KR101184370B1 - Method And Apparatus For Adapting Transmission Mode and Transmission Rate In MIMO System - Google Patents

Abstract

In the present invention, a MIMO system capable of transmitting a plurality of transport streams simultaneously specifies a signal-to-noise ratio (SNR) specific to each transmission mode, thereby adjusting MCS (Modulation and Coding Scheme) for transmission rate adjustment and optimally. A method and apparatus are provided for selecting a transmission mode having a throughput of.

Description

Method and apparatus for adjusting transmission mode and transmission rate in MIO system {Method And Apparatus For Adapting Transmission Mode and Transmission Rate In MIMO System}

The present invention relates to a method and apparatus for adjusting a transmission mode and a transmission rate in a multi-input multi-output (MIMO) system. Specifically, an SNR specialized for each transmission mode in a MIMO system in which a plurality of transport streams can be simultaneously transmitted. The present invention relates to a method and apparatus for specifying a signal to noise ratio to adjust a modulation and coding scheme (MCS) for transmission rate adjustment and to select a transmission mode having an optimal throughput.

In mobile communication, the radio channel may change over time, and thus, the modulation and coding scheme (MCS) is changed based on the feedback signal from the receiving side, compared to continuing the signal transmission at a constant transmission rate. It is common to perform rate adaptation by doing so. Meanwhile, the MIMO technology developed to efficiently increase data throughput enables selection of a transmission mode for diversity gain and a transmission mode for spatial multiplexing, which may also be selected according to radio channel characteristics. . However, when a plurality of streams are simultaneously transmitted in an existing MIMO system, the SNR (Signal to Noise Ratio) or SINR (Signal to Interference / Noise Ratio) for each stream is not considered in selecting MCS. Since it was performed through one metric considered, it was difficult to adjust the transmission rate optimized for the channel situation of each stream. In addition, since the SNR value calculation method for the transmission rate adjustment is different for each transmission mode, it is difficult to select a transmission mode that can have an optimal throughput at the same time as the transmission rate adjustment.

In one embodiment of the present invention for solving the above problems, a method and apparatus based on a technique for selecting an optimized MCS by considering SNR for each stream independently when a plurality of streams are simultaneously transmitted in a MIMO system To provide.

In addition, a method based on a technique for selecting a transmission mode that can have an optimal throughput at the same time as adjusting the transmission rate by adjusting the transmission rate for each transmission mode and comparing each other with the throughput that each transmission mode can have. To provide a device.

In addition, an embodiment of the present invention proposes an improved MCS combination that can maximize the advantages of UEQM even in a MIMO system that does not receive a CSI value, and proposes an improved MCS lookup table.

According to an aspect of the present invention for solving the above problems, a method of adjusting a transmission mode and a transmission rate in a multi-antenna system, the method comprising: performing channel estimation based on a received signal received from a transmitting side; Based on the channel estimate, a first signal-to-noise ratio for the first transmission mode in which the signal is transmitted on one stream, and a second on which N (N is a natural number of two or more) signals are transmitted on the stream. Calculating N second signal-to-noise ratios for the transmission mode; A first modulation and coding scheme (MCS) index and a first throughput corresponding to having the estimated first signal-to-noise ratio, and a second MCS index and a second corresponding to having the estimated N second signal-to-noise ratios Determining a throughput; And transmitting, to the transmitting side, an MCS index corresponding to a throughput having a larger value among the first throughput and the second throughput among the first MCS index and the second MCS index. We propose a transmission rate adjustment method.

In this case, the first MCS index may be any one of the first MCS index combination for the first transmission mode, and the second MCS index may be any one of the second MCS index combination for the second transmission mode. Preferably, the first MCS index combination and the second MCS index combination do not overlap each other.

The second MCS index combination may include an equal MCS index combination for applying the same modulation scheme to the N streams, and a differential MCS index combination for applying different modulation schemes to the N streams. In an embodiment wherein N is 2, the differential MCS index combination is a first combination in which the modulation order of the modulation scheme applied to the first stream is greater than the modulation order of the modulation scheme applied to the second stream, and the first stream. The modulation order of the modulation scheme to be applied to may include a second combination smaller than the modulation order of the modulation scheme to be applied to the second stream.

The differential MCS index combination may include an MCS index combination that applies Binary Phase Shift Keying (BPSK) to any one of the N streams.

Meanwhile, the second MCS index is the calculated N second signal-to-noise ratios in a table in which MCS indexes each having a maximum throughput in each of combinations of N second signal-to-noise ratios are predetermined. It can be selected as the MCS index corresponding to the combination in the case of having.

Further, the second throughput is a table in which predetermined combinations of MCS indexes having maximum throughputs and the maximum throughputs in each of combinations N N signal to noise ratios may have, respectively. It can be selected as the maximum throughput corresponding to the combination in case of having the N second signal to noise ratios.

The first transmission mode may include a mode for transmitting a signal using Space-Time Block Coding (STBC), and the second transmission mode may include a mode for transmitting a signal in a spatial multiplexing (SM) scheme. have.

On the other hand, in another aspect of the present invention for solving the problems described above, in the apparatus for adjusting the transmission mode and transmission rate in a multi-antenna system, a signal is transmitted through one stream from the transmitting side A receiver for receiving signals in one transmission mode or in a second transmission mode in which signals are transmitted over N streams (N being a natural number of two or more); Perform channel estimation based on the signal received by the receiver; Based on the channel estimate, calculate a first signal-to-noise ratio for the first transmission mode, and N second signal-to-noise ratios for the second transmission mode; A first modulation and coding scheme (MCS) index and a first throughput corresponding to the estimated first signal-to-noise ratio, and a second MCS index corresponding to the estimated N second signal-to-noise ratios; and Determine a second throughput; A processor for determining an MCS index corresponding to a throughput having a larger value among the first throughput and the second throughput among the first MCS index and the second MCS index; And a transmitter for transmitting the MCS index determined by the processor to the transmitting side.

