JP4576877B2 - Wireless communication system, wireless communication apparatus, wireless communication method, and computer program - Google Patents

Description

  The present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for realizing broadband wireless transmission between a plurality of wireless stations, such as a wireless LAN (Local Area Network), and in particular, a plurality of Transmitting by performing communication (MIMO (Multi Input Multiple Output) communication) in which a transmitter having an antenna and a receiver having a plurality of antennas are paired to form a plurality of logical channels using spatial multiplexing. The present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program whose capacity is increased.

  More specifically, the present invention relates to a radio communication system, a radio communication apparatus, and a radio that perform closed-loop type MIMO transmission using singular value decomposition (SVD) of a channel matrix whose elements are channels corresponding to transmission / reception antenna pairs. In particular, a wireless communication system, a wireless communication apparatus, and a wireless communication method that perform a communication operation with good transmission efficiency by reducing a difference in quality between a plurality of MIMO channels obtained by spatial multiplexing. And a computer program.

  Information network sharing and device resource sharing can be efficiently realized by computer networking such as a LAN. Here, a wireless LAN is attracting attention as a system for releasing users from the conventional wired LAN connection. According to the wireless LAN, most of the wired cables can be omitted in a work space such as an office, so that a communication terminal such as a personal computer (PC) can be moved relatively easily.

  In recent years, the demand for wireless LAN systems has increased remarkably with the increase in speed and cost. In particular, the introduction of a personal area network (PAN) has been studied in order to construct a small-scale wireless network between a plurality of electronic devices existing around a person and perform information communication. For example, different radio communication systems and radio communication apparatuses are defined using frequency bands that do not require a license from a supervisory agency, such as 2.4 GHz band and 5 GHz band.

  One standard for wireless networks is IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (for example, see Non-Patent Document 1), HiperLAN / 2 (for example, Non-Patent Document 2 or Non-Patent Document 2 or Non-Patent Document 2). (See Patent Document 3), IEEE 302.15.3, Bluetooth communication, and the like. Regarding the IEEE802.11 standard, there are extended standards such as IEEE802.11a (see, for example, Non-Patent Document 4), b, and g, depending on the wireless communication method and the frequency band to be used.

  Here, when a wireless network is constructed indoors, a multipath environment is formed in which the reception apparatus receives a superposition of a direct wave and a plurality of reflected waves / delayed waves. Multipath causes delay distortion (or frequency selective fading), and causes an error in communication. There is a problem that intersymbol interference occurs due to delay distortion.

  As a main countermeasure against delay distortion, a multi-carrier transmission system can be cited. For example, in IEEE802.11a, an OFDM (Orthogonal Frequency Division Multiplexing) modulation system, which is one of multicarrier transmission systems, is adopted as a standard for wireless LANs. In the OFDM modulation scheme, the frequency of each carrier is set so that the carriers are orthogonal to each other within a symbol period. At the time of information transmission, a plurality of data output by serial / parallel conversion of information sent serially for each symbol period slower than the information transmission rate is assigned to each carrier, and amplitude and phase are modulated for each carrier, By performing inverse FFT on the plurality of carriers, it is converted into a signal on the time axis and transmitted while maintaining the orthogonality of each carrier on the frequency axis. At the time of reception, the reverse operation, that is, FFT is performed to convert the time-axis signal into the frequency-axis signal and perform demodulation corresponding to each modulation method for each carrier, and parallel / serial conversion to the original Play back the information sent in the serial signal.

  The IEEE802.11a standard supports a modulation scheme that achieves a communication speed of 54 Mbps at the maximum, but a wireless standard capable of realizing a higher bit rate as a communication speed is required. For example, in IEEE802.11n, the next-generation wireless LAN standard is formulated with the aim of developing a high-speed wireless LAN technology with an effective throughput exceeding 100 MBPS.

  MIMO (Multi-Input Multi-Output) communication is attracting attention as one of the technologies for realizing high-speed wireless communication. This includes a plurality of antenna elements on both the transmitter side and the receiver side, and realizes a spatially multiplexed transmission path (hereinafter also referred to as “MIMO channel”), thereby expanding the transmission capacity and improving the communication speed. It is a technology that achieves improvement. Since MIMO communication uses spatial multiplexing, frequency utilization efficiency is good.

  In the MIMO communication method, transmission data is distributed and transmitted to a plurality of antennas in a transmitter, transmitted using a plurality of virtual MIMO channels, and received data is processed by signal processing from signals received by the plurality of antennas in a receiver. This is a communication method that utilizes the characteristics of the channel, and is different from a simple transmission / reception adaptive array.

  FIG. 4 conceptually shows the MIMO communication system. As shown in the figure, each transceiver is equipped with a plurality of antennas. On the transmission side, a plurality of transmission data are space / time code multiplexed and distributed to M antennas and sent to a plurality of MIMO channels. On the reception side, received signals received by N antennas via the channels. Can be received in space / time. The channel model in this case is composed of a radio wave environment (transfer function) around the transmitter, a channel space structure (transfer function), and a radio wave environment (transfer function) around the receiver. When signals transmitted from each antenna are multiplexed, crosstalk occurs, but each signal multiplexed by signal processing on the receiving side can be correctly extracted without crosstalk.

  Various schemes have been proposed as a configuration method for MIMO transmission. However, whether to exchange channel information between transmission and reception according to the antenna configuration is a major issue in implementation.

