JP2008098812A - Ofdm transmission method and ofdm transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM transmission system improving an SNR without exceeding the upper-limit specified value of a mean sending power in a sub-channel band when reducing a transfer rate. <P>SOLUTION: Modem devices A and B are connected to a power-line transmission line 1. In a transmission section 2 for the modem device A, 12 represents a serial-parallel conversion section, is controlled by a transmission control section 5, uses at least a partial sub-channel and series-parallel converts transmission data at a symbol unit by using at least the partial symbol period of the used sub-channel. The transmission control section 5 lowers the rate of the used sub-channel and/or the rate of the used symbol period in response to a setting lowering the transfer rate. An amplifying section 18 raises a transmission level in response to the rate of the used symbol period when a gain is controlled by the transmission control section 5. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an OFDM transmission method and an OFDM transmission apparatus used for a transmission line with a large variation in transmission quality, and particularly relates to a case where the transmission rate is lowered than during normal operation with good transmission quality.

2. Description of the Related Art In recent years, a PLC (Power Line Communication) data communication system that uses an indoor power line of a commercial power source and bidirectionally connects electrical products has been attracting attention in homes and offices.
As the PLC data communication system described above, one using OFDM (Orthogonal Frequency Division Multiplexing) transmission has been proposed (see Patent Document 1).
As for the transmission line characteristics of indoor power lines, noise generated from various electric products is large. However, since it is necessary to prevent radio wave leakage to a frequency band used for other communication systems and broadcasting, transmission average power is limited. In addition, since the frequency spectrum of the indoor power line changes frequently depending on the state of the power line load, the transmission quality such as the signal-to-noise ratio (SNR) varies greatly.

Generally, when transmission quality deteriorates, it is known to set the fallback mode to lower the data transmission rate than during normal operation (see Patent Document 2).
However, when the transmission quality deteriorates, the digital modulation method is changed to reduce the number of bits per symbol, or the same symbol is transmitted a plurality of times. Therefore, the SNR of the transmission path is not necessarily improved.

Even in the OFDM transmission system, when the transmission quality is lowered, it is considered that the transmission rate is lowered by changing the coding rate of the error correction code, the digital modulation system, the spreading factor, and the like (see Patent Document 3). ).
In the OFDM transmission method of Patent Document 3, in addition, in a transmission path with poor transmission quality, some subcarriers (for example, 80) of data symbols and pilot symbols are all subcarriers (for example, 80). For example, only 4) are used, and the total transmission power is concentrated on some of the subcarriers (transmission power is multiplied by 20) to increase the transmission power used for the data symbols.

However, the OFDM transmission method disclosed in Patent Document 3 makes constant the transmission power of an OFDM modulated signal obtained by synthesizing a transmission signal in a frequency band (hereinafter referred to as a subchannel band) corresponding to each subchannel of the OFDM transmission signal. It is assumed.
However, in a PLC data communication system, as a transmission standard based on laws and regulations, the spectrum energy of a transmission signal, in other words, each frequency within the frequency band used, taking into account interference with other communication systems and broadcasts, in other words, It has been studied to limit the maximum value of the average transmission power in.
Therefore, when the OFDM transmission system is used in the PLC data communication system, the transmission average power in each subchannel band constituting the OFDM transmission signal is limited.

As a result, the transmission power cannot be concentrated in a part of the subchannel bands as in the OFDM transmission method described in Patent Document 3 described above.
Therefore, when the OFDM transmission system is used in the PLC data communication system, a technique capable of stable transmission within a range that does not exceed the upper limit restriction value of the transmission average power of each subchannel band is required.

  In the OFDM transmission system, there is a system in which a null symbol is arranged at the head of one frame composed of a plurality of symbols, and all carriers are completely stopped during this null symbol period (see Patent Document 4). However, this null symbol is a symbol for the receiving apparatus to obtain frame synchronization, and is not for making the transmission rate variable.

On the other hand, in OFDM transmission, when the instantaneous carrier frequency of a subcarrier fluctuates, the orthogonality of the instantaneous carrier frequency is lost and intersubchannel interference occurs. In addition, even when an OFDM signal is output in a manner that does not insert a guard interval into the inverse Fourier transform output that is output in units of blocks, it is subject to intersubchannel interference due to waveform discontinuity between adjacent blocks. Intersubchannel interference is particularly large from adjacent subchannels.
In a transmission line with poor transmission quality, this interchannel interference cannot be ignored.
From the above, OFDM transmission used in PLC data communication systems cannot reduce the average transmission power of the subchannel band even if the noise level is high, and there is also intersubchannel interference, so the transmission rate is simply reduced. The transmission quality is not necessarily improved simply by making it happen.
JP 2003-188781 A Japanese Patent Laid-Open No. 2000-222771 JP 2005-27294 A JP-A-11-145929

  The present invention has been made to solve the above-described problems. When the transmission rate is lowered, the signal-to-noise ratio (SNR) is not exceeded without exceeding the upper limit limit value of the average transmission power in the subchannel band. It is an object of the present invention to provide an OFDM transmission scheme for improving the transmission rate and an OFDM transmission apparatus used for the OFDM transmission scheme.

