KR20090097763A - Method of transmitting pilot in multiple antenna system - Google Patents

Abstract

A method of transmitting pilot in a multiple antenna system is provided to improve channel estimation by independently, individually and adaptively adjusting the transmission power of a pre-coding pilot in each channel. A method of transmitting pilot in a multiple antenna system comprises the steps of: generating a pre-coding matrix including a pre-coding vector to which data stream to be transmitted is applied(S110); performing pre-coding for a pilot by using the pre-coding matrix(S120); and transmitting a pre-coding pilot(S130), wherein the transmission power of the pilot is specifically determined in a data string that is to be transmitted.

Description

Pilot transmission method in multi-antenna system {METHOD OF TRANSMITTING PILOT IN MULTIPLE ANTENNA SYSTEM}

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 is rapidly increasing, including the generalization of information and communication services, the emergence of various multimedia services, and the emergence of high quality services. Various wireless communication technologies have been studied in various fields to satisfy these demands.

Next generation wireless communication systems must be able to transmit high quality, high capacity multimedia data at high speed using limited frequency resources. In order to enable this in a bandwidth-limited wireless channel, it is necessary to overcome inter-symbol interference and frequency selective fading that occur during high-speed transmission while maximizing frequency efficiency. The most popular technologies developed to maximize frequency efficiency are orthogonal frequency division multiplexing (OFDM) and multiple input multiple output (MIMO).

OFDM uses multiple orthogonal subcarriers. OFDM uses orthogonality between inverse fast fourier transforms (IFFTs) and fast fourier transforms (FFTs). The transmitter performs IFFT on the data and transmits it. The receiver recovers original data by performing FFT on the received signal. That is, an IFFT is used to combine multiple subcarriers and a corresponding FFT is used to separate multiple subcarriers. According to OFDM, in a frequency selective fading environment of a wideband channel, the complexity of a receiver can be reduced, and the spectral efficiency can be increased through selective scheduling in the frequency domain by using different channel characteristics between subcarriers. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme based on OFDM. According to OFDMA, the efficiency of radio resources can be improved by assigning different subcarriers to multiple users.

The MIMO technique can be used for two main purposes. First, it can be used for the purpose of increasing diversity gain in order to reduce performance degradation due to channel fading environment. Secondly, it can be used to increase the data rate in the same frequency band. MIMO technology has the advantage of being able to send more data without increasing the frequency bandwidth compared to SISO (Single-Input Single-Output) systems using a single transmit / receive antenna.

The MIMO channel provided by the multiple antennas can be decomposed into independent channels. When the number of transmit antennas is Nt and the number of receive antennas is Nr, the number of independent channels Ni becomes Ni ≦ min {Nt, Nr}, and each independent channel may be referred to as a spatial layer. In general, the rank is defined as the number of non-zero eigenvalues in which the MIMO channel matrix is non-zero.

In a MIMO system, a transmitter performs precoding by multiplying a transmission signal by a weight. Precoding is a technique of preprocessing a transmission signal using a weight and transmitting the weight. The weight is selected or calculated based on a channel that the transmission signal will experience. In general, in a frequency division duplex (FDD) system, a receiver determines a weight and transmits the weight to a transmitter, and the transmitter determines a weight to be used for actual transmission based on the reported weight. In a time division duplex (TDD) system, a transmitter determines a weight through a sounding channel.

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

Here, H is a channel matrix, W is a weight, S is a signal vector, and N is a noise vector. The weight W may be in the form of a matrix or a vector. A common pilot is used to estimate the channel matrix H. Precoded pilots are used to estimate the HW . That is, the receiver uses the HW obtained in the precoded pilot to detect S.

Precoded pilots have the advantage of being able to detect S directly using equivalent channel HW . If a precoded pilot is used in rank 1 transmission, signal to noise ratio (SNR) gain can be expected in channel estimation, and thus reliability of channel estimation performance can be improved. If a precoded pilot is used in rank 2 or higher transmission, not only the signal-to-noise ratio gain in channel estimation, but also the yield improvement can be expected.

However, the multi-antenna system has a very complicated channel environment, and the signal-to-noise ratio gain and the yield improvement gain are not guaranteed for all such complex channels. Because each data stream input to precoding experiences a different channel and is transmitted to the receiver. In this case, the data stream that has undergone a very poor channel has a limitation in gaining a signal-to-noise ratio gain and a yield improvement. Because.

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

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

According to an embodiment of the present invention, a method for transmitting pilots in a multiple 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 a precoding on a pilot using the precoding matrix, and the precoding Transmitting a pilot. The transmit power of the pilot is determined specifically for the precoding vector.

According to another example of the present invention, an apparatus for transmitting a pilot in a multiple antenna system is provided. The method includes a precoding unit for performing precoding on the pilot. A power control unit for adjusting the transmission power of the precoded pilot, a subcarrier allocator for mapping the precoded pilot to a subcarrier, and an OFDM to generate an OFDM symbol by performing an IFFT on the precoded pilot mapped to the subcarrier A modulator, and multiple antennas for transmitting the OFDM symbol.

