Transmitter/receiver for controlling multiuser multiple input multiple output system and method thereof

Provided are a method of controlling a multi-user multiple input multiple output (MIMO) system and a transmitter/receiver used in the method. In the method, it can be determined whether feedback information is fed back according to the feedback information type rather than being indiscriminately provided to a transmitter, and then only a necessary feedback information type is transmitted, thereby increasing system capacity while reducing feedback load.

RELATED APPLICATIONS

This application is a 35 U.S.C. §371 national stage filing of PCT Application No. PCT/KR2008/003358 filed on Jun. 13, 2008, which claims priority to, and the benefit of, Korean Patent Application No. 10-2007-0058373 filed on Jun. 14, 2007 and Korean Patent Application No. 10-2008-0055849 filed on Jun. 13, 2008. The contents of the aforementioned applications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a method of controlling a multi-user multiple input multiple output (MIMO) system and a transmitter and receiver used in the method, and more particularly, to a method of controlling a MIMO system that can increase a system capacity without increasing a feedback amount of channel information, and a transmitter and receiver used in the method.

The present invention is supported by the Information Technology (IT) Research & Development (R&D) program of the Ministry of Information and Communication (MIC) and the Institute for Information Technology Advancement (IITA) [2006-S-001-02, Development of Wireless Connection and Transmission is Technology for Fourth Generation Mobile Communication].

BACKGROUND ART

A multiple input multiple output (MIMO) method using multiple antennas for a transmitter and a receiver is a technology that has received much attention in wireless/mobile systems, since MIMO performance is excellent due to its improved frequency efficiency and its diversity of transmission/receipt.

Singular value decomposition (SVD) is an example of a conventional MIMO method. SVD is a method in which a transmitter uses singular value decomposition of the channel matrix that is fed back from a receiver, thereby achieving maximum performance. However, generally, in a system such as frequency division duplexing (FDD), a receiver needs to notify a transmitter of information regarding a channel matrix so that a transmitter can know the channel matrix. In this regard, since the amount of information is very large, it is difficult to use the information in an environment where a channel varies according to a period of time.

In order to overcome this problem, research has been conducted on technologies in which the performance of a transmitter can be increased by feeding back partial channel information to the transmitter. Examples of such technologies include per antenna rate control (PARC) technologies, per stream rate control (PSRC) technologies, per unitary basis stream user rate control (PU2RC) technologies, etc.

In PARC technologies, an open loop channel capacity of a MIMO channel can be obtained theoretically by feeding back only signal-to-interference-plus-noise ratio (SINR) information for each antenna while using successive interference cancellation in a receiver.

In PARC technologies, data streams are transmitted to respective antennas. On the other hand, in PSRC technologies, data streams are precoded using a unitary matrix that is fed back from a receiving terminal so as to be transmitted. In addition, the receiving terminal feeds back a unitary matrix selected from among a plurality of unitary matrixes which can be used to precode data streams in a transmitting terminal, and then the data streams are precoded to respective column vectors of the unitary matrix so as to be transmitted. The receiving terminal also feeds back an SINR for each data stream to be precoded, and the transmitting terminal determines a data transmission rate of each stream by using the fed back SINR and transmits data.

Unlike in the case of PARC and PSRC technologies improving performance between transmitting and receiving terminals using multiple antennas, PU2RC technologies improve performance by using multi-user diversity when there are a plurality of terminals using multiple antennas. In PU2RC technologies, a base station transmits a plurality of data streams. Prior to transmitting the data streams, the data streams are precoded by respective column vectors of a unitary matrix. This method is the same as PSRC technologies except that respective data streams are transmitted to different users. The SINR for each data stream is fed back from respective users. A stream is assigned to a user having an optimum SINR from among the data streams so as to improve the performance of a MIMO system.