  In order to exchange channel information, it is easy to transmit known information (preamble information) only from the transmission side to the reception side. In this case, the transmitter and receiver perform spatial multiplexing transmission independently of each other. This is called an open-loop type MIMO transmission system. Further, as a developed form of this method, there is a closed-loop type MIMO transmission system that creates an ideal spatial orthogonal channel between transmission and reception by feeding back preamble information from the reception side to the transmission side.

  As an open-loop type MIMO transmission system, for example, a V-BLAST (Vertical Bell Laboratories Layered Space Time) system can be cited (for example, see Patent Document 1). On the transmission side, the antenna weight coefficient matrix is not particularly given, and signals are simply multiplexed and transmitted for each antenna. In other words, any feedback procedure for obtaining the antenna weighting coefficient matrix is omitted. The transmitter inserts a training signal for channel estimation on the receiver side, for example, for each antenna in a time division manner before transmitting the multiplexed signal. On the other hand, in the receiver, the channel estimation unit uses the training signal to perform channel estimation, and calculates a channel information matrix H corresponding to each antenna pair. Then, by skillfully combining zero-forcing and canceling, the SN ratio is improved by utilizing the degree of freedom of the antenna generated by canceling, and the decoding accuracy is increased.

  Also, as one of the ideal forms of closed-loop type MIMO transmission, there is known an SVD-MIMO scheme that uses singular value decomposition (SVD) of a propagation path function (for example, non-patent literature). 5).

FIG. 5 conceptually shows the SVD-MIMO transmission system. In SVD-MIMO transmission, a numerical matrix having channel information corresponding to each antenna pair as an element, that is, a channel information matrix H, is singularly decomposed to obtain UDV ^H and V is given as an antenna weighting coefficient matrix on the transmission side, and reception is performed. U ^H is given as the antenna weighting coefficient matrix on the side. Thus, each MIMO channel is represented as a diagonal matrix D having the square root of each singular value λ _{i as} a diagonal element, and signals can be multiplexed and transmitted without any crosstalk. In this case, a plurality of logically independent transmission paths that are spatially divided, that is, spatially orthogonally multiplexed, can be realized on both the transmitter side and the receiver side.

  According to the SVD-MIMO transmission method, the maximum communication capacity can theoretically be achieved. For example, if the transceiver has two antennas, the maximum transmission capacity can be doubled.

Here, the mechanism of the SVD-MIMO transmission scheme will be described in detail. If the number of antennas of the transmitter is M, the transmission signal x is represented by an M × 1 vector, and if the number of antennas of the receiver is N, the received signal y is represented by an N × 1 vector. In this case, the channel characteristic is expressed as an N × M numerical matrix, that is, a channel matrix H. The element h _ij of the channel matrix H is a transfer function from the jth transmit antenna to the ith receive antenna. The received signal vector y is expressed by multiplying the transmission signal vector by the channel information matrix and further adding the noise vector n as shown in the following equation (1).

  As described above, when the channel information matrix H is subjected to singular value decomposition, the following equation (2) is obtained.

  Here, the antenna weight coefficient matrix V on the transmission side and the antenna weight matrix U on the reception side are unitary matrices that satisfy the following expressions (3) and (4), respectively.

That is, the antenna weight matrix U ^H on the receiving side is arranged with the normalized eigenvectors of HH ^H , and the antenna weight matrix V on the transmitting side is arranged with the normalized eigenvectors of H ^H H arranged. Further, D is a diagonal matrix, and has the square root of the singular value λ _i of H ^H H or HH ^{H as} a diagonal component. The size is a small number out of the number M of transmitting antennas and the number N of receiving antennas, is a square matrix having a size of min (M, N), and is a diagonal matrix.

In the above description, the singular value decomposition using real numbers has been described. U and V are matrices composed of eigenvectors. However, even when an operation that normalizes the eigenvector to 1 is performed, that is, when normalization is performed, there is no single eigenvector, and there are an infinite number of eigenvectors with different phases. Exists. Depending on the phase relationship between U and V, the above equation (2) may not hold. That is, U and V are correct, but the phase is arbitrarily rotated. In order to make the phases completely coincide with each other, V is obtained as an eigenvector of H ^H H as usual, and U is obtained by multiplying both sides of the above equation (2) from the right by the following equation. .

On the transmitting side, weighting is performed using the antenna weighting coefficient matrix V, and on the receiving side, when weighting is performed using the antenna weighting coefficient matrix U ^H , U and V are unitary matrices (U is N × min ( M, N) and V are M × min (M, N)), as shown in the following equation.

  Here, the received signal y and the transmitted signal x are (min (M, N) × 1) vectors, not vectors determined by the number of transmitting antennas and receiving antennas.

  Since D is a diagonal matrix, each transmission signal can be received without crosstalk. Since the amplitude of each independent MIMO channel is proportional to the square root of the singular value λ, the power of each MIMO channel is proportional to λ.

Since the noise component n is also an eigenvector whose norm is normalized to 1 in the U column, U ^H n does not change its noise power. As the size, U ^H n is a (min (M, N)) vector, which is the same size as y and x.

  Thus, in SVD-MIMO transmission, it is possible to obtain a plurality of logically independent MIMO channels having no crosstalk while having the same frequency and the same time. That is, it is possible to transmit a plurality of data by wireless communication using the same frequency at the same time, and an improvement in transmission speed can be realized.

