JP5642046B2 - Receiving apparatus and receiving method in MIMO-OFDM transmission - Google Patents

Description

  The present invention relates to a receiving apparatus and a receiving method for receiving a transmission signal from a transmitting apparatus in MIMO-OFDM transmission, and in particular, a receiving apparatus that performs soft decision error correction decoding using soft decision bits based on a log likelihood ratio, and It relates to the receiving method.

  A service of a long term evolution system LTE (Long Term Evolution) of the third generation mobile communication system has been started. In LTE, particularly in downlink communication, an OFDM (Orthogonal Frequency Division Multiplexing) transmission scheme is used. The OFDM transmission scheme has advantages such as being strong against intersymbol interference and fading accompanying multipath transmission and having high frequency utilization efficiency.

  As one means for increasing the transmission capacity of a mobile communication system, there is a multiple input multiple output (MIMO) transmission system that uses a plurality of antennas for transmission between transmitting and receiving apparatuses. Spatial multiplexing can be performed by transmitting different data from the antenna. If the number of antennas on the receiving side is greater than or equal to the number of antennas on the transmitting side, spatial multiplexing for the number of antennas on the transmitting side is possible.

  Thus, in the MIMO transmission system, signals transmitted from a plurality of transmission antennas by spatial multiplexing are received by the reception antenna while being mixed on the radio channel. For this reason, it is necessary to separate the MIMO signal into signals for each transmission antenna on the receiving side, and the method of separating the MIMO signal is roughly divided into a method using linear signal processing and a method using nonlinear signal processing. . As a method using nonlinear signal processing, a method using maximum likelihood detection (MLD) is known. This method is a method of selecting the most probable signal point from all the candidate points of the transmission signal, and has an advantage that a good decoding characteristic can be obtained. Accordingly, since the amount of calculation increases exponentially, it is difficult to implement in practice. On the other hand, as a method using linear signal processing, there are methods using zero forcing (ZF) and least square error method (Minimum Mean Square Error: MMSE). However, since it can reduce the amount of calculation, it is actually widely used.

  In MIMO-ODFM transmission, which combines the above-described OFDM transmission and MIMO transmission, signal processing is performed in a composite region of a frequency region and a spatial region, so that communication quality and communication capacity can be improved. For this reason, it is adopted in a new generation mobile communication system such as LTE.

  In general, in a mobile communication system, forward error correction coding is performed on the transmission side in order to improve transmission reliability of signal transmission. As this forward error correction coding, a convolutional code or a turbo code is generally used. Used. In addition, methods using LDPC codes are also being studied. On the receiving side, Maximum Likelihood (ML) decoding and Maximum A Posteriori Probability (MAP) decoding are used to decode these forward error correction encoded signals. Maximum likelihood decoding is mainly used for decoding convolutional codes, and MAP decoding is used for decoding turbo codes and LDPC codes. As an algorithm for efficiently realizing maximum likelihood decoding, there is a well-known Viterbi algorithm. For turbo codes and LDPC codes, decoding is performed using a repetitive process such as the well-known BCJR algorithm, Max-Log-MAP algorithm, Sum-Product algorithm, or the like.

  The decoding includes hard decision decoding using hard decision bits as input data to the error correction decoder and soft decision decoding using soft decision bits. From the viewpoint of error correction capability, soft decision decoding is performed. Is called. However, in soft decision decoding, there is a problem with a method of generating soft decision bits from a received signal. In particular, in a MIMO system, the soft decision bit generation method differs depending on the transmission and reception methods.

  In addition, in the method of performing MIMO signal separation using the above-described nonlinear signal processing, a method of generating an optimum soft decision bit has been studied. For example, there are Patent Documents 1 and 2 below.

  Patent Document 1 describes a soft decision bit generation method in maximum likelihood detection (MLD) and a calculation method thereof, and Patent Document 2 discloses a soft decision bit generation method in a calculation amount reduction type MLD. Have been described.