PU2RC technologies are advantageous compared to PARC and PSRC technologies in that performance can be increased using multi-user diversity in a multi-user environment. However, PU2RC technologies are disadvantageous in that interference between data streams cannot be cancelled by using successive interference cancellation unlike in PARC or PSRC technologies.

On the other hand, PARC and PSRC technologies are advantageous compared to PU2RC technologies in that interference between data streams can be cancelled by using successive interference cancellation. However, PARC and PSRC technologies are disadvantageous in that performance cannot be increased using multi-user diversity in a multi-user environment.

Actually, comparing PU2RC technologies with PARC or PSRC technologies, performances are different according to the number of users and channel environment in a system. Generally, as the number of users is increased, and the channel environment is closed to a line of sight (LOS), the performance of PU2RC technologies is better. On the other hand, as the number of users is reduced and the channel environment is closed to a rich scattering environment, PARC or PSRC technologies are better.

In successive interference cancellation based per user stream rate control (S-PUSRC) technologies that have been newly suggested as technologies simultaneously overcoming disadvantages of PARC, PSRC and PU2RC technologies, a receiver always needs to feed back decoding order used in successive interference cancellation to a transmitter. S-PUSRC technologies are disadvantageous compared is to PARC or PU2RC technologies in that feedback load is increased.

DETAILED DESCRIPTION OF THE INVENTION

Technical Problem

The present invention provides a method of controlling a multi-user multiple input multiple output (multi-user MIMO) system and a transmitter and receiver used in the method.

The present invention also provides a method of controlling a multi-user MIMO system, in which better performance can be achieved compared to conventional per unitary basis stream user rate control (PU2RC), per antenna rate control (PARC) and per stream rate control (PSRC) technologies by overcoming problems of the conventional technologies having varying performance according to the number of users and a channel environment, and in which feedback load can be reduced compared to successive interference cancellation based per user stream rate control (S-PUSRC), and a transmitter and receiver used in the method.

The objects and advantages of the present invention will be explained in the is following description, which includes exemplary embodiments of the present invention. In addition, it can be easily understood that the objects and advantages of the present invention can be implemented with means disclosed in the appended claims and combinations thereof.

Technical Solution

According to the present invention, it is determined whether feedback information is fed back according to the feedback information type rather than being indiscriminately provided to a transmitter, and then only a necessary feedback information type is transmitted, thereby increasing system capacity while reducing feedback load.

Advantageous Effects

According to the present invention, it is determined whether feedback information is fed back according to the feedback information type rather than being indiscriminately provided to a transmitter, and then only a necessary feedback information type is transmitted, thereby increasing system capacity while reducing feedback load.

As such, according to a method of controlling a multi-user multiple input multiple output (multi-user MIMO) system and a transmitter and receiver used in the method, better performance can be achieved compared to conventional per unitary basis stream user rate control (PU2RC), per antenna rate control (PARC) and per stream rate control (PSRC) technologies by overcoming problems of the conventional technologies.

BEST MODE

According to an aspect of the present invention, there is provided a receiver controlling a multi-user multiple input multiple output (MIMO) system, the receiver comprising a channel estimator estimating a channel with respect to a plurality of pairs of transmission/receipt antennas; a feedback unit determining whether feedback information is generated according to the estimated channel and transmitting the generated feedback information to a transmitter; and a restorer receiving a signal encoded based on the feedback information from the transmitter and restoring a plurality of data streams by applying successive interference cancellation to the received signal.

According to another aspect of the present invention, there is provided a transmitter controlling a multi-user MIMO system, the transmitter comprising a encoder assigning a plurality of pieces of user data to respective streams and encoding the pieces of user data; an adaptive transmission controller controlling streams to which the pieces of user data are assigned and data transmission rate for each stream, according to a stream decoding order fed back and an SINR for each stream, which is calculated by cancelling interference between the streams according to the stream decoding order from a receiver; and a feedback period calculator determining a feedback period of the stream decoding order and a feedback period of SINR for each stream and transmitting the two periods to the receiver.