Since the transmission power of the precoding pilot is adaptively, individually and independently adjusted to each channel, 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 and packet data.

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

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 downlink, the transmitter may be part of the base station 20 and the receiver may be part of the terminal 10. In 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 uses orthogonality between inverse fast fourier transforms (IFFTs) and fast fourier transforms (FFTs). The transmitter performs IFFT on the data and transmits it. The receiver recovers original data by performing FFT on the received signal. The transmitter uses an IFFT to combine multiple subcarriers, and the receiver uses a corresponding FFT to separate multiple subcarriers. There is no limitation on the multiple access scheme applied to the wireless communication system. Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Single-Carrier FDMA (SC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), or other known modulation techniques. It can be based on the same multiple access techniques. These modulation techniques demodulate signals received from multiple users of a communication system to increase the capacity of the communication system. Multiple access schemes for downlink and uplink transmission 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 multiple antenna system may be a multiple-input multiple-output (MIMO) system. Alternatively, the multi-antenna system may be a multiple-input single-output (MISO) system or a single-input single-output (SISO) system or a single-input multiple-output (SIMO) system. May be a system. The 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 showing a pilot transmission apparatus according to an embodiment of the present invention.

2, the pilot transmitter 100 may include a data processing unit 110, a precoding unit 120, a subcarrier allocation unit 130, an OFDM modulator 140, and a power controller 150. Include. The pilot transmitter 100 includes Nt (Nt> 1) transmit antennas 160-1,..., 160-Nt.

The data processor 110 generates a codeword by performing channel coding on an input information bit, and generates a modulation symbol by constellation mapping. The information bits may be user data or control information.

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

The precoding unit 120 may perform precoding on a pilot as well as a modulation symbol. The pilot is a signal known between the pilot transmitter 100 and the receiver for channel estimation and is referred to as a reference signal. A signal obtained by performing precoding on a 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 without additional weight information. That is, the public pilot refers to a pilot transmitted for channel estimation, and the dedicated pilot refers to a pilot transmitted to estimate a value including a channel and additional weight information.

The subcarrier assignment unit 130 allocates a subcarrier to a precoded signal and a dedicated pilot.

The OFDM modulator 140 outputs OFDM symbols by OFDM modulating the input symbols. The OFDM modulator 140 may perform an inverse fast Fourier transform (IFFT) on the input symbol, and may further insert a cyclic prefix (CP) after performing the IFFT. The OFDM symbol is transmitted through each transmit antenna 160-1,..., 160 -Nt.

The power controller 150 may adjust the transmission power of the pilot precoded specifically for the data string or for the receiver. As an example, the power controller 150 adjusts the transmission power of the precoded pilot differently according to the data sequence. The precoding vector for each data stream is transmitted via a precoded pilot, which may also be tuned 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 indicates the index of the precoding vector. W _k is a precoding vector applied to the k-th data string. Assume that the channel through which the zeroth data string is transmitted is very degraded, and the channel through which the second data string is transmitted is good. The power controller 150 may determine a transmit power of the 0 precoding pilot to which W ₀ is applied as 3 dB and a transmit power of the second precoding pilot to which W ₂ is applied as 0 dB. That is, the transmit power of a precoded pilot that will experience a bad channel may be determined relatively high, and the transmit power of a precoded pilot that will experience a good channel may be determined relatively low.

Hereinafter, the method of determining the transmission power will be described with an equation.

Here, H is a channel matrix, W is a weight, S is a signal vector, and N is a 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 may be in the form of a matrix or a vector. The public pilot is used to estimate the channel matrix H. Dedicated pilots are used to estimate HW . That is, the receiver uses the HW obtained in the dedicated pilot to detect S.

When HW is H _eq , H _eq and W can be expressed as in Equation 3.

Here, n is the number of data streams, H _eq is N _R xn, and W _n is N _T x1. From equations (2) and (3), H _eqn = HW _n holds.

When alpha _n is the transmission power for the nth data string, the following equation holds.

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

There is no limitation on the method of power control by the power control unit 150, a closed loop (open-loop) method for adjusting the power without feedback information from the receiver or closed loop for controlling the power through the feedback information from the receiver (closed) -loop) method can be applied. In the open loop scheme, the pilot transmitter 100 directly determines the transmission power of the precoded pilot. In the closed loop scheme, the pilot transmitter 100 may receive channel quality information (CQI) from a receiver and adjust α _n in consideration of the channel quality information.

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

When the pilot transmitter 100 transmits data using a spatial multiplexing technique, there are two methods for the pilot transmitter 100 to configure a plurality of data streams. The first is a single codeword (SCW) method. In the SCW method, one data block is coded through one channel encoder and then divided into a plurality of data strings and transmitted through multiple antennas. The second is a multiple codeword (MCW) method. The MCW method divides one data block into a plurality of data strings first, performs coding through the channel encoder separately, and then transmits each coded data string through multiple antennas. In the SCW method, one block is coded and divided into a plurality of data strings. On the other hand, in MCW, one block is divided into a plurality of data strings before coding.

