KR101499255B1 - Method of transmitting pilot in multiple antenna system - Google Patents

Abstract

A method of transmitting a pilot in a multi-antenna system is provided. The method includes generating a precoding matrix including a precoding vector to be applied to each data stream to be transmitted, performing precoding on a pilot using the precoding matrix, And transmitting the transmitted pilot. The transmit power of the pilot is determined specifically for the data stream. The transmission power of the precoding pilot is adaptively, individually, and independently adjusted for each channel, so that the channel estimation performance can be improved.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of transmitting a pilot in a multi-

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to wireless communication, and more particularly, to a method of transmitting a pilot in a multi-antenna system.

The demand for communication services such as the universalization of information communication services, the appearance of various multimedia services, and the emergence of high quality services are rapidly increasing. Various wireless communication technologies are being investigated in various fields to satisfy this demand.

The next generation wireless communication system should be able to transmit high-quality, high-capacity multimedia data at high speed by using limited frequency resources. To enable this in a bandwidth-limited radio channel, it is necessary to overcome inter-symbol interference and frequency selective fading occurring during high-speed transmission while maximizing the frequency efficiency. Among the technologies developed to maximize the frequency efficiency are Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO).

OFDM uses a plurality of orthogonal subcarriers. OFDM utilizes the orthogonality property between IFFT (inverse fast Fourier transform) and FFT (fast Fourier transform). The transmitter performs IFFT on the data and transmits it. The receiver performs an FFT on the received signal to recover the original data. That is, an IFFT is used to combine multiple subcarriers, and a corresponding FFT is used to separate multiple subcarriers. According to OFDM, the complexity of the receiver can be reduced in a frequency selective fading environment of a wideband channel, and the spectral efficiency can be increased through selective scheduling in the frequency domain by utilizing different channel characteristics between subcarriers. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access method based on OFDM. According to OFDMA, the efficiency of radio resources can be improved by allocating different subcarriers to multiple users.

The MIMO scheme can be used for two purposes. First, it can be used for the purpose of increasing the diversity gain to reduce the performance degradation due to the channel fading environment. Second, it can be used to increase the data rate in the same frequency band. MIMO technology is advantageous in that it can send more data than a single-input single-output (SISO) system using one transmitting / receiving antenna without increasing the frequency bandwidth.

The MIMO channel provided by multiple antennas may decompose into independent channels. When the number of transmission antennas is Nt and the number of reception antennas is Nr, the number of independent channels Ni is Ni min (Nt, Nr), and each independent channel is a spatial layer. In general, the rank is defined as the number of non-zero eigenvalues in which the MIMO channel matrix is not zero.

In a MIMO system, a transmitter performs precoding that multiplies a transmission signal by a weight. Precoding is a technique of preprocessing and transmitting a transmission signal using a weight, and the weight is selected or calculated based on a channel to be experienced by the transmission signal. Generally, in a Frequency Division Duplex (FDD) system, a receiver determines a weight and transmits it to a transmitter, and the transmitter determines a weight to be used for an actual transmission based on the reported weight. In a TDD (Time Division Duplex) system, a transmitter determines a weight through a sounding channel.

The model for the received signal vector y can be expressed as follows.

Where H is the channel matrix, W is the weight, S is the signal vector, and N is the noise vector. The weight W can be in matrix or vector form. The common pilot is used to estimate the channel matrix H. A precoded pilot is used to estimate the HW . That is, the receiver uses the HW obtained in the precoded pilot to detect S.

The precoded pilot is advantageous in that it can directly detect S using an equivalent channel HW . If a precoded pilot is used in the rank 1 transmission, the signal to noise ratio (SNR) gain can be expected in the channel estimation plane, so that the reliability of the channel estimation performance can be enhanced. If a precoded pilot is used in transmission of Rank 2 or higher, a signal-to-noise ratio gain in the channel estimation plane as well as a yield improvement can be expected.