According to the present invention, in the invention described in claim 1, transmission data is serial-parallel converted into a plurality of subchannels in symbol units, OFDM-modulated with the transmission data subjected to serial-parallel conversion, transmitted, and a received signal is converted. In the OFDM transmission method of performing OFDM demodulation, parallel-serial conversion of received data of each subchannel obtained by OFDM demodulation in symbol units, and outputting received data, at least some of the subchannels in the plurality of subchannels The ratio of the subchannel used according to the setting for serially parallel converting the transmission data in symbol units using a partial symbol period in the used subchannel and reducing the transmission rate And at least the proportion of the symbol period used among the proportion of the symbol period used. In an arbitrary symbol period in an arbitrary subchannel where the transmission data is not subjected to serial / parallel conversion, OFDM modulation is performed without outputting a signal in a band corresponding to the arbitrary subchannel, and the transmission data is used. The OFDM-modulated signal is transmitted by raising the level of the OFDM-modulated signal according to the ratio of the used symbol period within a range where the average transmission power of the band corresponding to the subchannel does not exceed the upper limit regulation value. Then, the received signal is demodulated by OFDM, and the received data of each subchannel obtained by OFDM demodulation is subjected to parallel-serial conversion opposite to that when the transmission data is serial-parallel converted, and the received data is output. Is.
Therefore, the transmission power in the used symbol period increases by increasing the level of the OFDM-modulated signal as the ratio of the used symbol period is reduced according to the setting for reducing the transmission rate. To do. As a result, the SNR is improved.
At that time, there is a block period (period corresponding to the symbol period in each subchannel) in which the OFDM modulation signal is not output in the subchannel band to be used, so the average transmission power of each subchannel band exceeds the upper limit regulation value. Can not be. If the transmission is performed at the upper limit limit of the average transmission power during normal operation, the signal level can be increased to the inverse of the ratio of the symbol period used, ideally in terms of power. .
On the other hand, if the ratio of subchannels to be used is also reduced according to the setting to reduce the transmission rate, the ratio of subchannel bands that are used simultaneously in the same symbol period also decreases, thus reducing intersubchannel interference. As a result, the SNR is improved.

According to a second aspect of the present invention, in the OFDM transmission method according to the first aspect, the partial symbol period used in each of the used subchannels is between the used subchannels. Thus, the series-parallel conversion is performed so as to deviate from each other.
Therefore, even if the ratio of subchannels to be used is not reduced, the ratio of subchannels that are used simultaneously in the same symbol period is reduced, so that intersubchannel interference is reduced and SNR is improved.

According to a third aspect of the present invention, in the OFDM transmission method according to the first or second aspect, the transmission rate is changed, OFDM is modulated with test data instead of the transmission data, and the OFDM demodulation is performed. The transmission quality of the received signal of each subchannel obtained by the above is determined, and the number of subchannels determined to have good transmission quality is multiplied by the proportion of the symbol period used at the transmission rate. The transmission rate that maximizes the value is determined as the transmission rate for transmitting the transmission data.
Therefore, it is possible to determine the transmission rate set for changing the processing function so that the transmission rate of the transmission data using the subchannel that can be transmitted with good transmission quality is maximized.

In the invention of claim 4, transmission data is serial-parallel converted into a plurality of subchannels in symbol units, OFDM-modulated with the transmission data subjected to serial-parallel conversion and transmitted, and the received signal is OFDM demodulated, In an OFDM transmission method for outputting received data by parallel-serial conversion of received data of each subchannel obtained by OFDM demodulation in symbol units, using a part of subchannels in the plurality of subchannels, In any subchannel in which transmission data is serial-parallel converted in symbol units, the proportion of the subchannel used is reduced according to the setting for reducing the transmission rate, and the transmission data is not serial-parallel converted. OFDM modulation without outputting a signal of a band corresponding to the arbitrary subchannel, and the OF M-modulated signal is transmitted, received signal is OFDM demodulated, and the received data of each subchannel obtained by OFDM demodulation is subjected to parallel-serial conversion opposite to when the transmitted data is serial-parallel converted. The received data is then output.
Therefore, since the proportion of subchannels used is reduced according to the setting for reducing the transmission rate, the proportion of subchannels used simultaneously in the same symbol period is lowered. As a result, intersubchannel interference is reduced and SNR is improved.

According to a fifth aspect of the present invention, in the OFDM transmission method according to the fourth aspect, the transmission rate is changed, OFDM modulation is performed using test data instead of the transmission data, and transmission is performed by the OFDM demodulation. A transmission rate for maximizing the number of subchannels determined to have good transmission quality is determined as a transmission rate for transmitting the transmission data. To do.
Therefore, it is possible to determine the transmission rate set for changing the processing function so that the transmission rate of the transmission data using the subchannel that can be transmitted with good transmission quality is maximized.

In the invention according to claim 6, in the OFDM transmission apparatus that performs transmission / reception conversion of transmission data into a plurality of subchannels in symbol units, and performs OFDM modulation using the transmission data subjected to serial / parallel conversion, the transmission is performed. Using at least some subchannels in the channel and using some symbol periods in the subchannels used, the transmission data is serial-parallel converted in symbol units, and the transmission data is not serial-parallel converted. In an arbitrary symbol period in an arbitrary subchannel, a serial-parallel conversion unit that inserts data that does not output a signal of a band corresponding to the arbitrary subchannel, and a setting for reducing a transmission rate, Proportion of the subchannel used in the serial-parallel converter and before it is used Output from the OFDM modulation means within a range in which the average transmission power of the band of the used subchannel does not exceed the upper limit regulation value while at least reducing the ratio of the used symbol period in the ratio of the symbol period. OFDM modulation is performed by transmission control means for controlling the amplification factor so that the level of the signal to be increased in accordance with the ratio of the symbol period used, and transmission data of each subchannel output from the serial-parallel conversion means OFDM modulation means and amplification means for amplifying a signal output from the OFDM modulation means with the amplification factor controlled by the transmission control means.
Therefore, the transmission apparatus used for the OFDM transmission system according to claim 1 can be easily realized.

In the invention according to claim 7, in the OFDM transmission apparatus that performs transmission / reception conversion of transmission data into a plurality of subchannels in symbol units and performs OFDM modulation with the transmission data subjected to serial / parallel conversion, and transmits the plurality of subchannels. Using some subchannels in the channel, the transmission data is serial-parallel converted in symbol units, and in any subchannel where the transmission data is not serial-parallel converted, the band corresponding to the arbitrary subchannel Serial-to-parallel conversion means for inserting data to prevent the output of the signal, and transmission control means for reducing the ratio of the subchannels used in the serial-to-parallel conversion means according to the setting for reducing the transmission rate, O which performs OFDM modulation with the transmission data of each subchannel output from the serial-parallel conversion means And DM modulating means, and has an amplification means for amplifying a signal outputted from said OFDM modulation unit.
Therefore, the transmission apparatus used for the OFDM transmission system according to claim 4 can be easily realized.

