JP7219157B2 - Transmission device and transmission method - Google Patents

Description

The present invention relates to a data transmission system for multiplexing and transmitting a plurality of data by hierarchical division multiplexing.

As part of the transition to next-generation terrestrial digital broadcasting, studies are progressing on a layered division multiplexing (LDM) system that transmits broadcast waves of a system different from the current 2K broadcasting by sharing frequency and time. there is In Non-Patent Document 1, the upper layer (Upper Layer; UL) and the lower layer (Lower Layer; LL) are OFDM (Orthogonal Frequency Division Multiplexing) with the same number of FFT (Fast Fourier Transform) points. ) and multiplex UL and LL on the same subcarrier at different levels. Also, Non-Patent Document 2 proposes a method of OFDM-modulating UL and LL with different numbers of FFT points and multiplexing them in the time domain. Hereinafter, the former method (Non-Patent Document 1) will be referred to as “synchronous LDM”, and the latter method (Non-Patent Document 2) will be referred to as “quasi-synchronous LDM”.

In quasi-synchronous LDM, as shown in FIG. 3, for UL, for example, 8192 FFT points are used to generate effective symbols, and a 1024-sample guard interval is added to generate 9216-sample OFDM symbols. On the other hand, for LL, 32768 FFT points are used to generate effective symbols, and by adding the same guard interval of 1024 samples as in UL, OFDM symbols of 33792 samples are generated. In this case, the integer ratio of 11:3 between the UL symbol and the LL symbol is the lowest common multiple, and the start timings of the UL symbols and the LL symbols match at a predetermined cycle.

Thus, in quasi-synchronous LDM, the guard interval lengths of UL and LL are the same, and the effective symbol length of LL is longer than that of UL. However, LL can improve the bit rate. Taking advantage of this feature, Non-Patent Document 2 proposes applying the quasi-synchronous LDM to the next-generation digital terrestrial system. Although Non-Patent Document 2 describes the efficiency and characteristics of quasi-synchronous LDM, it does not clarify the detailed demodulation scheme of quasi-synchronous LDM.

Akihiko Sato, 11 others, "Study on application of LDM for next-generation terrestrial broadcasting", Institute of Image Information and Television Engineers Technical Report, vol. 41, no. 6, BCT2017-34, pp.45-48, February 2017 Hiromasa Okada, 6 others, “Study on improvement of various problems when applying LDM to terrestrial digital broadcasting -Study on new broadcasting method reception area expansion method and synchronization method-”, Institute of Image Information and Television Engineers Technical Report, vol. 42, no. 28, BCT2018-76, pp.13-16, September 2018

In synchronous LDM, the same FFT time window is provided for both the UL and LL received signals, and by performing FFT on the signal within the FFT time window, each subcarrier maintains the orthogonal relationship and the frequency domain is converted to a signal. However, since the number of FFT points differs between UL and LL in quasi-synchronous LDM, the orthogonal relationship between subcarriers is lost, and demodulation cannot be performed using the same method as in synchronous LDM.

Similarly to the synchronous LDM receiver, the quasi-synchronous LDM receiver generates a replica of the UL received signal and subtracts it from the received signal to extract the LL signal. When generating a replica of this UL received signal, it is necessary to estimate the characteristics of the transmission path with high accuracy. For channel estimation, a method of estimating from pilot signals distributed in the UL signal in the frequency direction and the time direction is generally used. However, since the LL signal in quasi-synchronous LDM behaves like noise for the UL signal, an equivalent CNR (Carrier to Noise Ratio) drop occurs in proportion to IL (Injection Level). IL is a value representing the ratio of the UL signal level to the LL signal level. Due to such a problem, the CNR of the pilot carrier is lowered and the accuracy of transmission path estimation is also lowered, so there is a problem that the replica of the UL received signal cannot be correctly generated on the receiving side.

SUMMARY OF THE INVENTION The present invention has been made in view of the conventional circumstances as described above, and it is an object of the present invention to provide a transmitting apparatus capable of reducing the interference that the LL signal component exerts on the pilot carrier of the UL signal. do.