However, the multi-antenna system has a very complicated channel environment and does not guarantee such a signal-to-noise ratio gain and a yield improvement gain for all such complex channels. Since each data stream input to precoding is transmitted to a receiver experiencing a different channel, a data stream that has undergone a very poor channel has a limitation in obtaining a gain in signal-to-noise ratio gain and yield improvement Because.

There is a need for a method of transmitting precoded pilots to improve the performance of channel estimation.

The present invention provides a method of transmitting a pilot for improving channel estimation in a multi-antenna system.

According to an example of the present invention, a method of transmitting a pilot in a multi-antenna system is provided. The method includes generating a precoding matrix including a precoding vector to be applied to a data stream to be transmitted, performing precoding on a pilot using the precoding matrix, And transmitting the pilot. The transmit power of the pilot is determined specifically for the precoding vector.

According to another aspect of the present invention, there is provided a transmission apparatus of a pilot in a multi-antenna system. The method includes precoding a pilot to perform precoding on the pilot. A power adjuster for adjusting transmission power of the precoded pilot, a subcarrier allocator for mapping the precoded pilot to a subcarrier, an OFDM receiver for performing OFDM on the precoded pilot mapped to the subcarrier to generate an OFDM symbol A modulator, and multiple antennas for transmitting the OFDM symbols.

The transmission power of the precoding pilot is adaptively, individually, and independently adjusted for each channel, so that the channel estimation performance can be improved.

1 is a block diagram illustrating a wireless communication system. Wireless communication systems are widely deployed to provide various communication services such as voice, packet data, and the like.

Referring to FIG. 1, a wireless communication system includes a user equipment (UE) 10 and a base station (BS) 20. The terminal 10 may be fixed or mobile and may be referred to by other terms such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, The base station 20 generally refers to a fixed station that communicates with the terminal 10 and may be referred to in other terms such as a Node-B, a Base Transceiver System (BTS), an Access Point Can be called. One base station 20 may have more than one cell.

Hereinafter, downlink (DL) means communication from the base station 20 to the terminal 10, and uplink (UL) means communication from the terminal 10 to the base station 20. In the downlink, the transmitter may be part of the base station 20 and the receiver may be part of the terminal 10. In the uplink, the transmitter may be part of the terminal 10 and the receiver may be part of the base station 20.

The wireless communication system may be an Orthogonal Frequency Division Multiplexing (OFDM) / Orthogonal Frequency Division Multiple Access (OFDMA) based system. OFDM uses multiple orthogonal subcarriers. OFDM utilizes the orthogonality property between IFFT (inverse fast Fourier transform) and FFT (fast Fourier transform). The transmitter performs IFFT on the data and transmits it. The receiver performs an FFT on the received signal to recover the original data. The transmitter uses an IFFT to combine multiple subcarriers, and the receiver uses a corresponding FFT to separate multiple subcarriers. There are no restrictions on multiple access schemes applied to wireless communication systems. (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Single-Carrier FDMA (SC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA) And may be based on the same multiple access schemes. These modulation techniques increase the capacity of the communication system by demodulating signals received from multiple users of the communication system. Multiple access schemes for downlink and uplink transmissions may be different. For example, OFDMA may be used for downlink transmission and SC-FDMA may be used for uplink transmission.

The wireless communication system may be a multiple antenna system. The multi-antenna system may be a multiple-input multiple-output (MIMO) system. Or multiple antenna systems may be implemented using a multiple input single-output (MISO) system or a single-input single-output (SISO) system or a single- ) System. A MIMO system uses multiple transmit antennas and multiple receive antennas. The MISO system uses multiple transmit antennas and one receive antenna. The SISO system uses one transmit antenna and one receive antenna. The SIMO system uses one transmit antenna and multiple receive antennas.

2 is a block diagram illustrating a pilot transmission apparatus according to an embodiment of the present invention.

2, the pilot transmission apparatus 100 includes a data processing unit 110, a precoding unit 120, a subcarrier allocating unit 130, an OFDM modulator 140, and a power control unit 150 . The pilot transmission apparatus 100 includes Nt (Nt > 1) transmission antennas 160-1, .., 160-Nt.