The apparatus according to an embodiment in this aspect may further include a memory for storing a table, each pre-determined MCS indexes to have a maximum throughput in each of the combinations that N second signal to noise ratios may have. In this case, the processor may select the second MCS index as an MCS index corresponding to a combination in the case of having the calculated N second signal-to-noise ratios in the table.

In addition, the apparatus according to an embodiment of the present invention predetermines a combination of maximum throughputs in each of combinations that N second signal-to-noise ratios may have, and a combination of MCS indexes to have the maximum throughputs. And a memory for storing a table, wherein the processor sets the second throughput as a maximum throughput corresponding to a combination of the second estimated signal-to-noise ratios in the table. You can choose.

1 is a diagram illustrating a system structure for applying an adaptive modulation and coding (AMC) technique, for example, of an orthogonal frequency division multiplexing (OFDM) system.
2 is a configuration diagram of a general multiple antenna (MIMO) communication system.
3 is a diagram illustrating a receiver structure for easily explaining a system using an Alamouti code.
4 is a diagram schematically illustrating a method of adjusting a transmission mode and a transmission rate according to an embodiment of the present invention.
FIG. 5 is a diagram for explaining an algorithm for generating an optimal MCS index for post-detection SNR value (s) and a mapping table for throughput at this time according to an embodiment of the present invention.
6 and 7 show FER curves for post-detection SNR vectors used for mapping table generation for STBC transmission mode and SM transmission mode in one embodiment of the present invention, respectively.
8 is a diagram for summarizing and explaining a rate adjustment and a transmission mode selection algorithm according to one preferred embodiment of the present invention.
FIG. 9 is a diagram showing a simulation result for comparing the processing rate when the rate adjustment and the transmission mode selection according to one embodiment of the present invention are applied and when one transmission mode is used.
10 and 11 are simulation results for explaining the performance improvement when using the extended MCS lookup table according to one embodiment of the present invention.

Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detailed description, together with the accompanying drawings, is intended to illustrate exemplary embodiments of the invention and is not intended to represent the only embodiments in which the invention may be practiced. The following detailed description includes specific details in order to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details.

In some instances, well-known structures and devices may be omitted or shown in block diagram form centering on the core functions of the structures and devices in order to avoid obscuring the concepts of the present invention. In addition, the same components will be described with the same reference numerals throughout the present specification.

In the following description, a transmission rate may be referred to as a data rate, and a concept that may correspond to a transport block size (TBS) transmitted at a unit time in terms of a physical layer. Refer. In addition, in the following description, the transmitting side and the receiving side may each be a base station or a mobile terminal. Specifically, in the following description, the transmitting side and the receiving side may be an access point (AP) and a station (STA) in an IEEE 802.11 series communication system, and a base station (BS) and a mobile station (MS) in an IEEE 802.16 series communication system. ), Or may be Node-B, UE (User Equipment) in the 3GPP family system. In addition, in the following description, the signal-to-noise ratio means SNR, but it is assumed to include a metric of equal concept such as SINR. In addition, in the present invention, the transmitting side and the receiving side has a device configuration such as a receiver, a transmitter, a processor, etc., the receiver and the transmitter are involved in transmitting and receiving signals with each other, and the processor is configured with hardware or software to perform an algorithm according to the present invention. Assume a module. In some cases, the transmitter and the receiver according to the present invention may store necessary information including a memory.

First, the rate adjustment technique applied to the present invention will be described.

IEEE 802.11a, a Wireless Local Area Network (WLAN) standard before MIMO technology was applied, defines a total of eight PHY modes (MCS index) for transmission, and one MCS index is set to correspond to one data rate. have. Table 1 shows eight MCS Indexes defined in IEEE 802.11a.

In general, the data rate has a property that is proportional to the product of the modulation order * coding rate. Therefore, the MCS index may be represented by a combination of modulation order and coding rate as shown in Table 1 above, or may be represented by a combination of modulation order and TBS (Transport Block Size) as shown in Table 2 below as defined in 3GPP LTE.

MCS Index
Modulation Order
TBS Index
Redundancy Version
0 2 0 0 One 2 One 0 2 2 2 0 3 2 3 0 4 2 4 0 5 2 5 0 6 2 6 0 7 2 7 0 8 2 8 0 9 2 9 0 10 2 10 0 11 4 10 0 12 4 11 0 13 4 12 0 14 4 13 0 15 4 14 0 16 4 15 0 17 4 16 0 18 4 17 0 19 4 18 0 20 4 19 0 21 6 19 0 22 6 20 0 23 6 21 0 24 6 22 0 25 6 23 0 26 6 24 0 27 6 25 0 28 6 26 0 29 reserved One 30 2 31 3

Rate adaptation refers to a technique for selecting and using the most suitable of a predetermined number of MCS indexes. In other words, adjusting the data rate means adjusting the MCS index.

In describing the 802.11a system as an example, two types of rate adjustment techniques have been considered. One is a loss-based rate adaptation technique, and the other is an SNR-based rate adaptation technique. The former is a method of determining the data rate in consideration of the loss information of the packet (packet), a method of lowering the data rate when the packet loss occurs, and increases the data rate when the packet is successfully transmitted. On the other hand, the latter is to measure the SNR value of the forward link to predict the Frame Error Rate (FER) according to this value, and select an appropriate data rate accordingly. Embodiments of the present invention described below are described assuming a case of using an SNR-based rate adjustment technique that can better measure the instantaneous channel change to select a data rate.

1 is a diagram illustrating a system structure for applying an adaptive modulation and coding (AMC) technique, for example, of an orthogonal frequency division multiplexing (OFDM) system.

As shown in FIG. 1, in a typical OFDM system, the transmitter 100 includes an encoder 101, a channel interleaver 102, a mapper 103, an IFFT module 104, and the like, together with an MCS lookup for AMC application. up) table 105, and the like.

Specifically, the encoder 101 is intended to reduce the effects on the channel or the effects of noise through coding that inserts extra bits into the data bits, and the channel interleaver 102 stores the bits coded by the encoder 101. It is intended to disperse intensive errors that can occur in the channel by shuffling (shuffling) bit by bit. In addition, the mapper 103 converts the bit information output by the channel interleaver 102 into a symbol through modulation, and the IFFT module 104 modulates the bit information into an OFDM symbol and transmits it to the channel 300. .