JP 2008-11461 A JP 2011-04142 A

  In the above-described signal separation method using linear signal processing (hereinafter referred to as linear spatial filtering), it is necessary to generate an optimum soft decision bit, but no optimum method has been proposed yet.

  An object of the present invention is to provide a receiving apparatus and a receiving method for generating soft decision bits by applying linear spatial filtering to a received signal, particularly on the receiving side of MIMO-OFDM transmission.

  The present invention is a receiving apparatus that receives a signal transmitted from a transmitting apparatus in MIMO-OFDM transmission in which a reference signal for channel estimation is multiplexed on a modulated signal for error correction coded user data transmission. Based on the received signal of the signal portion, a channel estimation unit that estimates a MIMO channel matrix and noise power density, and linear spatial filtering using the MIMO channel matrix separates and combines the received signal into signals for each transmission layer A signal separation unit; a reception SINR calculation unit that calculates a reception SINR (Signal-to-Interference Noise Ratio) of each transmission layer based on the MIMO channel matrix and noise power density for each OFDM subcarrier; and the separation / combination Channel log likelihood ratio (Log-Likelihood Rat) of each transmission layer based on the received signal and transmission signal candidate points. io: LLR) for each bit, a channel log likelihood ratio calculation unit, and a decoding unit that performs soft decision error correction decoding using the channel log likelihood ratio weighted by the received SINR. It is characterized by.

  Further, the present invention provides a signal receiving method for receiving a signal transmitted from a transmitting apparatus in MIMO-OFDM transmission in which a reference signal for channel estimation is multiplexed with a modulated signal for user data transmission subjected to error correction coding. A step of estimating a MIMO channel matrix and noise power density based on a received signal of a reference signal portion, a step of separating and combining signals for each transmission layer by linear spatial filtering using the MIMO channel matrix, and the MIMO channel Calculating the received SINR of each transmission layer for each OFDM subcarrier based on the matrix and the noise power density; and, based on the separated and combined received signal and the transmission signal candidate point, the channel logarithmic likelihood for each transmission layer. Calculating a ratio for each bit and weighting by the received SINR It was using the channel log-likelihood ratios, and having the steps of: performing a soft-decision error correction decoding.

It is a figure which shows the transmission side and reception side model which perform forward error correction encoding. It is a figure which shows the OFDM transmission / reception model in SISO (Single Input Single Output) transmission. It is a figure which shows the OFDM transmission / reception model in MIMO transmission. It is a figure which shows the equivalent parallel SISO transmission / reception model of the MIMO system based on this invention. It is a figure which shows the transmitter structure in a MIMO-OFDM system. It is a figure which shows the receiver structure in the MIMO-OFDM system based on this invention. It is a graph showing a block error rate characteristic.

  First, a method for separating and combining received signals in a MIMO system will be described with reference to FIG. FIG. 3 shows a transmission / reception model of a MIMO system in the OFDM transmission system, where reference numeral 31 is a serial-parallel converter, 32 is a forward error correction (FEC) encoder, 33 is a modulator, and 34 is a precoder. These are the transmitting side devices. Reference numeral 35 denotes a radio propagation path, which can be modeled as a MIMO channel matrix. 36 is a spatial filter in the receiving side device, 37 is a channel log likelihood ratio generator, 38 is a layer demapper, 39 is a soft decision error correction decoder, and 40 is a parallel-serial converter. In the MIMO system of this figure, when the number of transmission antennas is Nt, the number of reception antennas is Nr, and the number of transmission layers (the number of spatial multiplexing) is M, the number of reception antennas is the same as or larger than the number of transmission antennas (ie, Nt <Nr), the maximum value of the number of transmission layers M is the same as the number of transmission antennas. In LTE, rank adaptation for selecting the optimum number M of transmission layers according to the state of the radio propagation path can be applied. K represents a subcarrier number.

  In FIG. 3, transmission symbols are converted into parallel signals for the number of transmission layers by a serial-parallel converter 31, error-corrected by a forward error correction code (FEC) encoder 32, and modulated by a modulator 33. Done. Thereafter, in the precoder 34, a predetermined precoding matrix is multiplied and transmitted on the radio channel.