  According to the present invention, when setting to reduce the transmission rate in OFDM transmission, the upper limit regulation value of the average transmission power of the subchannel band is satisfied, and the SNR is improved.

FIG. 1 is a block configuration diagram of an OFDM transmission system for explaining an embodiment of the present invention.
The upper stage in the figure is a modem apparatus (modem / demodulator) A, and the lower stage in the figure is a modem apparatus (modem / demodulator) B, both of which are connected to the power line transmission line 1.
The modem devices A and B have the same configuration, and are directly connected to an indoor power line that is the power line transmission line 1 or incorporated into an electric product having a communication function as a core module of the modem.

The modem device A has a transmission unit 2 to which transmission data is input and a reception unit 3 to which reception data is output, and both are coupled to the power line transmission line 1 via a power line coupler 4. Both are controlled by the transmission control unit 5.
Similarly, the modem device B includes a transmission unit 6 to which transmission data is input and a reception unit 7 to which reception data is output, and both are coupled to the power line transmission line 1 via a power line coupler 8. Both are controlled by the transmission control unit 9.
The internal configuration of the reception unit 3 is the same as that of the reception unit 7, and the internal configuration of the transmission unit 6 is the same as that of the transmission unit 2.

In the transmission unit 2 of the modem device A, 11 is a buffer, and transmission data is input as a bit string. Since the present invention is a system in which the transmission rate (transmission speed) is variable, the input transmission data is stored in the buffer 11 until it is taken into the serial-parallel converter 12.
Reference numeral 12 denotes a serial-parallel converter, which distributes a bit string to a plurality of subchannels. Since each subchannel is distributed in symbol units, the number of unit bits to be distributed is determined according to the digital modulation method employed. The subchannel corresponds to orthogonal subcarriers in OFDM modulation, and is a channel that supplies data for digitally modulating the corresponding subcarriers.

In the serial-parallel converter 12, the transmission controller 5 controls the distribution of the bit string to a plurality of subchannels so that the transmission rate is optimized according to the transmission quality.
That is, transmission data is serial-parallel converted using at least some subchannels of the plurality of subchannels and using at least some symbol periods of the used subchannels.
The transmission control unit 5 reduces the ratio of the subchannels used in the serial-parallel conversion means and / or the ratio of the symbol periods used according to the setting for reducing the transmission rate. More specifically, three symbol arrangements can be considered, and details will be described later with reference to FIGS.
In an arbitrary symbol period in an arbitrary subchannel where transmission data is not distributed, data for preventing output of a signal in a subchannel band corresponding to the subchannel is inserted.

A symbol mapping unit 13 converts a plurality of bits in symbol units of each subchannel into symbol data (complex data) of each subchannel indicating signal points on a signal phase plane (complex plane) of digital modulation. .
Note that it is also possible to change to a configuration in which serial mapping is performed after symbol mapping is performed first.
An OFDM modulation unit 14 outputs an OFDM modulated signal (complex data) obtained by combining signals obtained by modulating each orthogonal subcarrier with the complex data of each subchannel described above for each orthogonal subcarrier. More specifically, it is realized by performing an inverse Fourier transform. A guard interval may be inserted in each block period of the OFDM modulated signal. One block period of the OFDM modulation signal corresponds to one symbol period in each subchannel.

Reference numeral 15 denotes an LPF (low-pass filter) that limits the band of the transmission signal waveform.
Reference numeral 16 denotes an orthogonal modulation unit, which orthogonally modulates the output of the LPF 15 with a carrier. As a result, the output of the LPF 15 is frequency shifted to the transmission band. The processing so far is performed by digital signal processing. For example, the processing is implemented as an LSI (Large Scale Integrated circuit).
The signal is converted into an analog signal by the D / A converter 17 and output to the amplifier 18.
The amplifying unit 18 may amplify the signal output from the OFDM modulation unit with an amplification factor controlled by the transmission control unit 5.

The transmission control unit 5 limits the average transmission power of the subchannel band to be used in conjunction with controlling the ratio of the symbol period used in the serial-parallel conversion unit 12 in accordance with the decrease in the transmission rate. The amplification factor is controlled so that the level of the signal output from the OFDM modulation unit 14 increases according to the ratio of the symbol period used within a range not exceeding the regulation value.
Reference numeral 19 denotes an analog LPF, which limits the band of the transmission signal and outputs it to the power line transmission line 1 via the power line coupler 4.

In the receiving unit 7 of the modem apparatus B, reference numeral 21 denotes an LPF which receives a transmission signal from the power line transmission line 1 via the power line coupler 8 and passes a necessary frequency band component.
An AGC (automatic gain control) unit 22 automatically controls the amplification gain so that the level of the received signal that has passed through the LPF 21 falls within a range suitable for subsequent digital signal processing.
An A / D converter 23 converts an analog signal output from the AGC 22 into a digital signal. The subsequent processing is digital signal processing, for example, implemented as an LSI (Large Scale Integrated Circuit).

An orthogonal demodulation unit 24 demodulates using a carrier having the same frequency as the carrier in the transmission unit 16 and shifts the output of the A / D conversion unit to baseband (complex data).
Reference numeral 25 denotes an LPF which passes a necessary frequency band component.
Reference numeral 26 denotes an OFDM demodulator and an equalizer.
First, the OFDM demodulator outputs complex data representing signal points on a signal phase plane (complex plane) as symbol data obtained by modulating each corresponding subcarrier for each subchannel.
Specifically, the OFDM demodulation is realized by Fourier transform. When the guard interval is inserted in the OFDM modulation unit 14 of the modem apparatus A, the effective symbol length is cut out and Fourier transformed.

Transmission path equalization is performed by the equalizer for the symbol data (complex data) of each subchannel. For example, in the training mode, the transmission path equalization is performed by changing the characteristics of the equalizer using the training signal transmitted instead of the transmission data in the transmission unit 2 of the modem apparatus A.
A symbol demapping unit 27 digitally demodulates symbol data (complex data) of each subchannel output from the OFDM demodulator and equalizer 26 and converts it into a plurality of bits.