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

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

In the MCW MU-MIMO scheme, a precoded pilot for each receiver may be used, and the precoded pilot of each receiver may 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 one resource grid defined for each transmit antenna. Transmission Time Interval (TTI) means a time when one subframe is transmitted. The radio frame may include a plurality of subframes. For example, one radio frame may include 10 subframes.

Referring to FIG. 3, common pilots T1, T2, T3, and T4 and precoded pilots P1 and P2 are distributed and arranged on a subframe. The first public pilot T1 is a public pilot transmitted through a first transmit antenna, the second public pilot T2 is a public pilot transmitted through a second transmit antenna, and the third public pilot T3 is a first pilot. The common pilot transmitted through the 3 transmit antennas, and the fourth shared pilot T4 is the public pilot transmitted through the fourth transmit antenna. The first precoded pilot P1 is a dedicated pilot transmitted via the first transmit antenna and the second precoded pilot P2 is a dedicated pilot transmitted via the second transmit antenna.

Although a common pilot for four transmit antennas and a dedicated pilot for two transmit antennas are shown here, the number of transmit antennas to which a shared pilot or a dedicated pilot is transmitted is not limited. In addition, the arrangement of the common pilot and the dedicated pilot is only an example and may be disposed on the subframe in various other forms.

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

Referring to FIG. 4, channel quality information is received (S100). The channel quality information includes some kind of criterion that reflects the state of the channel experienced by the signal transmitted by the transmitter. For example, the channel quality information may be SNR. A precoding matrix is generated (S110). The precoding matrix includes at least one precoding vector to be applied to the data string to be transmitted. The precoding vector is also called a weight vector, and the precoding vector may be obtained by a codebook-based Perup technique that reflects the state of the channel experienced by the received signal, or by an open loop technique determined by the transmitter regardless of the channel state. May be obtained

The precoding is performed on the pilot for the data string to be transmitted using the precoding matrix (S120). Precoding is performed by multiplying the input data and / or pilot with a precoding matrix. Here, the power to which the precoded pilot for each data string is transmitted is specifically determined for that data string. Channel quality information may be used to determine the transmit power level of the pilot. That is, in consideration of the channel quality information, a relatively low transmit power may be provided to a pilot that will experience a good channel, and a relatively high transmit power may be provided to a pilot that will experience a poor channel. As a result, each pilot may be transmitted according to an adaptively determined transmission power, and channel estimation performance may be improved. On the other hand, even if the transmission power is adjusted for each pilot, there may be a case where the total transmission power of the OFDM symbol should be kept constant. In this case, part of the transmission power used for data transmission may be used as the transmission power of the pilot. Alternatively, a part of subcarriers used for data transmission may be punctured. The precoded pilot is transmitted (S130). Here, the transmission power for the precoded pilot for each data string may be transmitted as separate control information. In this case, the transmission power of the precoded pilot for each data string may be provided as independent information, or may be provided as integrated information such as a transmission power ratio.

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

Although the present invention has been described above with reference to the embodiments, it will be apparent to those skilled in the art that the present invention may be modified and changed in various ways without departing from the spirit and scope of the present invention. I can understand. Therefore, the present invention is not limited to the above-described embodiment, and the present invention will include all embodiments within the scope of the following claims.

1 is a block diagram illustrating a wireless communication system.

2 is a block diagram showing an apparatus for transmitting 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 multiple antenna system according to an embodiment of the present invention.

Claims (9)

In a transmission method of a pilot in a multi-antenna system, 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 precoded pilot, wherein the transmit power of the pilot is determined specifically for the data string to be transmitted. The method of claim 1, Before transmitting the pilot, receiving channel information regarding a channel on which the data string to be transmitted is transmitted; And a transmission power of the pilot is determined using the channel information. The method of claim 2, Wherein the channel information is a signal to interference noise ratio (SINR). The method of claim 1, The transmission power of the pilot is to be transmitted as separate control information, the pilot transmission method. The method of claim 1, The transmission power of the pilot is applied as a weight for the precoding matrix, pilot transmission method. In a transmitter of a pilot in a multi-antenna system, A precoding unit for precoding each pilot to be applied to the plurality of streams; A power controller for adjusting the transmission power of each of the precoded pilots; A subcarrier allocator for mapping the precoded pilots to subcarriers; An OFDM modulator for generating an OFDM symbol by performing an IFFT on each of the precoded pilots mapped to the subcarriers; And A pilot transmission apparatus including multiple antennas for transmitting the OFDM symbol. The method of claim 6, And the power control unit adjusts the transmission power according to a quantized value. The method of claim 5, wherein And the power control unit adjusts receiver specific for each receiver. The method of claim 6, And the power control unit adjusts the transmission power using channel quality information fed back from the receiver. The method of claim 6, And the power control unit adjusts the transmission power to be receiver specific.

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