According to another aspect of the present invention, there is provided a method of controlling a receiver of a multi-user MIMO system, the method comprising estimating a channel with respect to a plurality of pairs of transmission/receipt antennas; determining whether feedback information is generated based on the estimated channel and transmitting the generated feedback information to a transmitter; and receiving a signal encoded based on the feedback information from the transmitter and restoring a plurality of data streams by applying successive interference cancellation to the received signal.

According to another aspect of the present invention, there is provided a method of controlling a transmitter of a multi-user system, the method comprising receiving feedback information regarding a stream decoding order and SINR for each streams, which is calculated by cancelling interference between the streams according to the stream decoding order, from a receiver; assigning a plurality of pieces of user data to respective streams according to a control signal generated based on the feedback information; encoding and transmitting the streams at a data transmission rate according to the control signal; and determining a feedback period of the stream decoding order and a feedback period of SINR for each stream and transmitting the two periods to the receiver.

According to another aspect of the present invention, there is provided a method of controlling a multi-user MIMO system comprising a transmitter having multiple antennas and a receiver having multiple antennas, the method comprising (a) estimating a MIMO channel, wherein the estimating is performed by the receiver; (b) determining whether a stream decoding order is fed back for optimum successive interference cancellation and feeding back the stream decoding order calculated based on the estimated channel when the stream decoding order is required to be fed back, wherein the determining is performed in the receiver; (c) determining whether an SINR for each stream is fed back, and feeding back the SINR for each stream, which is calculated by cancelling interference between streams based on the estimated channel when the SINR for each stream is required to be fed back, wherein the determining is performed in the receiver: and (d) determining a data transmission rate for each stream and a stream to be assigned to the receiver from among a plurality of streams by using feedback information regarding the SINR for each stream.

According to another aspect of the present invention, there is provided a computer readable recording medium having recorded thereon a program for executing a method of controlling a multi-user MIMO system, a transmitter and receiver.

MODE OF THE INVENTION

Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. In the drawings, like reference numerals denote like elements. Also, while describing the present invention, detailed descriptions about related well-known functions or configurations that may diminish the clarity of the points of the present invention are omitted.

When an element is “included”, it means that other elements may be further included, instead of excluding other elements unless there is any specific contrary description.

As described in the background art, in per unitary basis stream user rate control (PU2RC) technologies, appropriate streams are assigned to respective users in a multi-user multiple input multiple output (MIMO) environment. Each stream is multiplied by precode vectors corresponding to the stream so as to be transmitted via multiple antennas. In PU2RC technologies, streams are respectively precoded by unitary vectors, and then assigned to respective users, thereby improving the performance of a system by using multi-user diversity. However, PU2RC technologies are disadvantageous compared to per antenna rate control (PARC) or per stream rate control (PSRC) technologies in that interference between streams cannot be cancelled.

In the meantime, in successive interference cancellation based per user stream rate control (S-PUSRC) technologies, multi-user diversity can be achieved by overcoming the problems of the above-described technologies and a successive interference cancellation gain is achieved by cancelling interference between streams. However, S-PUSRC technologies have a problem with large feedback load compared to the above-described technologies.

In a method according to the present invention, the problems of S-PUSRC having large feedback load can be overcome while retaining all advantages of the above-described technologies.

FIG. 1is a block diagram of a multi-user MIMO system according to an embodiment of the present invention.

Referring toFIG. 1, the multi-user MIMO system according to the current embodiment of the present invention includes a transmitter110including multiple antennas and a plurality of receivers (i.e., receivers1through3) each of which includes multiple antennas. Hereinafter, the multi-user MIMO system of the current embodiment will be described in terms of the receiver1(hereinafter, referred to as a receiver150), which can also be applied to the receivers2and3.