The symbol data of each subchannel is also output to the transmission control unit 9. The transmission control unit 9 determines the transmission quality (transmission quality in the direction from the modem device A to the modem device B), for example, the SNR for each subchannel, and the serial / parallel conversion unit 12 of the modem device A according to the transmission quality. Transmission control data (modem device B to modem device A) for controlling the transmission unit 6 is transferred to the transmission unit 6, and the serial / parallel conversion unit (the same block as the serial / parallel conversion unit 12) of the transmission unit 6 transmits the transmission control data. (Modem device B to modem device A) is transmitted to modem device A.
The transmission control unit 9 also determines a transmission rate according to the determined transmission quality, and controls the parallel-serial conversion unit 28 in the reception unit 7.

  In the modem device A, the parallel / serial converter (similar to the parallel / serial converter 28 described later) in the receiver 3 extracts the transmission control data (the modem device A from the modem device B) and sends it to the transmission controller 5. Forward. The transmission control unit 5 determines a transmission rate according to the transmission control data, and controls the serial / parallel conversion unit 12 and the amplification unit 18.

28 is a parallel-serial conversion unit, which is controlled by the transmission control unit 9 according to the setting for reducing the transmission rate, and in the serial-parallel conversion unit 12 of the modem device A for a plurality of bits in symbol units in each subchannel. The original transmission bit string is restored by performing a conversion opposite to that performed when the series-parallel conversion is performed.
Since 29 is a buffer and the transmission rate of the transmission data is variable, the received bit string output from the parallel-serial converter 28 is temporarily stored so that it is output at a timing suitable for a utilization device (not shown). To.

The above description follows the processing when data is transmitted from the modem device A to the modem device B. Similarly, data can be transmitted from modem device B to modem device A.
This bi-directional transmission can be performed by half-duplex communication that alternately switches between the transmission side and the reception side, or can be performed by full-duplex communication by separately assigning the use band of OFDM transmission in the power line transmission path 1 to both directions. Is possible. A system can be constructed with three or more modem devices, one being a parent modem device and the other being a child modem device for multiple access.

In the above description, the transmission quality is the transmission quality from the modem device A to the modem device B. However, the transmission from the modem device B to the modem device A is similarly performed from the modem device B to the modem device A. The transmission quality to A is determined on the modem device A side, and the modem device A controls the serial / parallel conversion unit in the transmission unit 6 of the modem device B from the modem device A to the modem device B according to the transmission quality. Therefore, transmission control data for transmission is transmitted from the modem apparatus A to the modem apparatus B.
In addition, when it is estimated that each of the bidirectional transmissions has the same transmission quality, only one transmission quality may be determined.

In the above description, the data equalized on the receiving side during transmission (operation) of transmission data (which may insert a pilot signal) from an external device using this OFDM transmission system. (Or pilot signal) transmission quality is determined, transmission control data is transmitted to the transmission side, and the processing of the serial-parallel converter 12 and the parallel-serial converter 28 is switched between them synchronously. However, it is difficult to switch processing during operation.
Therefore, during a period when transmission data from an external device is not transmitted (during pause), test data is transmitted instead of transmission data, transmission quality is determined together with control of the equalizer, and transmission control data is transmitted. The processing of the serial-parallel converter 12, the amplifier 18, and the parallel-serial converter 28 can be switched.

Hereinafter, the symbol arrangement when the transmission data is serial-parallel converted according to the transmission rate will be specifically described.
FIG. 2 is an explanatory diagram showing symbol arrangement during normal operation with good transmission quality, which is a base point for fallback.
FIG. 2A shows a data bit string input from the buffer 11 to the serial-parallel converter 12. The data bit string is serial-parallel converted in units of the number of bits corresponding to one symbol of digital modulation. For this reason, this bit string in symbol units is represented by a sequence number.
For example, in the case of QPSK (4-phase phase modulation), since 2 bits are converted into 1 symbol, each number shown in the figure represents 2-bit transmission data.

FIG. 2B shows the symbol arrangement in the serial-to-parallel converter 12 shown in FIG. 1 during normal operation.
In the illustrated example, in order to simplify the description, it is assumed that the OFDM modulation unit 14 uses eight orthogonal subcarriers. In this case, a bit string 1, 2, 3,... In one symbol unit (2 bits in the case of QPSK) is distributed to 8 subchannels. In other words, the serial bit string is converted into 8 parallel data.

As a result, the bit sequence 1, 9, 17, 25, ... in symbol units is distributed to the subchannel 1, and the same applies to the following subchannels. The last subchannel 8 includes the bit sequence 8 in symbol units. , 16, 24, 32, ... are distributed.
In the symbol mapping unit 13 shown in FIG. 1, all eight subchannels are used, and distributed transmission data in symbol units is digitally modulated (QPSK) by each subcarrier. That is, the distributed bit sequence in symbol units is converted into signal points represented on the signal phase plane with the subcarrier as the reference phase.
The OFDM modulation unit 14 shown in FIG. 1 outputs an OFDM modulation signal in which a signal digitally modulated in each subchannel is combined for all subchannels in units of blocks. One block period corresponds to one symbol period in each subchannel.

In actual OFDM transmission, pilot symbols are inserted between data symbol sequences. However, since the present invention does not distinguish between data symbols and pilot symbols, description of pilot symbol insertion processing is omitted.
In an OFDM transmission system, a frequency band that is not allowed to be included in a transmission signal for other communication systems, broadcasting, or the like may be defined in a frequency band permitted for this system. In such a case, subchannels that use an unacceptable frequency band are registered in advance as unused subchannels, and the serial-parallel converter 12 does not allocate bit strings to the unused channels.
Similarly, in the frequency band allowed for this system, if a frequency band with poor transmission quality is known in advance, subchannels using this poor frequency band are also registered in advance as unused channels. Keep it.

FIG. 3 is an explanatory diagram showing symbol arrangement in the serial-parallel converter 12 shown in FIG. 1 during fallback (1/2 transmission rate). Three types of arrangement examples will be described.
A case where the transmission control unit 5 reduces the transmission rate to ½ during normal operation when the transmission quality deteriorates will be described. The serial-parallel converter 12 sets the symbol arrangement so that the transmission rate becomes 1/2 according to the instruction from the transmission controller 5.

