signal in the following frames.

Referring toFIG. 5C, if a plurality of subcarriers in an uplink frame are allocated for channel sounding of an MS, the MS transmits a channel sounding signal through a first antenna on one half of the subcarriers and through a second antenna in the other half of the subcarriers.

Referring toFIG. 6, the BS receiver includes a Tx-Rx switch603, a channel estimator605, a channel buffer607, a beamforming weight calculator609, a beamformer611, a modulator613, and a channel encoder615. Tx-Rx switch603switches a channel sounding signal received from the first or second antenna of an MS to channel estimator605. The MS switches the channel sounding signal generated from a sounding signal generator601to be transmitted to the BS.

Channel estimator605estimates channels between the antennas of the BS and the first/second antenna of the MS using the received channel sounding signal and provides the channel estimates hk(k is the index of an antenna in the MS) to channel buffer607.

Channel buffer607buffers channel estimates with respect to the respective antennas of the MS. When receiving all channel estimates for the MS, channel buffer607outputs the channel estimates to beamforming weight calculator609with a channel matrix H as shown in Equation (1).
H=[h1, h2]T(1)

Beamforming weight calculator609calculates beamforming weight values B based on the channel matrix H and provides the beamforming weight values B to the beamformer611.

Channel encoder615channel-encodes a data signal received from an upper layer at a predetermined code rate. Channel encoder615can be a convolutional encoder, a turbo encoder, a Low Density Parity Check (LDPC) encoder, or a Convolutional Turbo encoder (CTC), for example.

Modulator613modulates the coded data received from channel encoder615in a predetermined modulation scheme by mapping the coded data to complex signal points on a predetermined constellation. The modulation scheme can be Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 8-ary Quadrature Amplitude Modulation (8QAM), 16-ary QAM (16QAM), or 64-ary QAM (64QAM). One bit (s=1) is mapped to one complex signal in BPSK, two bits (s=2) to one complex signal in QPSK, three bits (s=3) to one complex signal in 8QAM, four bits (s=4) to one complex signal in 16QAM, and six bits (s=6) to one complex signal in 64QAM.

Beamformer611duplicates the modulation signal s to as many modulation signals for transmit antennas and performs beamforming for the respective antennas by multiplying the modulation signals by the beamforming weight values B. Tx-Rx switch603transmits the beams to the MS through the Tx antennas.

Referring toFIG. 7, the BS receiver includes a Tx-Rx switch703, a channel estimator705, a channel buffer707, a singular value decomposer709, a precoder711, a first to Nthmodulators713-1to713-N, and a first to Nthchannel encoders715-1to715-N.

Singular value decomposer709calculates a precoding matrix V by performing SVD on a channel matrix H associated with an MS received from channel buffer707. The SVD is given by Equation (2),
H=UDVH(2)

The SVD operation decomposes the channel matrix H into two unitary matrices U and V and a diagonal matrix D. The matrices U and V are the left and right eigenvector unitary matrices of the matrix H, respectively. The matrix D is a diagonal matrix with eigenvalues of the matrix H.

Precoder711generates a plurality of data signals x by multiplying the precoding matrix V by modulated data signals s received from the first to Nthmodulators713-1to713-N and provides the data signals in correspondence with a plurality of Tx antennas to Tx-Rx switch703. Tx-Rx switch703transmits the data signals to the MS through the respective Tx antennas.

While beamforming and SVD have been described with reference toFIGS. 6 and 7, SDMA and other precoding techniques are also available.

Referring toFIG. 8, the BS first makes a decision as to whether to transmit data in a closed-loop MIMO technique such as beamforming, SVD, SDMA, etc. The decision can be made by the MS and then the MS requests closed-loop MIMO transmission to the BS. Alternatively, the BS makes the decision based on information received from the MS. Once the closed-loop MIMO transmission is decided, the BS allocates part of uplink channels in a frame for use as sounding channels to the MS. The MS transmits a sounding signal through a first antenna and then through a second antenna on the allocated channels.

The BS sets the index M of an antenna of the MS to1in step801and monitors reception of a sounding signal from the Mthantenna of the MS in step803. Upon receipt of a sounding signal from the Mthantenna of the MS, the BS estimates channels hMbetween the Mthantenna of the MS and the antennas of the BS in step805. If the BS has four antennas, h1is a 1×4 vector.

In step807, the BS compares M with the number of antennas in the MS. If M is different from the number of antennas of the MS, the BS increases M by 1 in step809and returns to step803. On the other hand, if M is equal to the number of antennas in the MS, the BS generates a channel matrix H using all channel estimates hMand calculates a precoding matrix using the channel matrix H in step811. Then, the BS precodes data signals by multiplying them by the preceding matrix and transmits the precoded signals to the MS through the antennas in step813. The BS then ends the procedure of the present invention.

Referring toFIG. 9, the MS sets the index M of its antenna to 1 in step901and transmits a sounding signal through an Mthantenna on a sounding channel allocated by the BS in step903. In step905, the MS compares M with the number of its antennas. If M is different from the number of antennas of the MS, the MS increases M by 1 in step907and returns to step903. On the other hand, if M is equal to the number of antennas in the MS, the MS receives a data signal from the BS, considering that all sounding signals have been transmitted to the BS in step909. Then, the MS ends the procedure of the present invention.

Meanwhile, the present invention is also applicable to uplink MIMO transmission as well as downlink MIMO transmission. For instance, if a multi-antenna MS uses an antenna selection technique, a sounding signal received at the BS can be a criterion for selecting an MS antenna. That is, the BS selects an antenna with a good channel among channels estimated using the sounding signal and feeds back the antenna selection information to the MS. The MS then transmits a data signal through the selected antenna.

As described above, the present invention provides an uplink channel sounding apparatus and method in a wireless communication system. When an MS transmits a channel sounding signal to provide downlink channel information to a BS in a TDD-MIMO system, the present invention solves the problem that the BS lacks information for the channels between every pair of antennas, caused by the use of different numbers of Tx and Rx antennas in an MS. Further, since the present invention enables accurate downlink channel estimation, system performance is improved.