The data processing unit 110 performs channel coding on the input information bits to generate codewords and performs constellation mapping to generate modulation symbols. The information bits may be user data or control information.

The precoding unit 120 precodes the modulation symbol to output a precoded signal. Here, precoding can be performed by multiplying the modulation symbol by a precoding matrix. The precoding matrix includes at least one precoding vector. The precoding vector may also be referred to as a weight vector.

The precoding unit 120 may precode not only a modulation symbol but also a pilot. A pilot is a signal known to the pilot transmission apparatus 100 and a reception period for channel estimation, and is referred to as a reference signal. The signal obtained by precoding the pilot is called a precoded pilot or a dedicated pilot. The dedicated pilot is a pilot carrying additional weight information, and is distinguished from a common pilot not including additional weight information. That is, a public pilot refers to a pilot transmitted for channel estimation, and a dedicated pilot refers to a pilot transmitted so as to estimate a value including a channel and additional weight information.

The subcarrier allocator 130 allocates subcarriers to the precoded signal and the dedicated pilot.

The OFDM modulator 140 OFDM-modulates an input symbol and outputs an OFDM symbol. The OFDM modulator 140 may perform inverse fast Fourier transform (IFFT) on the input symbol, and may further insert a cyclic prefix (CP) after performing IFFT. The OFDM symbols are transmitted through the respective transmission antennas 160-1, ..., and 160-Nt.

The power adjuster 150 may adjust the transmit power of the pilot precoded specifically to the data stream or to the receiver. As an example, the power adjuster 150 adjusts the transmit power of the precoded pilot differently according to the data stream. The precoding vector for each data stream is transmitted on a precoded pilot, the transmit power of the precoded pilot for each such data stream may also be adjusted specifically to the receiver. This is because the precoded pilot is a UE specific pilot.

For example, if the precoding matrix P includes four precoding vectors W, then B = {W ₀ , W ₁ , W ₂ , W ₃ }. Here, the subscript denotes the index of the precoding vector. W _k is a precoding vector applied to the kth data stream. Suppose that the channel through which the zeroth data stream is transmitted is very degraded and the channel through which the second data stream is transmitted is good. The power control unit 150 may determine the transmission power of the 0 th precoding pilot to which W ₀ is applied to be 3 dB and the transmission power of the second precoding pilot to which W ₂ is applied to be 0 dB. That is, the transmission power of the precoded pilot to undergo a bad channel is relatively high, and the transmission power of the precoded pilot to undergo a good channel can be determined to be relatively low.

Hereinafter, a method for determining the transmission power will be described using the following equation.

Where H is the channel matrix, W is the weight, S is the signal vector, and N is the noise vector. The size of H is given by N _R × N _T , where N _T is the number of transmit antennas and N _R is the number of receive antennas. The weight W can be in matrix or vector form. The common pilot is used to estimate the channel matrix H. The dedicated pilot is used to estimate the HW . That is, the receiver uses the HW obtained in the dedicated pilot to detect S.

If HW is H _eq , H _eq and W can be expressed by Equation (3).

Where n is the number of data streams, H _eq is N _R × n, and W _n is N _T × 1. From equations (2) and (3), H _eqn = HW _n is established.

and? _n is the transmission power with respect to the n-th data stream, the following equation holds.

That is, the transmit power of the precoded pilot according to the precoding vector at the desired location can be determined by adjusting the weight [alpha] _n . The transmit power level may be given as a continuous value or may be given as a quantized value. In the latter case, the transmission power of each precoded pilot can be any one of {0 dB, 3 dB, 6 dB, ...}.

There is no limitation on the manner in which the power regulator 150 regulates the power, and there is no limitation in the open-loop regulator that regulates power without feedback information from the receiver, or a closed loop regulator that regulates the power through the feedback information from the receiver -loop) method can be applied. In the case of the open loop scheme, the pilot transmission apparatus 100 determines the transmission power of the direct precoded pilot. In the case of the closed loop scheme, the pilot transmission apparatus 100 can receive channel quality information (CQI) from a receiver and adjust α _n considering channel quality information.