Note that the number of MIMO channels obtained in the SVD-MIMO communication system generally corresponds to the smaller one of the number M of transmission antennas and the number N of reception antennas, min [M, n]. The antenna weighting coefficient matrix V on the transmission side is constituted by a transmit vector v _i for the number of MIMO channel _{(V = [v 1, v} 2, ..., v min [M, N]). Further, the number of elements of each transmission vector v _i is the number M of transmission antennas.

  In general, in a closed-loop MIMO scheme typified by SVD-MIMO, an optimal antenna weighting factor can be calculated on the transmitter side in consideration of transmission path information. Furthermore, it is said that more ideal information transmission can be realized by optimizing the coding rate and modulation scheme given to the bit stream of each transmitting antenna.

On the other hand, in order to introduce the closed loop type MIMO system as an actual system, there is a problem that, when the channel fluctuation is large due to the movement of the transceiver, the frequency of feedback from the receiving side to the transmitting side is required. In the SVD-MIMO communication system, it is not easy to perform singular value decomposition in real time, and a setup procedure is required in which the derived V or U ^H is transmitted to the other party in advance.

  Information amount of transmitting side antenna coefficient matrix V taking IEEE 802.11a, which is one of the LAN systems to which SVD-MIMO transmission is applied, that is, an OFDM (Orthogonal Frequency Division Multiplexing) communication system of 5 GHz as an example Let's consider about. If the number of transmitting / receiving antenna elements is three, the antenna coefficient matrix V on the transmission side is a 3 × 3 matrix, and the number of elements is nine. Assume that each element is represented by a real number and a complex number with 10-bit precision, and if it is required for 52 carriers, 9360 bits (= 9 (number of elements of matrix) × 2 (real part, imaginary part of complex number) × 10 (Bit) × 52 (number of OFDM subcarriers)) must be fed back from the receiver to the transmitter.

  Here, points that must be considered when configuring an actual SVD-MIMO transmission / reception system will be described.

In the basic form of the SVD-MIMO transmission method, the receiver performs singular value decomposition on the acquired channel matrix H to obtain a reception weight vector U ^H and a transmission weight vector V used by the transmitter. Is fed back to the transmitter. The transmitter uses this V as a transmission weight.

  However, since the amount of information of the transmission weight matrix V fed back to the transmitter side is large, when the V information is thinned out and transmitted, the orthogonal state between the MIMO channels is different due to an error from the true V information. It breaks and crosstalk occurs.

  Therefore, normally, after the transmission weight matrix V acquired on the receiver side is fed back to the transmitter side, the transmitter weights and transmits the reference signal using the matrix V, and the receiver side again determines the channel matrix. get. If the channel matrix is H, the receiver can obtain a channel matrix of HV from the reference signal weighted with V and transmitted.

On the receiver side, an inverse matrix of this HV is obtained and used as a receiving weight. Since H = UDV ^H , HV is as follows.

This is done by using the same U ^H as the normal SVD-MIMO for reception weight, and then multiplying the stream of each separated MIMO channel by a constant obtained from each diagonal element λ _{i of the} diagonal matrix D. is there.

  The configuration in which the matrix V is used as the transmission weight on the transmission side and the reception weight is used on the inverse side of the HV on the receiver side is the same as the performance of normal SVD-MIMO, There is no discrepancy between V on the receiver side and the receiver side. Therefore, such a configuration can be employed in practice.

Japanese Patent Laid-Open No. 10-84324 International Standard ISO / IEC 8802-11: 1999 (E) ANSI / IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layers (PH) ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part1: BasicControl ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2: Radio Link Control (LC) Supplement to IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5GHZ Band http: // radio3. ee. uec. ac. jp / MIMO (IEICE_TS). pdf (as of October 24, 2003)

  As described above, in the SVD-MIMO communication system, it is possible to obtain a plurality of independent logical earth pipes, that is, MIMO channels, that do not interfere (do not crosstalk) while having the same frequency and the same time. That is, a plurality of data can be transmitted by wireless communication using the same frequency at the same time. In this way, an improvement in transmission speed can be realized.

In the SVD-MIMO communication system, weighting is performed using the antenna weighting coefficient matrix V on the transmitting side, and weighting is performed using the antenna weighting coefficient matrix U ^H on the receiving side. It is expressed as the above formula (7). Since D is a diagonal matrix, each transmission signal can be received without crosstalk. Since the amplitude of each independent MIMO channel is proportional to the square root of the singular value λ, the magnitude of the power of each MIMO channel is proportional to the singular value λ.

  In this way, a plurality of MIMO channels having a quality proportional to the magnitude of the singular value λ can be formed. However, there is a problem of how power is distributed to these channels and what modulation scheme is allocated. .

If the power is equally distributed to the transmitting antennas on the transmitter side, the power ratio of each independent MIMO channel is the same, but by optimizing the power distribution to each MIMO channel, the power is evenly distributed to each MIMO channel. It is possible to obtain a larger communication capacity than the case of distribution. Specifically, by applying the water injection theorem, it is possible to increase the communication capacity of the entire system by allocating a large transmission power to a MIMO channel with good communication quality (ie, a large singular value λ _i ). .

