JP2014220776A - Radio communication device and radio transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radio communication device and radio transmission system capable of suppressing deterioration in throughput by improving accuracy of switching a rank based on a SINR in a radio transmission path using a MIMO radio transmission method.SOLUTION: A radio transmission system calculates information-theoretical capacity considering a transmission method and reception method in a radio transmission path for each of candidate values of a plurality of ranks in the radio transmission path, on the basis of an estimate of a transmission path response acquired from signals transmitted/received by radio transmission paths among a plurality of radio communication devices and an estimate of a Signal-to-Interference plus Noise power Ratio (SINR) calculated from an estimate of noise power; corrects the information-theoretical capacity by a weighting factor for each of the candidate values of the plurality of ranks; and determines to take a rank whose information-theoretical capacity is the largest of information-theoretical capacity calculated and corrected for each of the candidate values of the plurality of ranks as a rank to be used for next data transmission in the radio transmission path.

Description

  The present invention relates to a wireless communication apparatus and a wireless transmission system that can communicate via a wireless transmission path.

  2. Description of the Related Art Conventionally, a MIMO (Multi Input Multi Output) wireless transmission system that transmits and receives data with a plurality of antennas is known. This MIMO radio transmission system is adopted in the LTE (Long Term Evolution) communication standard defined by 3GPP (Third Generation Partnership Project), which is a standardization project for third generation (3G) mobile communication systems, and is also LTE- It is also adopted in the Advanced communication standard. In a mobile communication system that employs a MIMO wireless transmission scheme, a base station device (eNode-B) that is one of the plurality of wireless communication devices that perform data transmission and reception and a mobile station that is the other wireless communication device (eNode-B) By transmitting and receiving data with a plurality of different transmission layers (transmission streams) using a plurality of antennas with a UE (user equipment), wireless transmission by the MIMO spatial multiplexing scheme or the MIMO diversity scheme can be performed. . The MIMO spatial multiplexing scheme is a scheme in which different signals are transmitted in parallel from a plurality of antennas using the same radio resource (frequency and time), and the MIMO diversity scheme is the same signal transmitted from a plurality of antennas in space-time (or , Space-frequency) transmission. In LTE / LTE-Advanced, precoding is applied on the transmission side in both the MIMO spatial multiplexing scheme and the MIMO diversity scheme, but an open-loop type (Open-Loop MIMO) that does not use feedback control from the reception side, There is a closed loop type (Closed-Loop MIMO) using feedback information from the receiving side (see Non-Patent Document 1, Non-Patent Document 2, and Non-Patent Document 3).

  In the MIMO wireless transmission method, the rank (“number of transmission streams”, “number of transmission layers”) depends on the channel state such as the desired signal-to-interference plus noise power ratio (SINR) of the received signal. Alternatively, rank adaptation control that adaptively controls (a non-patent document 4 to 5) is generally applied. Here, a rank of 1 corresponds to the MIMO diversity scheme, and a rank of 2 or more corresponds to the MIMO spatial multiplexing scheme. In order to apply the rank adaptation control, the mobile station (UE) on the receiving side feeds back information (RI: Rank Indicator) on the rank determined in the mobile station to the base station (eNode-B) on the transmitting side. Thus, the base station performs dynamic rank control (see, for example, Non-Patent Documents 4 and 5).

  Non-Patent Document 6 discloses a MIMO radio transmission scheme that targets MIMO-OFDM combined with OFDM transmission, predicts an optimal transmission rank based on the SINR before signal separation and synthesis on the reception side, and feeds back to the transmission side. Has been. In this method, the average received power of all transmission antennas before signal separation / combining processing is averaged between the subcarriers and the receiving antennas, and the average received SINR before processing obtained by taking the ratio with the interference noise power. Then, an average reception SINR threshold for selecting each transmission rank is determined in advance from the relationship of throughput of each transmission rank, and a transmission rank to be used is selected based on the pre-processing average reception SINR. On the other hand, Patent Document 1 discloses a MIMO wireless transmission scheme in which an optimal transmission rank is predicted on the reception side based on SINR after signal separation / combination processing and fed back to the transmission side. Calculate the SINR (received SINR after signal separation / combination processing) for each subcarrier of each transmission stream of MIMO, and calculate the average channel capacity (Shannon capacity) for each of a plurality of ranks from the average received SINR after processing averaged between the transmission streams. A MIMO radio transmission scheme that calculates and selects a rank to be used based on the calculation result is disclosed. In the MIMO wireless transmission systems disclosed in Non-Patent Document 6 and Patent Document 1, for example, the transmission rank is controlled to a low transmission rank in an environment where the average reception SINR is low, and is controlled to a high transmission rank in an environment where the average reception SINR is high.

  However, in the conventional MIMO wireless transmission systems disclosed in Non-Patent Document 6 and Patent Document 1 described above, when the rank is switched according to SINR, the coding gain of the error correction code that can be realized, and the fading correlation between the antennas The rank cannot be accurately switched due to the influence of the presence / absence of the signal, the number of modulation levels, and the like, and the throughput in the wireless transmission path may be deteriorated. In addition, including rank control, modulation multi-level number and combination of coding schemes (MCS) control, and feedback delay and feedback information amount limitations in hybrid ARQ retransmission control, which greatly affect throughput characteristics in a MIMO wireless transmission system Although optimization is necessary, the above-mentioned Non-Patent Document 6 and Patent Document 1 do not disclose an optimization method that takes into account the effect of feedback information amount limitation or feedback delay.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a radio communication apparatus and a radio transmission capable of suppressing degradation of throughput by improving rank switching accuracy based on SINR in a radio transmission path of a MIMO radio transmission system. Is to provide a system.

A wireless transmission system according to the present invention is a wireless transmission system capable of transmitting and receiving data using a plurality of different transmission streams between a plurality of wireless communication devices, and transmitting and receiving on a wireless transmission path between the plurality of wireless communication devices. A plurality of ranks in the wireless transmission path based on the estimated value of the channel response obtained from the received signal and the estimated value of the desired signal-to-interference noise power ratio (SINR) calculated from the estimated noise power. For each candidate value, a means for obtaining an information theoretical capacity in consideration of a transmission method and a reception method in the wireless transmission path, and using a weighting factor for the information theoretical capacity for each candidate value of the plurality of ranks Means for correcting the rank of the information theoretical capacity obtained and corrected for each of the candidate values of the plurality of ranks, the rank having the maximum information theoretical capacity; And means for determining a rank to use for data transmission, a.
The wireless communication device according to the present invention is a wireless communication device on a data receiving side in a wireless transmission system capable of transmitting and receiving data using a plurality of different transmission streams between a plurality of wireless communication devices, the plurality of wireless communication devices. Based on an estimate of a transmission path response obtained from a signal transmitted and received on a wireless transmission path between communication devices and an estimate of a desired signal to interference noise power ratio (SINR) calculated from an estimate of noise power Means for obtaining an information theoretical capacity in consideration of a transmission scheme and a reception scheme in the wireless transmission path for each candidate value of the plurality of ranks in the wireless transmission path, and the information theory for each candidate value of the plurality of ranks. A means for correcting the dynamic capacity using a weighting factor, and a run in which the information theoretical capacity is maximized among the information theoretical capacity obtained and corrected for each of the candidate values of the plurality of ranks. The comprises means for determining a rank to use for the next data transmission in the radio transmission path, and means for notifying information ranks the determined to the wireless communication device of the data transmission side.
In this radio transmission system and radio communication device, in consideration of the influence of propagation path estimation accuracy and error correction coding to be used, for each candidate value of a plurality of ranks in the radio transmission path, a transmission scheme and a reception scheme in the radio transmission path Is corrected using a weighting factor for the information theoretical capacity obtained in consideration of In this way, the information theoretical capacity obtained and corrected for each of the candidate values of the plurality of ranks is used to calculate the accuracy of the plurality of ranks in consideration of the actual channel estimation accuracy, error correction coding gain, presence / absence of fading correlation between antennas, etc. The capacity for each candidate value can be accurately determined. Therefore, the rank used for the next data transmission can be accurately determined so as to suppress the deterioration of the throughput according to the SINR in the actual wireless transmission path. Therefore, it is possible to improve rank switching accuracy based on SINR in a MIMO wireless transmission system wireless transmission path and suppress degradation of throughput.

In the wireless transmission system and the wireless communication device, the means for correcting the information theoretical capacity performs clipping for setting an upper limit value for the information theoretical capacity in addition to correction using a weighting factor for the information theoretical capacity. May be.
In this wireless transmission system and wireless communication device, by setting an upper limit value for the information theoretical capacity, each rank is considered in consideration of the influence of the wireless transmission performance variation due to the environment change of the wireless transmission path such as the spread of multipath delay. The capacity for the candidate value can be calculated with high accuracy. Therefore, robust rank switching can be performed against changes in the environment of the wireless transmission path such as spread of multipath delay.

In the wireless transmission system and the wireless communication apparatus, the weighting factor is a loss factor corresponding to a transmission loss in the transmission stream, and the upper limit value in the clipping is a modulation scheme and encoding used in the transmission stream. You may set according to a system.
In this radio transmission system and radio communication apparatus, the capacity for the candidate value of each rank can be accurately calculated in consideration of the transmission loss in the transmission stream, the modulation scheme, and the coding scheme. Therefore, the rank switching accuracy based on SINR in the wireless transmission path of the MIMO wireless transmission scheme can be further increased.

