JP4138280B2 - OFDM communication device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an OFDM communication apparatus, and more particularly to means for improving communication quality by estimating a carrier-to-noise ratio (hereinafter referred to as CNR) related to a transmission path.
[0002]
[Prior art]
Power line communication uses the power lines that are installed to supply power such as outdoor distribution lines and indoor light lines, and does not require a new communication line to be installed. Since the cost can be reduced, various methods have been studied conventionally. While power line communication has the advantages as described above, a power line with poor transmission characteristics due to noise or the like is used. Therefore, it is necessary to use a communication system that is resistant to noise.
[0003]
Orthogonal Frequency Division Multiplexing (hereinafter referred to as OFDM) is a type of multi-carrier modulation method in which 1-channel data is distributed over a plurality of carriers and transmitted. The data is distributed over a plurality of carriers. Therefore, the probability of loss of all data due to noise is reduced, and therefore, it is known as a communication method suitable for power line communication.
[0004]
FIG. 7 is a functional block diagram showing a configuration example of a conventional OFDM communication apparatus in a power line communication apparatus. The power line communication apparatus shown in this figure uses an OFDM modulation unit 100 as a transmission system through a D / A converter (digital / analog converter) 110 and a low-pass filter 120 as an intermediate frequency / high frequency processing unit (hereinafter referred to as IF / RF). The IF / RF processing unit 130 as a receiving system via an antialiasing filter (low-pass filter) 140 and an A / D converter (analog / digital converter) 150. Connected to 200.
[0005]
The OFDM system is described in detail in, for example, “Makoto Itami, OFDM modulation technology, triceps, March 2000” and the like, so only the main points will be described here.
The OFDM modulation unit 100 includes a symbol mapper 101 that generates transmission data by dispersing the frequency data into a plurality of orthogonal carriers with some overlapping frequency components, and an S / S that converts serial data into parallel data. P conversion circuit 102, Inverse Fast Fourier Transform (hereinafter referred to as IFFT) 103 as an inverse Fourier transform means, P / S conversion circuit 104 for converting parallel data into serial data, and transmission path ( A transmission side guard interval circuit 105 that reduces the influence of multipath due to a reflected wave from the power line branch is sequentially connected.
[0006]
Further, the OFDM demodulator 200 obtains a demodulated signal by the reverse operation of the OFDM modulator 100 described above, so that a reception-side guard interval circuit 201, an S / P converter circuit 202, and a plurality of orthogonal carriers from the received OFDM signal. Fast Fourier Transform (hereinafter referred to as FFT) 203, P / S conversion circuit 204, and symbol demapper 205 for performing predetermined demodulation processing are sequentially connected as Fourier transform means for generating Configure.
[0007]
FIG. 8 is a diagram illustrating a spectrum of a signal output from the symbol mapper 101. In this example, a spectrum in the case of generating an OFDM signal using n carriers is shown, and each spectrum is arranged so as to overlap with a part of an adjacent spectrum in order to increase frequency use efficiency.
[0008]
FIG. 9 is a diagram illustrating an example of an OFDM signal (a signal obtained by multiplexing 16 carriers) output from the transmission-side P / S conversion circuit 104 when 16 (n = 15) carriers are used.
[0009]
Hereinafter, the operation of the OFDM communication apparatus shown in FIG. 7 including the operation of the entire power line communication apparatus will be described with reference to FIGS. First, as an operation of the transmission system, the symbol mapper 101 disperses the transmission data into a plurality of carrier waves having frequency components as shown in FIG. 8 and orthogonal to each other to obtain a predetermined modulated signal (for example, quadrature amplitude modulation (QAM) Alternatively, when phase modulation (PSK) is generated and output, the S / P conversion circuit 102 converts it into a parallel signal.
[0010]
This modulated signal is not guaranteed to be accurately orthogonal due to the occurrence timing shift (phase shift) of each carrier wave, but by converting each carrier wave to a signal in the time domain by IFFT converter 103, It is known that the occurrence timing shift is corrected, and an ideal OFDM signal is output as a multiplexed waveform as shown in FIG. This OFDM signal is converted back to a serial signal by the P / S conversion circuit 104, processed into a signal that is not easily affected by multipath by the transmission side guard interval circuit 105, and the D / A converter 110 and the low-pass filter 120 Is converted to an analog signal from which harmonics have been removed, subjected to predetermined processing in the IF / RF processing unit 130, and then sent to the transmission line.