  However, in reality, the number of modulation schemes that can be used in a wireless communication system is determined, so even if the MIMO channel with the highest quality is partially formed, the quality of the other MIMO channels is poor. If it exists, the system as a whole is meaningless. In other words, it is important to prepare a plurality of MIMO channels having different quality that can be allocated to a limited modulation mode.

  For example, in the case of a system that uses a modulation scheme capable of transmitting the maximum information by 64QAM3 / 4, the quality of the MIMO channel can be ensured if the quality that the bit error rate is considered to be sufficiently small can be secured with this modulation scheme. It doesn't make sense to get better than that.

  Here, consider the case where SVD-MIMO is applied to the OFDM modulation scheme in the IEEE 802.11a physical layer.

In IEEE 802.11a, singular value decomposition is performed for each of 52 subcarriers. In this case, the transmission weight vector V and the reception weight vector U ^H are acquired for each subcarrier. That is, by acquiring the channel matrix H itself after the FFT on the receiver side, H can be acquired for every 52 subcarriers. By performing singular value decomposition on the 52 channel matrices H, 52 transmission weights and reception weights can be obtained.

Here, the singular value decomposition is decomposed as H = UDV ^H. D is a diagonal matrix whose diagonal component is a singular value λ. The order of the singular values is changed in order from the largest to the smallest (normal singular value decomposition is in descending order). Each element vector of U and V is an eigenvector corresponding to this singular value. When the order of singular values is changed, the order of the eigenvectors is also changed. That is, the singular value decomposition includes an operation of changing the order of the singular values.

In SVD-MIMO, a plurality of independent MIMO channels that do not interfere with each other can be created, but the communication quality differs for each MIMO channel. For example, in the case of a 2 × 2 MIMO communication system, there are two independent MIMO channels. In this case, singular value decomposition is performed on ^H to obtain H = UDV ^H , and √λ ₁ and √λ ₂ are obtained as singular values.

Assuming that λ ₁ > λ ₂ , assuming that the average signal-to-noise ratio of each MIMO channel is S / N, a MIMO channel of λ ₁ S / N and a MIMO channel of λ ₂ S / N are created. That is, a high quality MIMO channel and a low MIMO channel. It can be said that the quality of each MIMO channel differs according to the square of the singular value.

  Here, the OFDM modulation scheme will be described. For each subcarrier, the order of singular values is rearranged by singular value decomposition. For each subcarrier, one MIMO channel 1 is always high in quality, and the other MIMO channel 2 is inferior in communication quality. Before performing Viterbi decoding, data of MIMO channel 1 of each subcarrier is collected into one integrated MIMO channel 1, and data of MIMO channel 2 is collected into one integrated MIMO channel 2, respectively. Is subjected to Viterbi decoding. That is, decoding is performed on two MIMO channels with good quality and those with poor quality. On the transmission side, a high modulation scheme (16QAM) is assigned to this high-quality MIMO channel, and a low modulation scheme (BPSK) is assigned to the low-quality MIMO channel.

  Actually, there are eight types of modulation schemes prepared by IEEE802.11a, such as BPSK1 / 2, BPSK3 / 4, QPSK1 / 2, QPSK3 / 4, 16QAM1 / 2, 16QAM3 / 4, 64QAM2 / 3, and 64QSM3 / 4. It ’s just not enough. For this reason, when there is a very large difference in quality between the two MIMO channels, a corresponding modulation scheme is not prepared, and a situation in which high communication quality is meaningless occurs. For example, QPSK3 / 4 may be used for a low-quality MIMO channel, but in reality, only a QQ3 / 4 is available for a high-quality MIMO channel even though it has a quality sufficient to send 256 QAM. No more than assigning 64QAM3 / 4. This is not to say that the transmission efficiency is used completely efficiently.

Therefore, in order to adjust it, it is assumed that the power distributed to the two MIMO channels is changed. MIMO Channel 1 singular value lambda ₁ and singular value lambda ₂ of the MIMO channel 2 respectively, lambda ₁ = 1.0, in the case of lambda ₁ = 0.1, in the two MIMO channel at the same level, MIMO The power should be allocated to channel 1 by 0.1 and MIMO channel 2 by 0.9 (the total power is set to 1). In this case, λ ₁ × power 1 = 1.0 × 0.1 = 0.1 and λ ₂ × power 2 = 0.1 × 0.9 = 0.09. However, both have gains of 0.1 or less.

  The present invention has been made in view of the above-described technical problems, and its main purpose is to expand transmission capacity by performing MIMO communication in which a plurality of logical channels are formed using spatial multiplexing. An object of the present invention is to provide an excellent wireless communication system, wireless communication apparatus, wireless communication method, and computer program that can be performed.

  It is a further object of the present invention to provide power distribution and modulation schemes for each MIMO channel in closed-loop type MIMO transmission using singular value decomposition (SVD) of a channel matrix having channels corresponding to each antenna pair for transmission and reception as elements. It is an object of the present invention to provide an excellent wireless communication system, wireless communication apparatus and wireless communication method, and computer program that can obtain a larger communication capacity in the entire system by optimizing the allocation.

  A further object of the present invention is to increase the communication capacity of the entire system by reducing the difference in quality of each MIMO channel in MIMO communication in which a plurality of logical channels are formed using spatial multiplexing. It is an object to provide an excellent wireless communication system, wireless communication apparatus and wireless communication method, and computer program capable of performing a communication operation with high transmission efficiency.