In the wireless transmission system and the wireless communication apparatus, a Shannon capacity or a signal point constrained capacity may be used as the information theoretical capacity.
In this wireless transmission system and wireless communication device, the capacity calculation process can be simplified by using a Shannon capacity that is easy to calculate. In addition, by using the signal point constrained capacity as the information theoretical capacity, it is possible to calculate the capacity taking into account the restrictions due to the signal point arrangement in the actual modulation scheme, so that the rank switching accuracy based on the SINR can be further improved.

In the wireless transmission system and the wireless communication apparatus, transmission / reception of signals in the wireless transmission path is performed via a plurality of subcarriers, and the means for obtaining the information theoretical capacity is provided for each of the candidate values of the plurality of ranks. In addition, for each of the plurality of subcarriers, an information theoretical capacity in consideration of a transmission scheme and a reception scheme in the wireless transmission path is obtained, and the means for correcting the information theoretical capacity is, for each candidate value of the plurality of ranks, The information theoretical capacity obtained by correcting the information theoretical capacity obtained for each of the plurality of subcarriers and determining the rank is calculated and corrected for each of the plurality of subcarriers for each candidate value of the plurality of ranks. Is added or averaged over all or a part of the plurality of subcarriers, and obtained for each candidate value of the plurality of ranks The rank sum value or the average value of the serial information theoretical capacity is maximum may be determined as a rank to use for transmitting the next data in the radio transmission path.
In the wireless transmission system and the wireless communication apparatus, it is possible to increase the rank switching accuracy based on the SINR in the wireless transmission path of the MIMO wireless transmission system via a plurality of subcarriers, and suppress degradation in throughput.

  In this specification, the “information theoretical capacity” is a communication path capacity obtained by modeling a wireless transmission path based on information theory. The “Shannon capacity” is a communication path capacity assuming a transmission signal having a Gaussian distribution as a transmission signal source capable of maximizing the communication path capacity. “Signal point constrained capacity” is not a condition of a transmission signal having a Gaussian distribution, but a signal space in modulation such as quadrature phase shift keying (QPSK), 16QAM, and 64QAM, as described in Non-Patent Document 7. It is a capacity obtained under the condition of a transmission signal in which signal points on the diagram are constrained, that is, Constellation Constrained Capacity or Constellation Constraint Capacity (CCC). In the “signal point constrained capacity”, in the case of a multi-level modulation scheme such as 16QAM or 64QAM, a plurality of bits of binary data of 0 or 1 after error correction coding are combined into a single modulation symbol. The loss of capacity that occurs when IQ mapping is performed using the above means may be considered (see Non-Patent Document 7).

  According to the present invention, it is possible to improve the rank switching accuracy based on the SINR in the wireless transmission path of the MIMO wireless transmission system and suppress the deterioration of the throughput.

The functional block diagram which shows an example of schematic structure of the radio base station and user apparatus in the downlink of the closed-loop type MIMO radio transmission system which concerns on one Embodiment which can apply this invention. The functional block diagram which shows an example of schematic structure of the radio base station and user apparatus in the downlink of the open loop type MIMO wireless transmission system which concerns on other embodiment which can apply this invention. The functional block diagram which shows an example of schematic structure of the radio base station and user apparatus in the uplink of the closed-loop type | mold MIMO radio | wireless transmission system which concerns on other embodiment which can apply this invention. The graph which shows the result of the computer simulation in the downlink of the open loop type MIMO radio | wireless transmission system when there is no fading spatial correlation between the antennas of a transmission side. The graph which shows the result of the computer simulation in the downlink of an open loop type MIMO radio | wireless transmission system in case there exists fading spatial correlation between the antennas of a transmission side. The schematic diagram which shows an example of the system model of a MIMO wireless transmission path when a rank (space multiplexing number) is two or more. The schematic diagram which showed the system model of the MIMO wireless transmission path of FIG. 6 with the approximate equivalent parallel SISO channel transmission model. The schematic diagram which shows an example of the system model of a MIMO radio-transmission path when a rank (space multiplexing number) is 1. The schematic diagram which showed the system model of the MIMO wireless transmission path of FIG. 8 with the approximate equivalent parallel SISO channel transmission model. The graph which compared the throughput calculated from the Shannon capacity | capacitance about the case where a rank is each 1 and 2, and the throughput measured with the actual radio | wireless transmission system. The graph which compared the throughput calculated from the signal point restraint capacity | capacitance (CCC) about the case where each rank is 1 and 2, and the throughput measured by the actual wireless transmission system. The graph which shows an example of parameter tuning of weighting coefficient (alpha) _1,1 and upper limit (beta) ₁ in the case of SFBC transmission diversity (reception system: MRC) whose rank is 1. FIG. The graph which shows an example of parameter tuning of weighting coefficient (alpha) _{L, l} and upper limit (beta) ₁ in the case of SDM (reception system: MMSE) combined with CDD precoding whose rank is 2. FIG. Maximum Doppler frequency shift fd is 5 information theoretical capacity in rank adaptation in the case of [Hz] _C L, the correction parameters used to correct the _{C 1} (weight coefficient α _{L, l,} α _1,1) graph showing the effect of . Maximum Doppler frequency shift fd is 70 information theoretical capacity in rank adaptation in the case of [Hz] _C L, the correction parameters used to correct the _{C 1} (weight coefficient α _{L, l,} α _1,1) graph showing the effect of . Graph showing an example of throughput when applying rank adaptation is not performed Clipping upper limit value in the information theoretical capacity C _L, the correction of the C ₁ when the delay spread is large. Graph showing an example of throughput when applying rank adaptation performing the delay spread is large when the information theoretical capacity C _L, clipping with the upper limit value is provided for the correction of C _1.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, an overall configuration of a MIMO wireless transmission system having a wireless communication apparatus to which the present invention can be applied will be described.
FIG. 1 is a functional block diagram illustrating an example of a schematic configuration of a radio base station and a user apparatus in a downlink of a closed-loop MIMO radio transmission system according to an embodiment to which the present invention can be applied. The MIMO wireless transmission system shown in FIG. 1 is based on control information (PMI: Precoding Matrix Indicator) indicating an index of a candidate data table (codebook) of an optimal transmission antenna weight matrix fed back from the user apparatus 10. This is a closed-loop MIMO wireless transmission system that multiplies a transmission signal by a transmission antenna weight that is different for each stream.
In the closed-loop MIMO wireless transmission system (Closed-Loop MIMO) of the present embodiment, a case of a configuration of a 2-transmission layer (number of ranks 2) conforming to the LTE communication standard will be exemplified. However, the present invention is not limited to this configuration.

  In FIG. 1, a user device 10 is a wireless communication device that can be used when a user uses various communication services, and is referred to as a “communication terminal” or a “terminal” or can be moved. Sometimes called a “station”, sometimes called a “radio”. The user device 10 may be a mobile communication terminal such as a mobile phone. As shown in FIG. 1, the user apparatus 10 includes a plurality of antennas 100, a downlink channel estimation unit 101, a downlink control signal demodulation unit 102, a data signal demultiplexing / combining unit 103, and a serial / parallel conversion unit (S / P) 104, a control information (RI / PMI / CQI) generation unit 105, and an uplink transmission unit 106.

  In the example of FIG. 1, the user apparatus 10 includes a plurality of antennas 100, but the number of antennas 100 included in the user apparatus 10 is not limited to a specific number. For example, the user apparatus 10 may be provided with a single antenna 100 or may be provided with a plurality of antennas 100 such as two or four. Further, when the user apparatus 10 includes a plurality of antennas 100, the user apparatus 10 may be capable of switching the number of antennas used during actual MIMO communication.

  Also, in the downlink channel estimation unit 101 of the user apparatus 10, the radio channel response between each antenna 200 of the base station apparatus 20 to each antenna of the user apparatus 10 is estimated and the MIMO channel response is acquired. The multiplexer 208 of the station 20 multiplexes a reference signal RS, which is a predetermined pilot signal, which is not shown in the figure. Here, in the downlink of the 3GPP LTE Release-8 / 9 standard, the reference signal RS normally used for data demodulation is a signal orthogonal to each other on the frequency axis and the time axis between a plurality of transmission antennas, and is specific to the cell. Since it is a signal, it is also called a cell-specific reference signal (CSRS). In the downlink of 3GPP LTE-Advanced Release-10 / 11 standard, the reference signal RS normally used for data demodulation uses signals orthogonal to each other on the code axis among a plurality of transmission layers, and is specific to each mobile station (UE). Therefore, it is also called a UE-specific reference signal.

  The downlink channel estimation unit 101 first divides the received signal from each antenna 100 into received signals of a reference signal RS part, a data signal part, and a downlink control signal part. Next, the MIMO channel response is estimated based on the received signal of the reference signal RS part and the transmission sequence of the predetermined reference signal RS. Then, the channel estimation value, which is the estimation result of the MIMO channel response, is output to the data signal demultiplexing unit 103 and the downlink control signal demodulating unit together with the received signals of the data signal portion and the downlink control signal portion. Further, the channel estimation value is output to control information (RI / PMI / CQI) generation section 105.