In the arrangement example shown in FIG. 3A, symbol periods used along the time axis are thinned out at a ratio of 1/2.
In the first symbol period, when the bit sequence 1 to 8 in symbol units is distributed to each subchannel, the bit sequence is not distributed to each subchannel in the next second symbol period. In the next third symbol period, the bit sequences 9 to 16 in symbol units are again distributed in the same manner as in the first symbol period, and thereafter, similarly, in the time axis direction, the symbol units in the ½ symbol period. Distributes bit strings.
Of all the subchannels, the ratio of the used subchannel is 1, but the ratio of the used symbol period in the used subchannel is reduced to 1/2. As a result, the transmission rate of the transmission data is 1/2. It becomes.

At this time, in a symbol period in which a symbol-unit bit string is not distributed to each subchannel, such as in the second symbol period, a special bit string (hereinafter referred to as a null symbol) that is not included in the symbol-unit bit string. Insert a bit string).
In the subsequent symbol mapping unit 13, when a bit string representing the null symbol is input, complex data having an amplitude of 0 (hereinafter referred to as a null symbol) is output in the symbol period. As a result of all subcarriers being modulated with null symbols having an amplitude of 0, the output level of the transmission waveform is suppressed in the block period corresponding to the symbol period in the output signal of the OFDM modulation unit 14.
In the OFDM modulation unit 14, when a guard interval is inserted in each block period, the output level of the transmission waveform is suppressed and is not output. In contrast, in the OFDM modulation scheme that performs block convolution, the output level of the transmission waveform is suppressed in the block period corresponding to the symbol period of the null symbol described above, but does not become zero.

In FIG. 3A, a bit string representing a null symbol is inserted into all subchannels in the second and fourth symbol periods, and the output of the OFDM modulation unit 14 is suppressed in a block period corresponding to the symbol period. Is done. As a result, the level of the transmission signal is suppressed from the transmission unit 2 at a rate of once per two symbol periods in accordance with the decrease in the transmission rate.
In fact, since there are LPF 15 and LPF 19, the symbol waveform slightly spreads in adjacent symbol periods, but is negligible.
In the above description, symbol periods in which transmission bit strings in symbol units are arranged and output transmission signals are provided at equal intervals. However, in order to reduce the transmission rate to 1/2, it is not always necessary to provide the transmission rate at equal intervals, and the rate of the symbol period used for outputting the transmission signal corresponds to the transmission rate reduction rate of 1/2. Just do it.

According to the transmission rate reduction by decimation along the time axis described above, the transmission average power is reduced (ideally 1/2) in each subchannel band. Therefore, even when an upper limit restriction value is set for the average transmission power of the subchannel band as in the PLC data communication system, the transmission level is increased (ideally, power conversion) in the symbol period in which the transmission signal is output. 2 times maximum).
Therefore, the transmission control unit 5 can increase the gain (power amplification factor) of the amplifying unit 18 shown in FIG. 1 when the transmission rate is reduced by thinning the time axis.
The transmission signal output from the amplifying unit 18 has a higher signal power than usual during the period in which the transmission signal is output. Therefore, even in the reception signal, the SNR increases and the transmission signal is buried in noise. Since the signal may be distinguishable from noise, transmission quality is improved.

There is another method for increasing the transmission level.
In the symbol mapping unit 13 of the transmission unit 2 shown in FIG. 1, when the symbol signal points are arranged on the signal phase plane, the amplitude level is increased, for example, 2 ^1/2 times. As a result, the transmission power is doubled as normal in terms of power.
However, since the transmission unit 2 performs digital signal processing, it performs fixed-point arithmetic using the maximum number of signal digits (bits) during normal transmission. There is no room to raise the ratio to 2 ^1/2 times. Therefore, when this method is employed, the accuracy is lowered because it is necessary to provide a margin for the number of digits (number of bits) to be calculated in advance during normal transmission.

Next, the arrangement example shown in FIG. 3B thins out the subchannels used along the frequency axis at a ratio of 1/2.
That is, in any symbol period, when the bit sequence 1 in symbol units is distributed to the first subchannel, the bit sequence in symbol units is not distributed to the next second subchannel. Similarly to the first subchannel, a bit string 2 in symbol units is distributed to the next third subchannel, and thereafter, in the frequency axis direction, according to the transmission rate 1/2 in all subchannels, etc. Distributes a bit string in symbol units at intervals.
As a result, in the first symbol period, the bit strings 1, 2, 3, 4 in symbol units are distributed only to the first, third, fifth, and seventh subchannels, respectively. The same applies to the symbol period.
In this arrangement example, the ratio of the symbol period to be used is 1, but the ratio of the subchannel to be used is 1/2, so that the transmission rate of the transmission data is 1/2.

As in the arrangement example shown in FIG. 3A, a bit string representing a null symbol is inserted into a subchannel to which a bit string in symbol units is not distributed, and the bit string representing this null symbol is input in the symbol mapping unit 13. Output a null symbol with an amplitude of zero. As a result, the transmission waveform is not output in the subchannel band in the output signal of the OFDM modulation unit 14.
In the above description, the subchannels that distribute the transmission bit string in symbol units are arranged at equal intervals (alternately) in accordance with the reduction of the transmission rate to 1/2. Therefore, interference between adjacent subchannels is reduced. However, the inter-subchannel interference can be reduced without necessarily arranging them at equal intervals.

In the transmission rate reduction by thinning along the frequency axis described above, the transmission level cannot be increased because the transmission average power does not change from the normal time in the subchannel band that outputs the transmission signal. Therefore, the transmission control unit 5 does not change the gain of the amplification unit 18.
However, the ratio of subchannels used at the same time in the same symbol period, in other words, the ratio of modulated subcarriers output at the same time is halved, thereby eliminating intersubchannel interference, resulting in SNR. Can be improved.
Since the ratio of the modulated subcarriers output at the same time becomes 1/2, the level when the amplitude of the OFDM modulated signal obtained by combining the transmission signals in each subchannel band becomes maximum is lowered. .