The present invention has been made in view of the above problems, and a first aspect of the present invention is a wireless transmission of data using a multicarrier modulation scheme using a plurality of spatially multiplexed communication channels between a transmitter and a receiver. A communication system,
Switch the combination of transmission weight and reception weight assigned to each communication channel for each subcarrier to adjust the communication quality of each communication channel.
This is a wireless communication system.

  However, “system” here refers to a logical collection of a plurality of devices (or functional modules that realize specific functions), and each device or functional module is in a single housing. It does not matter whether or not.

  In the wireless communication system according to the present invention, for example, a MIMO communication method is adopted, and a transmission capacity can be increased by using a plurality of spatially multiplexed transmission paths, that is, MIMO channels, so that the communication speed can be improved. In this case, each of the transmitter and the receiver includes a plurality of antennas, the transmitter distributes transmission data to a plurality of streams and performs weighted transmission from each transmission antenna, and the receiver weights the streams at each reception antenna. Receive.

  In the radio communication system according to the present invention, a closed loop MIMO communication system represented by SVD-MIMO transmission can be adopted. In this case, the transmitter obtains an optimal transmission antenna weighting factor based on feedback information from the receiver.

  Here, in a wireless communication system having a plurality of MIMO channels having a quality proportional to the magnitude of the singular value λ, in order to obtain a larger communication capacity, power distribution to each MIMO channel and modulation scheme allocation are performed. There's a problem.

  For example, by applying the water injection theorem, it is possible to increase the communication capacity of the entire system by allocating a large transmission power to a MIMO channel with good communication quality.

  However, in reality, since the number of modulation schemes that can be used in a wireless communication system is determined, it is meaningless for the entire system to form a MIMO channel with an extremely good quality.

  On the other hand, in the present invention, by switching the combination of transmission weight and reception weight assigned to each communication channel for each subcarrier, the difference in communication quality of each communication channel after integrating the subcarriers is reduced. Can be adjusted. Then, an appropriate modulation scheme is assigned to each communication channel according to the communication quality obtained by the combination of the assigned transmission weight and reception weight.

  That is, in the present invention, the difference in quality between MIMO channels is reduced by adjusting the order of allocation to each MIMO channel for each subcarrier used in a multicarrier modulation scheme such as OFDM. Then, by suppressing the difference in the quality of the MIMO channel within the quality range corresponding to the available modulation scheme, the communication capacity of the entire system is increased, and more efficient data communication is realized. This is more efficient than simply adjusting the distribution of transmit power to each MIMO channel.

In addition, the second aspect of the present invention is described in a computer-readable format so that processing for data transmission using a plurality of spatially multiplexed communication channels is performed on a computer system. A computer program,
A communication quality acquisition step of acquiring the communication quality of each communication channel;
Based on the communication quality of each communication channel, a communication quality adjustment step of switching the combination of transmission weight and reception weight assigned to each communication channel for each subcarrier and adjusting the communication quality of each communication channel;
A computer program characterized by comprising:

  The computer program according to the second aspect of the present invention defines a computer program described in a computer-readable format so as to realize predetermined processing on a computer system. In other words, by installing the computer program according to the second aspect of the present invention in the computer system, a cooperative action is exhibited on the computer system, and it operates as a communication device. By activating a plurality of such communication devices to construct a wireless network, the same operational effects as the wireless communication system according to the first aspect of the present invention can be obtained.

  According to the present invention, an excellent radio communication system, radio communication apparatus, and radio communication method capable of expanding transmission capacity by performing MIMO communication in which a plurality of logical channels are formed using spatial multiplexing. As well as computer programs.

  In addition, according to the present invention, in closed-loop type MIMO transmission using singular value decomposition (SVD) of a channel matrix whose elements are channels corresponding to transmission / reception antenna pairs, power distribution and modulation to each MIMO channel are performed. It is possible to provide an excellent wireless communication system, wireless communication apparatus and wireless communication method, and computer program capable of optimizing system assignment and obtaining a larger communication capacity in the entire system.

  As a feature of SVD-MIMO communication, it is easy to obtain a high-quality MIMO channel and a low-quality MIMO channel. According to the present invention, when a MIMO channel with an extremely high quality can be formed and there is no modulation scheme corresponding to the MIMO channel, the singular value λ of each subcarrier is replaced, thereby reducing the quality difference between the MIMO channels, The difference in quality of each MIMO channel can be suppressed within the quality range corresponding to the modulation schemes available in FIG. Thus, efficient communication can be performed.

  Other objects, features, and advantages of the present invention will become apparent from more detailed description based on embodiments of the present invention described later and the accompanying drawings.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  The present invention relates to a MIMO communication system in which a transmitter having a plurality of antennas and a receiver having a plurality of antennas are paired and communicated by spatially multiplexing signals. In this case, it is possible to increase the transmission capacity by using a plurality of spatially multiplexed MIMO channels and improve the communication speed.

Here, in a wireless communication system equipped with a plurality of MIMO channels having a quality proportional to the magnitude of the singular value λ, in order to obtain a larger communication capacity, power distribution to each substream and modulation scheme allocation are performed. There's a problem. For example, transmission efficiency can be improved by determining transmission power distribution according to communication quality. However, in reality, since the number of modes of modulation schemes that can be used in a wireless communication system is determined, it is meaningless for the entire system to form a substream with extremely good quality.