  The downlink control signal demodulator 102 demodulates the control signal from the received signal received from the downlink channel estimator 101, and MCS (Modulation and Coding Scheme), which is transmission scheme information necessary for demodulating and decoding user data. ), Rank, and T-PMI (Transmit-Precoding Matrix Indicator) are acquired as control information and output to the data signal demultiplexing and combining unit 103.

  The data signal demultiplexing / combining unit 103 demultiplexes and / or combines the received signals of the user data received from the downlink channel estimation unit 101 based on the control information received from the downlink control signal demodulating unit 102, and a predetermined number of codes It outputs to the serial / parallel conversion part (S / P) 104 as data consisting of words.

  The serial-to-parallel converter (S / P) 104 converts the data consisting of a predetermined number of code words received from the data signal separation / combination unit 103 into received data (user data) that can be used by the user according to the number of downlink ranks. ).

  Control information (RI / PMI / CQI) generation section 105 determines a rank for maximizing the data rate based on the channel estimation value received from downlink channel estimation section 101, and determines this rank as rank information (RI: Rank). (Indicator) and output. Also, the control information (RI / PMI / CQI) generation unit 105 determines a precoding matrix suitable for downlink data transmission based on the channel estimation value, and a PMI (Precoding Matrix Indicator) indicating this precoding matrix ) Is selected from the candidate data table (codebook) and output. Further, control information (RI / PMI / CQI) generation section 105 measures and outputs CQI (Channel Quality Indicator) as channel quality information based on the channel estimation value. The rank information RI, PMI, and CQI output from the control information (RI / PMI / CQI) generation unit 105 are respectively output to the uplink transmission unit 106 as control information, and the uplink control channel (PUCCH or PUSCH) is selected. Via the antenna 100 to the radio base station 20.

In particular, the downlink channel estimation unit 101 and the control information (RI / PMI / CQI) generation unit 105 also function as the following means (1) to (3).
(1) The desired received signal versus interference noise calculated from the estimated value of the channel response acquired from the signal transmitted and received on the radio channel between the user apparatus 10 and the radio base station 20 and the estimated value of the noise power Means for obtaining an information theoretical capacity in consideration of a transmission scheme and a reception scheme in the wireless transmission path for each candidate value of a plurality of ranks in the wireless transmission path based on an estimated power ratio (SINR).
(2) Means for correcting the information theoretical capacity using a weighting factor for each of the plurality of rank candidate values.
(3) Of the information theoretical capacities obtained and corrected for each of the candidate values of the plurality of ranks, a rank that maximizes the information theoretical capacity is determined as a rank used for the next data transmission in the wireless transmission path. means.

  The uplink transmission unit 106 also notifies the radio base station 20 of the rank information RI determined by the control information (RI / PMI / CQI) generation unit 105, that is, the control information (RI / PMI / CQI) generation unit 105. It also functions as rank information transmitting means for transmitting the rank information RI generated in step 1 to the radio base station 20.

The user apparatus 10 having the configuration of FIG. 1 estimates a MIMO channel response based on a reception result of a reference signal RS that is a known downlink pilot signal, obtains a channel estimation value, and obtains optimum rank information from the channel estimation value. The RI is determined, and the value of the rank information RI is fed back to the radio base station 20 together with the optimum PMI / CQI.
As in the specification of 3GPP LTE (Long Term Evolution), when the transmission timing of rank information RI overlaps with the transmission timing of PMI or CQI as other control information, transmission of rank information RI is given priority. The

  In FIG. 1, the radio base station 20 is a radio communication device that relays radio communication between the communication network side and the user terminal device 10, and is called a “base station device” or simply called a “base station”. Sometimes it is. Also, the radio base station 20 may be called “eNodeB (evolved Node B)” in the 3GPP and LTE specifications.

  As illustrated in FIG. 1, the radio base station 20 includes a plurality of antennas 200, an uplink reception unit 201, a downlink scheduler 202, a precoding weight generation unit 203, and a downlink control signal generation unit 204. A parallel conversion / modulation unit 205, a multiplier 206, an addition processing unit (Σ) 207, and a multiplexer 208 are provided.

  In the example of FIG. 1, the radio base station 20 includes a plurality of antennas 200, but the number of antennas 200 included in the radio base station 20 is not limited to a specific number. For example, the radio base station 20 may include a single antenna 200 or may include a plurality of antennas 200 such as two or four. Further, when the radio base station 20 includes a plurality of antennas 200, the radio base station 20 may be capable of switching the number of antennas used in actual MIMO communication.

  The uplink receiving unit 201 receives control information (RI, CQI, PMI) from the user apparatus 10 via the uplink control channel (PUCCH), and sends it to the downlink scheduler 202.

  The downlink scheduler 202 determines various control parameters used in the downlink with the user apparatus 10 based on the control information (RI, CQI, PMI) received from the uplink receiving unit 201. For example, the downlink scheduler 202 determines a rank number (Rank) indicating the number of spatially multiplexed transmission layers, a T-PMI that specifies a transmission precoding matrix, and an MCS that specifies a modulation and coding scheme. Then, the downlink scheduler 202 sends the determined MCS and Rank signals to the serial-to-parallel conversion / modulation unit 205, and sends the Rank and T-PMI signals to the precoding weight generation unit 203, and the MCS, Rank, and The T-PMI signal is sent to the downlink control signal generator 204.

  Based on the Rank and T-PMI signals received from the downlink scheduler 202, the precoding weight generation unit 203 transmits each of the plurality of antennas 200 based on the uplink reception quality in the resource block allocated to the user apparatus 10. Precoding weights for controlling the phase and / or amplitude of the signal are generated and sent to the multiplier 206.

  The downlink control signal generation unit 204 generates a reference signal RS and a control signal used in the downlink based on the MCS, Rank, and T-PMI signals received from the downlink scheduler 202, and sends them to the multiplexer 208.

  The serial-parallel conversion / modulation unit 205 distributes user data to be transmitted to the number of transmission layers based on the rank number (Rank) received from the downlink scheduler 202. The multiplier 206 includes three systems of multipliers 206a, 206b, and 206c corresponding to the three antennas 200. When user data is input, data received from the downlink scheduler 202 by the serial-to-parallel converter / modulator 205 is multiplied by transmission data by multipliers 206a, 206b, and 206c, and the phase and amplitude are respectively controlled. By (shifting), a transmission signal is generated for each of the plurality of antennas 200. Then, the transmission signals generated by being distributed to a plurality of transmission layers (two layers in the example of FIG. 1) are added by the adder (Σ) 207 for each transmission layer, and then the reference signal RS and the multiplexer 208 Control signals are further multiplexed and transmitted from each of the three antennas 200.

In the MIMO radio transmission system having the configuration shown in FIG. 1, the rank number Rank is determined by the radio base station 20 based on the rank information RI fed back from the user apparatus 10 as described above. For example, the rank number Rank = 1 when the rank information RI = 1, and the rank number Rank = 2 is normally selected when the rank information RI = 2. When the rank number Rank = 1, single transmission beam forming is performed, and when the rank number Rank ≧ 2, multi-beam transmission spatial multiplexing is performed.
As in the 3GPP specifications, since the radio base station 20 side has the final decision right of the rank number Rank, when determining the rank number Rank on the radio base station 20 side, the rank information RI of the user apparatus 10 is determined. It is not always necessary to follow the value of.

FIG. 2 is a functional block diagram illustrating an example of a schematic configuration of a radio base station and a user apparatus in the downlink of an open-loop MIMO radio transmission system (Open-Loop MIMO) according to another embodiment to which the present invention is applicable. It is.
The MIMO radio transmission system in FIG. 2 is an open-loop MIMO radio transmission system that feeds back rank information RI and CQI from the user apparatus 10 to the radio base station 20 but does not feed back PMI. In addition, the same code | symbol is attached | subjected about the structure similar to said FIG. 1, and detailed description is abbreviate | omitted.

  In FIG. 2, the control information (RI / CQI) generation unit 115 of the user apparatus 10 determines a rank that maximizes the data rate based on the channel estimation value received from the downlink channel estimation unit 101, and determines this rank. Generate and output as rank information RI. Control information (RI / CQI) generation section 115 measures and outputs CQI as channel quality information based on the channel estimation value received from downlink channel estimation section 101. The rank information RI and CQI output from the control information (RI / CQI) generation unit 115 are output as control information to the uplink transmission unit 106 and transmitted to the radio base station 20 via the antenna 100. Unlike the configuration of FIG. 1 described above, PMI determination and transmission are not performed.

In this way, the user apparatus 10 having the configuration of FIG. 2 estimates the MIMO channel response based on the reception result of the reference signal RS, which is a known downlink pilot signal, and obtains a channel estimate, and from the channel estimate The rank information RI is determined, and the value of the rank information RI is transmitted to the radio base station 20 via the uplink control channel (PUCCH or PUSCH) together with the optimum CQI and fed back.
As in the 3GPP specification, when the transmission timing of rank information RI overlaps with the transmission timing of CQI, which is other control information, transmission of rank information RI is given priority.