[0011]
On the other hand, as an operation of the receiving system, an OFDM signal obtained by removing unnecessary waves after being subjected to predetermined processing via an IF / RF processing unit 130, an anti-aliasing filter 140, and an A / D converter 150 and converted into a digital signal is OFDM demodulated. When the signal is input to the unit 200, the transmission side guard interval processing is canceled by the reception side guard interval circuit 201, converted into a parallel signal by the S / P conversion circuit 202, and supplied to the FFT 203. When the FFT 203 generates a plurality of orthogonal carrier waves (modulated signals) as frequency components from this signal and supplies them to the symbol demapper 205 via the P / S converter 204, the transmission data from the modulated signal here. A predetermined demodulation process is performed for reproduction.
[0012]
As shown in FIG. 8, in the OFDM signal, a part of the spectrum of each carrier wave overlaps with the adjacent spectrum, so that each carrier wave cannot be extracted (separated) by a filter. However, as is well known, signals can be separated by utilizing the orthogonality between the carrier waves. Since this is complicated, description is omitted (described in pp. 37-41 of the above document).
[0013]
As described above, since the OFDM signal transmits one channel signal using a plurality of carrier waves, even if data of a specific carrier wave is lost due to noise, it is unlikely that the data of the entire carrier wave is lost. By using a predetermined error correction technique or the like together, information data can be transmitted and received even when the power line is used as a transmission line.
[0014]
[Problems to be solved by the invention]
The conventional multicarrier communication apparatus (OFDM communication apparatus) as described above has the following problems. That is, the power line used as a transmission line generates noise whose level and frequency distribution vary depending on the type, number of connections, usage status, and power line laying status of the electronic equipment connected to the power line. Therefore, a carrier using a frequency with a high CNR characteristic (low noise level) can achieve good communication with few information transmission errors, but a carrier using a frequency with a low CNR characteristic (high noise level) can cause information due to noise. There was a problem that communication quality deteriorated due to frequent transmission errors. In the worst case, communication becomes impossible, and there is a problem that transmission power is wasted.
For such problems, Japanese Patent Laid-Open No. 2000-165304 discloses the selection of a modulation method that increases the transmission rate and reliability with respect to the received SNR (CNR) or the carrier in the power line communication device using the multicarrier method. There is a description about the selection. Japanese Patent Laid-Open No. 2000-216752 describes that in a multicarrier communication apparatus, only a carrier other than a band that is greatly affected by noise is used. However, the above publication does not disclose specific noise evaluation means or CNR evaluation means related to these devices (transmission paths).
In the Japanese Patent Application No. 2001-125916 filed on April 24, 2001, the applicant of the present invention samples a large number of received signals at the output of the Fourier transform means, and calculates noise power from the dispersion characteristics of the average signal (S) and noise components. We have proposed a power line communication device that obtains (N) and performs SNR (CNR) estimation. However, more efficient CNR estimation means has been required to reduce processing time.
[0015]
The present invention has been made to solve the above-described problems related to the conventional multicarrier communication apparatus (OFDM communication apparatus), and provides an OFDM communication apparatus having means for efficiently performing CNR estimation relating to a transmission line such as a power line. The purpose is to do.
[0016]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 of the OFDM communication apparatus according to the present invention receives an OFDM signal having a plurality of preamble signals composed of predetermined fixed data, and receives the received signal. At least two or more of the preamble signals are extracted and connected to generate an extended preamble signal, which is converted into frequency data using a Fourier transform means, whereby the carrier component related to the extended preamble signal and other noise The components are calculated all at once, and the CNR estimation of the carrier component is performed based on the result.