  Therefore, in the present invention, by adjusting the combination of transmission weight and reception weight assigned to each communication channel for each subcarrier, adjustment is made so that the difference in communication quality of each communication channel after subcarrier integration is reduced. be able to. An appropriate modulation scheme can be assigned to each communication channel in accordance with the communication quality obtained by the combination of the assigned transmission weight and reception weight.

  FIG. 1 shows a configuration of a wireless communication apparatus according to an embodiment of the present invention.

  The wireless communication apparatus shown in the figure includes two transmission / reception antennas 11a and 11b, and can perform data transmission by the SVD-MIMO method. That is, at the time of transmission, a transmission antenna weighting coefficient is given to each transmission signal to be multiplexed, space / time code is performed, and the two antennas 11a and 11b are distributed and transmitted to the channel. Received antenna weights are given to the multiplexed signals received by the antennas 11a and 11b, and space / time decoding is performed to obtain received data. However, the gist of the present invention is not limited to two antennas, and may be three or more.

  Each transmission / reception antenna 11a and 11b is connected in parallel with a transmission system and a reception system via switches 12a and 12b, respectively, and wirelessly transmits signals to other wireless communication devices on a predetermined frequency channel, or Collects signals sent from other wireless communication devices. However, the switches 12a and 12b connect the transmission / reception antennas 11a and 11b exclusively to one of the transmission system and the reception system, and cannot perform transmission and reception in parallel.

  Each transmission system includes a modulation coding unit 21, a transmission weight multiplication unit 22, an IFFT 23, a preamble / reference adding unit 24, a D / A converter 25, and a transmission analog processing unit 26.

  The modulation encoding unit 21 encodes transmission data transmitted from an upper layer of the communication protocol with an error correction code, and maps a transmission signal on a signal space by a predetermined modulation method such as BPSK, QPSK, or 16QAM. . At this point, a known data sequence may be inserted as a pilot symbol into the modulation symbol sequence according to the pilot symbol insertion pattern and timing. A pilot signal having a known pattern is inserted for each subcarrier or at intervals of several subcarriers.

The transmission weight multiplier 22 multiplies the encoded transmission signal by the transmission weight matrix V to obtain a plurality of substreams by spatial multiplexing.

The transmission weight matrix V is constructed based on feedback information sent from the communication partner, and is set in the transmission weight multiplier 22. This transmission weight matrix V uses eigenvectors corresponding to singular values as element vectors, but the channel characteristic acquisition and vector rearrangement unit 37 rearranges the transmission weight vectors as appropriate. Such rearrangement of transmission vectors corresponds to an operation of switching a combination of transmission weights and reception weights assigned to each substream , and thereby the communication quality of each substream can be adjusted. For example, it is possible to adjust so as to suppress a difference in quality of substreams within a quality range corresponding to available modulation schemes. Details of the operation of the channel characteristic acquisition and vector rearrangement unit 37 will be described later.

  In IFFT 23, the modulated serial signal is converted into parallel data corresponding to the number of parallel carriers in accordance with the number of parallel carriers and timing, and then subjected to inverse Fourier transform for the FFT size according to a predetermined FFT size and timing. Do. Here, in order to remove intersymbol interference, guard interval sections may be provided before and after one OFDM symbol. The time width of the guard interval is determined by the state of the propagation path, that is, the maximum delay time of the delayed wave that affects the demodulation. Then, the signal is converted into a serial signal, converted into a signal on the time axis while maintaining the orthogonality of each carrier on the frequency axis, and used as a transmission signal.

  The preamble / reference adding unit 24 adds a preamble signal or a reference signal to the head of a transmission signal such as an RTS, CTS, or DATA packet. The reference signal is weighted by a transmission weight matrix V given by the channel acquisition and vector rearrangement unit 37.

  This transmission signal is converted into an analog baseband signal by the D / A converter 25, further up-converted to an RF frequency band by the transmission analog processing unit 26, and then transmitted from the antenna 11 to each MIMO channel.

  On the other hand, each reception system includes a reception analog processing unit 31, an A / D converter 32, a synchronization acquisition unit 33, an FFT 34, a reception weight multiplication unit 35, a demodulation decoder 36, a channel characteristic acquisition and The vector rearrangement unit 37 is configured.

  The signal received from the antenna 11 is down-converted from a radio signal in the RF frequency band to a baseband signal in the IF frequency band by the reception analog processing unit 31 and converted into a digital signal by the A / D converter 32.

  Next, according to the synchronization timing detected by the synchronization acquisition unit 33, the received signal as serial data is converted into parallel data and summarized (here, signals for one OFDM symbol including up to the guard interval are collected), A signal corresponding to the effective symbol length is Fourier-transformed by the FFT 34 and a signal of each subcarrier is taken out to convert a time-axis signal into a frequency-axis signal.

The channel characteristic acquisition and vector rearrangement unit 37 first obtains a channel matrix H using a reference signal to which a weight is assigned for each signal multiplexed and transmitted by the communication partner. Furthermore, this channel matrix H can be decomposed into singular values to obtain a transmission weight matrix V, a reception weight matrix U ^H, and a diagonal matrix D. When a reference signal is sent from a communication partner at a predetermined interval, the channel characteristic acquisition and vector rearrangement unit 37 updates to a new channel matrix H each time and performs singular value decomposition.