In FIG. 2, the downlink scheduler 202 of the radio base station 20 uses various control parameters used in the downlink with the user apparatus 10 based on the control information (RI, CQI) received from the uplink receiving unit 201. To decide. For example, the downlink scheduler 202 determines a rank number (Rank) indicating the number of spatially multiplexed transmission layers and an MCS that specifies a modulation and coding scheme. Then, the downlink scheduler 202 sends the determined MCS and Rank signals to the serial-parallel conversion / modulation unit 205, sends the Rank signal to the precoding weight generation unit 203, and transmits the MCS and Rank signals to the downlink. It transmits to the control signal generation unit 204.
Note that the radio base station 20 having the configuration of FIG. 2 does not receive PMI from the user apparatus 10 and generate T-PMI based on the PMI reception result. The precoding weight generation unit 203 and the downlink control signal generation unit 204 use a precoding scheme that is predetermined for each number of ranks in the specification without using the PMI from the user apparatus 10.

In the MIMO radio transmission system having the configuration shown in FIG. 2, the rank number Rank is determined by the radio base station 20 based on the rank information RI fed back from the user apparatus 10 as described above. For example, the rank number Rank = 1 when the information RI = 1, and the rank number Rank = 2 is normally selected when the rank information RI = 2. When the rank number Rank = 1, transmission diversity using SFBC (spatial frequency block coding) is used, and when the rank number Rank ≧ 2, spatial multiplexing using CDD (cyclic delay diversity) is used.
In the MIMO radio transmission system having the configuration shown in FIG. 2 as well, the final decision on the rank number Rank is held by the radio base station 20 as in the 3GPP specifications, so that the rank number Rank is set on the radio base station 20 side. When determining, it is not necessary to follow the value of the rank information RI of the user device 10.

  In the MIMO radio transmission system of FIG. 1 and FIG. 2 described above, the determination of the optimum number of transmission layers for the downlink from the radio base station 20 to the user apparatus 10 has been described. The present invention can also be applied to the case of uplink for transmitting data to.

  FIG. 3 is a functional block diagram showing an example of a schematic configuration of a radio base station and user equipment in an uplink of a closed-loop MIMO radio transmission system according to still another embodiment to which the present invention is applicable. In the closed-loop type MIMO wireless transmission system of the present embodiment, a case of a configuration of a 2-transmission layer (rank number is 2) conforming to the LTE-Advanced communication standard is illustrated, but the present invention has this configuration. It is not limited. Further, detailed description of the same configuration as in FIG. 1 is omitted.

  As shown in FIG. 3, the user apparatus 30 includes a plurality of antennas 300, a downlink reception unit 301, a precoding weight generation unit 302, a serial / parallel conversion / modulation unit 303, a multiplier 304, and an adder ( Σ) 305, a multiplexer 306, and an uplink reference signal generation unit 307.

  In the example of FIG. 3, the user apparatus 30 includes a plurality of antennas 300, but the number of antennas 300 included in the user apparatus 30 is not limited to a specific number. For example, the user apparatus 30 may be provided with one antenna 300 or may be provided with a plurality of antennas 300 such as two or four. Further, when the user apparatus 30 includes a plurality of antennas 300, the user apparatus 30 may be capable of switching the number of antennas used during actual MIMO communication.

  In the user apparatus 30, the reference signal RS generated by the uplink reference signal generation unit 307 is multiplexed with another control signal by the multiplexer 306 and transmitted from the plurality of antennas 300.

  Also, the user apparatus 30 separates the control information (RI, MCS, PMI) included in the downlink control channel received from the radio base station 40 by the downlink receiving unit 301, and the PMI is sent to the precoding weight generating unit 302. The RI and the MCS are output to the serial / parallel conversion / modulation unit 303. Thereby, the user apparatus 30 transmits user data on the uplink data channel according to the RI, MCS, and PMI included in the received downlink control channel.

  As shown in FIG. 3, the radio base station 40 includes a plurality of (in this example, three) antennas 400, an uplink channel estimation unit 401, a data signal separation / combination unit 402, and a serial / parallel conversion unit (S / P) 403, an uplink scheduler 404, and a downlink transmission unit 405.

  In the radio base station 40, signals received by the plurality of antennas 400 are sent to the uplink channel estimation unit 401. Uplink channel estimation section 401 corrects the distortion of the signal from each antenna 400 and outputs the received signal after the distortion correction to data signal demultiplexing and combining section 402. Further, the uplink channel estimation unit 401 estimates a MIMO channel response based on the reference signal RS as a predetermined pilot signal received from the user apparatus 30, and sets a channel estimation value that is an estimation result of the MIMO channel response to the uplink. The data is output to the scheduler 404.

  The uplink scheduler 404 determines various control parameters used in the uplink with the user apparatus 10 based on the channel estimation value received from the uplink channel estimation unit 401. For example, the uplink scheduler 404 generates and outputs rank information RI indicating the number of spatially multiplexed transmission layers that maximizes the data rate in the uplink. Also, the uplink scheduler 404 determines a precoding matrix suitable for uplink data transmission / reception, and selects and outputs a PMI indicating the precoding matrix from the codebook. Also, the uplink scheduler 404 determines and outputs an MCS that specifies a modulation and coding scheme. Then, the uplink scheduler 404 sends the rank information RI to the serial / parallel conversion unit (S / P) 403, and transmits the RI information, MCS, and PMI to the data signal separation / combination unit 402 and the downlink transmission unit 405. The downlink transmission unit 405 transmits the rank information RI received from the uplink scheduler 404 to the user apparatus 30 via the downlink control channel (PDCCH) and feeds it back.

  In LTE-Advanced, since the radio base station 40 has the right to determine the transmission method of the user apparatus 30 for both downlink and uplink, the rank information RI calculated by the uplink scheduler 404 of the radio base station 40 remains as it is. It becomes the rank number Rank of the uplink.

In the uplink configuration of FIG. 3, the uplink channel estimation unit 401 and the uplink scheduler 404 also function as the following means (1) to (3).
(1) A desired received signal versus interference noise calculated from an estimated value of a transmission path response acquired from a signal transmitted and received on a radio transmission path between the user apparatus 30 and the radio base station 40 and an estimated value of noise power Means for obtaining an information theoretical capacity in consideration of a transmission scheme and a reception scheme in the wireless transmission path for each candidate value of a plurality of ranks in the wireless transmission path based on an estimated power ratio (SINR).
(2) Means for correcting the information theoretical capacity using a weighting factor for each of the plurality of rank candidate values.
(3) Of the information theoretical capacities obtained and corrected for each of the candidate values of the plurality of ranks, a rank that maximizes the information theoretical capacity is determined as a rank used for the next data transmission in the wireless transmission path. means.

  Further, the downlink transmission unit 405 notifies the user apparatus 30 of the rank information RI determined by the uplink scheduler 404, that is, rank information transmission for transmitting the rank information RI generated by the uplink scheduler 404 to the user apparatus 30. It also functions as a means.

  Next, an application example of rank adaptation for optimizing the rank in the MIMO wireless transmission system of FIGS.

4 and 5 are graphs showing the results of computer simulation in the downlink of the 3GPP LTE / LTE-Advanced open-loop MIMO radio transmission system (Open-Loop MIMO) shown in FIG. Hereinafter, in the computer simulation exemplified in this specification, the specifications shown in Table 1 are all assumed, and the PDU (protocol data) in the MAC (Medium Access Control) layer in the downlink from the radio base station 20 to the user apparatus 10 is assumed. -Unit) level throughput was evaluated by considering the influence of feedback control delay by link level simulation. In addition, in this computer simulation, in order to simulate the operation of a system that is closer to reality, parameters related to feedback control, which are considered to have a great influence on the performance of rank adaptation, specifically CQI feedback delay and feedback interval, hybrid Regarding parameters such as feedback delay and retransmission interval of ACK / NACK information in ARQ retransmission, feedback delay and feedback interval of RI when rank adaptation is applied, parameters that can be realized or set in an actual LTE / LTE-Advanced system were used. Further, regarding the computer simulation in the case of applying the rank adaptation, as in the actual LTE / LTE-Advanced specification, when the RI report timing overlaps with the CQI report timing, the RI transmission is given priority for this timing. The transmission of CQI is assumed to be dropped, and evaluation is performed in a manner that includes the effects of actual feedback delay and feedback information amount limitation. 4 and 5 are different from each other in fading spatial correlation on the transmission side. Specifically, FIG. 4 shows the result when there is no fading spatial correlation between antennas on the transmitting side (when fading spatial correlation coefficient ρt = 0), and FIG. 5 shows the case when there is fading spatial correlation between antennas on the transmitting side. This is a result of (when fading spatial correlation coefficient ρt = 0.7). 4 and 5, it is assumed that no fading spatial correlation between antennas on the receiving side has occurred. The horizontal axis in the figure is the average SINR per reception antenna (hereinafter referred to as “reception level” or “reception SINR”) [dB], and the vertical axis in the figure is the average throughput [Mbps].