3, transmission symbol streams s ₁ (k), s ₂ (k),..., S _M (k) interfering with each other on the radio channel are converted into a linear space by a spatial filter 36. Separation and synthesis by filtering process. As described above, it is possible to separate and synthesize received signals by nonlinear signal processing, but only more realistic linear spatial filtering processing is considered here. Also, here, as linear spatial filtering processing, a method based on zero forcing (ZF) in which the interference power between substreams is zero, and a least square error (MMSE) that minimizes the desired signal-to-interference noise power ratio SINR are used. A method based is described, but is not limited to these two methods.

  An equivalent transmission / reception model using the M independent SISO transmissions is shown in FIG. FIG. 4 is also a diagram for explaining the present invention. In FIG. 4, reference numeral 32 denotes a transmission-side forward error correction encoder, 33 denotes a modulator, 37 denotes a reception-side channel log likelihood ratio generator, and 39 denotes a soft decision error correction decoder. These configurations are the same for each of the M routes.

As shown in the above equation (11), the channel log likelihood ratio L _ch (of the l-th bit c _i (k, l) of the symbol s _i (k) transmitted on the k-th subcarrier of the i-th substream. In calculating c _i (k, l)), the log likelihood ratio is weighted by the received SINR of the symbol, whereby an optimal channel log likelihood ratio can be obtained for each substream.

  It should be noted that although the optimal channel log likelihood ratio can be obtained by performing weighting with the received SINR, weighting may be performed with a value related to the received SINR.

  The received SINR of the above equation (12) can be developed specifically as follows by the method of linear filtering.

  As described above, the optimum method for generating the channel log likelihood ratio in the MIMO-OFDM transmission has been described above. In the following, in order to supplement the above description, a method for calculating the channel log likelihood ratio using the SISO model will be described.

By using the above-mentioned channel log likelihood ratio L _ch (c _{l, p} ) as the soft decision bit value to be input to the soft decision error correction decoder 16, Pr (r ′ _{l, p} | c _{l, p} ) is obtained, and maximum likelihood decoding can be performed.

  As described above, in soft decision error correction decoding, the channel log likelihood ratio of each transmitted bit can be used as soft decision bits input to the soft decision error correction decoder. Next, a specific method for generating soft decision bits in the SISO system will be described.

  FIG. 2 shows an OFDM transmission / reception model in the SISO system, where reference numeral 11 is a forward error correction encoder on the transmission side, 13 is a modulator, 21 is a radio propagation channel response modeling a radio channel, and 14 is on the reception side. A channel log likelihood ratio generator 16 is a soft decision error correction decoder. Note that the interleaver and deinterleaver shown in FIG. 1 are omitted.

  The method for calculating the channel log likelihood ratio using the SISO transmission model has been described above. As described above, MIMO-OFDM transmission can be equivalently expressed as an independent SISO transmission model for each transmission layer. The channel log likelihood ratio of SISO transmission is calculated by weighting the difference between log likelihood A and log likelihood B by the reception SNR of the corresponding subcarrier as shown in equation (51). . For this reason, also in MIMO-OFDM transmission, as shown in Equation (11), in each substream (transmission layer), reception of a corresponding subcarrier is performed with respect to a difference between log likelihood A and log likelihood B. By weighting with the SNR, the channel log likelihood ratio can be calculated. However, it should be noted that in Equation (11), the interference SI component is also taken into consideration and weighted by the received SINR.