The reception weight matrix U ^H obtained by singular value decomposition of the channel matrix is set in the reception weight multiplication unit 35 of the own apparatus, and the transmission weight matrix V is fed back to the communication partner. However, instead of U ^H , D ⁻ U ^H as an inverse matrix of HV may be used as the receiving weight matrix (see the above-mentioned and equation (8)). Further, the obtained transmission weight matrix V is given to the transmission weight multiplier 22.

The reception weight matrix U uses eigenvectors corresponding to singular values as element vectors. In the present embodiment, the channel characteristic acquisition and vector rearrangement unit 37 appropriately rearranges the reception weight vector U. Such rearrangement of reception vectors corresponds to an operation of switching the combination of transmission weights and reception weights assigned to each substream , and thereby the communication quality of each substream can be adjusted. For example, it is possible to adjust so as to suppress a difference in quality of substreams within a quality range corresponding to available modulation schemes. Details of the operation of the channel characteristic acquisition and vector rearrangement unit 37 will be described later.

The reception weight multiplication unit 35 spatially separates the spatially multiplexed reception signal by multiplying the reception signal by the reception weight matrix U ^H or D ⁻ U ^H obtained by singular value decomposition of the channel matrix H. .

  The weighted received signal is further demodulated by a demodulator / decoder 36 in accordance with a predetermined method such as BPSK, QPSK, 16QAM, etc., and error-corrected and decoded to become received data. Passed to higher layers.

The channel characteristic acquisition and vector rearrangement unit 37 switches the combination of transmission weights and reception weights assigned to each substream for each subcarrier by switching the order of eigenvectors that are element vectors of the matrices V and U, and for each substream. Adjust the communication quality. Adjustment is performed so as to suppress a difference in quality of substreams within a quality range corresponding to an available modulation scheme.

  FIG. 2 schematically shows a functional configuration of the channel characteristic acquisition and vector rearrangement unit 37. As illustrated, the channel characteristic acquisition and vector rearrangement unit 37 includes a channel matrix acquisition unit, a singular value decomposition unit, and a vector rearrangement unit.

  The channel matrix acquisition unit obtains a channel matrix H using a reference signal weighted for each signal multiplexed and transmitted by the communication partner.

The singular value decomposition unit performs singular value decomposition on the obtained channel matrix H to obtain a transmission weight matrix V, a reception weight matrix U ^H, and a diagonal matrix D. The obtained reception weight matrix U ^H is set in the reception weight multiplication unit 35 of the own apparatus, and the transmission weight matrix V is fed back to the communication partner. However, instead of U ^H , D ⁻ U ^H as an inverse matrix of HV may be used as the receiving weight matrix (see the above-mentioned and equation (8)).

The matrices V and U use eigenvectors corresponding to singular values as element vectors, and the vector rearrangement unit rearranges these element vectors. This sort of such element vector corresponds to an operation of switching the combinations of transmit weights and reception weights assigned to each sub-stream, whereby it is possible to adjust the communication quality with each sub-stream. For example, it is possible to adjust so as to suppress a difference in quality between substreams within a quality range corresponding to an available modulation scheme.

  Here, as in the case described above, a case where SVD-MIMO is applied to the OFDM modulation scheme in the IEEE 802.11a physical layer will be considered as an example.

If the number of subcarriers is two, likewise, lambda ₁ is 1.0, if the lambda ₂ is 0.1, the sub-carrier 1, a singular value lambda ₁ to the sub-streams 1, singular values substream 2 Allocate λ ₂ respectively. On the other hand, the subcarrier 2, lambda ₂ to the sub-streams 1, allocates lambda ₁ to substream 2 (see FIG. 3).

In this way, when subcarriers are combined, in substream 1, λ ₁ ( substream 1) = 1.1 / 2 = 0.55, λ ₂ ( substream 2) = 1.1 / 2 = 0 .55. Therefore, when power is allocated by 0.5, a gain of 0.275 is obtained. This is simply because the two substreams are of higher quality than when they are adjusted with power to have the same quality substreams , and it is not necessary to use a modulation scheme that requires high accuracy.

  Hereinafter, an operation procedure for performing MIMO communication in the wireless communication system according to the present embodiment will be described.

(Step 1)
Prior to packet communication, a reference signal is transmitted from the transmitter. The reference signal is transmitted from each antenna in a time division manner so that the two antennas are transmitted at different times.

(Step 2)
The receiver receives a reference signal, obtains it after performing a FFT on the transfer function using the reference signal, and obtains a channel matrix H in which the transfer function between the transmission antenna and the reception antenna is a matrix. The channel matrix H in this case is a 2 × 2 matrix, and 52 subcarriers can be acquired. Acquire from H (1) to H (52). Here, i is a subcarrier number from i = 1 to 52, and H (i) is a channel matrix of the i-th subcarrier.

(Step 3)
In the receiver, the acquired H (i) is subjected to singular value decomposition, and U (i) D (i) V (i) ^H is acquired. The diagonal components of D (i) are rearranged in descending order of singular values. This is a function of normal singular value decomposition.

(Step 4)
For each subcarrier , it is determined whether to assign a small λ from a large λ to each substream . Since the matrices U and 5 are composed of a set of eigenvectors corresponding to the eigenvector λ, if the order of λ is changed for each subcarrier, the order of the eigenvectors that are the elements of the matrix 5 and U is correspondingly changed. Therefore, an operation for switching the communication quality of the substream is performed.