In the example of FIG. 4 and FIG. 5, as a transmission method, a “transmission diversity” method using SFBC (spatial frequency block coding) precoding and a “space division multiplexing” (CDD (cyclic delay diversity) precoding) ( Select one of the SDM) methods. By performing rank adaptation that optimizes the switching control of these transmission methods, the throughput can be maximized.
For example, when there is no fading spatial correlation between antennas on the transmission side in FIG. 4 (when fading spatial correlation coefficient ρt = 0), the threshold of the reception level for switching the rank is 22 [dB] indicated by arrow A in the figure. . In this case, the “transmission diversity” method is selected when the reception level is equal to or less than the threshold value 22 [dB], and the “spatial division multiplexing” (SDM) method is selected when the reception level is greater than the threshold value 22 [dB]. To control.
On the other hand, when there is fading spatial correlation between antennas on the transmitting side in FIG. 5 (when fading spatial correlation coefficient ρt = 0.7), the threshold of the reception level for switching the rank is 26 [dB] indicated by arrow A in the figure. It is. In this case, the “transmission diversity” method is selected when the reception level is equal to or lower than the threshold value 26 [dB], and the “spatial division multiplexing” (SDM) method is selected when the reception level is higher than the threshold value 26 [dB]. To control.
By optimizing the transmission system switching control in this way, it is possible to maximize the throughput. However, the optimal rank number (spatial multiplexing number) is not only the reception level but also the fading spatial correlation between antennas in the radio propagation path. The optimum rank number (spatial multiplexing number) varies depending on the presence or absence of fading spatial correlation between antennas. For example, in the range of reception levels of 22 to 26 [dB] in FIGS. 4 and 5, when there is no inter-antenna fading spatial correlation (ρt = 0), “spatial division multiplexing” (SDM) with a rank number of 2 The method is suitable, and when there is fading spatial correlation between antennas (ρt = 0.7), the “transmission diversity” method with a rank number of 1 is suitable.

However, considering the application of the rank adaptation to an actual wireless transmission system, the reception level (SINR) threshold value that differs for each value of the fading spatial correlation coefficient ρt for the reasons shown in (A) to (C) below. Having a table is unrealistic.
(A) In an actual radio propagation path, fading spatial correlation occurs not only on the transmission side but also on the reception side.
(B) The actual fading correlation coefficient ρt is not necessarily a real number and is generally a complex number.
(C) As the number of transmission / reception antennas increases, the number of combinations of fading spatial correlations on the transmission side and reception side becomes enormous.

  Therefore, in the present embodiment, in consideration of application of the rank adaptation to an actual wireless transmission system, a plurality of pieces of information are used without using fading spatial correlation information in a receiving-side wireless communication device (user device or wireless base station). Throughput (or frequency utilization efficiency) is predicted for each candidate value of the rank (spatial multiplexing number), and the rank (spatial multiplexing number) that is expected to maximize the predicted throughput (or predicted frequency utilization efficiency) is transmitted. To the wireless communication device (radio base station or user device) on the side.

  For example, in this embodiment, first, as an index value indicating a throughput (or frequency use efficiency) predicted for each rank (number of spatial multiplexing), an information theoretical capacity in consideration of a transmission method and a reception method in a wireless transmission path is set. Ask. Here, the “information theoretical capacity” is a communication path capacity obtained by modeling a wireless transmission path based on the information theory. As this information theoretical capacity, a “Shannon capacity” that is a channel capacity when a Gaussian-distributed transmission signal is assumed as a transmission signal source capable of maximizing the channel capacity may be used. “Capacity” may be used. Here, the “signal point constrained capacity” is not a condition of a transmission signal having a Gaussian distribution, but a signal space in modulation such as quadrature phase shift keying (QPSK), 16QAM, and 64QAM as described in Non-Patent Document 7. It is a capacity obtained under the condition of a transmission signal in which signal points on the diagram are constrained, that is, Constellation Constrained Capacity or Constellation Constraint Capacity (CCC). In the “signal point constrained capacity”, in the case of a multi-level modulation scheme such as 16QAM or 64QAM, a plurality of bits of binary data of 0 or 1 after error correction coding are combined into a single modulation symbol. Loss of capacity that occurs when mapping using may be considered (see Non-Patent Document 7). Further, as the “signal point constrained capacity”, a received coded bit information rate (RBIR), that is, a frequency utilization efficiency based on a mutual information amount per modulation symbol, may be used. Frequency utilization efficiency based on a mean mutual information per coded bit (MMIB) may be used. Since the throughput can be obtained by subtracting the overhead due to the reference signal, control signal, cyclic prefix, etc. from the product of “frequency utilization efficiency” and “transmission bandwidth”, generally the frequency utilization efficiency is the throughput. Is a relatively equivalent index. Therefore, when comparing the magnitude relationship of predicted throughput between ranks, it is equivalent to comparing the magnitude relationship of frequency utilization efficiency between ranks.

  The Shannon capacity as the information theoretical capacity when the rank (space multiplexing number) is 1 or 2 or more can be obtained, for example, as follows.

  FIG. 6 is a schematic diagram showing an example of a system model of a MIMO wireless transmission path when the rank (number of spatial multiplexing) is 2 or more. The transmission system of this model is a space division multiplexing (SDM) system, and the reception system is a linear MMSE system.

In the following description, data communication between the user apparatus and the radio base station is performed using a plurality of subcarriers. K is the number of transmission subcarriers or the number of modulation symbols per transmission interval, and k is the subcarrier number or modulation symbol number. Nt is the number of transmitting antennas, and Nr is the number of receiving antennas. Ps is the transmission power per antenna, L is the number of transmission substreams (the number of spatial multiplexing), and σ ² is the noise variance coefficient.

The frequency domain transmission line response, that is, the MIMO channel matrix H (k) in the k-th modulation symbol in the MIMO wireless transmission line of FIG. 6 is expressed by the following equation (1).

The transmission signal s (k) is encoded by the precoder P (k), transmitted from the transmission antenna, transmitted through the MIMO wireless transmission path H (k), and then received by the reception antenna. This received signal x (k) is expressed by the following equation (2). Here, n (k) is a noise signal added to the received signal, and P (k) is a precoding and transmission stream number control matrix (Nt × L matrix).

Here, the expected values of the transmission signals s (k) and s ′ (k) and the noise n (k) before and after encoding by the precoder P (k) are expressed by the following equations 3 to 5. The

The received signal y (k) after being processed by the receiving side spatial filter and the postcoder W ^H (k) is expressed by the following equation (6). W (k) is a spatial filtering weight vector W (k) expressed by Equation 7, and G (k) is a channel gain of the MIMO wireless transmission path.

FIG. 7 is a schematic diagram showing the system model of the MIMO wireless transmission path of FIG. 6 as an approximate equivalent parallel SISO channel transmission model. The expected value of the equivalent noise n ′ (k) in the figure is expressed by the following equation (8).

In addition, the channel gain G (k) of the approximate equivalent parallel SISO wireless transmission path of FIG. 7 can be approximated by the following equations 9 and 10, and the spatial filtering weight vector W (k) is approximated by the following equation 11. it can.

Based on the expected value of the equivalent noise n ′ (k), the channel gain G (k), and the spatial filtering weight vector W (k), the signal of the l-th substream of the k-th modulation symbol in the spatial multiplexing number L is represented as MMSE. The estimated value γ _{L, l} (k) of the desired signal to interference noise power ratio (SINR) when received by the method is expressed by the following equation (12).

Using the SINR estimate γ _{L, l} (k), the Shannon capacity (frequency utilization efficiency) C _L in the spatial division multiplexing (SDM) of the spatial multiplexing number L can be obtained by the following equation (13). .

  FIG. 8 is a schematic diagram illustrating an example of a system model of a MIMO wireless transmission path when the rank (number of spatial multiplexing) is 1. The transmission method of this model is transmission diversity by SFBC (Spatial Frequency Block Coding), and the reception method is MRC. Note that description of portions common to the example of spatial division multiplexing (SDM) in which the rank (number of spatial multiplexing) shown in FIGS. 6 and 7 is two or more is omitted.

In the MIMO wireless transmission path of FIG. 8, the transmission signal s (k) is encoded by the precoder P (k) and transmitted from the transmission antenna, transmitted through the MIMO wireless transmission path H (k), and then received by the reception antenna. The This received signal x (k) is expressed by the following equation (14).

Further, the received signal y (k) after being processed by the receiving side spatial filter and the postcoder W ^H (k) is expressed by the following equation (15).

FIG. 9 is a schematic diagram showing the system model of the MIMO wireless transmission path of FIG. 8 as an approximate equivalent parallel SISO channel transmission model. The expected value of the equivalent noise n ′ (k) in the figure is expressed by the following equation (16).

Based on the expected value of the equivalent noise n ′ (k), when SFBC transmission diversity is applied as a multi-antenna transmission scheme with a rank (spatial multiplexing number) of 1, the signal of the k-th modulation symbol is received by the MRC scheme. The estimated value γ _l (k) of the desired signal-to-interference noise power ratio (SINR) is expressed by the following approximate expression (17). Here, h _{nr, nt} (k) is the (n _r , n _t ) element of the MIMO channel matrix H (k).

Using the SINR estimate γ _l (k), the Shannon capacity (frequency utilization efficiency) C ₁ when SFBC transmission diversity is applied as a multi-antenna transmission scheme with a rank (spatial multiplexing number) of ₁ is given by It can be obtained by 18 approximate expressions.

