Signal detection method and receiving apparatus in MIMO system

This application claims priority to and the benefit of Korean Patent Application No. 10-2006-0122858 filed in the Korean Intellectual Property Office on Dec. 6, 2007, the entire contents of which are incorporated herein by reference.

(a) Field of the Invention

The present invention relates to a signal detection method and a receiving apparatus in a multiple input multiple output (MIMO) system. More particularly, the present invention relates to a signal detection device in a MIMO system using a spatial multiplexing (SM) method.

(b) Description of the Related Art

A fourth generation mobile communication-based wireless communication system requires high-speed data services for images and packets rather than services based on voice. Accordingly, a multiple input multiple output (MIMO) system is being spotlighted since the MIMO system uses a spatial multiplexing method in which high-rate data transmission can be performed by transmitting multiple data streams (here, each stream is also referred to as a “layer”) to satisfy such a requirement. In the MIMO system using the spatial multiplexing method, transmitting antennas respectively transmit data layers including different pieces of information, and a receiving terminal separates the transmitted data layers. A conventional maximum likelihood (ML) signal detection method for detecting a transmission signal vector having ML metric values of available combinations of transmission signal vectors to detect a transmission signal has optimum performance. However, the conventional ML signal detection method may not be realized since complexity is problematically increased according to the number of transmitting antennas and the number of constellation points. Accordingly, a linear signal detection method reducing the complexity (e.g., a zero forcing (ZF) method and a minimum mean square estimator (MMSE) has been suggested, but performance thereof is deteriorated compared to the ML method. In addition, non-linear signal detection methods include an ordered successive interference cancellation (OSIC) method that is known as a vertical Bell Lab layered space time (VBLAST) method. The VBLAST method may be simply realized, and performance thereof is improved compared to the performance of the ZF and MMSE methods but is not greater than the ML method.

Associated technology relates to a signal detection method in a multiple transmitting/receiving antenna system using the spatial multiplexing method. To detect a signal in the MIMO system, a received signal is detected in a ZF method, and a first detection section is established from the detected signal. The received signal is detected in the ML method in the first detection section, and a second detection section is established from the signal detected in the ZF method and the signal detected in the ML method. The received signal is detected in the ML method in the second detection section, and a final value is established. Accordingly, while this conventional art has lower complexity than the ML method and higher performance than the ZF method and the MMSE method, it has lower performance than the ML method.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

The present invention has been made in an effort to provide a signal detection method for achieving performance that is close to that of the maximum likelihood (ML) method while having lower complexity than the ML method in a multiple input multiple output (MIMO) antenna system using a spatial multiplexing method, and a receiving apparatus.

According to an exemplary embodiment of the present invention, a receiving apparatus of a MIMO system includes a layer arrangement unit and a symbol detection unit. The layer arrangement unit rearranges channel estimated layers for a received signal based on reliability. The symbol detection unit detects a transmission symbol vector according to the layers that are rearranged by the layer arrangement unit.

According to another exemplary embodiment of the present invention, in a receiving method of a multiple input multiple output (MIMO) system, a) channel estimated layers for a received signal are rearranged based on reliability, b) a rearranged channel matrix H is QR-decomposed, and c) all constellation points for a symbol of a layer having the lowest reliability among the layers are considered, and the symbols are detected by using a sequential interference elimination method.

FIG. 1 is a schematic block diagram of a multiple input multiple output (MIMO) system according to an exemplary embodiment of the present invention.

FIG. 2 is a flowchart representing a method for detecting a signal in the MIMO system according to the exemplary embodiment of the present invention.

FIG. 3 is a diagram representing an operational algorithm of a symbol detection unit in the MIMO system according to the exemplary embodiment of the present invention.

FIG. 4 shows a diagram representing bit error rates according to signal detection methods when four transmitting antennas, four receiving antennas, and a 16-quadrature amplitude modulation (QAM) method are used.

FIG. 5 shows a diagram representing bit error rates according to the signal detection methods when four transmitting antennas, four receiving antennas, and a quadrature phase shift keying (QPSK) modulation method are used.