The transmitter110determines a stream to be assigned to the receiver150from among a plurality of streams, according to feedback information (decoding order and signal-to-interference-plus-noise ratio (SINR) information for each stream), which is input from the receiver150, and determines a data transmission rate for each stream. In addition, the transmitter110calculates a feedback period of the decoding order and a feedback period of the SINR for each stream, to which successive inference cancellation is applied, and notifies the receiver150of the two periods.

User data is transmitted to the receiver150by using a stream assigned to a user from among a plurality of streams transmitted from the transmitter110.

The receiver150determines whether an order of successive inference cancellation (decoding order) is fed back and whether SINR for each stream, to which the successive inference cancellation is applied, is fed back. When it is determined that the decoding order and the SINR are required to be fed back, the decoding order and the SINR are calculated and are fed back to the transmitter110.

FIG. 2is a detailed block diagram of a multi-user MIMO system according to another embodiment of the present invention.

Referring toFIG. 2, the multi-user MIMO system according to the current embodiment of the present invention includes a transmitter210and a receiver250.

The transmitter210includes an encoder211, an adaptive transmission controller216and a feedback period calculator217.

The encoder211assigns a plurality of pieces of user data to respective streams and encodes the plurality of pieces of user data. The encoder211includes a user selector212, a plurality of channel encoders213, a plurality of symbol mappers214and a plurality of precoding/antenna mappers215.

The user selector212assigns the plurality of pieces of user data to the respective streams, according to a control signal of the adaptive transmission controller216. For example, when the number of antennas in a base station (a transmitter) is four, and the number of users (receivers) is three, a stream1is assigned to a user1, and streams2and3are assigned to a user2, and a stream4is assigned to a user3. Like in the case of the user2, when a plurality of streams are assigned to a single user, the user selector212divides data of the user2into two pieces of low speed parallel data and assigns the two pieces of low speed parallel data to the respective streams. That is, one piece of low speed parallel data is assigned to the stream2and the other piece of low speed parallel data is assigned to the stream3.

The channel encoder213encodes the streams. The symbol mapper214maps the encoded streams to symbols. The precoding/antenna mapper215multiplies the streams by precoding vectors corresponding to the respective streams so as to transmit the streams to a transmitting antenna, or alternatively, the precoding/antenna mapper215simply performs antenna-mapping the streams so as to transmit the streams to an antenna. The precoding vectors corresponding to the respective streams may each be a fixed predetermined vector, or alternatively, may be changed by a request of the adaptive transmission controller216according to a precoding vector fed back from the receiver250.

The adaptive transmission controller216determines how the streams are transmitted using feedback information regarding the decoding order and the SINR, which is received from the receiver250, and then controls the encoder211according to a determination result. That is, the adaptive transmission controller216receives feedback information regarding the decoding order and the SINR for each stream depending on the decoding order from the receiver250, and determines streams to which each of the plurality of pieces of user data is assigned and data transmission rate of the respective streams, according to the feedback information.

The feedback period calculator217may be optionally included in the multi-user MIMO system, and may determine a feedback period of the decoding order of the streams and a feedback period of the SINR of each stream, and transmits the two periods to the receiver250.

The receiver250includes a restorer251, a channel estimator255and a feedback unit256.

The restorer251restores a signal received from the transmitter210to a plurality of streams by applying successive interference cancellation. The received signal is a signal encoded according to the feedback information that is previously transmitted by the receiver250to the transmitter210. Also, the restorer251restores the received signal by applying successive interference cancellation, according to the feedback information. The restorer251includes a symbol detector252, a decoder253and a multiplexer254.

The symbol detector252detects a vector signal received via an antenna in a predetermined order. That is, the symbol detector252detects symbols of the received signal by applying successive interference cancellation, according to the decoding order of the feedback information. The decoder253decodes and restores the detected symbol to a plurality of data streams. The multiplexer254multiplexes the plurality of restored data streams. For example, when general successive inference cancellation is used, the decoder253restores a signal, which is first detected, to original information. The restored signal is encoded to a signal transmitted from an original transmitting terminal. Then, the encoded signal is multiplied by a corresponding channel and is deduced from the received signal. Thus, the signal that is first detected does not affect other remaining streams. According to the present invention, it is assumed that such successive inference cancellation is used in the receiver250.

