1.  Field of the Invention
The present invention generally relates to an apparatus and method for transmitting/receiving data in a mobile communication system.  More particularly, the present invention relates to a data transmission/reception apparatus and method for implementing a spatial multiplexing transmission in a mobile communication system using multiple transmit/receive antennas. 
2.  Description of the Related Art
Mobile communication systems are developing into high-speed, high-quality wireless data packet communication systems for providing a data service and a multimedia service as well as a voice service.  For example, the standardization for High-Speed Downlink Packet Access (HSDPA) ongoing in the 3rd Generation Partnership Project (3GPP) and the standardization for 1× Evolution Data and Voice (1×EV-DV) ongoing in the 3rd Generation Partnership Project 2 (3GPP2) can be regarded as evidence of the effort for finding a high-quality wireless data packet transfer service at more than 2 Mbps in the 3G mobile communication system.  On the other hand, the 4th generation (4G) mobile communication system serves to provide a higher-speed, higher-quality multimedia service. 
To provide a high-speed, high-quality data service in wireless communication, a spatial multiplexing transmission scheme has been proposed which exploits a Multiple-Input Multiple-Output (MIMO) antenna system with multiple antennas in transmitting and receiving stages.  The spatial multiplexing transmission scheme simultaneously transmits different data streams on a transmit antenna-by-transmit antenna basis.  It is known that the possible service data capacity linearly increases in proportion to the number of transmit/receive antennas as the number of transmit/receive antennas increases without an increase in an additional frequency bandwidth theoretically. 
When fading between the transmit/receive antennas is independent, the spatial multiplexing transmission scheme provides high capacity in proportion to the number of transmit/receive antennas.  The capacity is significantly reduced in an environment with a high spatial correlation of fading rather than an independent fading environment.  This is because fading from which signals transmitted from the transmit antennas suffer is similar and therefore it is difficult for the receiving stage to spatially distinguish a signal.  Possible transmission capacity is affected by a Signal to Noise Ratio (SNR).  As the received SNR decreases, the transmission capacity decreases.  Thus, a transmission data rate can be maximized when the number of data streams to be simultaneously transmitted and a transmission rate of each data stream are adjusted according to radio channel states, in other words a spatial correlation of fading and a received SNR.  If a transmission rate of data to be transmitted exceeds the transmission capacity supportable by a radio channel, many errors occur due to interference between data streams to be simultaneously transmitted and an actual data transmission rate decreases. 
To increase a transmission data rate in the spatial multiplexing transmission scheme, profound research on a precoding scheme has been conducted.  The precoding scheme multiplies data streams to be transmitted from a transmitter by transmission weights and transmits the data streams using information about a downlink channel from the transmitter to a receiver.  Thus, the transmitter is to know a state of a downlink channel from each transmit antenna of the transmitter to each receive antenna of the receiver.  For this, the receiver is to estimate the downlink channel state and feed back information about the downlink channel state estimated through a feedback channel.  However, the receiver is to transmit a large quantity of feedback data using an uplink feedback channel in order to feed back the downlink channel state information.  When a large amount of feedback data is to be transmitted, much time is taken to feed back the downlink channel state information from the receiver to the transmitter using the uplink feedback channel whose bandwidth is limited.  The conventional precoding scheme cannot be applied to an instantaneously varying wireless channel environment.  Accordingly, a real need exists for technology for maximizing a data transmission rate by precoding while minimizing an amount of feedback data needed to be transmitted from the receiver to the transmitter. 
A precoder codebook scheme has been proposed as the conventional technology for reducing an amount of feedback information.  In the precoder codebook scheme, the receiver selects a precode with the maximum transmission rate from among candidate precodes of a precoder codebook constructed by a limited number of precodes known to the transmitter and the receiver, and feeds back an index of the selected precode to the transmitter.  The transmitter sends data using a precode mapped to the fed-back index in the precoder codebook.  For example, when 4-bit feedback information is used, a precoder codebook constructed by a maximum of 24 (=16) precodes is preset between the transmitter and the receiver.  Because fading varies with time, the precode decision process is to be repeated in every time slot, such that the selected precode index is fed back to the transmitter in every time slot. 