In order to achieve the above objects, the present invention configures a transmitter as follows.
That is, in a transmitting apparatus used in a data transmission system that multiplexes and transmits a plurality of data by hierarchical division multiplexing, a first subcarrier signal containing first pilot carriers distributed in the frequency direction and the time direction a first generating means for performing IFFT processing with a first number of points on the subcarrier signal to generate a first modulated signal; and a second subcarrier signal with IFFT processing with a second number of points to generate a second modulated signal. second generating means for generating the first modulated signal and the second modulated signal, synthesizing means for synthesizing the first modulated signal and the second modulated signal so that their start timings match each other at a predetermined cycle, and the synthesizing means sending means for sending a combined signal, wherein the second generating means suppresses interference of the second subcarrier signal to the first pilot carrier by the second modulated signal. It is characterized by including an interference suppression carrier for.

Here, the second generation means allocates the interference suppression carrier to a subcarrier having a frequency closest to the first pilot carrier among a plurality of subcarriers included in the second subcarrier signal. may be configured.

Further, the second generation means may be configured to generate the interference suppression carrier by calculation shown in (Equation 8) or (Equation 9) described later. Note that this description is not intended to be limited to the same calculation as (Equation 8) or (Equation 9) described later, and other equivalent calculations (that is, (Equation 8) or (Equation 8) described later or It also includes generating the interference suppressing carrier by other calculations that can obtain the same result as the expression 9).

Advantageous Effects of Invention According to the present invention, it is possible to provide a transmitting apparatus capable of reducing interference exerted by LL signal components on pilot carriers of UL signals.

1 is a diagram showing a configuration example of a transmission device in a data transmission system according to an embodiment of the present invention; FIG. FIG. 2 is a diagram showing a configuration example of a dummy mapping unit in the transmission device of FIG. 1; FIG. FIG. 4 is a diagram showing an example of a modulated signal of quasi-synchronous LDM; FIG. 4 is a diagram showing an example of distributed arrangement of pilot symbols; FIG. 2 is a diagram for explaining the orthogonal relationship between subcarriers in OFDM; FIG. 4 is a diagram for explaining UL interference caused by LL signal components; FIG. 3 is a diagram for explaining a dummy mapping signal for reducing UL interference caused by LL signal components; FIG. 3 is a diagram showing an example of UL and LL subcarrier allocation;

A data transmission system according to an embodiment of the present invention will be described with reference to the drawings.
A quasi-synchronous LDM transmission system in which UL and LL are OFDM-modulated with different numbers of FFT points, multiplexed in the time domain, and transmitted will be described below as an example. FIG. 1 shows a configuration example of a transmitter used in a quasi-synchronous LDM transmission system.

The transmitting apparatus includes an error correction coding unit 11, a data mapping unit 12, a pilot mapping unit 13, a selection unit 14, a _NUL point OFDM modulation unit 15, a combining unit 16, and an error correction coding unit 17. , a data mapping unit 18 , a pilot mapping unit 19 , a dummy mapping unit 20 , a selection unit 21 , an _NLL point OFDM modulation unit 22 and an antenna 23 .

First, the UL signal generation method will be described. A general OFDM modulation method can be used to generate the UL signal, and the terrestrial digital broadcasting method is specified in ARIB STD-B31.

When this system is applied to terrestrial digital broadcasting, UL information code (for example, 2K broadcasting data) from an external video encoder or the like is input to the error correction encoder 11 . The error correction coding unit 11 applies error correction coding to the input UL information code. ARIB STD-B31, which defines the terrestrial digital broadcasting system, performs concatenated coding of RS (Reed-Solomon) code and convolutional code. As for the error correction encoding method, other error correction methods such as turbo code and LDPC (Low Density Parity Check) code may be used in addition to the above encoding method.

A signal resulting from error correction encoding by the error correction encoding unit 11 is input to the data mapping unit 12 . The data mapping unit 12 maps the UL error correction code onto the I/Q complex plane using 64QAM (Quadrature Amplitude Modulation), 16QAM, or the like. As for the mapping method, a method other than the above may be used.