Each precoded pilot experiences a different channel. However, if the precoded pilot is transmitted at a uniform transmission power, the channel estimation performance by the precoded pilot may be different from each other. As in the present invention, if the transmission power of the precoding pilot is adaptively, individually, and independently adjusted for each channel, the overall channel estimation performance can be improved.

There are two methods by which the pilot transmission apparatus 100 constructs a plurality of data streams when the pilot transmission apparatus 100 transmits data using the spatial multiplexing scheme. The first is a Single Code Word (SCW) scheme. The SCW scheme is a scheme of coding one data block through one channel encoder and dividing the data block into a plurality of data streams and transmitting through a plurality of antennas. The second is a Multiple Code Word (MCW) scheme. In the MCW scheme, a data block is divided into a plurality of data streams, and the data streams are separately encoded through a channel encoder, and each coded data stream is transmitted through multiple antennas. In the SCW method, one block is coded and divided into a plurality of data strings. On the other hand, in the MCW, a block is divided into a plurality of data strings and then coded.

For example, in a single user (SU) MIMO system using SCW, each channel experienced by N data streams can be estimated as a representative CQI (or average CQI) of each data stream. However, there may be a data stream (hereinafter referred to as a strong data stream) experiencing a good channel and a data stream (hereinafter referred to as a weak data stream) suffering a bad channel among the N data streams. If the representative CQI is informed as a precoded pilot, the channel estimation error of the equivalent channel of the weak data sequence may be increased. In this case, channel estimation performance can be improved by boosting the transmission power of the precoded pilot of the weak data stream.

This is also applicable to single-user MIMO systems or multiple user (MU) MIMO systems using MCW. The MCW MU-MIMO system is a system that improves frequency efficiency by allocating the same frequency / time resources by paring receivers located in different spaces. The MCW SU-MIMO selects a modulation and coding scheme (MCS) corresponding to the CQI of each data sequence. The data streams with low CQIs can be guaranteed a robust transmission by selecting a low MCS. Likewise, if the transmission power of a precoded pilot for a data sequence with a low CQI reported is increased, the equivalent channel estimation performance for data demodulation can be improved.

In the MCW MU-MIMO scheme, a precoded pilot may be used for each receiver. In this case, the precoded pilot of each receiver can perform power control for each receiver.

3 shows an example of a subframe. A subframe includes a plurality of OFDM symbols in the time domain and a plurality of subcarriers in the frequency domain. The subframe is a resource grid defined for each transmission antenna. Transmission Time Interval (TTI) is a time at which one subframe is transmitted. A radio frame may include a plurality of subframes. For example, one radio frame may include ten subframes.

Referring to FIG. 3, public pilots (T1, T2, T3, T4) and precoded pilots (P1, P2) are scattered on a subframe. The first public pilot Tl is a public pilot transmitted through the first transmission antenna, the second public pilot T2 is a public pilot transmitted through the second transmission antenna, the third public pilot T3 is a public pilot transmitted through the second transmission antenna, 3 transmission antenna, and the fourth public pilot T4 is a public pilot transmitted through the fourth transmission antenna. The first precoded pilot P1 is a dedicated pilot transmitted via a first transmit antenna and the second precoded pilot P2 is a dedicated pilot transmitted via a second transmit antenna.

Here, the common pilot for four transmission antennas and the dedicated pilot for two transmission antennas are shown, but the number of transmission antennas through which common pilots or dedicated pilots are transmitted is not limited. Further, the arrangement of the public pilot and the dedicated pilot is merely an example, and may be arranged on the subframe in various other forms.

4 is a flowchart illustrating a pilot transmission method in a multi-antenna system according to an exemplary embodiment of the present invention.

