JP3796420B2 - OFDM receiver circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an OFDM receiving circuit that receives and demodulates an OFDM (Orthogonal Frequency Division Multiplexing) signal mainly used for wireless communication, and more particularly to an OFDM receiving circuit that realizes high-speed signal transmission and high frequency utilization efficiency.
[0002]
[Prior art]
The conventional OFDM receiving circuit is configured as shown in FIG. 4 or FIG.
In the case of the OFDM receiving circuit shown in FIG. 4, the received OFDM signal is converted into a baseband complex OFDM signal by the carrier signal output from the oscillator 52 in the quadrature detector 51.
[0003]
The baseband complex OFDM signal is converted into a digital signal by A / D converters 55 and 56 after unnecessary wave components are removed by filters (LPF) 53 and 54.
The digital baseband complex OFDM signals output from the A / D converters 55 and 56 are subjected to discrete Fourier transform by a Fourier transformer (FFT) 57 and separated into modulated signals for each subcarrier. The modulated signal for each subcarrier is demodulated by a subcarrier demodulator 58. Data transmitted on the transmission side is obtained at the output of the subcarrier demodulator 58.
[0004]
In the OFDM receiving circuit shown in FIG. 5, the quadrature detection circuit is digitized. In the case of this OFDM receiving circuit, the received OFDM signal is frequency-converted by the frequency converter 71 using the local oscillation signal output from the oscillator 72 and converted to a carrier frequency for digital quadrature detection.
The OFDM reception signal frequency-converted by the frequency converter 71 is filtered by the A / D converter 74 after removing unnecessary wave components through the filter 73.
[0005]
The digital OFDM reception signal output from the A / D converter 74 is quadrature detected by a digital quadrature detector 75. A digital baseband complex OFDM signal is obtained at the output of the digital quadrature detector 75.
The baseband complex OFDM signal is discrete Fourier transformed by a Fourier transformer 76 and separated into modulated signals for each subcarrier. The modulated signal for each subcarrier is demodulated by a subcarrier demodulator 77. Data transmitted on the transmission side is obtained at the output of the subcarrier demodulator 77.
[0006]
[Problems to be solved by the invention]
In the OFDM receiving circuit as described above, a filter (LPF) for removing harmonics which are unnecessary waves is used to avoid aliasing before A / D conversion.
Further, in the conventional general OFDM receiving circuit, it is assumed that harmonics are removed using a filter having a steep filter characteristic as shown in FIG.
[0007]
However, in reality, a filter having such a steep filter characteristic cannot be realized. Therefore, in practice, a specific carrier located outside of a large number of subcarriers included in the OFDM signal is not used.
Thereby, since the substantial frequency interval with an adjacent channel or an adjacent band can be expanded, the characteristic requested | required of a filter is eased. However, since a specific carrier located outside cannot be used, frequency utilization efficiency is sacrificed. It is difficult to achieve both high frequency utilization efficiency and a steep filter feasibility.
[0008]
On the other hand, by causing the A / D converter to perform an oversampling operation and increasing the number of calculation points of Fourier transform (FFT), it is possible to avoid interference due to aliasing of unnecessary waves. In that case, the characteristics of the filter used to remove unwanted waves are relaxed as shown in FIG.
However, when transmitting a high-speed signal as an OFDM signal, it is necessary to use an A / D converter with a remarkably high operating speed in order to perform an oversampling operation of the A / D converter. There is a problem in terms of sex.
[0009]
In addition, high-speed signal transmission is possible by using a plurality of OFDM signals whose frequencies are shifted from each other in parallel. However, in this case, it is necessary to provide a guard band as shown in FIG. 6C so that a plurality of OFDM signals do not interfere with each other. If the guard band is provided, it is inevitable that the frequency utilization efficiency is lowered. An object of the present invention is to provide an OFDM receiver circuit capable of realizing both high-speed signal transmission and high frequency utilization efficiency.