(Step 5)
A transmission weight vector V (however, rearranged corresponding to the order of singular values λ for each subcarrier) or a reference signal weighted by V is transmitted to the transmission side.

(Step 6)
The transmitter transmits a packet using the received V as a transmission weight vector.

(Step 7)
In the receiver, the received packet is decoded using U ^H (however, rearranged corresponding to the order of λ for each subcarrier) as a reception weight vector.

  The present invention has been described in detail above with reference to specific embodiments. However, it is obvious that those skilled in the art can make modifications and substitutions of the embodiment without departing from the gist of the present invention.

  The present invention can be applied to various wireless communication systems that perform data transmission by spatial multiplexing, and the application range is not limited to a multiplex transmission system that is spatially divided, that is, spatially orthogonal, such as the SVD-MIMO system. In addition, the present invention can be suitably applied to other types of wireless communication systems in which either the transmission side or the reception side performs spatial multiplexing and performs weighted transmission / reception based on a channel matrix. it can.

  For example, the present invention can also be applied to a V-BLAST scheme that uses only reception weights and allocates different channels to different transmission antennas. In the MIMO communication system represented by the V-BLAST system, the inverse matrix of the channel matrix H is used as a reception weight, and no weight is used on the transmitter side.

  In short, the present invention has been disclosed in the form of exemplification, and the description of the present specification should not be interpreted in a limited manner. In order to determine the gist of the present invention, the claims section described at the beginning should be considered.

FIG. 1 is a diagram illustrating a configuration of a wireless communication apparatus according to an embodiment of the present invention. FIG. 2 is a diagram schematically illustrating a functional configuration of the channel characteristic acquisition / transmission weight separation unit 37. FIG. 3 is a diagram showing how the singular value λ is switched for each subcarrier. FIG. 4 is a diagram conceptually showing the MIMO communication system. FIG. 5 is a diagram conceptually showing the SVD-MIMO transmission system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Antenna 12 ... Switch 21 ... Encoder 22 ... Transmission weight multiplication part 23 ... IFFT
24 ... Breamble / reference adding unit 25 ... D / A converter 26 ... Analog processing unit for transmission 31 ... Analog processing unit for reception 32 ... A / D converter 33 ... Synchronization acquisition unit 34 ... FFT
35 ... Receiving weight multiplication unit 36 ... Decoder 37 ... Channel characteristic acquisition and vector rearrangement unit

Claims (7)

A wireless communication system using an SVD-MIMO-OFDM communication system in which a spatial channel for transmitting a substream is assigned to each subcarrier ,
When the spatial channel quality is accumulated in the subcarrier direction, a combination of transmission weights and reception weights assigned to each substream is set for each subcarrier so that the difference in accumulated channel quality is reduced among a plurality of substreams. Switch to adjust the communication quality of each substream,
A wireless communication system. An appropriate modulation scheme is allocated to each substream according to the communication quality obtained by the combination of the allocated transmission weight and reception weight.
The wireless communication system according to claim 1. A wireless communication apparatus for transmitting data using an SVD-MIMO-OFDM communication system in which a spatial channel for transmitting a substream is assigned to each subcarrier.
Receiving means for weighted reception using reception weights;
Transmission means for weighted transmission using transmission weights;
Communication quality acquisition means for acquiring communication quality of each communication channel;
When the spatial channel quality is accumulated in the subcarrier direction, a combination of transmission weights and reception weights assigned to each substream is set for each subcarrier so that the difference in accumulated channel quality is reduced among a plurality of substreams. A communication quality adjusting means for switching and adjusting the communication quality of each substream ;
A wireless communication apparatus comprising: The communication quality acquisition means comprises means for acquiring a channel matrix using a reference signal sent from the transmission side, and singular value decomposition means for singular value decomposition of the acquired channel matrix, the singular value Get the communication quality of each communication channel based on the singular value obtained by decomposition,
The wireless communication apparatus according to claim 3 . Modulation means allocating means for allocating an appropriate modulation scheme for each substream according to the communication quality obtained by the combination of the assigned transmission weight and reception weight;
The wireless communication apparatus according to claim 3 . A wireless communication method for transmitting data using an SVD-MIMO-OFDM communication scheme in which a spatial channel for transmitting a substream is assigned to each subcarrier.
A receiving step for weighted reception using reception weights;
A transmission step of weighted transmission using transmission weights;
A communication quality acquisition step of acquiring the communication quality of each communication channel;
When the spatial channel quality is accumulated in the subcarrier direction, a combination of transmission weights and reception weights assigned to each substream is set for each subcarrier so that the difference in accumulated channel quality is reduced among a plurality of substreams. A communication quality adjustment step of switching and adjusting the communication quality of each substream;
A wireless communication method comprising: A computer program described in a computer-readable format so that processing for data transmission using the SVD-MIMO-OFDM communication system that assigns a spatial channel for transmitting multiple substreams for each subcarrier is executed on a computer. For the computer,
A communication quality acquisition step of acquiring the communication quality of each communication channel;
When the spatial channel quality is accumulated in the subcarrier direction, a combination of transmission weights and reception weights assigned to each substream is set for each subcarrier so that the difference in accumulated channel quality is reduced among a plurality of substreams. A communication quality adjustment step of switching and adjusting the communication quality of each substream;
A computer program for executing

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