In addition, in the transmitter 100, the MCS lookup table 105 uses feedback information such as an MCS index fed back from the receiver 200, and a modulation rate and a coding rate corresponding to the corresponding MCS index in the MCS lookup table 105. Are used to determine the encoding of the encoder 101 and the mapping operation of the mapper 103, respectively.

Meanwhile, as shown in FIG. 1, the receiving end 200 includes the FFT module 201, the demapper 202, the channel deinterleaver 203, the decoder 204, and the AMC controller 205 for applying AMC. ) May be included.

Specifically, the FFT module 201 performs inverse transformation of the received OFDM symbol received through the channel 300 by the IFFT module 104, and the demapper 202 converts the converted symbol into bit information. do. In addition, the channel deinterleaver 203 serves to change the order of the shuffled bits back to the original bit order. In addition, the channel decoder 204 outputs the estimated data bits.

In addition, in the receiver 200, the AMC controller 205 measures the signal-to-noise ratio (SNR) of the received signal, determines feedback information such as an MCS index to be used for the AMC scheme, and feeds it back to the transmitter 100. do.

Meanwhile, the case where the above-described adaptive modulation and coding scheme is applied to the MIMO system will be described. The concept of the MIMO system is briefly described.

MIMO (Multi-Input Multi-Output) is a method using a plurality of transmission antennas and a plurality of reception antennas, and this method can improve the transmission and reception efficiency of data. That is, by using a plurality of antennas at the transmitting end or the receiving end of the wireless communication system, the capacity can be increased and the performance can be improved. In the following description, MIMO may be referred to as a 'multi-antenna'.

In a multi-antenna technique, it does not rely on a single antenna path to receive one full message. Instead, in multi-antenna technology, data fragments received from multiple antennas are gathered and merged to complete the data. Using multi-antenna technology, it is possible to improve the data rate within a cell area of a specified size or to increase system coverage while ensuring a specific rate of data rate. This technique can also be widely used in mobile communication terminals, repeaters, and the like. According to the multiple antenna technology, it is possible to overcome the transmission limit in the mobile communication according to the prior art, which used a single antenna.

2 is a configuration diagram of a general multiple antenna (MIMO) communication system.

The transmitting end is provided with N _T antennas, and the receiving end is provided with N _R antennas. When a plurality of antennas are used in both the transmitting end and the receiving end, the theoretical channel transmission capacity is increased as compared with the case where a plurality of antennas are used for either the transmitting end or the receiving end. The increase in channel transmission capacity is proportional to the number of antennas. Therefore, the transmission rate is improved and the frequency efficiency is improved. If the maximum transmission rate when using one antenna is R _o , the transmission rate when using multiple antennas can theoretically be increased by multiplying the above rate increase rate R _i by the following R _o .

For example, in a MIMO communication system using four transmit antennas and four receive antennas, it is theoretically possible to obtain a transmission rate four times higher than that of a single antenna system. Since the theoretical capacity increase of such a multi-antenna system was proved in the mid 90s, various techniques for substantially improving the data rate have been actively studied to date, and some of these techniques have already been developed for 3G mobile communication and next generation WLAN. It is reflected in various wireless communication standards.

Multi-antenna technology uses a spatial diversity or transmit diversity scheme that improves transmission reliability by using symbols that have passed through various channel paths, and multiple data symbols simultaneously by using multiple transmit antennas. It can be divided into a spatial multiplexing scheme for transmitting and improving a transmission rate. You can also combine these two approaches properly to get the benefits of each.

Each method will be described in more detail as follows.

First, in the case of the spatial diversity method, there is a space-time block code (STBC) sequence and a space-time trellis code sequence system that simultaneously uses diversity gain and coding gain. In general, the bit error rate improvement performance and the code generation degree of freedom are excellent in the trellis code method, but in terms of arithmetic complexity, a space-time block code can be easily implemented. The spatial diversity gain can be obtained by an amount corresponding to the product of the number of transmit antennas and the number of receive antennas. On the other hand, the "space time coding (STBC) method" can be regarded as the "frequency space coding (SFBC) method" in consideration of the time-domain new frequency domain, and the same coding method may be used as it is.

Secondly, the spatial multiplexing (SM) technique is a method of transmitting different data strings at each transmitting antenna, and at the receiver, mutual interference occurs between data transmitted simultaneously from the transmitter. The receiver removes this interference using an appropriate signal processing technique and receives it. Noise cancellation methods used here include maximum likelihood receiver, ZF receiver, MMSE receiver, D-BLAST, V-BLAST, and so on, especially when the transmitter can know the channel information. Decomposition: SVD) can be used.

Third, there is a combined technique of spatial diversity and spatial multiplexing as described above. If only the spatial diversity gain is obtained, the performance improvement gain is gradually saturated as the diversity order is increased, and if only the spatial multiplexing gain is taken, transmission reliability is deteriorated in the wireless channel. In order to solve this problem, methods for obtaining both gains have been studied. Among them, there are a space-time block code (Double-STTD) and a space-time BICM (STBICM).

Meanwhile, a method of using an Alamouti code for transmission diversity in the above-described MIMO system will be briefly described.

3 is a diagram illustrating a receiver structure for easily explaining a system using an Alamouti code.

Two kinds of matrix of Alamouti codes are considered.

In the matrices of Equations 2 and 3, each column represents time or frequency, and each row represents an antenna. Specifically, the matrix of Equation 2 is a general formula proposed by Alamouti in his paper, and Equation 3 represents a matrix used in the 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) standard. That is, Equation 3 is a matrix reconstructed such that the signal transmitted through antenna 1 is identical to the signal transmitted through antenna 1 in the Alamouti method in the single input single output (SISO) method in which only one antenna is used.

For reference, in the above equation, each column represents a time when the Alamouti code is used as a type of STBC (Space Time Block Code), and when each column represents a frequency, the Alamouti code represents the SFBC (Space Frequency). Block Code) is used as one type.