Note that the Shannon capacity (frequency utilization efficiency) C ₁ calculation method shown in FIGS. 8 and 9 and Equations 14 to 18 is based on the transmission diversity of the multi-antenna transmission method using STBC (space-time block coding). The same applies to the case.

FIG. 10 shows the throughput [Mbps] calculated from the Shannon capacity as the information theoretical capacity described with reference to FIGS. It is the graph which compared the throughput [Mbps] measured with the transmission system (code modulation communication system).
FIG. 11 shows the throughput [Mbps] calculated from the signal point constrained capacity (CCC) as the information theoretical capacity and the actual wireless transmission system (coded modulation communication system) for each of the ranks 1 and 2. 6 is a graph comparing the throughput [Mbps] measured in (1). As described above, the signal point constrained capacity (CCC) is not a condition for a transmission signal having a Gaussian distribution, but as described in Non-Patent Document 7, in modulation such as quadrature phase shift keying (QPSK), 16QAM, and 64QAM. This is the capacity obtained under the condition of the transmission signal that constrains the signal points on the signal space diagram.

  10 and 11, in calculating the throughput from the Shannon capacity as the information theoretical capacity, a loss due to overhead such as a control signal is taken into consideration. Further, the transmission scheme in the case of rank 1 is transmission diversity by SFBC, and the reception scheme is MRC. The transmission scheme in the case of rank 2 is a space division multiplexing (SDM) scheme, and the reception scheme is linear MMSE.

  As shown in FIG. 10 and FIG. 11, the throughput obtained from the information theoretical capacity such as Shannon capacity and signal point constrained capacity for each of the ranks 1 and 2 is the actual radio transmission system (coded modulation communication system). Is far from the measured throughput. Therefore, an optimum rank cannot be obtained by simply comparing information theoretical capacities such as Shannon capacity and signal point constraint capacity in each rank.

  Therefore, in the rank adaptation in the present embodiment, the information theory such as the Shannon capacity and the signal point constrained capacity for each of the rank candidate values so as to approach the capacity in the actual wireless transmission system (coded modulation communication system). The capacity is corrected using a weighting factor. In addition to the correction using the weighting factor, clipping for providing an upper limit value for the corrected information theoretical capacity may be performed.

For example, information theoretical capacity C _L in the space division multiplexing (SDM) of the spatial multiplexing number L calculated by the formula or the like of the number 13 of the foregoing, it can be corrected using the formula for a number of 19. Further, the information theoretical capacity C ₁ in SFBC transmission diversity obtained by the above equation 18 can be corrected using the following equation 20.
By

In the correction example using the equation (19) above, the weighting coefficient α _{L, l} for the information theoretical capacity C _{L, l} (k) for each of the plurality of subcarriers (k) and the plurality of substreams (l). Is multiplied. Then, the information theoretical capacity α _{L, l} · C _{L, l} (k) after multiplying by the weighting coefficient α _{L, l} is averaged for a plurality of subcarriers (k) and a plurality of substreams (l). doing. When this averaging process is performed, clipping is performed to provide an upper limit value β _l for the corrected information theoretical capacity α _{L, l} · C _{L, l} (k).

Note that C _{L, l} (k) in the equation (19) is the information theoretical capacity in the k-th modulation symbol in the l-th substream when rank (spatial multiplexing number) L is reached. Further, the weighting coefficient α _{L, l in} the equation (19) is a loss coefficient for the information theoretical capacity in the l-th substream when the spatial multiplexing number is L. Further, the upper limit value β _l is a clipping parameter considering the available MCS in the l-th substream.

Further, in the correction example using the above equation (20), the information theoretical capacity C _1,1 (k) is multiplied by the weight coefficient α _1,1 for each of the plurality of subcarriers (k). Then, the information theoretical capacity α _1,1 · C _1,1 (k) after being multiplied by the weighting factor α _1,1 is averaged for a plurality of subcarriers (k). When this averaging process is performed, clipping is performed in which an upper limit value β ₁ is provided for the corrected information theoretical capacity α _1,1 · C _1,1 (k). In this case, the weighting factor α _{L, l} and the upper limit value β ₁ are a combination of a modulation scheme and an error correction coding scheme used in adaptive modulation and coding (AMC) exemplified in Table 1 ( MCS) is set.

As for the information-theoretic capacitance C _L in the space division multiplexing (SDM) of the spatial multiplexing number L calculated by the formula or the like of the number 13 of the aforementioned corrected using the formula for a number 21, number 18 of the aforementioned The information theoretical capacity C ₁ in SFBC transmission diversity obtained by an equation or the like may be corrected using the following equation (22). In these correction example, before the correction information theoretical capacity C _L in the space division multiplexing (SDM), an averaging process for a plurality of subcarriers (k) and a plurality of sub streams (l), a plurality of subcarriers An averaging process for (k) is performed. Further, prior to the correction information theoretical capacity C ₁ in SFBC transmission diversity is performed an averaging process for a plurality of sub-carrier (k). Since the correction methods used in Equations 21 and 22 can reduce the number of multiplications of the weighting coefficient compared to the correction methods of Equations 19 and 20, there is an effect of reducing the amount of calculation for correcting the information theoretical capacity. .

After correcting using the formulas 19 and 20 and the formulas 21 and 22 for each of the plurality of rank candidate values, as shown in the following formula 23, the corrected information theoretical capacity C _The rank that maximizes _{L 1} and C ₁ is determined as the rank used for the next data transmission in the MIMO wireless transmission path. Then, as the determined rank information, a rank identifier (RI :) for identifying the rank is notified to be fed back to the wireless communication device (wireless base station, user terminal device) on the data transmission side.

The correction parameters (weighting factors α _{L, l} , α _1,1 and upper limit values β ₁ , β ₁ ) used for correcting the information theoretical capacities C _L and C ₁ are based on, for example, the following LTE link level simulation. It can be determined by parameter tuning.

FIG. 12 is a graph showing an example of parameter tuning of the weighting coefficients α _1,1 and the upper limit value β ₁ in the case of SFBC transmission diversity (reception method: MRC) with a rank of 1.
“●”, “■”, and “◆” in the figure indicate three types of multipath fading channel models (EPA: Extended Pedestrian-A model, EVA: Extended Vehicular-A) defined in Non-Patent Document 8. Throughput [Mbps] calculated by LTE link level simulation using each model, ETU (Extended Typical Urban model). The values of RMS (Root Mean Square) delay spread of EPA, EVA, and ETU are about 45 [ns], 357 [ns], and 991 [ns], respectively. Among these channel models, the delay spread is the smallest in EPA. , ETU is the largest. The maximum Doppler frequency in each channel model was 5 [Hz]. Further, it is assumed that there is no feedback delay time and fading spatial correlation of the rank identifier RI. Other conditions were set in the same manner as in Table 1 above.
Also, the broken line in the figure is the throughput [Mbps] obtained from the Shannon capacity C1 as the information theoretical capacity calculated by using the equation (22). For calculating the throughput, the following equation 24 was used. W in the equation is an effective transmission bandwidth. Here, the effective transmission bandwidth is obtained by correcting the bandwidth of the frequency used for transmission in consideration of the overhead due to the reference signal, the control signal, and the OFDM guard interval.

In FIG. 12, the weights in the above formula 22 corresponding to the broken lines in the figure are fitted so as to fit the throughput calculation results (“●”, “■”, and “◆”) calculated by the LTE link level simulation. The coefficient α _1,1 and the upper limit value β _{1 of} clipping are adjusted and determined. In the fitting example indicated by the broken line in FIG. 12, 0.655 is determined as the weighting coefficient α _1,1 , and the maximum transmission data rate per rank is set as the upper limit value β ₁ of clipping.

FIG. 13 is a graph showing an example of parameter tuning of the weight coefficient α _{L, l} and the upper limit value β ₁ in the case of SDM (reception system: MMSE) combined with CDD precoding with a rank of 2.
“●”, “■”, and “◆” in the figure are LTE link level simulations using each of the three types of multipath fading channel models (EPA, EVA, ETU) as in FIG. Throughput [Mbps] calculated by It is assumed that the maximum Doppler shift in each channel model is 5 [Hz], and that there is no feedback delay time and fading spatial correlation of the rank identifier RI. Other conditions were set in the same manner as in Table 1 above.
Also, the broken line in the figure is the throughput [Mbps] obtained from the Shannon capacity Cl as the information theoretical capacity calculated by using the equation (21).

In FIG. 13, the weights in the equation (21) corresponding to the broken line in the figure are fitted so as to fit the throughput calculation results (“●”, “■”, and “◆”) calculated by the LTE link level simulation. The coefficient α _{L, l} and the upper limit value β _{1 of} clipping are adjusted and determined. In the fitting example indicated by the broken line in FIG. 13, 0.475 is determined as the weighting coefficients α _2,1 and α _2,2 , and the maximum transmission data rate per rank is determined as the clipping upper limit values β ₁ and β _2. doing.

Next, the effect of the correction parameters (weighting coefficients α _{L, l} , α _1,1 and upper limit values β ₁ , β ₁ ) used for correcting the information theoretical capacity C _L , C ₁ in rank adaptation will be described.