[0010]
[Means for Solving the Problems]
An OFDM receiver circuit according to claim 1 includes: a demultiplexer that demultiplexes a received OFDM signal into a plurality of N sequences; N oscillators that output carrier signals whose frequency synchronization has been established; and the N oscillators. A baseband complex OFDM by inputting a reference signal oscillator for providing a common reference signal, each of N series of OFDM signals output from the duplexer, and a signal output from each of the N oscillators. N quadrature detectors for outputting signals, N pairs of filters for removing unnecessary wave components from the complex OFDM signal output by the quadrature detector, and N for converting the complex OFDM signal output by the filter into a digital signal A pair of A / D converters and a digital complex OFDM signal output from the pair of A / D converters are input to perform discrete Fourier transform, and a modulation signal for each subcarrier is obtained. In the OFDM receiving circuit, the N oscillators are provided with N Fourier transformers to be operated and a subcarrier demodulator for demodulating a modulation signal for each subcarrier output from the Fourier transformer. A carrier signal having a frequency interval corresponding to M times the subcarrier interval included in the received OFDM signal (M is a natural number) is output and output, and a band in which the N Fourier transformers perform discrete Fourier transform is determined based on the frequency interval. It is also characterized by widening.
[0011]
In claim 1, the band of the OFDM signal to be received is divided and the signal is processed for each band. As a result, it is possible to avoid the operation speed of the A / D converter from becoming extremely high.
However, as described above (see FIG. 6 (c)), it is necessary to provide a guard band so that a plurality of OFDM signals do not interfere with each other only by dividing the band of the OFDM signal on the transmission side. There is no problem.
[0012]
Therefore, in claim 1, the interval between the OFDM signals to be dividedly received is set to M times the subcarrier interval of the OFDM signal (M is a natural number), and consideration is given so that the OFDM signals do not overlap each other. Furthermore, frequency synchronization is established among a plurality of oscillators that generate carriers for orthogonal detection of a plurality of divided OFDM signals. Thereby, it is possible to divide the OFDM signal into a plurality of parts and process as if one broadband OFDM signal was demodulated. Therefore, it is not necessary to install a guard band, and high frequency utilization efficiency can be realized.
[0013]
An OFDM receiver circuit according to claim 2 includes: a demultiplexer that demultiplexes a received OFDM signal into a plurality of N series; N oscillators that output carrier signals that have established frequency synchronization with each other; and the N oscillators. N frequencies for frequency-converting each of the reference signal oscillator for providing a common reference signal and the N series of OFDM signals output from the duplexer using signals output from the N oscillators A converter, N filters for removing unnecessary wave components from the OFDM signal output from the frequency converter, N A / D converters for converting the OFDM signal output from the filter into a digital signal, The digital OFDM signal output from the A / D converter is input, digital quadrature detection is performed, and N digital signals that output a baseband complex OFDM signal are output. An AC detector, a discrete Fourier transform of the complex OFDM signal output from the digital quadrature detector and N modulated Fourier transform signals for each subcarrier, and a subcarrier output from the Fourier transformer. In the OFDM receiving circuit provided with a subcarrier demodulator for demodulating the modulation signal, the N oscillators correspond to M times the subcarrier interval (M is a natural number) included in the received OFDM signal. A carrier signal having a frequency interval is output and output, and a band in which the N Fourier transformers perform discrete Fourier transform is wider than the frequency interval.
[0014]
In the second aspect, similarly to the first aspect, the band of the OFDM signal to be received is divided and the signal is processed for each band. As a result, it is possible to avoid the operation speed of the A / D converter from becoming extremely high. Further, the interval between the OFDM signals to be divided and received is set to M times the subcarrier interval of the OFDM signal (M is a natural number), and consideration is given so that the OFDM signals do not overlap each other.
[0015]
Further, frequency synchronization is established among a plurality of oscillators that generate local oscillation signals used for frequency conversion of the plurality of divided OFDM signals. Thereby, it is possible to divide the OFDM signal into a plurality of parts and process as if one broadband OFDM signal was demodulated. Therefore, it is not necessary to install a guard band, and high frequency utilization efficiency can be realized.
[0016]
In the OFDM receiver circuit according to claim 1 or 2, a reference signal oscillator for providing a common reference signal to the N oscillators is provided.
By commonly using one reference signal output from the reference signal oscillator among the N oscillators, the frequencies of the signals output from the N oscillators can be synchronized with each other. For example, by configuring each oscillator using a PLL (phase-locked loop) circuit, signals having different frequencies and constant frequency intervals can be output from the N oscillators.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
One embodiment of the OFDM receiving circuit of the present invention will be described with reference to FIGS. This form corresponds to claim 1.