Now look at in detail with reference to FIG.

In general, the reception signal in the case where the Alamouti transmission diversity is used can be expressed as follows. Hereinafter, a description will be given on the assumption that STBC in which the columns of Equations 2 and 3 represent time, but the same mathematical modeling is possible in the case of SFBC in which the columns of Equations 2 and 3 represent frequencies. . The signals of the time 1 and the time 2 are represented by y1 and y2 as follows.

In Equation 4, n ₁ and n ₂ denote noise generated by each receiving antenna, s ₁ and s ₂ denote transmission signals at time 1 and time 2, and h ₁ and h ₂ denote transmission channel values for each antenna. Indicates.

The Alamouti-based STBC transmission scheme and the SM transmission scheme for spatial multiplexing are applied to the IEEE 802.11n standard and the 3GPP LTE standard using MIMO technology. However, when the above-described SNR based rate adaptation is applied in a system to which such MIMO transmission schemes are applied, the following problem may occur.

The SNR value changes according to the MIMO transmission technology even under a certain channel condition. The SNR used before the MIMO technology, such as 802.11a, does not contain the change according to the MIMO technology. It does not contain much information. In addition, since the data rate is influenced according to the MIMO transmission technology, that is, SM or STBC, it is necessary to select a transmission scheme suitable for a channel situation unlike the existing SNR-based rate adjustment scheme. Accordingly, an embodiment of the present invention is to provide a method for obtaining an SNR value for each stream reflecting the effects of the MIMO transmission technique, and selecting a data rate and a MIMO transmission mode accordingly.

The following embodiments are described assuming a 2 * 2 MIMO system that performs closed-loop rate adaptation under an open-loop MIMO system in an IEEE 802.11n WLAN for convenience of description. That is, in order to adaptively apply MCS to a transmission signal at the transmission side without receiving CSI (Channel Status Information) from the reception side to perform beamforming on the transmission signal at the transmission side of the MIMO system. Assume a system that feeds back an MCS index from a receiver. In addition, the transmission mode considers the STBC mode and the SM mode according to the IEEE 802.11n standard, and assumes that the STBC uses the Alamouti STBC that operates with two receiving antennas. In the case of SM, a system that detects a stream through MMSE at a receiver without BF (Beam Forming) is considered. However, in the following description, the number of antennas is exemplary and may be extended to a general N (N is a natural number of 2 or more) antenna system. In addition, it can be applied to the same / similar to various systems such as 3GPP LTE as well as IEEE 802.11n system.

The MCS index defined for 2 * 2 MIMO in the IEEE 802.11n standard is as follows.

Specifically, Table 3 shows MCS indexes applicable when one stream is transmitted, Table 4 shows applicable MCS indexes when two streams are transmitted, and Table 5 shows two streams. MCS indices applicable to a differential modulation (UEQM) scheme in which different modulations are applied to a first stream and a second stream are shown. That is, according to the IEEE 802.11n standard, MCS indexes 0-7 can be used when the transmission mode is STBC in a 2 * 2 system, and an MCS index 8-15 and 33-38 can be selected for an SM using two streams. It can be seen that it can be used.

Under this assumption, an SNR-based rate adaptation method according to an embodiment of the present invention will be described.

4 is a diagram schematically illustrating a method of adjusting a transmission mode and a transmission rate according to an embodiment of the present invention.

In the MIMO system, the receiving side may receive a signal from the transmitting side and estimate the channel based on this (S410). In an embodiment of the present invention, the SNR value for the SM mode and the SNR value for the STBC mode are separately calculated using the estimated channel values (S420 and S440), and the MCS index selection for each of them (S430 and 450). Suggest to do In addition, in consideration of the fact that a plurality of streams may be transmitted in the MIMO system, it is proposed that the SNR is separately calculated in consideration of channel status of each stream, which will be referred to as post-detection SNR.

That is, if the post-detection SNR for the SM mode is calculated (S420), an MCS index capable of representing the maximum throughput can be selected based on this (S430). In this case, when two streams are simultaneously transmitted according to the SM mode, the postfixed SNR is separately calculated for each of the first and second streams, and the MCS index selection S430 is applied to each of the first and second streams. It can be selected in consideration of the post-detection SNR values calculated separately for.

Independently, if the post-detection SNR for the STBC mode is defined using the channel estimation value (S440), an MCS index that may represent the maximum throughput may be selected based on this (S450).

When the MCS index for each mode is selected as described above, it is possible to compare which of the maximum throughputs for each mode may have a larger throughput (S460). If the throughput in the SM mode is greater than the throughput in the STBC mode, the receiver may select the SM mode and transmit the MCS index selected for the SM mode to the transmitter (S470). In addition, when the throughput according to the SM mode is less than the throughput according to the STBC mode, the receiver may select the STBC mode and transmit the MCS index selected for the STBC mode to the transmitter (S480). In this embodiment, since the range of MCS indexes that can be selected for the SM mode and the range of MCS indexes that can be selected for the STBC mode are distinguished from each other, when the receiving side feeds back only the MCS indexes to the transmitting side, the transmitting side decides until the transmission mode. Assume that you can.

Based on the algorithm described above, a method for calculating post-detection SNR for each transmission mode will now be described in detail.

The post-detection SNR may be referred to as an SNR value for each stream after MIMO detection, and has a different value according to the MIMO transmission mode as described above. In the case of 2 * 2 MIMO considered in the present embodiment, it is assumed that one stream is transmitted in the STBC transmission mode and up to two streams in the SM transmission mode. In this embodiment, it is proposed to calculate the post-detection SNR for each stream. Accordingly, it is proposed to calculate one post-detection SNR for the STBC mode and two post-detection SNRs for the SM mode as follows. First, for the STBC mode, one post-detection SNR as shown in Equation 5 below can be calculated.

In this case, hij represents a matrix value of row i and column j in the 2 * 2 channel matrix obtained as a result of channel estimation. On the other hand, in the SM mode, two post-detection SNRs as shown in Equations 6 and 7 can be calculated.