In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification.

A transmission signal transmitted through a plurality of transmitting antennas is received in a multiple input multiple output (MIMO) system through a channel and a plurality of receiving antennas. For convenience of description, while it is assumed that four transmitting antennas and four receiving antennas are provided according to an exemplary embodiment of the present invention, it is not limited thereto.

FIG. 1 is a schematic block diagram of the MIMO system according to the exemplary embodiment of the present invention.

As shown in FIG. 1 , the MIMO system includes a transmitting apparatus 100 and a receiving apparatus 200 . The transmitting apparatus 100 includes a signal processing unit 110 for processing interleaving and channel encoding operations, a symbol mapping unit 120 , a demultiplexer 130 , and four transmitting antennas 140 , and the receiving apparatus 200 includes a channel estimation and layer arrangement unit 210 , a symbol detection unit 220 , a multiplexer 230 , a symbol demapping unit 240 , a signal processing unit 250 , and four receiving antennas 260 .

In the transmitting apparatus 100 , the signal processing unit 110 performs transmission data processes including scrambling, error correction coding, and interleaving processes, and transmits processed data to the symbol mapping unit 120 . The transmission data transmitted to the signal processing unit 110 is binary data transmitted to a physical layer in a medium access control (MAC). The symbol mapping unit 120 converts the transmission data processed by the signal processing unit 110 to a symbol according to a modulation method. The symbols at high speed are divided to four low speed data layers through the demultiplexer 130 according to the number of transmitting antennas 140 , and the four data layers are simultaneously transmitted through the transmitting antennas 140 .

A signal transmitted to the receiving apparatus 200 through a channel is to received in parallel in the receiving apparatus 200 through the receiving antenna 260 , and transmitted to the channel estimation and layer arrangement unit 210 to estimate a MIMO channel and rearrange layers. When the layers are rearranged, a layer having the lowest reliability is arranged as a last layer, and remaining layers are arranged in order of a layer having the second lowest reliability to a layer having the highest reliability.

That is, the layer having the lowest reliability is arranged as the last layer, the layer having the highest reliability is arranged as a third layer, and layers having the next highest reliabilities are arranged as second and first layers. The reliability of layers is determined based on a signal-to-noise ratio (SNR). The symbol detection unit 220 determines a transmission symbol by using channel information received from a previous block and the rearranged signal. Subsequently, data are received through the multiplexer 230 , the symbol demapping unit 240 , and the signal processing unit 250 that respectively perform opposite functions of the signal processing unit 110 , the symbol mapping unit 120 , and the demultiplexer 130 of the transmitting apparatus 100 .

FIG. 2 is a flowchart representing a method for detecting a signal in the MIMO system according to the exemplary embodiment of the present invention.

As shown in FIG. 2 , a signal is received in the receiving apparatus 200 through the receiving antenna 260 , and it is transmitted to the channel estimation and layer arrangement unit 210 .

The channel estimation and layer arrangement unit 210 estimates and compensates signal distortion caused by multipath fading of a MIMO channel in step S 210 . In addition, the channel estimation and layer arrangement unit 210 arranges layers in step S 220 . In this case, when the layers are arranged, a layer having the lowest reliability is arranged as a last layer, and remaining layers are arranged in order from a layer having lower reliability to a layer having higher reliability. When {tilde over (H)} denotes a MIMO channel before rearrangement, a matrix G may be obtained by calculating an inverse matrix as given as Equation 1.

G→{tilde over (H)} −1   (Equation 1)

Here, each row norm value of the matrix G is calculated, layers are arranged in order of a layer having the highest value as x 4 , a layer having the lowest value as x 3 , and layers having second and third lowest values as x 2 and x 1 . A system model after the rearrangement is assumed to be the same as Equation 2.