In the arrangement example shown in FIG. 3 (c), in any subchannel, the symbol period used along the time axis number axis is thinned out at a rate of 1/2 according to the transmission rate 1/2, and the symbol used By shifting the period sequentially along the frequency axis, the subchannels used are shifted from each other.
In the first subchannel, bit sequences 1, 9, 17, and 25 in symbol units are distributed only in the first, third, fifth, seventh,... Symbol periods, respectively, and in the second subchannel, Only in the second, fourth, sixth, eighth,... Symbol periods, bit sequences 5, 13, 21, and 29 in symbol units are respectively distributed. Thereafter, in the same manner, the bit sequences in symbol units are divided in the frequency axis direction. The symbol period to be distributed is switched, but at any time, a bit string in symbol units is distributed to ½ symbol periods in all symbol periods.
Therefore, the ratio of the subchannels used in all the subchannels is 1, but the ratio of the symbol period used in each subchannel is reduced to 1/2. As a result, the transmission rate of transmission data is 1/2. Become.

  When a bit string in symbol units is not distributed in an arbitrary symbol period in an arbitrary subchannel, a bit string representing a null symbol is inserted into each subchannel, and the symbol mapping unit 13 inputs a bit string representing this null symbol. Output a null symbol. A transmission signal is not output in an arbitrary symbol period in an arbitrary subchannel band in which a null symbol is output in the output signal of OFDM modulation section 14.

In the above description, in the symbol arrangement along the time axis and the frequency axis, the symbol positions to which the bit string of the symbol unit is distributed are arranged at equal intervals along the time axis and the frequency axis.
However, in order to reduce the transmission rate to 1/2, it is not always necessary to arrange them at regular intervals.
The ratio of symbol positions for distributing the bit sequence in symbol units along the time axis corresponds to the transmission rate reduction rate 1/2 in any subchannel, and the bit sequence in symbol units is distributed along the frequency axis. The ratio of the symbol positions only needs to correspond to the transmission rate reduction rate 1/2 in any symbol period.

Also in the arrangement example of FIG. 3C, since the transmission rate is reduced by thinning along the time axis, the upper limit is set on the average transmission power of the subchannel band as in the arrangement example of FIG. Even in the case where a restriction value is provided, the transmission level can ideally be doubled in terms of power in the symbol period in which the transmission signal is output in each subchannel band.
Also, the ratio of subchannels used simultaneously in the same symbol period, in other words, the ratio of modulated subcarriers output at the same time becomes 1/2, so that subchannel interference is removed. SNR is improved.

In the description using FIG. 3, the case where the transmission rate is set to 1/2 in the fallback mode has been specifically described. A similar method can reduce the transmission rate at a fractional ratio.
FIG. 4 is an explanatory diagram showing an example of symbol arrangement in the serial-parallel converter 12 shown in FIG. 1 during fallback (1/4 transmission rate). The arrangement shown in FIG. 3C is a 1/4 transmission rate.

In any subchannel, there is a symbol position for distributing a bit string in symbol units at equal intervals of one symbol period out of four symbol periods.
The symbol position for distributing the bit string in symbol units is distributed with a shift of one symbol period from the adjacent subchannel along the frequency axis.
As a result, in any symbol period, for every four subchannels, there is a symbol position that distributes a bit string in symbol units at equal intervals of one subchannel.
Even when an upper limit restriction value is set for the average transmission power of the subchannel band, the transmission level is ideally quadrupled in terms of power in the symbol period in which the transmission signal is output in each subchannel band. can do.

In the above description, the transmission rate is reduced by either symbol thinning in the time axis direction or symbol thinning in the frequency axis direction. However, it may be realized by simultaneously using symbol thinning in the time axis direction and symbol thinning in the frequency axis direction.
For example, in FIG. 3A, if the subchannels 2, 4, 6, and 8 are not used, the transmission rate becomes 1/4.
The process for reducing the transmission rate after the transmission rate is determined in the fallback processing has been described above.
In the above description, the transmission rate is reduced by symbol thinning out in the time axis direction and / or symbol thinning out in the frequency axis direction. You may use the method of changing etc. together.

Next, an example of a method for determining a setting transmission rate used for changing the symbol arrangement and the transmission level will be described.
The modem device A and the modem device B shown in FIG. 1 do not always transmit data. Therefore, a time zone for determining the transmission rate in the idle time when communication is not performed at the time of line setting when the power is supplied and starting both, or after that, test data for determining the transmission rate is provided. Transmit and determine its transmission quality. At that time, using the test data, the transmission rate is actually changed, and transmission is performed, and the optimum transmission rate on the setting that maximizes the transmission rate of transmission data that can be transmitted with good transmission quality is determined.

FIG. 5 is a flowchart illustrating an example of processing for determining an optimal transmission rate in a time zone for determining a transmission rate.
In S31, the fallback degree N = 0. This is a normal transmission time and is the base point for fallback control.
Depending on the fallback degree N, for example, the set transmission rate is set to 1/2 ^{N of} normal transmission.
In S32, the symbol arrangement in N is determined. That is, as shown in FIGS. 3 and 4, in the symbol arrangement, the symbol position to which the bit string of the symbol unit is distributed is determined. Since the symbol arrangement is repeated along the time axis, the repeated basic pattern is stored in advance in the correspondence table, and the symbol arrangement is determined with reference to this correspondence table.

However, when OFDM modulation is used in the PLC transmission system, there may be subchannels that are not permitted by laws and regulations. In that case, the subchannel that is not allowed is not used. Also, a subchannel (mask subchannel) whose transmission quality is always known to be poor is not used.
These unacceptable subchannels and mask subchannels may be stored in the correspondence table.

In S32, the symbol period in which the bit string of the symbol unit is distributed in all the symbol periods in the fallback degree N, that is, the ratio R of the used symbol period is obtained from the correspondence table.
When the fallback degree N = 0, the ratio R of the used symbol period is R = 1.
In the symbol arrangement shown in FIGS. 3A and 3C, R = 1/2. In the symbol arrangement shown in FIG. 4, R = 1/4.
However, as shown in FIG. 3B, when the ratio of the symbol period to be used is determined to be R = 1, it is not necessary to acquire the symbol period.