FIG. 1 is a block diagram showing an OFDM receiving circuit of this form. FIG. 3 is a spectrum diagram showing the spectrum arrangement of a signal of this form.
[0018]
In this embodiment, the duplexer, the oscillator, the quadrature detector, the filter, the A / D (analog / digital) converter, the Fourier transformer, and the subcarrier demodulator according to claim 1 are respectively connected to the duplexer 11 and the oscillator (14). 15), quadrature detectors (12, 13), low-pass filters (16-19), A / D converters (20-23), Fourier transformers (24, 25), and subcarrier demodulator 26. The reference signal oscillator corresponds to the reference oscillator 27.
[0019]
Referring to FIG. 1, this OFDM receiver circuit includes a duplexer 11, quadrature detectors 12, 13, oscillators 14, 15, low-pass filters 16-19, A / D converters 20-23, Fourier transformers 24, 25, A subcarrier demodulator 26 and a reference oscillator 27 are provided.
In this embodiment, it is assumed that the received OFDM signal is divided into two systems and the signals are processed by the two systems. That is, as shown in the areas RL and RU shown by hatching in FIGS. 3A and 3B, the received signal band is divided into two upper and lower areas, and the signal is processed for each band.
[0020]
The demultiplexer 11 demultiplexes the received OFDM signal into two and outputs OFDM signals to the two signal processing systems. That is, one OFDM signal is input to the quadrature detector 12 and the other OFDM signal is input to the quadrature detector 13.
The quadrature detectors 12 and 13 perform quadrature detection of the OFDM signal using the carrier signals output from the oscillators 14 and 15, respectively. By this orthogonal detection, a baseband complex OFDM signal is obtained. That is, OFDM signals composed of a real component (I signal) and an imaginary component (Q signal) are output from the quadrature detectors 12 and 13, respectively.
[0021]
A predetermined frequency difference Δfd exists between the carrier signal output from the oscillator 14 and the carrier signal output from the oscillator 15. This frequency difference Δfd is an integral multiple of the subcarrier interval in the received OFDM signal. Further, the frequency difference Δfd is determined so that the divided signals of each band do not overlap each other.
[0022]
Further, frequency synchronization of the carrier signal is established between the two oscillators 14 and 15 so that the frequency difference Δfd does not fluctuate. In this example, the oscillators 14 and 15 are constituted by PLL circuits, and the reference signal output from the reference oscillator 27 is given to the two oscillators 14 and 15 as a common reference signal.
The baseband complex OFDM signal output from the quadrature detector 12 is input to the low-pass filters 16 and 17 in order to remove unnecessary wave components (harmonics). Similarly, the baseband complex OFDM signal output from the quadrature detector 13 is input to the low-pass filters 18 and 19 in order to remove unnecessary wave components.
[0023]
The complex OFDM signals output from the low-pass filters 16 and 17 are converted into digital signals by the A / D converters 20 and 21, and the complex OFDM signals output from the low-pass filters 18 and 19 are converted to the A / D converters 22, 23 is converted to a digital signal.
Further, the digital complex OFDM signals output from the A / D converters 20 and 21 are Fourier-transformed by the Fourier transformer 24 and separated into modulated signals for each subcarrier. Similarly, digital complex OFDM signals output from the A / D converters 22 and 23 are Fourier-transformed by a Fourier transformer 25 and separated into modulated signals for each subcarrier.
[0024]
The modulation signal output from the Fourier transformer 24 is a component of each subcarrier included in the half region RL shown in FIG. 3 in the received OFDM signal, and the modulation signal output from the Fourier transformer 25 is It is the component of each subcarrier included in the half region RU shown in FIG. 3 in the received OFDM signal.
The modulation signal of each subcarrier output from the Fourier transformers 24 and 25 is demodulated by the subcarrier demodulator 26 and output from the subcarrier demodulator 26 as output data.
[0025]
In this example, the number of calculation points (FFT window) in the Fourier transformers 24 and 25 is made larger than the band of each of the divided regions RL and RU. Therefore, the filter characteristics required for the low-pass filters 16 to 19 for removing unnecessary waves may be moderate characteristics as shown in FIG.