The pilot mapping section 13 maps a pilot signal whose amplitude and phase are known onto the I/Q complex plane in the same manner as the data mapping section 12 . Although BPSK (Binary Phase Shift Keying) is generally used for mapping pilot signals, other mapping may be used.

The selection unit 14 selects each output of the data mapping unit 12 and the pilot mapping unit 13 according to a predetermined rule. Specifically, as shown in FIG. 4, the selection unit 14 arranges pilot signals on subcarriers (black blocks in the figure) dispersed in the frequency direction and the time direction, and other subcarriers (black blocks in the figure). white blocks). A subcarrier to which a pilot signal is assigned is also referred to as a "pilot carrier", and a subcarrier to which a data signal is assigned is also referred to as a "data subcarrier". Although the details are omitted, in the receiving device arranged corresponding to the transmitting device in this example, by comparing the known pilot signals that are regularly distributed and arranged with the pilot signals actually received from the transmitting device, Estimate the characteristics of the transmission path between the transmitter and the receiver.

The UL subcarrier signal output from the selector 14 is converted from a frequency domain signal to a time domain signal by the N _UL point OFDM modulator 15 . Also, the N _UL point OFDM modulation unit 15 cyclically copies the second half of the time domain of the symbol to the head of the symbol as a guard interval signal. Thus, a UL OFDM modulated signal is generated.

Next, a method for generating the LL signal will be described.
As with UL, an LL information code (for example, 4K broadcast data) from an external video encoder or the like is input to the error correction encoder 17 . The error correction coding section 17 applies error correction coding to the input LL information code, similarly to the error correction coding section 11 . At this time, it is not necessary to use the same method as the UL error correction code.

A signal resulting from error correction encoding by the error correction encoding unit 17 is input to the data mapping unit 18 . The data mapping unit 18, like the data mapping unit 12, maps the LL error correction code onto the I/Q complex plane. Also, the pilot mapping section 19 performs mapping processing similar to that of the pilot mapping section 12 .

Next, the dummy mapping unit 20 will be described. The processing by the dummy mapping unit 20 is the main focus of the present invention, and aims to map the dummy signal on the I/Q complex plane so as not to cause interference with the UL pilot signal.
First, the frequency domain representation of the OFDM signal is described, and then the interference of the LL signal components on the pilot carriers of the UL signal is described. Finally, mention is made of dummy mapping processing for suppressing this interference.

A feature of OFDM is that the subcarrier signals of OFDM can be represented by a sinc function in the frequency domain, and the subcarriers are orthogonal (see FIG. 5). However, as described above, when different numbers of FFT points are used for UL and LL, the orthogonal relationship between subcarriers is lost between UL and LL. To explain using a specific example, if N _UL and N _LL have a ratio of 1:4, for example, LL subcarriers with subcarrier numbers that are multiples of 4 are orthogonal to UL subcarriers, and other LL subcarriers The carriers are not orthogonal to the UL subcarriers. Therefore, the component of the LL signal, which is non-orthogonal to the UL signal, mixes with the UL signal as interference (see FIG. 6).

In particular, in quasi-synchronous LDM, in order for the receiver to cancel the UL received signal from the received signal and extract the LL received signal, it is necessary to improve the accuracy of transmission path estimation. However, the LL signal component interferes with the pilot carrier of the UL signal required for channel estimation, and the estimation accuracy is degraded.

Therefore, the dummy mapping unit 20 in the present invention, in order to reduce the interference that the LL signal component exerts on the pilot carrier of the UL signal, the subcarrier (dummy carrier) indicated by the dotted line in the lower diagram of FIG. A dummy signal is mapped to cancel the interference component to .

The generation of this dummy signal will be described below using mathematical expressions.
First, the UL pilot signal is formulated. A subcarrier S _UL (ω _UL ) of the UL signal with the UL subcarrier number ω _UL is represented by the following (equation 1).

Here, sinc(.) is defined as the following (formula 2).