In comparison with the precoding scheme using the feedback channel state information, the precoder codebook scheme requires a smaller amount of feedback information.  Assuming that the number of transmit antennas and the number of receive antennas are nT and nR in the MIMO antenna system, respectively, a total of nT×nR complex channel coefficients must be fed back when the channel state information is fed back.  When Q bits are required to indicate one complex channel coefficient, a total of nT×nR×Q bits are required.  On the other hand, the precoder codebook scheme requires ┌log2K┐ bits when the number of precodes for providing a sufficient data rate is K, where ┌x┐ is an integer equal to or more than x. In the precoding scheme using the channel state information, an amount of feedback information increases in proportion to a product of the number of transmit antennas and the number of receive antennas.  However, in the precoder codebook scheme, an amount of feedback information depends on the number of precodes contained in the precoder codebook, in other words a size of the precoder codebook. 
The precoder codebook scheme is to include, in the codebook, ready-made precodes quantized in all possible cases at a spatial multiplexing transmission time.  The precoder codebook scheme can reduce an amount of feedback information using predefined precodes, and can also reduce the degree of freedom in a precoding matrix.  When the number of factors to be considered is large, the degree of freedom in the preceding matrix significantly increases the number of preset precodes, such that a size of the precoder codebook increases.  In the following two cases, a size of the precoder codebook significantly increases. 
First, the number of precodes to be considered increases at the ratio of geometrical progression because all precodes are to be considered according to a spatial correlation of a channel for an application in a channel environment with various spatial correlations.  An optimal precoder codebook differs according to a spatial correlation of a channel.  In the conventional precoder codebook technology, the precoder codebook is designed under the assumption that a fading channel does not have a spatial correlation.  However, a distribution of valid eigenvalues, in other words eigenvectors whose eigenvalues are large, differs, and therefore optimal precodes differ, according to the spatial correlation of the fading channel.  As a result, a large number of precoder codebooks optimized according to the spatial correlation of the fading channel are to be used to achieve a high data transmission rate. 
Second, the number of precodes to be considered increases at the ratio of geometrical progression because all precodes are to be considered according to the number of data streams to be simultaneously transmitted when the number of data streams to be simultaneously transmitted is adjusted according to a channel environment.  The number of data streams to be simultaneously transmitted varies from 1 to a maximum of min(nT,nR) (indicative of a minimum value between the number of transmit antennas and the number of receive antennas).  The number of columns of a precode matrix is to be varied according to the number of data streams to be simultaneously transmitted.  Because column vectors for constructing the precode matrix serving as weight vectors are multiplied by data streams, the number of column vectors of the precode matrix is to match the number of data streams to be simultaneously transmitted.  For example, when both the number of transmit antennas and the number of receive antennas are 4, the number of data streams capable of being simultaneously transmitted varies from 1 to 4.  There must be considered precodes in which the number of column vectors is 1, precodes in which the number of column vectors is 2, precodes in which the number of column vectors is 3, and precodes in which the number of column vectors is 4.  When the maximum number of data streams capable of being simultaneously transmitted increases as the number of transmit/receive antennas increases, a significantly increased amount of feedback information is required according to an increased number of precodes to be considered.  Thus, it is difficult for the precoder codebook scheme to be applied to the spatial multiplexing transmission scheme for obtaining the maximum transmission rate in an associated channel environment by varying a transmission data rate and the number of data streams to be simultaneously transmitted according to the channel environment.  In the precoder codebook scheme using a set of predefined precodes as described above, a size of a precoder codebook increases according to the number of transmit antennas and the number of data streams to be simultaneously transmitted, such that its actual application may be difficult. 
The number of antennas may be different between receivers communicating with one transmitter.  For example, when the number of base station antennas is 4 and the number of mobile station antennas is 1, 2, 3, or 4 according to a terminal type, the maximum number of sub-data streams capable of being transmitted becomes 1, 2, 3, or 4.  When the precoder codebook technology is applied, each precoder codebook based on the number of all available receiver antennas and a feedback channel based on each precoder codebook must be defined.  The receivers are to select and use a precoder codebook based on the number of antennas of an associated receiver and a feedback channel based on the precoder codebook.  A process for defining a precoder codebook and feedback information to be used between a transmitter and a receiver is required.  Thus, a flexible precoding scheme applicable to various transmit/receive antenna structures is required. 
Accordingly, there is a need for an improved and efficient precoding scheme and a feedback scheme that can be applied to a spatial multiplexing transmission scheme for adjusting the number of data streams to be simultaneously transmitted in a channel environment with various spatial correlations and can provide a high data transmission rate with a significantly small amount of feedback information.