The invention according to claim 2 of the OFDM communication apparatus according to the present invention distributes at least transmission data as a transmission system to a plurality of orthogonal carriers with a part of each frequency component overlapping, and a predetermined coverage based on a plurality of modulation schemes. A symbol mapper that generates a modulation signal; and an inverse Fourier transform unit that multiplexes the modulated signal in the time domain and outputs an OFDM signal, and a Fourier that generates the plurality of orthogonal carriers from the received OFDM signal as a reception system An OFDM communication apparatus comprising conversion means and a symbol demapper that performs predetermined demodulation processing,
The received OFDM signal has a plurality of preamble signals composed of predetermined fixed data, and at least two of the preamble signals are extracted and connected to generate an extended preamble signal, which is then used by Fourier transform means By converting into frequency data, the carrier component related to the extended preamble signal and the other noise components are calculated all at once, and the CNR estimation of the carrier component is performed based on the result.
The invention according to claim 3 of the OFDM communication apparatus according to the present invention is the OFDM communication apparatus according to claim 2, wherein transmission data is transmitted from the plurality of modulation schemes for each carrier based on the CNR value obtained by the CNR estimation. Communication control is performed to select the modulation method that optimizes the rate.
The invention according to claim 4 of the OFDM communication apparatus according to the present invention is the OFDM communication apparatus according to claim 2 or 3, wherein the carrier is not used when the CNR value obtained by the CNR estimation is lower than a predetermined value. Communication control was performed. .
The invention according to claim 5 of the OFDM communication apparatus according to the present invention is the OFDM communication apparatus according to claim 1, claim 2, claim 3 or claim 4, wherein the predetermined number of preamble signals are continuous. Thus, at least two of them are taken out continuously to obtain an extended preamble signal.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on the illustrated embodiment. The OFDM communication apparatus according to the present invention can be used for a power line communication apparatus that is a wired communication system, or a fixed or mobile wireless communication apparatus. As an example, a case where the OFDM communication apparatus is used for a power line communication apparatus will be described.
FIG. 1 is a functional block diagram showing an embodiment when the OFDM communication apparatus according to the present invention is used in a power line communication apparatus. A feature of the present invention is that the receiving system has CNR estimation means to be described later. First, the configuration of the entire apparatus will be described.
The power line communication apparatus shown in this example uses an OFDM modulation unit 10 as a transmission system via a D / A converter (digital / analog converter) 30 and a low-pass filter 40, and an intermediate frequency / high frequency processing unit (hereinafter referred to as IF / RF). And the IF / RF processing unit 50 as a receiving system through an anti-aliasing filter (low-pass filter) 60 and an A / D converter (analog / digital converter) 70. Configured to connect to 20.
[0018]
The OFDM modulation unit 10 includes a symbol mapper 11 that generates a predetermined modulated signal from a plurality of modulation schemes by dispersing transmission data into a plurality of orthogonal carrier waves with each frequency component partially overlapping, and serial data into parallel data. An S / P conversion circuit 12 for conversion, an inverse fast Fourier transform (inverse fast Fourier transform, IFFT) 13 as an inverse Fourier transform means, a P / S conversion circuit 14 for converting parallel data into serial data, A transmission side guard interval circuit 15 that reduces the influence of multipath due to a reflected wave from a transmission line (power line) branch is sequentially connected.
[0019]
Further, the OFDM demodulator 20 obtains a demodulated signal by the reverse operation of the OFDM modulator 10 described above, so that a reception-side guard interval circuit 21, an S / P converter circuit 22, and a plurality of orthogonal carriers from the received OFDM signal. A first fast Fourier transformer (FFT) 23, a P / S conversion circuit 24, and a symbol demapper 25 that performs predetermined demodulation processing are sequentially connected as Fourier transform means for generating In addition, a CNR estimator 26 for estimating the CNR characteristics related to the transmission path (power line) in the subsequent stage of the guard interval circuit 21, and a modulation scheme in the symbol mapper 11 and the symbol demapper 25 based on the estimation result, or used A modulation scheme / carrier control unit 27 for selecting a carrier wave is arranged.
[0020]
The basic operation of the power line communication apparatus including the OFDM modulation / demodulation unit is the same as that of the above-described prior art, and thus description thereof is omitted.