{tilde over (y)}=Hx+ñ   (Equation 2)

Here, {tilde over (y)}=[{tilde over (y)} 1 {tilde over (y)} 2 {tilde over (y)} 3 {tilde over (y)} 4 ] T denotes a received signal, x=[x 1 x 2 x 3 x 4 ] T denotes a rearranged transmission signal, ñ=[ñ 1 ñ 2 ñ 3 ñ 4 ] T denotes noise, and H=[h 1 h 2 h 3 h 4 ] T denotes a 4×4 estimated channel matrix after the rearrangement. Accordingly, x 4 denotes a symbol of the layer having the lowest SNR, x 3 denotes a symbol of the layer having the highest SNR, x 2 denotes a symbol of the layer having the second highest SNR, and x 2 denotes a symbol of the layer having the third highest SNR. The layers are rearranged by the channel estimation and layer arrangement unit 210 to consider all possible constellation points for the layer that is least reliable (i.e., x 4 ) when the symbol detection unit 220 detects the symbol.

The symbol detection unit 220 QR-decomposes the rearranged channel matrix H as shown in Equation 3 in step S 230 .

Here, Q denotes an orthogonal matrix, and R denotes an upper triangular matrix in which upper elements including diagonal elements have values. In addition, y denotes a signal obtained by applying a matrix QH to the received signal as shown in Equation 4.

y=Q H {tilde over (y)}  (Equation 4)

The symbol detection unit 220 considers all constellation points for the symbol x 4 of the layer having the lowest reliability in step S 240 , and detects the symbol in a sequential interference elimination method by using characteristics of an R matrix in step S 250 . The symbol detection unit 220 may detect a symbol by using the sequential symbol elimination method after the layer having the lowest reliability. That is, the symbol detection unit 220 may detect a symbol by performing sequential symbol elimination for the remaining layers. For example, with respect to x 4 , x 3 is given as Equation 5.

Accordingly, x 2 and x 1 are determined as Equation 6 and Equation 7. Here, a slicing operation is a mapping operation to closest constellation points.

When the above operation is performed for all the constellation points of x 4 , symbol vectors corresponding to a number C of final constellation points are obtained, and maximum likelihood (ML) metrics given as Equation 8 are applied to respective symbol vectors.

arg min∥y−Rx∥ 2   (Equation 8)

Accordingly, a symbol vector having the lowest ML metric is detected as a final transmission symbol vector. An operational algorithm of the symbol detection unit 220 is shown in FIG. 3 .

FIG. 4 and FIG. 5 are respectively graphs representing simulation results comparing a detection method according to the exemplary embodiment of the present invention and an ML detection method.

In FIG. 4 , bit error rates according to signal detection methods are shown when four transmitting antennas, four receiving antennas, and a 16-quadrature amplitude modulation (QAM) method are used.

As shown in FIG. 4 , the signal detection method according to the exemplary embodiment of the present invention reaches closely to the performance of the ML detection method. In addition, when complexity of the ML detection method and complexity according to the exemplary embodiment of the present invention are calculated with respect to a real number multiplier, it is required to provide 525312 real number multipliers in the ML detection method while it is only required to provide 1368 real number multipliers in the detection method according to the exemplary embodiment of the present invention. Accordingly, the signal detection method according to the exemplary embodiment of the present invention has 0.26% of the complexity of the ML detection method.

FIG. 5 shows bit error rates according to signal detection methods when four transmitting antennas, four receiving antennas, and a quadrature phase shift keying (QPSK) modulation method are used.

As shown in FIG. 5 , the signal detection method according to the exemplary embodiment of the present invention reaches closely to the performance of the ML detection method. Accordingly, when the signal detection method according to the exemplary embodiment of the present invention is applied to the MIMO system using a spatial multiplexing method, the complexity is lower than the ML detection method and the performance is close to that of the ML detection method.

While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

According to the exemplary embodiment of the present invention, without performing an exhaustive full search operation of the ML detection method that is know as an optimum detection method, performance that is close to that of the ML detection method may be achieved. That is, performance that is close to that of the ML detection method is achieved and the number of required calculations is reduced, and therefore it is easy to realize hardware.