The sinc function is a function that becomes 0 when the inside of the parenthesis is an integer other than 0, and becomes 1 when the inside of the parenthesis is 0. Also, ω indicates a frequency, and subcarriers are arranged at positions where ω is an integer in OFDM. That is, the value of sinc(.) is 1 only when .omega.=. _omega.UL , and the value of sinc(.) is 0 at other frequencies where .omega. is an integer. This indicates that the OFDM subcarriers shown in FIG. 5 are orthogonal. D _UL (ω _UL ) indicates mapping of data signals and pilot signals assigned to subcarrier number ω _UL .

Next, the subcarrier S _LL (ω, ω _LL ) of the LL signal with the LL subcarrier number ω _LL normalized by the UL subcarrier interval is represented by the following (equation 3).

In the above (Formula 3), when the LL subcarrier number ω _UL is an integral multiple of N _LL /N _UL , the brackets of the sinc function are integers, and the LL subcarriers are orthogonal to the UL subcarriers. Conversely, in other cases, it means that the LL subcarriers are not orthogonal to the UL subcarriers and are mixed as interference (see FIG. 6). If N _UL and N _LL are, for example, a 1:4 ratio, then the LL subcarriers are orthogonal to the UL subcarriers when ω _LL is a multiple of 4; Not orthogonal to subcarriers.

Here, the interference component I(ω _P ) of the LL signal to the subcarrier with the UL subcarrier number ω _P is the LL mapping value D _LL (ω _LL ) and the sinc function of It is represented by a convolution operation.

In the above (Equation 4), if a pilot signal is assigned to a subcarrier with the UL subcarrier number ω _P , the LL subcarrier number ω _X that is the LL subcarrier number that is orthogonal to ω _P and that is closest to ω _P is given by: (Expression 5).

A dummy signal for reducing the interference of the LL signal with the pilot carrier of the UL signal is assigned to this LL subcarrier number _ωX . Before deriving this dummy signal, the interference component of the UL signal to the pilot carrier is shown in the following (Equation 6). The following (formula 6) is obtained by substituting the above (formula 5) into the above (formula 4).

In order to set the amount of interference to the subcarrier with the UL subcarrier number ω _P to 0, I(ω _P )=0, and the above (formula 6) becomes the following (formula 7).

In the above (formula 7), if only the subcarriers with the LL subcarrier number ω _X are transferred and rearranged, the conversion is as shown in the following (formula 8).

From the above derivation, the dummy signal D _LL (ω _X ) for setting the amount of interference to the subcarrier with the UL subcarrier number ω _P to 0 can be calculated by the above (Equation 8). Thus, it can be seen that the dummy signal D _LL (ω _X ) is represented by convolution of the LL mapping signal D _LL (ω _LL ) and the sinc function.
Therefore, the dummy mapping unit 20 generates the dummy signal D _LL (ω _X ) that satisfies the above (Equation 8). The dummy signal D _LL (ω _X ) generated by the dummy mapping unit 20 is assigned to the subcarrier with the subcarrier number ω _X of LL by the selection unit 21 in the succeeding stage.

Here, in the above (Equation 8), the integration range ω _LL is the entire signal bandwidth and all subcarriers of the _LL signal _. calculation will be required. The frequency spread of the LL signal is represented by a sinc function, and as is clear from the characteristics of the sinc function, the amount of interference decreases with increasing distance from the pilot carrier frequency.

Therefore, even if only the subcarriers of the LL signal around the pilot carrier of the UL signal (the subcarrier of the UL subcarrier number ω _P ) are used, the influence of the UL signal on the pilot carrier can be reduced. Therefore, as shown in the following (Equation 9), the subcarriers of the LL signal used to generate the dummy signal D _LL ( _ωX ) are limited to those within a predetermined range α centered on the subcarrier number _ωX . By doing so, we aim to reduce the scale of calculation.

FIG. 2 shows a configuration example of the dummy mapping unit 20. As shown in FIG. The dummy mapping unit 20 has a shift register 31 , a multiplier 32 , an integrator 33 and an inverter 34 . The value of the LL data signal D _LL (ω _LL ) in the above (equation 9) is input to the dummy mapping unit 20 and set in the shift register 31 .