[0021]
In the present invention, the CNR estimator 26 estimates the CNR characteristics related to the transmission path, and selects an optimum scheme from a plurality of modulation schemes (for example, PSK / QPSK / 8PSK / 16PSK) for each carrier based on the estimated value. Thus, the symbol mapper 11 and the symbol demapper 25 are controlled via the modulation scheme / carrier control unit 27.
[0022]
In other words, BPSK (2-level PSK) / QPSK (4-level PSK) / 8PSK (8-level PSK) / 16PSK (16-level PSK) used as the modulation method has 1/2/3 information bits that can be transmitted by one symbol. / 4 bit. Here, as the number of information bits can be transmitted, the signal interval on the signal space diagram (phase shift interval in the PSK system) becomes narrower. For example, the phase shift interval of BPSK is 180 °, 90 ° for QPSK, 45 ° for 8PSK, and the more the number of information bits can be transmitted, the more likely signal errors due to noise occur, and the corresponding communication quality is obtained accordingly. Therefore, a high CNR characteristic (low noise characteristic) is required. On the other hand, BPSK cannot transmit a larger number of information bits per symbol, but the phase shift interval is 180 ° and transmission errors due to noise are small. Therefore, even with low CNR characteristics, communication quality is not hindered.
[0023]
Therefore, based on the estimated CNR value, BPSK with a small number of information bits that can be transmitted on a carrier with a low CNR value but a small transmission error is used, and 16PSK with a large number of transmission information bits is selected on a carrier with a high CNR value. Try to improve the transmission rate.
[0024]
As a result of estimation by the SNR estimation unit 26, if the CNR value of the carrier wave is smaller than the predetermined value, the communication quality deteriorates and communication becomes impossible.In this case, in order to improve the waste of transmission power, The symbol mapper 11 is controlled via the modulation scheme / carrier control unit 27 so as to turn off the carrier wave.
[0025]
Next, CNR estimation in the CNR estimation unit 26 characterizing the present invention will be described in detail. FIG. 2 is a functional block diagram showing a configuration example of a CNR estimation unit used in the OFDM communication apparatus according to the present invention. The CNR estimation unit 26 shown in this example sequentially connects a gate circuit 261 that extracts and concatenates a preamble signal described later from the received OFDM signal to generate an extended preamble signal, a second FFT 262, and a CNR calculation unit 263. The noise power detector 264 is arranged in parallel between the second FFT 262 and the CNR calculation unit 263.
[0026]
Prior to the description of the operation of the CNR estimation unit 26 shown in FIG. 2, first, the principle of CNR estimation and a transmission signal configuration example related thereto will be described. FIG. 3 is a diagram for explaining the principle of this CNR estimation. (A) in the figure shows a transmission preamble signal in which a guard interval signal is placed at the head and then n bits of fixed data are arranged in advance, and (b) in FIG. 4 shows this fixed data is 4 bits (n = 4). The FFT output signals when the guard interval signal GI is removed are shown respectively. Further, FIGS. 7C and 7D respectively show a signal format and an FFT output signal when two preamble signals of FIG. 9A are connected.
[0027]
The number of bits of the fixed data is set so as to coincide with the number of bits constituting the OFDM symbol. For example, when configured to 4 bits, the FFT output signal includes four carrier waves as shown in FIG. Are arranged at intervals of Δf. On the other hand, when two identical preamble signals are concatenated as shown in (c) of the figure, the number of bits of the signal is doubled to 8 bits. The frequency interval is Δf / 2 as shown in FIG.
[0028]
At this time, since the fixed data of 4 bits is repeated, each modulated signal does not vary from a constant value on the signal space diagram and is held in an unmodulated state (continuous sine wave signals are repeated). As is well known, since such a continuous sine wave signal is observed as a line spectrum, the spectrum spread as shown in FIG. 9 does not occur in FIG.