The OFDM receiving circuit in FIG. 1 can receive a broadband OFDM signal that combines the two regions RL and RU shown in FIG. In addition, since it is not necessary to provide a guard band between the two regions RL and RU, high frequency utilization efficiency is realized.
[0026]
Of course, it is also possible to divide the band of the received OFDM signal into three or more regions and process the signal for each region.
(Second Embodiment)
Another embodiment of the OFDM receiving circuit of the present invention will be described with reference to FIG. This form corresponds to claim 2.
[0027]
FIG. 2 is a block diagram showing an OFDM receiving circuit of this form. This embodiment is a modification of the first embodiment, and the configuration is changed so that quadrature detection is performed by a digital circuit.
In this embodiment, the duplexer, the oscillator, the frequency converter, the filter, the A / D converter, the digital quadrature detector, the Fourier transformer, and the subcarrier demodulator according to claim 2 are respectively connected to the duplexer 31 and the oscillator (34). 35), frequency converter (32, 33), low pass filter (36, 37), A / D converter (38, 39), digital quadrature detector (40, 41), Fourier transformer (42, 43). And the subcarrier demodulator 44. The reference signal oscillator corresponds to the reference oscillator 45.
[0028]
Referring to FIG. 2, the OFDM receiving circuit includes a duplexer 31, frequency converters 32 and 33, oscillators 34 and 35, low-pass filters 36 and 37, A / D converters 38 and 39, and digital quadrature detectors 40 and 41. , Fourier transformers 42 and 43, a subcarrier demodulator 44, and a reference oscillator 45.
In this embodiment, it is assumed that the received OFDM signal is divided into two systems as in the first embodiment, and the signals are processed by the two systems, respectively. That is, as shown in the areas RL and RU shown by hatching in FIGS. 3A and 3B, the received signal band is divided into two upper and lower areas, and the signal is processed for each band.
[0029]
The demultiplexer 31 demultiplexes the received OFDM signal into two and outputs the OFDM signals to the two signal processing systems, respectively. That is, one OFDM signal is input to the frequency converter 32 and the other OFDM signal is input to the frequency converter 33.
The frequency converters 32 and 33 use the carrier signals output from the oscillators 34 and 35, respectively, to convert the frequency of the OFDM signal into a carrier frequency used in digital quadrature detection.
[0030]
A predetermined frequency difference Δfd exists between the carrier signal output from the oscillator 34 and the carrier signal output from the oscillator 35. This frequency difference Δfd is an integral multiple of the subcarrier interval in the received OFDM signal. Further, the frequency difference Δfd is determined so that the divided signals of each band do not overlap each other.
[0031]
Further, frequency synchronization of the carrier signal is established between the two oscillators 34 and 35 so that the frequency difference Δfd does not fluctuate. In this example, the oscillators 34 and 35 are composed of PLL circuits, and the reference signal output from the reference oscillator 45 is given to the two oscillators 34 and 35 as a common reference signal.
The OFDM signal frequency-converted by the frequency converter 32 is converted into a digital signal by an A / D converter 38 after an unnecessary wave component is removed by a low-pass filter 36. Similarly, the OFDM signal frequency-converted by the frequency converter 33 is converted to a digital signal by the A / D converter 39 after the unnecessary wave component is removed by the low-pass filter 37.
[0032]
The digital OFDM signal output from the A / D converter 38 is quadrature detected by the digital quadrature detector 40 to become a digital baseband complex OFDM signal. The digital OFDM signal output from the A / D converter 39 is quadrature detected by a digital quadrature detector 41 to become a digital baseband complex OFDM signal.
[0033]
Further, the digital complex OFDM signal output from the digital quadrature detector 40 is Fourier transformed by a Fourier transformer 42 and separated into modulated signals for each subcarrier. Similarly, a digital complex OFDM signal output from the digital quadrature detector 41 is Fourier-transformed by a Fourier transformer 43 and separated into modulated signals for each subcarrier.
[0034]
The modulation signal output from the Fourier transformer 42 is a component of each subcarrier included in the half region RL shown in FIG. 3 in the received OFDM signal, and the modulation signal output from the Fourier transformer 43 is It is the component of each subcarrier included in the half region RU shown in FIG. 3 in the received OFDM signal.