In S33, SNR is measured for each subchannel band of the subchannel being used.
For example, in FIG. 1, the serial-parallel converter 12 of the transmitter 2 inserts test data at the symbol position determined in S32 by the transmission controller 5 of the modem device A.
The transmission control unit 9 of the modem apparatus B has an eye opening degree of an eye pattern exceeding a predetermined threshold for the equalized sub-channel symbol data (complex data) output from the OFDM demodulator & equalizer unit 26. If it exceeds, it is determined that the SNR of the subchannel band is good, and this subchannel is regarded as a subchannel (effective subchannel) with good transmission quality. The total number m of such effective subchannels is obtained.

In S34, a value obtained by multiplying the total number m of effective subchannels by the ratio R of the used symbol period is named an evaluation value, and this evaluation value is calculated and stored together with the value of the fallback degree N. However, as shown in FIG. 3B, when the ratio of the symbol period used is fixed to R = 1, the total number m of effective subchannels may be used as the evaluation value.
In S36, the value of the fallback degree N is incremented by 1, for example, so that the transmission rate becomes 1/2 of the current rate.

In S37, it is determined whether or not the fallback degree N exceeds a preset maximum value Nmax. If not, the process returns to S32 and repeats the processes of S32 to S37. Proceed with the process.
In S38, the value of the fallback degree N that maximizes the evaluation value that is calculated and stored in S35 is obtained, and this value is used as the fallback degree during operation for transmitting the transmission data input from the utilization device. Set.

In operation, in the modem apparatus A, the transmission control unit 5 performs symbol distribution in the serial-parallel conversion unit 12 with a symbol arrangement corresponding to the set fallback degree N.
In the modem device B, the transmission control unit 9 converts the symbol arrangement corresponding to the set fallback degree N into a bit string in the serial / parallel conversion unit 12.
The serial-parallel converter 12 distributes a bit string in symbol units except for unacceptable subchannels and mask subchannels stored in the correspondence table during operation.
In addition, in the set fallback degree N, only the subchannels determined as effective subchannels in S34 may be used, and the subchannels not determined as effective subchannels may not be used during operation.

Hereinafter, specific description will be given using numerical examples for explanation.
It is assumed that when the set fallback degree is N = 0 (the set transmission rate is 1), the symbol arrangement shown in FIG. 2B is used. Unacceptable subchannels and masked subchannels are not used.
Assuming that the total of unacceptable subchannels and mask subchannels is one, the actual transmission rate is (8−1) / 8 = 7/8.

Next, it is assumed that operation is performed using the symbol arrangement of FIG. 3A or FIG. 3C when the set fallback degree is N = 1 (the transmission rate on setting is 1/2).
Here, the total of the unacceptable subchannel and the mask subchannel is one, but when the fallback degree is N = 1, the optimum transmission rate determination control shown in FIG. 5 is performed. Suppose that the total number of effective subchannels is m = 8-1−2 = 5 because the SNR of the two subchannels is poor.

Here, if subchannels with poor SNR are also used, the ratio of subchannels used is (8−1) / 8 = 7/8, and the ratio of symbol periods used is R = 1/2. Therefore, the actual transmission rate during operation is (7/8) × (1/2) = 7/16.
On the other hand, if only the effective subchannels with a fallback degree of N = 1 are used, the ratio of subchannels used is m / 8 = 5/8, and the ratio of symbol periods used is R = Since it is 1/2, the actual transmission rate during operation is (5/8) × (1/2) = 5/16.

Next, it is assumed that the symbol arrangement of FIG. 3B is used when the set fallback degree is N = 1.
Here, although the sum of the unacceptable subchannel and the mask subchannel is one, it is assumed that these subchannels correspond to subchannels to be thinned out (or these subchannels are thinned out). Change to a symbol arrangement with a sub-channel usage rate of 1/2). When the fallback degree is N = 1, the SNR of one subchannel among the four subchannels 1, 3, 5, and 7 that should be used during the optimum transmission rate determination control shown in FIG. It is assumed that the total number of effective subchannels is m = 4-1 = 3 due to a defect.

Here, if subchannels with poor SNR are also used, the ratio of subchannels used is 4/8, and the ratio of symbol periods used is R = 1. The rate is (4/8) × 1.
On the other hand, when only effective subchannels are used, the proportion of subchannels used is 3/8, and the proportion of symbol periods used is R = 1, so the actual transmission rate during operation Is (3/8) × 1.
In either case, if the unacceptable subchannel and the mask subchannel correspond to subchannels that are not thinned out in the symbol arrangement shown in FIG. 3B, the transmission rate is further reduced.

In S37 described above, the evaluation value is obtained until the fallback degree exceeds a predetermined maximum value. However, if the evaluation value does not become better than the previous fallback degree, the process for obtaining the evaluation value may be stopped at that time, and the process may proceed to S38.
In the above description, the fallback degree N during normal operation was started from N = 0. However, the fallback degree N before and after the fallback degree N set during the current operation is used as a starting point. You may proceed.

According to the processing shown in FIG. 5, transmission data that can be transmitted with good transmission quality in consideration of the possibility that the SNR is improved and the number of effective subchannels may increase as a result of reducing the transmission rate by fallback control. The transmission rate for the setting can be determined so that the transmission rate of the maximum becomes the maximum.
The processing shown in FIG. 5 is performed when a conventional method other than symbol thinning out in the time axis direction and / or symbol thinning out in the frequency axis direction is used together to reduce the transmission rate, or transmission is performed using only the conventional method. It can also be applied when the rate is reduced.

In the above description, the case where the OFDM transmission method of the present invention is applied to a power line communication system has been described. Even in a poor environment such as indoor wiring in a general home, stable data communication can be performed. As a result, application in control applications such as remote control of information appliances is expected.
Also, since the OFDM transmission method is applied as a communication method for mobile phones and wireless LANs, the OFDM transmission method of the present invention can also be applied to a wireless transmission system, and similarly, the average of each subchannel band When the upper limit of transmission power is regulated, stable data communication can be performed even in a poor environment.