[0029]
By the way, since the frequencies f0 to f3 generated by the carrier waves C0 to C3 shown in FIG. 5B are determined in advance, the spectrum generated at other frequencies can be regarded as a noise component. However, as shown in the figure (d), noise components (N0, N2, N4, N6) are superimposed on each carrier, so the observed signals are C0 + N0, C1 + N2, C2 + N4, C3 + If it becomes N6 and each noise component level can be specified (estimated), the level of only the carrier wave can be specified. Therefore, when the noise level is evaluated as an average noise level, in this example, four noise components N1, N3, N5, and N7 are observed separately from the carrier wave, so each level is expressed as n1, n2, n3, and n4. Then average noise level = (n1 + n3 + n5 + n7) / 4 (1)
Can be evaluated as
[0030]
Therefore, the average noise level is calculated from the observation level (C0 + N0, C1 + N2, C2 + N4, C3 + N6) of each carrier, assuming that the noise represented by equation (1) is superimposed on each carrier. Is subtracted to estimate the carrier level. As a result, the carrier level and the noise level are calculated, so that the CNR can be estimated.
[0031]
FIG. 4 is a diagram showing a configuration example of a transmission signal used in the OFDM communication apparatus according to the present invention. In the transmission signal shown in this example, the preamble signal described above is arranged at the head portion of each frame, and then a predetermined number of OFDM symbol data are arranged.
[0032]
Hereinafter, the operation of the CNR estimation unit 26 shown in FIG. 2 will be described with reference to FIG. 3 and FIG. 4.For example, Japanese Patent No. 2722922 or a patent application filed by the same applicant as the present application (Japanese Patent Application No. 2001-201585). The symbol timing synchronization is already established based on
[0033]
First, when the transmission OFDM signal shown in FIG. 4 is input to the reception system, the guard signal GI is removed by the guard interval circuit 21 arranged in the preceding stage of the CNR estimation unit 26, and then supplied to the gate circuit 261. The gate circuit 261 counts the number of input OFDM symbols, extracts the preamble signals for each predetermined count, combines them, generates an extended preamble signal (see FIG. 4), and outputs it to the FFT 262. As shown in FIG. 3, from the FFT262, the frequency spectrum related to this extended preamble signal is a carrier (C0 + N0, C1 + N2, C2 + N4, C3 + N6) and noise components (N1, N3, N5, N7). In the noise power detector 264, the average noise level represented by the above equation (1) is calculated, and the CNR calculation unit 263 calculates the carrier level by the procedure described above. Estimate the CNR for each carrier.
[0034]
In this embodiment, the preamble signal is arranged at the head of each frame. In short, it is only necessary that two or more preamble signals are connected to generate an extended preamble signal, so that the preamble signal can be generated at an arbitrary position in the frame. May be arranged.
[0035]
Further, the transmission signal may be configured such that a concatenation of two or more preamble signals from the beginning is arranged at the head of each frame. FIG. 5 is a diagram showing another configuration example of a transmission signal used in the OFDM communication apparatus according to the present invention. In the transmission signal shown in this example, two preamble signals are continuously arranged at the head of the frame. If such a signal is used, it is not necessary to connect the preamble signals again in the gate circuit 261, so the processing in the gate circuit 261 becomes lighter, and therefore the hardware configuration of the gate circuit 261 can be simplified. is there.
[0036]
Note that the accuracy of CNR estimation can be further improved by increasing the number of preamble signals that constitute the extended preamble signal. FIG. 6 is a diagram for explaining an advantage of using an extended preamble signal in which the number of preamble signals is increased to three in the OFDM communication apparatus according to the present invention. FIG. 4A shows an extended preamble signal composed of three preamble signals, and FIG. 4B shows an FFT output signal when the guard interval signal GI is removed from the extended preamble signal. In this case, since the number of bits of the preamble signal is increased as compared with those shown in FIGS. 3 (c) and (d), the spectrum interval is further narrowed to Δf / 3 and can be observed from the above-mentioned FFT characteristics. The number of noise components increases.
[0037]
Here, assuming that the noise power is constant, the spectrum levels of the observation noises N0 to N11 are lower than those in FIG. 3 (d). Accordingly, the levels of the noise components (N0, N3, N6, N9) superimposed on each carrier wave are also reduced, and the influence of noise level estimation on the carrier wave is reduced accordingly, so that the accuracy of the CNR estimation value is improved.