The modulated signal of each subcarrier output from the Fourier transformers 42 and 43 is demodulated by the subcarrier demodulator 44 and output from the subcarrier demodulator 44 as output data.
[0035]
In this example, the number of calculation points (FFT window) in the Fourier transformers 42 and 43 is made larger than the band of each of the divided regions RL and RU. Therefore, the filter characteristics required for the low-pass filters 36 and 37 for removing unnecessary waves may be moderate characteristics as shown in FIG.
[0036]
The OFDM receiver circuit of FIG. 2 can receive a wideband OFDM signal that combines the two regions RL and RU shown in FIG. In addition, since it is not necessary to provide a guard band between the two regions RL and RU, high frequency utilization efficiency is realized.
[0037]
【The invention's effect】
As described above, according to the OFDM receiver circuit of the present invention, high-speed signal transmission and high frequency utilization efficiency can be realized without significantly increasing the operation speed of the A / D converter.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating an OFDM receiver circuit according to a first embodiment.
FIG. 2 is a block diagram illustrating an OFDM receiver circuit according to a second embodiment.
FIG. 3 is a spectrum diagram showing a spectrum arrangement of a signal according to the first embodiment.
FIG. 4 is a block diagram showing a conventional OFDM receiving circuit.
FIG. 5 is a block diagram showing a conventional OFDM receiving circuit.
FIG. 6 is a spectrum diagram showing a spectrum arrangement of a signal of a conventional example.
[Explanation of symbols]
11 demultiplexer 12, 13 quadrature detector 14, 15 oscillator 16, 17, 18, 19 low pass filter 20, 21, 22, 23 A / D converter 24, 25 Fourier transformer 26 subcarrier demodulator 27 reference oscillator 31 Demultiplexer 32, 33 Frequency converter 34, 35 Oscillator 36, 37 Low-pass filter 38, 39 A / D converter 40, 41 Digital quadrature detector 42, 43 Fourier transformer 44 Subcarrier demodulator 45 Reference oscillator

Claims (2)

A demultiplexer for demultiplexing the received OFDM signal into a plurality of N series;
N oscillators that output carrier signals whose frequency synchronization is established with each other;
A reference signal oscillator for providing a common reference signal to the N oscillators;
N quadrature detectors for inputting each of the N-sequence OFDM signals output from the demultiplexer and the signals output from the N oscillators and outputting a baseband complex OFDM signal; ,
N pairs of filters for removing unnecessary wave components from the complex OFDM signal output by the quadrature detector;
N pairs of A / D converters for converting the complex OFDM signal output from the filter into a digital signal;
N Fourier transformers that input a digital complex OFDM signal output by the pair of A / D converters, perform a discrete Fourier transform, and output a modulation signal for each subcarrier;
An OFDM receiver circuit comprising: a subcarrier demodulator that demodulates a modulation signal for each subcarrier output by the Fourier transformer;
The N oscillators output carrier signals having frequency intervals corresponding to M times (M is a natural number) a subcarrier interval included in the received OFDM signal,
An OFDM receiving circuit, wherein a band in which the N Fourier transformers perform discrete Fourier transform is wider than the frequency interval. A demultiplexer for demultiplexing the received OFDM signal into a plurality of N series;
N oscillators that output carrier signals whose frequency synchronization is established with each other;
A reference signal oscillator for providing a common reference signal to the N oscillators;
N frequency converters for frequency-converting each of the N-sequence OFDM signals output from the duplexer using signals output from the N oscillators,
N filters for removing unnecessary wave components from the OFDM signal output from the frequency converter;
N A / D converters for converting the OFDM signal output from the filter into a digital signal;
N digital quadrature detectors that input a digital OFDM signal output from the A / D converter, perform digital quadrature detection, and output a baseband complex OFDM signal;
N Fourier transformers that perform discrete Fourier transform on the complex OFDM signal output from the digital quadrature detector and output a modulated signal for each subcarrier;
An OFDM receiver circuit comprising: a subcarrier demodulator that demodulates a modulation signal for each subcarrier output from the Fourier transformer;
The N oscillators output carrier signals having frequency intervals corresponding to M times (M is a natural number) a subcarrier interval included in the received OFDM signal,
An OFDM receiving circuit, wherein a band in which the N Fourier transformers perform discrete Fourier transform is wider than the frequency interval.

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