1 is a block configuration diagram of an OFDM transmission system for explaining an embodiment of the present invention. It is explanatory drawing which shows the symbol arrangement | positioning at the time of normal operation | movement (fallback base point). It is explanatory drawing which shows the symbol arrangement | positioning at the time of fallback (1/2 transmission rate). It is explanatory drawing which shows the symbol arrangement | positioning at the time of fallback (1/4 transmission rate). It is a flowchart which shows an example of the process which determines an optimal transmission rate in the time slot | zone which determines a transmission rate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Power line transmission path, 2, 6 ... Transmission part, 3, 7 ... Reception part, 5, 9 ... Transmission control part, 12 ... Serial-parallel conversion part, 14 ... OFDM modulation part, 26 ... OFDM demodulation & equalizer part , 28 ... Parallel to serial converter

Claims (7)

Each subchannel obtained by performing serial-parallel conversion of transmission data into a plurality of subchannels in units of symbols, performing OFDM modulation with the transmission data subjected to serial-parallel conversion, transmitting, demodulating the received signal, and performing OFDM demodulation In an OFDM transmission method for outputting received data by parallel-serial conversion of received data in symbol units,
Using at least some subchannels of the plurality of subchannels and using some symbol periods in the subchannels used, the transmission data is serial-parallel converted in symbol units and the transmission rate is reduced. Depending on the setting to be performed, at least the ratio of the used symbol period is reduced among the ratio of the used subchannel and the ratio of the used symbol period, and the transmission data is not serial-parallel converted. In an arbitrary symbol period in an arbitrary subchannel, OFDM modulation is performed without outputting a signal in a band corresponding to the arbitrary subchannel, and the average transmission power in the band corresponding to the used subchannel is an upper limit regulation value. OFDM modulation depending on the proportion of the symbol period used Raising the level of the signal, and transmits the OFDM modulated signal,
OFDM demodulation of received signal,
For the reception data of each subchannel obtained by OFDM demodulation, the reception data is output by performing parallel / serial conversion opposite to when the transmission data is serial / parallel converted,
An OFDM transmission method characterized by the above. The partial symbol periods used in each of the used subchannels are serial-parallel converted so as to be shifted from each other among the used subchannels.
The OFDM transmission method according to claim 1, wherein: The transmission rate is changed, OFDM modulation is performed using test data instead of the transmission data, and the transmission quality of the received signal of each subchannel obtained by the OFDM demodulation is determined. A transmission rate that maximizes a value obtained by multiplying the number of subchannels determined to be a ratio of the symbol period used in the transmission rate as a transmission rate when transmitting the transmission data is determined.
The OFDM transmission method according to claim 1 or 2, characterized in that Each subchannel obtained by performing serial-parallel conversion of transmission data into a plurality of subchannels in units of symbols, performing OFDM modulation with the transmission data subjected to serial-parallel conversion, transmitting, demodulating the received signal, and performing OFDM demodulation In an OFDM transmission method for outputting received data by parallel-serial conversion of received data in symbol units,
Using some subchannels among the plurality of subchannels, the transmission data is serial-parallel converted in symbol units, and the ratio of the subchannels used is reduced according to the setting for reducing the transmission rate. And, in an arbitrary subchannel where the transmission data is not subjected to serial-parallel conversion, OFDM modulation is performed without outputting a signal of a band corresponding to the arbitrary subchannel, and the OFDM-modulated signal is transmitted,
OFDM demodulation of received signal,
For the reception data of each subchannel obtained by OFDM demodulation, the reception data is output by performing parallel / serial conversion opposite to when the transmission data is serial / parallel converted,
An OFDM transmission method characterized by the above. The transmission rate is changed, OFDM modulation is performed using test data instead of the transmission data, and the transmission quality of the received signal of each subchannel obtained by the OFDM demodulation is determined. Determining a transmission rate that maximizes the number of subchannels determined to be a transmission rate for transmitting the transmission data;
The OFDM transmission method according to claim 4. In an OFDM transmission apparatus that performs serial-parallel conversion of transmission data on a symbol-by-symbol basis to a plurality of subchannels, performs OFDM modulation with transmission data that has been serial-parallel converted, and transmits the data.
Using at least some of the subchannels of the plurality of subchannels, and using some symbol periods in the subchannels used, the transmission data is serial-parallel converted in symbol units, and the transmission data is Series / parallel conversion means for inserting data that does not output a signal of a band corresponding to the arbitrary subchannel in an arbitrary symbol period in an arbitrary subchannel that is not subjected to serial / parallel conversion;
According to the setting for reducing the transmission rate, at least the ratio of the used symbol period among the ratio of the subchannels used in the serial-to-parallel conversion means and the ratio of the used symbol periods is reduced, and Amplified so that the level of the signal output from the OFDM modulation means is increased in accordance with the ratio of the used symbol period within a range where the average transmission power of the used subchannel band does not exceed the upper limit regulation value. Transmission control means for controlling the rate;
OFDM modulation means for performing OFDM modulation with transmission data of each subchannel output from the serial-parallel conversion means;
Amplifying means for amplifying the signal output from the OFDM modulation means at the amplification factor controlled by the transmission control means;
An OFDM transmitter characterized by comprising: In an OFDM transmission apparatus that performs serial-parallel conversion of transmission data on a symbol-by-symbol basis to a plurality of subchannels, performs OFDM modulation with transmission data that has been serial-parallel converted, and transmits the data.
Using some subchannels of the plurality of subchannels, the transmission data is subjected to serial-parallel conversion in units of symbols, and the arbitrary subchannel is used in any subchannel where the transmission data is not subjected to serial-parallel conversion. Serial-to-parallel conversion means for inserting data that does not output a signal in a band corresponding to
Transmission control means for reducing the ratio of the subchannels used in the serial-parallel conversion means according to the setting for reducing the transmission rate;
OFDM modulation means for performing OFDM modulation with transmission data of each subchannel output from the serial-parallel conversion means;
Amplifying means for amplifying the signal output from the OFDM modulation means;
An OFDM transmitter characterized by comprising:

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