In Equations 6 and 7, H _1T ² , H _2T ^2, and H _T may be calculated as follows.

Hereinafter, a method of selecting an MCS index for each transmission mode using the calculated post-detection SNR will be described.

In one embodiment of the present invention, MCS index selection assumes a manner of selecting a predetermined MCS index according to a possible post-detection SNR value or a combination of post-detection SNR values. It is proposed that a predetermined MCS index according to such a possible post-detection SNR value or a combination of post-detection SNR values is predetermined as an MCS index having a maximum throughput according to a possible post-detection SNR value or a combination of post-detection SNR values.

FIG. 5 is a diagram for explaining an algorithm for generating an optimal MCS index for post-detection SNR value (s) and a mapping table for throughput at this time according to an embodiment of the present invention.

)에 대해 각각 최대 처리율을 나타내는 MCS 인덱스(i) 및 이때의 처리율(THR) 조합을 미리 결정하여, 이를 매핑 테이블 형태로 생성하는 것을 제안한다. First, as described above, when N post-detection SNRs for a specific transmission mode (N is a natural number of 2 or more) are calculated, the MCS index and transmission mode can be selected using the calculated postdetection SNR (combination). To this end, in this embodiment, all possible post-detection SNR vectors (

It is proposed to determine in advance the combination of MCS index (i) representing the maximum throughput and the throughput THR at this time, and generate them in the form of a mapping table.

)에 대해 전체 이용 가능한 MCS 인덱스들(i)에 대한 처리율을 산정하고, 이들 중 최대값을 가지는 처리율과 이때의 MCS 인덱스를 결정해 둘 수 있다. 이때, 특정 MCS 인덱스 i에 대한 처리율은 도 5에 도시된 바와 같이 데이터 레이트(R_i)와 후검출 SNR 벡터(

)를 가질 경우 전송 성공율에 대응하는 FER(Frame Error Rate)을 이용하여 산정할 수 있다.All possible postdetection SNR vectors (as shown in FIG. 5)

For the total available MCS indexes (i) can be estimated, and the throughput with the highest value among them and the MCS index at this time can be determined. At this time, the throughput for a particular MCS index i is shown in Figure 5 is the data rate (R _i ) and post-detection SNR vector (

) Can be calculated using the FER (Frame Error Rate) corresponding to the transmission success rate.

)에 따라 FER_i 값이 어떻게 될 것인지를 예측하여야 THR_i을 계산하게 된다. 이는 다음과 같은 시뮬레이션 결과를 통해 결정될 수 있다.However, in the process of calculating the throughput THR _i for a specific MCS index i, the post-detection SNR vector (

We need to predict what the FER _i value will be in order to calculate THR _i . This can be determined through the following simulation results.

6 and 7 show FER curves for post-detection SNR vectors used for mapping table generation for STBC transmission mode and SM transmission mode in one embodiment of the present invention, respectively.

6 and 7 show FER values corresponding to each post-detection SNR vector as a result of performing a bitwise simulation in consideration of the IEEE 802.11n situation in the 2 * 2 MIMO system described above. In FIG. 6 and FIG. 7, the elements of the post-detection SNR vectors are considered to have a value between -3.75 and 33.75 dB in dB units, which can be quantized using the post-detection SNR index as shown in Table 6 below.

Post detection SNR index Ranges (dB) Representative value (dB) One -5.0 to -2.5 -3.75 2 -2.5 to 0.0 -1.25 3 0.0 to 2.5 1.25 4 2.5 to 5.0 3.75 5 5.0 to 7.5 6.25 6 7.5-10.0 8.75 7 10.0 to 12.5 11.25 8 12.5-15.0 13.75 9 15.0-17.5 16.25 10 17.5-20.0 18.75 11 20.0 to 22.5 21.25 12 22.5-25.0 23.75 13 25.0-27.5 26.25 14 27.5 to 30.0 28.75 15 30.0 to 32.5 31.25 16 32.5 to 35.0 33.75

FIG. 6 shows FER curves for each MCS index in the STBC transmission mode. Based on the quantization index of Table 6 and the simulation results of FIG. 6, the MCS index indicating the maximum throughput and the throughput at this time may be determined corresponding to each post-detection SNR index, and Table 7 shows an example thereof.

For example, when the post-detection SNR value calculated as shown in Equation 5 is 11 dB, this corresponds to the post-detection SNR index 7, in which case the MCS according to Table 7 determined in advance by optimizing for each post-detection SNR candidates. Index 2 and throughput 19.4 Mb / s will be selected as the MCS index and throughput combination for STBC mode.

7 illustrates the FER plane as an example of the case of MCS index 15 in the SM transmission mode. However, it is apparent to those skilled in the art that the FER plane can be obtained in the same manner for other MCS indexes that are available for the SM transmission mode although not shown in FIG. 7. Based on the quantization index of Table 6 and the simulation results of the principle as shown in FIG. 7, the MCS index representing the maximum throughput and the throughput at this time may be determined corresponding to each post-detection SNR index combination. An example is shown.

Table 8 shows that the MCS index representing the maximum throughput is determined corresponding to each post-detection SNR index combination. The maximum throughput corresponding to each post-detection SNR index combination may also be represented as a mapping table, which is obviously omitted here.

Thus, using the MCS index and the throughput combination corresponding to each post-detection SNR index corresponding to the STBC mode, and the MCS index and the throughput combination corresponding to each post-detection SNR index combination corresponding to the SM mode, the transmission mode is as follows. Selection and transmission of MCS indexes accordingly can be performed.

8 is a diagram for summarizing and explaining a rate adjustment and a transmission mode selection algorithm according to one preferred embodiment of the present invention.

Basically, the algorithm of FIG. 8 is intended to describe the basic algorithm of FIG. 4 more clearly.

First, in order to perform rate adjustment and transmission mode selection according to the present embodiment, the receiving side may perform channel estimation (S810). Such channel estimation is preferably performed using sounding packet reception as shown in FIG. Through this channel estimation, for example, a 2 * 2 channel matrix H of a 2 * 2 MIMO system is calculated.