14 and 15 are graphs showing the effect of the correction parameters (weighting factors α _{L, l} , α _1,1 ) used for correcting the information theoretical capacities C _L , C ₁ in rank adaptation, respectively.
“●” in the figure indicates the information theoretical capacity C _L , corrected with the correction parameters (weighting coefficients α _{L, l} , α _1,1 and upper limit values β ₁ , β ₁ ) in the above formulas 21 and 22. shows the average throughput [Mbps] when executing the rank adaptation based on C _1.
Further, “▲” and “■” in the figure are average throughputs [Mbps] for ranks 1 and 2 when rank adaptation is not executed, respectively. The horizontal axis in the figure is the average SINR [dB] per receiving antenna.

  The average throughput was calculated by LTE link level simulation using EVA as a multipath fading channel model. Here, the feedback period of CQI was 20 [ms], the feedback period of rank identifier RI was 160 [ms], and the value of fading spatial correlation coefficient ρt was 0.9.

  FIG. 14 shows a case in which the maximum Doppler frequency shift fd is 5 [Hz] and a moving speed of about walking speed is assumed in the 2 GHz band. On the other hand, FIG. 15 shows a case where the maximum Doppler frequency fd is 70 [Hz] and the moving speed of the vehicle on a general road is assumed in the 2 GHz band.

In the examples of FIGS. 14 and 15, 0.655 is used as the weighting coefficient α _1,1 for correcting the information theoretical capacity C ₁ of rank 1, and the upper limit β ₁ of clipping is infinite (no clipping). It was. Further, 0.475 is used as the weighting coefficients α _2,1 and α _2,2 for correcting the information theoretical capacity C ₂ of rank 2, and the maximum transmission data rate per rank is used as the upper limit value β ₂ of clipping. .

14 and FIG. 15, the information theory of ranks 1 and 2 corrected by the correction parameters (weighting coefficients α _1,1 , α _2,1 , α _2,2 and upper limit values β ₁ , β ₂ ). Based on the target capacities C ₁ and C ₂ , 30 [dB] is set as the threshold value of the average SINR per receiving antenna for switching between ranks 1 and 2. Based on this threshold, in the data at the time of rank adaptation execution of “●” in the figure, the average SINR per receiving antenna is transmitted in rank 1 in the range of 30 [dB] or less, and the average SINR is higher than 30 [dB]. In a large range, transmission is performed at rank 2.

14 and 15 show the results when the fading spatial correlation between the transmitting antennas (fading spatial correlation coefficient ρt) is 0.9 and the Doppler frequency shift fd = 70 [Hz]. The coefficients α _1,1 , α _2,1 , α _2,2 and the upper limit values β ₁ , β ₂ ) are 0 in the fading spatial correlation between the transmitting antennas in FIGS. 12 and 13, and the Doppler frequency shift fd = 5 [Hz. ] Optimized values are used. As shown in the results of FIGS. 14 and 15, it can be seen that substantially appropriate rank control is achieved even in an environment of fading correlation and Doppler frequency shift different from the optimization premise. In this way, by appropriately selecting and setting the correction parameters (weighting factors α _1,1 , α _2,1 , α _2,2 and upper limit values β ₁ , β ₂ ), the transmission antennas in the MIMO wireless transmission path It is possible to realize rank adaptation (rank prediction) that is robust against changes in propagation environment such as fading spatial correlation (fading spatial correlation coefficient ρt) and moving speed (Doppler frequency shift fd). 14 and 15 show the feedback information (RI, CQI, ACK / NACK) for rank control, MCS control, and hybrid ARQ retransmission control that greatly affect the throughput characteristics as shown in Table 1. Since the evaluation result includes the influence of the feedback delay and the feasible feedback interval, by appropriately selecting and setting the correction parameters (upper clipping values β ₁ and β ₂ ), the feedback delay and It can be seen that rank adaptation (rank prediction) that is less susceptible to the feedback information amount limitation is also realized.

FIGS. 16 and 17 each use ETU (delay spread: 991 [ns]) as a multipath model, and are used for correcting the information theoretical capacities C _L and C ₁ in rank adaptation when the delay spread is relatively large. 10 is a graph showing the effect of the correction parameters (upper clipping values β ₁ , β ₁ ). FIG. 16 is a graph showing an example of throughput when rank adaptation that does not perform clipping with an upper limit value applied to the correction of the information theoretical capacities C _L and C ₁ is applied. FIG. 17 is a graph illustrating an example of throughput when rank adaptation is performed in which clipping with an upper limit is applied to the correction of the information theoretical capacities C _L and C ₁ .

“●” in the figure indicates the information theoretical capacity C _L , corrected with the correction parameters (weighting coefficients α _{L, l} , α _1,1 and upper limit values β ₁ , β ₁ ) in the above formulas 21 and 22. shows the average throughput [Mbps] when executing the rank adaptation based on C _1.
Further, “▲” and “■” in the figure are average throughputs [Mbps] for ranks 1 and 2 when rank adaptation is not executed, respectively. The horizontal axis in the figure is the average SINR [dB] per receiving antenna.

  The average throughput was calculated by LTE link level simulation using ETU (Extended Standard City Model) as a multipath fading channel model. The value of the fading spatial correlation coefficient ρt was set to 0.9.

  16 and 17 both show a case where the maximum Doppler frequency shift fd is 70 [Hz] and a moving speed of about the vehicle speed is assumed in the 2 GHz band.

In FIGS. 16 and 17, 0.655 is used as the weighting coefficient α _1,1 for correcting the information theoretical capacity C ₁ of rank 1, and the weighting coefficient for correcting the information theoretical capacity C ₂ of rank 2 is used. As α _2,1 and α _2,2 , 0.475 was used.

In FIG. 16, the upper limit β _{1 of} clipping for correcting the information theoretical capacity C ₁ of rank 1 is set to infinity (no clipping), and the upper limit of clipping for correcting the information theoretical capacity C ₂ of rank 2 is set. value β ₁ was also infinite (no clipping). On the other hand, in FIG. 17, the upper limit value β _{1 of} clipping for correcting the information theoretical capacity C ₁ of rank ₁ is 6.20, and the upper limit value of clipping for correcting the information theoretical capacity C ₂ of rank 2 is set. β ₂ was set to 5.55.

In the examples of FIGS. 16 and 17, the information theories of ranks 1 and 2 corrected with the correction parameters (weighting coefficients α _1,1 , α _2,1 , α _2,2 and upper limit values β ₁ , β ₂ ) are used. Based on the target capacities C ₁ and C ₂ , 30 [dB] is set as the threshold value of the average SINR per receiving antenna for switching between ranks 1 and 2. Based on this threshold, in the data at the time of rank adaptation execution of “●” in the figure, the average SINR per receiving antenna is transmitted in rank 1 in the range of 30 [dB] or less, and the average SINR is higher than 30 [dB]. In a large range, transmission is performed at rank 2.

As shown in the result of FIG. 17 with respect to FIG. 16, by appropriately selecting and setting the correction parameters (upper clipping values β ₁ and β ₂ ), it is possible to avoid overestimating the capacity in low rank transmission, and MIMO. It is possible to realize rank adaptation (rank prediction) that is robust against changes in delay spread in the wireless transmission path. In addition, as shown in Table 1, the results of FIG. 17 show feedback delays and realization of feedback information (RI, CQI, ACK / NACK) in rank control, MCS control, and hybrid ARQ retransmission control that greatly affect throughput characteristics. Since the evaluation result includes the influence of the feedback interval, the above correction parameters (upper clipping values β ₁ , β ₂ ) are appropriately selected and set so that the feedback delay and the feedback information amount limit can be reduced. It can be seen that rank adaptation (rank prediction) that is not easily affected is also realized.

As described above, according to the present embodiment, it is possible to accurately determine the rank used for the next data transmission in accordance with the change in the propagation environment based on the SINR in the actual MIMO wireless transmission path, and to suppress the degradation of the throughput. can do. Therefore, it is possible to improve rank switching accuracy based on SINR in a MIMO wireless transmission system wireless transmission path and suppress degradation of throughput.
In particular, according to the present embodiment, by setting an upper limit value β in the information theoretical capacity, the influence of fluctuations in wireless transmission performance due to changes in the environment of the wireless transmission path such as the spread of multipath delay in the transmission stream is taken into consideration. The capacity for the candidate value of each rank can be calculated with high accuracy. Therefore, robust rank switching can be performed against changes in the environment of the wireless transmission path such as spread of multipath delay.
Furthermore, since the capacity for each rank candidate value can be accurately calculated in consideration of the transmission loss in the transmission stream, the modulation scheme, and the coding scheme, the rank of the rank based on the SINR in the radio transmission path of the MIMO radio transmission scheme can be calculated. Switching accuracy can be further increased.
In addition, since the evaluation result includes the influence of feedback delay in rank control, MCS control, and hybrid ARQ retransmission control that greatly affects the throughput characteristics, the correction parameter (upper limit values β ₁ and β _{2 of} clipping) are used. It can be seen that rank adaptation (rank prediction) that is not easily affected by feedback delay is realized at the same time by appropriately selecting and setting ().