The shift register 31 performs a shift operation at the timing of the subcarrier of the LL signal. Of the taps in the shift register 31, the taps in the range of α before and after the subcarrier number of the dummy carrier are subject to calculation. The dummy mapping unit 20 multiplies each tap of the shift register 31 excluding the center tap by a predetermined coefficient corresponding to “sinc(·)” in the above (Equation 9) by the multiplier 32 , and then integrates by the integrator 33 . I do. Here, the fact that the center tap of the shift register 31 does not contribute to the calculation corresponds to "ω _LL ≠ω _X " in the above (Equation 9). The output of the integrator 33 is input to the inverter 34, where it is multiplied by "-1" to invert the sign. The processing of the inverter 34 corresponds to "-" in the above (Equation 9).

The above is the description of the processing of the dummy mapping unit 20, which is the main focus of the present invention. By using the dummy mapping unit 20 that performs such processing, it is possible to reduce the interference of the LL signal component with the pilot carrier of the UL signal.

Each output of the data mapping section 18 , the dummy mapping section 20 , and the pilot mapping section 19 is input to the selection section 21 . The selector 21 selects each input signal so as to achieve a predetermined subcarrier arrangement (see FIG. 8). In the example of FIG. 8, the dummy carriers of the LL signal are arranged so that their start timings match the pilot carriers of the UL signal.

During the transition period from 2K broadcasting to 4K broadcasting, the IL of the UL signal that broadcasts 2K resolution video and the LL signal that broadcasts 4K resolution is made variable, and the IL is gradually reduced to complete the transition to 4K broadcasting. At this stage, the UL signal disappears and only the LL signal remains. During the period when the UL signal exists, the pilot signal of the LL signal is unnecessary because the channel estimation is performed using the pilot signal of the UL signal. Further, when the UL signal is no longer present, the dummy signal becomes unnecessary, so it becomes possible to allocate the pilot signal, which is a known signal, to the subcarriers to which the dummy signal has been allocated. Therefore, the selection unit 21 may switch the method of selecting each output of the dummy mapping unit 20 and the pilot mapping unit 19 according to the transition stage.

The _LL subcarrier signal output from the selector 21 is converted from a frequency domain signal to a time domain signal by the NLL point OFDM modulator 22 . Also, the _NLL point OFDM modulation unit 22 cyclically copies the second half of the time domain of the symbol to the head of the symbol as a guard interval signal. Thus, an LL OFDM modulated signal is generated.

After that, the combiner 16 generates an LDM signal by weighting and combining the outputs of the N _UL point OFDM modulation unit 15 and the N _LL point OFDM modulation unit 22 according to IL. The LDM signal output from the synthesizing unit 16 is sent out into space via the antenna 23 . As described above, the transmission process in quasi-synchronous LDM is completed.

As described above, in the transmitting apparatus of this example, the UL processing units such as the error correction coding unit 11 to the N _UL point OFDM modulation unit 15 generate UL pilot carriers distributed in the frequency direction and the time direction. N _UL point IFFT processing is performed on the UL subcarrier signal including the UL OFDM modulated signal, and the LL processing unit such as the error correction coding unit 17 to the N _LL point modulation unit 22 performs the LL sub The carrier signal is subjected to N _LL point IFFT processing to generate an LL OFDM modulated signal, and the synthesizing unit 16 adjusts the timing of these OFDM modulated signals so that the start timings of the OFDM modulated signals coincide with each other at a predetermined cycle. Then, the LDM signal obtained by this combination is sent to space from the antenna 23 . At this time, in the LL processing unit, the LL subcarrier signal includes a dummy carrier, which is an interference suppression carrier for suppressing interference with the UL pilot carrier by the LL OFDM modulated signal.

Even if the LL OFDM modulated signal is combined with the UL OFDM modulated signal at a predetermined IL by the transmitting device having such a configuration, it is possible to greatly reduce the interference of the LL OFDM modulated signal to the UL pilot carrier. becomes. In addition, data can be assigned to LL subcarriers to which dummy carriers are not assigned, and transmission efficiency can be improved.