  Next, the MIMO-OFDM transmitter configuration in the present invention is shown in FIG. 5, and the receiver configuration is shown in FIG. Since the transmitter configuration is the same as that of the conventional method, detailed description is omitted. Here, only the data is precoded by the precoder 34, and the reference signal generated by the reference signal generator 54 is not precoded. At the receiver, the channel estimator 79 estimates the MIMO channel matrix and noise power. Using the estimated MIMO channel matrix and noise power, the spatial filter coefficient generator 80 generates spatial filter coefficients. The spatial filter 36 performs linear filtering such as MMSE and ZF, and separates and combines the received signals. The spatial filter output SINR calculation unit 81 calculates a reception SINR corresponding to the type of linear filtering, as shown in Expression (17). The channel log likelihood ratio calculation unit 37 calculates the channel log likelihood ratio as in the conventional method, and then weights each substream with the received SINR output from the output SINR calculation unit 81. The weighted channel log likelihood ratio is input to the soft decision error correction decoder 39, and the received signal is decoded by the soft decision.

The effect of the present invention is shown in the graph of FIG. FIG. 7 shows the average block error rate (BLER) with respect to the average E _s / N ₀ (symbol power per noise power) per receiving antenna obtained by computer simulation. Table 1 shows the simulation conditions. The MIMO antenna configuration is 2 × 2 (2 transmission antennas, 2 reception antennas), and MMSE is used as linear filtering. Error correction coding is performed by a turbo code, and the decoder performs Max-Log-MAP decoding with 8 repetitions.

In FIG. 7, a dotted line (left end) is a characteristic by maximum likelihood determination (MLD) and shows an ideal characteristic. The solid line shows the characteristics in the present invention, and the channel log likelihood ratio is weighted by the received SINR. A broken line (right end) is a characteristic in the conventional method, and weighting is not performed on the channel log likelihood ratio. That is, the weighting coefficient in the present invention is set to 1. At the received E _s / N ₀ that achieves an average BLER = 10 ^{−1, the performance according} to the present invention is improved by about 5 dB compared to the performance by the conventional method (reduced from about 19 dB to 14 dB). ). That is, in the present invention, an average BLER characteristic equivalent to that of the conventional method can be obtained in a propagation path environment in which the received E _s / N ₀ is 5 dB lower than that of the conventional method. In addition, it can be seen that only about 2 dB of degradation can be seen compared to the characteristics of MLD.

  As described above, according to the present invention, the soft decision bits input to the soft decision error correction decoder are generated by weighting the channel log likelihood ratio with the received SINR for each substream, thereby generating the conventional method. Compared with, significant improvement in characteristics is obtained, and the effectiveness of the present invention can be confirmed.

Claims (2)

A receiving apparatus for receiving a signal transmitted from a transmitting apparatus in MIMO-OFDM transmission in which a reference signal for channel estimation is multiplexed on a modulated signal for user data transmission that has been subjected to error correction coding,
A channel estimation unit for estimating a MIMO channel matrix and noise power density based on the received signal of the reference signal portion;
A spatial filter unit that separates and combines a received signal into a signal for each transmission layer by linear spatial filtering using the MIMO channel matrix;
Based on the MIMO channel matrix and noise power density , for each type of linear filtering, an output SINR calculation unit that calculates the reception SINR of each transmission layer for each OFDM subcarrier;
A channel log likelihood ratio calculation unit that calculates a channel log likelihood ratio of each transmission layer for each bit based on the separated and combined received signal and transmission signal candidate points;
A decoding unit that performs soft decision error correction decoding using the channel log likelihood ratio weighted by the received SINR calculated for each type of linear filtering ;
A receiving apparatus comprising: A signal reception method for receiving a signal transmitted from a transmission apparatus in MIMO-OFDM transmission in which a reference signal for channel estimation is multiplexed on a modulation signal for user data transmission that has been subjected to error correction coding,
Estimating a MIMO channel matrix and noise power density based on the received signal of the reference signal portion;
Separating and combining received signals into signals for each transmission layer by linear spatial filtering using the MIMO channel matrix;
Calculating the received SINR of each transmission layer for each OFDM subcarrier , for each type of linear filtering , based on the MIMO channel matrix and noise power density;
Calculating a channel log likelihood ratio for each transmission layer for each bit based on the separated and combined received signal and transmission signal candidate points;
Performing soft decision error correction decoding using the channel log likelihood ratio weighted by the received SINR calculated for each type of linear filtering ;
A signal receiving method comprising:

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