The channel estimator255estimates a MIMO channel with respect to a plurality of pairs of transmission/receipt antennas by using a pilot symbol. The estimated channels are input to the symbol detector252so as to be used to detect the respective streams.

The feedback unit256determines whether feedback information is generated, according to a matrix of the channels estimated by the channel estimator255, and transmits the generated feedback information to the adaptive transmission controller216of the transmitter210. The feedback unit256includes a decoding order feedback unit257and an SINR feedback unit258.

The decoding order feedback unit257determines whether a new decoding order is required to be fed back. When the new decoding order is required to be fed back, the decoding order feedback unit257calculates an optimum decoding order and feeds back the optimum decoding order to the transmitter210. The SINR feedback unit258determines whether SINRs for each respective stream, to which successive interference cancellation is applied, are required to be fed back. When the SINRs are required to be fed back, the SINR feedback unit258feeds back the SINRs to the transmitter210. Determining whether the optimum decoding order feed back is fed back and determining whether the SINR is fed back are independently performed. That is, even if the decoding order is not fed back, the SINR may be fed back. When the optimum decoding order and the SINR for each stream are simultaneously fed back, the SINR feedback unit258feeds back the SINR for each stream, to which successive interference cancellation is applied, according to the optimum decoding order. When it is determined that the decoding order is not fed back and only the SINR is fed back, the SINR feedback unit258feeds back the SINR for each stream, to which successive interference cancellation is applied, according to a decoding order that has previously been fed back.

The transmitter210and the receiver250feed back the SINR, to which an order of successive inference cancellation (the decoding order) and successive inference cancellation are applied. On the other hand, in per unitary basis stream user rate control (PU2RC), a decoding order is not fed back. In addition, SINR, to which successive inference cancellation is not applied, is fed back.

For example, the optimum decoding order may be calculated as follows. However, the present invention is not limited thereto.

First, SINRs for respective streams are calculated using estimated channel coefficients. A stream having the greatest SINR is determined as a first decoding stream. Interference due to the first decoding stream is cancelled in a received signal, and then SINRs for other remaining streams are newly calculated. Then, a stream having the greatest SINR from among the SINRs for other remaining streams is determined as a second decoding stream. The process is repeated up to the last stream, and thus the optimum decoding order can be determined.

In the decoding order feedback unit257and the SINR feedback unit258, it is determined whether the decoding order and the SINR are fed back as follows. However, the present invention is not limited to the following algorithm.

The optimum decoding order is fed back for every predetermined period T1, and SINR for each stream is fed back for every predetermined period T2. T1and T2may be the same or different from each other.FIG. 3illustrates the case where an SINR feedback period, i.e., T2is 4 time slots and an optimum decoding order feedback period, i.e., T1is 20 time slots. InFIG. 2, the decoding order feedback unit257operates a timer for every hour so that the optimum decoding order is fed back to the transmitter210for every 20 time slots.

The feedback periods T1and T2may be determined by the transmitter210or the receiver250. When the transmitter210determines the feedback periods T1and T2, the transmitter210determines load of a feed back channel, according to the number of users in the MIMO system and a speed of channel change of the user. The feedback period calculator217of the transmitter210calculates the feedback periods T1and T2and then notifies the receiver250of the feedback periods T1and T2. When the receiver250determines the feedback periods T1and T2, the receiver250determines the feedback periods T1and T2according to the speed channel change of the user, and notifies the transmitter210of the feedback periods T1and T2.