Thereafter, the post-detection SNR vector can be calculated for each transmission mode using the channel matrix. In the STBC mode, one post-detection SNR value may be calculated using channel matrix elements as shown in Equation 5 (S820), and two post-detections are performed in the SM mode through the same principle as in Equations 6 to 8 above. The SNR value can be calculated (S840). Of course, the post-detection SNR value calculated according to the number of transmit antennas may be larger than this.

MCS index and throughput combination may be selected using the post-detection SNR vector calculated for each transmission mode (S830 and S850). Specifically, in the STBC mode, the combination of the MCS index and the throughput may be selected according to Table 7 using the post-detection SNR index determined by post-detection SNR quantization as shown in Table 6 (S830). In the SM mode, a combination of the MCS index and the throughput can be selected using the post-detection SNR index determined by the post-detection SNR quantization as shown in Table 6, using Table 8 and the corresponding throughput values. (S850).

By using the combinations selected as described above, the throughput of each transmission mode may be compared in step S860. That is, as illustrated in FIG. 8, it is determined whether or not the throughput selected in the SM mode is greater than the throughput selected in the STBC mode. If the throughput selected in the SM mode is larger, the MCS index selected in the SM mode is transmitted to the transmitting side. If not, in step S880, the MCS index selected in the STBC mode may be transmitted to the transmitter. In this embodiment, since the MCS index range is divided for each transmission mode, the transmitting side can determine the transmission mode through the MCS index fed back by the receiving side, thereby simultaneously adjusting the rate adjustment and the transmission mode selection which can have the maximum throughput. Can be done.

FIG. 9 is a diagram showing a simulation result for comparing the processing rate when the rate adjustment and the transmission mode selection according to one embodiment of the present invention are applied and when one transmission mode is used.

로 식별되는 곡선은 도 8과 관련하여 상술한 실시형태에 따라 레이트 및 전송 모드를 조정하는 알고리즘에 따른 것이며,

로 식별되는 곡선은 SM만을 적용하는 경우를,

로 식별되는 곡선은 STBC만을 적용하는 경우를 나타내었다. Of the three curves shown in FIG. 9

The curve identified by is according to an algorithm for adjusting the rate and transmission mode according to the embodiment described above with respect to FIG. 8,

The curve identified by applies only SM

The curve identified by indicates a case where only STBC is applied.

First, in comparison with the case of applying only SM and the case of applying only STBC, the STBC method which can obtain the transmission diversity gain in the channel condition with low SNR can obtain higher throughput and the channel condition with high SNR. It can be seen that in the SM scheme for increasing transmission efficiency, higher throughput can be obtained.

On the other hand, when comparing the embodiments according to the present invention with these, it can be seen that while having a high throughput similar to that of STBC in the state of low SNR, while showing a higher throughput than that of SM in the state of high SNR. have. This is because the STBC transmission mode is selected in the low SNR situation, which has a similar throughput as that of the STBC, the SM transmission mode is selected in the high SNR situation, and the SNR consideration in the existing SM transmission mode is based on the one-dimensional SNR value. While the MCS index is selected, the MCS index is selected based on the two-dimensional (N-dimensional depending on the embodiment) SNR value combination, and thus, it can be seen that the throughput can be higher than that of the conventional SM mode. have.

Specifically, the performance improvement effect compared to the SM mode as described above is the differential modulation (UEQM) as illustrated in Table 5 in which the modulation scheme for the first stream and the modulation scheme for the second stream of the two streams are different from each other. Can also be affected by the use of this method. Hereinafter, the use of the differential modulation scheme will be described in more detail.

When multiple streams are simultaneously transmitted in a MIMO system, channel conditions may be different for each stream. Assuming such a situation, MCS indexes for differential modulation that apply different modulation schemes to each stream as shown in Table 5 above may be used. It is provided. However, the MCS set for differential modulation currently provided in IEEE 802.11n may not be sufficient for utilization in a system that does not receive CSI information as in the present embodiment. Another aspect of the present invention is to provide an improved MCS lookup table as a way to increase the utilization of the UEQM in a system using the open-loop MIMO scheme as described above.

When the MCS index set according to the post-detection SNR combination is selected in the 2 * 2 MIMO system as shown in Table 8, these are divided into equal modulation MCS indexes and differential modulation MCS indexes as shown in Table 4 below. Same as 8.

In Table 9, the yellow part represents a case in which the MCS index is selected from the MCS set for the uniform modulation scheme (Table 4), and the red part represents a case in which the MCS index is selected in the MCS set (Table 5) for the differential modulation scheme. Indicates. Considering Table 8, two problems can be found.

First, the differential modulation scheme cannot be applied to the case where the post-detection SNR value of the second stream is larger than the post-detection SNR value of the first stream.

Second, even if the post-detection SNR value of the first stream is very large, the differential modulation scheme cannot be applied even if the post-detection SNR value of the second stream is not large enough.

The cause of such a problem can be found in the structure of the UEQM MCS set defined in the IEEE 802.11n standard as shown in Table 5. First, referring to the UEQM MCS set shown in Table 5, only the combination whose modulation order for the first stream is larger than the modulation order for the second stream is included. This assumes a system that can be efficiently operated while reducing the overhead, assuming a case where the transmitting end receives the CSI information from the receiving end and sets the first stream with the better channel condition. Considering a module capable of switching a differentially modulated stream in a system receiving CSI information, Table 8 may also be represented as Table 10 below.

On the other hand, in the open loop MIMO scheme in which the receiving side does not feed back the CSI information to the transmitting side as in the present embodiment, the channel state for the second stream is better than the channel state for the first stream. To make.

Another problem is that, when looking at the modulation schemes defined for the UEQM MCS as shown in Table 5, it can be seen that Binary Phase Shift Keying (BPSK) is missing. Since the BPSK is missing, it can be inferred that UEQM cannot be applied if the post-detection SNR value of the second stream is very small even if the post-detection SNR value of the first stream is very large.

In order to solve these problems, the inventor of the present invention proposes to add the following UEQM MCS combination to the UEQM MCS combination of Table 5 above.