[0038]
As described above, since the OFDM communication apparatus according to the present invention operates, it is possible to select an optimum modulation method according to the CNR characteristics of each carrier wave, thereby improving communication quality degradation and communication. Since a carrier wave that cannot be used for the first time is not used from the beginning, useless transmission power consumption can be prevented.
[0039]
In addition, when performing the modulation method control and the carrier wave OFF control described above based on the CNR information obtained on the receiving side, it is necessary to report information related to this to the transmission partner side device. In addition to the preamble signal, necessary information may be arranged in the header portion).
[0040]
【The invention's effect】
As described above, the present invention estimates the CNR characteristics related to the transmission path and selects the optimum modulation method for each carrier wave, so that the deterioration of communication quality can be improved, and the carrier wave that becomes unable to communicate is first selected. Therefore, it is effective in realizing an OFDM communication apparatus that can improve waste of transmission power.
[Brief description of the drawings]
FIG. 1 is a functional block diagram showing an embodiment of an OFDM apparatus according to the present invention. FIG. 2 is a functional block diagram showing a CNR estimation unit used in the OFDM apparatus according to the present invention. FIG. 4 is a diagram illustrating a configuration example of a transmission signal used in an OFDM apparatus according to the present invention. FIG. 5 is another diagram of a transmission signal used in the OFDM apparatus according to the present invention. FIG. 6 is a diagram illustrating an advantage of using an extended preamble signal with an increased number of preamble signals in the OFDM device according to the present invention. FIG. 7 is a functional block diagram illustrating a configuration example of a conventional OFDM device. 8] Diagram explaining the spectrum of OFDM signal [Fig. 9] Schematic diagram showing multiplexed waveform of OFDM signal using 16 carriers [Explanation of symbols]
11 .. Symbol mapper
12. ・ S / P converter
13. ・ High-speed inverse Fourier transformer
14.P / S converter
15. ・ Transmission side guard interval circuit
21 ・ ・ Guard interval circuit on the receiving side
22. ・ S / P converter
23..First Fast Fourier Transform
24 ・ ・ P / S converter
25. Symbol demapper
26 ・ ・ CNR estimation part
27 ・ ・ Modulation system / Carrier control unit

Claims (5)

An OFDM signal having a plurality of preamble signals composed of predetermined fixed data is received, and at least two or more preamble signals are extracted from the received signals and connected to generate an extended preamble signal, which is Fourier transformed By converting into frequency data using a conversion means, the carrier component related to the extended preamble signal and other noise components are calculated in a lump, and based on this result, CNR estimation of each carrier component is performed. A featured OFDM communication device. As a transmission system, at least transmission data is distributed to a plurality of orthogonal carriers with each frequency component partially overlapping, and a symbol mapper for generating a predetermined modulated signal based on a plurality of modulation schemes, and the modulated signal in the time domain And an inverse Fourier transform means for multiplexing and outputting an OFDM signal, a Fourier transform means for generating a plurality of orthogonal carriers from the received OFDM signal as a receiving system, and a symbol demapper for performing a predetermined demodulation process An OFDM communication device,
The received OFDM signal has a plurality of preamble signals composed of predetermined fixed data, and at least two of the preamble signals are extracted and connected to generate an extended preamble signal, which is then used by Fourier transform means By converting the frequency data into the frequency data, the carrier component related to the extended preamble signal and other noise components are calculated at once, and the CNR estimation of each carrier component is performed based on the result. Communication device. 3. The OFDM communication according to claim 2, wherein communication control is performed to select a modulation scheme that optimizes a transmission rate of transmission data from the plurality of modulation schemes for each carrier based on a CNR value obtained by the CNR estimation. apparatus. 4. The OFDM communication apparatus according to claim 2, wherein communication control is performed so that the carrier wave is not used when a CNR value obtained by the CNR estimation is lower than a predetermined value. The predetermined number of the plurality of preamble signals are continuous, and at least two of them are extracted so as to be an extended preamble signal. The OFDM communication apparatus described.

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