Referring to FIG. 4, channel quality information is received (S100). The channel quality information includes any kind of criteria that reflects the state of the channel experienced by the signal transmitted by the transmitter. For example, the channel quality information may be an SNR. And generates a precoding matrix (S110). The precoding matrix includes at least one precoding vector to be applied to the data stream to be transmitted. The precoding vector is also referred to as a weight vector, and the precoding vector may be obtained by a codebook-based Peruf technique that reflects the state of the channel experienced by the received signal and may be obtained by an open loop technique determined by the transmitter, May be obtained

And precodes the pilot for a data stream to be transmitted using the precoding matrix (S120). Precoding is performed by multiplying the input data and / or pilot by a precoding matrix. Here, the power to which the precoded pilot for each data stream is transmitted is determined specifically for that data sequence. The channel quality information may be used to determine the transmit power level of the pilot. In other words, considering the channel quality information, it is possible to provide a relatively low transmission power for a pilot that will undergo a good channel and a relatively high transmission power for a pilot that will suffer a poor channel. Thereby, each pilot can be transmitted according to the adaptively determined transmission power, and the channel estimation performance can be improved. On the other hand, even if the transmission power is adjusted for each pilot, the total transmission power of the OFDM symbol may be maintained constant. In this case, a part of the transmission power used for data transmission can be used as the transmission power of the pilot. Or puncturing some of the subcarriers used for data transmission. The precoded pilot is transmitted (S130). Here, the transmission power for the precoded pilot for each data stream may be transmitted as separate control information. At this time, the transmission power for the precoded pilot for each data column may be provided as independent information, or may be provided as integrated information such as a transmission power ratio.

All of the functions described above may be performed by a processor such as a microprocessor, a controller, a microcontroller, an application specific integrated circuit (ASIC), etc. according to software or program code or the like coded to perform the function. The design, development and implementation of the above code will be apparent to those skilled in the art based on the description of the present invention.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention. You will understand. Therefore, it is intended that the present invention covers all embodiments falling within the scope of the following claims, rather than being limited to the above-described embodiments.

1 is a block diagram illustrating a wireless communication system.

2 is a block diagram illustrating a transmitting apparatus for a pilot according to an embodiment of the present invention.

3 shows an example of a subframe.

4 is a flowchart illustrating a pilot transmission method in a multi-antenna system according to an exemplary embodiment of the present invention.

Claims (9)

A method for pilot transmission by a base station in a multi-antenna system, Generating a precoding matrix including precoding vectors to be applied to each data stream to be transmitted; Determining a weight to be applied to each of the precoding vectors; Multiplying the weight and the precoding vectors, respectively; Performing precoding on a pilot using the precoding vectors multiplied by the weights; And And transmitting the precoded pilot, The precoding vectors are selected from a predetermined codebook, the transmit power of the pilot is determined by the weight, Wherein the weight is set low when the channel environment is good and is set high when the channel environment is poor. The method according to claim 1, Further comprising receiving channel information regarding a channel through which the data sequence to be transmitted is transmitted before transmitting the pilot, And the weight is determined using the channel information. 3. The method of claim 2, Wherein the channel information is a signal to interference noise ratio (SINR). The method according to claim 1, And the transmission power of the pilot is transmitted as separate control information. delete A transmission apparatus of a pilot in a multi-antenna system, A precoding unit for precoding each of the pilots to be applied to the plurality of streams; A power adjuster for adjusting transmit power of each of the precoded pilots; A subcarrier allocator for mapping the precoded pilots to subcarriers; An OFDM modulator for performing an IFFT on each of the precoded pilots mapped to the subcarriers to generate an OFDM symbol; And A plurality of antennas for transmitting the OFDM symbols, The power adjuster determines a weight to be applied to each of the precoding vectors, Wherein the precoding unit generates a precoding matrix including the precoding vectors to be applied to each data stream to be transmitted, Multiplying the weight and the precoding vectors, respectively, Performing precoding on a pilot using the precoding vectors multiplied by the weights, Wherein the precoding vectors are selected from a predetermined codebook, the transmit power of the precoded pilot is determined by the weight, Wherein the weight is set low when the channel environment is good and is set high when the channel environment is poor. The method according to claim 6, Wherein the power adjuster adjusts the transmit power according to a quantized value. The method according to claim 6, Wherein the power controller adjusts the transmission power using feedback channel quality information from a receiver. The method according to claim 6, Wherein the power adjuster adjusts the transmit power for each receiver.

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