Briefly describing the MCS combinations added according to Table 11 above, a combination of applying higher order modulation to the second stream by the number corresponding to the combinations of Table 5 above was added. We also added MCS combinations where BPSK is applied to each stream.

The IEEE 802.11n standard defines a total of 76 MCS indexes and uses 7 bits for its feedback. However, since signaling using 7 bits can represent a total of 128 MCS indexes, signaling overhead does not occur even when additionally defining a total of 52 MCS combinations from 77 to 127. Therefore, the signaling overhead problem does not occur even if the MCS combinations shown in Table 11 are added, but UEQM MCS can be used more efficiently in an open loop MIMO system.

When configuring the MCS lookup table including the added MCS combination as described above, the MCS index selection in the embodiment of Table 8 may be changed as follows.

In Table 12, the yellow area indicates the case where the EQM MCS index is selected, the red area and the blue area indicate the case where the UEQM MCS index is selected, and the blue area indicates the case where the BPSK is applied. As can be seen from Table 12, it can be seen that the UEQM is applied until the post-detection SNR value of the second stream is larger than the post-detection SNR value of the first stream. If the detection SNR is large enough, it can be confirmed that the data rate can be increased through the UEQM even though the post-detection SNR value of other streams is small.

10 and 11 are simulation results for explaining the performance improvement when using the extended MCS lookup table according to one embodiment of the present invention.

로 식별되는 곡선은 상기 도 8과 관련하여 상술한 실시형태에서 상기 표 11과 같이 추가된 MCS 인덱스를 포함하는 확장된 MCS 룩업 테이블을 이용하여 상기 표 12와 같이 MCS 인덱스가 선택되는 경우의 처리율을 나타내며,

로 식별되는 곡선은 상기 도 8과 관련하여 상술한 실시형태에 따르되, 기존 MCS 룩업 테이블을 이용하여 MCS 인덱스가 상기 표 8과 같이 선택되는 경우의 처리율을 나타낸다. 또한,

로 식별되는 곡선은 기존 MCS 룩업 테이블을 이용하고 SM만을 적용하는 경우를,

로 식별되는 곡선은 STBC만을 적용하는 경우를 나타내고 있다.Of the four curves shown in FIG. 10

The curve identified by denotes the throughput when the MCS index is selected as shown in Table 12 using the extended MCS lookup table including the MCS index added as shown in Table 11 in the embodiment described above with reference to FIG. Indicates,

The curve identified by denotes the throughput in the case where the MCS index is selected as shown in Table 8 according to the embodiment described above with reference to FIG. 8, using an existing MCS lookup table. Also,

The curve identified by uses the existing MCS lookup table and applies only SM,

The curve identified by indicates the case where only STBC is applied.

First, in case of using the extended MCS lookup table and following the above-described embodiment with respect to FIG. 8, following the above-described embodiment with respect to FIG. 8 as shown in FIG. 9, but using an existing MCS look-up table. It can be seen that the throughput is further improved. The simulation results assume a 2 * 2 MIMO system, and if the number of antennas on the transmission / reception side increases, the performance difference according to both methods may increase accordingly.

Meanwhile, the expansion of the MCS lookup table as described above may have meaning separately from the above-described embodiment with reference to FIG. 8, which will be described with reference to FIG. 11.

로 식별되는 곡선을 추가한 것으로서,

로 식별되는 곡선은 상기 표 11과 같이 추가된 MCS 인덱스를 포함하는 확장된 MCS 룩업 테이블을 이용하며, 전송 모드는 SM만 적용하는 경우에 대한 것이다. 이를 기존 MCS 룩업 테이블을 이용하면서 SM만 적용하는 경우와 비교하여 보면, 확장된 MCS 룩업 테이블을 이용하는 경우의 처리율이 기존 MCS 룩업 테이블을 이용하는 경우에 비해 UEQM을 보다 적극적으로 활용함으로써 처리율이 약간 향상된 것을 확인할 수 있다. 본 시뮬레이션 결과는 2*2 MIMO 시스템을 가정한 것이며, 만일 송수신측의 안테나 수가 증가하는 경우 이에 따라 양 방식에 따른 성능 차이는 더 커질 수 있다.11 is in FIG. 10.

With the curve identified by

The curve identified by using the extended MCS lookup table including the added MCS index as shown in Table 11, the transmission mode is for the case of applying only SM. Compared to the case of applying only SM while using the existing MCS lookup table, the throughput of using the extended MCS lookup table is slightly improved by using UEQM more actively than using the existing MCS lookup table. You can check it. The simulation results assume a 2 * 2 MIMO system, and if the number of antennas on the transmission / reception side increases, the performance difference according to both methods may increase accordingly.

In addition, when using the extended MCS lookup table including the MCS index added to the proposed algorithm as shown in Table 11, it can be seen that the throughput is significantly higher than the other cases.

Simulation for the performance improvement as described above is exemplary, and the effect according to the present invention can also be confirmed through comparison of the treatment rate and the like observed in the actual product.

Although the above embodiments have been described assuming a 2 * 2 MIMO system, the proposed scheme may be extended to a system using N antennas, such as a 4 * 4 MIMO system. For example, when applied to a 4 * 4 MIMO system, in addition to the transmission mode selection and rate selection as shown in FIG. 8, the transmitter determines information on the number of streams (or the number of antennas) to use for signal transmission and transmits the information. A procedure may be performed to feed back. Such feedback information may also be referred to as RI (Rank Indicator) according to the applied standard.

In addition, in some cases, information for turning off a specific antenna may be additionally transmitted. Unlike the above-described embodiments, assuming that a transmitter performs precoding on a transmission signal and transmits the signal, turning off a specific antenna among predetermined precoding matrices between the transmitter and the receiver. By defining a precoding matrix for the receiver, the receiver may perform an index for the precoding matrix for turning off a specific antenna through the received signal to the transmitter.

Meanwhile, in the above-described embodiment with reference to FIG. 8, the case in which the two transmission modes are STBC and SM has been described as an example. However, not only the STBC and SM but also the general SISO (single input mode) are selected as the transmission modes. It can be various transmission modes such as Single Output mode, SFBC transmission mode.