Unlike in the above-described method, the receiver250calculates the difference between performances of two cases, wherein a first case is the case where the optimum decoding order is used and a second case is the case where a decoding order that is previously fed back is used. When the difference is equal to or greater than a predetermined threshold value, the optimum decoding order and the SINR for each stream depending on the optimum decoding order are newly notified to the transmitter210. When the difference is smaller than the threshold value, the optimum decoding order may not be notified to the transmitter210. In addition, the receiver250calculates the difference between performances of two cases, wherein a first case is the case where a current SINR is used and a second case is the case where a SINR that is previously fed back is used. When the difference is equal to or greater than a predetermined threshold value, a new SINR for each stream is notified to the transmitter210. When the difference is smaller than the threshold value, the new SINR may not be notified to the transmitter210.

T1and T2as optimum feedback periods are determined by the transmitter110, according to the number of users in the MIMO system and the speed of channel change of users, and are notified to the receiver150. The receiver150may feedback the optimum successive interference cancellation based decoding order and the SINR for each stream, to which successive interference cancellation are applied.

Alternatively, the receiver150may determine the optimum feedback periods T1and T2, according to the speed of channel change of user, and may notify the transmitter110of the optimum feedback periods T1and T2. The receiver150may feedback the optimum successive interference cancellation based decoding order and the SINR for each stream, to which successive interference cancellation are applied.

FIG. 4is a flowchart showing a method of controlling a receiver of a multi-user MIMO system, according to an embodiment of the present invention. The same description as inFIGS. 1 through 3will not be repeated.

Referring toFIG. 4, the receiver estimates a MIMO channel with respect to a plurality of receiving antennas by using a pilot symbol (operation S410).

The receiver determines whether feedback information is generated, according to the estimated MIMO channel (operation S430). If necessary, the generated feedback information is transmitted to a transmitter (operation S450). The receiver independently determines whether a SINR of a stream, in which interference is cancelled, according to a predetermined decoding order and whether a decoding order of streams are fed back. A feedback period of the decoding order and a feedback period of the SINR may be differently determined. The feedback period of the decoding order may be determined so as to be greater than the feedback period of the SINR.

At least one stream decoded according to the feedback information is received from the transmitter, and the stream is restored applying successive interference cancellation technique (operation S470). When the new feedback information is not generated, the stream is restored according to the feedback information that is previously transmitted (operation S490).

FIG. 5is a flowchart showing a method of controlling a transmitter of a multi-user MIMO system, according to an embodiment of the present invention. The same description as inFIGS. 1 through 4will not be repeated.

Referring toFIG. 5, the transmitter receives feedback information including a decoding order of data streams and an SINR for each stream from a receiver (operation S510).

The transmitter assigns a plurality of pieces of user data to respective streams, according to a control signal generated based on the received feedback information (operation S530).

The transmitter encodes and transmits the assigned streams to the receiver at a data transmission rate according to the control signal generated based on the feedback information (operation S550).

The transmitter determines whether a feedback period of the decoding order of streams and a feedback period of the SINR for each stream depending on the decoding order are required to be determined (operation S570). If necessary, the feedback period of the decoding order of streams and the feedback period of the SINR for each stream depending on the decoding order are transmitted to the receiver (operation S590).

FIGS. 6 and 7are graphs for comparison of performance between a conventional MIMO system and a MIMO system for which feedback information is periodically provided, according to an embodiment of the present invention.

InFIGS. 6 and 7, transmission data rates are respectively inferred according to is a theoretical channel capacity formula and a real adaptive modulation and coding (AMC) table. T is a feedback period of a decoding order, and a feedback period of SINR is set to 1.

Referring toFIGS. 6 and 7, it can be seen that the system capacity of the present embodiment is greater regardless of the number of users and the amount of feedback can be considerably reduced without performance difference in the range of T=1 to T=100.

The present invention has been described in greater detail with reference to the exemplary embodiments. The terms used to describe the present invention are for descriptive purposes only and are not intended to limit the scope of the invention.