  In this specification, for the sake of simplicity, the maximum transmission rank is 2, the number of transmission antennas is 2, the number of reception antennas is 2, the precoding scheme is fixed for each transmission rank, and PMI feedback is not required. 3GPP LTE / LTE -The case of an advanced downlink open-loop MIMO radio transmission system (Open-Loop MIMO) has been described in detail. Since the rank prediction means shown in this specification applies a plurality of precoding patterns for each transmission rank, it can be similarly applied to a closed-loop MIMO wireless transmission system (Closed-Loop MIMO) using PMI feedback and an uplink. And can be applied to various MIMO wireless transmission systems to which rank adaptation is applied.

  Note that the processing steps and the components of the wireless communication system and the wireless communication device described in this specification can be implemented by various means. For example, these steps and components may be implemented in hardware, firmware, software, or a combination thereof.

  For hardware implementation, means such as a processing unit used to realize the above steps and components in an entity (for example, various wireless communication devices, Node B, terminal, hard disk drive device, or optical disk drive device) One or more application specific ICs (ASICs), digital signal processors (DSPs), digital signal processing units (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors , A controller, a microcontroller, a microprocessor, an electronic device, other electronic units designed to perform the functions described herein, a computer, or a combination thereof.

  Also, for firmware and / or software implementation, means such as processing units used to implement the above components may be programs (eg, procedures, functions, modules, instructions) that perform the functions described herein. , Etc.). In general, any computer / processor readable medium that specifically embodies firmware and / or software code is means such as a processing unit used to implement the steps and components described herein. May be used to implement For example, the firmware and / or software code may be stored in a memory, for example, in a control device, and executed by a computer or processor. The memory may be implemented inside the computer or processor, or may be implemented outside the processor. The firmware and / or software code may be, for example, random access memory (RAM), read only memory (ROM), nonvolatile random access memory (NVRAM), programmable read only memory (PROM), electrically erasable PROM (EEPROM) ), FLASH memory, floppy disk, compact disk (CD), digital versatile disk (DVD), magnetic or optical data storage, etc. Good. The code may be executed by one or more computers or processors, and may cause the computers or processors to perform the functional aspects described herein.

  Also, descriptions of embodiments disclosed herein are provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to the present disclosure will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. The present disclosure is therefore not limited to the examples and designs described herein, but should be accorded the widest scope consistent with the principles and novel features disclosed herein.

DESCRIPTION OF SYMBOLS 10, 30 User apparatus 20, 40 Radio base station 101 Downlink channel estimation part 102 Downlink control signal demodulation part 103 Data signal demultiplexing / combination part 104 Serial / parallel conversion part 105 Control information (RI / PMI / CQI) generation part 106 Uplink Transmission unit 201 Uplink reception unit 202 Downlink scheduler 203 Precoding weight generation unit 204 Downlink control signal generation unit 205 Serial parallel conversion / modulation unit 206 Multiplier 208 Multiplexer 301 Downlink reception unit 302 Precoding weight generation unit 303 Serial parallel Conversion / Modulation Unit 304 Multiplier 307 Uplink Reference Signal Generation Unit 401 Uplink Channel Estimation Unit 404 Uplink Scheduler 405 Downlink Transmitter

JP 2012-105271 A

3GPP TS36.211 V10.6.0, "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10)," Dec. 2012. 3GPP TS36.212 V10.7.0, "Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding (Release 10)," Dec. 2012. 3GPP TS36.213 V10.8.0, "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 10)," Dec. 2012. H. Taoka, S. Nagata, K Takeda, Y. Kakishima, X. She, and K. Kusume, "MIMO and CoMP in LTE-Advanced," NTT DoCoMo Technical Journal (English Edition), vol.12, no.2 , pp.20-28, Sept. 2010. E. Dahlman, S. Parkvall and J. Skold, "4G LTE / LTE-Advanced for Mobile Broadband," Academic Press, 2011. X. Yang, Y. Xiong and W. Zhao, "Cross-layer design of MIMO OFDM with mode switching and hybrid ARQ," in Proceedings of the 5th International Conference on Wireless Communications, Networking and Mobile Computing 2009 (WiCOM2009), Beijing, China, Sept. 2009. IEEE 802.16 BWA Working Group, "IEEE 802.16m Evaluation Methodorogy Document (EMD)," IEEE 802.16m-08 / 004r2, July 2008. 3GPP TS36.101 V11.3.0, "Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception (Release 11)", Dec. 2012.

Claims (10)

A wireless transmission system capable of transmitting and receiving data using a plurality of different transmission streams between a plurality of wireless communication devices,
Estimated value of desired signal-to-interference noise power ratio (SINR) calculated from an estimated value of transmission path response and an estimated value of noise power acquired from signals transmitted and received on a wireless transmission path between the plurality of wireless communication devices Based on the above, for each of a plurality of rank candidate values in the wireless transmission path, a means for obtaining an information theoretical capacity considering the transmission scheme and reception scheme in the wireless transmission path;
Means for correcting the information theoretical capacity using a weighting factor for each of the plurality of rank candidate values;
Means for determining a rank that maximizes the information theoretical capacity among information theoretical capacities obtained and corrected for each candidate value of the plurality of ranks as a rank to be used for next data transmission in the wireless transmission path;
A wireless transmission system comprising: The wireless transmission system of claim 1,
The wireless transmission system according to claim 1, wherein the means for correcting the information theoretical capacity performs clipping that provides an upper limit value for the information theoretical capacity in addition to correction using a weighting factor for the information theoretical capacity. The wireless transmission system of claim 2,
The weighting factor is a loss factor corresponding to a transmission loss in the transmission stream;
The radio transmission system according to claim 1, wherein the upper limit value in the clipping is set according to a modulation scheme and a coding scheme used in the transmission stream. The wireless transmission system according to any one of claims 1 to 3,
A wireless transmission system using a Shannon capacity or a signal point constrained capacity as the information theoretical capacity. The wireless transmission system according to any one of claims 1 to 4,
Transmission and reception of signals in the wireless transmission path is performed via a plurality of subcarriers,
The means for obtaining the information theoretical capacity obtains an information theoretical capacity in consideration of a transmission scheme and a reception scheme in the wireless transmission path for each of the plurality of subcarriers for each of the plurality of rank candidate values,
The means for correcting the information theoretical capacity corrects the information theoretical capacity obtained for each of the plurality of subcarriers for each of the plurality of rank candidate values,
The means for determining the rank is:
For each candidate value of the plurality of ranks, the information theoretical capacity obtained and corrected for each of the plurality of subcarriers is added or averaged over all or part of the plurality of subcarriers,
The rank in which the added value or average value of the information theoretical capacity obtained for each candidate value of the plurality of ranks is maximized is determined as a rank used for the next data transmission in the wireless transmission path. Wireless transmission system. A wireless communication device on a data receiving side in a wireless transmission system capable of transmitting and receiving data by a plurality of different transmission streams between a plurality of wireless communication devices,
Estimated value of desired signal-to-interference noise power ratio (SINR) calculated from an estimated value of transmission path response and an estimated value of noise power acquired from signals transmitted and received on a wireless transmission path between the plurality of wireless communication devices Based on the above, for each of a plurality of rank candidate values in the wireless transmission path, a means for obtaining an information theoretical capacity considering the transmission scheme and reception scheme in the wireless transmission path;
Means for correcting the information theoretical capacity using a weighting factor for each of the plurality of rank candidate values;
Means for determining a rank that maximizes the information theoretical capacity among information theoretical capacities obtained and corrected for each candidate value of the plurality of ranks as a rank to be used for next data transmission in the wireless transmission path;
Means for notifying information of the determined rank to the wireless communication device on the data transmission side;
A wireless communication apparatus comprising: The wireless communication apparatus according to claim 6.
The wireless communication apparatus characterized in that the means for correcting the information theoretical capacity performs clipping that provides an upper limit value for the information theoretical capacity in addition to correction using a weighting factor for the information theoretical capacity. The wireless communication apparatus according to claim 7.
The weighting factor is a loss factor corresponding to a transmission loss in the transmission stream;
An upper limit value in the clipping is set according to a modulation scheme and a coding scheme used in the transmission stream. The wireless communication device according to any one of claims 6 to 8,
A wireless communication apparatus using a Shannon capacity or a signal point constrained capacity as the information theoretical capacity. The wireless communication device according to any one of claims 6 to 9,
Transmission and reception of signals in the wireless transmission path is performed via a plurality of subcarriers,
The means for obtaining the information theoretical capacity obtains an information theoretical capacity in consideration of a transmission scheme and a reception scheme in the wireless transmission path for each of the plurality of subcarriers for each of the plurality of rank candidate values,
The means for correcting the information theoretical capacity corrects the information theoretical capacity obtained for each of the plurality of subcarriers for each of the plurality of rank candidate values,
The means for determining the rank is:
For each candidate value of the plurality of ranks, the information theoretical capacity obtained and corrected for each of the plurality of subcarriers is added or averaged over all or part of the plurality of subcarriers,
The rank in which the added value or average value of the information theoretical capacity obtained for each candidate value of the plurality of ranks is maximized is determined as a rank used for the next data transmission in the wireless transmission path. Wireless communication device.