The detailed description of the preferred embodiments of the invention disclosed as described above is provided to enable any person skilled in the art to make and practice the invention. While the above has been described with reference to preferred embodiments of the present invention, those skilled in the art will understand that various modifications and changes can be made without departing from the scope of the present invention. For example, those skilled in the art can use each configuration described in the above embodiments in a manner that combines with each other.

Accordingly, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

The detailed description of the transmission rate and transmission mode selection as described above has been described with reference to the example of the IEEE 802.11n system. However, except for a special case of the IEEE 802.11n system, the same method is applied to various mobile communication systems to which the MIMO technology is applied. Can be applied as

Claims (20)

In the method of adjusting the transmission mode and transmission rate in a multi-antenna system,
Performing channel estimation based on the received signal received from the transmitting side;
Based on the channel estimation, the first signal-to-noise ratio for the first transmission mode in which the signal is transmitted over one stream and the second transmission in which the signal is transmitted over N streams, where N is a natural number of two or more. Calculating N second signal-to-noise ratios for the mode;
Determining a first MCS index and a first throughput corresponding to having the estimated first signal-to-noise ratio and a second MCS index and a second throughput corresponding to having the estimated N second signal-to-noise ratios step; And
Transmitting an MCS index corresponding to a throughput having a greater value of the first throughput and the second throughput of the first MCS index and the second MCS index to the transmitting side,
The first MCS index is any one of the first MCS index combinations for the first transmission mode, and the second MCS index is any one of the second MCS index combinations for the second transmission mode,
The first MCS index combination and the second MCS index combination do not overlap each other,
The second MCS index combination includes a uniform MCS index combination that applies the same modulation scheme to the N streams and a differential MCS index combination that applies different modulation schemes to the N streams. . delete delete The method of claim 1,
N is 2,
The differential MCS index combination is
A first combination in which the modulation order of the modulation scheme applied to the first stream is greater than the modulation order of the modulation scheme applied to the second stream and
And a second combination, wherein a modulation order of a modulation scheme applied to the first stream is less than a modulation order of a modulation scheme applied to the second stream. The method of claim 1,
The differential MCS index combination includes an MCS index combination that applies Binary Phase Shift Keying (BPSK) to any one of the N streams. The method of claim 1,
The second MCS index,
MCS corresponding to the combination in the case of having the calculated N second signal-to-noise ratios in a table in which MCS indexes each having a maximum throughput in each of the combinations of N second signal-to-noise ratios may have a predetermined value A transmission mode and a transmission rate adjusting method selected as an index. The method of claim 1,
The second throughput is
The estimated N second signal bands in the table, each of which determines a combination of the maximum throughputs in each of the combinations of N second signal-to-noise ratios and the MCS indices to have the maximum throughputs, respectively. A transmission mode and a transmission rate adjusting method selected as the maximum throughput corresponding to the combination in case of having noise ratios. The method of claim 1,
And transmitting information indicating the number of streams or the number of antennas to be simultaneously transmitted at the transmitting side determined based on the channel estimation to the transmitting side. The method of claim 1,
The first transmission mode includes a mode for transmitting a signal using Space-Time Block Coding (STBC). The method of claim 1,
The second transmission mode includes a mode for transmitting a signal in a spatial multiplexing (SM) scheme. An apparatus for adjusting a transmission mode and a transmission rate in a multi-antenna system,
A receiver for receiving a signal in a first transmission mode in which a signal is transmitted from a transmitting side through one stream or in a second transmission mode in which a signal is transmitted in N (N is a natural number of two or more) streams;
Perform channel estimation based on the signal received by the receiver, calculate a first signal-to-noise ratio for the first transmission mode and N second signal-to-noise ratios for the second transmission mode based on the channel estimate, Determine a first MCS index and first throughput corresponding to having the estimated first signal to noise ratio and a second MCS index and second throughput corresponding to having the estimated N second signal to noise ratios; A processor for determining an MCS index corresponding to a throughput having a greater value of the first throughput and the second throughput of the first MCS index and the second MCS index; And
A transmitter for transmitting the MCS index determined by the processor to the transmitting side,
The first MCS index is any one of the first MCS index combinations for the first transmission mode, and the second MCS index is any one of the second MCS index combinations for the second transmission mode,
The first MCS index combination and the second MCS index combination do not overlap each other,
The second MCS index combination includes an equal MCS index combination for applying the same modulation scheme to the N streams and a differential MCS index combination for applying different modulation schemes to the N streams. . delete delete 12. The method of claim 11,
N is 2,
The differential MCS index combination is
A first combination in which the modulation order of the modulation scheme applied to the first stream is greater than the modulation order of the modulation scheme applied to the second stream and
And a second combination, wherein the modulation order of the modulation scheme applied to the first stream is smaller than the modulation order of the modulation scheme applied to the second stream. 12. The method of claim 11,
And the differential MCS index combination comprises an MCS index combination that applies Binary Phase Shift Keying (BPSK) to any one of the N streams. The method of claim 11,
And further comprising a memory for storing a table in which MCS indexes each having a maximum throughput in each of the combinations of the N second signal-to-noise ratios may be pre-determined.
And the processor selects the second MCS index as an MCS index corresponding to the combination of having the estimated N second signal-to-noise ratios in the table. The method of claim 11,
And further comprising a memory for storing a table in which a maximum of the throughputs in each of the combinations of the N second signal-to-noise ratios and a combination of the MCS indices to have the maximum throughputs are pre-determined, respectively;
And the processor selects the second throughput as the maximum throughput corresponding to the combination of having the estimated N second signal-to-noise ratios in the table. The method of claim 11,
And the processor further determines information indicating the number of streams or antennas to be simultaneously transmitted at the transmitting side determined based on the channel estimation. The method of claim 11,
The first transmission mode includes a mode for transmitting a signal using Space-Time Block Coding (STBC). The method of claim 11,
And the second transmission mode includes a mode for transmitting a signal in a spatial multiplexing (SM) scheme.

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