Further, in this example, the LL processing unit is configured to allocate dummy carriers to subcarriers having frequencies closest to UL pilot carriers among a plurality of subcarriers included in LL subcarrier signals. With such a configuration, it is possible to efficiently reduce the interference of the LL OFDM-modulated signal component with the pilot carrier of the UL OFDM-modulated signal.

Further, in this example, the LL processing unit is configured to generate dummy carriers by the calculations shown in the above (Equation 8) or (Equation 9) (or equivalent calculations). With such a configuration, it is possible to reduce the computation scale for calculating dummy carriers.

Here, in the above description, two types of data are transmitted using two layers, the upper layer (UL) and the lower layer (LL). A quasi-synchronous LDM data transmission system may be used. For example, when three types of data are transmitted using three layers each having a power difference, the present invention is applied to at least one of the relationship between the first and second layers or the relationship between the second and third layers. Is possible.

In the above description, the communication between the transmitting device and the receiving device is performed wirelessly. It is possible to

In the above description, data is transmitted by semi-synchronous LDM in which the number of FFT points of _UL (NUL points) and the number of FFT points of _LL (NLL points) are different. It does not preclude the configuration of matching numbers (ie applying the invention to synchronous LDM).

Although the present invention has been described in detail above, it is needless to say that the present invention is not limited to the above embodiments, and can be widely applied to applications other than those described above.
Further, the present invention can also be provided as, for example, a method or system for executing the above processes, a program for realizing such a method or system, a storage medium for storing the program, or the like.

INDUSTRIAL APPLICABILITY The present invention can be used in a data transmission system that multiplexes and transmits a plurality of data using hierarchical division multiplexing.

11: error correction coding unit 12: data mapping unit 13: pilot mapping unit 14: selection unit 15: N _UL point OFDM modulation unit 16: combining unit 17: error correction coding unit 18: data Mapping units 18, 19: Pilot mapping unit 20: Dummy mapping unit 21: Selection unit 22: N _LL point OFDM modulation unit 23: Antenna 31: Shift register 32: Multiplier 33: Integrator 34 : Inverter

Claims (4)

In a transmission device used in a data transmission system that multiplexes and transmits a plurality of data by hierarchical division multiplexing,
a first generating means for generating a first modulated signal by performing IFFT processing of a first number of points on a first subcarrier signal including first pilot carriers distributed in the frequency direction and the time direction;
a second generating means for performing IFFT processing of a second number of points on the second subcarrier signal to generate a second modulated signal;
synthesizing means for synthesizing the first modulated signal and the second modulated signal by adjusting the timing so that the respective start timings coincide with each other in a predetermined cycle;
a sending means for sending a signal synthesized by the synthesizing means,
wherein the second generating means includes, in the second subcarrier signal, an interference suppression carrier for suppressing interference with the first pilot carrier by the second modulated signal. Device. The transmitting device according to claim 1,
The second generation means assigns the interference suppression carrier to a subcarrier having a frequency closest to the first pilot carrier among a plurality of subcarriers included in the second subcarrier signal. Transmitting device for In the transmitting device according to claim 1 or claim 2,
The transmitting apparatus, wherein the second generating means generates the interference suppressing carrier by calculation of the following (Equation A).

Here, NUL is the first number of points, _NLL is the second number of points, _ωX is the subcarrier number of the interference suppression carrier, and _ωLL is a predetermined number _centered on _ωX . , _{DLL (ω LL )} is the subcarrier of the subcarrier number ω _LL , and _DLL (ω _X ) is the interference suppression carrier. In a transmission method executed by a transmission device of a data transmission system that multiplexes and transmits a plurality of data by hierarchical division multiplexing,
performing IFFT processing with a first number of points on a first subcarrier signal including first pilot carriers distributed in the frequency direction and the time direction to generate a first modulated signal;
subjecting the second subcarrier signal to IFFT processing with a second number of points to generate a second modulated signal;
a step of synthesizing the first modulated signal and the second modulated signal by adjusting the timing so that the respective start timings match at a predetermined cycle;
and sending a signal synthesized by the synthesizing step ,
In the step of generating the second modulated signal, the second subcarrier signal includes an interference suppression carrier for suppressing interference of the second modulated signal to the first pilot carrier. Characteristic transmission method.

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